Research On The Preparation Of High-purity Silicon By Recycling Diamond Wire Silicon Wafer Cutting Waste

With the continuous reduction of power generation cost,the photovoltaic power generation technology based on crystalline silicon solar cells has developed rapidly in recent years,which leads to the increase of demand for crystalline silicon in the upstream and the waste emission of crystalline silicon manufacturing process is increasing year by year.About 35%of high-purity silicon is lost into the diamond wire saw silicon powder(DWSSP)waste in the production of crystalline silicon solar cells with diamond wire saw,which causes the ecological environment pollution.Due to the high content of high-purity silicon and low content of metal impurities contaminants,DWSSP has a potential for high-purity silicon remanufacturing.In order to realize the high-value utilization of DWSSP,a new idea for high-purity silicon preparation from DWSSP was proposed in this research.This research focuses on the Al,Fe,Ni metal impurities removal and surface SiO₂ layer elimination from DWSSP,the main research issues as follows:(1)The characteristic of raw DWSSP from different sources were studied by various analysis and characterization methods.The main common metal impurities Al,Fe,Ni and surface oxidation layer in the raw DWSSP were determined.It provides basic information of raw DWSSP for the process selection.(2)Combined with the working condition of diamond wire saw,the trace analysis of metal impurities was completed,and the source of metal impurities was determined.Under the optimum conditions of hydrochloric acid concentration of 4 M,leaching temperature of 333 K,leaching time of 3 h and liquid-solid ratio of 10:1,the removal efficiency of Al,Fe,Ni,Ca and Mg are 95.10%,95.93%,83.55%,90.42%,70.81%respectively,and the total removal efficiency is 93.37%.Taking the typical metal impurity Al as an example,the removal mechanism of metal impurities in hydrochloric acid leaching was studied.It was found that removal of Al is spontaneous process,and the dissolved metal impurities were located on the surface of silicon micro particles in hydrochloric acid leaching.The reaction rate was fast in the early stage of the reaction,which was in line with the homogeneous reaction kinetic model,and the later reaction rate was slow,which was in line with the Avrami model of diffusion control steps,and then the retention of metal impurities was difficult to eliminate.The mechanism of Al removal in HCl leaching process points out the need of enhancing acid leaching for the metal deep removal.(3)Combined with the storage conditions of DWSSP,the cause of retention of residual metal impurities in hydrochloric acid leaching was revealed.It is found that the surface oxidation of silicon particles under the storage conditions forms amorphous SiO₂oxide layer and grows up gradually.In this process,some metal impurities migrate to the inner of the oxide layer and stay in the oxide layer,which leads to fail removal by the conventional hydrochloric acid leaching process.In the presence of hydrofluoric acid,the corrosion of hydrofluoric acid on the oxide layer exposes the retained metal impurities,which promotes the dissolution and removal of metal impurities.Therefore,the mixed acid leaching of hydrofluoric acid and hydrochloric acid can improve the removal efficiency and silicon powder purity.The experimental results show that the optimum conditions of two-stage mixed acid leaching are hydrochloric acid concentration 2 M,hydrofluoric acid concentration 2.5M,leaching temperature 333 K,leaching time 3 h,liquid-solid ratio 10:1.The removal efficiency of metal impurities Al,Fe,Ni,Ca and Mg are 99.93%,100%,90.14%,100%and 100%,respectively.The total removal efficiency is 99.85%.This process can be used to 4-5N grade silicon powder preparation.The relationship between the removal efficiency of different metal impurities and the thickness reduction of SiO₂ layer was quantified by acid leaching experiment and linear regression analysis.The results show that the removal efficiency of metal impurities increases with the thickness of oxide layer decreasing.Moreover,the removal efficiency of Al,Fe and Ni have a significant linear correlation with the thickness of the oxide layer compared with the Ca and Mg,which proves that the metal impurities Al,Fe and Ni have a tendency of migration and enrichment into the oxide layer than Ca and Mg.Based on this,the point of view of source control impurities and comprehensive prevention of oxidation was proposed.(4)The SiO₂ layer on the surface of silicon micro particles was removed by carbothermal reduction in vacuum.The results show that the reduction temperature could be decreased in vacuum,surface SiO₂can be reduced by carbothermal reduction under the conditions of melting temperature 1753 K,vacuum degree1.1×10^-1-2.2×10^-2Pa and time of 3 h with activated carbon as reducing agent.However,the volatilization loss of gaseous reduction product SiO and the accumulation of intermediate reduction product Si C under vacuum condition will lead to lower yield of final silicon product.(5)High Al-DWSSP was treated by low temperature sintering and high temperature refining with Na₂CO₃-CaO addition,and the surface SiO₂ layer and metal impurity Al were removed simultaneously.The results show that Na₂CO₃ can digest SiO₂ and remove Al,and CaO can stabilize SiO₂ and Al removal products into slag.The experimental conditions are refining temperature 1823 K,holding time 2 h and feed composition of DWSSP:Na₂CO₃:CaO=300.0:38.4:5.1,the yield of silicon is55.5%,and the purity is 99.31%.In further study,the addition of NaCl and Na₃AlF₆can significantly improve the recovery and purity.The results show that the recovery of silicon is 76.39%and 79.25%,and the purity is 99.985%and 99.986%under the furnace feed composition of DWSSP:Na₂CO₃:CaO:NaCl=100.0:12.8:1.7:8.6 and DWSSP:Na₂CO₃:CaO:Na₃AlF₆=100.0:12.8:1.7:8.6.(6)High Fe-DWSSP was treated by CaO and eutectic NaCl-MgCl₂addition,and the surface SiO₂ layer and metal impurities Fe and Ni were removed at the same time.The results show that CaO can absorb SiO₂ to form stable slag phase,eutectic NaCl-MgCl₂ has outperformed effect for Fe and Ni removal,and can improve the recovery of silicon.The experimental conditions are refining temperature 1823 K,holding time 2 h and furnace feed composition of DWSSP:CaO:NaCl:MgCl₂=150.0:23.42:7.50:8.85,the recovery rate and purity of silicon are 98.96%and 99.83%,respectively.Based on the research on the high-purity silicon preparation from DWSSP,the mechanism,process exploration and process concept design of the hydrometallurgical remanufacture of DWSSP to preparetion 4-5N grade silicon powder are completed,it lays a theoretical foundation for the process optimization and development of high purity silicon powder prepared by acid leaching.At the same time,different process methods and refining reagent to preparation 3-4N industrial grade silicon from DWSSP with pyrometallurgical process are developed,which provides theoretical guidance and technology roadmap for the high-purity silicon remanufacture and preparation for engineering application.