Study On Impurity Removal Effect And Mechanism Of Adding Sodium Chloride In Producing Industrial Silicon By Metallurgical Method

At present,due to the increasing depletion of fossil fuels,provoking the use of solar energy to replace the traditional fossil energy sources of the task is imminent.Compared with the traditional technology such as the SIEMENS method and the silane method,the method of the metallurgical method has the advantages of low cost,environment friendliness,low energy consumption and so on,which is more suitable for the development of our country.Metallurgical method for preparing industrial silicon can be divided into traditional metallurgical method and new metallurgical method.In traditional metallurgical method,smelting comes first,and then a series of procedures for the production of industrial silicon,impurity removal will be conducted.In contrast,when it comes to new metallurgy method,the raw materials will be acid-leached to remove impurities before smelting.So it shortens the subsequent cleaning process,but the consequent wastewater treatment brings great pressure on the environment.Thus exploring a route which contains short process and environmentally friendly is particularly important.In industrial preparation of green silicon carbide,NaCl will be added in the raw materials to gain high qualified SiC products.Inspired by this method,if the process can be applied in metallurgical process,it can shorten the process flow,reduce the cost and energy consumption,and also be more environmentally friendly.This paper mainly studies the impurity removing effect and impurity removal mechanism of adding NaCl as additive in the process of preparing industrial silicon by metallurgical method.The main contents are as follows:smelting exploration,thermodynamic calculation of silicon refining reaction system furnace structure analysis,XRD and XRF analysis of furnace products and mechanism of the reaction between NaCl and impurity.The main conclusions of this paper are as follows:(1)Through smelting pre-experiment,the basic process of smelting is determined,furnace conditions and related operations are recorded,including how to pound the furnace,when feeding,etc.,and also the furnace dissection method.(2)Quartz sand and petroleum coke,quartz sand and petroleum coke as well as NaCl,quartz sand and carbonized rice husk,quartz sand and carbonized rice husk and NaCl as the raw materials were smelted,and the smelting phenomena were recorded and analyzed.The use of petroleum coke as carbon reductant caused the worst furnace conditions,accompanied by a large number of white smoke.After adding NaCl reagent furnace condition improved significantly with less white smoke,and raw material crust layer became soft,and it was conducive to pound furnace,and also feeding became faster.When using carbonized rice husk as carbon reductant,furnace condition was better,with a small amount of white smoke.After adding NaCl,the raw material crust layer was the most soft,and feeding was the fastest,with almost no white smoke.(3)Using FactSage software,the starting temperature of each reaction in the reaction system of silicon smelting furnace was calculated,which provided the basis for the following up theoretical analysis,and the calculated results could be used to exclude some reactions that could not happen in reality.(4)Through furnace dissection,furnace structure from top to bottom are divided into:top bulk layer,the top crust,vitreous body,wall SiC layer,bottom SiC layer,and silicon producing areas.XRF and XRD detections were performed on each sample.The XRF results show that the purity of silicon is 92.54%when petroleum coke is used as carbon reductant and the purity of silicon is 96.46%after adding NaCl.At the same time,the impurity in furnace is enriched upwards.The purity of silicon was 94.96%when carbonized rice hull was used as carbon reductant,and the purity of silicon was 97.88%when NaCl was added,but the impurity of both was up-enriched.Indicating that NaCl promotes the overall shift of impurities in the furnace.The results of XRD and thermodynamic analysis showed that the addition of NaCl had little effect on the phase distribution of the furnace.The difference of impurity in the silicon surface and the surface of the silicon surface was analyzed by SEM,and the former has less impuities.(5)The binary phase diagrams of NaCl and Fe2O3 were calculated using FactSage software and a high temperature experimental scheme was designed.It was confirmed that NaCl did not react with Fe2O3 below 1200℃.And Fe2O3 and SiO2 were mixed with NaCl at 1:1,2:1 respectively,then heated at 2000℃ for 5 min,10 min and 15 min respectively.At this point,all the rapid evaporation of NaCl,Fe2O3 in the CO atmosphere was reduced to iron,and the formation of SiO2 became porous structure,thus excluding the possibility of purification through the reaction between NaCl and impurities.The results show that NaCl can promote the enrichment of impurity in the furnace under high temperature by increasing the gas permeability of the charge,so as to achieve the purification effect.