Indian Journal of Chemical Technology Vol. I 0. November 2003 , pp. 593-597

Articles

Fabrication of a laboratory scale flotation cell device for bio-deinking of waste papers

Santosh Vyas", S R Sainkarb & Ani! Lachke a•

"Division of Biochemical Sciences, 2Center for Materi als Characterization National Chemical Laboratory, Pune 411 008, India

Received 11 October 2002; revised received 28 May 2003; accepted 2 July 2003

Fabrication of a cost effective, laboratory scale flotation device that can be used for deinking of various grades of wastepaper has been described. The unit consists of aeration device, sparger, baffies for high air to stock ratios and high shear mixing. The sparger is designed in such a way, that, it gives micro ·turbulent airflow necessary for removal of smaller ink particles. The deinking experiments using alkaline active cellulases from an alkalotolerant Fusarium sp have been reported. The enzyme preparation showed different cellulolytic and xylanolytic activities. Successful application of cellulases in enzymatic deinking of mixed office waste paper has been described.

Recycling of waste paper into high-grade writing paper is the new trend in paper industry. The major problem in recycling of waste paper is the removal of contaminants mainly the inks. Normally the ink contains non-polar pigments or dyes and an agent that canies the ink to paper and facilitates in binding. These agents are typically water insoluble oils or polymers as in case of toners. Thus, their removal is not an easy process. Washing and flotation are two most commonly used methods of deinking. Other methods I ike heat decolourization 1, irradiation2

,

organic solvent-based dei nking3, and magnetic

deinking4 are sti ll under experimental conditions. The choice of method depends on the type of ink to be removed and the desired quality of pulp. Black newspaper inks are generally removed by washing, while coloured magazines are deinked by flotation . Both the methods use chemicals that are highly toxic. Microbial enzy mes can prove to be a valuable alternative in order to reduce the use of these toxic chemicals. Microbial cellul ases5

·6

, hemicellulases 7,

amylases8·9

, lipases 10 have shown promising results in increasing brightness of the fibres when used in combination with fl otation deinking. The availability of efficient flotation device is always one of the limi ting fac tors in evaluating performance of different enzyme preparations for deinking. In thi s com municat ion , fabrication of the laboratory scale flotation device is described. The alkaline active

*For correspondence (E-mail: lachke @da lton.ncl.res.in ; Fax: +91-20-5894032)

cellulase preparation was obtained from the alkalotolerant Fusarium sp and its application in deinking of mi xed office waste paper.s has been explored.

Experimental Procedure

Chemicals Sodium salt of carboxymethyl cellulose (CMC) and

oat spelt xylan were purchased from Sigma Chemical Co. , U.S.A. Cellulose-123 was purchased from Carl Schleicher & Schull Company, Germany. Wheat bran. rice bran, corncob and bagasse pith were obtained locally . All the buffer salts and microbial media components were procured from the standard commercial sources and of highest quality available .

Enzyme production The fungus, alkalotolerant Fusarium sp strain was

grown in M-1 medium 11 enzyme production . The M-1 medium supple mented with 0.5 % wheat bran was inoculated with vegetative mycelia from 7-day-old sporulating slant on PDA. The culture was grown for 3 days and used as the innoculum. Enzy me producti on was carried out in 250 mL Erlenmeyer tlask containing 50 mL M-1 medium with cellulose powder (I %) as the sole carbon source. The culture was incubated at 30°C on a rotary shaker at 200 rpm . The samples were withdrawn at regular interval s. The mycelium was removed by centrifugation at 7000 rp m at 4°C to obtai n a clear crude broth. This preparati on was used for the measurement of enzyme acti viti es.

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Results given are the mean of at least two observations.

Enzyme assays The activity of endoglucanase (1,4-/3-D-glucan- 4-

g lucanohydrolase, EC 3.2. 1.4), xylanase (1,4-/3-D­xylan xylanohydrolase, EC 3.2 .1.8), filter paper degradi ng activity (FPase) were measured in terms of release of reducing sugars by 3,5-dinitrosalicy lic acid (DNSA) method 12. The enzyme assays were carried out as described earlier 13 .~ 4 .

One lU of enzyme activity was defined as the

amount of enzyme that liberates l,umole glucose/min (for endoglucanase and FPase) or xylose/min (for xylanases) under the standard assay conditions. I IU of /3 -g lucosidase and {3-xylosidase has been defined as the amount of enzyme, which liberates l,umole of p-nitrophenol/min under the standard assay conditions.

