1

CHEMISTRY OF STRAW BASED, CHEMI-THERMO-MECHANICAL PULPING BLACK LIQUOR AND ITS

CHEMICAL AND BIOLOGICAL TREATMENT

Muhammad Ikram Reg. No. 24-PhD-CHEM-2004

Session 2004-2008

DEPARTMENT OF CHEMISTRY

GC UNIVERSITY, LAHORE.

2

CHEMISTRY OF STRAW BASED, CHEMI-THERMO- MECHANICAL PULPING BLACK LIQUOR AND ITS

CHEMICAL AND BIOLOGICAL TREATMENT

Submitted to GC University, Lahore in partial fulfillment of the requirements for the award of degree of

DOCTOR OF PHILOSOPHY IN

CHEMISTRY

By

Muhammad Ikram REG. 24 – PhD-CHEM – 2004.

DEPARTMENT OF CHEMISTRY

GC UNIVERSITY, LAHORE.

3

Certified that Mr. Muhammad Ikram (Reg. No. 24-PhD-CHEM-2004, Ph.D. Chemistry) has completed the research work contained in this thesis entitled “CHEMISTRY OF STRAW BASED, CHEMI-THERMO-MECHANICAL PULPING BLACK LIQUOR AND ITS CHEMICAL AND BIOLOGICAL TREATMENT ” under my supervision in laboratories of Chemistry, Government College University, Lahore and Research and Development Laboratories of Packages Limited Lahore. Dated: March 26, 2007 Research Supervisor, Co-Supervisor, Dr. Babar Ali Dr. Muhammad Akram Kashmiri Development Manager, Chairperson, Department of Chemistry, Packages Ltd. Lahore GC University, Lahore. Submitted through Dr. Muhammad Akram Kashmiri Controller of Examinations, Chairperson, Department of Chemistry, GC University, Lahore. GC University, Lahore.

4

I, Mr. Muhammad Ikram, Reg. No. 24-PhD-CHEM-2004, student of

Ph.D. in Chemistry, hereby declare that the matter printed in this

thesis entitled,

“CHEMISTRY OF STRAW BASED, CHEMI-THERMO- MECHANICAL PULPING BLACK LIQUOR AND ITS CHEMICAL AND BIOLOGICAL TREATMENT” is my own work and has not been printed, published and submitted

as research work, thesis or publication in any form in any University,

Research institution or Journal etc., in Pakistan or abroad.

Dated: March 26, 2007 Muhammad Ikram Department of Chemistry GC University, Lahore.

5

6

All praises and thanks are for Almighty Allah, the source of all knowledge

and wisdom endowed to mankind, Who guides us in darkness and helps us in

7

difficulties; and, all respects are for His last Holy Prophet Muhammad (Peace Be

Upon Him) who enabled us to recognize our Creator.

I am grateful for the efforts and vision of worthy Vice Chancellor Prof. Dr.

Khalid Aftab for providing a platform of research and academic career.

I am thankful for Higher Education Commission of Pakistan for providing

funds for my PhD research. Without this, it really would not have been possible.

I have sense of obligation to the reverend teacher and my supervisor, Dr.

Muhammad Akram Kashmiri (Chairperson, Department of Chemistry), for

providing me an opportunity to learn and prosper. I am also thankful for his

keen interest and valuable suggestions during my studies and research work.

I have the honour to express my heartiest gratitude and indebtedness to

Dr. Babar Ali (Manager Research and Development Packages Limited) for his

skillful suggestions during the course work and research project. He encouraged

me to drill out the facts while working at Packages Research Laboratories.

I can also boast of in having the patronage of Higher Management of

Packages Limited. It will be a privilege for me to mention the names of Dr. Amir

Said (Packages Limited Lahore) and Mrs. Asma Javed (HR Manager Packages

Limited) who helped me to conduct my PhD while I am part of packages.

I would like to take this opportunity to extend my thanks to Mr.

Muhammad Hussain. His keen and painstaking care during this project made

me able to complete this work with dignity, serenity and sagacity.

I can’t neglect the affectionate and loving suggestions of Dr. Ahmad

Adnan and Dr. Waheed Ahmad, Dr. Iftikhar Ahmad and Dr. Zahid Qureshi who

helped me in various issues in carrying out my research at Biochemistry

Research laboratories of GC University Lahore.

8

I really appreciate the co-operation and support of Sir Javed Iqbal Lodhi,

Ishtiaq-ur-Rehman Sahib, Mr. Atif Zia, Mr. Waqar, Madam Samina Bano,

Khadija Shibbiri and unlimited list of respected fellows, friends and relatives.

In end, I am thankful to every body, who gave me tough time in my life

and I always took it as a challenge for reaching to even better stage of knowledge

and prosperity.

Muhammad Ikram Aujla.

9

Following research papers have been extracted from this research work and

have been selected to forward for publications.

1. M. Ikram and B. Ali. “Desilication of the green liquor, Modern plant trial

strategies” IPPTA journal Volume 18 (1), January 2006, page 39-43.

2. M. Ikram, M. Akram Kashmiri and B. Ali. “ Chemistry of wheat and rice

straw based CTMP Black liquor” TAPPI Journal, Vol 6; No 4, April 2007,

pp. 1-6.

3. Muhammad Ikram, M. Akram Kashmiri and Babar Ali. “Determination of

chemical changes in the straw based black liquor during its storage at high

temperatures” Proceedings of 56th Chemical Engineer Conference Canada,

Oct 15-18, 2006, Ref # 388.

4. Muhammad Ikram, Babar Ali and Shahid ul Haq. “Mitigation of chemical

oxygen demand of green liquor sludge emerging from the chemical

recovery plant” Presented in 56th Chemical Engineer Conference Canada,

Oct 15-18, 2006, Ref # 390.

5. Muhammad Ikram , M. A. Kashmiri and M. Z. Qureshi. “Mechanism for

silica precipitation in the chemi-thermo-mechanical pulping black liquors”

Presented in Pakistan Chemical Society Conference, Feb 26-28 2007.

6. Muhammad Ikram, Muhammad Akram Kashmiri, Babar Ali and Ahmad

Adnan. ” State-of-the-art studies of chemical changes in the non-fibrous

10

pulping black liquors during storage” IPPTA Journal, Vol. 19, No.2. March

2007, page 67-69.

7. Muhammad Ikram aujla, Muhammad Akram Kashmiri and Babar Ali.

“Activated sludge and chemical treatment process for chemi-thermo-

mechanical pulping black liquors” Accepted for publication in

International Journal of Environment and waste management UK, in 2008.

11

LIST OF ABBREVIATIONS

Following abbreviations have been used in the thesis.

CTMP : Chemi-thermo-mechanical pulping

CMP : Chemi-mechanical pulping

BCTMP : Bleached CTMP

CRMP : Chemi Refiner Mechanical Pulp

DWS : Dilution Water Sulfonation process

TMP : Thermo-mechanical pulp

LFCMP : Long Fiber CMP

CTLF : Chemically Treated Long Fibers

SLF : Sulfonated Long Fibers

G-CTMP : Ground wood CTMP

CTMPR : Reject CTMP

APMP : Alkaline Peroxide Mechanical Pulp

APTMPTM : Alkaline Peroxide Thermo Mechanical Pulp

SCMP : Sulfite or Sulfonated Chemi-mechanical Pulp

UHY : Ultra High-Yield Sulfite Pulp

VHY : Very High-Yield Sulfite Pulp

12

ABSTRACT

Pulping of non-wood raw materials available in abundance by Kraft, soda

and chemi-thermo-mechanical processes produces good quality pulp in short

processing time. As these process involve less chemical doses hence the

present research work was designed for the chemical recovery from the black

liquors to make the waste environment friendly.

In the first step of the chemical recovery of the effluents, series of experiments

were conducted to determine behavior and chemistry of this straw based black

liquors. Chemical composition of the chemi-thermo-mechanical pulping black

liquor was determined by fractional and composite sample analysis which

showed extensively low dry solids and physical and chemical properties of the

black liquor tend to vary with organic to inorganic ratio of the solid contents.

In the second step of the chemical recovery, desilication of the black and green

liquors obtained from the chemi-thermo-mechanical pulping was conducted

through the pH reduction by carbonation (80%) and sulphuric acid (85%). The

green liquor sludge was found to be extremely high in COD, BOD with high

reduction value. Sodium (95%) and sulphur (97%) were recovered from the

sludge on dilution and addition of coagulating polymers.

Second part of this thesis discusses treatment of the black liquor by chemical

and/or microbiological means. In the chemical treatment process some

flocculating and coagulating chemicals were employed. Addition of 200ppm alum,

Buflok polymer 5425 (Buckmans) and setting of two hours yielded 82% COD

reduction and solids in the range of 292ppm. Increase of alum to 1,000ppm and

polymer dose of only 1ppm yielded similar results. The treatment of black liquor

with hydrogen peroxide, ferric chloride, polyaluminium chloride (PAC), other

13

flocculating polymers like HMW 110, HMW 130 by GEBetz and 8086 by BALCO

gave no appreciable results.

Activated sludge treatment was then employed to reduce the BOD of the black

liquors in a series of experiments and the sludge yielded 50-60% reduction in

COD, 60-65% reduction in BOD, 80-90% reduction in suspended solids and 58-

60% reduction in total solid contents. Isolation and characterization of the micro-

organisms from the activated sludge was also carried out. Results showed that

the degradation activity of the activated sludge was high after 12 hours and was

at peak after 24 hours of aeration because enzyme activity was maximum at this

stage. Finally a combination of activated sludge and chemical treatment was

designed which produced reducation of 90-95% in COD, 90-95% in BOD.

Suspended solids and dissolved solids were within the National Environment

Quality Standard (NEQ’s) limits.

14

Table of Contents:

Chapter 1. Introduction……….…………………………..….…………………………………….....8 1. 1 Pulp and paper industry in Pakistan:..................................................................................21 1. 2 Wheat and rice straw as pulping source: .............................................................................21 1. 3 Chemical composition of wheat straw:......................................................................................23 1. 4 Chemi-thermo-mechanical pulping: ....................................................................................25 1. 5 The Chemical stage in digestion of straw: ...........................................................................26 1. 6 Chemical properties of the black liquors: ............................................................................27 1. 7 Acid-base properties of black liquor: ...................................................................................28 1. 8 Elementary analysis of black liquor dry solids:...................................................................29 1. 9 Critical solid content:............................................................................................................30 1. 10 The Chemical recovery process: .........................................................................................30 1. 11 Silica problem and desilication: ..........................................................................................36 1. 12 Various chemical treatments:..............................................................................................37 1. 13 Microbial treatment of black liquors: ................................................................................38

Chapter 2 Literature Survey: …………………….………………..………………...………….…30 2. 1 Chemistry of Black Liquors:..................................................................................................43 2. 2 Chemical Recovery of Black Liquors:.......................................................................................49 2. 3 Chemical Treatment of the Black Liquors: ................................................................................57 2. 4 Literature about black liquor biological treatment: ............................................................59

Chapter 3. Experimental procedures:………….………....……………………….…………52

3. 1 Chemical investigation of the black liquors:.......................................................................64 3. 2 Determination of pH of the weak black liquor:...................................................................64 3. 3 Density determinations: ........................................................................................................64 3. 4 Determination of total solids:................................................................................................64 3. 5 Total alkali: .............................................................................................................................65 3. 6 Determination of active alkali: ............................................................................................65 3. 7 Determination of effective alkali: .........................................................................................66 3. 8 Determination of total sulfur as sulfate:...............................................................................66 3. 9 Determination of sodium sulfide in green liquors: .............................................................67 3. 10 Determination of silicon dioxide in the black liquor:........................................................67 3. 11 Determination of sodium: ...................................................................................................68 3. 12 Determination of chemical oxygen demand:.....................................................................69 3. 13 Determination of biological oxygen demand: ...................................................................69

15

3. 14 Isolation and purification of micro-organisms: .................................................................70 3. 15 Determination of biodegradation activity:.........................................................................71 3. 16 Determination of dry cell mass: .........................................................................................71 3. 17 Enzymatic Extraction and determination of its activity:..................................................71 3. 18 Protein estimation: ..............................................................................................................72

Chapter 4. Results and discussion:……………..………….……….………………………..66 4. 1 Chemical composition of the black liquors:.........................................................................75 4. 2 Relation of black liquor composition with its properties:...................................................78 4. 3 Relation of dry solids with density:......................................................................................78 4. 4 Relation of dry solids with viscosity: ..................................................................................88 4. 5 Effect of temperature on viscosity: .......................................................................................94 4. 6 Dependence of viscosity on silica contents: .........................................................................94 4. 7 Solubility of sodium carbonate and sodium sulfate:...........................................................96 4. 8 Alkalinity values of the black liquor: ...................................................................................99 4. 9 Desilication of the green and black liquors:.......................................................................101 4. 10 Silica solubility: .................................................................................................................101 4. 11 Foaming characteristics of the spent Liquors: .................................................................102 4. 12 Removal of silica by sarbonation: .....................................................................................103 4. 13 Growth of silica particles, silica sols by carbonation:.....................................................104 4. 14 Silica precipitation by carbonation: .................................................................................105 4. 15 Filtration during carbonation process: .............................................................................105 4. 16 Changes in chemical composition during carbonation process: ....................................105 4. 17 Prevention of silica scale: ..................................................................................................106 4. 18 Natural desilication: ..........................................................................................................106 4. 19 Green liquor desilication: ..................................................................................................106 4. 20 Desilication by lime addition: ...........................................................................................107 4. 21 Mechanism of silica precipitation by lowering pH: ........................................................108 4. 22 Direct carbonation (bubbling of CO2):..............................................................................108 4. 23 Laboratory desilication with sulfuric acid: ......................................................................109 4. 24 Laboratory desilication with carbon dioxide:..................................................................111 4. 25 Laboratory desilication with Hydrated Lime: .................................................................112 4. 26 Plant scale desilication with hydrated lime: ....................................................................115 4. 27 Advantages of desilication: ...............................................................................................116 4. 28 Chemical Treatment of sludge of recovery plant to recover sodium and sulfur: ........117 4. 29 Chemical treatment of the black liquors: .........................................................................121 4. 30 Biological treatment of the black liquors:.........................................................................140 4. 31 Isolation and screening of micro-organisms:...................................................................150 4. 32, Cellulase production by the Trichoderma sp. for black liquor degradation: ...............151 4. 33 Cellulase production by Clostridium sp: .........................................................................154 4. 34 Fate of the chemi-thermo-mechanical black liquors:..............................................................158

16

Chapter 5. References:…..……………………………………………………………..…………….145

17

List of Tables:

Table 1. 1 Province wise production capacity ............................................................................21 Table 1. 2 Chemical composition of wheat straw.......................................................................22 Table 1. 3 Morphology of wheat straw: .....................................................................................23 Table 1. 4 Physical Contents of Wheat. .......................................................................................23 Table 4. 1, Chemical composition of the black liquors: .............................................................76 Table 4. 2, Chemical composition of the chemi-thermo-mechanical pulping black liquor.....77 Table 4. 3, Analysis results of green liquor.................................................................................77 Table 4. 4, Comparison of densities and viscosities of the black at different dry solid

contents. ................................................................................................................................79 Table 4. 5, Comparison of viscosity and density at 70˚C at various dry solid contents. .........80 Table 4. 6, Comparison of Density and viscosity at different dry solid contents. ...................81 Table 4. 7, Comparison of density and viscosity at high dry solid contents............................82 Table 4. 8, Comparison of density at various temperatures and dry solid contents. .............83 Table 4. 9, Comparison of density and viscosity at various dry solids. ...................................88 Table 4. 10, Comparison of density and viscosity at 70˚C dry solids. ......................................89 Table 4. 11, Comparison of density and viscosity at various dry solid contents at 75˚C. .......91 Table 4. 12, Comparison of viscosity with increase in dry solid contents. ...............................92 Table 4. 13, Comparison of solubility of Na2CO3 and Na2SO4 at various dry solid

contents. ................................................................................................................................96 Table 4. 14, Comparison of total alkalinities of the weak black liquor. ...................................99 Table 4. 15, Desilication by Sulfuric acid addition. ..................................................................110 Table 4. 16, Desilication results with sulfuric acid and settling. .............................................110 Table 4. 17, Desilication with carbon dioxide.........................................................................111 Table 4. 18, Lime addition at various doses and desilication results:.....................................112 Table 4. 19, A Comparison of black liquor before & after lime addition:...............................112 Table 4. 20, Results of the lime addition – desilication trial. ...................................................113 Table 4. 21, Results of the lime addition – desilication trial. ...................................................113 Table 4. 22, Findings of plant trial. ............................................................................................115 Table 4. 23, Effect of 2 times dilution on the removal of COD and recovery of sodium &

sulfur ...................................................................................................................................117 Table 4. 24, Effect of.4 times dilution on the removal of COD and recovery of sodium &

sulfur but with the addition of animal glue .....................................................................119 Table 4. 25, Effect of.4 times dilution on the removal of COD and recovery of sodium &

sulfur but at temperature 80°C .........................................................................................119 Table 4. 26, Chemical treatment of the black liquors ..............................................................121 Table 4. 27, Results of the batch treated with alum and different doses of anionic

polymer (Bufloc 5425) and by passing the sample through activated carbon...............122 Table 4. 28, Results of the batch treated with alum and different doses of anionic

polymer (Bufloc 5425). .......................................................................................................124

18

Table 4. 29, Chemical treatment of batch fourth of the CTMP black liquors. ........................124 Table 4. 30, Chemical treatment and after passing the black liquor sample through ion

exchange resin. ...................................................................................................................125 Table 4. 31, Chemical treatments at various pH.......................................................................126 Table 4. 32, Results of the treatment trials. ...............................................................................128 Table 4. 33, Results after treatment with hydrogen peroxide .................................................128 Table 4. 34, Results of COD reduction after treatment with Hydrogen per oxide (H2O2) ....129 Table 4. 35, Reduction in COD by treating with iron salts. .....................................................129 Table 4. 36, Results of chemical treatment, polymer and activated carbon. ..........................129 Table 4. 37, Reduction in COD after chemical treatment at pH 2.2. .......................................130 Table 4. 38, Treatment by acidification and then neutralization.............................................131 Table 4. 39, Reduction in COD after chemical treatment. .......................................................131 Table 4. 40, Chemical treatment of the black liquor.................................................................133 Table 4. 41, Treatment of black liquor with iron salts..............................................................133 Table 4. 42 Treatment results of Packages composite sample.................................................134 Table 4. 43 Results of waste water chemical treatment after dilution. ...................................134 Table 4. 44, COD reduction after treatment with ferric chloride and ferrous sulfate. ..........135 Table 4. 45, Iron salts treatment at various pH ranges. ..........................................................136 Table 4. 46, Settling efficiency of Iron salts enhanced with the flocculating polymer...........136 Table 4. 47, Settling efficiency of alum, iron salts and polymer.............................................137 Table 4. 48, Results of further chemical treatment ..................................................................138 Table 4. 49, Results of treatment with Alum, PAC and flocculating polymer. ......................139 Table 4. 50, Activated sludge treatment trial, of first week.....................................................140 Table 4. 51, Degradation efficiency of the activated sludge per week basis. ........................145 Table 4. 52, Activity of Trichoderma GCU-110 for degradation: ..........................................152 Table 4. 53, Cellulase production comparison by Clostridium GCU-111: ............................155

19

List of Figures:

Figure 1. 1 Ash contents ...............................................................................................................24 Figure 1. 2 Acid-insoluble ash contents ......................................................................................25 Figure 1. 3 Simplified flow sheet of a typical TMP plant. .........................................................26 Figure 4. 1, Dry solids verses density relations at 65 ˚C. ...........................................................80 Figure 4. 2, Density verses dry solids at 70˚C for the weak black liquors. ...............................81 Figure 4. 3, Dry solids verses density of the black liquor at 75˚C. ...........................................82 Figure 4. 4, Density verses dry solids at 80˚C.............................................................................83 Figure 4. 5, (a) Dry solids verses density at 60˚C, for the entire range of dry solid

contents. ................................................................................................................................85 Figure 4. 6, Effect of temperature on the density of the weak black liquors. ...........................86 Figure 4. 7, Illustrates variation of density with increasing dry solid contents. Initially the

density increases and then starts decreasing with rise in dry solids. ..............................87 Figure 4. 8, Relation of dry solids with viscosity at 65˚C. .........................................................89 Figure 4. 9, Relation of dry solids with viscosity at 70˚C. .........................................................90 Figure 4. 10, Rate of change of viscosity with increase in dry solid contents. .........................91 Figure 4. 11, Relation of dry solids with viscosity for Thick Black Liquor at 80°C. ................93 Figure 4. 12, Viscosity variations with temperature. Upper line gives viscosity at 65 ºC,

middle line gives viscosity at 70 ºC and lower line provides viscosity information at 75 ºC. .....................................................................................................................................94

Figure 4. 13, Shows effect of silica on viscosity. First line gives viscosity with silica contents and second line gives viscosity after desilication. ..............................................95

Figure 4. 14, Solubility of sodium carbonate and sodium sulfate. Lowering of the graph lines indicate the critical solid points..................................................................................97

Figure 4. 15, Effect of aging on total alkalies of the black liquors.............................................99 Figure 4. 16, Mechanism of silica precipitation from the black liquors. ................................104 Figure 4. 17, Desilication of black liquor with sulfuric acid addition.....................................111 Figure 4. 18, Percentage reduction in the silica contents of the green liquor.........................115 Figure 4. 19, Reduction efficiency of activated sludge for various parameters; ....................146 Figure 4. 20, Reduction efficiency of activated sludge for total solids. ..................................147 Figure 4. 21, Suspended solids reduction efficiency of the activated sludge plant. ..............147 Figure 4. 22, Percent reduction in dissolved solids by activated sludge plant. .....................148 Figure 4. 23, BOD reduction efficiency of the activated sludge plant. ...................................148 Figure 4. 24, COD reduction efficiency of the activated sludge treatment plant:..................149 Figure 4. 25, Mycelium dry mass production at various carbon source concentrations:...................153 Figure 4. 26, Enzyme activity for degradation of carboxymethyl cellulose: ..........................153 Figure 4. 27, Specific enzyme activity with increasing glucose concentrations. ...................154 Figure 4. 28, Bio-mass production at various carbon source concentrations: .......................156 Figure 4. 29, Cellulase activity for degradation of carboxymethyl cellulose: .......................156 Figure 4. 30, Specific enzyme activity; with increasing glucose concentrations. ..................157

20

Figure 4. 31 Flow sheet diagram of activated sludge and chemical treatment plant. ...........161 Figure 4. 32 Settling zones for activated sludge. ......................................................................162

Introduction

21

1. 1 Pulp and paper industry in Pakistan:

At the time of independence in 1947, Pakistan was totally dependent on imports for its paper requirement. Today there are over sixty small and large paper and paperboard mills with an annual installed capacity of nearly 400,000 tons. In 1996, their total production was 250,000 tons. About 175,000 tons of paper and board was imported to meet the total consumption of 425,000 tons in 1996. The major share of imported paper consists of high quality paper and newsprint. The first paper industry in Pakistan was set up by the Adamjee Industries Limited at Nowshera. Most of the paper mills are located in the Punjab. The province wise distribution of production capacity of paper and board industry is shown in the following table.1

Table 1. 1 Province wise production capacity

Province % Share Punjab 70 Sindh 20 NWFP 10

The industry consists of about 43 paper and board manufacturing plants,

37 kraft paper plants, and 15 writing and printing paper plants. About 35 paper and board manufacturing units of various production capacities range from 20 to 120 tons/day. There are about 17 units with capacity in the range of 20 to 30 tons, while there are only few units with a capacity above 60 tons/day. The average capacity of a Pakistani pulp & paper mill is much lower than those in the developed countries. The largest mill has installed capacity of 68,000 tons per annum. The local paper industry can not compete in the international market, as costs of production are high especially for superior quality products. At present the installed capacity is more than the local demand and there is strong competition amongst the paper manufacturers.

1. 2 Wheat and rice straw as pulping source:

Non-wood fibers have a long history as a raw material for papermaking. The use of this raw material declined in Europe and North America during the first half of this century as the amount of inexpensive and readily available wood fiber increased. Currently China produces about one-half of the world’s non-wood pulp while Europe and North America are relatively small contributors2. These two regions consume about 60% of the world pulp and paper production. Only four modern straw/grass fiber production sites exist in Europe and none in the United States. In some situations however, non-wood plants may prove a viable fiber source in these industrialized regions.

22

Environmental and population growth pressures are contributing to long-range changes in forest land management practices which reduce harvest of wood for wood products and for pulp and paper manufacture3. At the same time cereal grain crop production in the United States generates tremendous quantities of straw. For example, three million acres of wheat are grown in Washington state each year producing about three tons of straw per acre. While 0.5 tons of straw per acre are required to be maintained on the soil surface for erosion control of steeply sloped ground4, the excess straw often presents problems for subsequent field operations such as no-till seeding. Therefore, straw may represent a significant fiber substitution opportunity. For example, pulp from cereal grain straw may partially substitute for wood fiber in a range of paper and paperboard products.

Properties of papermaking fibers from wood or from annual crops can be influenced by both growing conditions and genetic manipulation. For example, many studies show that wood morphology and chemical composition vary with location, genetics, and growth conditions. The chemical composition of both eucalyptus5, and rice straw6 has also been found to vary with growing location. Similar trends occur when comparing wheat straw chemistry and morphology from different sources. For example, reports shown in Table 1.2 show that carbohydrate contents vary about 0.5% (absolute), lignin 2%, ash 3%, and silica and extractractables in similar amounts.

Table 1. 2 Chemical composition of wheat straw

Pulp parameters Pakistan7 Illinois8 India9 Norway10 American11 Denmark11

Holocellulose 58.5 72.9 Ά-celluloses 33.7 34.8 29-35 39.9 41.6

Hemicelluloses 25.0 27.6 28.9 26-32 28.2 31.3 Lignin 16-17 20.1 23.0 16-20 16.7 20.5

Ash 7.5-8.5 8.1 9.99 4-9 6.6 3.7 Silica and

silicate 5.5-5-5 6.3 3-7 2.0

EtOH-Benzene Extract 5.8 4.5 4.7 3.7 2.9

The origin of these variations may be due to genetics or growth conditions

but is not apparent in the reported work. Like chemical content, straw cell dimensions are believed to vary with soil and growth conditions 12. Table 1.2 lists some of the reported wheat cell dimensions. Clearly, wide ranges of properties occur in the published literature.

23

Table 1. 3 Morphology of wheat straw:

Author name Avg. Length (mm)

Avg. Diam. (µm)

NAFL* (mm)

WAFL ** (mm)

MWAFL *** (mm)

Atchison and McGovern13 1.5 15 - - -

Cheng et al14. - - 0.26 0.63 1.09 Hua and Xi15 - 12.9 1.32 1.49 - Mohan et al16. 1.5

0.7 - 3.1 13.3

6.8 – 24.0 - - -

Utne and Hegbom17

1.3 13 - - -

* Numerical average fiber length ** Weighed average fiber length *** Mass weighed average fiber length

Some may be real, but some may depend on measurement technique. For example the difference in fiber length between18 Cheng and coworkers and Hua and Xi is19 extreme. A possible explanation for the difference in values could be that Cheng counted all of the cells while the other authors only included the fibers in their measurements. Many of the literature reports are limited to the description of whole plant morphology and chemical differences. In addition, leaf, node and stem fractions may have different composition. The plant parts contribute significantly different mass as shown in Table 1.4.

Table 1. 4 Physical Contents of Wheat.

Contents Mass Percent* Internodes 68.5 Leaves - Sheaths 20.3 Leaves - Blades 5.5 Nodes and Fines 4.2 Grain and Debris 1.5

* Dry mass percent

1. 3 Chemical composition of wheat straw:

The last portion of Phase 1 was a comparison of the chemical composition within the straw and between commercial cultivars. The chemical composition of the different straw fractions may provide some insight into the ease of pulping different fractions and the source of troublesome components like silica. Identification of any variation between cultivars may aid in identifying hybrids

24

which are easier to pulp or contain less non-process elements. The Total (%) column should total 100, but in several cases does not lower results may be due to a variety of factors. (1) A comprehensive ash balance was not done to determine which components contained ash (like holocellulose). Reporting ash-free carbohydrate and lignin contents may aid in solving this discrepancy. (2) The extractives may not all be counted. (3) Some carbohydrates may have been lost from the holocellulose if conditions for acid chlorite treatment were too severe. Further investigation would need to be conducted to resolve this discrepancy.

The extractives content is quite low compared to the ethanol-benzene extractives contents. Two factors may have caused our results to be low. Table 8 lists acetone extractives. More extractives are soluble in ethanol-benzene mixtures than acetone. Another possibility is that the eight cycles on the soxlet may not have been sufficient to remove all of the acetone extractives from the straw. The ash content of the leaves and nodes appeared to be similar while the ash content of the internodes was lower than the leaves and often the nodes. When comparing acid insoluble ash, the nodes and internodes were in the same range.

However, the leaves contained much more silica and silicates. The similar silica contents in the nodes and internodes match the findings of Roy and coworkers20 who found silica in rice straw concentrated all along the stem rather than being confined to the node.

Figure 1. 1 Ash contents

25

Figure 1. 2 Acid-insoluble ash contents

When comparing commercial cultivars, Cashup had higher silica contents

in the nodes and internodes. The leaves in Eltan straw seemed to have less silica than the other major commercial cultivars (Madsen and Stephens). These results suggest that cultivar fractionation may be beneficial if silica content is a limiting factor in a pulp production facility. Whole straw lignin content have been reported between 16-23%. The lignin contents of the different plant sections listed in Table 8 would support total lignin contents in that range. Zhang and coworkers21 found the lignin content of the nodal sections (23.22%) to be higher than the leaves (17.48%). Our Moses Lake straw seems to show the opposite trend with the nodes containing similar or less lignin than the leaf sections. The inter-nodal section, which contains the more promising fiber length distribution, contained lignin contents of the same order of magnitude as the leaves and slightly higher than the nodes. While differences in density between nodes and internodes probably are the major influence on pulping kinetics, the lower lignin contents of the nodes may impact the pulping of the nodes.

1. 4 Chemi-thermo-mechanical pulping:

Originally thermo-mechanical pulping meant a process with pressurized preheating of chips followed by either pressurized or open discharge refining. It was believed that the preheating at elevated temperatures, which softens the wood, was the key to the exceptional strength properties of TMP. But at the end of the 1970s, it was determined to be essential that the refining should be performed at elevated temperature. Since the early 1980s, this idea has led toward the development of the TMP process toward lower temperatures in preheating and higher temperatures in the refiner, also known as the PRMP process (pressurized refiner mechanical pulping)22. However, the name has

26

remained TMP. Rice straw pulping can also be carried out by soda-oxygen-anthraquinone sequence23.

Figure 1.5 shows a simplified flowsheet of a typical TMP plant for producing pulp for mechanical printing papers. The wood logs are debarked and cut down to chips of a certain size. The chips are steamed in an atmospheric steaming vessel at 100°C, after which the chips are washed in hot circulating water. The warm and wet chips are fed by a plug screw feeder into a pressurized preheater with relatively low pressure and temperature. After the preheater, the chips pass a second plug screw feeder on their way into the first refiner, which operates at relatively high pressure and temperature (usually 300-500 kPa over pressure and 143°C-158°C temperature). After this first-stage refining, the coarse pulp is blown to a steam separator from which it is fed to the second-stage refiner. From the second-stage refiner, which operates at about the same pressure and temperature as the first-stage refiner, the pulp is blown to a second steam separator.

Figure 1. 3 Simplified flow sheet of a typical TMP plant.

After passing the plug screw feeder at the bottom of the second steam

separator, the pulp falls down into a pulper for removal of latency. Then follows screening and reject refining, and usually also bleaching before storage and transport to the paper mill.

