Effect of Fe and Al on phosphorus distribution characteristics during solidification of Si-Fe-Al alloy melt

Phosphorus (P) in metallurgical grade silicon (MG-Si) affects the production of downstream silicon materials. By ignoring the influence of minor elements on P, MG-Si can be approximated to a Si-Fe-Al system, and the distribution characteristics of P directly affect its separation. This study investigated the effect of Fe and Al on the P distribution characteristics during solidification of the Si-Fe-Al alloy melt. In an experiment, MG-Si and the Si-Fe-Al alloy, with similar composition and solidification route, were subjected to the same in situ corrosion and acid leaching. Their comparison shows that the total concentration of Fe and Al increased from 0.59 to 6.35 wt.%, while the removal efficiency of P only increased from 7.23 to 25.00%. The residual percentage of P in the Si matrix of MG-Si and the Si-Fe-Al alloy were 92.77 and 75.00%, respectively. Additionally, the dissolution of AlP at the grain boundary is the main reason for the separation of P by acid leaching. At 1,685 K, the activity interaction coefficients εPFe and εPAl are -3.1348 and -15.5498, respectively, as calculated by the molecular volume interaction model. During the initial crystallization of Si in the Si-Fe-Al system, Fe and Al can reduce the segregation coefficient of P and transform it into AlP, thereby theoretically promoting the separation of P from Si. However, AlP and Si have similar crystal structure, and the low mismatch factor and strong binding force between them facilitate the capture of AlP by primary Si, which hinders the enhanced separation of P from Si. An accurate understanding of the formation mechanism of the distribution characteristics of P in MG-Si is of great significance to the optimal design of MG-Si purification scheme.