| Solar energy is considered to be one of the most promising energy in the 21st century because of its abundance, environmental friendliness, safety, convenience, andother advantages. Compared with traditional chemical method, the metallurgical technology to produce solar grade polysilicon (SoG-Si) showed great potential in reducing cost and energy consumption. However, the acid-leaching has more challenge in removing impurity deeply in the traditional physical metallurgy process. This paper proposes a novel hydrometallurgical purification method for industrial silicon, which combines the metal-assisted chemical etching (MACE) and the traditional acid leaching purification techniques, aiming to achieve deep removal of impurity in industrial silicon for the production of low-cost and high-quality SoG-Si.(1) By investigating the impurities existence form in theindustrial silicon bulk, it is found that main impurity phases, occurred like island, are sandwiched in the silicon substrate with existence form of alloy phase. The metal impurities mainly contain Fe, Al, Ca, Ti, V, and small amount of Ni and Cu.(2) By comparing the morphology of porous silicon powder prepared in different methods, it is found that both of the traditional stain etching and the metal assisted chemical etching can successfully introduce porous structure in silicon powder; and the pore sizes and the distribution uniformity are increasing with the oxidant concentration in the etchant. However, metal assisted chemical etching method is more outstanding in the controllability of growth rate and morphology of porous structure. Under the suitable oxidant species (H2O2) and concentration (5 mM-15 mM) conditions, porous industrial silicon powder with pore diameter about 50-300 nm, depth about 0-60 μm can be obtained.(3) By researching the effect of various parameters on the morphology and impurity removal during the deposition process, the optimal experimental conditions can be obtained with HF concentration 4.6 M, AgNO3 concentration 10 mM and deposition time 60 s. The best impurities removal efficiency is Fe 99.51%, Al 92.90%, Ca 80.17%, Ti 89.49%, B 52.67%, and P57.70%. The impurity removal depends mainly on the impurities reaction and dissolution with HF.(4)Among the various influence factors in etching process, the etching time (0.5～ 8h) and HF concentrations (2.3～9.2M) have important influence on impurities removal, but not on porous structure. The etching solvent species (H2O～Alcohol) affect the morphology of porous structure; Ethanol addition is beneficial for reducing silicon loss during the etching process. Etching temperature (40～80℃) and H2O2 concentrations (2.5～15mM) have great influence on both porous morphology and impurity removal. The results show that the best impurity removal efficiencies are:Fe 99.7%, Al 96.83%, Ca 96.83%, Ti 94.35%, B 78.55%, and P 81.33%. By synthetically considering silicon loss and impurity removal efficiency, the optimal etching conditions are HF concentration 4.6 M, H2O2 concentration 75 mM, etching temperature 25℃, etching time 2 h, and 25% ethanol addition. The research results indicate that part of the impurities can directly react with HF species and be dissolved. Meanwhile, the dissolution of silicon bulk can prompt the exfoliation of impurities phase, which may be responsible for B and P removal during the MACE process.(5)The impurity removal rate increases with the leaching time and nitrate concentration during the silver particles removal process under nitric acid immersion. The best impurity removal efficiencies are Fe 99.61%, Al 94.1%, Ca 84.41%,Ti 88.04%, B 55%,and P 56%. The exposed impurities can further reacts with nitric acid which enhances the impurity removal. |
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