Biological deinking technology for the recycling of office waste papers

blOl O[ll (l Tl CtlflOLO(iY

ELSEVIER

Bioresource Technology67 (1999) 255-265

Biological deinking, technology for the recycling of office waste papers U. Viesturs*, M. Leite, M. Eisimonte, T. Eremeeva, A. Treimanis Latvian State Institute of Wood Chemistry, 27 Dzerbenes Str., LI/-1006 Riga, Latvia

Received 27 August 1997; revised and accepted 29 June 1998

Abstract Results of the enzymatic deinking approach were demonstrated on a laboratory scale to provide a good quality deinked pulp from laser-printed alkaline office paper wastes. It has been suggested that, for alkaline papers, the majority of inks are localised on the paper coatings and fillers consisting mainly of CaCO3. Enzyme treatment improved by stock acidification and dissolution of CaCO3 prior to flotation resulted in effective detachment and dispersion of toner specks ensuring a high deinking effectiveness. Highly dispersed toner particles separation was promoted by acidic flotation in the presence of a surfactant, hydrocarbon oil, applied to improve microink agglomeration and hydrophobicity of the toner particles and agglomerates. A minimal level of visible dirt was observed for optimised enzyme-treated and acidically floated pulps. Both enzyme preparations significantly favoured deinking effectiveness relative to control stocks. Only minor differences were observed between cellulase and resinase (lipase) preparations. The lowering of pH prior to flotation considerably improved the cleanliness not only of the enzyme-treated, but also the control pulps relative to alkaline floated stocks. The addition of an appropriate surfactant, such as hydrocarbon oil proved to be a necessary factor to prevent redeposition of microink particles on the fibre surfaces and promote the separation of highly dispersed toner particles from the fibre network. © 1998 Elsevier Science Ltd. All rights reserved. Keywords: Cellulase; Resinase; Lipase; Biological deinking; Toners; Alkaline papers; Flotation

1. Introduction

The use of recycled fibres in the paper industry has significantly increased during the last 10 y. Imports of paper and paperboard, the source of recycled fibre in Latvia, are fairly high. An increase in the percentage of the use of recovered paper from recycled fibres results in end product quality problems. Reduced dewatering properties of mixed recovered paper are among the drawbacks experienced during the use of mixed waste paper for newspaper production. Another major problem is connected with printed office waste papers. In Europe, alkaline sizing processes are used to produce 50-60% of all the paper and paperboard. It is widely recognised that papers, including xerographic and laser printed ones, are difficult to deink using conventional methods. Traditional oil-based ink consists of a pigment and a solvent vehicle for printing fluidity. The vehicle can be saponified by alkali,

*Corresponding author.

releasing the pigment. The pigment breaks up into fine particles that can be removed by conventional flotation and washing (McCool and Silveri, 1987). The toners used in xerographic and laser printers consist of thermoplastic resin pellets, such as copolymers of acrylate and styrene or polyester along with pigments, such as carbon black. Toner pellets are fixed by fusion and placed electrostatically on a sheet of paper, where they commingle with the fibres (Carr, 1991). In contrast to traditional ink formulations, toners cannot be broken into individual pigment particles. In the course of conventional repulping, toners are released from the fibres in fiat, plate-like particles, which are difficult to disperse and remove, by conventional processes of washing and flotation. Conventional flotation is size dependent and most effective in the 10150/~m ranges. Flat plate-like toner particles float poorly and remain in the final product as residual specks (McCool and Silveri, 1987). Specks in a diameter range of 40-400/~m are visible to the naked eye. Fibres containing specks downgrade the quality of the final product below the quality of the fibres present

0960-8524/99/$ - - see front matter © 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S0960-8524(98)00119-9

