The application of colloidal gas aphrons in the recovery of fine cellulose fibres from paper mill wastewater

Bioresource Technology 64 (1998) 199-204 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0960-8524/98 $19.00 ELSEVIER

PII:S0960-8524(97)00169-7

THE APPLICATION OF COLLOIDAL GAS APHRONS IN THE R E C O V E R Y OF FINE CELLULOSE FIBRES FROM PAPER MILL WASTEWATER Mohd. Ali Hashim & Bhaskar Sen Gupta*

Institute of Post Graduate Studies and Research, University of Malaya, 50603 Kuala Lumpur, Malaysia (Received 23 September 1997; revised version received 27 October 1997; accepted 3 November 1997)

(CGAs) is an effective method in recovering fine particles from a suspension. Flotation of fine particles by CGAs is recognised as an inexpensive method of separation. The various applications of CGAs are as follows (Sebba, 1987; Subramaniam, 1988):

Abstract Colloidal gas aphrons (CGAs) are micron-sized gas bubbles of 25-30 kon in diameter produced by a highspeed stirrer in a vessel containing dilute surfactant solution. These bubbles, because of their small size, exhibit some coUoidal properties. In this work, CGAs were used to separate fine fibres from a lean slurry of cellulosic pulp in a flotation column. The pulp fibres were recovered as foamate from the top. Sodium dodecyl sulphate at a concentration of 2.0 kg/m 3 was used as a surfactant to generate the CGAs in a spinning disc apparatus. The results indicated that up to 70% flotation efficiency could be obtained within a short column height of 0.3-0.35 m. This technique can be applied to recover fine cellulosic pulp from papermachine backwater © 1998 Elsevier Science Ltd. All rights reserved

1. separation of finely divided suspensions of solids from water 2. removal of finely dispersed oil droplets from water 3. ion and precipitate flotation 4. removal of ash-forming materials from coal 5. harvesting of microorganisms from a culture 6. clarification of wastewater.

The separation of fine particles, of the order of a few microns or less, from aqueous media is a requirement in many chemical and biochemical processes. Flotation and filtration are the two common techniques used for this purpose. For example, a membrane filtration is quite versatile as it can be employed either to recover the liquid as permeate or to concentrate the fine particles in the reject stream. However, it is an expensive process and is not suitable if recovery of fine particles from an aqueous medium is the main objective. On the other hand, flotation is a more economical process for harvesting fine particles, such as microorganisms from an aqueous slurry. To this end, the use of microfoams of colloidal gas aphron dispersions

A significant development has taken place in the last decade on this flotation technique and applications have been reported in the literature (Subramaniam et al., 1990; Hashim et al., 1995a,b; Honeycutt et al., 1983; Save and Pangarkar, 1995). Colloidal gas aphrons can be described as micronsized gas bubbles, of 25-30/~m in diameter, which are produced by a high-speed stirrer in a vessel containing dilute surfactant solution. The system was first termed as 'microfoams' because of the minute size of the bubbles. These bubbles, because of their small size and like charges, exhibit some colloidal properties such as high stability and low coalescence rates. Hence, microfoams are more commonly designated as 'colloidal gas aphron dispersions' or CGAs. The CGAs carry an electrical charge depending on the surfactant used and combine well with oppositely charged particles to form a particlebubble aggregate, more commonly known as 'foamate', which can be recovered as an overflow. The entrapped liquid can be separated and recycled for further CGA generation. The CGAs have the following properties which are exploited in flotation processes.

*Author to whom correspondence should be addressed.

