Silicon purification by acid leaching and slag refining techniques

Abstract

Solar energy has become the most popular and fastest-growing renewable energy resource worldwide. However, it still requires the current solar-grade silicon (SoG-Si, purity 99.9999%) production with a larger manufacturing scale, lower cost, and increasing sustainability to embrace the upcoming photovoltaic era.

Owing to the low energy consumption and low carbon footprint features, the emerging metallurgical route of SoG-Si production has received increasing attention. As one of the recent successfully commercialized processes, the Elkem Solar process, which is operated by REC Solar Norway, is known to purify crude metallurgical-grade Si (MG-Si, purity 99%) through the combination of a series of metallurgical techniques, including slag refining, acid leaching, and direction solidification.

This thesis aims to further investigate the Elkem process by focusing on the acid leaching and slag refining techniques and targets on the alloying effect on the most problematic impurities to be removed in MG-Si: phosphorus and boron.

In the study of the alloying-leaching process for phosphorus removal, it was found the alloying elements significantly affect the silicon microstructure, leaching behaviour, impurity segregation, and silicon purification efficiency. Compared to the binary Si-Ca and Si-Mg alloying-leaching systems, the novel Si-Ca-Mg ternary system exhibited cleaner and more sustainable leaching features, and kept a good balance on high phosphorus removal, high leaching kinetics, and high Si recovery. Based on the principle of Gulliver-Scheil solidification and thermodynamic analysis, phosphorus removal model was established. The effect of alloy concentration and metal-phosphorus affinity on phosphorus removal was theoretically quantified and validated with experimental results. The averaged interaction coefficient of different alloying metals (Ca, Mg, and Al) on phosphorus was fitted and obtained as, respectively, -19.2, -10.8, and -1.8. The mathematic relationship among the critical process parameters, including P concentration, alloying metal concentration, and the operation repeating times, was also obtained.

In the study of the slag refining process, boron removal was improved by La2O3 addition in CaO-SiO2 slag and by Sn alloying with Si. The slag structure evolution was investigated by classical molecular dynamics simulation with a novel proposed oxygen classification method for slag systems containing mixed network modifiers. A thermodynamic model was derived to theoretically understand the alloying effect on impurity distribution in slag refining. It further highlights the positive interaction coefficient and high alloying concentration are the preferred conditions for enhancing slag refining performance.