256

U. Viesturs et al./Bioresource Technology 67 (1999) 255-265

in these papers (Carr, 1991; Shrinath et al., 1991; Fergusson, 1992; Borchardt and Rask, 1994). Progress is being made in unconventional and modified approaches to the deinking of toners. The mechanical dispersion (steam explosion inclusive) and chemical agglomeration of ink particles, followed by screening and forward cleaning techniques, such as pressure screens and centrifugal cleaners, are the main approaches undertaken to improve the deinking effectiveness of toners-printed office papers (Darlington, 1989; Carr, 1991; Snyder and Berg, 1994; Olson et al., 1993). An alternative to chemical deinking and mechanical dispersion is the enzymatic treatment of office waste papers. There are two principal approaches for enzyme use. One method is the employment of cellulases, xylanases or pectinases for a limited hydrolysis of outer layers of cellulose fibres to release the ink from the fibre surfaces (Ow, 1992; Prasad et al., 1992, 1993; Zeyer et al., 1994; Jeffries et al., 1994). The other approach is the use of lipase to hydrolyse soy-based ink carriers (Sharyo and Sakaguchi, 1990; Ow et al., 1996). It has been demonstrated (Prasad, 1993; Jeffries et al., 1994, 1996; Yang et al., 1996) that commercial acidic or alkaline cellulase preparations can effectively remove and disperse laser and xerographic toners, and enzymes appear to be promising for practical application in office paper deinking. However, information about the best way for removal of the dispersed ink from the fibre network is still lacking. The mechanism by which cellulase can improve the deinkability of waste paper is widely discussed in the literature (Prasad et al., 1992, 1993; Jeffries et al., 1996; Yang et al., 1996). The enzyme action includes a limited hydrolysis of the surface layers of pulp fibres to which the ink adheres during the printing process. Additionally, for alkaline papers, it has been suggested that the major portion of the ink is Iocalised on the calcium carbonate-containing coating and/or fillers (Mahagaonkar and Banham, 1995). The hydrolysis of the surface layers, as well as removal of coatings and fillers can facilitate the detachment and dispersion of the toner specks. The effect of enzymes on the removal of coatings and the influence of this action on deinking, as well as the action of lipase on the ester constituents of toners, such as acrylic and other synthetic resins has not been extensively addressed in the literature. Studies on enzymatic deinking of toners-printed office waste paper (OWP) reveal a large number of variables influencing the deinking effectiveness, such as the type and amount of toners, as well as the amount of sizings and other contaminants. Enzyme treatment is connected with disintegration of pulp and is followed by cleaning (screening) operations, which makes it difficult to evaluate the exact role of enzymes, because all operations are extremely sensi-

tive to the conditions under which they occur. The objective of this study was to investigate the effectiveness of the enzymatic deinking of toners-printed alkaline office waste paper (AOWP) and reveal the best way for an effective removal of the enzymatically dispersed ink from the fibre network. The enzymes used were commercial preparations of cellulases and resinases (lipase). A comparative study of the effects of the pulping time and alkaline (pH 8) or acidic (pH 5 and pH 3) flotation conditions was performed for cellulase and lipase-treated, as well as control pulps. The parameters analysed were residual ink count, ink particle size distribution, as well as changes in brightness and ash content during the processing of pulps.

2. Methods

2.1. Source of paper

Alkaline paper (80 g/m2) produced by the Modo Paper Husum mill (Sweden) printed with toners on a laser printer on approximately 50% of the area of one side of each sheet was used in all experiments. 2.2. Source of enzymes

The enzymes used were commercial preparations supplied by Novo Nordisk AS (Denmark): Celluclast 1.51 (declared activity 1500 IU/g of cellulases) and Resinase A 2X (declared activity 100 KLU/g of lipase). 2.3. Enzyme treatments

Treatments with enzymes were performed during AOWP repulping in a laboratory disintegration device (a total volume of 4.0 1) at 50°C and a mixing speed of 3000 rpm. The consistency of the pulp in the disintegrator was 2% based on the air-dry weight of the pulp. Both enzyme preparations were diluted 1:100 prior to application to ensure an even enzyme distribution on the pulp fibre. The amount of diluted enzymes was 1% (based on the air-dry weight of the pulp). This dosage was established based on preliminary experiments which were made to determine the amount of enzymes necessary to ensure a reasonable deinking level of pulp, i.e. maximum deinking of pulp at minimum losses in fibres strength, especially when cellulase was added. The enzymatic treatment was optimised for parameters influencing the activity of enzymes. Prior to disintegration, the AOWP was torn into small pieces manually and pre-soaked in distilled water at 55°C for 10 rain. The pH of the alkaline-sized paper slurry was 7.5-9.5, which was not compatible with acidic cellulases (Celluclast 1.51) having an optimum activity at pH 4.8-5.5. Therefore, prior to

U. Viesturset al./Bioresource Technology67 (1999) 255-265

disintegration of the stock with addition of cellulases, the pH was adjusted to 5.0 by 0.1 M HCI. Lipase was found to be active within a wide range of pH. Therefore, no pH correction was made to the stock prior to addition of lipase, as well as for control experiments. 2. 4. Flotation