1. 5 The Chemical stage in digestion of straw:

Sulfonation chemistry: The wood matrix can be chemically modified in various ways that affect

the behavior in refining and change the fiber properties. Lignin swelling can be improved by addition of hydrophilic groups like sulfonate and carboxylic groups. Also the carbohydrates can be chemically modified through deacetylation, hydrolysis, partial dissolution, etc. Much research has been carried out over several decades to understand conventional low-yield sulfite pulping.

27

However, the factors which affect the initial, relatively rapid lignin sulfonation taking place in chemimechanical pulping have been less thoroughly investigated.

Figure depicts the major overall reaction introducing sulfonate groups into lignin.The reactive groups in lignin can be divided into various categories: B, X, and Z . The groups of the B type are sulfonated in the pH range of 1 to 2, whereas the groups of the X and Z type can be sulfonated in the pH range of 4 to 924.

The scheme of basic reaction introducing sulfonate groups into lignin25.

Formulae of different basic groups in lignin that can be sulfonated under suitable conditions.

The waste effluent of the pulping is termed as black liquor and is treated in chemical recovery plant to recover sodium and sulfur.

1. 6 Chemical properties of the black liquors:

Composition of black liquor: Black liquor contains water, organic residue from pulping and inorganic

cooking chemicals. The solid content of B.L. from NFL is around 9-10%, organics 60-70% and inorganics 30-40%. Strong black liquor contains between 50 and 70% solids, with the remainder being water. The solids are comprised of a complex mixture of both inorganic and organic constituents26. Inorganic Constituents:

The inorganic constituents in black liquor are derived from the cooking liquor which is used to pulp the wood chips, and are comprised of sodium

28

hydroxide (NaOH), sodium sulfide (Na2S), sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), sodium thiosulfate, (Na2O and sodium chloride (NaCI). Collectively, inorganic salts constitute between 18 and 25% of the solids in black liquor. Organic Constituents:

The organic compounds found in black liquor are derived from wood. They are either 1) natural wood extractives (or their reaction products) that are released as a result of the pulping process, or 2) materials formed through the reactions of the pulping liquors with the lignin or cellulose components of wood. Therefore, the compounds can be classified as lignin derived, cellulose derived, or extractives derived. Typical content ranges in kraft liquor are:

- Lignin derived (39-54%; primarily consisting of polyaromatic macromolecules with lesser amounts of loti molecular weight alcohols, aldehydes and simple phenolic compounds such as phenol, p-methyl phenol, catechol and guaiacol),

- Cellulose derived (25-35%; primarily a mixture of carboxylic acids such as formic, acetic, glycolic, lactic and glucoisosaccharinic), and Extractive derived (3-5%; primarily resin acids and fatty acids which are converted to salts at the high pH of the mixture).

In sum, spent pulping liquor can have hundreds of consituents. The acetyl bromide assay was also developed to provide a rapid and sensitive method for quantifying lignin in woody plant species. 27

1. 7 Acid-base properties of black liquor:

Black liquor is distinctly alkaline (caustic), with Ph ranging from 11.5 to 13.5 (Various company Material Safety Data Sheets.) Due to the presence of three distinct buffer systems, black liquor is highly buffered. These buffer systems and their pKa values (representing their potential for dissociation) are: Sulfide buffer: Na2S + H 2 O + NaHS + NaOH pKa 13-13.5

Phenolic buffer: R-OH + NaOH -+ R-ONa + H2O pKa , 9.4-l 0.8 Carbonate buffer: Na2CO2 + H2O + NaHCO3 + NaOH pKa 10.2 The high alkalinity is largely responsible for solubilizing the various organic constituents. If the pH is reduced, various organic constituents will precipitate, beginning with the components with low pKa values (e.g. the phenolics) and eventually those with higher pKa values (e.g. the carboxylic acids). Thus, the soluble component would vary as pH is reduced. Consequently, if the pH is adjusted in order to perform certain tests, the nature and composition of the test material will necessarily change.

The determination of aliphatic carboxylic acids was done only for acidified liquors originating form Kraft black liquor with 25% sulfidity. The

29

concentration of these acids in the acidified liquors slightly decreased as the pH of the black liquor decreased. The overall loss of volatile acids was 2-3%c and the loss of non-volatile acids was as 1-11% when the black liquor was acidified to pH 7. Further acidification to pH 2 resulted in the liberation of acids, and the loss of volatile and non-volatile acids was 8% and 13% respectively. These losses were caused by the retention of acids in the precipitates. Acid washing of the precipitates resulted in the release of 45-80% of non-volatile carboxylic acids into the washing liquor28.

1. 8 Elementary analysis of black liquor dry solids:

It is used for material and energy balance calculations. The ultimate analysis of black liquor is attached. Heating Value signifies the amount of heat that fuel can produce upon incineration. Different terminologies used are:

HHV: Involves oxidization in calorimeter and water produced as a result of chemical reactions is condensed and products are cooled to reference temperature and pressure. Resulting chemicals in ash are fully oxidized species (Na2CO3 and Na2SO4) and gas contains non-condensable combustibles with some water vapor.

LHV: It results by subtracting the heat of evaporation of water generated by combustion of hydrogen and water in the fuel from HHV.

LHV = HHV – I 25 [(MH2O/ MH2) CH + (1-X)/X] NHV: It reflects the heat available for steam generation by subtracting the

heats of evaporation and heat required for reduction reaction from the HHV. NHV = HHV – I 25 [(MH2O/ MH2) CH + (1-X)/X] – (78/32) DhR CShred

I 25 Heat of evaporation of water at 25oC , (2.443 MJ/kg ) CH Hydrogen content of dry solids (kg/kg dry solids)

MH2O Molar mass of water ( 18.015 kg/ kgmol) MH2 Molar mass of hydrogen ( 2.016 kg/ kgmol) X Dry Solids Concentration (kg dry solids/ kg fuel) CS Sulfur Contents of dry solids (kg/kg dry solids) DhR Heat of Reduction, MJ/kg ( 13.1 MJ/kg Na2S) hred Reduction Efficiency in the smelt Viscosity: Viscosity of black liquor increases with increase in dry solid content. In

certain cases it increases sharply after a certain solid content. E.g. viscosity of NFL black liquor increases sharply after 50% solids. Viscosity decreases with

30

increase in temperature. There is no direct relation to estimate liquor viscosity since it is specific for each liquor.

Viscosity can also be reduced by heat treatment at 180oC and 30 min, which breaks long organic molecules into shorter forms and results in irreversible reduction of liquor viscosity. Viscosity of black liquor decreases with increase in RAA. RAA concentration should be at least 5-6% as Na2O based on black liquor solids. Below 4% as Na2O may affect viscosity. However beyond a certain optimized level the effect is minimal or negative.

Viscosity of black liquor also decreases with increase in sulfidity. Higher sulfidity on the other hand increases S/Na2 ratio in flue gas, which results in increased corrosion and sulfur losses.

Surface tension:

It increases with an increase in dry solids contents and decreases with increase in temperature. Low surface tension increases the tendency to foam. Therefore foaming is usually a problem in the evaporator effects operating at low consistency.

1. 9 Critical solid content:

Black liquor critical solid is the total solid concentration at which precipitation of the inorganic compounds in the liquor can begin. Typical liquors values are 45% to 55% total solids. To understand the phenomenon and a point of critical solid content it is very important to know the liquor chemistry.

As liquor approaches concentration near critical solid content burkeite starts precipitating. Since burkeite contains two moles of sodium sulfate for every mole of sodium carbonate, the sulfate in solution is depleted rapidly until a second critical point occurs. At this point, the sulfate concentration is sufficiently low and now sodium carbonate begins to precipitate. Beyond this point both sodium carbonate and burkeite precipitates. Therefore there are two critical solid content points.

Both these points can be avoided by adding seed crystals at first critical solid content and by addition of sufficient quantity of sodium sulfate to avoid the precipitation of sodium carbonate i.e. elevating the second critical content point.

1. 10 The Chemical recovery process:

Evaporators: Black liquor at 65-70oC and 9-10% solids comes from NFL hot liquor tank

to settling tank (4T-02A) of CRP. This tank provides about 10 hr retention time

31

for the settling of black liquor mud and the supernatant overflows to 4-T01 & 4-T02, which are connected from bottom. The settled mud is discharged in drain. The settling tank has a volume of 400m3 while rest has a capacity of 250 m3 each. The plant a has a five stage backward feeding falling film evaporation battery. Liquor solids are increased from 9-10% to 48-52%. Liquor flows from low pressure to high pressure effects with the help of pumps. Live steam is introduced in the 1st effect where the concentration of liquor is maximum and secondary steam moves to lower pressure side. The energy for water evaporation is taken from condensing steam. Vacuum is created in the evaporation section by means of ejectors and surface condenser after the 5th stage of evaporation battery.

Liquor is pumped through TAM-312 or 401 to the upper liquor space of 5th stage. Black liquor is measured and controlled with FIC-305. In the evaporators black liquor is circulated from lower liquor space to upper liquor space. From upper space it overflows from circular weir and then comes onto the distribution plate. The function of distribution plates is to direct the liquor equally in all tubes. In tubes, liquor forms a downward falling film on the inner surfaces. During its down movement the liquor absorbs latent heat of steam and water begins to evaporation. From tubes, the liquor falls into the lower liquor space. The steam being separated from liquor in the tubes flows downward and is lead to a central pipes which takes it into separator. In the separator the flow velocity of steam reduces and the bulk of secondary steam moisture is separated by gravity. The separated liquor droplets fall on the liquor surface on the liquor distribution plate.

Part of liquor being circulated is directed through level controller from pressure side of circulation pump to the suction side of next stage. In 4th , 3rd and 2nd stages of evaporation battery liquor is introduced in the suction side and recirculated while in 1st stage liquor is introduced in the discharge side of circulation pump. From 1st stage liquor is pumped to the expansion vessel which is operating at a pressure as that of 3rd effect evaporation stage. After expansion vessel, thick liquor at 50% solids is then pumped to storage (4-T03) through LIC-308. Thick black liquor has a storage capacity of 150 m3. 1st, 2nd and 3rd stage of evaporation battery operate under pressure of (1.5-2.5) bar, (0.5-1) bar, (-0.1 to 0.2) bar. The operating pressures increases with increase in scaling. When pressure in the 1st stage evaporator reaches 2.5-3 bar then evaporators are cleaned with caustic and SO2 water to remove the scales. There is also provision to bypass the 1st effect and while operating the evaporation battery with four evaporators. 4th and 5th evaporators operate under vacuum of (–0.6 to –0.4) bar and (-0.8 to –0.6) bar.

32

Types of Evaporators: Both are rising film evaporators but differ in tube length. Tube length of short tube evaporators is 2m while that of long tube evaporators is around 7m. In rising film evaporators liquor and vapor flow together up the tubes. Flow is large and turbulent bubbles carry the liquid up the tubes ahead of them giving churn flow pattern.

The formation of aliphatic carboxylic acids during soda-AQ pulping of kenaf bark was studied. In addition to formic and acetic acids, a variety of hydroxy monocarboxylic and dicarboxylic acids were monitored. The results showed that the formation of hydroxy acids and formic acid significantly depend, in contrast to acetic acid, on the cooking conditions employed. Detailed gas chromatographic studies revealed that the most abundant hydroxy carboxylic acids were glucoisosaccharinic, lactic, glycolic, 3-deoxypentonic, 2-hydroxybutanoic, xyloisosaccharinic, 3,4- dideoxypentonic, 2-hydroxyglutaric, and glucoisosaccharinaric acids.The total amount of aliphatic carboxylic acids corresponded to 12–16% of o.d. kenaf bark.29. The recovery boiler: The two main functions of recovery boiler are; · To recover the in-organic cooking chemicals used in the pulping process · To use make of the chemical energy in the organic portion of the liquor to generate steam for the mill. The smelt from a recovery boiler consists of approximately two-thirds Na2Co3 and one-third Na2S with small amounts of Na2SO4 other sodium/sulfur compounds, and unburned carbon. Production of Na2S in the furnace instead of Na2SO4, or any other partially oxidized sodium/sulfur compound, requires both a reducing agent and a physical layout that limits the contact of the smelt with air. At the same time, the organic portion of the black liquor can only be fully consumed and converted to useful energy by intense mixing and combustion with air (or more precisely oxygen). Operating strategy: Water from any source must not contact the char bed. There is a well-documented, violently explosive reaction b/w water and smelt. The reaction is physical in nature and involves very rapid generation of vapor from water due to contact with hot smelt. The volume expansion of water as it vaporizes pushes the surrounding furnace gas away so rapidly that a highly destructive detonation wave is generated. Contact of water with smelt can cause massive damage to the boiler.

33

Aside from the risk of smelt-water contact wet black liquor reaching the bed causes local blackout of a portion of the bed surface. Because the liquor must be dry and hot to burn, the bed will not resume burning until heat from the bed, or from the combustion above the bed dries and pyrolizes the wet liquor. Ignition of the pyrolysis gases above the bed depends in part on the temperature of these gases leaving the bed, so the two processes are interdependent. Wet black liquor reaching the bed in sufficient quantity can break this feedback and cause a general blackout of the furnace. In this situation, pyrolysis of the black liquor can continue due to the heat stored in the bed, filling the furnace with combustible gas. A serious combustible gas explosion hazard can exist until stable ignition is re-established. Recovery boiler equipment: Recovery boilers have two main sections: · A furnace section · A convective section Recovery boiler chemistry: In the recovery boiler furnace a number of entirely separate physicochemical processes take place simultaneously. - Air injection and mixing with the furnace gases. - Black liquor spraying and droplet formation. - Drying of the black liquor droplets. - Pyrolysis of the black liquor and combustion of the Pyrolysis gases. - Gasification and combustion of the char residue. - Reduction of the sulfur compounds to sulfide. - Tapping the molten salt mixture of sodium sulfide and sodium carbonate from the furnace bottom - An undesired side effect during combustion is the vaporization or fuming of sodium and sulfur compounds (and to a lesser extent of chlorine and potassium compounds). The vapors and fume escape with the flue gases. Overall sodium and sulfur balance: In the recovery boiler, the sulfur occurs in the molten salt as both sulfide and sulfate. A substantial part of the sulfur and sodium is carried by the combustion gases into the boiler flue, mainly in the form of sodium sulfate dust and sulfur containing gases. These compounds cause fouling of heat transfer surfaces and corrosion.

34

Most of the sodium and sulfur dust falls out of the flue gases into dust hopper. This dust is fed back into the boiler by mixing it with the incoming virgin black liquor. Roughly 10 % of the sodium contained in black liquor is typically in this circulating load. The part of the sulfur carried by the combustion gases and fume ranges from 20-40 %. A small portion of both sodium & sulfur leaves the process as flue gas emissions. The emission components are Na2SO4, H2S, SO2 and CH3SH, (methyl mercaptans), COS. (especially at low bed temperature), NaOH & metallic sodium. Two of the conditions in the lower furnace that have major impact on sulfur and sodium distribution are the temperature of the lower furnace and the temperature of the char bed. These temperatures are chiefly influenced by liquor spraying technique, and the introduction of combustion air into the furnace. Also the black liquor properties, particularly its dry solid contents, affect the temperatures in the furnace. Solid compounds begin to form in the bed when the temperature drops below approximately 8000C (14750F). In principle, the sulfate contained in a partly burned black liquor droplet entering the bed may be reduced by either of the reducing gases (CO or H2) or solid carbon (char) in the bed. One comparison shows that the reduction of sulfate by means of char is about two order of magnitude faster than the corresponding reduction with reduction gases. Gas reactions become dominant in the reduction process only after 99% of the char in the smelt bed has been consumed. At this stage, however, the whole reduction process is very slow, essentially insignificant for practical purposes. So a pre-requisite for efficient smelt reduction is a sufficient amount of char continuously on the bed surface. This means that black liquor should be sprayed in such a way that a significant fraction of the droplets hit the bed surface while char burning is still incomplete. The limit would naturally be the capacity of the bed to burn the char. An excessive flow of char onto the bed would result in unwanted growth of the bed. Another important factor for efficient smelt reduction efficiency is temperature, so as to provide activation energy. The reduction rate roughly doubles whenever the temperature increases by 50-600C. The reduction of the smelt is to some extent counteracted by re-oxidation of fine smelt droplets in flight, and re-oxidation of exposed smelt near the furnace walls and smelt spouts. In normal practice, however, such re-oxidation is insignificant. Sodium and sulfur balance during black liquor burning: In the lower furnace, part of the sodium and sulfur is transferred into the flue gases and does not return to the char bed. The main factor controlling the

35

vaporization of sulfur and sodium compounds is the main temperature. It is generally known that the concentration of gaseous sodium compounds increases with increasing temperature, while the concentration of sulfur compounds decreases with increasing temperature. When the boiler is operated at a low temperature the flue gas contain higher concentrations of sulfur gases. The low temperature may be due to low heat value of black liquor. So increasing the temperature increases the rate of dust circulation in the combustion gases and reducing the concentration of sulfur gases. The char burning stage releases significant amount of sodium vapor. Sodium volatilization during char burning seems to be due to reactions b/w the inorganic salts, mainly sodium carbonate and the char carbon at temperature where the salts are in a molten state. The reactions yield sodium in vapor form and gaseous oxides of carbon. They are kinetically controlled reactions so sodium release increases with higher particle temperature and longer reaction time. Laboratory measurements indicate that these reactions become significant when particles temperature exceed 900°C. Formation of the green liquor:

The black liquor after the first effect evaporator is taken to the thick black liquor storage. From the black liquor storage it is taken to the direct contact evaporator, where it is contacted directly with flue gases coming from the economizer. Here the solid contents of the black liquor are raised to 60%. The discharge from the direct contact evaporator, after passing through the screw pump, has two ways, one is re-circulation lining and the other is going to the furnace for the burning purposes. After the formation of smelt in the boiler, it goes to the smelt dissolving tank, where water is mixed with it, The tank is provided with agitator for a better mixing. For water coming in there are two lines, one for H2S condensate and the other for raw water. Sulfur dioxide absorption: In the absorption tower, SO2 is absorbed in sodium sulfite solution. Already prepared sodium sulfite is recycled from the tank 4T-04 to absorb SO2 . The reaction is as follows. Na2SO3 + SO2 + H2O + 2 NaHSO3 Sodium sulfite solution is introduced from the top of absorption tower with the help of nozzle, which sprays the solution. Sulfur dioxide enters from the

36

bottom of tower. The absorption tower has two independent internal circulations. Upper half and lower half of tower has separate upward spraying nozzles. Sodium bisulfite produced in the absorption tower has a strength of 500-550 g/L (expressed as NaHSO3) and pH of around 5.4-5.9 is transferred to storage through LIC-239. The exit gases from absorption tower are directed to the third washing of conversion tower where sodium carbonate solution is sprayed. After 3rd washing, flue gases are discharged into the main stack. Sulfitation: Sodium sulfite is produced in the sulfitation section where sodium carbonate and sodium bisulfite solution react as follows. 2NaHSO3+Na2CO3 2Na2SO3 + CO2 + H2O Typical flows of bisulfite and carbonate solutions is around 9.5-10m3/hr and 4.5-5 m3/hr, which are controlled through FFC-262 and FFC-260. Both the solutions are introduced from the top of sulfitation column through a common distribution pipe. Sulfitation column is filled with polypropylene rings to increase the surface area of the liquid and to liberate carbon dioxide. The pressure in the sulfitation section is around 1.6-1.8 bar. The carbon dioxide produced in the sulfitation is led to CO2 accumulator from where it goes in the carbonation stage and excess is vent in the flue gas stack. The condensate from the carbon dioxide accumulator flows to the green liquor clarifier. Sodium sulfite produced in the sulfitation has a strength of 200-230 g/L and a pH of 7.2-7.5. It is pumped to the storage tank (4T-04) from where part of it recycled in the absorption tower and excess is transferred in the mixing tank( 4T-06). From mixing tank sodium sulfite solution is pumped to chemical house.

1. 11 Silica problem and desilication:

Straw pulping black liquor usually contains a high level of silica30. However, a cheap industrial byproduct consisting mainly of aluminum compounds can be used as a desilication agent and produces black liquor with over 90% of the silica removed. The process is relatively simple. Yield, permanganate number and beatability are not adversely affected, nor are sharp hanges in pH, total alkalinity or heat value recorded. The resulting increase in viscosity has favorable implications for the evaporation and combustion of weak black liquor in the chemical recovery system. In a joint venture with Lurgi Co., the company has developed an efficient silica removal system that allows the subsequent recovery of heat and chemicals from rice straw black liquor31.

37

Silica poses several problems in all unit operations of the chemical recovery cycle i.e. evaporation, combustion and causticization. Multiple effect evaporation and causticization are mostly affected by the presence of silica in the system. The average black liquor flow rate in the evaporator was observed to be only 85% of the design capacity due to silica scaling. 32

Previous research work designed to overcome the problems of silica in black liquor is briefly reviewed. Carbon dioxide in flue gases has been used to reduce the pH of black liquor. Because of fear of decreased heating value of the desilicated black liquor due to co-precipitation of the organic matter, the ph drop was limited to a pH window; where silica was suppose to precipitate without significant precipitation of organic matter33. A project undertaken in this field by the Egyptian company, RAKTA, and the German firm, BKMI Industrieanlagen GmbH is described34. Criteria to be satisfied by the project are outlined, and the pilot plant process developed which has successfully treated 4 m3/hr of black liquor from non-wood fiber pulping operations is reported. Results achieved are outlined, and said to be so successful that a large scale (150 m3/hr) desilication plant is now under construction for the treatment of mixed black liquor from bagasse and rice straw pulp at RAKTA’s Alexandria mill.

The problems of recovering chemicals from the spent black liquor in non-wood pulping are discussed. So far, no pulp mill using rice straw has successfully operated a recovery plant.35 Investigations into the desilication of rice straw black liquor have been carried out in co-operation with the Ratka General Co. of Alexandria, Egypt, which runs the world’s largest rice straw pulp and paper mill. Details are given of a semi-industrial scale pilot-plant, which was put into operation in 1985 on the basis of the results of the investigations. Experience with the pilot-plant has shown that it is technically feasible to desilicate black liquor from rice straw and that it is an economic proposition.

1. 12 Various chemical treatments:

Chemical coagulation is combined with floc formation in a single process that involves rapid mixing in a flocculation tank having 1-3 compartments. The idea is to bring together small solid particles to form larger flocs, which are easier to remove. The addition of a polyelectrolyte during the rapid mixing stage assists flocculation. pH control is also important.

Vigorous mixing ensures that the chemicals required are mixed as completely as possible into the water before flocculation. The residence time of the effluent in the rapid mixing tank is 0.5-3 minutes.

Effluent in the flocculation tanks is kept moving by means of gentle stirring. Under the right conditions, solid particles coalesce to form larger

38

particles, or flocs. Mixing must be performed gently because of the delicate nature of the flocs. The residence time of the water in the flocculation tanks is 20-40 minutes. After chemical coagulation, the treated effluent is led into the clarification unit or other equipment designed for solids removal. Proper design of the system usually requires some prior experimentation. The main chemicals used for coagulation are:

- Aluminium salts such as Al2(SO4)3 and Aln(OH)mCl3n-m - Iron (III) salts such as FeCl3 and Fe2(SO 4)3 - Iron (II) salts such as FeSO4 - Lime.

To achieve optimum flocculation results, it is often necessary to feed in a suitable polymer during the slow mixing stage.

1. 13 Microbial treatment of black liquors:

Activated Sludge: Activated sludge consists of a mixed community of microorganisms that

metabolize and transform organic and inorganic substances into environmentally acceptable forms. The typical microbiology of activated sludge consists of approximately 95% bacteria and 5% higher organisms (protozoa, rotifers, and higher forms of invertebrates). The term "activated sludge" refers to a biological process. This process cannot be monitored without using a biological tool: the microscope. Activated sludge can be defined as "a mixture of microorganisms which contact and digest bio-degradable materials (food) from wastewater." To properly control the activated sludge process, you must properly control the growth of microorganism. This involves controlling the items which may affect those microorganisms. Bacteria: Make up about 95% of the activated sludge biomass. These single celled organisms grow in the wastewater by consuming (eating) bio-degradable materials such as proteins, carbohydrates, fats and many other compounds. The role of enzymes:

Enzymes are compounds that are made by living organisms. Their purpose is to help biochemical reactions to occur. Almost all biochemical reactions require the presence of enzymes to cause the reaction to occur. Enzymes help bacteria in the process of breaking down nutrients, and in rebuilding broken down nutrients into the new compounds that they require for

39

growth and reproduction. Enzymes only do what they are supposed to when environmental conditions are right. If the conditions are not right the enzymes will not function properly, thus, the bacteria will not function properly, and they will not survive. If conditions are right the bacteria will live and prosper.

Recent developments in molecular breeding and directed evolutions have promised great developments in industrial enzymes as demonstrated by exponential improvements in β-lactamase and green flourescent protein. Detection of and screening for improved enzymes are relatively easy if the target enzyme is expressible in a suitable-highthroughput screening host and a clearly defined and usable screen or selection is available, as with GFP and β-lactamase. Fungal cellulases, however, are difficicult to measure and have limited expressibility in heterologous hosts. Furthermore, traditional cellulose essays are tedious and time consuming. Multiple enzyme components, an insoluble substrate, and generally slow reaction rates have plagued cellulase researchers interested in creating cellulase mixtures with increased activities and/or enhanced biochemical properties. 36 Evaporation:

The evaporation or distillation of effluents has usually been considered too expensive to be a realistic option. However, evaporation is used for barking effluents and for some bleaching effluents in full scale. The method apperars to have good potential but more process and equiopment development will be needed. Lignin removal process:

The lignin removal process (LRP) is based on the ability of acidified fiber sludge to cause organic material in effluent to precipitate on the surfaces of the fibers. The method has been used in full scale for effluents from non-wood pulp and paper mill. The process consists of three main stages:

- Acidification of fiber sludge - Reaction stage - Removal of fiber sludge from the effluent.

The fiber sludge used in this process is acidified to bring the zeta potential of

the sludge slurry close to zero. In the reaction stage, the acidified sludge is mixed with effluent. The result is precipitation of organic matter onto the fiber surfaces. The method is cheaper than conventional chemical treatment. The sludge treatment is also more easy. The sludge can be reused for making of board or for concrete or tile products.

40

Stripping:

Stripping is a way of removing volatile compounds from effluent. Heat and/or pH adjustment may be used to provide the right stripping conditions for the compounds in question. Stripping takes place in a column. The effluent is introduced at the top and a gas (usually steam or air) at the bottom, so that the operation is counter-current. The effluent passes via a heat exchanger out of the base of the column.

The column may be packed with some inert material, or it may contain a variety of plate arrangements. Packed columns are used mainly for removal of malodorous sulfur compounds and have lower capacities. The packing particles are either plastic or stainless steel. Columns with bell or aperture plate arrangements operate at high efficiency over a wide capacity range. Pressure loss is constant for a given steam loading. In the forest industry, stripping is used for:

- Cleaning up contaminated condensates - Removal of SO2 from effluents - Removal of NH3 from effluents. Stripping with air or steam is used to remove reduced sulfur compounds and

volatile organics from contaminated condensates from chemical pulp digesters and evaporation units. At sulfite pulp mills, sulfur dioxide stripping is used to treat condensates before anaerobic treatment. Stripping also can be used to remove ammonia from effluents in which the ammonia content is high. Ion exchange:

Ion exchange is a process whereby ions bound by electrostatic attraction to the functional groups of a solid-state compound are exchanged for ions in the surrounding solution. Ion exchange is used to remove specific ions from effluent. An ion exchange process for the treatment of bleaching effluents was developed in Sweden in the early 1970s. The process removes colored compounds from the effluents very efficiently. Effluent is led through a column packed with a suitable ion-exchange resin. When the resin becomes saturated with the ion concerned, it is regenerated with caustic soda followed by activation with sulfuric acid from the chlorine dioxide plant or effluent from the chlorination stage of the bleaching sequence. The volume of eluent used to regenerate the column is about 5% of the volume of effluent treated. The eluate from the column can be combined with black liquor or led into the chemicals recovery system. Ion exchange is used mainly for the treatment of bleaching effluents and for effluents with high COD loads.

41

42

Literature Survey

43

2. 1 Chemistry of Black Liquors: Following literature have been consulted to draw chemistry of black liquors:

Zhinan et al37 worked on the formation of aliphatic carboxylic acids during soda-AQ pulping of kenaf bark was studied. In addition to formic and acetic acids, a variety of hydroxy monocarboxylic and dicarboxylic acids were monitored. The results showed that the formation of hydroxy acids and formic acid significantly depend, in contrast to acetic acid, on the cooking conditions employed. Detailed gas chromatographic studies revealed that the most abundant hydroxy carboxylic acids were glucoisosaccharinic, lactic, glycolic, 3-deoxypentonic, 2-hydroxybutanoic, xyloisosaccharinic, 3,4- dideoxypentonic, 2-hydroxyglutaric, and glucoisosaccharinaric acids.The total amount of aliphatic carboxylic acids corresponded to 12–16% of o.d. kenaf bark.

Greg et al38. described a new solubility model for black liquor using NAELS, a chemical equilibrium calculator. This model has use to determine the solubility of inorganic compounds in black liquor systems over a wide range of temperatures and concentrations. The model was developed by deriving thermodynamic data from solubility data for inorganic compounds of interest in black liquor. Addition of a hypothetical organic species that competes for sodium in solution was necessary to allow accurate predictions of solubility at high liquor solids levels. Several case studies show application of the model to calculate solubility of inorganic compounds in black liquor. Model results show that black liquor exhibits two critical solids points over the range of solids concentrations in typical concentrators. Burkeite begins precipitating at the first critical point, and sodium carbonate begins precipitating at the second point.Temperature and liquor chemistry influence the solubility of burkeite and sodium carbonate in black liquor.

Sricharoenchaikul et al39 performed experiments to investigate tar formation during devolatilization of black liquor at high heating rates, at temperatures from 700 to 1000 °C and 1 bar of total pressure. The tar compounds detected were grouped into two categories, semivolatiles and nonvolatiles, based on their molecular weights and boiling points. The semivolatile tar collected ranged from 0.1% to 5% of the carbon in black liquor, while the nonvolatile tar collected ranged from 0.02 to 1%. However, carbon balances suggested that tar may have accounted for 20% or more of the carbon in black liquor at 900 °C and below. Tar characterization revealed a similarity between the lignin substructure and some of the tar compounds produced. This indicates that kraft lignin was the

44

source of these tar compounds. Tar yields were controlled by a number of competitive formation and decomposition reactions, which are influenced by the residence time, temperature, and gas composition. Low temperatures favored the formation of more aromatic tar species with diversified substituted groups such as toluene and xylenes. These compounds were formed at a finite rate, and their concentrations were increased with gas/particle residence time. Tar species formed more rapidly and decomposed more rapidly at higher temperatures. Nonsubstituted aromatics such as benzene were more stable at higher temperatures and were formed by decomposition of substituted aromatics. Oxidizing gases enhanced both the formation and destruction of tar species, depending on the temperature and residence time.

Zhu et al40 experimentally quantified the formation of organic sulfur compounds in a commercial Super Batch kraft pulping process using a laboratory pilot-scale digester. The results indicate that wood chips not only can adsorb HS- but also methyl mercaptan (MM) in the black liquor used for pretreatment during the two pretreatment stages. The absorption rate of MM is much faster than that of HS-. In the third stage of SuperBatch pulping, the rate of formation of MM and dimethyl sulfide (DMS) is very similar to that found in conventional batch kraft pulping processes. The study examined the applicability of the phase transition cooking (PTC) concept for reducing organic sulfur compounds in pulping, previously developed in laboratory batch pulping using a bomb-type digester, in a SuperBatch process. It was confirmed that there is a phase transition point (PTP) corresponding to PTC in SuperBatch pulping beyond which further delignification significantly increases the formation of volatile organic sulfur compounds. The results indicate that a 40% reduction of TRS formation can be achieved by using PTC.

