Recovery of Phosphoric Acid and Calcium Phosphate from Dephosphorization Slag

We previously reported that by adding aqueous ammonia to the nitric acid extract of dephosphorization slag, a solid with relatively high concentrations of calcium and phosphorus could be obtained. However, the solid contains considerable amounts of manganese and iron, which hinder its application in production processes. In the present study, the obtained solid was again dissolved in nitric acid, and the resultant filtrate was passed through a cation exchange resin that removed most unwanted cations from the aqueous phosphoric acid solution. The recovery of phosphoric acid was confirmed via phosphorus nuclear magnetic resonance. Furthermore, when calcium nitrate was added to this aqueous solution, calcium hydroxyapatite was obtained, which was converted into calcium phosphate after calcination at 1073 K. Phosphoric acid, calcium hydroxyapatite, and calcium phosphate are raw materials used to produce various industrial products containing phosphorus, and our suggested process significantly improves the technology for recovering phosphorus-containing materials, which are mostly used as fertilizers.