Effect of low aluminum silicon on the direct process

GE Silicones ran a commercial direct process production unit with low aluminum content silicon, and very poor reaction yields were obtained. The direct process is used to react silicon with methyl chloride to produce methylchlorosilanes, which are used to produce silicone materials. Laboratory experiments were conducted that allowed GE to generate a plausible hypothesis to explain the commercial failure. The commercial and laboratory results have been used by GE to diagnose additional direct process reaction problems. Aluminum is an important impurity found in chemical-grade silicon. Aluminum acts as one of many promoters in the direct process. Laboratory experiments indicated that the presence of aluminum is required to increase the vapor pressure of the zinc promoter at direct process reaction conditions. Increased vapor pressure allows movement of zinc within the solid reaction mass present in the fluid-bed reactor. Insufficient zinc vapor pressure causes severe problems with raw material yields and product quality that translates to uneconomic operation of the commercial direct process reaction unit.