One-step wet-process phosphoric acid by-product CaSO4 and its purification

In this study, phosphate ore powder was used as raw material, and anhydrous calcium sulphate (CaSO4) with P2O5 and F− contents of only 0.01% and 0.03% was prepared by one-step acidification at atmospheric pressure through regulating the sulphuric acid concentration and reaction conditions. Experimentally received: when the sulfuric acid concentration is 35 vol%, the liquid–solid ratio is 3.5 mL/g and the reaction time is 2 h, the made calcium sulfate is CaSO4, and its P2O5 and F− content is just 0.03% and 0.05%. On the other hand, H3PO4 extraction rate of the corresponding filtrate can reach to 72.35%, and the F− content in H3PO4 remains only 0.02%. After including 3 wt% of OA in this reaction, the elimination of SiO2 impurities reached 89.62%, leading to the final purity of CaSO4 of 96.65%. We also investigated the mechanism for the removal of impurities such as P, F, and Si during the system and explained the mechanism for the formation of CaSO4. This study not only improves the yield of H3PO4 but also the availability of phosphogypsum by producing CaSO4. This can be considered as a reference pathway to enhance the benefits of wet- process phosphoric acid.