Toners separation from the fibre net was carried out in a laboratory flotation cell, which was a modified version of the FS-8 bioreactor (Viesturs et al., 1990, 1998). The bioreactor consisted of a closed cylindrical body of diameter 140 mm with an aeration system and an upper glass part with froth overflow channels of the same diameter. The total height of the flotator was 350mm. An FS-8 air conditioning system with compression and a rotameter for flow rate control was used. The working flow rate of the air was 7.0 l/rain, which can be interpreted as an air/stock volumetric ratio of 2.8vv/min(volume volume/minute). As the repulping was accomplished, each sample was diluted to a 1% consistency and floated twice at 45°C for 15 min in the presence of hydrocarbon oil (0.35% based on the air-dry weight of the pulp). It has been established (Snyder and Berg, 1994, 1996) that hydrocarbon oil selectively wets the toner particles, because they are hydrophobic, and not the fibres, because they are hydrophilic. The oil-coated toner particles are collected together, adhere more readily to the air bubbles, and are lifted to the top of the flotation device, from where they are removed. Oil serves as an effective agglomerating agent for microink aggregation and removal. In some respects, the process used was similar to the liquid-bridge agglomeration technique described by Snyder and Berg (1994, 1996). The only difference was that, instead of large agglomerates which could be separated by centrifugal cleaning or screening, smaller agglomerates were formed (by using an appropriate oil/toner volumetric ratio), which could be removed by flotation. To assess the most effective way to perform the flotation, each sample, after repulping for 5, 10 and 20 rain, was divided in two parts and floated in the two flotation runs: (1) alkaline flotation without pH corrections after repulping (at pH 8 - - lipase and control and pH 5 - - cellulase); (2) after stock acidification to pH 3 - - low acidic flotation. The acidification of the pulp slurry from pH 8 to pH 3 (in the case of cellulase, from p H 5 to p H 3 ) was performed with 0.5 N HC1 after repulping. 2.5. Evaluation of pulp properties

Handsheets (3.0 g 20 × 20 cm) were made from the pulp slurry according to the TAPPI method T 205 om-88 and analysed for residual ink area and ink particle size distribution using three randomly selected

257

areas on each side of the handsheets. The reported results are an average of these values. For ink particle count analysis, a microspectrophotometer was used, and analysis was made according to the TAPPI dirt count analysis method T 213 om-89. Due to the fact that small size ink particles were of special interest, and the TAPPI dirt count excludes particles smaller than 0.04ram 2, the measurement was done at 11 x magnification to include particles down to 0.005 mm 2 The residual ink area measurement was done in units of ppm (parts per million) or mm2/m 2 (mm 2 of toner particles per m 2 of handsheets). The handsheets made using the pulp slurry after 3-min disintegration were the blank ones used for determining flotation effectiveness. The comparison of the blank handsheets with those from the flotation runs allowed to determine the percentage of the toner removal in each size class in the particle size range from 10 to 500/~m. In size distribution histograms, the ink particles were distributed in three size classes with the mean ink particles sizes 25, 120 and 350/~m. The ISO brightness of handsheets was measured by the TAPPI method T 452 om-92. The TAPPI method T 211 om-93 was used for determining ash content in the pulp. The release of the reducing sugars was analysed using the DNS method (Miller, 1959). 2.6. Statistical anakysis and experimental design

Statistical analysis of data (Descriptive statistics and ANOVA: single-factor) was provided using the computer software MS Excel, 97, Data Analysis Toolpak. All experiments were conducted using a randomised block design.

3. Results and discussion

3.1. Residual ink area

Analysis of handsheets made after 5, 10 and 20 min repulping of the stock in the disintegrator as separate experimental variants, each of them followed by cleaning operations, revealed the three factors having a pronounced effect on the ink removal effectiveness: (1) repulping time; (2) the presence of enzymes; (3) acidification of pulp slurry to a low pH of 3 prior to flotation. The data presented in Fig. 1 show that, with an increase in the pulping time, the residual ink area gradually decreases, reaching the minimum after 20 min. The values of the residual ink area after 5, 10 and 20 min repulping followed by flotation differed significantly (P< 0.05) for all the experiments performed (Tables 1 and 2). The presence of enzymes

U. Viesturs et al./Bioresource Technology 67 (1999) 255-265

258

improved ink removal relative to the control during repulping and, correspondingly, favoured the subsequent cleaning of the pulp in flotation. In addition, the acidification Of the pulp slurry to pH 3 after repulping preceeded the lowering of the residual ink area, improving t h e flotation effectiveness significantly (P<0.01) relative to alkaline (pH8) or weak acidic (pH 5) flotation (P < 0.05). In the case of lipase (Figs 1 and 2), a comparison of both enzymes for deinking effectiveness after 20 min repulping indicated a small, LIPASE 1200

statistically non-significant (P>0.07) increase in ink removal. This was similar 1~o the results obtained and discussed by Ow et al, (1996) in a comparative study of lipase and cellulases in the enzymatic deinking process of old newsprint. However, the observed difference between both the enzymes was surprisingly negligible, despite a possible difference in their mechanism action. The reducing sugars (RS) analysis of the cellulasetreated pulp, as well as high ink removal effectiveness for both the enzymes revealed 20 min as the optimum for the stock repulping time in the disintegrator. The prolongation of the treatment time from 20 to 30 min showed a statistically significant (P<0.0024) decrease in the RS release (Fig. 3), possibly owing to a partial inactivation of cellulases during disintegration.

