Purification of Sodium Sulfate by Crystallization

Sodium sulfate can be recovered from a dilute solution by concentrating it by conventional water treatment techniques like reverse osmosis and electrodialysis; however, those processes also concentrate impurities like arsenic. The sodium sulfate would have commercial use if it can be recovered in an arsenic-free form. This study demonstrated that sodium sulfate could be separated from impurities using slow cooling crystallization in a novel apparatus developed and designed during the experimentation. This device is called a heat exchanger crystallizer and allows slow cooling crystallization in order to grow pure crystals of sodium sulfate decahydrate. Crystals grown in the heat exchanger crystallizer from a sodium sulfate solution with 1 mg/L arsenate had arsenic levels below the detection limits of IC and ICP. In contrast, crystals grown under vacuum in a rotary evaporator had arsenic levels about the same as the feed solution due to entrapment of mother liquor in the slurry of tiny crystals. These results offer new insight into how sodium sulfate can be recovered and purified by crystallization to the extent that arsenate is not detectable in the crystals