Demirbas et al41 characterized major byproducts of delignification processes of lignocellulosic biomass include lignin degradation products. Lignin and its degradation products have fuel values. The yields of liquid and gaseous products from the solid waste pyrolysis increase from 23.8 to 49.7% with temperatures increasing from 584 to 683 K. The yields of liquid and gaseous products from the black liquid pyrolysis increase from 7.3 to 55.0%, with temperatures increasing from, 500 to 800 K. The yields of liquid and gaseous products from pyrolysis of the solid waste and black liquor samples increase with increasing temperature. This increase is very sharp for the yields of liquid products from the solid waste at the same temperature range.

45

Das et al42 collected data from a large number of pulp and paper mills across the U.S and showed that although there has been a significant increase in pulp production during the last 20-25 years, discharges to the environment have gone down. Over this period total reduced sulfur (TRS) emissions have been reduced by 85%, SO2 emissions have gone down by over 40%, effluent flow has decreased by over 30%, and biochemical oxygen demand (BOD) and total suspended solids (TSS) have decreased by 75% and 45%, respectively. Paper recovery has increased from 20% to 50%. The recently promulgated Cluster Rule which links air and water standards, will further reduce environmental releases of hazardous pollutants and chlorinated organics. New and emerging technologies, such as enzyme bleaching, black liquor gasification, cogeneration, production of ethanol from wastewater treatment plant sludges, and beneficial use--including land application--of solids residuals are expected to further reduce emissions.

Golike et al43 used thermodynamic equilibrium calculations to describe the solubility of inorganic compounds in black liquor in kraft pulp mills is reported. The effects of black liquor chemistry on precipitation of inorganic compounds are considered. An attempt is made to solve soluble scaling issues in black liquor concentrators. Wasik et al44 used methods of controlling brownstock washing flows during production rate changes. Wash water flow to a brownstock washing system affects both pulp washing loss and black liquor evaporation demand. The complexity of brownstock washing systems has increased with the application of new technology, the increasing demands for improved washing, and the inclusion of oxygen delignification systems. Storage and mixing tanks create process lags and dead times, which need to be considered when controlling wash water flows. An optimum operating point exists; this point corresponds to the minimum of the combined costs associated with washing loss and evaporation. To ensure that operation is sustained as close to this optimum point as possible, a system requires stable control of stock flows and tank levels and feed-forward dilution factor control, as well as feedback control based on washing loss measurements. Sufficient filtrate surge capacity is recommended to balance accumulations in pulp storage tanks, especially during production rate changes. The impacts of production rate disturbances and the consequences of specific control strategies are demonstrated with dynamic simulations of a brownstock washing system. The methods studied range from simple feed-forward PID control to more complex filtrate management strategies, including multivariable fuzzy logic control.

46

Larson et al45 used Kraft pulp and paper mills generate large quantities of black liquor and byproduct biomass suitable for gasification. These fuels are used today for onsite cogeneration of heat and power in boiler/steam turbine systems. Gasification technologies under development would enable these fuels to be used in gas turbines. This paper reports results of detailed full-load performance modeling of pulp-mill cogeneration systems based on gasifier/gas turbine technologies and, for comparison, on conventional steam-turbine cogeneration technologies. Pressurized, oxygen-blown black liquor gasification, the most advanced of proposed commercial black liquor gasifier designs, is considered, together with three alternative biomass gasifier designs under commercial development (high-pressure air-blown, low-pressure air-blown, and low-pressure indirectly-heated). Heavy-duty industrial gas turbines of the 70-MWe and 25-MWe class are included in the analysis. Results indicate that gasification-based cogeneration with biomass-derived fuels would transform a typical pulp mill into a significant power exporter and would also offer possibilities for net reductions in emissions of carbon dioxide relative to present practice.

Zaman et al46 reported the influence of black liquor composition and solids

concentrations on the Newtonian viscosity of slash pine black liquors over wide ranges of temperature (up to 140°C) and solids concentrations (between 50% and 83% solids) has been studied. It was found that the zero shear rate viscosity of high solids black liquors depends strongly on the cooking conditions and/or black liquor composition. Not only is high solids viscosity affected by lignin molecular weight and lignin concentration in the liquor but other organic and inorganic constituents of black liquor also make a significant contribution to viscosity. The dependency of zero shear rate viscosity on solids concentrations, and temperature is defined. The Newtonian viscosities vary over a wide range depending on temperature, solids concentrations and solids composition. The results indicate that, at fixed levels of effective alkali and sulfidity, the zero shear rate viscosities can be described as a function of both lignin concentration and lignin molecular weight. The viscosity of black liquor is an increasing function of the organics-to-inorganics ratio and is a decreasing function of the concentration of sodium and chloride ions and pH of the liquor. Zaman et al47 developed methods for precise density measurements of black liquors at 10-100 solids and for thermal expansion are available. Black liquors exhibit two second order thermodynamic transitions at 25°C over a range of solids. Linear extrapolation of low solids data does not determine black liquor

47

density. Variation in density for different kraft liquors comes not only from variations in the organic to inorganic ratio. At solids concentrations below 65, thermal expansion correlates successfully as a reduced variable quantity.

Zaman et al48 developed the newtonian (zero shear rate) viscosities of four different softwood kraft black liquors from a four variable-two level factorially designed experiment for pulping slash pine were determined for solids concentrations up to 84 and temperatures up to 140°C (413.2 K). Methods of measurement and estimation of zero shear rate viscosities from viscosity-shear rate data have been described and compared. The combination of the absolute reaction rates and free-volume concepts were used to express the relationship between the Newtonian viscosity and temperature. Attempts were made to obtain a generalized correlation for Newtonian viscosity as a function of temperature and solids concentrations. The results of this model and results of our previous empirical correlation have been compared and discussed. Zaman et al49 gave an evaluation of the utility of various fundamentally based models for correlating viscosity data of black liquors. The evaluation is conducted as a function of temperature and concentration of nonvolatile components in the region in which the liquors behave as Newtonian fluids. A model based on free volume theory is likely to be the best, both for defining the viscosity of a liquor as a function of temperature and solids contents and for extrapolating to higher temperatures at a fixed concentration. Ramamurthy et al50 developed simple techniques to measure the viscosity and thermal conductivity of high-solids black liquor. The method using a Brookfield viscometer for measuring viscosity was modified by applying a thin film of silicone oil on the surface of black liquor to prevent evaporation and skin formation. These two phenomena take place at high temperatures and high solids contents, affecting the viscosity measurements when the conventional method is used. The results of the modified method compared well with the vibrating blade and coaxial cylinders viscometers for a hardwood liquor at 67.7 solids level. A line source method was used to measure the thermal conductivity of black liquor up to 83 solids. We found that the data published earlier by Harvin and Brown (1) could be extrapolated to high-solids contents. Myreen et al51 found that Silicate ions in black liquor from soda cook of non-wood fiber materials cause problems in the recovery of sodium and energy from the liquor. When adding carbon dioxide to the liquor, the pH of the liquor decreases and the silicate solidifies as silica that can be separated from the liquor

48

by filtration. The new approach differs from previous attempts in three ways: The black liquor is saturated with carbon dioxide, technically pure carbon dioxide is used, and the precipitate is separated from the liquor in efficient variable-chamber filter presses. Black liquor burnt in an oxidizer with oxygen instead of air generates the required carbon dioxide. The oxidizer can be used to burn all the black liquor generated at the mill, or only about 10 % of the liquor that is sufficient to generate the carbon dioxide required for desilication of the total amount of black liquor. Jain et al52 developed lignin removal process at CPPRI for selective separation of high molar mass fraction of lignin from agro based black liquors, so that the liquor after removal of lignin becomes more amenable to biological treatment. The process has been up scaled to pilot plant, installed in an agro based mill to process 15-20m3 of black liquor. The continuous operation of the pilot plant for a period of over four months has shown satisfactory results in terms of lignin removal and subsequent COD reduction. With the incorporation of the lignin removal process in the existing biomethanation pilot plant, an overall COD reduction of 80-85% could be achieved against 50% in conventional treatment systems, thus making it possible to reach closer to the direct standard norms. Utilization of precipitated lignin as sealable phenolic derivative will make the process more attractive for commercialization in small agro based pulp and paper mills in India and abroad.

49

2. 2 Chemical Recovery of Black Liquors:

Following references were found on the chemical recovery of the back

liquors: Thomas et al53 worked on technologies to improve the chemical recovery

efficiency of the recovery boiler. The work described the mill’s approach in achieving the global distinction of high chemical recovery efficiency of plus 98% and almost self-sufficiency in energy with implementation of the state-of-the-art cleaner technologies. Some of the major cleaner technologies implemented are black liquor crystallization in evaporator, high dry solids firing in recovery boiler, continuous up-gradation of the recovery boiler combustion technology, slow motion slaking in causticizing, modified lime mud clarification, two stage sedimentation type dregs washing system, four stage counter current brown stock washing, closed compact pressure knotter, microprocessor based electro static precipitator for soda recovery boilers and lime kiln, process automation with DCS and conductivity controlled recovery pits.

V. Janbade et al54 worked on the troubleshooting of the Kraft recovery system using modern analytical tools. Their work describes that with the closure of the recovery cycle, the presence of npn-process elements in the black liquor have assumed greater significance. Elements that were relatively benign have become potential source of deposit formation and corrosion in partially of completely close cycles. Na2SO4, Na2SO3, Na2S2O3 and oxalates in black liquor contribute to build up of dead load in the recovery boilers and load to the deposit problem in the digesters and evaporators while chlorides lower the smelting temperature and cause corrosion problems. It is therefore necessary to incorporate a rapid process monitoring tool for the analysis of such ions that are detrimental to the recovery operations.

Gae et al55 investigated the use of non-woody biomass as a raw material for pulp and paper production is growing worldwide. Moreover, new technologies are being developed to solve the problems caused by the silica present in alkaline black liquor (ABL) from straw in heat recovery systems. Because, among the various possible technologies, gasification is becoming attractive as a new alternative recovery system for ABL, in-depth studies of ABL gasification are therefore required. The present work is focused on the study of CO2 gasification of a char obtained from the ABL pyrolysis. Specifically, the influence of CO and CO2 on the gasification rate at different gasification temperatures has been studied. The process may be described by a Langmuir-

50

Hinshelwood equation for a specific range of operating conditions. The data obtained suggest that this operational range could be broader if the number of catalytic sites is considered as a function of CO and the final pyrolysis temperature rather than as constant.

Chen et al56 worked on scaling in conventional black liquor evaporators has presented problems for decades, impeding the improvement of productivity in paper mills. Recent investigations suggest that falling film technology may effectively minimize black liquor fouling and improve productivity in a paper mill. This finding motivates the current work to analyze the transport phenomenon, enrichment and scale fouling of black liquor in a falling film evaporator. In the paper, a mathematical model based on a turbulent two-phase flow with multiple components is presented to investigate the transport processes of black liquor in a falling film evaporator. A phenomenological model of crystallization fouling is used to predict the fouling process. The results show the relationship between heat and mass transfer occurring within a very thin viscous sublayer close to the heat transfer surface, and the influence of soluble solids concentration and thermal boundary condition on the enrichment and scale fouling of black liquor.

Gea et al57 worked on the usual problems experienced with common recovery boilers are aggravated in processes involving alkaline black liquor derived from straw. The consequent need to develop alternative processes for the use of black liquor for energy purposes, such as pyrolysis or gasification, requires a good understanding of the thermochemical behavior of this substance. There is, however, little information available. Therefore, studied the thermal degradation of alkaline black liquor from straw in a thermogravimetric system. The present work also focuses on its pyrolysis. In particular, the influence has been analyzed of the final pyrolysis temperature (250-900 °C) and the heating rate (5-30 °C/min) on the product yields, gas composition, and specific surface area of the resulting char in a fixed-bed reactor. The results obtained show that both the energy recovery and the specific surface area of the char increase with a rise in the final pyrolysis temperature and the heating rate. Raberg et al58 worked on improvement of the binary phase diagram Na2CO3-Na2S, particularly on the Na2CO3 side of the system, which is the region of interest concerning black liquor combustion and gasification and also the region with the greatest uncertainties, is presented. Measurements were taken in a dry inert atmosphere at temperatures from 25 to 1,200°C using high-temperature microscopy (HTM) and high-temperature X-ray powder diffraction.

51

The influence of pure CO2 atmosphere on the melting behavior was studied by means of HTM experiments in the same temperature interval. New data complementing earlier published data on the binary phase diagram Na2CO3-Na2S are presented. Frederik et al59 made investigation of the composition and sintering characteristics of deposits of alkali metal salt particles produced in situ in a kraft recovery boiler is presented. The deposits were produced on air-cooled probes introduced into the convective gas passages of an operating kraft recovery boiler. Their microstructure was analyzed using scanning electron microscopy. The data revealed that, in regions of the boiler where the alkali metal salt particles are no longer molten, 2 types of deposits can be produced. Jones et el60 presented their results for a study on the dissolution behavior of salt mixtures formulated to simulate smelt deposits inside pulp mill recovery boilers. Solubilities of carbonate-rich NaOH-Na2CO3-Na2SO4 and NaOH-Na2CO3-Na2SO4-Na2SO3-Na2S2O3-Na2S mixtures in water at 25°C and 1 bar exhibited an increase in the concentration of Na2CO3(aq) and Na2SO4(aq) with an increasing amount of water added. An ion-interaction solution model explains this behavior as resulting from the suppressive effect of NaOH on the solubility of other salts; dilution of NaOH increases the solubility of Na2SO4 and Na2CO3. In addition, the formation of hydrated salts of Na2CO3 and Na2SO4 reduced the amount of liquid water present and thus had a concentrating effect on NaOH(aq). These results are applied to understanding the role of smelt-water mixtures in corrosion of stainless steel in boiler floors. Nassar et al61 worked on thermal analysis, TGA and DTA, of the bagasse kraft black liquor was carried out under an oxidized atmosphere (combustion) and an inert atmosphere (pyrolysis). It was found that dried black liquor evaporated and pyrolyzed over a broad range with considerable loss between 70 and 800°C. There are indications of peak activity at 50, 273, and 630°C under oxidizing atmosphere and 70, 292, and 637°C under inert atmosphere. The activity at lower temperatures is most likely due to volatilization of organic and breakdown of sulfur compounds. The activity at higher temperatures is probably due principally to pyrolysis of organic. During pyrolysis an additional, final, step covers a temperature range of 800 to 1000°C; this is due to fusion of inorganic sodium salt. During combustion of dried black liquor, the oxidation condition is favorable at the beginning of the combustion operation, while the reducing condition is favorable at the end.

52

Shi et al62 worked on primary nucleation of burkeite, which is a crystalline solid solution of sodium carbonate and sodium sulfate having the stoichiometry Na2CO3.2Na2SO4, was monitored using an on-line particle-size analyzer. Observations of unusual nucleation behavior led to explorations that identified specific impurities in the system that affected nucleus formation. By varying the conditions under which nucleation occurred, calcium ions were found to inhibit nucleation, and further research showed that the substitution of calcium ions into the burkeite crystal lattice was the reason for the observed behavior.

Stefanov et al63 worked on phenomenological modeling of black liquor evaporators, found in the pulp and paper industry, presents a very challenging task. The physicochemical phenomena that occur do not lend themselves readily to known mechanisms, and in many instances, data to support these hypotheses are difficult to obtain. In this work, a distributed-parameter model (a system of partial differential equations) based on first-principles knowledge about the fluid dynamics and heat-transfer processes is developed for a falling film lamella type evaporator. Primarily, the model describes falling film evaporation on one lamella. The model is solved using orthogonal collocation on finite elements in the presence of scaling and disturbances in the mass feed rate, feed dry solids content, and wall temperature. Wessel et al64 worked on the Algorithms for alkali-salt deposition were incorporated into a 3-D numerical model of a recovery boiler. The algorithms are combined with models for turbulent flow, black liquor combustion, and heat transfer in the boiler. Model predictions for particle deposition in the convection pass of a commercial recovery boiler are presented.

Sricharoenchaikul et al65 made experimental measurements of the carbon-containing gases produced during gasification and pyrolysis of dry solids from two black liquors were made at high heating rates, at 700-1100°C. The product gases were analyzed for carbon-containing species by Fourier transform infrared spectrometry. The twelfth most abundant carbon-containing light product gases were quantified. Devolatilization of black liquor proceeded rapidly and was complete within the shortest sampling interval (0.3 s) in all of the laminar entrained-flow reactor experiments. Less than 15% of the carbon in the black liquor solids was converted to light gases during devolatilization. In a nitrogen atmosphere, additional carbonaceous material from tar was converted to light gases via secondary reactions after devolatilization was complete, and fixed carbon was gasified by reduction of Na2SO4 and Na2CO3. Neither the presence of water vapor nor the composition of the black liquor solids had a large effect on

53

the transformation of organic matter to light gases during devolatilization. These parameters also had little impact on the evolution of light gases during secondary cracking and reforming reactions at low temperatures (700-800°C). However, at higher temperatures (900-1100°C), these variables had a greater impact. The water gas shift reaction apparently played a major role in the distribution of carbon between CO and CO2 during secondary reactions at longer residence times and higher reactor temperatures.

Gea et al66 found that both pyrolysis and gasification can be considered as alternatives to conventional boilers for recovering chemicals and energy from black liquor. The present work is focused on the study of pyrolysis of alkaline black liquor from straw, and its interest is enhanced by the fact that the material used has hardly been studied before. The influence has been studied of the final pyrolysis temperature (500-900°C), the heating rate (5-30 °C/min), and the addition of a determined CO concentration in the N2 atmosphere (5-40% vol) on both the final solid conversion and the devolatilization rate. The results show that the thermal decomposition of the organic matter fraction of black liquor takes place at temperatures below 550 °C in N2 atmosphere. The weight loss observed at temperatures higher than 550 °C is mainly due to reduction reactions of alkaline compounds by the carbon. The final solid conversion and the devolatilization rate are also noticeably influenced by the addition of a certain CO flow rate in N2 atmosphere. Wartena et al67 worked on recycling of inorganic chemicals from the kraft recovery black liquor 68and molten salt mixtures containing sodium carbonate, sodium sulfide, and sodium sulfate have been electrolyzed to generate sodium oxide and sodium sulfide while removing carbon from the system in the form of gas and maintaining a sulfur balance in the melt. This investigation leads toward the development of an electrolysis-based recycle process for pulping chemicals. The molten salts are presently found in the chemical recovery process of kraft pulping. Electrolysis was performed in a divided melt/undivided atmosphere and divided melt/divided atmosphere to avoid consumption of the oxide and sulfide products. The anodic reaction was carbonate oxidation to carbon dioxide and oxygen while sulfide oxidation to polysulfides occurred to a lesser extent at less positive potentials; sulfate oxidation was not observed to occur. The cathodic reaction was sulfate reduction to sulfide and oxide, the desired molten precursors for recycled pulping liquors.

Wartena et al69 investigated an electrochemical pathway for the efficient recycling of inorganic chemicals using the kraft pulping process. The

54

electrochemical behavior of sodium carbonate-sulfate melts, under conditions similar to those of kraft smelts, are examined. Results indicate the potential for a process that causticizes kraft smelt by electrolysis in a divided electrochemical cell. Cyclic voltammetry suggests sodium oxides and additional sulfides can be formed in the melt while the carbon is released in gaseous form from mixtures of sodium carbonate and sulfate. Hupa et al70 presented initial results concerning the influence of metaborate addition on liquor burning properties, smelt bed melting behavior, and fireside deposit characteristics in superheaters. The data are obtained from multiphase chemical equilibrium calculations of the furnace processes and from results obtained by employing the Abo Akademi single droplet burning test facility. The presence of boron is found to lower the melting temperature range of the smelt and the carry over liquor residue particles.

Adams et al71 evaluated sodium salt scaling in black liquor evaporators and concentrators. The control of Na2SO4 content of liquor avoids heterogeneous nucleation by ensuring a continuous supply of the sodium salt crystals that can form from black liquor. Five ways of reducing sodium salt scaling are listed. Bujanovic et al72 found that borate can be used as either a total or partial autocausticizing agent for conversion of Na2CO3 to NaOH. For every mole of Na2CO3 converted, a mole of NaBO2 is generated and remains in the black liquor. The objective of this study is to determine the effect of this additional NaBO2 on the boiling point rise of the resulting black liquor. Commercial black liquor was obtained, and borate was added at levels equivalent to 30, 60, and 120% autocausticizing. With borate present, the boiling point rise observed during evaporation was greater than that observed during dilution. This phenomenon was not seen for black liquor without borate added nor for black liquor with only Na2CO3 added. It is proposed that this difference is due to the formation of a complex between kraft lignin and borate. Borate was also found to be totally soluble in black liquor and to increase the solubility of the other components. The addition of borate had little effect on the boiling point rise of the black liquor. The largest effect of borate was observed at an addition level equivalent to 120% autocausticizing and at over 70% solids, where borate increased the boiling point rise by about 5 K. Shi et al73 produced data for describing that Sodium carbonate and sodium sulfate were the major inorganic components in black liquor produced from alkali pulping. And these two compounds had been shown to be the major

55

contributors to the formation of encrustations on the heat-transfer surface of black-liquor evaporators. Therefore, a determination of the variables affecting the nucleation and growth of these species is an important step in elucidating mechanisms by which such encrustations are formed. The metastable zone width for crystallization in the Na2CO3-Na2SO4-H2O system was determined as a function of the total solute concentration and the ratio of sodium carbonate to sodium sulfate. The polythermal method was used in the measurements, and supersaturation was generated by heating the system contents to increase the system temperature at a constant rate. Classical nucleation theory was combined with the metastable limit to correlate the effects of temperature and solute concentration on primary nucleation kinetics.

Rudie et al74 discussed the tendency of calcium to form calcium scales in digester heaters and black liquor evaporators is discussed. Calcium carbonate scale formation is so pervasive in digesters and evaporators that systems are designed to operate when 1 unit is taken off line for descaling. Scale formation occurs either by continuous growth/crystallization or by aggregation of suspended crystals into a surface deposit. For precipitation to occur, the cation and anion must be present in solution at concentrations that exceed the solubility product, conditions that are met in the kraft process.

Severtson et al75 found that buildup of scale in the kraft digester and black-liquor evaporators is a major contributor to lost pulp mill productivity. Scale deposition occurs in areas such as the heaters and extraction screens of continuous digesters and the liquor side of heat-transfer surfaces in the evaporators. This reduces the efficiency and control of pulping and evaporation processes and eventually forces the costly cleaning of equipment. Since kraft pulping conditions require high temperatures and a high concentration of calcium and carbonate in alkaline liquors, the precipitation of CaCO3 is inevitable. Traditionally, scaling tendencies have been managed using process control methods to reduce deposition rates. This paper examines the thermodynamic principals behind such process modifications while introducing the kinetic steps of the overall deposition pathway. Chemical additives that interfere with and retard the individual steps of the overall scaling mechanism can help to further reduce deposition rates. Rosier et al76 calculated the ability to predict burkeite precipitation in the evaporator train is an important consideration in the design and operation of this equipment. With knowledge of the factors affecting precipitation of burkeite and sodium carbonate, the papermaker can predict the level of black-liquor solids

56

(BLS) at which precipitation begins. This paper presents a simple model that uses white-liquor composition and kraft digester operating parameters to predict the level of BLS (critical BLS) at which burkeite precipitation will begin. The model illustrates how critical BLS responds to changes in causticizing efficiency, to addition of spent acid (sesquisulfate) from a ClO2 generator to weak black liquor, and to addition of makeup chemicals. Roberts et al77 discovered a new approach for reducing black liquor viscosity is presented. Black liquor causes problems for the paper industry by hindering pulp processability. In the proposed approach, viscosity reduction is achieved by "salting-in" black liquor through the addition of thiocyanate salts. Several thiocyanate salts capable of reducing liquor viscosity by more than two orders of magnitude are identified. Various comparisons show that GuSCN is the best viscosity-reducing agent. Moreover, lignin is found to play an important role in the viscosity behavior of both unmodified and salted-in black liquor at high solids concentrations. Roberts et al78 discovered a new approach for concentrating black liquor without increasing its viscosity involves addition of thiocyanate salts. This induces enhanced solubilization of the polymeric components through salting-in. A viscosity reduction of more than two orders of magnitude occurs despite wood type or liquor solids concentration. Viscosity reduction is greatest at high solids contents. The salting-in technique is also more effective in controlling black liquor viscosity than heat treatment. Computer simulations ascertain the ramifications of the change in sodium to sulfur balance in commercial pulp mill.

Tucker79 developed the main obstacles to commercial black liquor (BL) and biomass gasification/combined cycle power generation (GCC) by paper mills are outlined. For low temperature BLGCC, the uncertainty over the carbon conversion level in kraft-liquor applications, the need for a computational fluid dynamics model, and the need for an autocausticization process to prevent some sodium carbonate from forming are the major areas for further research. For high temperature BLGCC, reactor vessel lining materials must be identified that can survive the high temperatures and alkalinity, along with methods to reduce the cost of the higher causticizing load. Biomass GCC needs more research into tar management both by catalytic and non-catalytic systems.

57

2. 3 Chemical Treatment of the Black Liquors: Following references were found on the chemical treatment of the back

liquors: Gea et al80 characterized the swelling properties of black liquors and assess

their dependence on the temperature and gas composition of their combustion or gasification environment. Measurements are made using an industrial black liquor derived from the soda pulping of wheat straw. The tests are conducted in inert, oxidizing, and gasifying environments at temperatures ranging from 500 to 900°C. It is found that the straw liquor swells to a much greater extent than most kraft or soda liquors produced by the pulping of wood. Liquor droplets subjected to hot gas atmospheres, containing O2 and/or H2O(v), swell less than droplets in hot N2 atmospheres. Results imply that straw soda black liquor burns more rapidly than most kraft liquors because of its large surface area following swelling. This causes fluidization problems when the liquors are injected in a laboratory-scale fluidized bed gasifier.

Swann81 assessed the potential for development of new pulping processes

and pulping control systems. Pulping different species requires enhanced flexibility in the chemical fiber-line equipment. Mills around the world are installing new bleaching sequences, and the move away from traditional chlorine bleaching is almost complete. Some mills are experimenting with enzyme treatments to improve pulp bleaching, and research on enzymes is continuing. There is increased interest in black liquor gasification systems, and improvements are also being effected in mechanical pulping systems.

Howell et al82 developed a method to recover hardwood kraft lignin by acidification of black liquor using waste acid from a Mathieson chlorine dioxide generator is proposed. Optimum reaction conditions to maximize the lignin yield and minimize acidification costs were determined. To analyze the effects of the major variables, a 23 factorial model describing the effects of acidification temperature, degree of agitation, and rate of waste acid addition was developed. Increasing the acidification temperature improved the lignin precipitation and filterability. However, the maximum practical temperature was 70 °C because the lignin precipitate starts to form a tarlike substance at approximately 80 °C. Also, the rate of acid addition should be minimized. In practice, this will be determined by the mill reaction vessel size, which depends on the black liquor flow rate to be acidified. Last, the stirring rate should be kept as low as possible, although some agitation is still required to uniformly mix the acid and black

58

liquor. A steady-state computer simulation of incorporating a proposed 21 ton/day hardwood lignin recovery plant to the kraft liquor cycle showed no adverse effects in the chemical balance of the mill.

De et al83 developed the recovery of water with inorganic compounds in kraft black liquor by pressure carbonation followed by different combinations of membrane separation processes. Experiments were carried out to observe the effect of carbonation on black liquor, in terms of conversion of lignin-bound alkali compounds into carbonate salts. Further, the reaction rate during carbonation was studied and a correlation for decrease of active alkali with time is presented, which is equivalent to a first order rate equation. Different combinations of membrane processes used were (a) ultrafiltration of the carbonated liquor followed by nanofiltration, (b) nanofiltration of the carbonated liquor, and (c) direct nanofiltration of untreated black liquor. Comparisons were made in terms of recovery of the inorganics, quality of the permeate, and permeate flux. Of the three combinations, the first scheme yields higher rejection and better recovery as well as reasonable permeate flux. Recovery was higher for higher feed concentrations. Erich Gruber et al84 characterized the state of flocculation by image analysis and by the scattering of laser light in their study on the flocculation behavior of micro-particles retention system in different furnishes. The dosage of cationic polymer was found to influence the floc size strongly, regardless of the kind of microparticle system used. The dosage of the microparticle component has an impact on the floc size only in the case of the cationic polyacrylamide – bentonite system. The floc index, measured by light scattering, in both cases by the dosage of polymer and microparticle component. The floc index correlated well with filler retention during hand sheet formation. Satu Ojala et al85 worked on the catalytic oxidation to abate VOC and odorous emissions form pulp mill. In their study on catalytic incineration of chip bin emissions, they followed the operation of catalytic pilot process by measuring emissions. The catalysts were also tested in the laboratory work. It was found that catalytic incineration can treat these odorous emissions, and the conversions can even be close to complete oxidation. However, some problems occurred because of black composites, which blocked the heat exchangers of the pilot incinerator.

59

2. 4 Literature about black liquor biological treatment:

Grover et al86 worked on the methanogenesis of black liquor from a pulp and paper mill was achieved in a 2-stage biphasic reactor system using an immobilized cell system. The most effective organic loading rate for the anaerobic treatment of black liquor was 8.0 kg/m3 per day, leading to 55 percent chemical oxygen demand removal, 11 l of biogas production per day, and a biogas methane content of 71 percent. The 2-stage reactor performed better than a monophasic system, probably because the former has efficient process control and removes carbon dioxide during the acidogenic phase.

Olazar et al87 examined the thermal processing, at low temperature, of

straw black liquor in two different bench scale reactors. In the fluidized bed, the loss of fluidization due to bed agglomeration was the main problem encountered. The second reactor utilized a spouted bed and was tested in order to overcome the agglomeration observed in the first reactor. Experiments under various operating conditions were carried out in order to gain basic knowledge concerning the behavior of this residue during pyrolysis, gasification, and combustion processes. In order to work below the melting point of the black liquor inorganics, the reaction temperature was maintained under 600°C. Liquid black liquor and dry black liquor were employed as feedstocks. Air, nitrogen, and nitrogen-oxygen mixtures were considered as reaction atmospheres.

Philippe et al88 determined compact anaerobic/aerobic waste water treatment at the Minguet and Thomas recycled paper mill in Franace. They developed new biological wastewater treatment facility who met the Government’s new discharge limits. The plant consists of an anaerobic pretreatment step using the high rate BIOPAQ UASB (upflow anaerobic sludge banquet) technology followed by a small activated sludge plant for polishing. Buisman et al89 worked on biotechnological sulphide removal from the effluents. They reported a new system for sulphide removal. Sulphide is converted into elemental sulfur using colourless sulfur bacteria. A 4 m3 biorotor reactor has been tested using sulphide containing anaerobically treated paper mill wastewater. Sulphide removal efficiencies ewll above 90% were achieved at a hydraulic retention time of 19 minutes. Using Pall rings of 2.5cm Bio-Net 200 as carrying materials. It was also found that reticulated polyurethane is not suitable as carrying material for the sulphide removal process in the presence of fatty acids.

