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Influence of addition of calcium salts on extraction of potassium from phosphorus- potassium associated mineral

In this article, calcium salts were added into the hydrofluoric acid-sulfuric acid (HF-H2SO4) leaching system, and their effects on the extraction of potassium from phosphorus-potassium associated mineral (mineral deposits containing Ca5(PO4)3F (fluorapatite) and KAlSi3O8 (potassium feldspar)) were investigated. KAlSi3O8 was converted into water-soluble potassium fertilizers, and important factors affecting the extraction, including dosages of H2SO4, types and dosages of calcium salts were studied. The results showed that the maximum extraction of 75.3 wt.% was obtained with H2SO4 (70 wt.%) dosage of 5 mL. When the reaction was added with 0.15 g CaSO4, the extraction rate of K of 95 wt.% was achieved, which was an increase by 19.7 wt.% compared with that without calcium salts. Moreover, the Eh-pH diagrams for K-Si-(P)-(F)-(Ca)-H2O systems were plotted and applied to analyze the mechanism of potassium extraction in the HF-H2SO4 reaction system. The analysis demonstrated that with the presence of F-, the formation of fluoro-complexes enlarged the stable regions of soluble phosphorus (KPF6) and silicon (K2SiF6). However, when calcium salts were added, such region decreased with the increase of [Ca2 +]; and such trends suggested that [K+] could be enhanced by adding [Ca2 +] during the leaching process.