~

1000

| 00o

3.2. Particle size distribution

.~

6OO

i

4oo

The toner particle size distribution analysis was performed in the handsheets made after repulping and acidification, and the values obtained for the enzymetreated and control stocks were compared. Based on the data shown in Table 3 it is apparent that treatments with enzymes and a low pH are the two factors essentially influencing the toner particle size. The data presented reveal a statistically significant increase (P<0.05) in the toner breakdown during the enzymatic treatment relative to the control for the stocks treated at pH 3. However, the effect of enzymes was not significantly different for unacidified (pH 8) pulps. Pulp acidification to pH 5 and even to pH 3 further enhanced the breakage of the toner particles relative to the unacidified stock (Table 3). There were significant differences in the average particle diameters between pulps treated at pH 8 and pH 3 (P < 0.0078 for runs with the addition of lipase, and P<0.05 for control samples), and a negligible difference between repulping at pH 8 and pH 5 for treatments with the addition of cellulase (P > 0.072). The synergistic effect of enzymatic hydrolysis and acidification of the pulp slurry led to a decrease in the average particle size of the toner, resulting in an increased deinking effectiveness at the flotation stage. A reduction in the residual ink area in the handsheets made after the repulping stage prior to flotation (Fig. 1) indicates that a portion of the small size toner particles was broken into microscopically smaller particles and removed by limited washing during handsheets preparation. It has been demonstrated (Ow et al., 1996) that the breakdown and dispersion of middle and small size particles occurs more rapidly than in the case of large ones. Some limitation, such as the substrate accessibility to the enzyme exists, possibly restricting the enzyme action to the size reduction of large and thick toner specks. The addition of lipase during repulping results in a lower average particle size and higher ink removal effective-

"o

'~

n,.

200

a

b

c

d

•

f

Pmum

(J

•

f

Pm" °

a

•

f

CELLULASE 1200

~1000

| 00o ..~ ._c

000

~m 200 a

b

c CONTROL

12000

dig

.S 600

i

= 400

lID

~

200 o a

b

c

Fig. 1. Residual ink area. Repulping time 5 min (a); 10 rain (c); 20 min (e), followed by flotation (b,d,f). Each value is a mean of three replicates. Statistical analysis of data is presented in Tables 1 and 2.

U. Viesturs et al./Bioresource Technology 67 (1999) 255-265

259

Table 1 Residual ink area - - data base for analysis of variance (ANOVA) Sample

Control Cellulase Lipase

pH

8 3 5 3 8 3

Repulping, 5 min

Flotation

Mean

SE

Mean

1192 1092 740 712 980 856

55.6 50.4 34.2 32.8 34.0 40.1

668 367 362 256 296 144

Repulping, 10 min

Flotation

SE

Mean

SE

Mean

SE

Mean

SE

Mean

SE

30.8 16.9 15.9 11.8 10.2 6.6

1044 933 584 536 724 520

48.2 30.1 27.0 21.8 25.1 24.0

492 294 137 104 204 93

17.0 13.6 7.6 5.1 9.9 4.3

752 696 384 344 498 294

34.7 32.2 17.7 15.8 17.2 13.6

420 176 121 68 113 19

19.4 8.1 5.6 3.4 4.0 1.1

Repulping, 20 min

Flotation

Each value is a mean followed by the standard error (SE) of three replicates.

ness relative to cellulase-treated or controls samples. However, as mentioned above, the differences were statistically significant only for acidified pulps and non-significant for pulps treated at pH 8. However, the necessity of adjusting the pH of the stock to 5 prior to cellulase addition makes the comparison of the effect caused by both the enzymes problematic. The mechanism of the action of progressive acidification in the toner dispersion is not fully understood. The results presented have revealed a possible similarity to the phenomena noted by Dorris and Nguyen (1995) in experiments with various approaches to improve the flotation effectiveness of flexo inks. Acidification to a low pH of three of the carboxylic groups of the acrylic resins used as binders in flexo inks led to destabilising of flexo inks particles and higher flotation rates. Acidification to a weak acidic pH of 5 had little influence on the flexo ink deinkability in flotation.

