New insights into silicon purification by alloying-leaching refining: A comparative study of Mg-Si, Ca-Si, and Ca-Mg-Si systems

In the present work, a comparative study of the silicon alloying-leaching purification process was carried out on the recently developed Mg-Si system, the Ca-Si system, and the novel ternary Ca-Mg-Si system. Insights were provided into the integrated process from aspects of thermodynamic assessment, microstructural analysis, experimental observation, computational simulation, and analytical modeling. The main silicide precipitates of the three Mg-Si, Ca-Si, and Ca-Mg-Si alloys studied were determined as Mg2Si, CaSi2, and ternary Ca7 Mg7.5±δSi14, respectively. Other metallic impurities were found to form complex silicides embedded inside the main precipitate, where P also segregated and precipitated according to the interaction with the alloying elements. All of the impurities were further carried away with the removal of the main precipitates through the subsequent leaching process. It was found that the ternary Ca-Mg-Si alloy exhibits a cleaner leaching process due to the unique crystal structure of Ca7Mg7.5±δSi14. A novel cracking-shrinking principle-based kinetics model was developed to further describe the impurity removal process. The segregation behavior of P was also modeled through a thermodynamic approach and Ca was found to have stronger P affinity compared to Mg. It was finally concluded that the novel Ca-Mg-Si ternary alloy system exhibited better performance overall as compared to the other two binary alloys.