| Solar energy already has shortly been in great demand since it is inexhaustible and cleaner than any conventional energy resources. At present, an expensive grade silicon for semiconductor is used for a solar cell to convert solar energy into electri-city. However, the amount of supply is limited and we have to develop an innovative process, which using metallurgical grade silicon (MG-Si) as a starting material for production of solar grade silicon (SOG-Si) is believed to be one of the ways to make solar cells less expensive.According to the distribution of the impurities in metallurgical grade silicon,a new method for production of solar grade silicon at low cost from MG-Si via vacuum metallurgical technique was proposed by the research team consisted of the author, which is a promising process to feedstock solar grade silicon material independence of traditional Siemens method. In this procedure, pre-treatment of MG-Si using hydro-metallurgical route to remove metallic impurities such as iron and aluminum to reduce the production cost is an important process. One hand, it could remove mostly of metallic impurities for reduce the difficulty of follow-up vacuum melting process of this new method. On other hand, it is a process of blending impurities for being con-ductive to remove impurities in the vacuum melting process.In this paper, a novel process for pre-treatment of MG-Si on removal metallic impu-rities under hydrometallurgical route was proposed according to the distribution of the impurities in metallurgical grade silicon from Yunnan province, and the related theory was investigated.(1) The distribution of impurities in different positions in MG-Si was analyzed by SEM. The results of SEM showed that most of metallic impurities at the periphery of MG-Si ingot are mainly randomly distributed in the edge of silicon ingot as the small granular groups, and the mostly metallic impurities at the middle of MG-Si ingot mainly deposit in some narrow band of impurities or form some big granular groups. It can make more impurities have enough time to spread to the center of silicon ingot or form large particle size of impurities by the means such as increasing the thickness of silicon ingots, reducing ingot size, speeding up the casting speed when the silicon fluid is cooling. After silicon ingot crushing, most of impurities can expose and have the relation with acid. The chemical content of MG-Si from a MG-Si factory in Yun-nan province showed that the content of Fe, Al, Ca, Ti in MG-Si was 1500 ppmw, 1082 ppmw,320 ppmw,120 ppmw, respectively.(2) Aluminum in silicon cannot form new phase with the cooling of silicon fluid, but deposit in the grain boundary with metal phase or distribute in silicon ingot ran-dom as graininess.The thermodynamics of removal of iron and aluminum from MG-Si with hydrometallurgical route was analyzed. The solubility of Fe2+ in solution of acid would increase with the increasing of concentration of hydrochloric aicd. The solubi-lity of Al3+ in solution of acid would increase with the increasing of concentration of hydrochloric aicd.(3) With the scale of experimental extended constantly, the influences of many factors such as hydrochloric acid, reaction temperature and reaction time in the atmos-pheric pressure on the removal efficiencies of iron and aluminum impurities was sys-tematic studied. The optimum operating conditions for leaching in hydrochloride was acid concentration 6 mol/L at 60℃for 3days with 50μm in diameter for raw material ground and with liquid to solid ratio of 2:1. The optimum operation conditions of in-fluences on the removal efficiency of iron and aluminum impurities were up to 70% and 60%, respectively.(4) The influences of many factors such as hydrochloric acid, reaction tempe-rature, reaction pressure, liquid to solid ratio and reaction time in the pressure leach-ing on the removal efficiencies of some impurities, such as Fe, Al, Ca and Ti, was systematic studied. The optimum operating conditions for leaching in hydrochloric was acid concentration 4mol/L at 423～433K for 2 hours with 50μm in diameter for raw material ground and with liquid to solid ratio of 4:1 in the pressure of 1.5～1.6MPa. The optimum operation conditions of influences on the removal efficiency of Fe, Al, Ca were up to 80%,75% and 90%, respectively. But Ti was not well removed in the pressure leaching. The residual content of Fe, Al and Ca in silicon powder after pres- sure leaching of reproducible experiment was less than 290 ppmw,295 ppmw and 30 ppmw, respectively.(5) The kinetics of pressure leaching of MG-Si powders was studied. The kinetics equations of particle size, temperature,pressure and concentration of hydrochloric acid are found respectively. The apparent reaction activation energy of removal of iron & aluminum was Ea Fe=46.908kJ/mol, Ea Al=34.067kJ/mol, respectively. The apparent reaction order of removal of iron & aluminum with pressure leaching in hydrochloric was 0.899,0.346, respectively. The kinetics model of Fe&Al extraction from MG-Si by pressure leaching was found respectively:The models were proved by 80experiments, respectively. They indicate that the calculated consequences and experimental consequences were inosculation very well.(6) The economy analysis on the pre-treatment of MG-Si(production of upgrade metallurgical grade silicon) showed that the production cost was controlled less than 1.73×104 RMB/ton.It can get great economic benefit for production of upgrade metallurgical grade silicon using hydrometallurgical routes in industry.
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