Abstract
The effects of inorganic impurities on the crystallization of calcium sulfates in strong HCl (6.3 mol L–1)-CaCl2 (1.8 mol L–1) solutions were investigated. The impurities considered relate to hydrochloric acid leaching of apatite-type ores for the extraction of rare earth elements. The impurities investigated were K+, Mg2+, Sr2+, Ba2+, Al3+, Fe2+, Fe3+, La3+, Y3+, F– (fluoride), and PO43– (phosphate). The investigation was done in the context of a continuous steady-state crystallization process. Therefore, temperature-controlled, semibatch crystal growth experiments with regulated reagent addition, to ensure nearly constant supersaturation, were performed. The experiments were conducted at 40 and 80 °C corresponding, respectively, to crystallization of calcium sulfate dihydrate (DH) and calcium sulfate hemihydrate (HH). Among all impurities investigated, phosphate and strontium were found to have the most significant effects, with La3+ and Y3+ having some modest effects. Phosphate (added as phosphoric aci...
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