| With the recent sharp drop in the cost of photovoltaic modules,it is very possible for the usher to meet the era of affordable photovoltaic power generation in the next two or three years.And then,there will be an unprecedented huge demand for polysilicon raw materials in photovoltaic market.However,due to the inevitable cutting loss caused by the wildly used diamond-wire cutting technology during wafer slicing process,a large amount of high-purity silicon will be converted into the silicon powder waste which is difficult to be recycled by far,causing severe environmental pollution and resource waste.Therefore,it is imperative to do research on the recycling of the diamond-wire cutting silicon powder waste,which is vital to green and efficient utilization of silicon resource and the reduction of the total cost of solar cell preparation.At present,the metallurgy method has been considered as the most possible solution to the large-scale recovery of the diamond-wire cutting silicon powder waste.However,the extremely low silicon yield hinders its rapid development.Therefore,the main work of this article will focus on the analysis of low silicon yield problem and the improvement of the silicon yield through a series of efficient methods.In order to solve the practical problems more specifically,this article systematically analyzed the basic characteristics of the diamond-wire cutting silicon powder waste,determined the optimal raw material,the detection method of key impurity elements such as 0,and the optimal pretreatment process before smelting.The causes of the low silicon yield problem during conventional smelting were revealed,and two crucial starting points for solving the recycling problem of the diamond-wire cutting silicon powder waste were pointed out by means of the combination of experiments and theoretical analysis:1.How to reduce the specific surface area of the silicon powder by certain methods with high yield of silicon,no environmental pollution,and high energy utilization,so as to make it suitable for traditional large-scale smelting process and finally obtain high purity silicon;2.How to obtain industrial silicon effectively by certain high energy smelting methods with some reducing agent added to reduce the oxidation part of the silicon powder during smelting process.It is expected to use low cost,diversified and more concise technical methods to improve the silicon yield.And the technical analysis and in-depth discussion of different efficient smelting methods is proceeded to provide an effective phased solution and theoretical basis for subsequent researches and large-scale recovery of the diamond-wire cutting silicon powder waste.The details are shown as follows:(1)A laser granulation technique for recycling diamond-wire cutting silicon powder waste was proposed.The silicon block obtained by laser granulation can be easily melted into a betterquality silicon ingot at a relatively low temperature in a traditional vacuum smelting furnace.Moreover,laser granulation can significantly improve the filling efficiency of the subsequent smelting process and the final silicon yield,and the corresponding experimental statistics can reach 65.7%and 94.7%,respectively.During the process of laser granulation,the removal efficiency of C contamination can reach 74.2%,which can finally reach 86%after 30 min of vacuum smelting.In addition,most kind of metal impurities such as Al,Na,Ca and Mg can be removed well in the laser granulation process without introducing any other impurities.And the residual impurities in the silicon block can be further removed by a general vacuum directed solidification process.The experiment revealed that the out capacity of the silicon block melted by a single laser gun was 1.91 kg/h,the energy consumption of laser melting was 2.63 kWh/kg,and the yield of the silicon block exceeded 97%at a laser power of 2000 W,a laser scanning interval of 5 mm,and a scanning speed of 6 mm/s.Therefore,laser granulation method has been proved to be the simplest and most efficient high energy density granulation technique at present.(2)A non-transfer arc granulation technique for recycling diamond-wire cutting silicon powder waste was presented.The final yield and filling rate of silicon could reach 93.9%and 73%,respectively.The formation mechanism of a hollow silicon block,removal efficiency of impurities,energy consumption and the effect of subsequent vacuum smelting were analyzed in detail to offer profound insights into a non-transfer arc granulation method.The critical conditions for continuous and efficient preparation of a solid silicon block were further confirmed through orthogonal experiments with 7.5 kW welder output power,3 mm vertical scanning height,and 5 mm/s scanning speed.The yield of the silicon block was more than 96%.Meanwhile,the granulating process demonstrated a favorable effect on removing most impurities,such as C,Al,and Na,without introducing any other impurities.The residual impurities could be further removed by vacuum directional smelting.Given the extremely low equipment cost and huge room for improvement,this work is crucial for large-scale recovery of silicon powder waste.This method has been proved to be the most possible way to achieve industrialization among high energy density granulation technologies in a short time.(3)We successfully achieved the continuous melting of silicon powder in an experimental vacuum electron beam melting equipment.The characteristic of electron beam melting of silicon powder was revealed by comparing the three different approaches to prefabricate the molten silicon pool.The application of high purity silicon block can be regarded as the most efficient method to prefabricate a stable molten silicon pool.The theoretical feasibility analysis of continuous melting of silicon powder in a prefabricated silicon pool shows that ultrafine silicon powder is mainly melted on the surface of the silicon pool,which is very beneficial to remove volatile impurities such as P.In the continuous powder adding process,the actual removal rate of P can reach up to 46.5%.Although the P removal efficiency decreases with increasing powder feeding rate during the powder feeding process,an appropriate increase in the powder feeding rate is not only beneficial to improve the melting efficiency of silicon powder but also conductive to the improvement in uniformity of the obtained silicon ingot.Therefore,the powder feeding rate of 10 g/min was the best option in this study,the corresponding removal efficiency of P was 46.5%.Compared with the melting of silicon block,which has been wildly applied,this continuous powder feeding process can save nearly 27%of the smelting time,which can be beneficial to saving smelting time and the total cost.(4)The feasibility of using diamond-wire cutting silicon powder directly in arc smelting has been proved by the experiment in an arc smelting furnace with 100 kVA apparent power.The actual operation shows that to stabilize the arc initiation,two basic conditions must be met:1.During the initial packing process,the interference of non-conductive silicon powder to the discharge arc area should be minimized;2.A liquid silicon layer should be formed between the electrode and the discharge floor as soon as possible to avoid excessive ablation of the discharge floor.To achieve continuous and stable smelting of diamond-wire cutting silicon powder,the output power of this type of arc smelting furnace should be above 39.5 kW.When the output power was stable at 39.5 kW,the actual silicon yield was greater than 92.2%,and as continuous smelting proceeding,the final silicon yield will approach to the ideal value of 97.9%.The obtained industrial silicon was 1101 grade,which means that arc smelting presents a very good prospect of large-scale industrialization. |
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