influence of final bleaching stage on ecf pulp … · 2012. 7. 17. · bandeira azevedo, oldair...

INFLUENCE OF FINAL BLEACHING STAGE ON

ECF PULP BRIGHTNESS DEVELOPMENT, REFINABILITY

AND PULP PROPERTIES

Dr. V.R. (Perry) Parthasarathy*

Canexus North America Ltd, Houston, Texas, USA

and

Dr. Jorge L. Colodette

Federal University of Vicosa, Vicosa, MG, Brazil

*Now with Weyerhaeuser Company, USA.

3rd International Colloquium on Eucalyptus Pulp,

Belo Horizonte, Brazil, March 4-7, 2007

ACKNOWLEDGEMENT

The authors (VRP and JLC) acknowledge the financial support by Canexus NA

and Canexus Quimica Ltda Brasil for this project.

Acknowledgements are also due to Gary Kubera, Brian Bourgeois and Mike

Christian of Canexus NA and Pericles Dos Santos and Eduardo Chow of Canexus

Quimica Ltda Brasil.

The authors also wish to place in record their acknowledgement and appreciation

to the laboratory staff at the Cellulose and Pulp Laboratory at the Department of

Forest Engineering, Federal University of Vicosa, MG, Brazil for the excellent

work - part of the data of that work is used in preparing this manuscript.

Objective

Yesterday, Goran Gellerestedt, Vuorinen et al.,

Sevastyanova, et al., and Litia et al., explained the

fundamental aspects of brightness development, and the

role of various wood components including HexA’s role

in the brightness stability of pulps subjected to ECF and

TCF bleaching under different bleaching sequence. In

this paper, we have taken an approach to see what effect

the final stages of bleaching has on brightness

development and brightness stability, refinability and

properties.

Objective

The process used in the final stages of bleaching is

important for reaching high brightness in pulps. High

brightness and optically balanced pulp (as to their

whiteness (L*) and a* and b* values) is imperative to

produce ultra high brightness (>92 %ISO) and high

whiteness (>100%) paper products with the use of as

little Optical Brightening Agents (OBA) as possible. The

two distinct final bleaching stages, DD and DP are

compared with respect to their ability to produce not only

very bright and optically balanced pulps but also pulps

with low brightness reversion.

Approach

The paper is divided into four sections…..

1. “Color” Chemistry and Metrics Used to Track

Optical Changes in Pulp

2. The Bleaching Chemistry of Chlorine Dioxide and Hydrogen Peroxide in Brightness Development

and on Brightness Stability

3. Results and Discussion

4. Conclusions with Recommendations

Brief Review on “Color” Chemistry

Brightness is a measurement of pulp’s ability to reflect light in

a very narrow blue band (457 nm )(Popson and Malthouse

1990) and in bleached pulps, it is a function of lightness or

whiteness (L*), because the chromacity of these substrates

are rather low.

Chromacity is defined by CIELAB’s X-Y coordinates (in Hunter

Space, it is a* (green-red) and b* (blue-yellow) values that

defines the color space (Popson and Malthouse 1990)).

On the other hand, whiteness is a measurement of a pulp’s

ability to reflect light across the entire color spectrum, color is

defined by value, hue, and chroma (Judd and Wyscezi 1975).

CIE Chromacity Diagram

a*

b*


For white and near-white papers, brightness is a single

optical measurement. However, when pulps are bleached

to very high brightness, a* (green-red) and b* (blue-

yellow) numbers do change. They have to be tracked so

as to understand what influence they will have on the

“Chromacity Value”.

The a* and b* values have to be optically balanced to

minimize the “Grayness” of the pulp or sheet.

In this work, chromacity values were used to track the

efficiency of ECF bleaching and to measure the

brightness reversion of DD and DP bleached pulps under

different humidity conditions.


TAPPI’s TAPPI T 524 om-94 sets the criteria for white or

near-white paper, having L*≥ 84.0 and a chromacity value

of (√ (a*2 + b*2) ² ≤ 10.0. This specification of L*a*b* is not

adequate to define pulps bleached to very high brightness.

A unified measurement called “Chromacity” defines the

changes in color on the Hunter’s color space.

An extension to the “Chromacity” measurement is “Color

Index (CI)” that takes into account the difference in the

lightness, darkness, and the chroma of the sample and the

standard.


