Article pubs.acs.org/IECR
Chemical Cracking Effect of Aqua Regia on the Purification of Metallurgical-Grade Silicon Hu Zhang,†,‡ Zhi Wang,*,† Wenhui Ma,‡ Keqiang Xie,‡ and Lei Hu† †
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650000, China ABSTRACT: The chemical cracking effect on metallurgical-grade silicon (MG-Si) during leaching by aqua regia was investigated. The results showed that the granularity of the particles was reduced by acid leaching, and the phenomenon was more noticeable with the increase in the silicon particle size. The cracking was essentially ascribed to the dissolution of the major impurities that depended on the composition and distribution of the impurity phases in MG-Si. In addition, the removal efficiency of the main metallic impurities with the occurrence of the chemical cracking effect during the leaching process was studied. The remarkable removal efficiency was obtained from the coarse particle size, where 79% Fe, 77% Al, 45% Ca, and 63% Ti were removed. The reason could be attributed to the strong oxidation of aqua regia that promoted the dissolution of the Si− Al−Fe soluble phase, which resulted in the enhanced removal effect of acid on the impurities.
1. INTRODUCTION Recently, solar-grade silicon (SOG-Si) has received a large amount of attention because of a growing demand for the photovoltaic (PV) industry. At present, there are two major SOG-Si-producing technologies: a modified Siemens process and a fluidized bed reactor process. However, their main disadvantages are the high production cost and consequent environmentally hazardous substances, which hinder the widespread use of silicon solar cells.1,2 In this context, the metallurgical process for producing SOG-Si from metallurgicalgrade silicon (MG-Si) has attracted substantial interest because of its low cost and level of energy consumption.3 The process is comprised of acid leaching,4 oxidization refining,5 vacuum treatment,6 directional solidification,7 etc. As the pretreatment process, acid leaching is an indispensable hydrometallurgical method of refining silicon with the advantages being a low level of energy consumption, simple equipment, easy scale production, and the prominent effect on the removal of metal impurities. The process generally comprises inorganic acids leaching in single or multiple steps. Wang et al.8 upgraded the purity of crushed MG-Si from 98 to 99.93% through leaching with various acids, and HF was found to be more efficient on purification than HCl. Chu9 investigated the purification of pulverized MG-Si by three types of acids. The results showed that the efficiency of removal of impurities decreased in the following order: aqua regia > equivalent mixture of nitric acid and sulfuric acid > hydrochloric acid. Santos et al.10 obtained 99.9% pure Si after leaching by combining hydrochloric acid (16%, 5 h, 80 °C) with hydrofluoric acid (2.5%, 2 h, 80 °C). Because most of the metallic elements have a lower solubility in solid silicon than in the liquid state, it is easy for metallic impurities to precipitate as minor phases in the grain boundaries during the solidification process of MG-Si.11 When MG-Si lumps are crushed to particles, the phases carrying impurities are exposed and set free on the surface of the grains. Thus, it is necessary to crush the © 2013 American Chemical Society
MG-Si into appropriate particle sizes to improve the efficiency of removal of impurities by acid leaching. Juneja12 considered the crushed particle size of MG-Si must be 90% for MG-Si with an average particle size of
