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

Abstract

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%.