Color Index =√ (∆L*2+∆a*2+∆b*2)

where ∆L* = Delta of whiteness before and after aging

∆a* = Delta of a* before and after aging

∆b* = Delta of a* before and after aging

Chlorine dioxide is a very specific oxidant. It prefers to react

with residual lignin instead of cellulose fractions of pulp.

Certain decomposition and disproportionation products of

ClO2 give rise to a number of oxidative species, including

radical species like ClO. and Cl.. These are indiscriminate

oxidants and attack both lignin and carbohydrate structures

in pulp.

The attack on lignin by these radicals does result in the

generation of quinones whereas the attack on carbo-

hydrates does not result in the oxidation of C2 or C3 carbon

in the cellulose molecule to the formation of carbonyl

groups but the oxidation of C6 carbon to glucouronic acid

(Stewart and Smelstorius 1968 and Miyazaki 1971).

Brief Review of Bleaching Chemistry

The decomposition products of H2O2 - the hydroxyl and

hydro peroxyl anions react mostly with lignin structures but

the hydroxyl radicals, a strong and indiscriminate oxidant

(electrophile) reacts with both residual lignin as well as

carbohydrates.

Reactions of hydroxyl radicals with the carbohydrates result

in the oxidation of C2 and C3 carbon positions in the

cellulose and hemicelluloses structures to generate

carbonyl groups.

The excessive presence of carbonyl groups in cellulose

structures (Andrady and Parthasarathy (1991) is attributed

for the thermal yellowing (brightness reversion) of hydrogen

peroxide bleached pulps of high lignin content.



In dye chemistry, the fastness of color using a particular dye

is due to the extent of changes in quinone (color) structures

to quinhydroxy (colorless) structures (Finar 1956).

Lachanel and Chirat (2004 and 2005) showed that the

residual lignin in unbleached pulps differed in their quinone

concentration and that the pulps which were more difficult

to bleach were more colored and richer in quinone

structures, with a more pronounced reddish hue (a* (green-

red coordinates) in the L*a*b* space).

It is theorized by Lachanel and Chirat (2005) that since

quinones are not easily degraded by ClO2, pulp bleachability

would be related to the amount of quinone groups and by

proxy by tracking the value of a* in the L*a*b* space.

Brightness reversion of pulp is a complex phenomenon

having initiated by light and thermal radiation and a

combination of both. Also, the extent of brightness

reversion depends not only on the intensity of the light or

heat but also on the ambient moisture conditions (Relative

Humidity)

Market Pulps shipped to different destinations are subject to

cyclical changes of both heat and relative humidity resulting

in changes in the brightness at delivery points. It is

therefore important for market pulps that not only the

measurement of final brightness but also the brightness

stability is critical.


The objective is to verify the assertion by Lachanel and

Chirat (2005) that peroxide is better in reducing the quinone

(color) structures in pulps to quinhydroxy (colorless)

structures than chlorine dioxide which then implies that

bleach plants with final P-stage are better suited to produce

high brightness pulps than D-stage.

To compare the efficiency of a bleaching process,

particularly in providing very high brightness and

brightness stability in Eucalyptus market pulps, it is

important to have a unified measurement. In this study we

have used the chromacity measurements to track the

efficiency of bleaching of pulps to very high brightness.


Properties of Oxygen Delignified Pulp

Oxygen Delignified Eucalyptus Kraft Pulp

Brightness, % ISO

Kappa Number

Viscosity, cP

HexA, mmol/kg

COD, kg O2/t

47.6

12.5

31.0

69.5

7.6

Chlorine Dioxide Delignification Conditions

Two Different Initial Chlorine Dioxide Delignification of Oxygen

Delignified Pulps were employed. (1) Conventional Chlorine

Dioxide (Do) and (2) Hot Chlorine Dioxide Delignification (DHT).

