A Comparison Study on Formation and Stabilities of Chromium Bearing Spinels

Chromium, broadly present in metallurgical systems such as stainless steel slags and vanadium-bearing slags, tends to be in a trivalent form bound in spinel phases due to their ultrahigh stability. To explore the stability of chromium bearing spinels with different divalent cations, herein ACr₂O₄ (A=Mg, Zn, Mn) spinels were synthesized and stability-tested under both alkaline and acidic environments. X-ray diffraction (XRD) and refinement results reveal that MnCr₂O₄ exhibited the largest lattice parameters. X-ray photoelectron spectroscopy (XPS) results show that MnCr₂O₄ had the highest oxygen vacancy concentration, potentially triggering structural instability, consistent with the Fourier-transform infrared (FTIR) and Raman analyses. To further investigate the stability of ACr₂O₄ spinels in alkaline and acidic environments, we introduced CaO and SiO₂, respectively, for high temperature calcinations. XRD analyses reveal that ACr₂O₄ spinels exhibited a high instability under an alkaline condition, with an increasing degree of instability for MgCr₂O₄, ZnCr₂O₄, and MnCr₂O₄ successively. We further quantified the relationship between the oxygen vacancy concentrations and the divalent cations containing in spinels as well as the spinel stabilities under an alkaline environment. Under an acidic condition, all ACr₂O₄ spinels demonstrated a relatively strong structural stability. The understanding of formation and stability of chromium bearing spinels under different conditions could contribute to potential modifications and applications of spinels especially with regard to resource recycling in metallurgy.