Study of solubility in the systems CaO-P2O5-SO3-H2O and CaO-P2O5-H2O on the example of sulfuric-acid and phosphoric-acid decomposition of phosphoric slag

Abstract

The results of the theoretical graphical analysis of the decomposition of sulfuric and phosphoric acids from the mass wastes of phosphoric production – slag, the accumulated reserves of which are of industrial importance in terms of quantity, are presented. A representative sample of phosphoric slag from Kainar LLP was obtained for the research. The research showed that the main products of the phosphoric slag's sulfuric decomposition stage are a solution of phosphoric acid with a concentration of 14-17% P2O5 and a cake – a precipitate containing mainly calcium sulfate and insoluble impurities (SiO2). Thus, in the soft dihydrate mode (60°C, calcium sulfate dihydrate formation), a weak phosphoric acid is obtained, sent to the second phosphoric acid decomposition stage of a new portion of phosphoric slag in order to obtain a calcium phosphate product. Potassium with phosphoric acid forms K2HPO4, which, entering into exchange with Ca(H2PO4)2, can pass into a more acidic salt – K2PO4, and the phosphate anion released in this case with a calcium ion forms a hardly soluble CaNRO4. As a result, the final product after the phosphoric acid decomposition contains mainly Ca(H2O4)2, in small amounts CaNRO4 and water-soluble phosphates of magnesium, potassium, as well as an insoluble residue (SiO2). The research results allow to conclude that phosphoric slag is a valuable secondary phosphate raw material, quite suitable for processing into phosphorus, potassium-containing complex mineral fertilizer.