The Delignification Conditions are Detailed Below:

Temperature, oC

Time, minutes

Acid Used

End pH at the D-Stage

ClO2, as % Active Cl2

H2O2, %

NaOH, %

Acid Charge, %

Total Active Cl2, %

60, 90, 95

15, 30, 60, 90, 120

H2SO4

2.50 3.0

2.55 2.55

0.50 0.50

1.20 1.20

0.30 0.40

3.60 3.60

Bleaching Conditions

Chlorine Dioxide delignified (Do and DHT) pulps were bleached

by E(P)DD, E(P)DP and E(P)DDP processes. Detailed bleaching

conditions are listed elsewhere*

“Final D-stage versus Final P-stage for high brightness ECF bleaching of

eucalyptus Kraft pulps, which is the best?”. Submitted to Canexus Limited 450 Gears Road, Suite 400 , Houston Texas USA.

Report # 027/2006 by Jorge Luiz Colodette, Rubens Chaves de Oliveira, Marcos Antônio

Bandeira Azevedo, Oldair Cristino de Paula, Luciano Sabioni, and Carla L. Lopes Motta

Brightness Reversion Measurements

Brightness reversion was tested under two different aging

conditions:

(1) Wet (60ºC, 7 days, 90% RH) (Finnish Pulp and Paper

Institute) (Modified).

Finnish Pulp and Paper Institute’s (KCL) suggested method

for the wet reversion test (Modified). For the wet reversion

test, the original KCL procedure calls for subjecting the pulp

sheets at 100% RH at 100oC for one hour. In this study, the

procedure was modified and the exposure time prolonged to

7 days at lower humidity levels (90% RH) and lower

temperature (60oC) to simulate the conditions that the market

pulps would be subjected to when shipped overseas.

(2) Dry (105ºC, 4 hours, 0% RH) (TAPPI Useful Method)

RESULTS AND DISCUSSION

Relationship Between Brightness and Whiteness of

Eucalyptus Pulps

Brightness Versus Whiteness

y = 0.2235x + 77.697

R2 = 0.8725

97.85

97.90

97.95

98.00

98.05

98.10

90.20 90.40 90.60 90.80 91.00 91.20

Brightness, %ISO

Wh

iten

ess

Brightness and Whiteness of DD and DP Bleached

Eucalyptus Pulps (Do Delignification)

Conventional Chlorine Dioxide Delignification (DO) and Multi-stage Bleaching

Bleaching

Sequence

Final Stage

Temperature oC

End pH Brightness %

ISO

Lightness or

Whiteness

(L*)

DOE(P)D1P 70.0 9.0 90.60 98.00

10.0 90.40 97.92

11.0 90.50 97.89

90.50 97.94 Average

90.0 9.0 90.00 97.80

10.0 90.60 97.93

11.0 90.50 97.91

90.37 97.88 Average

DOE(P)D1D2 70.0 4.5 90.60 97.93

6.0 90.70 98.08

9.0 90.30 97.85

90.53 97.95 Average

90.0 4.5 91.50 98.13

6.0 91.10 98.11

9.0 90.80 98.01

91.13 98.08 Average

Brightness and Whiteness of DD and DP Bleached

Eucalyptus Pulps (DHT Delignification)

Hot Chlorine Dioxide Delignification (DHT) and Multi-stage Bleaching

Bleaching

Sequence

Final Stage

Temperature oC

End pH Brightness %

ISO

Lightness or

Whiteness

(L*)

DHTE(P)D1P 70.0 9.0 91.00 97.97

10.0 90.90 97.95

11.0 90.90 97.95

90.93 97.96 Average

90.0 9.0 91.00 98.06

10.0 91.10 98.03

11.0 91.30 98.08

91.13 98.06 Average

DHTE(P)D1D2 70.0 4.5 91.50 98.13

6.0 91.20 98.14

9.0 90.70 97.95

91.13 98.07 Average

90.0 4.5 91.50 98.13

6.0 91.10 98.11

9.0 90.80 98.01

91.13 98.08 Average

Brightness Reversion (PC Numbers) For DD and DP

Bleached Pulps


Bleaching

Sequence

Final Stage

Temperature oC

Brightness % ISO

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

DHTE(P)D1P 70.0 90.93 88.87 89.17 0.212 0.196

90.0 91.13 89.13 89.63 0.208 0.181

Average 91.03 89.00 89.40 0.210 0.188DHTE(P)D1D2 70.0 91.13 88.23 88.10 0.250 0.256

