Oxidation reaction behavior of Cr-hosting spinels during heating of solid wastes containing Cr

Cr-hosting spinels are frequently formed during heating of solid wastes containing multiple metals, and its oxidation reaction (Cr(III) → Cr(VI)) is closely related with the toxicity of products. This study examined the reaction behaviors of Cr-hosting spinels (ZnCr₂O₄, CuCr₂O₄ and NiCr₂O₄) at high temperature and proposed possible oxidation mechanism. Cr-hosting spinels alone usually exhibit good thermal stability at high temperature. However, CaO can trigger the oxidation of Cr(III) in Cr-hosting spinels at 500-900 °C and ZnCr₂O₄ is easier to be oxidized than NiCr₂O₄ and CuCr₂O₄ at same condition. The oxidation of Cr-hosting spinels is accompanied with the formation of CaCrO₄ and divalent metal oxides (ZnO, NiO and CuO). The broken and rebuilding of CrO bonds are key steps for Cr-hosting spinels oxidation, blocking the combination of free Cr with Ca and O atoms maybe more effective approach for suppressing Cr(III) oxidation. Furthermore, CaO can trigger the reduction of CaCrO₄ into a new Cr(V) compound (Ca₅(CrO₄)₃O_0.5) at 900-1200 °C. As the temperature rising to 1300 °C, CuO reacts with CaCrO₄ to form CuCrO₂, in which Cu(II) and Cr(VI) are reduced into Cu(I) and Cr(III) respectively. This study provided some new knowledge for the reaction behavior of Cr-hosting spinels when solid wastes containing Cr were treated at high temperature.