High purity silane and silicon production

An improved process for the production of polycrystalline silicon from metallurgical grade silicon comprising:

• (a) reacting metallurgical silicon with hydrogen and silicon tetrachloride in a reaction zone maintained at a temperature of from about 400.degree. C. to about 600.degree. C. and at a pressure above the critical pressure of the mixture of hydrogen and silicon tetrachloride vapor and in the range of from about 300 to about 600 psi to form trichlorosilane and dichlorosilane said reaction zone comprising an iron-containing reactor susceptible to corrosion in the presence of said reaction mixture;
• (b) separating said trichlorosilane as overhead from unreacted silicon tetrachloride in a distillation zone;
• (c) recycling separated silicon tetrachloride from said distillation zone to said reaction zone for reaction with additional quantitites of metallurgical silicon and hydrogen;
• (d) subjecting said trichlorosilane to a temperature capable of causing the disproportionation thereof in a disproportionation reaction zone containing a disproportionation catalyst, said disproportionation constituting reactions resulting in the formation of silane gas and mono-, di- and trichlorosilane and silicon tetrachloride;
• (e) recycling said mono, di-, and trichlorosilane and silicon tetrachloride and unreacted trichlorosilane to said distillation zone and said disproportionation reaction zone for further separation and disproportionation therein;
• (f) passing silane formed in said disproportionation reaction zone to a silane decomposition zone maintained at a temperature within the decomposition temperature range of silane, thereby causing said silane to decompose and to form high purity polycrystalline silicon and by-product hydrogen;
• (g) separating said high purity silicon from said by-product hydrogen, whereby said reaction pressure enables an advantageous combination of desirable trichlorosilane yield and minimum undesired corrosion effects to be achieved in the reaction zone, facilitating the overall production of high purity silicon from metallurgical grade material in practical commercial operations.