3.3. Brightness In the present work, brightness measurements after enzymatic repulping, flotation and bleaching were performed, and the values obtained were compared to the values for the initial unprinted paper, as well as the water control stock. The unprinted initial alkaline paper stock had a brightness of 93% ISO. A dramatic darkening of the pulp was observed as the repulping time increased, followed by acidification of the stock (P<0.05) for all the pulps treated (Fig. 4). Some brightness restoration was observed during flotation and bleaching. However, in this case, the restoration of the full brightness of unprinted paper was not achieved, possibly owing to the loss of a large portion of white pigment-containing coatings and fillers. The dynamics of the changes in pulp brightness revealed diametrically opposite tendencies for pulps after alkaline (pH 8) and acidic (pH 5 and pH 3) flota-

Table 2 Residual ink area - - statistical analysis data (ANOVA) Data sets compared

Lipase, repulping 5 min vs lipase, repulping 20 min, pH 8 Lipase, repulping 5 min vs lipase, repulping 20 min, pH 3 Ceilulase, repulping 5 rain vs lipase, repulping 20 min, pH 5 Cellulase, repuiping 5 rain vs cellulase, repulping 20 min, pH 3 Control, repulping 5 min vs control, repulping 20 min, pH 8 Control, repulping 5 min vs control, repulping 20 rain, pH 3 Lipase, pH 8 vs lipase pH 3, repulping time 20 min Cellulase, pH 5 vs cellulase, pH 8, repulping time 20 min Control, pH 8 vs control pH 3, repulping time 20 min Lipase, pH 8 vs control, pH 8, repulping time 20 min Lipase, pH 8 vs control, pH 3, repulping time 20 min Cellulase, pH 3 vs control, pH 8, repulping time 20 min Cellulase, pH 3 vs control, pH 3, repulping time 20 min Lipase, pH 8 vs cellulase pH 5, repulping time 20 min Lipase, pH 3 vs cellulase, pH 3, repulping time 20 min

P-value Repuiping

Flotation

0.00024 0.00017 0.00076 0.00054 0.00249 0.00269 0.01621 0.4361 0.30221 0.00366 0.00031 0.0007 0.0006 0.35456 0.07501

0.00011 *n.s.a. 0.00016 0.00052 0.00244 0.00053 n.s.a. 0.00116 0.00032 0.00010 0.00012 0.00012 n.s.a. 0.03721 0.00011

n.s.a. = not statistically analysed, because the variance between the data sets was more than five times. Significant differences are assumed to be those with P < 0.050.

260

u. l,r~esturset al./Bioresource Technology 67 (1999) 255-265

tion stages (Fig. 4). Repulping at pH 8, followed by flotation without a reduction in pH, increased pulp brightness linearly, with an increase in the repulping time from 5 to 20 min, reaching the maximum after 20 min. An increase in ISO brightness by some units was observed for the stock treated with lipase in

alkaline conditions relative to cellulase-treated (P < 0.00012) and control (P<0.039) pulps. Acidification of the stock to pH 3 at the same time interval progressively decreased pulp brightness proportionally to the brightness values obtained after repulping (Table 4). For all experiments, analysis of variance, as

l i pH8 (pH6 J CELLULASE) C]pH3

a) repulping 100

Blank 1300 ppm

90 80 ..~

70 6O 50

E E 4o J¢ c --

30 20 10 6min

lOmln 20rain

6nalin 10rain 20rain LIPASE

CONTROL

6ndn

CELLULASE

• pH8 (pH5 CELLULASE [] pH3

b) r e p u l p i n g + f l o t a t i o n 100

Blank 1300 ppm

10rain 20rain

98.8

90 80

,--- 7O -m > O

60

E 40 Jc --= 30 20 t0

0 Bmin - 10rain 20rain CONTROL

Emln 10rain 20 mln LIPASE

Fig. 2. Ink removalefficiency.Statistical analysisof data is presented in Table 2.

6rain

lOndn 20rain

CELLULASE

U. Viesturs et al./Bioresource Technology 67 (1999) 255-265

2.2

1.8

i~I~

~

~1

1.6 1.4 ""

1.2

W 0.8 0.6 0.4 i'

'

!!~

0.2 0 5

min

10 min

30 rain

20 rain

Repulping time

Fig. 3. Release of reducing substances (RS) during repulping of toner-printed office waste paper (AOWP) with the addition of cellulase (Figures in % based on DW). Each value is a mean of three replicates. Vertical bars denote the standard error (SE) of the mean. Differences in RS release were non-significant (P > 0.05) for 5 vs 10 min, as well as 10 vs 20 min and were statistically significant for 20 vs 30 min (P - 0.0342).

