Gypsum Crystallization in the Presence of Cr 3+ and Citric Acid

Reactive crystallization of calcium sulfate dihydrate (gypsum) was investigated at pH 3.5 and 65 °C in the presence of citric acid and Cr 3+ ions. Concentrations of both additives varied between 0 and 4000 ppm. The combined effect of additives on crystal size distributions, filtration characteristics, and crystal morphology were investigated and discussed. It was found that the effect of citric acid on gypsum morphology was suppressed in the presence of Cr 3+ ions. The change of morphology is related to the complex formation between Cr 3+ ions and citric acid at high ion concentrations.