Study On Removal Of Nonmetallic Impurity Boron And Phosphorus In Metallurgical Grade Silicon Based On Si-Cu Solvent Refining

Metallurgical production of solar grade silicon(SoG-Si)has been extensively studied due to its low cost,low investment,and high productivity.The typical metallurgical route is designed based on the physical characteristics of metallurgical grade silicon(MG-Si).Metallic impurities such as iron(Fe),aluminum(Al),calcium(Ca)are concentrated in the Si grain boundary,which can be easily removed by acid leaching or directional solidification.While nonmetallic impurities boron(B)and phosphorus(P)are distributed in the Si matrix attributed to their large segregation coefficients.Although slag treatment and electron beam melting are applied to remove B and P,high energy consumption and low production efficiency have blocked the sustainable development of metallurgical methods.Solvent refining has proven to be effective to redistribute impurities in MG-Si by altering the segregation behavior of impurities.The combination of solvent refining and traditional metallurgical methods offers an alternative perspective on the purification of MG-Si.In this thesis,Si-Cu solvent refining is combined with slag treatment,acid leaching,and impurity getters to enhance the removal efficiency of B and P from MG-Si.The main results are summarized as follows.(1)A combination of Si-Cu solvent refining and CaO-SiO2-CaCl2 slag treatment is used to the purification of MG-Si.The target of this method is to collect impurities B and P in the Si-Cu alloy phase,thus facilitating the removal of B and P by CaO-SiO2-CaCl2 slag treatment.The high-purity Si can be obtained after eliminating Si-Cu alloy phase via selective acid leaching.Influences of slag composition(including CaO/SiO2 mass ratio and CaCl2 content)and Cu content on the removal of B and P were studied.The optimal results were obtained when a Si-50wt%Cu alloy refined with 45wt%CaO-45wt%SiO2-10wt%CaCl2 slag for 30 min at 1550 ? under air atmosphere.Compared to MG-Si,the removal efficiency of B and P from Si-50wt%Cu alloy was increased from 72 to 89%and 33 to 58%,respectively.The reaction kinetics indicates that the removal of B and P from Si-Cu alloy during slag treatment process follows a first-order rate law,and were dominated by mass transfer in the slag phase with overall mass transfer coefficients of 6.25×10-4cm/s and 2.55×10-4 cm/s,respectively.(2)A purification method using Si-Cu solvent refining and selective acid leaching is studied to upgrade MG-Si.Impurities B and P were concentrated in the formed Si-Cu alloy phase in the Si-50wt%Cu alloy,providing a favorable condition for selective acid leaching.Selective acid leaching process is designed to optimize the separation of Si-Cu alloy phase from Si grains,and then enhances the extraction of B and P from MG-Si.The acid sensitivity of the Si-Cu alloy phase was tested by leaching with aqua regia,HNO3,and HCl,respectively.Additionally,the optimal parameters were explored.The results show that aqua regia can separate Si-Cu alloy phase from Si grains by forming CuCl and CuCl2 and reach highest removal efficiencies of B and P.Consequently,a three-step sequential acid leaching process was applied.Firstly,2 M aqua regia is used to separate Si-Cu alloy phase;Then:a mixed acid(2 M HNO3 + trace HF)is used to eliminate residual Cu3Si;Finally,1 M HNO3 is used to liberate gases and impurities from the surface of the particle.Furthermore,the effect of particle size and aqua regia leaching parameters(including leaching time,leaching temperature,and leaching environment)on the extraction of B and P from Si-Cu alloy were investigated.The best removal efficiency was obtained when Si-Cu powders with a size range 74-106 ?m leached at 70 ? for 5 h via a three-step sequential acid leaching procedure under magnetic stirring.Compared with MG-Si,the extraction yield of B and P from Si-50wt%Cu alloy was increased from 21 to 59%and 23 to 42%,respectively.Based on the cracking shrinking model,the extraction of B and P from Si-50wt%Cu alloy using aqua regia is found to be joint-controlled by interfacial transfer and product layer diffusion.(3)Addition of impurity getters titanium(Ti)and calcium(Ca)to Si-Cu alloy system is investigated to improve the removal efficiency of B and P,respectively.Selected impurity getter is melt with Si-50wt%Cu alloy to form the stable compound,which would be precipitated in the Si-Cu alloy phase and can be eliminated by the following selective acid leaching process.Effects of impurity getters content on the impurity distribution and phase reconstruction of Si-Cu alloy are investigated.Addition of 1-5%Ti led to the formation of secondary phases including TiB2 and TiSi2 in the Si-Cu alloy phase.Following selective acid leaching,B removal efficiency was increased from 23 to 85%reached when 5%Ti was added to a Si-50wt%Cu alloy.Additionally,TiSi2 appeared to be attractive to impurities P and Al.Addition of 1-5%Ca resulted in the phase reconstruction of Si-Cu alloy by forming CaCu2Si2 and CaCu11Sis.P was found to be collected by CaCu2Si2 phase,and its extraction was enhanced with increasing Ca content in Si-50 wt%Cu alloy.The removal fraction of P was increased from 27 to 82%when 5%Ca is introduced.The corresponding mechanism was investigated,confirming that the addition of impurity getter(Ti or Ca)could facilitate the formation of B/P-containing phases(TiB2 or P-CaCu2Si2)and decrease the segregation coefficient of B/P between solid Si and the Si-Cu melt.It is concluded that Ti and Ca can be used as impurity trappers to facilitate the B and P removal in the Si-Cu alloy.