Crystallization of calcium sulfate in concentrated phosphoric acid

In a continuous crystallization procedure calcium sulfate hemihydrate (HH) crystals were formed by feeding a calcium dihydrogen phosphate (CDHP) solution in concentrated phosphoric acid (5.6 and 6.6 mol H₃PO₄ per kg), simultaneously with a sulfuric acid solution, into a 1 litre crystallizer at 92°C. The slurry removal occurred unclassified. Residence times of 20 to 80 min were maintained. The solid over liquid weight ratio depended on the calcium content of the CDHP solution. By slight adjustment of the feed streams the molar calcium over sulfate ratio in the solution was varied between about 0.05 and 8. A cadmium concentration of 30 ppm was maintained in the crystallizing solution. The shape of the HH crystals varied with increasing sulfate concentrations from mainly aggregates to mainly needles. During crystallization of the HH crystals, cadmium as well as phosphate ions are incorporated into the lattice. The degree of uptake strongly depends on the operating conditions. The phosphate uptake decreases with increasing sulfate concentrations in the crystallizer, while the cadmium uptake increases. The molar phosphate over sulfate ratio in the crystals is found to be proportional to the same ratio in the solution. A comparable linear relationship is also observed for the molar cadmium over calcium ratios at a residence time of 20 min. At high sulfate concentrations, long residence times and high temperatures (about 92°C), the stable anhydrite modification (AH) tends to develop next to the metastable HH phase. The cadmium uptake in AH is at least 10 times higher than in HH, while the phosphate uptake is not influenced by a small amount of AH in the crystals. The cadmium incorporation in the HH lattice can be reduced by adding halogenides to the solution.