Abstract
The effect was examined of AlF3 on the formation of calcium sulfate during phosphoric acid processing. The solubility and crystallization of calcium sulfate hemihydrate (HH) and dihydrate (DH) were examined in concentrated phosphoric/sulfuric acid solutions at 75 and 90 deg C in the presence of small amounts of AlF3 and HF. Both AlF3 and HF increase the solubilities of HH and DH through their effect on the activity coefficients of Ca and sulfate. AlF3 retards the growth of HH and DH, and an AlF3 concentration of 95 mmol/kg increases the calcium sulfate concentration far above the DH solubility line, leading to DH formation under conditions where HH is normally formed. The presence of AlF3 also results in modification of the crystal habit and agglomeration of the HH and DH crystals formed.