Low-Sulfur and Low-Temperature Microwave Roasting Process for Antimony Production Utilizing Biochar as Reductant and Carbide Slag as Desulfurization Agent

The smelting process of stibnite concentrates is cumbersome, characterized by high energy consumption, significant alkali usage, and a low rate of solid sulfur recovery. A low-sulfur and low-temperature microwave roasting process has been developed for antimony production, utilizing biochar as the reductant and carbide slag as a sulfur-fixing agent. The method aims to reduce operating costs and add value to solid waste. Fundamental thermodynamic analysis indicates that reactions among the main components of stibnite concentrates, carbide slag, and biochar are feasible at low temperatures, theoretically confirming the viability of this approach. A comparison of the thermogravimetric and microwave response characteristics of different types of biochar showed that coconut shell charcoal is a superior reductant with effective microwave adsorption properties. Therefore, selecting coconut shell charcoal as a reducing agent is advantageous for converting antimony sulfide to metallic antimony. The reaction process of stibnite concentrates under various roasting conditions has been explored. The findings confirmed that metallic antimony is formed at 400 °C, the Sb₂S₃ phase is no longer present in the slag phase at 500 °C, consistent with the results of low-temperature roasting from thermodynamic analysis. Analysis of the roasting mechanism revealed that the thermal decomposition of the carbide slag produces CaO, which fixes the sulfur elements in the slag phase as CaS, achieving low sulfur emissions during roasting. Metallic antimony is deposited on the surface of the roasting slag to form antimony metal particles, which gradually grow in size with increasing temperature. The efficient utilization of solid wastes such as carbide slag and biochar in producing antimony holds significant economic benefits for the utilization of stibnite concentrates and provides a foundation for the advancement and implementation of the antimony smelting process.