Effects of acid and Na₂SiO₃ modification on the dissolution behavior of 2CaO·SiO₂-3CaO·P₂O₅ solid solution in aqueous solutions

Steelmaking slag with high P₂O₅ content is considered as an alternative resource of P. In such slag, P is mainly concentrated in 2CaO·SiO₂-3CaO·P₂O₅ (C₂S-C₃P) solid solution, which has higher water solubility than other mineralogical phases, which indicates the possibility of selective leaching of P. In order to recover P from steelmaking slag, its dissolution ratio from C₂S-C₃P into aqueous solutions must be increased. In this study, the effects of acid as leaching agent, high temperature modification of C₂S-C₃P by Na₂SiO₃, and pH on the dissolution behavior of C₂S-C₃P in aqueous solutions have been investigated. The obtained results show that when oxalic acid (H₂C₂O₄) or citric acid (H₃C₆H₅O₇) was used to leach P from C₂S-C₃P, dissolved Ca²⁺ ions were removed by CaC₂O₄ precipitation or by the formation of CaC₆H₅O₇^- complex, respectively. As a result, the precipitation of phosphates was suppressed, and P dissolution increased dramatically. On the other hand, the P-rich phase changed from the original C₂S-C₃P into 2CaO·SiO₂-2CaO·Na₂O·P₂O₅ with higher water solubility after Na₂SiO₃ addition, thus improving P dissolution in the H₂C₂O₄ and H₃C₆H₅O₇ solution. At higher pH (pH ≥ 7), H₂C₂O₄ was beneficial for P dissolution from the modified C₂S-C₃P. However, at lower pH (5 ≤ pH < 7), H₃C₆H₅O₇ was an optimal leaching agent for extracting soluble P. At pH 5, the dissolution ratio of P from the modified C₂S-C₃P in the H₃C₆H₅O₇ solution reached 85.7%.