Effects of cooling rate and acid on extracting soluble phosphorus from slag with high P₂O₅ content by selective leaching

With the use of low-grade iron ores with high P content, slag with high P₂O₅ content generated after dephosphorization is considered a great potential source of P. Because of the solubility difference between the solid solution and matrix phase, it is possible to extract the P-rich solid solution selectively from slag by leaching. The soluble P obtained is suitable to produce phosphate fertilizers. To achieve selective leaching of P and increase its dissolution ratio, the effects of the cooling rate and acid on dissolution of the slag in aqueous solutions at pH 5 and 7 were investigated. This study found that during solidification, slow cooling facilitates coarsening of the solid solution and formation of the magnesioferrite phase. The solid solution was dissolved preferentially. At pH 7, the air-cooled slag showed the highest dissolution ratio of P. When the pH was decreased to 5, slag dissolution was significantly promoted. As the cooling rate decreased, the dissolution ratio of P increased. Slow cooling not only enhanced dissolution of the solid solution but also suppressed dissolution of the matrix phase. Citric acid performed better in promoting dissolution of slag. At pH 5, almost all of the solid solution was dissolved from the furnace-cooled slag. However, the dissolution ratio of Fe was also high. When nitric acid was used, 66.8% of the solid solution was dissolved, without dissolution of large amounts of the matrix phase. After leaching, the P₂O₅ content in the residue reduced and the Fe₂O₃ content increased.