High-value recycling of photovoltaic silicon waste: Accelerated removal of impurity boron through Na3AlF6-enhanced slag refining

Recycling of environmentally hazardous silicon powder waste (SPW) is conducive to achieving “carbon neutrality”. However, the high-cost and low-efficiency impurity removal limit the industrial recovery of SPW. Herein, a combination strategy of vacuum sintering and Na3AlF6-enhanced slag refining is demonstrated to upgrade the traditional recycling process. The cost-effective vacuum sintering can remove 89.44% of oxygen in silicon waste, indicating that the oxide layer of SPW is removed effectively. In the Na3AlF6-enhanced CaO–SiO2 slag, the optimal Na3AlF6 content and CaO/SiO2 mass ratio are set to 20 wt% and 1.6 based on thermodynamic simulation. Na3AlF6 reduces the liquidus temperature and increases the interfacial tension of the slag system. Moreover, in Na3AlF6-containing slag, the diffusion pathway of BO33- is dredged. As a result, the silicon-slag interface is adjusted from a half-spherical to a cylindrical surface, and the interface area has increased by 14.69%. The boron removal rate by Na3AlF6-strengthened slag refining is 40.92% faster than that of traditional slag. This work improves the removal efficiency of key impurity boron, reducing the cost of SPW recovery. Based on economic evaluation, this strategy offers a commercially available way to achieve the high-value recycling of silicon waste.