well as regression trendlines, added to ISO brightness data series (Fig. 4) indicated a positive non-significant (P > 0.05) linear trend for pulps repulped at pH 8, as well as a strong (P< 0.05) inverse linear trend for pulps acidified to pH 3. In contrast to alkaline treated pulps, brightness values for acidically treated pulps yielded the maximum value after 5 min repulping (Fig. 4). It is generally recognised (Ow et al., 1996; Prasad et al., 1992; Jeffries et al., 1994) that brightness drops with a decrease in the ink particle size, and an essential success in Table 3 Effect of enzymes and pH on the average diameter of toner particles (repulping time: 20 min in a disintegrator) Enzyme

Average diameter of particles (/lm) pH 8

Lipase Cellulase Control

pH 3

Mean

SE

Mean

SE

285 257"a 301

13,2 11.9 13.9

205a 219 251a

9.5 10.1 11.6

*pH 5.0. Values represent a mean followed by the standard error (SE) of three replicates. Means followed by the same letter (a) are not significantly different (P > 0.05).

261

deinking is very much dependent on the effective separation of microscopically small ink particles, immediately after detachment. The particles not removed immediately after their detachment from the fibre surface are likely to be redeposited on the fibre during further treatment (Sain et al., 1996; Jeffries et al., 1996; Dorris and Nguyen, 1995). A comparison of the residual speck area and particle size distribution after alkaline and acidic flotation showed a reduced breakdown of specks in alkaline conditions. According to data presented in the literature (Ow et al., 1996), fibres are swollen to a greater extent at pH 8, as compared to fibres at trH 5 or pH 3. The swelling may cause a shearing force between the fibre surfaces covered with inks and the ink particles, owing to possibly different degrees of hydration (Ow et al., 1996). This gives a large detached ink area without their breaking into small particles. It is generally recognised that large specks have a minor influence on the pulp brightness. Therefore, after alkaline repulping and flotation, brightness increases. However, in contrast to acidically floated pulps and despite yielding the highest brightness values, alkaline floated pulps showed a significantly (P<0.05) decreased cleanliness. It should be mentioned that the cleanliness of the deinked pulp is often more critical than brightness to meet pulp quality requirements. Brightness values do not necessarily represent the actual extent of ink removal. Based on data presented in the literature it appears that, in general, enzyme-treated handsheets have a slightly higher brightness at pH4.7-5.5, than at pH 8.0-9.0 (Prasad et al., 1992; Prasad, 1993; Jeffries et al., 1996), The brightness drop observed after pulp acidification in the present work is in disagreement with the results of above-mentioned researchers. However, it has also been found that some conditions, such as high enzyme loading lead to a brightness reduction (Jeffries et al., 1994). The darkening of pulp is dependent on the microscopic particle resorption on the fibre surfaces. This process can be accelerated considerably by manipulating factors that enhance resorption, mainly increasing mixing or aerating more intensively during the pulp processing. To diminish the effect of mixing, we optimised the treatment time with enzymes by reducing the stock repulping time in the disintegrator to 5 min and continued the enzymatic treatment for the remaining 15 min at an appropriate temperature, although without mixing. Such an approach allowed a significant (P<0.05) elimination of the darkening of pulp, caused by redeposition of ink, as compared to pulps treated 20 min at pH 3 in the disintegrator with mixing, and made the difference in pulp brightness between acidically and alkali-repulped stocks non-significant (P>0.05) (Table 2). The best results obtained in such conditions are presented in Table 5. Minimal levels of visible dirt and 91.5-92.3%

262

u. lPtesturset al./Bioresource Technology 67 (1999) 255-265

ISO brightness was observed for optimised enzymetreated and acidically floated pulps. A comparison of the means presented in Table 5 indicate significant differences (P<0.05) between the enzyme-treated and control pulps in the residual ink area after repulping, as well as a 10-fold improvement in cleanliness of the

enzyme treated pulps relative to control pulps after flotation. Differences in the effect between both enzymes on the residual ink area and ISO brightness of pulps were non-significant (P>0.0883 and P~0.0524, respectively) (Table 5). The lowering of pH prior to flotation considerably (P<0.05) improved the cleanli-

a)CONTROL 89 88 A

87

|u •c

86

.c

84

i

4d

o;

'c 83 m 82

pH3 pH8 Linear (pH3) Linear (pHS)

81

5 min

10 min

20 min

b) LIPASE 91 90 A

se v

e) e0 Q c ,4,,; J: o) "c m

89 88

/

87 86 86 84

pH3 pH8 Linear (pH3) Linear (pH8)

83 82

S min

10 min

20 min

c) C E L L U L A S E 92 9O " " 88

|.

