Research On Recycling And Purifying Silicon By Oxidative Refining Of Crystalline Silicon Cutting Waste And Vacuum Directional Solidificatio

The substantial increase in the amount of mono and polysilicon needed is due to the rapid growth of the photovoltaic industry.The amount of waste containing Siproduced in the process of manufacturing PV wafers is increasing every year.Diamond wire sawing silicon powder（abbreviated as DWSSP）is one of the waste-containing Sigenerated in the production process of photovoltaic companies.DWSSP’s production in China in 2022 was about 210,000 tons.The main component of DWSSP is the monolithic silicon that is ground down during the wafer cutting process.Other types of impurities Fe,Al,Ca,Ti,Ni,B,P,C,and O are introduced during the cutting process.Recycle and purify the valuable silicon in DWSSP to obtain regenerated silicon,which can be used as a raw material to prepare monocrystalline silicon.Compared to the conventional metallurgical purification MG-Sifor the production of monocrystalline silicon,the purification process has great economic benefits and is of great social significance for the reduction of pollution and carbon emissions in the photovoltaic industry in our research.The research object of the paper is DWSSP and is generated in the process of cutting wafers from monocrystalline silicon.Determination of the type,form and distribution of impurity elements in the DWSSP.The selection of a suitable method to remove impurities from DWSSP by comparing the differences in physical and chemical properties between impurities and Si.Selection of a suitable method for removing impurities from DWSSP is based on the differences in physical and chemical properties of impurities and Si.A process route for recovery and purification of silicon from DWSSP using the oxidative refining-vacuum directional solidification method is proposed.By conducting theoretical research and practical experimentation,we successfully produced silicon with a 99.999%purity level.The research work carried out in this thesis is shown below:（1）Understanding of the entire process of slicing monocrystalline silicon in order to grasp the use of materials and updates in each step of the silicon cutting process,the control and optimization measures are proposed for the introduction of impurities in DWSSP.The introduction of impurities in DWSSP from the source is reduced by these measures.Based on the analysis of raw materials to comprehend the differences in the physical and chemical properties between impurities and silicon,a research idea to remove impurities from DWSSP using oxidative refining-vacuum directional solidification is targeted.The impurities in the DWSSP were removed in steps.（2）The removal of the oxygen-friendly impurities Ca,Al,B,and the oxide layer on the surface of silicon particles in DWSSP is investigated using an oxidative refining method.The research indicates that the addition of alkaline components such as CaO,CaF₂,Na₂CO₃,and Na₃Al F₆ in the slagging agents,the impurities from DWSSP can be elimated and oxidized layer on the surface of silicon particles can be effectively dissolved.When the slagging agent chosen is CaO-SiO₂-Na₂CO₃-CaF₂,the slag-silicon ratio is 0.6,and the melting time is 1h in the experiment,the removal of impurities Al,Ca,P,and B were 84.4%,54.3%,58.9%,and 52.6%,respectively.The removal rate of oxygen in DWSSP at atmospheric pressure was 99.8%after three induction melts and the residual oxygen content in silicon was 570ppmw.It has been found that SiO_X in the oxide layer on the surface of DWSSP is reduced to gaseous SiO.The gaseous SiO reacts with the inner wall of the graphite crucible to form a white adherent composed of Si,SiC and SiO₂.Since gaseous SiO has vaporized,the formation of white adherents builds up at the crucible mouth and forms a sealed slag layer.The inability to volatilize SiO gas increases the pressure in the crucible and affects the migration behavior of impurities in the silicon melt.In the experiment of blowing refining of DWSSP,the residual amount of P in the regenerated silicon ingot obtained by blowing 50 m L/min O₂（g）+50 m L/min H₂O（g）to refine the DWSSP was at least 52.7 ppmw,the removal efficiency of impurity P was63.9%.At the same time,the residual amount of B in the regenerated silicon ingot was at least9.8 ppmw,the removal efficiency of impurity B was 83.3%.（3）Vacuum directional solidification was utilized to investigate the removal of impurities such as volatile Ca,Al,and P as well as Fe,Ni,and Ti which have small partition coefficients from DWSSP.The efficiency of impurity removal in directional solidification experiments depends on the effective separation coefficient of impurity elements in DWSSP and the temperature gradient at the solid-liquid solidification front.In this chapter,we conduct vacuum directional solidification experiments are performed on silicon ingots obtained attained from the induction melting of DWSSP.The effects of pull-down speed,holding time before directional solidification,and crucible size on the influence of impurity removal were investigated.It was shown that the variation of the pull-down velocity in the range of 3-15μm/s has a small effect on the effective separation coefficient.The total amount of impurities in the bottom silicon ingot was less than 50ppmw at a pull-down speed of 3μm/s in the directional solidification experiments.Changes in melting time with a range of 0-2h before directional solidification and crucible size during refining do not have a significant effect on the effectiveness of impurity removal.The study of vacuum directional solidification found that high vacuum is favorable for volatilization removal of impurities from melted DWSSP.With the experimental conditions of melting temperature of 1550℃and pressure of 5×10^-3Pa inside the furnace,the directional solidification was repeated twice at a pull-down speed of 1μm/s to purify the silicon ingots,and the residual amount of Fe and Ni in the regenerated silicon obtained from the experiments was 3ppmw and 2ppmw,respectively.The purity of the regenerated silicon ingots was 99.999%and the residual amount of impurity oxygen in the ingots was 12.3ppmw,the recovery rate of silicon is 56.4%.（4）The impurity phase in DWSSP is characterized by complex composition and low content.It is difficult to grasp the mechanism of DWSSP melting and the change of impurity phase during the melting process.In this paper,the Equilib module of Fact Sage software was used to calculate the composition and variation of impurity phases in DWSSP during the melting process.Equilibrium calculations of the composition and change of phases during the melting process were performed to infer the composition in the silicon melt at high temperatures.The phase transition and related reactions of DWSSP at different temperature intervals during the melting process are clarified.and analyzed in comparison with the experimentally obtained silica and slag phases,and the results verify the reliability of the simulated equilibrium calculations.The results of the equilibrium calculation of the silica slag phase formation were compared with silica and slag phases obtained by experiments,the results obtained from the analysis verified the reliability of the simulated equilibrium calculations.The main research object of this paper is the DWSSP.A combined oxidative refining-vacuum directional solidification process for recovery and purification of silicon from DWSSP is proposed.The best process conditions for oxidative refining of DWSSP were optimized to prepare regenerated silicon with a purity of 4N.Mechanism of removing the oxide layer on the surface of DWSSP particles by induction melting established.Regenerated silicon with a purity higher than 5N and a residual O content of 12.3ppmw was prepared under high vacuum and low pulling speed conditions.