ELSEVIER PII:S0960-8524(97)00169-7 Bioresource Technology 64 (1998) 199-204 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0960-8524/98 $19.00 THE APPLICATION OF COLLOIDAL GAS APHRONS IN THE RECOVERY 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) Abstract Colloidal gas aphrons (CGAs) are micron-sized gas bubbles of 25-30 kon in diameter produced by a high- speed 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 paper- machine backwater © 1998 Elsevier Science Ltd. All rights reserved Key words: Flotation, colloidal gas aphrons, cellulose fibres. INTRODUCTION 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 *Author to whom correspondence should be addressed. 199 (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): 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. 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 micron- sized 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 desig- nated 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 particle- bubble 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. 1. Large surface area for particle-bubble contact