1. Large surface area for particle-bubble contact

Key words: Flotation, colloidal gas aphrons, cellulose fibres. INTRODUCTION

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M. A. Hashim, B. S. Gupta

2. No significant coalescence in transportation by pumping 3. Adherence of particles to the outer shell of the bubbles by coulombic forces. In a paper-making process the paper forms on a moving wire in the wet section of the machine. The wet paper is dried in hot presses and over steamheated rollers in the drier section of the machine. The water that passes through the wire contains fine

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pulp fibres and other loading chemicals. Similarly, in the pulp mill, a large volume of fine fibres is lost while washing the bleached pulp. Although much of the fibre is recovered in the process, the total amount of fibre lost is substantial. The fibres increase both the BOD and COD of the effluent. Dey and Sen Gupta (1992) have reported the effluent characteristics from a 200 tonnes per day pulp and paper mill. The suspended solids (SS) content, BOD and COD of the pulp mill effluent were 944, 745 and 1220mg/1, respectively. The corresponding values of SS, BOD and COD of the paper mill effluent were 778, 131 and 469mg/1, respectively. These values indicate that recovery of fibres would be beneficial for the process as well as for pollution control. Although the CGA dispersions are fairly stable, they will slowly 'cream' and separate into a clear liquid region and a froth layer, where the aphrons are more closely packed than they were in the original dispersion. The extent of the closeness of packing can be expressed by the void fraction of the continuous phase. Amiri and Woodbum (1990) have reported extensive experimental results on the stability and drainage pattern of the CGAs generated with various concentrations (in the range of 0.15-13.5mM) of tetradecyltrimethyl ammonium bromide (TTAB). In the present work, the stability of the CGAs using sodium dodecyl sulphate (SDS), an anionic surfactant was investigated for its effectiveness in recovering cellulosic pulp fibres from a lean slurry. METHODS

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Time (s)

Fig. 2. Height of foam surface vs time. CGAs generated at 6000 rpm; SDS concentration 1.0 kg/m 3.

Fibre suspension A pulp slurry, of concentration 1.0 kg/m3 (1000 mgh) fibre, was used as the feed stream to recover pulp

Application of colloidal gas aphrons

201

fibres. This suspension resembled the machinebackwater in fibre consistency.

showed that an rpm range of 6000-6500 was more appropriate from the standpoint of stability.

Generation of CGAs Colloidal gas aphrons can be generated from aqueous solution with a variety of surfactants. The technique was first reported by Sebba (1987), where a dilute surfactant solution was forced through a venturi throat along with air. Later, he developed a more efficient and simple way of generating CGAs using a spinning disc. This technique, with some modifications, was adopted in the present work. CGAs can be obtained from an aqueous surfactants solution of concentration 1.0 x 10 -3 kg/m 3 and above using a baffled spinning disc apparatus. The speed of rotation of the disc is a critical factor in the generation of CGAs. In this work, the critical speed for bubble formation was found to be 4000 rpm. The drainage characteristics of the static foam later

Flotation trial In the conventional flotation processes, the CGAs and the feed slurry are passed in a counter-current direction. Owing to their small sizes, the C G A bubbles ascend slowly in a flotation column and facilitate attachment of the oppositely charged particles to their outer surfaces. The experimental scheme employed here has been reported elsewhere (Subramaniam et al., 1990; Hashim et al., 1995a,b). A schematic diagram of the flotation cell is given in Fig. 1. The flotation cell was made of a Perspex tube, 0.05 m in diameter and 1 m in height. The system worked in a counter-current fashion, where the feed was introduced at the top and the CGAs were sparged from the bottom. The CGAs were pumped by a peristaltic pump (Autoclude VL). They

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Fig. 4. (a) Concentration of fibre vs time. Flow rates of CGAs and pulp were 2.00 × 10 - 7 m3/s and 2.67 × 10 7 m3/s, respectively. Other conditions: pH, 7.6; temperature 25°C; SDS concentration, 1.0 kg/m 3. -----o---, foamate; = , tailings. (b) Same as (a) except flow rates of CGAs and pulp were 2.33 x 10 - 7 m3/s and 2.67 x 10 7 m3/s, respectively. (c) Same as (a) except flow rates of CGAs and pulp were 2.67 x 10 - 7 m3/s and 2.67 × 10 - 7 m3/s, respectively. (d) Same as (a) except flow rates of CGAs and pulp were 3.00 x 10 - 7 m3/s and 2.67 x 10 - 7 m3/s, respectively.