Fabrication of flotation cell device The efficiency of flotation depends on physical

interaction between ink particles, ai r bubbles and hydrodynamic characteristics of the suspension 15•

Considering these prerequisites, a simplified flotation cell was designed (Figs I &2). The working volume of the flotation cell was 10 L, with actual dimensions as L=260 mm, B= 260 mm and H=220 mm. The cell was constructed with mild steel with standard thickness of 2.5 mm. Joints were welded with an efficiency of 0.85. A hollow cylinder pipe of diameter 62.5 mm is fitted on the cylindrical sparger hav ing diameter 120 mm. The shaft is introduced with impeller blade diameter of 30 mm in hollow cylinder. This shaft is rotated at 1500 rpm using 1.5 Hp motor. The sparger hole has the diameter of 5 mm. For the remova l of the foam containing floated ink partic les the outflow is provided as a slope from the top of one side of flotation unit. The flotation efficiencies were tes ted during enzymatic deinking trials with cellulases, on mixed office waste paper. The enzyme treatments were followed by flotation run at I % consistencies in the same cell.

Deinking trials The waste paper used was a mixture of photocopier

waste papers that were coated with toner and laser printouts. Each of them was cut in to small pieces and immersed in the warm water at 50°C for at least 2 h. This paper was fiberized in medium consistency water jacketed pulper for 10-15 min in the presence ofO.l %

594

Indian J. Chern. Techno!.. November 200:;

~ 30nn

Dio. of hole =5 nM.

I ,1. ~ 1,. I 70 ""' I 120rm I 70 ,., ..,

Fig. ! - Laboratory scale fl otat ion cell dev ice-front vie\\'

-------------------------------------, L=26G MH - ------11 TO? V}~\/

0

Fi g. 2 - Laboratory scale fl otation cell device--top view

(vlv) nonionic surfactant. The temperature during

pulping was kept at 55°C and pH was adjusted to 8.5 with IN NaOH . The suspension was mixed until paper particles were no lo nger visi ble. The enzyme treatments were carried out at I 0% final consistency

at pH 8.0-8.5 at 55°C for 20 min . The enzyme do~c was selected as I 00 IU CMCase/1 OOg of oven dri ed pulp. Prior to flotation the pulp samples were disintegrated in universal laboratory d isintegrater at 5000 rpm for 5 min . The pulp was diluted to 17r consis tency by using distilled water. To separate ton er particles from the fibres, all the enzy me treatments were followed by lO min flotation run in a 10 L capacity laboratory flotation unit at room te mperature. At the end of flotation , the deinked fi bres were recovered on a laboratory mesh from the drain va lve of the flotation cell. Control runs were taken under

Vyas et a!. : Fabrication of flotati on cell device for bio-deinking of waste paper Articles

identical conditions replacing active enzyme by heat denatured enzyme preparation. After flotation, pulp consistency was determined for each sample. From every sample, 5 handsheets were made. The handsheets were dried at room temperature. Handsheets from enzyme treated pulp, heat denatured control runs and handsheets from the pulp without flotation were compared for different optical properties. The ink specks in visible (220 to 80 J..l.m) range were counted according to Tappi Standard method (TAPPI T 213). Brightness was measured at different places on handsheet according to (T APPI T 452 om 92) and values are expressed as the average % value. The efficiency of deinking was calculated as per the following formula,

(No. of ink specks in blank sampl e

• -No. of ink specks in test sample) Deinking efficiency % = x 100

No. of ink specks in blank sample

Scanning electron microscopic studies The sediment of short fibres and ink particles from

rejected foam after flotation was resuspended in 1.0 mL of distilled water. 0 .1 mL of it, was dried on aluminium foil and coated with thin gold layer. The short cellulose fibres and toner particles released due to the action of endoglucanase was determined using a Leica Stereoscan 440 scanning electron microscope.

Results and Discussion

Enzyme properties The crude culture filtrate showed different

extracellular cellulolytic and xylanolytic activities as shown in Table 1. The refined enzyme preparation is active in a pH range of 4 to 10 with pH optima at 5.0 at 60°C. The enzyme is stable 16 in an alkaline pH range of 8-0 at 50°C. The half-life of the enzyme at pH 8.5 at 50°C was found to be 10 h. The enzyme showed maximum activity at 60°C and retained 17 80% of the maximum enzyme activity at 70°C. The deinking operations can be well performed under the alkaline conditions . Thus, the enzyme preparation was found to be alkaline active and alkali stable indicating their potential for deinking process.

