Abstract: Sodium sulfate crystals, also known as salt cake, are obtained as a side stream in the manufacture of chlorine dioxide by the reduction of sodium chlorate with chloride ions in an aqueous, acidic medium containing sulfate ions. The sodium sulfate crystals are contained in a mother liquor saturated with sodium sulfate and contaminated with chloride ions. The process provided by the present invention purifies the salt cake crystals by separating the crystals from the mother liquor without evaporation of the mother liquor until a critical dryness level is achieved and then the remaining mother liquor is evaporated to result in purified salt cake crystals having substantially reduced chloride ion content.