EFFECT OF BASICITY AND IMPURITIES ON CR-SPINEL STABILITY

Chromium leaching is still the most concern of Electric Arc Furnace slag both from carbon and stainless steels production. In the recent years, many researches aimed at understanding which properties (basicity, cooling speed) and phases (spinels, wustite, silicates) control the Cr leaching, defining different indexes (sp-factor, cs-factor) to forecast the slag behavior respect to the leach of this toxic metal. The literature suggests that spinel formation is usually a good way to fix Cr and prevent its leaching. However, in some conditions (high basicity, low amount of spinel-forming species) soluble Cr-bearing phases can be formed, i.e. Ca-chromite or unstable spinel. In these conditions, Cr can be leached easily, even if it is bound in a spinel structure. In this paper, the effect of basicity and impurities (Ca, Si) on the instability of Cr-spinel was investigated. Different samples of carbon steel slag suspected of forming unstable spinels, belonging to different steel grade production, were investigated. Similar analysis was also conducted on Mg-wustite, a common mineral in EAF slag, that might contribute to the leaching of Cr. Experimental analysis were carried out by SEM-EDS to measure the local chemical composition of Cr-spinel and Mg-wustite. These data were correlated with slag basicity (by XRF), spinel fraction (by XRD) and Cr leaching (by ICP-MS). The main results indicate that the increase of the slag basicity implies an increase of the impurity content (Ca, Si) in the spinel, that are also over-saturated by Cr. This aspect, coupled with spinel geometrical features, seems to justify the unexpected Cr leaching of some slag samples. Wustite was instead excluded as an additional Cr leaching source.