Extraction of Phosphorus and Recovery of Phosphate from Steelmaking Slag by Selective Leaching

At the refining temperature of steelmaking slag, phosphorus is distributed between the liquid phase and solid solution phase of 2CaO·SiO2-3CaO·P2O5. By exploiting the differences in water solubilities between the solid solution and other phases, we are developing an acid leaching process to separate out phosphorus. In this paper, we determined the optimum conditions of leaching with nitric acid by investigating the control over Fe valency in the slag and the recovery of phosphorus from the leachate. pH of 3 was found to dissolve the solid solution, while a solid solution containing FeO showed a lower phosphorus dissolution ratio. To avoid the formation of a glassy phase, slow cooling was necessary which suppressed the dissolution of other phases at this pH. Leaching was further studied in artificially prepared steelmaking slags of compositions representing commercial slag fertilizers. The dissolution ratio of phosphorus reached about 91% while the phosphorus content in the residue was sufficiently low. After separating from the residue, the pH of the leachate was increased to precipitate the phosphate. At a pH of 7, over 80% of the phosphorus in the leachate was precipitated, and the phosphate content of the precipitate was approximately 25% after calcination.