Enhanced Slag–Metal Separation for Preparation of Al-Si Alloys from Crystalline Silicon Cutting Waste by a CaO-Al2O3-CaF2 Slag Treatment

Crystalline silicon cutting waste (SCW) is a promising secondary silicon resource for Al-Si alloy production, but high-melting Al2O3 generated during aluminothermic reduction hinders alloy droplet coalescence and slag–metal separation. In this study, a CaO-Al2O3-CaF2 slag treatment strategy was developed to regulate in situ formed Al2O3 and improve Al-Si alloy recovery from SCW. The effects of slag composition, smelting temperature, and holding time on separation behavior, massive alloy yield, and Si recovery were investigated. Increasing CaF2 content decreased the liquidus temperature and calculated the viscosity of the slag, thereby improving slag fluidity and promoting the migration and coalescence of Al-Si droplets. The optimal conditions were a smelting temperature of 1400 °C, a holding time of 10 min, and a slag composition of 20 wt.% Al2O3-20 wt.% CaO-60 wt.% CaF2. Under these conditions, the massive alloy yield and Si recovery reached 89.32% and 83.43%, respectively. Mechanistic analysis indicated that the process involved alloying, slagging, and slag–metal separation. CaF2 promoted the formation of a low-melting, low-viscosity liquid slag, which facilitated slag aggregation, droplet migration, and efficient slag–metal separation. This work provides a low-cost and efficient technical approach for the high-value recycling of SCW, showing potential for future industrial application.