Mechanism Research On Slag Refining Reinforced By Si-Al-Ca Solvent Refining For Removing Key Impurity From Silicon

In order to solve the key issues of removing boron,phosphorus and metal impurities from metallurgical grade silicon(MG-Si)by metallurgical method,this paper adopted solvent refining technique to strengthen slag refining for removing key impurities in MG-Si,and through hydrometallurgical separation technology for recovering the refining silicon with low impurity content.Main work was carried out under the following four aspects:First,the leaching behaviors of various impurity phases in MG-Si was investigated,and the leaching kinetics of MG-Si in HF+H2O2 was studied,and the corrosive mechanism of leaching agent was further discussed.At last,the anti-corrosion measure was proposed;Secondly,the Si-Al-Ca phase-reconstruction technology was put forward for strengthening hydrometallurgy method to remove impurities in MG-Si,particularly phosphorus impurity,and the reinforcement mechanism was analyzed by calculating the thermodynamic properties of phosphorus in silicon phase and alloy phase;In addition,the Li2O-SiO2 slag system with high basicity,low melting point was used to explore an effective method for removing boron impurity in MG-Si,and the mechanism of boron removal and the dynamics model of boron migration were analyzed.Finally,for enhance phosphorus removal,a combining method with solvent refining and slag refining was conducted,and the phosphorus removal mechanism was further researched.The main results are as follows:1.The boron and phosphorous impurities in MG-Si are distributed uniformly in the silicon matrix,while the metal impurities are deposited in the interface with the form of impurity phases,and the size of the precipitations is not more than 100 ?m.Using etching technology,the impurity phases in MG-Si has different leaching behaviors in HF and HF+H2O2 etchant,but compared with HF,HF+H2O2 leaching agent could dissolve all the impurity phases in MG-Si.On the analysis of the different size of MG-Si before and after etching in HF+H2O2 found that the particle size ranged of 74-154 ?m featured an obvious cracking shrinking behaviour.The leaching process was well in accordance with cracking shrinking model,and according to the model,the leaching process was under controlled by interfacial chemical reaction.2.The HF+ H2O2 leaching agent can effectively remove the metal impurities in MG-Si,and there is also obvious corrosion of the silicon substrate,which will reduce the recovery of silicon.Based on Raman and TEM,the corrosion mechanism of the silicon matrix was studied,and the silicon was first oxidized by H2O2,and the Si-Si bond on the surface of the silicon was greatly reduced,forming an O terminal structure.Then,the Si-O bond is dissolved by the active components in etchant,forming Si-F bond and Si-H bond.Finally,based on the quantum confinement effect,a mechanism of corrosion silicon was constructed.However,at leaching process,adding glycol could effectively protect the silicon substrate from corrosion,and adding glycol did not affect the leaching efficiency of leaching agent,and upon leaching with 1 mol/L HF+ 2 mol/L H2O2+0.5 mol/L(CH2OH)2 for only 2 h at 55 ?,the purity of MG-Si with particle size of 74?154 ?m could be upgraded from 99.74%to 99.99%.3.After Si-Ca solvent refining,the main impurity phases in MG-Si changed from Si-Fe phase,Si-Ti-Fe phase into Si-Ca phase,and found a large number of phosphorus only in Si-Al-Ca phase,and all impurity phases in Si-Ca alloy could be dissolve in HCl + HF leachant.Therefore,hydrometallurgical method could not only effectively remove metal impurities,but also remove phosphorus impurity.Leaching Si-Ca alloy by HCl + HF were under chemical reaction control,which was calculated by using the cracking shrinking model.4.It was found that CaA12Si2 phase reconstruction could enhance separation of phosphorus from MG-Si.Through investigating effects of cooling rate on impurity removal efficiency by Si-Al-Ca solvent refining,it was determined that when higher than the eutectic temperature,slow cooling rate was advantageous to the impurity elements to get enough time to segregate to alloy phase;On the contrary,when lower than the eutectic temperature,the rapid cooling was conducive to obtain high purity primary silicon,because higher cooling rate inhibited the anti-diffusion of impurity elements.In this experiment,as high as 99.99%pure silicon could be achieved when quenching above 100 ? of the eutectic temperature.The activity coefficient of phosphorus in solid state silicon and liquid alloy phase were calculated as follows by phase balancing technology:In?P(s)insolidSi ?425884.6/T-294.7(1273?1473 K)In?p(l)insi-Al-camelt =417865.2/T-290.5(1273?1473 K)The phosphorus impurity had a relatively lower activity coefficient in Si-Al-Ca melt than that in solid silicon phase indicating that phosphorus impurity were rejected to the growth front.In other words,the more quantity of phosphorus was trapped in the alloy phase during the precipitation of the silicon dendrites.5.Boron removal from MG-Si by Li2O-SiO2-CaF2 slag refining under an air atmosphere was experimentally investigated,and found that the boron impurity was first oxidized by SiO2 to form B2O3,and then reacted with Li2O to generate gaseous LiBO2 volatilizing into the air or to generate solid-state Li4B2O5 phase remaining in the slag phase.The boron concentration in MG-Si was successfully reduced from 8.6 ppmw to 0.4 ppmw after slag refining for 0.5 h at 1700 ? when the mass ratio of the 60 wt.%Li2O-40 wt.%SiO2 slag to MG-Si was 3,which was quite closed to the value required by SOG-Si.6.The effect of Si-Al-Ca solvent refining on phosphorus removal of CaO-CaF2 reducing slag was investigated.Phosphorus impurity removal included two ways:First,phosphorus impurity was reduced by CaO-CaF2 reducing slag;Second,phosphorus impurity dissolved directly in the CaA12Si2 phase.After CaO-CaF2 slag refining,the main phase in SiAlCa alloy was changed from CaAl2Si2 phase to CaAl2Si2 and CaSi2 phase,but phosphorus impurity only dissolved in CaAl2Si2 phase.In addition,after combination of Si-Al-Ca solvent refining and CaO-CaF2 slag refining,increasing the amount of metal getter was beneficial to phosphorus removal.In the same way,multiple slag refining could also reduce the phosphorus impurity in the alloy.To investigate the effect of numbers of slag treatment on phosphorus removal,a consecutive slag refining experiment was performed at 1500 ?.After first run,the phosphorus concentration in refined silicon was reduced from 35 ppmw to 1 ppmw with removal efficiency of 97.1%,while after second run,the phosphorus concentration could further decrease to 0.5.Through XPS and XRD analysis,it was found that the phosphorus was reduced to Ca3P2 by reduction reaction,while Ca3P2 was easily oxidized in the air.