Cadmium removal from calcium sulphate suspension by liquid membrane extraction during recrystallization of calcium sulphate anhydrite

The removal of Cd2+ ions, incorporated in calcium sulphate anhydrite (AH), was performed by means of liquid membrane (LM) extraction. The LM extraction was performed during the process of solution-mediated transformation (recrystallization) of the cadmium-contaminated AH into the stable calcium sulphate dihydrate (DH). This transformation process involves the dissolution of AH and the crystal growth of DH. The extractants that transport Cd2+ selectively over Ca2+ ions (Alamine 304, tridodecylamine; and Alamine 336, trioctylamine), diluted in the appropriate organic solvent (kerosene), were applied. Liquid membrane, designed in a form of the so-called bulk liquid membrane (BLM), was found to be the most suitable configuration for the treatment of calcium sulphate suspension. Cadmium ions were successfully removed from the feed suspension and no cadmium was found incorporated in the stable, DH, phase. The mechanism of Cd2+ transport through the membrane is proposed.