The crystallization of Calcium sulfate dihydrate produced by the reaction between pure Ca(OH) 2 suspension and H 2 SO 4 solution was investigated at different pH values, temperatures and citric acid concentrations. Crystal size distributions, filtration rates and zeta potentials of gypsum were determined as a function of citric acid concentrations at pH 3.5 and 65°C. The influence of citric acid on the morphology of gypsum was also investigated and discussed. The average particle size of gypsum was reached to maximum in the presence of approximately 2500 ppm citric acid concentration, where the minimum cake resistance and maximum filtration rate were obtained. In the presence of citric acid, various crystal morphologies such as tabular, platelike, double-taper leaf-like and flower-like, etc., were obtained. The change of morphology is related to the preferential adsorption of citric acid on different crystallographic faces.

The effect of Fe 2+, Fe 3+ , and Cr 3+ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe 2+, Fe 3+, and Cr 3+ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe 3+ or Cr 3+ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe 3+ and Cr 3+ ion concentrations. The change of morphology is related to the complex formation between Fe 3+ or Cr 3+ ions and citric acid at high ion concentrations.