Study On Purification Of Polysilicon Head-end Wastes And Sawing Wastes

The issue of energy is becoming increasingly prominent around the world.As a renewable energy,solar energy has great potential for development.Crystalline silicon solar cells,as the leading products in the photovoltaic market,occupy a dominant position in the solar energy application market.In the production process of solar-grade polysilicon,there are mainly three kinds of wastes:head-end waste,sawing waste and other wastes.The head-end waste produced during the directional solidification of polycrystalline silicon accounts for about 10%of the mass of silicon ingot.The sawing waste is divided into mortar sawing waste and diamond-wire sawing waste.At present,the main sawing waste in the market is diamond-wire sawing waste,which accounts for 30%～40%of the silicon ingot.Other wastes include scrap and semifinished products.In addition to other wastes,head-end waste and sawing waste can only be reused after being purified.Therefore,the recycling of head-end waste and sawing waste is particularly important.In this paper,the polysilicon head-end waste and diamond-wire sawing waste,provided by Jiangsu Jinhui Photovoltaic Co.,were used as raw materials to study the purification of the two raw materials and explore the better purification conditions.At the same time,the iron leaching rate in the reactions was analyzed to provide theoretical guidance for reuseing of waste materials.The main research contents are as follows:（1）The properties of polysilicon head-end waste were studied.The XRF analysis shows that Si is the most abundant element on the surface of head-end waste,and its purity is more than 99%.In addition,there are some metal elements such as Fe,Al and Ca.XRD analysis shows there are SiC and SiO2,which indicates that the silicon of head-end waste is covered by SiC and SiO2 layers.The results of SEM and EDS analysis show that the surface of head-end waste is mainly composed of SiC and Si3N4,and the thickness of the thickest impurity layer is about 500 μm.（2）The purification of polysilicon head-end waste was studied,and the optimized conditions of ultrasonic purification were explored.The ultrasonic optimized conditions were determined as follows:ternary mixed acid of HF+HNO₃+H₂SO₄,temperature of 80℃,ultrasonic frequency of 45 kHz,ultrasonic power of 240 W.Under these conditions,the corrosion rate of polysilicon head-end waste was 10.0567 g/（m2·s）,and the leaching rate of Fe was 76.33%.（3）The properties of polysilicon diamond-wire sawing waste were studied.XRF analysis shows that Si is the most abundant element with the purity of 97.70%in the sawing waste.There are also some metallic impurities and non-metallic impurities.The XRD results show that the main phase in the sawing waste is silicon.The infrared spectrum analysis shows that the silicon oxide is contained.And the particle size analysis shows that the particle size of the sawing waste is micron.SEM analysis shows that the cutting waste is of micron size.The morphology is lamellar.（4）The purification of polysilicon sawing waste was studied.The ultrasonic metal assisted method was selected.The optimized conditions for removing impurities from polysilicon sawing waste were determined as follows:hydrofluoric acid concentration of 3.6 mol/L,hydrogen peroxide concentration of 0.36 mol/L,ultrasonic frequency of 45 kHz,temperature of 60℃.Under these conditions,the leaching rate of Fe was 96.91%.