The effect of aluminium fluoride on the formation of calcium sulfate hydrates

Abstract

Different phases, hydrates or polymorphs are often developed in mineral ore conversion processes where sparingly soluble salts are formed. Their development not only depends on the operating conditions but also on the specific composition of the ore. In this study the effect of aluminium fluoride, a constituent encountered in fluorapatite rock in varying concentrations, on the formation of calcium sulfate hydrates during phosphoric acid processing is presented. Solubility experiments and continuous crystallization experiments of calcium sulfate hemihydrate ( CaSO 4 · 1 2 H 2 O , HH) and calcium sulfate, dihydrate (CaSO 4 · 2H 2O, DH) were performed in concentrated phosphoric/sulfuric acid solutions at 75°C and 90°C in the presence of small amounts of aluminium fluoride or hydrogen fluoride. Both AlF 3 and HF increase the solubilities of HH and DH via their effect on the activity coefficients of calcium and sulfate. Aluminium fluoride retards the HH and DH growth rate and an AlF 3 concentration of 95 mmol (kg solution) −1 increases the calcium sulfate concentration far above the DH solubility line. This leads to DH formation under conditions where normally HH is formed. The presence of aluminium fluoride also results in crystal habit modification and agglomeration of the HH and DH crystals formed.