Silicon Refining Through Alloying by Calcium and Magnesium with Controlled Solidification and Acid Leaching

There is an increasing need worldwide for environmentally friendly routes to produce solar-grade silicon (SoG-Si) feedstock. This study addresses an underexplored gap by linking the cooling rate to the microstructure of Si–Ca–Mg alloys and their impurity removal efficiency during acid leaching. A cast master alloy and remelted samples with controlled cooling rates (3–40 °C min-1) were analyzed. Microstructural analyses were conducted using optical microscopy, scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy, and electron probe microanalysis, after which acid leaching experiments were performed on milled samples. The results reveal that slower cooling led to coarser primary Si grains, with grain size inversely proportional to cooling rate. Leaching results revealed element-dependent impurity removal. Notably, P removal reaches its maximum at intermediate cooling. This behavior arises from a balance between enhanced segregation into leachable phases and depletion of alloying elements during extended solidification. Theoretical analysis indicates that higher cooling rates reduce back diffusion, increasing deviation from equilibrium partitioning. Furthermore, higher initial alloying content improved tolerance to rapid cooling, maintaining effective impurity segregation. These findings highlight the potential to jointly optimize the thermal history and alloy composition to enhance efficiency and robustness in metallurgical refining of silicon.