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M. A. Hashim, B. S. Gupta

were generated by stirring a surfactant solution containing 2.0 kg/m 3 SDS at 6000 rpm by a Silverson stirrer (model LR1) fitted with a four blade impeller, 0.03 m in diameter. SDS was obtained from BDH laboratory supplies, UK. In order to study the stability of the CGA dispersions, the static drainage of the liquid through the plateau border was observed in a measuring cylinder. The fall in the upper surface of the foam layer and the rise in the foam-liquid interface were observed. Commercial cellulosic pulp was dried in an oven at 70°C for 24 h and stored in a sealed polythene envelope. The feed slurry was prepared by grinding a known amount of pulp in deionised water in a

high-speed blender at 1000 rpm for 10min. The slurry was diluted to the desired concentration and transferred to the feed tank. It was kept in a state of constant agitation by a magnetic stirrer to ensure uniform suspension of fibres. The column was filled with a dilute feed slurry of 0.5 kg/m 3 fibre concentration before sparging the CGAs. The CGAs flow rates were varied from 2.0 x 10 - 7 m3/s to 3.0 x 10 - 7 m3/s. The fibre concentration in the feed was 1.0 kg/m3 and the flow rate was 2.67 x 10 - 7 m3/s. Since the flow rate of the CGA dispersions and the pulp slurry were nearly equal, it was envisaged that the foamate and tailing concentrations would stabilise much earlier if the initial concentration of fibres in the column was reduced to

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Fig. ~;. (a) Fibre size distribution in the feed. Peak analysis by intensity: Peak 1, area ]00.0, mean 1702.2, width 3126.2. (b) Fibre size distribution in the foamate. Peak analysis by intensity: Peak 1, area 100.0, mean 2204.3, width 4048.2. (c) Fibre size distribution in the tailings. Peak analysis by intensity: Peak 1, area 100.0, mean 1412.9, width 2594.8

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Fig. 6. Height vs separation efficiency.Conditions were: fibre concentration in the feed, 1.0kg/m3; feed rate, 2.67 × 10 - 7 m3/s; CGA sparging rates: ( • ), 2.00 × l0 7 m3/s; ( .--), 2.33 × 10 7 m3/s; ( + ), 2.67 x 10 7 m3/s; ( • ), 3.00 × 10 - 7 m3/s. half. As soon as the CGA foam had reached the overflow level, the feed pump P2 (Autoclude V) was switched on to start the flotation process. Particle size measurement The size distributions of particles were measured by Zetasizer4 (Malvern Instruments, UK). This instrument works on the principle of laser beam diffraction and can measure zeta potential and particle size with a high degree of precision. Representative samples from the feed, foamate and the tailings were analysed to determine the size distribution of particles. RESULTS AND DISCUSSION

The experiments were designed to study the effect of two very important operating parameters namely, CGAs sparging rate and column height, on separation efficiency of cellulose particles. The stability of the CGA foam was also observed in a measuring cylinder. One such representative plot, similar to what is reported by Amiri and Woodburn (1990), is shown in Fig. 2. The pH of the feed influences the attachment between a fibre and a bubble, which is coulombic in nature. In order to determine the optimum pH of the feed, a series of experiments were conducted. The fibre concentration in the foamate was maximum at a pH of 7.6, as shown in Fig. 3, when the flow rates of the feed as well as the CGA dispersions were maintained at 2.67x10-7m3/s. The system was operated with a continuous countercurrent flow of feed-pulp slurry and the CGAs. CGAs flow rate was varied from 2.00 × 10 - 7 m3/s to 3.00 × 10 - 7 m3/s and the pulp flow rate was maintained at 2.67 x 10 - 7 m3/s. The foamate and the

