Research On Preparation Of High Purity Silicon By New Type Electrothermal Metallurgy Process

With exhaustion of traditional energy sources,the requirement of new type renewable energy is gradually increasing.Solar energy is one of the new type clean renewable energy and is deemed to be one way to solve global energy crisis.The solar energy cells convert solar energy into direct current electricity via the photovoltaic effect.And 85%solar energy cells are silicon-based solar energy cells.The producing process of solar grade silicon is divided into the chemical(modified Siemens process,silane method and silane fluidized bed method)and metallurgical routes.Metallurgical grade silicon(MG-Si)is the main raw material to produce solar grade silicon(SoG-Si)for photovoltaic cells.Improving the quality of the MG-Si can reduce the cost of subsequent purification steps and energy consumption.New type electrothermal metallurgy method is reducing impurity content of raw material for preparing metallurgical grade silicon.This paper prepared metallurgical grade silicon with high purity raw materials by electrothermal metallurgy process.The main contents of this paper include:acid purification of raw material,analyzing thermodynamics and mechanism of preparation silicon in electric arc furnace,researching the sample from different parts of crucible.And the conclusions are as following:(1)The optimal conditions of acid leaching of the petroleum coke with hydrochloric acid under ultrasound-assisted were obtained as follows:hydrochloric acid 12%,reaction time 35 min,temperature 50 ?,the ratio of leaching liquid to solid 10 mL/g,ultrasound power 240 W and frequency 80 kHz.The iron removal rate reached 90.4%in this conditions.(2)The optimal conditions of acid leaching of the silica with mechanical stirring were obtained as follows:hydrochloric acid 12%,temperature 40 ?,stirring speed 200 r/min,particle size less than 75 ?m.The iron removal rate reached 86.7%in this conditions.The optimal conditions of acid leaching of the silica with ultrasound-assisted mechanical stirring were obtained as follows:hydrochloric acid 12%,temperature 40 ? stirring speed 400 r/min,particle size less than 75 ?m,ultrasound power 270 W and frequency 80 kHz.The iron removal rate reached 98.7%in this conditions.And the kinetic analysis showed that the control step of steep was film diffusion control;and the control steps of mechanical stirring,ultrasound and ultrasound-assisted mechanical stirring were product layer diffusion control.(3)The optimal conditions of acid leaching of the rice husk ash with ultrasound-assisted mechanical stirring were obtained as follows:stirring speed 300 r/min,temperature 40 ?,hydrochloric acid 14%,particle size 75-106 ?m,ultrasound power 270 W and frequency 45 kHz.The phosphorus removal rate reached 90.1%in this conditions.The kinetic data fitted a new variant of the shrinking core model,in which both the interfacial transfer and diffusion through the product layer affected reaction rate.(4)Through summing up the reaction equation in electric arc furnace,all the reaction equations can be divided into three types:the formation of SiC,the formation of Si and other chemical reaction equations.The optimum conditions of preparing pellets with petroleum coke and silica were obtained as follows:holding time 90 s,pressing pressure 25 MPa,agglomerant content 2.5%.At the optimum conditions,the compression strength is 10.2 MPa and porosity is 27.6%.And the crucible can be divided into four parts:loose granular zone,skull zone,cavity zone(including crucible wall)and product zone.Phase change of quartz and graphitization of petroleum coke happened in the loose granular zone.In the skull zone,a great number of reactions happened including the formation of Si and SiC.The cavity should be full of the SiO and CO.The closer to the bottom the material is,the more Si it has.The product zone is filled with polycrystalline silicon.And the silicon melt reacted with CO to produce SiC and SiO during the crucible cooling.The product Si meets Si 1101 of GB/T 2881-2008 and A grade of ASTM A 922-05 demand.(5)The optimum conditions of preparing pellets with rice husk ash and silica were obtained as follows:holding time 120 s,pressing pressure 30 MPa,agglomerant content 3.5%.At the optimum conditions,the compression strength is 20.7 MPa and porosity is 39.7%.The crucible also can be divided into four parts:loose granular zone,skull zone,cavity zone(including cavity wall)and product zone.The residue was formed on cavity wall near crucible bottom,and the constituents of residue were calcium silicate and aluminium silicate.The product Si meets Si 4110 of GB/T 2881-2008 and A grade of ASTM A 922-05 demand.In this thesis,the high purity silicon has been obtained by new type electrothermal metallurgy process.This technology will lay a foundation for the preparation of solar grade polysilicon in next stage.