Immobilization of chromium in slags using MgO and Al₂O ₃

Chromium containing slags from stainless steelmaking may be leached by acidic environments, therefore they should be treated before being stockpiled or land filled. In this work, synthetic slags were prepared and the effect of CaO/SiO₂, Cr₂O₃, MgO and AI₂O ₃ contents on the stability of the mineralogical species formed was analysed. The morphology and composition of the slags were determined by XRD and SEM-EDS, whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that CaCr₂O₄ and CaCrO₄ are present in slags prepared with neither MgO nor AI ₂O₃. The Al₂O₃-based slags mainly produced Ca₂Al₂SiO₇ and the Cr(VI)-containing oxide complex Ca₄Al₆CrO₁₆, whilst MgO-based slags produced Mg Cr₂O₄ as main mineralogical species. Additionally, Eh-pH diagrams for the Ca-Cr-H₂O and Mg-Cr-H ₂O systems at 25°C were constructed. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to MgO-based slags owing to the stable binding of chromium in spinel with MgO. It was also observed in the Al₂O₃-based slags that when increasing the slag basicity from 1 to 2, the leachability of the slags was notably increased.