90.0 91.13 88.23 87.97 0.250 0.261

Average 91.13 88.23 88.03 0.250 0.259


Bleaching

Sequence

Final Stage

Temperature oC

Brightness % ISO

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

DoE(P)D1P 70.0 90.50 87.90 88.40 0.238 0.214

90.0 90.37 87.87 88.10 0.234 0.223

Average 90.43 87.88 88.25 0.236 0.218DoE(P)D1D2 70.0 90.53 86.43 87.90 0.298 0.239

90.0 91.13 88.23 87.97 0.250 0.261

Average 90.83 87.33 87.93 0.274 0.250

*Wet Reversion Test: 60ºC, 7 Days, 90% Relative Humidity

**Dry Reversion Test: 105ºC, 4 Hours, 0% Relative Humidity

Brightness

Reversion

Brightness

Reversion

Post-Color Number

Brightness Reversion

Summary of Color Index (CI) Values for DD and DP

Bleached Pulps


Bleaching

Sequence

Average Delta1

(between

DP and DD

bleaching)

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

DHTE(P)D1P Before Aging 98.01 98.01 -0.10 -0.10 2.78 2.78

After Aging 97.40 97.71 -0.10 -0.10 3.17 3.50 0.72 0.78 0.75

DHTE(P)D1D2 Before Aging 98.08 98.08 -0.08 -0.08 2.86 2.86

After Aging 97.19 97.29 0.28 -0.02 3.43 3.80 1.12 1.23 1.17 -0.42


Bleaching

Sequence

Average Delta1

(between

DP and DD

bleaching)

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

DoE(P)D1P Before Aging 97.9 97.91 -0.09 -0.09 3.07 3.07

After Aging 97.1 97.35 0.21 -0.02 3.60 3.83 0.99 0.95 0.97

DoE(P)D1D2 Before Aging 98.0 98.02 -0.10 -0.10 3.00 3.00

After Aging 96.9 97.20 0.33 0.09 3.65 3.86 1.33 1.20 1.27 -0.30

*Wet Reversion Test: 60ºC, 7 Days, 90% Relative Humidity

**Dry Reversion Test: 105ºC, 4 Hours, 0% Relative Humidity

1. Delta is calculated between DP and DD bleaching between likely testing events (before aging and after aging events)

b*

Chroma Values

Whiteness, %

(L*) a*

(L*)

Whiteness, %

a*

Color-Index

Color-Index

Chroma Values

b*

Properties of Bleached Pulps from

DHTE(P)D1D2P and DHTE(P)D1P Processes.

Bleaching

Sequence

Brightne

ss, %ISO

Average

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

Wet

Method*

Dry

Method**

D HT

E(P)D 1 D

2 P Before Aging 92.0 98.31 98.41 -0.11 -0.09 2.74 2.54

After Aging 89.7 97.59 98.01 -0.09 -0.10 2.93 3.20 0.74 0.77 0.76

D HT

E(P)D 1 P Before Aging 91.0 98.01 98.01 -0.10 -0.10 2.78 2.78

After Aging 89.2 97.40 97.71 -0.10 -0.10 3.18 3.50 0.73 0.78 0.75

Whiteness, % Chroma Values

(L*) a*

Color-Index

b*

CONCLUSIONS

Final brightness of HW pulps, in particular, eucalyptus pulps is

dictated by the type of delignification employed prior to bleaching.

Hot chlorine dioxide delignification (DHT) resulted in higher brightness

pulps over conventional Do pulps.

At a given OXE, DD can give a higher brightness pulps over DP with

statistically insignificant changes in reversion, CI and chromacity

values.

Adding a peroxide polishing step to the DD bleaching will lead to not

only high brightness pulps but a pulp optically balanced in its a* and

b* values and that the papermakers need to use less OBA to reach

“pure” white paper (>100% Whiteness)

It is estimated that between DD and DP bleaching, the former can

bleach pulps to >92 %ISO brightness at US$ 2 to US$ 3 per to less than

the latter, a significant cost advantage of having a final D-stage.

CONCLUSIONS

Refining the pulps bleached to high end pH is easier than pulps

bleached to low end pH.

Between DD and DP bleached pulps, the former provided a better

tensile and tear index at a given CSF than the latter but the

difference is statistically insignificant (Table 12).

OBrigado

influence of final bleaching stage on ecf pulp … · 2012. 7. 17. · bandeira azevedo, oldair...

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