Study Of Equilibrium Phases In CaO-MgO-SiO₂-Cr₂O₃ Slags

In pyrometallurgical processes, slags are used to prevent oxidation of the metal through contact with air, to limit heat losses through radiation and to remove impurities from the molten metal. The thermodynamics of molten slags is an important area in the field of process metallurgy. In order to optimize the metallurgical process, it's necessary to investigate the properties of industrial slag systems.Chromium is the metal in slag that is given the most attention when it comes to leaching. Several types of slag contain a significant fraction of oxidized chromium. This is especially the case for ferrochromium and stainless steel slags. Knowledge of the thermodynamic properties of chromium oxides-containing slags is of great importance to chromium retention in stainless steel production, in view of chromium impacts on economic costs and environmental protection.In the present study, phase equilibrium of CaO-MgO-SiO2-Cr2O3 synthetic slag system was investigated. The influential factors on equilibrium phases in this system such as slag basicity, heat treatment and addition of Al2O3 were studied, respectively. Chromium distribution, phase composition and microstructures in the sample were analyzed using SEM, XRD, EDS techniques. Calculation results from FactsageTM software were also used to predict equilibrium information for some slag compositions.It is shown that a certain amount of spinel phase precipitated in slag samples with basicity 1.4 and 1.6 which were quenched directly from 1600℃. When basicity is less than 1.2, no spinel was detected in the sample while the matrix had a relatively high amount of chromium at the same time.Spinel phase also precipitated in the samples with even higher basicity. But EDS analysis showed that there are some calcium and silicon impurities in this phase. In addition, chromium tends to form solid solution or calcium chromate combined with periclase and merwinite in slags with high basicity. This probably leads to leaching of chromium.If the samples were slowly cooled to 1400℃from 1600℃and then quenched, there were more spinel precipitated. Especially when basicity equals 1.2, big spinel crystals with size of 150μm appeared in the sample. Addition of Al2O3 also favored the formation of spinel which was in the formula of Mg(Al,Cr)2O4. With increasing addition of Al2O3, chromium bounded in spinel phase was replaced by aluminum. When 12wt% of Al2O3 was added to the sample with basicity 1.6, alumina-magnesia spinel, MgAl2O4, was formed.