Formation Mechanism of ZrB2 in a Si–Cu Melt and Its Potential Application for Refining Si and Recycling Si Waste

Based on (i) the low liquidus temperature of Si–Cu systems, (ii) low solubility of Cu in solid Si, and (iii) a strong affinity of Zr to B for the enhanced boride formation, a novel method to remove B from Si using Zr as a trapping agent via a Si–Cu solvent was developed in this study. B-bearing polygonal ZrBx precipitations found at the bottom of test samples were confirmed as ZrB2 by electron probe microanalysis. Thermodynamic analysis revealed that the solubility products of ZrB2 in a Si–Cu melt were 2.50 × 10–12 (1258 K, Si-57.0 atom % Cu) and 4.11 × 10–12 (1345 K, Si-50.0 atom % Cu). Finally, the compositions of Si–Cu alloys that can form stable ZrB2 particles were estimated by calculating the solubility products of ZrB2 and the equilibrium contents between B and Zr at different corresponding liquidus temperatures. The proposed method provides the potential for B removal via Si–Cu solvent refinement to realize the large-scale production of solar-grade silicon, thereby overcoming the key limitation to the sustainable utilization of solar energy.