Removal of Phosphorus from LD Converter Slag by Floating of Dicalcium Silicate during Solidification

Abstract

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 melt 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. Using this phenomena enabled us to separate LD converter slag into two layers, top and bottom in a vessel. As a result of slow cooling, CaO, SiO₂ and P₂O₅ were enriched in the top, and FeO, Fe₂O₃ and MnO, were in the bottom.
Dicalcium silicate was found to be 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.