Effect of MgO and CaO/SiO₂ on the immobilization of chromium in synthetic slags

This work investigated the chemical and mineralogical properties of CaO-SiO₂-Cr₂O₃-CaF₂-MgO slags. Synthetic slags were prepared and the effect of the slag basicity (mass ratio CaO/SiO₂) and MgO contents on the stability of the mineralogical species formed was analyzed. The morphology and composition of the slags were analyzed by X-ray powder diffraction (XRD) and scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that in slags with CaO/SiO₂ = 1, the main Cr-compound was MgCr₂O₄ spinel, which forms octahedron crystals. Small amounts of CaCr₂O₄ and CaCrO₄ were also observed. It was found that with increasing the slag basicity from 1 to 2 the compounds MgCr₂O₄ and CaCr₂O₄ were formed together with the Cr(V)-containing compound complex Ca₅(CrO₄)₃F which forms hexagonal crystals. The results showed that the highest Cr concentration levels in the leaching liquors corresponded to slags with CaO/SiO₂ = 2, probably owing to the formation of CaCrO₄ and Ca₅(CrO₄)₃F, whilst the lowest chromium concentration levels corresponded to MgO-based slags owing to the stable binding of chromium in spinel with MgO. Additionally, potential-pH diagrams for the Ca-Cr-H₂O and Mg-Cr-H₂O systems at 25 °C were calculated.