Research On Preparation Of High Purity Silicon From Solar-grade Silicon Cutting Waste

With the energy crisis and environmental pollution occurring, as an abundant renewable energy, solar energy has unique advantages and great potentiality of exploitation. In the process of preparating solar cells, cutting solar grade polycrystalline silicon is necessary.There are amount of silicon carbide, silica and silicon in the cutting waste. Therefore, it is significant to recycle cutting waste for improving economic efficiency, reducing the environmental pollution and cost of solar cells. In this thesis, the cutting waste from Jinzhou Sunshine New Energy Company is investigated, the results are as follows:(1) Analysis of the main ingredient and content of cutting waste. The main ingredient of cutting waste are silicon carbide, silicon, silica and iron oxide. The slurry is composed of silicon carbide 63.90wt%, silicon 11.80wt%, silica 19.13wt% and the other impurities 5.17wt%. Iron, as the main metallic impurity, its content reaches 3.771%. The content of phosphorus reaches 0.0129%.(2) Reseach on purification of cutting waste. Purify the pretreated cutting waste by ultrasonic acid leaching technology. Experimentally determine optmal parameters of ultrasonic acid leaching. The results show that the optimum conditions are as follows: mixture acid (HCl and HF) is leaching agent, leaching time 60min, leaching temperature 50℃, liquid-solid ratio 4:1, ultrasonic power 240W, ultrasonic frequency 45kHz. After purification by ultrasonic acid leaching, the content of ingredient are:SiC 76.36%, Si 15.74%, SiO270.24%. Meanwhile, up to 99.81wt% Fe and 89.81% Al can be removed, respectively. The total removal efficiency of impurities reaches 98.84%.(3) Preparation pellets using cutting waste after purifying. The measurement of physical properties of pellets which use HPMC as binder. The result showed that: palletizing pressure 30MPa, binder adding amount 1.5wt%, water adding amount 13wt%, the compressive strength of the pellets under this condition is 17.58MPa and the porosity 14.36wt%. The effect of calcinations conditions on the compressive strength and porosity of the pellets were investigated. The results show that the compressive strength and porosity increase over time and temperature. This illustrated that during smelting process, pellets in low temperature area can ensure enough compressive strength and porosity.(4) The preparing silicon using cutting waste and high purity silica. The content of silicon obtained from raw materials without puritying is 96.37%, the content of silicon obtained from raw materials after puritying is 99.82% where the content of iron reduced by 98.77%, the content of alumina reduced by 86.85%, the content of phosphorus reduced by 76.92%. Comply with national standard of A grade industrial silicon. It is proved that purify raw materials by altrasonic acid leaching can improve purity of the product.In this thesis, the high purity silicon has been obtained by electrothermal metallurgy process cutting waste. This technology will lay a foundation for the preparation of solar grade polysilicon in next stage.