Mineralogical study of LD converter slag was carried out by means of microscopic and EPMA examinations and phosphorus was found to exist only in dicalcium silicate as solid solution. This led to the study of separation of dicalcium silicate from LD converter slag in order to remove phosphorus.
When liquid slag was solidified slowly, most dicalcium silicate particles accumulated in the top part of the crucible and fewer in the bottom. The phenomena can be interpreted as follows; on solidification, dicalcium silicate is crystallized primarily and floats up owing to the difference of density between dicalcium silicate and residual liquid. By using this phenomena, we can separate LD converter slag into two layers, top and bottom in a vessel. As a result of slow cooling, CaO, SiO₂ and P₂O₅ are enriched in the top, and FeO, Fe₂O₃ and MnO in the bottom.
Dicalcium silicate is apt to separate more efficiently with higher total iron content in slag, and at higher start temperature of cooling, close to liquidus temperature. The efficiency of separation was improved by blowing oxygen into the molten slag before cooling.