~ i

U

I

82 .i= 0 ~ 8 "E 0 m 78

pH3 pH6 Linear (pH3) Linear (pHS)

76 74 5 rain

10 rain

20 min

Fig. 4. Effect of repulping time and pH in flotation on pulp bdghtness after flotation and peroxide bleaching. Each value is a mean of three replicates. Vertical bars denote the SD of the mean and lines-regressiontrendlines.

U. Viesturs et al./Bioresource Technology 67 (1999) 255-265

ness not only of enzyme-treated, but also control pulps, relative to alkaline floated stocks. 3.4. A s h content

The alkaline paper has been defined as a paper containing 20% (or above) of ash, consisting mainly of calcium carbonate (Crouse and Wimer, 1991). Calcium Table 4 Brightness of treated samples after the repuiping stage (prior to flotation) Treatment

Repulping time (rain)

pH

ISO brightness

(%)

Control

Control

Control Lipase

Lipase

Lipase Cellulase

CeUulase

Cellulase

5 10 20 5 10 20 5 min in disintegrator + 15 min without agitation 5 10 20 5 10 20 5 rain in disintegrator + 15 min without agitation~ 5 10 20 5 10 20 5 rain in disintegrator + 15 rain without agitation

Mean

SE

8 8 8 3 3 3 3

83.2a 83.2a 83.5a 76.3b 75.3b 70.7g 82.8a

0.75 0.70 0.72 0.91 0.80 0.84 0.72

8 8 8 3 3 3 3

85.2c 86,2c 86.6c 75.6b 73.8d 70.8g 83.3a

0.72 0.75 0.72 0.86 0.83 0.79 0.72

5 5 5 3 3 3 3

72.9d 70.8g 70.2g 74.6d 74.0d 70.0g 78.1h

0.84 0.83 0.80 0.85 0.83 0.80 0.91

Each value is a mean followed by the standard error (SE) of three replicates. There is no significant difference (P > 0.05) between the means followed by the same letter (a-h).

263

carbonate is generally used as a white pigment to increase the brightness and opacity of pulp. Pigment particles occupy the voids between the fibres, which leads to an increased number of fibre-air and pigment-air interfaces (Mahagaonkar and Banham, 1995). As a result, the optical properties of the pulp are increased with increasing the ash-containing white pigment. It has been suggested for alkaline papers that the major portion of inks is localised on the paper coatings and fillers. The removal of the coatings and fillers leads to effective ink detachment and dispersion, as well as an increased effectiveness of ink removal (Table 3). On the other hand, the removal of the white pigment during deinking inevitably leads to a reduction in pulp brightness. The present study revealed that the darkening of pulp was significantly increased after the removal of large portions of ash from the fibre surfaces, resulting from the acidification and/or enzyme treatment of the pulp. However, a drop in brightness cannot be fully attributed to the ash loss. It is possible that the removal of coatings and fillers from the voids and fibre surfaces can, to a certain extent, serve as a factor increasing the resorption and occupation of fibre surfaces and voids by highly dispersed small size ink particles, resulting in a significant darkening of pulp handsheets. As mentioned previously (Fig. 4), despite a high deinking effectiveness, a progressive brightness drop was observed with an increase in the repulping time, followed by stock acidification. To reveal the effect of ash on deinking effectiveness, determination of the ash content after repulping and flotation stages was performed, and the values obtained were compared with the initial stock of AOWP. The total amount of ash for the alkaline paper tested in our experiments was found to be 21.1%. Based on data presented in Table 6 it is apparent that large amounts of ash are lost during acidification of the pulp prior to flotation. Substantial foaming was observed for acidified pulp owing to the evolution of carbon dioxide,

Table 5 Deinking effectiveness and pulp brightness after 5 min repulping at pH 3 in a disintegrator plus 15 min retaining at 50°C without mixing, followed by acidification to pH3, flotation and bleaching Enzyme

Residual ink area (ppm) Repulping

Lipase Cellulase Control

Ink removal effectiveness* (%)

Flotation(%)

Mean

SE

Mean

SE

686a 592a 872

31.7 27.3 40.2

9b llb 91

0.40 0.51 4.20

34.6 43.9 17.0

99.1 98.9 91.4

ISO brightness (%) Repulping

Flotation

Mean

SE

Mean

SE

82.6c 77.9 82.4c

1.44 1.32 1.39

92.3 91.5 90.8

0.75 0.77 0.76

*Blank = 1050 Values followed by the standard error (SE) represent the mean of three replicates. In each column and row, means followed by the same letter (a-c) are not significantly different (P > 0.05).