60

Font et al90 investigated the treatment of black liquors from a soda pulping mill with the white-rot fungus Trametes versicolor pellets. Treatment with the T. versicolor pellets reduces color and aromatic compounds by 70-80 percent and chemical oxygen demand by 60 percent. Experimental results suggest a relationship between laccase production and toxicity reduction. Kim et al91 evaluated control pollutant emissions from coal combustion in some developing countries, biocoalbriquette and an artificially produced solid fuel. Both the breaking strength and production costs of the bio-coal-briquette have become essentially the most important factors in popularizing it in these countries. To increase the breaking strength and decrease the production costs, it is proposed in this study to use pulp black liquor, a byproduct from the pulp production industry, as a binder. The influences of pulp black liquor on the briquetting and combustion characteristics were investigated. Furthermore, the desulfurization characteristics of pulp black liquor were also evaluated through combustion experiments. The study results show that the briquetting pressure has a limited effect on the breaking strength. An increase in the briquetting pressure yields greater breaking strength of up to the 50 MPa. Above 50 MPa, the breaking strength changes very little with the briquetting pressure. The use of pulp black liquor has had a greater effect on increasing the breaking strength than on changing the briquetting pressure and also on improving the combustion characteristics of the biocoalbriquette. On the other hand, pulp black liquor has some desulfurization capabilities. When used as a binder, it not only increases the breaking strength and decreases the necessary briquetting pressure, but it also improves some characteristics of the combustion and reduces the pollutants emission. Gupta et al92 analyzed the treatment of pulp mill wastewater by Aeromonas formicans. Data from batch studies indicated the ability of the strain to remove 71 percent and 78 percent of chemical oxygen demand (COD) and lignin, respectively, while the color removal efficiency was approximately 86 percent in 10 days of retention time. The analysis of lignin degradation products following 20 days of incubation showed phenolic acid formation, which accounted for the pH reduction during batch studies. The removal efficiencies of COD, color, and lignin obtained in continuous reactor studies were 73, 88, and 77 percent respectively for a detention period of 8 days. These efficiencies closely resembled those in batch studies. Bergeron et al93 developed two five-litre activated sludge (AS) bioreactors were operated for several months to demonstrate potential mill applications of a

61

four-assay set, which has been proposed as a tool for monitoring the health of the AS microbial population. The set consists of three specific oxygen uptake rate (SOUR) determinations at different substrate concentrations (SOURat, SOURnmax and SOURtox where at, nmax and tox are defined as aeration tank, near maximum, and toxic, respectively), and a specific adenosine triphosphate (SATP) assay. Two disturbances were applied at different times to an AS system treating kraft effluent. First, temperature was increased from 25 to 40°C, and second, a black liquor spill was simulated. The data before, during, and after these disturbances were statistically analyzed. From this analysis, we concluded that the four-assay set could be used as a microbial health characterization (MHC) tool. It allows an operator to correlate microbial changes with operating data over a medium-term time scale. We compared the values obtained during periods of upset in the system treating the kraft effluent, to the baseline data set determined from stable operation periods. This demonstrated how the four-assay set could be used as a biological early warning (BEW) tool. It allows a treatment system operator to make appropriate adjustments immediately after detecting values that fall outside the baseline data range. Berube et al94 used over 99% of the reduced sulphur compounds (RSC) contained in a synthetic foul evaporator condensate. At a neutral pH, the removal of the RSC was entirely due to stripping by the aeration system. It was possible to reduce the amount of RSC that was stripped to the atmosphere by promoting the biological oxidation of RSC through pH adjustment. A pH of less than approximately 4.5 was required to establish biological oxidation of RSC in the MBR. However, even at a pH of 3, which has been reported by others to be the optimal pH for the growth of thermophilic sulphur-oxidizing microorganisms, biological oxidation accounted for only approximately 50% of the RSC removed during treatment. The removal of the remaining 50% of the RSC removed during treatment was still due to stripping by the aeration system. The results further suggested that the long-term stability of a high temperature MBR operated at a low pH is questionable. In addition, the biological oxidation of methanol, which is considered to be the primary contaminant of concern contained in evaporator condensate, was significantly inhibited at a pH of less than approximately 4.5. Consequently, the simultaneous biological removal of methanol and RSC from foul evaporator condensate using a high temperature MBR was concluded to be impractical. Dufresne et al95 of Domtar Papers pulp and paper mill in Windsor, Quebec, Canada, investigated the potential for anaerobic treatment of contaminated kraft mill condensates. The objectives of this project were to assess

62

the technical feasibility of replacing the steam stripper with anaerobic treatment, to provide basic information for the design of an anaerobic treatment process for condensates, and to provide information on treated condensate quality for eventual reuse. The project involved extensive chemical characterization of condensates, followed by treatability tests. The tests included laboratory bench-scale tests and on-site pilot testing using direct feed from the process. Characterization showed that the organic content of the condensates was essentially methanol, as expected, but that foul evaporator condensates had high sulfide contents. It was found that undiluted foul condensates at the Windsor mill are toxic to the anaerobic biomass because of these high concentrations of sulfides. Treatment of combined condensates is possible at an approximate volumetric loading of 10 to 12 g/L d chemical oxygen demand (COD) with good production of biogas (0.35 L/g of COD removed) and excellent methanol removal (better than 95%). The biogas produced is of excellent fuel quality with close to 90% methane, but with a high sulfide content (close to 4%). Marwaha et al96 discovered three different support materials for immobilization of Phanerochaete chrysosporium for biobleaching of anaerobically digested black liquor were tested. Specifically, the effectiveness of jute rope, cotton, and wheat were examined. Dangcong et al97 developed the anaerobic digestion of alkaline black liquor was investigated in batch and in continuous systems to establish its toxicity and treatability. The alkaline black liquor is the spent condensate liquor from alkaline pulp-making plants. There is much interest in treating this kind of wastewater, as the alkaline black liquor is considerably toxic to the environment. Findings indicated that lignin and related compounds in the alkaline black liquor, which were the major inhibitory substances, could not be decomposed by anaerobic bacteria. In the continuous systems, the up-flow anaerobic sludge blanket reactor obtained 50-60 percent chemical oxygen demand (COD) removal efficiencies at organic loading rates of 5-10 kg COD m3 day-1. Gas production was found to be 2-3 dm3 per dm3 of alkaline black liquor. Granular sludge and cluster-like sludge were found in the reactor; the latter possessed good settling ability. Mikulasova et al98 determined the efficiency of a-D-glucoisosaccharinic acid (GISA) degradation by M. lylae, and the ability of this strain to utilize other black liquor hydroxyacids. GISA is a sulfate black liquor that represents around 33 percent of the total amount of hydroxyacids formed during the sulfate cooking of softwoods. The ability of M. lylae to utilize individual basic acids of

63

GISA was determined by isotachophoretic analysis. It was found that the presence of acid in the environment influenced the ability of M. lylae to utilize GISA. This information may be applied in the biotechnological utilization of the hydroxyacids fraction of black liquor and in the biodegradation of wastewater hydroxyacids obtained at the different bleaching stages of pulp production.

Experimental Work

64

3. 1 Chemical investigation of the black liquors: Chemistry of the black liquor was investigated by its complete chemical analysis. Different parameters of the black liquor were analyzed by adopting standard methods. Some methods employed were of internal origin.

3. 2 Determination of pH of the weak black liquor:

The pH is determined by following procedure: - Calibrate the pH meter by immersing the electrodes t approximately 3cm

in a buffer solution of pH 7.0 in a beaker - Wash out the electrode with de-ionized water - Adjust the temperature of the sample to that of buffer solution. - Take the sample in a beaker and immerse the pH electrode in it to approx

3cm. - Note the reading when it is stable.

3. 3 Density determinations:

Weak black liquor was maintained at required temperature for 30 min before the density measurements. Black liquor was transferred into a pre-weighed, dry and cleaned measuring cylinder of 1 dm3. Right after settling of the liquor, the cylinder was weighed. Mass of the black liquor was taken by difference and volume by reading on the cylinder.

Density was determined by dividing mass with volume. Calculations were made to four decimals to get more precision and reliability.

3. 4 Determination of total solids:

Measure 10 ml of the liquor into a tared platinum evaporating dish at least 3 inches in diameter and weigh quickly to the nearest 5mg. Place it in an oven at

65

105 ± 3 °C for 24 hrs and cool in a desiccator to room temperature. Weigh quickly to nearest 5mg and obtained loss in weight26. Percentage of Total Solids = (Residue Weight / Sample weight) x 100

3. 5 Total alkali:

This is essentially the conductometric method of McElhinney99 . Dilute 10 ml of black liquor to 100 ml in a volumetric flask. Pipet 20 ml of diluted liquor into a 250 ml beaker containing 150 ml of water. Immerse the electrodes of the conductance bridge in the solution, stir continuously with a mechanical or magnetic stirrer and obtain the initial bridge reading. For most black liquors a 1-cm spacing of the electrode disks is most suitable. Add 0.1N HCl in fixed increments of volume and take a reading after each addition, keeping the temperature constant. Continue the titration until the change in the bridge reading remains constant for five or six additions of titrant. Plot the data on linear coordinates with reciprocal ohms as ordinate and milliequivalents of acid as abscissa. The curve obtained will usually resemble a V with apex flattened or broadly rounded. To locate the end point, extrapolate the arms of the V in a straight line until the extrapolated segments intersect. The point of intersection projected on the abscissa will give the milli-equivalents of acid corresponding to end point.

The titration measures the NaOH, Na2CO3, organic sodium salts, silicates and Na2SO3 (to NaHSO3). Na2SO4 and Na2S2O3 are not measured. A value for total recoverable sodium contents of the liquor is obtained from the sum of the total alkali, Na2SO4, Na2S2O3, and one half the Na2SO3.

Total alkali as Na2O = milli-equivalents of acid x 15.5

3. 6 Determination of active alkali: Pipette 100 ml black liquor into a 500 ml volumetric flask containing about 100 ml of water and allow the pipette to drain for its calibration period. Add 100 ml of 0,5M BaCl2, stir and allow to settle until partially clear. By means of a glass stirring rod, transfer a drop of clear supernatant liquid to a small test tube containing about 1ml of 1.0N H2SO4. if no white precipitate is observed , add an additional 20 ml of BaCl2 to the flask. When there is an excess of BaCl2, dilute the contents of the flask to mark, mix well and allow to settle until 200 ml of clear liquid stands above the precipitate. Pipette 10 ml of the supernatant liquid into a 250 ml beaker equipped with a mechanical or magnetic stirrer, add 5 ml of 40% formaldehyde, and titrate immediately with 0,5N HCl to a pH of about 3.5.

66

Follow the course of titration potentiometrically on the pH range of a pH meter, using the saturated calomel-general purpose glass electrode pair. Add the titrant in increments of fixed volume, record the pH after each addition, and plot the data on linear coordinates with pH as the ordinate and milli-equivalents of HCl as abscissa. Connect the plotted points in a smooth curve. Obtain the milli-equivalents of acid corresponding to the end point by projecting the inflexion point near pH 8.3 to abscissa. From this value subtract the acid equivalent, if any, of the 5ml formaldehyde as determined in a separate test, using phenolphthalein as indicator. The difference is the milli-equivalents of acid consumed by the liquor.

Active Alkali as Na2O = milliquivalents of acid x 1.55

3. 7 Determination of effective alkali:

The concentration of strongly alkaline constituents is determined by titration of a sample of the liquor with strong with the first inflexion point according to the following procedure.

Chose the sample volume (v ml) so that about half the burette capacity is used. Normally a sample volume of 20% of the burette capacity is suitable. With the aid of a calibration pipette or equivalent device, transfer the chosen volume of the sample to the titration vessel. The sample volume should be known with a precision of at-least 1%. Dilute the sample with distilled water to volume of acid as follows: Volume consumed at the first inflexion point = a ml Total volume consumed at the second inflexion point = b ml Total volume consumed at the third inflexion point = c ml Conc. of HCl in moles per liter = m Volume of the sample taken in milliliters = v ml

Effective Alkali = a . m/v

3. 8 Determination of total sulfur as sulfate:

Take a china dish and add 1 ml of sample, add 1 ml of conc. NaOH solution, add 5 ml of saturated KMnO4, dry to moisture less. Add dilute HCl while heating slowly. Transfer the dish contents to 500 ml distilled water in measuring flask and filter. Take 25 ml of this sample in titration flask, add 5 ml of conditioning reagent, add one small spoon of BaCl2. Leave the solution for 10 minutes. Check

67

for transmittance at 410 nm at UV / VIS spectrophotometer. Find the SO4-2 contents by comparing with the graph.

3. 9 Determination of sodium sulfide in green liquors:

Titrate 50 ml of the clear solution with 1.0 N HCl using phenolphthalein indicator to a colorless end point, and record this titration as “B”. Continue the titration on the same solution using methyl orange indicator to a red end point. Record this titration (from the beginning) as “C”100. Calculate sodium sulfide as:

Na2S (g/1,000 ml) = (C-B) x 15.6 Calculate amount of Na2O as:

Na2O = Na2S x 0.794 Record the result as “b”. Calculate Percent Reduction as:

Reduction, % = [a/(a+b)] x 100 Whearas: a is amount of Na2O present in sodium sulfate test, b is amount of Na2O present in sodium sulfide test.

3. 10 Determination of silicon dioxide in the black liquor:

Determination of silica in the black liquors can be made by following method;

1. Take 1 ml. Of the filtered sample in a 500 ml. Beaker, containing 200 - 250 ml. dis. Water.

2. Adjust the pH of the Sol. to 4.5 - 5.0 with the drop wise addition of 4N H2SO4 while continuous stirring.

3. Pour the Sol. in one liter volumetric flask and make the Vol. upto the mark with dis. water.

4. Take 10 ml. of the prepared Sol. in 100 ml. volumetric flask and make the Vol. up-to the mark with dis. water (10 times dilution).

5. Take two titration flasks. In first flask take 25 ml. of the diluted sample and in the second titration flask take 25 ml. of dis. water (Blank).

68

6. Add 1 ml. of soln. A in both flasks and allow to stand for 5 minutes after shaking well.

7. Add 1 ml. of Sol. B in both flasks and wait for 3 minutes. 8. Finally add 1 ml. of soln. C in both flasks and allow standing for 20

minutes. 9. Set the wavelength of spectrophotometer at 820 nm 10 minutes before

running the samples. 10. Adjust the transmittance at 100% with the blank Sol. 11. Read the transmittance for the sample (wavelength 820 nm). 12. Read the reading of Silica as SiO2 from the graph at the desired

transmittance. 13. Silica as SiO2, g/l is calculated as:

Silica as SiO2, g/l = Reading in ppm from the Graph * (Dilution Factor)

The Vol. of G. L sample taken As in above case

14. Silica as SiO2, g/l = Reading in ppm from the Graph * 10

15. If the transmittance reading does not fall in the range then appropriate dilution of the Sol. (Step 3) can be made.

3. 11 Determination of sodium:

Preparation of Calibration Solution: Prepare a series of at least 4-calibration solution by diluting v ml of sodium standard solution to 100 ml with water in volumetric flask. Select the volume v so that the working range of the flame photometer is covered, this range is normally between 1 mg/l and 10 mg/l. Use distilled water as zero solution101. Determination of Sodium: Dilute the sample solution stepwise until the sodium concentration is within the range covered by the calibration solution. Operate the flame photometer as instructed by the manufacturer. Adjust the instrument reading to zero using water. Measure the emission of the calibration solution and the sample solution at 589 nm. Determine the solution concentration of the sample solution, noted as a mg/l, from a calibration graph obtained by plotting the emission against the sodium content of the calibration solution. Calculate the sodium contents of the as grams per liter from the expression;

X2 = a v f / m2 Whereas:

69

X2 is the sodium contents of the original black liquor in grams per liter, a is the concentration of the sodium in sample solution, obtained from the calibration graph, in milligrams per liter, v is the volume of the sample solution in liters, m2 is the mass of the black liquor taken for analysis, in gram.

3. 12 Determination of chemical oxygen demand:

Chemical oxygen demand of the weak black liquor is determined by the following the steps given below;

- Weigh 0.5 – 1.0 g of the well mixed sludge in the quickfit flask. - Add I g of mercuric sulfate followed by the addition of 5 ml of sulfuric

acid. - Add 50 ml of 0.25 N potassium dichromate - Connect the flask with the reflux condense and add 70 ml of silver sulfate

– sulfuric acid reagent. - Reflux for two hours on a hot plate. Cool the flask - Titrate it with standard ferrous ammonium sulfate solution using 0.5 ml of

ferroin indicator till the color changes from green blue to reddish. Note the mls. consumed in the titration. (A)

- Run a blank sample along with it. (B) - Calculation:

COD mg/kg = (B – A) × N× 8000 (Sample weight) Where N is normality of ferrous ammonium sulfate

3. 13 Determination of biological oxygen demand:

Biological oxygen demand was determined by standard 5-days BOD procedure. Procedure is as follows: 1. Slowly siphon three portions of aerated dilution water into three separate BOD

bottles. Avoid adding atmospheric O2 to dilution water. 2. To two of the three BOD bottles, add 1 ml MnS04 solution, followed by 1 ml

alkali-iodide-azide reagent. Submerge pipette tips in sample when adding reagents. Rinse tips well between uses.

3. Stopper carefully to exclude air bubbles; mix by inverting bottle several times.

70

4. When precipitate has settled to about half the bottle volume, carefully remove the stopper and add 1.0 ml conc. sulfuric acid. Re-stopper and mix by gentle inversion until the iodine is uniformly distributed throughout the bottle.

5. Transfer 203 ml of sample into a white 500 ml casserole dish and titrate with

0.0250N sodium thiosulfate to a pale straw color. Add 1-2 ml of starch solution and continue to titrate to first disappearance of the blue color. (200 ml of original dilution water is equal to 203 ml of dilution water plus reagents.)

6. Titrate two of the three samples. Results should be within 0.1 mL if using a buret

with increments of 0.05 mL. Calibrate the DO probe with the third bottle. Calculation of BOD in sample:

BOD5 = BOD mg/l = [(IDO -DO5) - seed correction] x dilution factor * dilution factor = 300 sample size (mL)

3. 14 Isolation and purification of micro-organisms:

Biodegrading micro-organisms were isolated from the final effluent of treatment plant of Packages Limited Lahore. Both bacteria and fungi were isolated using nutrient agar and potato-dextrose-agar respectively.

Growth medium for bacteria and fungi was prepared by mixing following components. Nutrient agar medium:

Peptone 0.5% Yiest Extract 0.25% NaCl 0.05% Agar 2%.

Potato-dextrose-Agar: Potato extract 100 ml. Glucose 2% Agar 2%.

71

Both media were sterilized by autoclaving for 15 min at 121.21°C and then cooled to room temperature. Sterilized media were then transferred into pre-sterilized petri plates, aseptically. I each case, 0.05ml sample of the effluent was mixed with the agar medium and then the agar medium was allowed to set. The nutrient medium containing dishes were incubated at 37°C and potato-dextrose-agar medium containing dishes were incubated at 30°C. Both bacterial and fungal colonies appeared after 24 hours incubation. The isolated cultures were purified by streak plate method. Pure culture obtained were then maintained on agar slants and stored at 4°C. These cultures were revived monthly. Selected strains were evaluated for their bio-degradation activity.

3. 15 Determination of biodegradation activity: Medium preparation: Medium was prepared by adding following components. Corn starch: 28 g/L, glucose: 5 g/L, peptone: 18 g/L, KCl: 0.5 g/L, MgSO4.7H2O: 1.5 g/L, KH2PO4 :1 g/L, CaCl2.2H2O: 2 g/L. The media components were boiled for 30 min in order to get homogenous mixture. Then the solution was shifted to 250 ml conical flasks (50 ml to each flask). The flasks were sterilized in autoclave for 15 minutes at 121.21°C. After sterilization the flasks were cooled and inoculated with the selected cultures. Incubation: After inoculation, the fermentation flasks were incubated in orbital shaker set at 150 rpm. The incubation temperature for bacteria was 37°C and that for fungi was 30°C. Bacterial fermentation was carried out for 36 hours and mold fermentation for 60 hours. After termination of the fermentation, the flasks were analyzed for the production of microbial biomass, cellulose and total protein contents.

3. 16 Determination of dry cell mass:

After fermentation the biomass was collected by filtration, washed with 0.1% saline water in dried in oven at 105°C for 24 hours.

3. 17 Enzymatic Extraction and determination of its activity:

After fermentation the biomass was collected by filtration and washed with 0.1 % saline water. The biomass was then weighed. The biomass was crushed in pestle mortar. Crushed contents were filtered and the filtrate was collected for enzymatic activity determination.

72

Method applied to determine the activity of enzyme was cmc (carboxymethylcellulase) degradation. Took 2ml of the enzyme extract (filtrate) in a clean test tube and add 0.8 ml of citrate buffer, then added 1ml of 1% cmc solution. Incubate for 2 hours, add 0.2 ml of saturated DNS (Dinitrosalicylic Acid) reagent and make up volume upto 20ml by the addition of double distilled water. The sample was analyzed for absorbance at 580 nm, the cmc degraded was determined by comparing with calibration curve. An increase in absorbance at 580nm was recorded for 1-2 minutes. This rate was used to calculate the enzyme activity as follows; Enzyme activity = ΔA/min x V / Ix AmM580x V µ moles HQ/min/ml Whereas; V = 4.0 ml, volume of reaction mixture. µ = 0.05 ml, cmc solution added. AmM580 = 2.31, molar absorption, co-efficient of HQ (hydroquinone) at λmax = 580 nm. I = 2 cm, the path length of cell. Above equation becomes: Enzyme activity = 17.316 x Δ A/min μ moles HQ/min/ml. Specific Activity = 17.316 x A per min / mg protein per ml µ moles HQ/min/mg protein.

3. 18 Protein estimation:

Proteins were estimated according to the method described by Moss and Bond102 method is described as under. Standard bovine serum albumin solution: 5.0 mg of Bovine Serum Albumin was dissolved in 10ml of distilled water. The serum albumin dissolved very slowly. The standard was made two days before use and kept at 4°C. Procedure: 0.5 ml NaOH (1N) was added in each tube containing protein sample (250 – 750 µg) and shook in water bath (37°C) for 30 minutes. After cooling 2.5 ml of solution A and 2.5 ml of solution B, were added in each sample as well as in standard test tube. The solution in each tube was thoroughly mixed and incubated at 37°C for 30 minutes. Then to each tube, 0.5 ml of solution C was

73

added; mixed well and kept for 20 minutes at room temperature. Standard Bovine serum albumin (0.05 – 1.5 ml) was simultaneously used in experiment. The optical density was measured at 661 nm on spectrophotometer and a standard calibration curve was drawn and with its help the proteins were estimated.

Standard Curve for Proteins

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4

mg of Proteins

Opt

ical

Den

sity

Figure 3. 1 Standard curve for protein estimation

Further calculations were made by regression equation. As follows;

Y=1.6313x – 0.0105 R= 0.9987

Whereas, Y = optical density, X = mg of proteins,

74

Results and Discussion

75

4. 1 Chemical composition of the black liquors:

Black liquor is produced as a course of pulping process. For each ton of pulp nearly 100m3 black liquor is produced. Spent pulping liquor is a complex mixture of inorganic and organic salts suspended or dissolved in water. Strong black liquor contains between 50 and 70% solids, with the remainder being water. The solids are comprised of a complex mixture of both inorganic and organic constituents. A test for water solubility could be performed on the test material, but it would result in multiple values for individual constituents. Due to the lack of a suitable analytical method for the complex mixture, it is not feasible to measure the water solubility for the mixture.

Weak black liquor is concentrated to 50 – 70% dry solids in multi effect evaporators of the recovery plant. Thick black liquor contains 50-70% solids. At solids contents below 50%, the inorganic salts contained in spent pulping liquor are completely dissolved in the aqueous portion of the liquor. Often, the 50% solids point (the point where the salts start precipitating) is referred to as the “solubility limit.” At solids levels greater than 54%, Burkeite (2Na2SO4 - Na2C03) is the only salt that precipitates. Thus, between 50 to 75% solids, spent pulping liquor is essentially a water/organic-inorganic suspension.

Chemical analysis of the spent weak black liquor from soda pulping is given in table 4.1. It was found that liquor is alkaline in nature and contains solid contents between 8.5-10%. Around 60-75% of the dry solids contents of this liquor are organic in nature. The organic compounds found in black liquor are derived from wood. They are either: 1) natural wood extractives (or their reaction products) that are released as a result of the pulping process, or 2) materials formed through the reactions of the pulping liquors with the lignin or cellulose components of wood. Therefore, the compounds can be classified as lignin derived, cellulose derived, or extractives derived. Typical content ranges in kraft liquor are:

Lignin derived (39-54%; primarily consisting of polyaromatic macromolecules with lesser amounts of low molecular weight alcohols, aldehydes and simple phenolic compounds such as phenol, p-methyl phenol, catechol and guaiacol, Cellulose derived (25-35%; primarily a mixture of carboxylic acids such as formic, acetic, glycolic, lactic and glucoisosaccharinic) and Extractive derived (3-5%; primarily resin acids and fatty acids which are converted to salts at the high pH of the mixture).103

76

Table 4. 1, Chemical composition of the black liquors:

Parameters Unit Concentration (% w/w) PH 8.9 Solids % ( OD Basis) 3.5-10 Density g /cm3 1.03 Sodium %( OD Basis) 13.1 Sulfur %( OD Basis) 5.5 Silica %( OD Basis) 0.7 Organics %( OD Basis) 68

Chemical composition of the CTMP black liquor was determined by fractional and composite sample basis. Wasik et al (44) also used fractional composition analysis to evaluate lignin degradation products and addition of organic components into the black liquor at every stage. Information obtained from chemical analysis of the black liquor is given in the table 4.3. Chemical composition of the CTMP black liquor shows many variations from that of sulfite or kraft pulping. A big difference was found in the dry solids contents. CTMP liquor has extensively low dry solids, which is not suitable to get concentrated by commonly used concentrators. High energy will be required to concentrate it to 60 to 70%, which is minimal requirement of the recovery boiler. Organic to inorganic ratio also derives noticeable variation of chemical properties. Rate of change of chemical properties with rise in dry solid contents will be considerably determined by organics to inorganic ratio. Viscosity will also be affected. It is obvious that the organic to inorganics ratio will affect most of the properties of the black liquor towards chemical recovery.

77

Table 4. 2, Chemical composition of the chemi-thermo-mechanical pulping black liquor.

Tests After De-fiberator

After Twin Roll Press

Screw drainer Composite sample

Dissolved Solids %

7.7 0.6 1.83 1.04

Suspended Solids % (OD basis)

0.82 0.2 0.05 0.33

pH 8.4 7.86 6.3 6.0 Organics to inorganics ratio

1.80 1.52 1.95 1.85

Total Sodium % (OD basis)

11 13.9 8 10.96

Total Sulfur % (OD basis)

3.01 2.81 2.04 2.42

Chlorides % (OD basis)

0.36 075 0.42 0.48

Silica % (OD basis)

0.58 0.96 1.22 0.93

BOD %

0.32 0.82 0.65 0.45

COD %

1.65 1.34 2.24 1.34

%= % w/w. This black liquor is concentrated in the multiple effect evaporators (thick

black liquor with 60-70% dry solids) and is then burnt in the recovery boiler (10m3 of weak black liquor is concentrated to 0.8-1 m3). Incineration in recovery boiler results flue gases and molten sodium and sulfur known as smelt. The solution obtained after dissolving the recovery boiler smelt in water is termed as green liquor. 3.20-4.00 dm3 of thick black liquor produces 1dm3 of green liquor (Wartena et al; 67). Green liquor contains recoverable components of the black liquor e.g. sodium and sulfur. Analysis of the green liquor is given as under in table 4.2.

Table 4. 3, Analysis results of green liquor

Parameters Unit Concentration pH value 13.6 Density g /cm3 1.14 – 1.22 Sodium as Na2O g/l 147 Na2S g/l 90 Na2SO4 g/l 18 RE* as Na2O % (w/w) 90 * Reduction efficiency.

78

4. 2 Relation of black liquor composition with its properties:

In order to draw comprehensive relation of chemical properties of the black liquor with its composition, various determinations have been made. Most of the properties have direct or indirect relationship on the chemical composition of the black liquor. The standard parameter to determine these relations is dry solid contents of the black liquor. Relations have been derived with dry solids and other physico-chemical properties. This approach helps to determine the behavior of the black liquor towards chemical recovery plant (Greg et al;38). The solutions to various problems encountered by this black liquor, when fed to the chemical recovery plant can also be found.

4. 3 Relation of dry solids with density:

Zaman et al (48) determined the density of black liquor at various dry solid contents (10-100%) for wood based black liquors. Relation of density with dry solid contents for non-wood pulping black liquors have been determined. There are certain variations in rate of change of density with dry solids content of the black liquor. Results of viscosities and densities variations with the black liquor are given in table 4.4. At very low dry solid contents the density of black liquor is 1.009 gm/cm3. Density goes on rising with dry solids until 20% DS and then tends to decrease with increase in the dry solids. This non-uniform behavior in the density is different from other chemical pulping black liquors. Initially rise in the density is due to increase in the solid contents, but after 20% dry solids exceptional decrease in density is due to high organic contents. Though temperature is also a major defining factor of density but its effect in directly with water contents and conversely on expansion of black liquor. If we keep temperature constant and determine density with rise in dry solids then this depends wholly upon chemical composition of the black liquor and off course upon the organic contents.

At high dry solid contents organic contents also rises. Organic contents include micro fibers, hemicelluloses, celluloses, lignin and organic derivatives of wood origin organic compounds of pulping chemical action. Organic components are always lower in density than density of inorganic components and as its factor increases the density decreases and rate of change of density also increases. At low dry solid contents rate of change is around 0.006 with every point rise in dry solid contents this rate approaches to 0.001 at 17% DS and then increases towards lower side ultimately reaches 0.022 at 70% DS contents, thus rising up to three times of the original rate.

79

Table 4. 4, Comparison of densities and viscosities of the black at different dry solid contents.

Dry solids verses density data obtained at 65 °C

Sr. No. Dry Solids* Density** Viscosity*** 1 3.2525 1.0008 10.25 2 4.4789 1.0108 11 3 5.1513 1.0148 11.25 4 5.7296 1.0188 12 5 6.8709 1.0207 13.25 6 6.8738 1.0315 14.5 7 10.306 1.0413 15.5 8 10.9465 1.0453 17.25 9 11.442 1.0473 18.25 10 11.9876 1.0513 18.25 11 12.3026 1.0523 19.25 12 16.2246 1.0573 20.5

* percent of dry mass ** grams/cm3 *** milli pascal per second. The effect of the dry solid contents on density is plotted in figure 4.1. It is obvious form the graph that density increases with increase in the dry solid contents of the black liquor. Rate of change is higher in the start but it decreases with increase in the dry solid contents. Trend line becomes parallel to x-axis as it approaches to 17-20% dry solid contents. This variation in the density is due to exceptionally higher organic contents of the CTMP based black liquors. The decrease in density is clear from the figure 4.9. The graph shows the practical decrease in the density of the black liquor at high concentrations and at 65°C.

80

y = -0.0003x2 + 0.0098x + 0.9719

R2 = 0.9821

0.99

1

1.01

1.02

1.03

1.04

1.05

1.06

1.07

0 5 10 15 20

Dry Solids (% OD Basis)

De

ns

ity

(k

g/d

m3

)

Figure 4. 1, Dry solids verses density relations at 65 ˚C.

Following data (Table 4.5) shows the density and viscosity variations at 70˚C. As the temperature rises both density and viscosity increase but the trend is not linear.

Table 4. 5, Comparison of viscosity and density at 70˚C at various dry solid contents.

DS verses Density Curves at 70 °C Experimental Values

Sr. No. Dry Solids* Density** Viscosity*** 1 2.59 1.0011 9 2 3.39 1.0012 9.25 3 4.25 1.0032 9.5 4 6 1.0036 10 5 6.5 1.0098 10.25 6 7.37 1.01274 12.25 7 10.51 1.02344 12.25 8 10.77 1.0324 13.4 9 11.04 1.0374 14.4

10 12.43 1.0405 14.56 11 12.65 1.0464 15.6 12 13.71 1.0504 16.5 13 14.04 1.0512 16.9

* percent of dry mass, ** grams/cm3 *** milli Pascals per second. Following is the graphical presentation of the densities of the weak black liquor at 70˚C (figure 4.2). There is gradual increase in the density as in the case of 65˚C but the density is at lower side than that of 65°C.

81

y = 0.0003x2 - 0.0004x + 0.9989

R2 = 0.9788

0.99

1

1.01

1.02

1.03

1.04

1.05

1.06

0 2 4 6 8 10 12 14 16

Dry Solids (% OD Basis)

De

ns

ity

(k

g/d

m3

)

Figure 4. 2, Density verses dry solids at 70˚C for the weak black liquors.

