Pressure oxidative leaching of chromite in the NaOH-NaNO3-H2O binary sub-molten reaction medium

The liquid-phase oxidation of chromite ore in sub-molten medium is regarded as a clean and efficient method for the preparation of chromate. However, its industrial application is limited by the need for specialized equipment, stringent reaction conditions, and complex chromate-alkali separation processes. To enhance the leaching of chromite and achieve milder reaction conditions, this study introduces a novel NaOH-NaNO₃-H₂O binary sub-molten salt reaction system. Effects of reaction parameters on the leaching ratio of chromium were systematically studied. The results showed that a high chromium extraction rate up to 99 % was obtained within 240 min under the optimal reaction conditions: a reaction temperature of 513.15 K, a NaNO₃-to-ore mass ratio of 0.8:1, an alkaline-to-ore mass ratio of 4:1, an oxygen partial pressure of 2.0 MPa, a stirring speed of 800 rpm, and an alkali concentration of 50 wt%. Kinetics analysis revealed that the leaching process of chromite was controlled by the surface chemical reaction, with an apparent activation energy calculated as 31.58 kJ·mol⁻¹. Moreover, it was also found that the addition of NaNO₃ in the reaction system served a dual role purpose: diluting the viscosity of the reaction medium and acting as an oxygen carrier, which significantly enhanced the leaching efficiency of chromium. Additionally, the binary reaction medium exhibited stable usage performance. During 6 consecutive leaching runs, a chromium leaching rate of around 97 % could be maintained. Therefore, the NaOH-NaNO₃-H₂O binary sub-molten system shows great potential for applications in the highly efficient leaching of chromite ore.