Flotation cell The increasi ng vanet1es in the characteristics of

printing inks make new demands on the function of flotation cell. The flotation cell must be capable of

Table 1-Enzyme activities in cu lture supernawni

Activity

CMCase [Endo (1~4)-~-D-glucan­glucanohydrolase (EC 3.2. 1.4)] FPase (Fi lter paper degrading ac tivity)

I ,4-~-D-glucan-glucohydrolase (EC 3.2.1.21)

~- D-xylanase (EC 3.2. 1.8)

~-I ,4-D-xylan xylanohydrolase (EC 3.2. 1.37) Protease

N.D.= Not detected

IU/mL

R.SO

0 . .52 13 1

0.72

0.32

N.D.

removing ink particles that lie under the visible as well as sub visible particle limits. Collectively they determine the brightness levels and the optical cleanliness 18

• The flotation cell must provide an environment where ink particles have a high probability of colliding with air bubble. Thi s helps ink particles in attaching to an air bubble that tloat to cell's top surface and finally leaving the fibres in the pulp suspension. The basic conditions required for this high probability of collision include high air to stock ratios and high shear mixing rates . Thi s was achieved in the fabricated flotation device. During the flotation process the high vacuum is created in ho ll mv cylindrical pipe. High speed of impeller blade ensures high air generation. Impeller blades produce hi gh shear mixing.

High air to stock addition rates ensure that there is large population of air bubbles to allow the ink particles to have a high probability of colliding with air bubble and floating to cell's surface. General! y. the air introduced is up to I 0 times the stock volume 19

• High shear mixing results in dispersion or large quantity of air into stock. Also, high shear increases the probability, that, ink particle comes in contact with an air bubble. This prevents ink particle from being trapped in the fibres 15

• Finally high shear mixing ensures that fibres stay suspended. In order to separate the ink particles from the fibres , the ink particles must be hydrophobic so that they can adhere to the air bubble. This was achieved by using non­cationic surfactant.

Small ink particles (< 200 J..l.m) require a hi gh relative velocity in order to break out if the stream line around the bubble come into contact with bubble itself. Thus, the flotation of small particles , requires a micro turbulent flow that is high enough to supply the necessary contact energy. At the same time, the size of the bubble is determined by the turbulent shear field . Smaller the bubble size, higher the degree of turbulence 19

. The attachment probability increases as

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the diameter of air bubble decreases, since the particle can approach the air bubbles more easily. In the present flotation cell the micro turbulent airflow, was achieved by selecting sparger hole diameter of 5 mm. The data on ink counts and size of the ink particles suggested, that, smaller ink particles were removed preferentially than the larger ink particles. This also confirms the generation of microturbulent airflow that resulted in preferential removal of smaller ink particles . This was also observed during flotation of enzyme treated samples (Fig. 3) . The effect of ink particles on the brightness of paper has been evaluated by McKinney 18

• Generally, enzyme action breakdown larger fibrils into smaller microfibrils which are easily removed by flotation process. This results in further increase in brightness as well as reduction in ink counts (Table 2) . The blockage of air suction device by foam and stock deposit can become a serious problem during flotation process. While fabricating the flotation cell, this was avoided as there are no individual air hoses and the entry of compressed air takes place via a hollow cylindrical pipe that is installed outside the cell tank. The simplified air input device is introduced instead of costly air compressors used in commercial flotation ce ll s. The novel sparger unit is designed to generate microturbulant airflow. Absence of individual air hoses that can result in blockage of air device are the key highlights of fabricated flotation cell over commercially available flotation cells.

Deinking trials In order to evaluate the performance of the

fabricated flotation cell , flotation deinking trials were carried out for the pu lp samples. The hand sheets made from treated pulp and control pulp samples were compared with those of blank samples for brightness as well as residual ink particles. The fl otation deinking process resulted in an increase in bri ghtness as compared to blank pulp samples that are not processed by flotation . Similarly, improved brightness by 3-4 points was shown by enzyme treated pulp over heat denatured control pulp samples. The increase in brightness points can be attributed to combination of enzymatic action and flotation deinking process that yielded reduction in residual ink specks. The enzyme treatment of mixed office waste paper increased removal of ink in the presence of nonionic surfactant. Enzyme treatment resulted in reduction of ink specks after flotation (Table 2). The smaller ink particles (< 200 J .. un) were removed more

596

Indian J. Chern . Techno l. , November 2003

Fig . 3 - Scanning electron microscopic image show ing cc llul m.c· microfibrills with ink particles desi gnated by arrow ( ._ )

Table 2 - Quality control tes ts for recycled paper handshects made from enzyme de inked pulp

Q .C. Test Blank Contro l Test Run

Brightness (%) 66.8 69.6 72.X Ink specks/m2

>220 J..Lm 492 242 1)7

160 J..Lm 564 41 3 1.17

80- 160 J..Lm 432 144 58

10-80 J..Lm 628 278 63

Handsheets: made from mixed office waste paper pulp hy enzymatic treatment. The treatment was carri ed out in the presence of surfactant (0.1 %) concentrati on at I 0% consistency in the pulper for 30 min. Enzyme dose: 100 !U was se lec ted f() r treating I 00 g of pulp sample. Each treatme nt was fo ll owed by I 0 min floatati on run at I% consistency .