Density of the black liquor was also determined at 75˚C. Objective of this study is to evaluate the variations in the density at different temperatures. Results are given in the table 4.6. It is obvious that the density rises with the rise in the dry solid contents. But at the same dry solid contents the density is inversely proportional to the temperature. As the temperature rises the density decreases. Both of the factors; increase in the organic components and rise in temperature are responsible for lowering the rate of change of density with dry solid contents.

Table 4. 6, Comparison of Density and viscosity at different dry solid contents.

Dry Solids verses Density Curves at 75 °C (Experimental Values)

Sr. No. Dry Solids* Density** Viscosity*** 1 4.3977 1.0089 9.5 2 5.4294 1.0158 10.5 3 5.9933 1.0175 10.8 4 6.5637 1.0197 11.4 5 7.4106 1.02374 12.9 6 7.9596 1.02743 13.1 7 9.5 1.03275 13.5 8 10.3948 1.03574 14.75 9 10.7276 1.03574 14.8

10 13.5261 1.03892 14.9 11 13.9032 1.04255 14.75 12 14.2982 1.045 15 13 14.9888 1.05694 15.2 14 15.463 1.06587 15.5

82

* percent of dry mass ** grams/cm3 *** milli Pascal’s per second.

The changes in the density of the black liquor have also been plotted verses dry solid contents (graph 4.3). Trend line gives behavior of the density with increasing dry solid contents. Experimental values give some variations which are caused by the bubble formation property of the black liquor at high temperatures. Bubbles are caused by degradation of high organic components of the black liquor.

y = -8E-05x2 + 0.0052x + 0.9895

R2 = 0.9442

1

1.01

1.02

1.03

1.04

1.05

1.06

0 2 4 6 8 10 12 14 16

Dry Solids (% OD Basis)

De

ns

ity

(k

g/d

m3

)

Figure 4. 3, Dry solids verses density of the black liquor at 75˚C.

In this experiment, the relation of density with viscosity of the concentrated black liquor was determined. Purpose is to determine the density at both high temperature and high dry solid contents. By comparing the values given in table 4.7, it is concluded that density rises with increase in the dry solid contents. High dry solids black liquor (thick) contains high proportions of solid materials which yield high density. Viscosity of the thick black liquor increases with increase in dry solids but it exceeds the experimental limits above 70% dry solids.

Table 4. 7, Comparison of density and viscosity at high dry solid contents.

Dry solids verses Density Curves at 80 °C (TBL) (Experimental Values)

Sr. No. Dry Solids* Density** Viscosity*** 1 45.5975 1.1919 44 2 51.4212 1.2112 68 3 54.4118 1.2256 120

83

4 57.7023 1.2283 157.6 5 59.5238 1.2456 224 6 69.3548 1.2644 280 7 73.0965 1.2844 - 8 78.6036 1.3077 - 9 81.1321 1.3176 -

* percent of dry mass ** grams/cm3 *** milli pascals per second. Density of the thick black liquor was plotted against dry solid contents in the figure 4.4. At high dry solid contents (40-80%) the rate of increase of the density is less than at lower dry solids. In fact, at higher dry solids, the organic portion is increased which yields low density as compared to the inorganic components.

y = 1E-05x2 + 0.0017x + 1.0881

R2 = 0.9895

1.18

1.2

1.22

1.24

1.26

1.28

1.3

1.32

1.34

40 50 60 70 80 90

Dry Solids (% OD Basis)

De

ns

ity

(k

g/d

m3

)

Figure 4. 4, Density verses dry solids at 80˚C.

Following table (4.8) and graphs 4.5 (a-d) show the relation of dry solids

with densities over temperatures 60˚C, 70˚C, 80˚C and 90˚C. There is an increase in the density with rise in dry solids but density decreases with increase in temperature at the same dry solid contents. The rate of change in density at various temperatures is given in figures 4.5, a, b, c and d.

Table 4. 8, Comparison of density at various temperatures and dry solid contents.

S No. DS* 60°C 70°C 80°C 90°C 1 12.5748 1.01225 1.02653 1.02526 1.02538 2 18.2427 1.0505 1.05668 1.04751 1.0463 3 28.0318 1.112 1.1118 1.1128 1.10025 4 51.3219 1.21085 1.2036 1.2136 1.21054

84

5 56.3107 1.2666 1.2649 1.2575 1.2444 6 59.129 1.3042 1.3062 1.3025 1.2994

* percent of dry mass

85

y = 2E-05x2 + 0.004x + 0.9659R2 = 0.9835

0.8

0.9

1

1.1

1.2

1.3

1.4

0 10 20 30 40 50 60 70

Dry solids ( %OD basis)

Den

sit

y (

kg

/dm

3)

Figure 4. 5, (a) Dry solids verses density at 60˚C, for the entire range of dry solid contents.

y = 5E-05x2 + 0.0019x + 1.0027R2 = 0.9783

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

0 10 20 30 40 50 60 70

Dry solids ( %OD basis)

Den

sit

y (

kg

/dm

3)

Figure 4.5, (b) Dry solids verses density at 70˚C, for the entire range of dry solid

contents.

y = 3E-05x2 + 0.0031x + 0.9839R2 = 0.9853

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

0 10 20 30 40 50 60 70

Dry solids ( %OD basis)

Den

sit

y (

kg

/dm

3)

Figure 4.5, (c) Dry solids verses density at 80 °C, for the entire range of dry solid contents.

86

y = 5E-05x2 + 0.0018x + 0.9986R2 = 0.986

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

0 10 20 30 40 50 60 70

Dry solids ( %OD basis)

Den

sity

(kg

/dm

3)

Figure 4.5, (d) Dry solids verses density at 90˚C, for the entire range of dry solid contents.

It can be concluded that high masses of this black liquor contains low

recovery chemicals (mainly sodium and sulfur) because the organic portion is major and it imparts no recoverable chemical / compound. But it has high heat value and facilitates the auto-burning of the black liquor in the recovery boiler and increases recovery in terms of steam. Comparison of density of black liquor at various temperatures has been given in fig 4.6. The density of the black liquor changes with change in temperature. At lower dry solid contents, black liquor shows lower density at relatively higher temperatures. At higher dry solid contents the effect of temperature on density is not clearly demonstrated by the graph, this is because higher dry solid contents causes bubbling and foaming in the black liquor. Hence these both phenomenons (temperature and bubble formation) disturb normal trend in the density of the black liquor.

Effect of Temperature on Density

1.001.011.021.031.041.051.061.071.081.09

0 2 4 6 8 10 12 14 16 18

Dry Solids (%OD basis)

Den

sity

(kg

/dm

3)

Density 65 ºC

Density 70 ºC

Density 75 ºC

Figure 4. 6, Effect of temperature on the density of the weak black liquors.

87

Organic components impart lower density than inorganic components. As organic proportion increases, the density of the black liquor decreases. At low dry solid contents, the rate of change is around 6x10-3 gm/dm3 with every point rise in dry solid contents. This rate approaches 1x10-3 gm/dm3 at 17% dry solids and then increases. Ultimately, it reaches 2.2x10-2 gm/dm3 at 70% dry solid contents. Thus, it reaches to three times of the original rate. The rate of change of density with dry solids is demonstrated in fig 4.7. Graph shows decrease in density of the black liquor as the dry solid contents rise. Second important factor which determines density is the expansion of the black liquor (due to organic components) with rise in temperature. Foaming and bubbling causes expansion with reduce the density of the black liquors.

Figure 4. 7, Illustrates variation of density with increasing dry solid contents. Initially the density increases and then starts decreasing with rise in dry solids.

88

4. 4 Relation of dry solids with viscosity: Viscosity of the black liquor is key factor in determining behavior of black liquor towards chemical recovery plant. Flow of the liquor in recovery channels and its firing in the recovery boiler is determined by its viscosity. The relation of dry solids with density and viscosity is given in table 4.9. Relation of the dry solids with viscosity has been demonstrated in figure 4.8. At lower temperatures viscosity of the black liquor rises consistently with increase in dry solids. Rate of increase of viscosity is almost uniform throughout the dry solids range. This rate of change of viscosity is almost independent of temperature, at relatively lower temperatures. Viscosity of the black liquor is just 10 m.pa.s (milli Pascal second / milli poise) but at high dry solids it reaches upto 120 m.pa.s. Zaman et al (46) provided viscosity data with temperature and dry solids for kraft based black liquors. Objective of this data for straw based black liquors is to help the recovery mills based on this liquor.

Table 4. 9, Comparison of density and viscosity at various dry solids.

Dry solids verses density and viscosity at 65 °C (Experimental Values)

Sr. No. Dry Solids* Density** Viscosity*** 1 3.2525 1.0008 10.25 2 4.4789 1.0108 11 3 5.1513 1.0148 11.25 4 5.7296 1.0188 12 5 6.8709 1.0207 13.25 6 6.8738 1.0315 14.5 7 10.306 1.0413 15.5 8 10.9465 1.0453 17.25 9 11.442 1.0473 18.25

10 11.9876 1.0513 18.25 11 12.3026 1.0523 19.25 12 16.2246 1.0573 20.5

* percent of dry mass ** grams/cm3 *** milli Pascal per second.

89

y = -0.024x2 + 1.3295x + 5.6035R2 = 0.9665

0

5

10

15

20

25

0 2 4 6 8 10 12 14 16 18

Dry solids ( %OD basis)

Vis

cosi

ty (

mp

a/s)

Figure 4. 8, Relation of dry solids with viscosity at 65˚C.

Relations of dry solids with density and viscosity have been determined at 70°C. Experimental values are given in table 4.10. Trend in rising of the density and viscosity is comparable to that of values obtained at 65˚C, except of some decrease in density and viscosity. Both of these factors depend upon temperature. Temperature increases the energy contents of the system which effects movement of the molecules and reduce their friction to flow. The lowering of viscosity and solubility of inorganic compounds is similar as described by Golike et al 43 . Rate of change of viscosity have been given in figure 4.9.

Table 4. 10, Comparison of density and viscosity at 70˚C dry solids.

Dry solids verses Density and viscosity at 70 °C (Experimental Values)

Sr. No. Dry Solids* Density** Viscosity*** 1 2.59 1.0011 9 2 3.39 1.0012 9.25 3 4.25 1.0032 9.5 4 6 1.0036 10 5 6.5 1.0098 10.25 6 7.37 1.01274 12.25 7 10.51 1.02344 12.25 8 10.77 1.0324 13.4 9 11.04 1.0374 14.4

10 12.43 1.0405 14.56 11 12.65 1.0464 15.6 12 13.71 1.0504 16.5 13 14.04 1.0512 16.9

* percent of dry mass ** grams/cm3

90

*** milli Pascal per second. Relation of dry solids with viscosity at 70°C has been demonstrated in the

following figure (4.9).

y = 0.0381x2 + 0.0413x + 8.6717R2 = 0.9632

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16

Dry solids ( %OD basis)

Vis

co

sit

y (

mp

a/s

)

Figure 4. 9, Relation of dry solids with viscosity at 70˚C.

Viscosity relationships wit dry solids at 75˚C have also been determined.

Experimental values are given in table 4.11, and trends in change of viscosity have been demonstrated in figure 4.10. It is obvious form the data and graph that the viscosity increases with increase in dry solid contents but decrease with temperature. Roberts et al (77) described the problems encountered by high viscosity. Viscosity increases with rise in dry solid contents but the rate of increasing decrease above 15% dry solids.

91

Table 4. 11, Comparison of density and viscosity at various dry solid contents at 75˚C.

Dry solids relationship with Density and viscosity 75 °C (Experimental Values)

Sr. No. Dry Solids* Density** Viscosity*** 1 4.3977 1.0089 9.5 2 5.4294 1.0158 10.5 3 5.9933 1.0175 10.8 4 6.5637 1.0197 11.4 5 7.4106 1.02374 12.9 6 7.9596 1.02743 13.1 7 9.5204 1.03275 13.5 8 10.3948 1.03574 14.75 9 10.7276 1.03574 14.8

10 13.5261 1.03892 14.9 11 13.9032 1.04255 14.75 12 14.2982 1.045 15 13 14.9888 1.05694 15.2 14 15.463 1.06587 15.5

* percent of dry mass ** grams/cm3 *** milli Pascal per second.

Relation of dry solids with viscosity at 75°C has been demonstrated in the following figure.

Viscosity Verses Dry Solids at 75 °C

y = -0.0385x2 + 1.0336x + 8.3703

R2 = 0.9777

0

2

4

6

8

10

12

14

16

18

4.4 5.43 5.99 6.56 7.41 7.96 9.5 10.4 10.7 13.5 13.9 14.3 15 15.5

Dry Solids ( %OD basis)

Vis

co

sit

y (

m.p

a.s

)

Figure 4. 10, Rate of change of viscosity with increase in dry solid contents.

Zaman et al (50) reported that viscosity have relation with cooking conditions and composition of the black liquor. Viscosity of the thick black liquor (concentrated black liquor) was also determined with various dry solids. Experimental values are given in table 4.12. For thick black liquor the viscosity increases tremendously, this is due to emergence of gum like properties of the

92

organic components. At dry solids greater than 70% black liquor almost ceases to flow. At low temperature the viscosity is too high that it can’t be determined with common viscometer. In order to blow this liquor through recovery channels, temperature is increased beyond 300°C. High temperature decreases the viscosity to such extant that the flow of the thick black liquor through the recovery channels become possible. Relations of viscosity with dry solids for thick black liquor have been demonstrated in figure 4.11. Graph shows that viscosity rises very rapidly with increase in dry solids for thick black liquor. Results have been plotted up to 70% dry solid contents because beyond this point viscosity becomes too high to be determined.

Table 4. 12, Comparison of viscosity with increase in dry solid contents.

Dry solids verses density and viscosity at 80 °C (TBL) (Experimental Values)

Sr. No. Dry Solids* Density** Viscosity*** 1 45.5975 1.1919 44 2 51.4212 1.2112 68 3 54.4118 1.2256 120 4 57.7023 1.2283 157.6 5 59.5238 1.2456 224 6 69.3548 1.2644 280 7 73.0965 1.2844 - 8 78.6036 1.3077 - 9 81.1321 1.3176 -

* percent of dry mass ** grams/cm3 *** milli Pascal per second.

Relation of dry solids with viscosity at 80°C has been demonstrated in the following figure.

93

y = -0.0321x2 + 14.597x - 569.85R2 = 0.9342

0

50

100

150

200

250

300

350

400

450

40 45 50 55 60 65 70 75 80 85

Dry solids ( %OD basis)

Vis

cosi

ty (

mp

a/s)

Figure 4. 11, Relation of dry solids with viscosity for Thick Black Liquor at 80°C.

94

4. 5 Effect of temperature on viscosity: Viscosity decreases with increase in temperature at given dry solid contents. Figure 4.12 shows the effect of temperature on viscosity. At low dry solid contents the variations in viscosity are less pronounced. In fact at low dry solids, dry solid contents are least responsible for the viscosity, and viscosity is comparable to that of water. At high dry solid contents the viscosity increases. Organic part and silica of the inorganic part are major contributors of high viscosity. At given dry solid contents the viscosity decreases with increase in temperature.

Effect of Tepmerature on Viscosity

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18

Dry Solids (% OD basis)

Vis

cosi

ty (

m.p

a.s)

65 ºC

70 ºC

75 ºC

Figure 4. 12, Viscosity variations with temperature. Upper line gives viscosity at 65 ºC, middle

line gives viscosity at 70 ºC and lower line provides viscosity information at 75 ºC.

4. 6 Dependence of viscosity on silica contents:

Reduction of viscosity by reducing silica contents was studied. Black liquor was concentrated before and after desilication. The viscosity of the samples at increasing dry solid contents was determined. In figure 4.13 the influence of the silica on the viscosity is demonstrated. The curve with the lower viscosity values was obtained with the desilicated black liquor. After desilication still high viscosity of the black liquor is attributed to the high concentration of hemicellulose (pentosans) in the liquor. Because the desilication takes place at a temperature well below the temperature at which pentosans (form of the hemmi-celluloses in the black liquor) degrade, it is obvious that the removal of silica caused a remarkable reduction in thick black liquor viscosity. Viscosity shown by the black liquor is due to organic components particularly hemicelluloses. Viscosity reduction by desilication is better than ‘salting in’ method discovered by Roberts et al (78) because addition of salts reduces the efficiency of chemical recovery plant.

95

Figure 4. 13, Shows effect of silica on viscosity. First line gives viscosity with silica contents and second line gives viscosity after desilication.

96

4. 7 Solubility of sodium carbonate and sodium sulfate: Solubility of sodium carbonate and sodium sulfate is important parameter in prediction of scale formation in the concentrators. Solubility of Na2CO3 and Na2SO4 is given in table 4.13. The concentration of the black liquor from 1-2 % dry solids contents gives increased solubility of both of sodium sulfate and sodium carbonate (Jones et al 60). In fact, this is not the solubility but is concentration under solubility limits. Sodium sulfate and sodium carbonate concentration rises in the black liquor with increasing dry solid contents. Solubility limit appears at 53-54% dry solids and then decreases. Solubility of sodium carbonate decreases from 11.9% at 54% dry solid contents to 8.5% up to just 67% dry solid contents (figure 4.14). Solubility model for sodium carbonate and sodium sulfate was presented by Greg et al (38) but both critical solid points are different for straw based black liquors due to difference in chemical composition from that of kraft based black liquors.

Table 4. 13, Comparison of solubility of Na2CO3 and Na2SO4 at various dry solid contents.

Dry Solids % Na2CO3 (g) Na2SO4 (g) 50 12 - 51 12 8 52 11.8 8 53 11.85 7.6 54 11.75 7.1

55.5 11.65 6.7 58 11.2 5.8 59 11 5.4

60.8 10.7 5.2 62.5 9.75 5 64.3 8.5 2.9 67 8.2 2.7 69 7.4 2.6 72 6.6 2.5 75 6 2.4

97

Solubility of Sodium Carbonate and Sulfate

0

2

4

6

8

10

12

14

50 55 60 65 70 75 80

Total Solid Contents (%)

Sol

ubili

ty (%

of D

ry

Sol

ids)

Na2CO3

Na2SO4

Figure 4. 14, Solubility of sodium carbonate and sodium sulfate. Lowering of the graph lines indicate the critical solid points.

Same is the case with sodium sulfate, its solubility starts decreasing from 54 % dry solids (8 gm/100cm3 of black liquor) and reaches to very lower limit of 2.9 gm/100cm3 of black liquor at 64% total dry solid contents. Solubility of both of the sodium carbonate and sodium sulfate decreases up to 64 % dry solid contents at 110 °C and this becomes critical solid limit for both sodium carbonate and sodium sulfate. This limit predicts the precipitation of double salt of sodium carbonate and sulfate. This double salt is termed as burkeite (Shi et al 62). Further concentration of the liquor causes more burkeite to precipitate. The 60% dry solids determine first critical solids point for the CTMP black liquors. Thus the precipitation of burkeite starts and scaling appears in the concentrators. Typical scales of these two salts comprised of two moles of sodium sulfate for every mole of sodium carbonate. The sulfate in the black liquor gets depleted rapidly until second critical point occurs. At 65 % dry solids the sulfate contents is sufficiently low and at this point sodium carbonate begins to precipitate. This is second critical point for the liquor at which maximum of the sodium carbonate precipitates. These critical solid points are higher than that of kraft based black liquors as described by Rosier et al (76). If we compare this data with that of kraft based black liquor. Some variations in solubility contents of the CTMP liquor from the kraft based liquor were observed. One basic difference is in first and second critical point on the basis of dry solid contents. The first critical point appears at 60% dry solids contents in case of CTMP liquor rather than 52% dry solids contents of the kraft base. This difference rises from the difference in chemical composition of the two

98

liquors. Extremely high values of principle organic components in the CTMP black liquor results in relative high viscosity of the liquor. High silica contents also enhance the viscosity value. High viscosity causes an increase in solubility of the sodium carbonate and sodium sulfate and hinders its precipitation at the given temperature. At high temperatures the viscosity lowers, the solubility decreases and ultimately results in rapid precipitation of the two salts.

99

4. 8 Alkalinity values of the black liquor: Black liquor emerging from CTMP is alkaline in nature. It contains sodium salts of hemicelluloses and of other organic products with pulping chemicals. Sum of all the alkaline contents (hydroxides, carbonates, bicarbonates and sulfides) determine total alkali contents of the black liquor. There is considerable decrease in total alkali contents of the black liquor with time. This effect is greater in first 50 hours of storage and then no effect is found with its further storage. The reduction in total alkaline contents is due the neutralization of the alkalies with the acid contents generated from the degradation of the organic components as given by Demirbas et al (41). Settling also causes a decrease in solid contents in the supernatant portion of the black liquor. The degradation along with settling of dry solid contents (sodium and sulfate) causes reduction in total alkalies. Results of total alkalies are given in table 4.14 and Effect of aging on the total alkali contents of the black liquor is demonstrated in figure 4.15.

Table 4. 14, Comparison of total alkalinities of the weak black liquor.

Total Alkalies Hours Supernatants At Bottom

0 0.216 0.216 24 0.176 0.196 48 0.168 0.172 72 0.176 0.176 96 0.172 0.172

120 0.176 0.18 144 0.176 0.18

Total Alkali

0.16

0.17

0.18

0.19

0.2

0.21

0.22

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Time (Hours)

Tot

al A

lkal

i

Supernatants

At Bottom

Figure 4. 15, Effect of aging on total alkalies of the black liquors.

100

Another important factor that contributes in reduction of total alkalies is degradation of the hemicelluloses and lignin contents into their respective carboxylic acids as described by Zhinan et al (37). The production of acid contents is causes reduction of pH up-to certain level. After 4 – 5 days the degradation rate of the lignolytic and cellulosic contents decreases and total alkali contents become stable between 0.17 – 0.18 % of the total dry solid contents of the black liquor.

101

4. 9 Desilication of the green and black liquors:

In some part of the world, like China, India, Pakistan, Vietnam, Egypt; agricultural residues like wheat straw, rice straw, bagasse (non wood fiber) are being used for the manufacturing of pulp, while rest of the world relies on wood fiber for pulp manufacturing. Pulping based on wood fiber has its own problems, while pulping from non wood fiber is associated with entirely different problems.

In Pakistan mainly the raw materials used for the manufacturing of the pulp are wheat and rice straw. It is cheap and abundantly available. Silica is present in these raw materials both in combined and free states. Silica in free state comes as lumps of mud and sand. This silica is added by harvesting process and dust, enters into the pulping process. Most of this silica is removed by dry and wet cleaning procedure of the pre-pulping process. Combined silica, is the part of the straw that enter through assimilation from the roots. This silica is not removed by ordinary dry or wet cleaning processes. It enters into the pulping process and is extracted in the black liquor after pulping actions. Black liquor enters into the recovery plant and causes silica scaling in very channel and tubings.

Silica may be precipitated in one of the two ways; either by the addition of metal ions like aluminium, magnesium or calcium in soluble forms as hydroxides or oxides, or by the lowering of the pH of the black liquor by adding acids. Except for the use of anhydride CO2 from carbonic acid all these methods need additional chemicals and produce more waste than necessary to purge the silica104.

As long the black liquor is in diluted form (weak black liquor) the silica scaling is low but as the weak black liquor is concentrated gradually, precipitation occurs and the scaling appears as the major problem of the recovery plant.

Lime treatment is highly expensive, high organic and alkaline loss and calcium insolubility causes calcium scaling. Calcium carbonate solubility in black liquor is ~0.3 g/l, i.e., 20 times as high as solubility in water.

4. 10 Silica solubility:

Amorphous silica is surprisingly soluble, the solubility depending somewhat on the size of the primary particles. At pH7 (and below) and at room temperature the solution contains about 0.15 g/l silica rising at 90°C to about 0.3 g/l all in the form of monomolecular silicic acid. Such material can easily pass

102

through cell membranes of plant or animal. At pH 10, solubility rises from 0.28 g/l at room temperature to 0.4 at 90°C.

Silica solubility in black liquor is hardly different from water solubility. In fact almost everybody working on desilication has arrived at the limit of 0.3-0.4 g/l residual dissolved silica, independently of the starting concentration of silica. With the same lower limit the degree of desilication achieved, solely depends on the initial silica content. With the critical lignin precipitation pH, best results were achieved at demonstration plant, final pH 9.8.

Laboratory trials have shown straw lignin to be very stable, so that excellent desilication is possible. Aluminum considerably lowers silica content in well water, in soil solutions and boiler feed water at least around neutral pH. However, under no circumstances must soluble aluminum is left in black liquor, in order not to cause aluminum silicate scaling on evaporator tubes, which causes the most dreadful form of scaling.

Silica precipitation by pH reduction is only possible if silica is present in monomolecular form dispersion and at least partly in the ionic form i.e., in black liquor with a pH over 10.6. Similar to silica sols, the lignin stability in dilute and in concentrated black liquor depends on the negative electric charge of its colloidal particles, resulting from ionization of its sodium salts. About 90% of the acidic function of black liquor lignin is phenolic, the rest is carboxylic.

Less than 50% of the silica in the raw material gets dissolved during pulping, in the case of straws because the pulping conditions employed are milder, i.e., lower alkali concentration (20 g/dm3) and lower cooking temperature (150°C). With white liquor concentration of 120 g/ dm3 the evaporator cleaning was to be done every week. When the concentration was brought down to 80 g/ dm3 the evaporator cleaning was needed once in 20 days. Myreen et al (33) described that silica ion in the black liquor creates problems in recovery of sodium and sulfur. He proposed carbonation process to remove silica from the black liquor.

4. 11 Foaming characteristics of the spent Liquors:

Desilication employing carbonation with flue gas involves gas-liquor transfer operation and create excessive foam. Most of the black liquors contain high proportion of organic acid salts of sodium which determine the foaming tendency. Foaming creates problems during desilication in the submerse bubble reactor and also affects the mass-liquor transfer during carbonation. The excessive foam generated flood the column reduction the effective tank volume. Foam breakers and foam tanks had to be provided in the desilication unit to

103

recover the black liquor converted into foam. Foam also creates problems in other operation like filtration and washing of carbonated black liquors.

Carbon dioxide with water forms a weak acid, i.e., carbonic acid; a convenient agent for reduction the pH of black liquor without causing local “over-acidification”, which results in lignin precipitation along with silica thereby making the separation of silica sludge difficult. CO2 reacts first with the free alkali in black liquor and then neutralizes it forming Na2CO3 and NaHCO3. The lowering of pH coupled with the presence of inorganic and inorganic salts in black liquor helps silica to form higher agglomerates of silicic acid acids and the agglomerates settle down. Schwalbe(105) obtained filterable SiO2 particles by heating the colloidal silica. Jayme(106) used a mixture of CO2 and air, and precipitated SiO2 at a pH of 10.0 from straw black liquor. Kuna and Graber(107) used a mixture of air and CO2 (10%) and got precipitates with easy filterability (1.5 m3 of black with 15% solids per sq. meter of filter area per hour) and with a pressure difference of one atmosphere at 65°C. They achieved a concentration of 0.2-0.4 g silica per kg of black liquor.

4. 12 Removal of silica by sarbonation:

Franzreb108 reported that when carbonating kraft straw based black liquor slowly with recovery boiler flue gas, an easily filterable and washable precipitates of silica results. Rather sturdy gel-like granules are obtained that sediment quickly, filter well and can be rinsed quickly of the surrounding black liquor by washing on the filter. The mechanism of silica precipitation is demonstrated as under in figure 4.17.

104

Figure 4. 16, Mechanism of silica precipitation from the black liquors (109).

4. 13 Growth of silica particles, silica sols by carbonation:

According to Ralph K.Iller(110) due to weak acidic character of silica acid, silicate solutions of even high alkalinity are partially hydrolyzed into monomolecular silicic acid and caustic soda. On pH-reduction hydrolysis proceeds and condensation (polymerization) to larger silica units commences, catalyzed by the OH ions present, under the exclusion of water SiOH (silanol) groups link up to Si-O-Si, forming poly-silicic acids starting with di- and trimers, then forming chains and networks, three dimensional oligomers and finally submicroscopic particles of 5-10 nanometer diameter containing millions of silica elements. In the course of this process, water envelopes associated with silica molecules are displaced by forming silica links. Sovertsen et al (111) proposed that CaCO3 deposition is deadly form of scaling in the evaporation unit.

These “primary particles” are closely packed but remain amorphous because, other than in salts, the covalent linkage prevents mobility necessary for crystal formation. In the solid state that amorphous character is very stable: it takes millions of years for microcrystals to develop, e.g., in deposits of diatomaceous earth or in flint both derived from silica skeletons or organisms

105

that in conjunction with silicates operate to keep the silica level of the oceans well below silica solubility.

With the surface silanol groups having acidic character, the primary particles are negatively charged and repel each other forming a stable sol. Further particle growth takes place by condensation, this time by primary particles forming chains and networks. Finally solid or semi-solid gels are formed enclosing the whole agneous phase, even at silica concentrations lower than 1%.

4. 14 Silica precipitation by carbonation:

De et al (83) discovered the precipitation of silica by carbonation. Due to electric charge stabilization, precipitation of denser aggregates only can occur in silica solids in the presence of flocculants. Salt solutions of at least 0.2-0.3 molar concentration, corresponding to pulping spent liquors of 3-5% total solids act as flocculants. The precipitated silica strongly retains alkali, partly by inclusion, partly as counter-ion and partly by alkali ions acting as bridges between the single primary particles. This strong alkali sorption has led to speculate that part of the silica is precipitated as alkali silicate. Also co-precipitating hemicelluloses or lignins may act as flocculant during silica precipitation (Jain et al 52).

4. 15 Filtration during carbonation process:

Although filter drums as applied in the pulp industry can be used; an endless band filter was preferred, particularly so as to be able to apply controlled washing with minimal dilution. Although the submerse reactor silica-mud sediments easily and quickly, the inclusion of an intermediate sedimentation stage for sludge concentration was avoided to minimize complication.

4. 16 Changes in chemical composition during carbonation process:

The spent pulping liquors are alkaline solutions containing dissolved organic and inorganic components. The free alkali (NaOH + Na2S) present in the black liquors undergoes neutralization during carbonation resulting in lowering the pH of the black liquor which is the key step for precipitation of silica. The other inorganic components particularly the sulfur compounds like Na2S and Na2SO3 also undergo chemical changes during carbonation. For instance, part of the Na2S may get stabilized by oxidation to Na2S2O3 and part of the sulfur of the Na2S may be stripped off H2S due to lowering of pH. The later reaction takes place only in the final stages of the carbonation process and the former reaction

106

takes place in the initial phase of carbonation. Thus the extent of sulfur stabilization in the initial stage of carbonation is important.

The most important organic components of the black liquor are the sodium salts of lignin and organic acids. These components also undergo chemical changes due to neutralization and oxidation reactions which take place concomitantly during carbonation with flue gas containing 10-12% CO2 and 5-8%O2.

4. 17 Prevention of silica scale:

An effective method to prevent scale formation is to maintain an optimum level of active alkali in the black liquor by addition of either caustic soda, green liquor or with a mixture of both. This procedure not only limits silica scales in evaporator tubes but also helps for better burning of black liquor solids and forming a softer bed in the furnace hearth. Another practical method of control is to maintain the inorganic to organic ration of 1.5 to 1.65 for rice and wheat straw black liquor to minimize the silica scale formation in the evaporator tubes.

4. 18 Natural desilication:

It has been observed and reported that straw black liquors on storage for 8 hours or so at 80°C, precipitate SiO2 and organics. This is due to reduction of pH in black liquor. The thick precipitate contains high amounts of silica along with organics and the clear decanted liquor is freed of silica. In the process of storage desilication efficiency could be as high as 90%. Of course, organic contaminations are high. This could form the simplest method of natural desilication of many of the straw black liquors and could be adopted by small straw-based mills as a first step of desilication.

4. 19 Green liquor desilication:

During pilot plant investigations on precipitation of silica from black liquor, it was earlier observed that co-precipitation of lignin along with silica could not be always avoided and the extent of lignin-precipitation was dependent on pH and rate of pH reduction. Filtration of silica sludge in cases of higher-than-allowable-lignin precipitation was very slow, which could result either in too much loss of organic matter (fuel value) or too much wash water (degree of dilution) entering into the system. The second disadvantage of black liquor carbonation was the uncontrolled foaming of black liquor in the absorption towers. Black liquor carbonation for silica precipitation is more sensitive to the pH or the active alkali content in black liquor.

