Removal of fluoride from spent pot liner leachate using ion exchange

Spent pot liner (SPL), a hazardous solid waste produced at cell houses of aluminum smelters, is a potential source of fluoride pollution. Leachates collected from SPL disposal sites were found to contain fluoride at considerable concentration levels (up to 575 mg/L). This paper reports a study of selective fluoride removal following laboratory-prepared, ion-exchange treatment. Spent pot liner leachates were pretreated with lime to bring the fluoride level down to approximately 10 mg/L for economic and effective working of the ion exchanger. The detailed ion-exchange treatment study for removal of fluoride was carried out on synthetic SPL leachates and the optimum treatment thus developed was applied on natural SPL leachates. Bench-scale studies were carried out at various flow rates and pHs and in the presence of other ions commonly available in the SPL leachates. The prepared exchanger reduced the level of fluoride from approximately 10 mg/L to less than 1 mg/L. Results indicate that the extraction was 100% up to 6-mL/min flow rate through the ion exchanger and it works efficiently in the pH range of 7 to 10. There is no effect of the other ions present in leachates on removal of fluoride. The exchanger has good capacity to exchange and can be recharged by eluting fluoride sorbed on the exchanger using two molar hydrochloric acid.