Research On The Characteristics Of Impurities In Metallurgical Silicon And Its Efficient Wet Purificatio

With the development of Photovoltaic industry,the demand of Solar-grade silicon(SOG-Si)as the main material increases sharply.At present,the SOG-Si-producing technolgoies gave priority to a modified Siemens process,a fluidized bed reactor process,etc.However,their main disadvantages are high production cost and energy consumption,which hinders the development and use of silicon solar cells.In this context,metallurgical process for producing SOG-Si from metallurgical-grade silicon(MG-Si)has attracted substantial interests owing to environmentally friendly,low cost and energy consumption.Hydrometallurgy purification,the pretreatment process in metallurgical method,is preliminary purification of MG-Si by acid leaching and has many advantages as follows:the prominent effect on removing impurity,simple equipment and easy scale production,etc.Nevertheless,it is difficult to upgrade the purity of Si to 4N and reduce the content of typical impurities to an ideal level due to the limitation of current acid leaching technology.In this paper,according to the physical-occurrence properties of the impurities in MG-Si,the theoretical basis of removing impurities by acid leaching,the strengthen effect of addtives and mixed acid in compound leaching system were studied.A high quality and efficiency combined leaching process of general applicability apply was established.In addition,the removal effect and mechanism of B,P by combining the reconstruction of impurity phase with acid leaching were investigated.(1)Metal impurities in MG-Si are segregated in the grain boundary by the formation of alloy phases which depend on the type and content of elements.In High-Fe and high-Al type MG-Si(Fe/Al≈1),Al was a Si-Al-Fe phase form,Fe was predominantly distribute in Si-Al-Fe phase,Si-Fe phase and Si-Fe-Ti phase;In High-Fe and low-Al type MG-Si(Fe/Al>>1),Al was distribute dispersedly in the impurity phases,not the form of main phase,Fe was mostly distribute in Si-Fe phase.B and P were mainly the form of solid solution in Si and partly binary/multiple compounds with Si,metal impurities.When MG-Si was crashed and classified,metallic impurities and B segregated at grain boundaries enriched into finer particle,but the distribution of P in different particles was invariable.(2)Si-Fe,Si-Fe-Ti and Si-Al-Fe phase were corroded by hydrofluoric acid while only Si-Al-Fe phase was removed at different degrees by other acids.Due to the difference of corrosion-resisting properties of the impurity phases,the impurity removal efficiency of various acids accorded with their corrosion strength to impurity phases,and was in the order of HF>aqua regia>HCl>H2SO4>HNO3.The influence law of leaching time,temperature,Si particle sizes to the removing impurities effect of HCl acid,aqua regia,HF acid was not completely consistent.HCl acid was greatly influenced by these three factors,however,the influence of particle sizes on aqua regia and time on HF acid were not obvious.The type of MG-Si had a greater influence on the removing impurities effect of acids,for high-Fe and high-Al type MG-Si,all of HCl acid,aqua regia and HF acid had significant leaching effect.The removal efficiency of Fe was 53%,80.3%,91.9%respectively;for high-Fe and low-Al type MG-Si.Only hydrofluoric acid removing impurity effect was observable,the removal of Fe was 93.4%,but only 7.3%,18.8%for HCl acid and aqua regia respectively.(3)There was a strengthen effect on the removal of metal impurities with the adding of SDS and ferric chloride to the hydrochloric acid system.Because of its electrochemical corrosion to impurity phase,the effect of ferric chloride was better,and the extraction of Fe,Al,Ca and Ti improved to 81.2%,79.2%,81.5%and 61.2%respectively.However,the strengthen effect of addtives had certain limitation,it applied to only high-Fe and high-Al type MG-Si.The aqueous solution of HF and HCl acid was not restricted by metallurgical silicon type,for high-Fe and high-Al type MG-Si,the removal of Mg was 82.6%,the extraction of other metallic impurities were more than 90%;for high-Fe and low-Al type MG-Si,the removal efficiency of Mg,Cr,Cu were over 80%,the extraction of Fe,Al,Ti,Mn,Ni,V were beyond 90%.In addition,there was a phenomenon of "black silicon" after MG-Si was leached by the aqueous solution of HF and HCl acid.(4)The optimum leaching conditions of HF+HCl were obtained after a large number of experiments:the concentration of HF,HCl was 3-4wt%,13-14wt%,and time,temperature,Si particle sizes was 6h,70℃,74-150μm respectively.Combining with the characteristics of removing impurity and effects of different acids,combined leaching process including the aqueous solution of HF and HCl acid,aqua regia,HF acid was established.After using the acid leaching process,the removal of total impurities in HN-Si,YN-Si,FJ-Si and NX-Si from different producing areas was 92.5%,98.2%,97.1%,96.8%,the purity of Si was up to 99.98%,99.993%,99.992%,99.98%,the content of Fe was dropped down to 29ppm,24ppm,30ppm,52ppm respectively.(5)Combining the remelting of MG-Si and Ca/Ti with acid leaching,the content of B reduced effectively,and the removal of P was up to 80%.After solvent refining with Sn-Si and Al-Si,the content of B in MG-Si dropped to 3.12ppm,1.47ppm from 10.3ppm,the content of P was ruduced to 22.9ppm,7.9ppm from 118ppm.