107

Disadvantages:

· The operational problems in evaporators and recovery furnace do not get solved.

· Carbonation can lead to loss of sulfur in the form of H2S from the Na2S in the green liquor of sulfate pulp mill.

· A part of sodium hydroxide gets converted into bicarbonates and carbonates, which results in higher consumption of lime in the subsequent causticizing operation.

4. 20 Desilication by lime addition:

Silica-rich black liquor, when treated with lime, forms calcium silicate. Slaked lime forms calcium hydroxide with water, reacts with silicates forming calcium silicates, as a precipitate. This is filtered out from the black liquor.

In the year 1953, Gruen(112) patented his so-called Gruco process for precipitating silica from black liquor by addition of lime or milk of lime. The lime treatment in this process is carried out for 5-10 minutes at 90-200°C with dilute black liquor with silica content of 7-10 g/dm3. The desilication efficiency is 90%. The precipitation reaction is rapid. However, with longer reaction period, the organic acids in black liquor also react with lime forming calcium organic compounds and tend to precipitate with calcium silicates.

A part of lime also reacts with sodium carbonate and sodium sulfate. Rudie et al (74) gave the mechanism and composition of calcium scales. In this process lime must be added in considerable excess of the stoichiometric quantity. Jayme(106) further modified this process by treating partially concentrated black liquor (30% solids) with only twice the stoichiometric quantity. This method has the following advantages:

- Less lime requirement - Less lime sludge generation and less loss of organics - Loss of sulfate ion is reduced as side reaction are suppressed, and - Equipment costs for handling lime and filtering the silica sludge are less

due to smaller size. For filtration, he recommended a rotary vacuum filter with synthetic filter

cloth. West Coast Paper Mills of India used press filter. They suggested that there should be two Kelly Leaf Filters working alternately one for cleaning filter cloth and the other for filtration. It was estimated that for 1000 m3 weak black liquor containing 5 tons of silica, approx. 9 tons of lime (85% available CaO) would be required and the desilicated black liquor would contain 0.5 tons of silica only.

108

To remove one ton of silica, one generates two tons oven dry solids or more than 4 tons in wet state, which must be precipitated and disposed off as a land-fill. Besides, organic acids in black liquor combine with lime, which can form scales in the evaporator tubes. The method is, however, simple. Massive quantities of lime were required and it has been reported that as high as 200-600% of CaO on silica is required, depending on method of treatment. Pilot plant studies on desilication using lime were carried out but they had problems in filtration of calcium silicate precipitate. Thus the lime process did not materialize.

4. 21 Mechanism of silica precipitation by lowering pH:

Lowering the pH of black liquor converts silicates into insoluble polymeric anhydrides of silicic acid. The solution in equilibrium with amorphous silica at ordinary temperatures contains monomeric-monosilicic acid, Si(OH)4. Lowering the pH of alkali silicate solutions, results first in formation of monosilicic acid. This monomer undergoes polymerization at a concentration greater than the solubility of amorphous silica (100-200 ppm), forming higher molecular weight species. The mechanism of polymerization is ionic and is proportional to –OH concentration, above pH-2. Above pH-7 stabilized particles (Sols) grow to a size of about 100 nm. On the contrary, when salts are present, to neutralize the charge on growing particles, aggregation of particles occurs with the formation of chains and ultimately, three dimensional gel net works. Thus the gel or sol formation depends on the medium containing silicic acid monomer.

Black liquor is a complex colloidal system. If the behavior of the black liquor, during carbonation is to be understood, its colloidal nature must be realized along with the reasons for the stability of the colloidal state. Lignin which constitutes nearly 50% of the total organic residues in black liquor exists in the form of colloidal macromolecules. Phenolic –ONa and –COONa groups are the hydrophilic groups, which determine the total charge of colloidal lignin macromolecules. Lowering of pH, results in charge neutralization affecting the stability of lignin macromolecules. Merewether precipitated as much as 51% of the total lignin by carbonating the eucalyptus Kraft black liquor to a pH level of 7.0. Thus it becomes important to understand the colloidal stability of black liquors during carbonation to achieve selective precipitation of silica. From the previous studies conducted it is evident that the pH values at which silica precipitates almost quantitatively, is close to the pH value at which lignin starts precipitating. Thus one of the main tasks of desilication by carbonation was to attain the end-pH very close to silica precipitation without over-carbonating the black liquor, which would lead to the beginning of lignin precipitation.

4. 22 Direct carbonation (bubbling of CO2):

109

As per studies carried out, the flue gas was drawn from the chemical recovery stack which contained on an average about 12% CO2 and 5% O2. The reactor tank had a capacity of about 800 liters and was equipped with a high speed agitator. The black liquor was fed into tank tangentially by a 2” pipe. The flue gas from a separate blower entered the black liquor in the form of bubbles. About 20 experiments were carried out on batch scale with 200-300 liters of black liquors in each trial. The experiments included; desilication of bamboo and mixture of bamboo and bagasse black liquors. Over 70% desilication was achieved by lowering the pH to a level around 10.1 incase of weak black liquor and around 10.2-10.5 increase in case of semi concentrated black liquors. Filtration rate was reduced when the pH of carbonated black liquor was below 10.0, and also when the concentration of black liquor was on higher side. The flue gas flow was around 400m3/hr and carbonation was accomplished in about 100-150 minutes for black liquor quantity of nearly 200-300 liters. The average carbonation rate was about 2 lit/min of black liquor. When CO2 content goes below 5%, the rate of carbonation significantly reduces, consequently requiring much more time. Also, carbonation time varies widely with variation in free alkali content.

Howell et al (82) proposed the lowering of pH by carbonation but he himself used this technique to remove lignin from the black liquors. The silica sludge obtained in laboratory by carbonating about 500 ml of black liquor with 100% CO2 invariably, contained as high as 40% organic matter despite the end-pH being around 10.0. The laboratory carbonation experiments at CPPRI and carbonation experiments at packages chemical recovery plant differed, primarily with respect to quantity of black liquor handled and CO2 concentration during carbonation. In the former case the carbonation was much gentler as compared to laboratory experiments. At this stage, the idea of gentle carbonation to attain desired end-pH and selective separation of silica was conceived.

The results also reveal that after pH 10.8 the decrease in pH was slower till pH 10.0, compared to faster decrease in pH in the initial stage of carbonation. In the initial stage the neutralization of major portion of residual alkali takes place and after pH 10.8, the hydrophilic groups like –COONa and –ONa groups, undergo changes resulting in free –COOH and –OH groups which exert buffering action, resulting in slow shift of pH values.

4. 23 Laboratory desilication with sulfuric acid:

Selective precipitation of silica from black liquor is possible by pH reduction; even some ligninates also precipitate first due to their weaker acidic

110

function than silicic acid. The phenol group of lignin in black liquor have an acidity of pK = 9.4 to 10.5.while silicic acid acidity is intermediate between these values i.e. pK = 9.8. Desilication at the lab scale was planned with the addition of Sulfuric acid. Following parameters were selected for the sulfuric acid addition to the green liquor;

Table 4. 15, Desilication by Sulfuric acid addition.

Following table give results of desilication obtained by Sulfuric acid addition and settling.

Table 4. 16, Desilication results with sulfuric acid and settling.

Desilication with sulfuric acid gives best results at the settling time of 48

hrs. Results are satisfactory even at 60 minutes settling gives 90% desilication which is enough for any of the process, but if applicable at the plant scale. If we plot a graph for the desiccation by sulfuric acid we find the following curve:

Parameters Values Sample Volume 1 dm3 Initial pH of the sample 10.92 Initial carbonate concentration in the sample 250 g/ dm3 Final carbonate in the sample 86 g/ dm3 Sodium Sulfide in the sample 15 g/ dm3 Total acid added 85 cm3 Concentration of acid 98 %

Time PH Silica as SiO2 Reduction in silica Minutes g/ cm3 %

Blank 10.92 8.40 0 0.00 9.48 1.50 82 15.00 9.53 1.29 85 30.00 9.63 1.15 86 45.00 9.57 0.96 89 60.00 9.73 0.92 90

48 ( Hours) 9.84 0.43 96

111

desilication by pH reduction

0

20

40

60

80

100

120

0 0 15 30 45 60 72

Time ( hours)

Desi

licat

ion

(%of

sili

ca)

Figure 4. 17, Desilication of black liquor with sulfuric acid addition.

Desilication by sulfuric acid gives encouraging results even at the settling

time of 15 min i.e. 85 % which is raised to 90 % if the settling time is increased by 60 minutes.

4. 24 Laboratory desilication with carbon dioxide:

Desilication by pH reduction is also carried out by carbon dioxide addition (carbonation). Pure CO2 was added to the green liquor and its desilication characteristics were observed.

Table 4. 17, Desilication with carbon dioxide

Hence, carbon dioxide reduces the pH of the black liquor and causes the desilication of the black liquor. Results of three experiments given above depict the reduction in silica by pH reduction. At pH 10.04 just 64% reduction in the

Green Liquid taken

pH Silica

cm3 Initial Final Initial g/ dm3

Final g/ dm3 Reduction %

1000 10.69 9.49 4.00 0.67 83

1000 10.78 10.04 4.00 1.44 64

1000 10.74 9.74 4.00 1.08 73

112

silica is observed. As pH reaches to 9.74, around 73% of the silica precipitates out. However, 83% reduction in silica is found as the pH reaches to 9.49. Carbonation has appreciable effect on the precipitation of silica from the black liquor. Lowering of pH from 10.74 to 9.5 causes almost 80% of the silica to precipitate out. 4. 25 Laboratory desilication with Hydrated Lime: However, desilication can be affectively carried out by carbonation. Lowering the pH causes most of the silica to be precipitate out. But the carbonation process needs special equipments for carbon dioxide addition, removing the settled silica and it removes lignin components from the black liquor, because at low pH, lignin precipitates out. Second option for desilication, which was used by the early researchers, was the addition of lime. Lime reacts with the silica and forms insoluble calcium silicate.

Table 4. 18, Lime addition at various doses and desilication results:

Trial – 1 Trial –2 Trial - 3 Trial -4

Liquor , cm3 3000 3200 3500 3500

Lime , g 43 46 50 50 Reaction temp. °C 90 90 – 94 90 80 Stirring time, minutes 10 10 30 30 Tests pH value 8.9 9 9 8.9 Total solids, % 10 9.8 10 10

Total Silica, % on dry solids basis As such After lime addition & Settling for 2 hrs 12 hrs 24 hrs

0.4

nil

0.3

nil

0.4

0.113 0.037

0.41

0.12 0.04

Table 4. 19, A Comparison of black liquor before & after lime addition:

Trial – 1 Trial – 2

Tests Before

treatment After

treatment Before

treatment After

treatment pH 8.9 12.1 8.8 12.1

Solids, % 8.5 9.5 8.5 9.5 Sodium, % 13.0 13.1 13 13.2 Calcium, % 10.3 14.3 - - Sulfur, % 5.3 5.7 6.2 6.2

113

Sulfated Ash, % 39 38.9 - - Residual alkali as Na2O 5.1 6.5 - -

To 100 liters of liquor 80 °C, 1500 grams lime was added and the reaction

mixture was stirred for 20 minutes. Then the desilication characteristics were studied after different settling times. The results are given in table 4.21. The result chart shows that at pilot plant scale about 84% desilication can be achieved.

Table 4. 20, Results of the lime addition – desilication trial.

Black liquor For 1 hr For 4 hrs For 24 hrs Temperature, ºC 80 70 55 26 Total solids, % 8 8.4 8 8 Silica as SiO2, % 0.46 0.3619 0.1125 0.075 De-silication % - 22.6 76 84 100 liters of liquor was taken in the tank, 150 grams of lime was added to it and stirred for 20 minutes. Initial temperature of the liquor was 85°C. The desilication results of this sample are given in table 4.22.

Table 4. 21, Results of the lime addition – desilication trial.

Black liquor After 4 hrs After 24 hrs Temperature, ºC 85 65 26 Total solids, % 8.7 8.9 9 Silica as SiO2, % 0.4850 0.2696 0.222 De-silication, % - 44 44.6

100 liters of liquor was taken in the mixing tank with initial temperature of 65 °C. 500 grams lime was added to it and stirred for 20 minutes around 41% desilication was achieved. In these trial 1,000 grams of lime was added to 100 liters of liquor. By increasing lime addition, desilication was improved; it reaches to 64.7% after 4 hrs settling of the treated liquor. Lime addition is the most common desilication method of desilication due to availability and low cost of lime. Lime treatment was first carried out at laboratory basis. 500 grams of lime (index 60%) was added to 100 liters of the liquor, black liquor was stirred for 20 minutes and then desilication efficiency was analyzed after settling. Trials were planned at pilot plant basis and yielded

114

65% desilication. Initial silica contents were 0.59 % (dry solids basis) and it reduced to 0.21%. In another trial 100 liters of liquor was taken at 85°C and 150 gm lime (Index 60%) was added. After 20 minutes stirring the desilication was determined. Results after 4 hrs and 24 hrs settling are given in table 4.22b.

Table 4.22(b), Desilication after 0.15% lime addition.

In another trial 1500 grams of lime (index 60%) were added to 100 liters of liquor at 80°C. liquor was stirred for 20 minutes and then desilication was determined after 1, 4 and 24 hours settling time. Results of the trial are given in the table . It was observed that settling cause mark able effect on desilication. Because of turbidity and thickness of the liquor, settling requires time, which is maximum after 24 hours. After 1 hour only 22.6% silica reduction was found, this reduction was increased to 76% after 4 hours. But after 24 hours around 84% of silica was removed from the liquor. Results for dry solids and total solids reduction along with silica are given in the following table 4.22c.

4.22(c), Desilication results with 1.5% lime addition.

Reduction in silica contents with various lime additions has been plotted in figure 4.18.

Parameters Black Liquor Lime addition & settling 4Hrs

Lime addition & settling 24 Hrs

Temperature °C 85 65 26 Total Solids % 8.7 8.9 9 Silica as SiO2 % 0.485 0.270 0.222 Desilication % -- 44 44.6

Parameters Black Liquor Lime addition & settling 1Hrs

Lime addition & settling 4

Hrs

Lime addition & settling 24

Hrs Temperature °C 80 70 55 26 Total Solids % 8 8.4 8 8 Silica as SiO2 % 0.46 0.362 0.113 0.075 Desilication % -- 22.6 76 84

115

Desilication of green liquor

0.0

20.0

40.0

60.0

80.0

100.0

1 2 3 4 5

Lime addition, times

Red

uct

ion

in s

ilica

, %

Figure 4. 18, Percentage reduction in the silica contents of the green liquor.

4. 26 Plant scale desilication with hydrated lime:

Lime addition at plant scale was carried out by modified lime addition process. If lime is added at conventional process, the lime settles in the lime slurry tanks. But by the modified lime with uniform consistency was added to the green liquor. The system is modified by installing variable speed screw pump for feeding of lime into dissolver and dilution water line with Rota meter. Fresh lime sample for high lime index was chosen for the plant trails. Lime was used in 1:3 and 1:4 ratios of slurry. Lime slurry was prepared with lime of 74% lime index and 70% water. After addition of lime the green liquor was fed to the settling tank, with average settling time of 10-12 hours. Green liquor contained 7.0 g/l of silica. Followings were the finding of the plant trial.

Table 4. 22, Findings of plant trial.

Sr. No Lime feeding rate

Density of Green Liquor

Lime / Silica ratio

Silica Results Reduction in silica

l/h g/l g/l % 1 696 1.14 5.4 1.5 79 2 400 1.18 3.1 5.8 17 3 400 1.21 3.1 5.2 26 4 433 1.15 3.53 5.5 21 5 390 1.15 3.18 5.0 29

116

4. 27 Advantages of desilication:

Major advantage of desilication by sulfuric acid is that the deposition of silica and sulfuric acid application is rather simple. It causes desilication by both means of pH reduction and forming insoluble sulfates of silica. Its disadvantages include high cost of sulfuric acid at plant scale and Sodium losses due the formation of Sodium Sulfate, which acts as inert material in the recovery loop.

Desilication with carbon dioxide have not given some extra-ordinary results. Up to 83% desilication have been achieved by carbonation. One advantage of this process is the utilization of the flue gases, which are costless. But this method of desilication has been developed for black liquors and it requires filtration and causes foaming in the carbonation column and restricts the passage of flue gases leaving the column.

Finally the desilication with lime have given significant results. Desilication in the laboratory trails have been found up to 84%. Lime addition have many advantages like low cost, less lime requirements, less lime sludge generation, less loss of organics, loss of sulfate ions is reduced as side reactions are suppressed. Therefore lime treatment has been selected for the plant trials. By considering work of previous scientists and plant base lime addition problems new method for lime addition has been developed. Lime addition have not proven as better results as that by laboratory trials but this method subsidizes better lime addition and desilication and can be adopted for future plant development particularly lime treatment and ordinarily additions of settling chemicals like lime.

117

4. 28 Chemical Treatment of sludge of recovery plant to recover sodium and sulfur: During burning of the black liquor in the recovery boiler, the green liquor produced contains black particles. These black particles are problematic due to their deposition in the strippers and absorption towers. Black Particles in the green liquor are found to be 35 – 40 % Silica, 60 – 64 % un-burnt organic matter and around 2 – 4 % adsorbed alkali. These particles are insoluble in carbon tetrachloride (CCl4), benzene (C6H6), and water, which indicates its organic polymeric nature (cellulosic / lignin) nature. Silica is bound and / or entrapped with these organic particles. Due to siliceous composition these particles have high density than water but remain suspended due to high viscosity of the green liquor. The removal and treatment of these black particles is of serious concern and it involves settling and removal processes. Setting itself is not efficient in removing all the black particles.

Table 4. 23, Effect of 2 times dilution on the removal of COD and recovery of sodium & sulfur

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 85700 15650 82 - Total Sodium as

Na2O, (g/l) 199 105 - 53

Sulfur as Na2S, (g/l) 136 69 - 51 Table 4.24 (a), Effect of 3 times dilution on the removal of COD and recovery of sodium & sulfur

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 90110 14300 84 - Total Sodium as

Na2O, (g/l) 199 106 - 53

Sulfur as Na2S, (g/l) 136 73 - 54

Table 4.24 (b), Effect of 4 times dilution on the removal of COD and recovery of sodium & sulfur

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 85700 15650 82 -

118

Total Sodium as Na2O, (g/l)

148 113 - 77

Sulfur as Na2S, (g/l) 77 54 - 70

Table 4.24 (c), Effect of 4 .5 times dilution on the removal of COD and recovery of sodium & sulfur

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 85700 14440 83 - Total Sodium as

Na2O, (g/l) 148 116 - 78

Sulfur as Na2S, (g/l) 77 58 - 75 Table 4.24 (d), Effect of 5 times dilution on the removal of COD and recovery of sodium & sulfur

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 71793 13174 82 - Total Sodium as

Na2O, (g/l) 234 202 - 86

Sulfur as Na2S, (g/l) 130 108 - 83 Table 4.24 (e), Effect of 4 times dilution on the removal of COD and recovery of sodium & sulfur

but with the addition of polymer

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 80530 12540 85 - Total Sodium as

Na2O, (g/l) 148 127 - 86

Sulfur as Na2S, (g/l) 77 63 - 82

Table 4.24 (f), Effect of.4 times dilution on the removal of COD and recovery of sodium & sulfur

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 92904 25373 73 - Total Sodium as

Na2O, (g/l) 147 111 - 75

Sulfur as Na2S, (g/l) 80 59 - 74

119

Table 4. 24, Effect of.4 times dilution on the removal of COD and recovery of sodium & sulfur but

with the addition of animal glue

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 80530 50353 37 - Total Sodium as

Na2O, (g/l) 148 29 - 20

Sulfur as Na2S, (g/l) 77 15 - 20

Table 4. 25, Effect of.4 times dilution on the removal of COD and recovery of sodium & sulfur but at temperature 80°C

Parameters Before dilution After dilution

Reduction in COD

%

Recovery of active

ingredient %

COD, (ppm) 90595 17345 80 - Total Sodium as

Na2O, (g/l) 160 152 - 95

Sulfur as Na2S, (g/l) 77 70 - 91 Effect of dilution on the removal of COD loading and recovery of chemicals of green liquor sludge has been determined. Two to three times dilution was carried out at stirring time and speed of 2 hour and 450 revolutions per minute (rpm) respectively. Whereas stirring time and speed was kept 1 hour and 1000 rpm when dilution varied from 4 to 5 times.

Two times dilution resulted in 82 % reduction of COD. It was apparent that beyond 2 times dilution there was no significant removal of COD loading. However, recovery of active ingredients such as sodium (Na2O) and sulfur (Na2S) was increased with an increase in no. of dilution. Recovery of total sodium and sulfur at 5 times dilution was found to be 86 and 83 % respectively.

Effect of coagulant and stirring time on the removal of COD and recovery of the chemicals was also determined in the following experiments. An attempt was made to increase the removal the COD and recovery of chemicals by adding polymer coagulant. The idea was that addition of polymer will accelerate the

120

sedimentation rate of suspended particles, hence resulting in a reduction of COD contents and increase in the recovery of the chemicals.

Settling efficiency was studied by varying the stirring time from 0.5 to 1 hour at 1000 rpm. The addition of polymer coagulant showed little increase in the reduction of COD contents of green liquor sludge. However, recovery of sodium and sulfur was found to be 9 to 12 %.

It is worth mentioning that a half hour decrease (from 1 hr to 0.5 hr) in stirring time had significantly effected the removal; of COD and recovery of the chemicals as apparent from tables. Both COD and recovery of the chemicals were reduced. Analysis results given in tables 4.24(a-h) showed that animal glue, in contrary to polymer coagulant, had badly failed to remove COD load and recover the chemicals of the green liquor sludge. Effect of temperature on the removal of COD and recovery of the chemicals reflected the effect of temperature on the removal of COD and recovery of the chemicals. Four times dilution was carried out at temperature 80oC while stirring time and speed were 1 hr and 1000 rpm respectively. Temperature did not show any increase in the removal of COD content. However, chemical recovery (i.e. 95 % sodium and 91 % sulfur) was about 9 % higher than the recovery values.

121

4. 29 Chemical treatment of the black liquors: Chemical treatment of the black liquor involved the addition of various chemicals and flocculating agents. Various flocculating agents were found having different abilities of removing the COD and BOD. The black liquor contains dissolved solids, suspended solids, suspended fibers and other products of pulping. The extent of treatment and efficiency of the process was determined by two critical parameters; chemical oxygen demand and suspended solids. Erich Gruber et al (84) used cationic polymer to remove the suspended solids from the black liquor. Chemical treatment results are discussed under batch wise portions. First batch: Black liquor was treated with two flocculants. First of all the black liquor was determined for its natural settling. Natural settling was compared with chemical dozing. Results are given in table 4.27. Setting alone reduces the COD up to 50% of the original and if the settled liquor is filtered the reduction in COD is increased by 5% and the value in reduction reaches to 55%. Addition of 200 ppm of alum and 5 ppm flocculating polymer yields no significant results. Just, 48% settling after the addition of the chemicals and 55% reduction after filtering the treated sample. The test shows that the combination employed is insufficient to get low COD and suspended solids.

Table 4. 26, Chemical treatment of the black liquors

Sample Reference / Parameter COD Initial (ppm) COD Final (ppm)

Reduction in COD

Black Liquor Sample 2928 - - After 2 Hours Settling 2928 1470 50% After 2 Hours Settling and filtration 2928 1318 55%

Addition of Alum 200 ppm plus Anionic polymer 5425, 5 ppm and 2 hours settling

2928 1527 48%

Addition of Alum 200 ppm plus Anionic polymer 5425 (5 ppm), 2 hours settling and filtration

2928 1333 55%

Because of additional alum and anionic polymer, the settling is better even when compared with that given by Erich. Second batch:

Chemical treatment was carried out with alum aided with an anionic polymer. Alum alone was able to reduce COD to 40% after 2 hours settling and the settling reached to 52% after 12 hours of settling. Results are given in the

122

following table 4.28. However the COD reduction reached to 75% when the treated sample was passed through activated carbon bed. It means activated carbon can stop most of the COD imparting ions and can subsidize the chemical oxygen demanding load of the black liquor. Table 4. 27, Results of the batch treated with alum and different doses of anionic polymer (Bufloc

5425) and by passing the sample through activated carbon.

Sample Reference / Parameter COD Initial (ppm) COD Final (ppm)

Reduction in COD

Original Sample 1379 - - Addition of Alum 200 ppm And 2 Hours Settling

1379 827 40%

Addition of Alum 200 ppm And 12 Hours Settling

1379 660 52%

Addition of Alum 200 ppm plus 15 ppm Anionic Polymer 5425 And 2 Hours Settling

1379 585 57%

Addition of Alum 200 ppm plus 15 ppm Anionic Polymer 5425 And 2 Hours Settling

1379 587 57%

Passing the treated sample (2 hrs settling) through the bed of sand & Activated carbon

1379 372 75%

Passing the treated sample (12 hrs settling) through the bed of sand & Activated carbon

1379 364 74%

Third batch Black liquor selected for these chemical treatment trials was containing 1637 ppm of the COD, 2780ppm of total dissolved solids and 1120ppm of total suspended solids. The chemi-thermo-mechanical pulping liquor contains relatively low COD, Suspended solids and dissolved solids as compared to the sulphite and kraft liquors. Natural settling and filtration of black liquor causes around 55% reductions in the COD and other polluting chemicals. Addition of 200 ppm alum and 10 ppm flocculating anionic polymer yields 82% reduction in the Chemical Oxygen Demand. Sample was also passed through the bed of sand and activated carbon, the reduction in the Chemical Oxygen Demand was almost 79%. The reduction in chemical oxygen demand with chemicals additions and then providing settling is comparable with that of the reduction by passing through activated carbon. Results are manipulated in table 4.29. Two results are given for a sample with too high values of COD i.e. 6501ppm, addition of alum

123

and flocculating polymer causes only 34% reduction in COD and passing this sample through activated carbon and sand bed causes 45% reduction in the COD.

124

Table 4. 28, Results of the batch treated with alum and different doses of anionic polymer (Bufloc 5425).

Sample Reference / Parameter COD Initial (ppm)

COD Final (ppm) Reduction in COD

Original Sample 1637 - - After 2 Hours Settling 1637 670 59% After 2 Hours Settling and filteration

1637 724 54%

Addition of Alum 200 ppm plus Anionic polymer 5425, 10 ppm and 2 hours settling

1637 292 82%

Adding 200 ppm alum and 5 ppm of polymer 5425 and settling for 2 Hrs.

6501 4270 34%

After passing the original sample through activated sludge 1637 345 79%

After passing as such sample through bed of sand + activated carbon

6501 3564 45%

Reduction in COD obtained through this experiment was not so high. Fourth batch: Black liquor used in this experiment contained 2780 ppm of total solids, 1120 suspended solids and 1300ppm COD. It was treated with 200 ppm alum and 10ppm anionic polymer BUFLOC 5425. Reduction in COD by this treatment was not so high. Finding of the experiments are given in table 4.30. This sample when left for settling there was 34% reduction in COD, when this natural settling was aided with filtration the reduction in COD was 39%. The reduction is not enough to through this liquor directly into the stream. This liquor was treated with 200 ppm alum and 10 ppm anionic polymer Bufloc 5425 (table 4.30) and was analyzed after 2 hours of settling, reduction in COD was found to be only 40%. This shows that there is soluble COD in the black liquor which can not be removed by flocculation and settling. Anyhow, when this liquor was filtered through bed of activated charcoal, reduction in COD was found to be risen to 50%. Still this reduction is not enough to be considered suitable for final treatment.

Table 4. 29, Chemical treatment of batch fourth of the CTMP black liquors.

Sample Reference / Parameter COD Initial (ppm) COD Final (ppm)

Reduction in COD

Original Sample 1300 - - After 2 Hours Settling 1300 854 34% After 2 Hours Settling and 1300 786 39%

125

filteration Addition of Alum 200 ppm plus Anionic polymer 5425, 10 ppm and 2 hours settling

1300 772 40%

After passing the original sample through activated sludge

1300 653 50%

Fifth batch: Liquor selected for analysis was high in COD (2500ppm). Settling causes minute reduction in COD (9%). Addition of 100 ppm Alum and 1ppm of anionic polymer (Bufloc 5425) results in 49% reduction in COD, still the COD of the treated liquor sample is too high. Another method employed for treatment was settling under high and low pH values. pH was lowered with sulfuric acid addition and increased with addition of caustic but both of changes caused only 46% reduction in COD. This approach depicts that the settling is enhanced with change in pH but this effect is again too far from the desired results. A unique approach was used here. The liquor was passed through ion exchange resin. There was reduction of 66 % in COD after passing the sample through ion exchange resin. This showed that the liquor contained soluble COD which is non-settle able by ordinary means but can be removed if the sample is passed through the ion exchange resins. Results of these trials are given in table 4.31. Table 4. 30, Chemical treatment and after passing the black liquor sample through ion exchange

resin.

Sample Reference / Parameter COD Initial

(ppm) COD Final (ppm)

Reduction in COD

As such sample 2515 - - As such after settling for 2 hrs 2515 2282 9 As such sample after filtration 2515 - - As such sample after alum (100 ppm) & polymer 5425 (1 ppm) and settling for 2 hrs.

2515 1278 49

As such sample after lowering down the pH 2.2 with Sulphuric acid & settling for 2 hrs.

2515 1370 46

As such sample after lowering down the pH 2.2 with Sulphuric acid and then neutralizing with caustic at pH 7.1, with settling for 2 hrs.

2515 1374 46

As such sample after passing through ion exchange resin

2515 861 66

126

Sixth batch: Another sample with high COD was selected for treatment. Sample was high in pH (8) total solids, suspended solids and COD in the range of 2403, 1474 and 2485 respectively. Three methods were employed for its treatment. First was the reduction in COD and solids with natural settling. Natural settling resulted only 28% reduction in COD and this settling when aided with Alum and flocculating polymer the reduction in COD was 30%. Reduction in pH caused 50% removal of the soluble and insoluble pollutants. Again when this liquor was passed through ion exchange resin the reduction in COD was 68%. This gives information about the presence of soluble inorganic and organic compounds which are responsible for the COD and pollution. Analysis of the sample used for treatment and chemical treatments are given in the table 4.32.

Table 4. 31, Chemical treatments at various pH.

Sample description COD ppm

Reduction, %

As such sample 2485 - As such after settling for 2 hrs 1790 28 As such sample after filtration As such sample after alum (1 ppm) & polymer 5425 (1 ppm) and settling for 2 hrs. 1730 30

As such sample after lowering down the pH 2.2 with Sulphuric acid & settling for 2 hrs. 1122 55

As such sample after lowering down the pH 2.2 with Sulphuric acid and then neutralizing with caustic at pH 7.1, with settling for 2 hrs.

1772 29

As such sample after passing through ion exchange resin 792 68 Sixth batch: Previous sample (fifth batch) was selected for treatment with high concentration of alum, high concentration of flocculating polymer and treatment with hydrogen peroxide. Initially sample was at 7.8 pH, 2574 ppm of dissolved solids, 1508 ppm of suspended solids and 2485 ppm of COD. Addition of 3000ppm alum resulted in 43% reduction of COD and by enhancing treatment with 1 ppm of anionic polymer results in 46% reduction of COD. When this liquor sample was treated with 3000 ppm of hydrogen peroxide, very poor reduction in COD was found. Reduction was just 12% which is insufficient for the liquor to be thrown into the effluent channel. Anyhow, when the sample was treated with alum 1000 ppm and anionic polymer Bufloc 5425 1ppm, after settling for 2 hours, the liquor was found to be low in COD upto 80%. This

127

provides information that the removal of dissolved and suspended particles with alum and flocculating polymer needs settling time along with chemicals. Results of the chemical treatment are given in table 4.33.

