P removal from Si by Si-Ca-Al alloying-leaching refining: Effect of Al and the CaAl2Si2 phase

Abstract

• Al 2 O 3 crucible led to P removal degradation for Ca-containing Si alloys. • Increasing Ca/Al ratio and total alloying amount (Ca + Al) promote P removal. • A model for P removal prediction was obtained for Si-Ca-Al alloy system. • Ca concentration in liquid phase found as the essential driving force for P segregation. • CaAl 2 Si 2 exhibits higher P solubility than CaSi 2 but not decisive for P segregation. Phosphorus (P) is a critical impurity in metallurgical-grade Si that needed to be removed for solar-grade Si production. The Si-Ca-Al-P system is one of the most crucial alloy systems for Si refining, where P can be separated by either the Si-Al solvent refining or the Si-Ca alloying-leaching process. In this work, a series of Si-Ca-Al alloys were obtained to investigate the role of alloy composition and the CaAl 2 Si 2 phase in P segregation and separation. The used alumina crucible led to heavy Al contamination and resulted in unexpected Si-Ca-Al alloy compositions. CaAl 2 Si 2 and CaSi 2 were found as the main precipitates, and relatively higher P content was detected in the CaAl 2 Si 2 phase. Leaching experiments indicate that P removal degree increases with increasing Ca/Al ratio and increasing total alloying amount (Ca + Al). Effects of alloy composition were further analyzed by statistical methods, which indicates Ca firmly plays a strong, positive, and monotonic role in P removal while no strong attraction between Al and P. Theoretical P removal model for the Si-Ca-Al system was established and in good agreement with measured results. Interaction coefficients of Ca and Al to P were fitted as ε Ca P = - 19.2 and ε Al P = - 1.8 , which further confirms Ca is the essential driving force for P segregation. Finally, the possibility of P solid solution formation was verified by first-principle simulations that both CaAl 2 Si 2 and CaSi 2 are able to dissolve P, especially through the Si site, but the CaAl 2 Si 2 phase is more favourable for P dissolution than CaSi 2 , which explains the reason of the often detected high P content in CaAl 2 Si 2 .