Process for the production of ultrahigh purity silane with recycle from separation columns

Claim 1

In the process for the production of silane from metallurgical grade silicon which process includes the steps of (i) reacting metallurgical grade silicon with hydrogen and silicon tetrachloride at elevated temperature and pressure in a hyddrogenation reaction zone to form trichlorosilane and dichlorosilane; (ii) separating tri- and dichlorosilane as an overhead stream and unreacted silicon tetrachloride as a bottom stream in a first distillation zone; (iii) separating said overhead stream of tri- and dichlorosilane in a second distillation zone to form a dichlorosilane-rich overhead stream and a trichlorosilane-rich bottom stream; (iv) recycling said unreacted silicon tetrachloride bottom stream to said hydrogenation reaction zone; (v) subjecting said dichlorosilane-rich overhead of (iii) to temperature and pressure conditions capable of causing the disproportionation thereof in a first disproportionation reaction zone containing an ion exchange resin capable of catalyzing said disproportionation reaction, thereby forming a mixture of silane and chlorosilanes; (vi) subjecting said trichlorosilane-rich bottom stream of (iii) to temperature and pressure conditions capable of causing the disproportionation thereof in a second disproportionation reaction zone containing an ion exchange resin capable of catalyzing said disproportionation reaction, thereby forming a mixture of chlorosilanes and silicon tetrachloride; (vii) recycling said mixtures of chlorosilanes and silicon tetrachloride of (vi) to said first distillation zone; (viii) separating said mixture of silane and chlorosilanes of (v) in a third distillation zone to form a product silane overhead stream and a chlorosilane-rich bottom stream; (ix) recycling said chlorosilane-rich bottom stream from said third distillation zone to said second distillation zone; and (x) recovering silane product from said third distillation zone, the improvement which comprises bleeding a portion of the trichlorosilane-rich bottom stream of (iii) said bleed portion containing one or more of BCl 3 , PCl 3 and AsCl 3 impurities and adding said portion to the unreacted silicon tetrachloride bottom recycle stream of (iv) and bleeding a portion of the chlorosilane-rich bottom stream of (viii) said bleed portion containing one or more of B 2 H 6 , PH 3 and AsH 3 impurities and adding said portion to the unreacted silicon tetrachloride bottom recycle stream of (iv), wherein the respective bleed portions of the trichlorosilane-rich bottom stream of (iii) and chlorosilane-rich bottom stream of (Viii) are 0.01 to 0.1 percent of their respective bottom streams.