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 .
Highlights
Solar energy is a crucial renewable energy source to reduce green house emissions and limits global warming
A series of Si-CaAl alloys were obtained to investigate the role of alloy composition and the CaAl2Si2 phase in P segregation and separation
The role of alloy composition and the ternary CaAl2Si2 phase in the Si alloying-leaching process was further investigated with the focus on P removal
Summary
Solar energy is a crucial renewable energy source to reduce green house emissions and limits global warming. As the dominating feedstock material in the PV industry, solar grade silicon (SoG-Si, purity 99.9999%) has attracted increasing attention globally. Most of the SoGSi is currently produced by the modified Siemens process and the fluidized bed reactor process. Both methods are energyintensive and face the potentially severe environmental issue that the hazardous by-products may leak into the atmosphere, such as chlori nated gases and silane [2]. Many efforts so far have been devoted to refine the metallurgical-grade Si (MG-Si, purity 99%) with lower energy consumption and carbon footprint through the metallurgical techniques like slag refining [3,4,5,6], solvent refining [7,8,9,10,11,12,13], acid leaching [14,15,16,17,18,19], vacuum refining [20,21,22,23], gas refining [24,25], directional so lidification [26], and their combinations [27,28,29,30]
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