Microstructure of metallurgical grade silicon during alloying refining with calcium

Refining of metallurgical grade silicon is currently one of the promising routes to low-cost solar grade silicon for solar cells. Alloying with calcium followed by acid leaching is considered as the first step in the refining process. Efficiency of refining is dependent on process conditions and microstructure of the metallurgical grade silicon. In the present work a detailed analysis of the transformation of impurity phases in the grain boundaries has been carried out, focusing on the behaviour of impurities affected by calcium addition and solidification conditions. Various intermetallic silicide and non-silicide phases have been detected in grain boundaries and the mechanism of their formation is discussed. It was shown that calcium addition promotes segregation of impurities in metallurgical grade silicon. It was determined that some impurity phases contain significant phosphorus content after alloying with calcium, which could be useful for the phosphorus removal.

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► Mechanism of alloying refining of metallurgical silicon using calcium is presented.
► Phosphorus dissolves in ternary impurity phases during Ca-alloying.
► Cooling rate influences microstructure of MG-Si and composition of grain boundaries.
► Ca concentration influences microstructure of MG-Si.