128

Table 4. 32, Results of the treatment trials.

Treatment sequences: COD ppm

Reduction, %

As such sample 2485 - After addition of 3000 ppm alum 1418 43 After addition of 2000 ppm alum + 1 ppm cationic polymer Bufloc 5425.

1353 46

After addition of 3000 ppm Hydrogen peroxide 2191 12 After addition of 1000 ppm alum + 1 ppm polymer 5425 and settling for 2 hours.

396 80

Seventh batch: Following is the COD reduction after treatment with Hydrogen per oxide (H2O2) at different dosages. Hydrogen peroxide caused no significant reduction in the COD of the black liquor. Initially black liquor contained 7.2 pH, 2633 ppm of total dissolved solids, 1735 ppm of suspended solids and 2142 ppm of COD (table 4.34). The liquor by simple settling caused 43% reduction in COD but after addition of 1000 ppm of hydrogen peroxide the reduction was 21%, by doubling the concentration of hydrogen peroxide the reduction was just 27% and when 3000ppm of hydrogen peroxide were added net reduction in COD was just 29%. This reductions in COD in not enough, to be employed for the effluent treatment process, as a single step only. Sample sieved through 400 mesh sieve before treatment, No significant reduction, if waste water treated with Hydrogen per oxide. Table 4.34 gives results of treatment with hydrogen per-oxide.

Table 4. 33, Results after treatment with hydrogen peroxide

COD (ppm) Dosage of (H2O2)

Before treatment

After treatment

Reduction in COD, %

Blank – without H2O2

2516 1429 43

1000 ppm H2O2 1833 1456 21 2000 ppm H2O2 2411 1763 27 3000 ppm H2O2 2834 2023 29

Below is the treatment with hydrogen peroxide at various levels of COD

and suspended solids. Sample selected for this treatment was too high in dissolved solids 19332 ppm, 3716 ppm suspended solids and 9158 ppm of COD. Sample was sieved through 400 mesh sieve before treatment, then the sample was treated with hydrogen peroxide solution. Like previous experiment, it is

129

obvious form the results given in table 4.35, that every time the reduction in COD is high when the sample is left for natural settling and there is only 20-30% reduction when treated even with very high doses of hydrogen peroxide.

Table 4. 34, Results of COD reduction after treatment with Hydrogen per oxide (H2O2)

COD (ppm) Reduction in COD, % Dosage of (H2O2)

First day Second day First day Second day

Blank – without H2O2

10320 6443 22 30

1000 ppm H2O2 10392 6508 31 29 2000 ppm H2O2 10192 6926 33 25 3000 ppm H2O2 9390 7080 29 23

Eighth batch:

Ferric and aluminium salts have been found effective coagulants. Ferric salts i.e. ferric chloride and ferric sulfate were employed for removal of suspended solids and COD from the effluent. 72 % COD, when waste water from sedimentation tank 1 is treated with 2000 ppm ferric chloride, while 69 % reduction has seen while using 2000 ppm of ferrous sulfate but it lower down the pH to almost 3.0. In table 4.36, very high dose of the coagulants was used to remove the COD and suspended solids form the black liquor.

Table 4. 35, Reduction in COD by treating with iron salts.

As such sample

Settling for 2 hrs. Ferric chloride 2000 ppm

Ferrous sulfate 2000 ppm

COD, ppm 2427

1046 674 750

COD reduction, %

57 72 69

Ninth batch: This batch gave similar treatment results but for the liquor with different loads of COD (4489ppm) and dry solids (dissolved solids 7123ppm and suspended solids 3512ppm). Changing sequence of polymer and alum achieves up to 60% reduction in COD (table 4.37) but by passing this sample through the bed of activated carbon, the reduction in COD in 72%.

Table 4. 36, Results of chemical treatment, polymer and activated carbon.

130

Sample description COD, ppm Reduction, % As such sample 4489 - As such sample after settling for 2 hrs 2139 52 As such sample after settling for 2 hrs and filtration 2182 51

Addition of 200ppm alum and 15 ppm of Bufloc 5425 and settling for 2 Hrs. 1782 60

After passing as such sample through bed of sand + activated carbon

1267 72

Tenth batch:

Chemical treatment was evaluated at low pH. Sample selected for treatment was 6.7 in pH, 7123 ppm in dissolved solids, 3512 ppm in suspended solids and 4489 ppm in COD. Treatments were carried out by lowering the pH to 2.1-2.2. 71 % reduction in COD was observed after lowering down pH to 2.2. After lowering the pH down to 2.2, there is marked effect of the alum and/or flocculating polymer on the COD removal and settling of the black liquor (table 4.38). 72% removal in COD was achieved by treating this liquor with 200 ppm alum and 10 ppm of flocculating polymer Bufloc 5425.

Table 4. 37, Reduction in COD after chemical treatment at pH 2.2.

Sample description COD ppm

Reduction %

As such sample after lowering down the pH 2.2 with Sulphuric acid and settling for 10 minutes

1307 71

As such sample after lowering down the pH 2.2 with sufuric acid and adding alum & polymer, settling for 10 minutes Alum 200 ppm

Polymer 5425 5 ppm

1346 70

As such sample after lowering down the pH 2.2 with Sulfuric acid and adding alum & polymer, settling for 10 minutes Alum 200 ppm

Polymer 5425 10 ppm

1257 72

After lowering the pH down to 2.2, there is marked reduction in COD by simple settling. Suspended solids were removed 100% after 10 minutes settling. Dissolved solids were also removed to 45% after settling.

131

Eleventh batch: Sample selected for treatment was 6.8 in pH, 2783 ppm in dissolved solids, 2548 ppm of suspended solids and 3300 ppm of COD. This sample was treated with both flocculating polymer at same pH, then the sample was treated with sulphuric acid and after lowering the pH to 2.2 (table 4.39). Again neutralizing with caustic to pH 7. Neutralization caused markable effect in lowering the COD (73%).

Table 4. 38, Treatment by acidification and then neutralization.

Sample description COD ppm

Reduction %

As such sample 3300 - As such after settling for 2 hrs 1319 60 As such sample after filtration 1140 65 As such sample after alum (200 ppm) & polymer 5425 (10 ppm) and settling for 2 hrs.

1182 64

As such sample after lowering down the pH 2.2 with Sulphuric acid and settling 10 minutes

1081 67

As such sample after lowering down the pH 2.2 with Sulphuric acid and then neutralizing with caustic at pH 7.1, with no settling time

884 73

Twelfth batch: Black liquor selected for treatment in this batch contained 7533 ppm of dissolved solids, 2878ppm of suspended solids and 5900 ppm in COD. Different doses of alum and flocculating polymer were added and COD reduction was analyzed. Polymer caused 63% reduction in COD. Lowering in pH caused 69% reduction in COD and this effect increased when the liquor was treated with caustic to neutralize the liquor. Neutralization caused 71% reduction of COD. Results of chemical treatment are given in table 4.40.

Table 4. 39, Reduction in COD after chemical treatment.

Sample description COD ppm

Reduction %

As such sample 5900 - As such after settling for 2 hrs 2260 62 As such sample after filtration 2192 63 As such sample after alum (200 ppm) & polymer 5425 (10 ppm) and settling for 2 hrs. 2200 63

As such sample after lowering down the pH 2.2 with Sulfuric acid & settling for 2 hrs. 1844 69

As such sample after lowering down the pH 2.2 with 1700 71

132

Sulfuric acid and then neutralizing with caustic at pH 7.1, with settling for 2 hrs.

133

Thirteenth batch: Black liquor selected for analysis contained 13,462 ppm of dissolved solids, 2236 ppm of suspended solids and 4228 ppm of COD. This sample contained so high values of suspended and dissolved solids that normal treatments caused a little reduction in COD and solids. A different technique was tried to treat this liquor, which is to pass the liquor through ion exchange resin bed. Ion exchange resin caused double effect in reduction COD, at normal treatment methods this reduction was just 24 and 28 percent (table 4.41). But this reduction was almost doubled to 53% when the liquor was passed through ion exchange resin.

Table 4. 40, Chemical treatment of the black liquor.

Sample description COD ppm

Reduction %

As such sample 4206 - As such after settling for 2 hrs 4010 5 As such sample after filtration As such sample after alum (1 ppm) & polymer 5425 (1 ppm) and settling for 2 hrs.

3795 10

As such sample after lowering down the pH 2.2 with Sulfuric acid & settling for 2 hrs.

3044 28

As such sample after lowering down the pH 2.2 with Sulfuric acid and then neutralizing with caustic at pH 7.1, with settling for 2 hrs.

3240 24

As such sample after passing through ion exchange resin

1983 53

Fourteenth batch: Black liquor selected for analysis contained 13,462 ppm of dissolved solids, 2236 ppm of suspended solids and 4228 ppm of COD. When this liquor was treated with 2,000 ppm of ferric chloride; 42 % reduction in COD, while 34 % reduction was observed (table 4.42). While using 2000 ppm of ferrous sulfate but it decreased pH to almost 3.0

Table 4. 41, Treatment of black liquor with iron salts.

As such sample After settling for 2

hrs.

After treating with ferric chloride

2000 ppm

After treating with ferrous sulfate

2000 ppm COD, ppm 7110

4852 4104 4717

COD reduction, 32 42 34

134

% Fifteenth batch:

These experiments were carried out to confirm the treatment results of the sequence which works well for the CTMP liquors. A composite sample was obtained form the Packages composite waste water – just before outside of the boundary wall. The effluent was at pH 7.0, dissolved solids 3540 ppm, suspended solids 3231 ppm and COD 2043 ppm. This sample was treated with alum and flocculating polymer’s sequences. The results were verified after settling. Sequences and treatment results are given in table 4.43.

Table 4. 42 Treatment results of Packages composite sample.

Sample description COD ppm

Reduction, %

After addition of 1000 ppm alum + 1 ppm cationic polymer Bufloc 5425 , settling for ½ hr

1438 30

After addition of 1000 ppm alum + 1 ppm cationic polymer Bufloc 5425, settling for ½ hr , 1:1 dilution with process water

637 69

1 : 1 blending Treated Sed. Tank 1 Final discharge as

such Alum 1000 ppm + 1 ppm Polymer 5242

- 1172 43

1 : 1 blending Treated Sedimentation Tank 1

Treated composite

Alum 1000 ppm + 1 ppm Polymer 5242

Alum 1000 ppm + 1 ppm Polymer 5424

937 54

Sixteenth batch: This trial was carried to enhance the treatment by diluting the waste water with process water. Waste water selected for the treatment was at pH 7.0, dissolved solids in the average of 3540 ppm, suspended solids in the range of 3231 ppm and COD in the average 0f 2043 ppm. By diluting this water four times with process water 79 reduction in COD was achieved but this reduction was increased to 85% when the diluted sample was treated with flocculating polymers. The results of the treatment are given table 4.44.

Table 4. 43 Results of waste water chemical treatment after dilution.

135

Sample description COD ppm

Reduction, %

As such sample 2043 - After dilution of waste water with process water, 4 times

435 79

After dilution of treated waste water of sedimentation tank (1000 ppm alum + 1 ppm cationic polymer Bufloc 5425) bearing 4 times dilution with process water

313 85

Results of sixteen batch are better that that of given by Grover et al (86). Seventeenth batch:

Following liquor sample was treated with high doses of iron salts like ferric chloride and ferrous sulfate. The liquor contained 3976 ppm dissolved solids, 1160 ppm of suspended solids, 2861 ppm COD and 7.6 pH. Ferric chloride has shown a better reduction of 38 % in COD. While comparing Ferrous sulfate produce reduction of about 28 % at 2000 ppm concentration, (table 4.45), but it lower down pH to acidic, while on settling no reduction in COD has come to observation.

Table 4. 44, COD reduction after treatment with ferric chloride and ferrous sulfate.

Sample description COD ppm

Reduction %

With 2000 ppm FeCl3 1784 38 With 4000 ppm FeCl3 1662 42 With 6000 ppm FeCl3 1882 34 With 2000 ppm FeSO4 2053 28 With 4000 ppm FeSO4 2119 26 With 6000 ppm FeSO4 2126 26

136

Eighteenth batch: In this trial same black liquor was used but the settling efficiency of iron salts was verified at both pH 6.9 and at lower pH (by lowering by the addition of acid). 50 % reduction in COD has observed while using 2000 ppm of ferric chloride at pH 3.3, but this is less at high pH; just 25% reduction as in table 4.46.

Table 4. 45, Iron salts treatment at various pH ranges.

Ferric chloride treated Ferric sulfate treated As such sample

After 2 hr settling

1000 ppm 2000 ppm 1000 ppm 2000 ppm

pH COD (ppm)

pH COD (ppm)

pH COD (ppm)

pH COD (ppm)

pH COD (ppm)

6.9 1532

1357

4.7 1148 2.9 765 4.4 1061 3.0 921

Reduction in COD, % 11 25 50 31 40

Nineteenth batch:

Addition of 100 ppm of alum, 500 ppm of ferric chloride and 4 ppm of the flocculating polymer produced quick settling and relatively clear solution left behind. In second trial (table 4.47) where alum was added 40ppm, ferric chloride 200 ppm and polymer as 4ppm settling was very slow and the treated sample was not so clear.

Table 4. 46, Settling efficiency of Iron salts enhanced with the flocculating polymer.

COD (ppm) pH Sample Reference / Parameter Initial Final Initial Final

Reduction in COD

Original Sample 2800 - 7 - - Addition of Alum 100 ppm, FeCl3 500 ppm and Polymer 4 ppm.

2800 1691 7 4.2 40%

Addition of Alum 40 ppm, FeCl3 200 ppm and Polymer 4 ppm.

2800 2548 7 5.5 9%

Addition of Alum 100 ppm, FeCl3 300 ppm and Polymer 4 ppm.

2800 1764 7 4.5 37%

Addition of Alum 80 ppm, FeCl3 200 ppm and Polymer 4 ppm.

2800 2209 7 5.0 21%

Addition of Alum 60 ppm, FeCl3 500 ppm and Polymer 6 ppm.

2800 2436 7 5.3 13%

137

Twentieth batch: The sample selected for treatment contained 2939ppm of the dissolved solids and was treated with different sequences lime, polymer and varying pH of the liquor. First trial was made by 15 minutes stirring, by using the above combination at pH 4.2, second trial was made by 30 minutes stirring for the above combination at pH 4.2, third combination was selected for 15 minutes stirring, by using the above combination by adjusting pH 6.5 with NaOH & settling and then by 15 minutes stirring, by using the above combination by adjusting pH 8.5 with lime & settling. The results of these trials are given in table 4.48.

Table 4. 47, Settling efficiency of alum, iron salts and polymer.

COD pH Sample Reference / Parameter Initial Final Initial Final

Reduction in COD

Original Sample 2468 - 6.8 - - Alum 100 ppm, Ferric chloride

500 ppm and polymer 5425, 4ppm

2468 1594 6.8 4.1 35%

Alum 1000 ppm, Ferric chloride 500 ppm and polymer 5425,

4ppm 1782 1023 6.8 4.2 43%

Alum 1000 ppm, Ferric chloride 500 ppm and polymer 5425,

4ppm, 15 min settling at pH 4.2 1782 9 78 6.8 4.2 45%

Alum 1000 ppm, Ferric chloride 500 ppm and polymer 5425,

4ppm, 30 min settling at pH 4.2 1782 964 6.8 4.2 46%

Alum 1000 ppm, Ferric chloride 500 ppm and polymer 5425,

4ppm, 15 min settling at pH 6.5 1782 858 6.8 6.5 52%

Alum 1000 ppm, Ferric chloride 500 ppm and polymer 5425,

4ppm, 15 min settling at pH 8.5 1782 1455 6.8 8.5 18%

138

Twenty first batch: The sample selected for the chemical treatment contained 6939 ppm dissolved solids and 4414 ppm of the chemical oxygen demand. If this sample is left for settling; COD and dissolved solids drop to 4310 ppm and 6916 ppm respectively. Effects have been shown in table 4.49.

Table 4. 48, Results of further chemical treatment

COD (ppm) Final Sample Reference / Parameter

Initial Final pH DS* Reduction in

COD Original Sample 4414 - 5.5 6939 -

Alum 1000 ppm, PAC 1000 ppm, Polymer 4 ppm

4414 2753 5.1 6250 37%

Alum 400 ppm, PAC 1400 ppm, Polymer 4 ppm

4414 3350 5.4 6502 25%

Alum 1000 ppm, PAC 2000 ppm, Polymer 4 ppm

4414 3165 4.6 6298 28%

Alum 1000 ppm, PAC 3000 ppm, Polymer 4 ppm

4414 3132 4.6 6722 29%

PAC 2ooo ppm and polymer 4 ppm

4414 3307 5.1 6134 25%

Alum 1000 ppm, PAC 1400 ppm and polymer 4ppm 4414 3096 5.1 6034 30%

Alum 1000 ppm, PAC 1400 ppm and polymer 4 ppm, pH

adjusted with caustic 4414 3301 6.0 6787 25%

Alum 1000 ppm, PAC 1400 ppm and polymer 4 ppm, pH

adjusted with chlorine 4414 2880 6.0 10000 35%

* Dry solids on oven dry basis. Alum with 1000 ppm, 1400 ppm of PAC and 4 ppm of the flocculating polymer yields good settling and good clarity.

139

Twenty second batch: This study was made to optimize the polyaluminium chloride addition. Settling efficiency of polyaluminium chloride was enhanced by addition of alum and flocculating polymer 5424. Maximum COD reduction was found to be 36% (table 4.50), when 1,000 ppm alum, 1,000 ppm PAC and flocculating polymer 4 ppm were employed at pH 6.0 (adjusted with hypochloride).

Table 4. 49, Results of treatment with Alum, PAC and flocculating polymer.

COD (ppm) pH Sample Reference / Parameter Initial Final Initial Final

Reduction in COD

Original Sample 2965 - 6.8 - - Alum 1000 ppm, PAC 1000 ppm, Polymer 4 ppm

2965 2091 6.8 4.7 30%

Alum 1000 ppm, PAC 1400 ppm, Polymer 4 ppm

2965 2154 6.8 4.4 27%

Alum 1000 ppm, PAC 1000 ppm, Polymer 4 ppm, pH adjusted to 6.0 with caustic

2965 2098 6.8 6.0 29%

Alum 1000 ppm, PAC 1000 ppm, Polymer 4 ppm, pH adjusted to 6.0 with Hypochloride.

2965 1905 6.8 6.0 36%

140

4. 30 Biological treatment of the black liquors: Biological treatment of the CTMP based black liquor was carried out in pilot plant made at Packages Limited. The plant consisted of two major tanks first one the aeration tank provided with the active sludge from the existing batch every time. Second tank was used as settling tank and the results obtained were all from the supernatant liquor taken form this tank. Work of defining the biological treatment was carried in two stages;

· Determination of activity of activated sludge by direct aeration and settling.

· Isolation of all the microbes in the activated sludge and their biodegradation activity on their enzyme production basis.

Following are the results of treatment of the weak black liquor form chemi-

thermo-mechanical pulping with activated sludge. Fresh air was bubbled through and the sludge was provided with optimum conditions to grow and mature. Micro-organisms which can get their nourishment from the nutrients present in the sludge grow well and accumulate in the sludge but those which enter from the surroundings get distributed by the same ratio. Maturation of the sludge increases degrading micro-organisms and retains their level for the next incoming liquor. Modelling of the reactor was according to the concept of batch reactor by Olazar et al (87). Table 4.51(a-m) provides results of activated sludge treatment on weekly basis.

Table 4. 50, Activated sludge treatment trial, of first week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

7.81 212940 1600 211340 1913 2391

After Treatment

7.6 144776 400 144376 711 1345

Percent Reduction

32% 75% 32% 63% 44%

141

Table 4.51 (a), Activated sludge treatment trial, of second week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

7.84 773196 7000 766195 3360 4014

After Treatment

7.16 564917 3000 561917 1680 1461

Percent Reduction

27% 57% 27% 50% 46%

Table 4.51 (b), Activated sludge treatment, trial of third week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

9.06 396825 1600 295225 2024 3099

After Treatment

7.64 149925 800 149125 816 1550

Percent Reduction

62% 50% 49% 60% 50%

Table 4.51 (c), Activated sludge treatment, trial of fourth week. pH

Total Solids (ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

8.90 298804 2600 295704 1080 2592

After Treatment

8.16 145621 1351 144270 480 1479

Percent Reduction

51% 48% 51% 55% 43%

142

Table 4.51 (d), Activated sludge treatment, trial of fifth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

7.66 811248 1900 809348 3142 2249

After Treatment

7.19 301951 200 301751 1497 1128

Percent Reduction

63% 89% 63% 52% 50%

Table 4.51 (e), Activated sludge treatment, trial of sixth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

7.68 256850 2400 254450 1496 3834

After Treatment

7.40 146951 1189 145762 841 1308

Percent Reduction 43% 50% 43% 44% 62%

Table 4.51 (f), Activated sludge treatment, trial of seventh week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment 7.97 341909 1000 340909 1397 3956

After Treatment 7.61 132187 200 131987 960 1835

Percent Reduction 61% 80% 61% 31% 54%

143

Table 4.51 (g), Activated sludge treatment, trial of eighth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

7.24 14451 1375 13076 1290 660

After Treatment

7.84 6144 142 6002 420 468

Percent Reduction

57% 90% 54% 67% 29%

Table 4.51 (h), Activated sludge treatment, trial of ninth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

8.27 13107 427 12680 2040 10990

After Treatment

8.2 6577 125 5552 840 4622

Percent Reduction

57% 71% 56% 59% 58%

Table 4.51 (j), Activated sludge treatment, trial of the tenth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment 7.8 9708 416 9292 1920 9425

After Treatment 7.68 4174 104 4070 720 7342

Percent Reduction

57% 75% 56% 63% 22%

144

Table 4.51 (k), Activated sludge treatment, trial of eleventh week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment 7.65 8435 318 8117 1800 7219

After Treatment 7.5 3711 121 3590 867 5054

Percent Reduction 56% 62% 57% 52% 30%

Table 4.51 (l), Activated sludge treatment, trial of twelfth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment

7.96 8832 502 8330 1430 6180

After Treatment

7.4 3974 302 3672 785 4128

Percent Reduction

55% 40% 55% 45% 33%

Table 4.51 (m), Activated sludge treatment, trial of thirteenth week.

pH Total Solids

(ppm)

Suspended Solids (ppm)

Dissolved Solids (ppm)

Biological Oxygen Demand

(ppm)

Chemical Oxygen Demand

(ppm) Before Treatment 7.83 5739 652 5086 2542 9315

After Treatment 7.53 2457 198 2259 876 6414

Percent Reduction 58% 70% 56% 65% 31%

Biodegradation of activated sludge changes with maturation of the sludge. There are certain variations in the process which affect the incoming liquor from

145

time to time. This affects the degradation efficiency of the sludge. The maturation of the sludge was studied initially for thirteen weeks. Pilot was let running for thirteen weeks and every week the degradation efficiency was determined as per discussed in the above section. Rate in change in the efficiency of the activated sludge are given in the table 4.52. Following the table, the rates have been plotted against the degradation efficiency per-week of the sludge.

Table 4. 51, Degradation efficiency of the activated sludge per week basis.

Degradation Efficiency of Activated Sludge on percent reduction basis Time of Aging

Total Solids (% Reduction)

Suspended Solids (% Reduction)

Dissolved Solids (% Reduction)

Biological Oxygen Demand (% Reduction)

Chemical Oxygen Demand (% Reduction)

1st week 32 75 32 63 44 2nd week 27 57 27 50 46 3rd week 62 50 49 60 50 4th week 51 48 51 55 43 5th week 63 89 63 52 50 6th week 43 50 43 44 62 7th week 61 80 61 31 54 8th week 57 90 54 67 29 9th week 57 71 56 59 58 10th week 57 75 56 63 22 11th week 56 62 57 52 30 12th week 55 40 55 45 33 13th week 58 70 56 65 31 Bergeron et al (93) developed a 5-litres activated sludge treatment pilot plant, but he could not produce the appreciable results. Aging of activated sludge was plotted with reduction efficiency. Activated sludge caused more or less similar degradation even after thirteen weeks of aging. There is certain enhancement in total solids reduction by activated sludge but certain variations are due to waste water composition instabilities. Some how waste enters into biological treatment plant with high loads of solids, COD and BOD; at that time efficiency is reduced and when it enters with less loads the reduction efficiency of the activated sludge increases (Fig 4.19). Following figure indicates the effects of degradation of the activated sludge on the total solids, suspended solids, dissolved solids, BOD and COD. As the sludge is produced, its efficiency is lower in the initial period, and then it increases as the sludge maturation occurs. But there are certain variations in the activity of the sludge which emerge due to

146

variations in the load of the effluent entering into the treatment plant. Font et al (90) was able to reduce COD to 60%. Gupta et al (92) treated the pulp mill waste water with Aeromonas formicans. He was able to reduce COD to 73% but at very high retention time of 8-days.

Percent Reduction

0

20

40

60

80

100

0 2 4 6 8 10 12 14

Activated Sludge Aging (weeks)

Redu

ctio

n in

Tot

al S

olid

s (%

)

Tota l Solids

Su spen dedSolids

Dissolv edSolids

Biolog ica lOx y g enDem a n dCh em ica lOx y g enDem a n d

Figure 4. 19, Reduction efficiency of activated sludge for various parameters;

Following figure (4.20) indicates the reduction effect of the activated sludge on the total solids of the effluent entering into the treatment plant. Initial efficiency of the plant is 30% reduction, this efficiency increases with the aging of the sludge. As the sludge matures the micro-organisms responsible for degradation increase in number and the reduction efficiency increases. After 7th week the reduction efficiency reaches to 58-60% of total solids in the black liquors.

Percent Reduction in total Solids

0

20

40

60

80

0 2 4 6 8 10 12 14

Activated Sludge Aging (weeks)

Red

uctio

n in

Tot

al S

olid

s (%

)

147

Figure 4. 20, Reduction efficiency of activated sludge for total solids.

Reduction in suspended solids has been found variable throughout the life of activated sludge. Initially around 70% of the suspended solids were removed by the sludge treatment plant but after weeks there are variations, these are due to instabilities in the loads of the black liquor entering into the plant. Figure 4.21 shows reduction efficiency of the activated sludge plant for suspended solids.

Percent Reduction in Suspended Solids

0

20

40

60

80

100

0 2 4 6 8 10 12 14

Activated Sludge Aging (weeks)

Red

uct

ion

in

Su

spen

ded

S

oli

ds

(%)

Figure 4. 21, Suspended solids reduction efficiency of the activated sludge plant.

Reduction efficiency in dissolved solids by the activated sludge plant have been plotted in the following figure 4.22. Initially 30% dissolved solids were removed by the activated sludge plant. Some variations have been found but there is increase in reduction of the dissolved solids with aging of the sludge. After around seven weeks around 55-60% of the total solids are constantly being removed by the activated sludge treatment plant.

148

Percent Reduction in Dissolved Solids

0

20

40

60

80

0 2 4 6 8 10 12 14

Activated Sludge Aging (weeks)

Red

uct

ion

in

Dis

solv

ed S

oli

ds

(%)

Figure 4. 22, Percent reduction in dissolved solids by activated sludge plant.

Biological oxygen demand of the black liquor is key factor influencing the efficiency of the activated sludge plant. Around 60% of the total biological oxygen demand of the black liquor is reduced in the initial which is retained even after 14 weeks of the maturation of the sludge. Certain variations in the reduction efficiency are due to variations in the incoming load of the black liquor.

Percent Reduction in Biological Oxygen Demand

0

20

40

60

80

0 2 4 6 8 10 12 14

Activated Sludge Aging (weeks)

Red

uct

ion

in

BO

D

(%)

Figure 4. 23, BOD reduction efficiency of the activated sludge plant.

Chemical oxygen demand of the effluent coming out from the pulping plant is also treated with activated sludge plant. The reduction in COD is not appreciable by the plant. Reduction reaches to 60% at 6th and 9th week of the sludge maturation. The reduction efficiency is demonstrated in the following figure.

149

Percent Reduction in Chemical Oxygen Demand

0

20

40

60

80

0 2 4 6 8 10 12 14

Activated Sludge Aging (weeks)

Red

uct

ion

in

CO

D

(%)

Figure 4. 24, COD reduction efficiency of the activated sludge treatment plant:

150

4. 31 Isolation and screening of micro-organisms: Six different strains of fungi were isolated from Packages effluent. The strains were purified on potato dextrose agar slants and stored on PDA slants at 4°C. The isolated fungal strains were then identified up to genera level using standard fungal identification method as described by Font et al (90), who also investigated the treatment of soda pulping black liquors with white-rot fungus.

Following morphological characteristics were examined for the identification purposes.

1) Trichoderma sp. produced green mass turning into dark lichen like appearance after two days. After a week the mycelium converted into dark green thick mass, with abundant spores wet with water droplets, which disappeared after some days. The green mass turned into brownish green after 20 days.

2) Coniophora sp. produced brown mass like embrioded on wood, having dark brown spots in center of the colonies. With time the mycelial mass increased and spores appeared in the colonies. After 20 days the colonies remained with the same brownish look and spores existed all the times.

3) Phlebia sp. produced white hypheae in high amounts, even the hypheae filled the test tube (slant) and the petri plate. After 7-days the slant was filled completely with the hypheae and turned into off-white mass. After 20-days entire surfacing of the media was covered.

4) Myrothecium sp. colonies producing black sporangium turning into thick mass of black sporangia. After 5 days black mass of mycelium increased and it produced cracks in the potato-dextrose-agar slants. Colonies remained black after 20-days growth.

5) Fusarium sp. colonies producing off white to brown mass, turning into tourney layer of the fungus. After 5 days, the mass increased and turned into brownish in color. All surface of the slaunt was covered with the mycelium.

6) Basidiomycetes sp. with white hypheae in the initial and then appeared reddish pigments in upper of potato slants. After 5-days for growth, the colonies converted into dark red appearance.

Four different bacterial strains were isolated from the black liquor source

using agar nutrient medium by replica plate method. Bacterial strains were identified for their nomenclature up to genera level (Holt et al 113). Clostridium sp., Ruminococcus sp., Streptomyces sp. and actinomycetes sp. were screened and identified from the isolated strains.

151

Of all the six-fungal cultures Trichoderma sp. was selected for its adoptability on the higher concentrations of the black liquor. Similarly among bacterial strains Clostridium sp. was selected for its adoptability on the black liquor.

4. 32, Cellulase production by the Trichoderma sp. for black liquor degradation:

Micro-organisms present in the black liquor were found to hydrolyze cellulosic materials. The degrading activity of the Trichoderma sp. was determined on the basis of hydrolysis of carboxymethyl cellulose. Trichoderma sp. was found to be the most potent fungus as compared to other organisms. Sanjay et al (114) reported cellulase production by Trichoderma reesi, growth rate was 38 mg/mL and enzyme production was 4.75 units/mL. The selected Trochoderma sp. strain was identified according to standard fungal identification method and named as Trichoderma GCU-110. Fermentation behaviour of Trichoderma GCU-110 was studied in shake flasks.

It was observed that cellulase production was increased by increasing the concentration of glucose. It reached a maximum value of 2.281 units/min when the concentration of the glucose was 4%. At this concentration the biomass was also at its maximum level i.e. 12.86 g/L. Aiello et al (115) reported biomass of Trichoderma reesei as much as 0.78g/g (31.2 g/L) for production of cellulase. A further increase in the concentration of the glucose however, decreased the production of cellulase. This may be due to increased viscosity of the medium which hindered the oxygen and mass transfer. Results of the experiments are given in table 4.52. Higher protein production (23mg/mL) and cellulase activity (19.5units/mL) by Trichoderma reesei was also reported by FAO agriculture service bulletins (116).

152

Table 4. 52, Activity of Trichoderma GCU-110 for degradation:

Effect of glucose concentration on the production of cellulase by Trichoderma sp.

