Dissolution of chromite in slags

Abstract

The dissolution of chromite from Bushveld Complex of South Africa in liquid slags was studied experimentally. The slag compositions chosen were representative of ferrochromium production and stainless steel making. The dissolution process was monitored by chemical analysis of samples taken from the melt at predetermined time intervals and SEM–EDAX analysis of reacted chromite cylinders. The experimental procedure involved the use of the rotating cylinder technique in an inductively heated quartz reaction tube under the flow of argon in the temperature range 1550–1665°C. Empirical relations between the slag composition and the dissolution of chromite and the effect of different degrees of reducing conditions were established by using a statistical model. With this, a large amount of information in regard to the effect of slag composition on the dissolution of chromite was made available over a large range of slag compositions for practical applications.The empirical relations have shown that the chromite dissolution in silica–alumina–calcium oxide–magnesium oxide slag systems increased with increasing alumina content, increasing basicity, and decreasing oxygen partial pressure. The dissolution of chromite also increased with increasing rotational speed indicating the possibility of diffusion control of the process. SEM–EDAX studies on the reacted chromite cylinders showed that coring took place within the chromite grains subjected to dissolution reactions for a sufficient length of time. Chromium and iron concentrations in the chromite phase decreased when moving from the centre towards the surface of the grains, whereas aluminium and magnesium concentrations increased in the same direction indicating the opposite diffusion of these species. The slag also penetrated into the chromite grains increasing the silica content of the grains together with alumina and magnesium oxide.