Selective Leaching of P from Steelmaking Slag in Sulfuric Acid Solution

Steelmaking slag contains considerable quantities of valuable elements such as P and Mn; however, its intrinsic value has not been fully exploited. In steelmaking slag, P is primarily distributed in the 2CaO·SiO₂–3CaO·P₂O₅ solid solution, which can be dissolved readily in the aqueous solution compared with other phases. To separate and recover P from steelmaking slag, selective leaching of the P-containing solid solution was proposed. In this study, sulfuric acid was used as the leaching agent to reduce production costs. The effects of the cooling method of the molten slag, valence of Fe in slag, pH, and particle size on the dissolution characteristic of steelmaking slag were investigated. To achieve an improved selective leaching of P, the molten slag should be oxidized into the Fe₂O₃-containing slag and cooled slowly. As the pH decreased, the dissolution of the slag was promoted, resulting in a higher extraction efficiency of P. The majority of the solid solution was separated without dissolving the Fe-rich magnesioferrite at pH 3, indicating an improved selective leaching of P. Through selective leaching, the P₂O₅ content in the residue decreased significantly. If the formation of CaSO₄ can be suppressed, this residue has the potential to be reused in the steel plant. Decreasing the particle size increased the dissolution rate of slag. A shrinking core model was used to describe the kinetics of slag dissolution. The dissolution of P in the sulfuric acid solution was controlled by diffusion in the residue layer.