Glucose

%

Biomass (dry) (g/L)

Cellulase Activity

units/min

Total Protein contents (mg/ml)

Specific cellulase activity

units/mg

1 7.00 0.412 0.417 0.988

1.5 8.10 0.727 0.621 1.170

2 10.40 0.796 0.627 1.270

2.5 11.72 0.961 0.638 1.506

3 12.34 1.092 0.712 1.534

3.5 12.71 2.076 0.727 2.855

4 12.86 2.281 0.666 3.425

4.5 11.81 2.007 0.66 3.041

5 10.22 0.892 0.591 1.509

5.5 7.10 0.58 0.53 1.094

153

Biomass Production

0

24

68

1012

14

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Glucose Concentration (%)

Bio

ma

ss

(g

/L)

Figure 4. 25, Effect of glucose concentration on biomass production.

Cellulase activity by Trichoderma GCU-110 was found to be maximum of

2.281 units/min (1 µ moles/min = 1 unit) at 4% glucose concentrations. Enzyme activity has been plotted in figure 4.26;

Cellulase Activity

0

0.5

1

1.5

2

2.5

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Glucose concentration (%)

En

zym

e A

ctiv

ity

(un

its/

min

)

Figure 4. 26, Effect of glucose concentration on cellulase activity. Figure 4.27 shows rate of specific enzyme activity, it increases with

increase in concentration of glucose. Activity was noted to be maximum of 3.425 units/mg at 4% glucose concentrations. Cellulase activity starts decreasing with increasing glucose concentrations.

154

Specific Enzyme Activity

00.5

11.5

22.5

33.5

4

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Glucose Concentration (%)

Sp

ecif

ic E

nzy

me

Act

ivit

y (u

nit

s/m

g)

Figure 4. 27, Effect of glucose on Specific enzyme activity.

4. 33 Cellulase production by Clostridium sp:

The degrading activity of Clostridium sp. was determined on the basis of hydrolysis of carboxymethyl cellulose (cmc). The strain was identified according to standard bacterial identification method and was named as Clostridium GCU-111. Growth characteristics of of Clostridium GCU-111 were monitored in shake flasks. Glucose was added as carbon source and the enzyme extracted was subjected to carboxymethyl cellulose degradation.

It was observed that cellulase production was increased by increasing the concentration of glucose in the growth medium. It reached the maximum value of 2.49 units/ml when the concentration of the glucose was 4% (table 4.53). At this concentration, the biomass was also at its maximum level i.e. 14.10 g/L. A further increase in the concentration of glucose however, decreased the production of cellulase. Johnson et al (117) reported cellulase production by Clostridium thermocellum, the cellulase activity was found 6.1 units/mL at pH 5.7 and 70 °C temperature. Thomas K.and Zeikus J. G. (118) reported that wild species of Clostridium showed cellulase activity of 0.4/mL.

155

Table 4. 53, Cellulase production comparison by Clostridium GCU-111:

Effect of glucose concentration on the production of cellulase by Clostridium GCU-111.

Glucose concentration

%

Bio-mass (Dry) (g/L)

Cellulase Activity

(units/ml)

Total Protein (mg/ml)

Specific Enzyme activity

units/mg

1 6.80 0.308 0.047 6.553

1.5 10.38 0.828 0.058 14.275

2 11.01 0.897 0.059 15.203

2.5 12.17 1.03 0.065 15.846

3 13.81 1.036 0.072 15.388

3.5 12.62 2.108 0.073 28.877

4 14.10 2.49 0.066 37.727

4.5 11.00 2.246 0.063 35.65

5 8.70 0.757 0.058 13.05

5.5 7.00 0.48 0.051 9.41

156

Biomass Production

02468

10121416

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Glucose Concentration (%)

Bio

ma

ss

(g

/L)

Figure 4. 28, Effect of glucose concentration on bio-mass production.

Cellulase activity of 2.49 units/ml was obtained at 4% glucose concentrations. However, there was decline in the growth and activity at higher concentrations of glucose. Cellulase activity by Clostridium GCU-111, has been plotted in the figure 4.29;

Cellulase Activity

0

0.5

1

1.5

2

2.5

3

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Glucose concentration (%)

En

zym

e A

ctiv

ity

(un

its/

min

)

Figure 4. 29, Effect of glucose concentration on cellulase activity.

Figure 4.30 shows the specific enzyme activity. Specific enzyme activity increases with increase in glucose concentrations and it reaches to its maximum of 37.727 units/mg at 4% glucose concentration. Specific enzyme activity then starts decreasing with increasing glucose concentrations.

157

Specific Enzyme Activity

05

10152025303540

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Glucose Concentration (%)

Sp

ecif

ic E

nzy

me

Act

ivit

y (u

nit

s/m

g)

Figure 4. 30, Effect of glucose on specific enzyme activity:

158

4. 34 Fate of the chemi-thermo-mechanical black liquors:

Chemistry of the chemi-thermo-mechanical pulping black liquors was determined in order to draw its fate about its recovery or treatment. Both of the possible processes were studied. Parameters extracted by this research are as follows.

I. Run into the chemical recovery plant II. Biological treatment

III. Biological treatment aided with chemical treatment. I) Run into the chemical recovery plant

Chemical recovery process is state of the art technology running on the kraft and soda pulping effluents for the recovery of valuable sodium and sulfur. The plant is running effectively and set parameters have been managed for those liquors. CTMP liquor differs in basic chemistry form other effluents. This needs change in process variables in order to run the plant smoothly. Detailed chemistry of the CTMP black liquors have been described in the previous sections.

The chemical properties of the CTMP black liquor provide information about the behavior of black liquor towards chemical recovery plant. There are considerable differences in the chemical composition and chemical properties of the CTMP black liquor from that of other pulping processes such as kraft and sulfites. We can draw the following conclusions from the research data: § Chemi-thermo-mechanical pulping black liquor contains high values of

organic components and silica than that of other pulping processes. It will be of major concern by plant operating point of view.

§ High organic components determine high heating values of the black liquor and govern its auto burning in the recovery boiler. It will result in production of high in the recovery boiler. The high produced will yield greater quantity of steam.

§ Initial lower solid contents predict that high energy is required in the concentrators in order to take the liquor to the required dry solid value. This also predicts that the liquor will spend more time in the concentrators or it can be expressed in the way, low volume of the liquor will be concentrated in given time.

§ Low inorganic components determine lower recovery at the cost of high energy; this energy loss may be compensated in terms of high steam generated from the recovery boiler.

159

§ Density of the concentrated liquor will be lower than that of liquor from kraft or sulfite pulping process. This lower value owes its existence to higher organic contents of this black liquor.

§ First and second critical solid values are higher for CTMP black liquor, which offers a good margin of higher dry solids black liquor to be fed in the recovery boiler and predicts low scaling in the concentrators.

§ High silica contents also determine high viscosity. The second major problem, which has not been discussed in this paper, is the high scaling of silica in the conversion section.

§ Desilication will impart major problems in the plant both before and after the recovery boiler. Desilication in the concentrators is somewhat challenging but can be achieved by lowering the pH up to certain level.

II) Activated sludge (biological) treatment:

It is obvious form the above experiments in this research, that chemical or microbiological treatment alone is insufficient to remove all the COD and BOD form the black. Rather it needs combination of both. Factors like addition of various flocculating polymers how affects the settling rate and what are their impacts on removing COD and BOD have been determined. Biological treatments needs enough time for aeration (8 hours) and continuous re-circulation of the activated sludge. Liquor needs to be stayed there for that time and the liquor to be treated in secondary clarifier. Aeration time, sludge produced, BOD reduction rate and volume of the liquor produced are major factors which determine the treatment plant design. Following are key parameters to be considered in setting plant operation and conditions. Activated sludge treatment parameters: Black liquor volume : 25 dm3 Optimum aeration time : 12 hours Air volume fed in : 10 m3 per hour for the given liquor volume. Reduction in total solids : 30 – 60% Reduction in suspended solids: 40 – 90%

160

Reduction in dissolved solids : 27 – 61% Reduction in BOD : 30 – 67% Reduction in COD : 25 – 62% Sludge volume produced : 1 dm3 Chemical treatment parameters: Volume of the liquor : Concentration of flocculants used: 1 % solution in water after mixing for 30 min. Type of flocculants used : cationic and anionic Inorganic flocculants used in alternative: Alum / ferric salts Concentration of inorganics used : 100 – 200 ppm Settling time for organic flocculants : min 10 minutes Settling time for inorganic flocculants: min 10 minutes Sludge produced : 1 % of original liquor volume. Plant carrying both activated sludge and biological treatment will be having following stages as an important.

161

Proposed design for treatment plant: Following is flow sheet for activated sludge plus chemical treatment plant for chemi-thermo-mechanical pulping black liquors.

Figure 4. 31 Flow sheet diagram of activated sludge and chemical treatment plant.

Aeration Tank,

Settling zone. Sludge collection zone

Aeration Pump

Flocculant mixing Tank

Balmer Press

Sludge Handling

Mixer

Clarifier

Clear Liquor Out

Sludge Collection:

Flocculants Doze

Sludge Recycling

Black Liquor In

90% Sludge 10% Sludge recycled

162

III) The secondary clarifier: settling of activated sludge by chemical

treatment: Continuous sedimentation is a basis for settler design and operation in

activated sludge systems. A settler used to separate flocculent, compressible particles, as those found in activated sludge systems, is usually divided into four zones, referred to as the discrete particle, flocculent, hindered settling and compression zones, Figure 4.32.

Figure 4. 32 Settling zones for activated sludge.

The compression phase begins when the critical concentration, a characteristic of the suspension, is reached119. In this region, the settling velocity is drastically reduced due to the high concentration of solids. The thickening of the sludge is in turn influenced by a number of factors120, such as: · nature of the mixed liquor particles (density, shape, floc structure, type of

microorganisms, electrostatic charges, etc.); · dissolved substances in the substrate; · temperature; · depth of the sludge blanket; · surface area of the sludge blanket; · effects due to mechanical actions, vibrations, pressure, etc.;

163

· concentration of settleable solids in the mixed liquor. The concentration at the bottom of the settler is also affected by the time allowed for compaction.

In a fundamental work by Kynch121 a theoretical analysis of sedimentation was made, based on the theory of Coe and Clevenger122. Kynch concluded that the concentration of settleable solids in the mixed liquor was of the utmost importance when describing the settling process, that is, he focused on only one of the many factors listed above. The settling in batch reactors was analysed as a process where levels of constant concentrations moved upwards due to the downward movement of particles.

Kynch’s theory for batch reactors was later extended for continuous reactors by Yoshioka123. The main four assumptions of Kynch’s theory are that the settling velocity of a particle depends only on the local concentration of particles; · all the particles have the same shape, size and density; · the particle concentration is constant within each horizontal cross-section of

the settler; · in continuous sedimentation the total settling velocity is a function of both the

settling rate of particles relative to the liquid and of the downward flow of the suspension due to the underflow withdrawn from the bottom of the thickener.

The first assumption is the fundamental one. This means that all other forces

acting on a particle are in equilibrium. Dixon124 found that inertial effects cannot be ignored by comparing simulation models of Kynch’s continuum theory and discrete settling theory and thus questioned the validity of Kynch’s assumptions. Another study125 showed how flocculent suspensions did not strictly follow Kynch’s assumptions. In a work by Dick126 it is stated that Kynch’s theory is highly idealized and requires an ‘ideal slurry’ to be directly applicable – which activated sludge cannot be said to be. However, in Dick127 and Dick and Young128 it was concluded that the massflow concept could be applied to a flocculent suspension, such as activated sludge, as a reasonable approximation.

164

Sludge handling and withdrawl:

1. Mechanical sludge collection and withdrawal equipment is required and shall provide complete and continuous removal of settled sludge for intermediate and final clarifiers. The sludge collection equipment and the drive assembly shall be designed to withstand the maximum anticipated loads of transporting sludge to a hopper. The peripheral speed for circular flight mechanisms should be in the range of 0.02 to 0.05 revolutions per minute but shall not exceed 8 feet per minute in final clarifiers. The straight line flight speed should be in the range of 2 to 4 feet per minute but shall not exceed 1 foot per minute in final clarifiers.

2. Positive displacement pumps shall be provided for pumping primary sludge intermittently and continuously. A positive head should be provided on pump suctions. If motor driven return sludge pumps are used, the maximum return sludge capacity shall be obtained with the largest pump out of service. Pumps shall have at least 3 inch suction and discharge openings. Automatic controls shall be provided to separately activate sludge pumps and sludge collection mechanisms. Sludge pumping in large plants should be controlled by timers and valve activators to provide continuous "on-off" operation. A means of measuring the sludge withdrawal rate shall be provided for each unit. It is recommended that sludge-pumping stations have a standby pump and serve two or more units. Air lift systems for sludge removal shall not be used for removal of primary sludge.

3. Rapid sludge withdrawal pipes shall return sludge to a sludge well at the water surface that enables visual observation of the flow. The return sludge withdrawal pipes shall be at least 4 inches in diameter with a hydraulic differential between the clarifier water level and the return sludge well level sufficient to maintain a velocity of 3 feet per second. The discharge piping should be designed to maintain a velocity of at least 2 feet per second when return sludge facilities are operating at normal return sludge rates. Each sludge withdrawal pipe shall be accessible for rodding or backflushing when the clarifier is in operation. Cleanouts and couplings shall be provided in sludge piping to facilitate pipe cleaning and removal of pumping equipment. High points in piping shall be provided with air releases. All sludge piping shall be metallic material.

4. Sludge recirculation from the secondary clarifier to the aeration tank shall be variable within 25 to 100 percent of the average design flow. Sludge wasting from the activated sludge process may be from the mixed liquor

165

or the return sludge. Sludge wasting shall be variable to enable zero wasting to 50 percent of the total system solids daily.

5. The minimum permissible return sludge rate of withdrawal from the final clarifier is a function of the concentration of suspended solids in the mixed liquor (MLSS) entering it, the sludge volume index (SVI) of these solids, and the detention time that these solids are retained in the clarifier. The rate of sludge return for activated sludge processes expressed as a percentage of the average design flow of wastewater should generally be variable between the limits set forth as follows:

Minimum % Maximum %

Standard rate, step aeration and carbonaceous stage of separate stage nitrification

15 75

Contact stabilization and extended aeration 50 150

6. The rate of sludge return shall be varied by means of variable speed motors, drives, or timers to pump sludge as set forth in this section.

7. Waste sludge control facilities should have a maximum capacity of not less than 25 percent of the average rate of wastewater flow and function satisfactorily at rates of 0.5 percent of average wastewater flow or a minimum of 10 gallons per minute, whichever is larger.

8. Sludge wells/scum pits shall be provided adjacent to the tank and equipped with suitable devices for viewing, sampling and controlling the rate of sludge withdrawn. Metering devices shall be installed and located to indicate flow rates of all influent and effluent points, return sludge and waste sludge lines. Meters shall be accurate to within + 5 percent of the actual flow rates.

Conclusively, most effective treatment for this liquor will be combination of

activated sludge treatment with the chemical treatment. Chemical treatment will involve use of effective flocculants to remove all the COD and BOD form the black liquors.

166

References

167

1 Khan, M. H. Report Environment Technology Program for Industry of

Pakistan. March 1999; 6. 2 FAO Report, April 1995; 16-38. 3 Bruenner, R. TAPPI Pacific Section Seminar,Seattle, 1994. 4 Veseth, R. STEEP Extension Program; 1987. 5 Beadle, C. L., C. R. A. Turnbull, and G. H. Dean. Appita 1996; 49(4): 239-242. 6 Kuo, L. S., and S. Y. Shen. Second International Nonwood Pulping Conference

1992, 351. 7 Ali, S. H., S. M. Asghar, A. U. Shabbir, " Tappi Pulping Conference

Proceedings, Tappi Press, Atlanta, GA. 2001. 51. 8 Aronovsky, S. I.; A. J. Ernst.; H. M. Stucliffe, and G. H. Nelson. Paper Trade

Journal, 1984; 126(26): 78-82. 9 Mohan, R.; Prasad R.; Yadav, K.;p Ray K. and N. J. Rao. First International

Nonwood Pulping Conference, Bejing, 1988; 339. 10 Utne, B. and L. Hegbom. Second International Nonwood Pulping Conference,

Beijing, 1992. 11 Misra, D. K. “. TAPPI, Atlanta, GA. 1987. 12 Utne, B. and L. Hegbom. Second International Nonwood Pulping Conference,

Beijing, 1992. 13 Atchison and McGovern, . TAPPI, Atlanta, GA. 1987. 14 Cheng Z.; J. Leminen, K.; Ala-Kaila, H.; Paulapuro. Tappi Pulping Conference,

1994; 735. 15 Hua W. J. and H. N. Xi. Appita 1988; 1(5): 365-374.

168

16 Mohan, R.; R. Prasad, R.; Yadav, K.; K. Ray and N. J. Rao. First International

Nonwood Pulping Conference, Bejing, 1988; 339. 17 Utne, B. and L. Hegbom. Second International Nonwood Pulping Conference,

Beijing, 1992. 18 Cheng Z., J. Leminen, K. Ala-Kaila, H. Paulapuro. Tappi Pulping Conference,

1994; 735. 19 Hua W. J. and H. N. Xi,. Appita 1988; 41(5): 365-374. 20 Roy, T. K.; R. Pant; A. Panda and D. Fengel. CTAP Seventh International

Symposium on Wood and Pulping Chemistry Proceedings, Beijing, 1993; (1) 455.

21 Zhang, D.; X. Liu and Z. Li. China Pulp and Paper, 1990; 16-21. 22 Sundholm, J. and Mannström, B. Paperi ja Puu 1982 64(1): 8. 23 Ahmat Tutus; Ilhan Deniz and Hudaverdi Eruglu. Pakistan Journal of

Biological Sciences 2004; 7 (8): 1350-1354. 24 Adler, E.; Lindgren, B. O.; Saedén, U.; Svensk Papperstid. 1982; 55(7): 245. 25 Beatson, R.; Heitner, C.; Atack, D. J. Pulp Paper Sci. 1994; 10(1): 12. 26 TEST PLAN for SPENT PULPING LIQUOR, CAS No. 66071-92-g B Y The

American Forest & Paper Association, HPV Work Group, 1998. 27 Hatfield R.D.; J. Grabber, J.; Ralph and K. Brel. J. Agric. Food Chem. 1999;

(47): 628-632. 28 Thi Hong man Vu; Raimo alen and Hannu Pakkanen. IPPTA journal . 2005; 17

(1): 29-36. 29 Zhinan Feng; Raimo Alén1 and Klaus Niemelä. Holzforschung 2002; 56: 388–

394.

169

30 Wang P.; Hu J.; Cui L-F.; Wu Q.; Liang Y-Z.; Zhang K. Third International

non-wood fiber pulping and papermaking conference-INWFPPC, Beijing, China, 1996; 2: 646-653.

31 Ibrahim H. “TAPPI pulping conference,New Orleans, USA, 1988; 3: 561-563. 32 Abha Gupta; Rajeev M. Mathur and Arvind G. Kulkarni. Appita Journl, 2001;

54 ( 6): 50-54. 33 Myreen, B. TAPPI International Chemical Recovery Conference, Tampa,

Florida, 1998; 72-75. 34 Ebiary M A.; Hackl H J. TAPPI, 2003; 13: 69-73. 35 Marcks R.; Schildhauer G. . Pulp Pap. Int. 1986; 28 (11): 48-49. 36 Stephen R.; Williams S.; Adney Edward Jennings; Toddy B.; Vinzant and

Micheal E. Himmel. Journal of Applied Biochemistry and biotechnology 2003: 105 – 108: 689-703.

37 Zhinan Feng,; Raimo Alén and Klaus Niemelä. Holzforschung 2002; 56: 388–

394. 38 Greg P. Golike; Qiusheng Pu; Kermit L. Holman; Kenneth R. Carlson; Paul

C.Wollwage; Harry G. Folster; and Scott C. Rankin. TAPPI Journal 2000: 83 (7): 1-10.

39 Sricharoenchaikul,-Viboon; Frederick,-Wm.-James, Jr; Agrawal,-Pradeep.

Industrial and Engineering Chemistry Research 2002; 41 (23): 5650-8. 40 Zhu,-J.-Y; Chai,-X-S; Pan,-Z.-J. Environmental Science and Technology 2002;

36 (10): 2269-72. 41 Demirbas,-Ayhan. “. Energy Sources 2002; 24(3): 205-13. 42 Das,-Tapas-K; Jain,-Ashok-K. Environmental Progress 2001; 20 (2): 87-92.

170

43 Golike,-Greg-P; Pu,-Qiusheng; Holman,-Kermit-L Tappi Journal 2000; 83 (7):

68. 44 Wasik,-Larry-S; Mittet,-Gerald-R and Nelson,-Doug-J. Tappi Journal 2000; 83

(3): 94-101. 45 Larson,-E.-D; Kreutz,-T.-G; Consonni,-S. Journal of Engineering for Gas

Turbines and Power 1999; 121 (3): 394-400. 46 Zaman,-A.-A; Fricke,-A.-L. Industrial and Engineering Chemistry Research

1996; 35: 590-597. 47 Zaman,-Abbas; Wight,-Matthew-O; Fricke,-Arthur-L. Tappi-Journal. 1994; 77:

175-181. 48 Zaman,-Abbas-A; Fricke,-Arthur-L. Industrial-and-Engineering-Chemistry-

Research. 1994; 33: 428-435. 49 Zaman,-Abbas-A; Fricke,-Arthur-L. AIChE-Journal. 1994; 40: 187-192. 50 Ramamurthy,-Pritham; van-Heiningen,-Adriaan-R.-P; Kubes,-George-J.

Tappi-Journal. 1993; 76: 175-179. 51 Bertel Myréen. TAPPI 2001 Pulping Conference, 2001; 12-15. 52 Jain R.K., Abha Gupta, R.m. Mathur and A.G. Kulkarni. Paperex India 2001;

189-197. 53 Thomas Vargese and Manjunath. IPPTA Convention issue, 2005; 93-96. 54 Janbade V.; Anuradha, M.P.Singh,; Thapliyal, R.M. Mathur and A.G.

Kulkarni. IPPTA convention issue 2005; 97-101. 55 Gea,-Gloria; Sanchez,-Jose-L; Murillo,-Maria-B. Industrial and Engineering

Chemistry Research 2004; 43 (13): 3233-41. 56 Chen,-Fang-C; Gao,-Zhiming. International Journal of Heat and Mass Transfer

2004; 47 (8/9): 1657-71.

171

57 Gea,-G; Murillo,; -M.-B; Sanchez,; -J.-L. Industrial and Engineering Chemistry

Research 2003; 42 (23): 5782-90. 58 Raberg,-Mathias; Bostrom,-Dan; Nordin,-Anders. Energy and Fuels 2003; 17

(6): 1591-4. 59 Frederick,-Wm.-James, Jr; Vakkilainen,-Esa-K. Energy and Fuels 2003; 17(6):

1501-9. 60 Jones,-Camille-Y; Chen,-Gang; Dai,-Sheng. Industrial and Engineering

Chemistry Research 2003; 42 (18): 4228-33. 61 Nassar,-Mamdouh-M. Energy Sources 2003; 25 (8): 837-44. 62 Shi,-Bing; Frederick,-W.-James, Jr; Rousseau,-Ronald-W. Industrial and

Engineering Chemistry Research 2003; 42 (12): 2861-9. 63 Stefanov,-Z; Hoo,-K.-A. Industrial and Engineering Chemistry Research 2003;

42 (9): 1925-37. 64 Wessel,-Richard-A; Baxter,-Larry-L. Solutions (Norcross, Ga) 2003; 86 (2): 39. 65 Sricharoenchaikul,-Viboon; Frederick,-Wm.-James, Jr; Agrawal,-Pradeep.

Industrial and Engineering Chemistry Research 2002; 41 (23): 5640-9. 66 Gea,-Gloria; Murillo,-Maria-B; Arauzo,-Jesus. Industrial and Engineering

Chemistry Research 2002; 41 (19): 4714-21. 67 Wartena,-Ryan; Winnick,-Jack; Pfromm,-Peter-H. Journal of the

Electrochemical Society 2002; 149 (9): 125-131. 68 Wartena,-R; Winnick,-J; Pfromm,-P.-H. Journal of Applied Electrochemistry

2002; 32 (7): 725-33. 69 Wartena,-R; Winnick,-J; Pfromm,-P.-H. Journal of Applied Electrochemistry

2002; 32 (4): 415-24. 70 Hupa,-Mikko; Forssen,-Mikael; Backman,-Rainer. Solutions (Norcross, Ga)

2002; 85 (3): 62.

172

71 Adams,-Terry-N, Tappi Journal 2001; 84 (6): 70. 72 Bujanovic,-Biljana; Cameron,-John-H, Industrial and Engineering Chemistry

Research 2001; 40 (11): 2518-24. 73 Shi,-Bing; Rousseau,-Ronald-W. Industrial and Engineering Chemistry

Research 2001; 40 (6): 1541-7. 74 Rudie,-Alan. Tappi Journal 2000; 83 (12): 36-7. 75 Severtson,-Steven-J; Duggirala,-Prasad-Y; Carter,-Phillip-W. Tappi Journal

1999; 82 (6): 167-174. 76 Rosier,-Mark-A. Tappi Journal 1997; 80: 203-9. 77 Roberts,-Joanna-E; Khan,-Saad-A; Spontak,-Richard-J. AIChE Journal 1996; 42:

2319-26. 78 Roberts,-Joanna-E; Spontak,-Richard-J; Jameel,-Hasan. Tappi Journal 1996; 79:

167-174. 79 Tucker,-Paul. Solutions (Norcross, Ga) 2002; 85 (2): 34-8. 80 Gea,-G; Murillo,-M.-B; Arauzo,-J. Energy and Fuels 2003; 17 (1): 46-53. 81 Swann,-Charles-E. PIMA's Papermaker 2000; 82 (10): 45-7. 82 Howell,-John-F; Thring,-Ronald-W. Industrial and Engineering Chemistry

Research 2000; 39 (7): 2534-40. 83 De,-Sirshendu; Bhattacharya,-Prashant-K. Tappi Journal 1996; 79: 103-111. 84 Erich Gruber and Peter Muller TAPPI Journal, 2004; 3 (12): 17-23. 85 Satu Ojala, Ulla Lassi, Reeta Yolonen, Riita Keiski, Ilkka lakkso, teuvo

Maunula and Reijo Silvonen. TAPPI 2005; 4 (1): 9-13.

173

86 Grover,-Rajesh; Marwaha,-Satwinder-S; Kennedy,-John-F. Journal of Chemical

Technology and Biotechnology 2001; 76 (3): 251-6. 87 Olazar,-M; Aguado,-R; Sanchez,-J.-L. Energy and Fuels 2002; 16 (6): 1417-24. 88 Philippe Mermillod, leo H.A. Habets, Erik F. van Driel and L. de Vegt. TAPPI

Journal 1992; 24 (2): 177-180. 89 Buisman C.J.N., G. lettinga, C.W.M. Paasschens and L.H.A. Habets. Wat. Sci.

Tech. 2001; 24 ( ¾): 347-356. 90 Font,-Xavier; Caminal,-Gloria; Gabarrell,-Xavier. Journal of Chemical

Technology and Biotechnology 2003; 78 (5): 548-54. 91 Kim,-Heejoon; Lu,-Guoqing; Li,-Tianji. Environmental Science and

Technology2002; 36 (7): 1607-12. 92 Gupta,-V.-K; Minocha,-A.-K; Jain,-Neeraj. Journal of Chemical Technology

and Biotechnology 2001; 76 (6): 547-52. 93 Bergeron,-Janique; Paice,-Michael. Water Quality Research Journal of Canada

2001; 36 (4): 659-85. 94 Berube,-Pierre-R; Hall,-Eric-R. Water Quality Research Journal of Canada

2001; 36 (1): 94-104. 95 Dufresne,-Robert; Liard,-Alain; Blum,-Murray-S.Water Environment Research

2001; 73 (1): 103-109. 96 Marwaha,-Satwinder-S; Grover,-Rajesh; Prakash,-Chand. Journal of Chemical

Technology and Biotechnology 1998; 73 (3): 292-296. 97 Dangcong,-Peng; Qiting,-Jin. Journal-of-Chemical-Technology-and-

Biotechnology. 1993; 58 (1): 89-93. 98 Mikulasova,-Maria; Pekarovicova,-Alexandra; Rybarikova,-Daniela. Journal-of-Chemical-Technology-and-Biotechnology. 1993; 57 (4): 309-312.

174

99 TAPPI Test Method “determination of Total Alkali” TAPPI 625-Cm-1998; 432-

35. 100 Analysis of Green Kraft Liquors, TAPPI Test method, T-606 om-00, 2000; 1-4. 101 Scan test method, Scandinavian Pulp, Paper and board testing Committee.

SCAN-N 37:98, 1998; 1-4. 102 Moss R. D. and R. C. Bond. Manual of Microbial methods. 2000; 181. 103 Test plan for spent pulping liquor. SKANS, CAS No. 66071-92-g. 2001; 7. 104 Roland Macks and Gunter Schildhauer. TAPPI, 2000; 14 (2): 48-49. 105 Schwalbe C.G. German patent 1990; 522 730. 106 Jayme A. G. DBP 1046, 1990; 465. 107 Kuna, L. and Graber. Zellstoff and Papier, 1990; 9-13. 108 Franzreb J.P. Doctoral thesis. Institute of Cellulose Chemistry Darmstadt

1998; 170-174. 109 Kulkarni A.G., Pant R. and A. panda. Proceedings of the international

seminar and workshop on desilication, UNIDO 1991; 58-79. 110 Ralph K.iller “The Chemistry of Silica, Solubility, Polymerization, Colloid

and Surface Properties and Biochemistry” Willey Publishers 1980; 230-280. 111 Severtsen , Steven j., Duggirala Rrasad Y. and Carter Phillip W. , TAPPI Journal

1999; 82 (6): 167-174.

112 Gruen, B. USP 2628, 155, 1995. 113 Berge’s Manual of Determinative Bacteriology. 4th Ed. Willey sons, UK. 1994. 114 Sanjay Kar, Asish Mandal, Pradeep K., Keshab C. and Bikash R. Brazillian Journal of Microbiology 2006; 37: 462-464.

175

115 Aiello C., Ferrer A. and Ledesma A. Bioresource Technology 1996; 57: 13-18. 116 FAO Corporate document by Agriculture and Consumer Protection. 2004, reference W7241e08. 117 Johnson E.A., Sakajoh M., Halliwell G., Madia A. and Demain A.L. Appl. Environ. Microbiol. 1990; 43 (5): 1125-1132. 118 Thomas K.and Ziekrus J. G. Appl. And Environ. Microbiol. 1981; 42 (2): 231-240. 119 Eckenfelder, W.W., Melbinger, N. Sewage and Industrial Wastes, 1997; 29

(10): 1114-1122. 120 Ingersoll, A.C., McKee, J.E., Brooks, N.H. ASCE 1995; 81: 590. 121 Kynch, G.J. Trans.Faraday Soc. 1992; 48: 166-176. 122 Coe, H.S., Clevenger, G.H. Transactions, Am. Inst. Of Mining Eng. 1996; 55:

356. 123 Yoshioka, N., Hotta, Y., Tanaka, S., Naito, S., Tsugami, S. Kagaky Kogaku

(Chem. Eng.) 1987; 21 (2): 66-74. 124 Dixon, D.C., Souter, P., Buchanan, J.E. Chem. Eng. Sci. 1996; 31: 737-740. 125 Hultman, B., Hultgren, J. Trib. Cebedeau, 1980; 33 (441-442): 375-389. 126 Dick, R.I., Ewing, B.B. ASCE, 1994; 93 (SA4): 9-29. 127 Dick, R.I. J. San. Div., ASCE, 2002; 96 (SA2): 423-436. 128 Dick, R.I., Young, K.W. Proc. 27th Industrial Waste Conf., Purdue University,

Lafayette, Indiana, USA. 1992; 10-18.