Research On Metallurgical Purification Of Solar Energy Polysilicon

Facing the situation of traditional energy resource exhaustion and continuing oilprice rise, solar energy,as a renewable clean energy, has been the special focus of worldattention. Solar photovoltaic (PV) technology is the significant channel on putting solarenergy into electricity. In all kinds of solar battery, polysilicon is main raw material,which accounts for50%of the total cost. Therefore, reducing cost of polysiliconproduction is the key factor on developing solar photovoltaic.To optimize the photovoltaic purification process of the certain enterprise, thispaper researched two important issues. The one is that silicon can separate from slag ondiscussing the feasibility and technology of high temperature melting, which iseliminated boron from slagging at high temperature in the induction furance. The otheris that after eliminating boron element, the silicon powder was purifies byhydrometallurgy leaching, and the technological parameter of impurities removal can beacquired by using kinds of acid, alkali and group technology.The influences of smelting temperature, holding time, tapping temperature, furnaceatmosphere and lining materials on the separation effect of molten silicon and slag wereexamined. The experiments results showed that1620℃of molten temperature,2.5h ofholding time and200℃of trapping temperature can be achieved the separation onmolten silicon and slag. The design furnace of special structure is at the hightemperature to release the slag and silicon separately.Factors such as the leaching agent mass fraction, the reaction temperature, thereaction time, and the particle size of silicon powders were investigated in theexperiments to achieve the remove efficiency of the impurities including themetallurgical-grade silicon. Three pickling technologys were respectively carried out: The first is the only hydrochloric pickling; The second is alkali wash—mixed acidgroup technology; The last is aqua regia pickling.It was found that the hydrochloric acid pickling method could remove the most ofmetallic impurities in the experiments. Combining the industrial productionrequirements, The appropriate conditions were comprehensively obtained thought thesingle factor experiments and orthogonal experiments, which is that hydrochloric massfraction is5%, reaction temperature is70℃, reaction time is5h and the particle size is60mesh(120μm). The impurities removel effect are as follows: the Fe and Ca is above80%, Al is above60%, and non-metallic impurity B is above15%. Single hydrochloriccannot achieve better impurities removel effect, and the process is simple.Some experiments were studied by alkali wash mixed acid pickling. Thecondition is as follows: NaOH mass fraction is8%, HNO3is11%, the mixed acid of HFand HCl is separately5%and5%.The impurities removel effect are that the Fe and Cais about90%, Al is about60%, and B is about15%.The experiments results show thatthe alkali wash acid pickling process had best removal efficiency. However, theprocess is complex.Aqua regia had better removal efficiency by acid pickling experiments. The resultsis that Ca is especially about94%, Al is about80%,Fe is about70%, and B is about26%.But, aqua regia have the serious environment pollution problem.Acid pickling process studied can be used as a pre-treatment in order to eliminateimpurities, which is the basis for the follow-up purification of solar-grade polysiliconsuch as vacuum melting and directional solidification. For reducing the solar gradepolysilicon industrialization cost, this paper provide the raw materials and theoreticalbasis by experiments. The better pickling effect, the more simply follow-up process, thelower production cost.