Study On The Synergistic Mechanism And Separation Of Impurities P And Fe, Al, And Ca In Industrial Silico

Metallurgical grade silicon(MG-Si)is the main raw material in the photovoltaic,silicone and silicon alloy industries,and the impurities in it have a significant toxic effect on the downstream product production process.Among these impurities,non-metallic impurity P has a particularly notable influence,as excessive P content can impair the photoelectric conversion efficiency of photovoltaic products,the selectivity of silicone monomer synthesis reactions,and the mechanical properties of silicon alloys.However,previous research has predominantly focused on secondary refining separation methods for P while neglecting to investigate its formation mechanism within industrial silicon.By conducting systematic analysis based on the industrial silicon production process,we have obtained insights into the cooperative behavior mechanism of P with Fe,Al,and Ca during production.Furthermore,this study verifies the synergistic behavior mechanism of P with Fe,Al,and Ca through acid leaching and kinetics analysis of MG-Si,micro-analysis and acid leaching treatment of Si-Ca-Al-Fe-P,Si-Ca-P,and Si-Fe-P alloys.The main conclusions derived from this research are as follows:First of all,by analyzing primary industrial silicon,micro-silicon powder,refined industrial silicon,and refining slag,the synergistic behavior of P,Fe,Al,and Ca during the production process of industrial silicon was elucidated.During the formation of primary industrial silicon,there was a synergistic increase in the concentrations of P and Fe as well as Al and Ca.In micro-silicon powder,only the concentrations of P and Ca exhibited a synergistic increase.During oxidation refining,Al and Ca were separated from primary industrial silicon and enriched into the slag while it was difficult for Si to co-retain Fe and P in refined industrial silicon.In the solidification process of molten silicon,there was evident synergistic segregation of Al,Ca,and P towards grain boundaries whereas weak synergistic segregation occurred between Fe and P due to their weaker affinity compared to that of Ca-Al-P.The concentration rate of P with co-enrichment by Ca-Al in primary industrial silicon reached 60%,while in refined industrial silicon it was only about 15%at grain boundaries with approximately 9.09%being enriched in Si-Fe-Al-Ca phase;meanwhile,the main impurity phase FeSi2 accounted for only 4.55%.Secondly,in order to explore the synergistic behavior mechanism of Ca and P,the leaching behaviors of Si-16.39wt.%Ca alloy,Si-6.16wt.%Ca alloy,Si-5.51wt.%Ca alloy and Si-0.84wt.%Ca alloy in 2mol/L HCl and aqua regia were studied.In the HCl system,the removal rates of P in the above 4 alloys are 65.31%,75.24%,74.90%and 22.13%,respectively,which indicates that Ca can effectively strengthen the precipitation of P at the grain boundary,and the optimal addition amount of Ca is about 6.16wt.%.Then the orthogonal experiment and range analysis of the prepared Si-5.157wt.%Ca alloy were carried out systematically.The results showed that the influence of the acid leaching conditions on the P removal rate was as follows:the acid leaching system,the time of the acid leaching,the stirring rate,the temperature of the acid leaching,the particle size of the powder and the acid concentration.The optimal experimental parameters for removing P by acid leaching of Si-5.157wt.%Ca alloy are as follows:powder particle size 80-120 mesh,acid leaching system HCl,acid concentration 5mol/L,acid leaching temperature 60 ℃,acid leaching time 3h,liquid-solid ratio 4:1,stirring rate maintained at 800r/min,P removal rate up to 92.00%.Finally,the behavior mechanism of P during the solidification of Si-Fe-P alloy melt was experimentally investigated,considering that Fe is the main impurity in industrial silicon products.Successive occurrences during the solidification process included formation of Si,eutectic formation of Si with Fe3Si7(A-Fe3Si7),eutectic formation of Fe3Si7 and Si4P4Fe,and decomposition of Fe3Si7 into FeSi2 phase.The average concentration of P in different phases was found to be 1.354wt.%in Si matrix,3.249wt.%in Fe3Si7 phase with Si,11.147wt.%in Fe3Si7 phase with Si4P4Fe,and 36.570wt.%in Si4P4Fe phase.The equilibrium distribution coefficient for P between the Si phase and A-Fe3Si7 phase was determined to be 0.4167,which is higher than the separation coefficient for P in pure Si system(0.35).Overall,these findings indicate that activity interaction between components inhibits segregation of P during melt solidification of Si-Fe-P alloy.The acid leaching experiment on precipitated phases revealed that HCl is suitable for separating out the Si-Fe-P phase while HC1+HF can remove both Fe3Si7 phase and associated P content effectively;HNO3 and aqua regia are appropriate for isolating the Si4P4Fe phase from other components.Furthermore,leaching kinetics analysis was conducted on Fe and P under HNO3+HF system conditions:it was observed that chemical reaction controlled the leaching separation rate step for Fe at low temperature stages while diffusion through a solid product layer governed the overall process;meanwhile,diffusion through a solid product layer primarily controlled the leaching separation rate step for P as well.These results further confirm that segregation of P is inhibited during solidification process of Si-Fe-P alloy melt with most remaining within the Si matrix.