| With unique properties such as electrical insulation,high and low temperature resistance,water resistance and anti-adhesive properties,silicones are widely used in electrical and electronic,aerospace,medical,chemical and petroleum industries,and are essential and important materials in the fields of science and technology,industrial production and national defence construction.At present,the silicone industry mainly focuses on the production of silicone monomers,of which dimethyl dichlorosilane is the most important silicone monomer.Reaction selectivity and reactivity are considered to be the main indicators for assessing the production process of dimethyldichlorosilane.The type and content of trace elements in silicon metal powders,which are used as raw materials and also as catalyst carriers,is one of the key factors in establishing the optimum dynamic equilibrium conditions for the synthesis of dimethyldichlorosilane.High levels of lead and bismuth in silica metal powders can inhibit the reactivity and selectivity of the dimethyldichlorosilane synthesis process and can lead to catalyst poisoning.Therefore,in order to improve the efficiency and quality of dimethyldichlorosilane monomers,it is necessary to remove lead and bismuth impurities from silicon metal powders.Wet metallurgical processes are favoured by researchers for their relatively low energy consumption and low production costs,as well as their simplicity and ease of production on a large scale.It would be beneficial to the silicone industry if lead and bismuth impurities could be completely removed from metallurgical grade silicon using a wet metallurgical process.Therefore,in this paper,the leaching behaviour of lead and bismuth impurities in three commonly used acids,hydrochloric acid and aqua regia hydrofluoric acid,was firstly studied to obtain the order of sensitivity of lead and bismuth impurities to other impurity phases in the three acids.The effects of acid concentration,acid leaching time and acid leaching temperature on hydrochloric acid and aqua regia hydrofluoric acid were discussed respectively.It was found that there were differences in the patterns of influence of acid concentration,leaching time and leaching temperature on the effectiveness of hydrochloric acid,aqua regia and hydrofluoric acid for the removal of lead and bismuth impurities.In the hydrochloric acid leaching system,the influence of leaching time and temperature on the removal of lead and bismuth impurities was greater,and the increase of both acid leaching time and temperature brought about a relatively large increase in the removal ratio;In the aqua regia leaching system,the effect of leaching temperature on the removal effect of lead is more obvious,and the removal effect of bismuth is not obvious.In the hydrofluoric acid system,the effect of temperature on the removal of lead and bismuth impurities was not significant,but the effect of time was more prominent,however,with the extension of time,the removal ratio gradually became slower.The optimum acid leaching conditions for the removal of lead and bismuth impurities by hydrochloric acid,aqua regia and hydrofluoric acid were:acid concentration of 6 mol/L,acid leaching time of 6 h and acid leaching temperature of 80℃.The removal ratios of lead and bismuth were 84.76%,88.85%and 57.73%,respectively,and 85.98%,89.42%and 42.08%,respectively,under these conditions.The enrichment pattern of lead and bismuth impurities in silicon was studied by remelting silicon,and it was found that very little lead and bismuth appeared in the Ca Si2 impurity phase.The natural logarithms of the activity coefficients of lead and bismuth impurities in the Si-5%Ca-0.1%Pb,Si-5%Ca-0.1%Bi,Si-0.1%Pb,and Si-0.1%Bi systems were calculated according to the molecular mutual volume model as:The partition coefficients of lead impurities in the Si and Si-Ca systems were2.6971×10-8 and 3.3975×10-10,respectively;the partition coefficients of bismuth impurities in the Si and Si-Ca systems were 3.3975×10-10 and 2.4910×10-12,respectively.The results demonstratio that the Pb and Bismuth impurities are more likely to be partitioned in the Ca Si2 impurity phase than the Si matrix.The Ca-Si system was constructed to be used to enhance the removal of lead and bismuth impurities.The alloy composition,parting stage,solidification ratio during the refining process of Ca-Si melt and the subsequent wet process in terms of acid concentration,leaching temperature and leaching time had important effects on the removal of lead and bismuth impurities.Therefore,the removal patterns of lead and bismuth impurities under different calcium additions,different cooling stages,different cooling ratios and different acid leaching conditions were investigated.It was found that under the hydrochloric acid system,the removal ratio of lead and bismuth impurities after refining with Si-5%Ca melt was higher than that of direct acid leaching of silicon metal.The highest removal ratios of lead and bismuth were 94.11%and 96.56%respectively in the Si-5%Ca system at 6 mol/L,6 h and 80°C.The highest removal ratios of Pb and Bismuth were enhanced by 9.35%and 10.58%.The effect and mechanism of cooling ratio on the partitioning behaviour of Pb and Bb impurities in different solidification stages of the Si-5%Ca system were determined:at the initial stage of solidification（1500-1024°C）,the removal ratios of Pb and Bb impurities were inversely related to the cooling ratio,with the highest removal ratios of 88.92%and90.55%,respectively;at the end of solidification（1024°C-room temperature）,the removal ratios of Pb and Bb impurities were positively related to the cooling The highest removal ratios were 87.54%and 88.21%respectively. |
