Extraction of phosphorus from steelmaking slag by selective leaching using citric acid

An approach to leach phosphorus from slag was applied to commercial fertilizers made from steelmaking slag, which has relatively coarse grains. After leaching at pH 3 with citric acid, about 60% of the P₂O₅ had been dissolved, but the dissolution ratio of MnO was less than 30%. The separation index, calculated from the dissolution ratio of P₂O₅ divided by that of MnO, was 2.1. In addition, the dissolution ratio of the solid solution was 90% or greater. However, 25% or more of the matrix was also dissolved. To improve the selectivity of leaching, the mineralogical phases comprising the matrix were synthesized, and their leaching behaviors were investigated. The results indicate that it is important to eliminate the glassy phases and to change the valence state of iron in its oxides to 3+ to suppress the dissolution of the matrix. Finally, a synthesized slag, containing only Fe₂O₃ as the only iron oxide, was prepared by slow cooling, and leaching tests were conducted. About 99% of the P was dissolved, but the dissolution ratio of Mn was less than 10%. In addition, the separation index increased to 14.1. In this case, the dissolution of the matrix was suppressed, and almost all the solid solution phase was dissolved.