Study on the Removal Mechanism of Iron Impurities in Raw Coal for Metallurgical-Grade Silicon Smelting

Iron impurities in metallurgical-grade silicon (MG-Si) seriously restrict the improvements in of its quality, and they are difficult to remove in the current smelting process. Coal is one of the main raw materials for MG-Si smelting and the main source of iron impurities in MG-Si. Removing iron impurities in coal is the key to improve the quality of MG-Si. However, the occurrence form of iron impurities in raw coal used in MG-Si smelting is still unclear, and there is no relevant study on the removal mechanism of iron impurities. In this paper, the occurrence form of iron impurities in raw coal is determined and the iron impurities in raw coal are removed by direct nitric acid leaching. It is found that when the HNO3 concentration, temperature, time, and liquid–solid ratio was are respectively 3.5 mol·L−1, 333 K, 3 h, 5:1, and the removal efficiency of iron impurities from raw coal reached 87.07%. The kinetic analysis results show that the leaching process of iron impurities in raw coal conforms to a new shrinking core model, which is controlled by interfacial transfer and diffusion across the product layer. The reaction order n was 0.94, the apparent activation energy Ea was 28.28 kJ·mol−1, and the frequency factor A was 13.43. In addition, based on the material characterization, the dissolution mechanism of iron impurities in raw coal is revealed. It is also deduced that the gasification reactivity of the raw coal is significantly enhanced by nitric acid leaching, facilitating the intensification of the MG-Si smelting process. This study has important guiding significance for the improvement of product quality and production efficiency of MG-Si.