203

tailings were analysed for fibre content at every 500s. Each run was replicated and the results showed a maximum variation of 5% of the mean value. The average fibre concentrations are shown in Fig. 4(a-d). The foamate and the tailing concentrations tended to a steady state after 3000s. In addition, there was a marginal improvement in fibre concentration in the foamate when the CGAs flow rate exceeded 2.67 x 10 - 7 m3/s. Samples were drawn from equidistant points, 0.05 m apart, along the operating height of the column (0.35 m) to analyse the fibre content after 4000 s from start up. The analyses of foamate and tailings indicated that the system had attained a steady state by that time. The zeta potentials of the feed and the CGA dispersions were found to be - 4 and + 1 3 m V , respectively. Thus, a potential difference of 17 mV influenced the particle-bubble attachment. The average sizes of the fibres in the feed, foamate and tailings were 1702.2, 2204.3 and 1412 nm, respectively, as indicated in Fig. 5(a-c). Relatively large pulp fibres were recovered in the process. Figure 6 illustrates the effect of height on separation efficiency (E) for four different CGAs sparging rates. The separation efficiency is defined as follows (Subramaniam, 1988): E=

Initial total solids-final total solids Initial total solids-dissolved solids

x 100

The separation efficiency improved with increasing flow rate of the CGAs. However, the improvement was marginal if the CGAs flow rate exceeded 3 . 0 × 1 0 - 7 m 3 / s , as indicated in Fig. 6. Some aberrations in data points were observed at a height 0.15-0.25 m. The cause of this phenomenon was unclear. In general, the separation efficiency improved with height. However, there was no significant improvement after 0.30 m. This also indicates that up to 70% separation efficiency could be achieved within a short column height of 0.3-0.4 m. CONCLUSIONS The application of CGAs in recovering fine pulp fibres is an inexpensive method of separation. These fine fibres have poor settling characteristics and flotation is an effective method of harvesting them. This will also reduce the oxygen demand and the volume of sludge in a waste treatment plant. Since the fibres contain some absorbed surfactant, they may consume more antifoam chemicals if reconstituted into a feed stream. The processed fibre in the machine backwater is a high value material from the standpoint of process economics. Currently there is a lack of appropriate technology to recover these fines and reuse them as feeedstock supplement. The technology is appro-

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M. A. Hashim, B. S. Gupta

priate on a laboratory scale. It is necessary to undertake a pilot-plant study before it can be implemented on a commercial basis. ACKNOWLEDGEMENTS The authors would like to acknowledge the assistance of S. Chakraborty and A. Aziz in conducting some of the experiments. REFERENCES Amiri, M. C. & Woodburn, E. T. (1990). A method for the characterisation of collodial gas aphron dispersion~ Trans. I Chem. E. Part A, 68, 154-160. Dey, A. & Sen Gupta, B. (1992). Pollution abatement in Indian pulp and paper industry. The Environmentalist, 12(2), 123-129.

Hashim, M. A., Sen Gupta, B. & Subramaniam, M. B. (1995). Investigations on the flotation of yeast cells by colloidal gas aphrons (CGA) dispersion~ Bioseparation, 5, 167-173. Hashim, M. A., Sen Gupta, B. & Vijaya Kumar, S. (1995). Clarification of yeast by colloidal gas aphron~ Biotechnology Techniques, 9, 403-408. Honeycutt, S.H., Wallis, D.A. & Sebba, F. (1983). A technique for harvesting unicellular algae using collodial gas aphron~ Biotechnology and Bioengineering Syrup. Ser. No., 13, 567-575. Save, S. & Pangarkar, V.G. (1995). Harvesting of Saccharomyces cerevisiae using colloidal gas aphron~ J. Chem. Tech. Biotechnol., 62, 192-199. Sebba, F. (1987). Foams and Biliquid Foams - - Aphrons, John Wiley and Sons, Chichester. Subramaniam, M. B. (1988). Clarification of suspensions using colloidal gas aphrons, M. Eng. Sc. dissertation, University of Malaya, Malaysia. Subramaniam, M. B., Blakebrough, N. & Hashim, M. A. (1990). Clarification of suspensions by colloidal gas aphrons J. Chem. Tech. Biotechnol., 48, 41-60.