Enhancing molten salt oxidation sustainability: thermodynamic insights for spent salt reuse and carbonate cycle replenishment

Abstract

The Li₂CO₃–Na₂CO₃–K₂CO₃ carbonate system exhibits superior sulfur interception capacity and thermodynamic stability in the molten salt oxidation (MSO) of cation exchange resins (CERs) with high sulfur content. However, the spent salt from the MSO process cannot be reused to reduce the operating cost and further improve the reduction ratio. Therefore, the optimal oxygen equivalent for treatment is obtained by thermodynamic equilibrium analysis of the MSO process of CERs as 1.08 multiples of the theoretical oxygen demand and the corresponding approximate chemical formula in this work. The results of the equilibrium analysis are used as the basis to propose a replenishment cycle for the MSO process (MSO-RC), which replenishes the consumed carbonate to restore the SO₂ adsorption capacity and treatment capacity of the spent salt. During the experiments, the oxidation efficiencies of MSO-RC are higher than 99%, and the volume reduction ratio is improved by 12% compared with the traditional MSO. In addition, the MSO-RC maintains sulfur interception rates above 80% across multiple cycles and preserves over 94.7% Li₂CO₃ by adding cost-effective Na₂CO₃ and K₂CO₃ into the molten salt system. The MSO-RC process significantly improves the efficiency of process operation with lower operational costs.