Control: Under above conditions repl ac ing ac ti ve enzy me hy same volume o f heat denatured e nzy me preparation. Blank: Pulp sample neither treated w ith enzy me and a lso nor processed by fl o tati on.

effectively than larger ink particles (>220 pm ). However, the reduction in larger s ize particles is al so observed. It can be possible, that , the larger ink particles might have been broken down into small er particles due to enzyme action and small particles arc effectively removed by flotation. Wel t and Dinus

211

have also recorded similar observations. The effect o f enzymatic deinking on strength properties of the waste paper pulp has been described previousli 1

• The data indicated that there is marginal improvements in burst index and breaking length with reducti on in tear index values in enzyme treated handsheets over control.

Efficiency of deinking The efficiency of deinking was determined as

explained under experimental section . The hi ghe~ t

Yyas et al. : Fabrication of flotation cell dev ice for bio-deinking of waste paper Articles

efficiency of flotation was found to be around 80 %. The results indicated that the highest efficiency of deinking was possible when enzyme trials were followed by flotation run . During flotation, the air bubble rises to the top carrying hydrophobic toner particles along with them. The large toner particles possess cellulose fibrils entrapped within them. These hairy toner particles affect flotation efficiencies severely by generating hydrodynamic drag on air bubble that is rising to the top. Cellulases separate toner particles from entrapped fibers . Now these non-hairy hydrophobic particles are easily separated by flotation. Thus, the efficiency of flotation increases dramatically by application of cellulases.

Conclusion Microbial enzymes have shown promising results

when used in combination with flotation deinking during recycling of waste paper. The availability of efficient laboratory flotation device is always one of the limiting factors in evaluating performance of different enzyme preparations for deinking. The laboratory scale flotation device described herein is cost effective, and has been found to be efficient in biodeinking of mixed office waste papers. This unit can be used for studies on deinking of various grades of waste paper and also for the evaluation of different enzyme preparations at laboratory scale level.

Acknowledgements S.V. is thankful to the Council of Scientific and

Industrial Research (CSIR), Government of India, for financial assistance.

References 1 Fujioka S, Naito K, Okuyama T. Sano K & Takayama S. / IS

Pat. 5922115, 1999. 2 Macdonald J G & Nohr R S, US Pat. 5721287.199X. 3 Fujioka S, Machida S, Naito K, Okuyama T . Sano K &

Takayama S, US Pat. 6017386. 2000. 4 Gubitz G M, Mansfield S D, Bohm D & Saddler J N . ./

Biotechnol, 65 ( 1998) 209. 5 Sreenath H K, Yang V W, Burdsa ll H & Je ffri es T W. in

En zymes for Pulp and Paper processing Ser. 655. ed ited by Jeffries T W & Yiikari L(American Chemi cal Society. Washington, D.C, USA), 1996.207.

6 Jeffries T W, Klungness J H, Marguerite S & Crop,ey K R. Tappi 1, 77 (1994) 173.

7 Morkbak A L & Zimmermann W, Prog Pap Reeve/. 7 ( 199X) 14.

8 Zollner H K, Schroeder & Leland R. Tap pi 1. 8 1 ( 1998) 16h. 9 Elegir G, Panizza E & Canetti M. Tappi 1, 83 ( II ) (2000 ) 7 1.

lO Morkbak A L, Degn P & Zimmermann W. 1 Biotl'cluwl. 67 (1999) 229.

11 Reese E T & Mandels M. in Methods in Car/)()/n ·drm ,· Chemistry, vol. 3 edited by Whistler L (Academic Pre". New York, London), 1963, 139.

12 Miller G L, Blum R, Gelnnon W E & Burton A. A1111i Biochem, 2 (1960) 127.

13 Gole A, Yyas S, Phadtare S, Lachke A & Sastry M. Colloid' and Suifaces B: Biointerfaces, 25 (2002) 129.

14 Lachke A H, in Methods in Enzymology. vol. 160A edited by Wood W A & Kellogg S ( Academic Press. Inc., San Di ego. USA), 1988,679.

15 Herbert Britz, PAPERASIA , Nov. 1993,37. 16 Yyas S, Ahmad A & Lachke A, in Microbio/ogr a11d

Biotechnology for Sustainable Development B: 12. edited by Jain PC (CBS Publishers, New De lhi ), (in press) .

17 Gole A,Vyas S, Sainkar S, Lachke A & Sastry M. Lllllg11111ir. 17 (200 I) 5964.

18 McKinney R W J, in Pulping Conference 1987. TAPPI Proceedings Book, Yol.l (TAPPI Press, Washington. D C ). 29.

19 Gilkey M W, PAPERASIA , Nov. 1993,32. 20 Welt T & Dinus R J, Wochenblall Fiir Papierjabrikario11 . 9

(1998) 396. 21 Yyas S & Lachke A, Enz Microb Techno/, 32 (2003 ) 236.

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