(195e) Separation of Lignosulfonate from Aqueous Solution Using Liquid Membrane

The pulp and paper industry consumes a large volume of water and subsequently discharges enormous amount of effluent containing organic and inorganic pollutants such as lignosulfonate, sucrose, polyose, cooking chemicals and silica [1]. Lignosulfonate is a polydispersed polyelectrolyte that has a spherical macromolecular structure [2]. Extensive uses of commercial grade lignosulfonate include production of vanillin, animal feed pellet binders, dispersant for dye, pesticides, carbon black etc. It has applications in water treatment and industrial cleaning operations too. Attempts to utilize the paper mill waste stream as a source of lignosulfonate requires an efficient separation technology which will ensure effective separation on one hand and clear reusable water on the other.

Howard process is a well known industrial unit for separation of lignosulphonate from aqueous solution however it uses huge quantity of lime for the separation. Several other methods exist such as ultra filtration, ion-exchange, dialysis, electro dialysis and extraction with amines. Nevertheless, they are still in their lab-scale operation stage and are yet to be economically viable for industrial applications. Membrane technology has emerged out as a practical alternative for concentration and purification of macromolecular species in aqueous solutions [1]. Although its application in paper industries is still not possible, many researchers are putting their consistent effort to develop an efficient membrane separation process [3-8].

The liquid membrane technology, on the other hand, has so far found its application mainly in the field of metal ions separation. It is a separation method in which an organic phase, immiscible with water, containing a complexing agent (carrier) selective towards the target element is interposed between two aqueous phases i.e. source and receiving phases. The complex element is formed in the source phase, diffuses across the liquid membrane (organic phase) due to concentration gradient and releases the target element in the receiving phase [9]. Thus, target element can be extracted and stripped simultaneously in a single step. Very few researchers applied this concept of liquid membrane to pulp and paper mill effluent. Kontturi et al [2] studied liquid-liquid extraction of lignosulfonate from paper mill effluent using various solvents such as alcohols, ketones and cyclohexane and found that the process was not suitable for large scale application due to difficulties in separating the components and loss of solvent. Later on they developed a supported liquid membrane comprising of Decanol-Trilaurylamine and polytetrafluoroethylene and also explained their experimental results with a simple mass transfer model [10]. They observed that the membrane was stable for about 48 hours but the flux of lignosulfonate was too low for practical applications.

In this present work an effort has been made to look for a more efficient liquid membrane for the separation and recovery of lignosulfonate from its aqueous solution. Out of various liquid membrane configurations, bulk liquid membrane is the simplest one and it has been chosen to test the feasibility of the desired application. Two phase equilibrium study has been conducted first to choose a suitable membrane and also to study the effect of various parameters such as pH, temperature etc. on the equilibrium distribution of lignosulfonate. Dichloroethane has been found to be a good organic solvent that can be used as the liquid membrane along with tri-octylamin as a carrier agent. With the membrane chosen based on equilibrium study, three phase separation process has been carried out and the effects of various parameters such as stirring speed, carrier concentration, temperature, feed concentration, etc. have been investigated.

It has been observed that stirring of feed and organic phases and an increase of temperature enhanced the transport of lignosulfonate, whereas stirring of stripping phase and initial feed concentration below 1000 mg/l have no appreciable effect on the separation of lignosulfonate. The increase of rate of lignosulfonate transport with increase in stirring speed of the feed phase implies that the extraction process is basically controlled by diffusion. Stripping process is not affected much by stirring, but it is influenced by the chemical reaction at the membrane-strip interface. The feed concentration has no impact on the transport of lignosulfonate at lower concentration, which is a merit for this separation process. The separation achieved under the experimental conditions studied is around 90-98%, but recovery is only 5-10%. Further optimization of the strip phase condition, specially the chloride concentration, would enhance the recovery. Further research is going on in this direction.

References

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[8] B.Chakravorty, R.N Mukherjee, S. Basu, Studies on synthesis of ion exchange membrane for electrodialytic treatment of bleaching plant effluent, journal of Environmental studies, 27( 1986) 173-181.

[9] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1991.

[10] Kontturi.A.K. Kontturi.K, Niinikoski.P, Sundholm.G, Extraction of a polyelectrolyte using SLM ?II extraction and fraction of lignosulfonate, Acta chemical Scandinavica 44 (1990) 883-891.