Recycling Of Silicon And Silicon Carbide From SOG-Crystalline Silicon Cutting Loss Slurry

The supply of high cost crystalline silicon has failed to meet the growing demand of global solar industry. Nearly50%of the crystalline silicon are lost in the cutting process and become Cutting Loss Slurry (CLS). The polyethylene glycol (PEG) and silicon carbide in the CLS have been recycled to different extent. However, the recycling of the crystalline silicon, which is more valuable in the CLS, is still an open question. Therefore, how to utilize the CLS, especially how to obtaine solar grade crystalline silicon (SOG-Crystalline silicon) from the CLS, are of significance in solving the shortage of the SOG-Crystalline silicon in China, in reducing the imports of the SOG-Crystalline silicon, and in re-using of the waste. In this thesis, a novel technology for recycling silicon and silicon carbide from the CLS (REC Company, Norway) has been developed. In addition, the physical property of the CLS, pickling technology to remove impurities, technology to enrich Si and SiC from the CLS, and purification processes for Si and SiC have also been investigated.1. The property of the CLS was studied, and a pickling technology to remove impurities from the CLS was developed to provide the high-quality raw materials for the subsequent process. It has been found that the CLS mainly consist of silicon (36.000%, wt%, weight percent, the same below), silicon carbide(58.188%), iron(4.170%) and a small amount of impurities(1.642%). The size of silicon and silicon carbid are between1.12μm and25.7μm. The optimal condition of the pickling technology using HCl acid are:the liquid-solid ratio is4:1, pickling time is3h, pickling temperature is70℃and pickling solution concentration is15%. After the pickling process, the silicon in the slurry is increased to39.561%, the silicon carbide is increased to60.204%, the iron is reduced to0.210%, and the other impurities is decreased to0.025%. The removing rate of iron and impurities are95%and98%, respectively.2. Three sedimentation processes, i.e. water-based liquid phase, double layers organic solution sedimentation and column flow separation, are studied to enrich Si and SiC from the CLS. Technology1(Sedimentation process in the water-based liquid phase):The effects of sedimentation time, liquid-solid ratio and dispersant agent on the sedimentation process are investigated. The best conditions for the water-based liquid phase process are:sedimentation time is5h, liquid-solid ratio is2:25and water solution containing PEG. The sample obtained after the process mainly contain silicon80%and silicon carbide19%. The recycling rate of Si is40%. The process is a simple, low cost and effective method to enrich silicon micro powder from the CLS.Technology2(Sedimentation process in double layers organic solution):The sedimentation velocity of the silicon and silicon carbide micro powder in organic solution having different density and polarity has been studied, and the optimal double layers organic solution has been found. It has been found that the upper solution is chloroepoxy propane(low density and high polar) and the lower solution is carbon tetrachloride(high density and small polar). After the process, the silicon enrich sample obtained contain silicon95.040%and silicon carbide4.357%. the recycled rate of Si is90%. Meanwhile, the silicon carbide enrich sample obtained contain silicon5.027%and silicon carbide94.730%. Compared with technology1, this technology can not only separate silicon and silicon carbide micro powder effectively, but also shorten the sedimentation time greatly, realizing the fast separation and short cycle.Technology3(column flow separation process):The column flow separation method is used to enrich silicon and silicon carbide micro powder. According to the extraction theory, the home-designed column flow device is used to simulate countercurrent extraction process. The separation feasibility and the amount of Si and SiC at different position were investigated. The results showed that the sample obtained has80.9%silicon,18.8%silicon carbide, and no more than0.3%impurities. The reccyled rate of Si is60%. Tthe empirical formula obtained could provide theoretical guidance for industrial production. The method is appropriate for large-scale processing, low energy consumption and continuous feeding.3. The enrich silicon has been further refined and purified. Refining of the silicon sample obtained using Technology1to3can be performed using melting electric furnace. The results shows that the silicon of the refined sample is increased from80%(water-based liquid phase),95%(double layers organic solution) and80%(column flow separation) to99%at least. The purification experiment of silicon using Mg-Zn alloy shows that the best condition is600℃, Mg-Zn quality ratio3:2and vacuum distillation at1100℃. Using the method, the silicon of the purified sample is increased from80%to96.79%. The results demonstrate that purification of silicon by Mg-Zn alloy is feasible. Meanwhile, the surplus SiC has been recycled and purified by hydrometallurgy. The result shows that the purity of silicon carbide is changed from94.73%up to99.61%. the recycling of silicon carbide is realized.The new developed technology could be used to recycle and obtain practical products of silicon and silicon carbide from the SOG-Crystalline silicon CLS effectively. At the same time, the technology of recycling process not only reduces the pollution of the environment but also satisfy the requirement of industry production, resource utilization and environmental protection.