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

Abstract

In steelmaking slag, phosphorus is distributed between liquid phase and solid solution of 2CaO·SiO₂ and 3CaO·P₂O₅ at refining temperature. Using the difference in water solubility of the solid solution and the other phases, we are developing the process to separate phosphorus by acid leaching. In this paper, the optimum condition of leaching by nitric acid by the control of valency of Fe in slag was investigated and the recovery of phosphorus from leachate was also conducted. To dissolve the solid solution, pH has to be decreased to 3 and the solid solution containing FeO showed lower dissolution ratio. To suppress the dissolution of other phases in slag at this pH, slow cooling to avoid the formation of glassy phase was necessary. The artificially steelmaking slag with the appropriate composition was made and subjected to the leaching experiment. The dissolution ratio of phosphorus has reached about 91% and the phosphorus content in the residue was enough low. After the separation of residue, pH of the leachate increased to precipitate phosphate. At pH=7, 80% or more of the phosphorus in the leachate was precipitated and the phosphate content of the precipitate was about 25% after calcination.