264

U. IPtesturs et aL/Bioresource Technology 67 (1999) 255-265

Table 6 Effect of deinking on ash content Enzyme

Ash extraction (%)* Repulping pH 8

pH 5

Mean Lipase pH 3 Lipase pH 8 Cellulase pH 3 Cellulase pH 5 Control pH 3 Control pH 8

9.2 . 2.4 .

SE .

.

0.26 . 0.08 .

Flotation

pH 3

pH 8

pH 3

Mean

SE

Mean

SE

Mean

SE

Mean

SE

Mean

SE

-

-

.

-

0.70 0.61 0.73a 0.24b 0.74

0.03 0.03 0.01 0.03

0.01

0.57

0.34 0.38 0.01 0.03 0.54 0.55

0.24b

20.3 -

11.06 11.40 0.20 0.81at 17.8 18.1

0.22

0.01

0.28

0.02

.

.

Acidification

Ash content in final handsheets (%)

-

.

.

.

*Total ash content for furnish tested was 21.1%. tpH 5.0. Each value is a mean of three replicates followed by the standard error (SE). In each column and row, means followed by the same letter (a-b) are not significantlydifferent. resulting from the calcium carbonate-acid interaction. At the same time, some accelerating effect of enzymes on ash removal during the repulping stage was observed. The content of the ash extracted from the stock repulped for 20 min with the addition of lipase was four-fold higher as compared to the ash removed from the sample of the control pulp. Even a weak acidification of the stock (pH adjusted to 5.0), followed by cellulase-treatment, resulted in almost complete loss of ash (up to 95% from the initial ash content). It is possible that the stage when the major ash removal takes place has some influence on the flotation deinking effectiveness. Acidification of the stock removed the major portion of the ash prior to the flotation stage, significantly increasing the ink removal effectiveness. For alkaline treated stocks, the major ash loss was observed to occur during the flotation of pulp. The presence of CaCO3 in the suspension appears to reduce the fioatability of the toners owing to the consumption of oil for the agglomeration and flotation of the suspended CaCO3 particles, and not the toner particles. Similar results were obtained and discussed by Dorris and Nguyen (1995) in the study of fiexo inks flotation at alkaline pH with the addition of CaCO3. At alkaline pH, the addition of CaCO3 to a flexo ink sodium soap suspension led to a partial flotation of CaCO3, and not the flotation of ink. In an analogous study, Snyder and Berg (1996) found that cationicstarch, present in practically all the stocks, was partly r~dissolved during repulping and disrupted the use of hydrocarbon oil for ink agglomeration. However, in the study by Read (cit, after: Mahagaonkar and Banham, 1995), for flotation deinking of newsprint with the addition of about 10% of ash from coated magazines waste papers, it was found that deinking was far more

effective when the waste paper had a significant ash content. It is possible that during alkaline flotation, the floating capability of oil is partially consumed for the aggregation and flotation of calcium carbonate, thereby decreasing the flotation effectiveness of ink. It is believed (Mahagaonkar and Banham, 1995) that the dissolution of CaCO3 at an acidic pH facilitates ink particles flotation to some extent, because, in the absence of CaCO3 particles, the whole oil added to the pulp slurry can be used just for toner flotation. However, a comparison of flotation rates at alkaline and acidic pH-values is difficult because numerous variables affect the rate of flotation. At an acidic pH, the floated particles tend to fall back to the agitated part of the flotation device, owing to the instability of the froth at a low p H of 3. The addition of hydrocarbon oil as a water dispersion to the pulp slurry during flotation allows elimination of this problem by increasing the hydrophobicity of the toner particles and making their attachment to the air bubbles more successful in acidic, as well as alkaline conditions.

4. Conclusions

These studies have shown that both the enzyme preparations, progressively with an increase in the treatment time, decrease the average ink particle size and destroy the fibre-toner connection, improving ink removal effectiveness relative to the control. Only minor differences were observed in the ink removal efficiency between cellulase and lipase during repulping. Enzyme treatment, favoured by stock acidification to p H 3, improved the reduction of the ink particle size, as well as the ink removal effectiveness

U. Hesturs et aL/Bioresource Technology 67 (1999) 255-265

relative to unacidified (pH 8) stocks. However, despite a very low residual ink area, enzyme-treated acidified pulps showed a progressive decrease in brightness relative to alkaline treated stocks. Optimisation of the treatment time, as well as application of a surfactant (hydrocarbon oil) allowed elimination of the darkening of pulp and significantly improved the cleanliness of recycled toner-printed alkaline office waste paper.

Acknowledgements The authors express their gratitude to the company Novo Nordisk A/S for providing samples of the enzyme preparations: Celluclast and Resinase A2X (Lipase). This research was financed from the Latvian budget, grant No. 578 (Biotechnology).

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