Thermodynamic Study on Zinc Reduction Process for Production of Polycrystalline Silicon

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The cost of solar-grade silicon can not be reduced in Siemens process which is the major process of producing polycrystalline silicon with the current technologies. Therefore the zinc reduction process which can reduce the solar-grade silicon in a cost effective manner should be redeveloped under the circumstances. The thermodynamics in zinc reduction process is studied in this paper. When the reactions reach equilibrium, the equilibrium gas phase compositions have been analyzed with temperature range of 1184 K to 1500 K at nSiCl4/nZn of 1/2, 1/4, 1/8 and P = 1 atm, 3 atm, 6 atm. Furthermore, the relations of productive ratio of silicon and pressure, temperature and the feeding molar ratio (nZn/nSiCl4) are investigated and the diagrams are plotted. The major side-reactions which affect the productive ratio of silicon have been decided as: SiCl4(g) + Zn(g) → SiCl2(g) + ZnCl2(g) and SiCl4(g) + Si(s) → SiCl2(g). Finally the suitable operation conditions in the real poly-silicon manufacture process have been decided. 1200 K, 2 atm and the feeding molar ratio (nZn/nSiCl4) of 4 in the entrance is the best condition. The productive ratio of silicon is 90.3% under the condition. Furthermore, 1184 K, 2 atm and the molar ratio (nZn/nSiCl4) of 4 is ideal condition. Under the condition, the theoretical yield is 98%.
2011-07-01