Many methods are available for the separation of liquid mixtures and metal ions. Among them, foam separation technique holds great promise especially when the concentrations involved is very low. It is a simple and low cost method with less maintenance as there are no moving parts. It comes under adsorptive bubble separation techniques. In foam separation, the more surface active compounds are attracted by adsorption to gas bubbles, which then rise to the top of the liquid at the surface, the less surface active molecules remained entrapped in the interstitial liquid. When these bubbles rise out of the solution, foam is formed. Only a small fraction of the liquid is carried with the bubbles into the foam phase due to gravity drainage. The foam phase can be collapsed into a new liquid foamate by releasing the gas bubbles.

The concentration of the surface active solute in the new liquid is usually several times of that in the initial liquid solution. The success of this technique depends on the stability and characteristics of foam. In this study we designed and fabricated a single continuous foam fractionator and investigated a continuous operation in the recovery of Zinc (II) anions from synthetic and industrial effluent using sodium lauryl sulphate (SLS) as foaming agent. The effects of conditions of the feed solution and surfactant (pH and concentration) and operational parameters of the column (air and feed flow rate, liquid pool height) on the separation characteristics were investigated and enrichment ratio (E) was calculated.

‘Enrichment ratio' or ‘Separation factor' E is defined as the ratio of the concentration of Zinc (II) ions in the foamate (CP) to the concentration of zinc (II) ions in the bulk liquid (Cb) from which the foam has been generated.From these studies it has been observed that as the flow rate of air increases from 1.0 to 10 LPM, the separation factor decreases which is due to the fact that more solution drainage occurs at low flow rates which results in dry foam. The enrichment ratio is observed to be high whenever the foam is dry. The increasing concentrations of the surfactant and zinc (II) ions from 0.1 to 0.5% and 50 to 100ppm respectively resulted in the reduction in enrichment ratio. It is caused by lower surface tension and higher surface liquid viscosity which causes decrease in the rate of film drainage. As the liquid pool height increases from 10 cm to 30 cm, the separation factor increases which is due to the fact that the residence time of bubbles in the liquid pool is more. It takes longer time for bubbles to rise through the solution and adsorption of Zn (II) ions at the gas-liquid interface could approach an equilibrium level more closely, and raise the enrichment ratio. As the pH of the solution determines the sign and the magnitude of the charge on a variety of inorganic particulates, the extent of removal of the particulates by foam separation technique is controlled by pH of the solution. The optimum pH obtained from the experiment is around 7.3 which reveals that Zn (II) reverses its charge at pH 7.3(isoelectric point).

Key Words: Foam Fractionation, Zinc (II), Enrichment ratio, Foaming agent, Sodium lauryl sulphate.