1. Field of the Invention
The invention relates to a process for the direct synthesis of chlorosilanes and methylchlorosilanes in a fluidized bed, in which process silicon-containing dust formed is introduced in the form of a suspension in a liquid into the fluidized bed.
2. Background Art
In the Mxc3xcller-Rochow direct synthesis, chloromethane is reacted with silicon in the presence of a copper catalyst and suitable promoters to produce methylchlorosilanes. In this process, very high selectivity to the target product dimethyldichlorosilane is necessary. Dimethyldichlorosilane is required, for example, for the preparation of linear polysiloxanes.
In the direct synthesis of chlorosilanes, silicon is reacted with hydrogen chloride, in the presence or absence of a copper catalyst, to form trichlorosilane and tetrachlorosilane. Chlorosilanes are required, for example, for the preparation of pyrogenic silica. Pyrogenic silicas can also be prepared from methylchlorosilanes and mixtures of methylchlorosilanes and chlorosilanes without the product quality being impaired.
In both processes, not only a very high productivity, measured as the amount of silanes formed per unit time and per reaction volume, but also a very high silicon conversion combined with reliable and flexible operation of the overall plant are required.
Both processes can be carried out batchwise, continuously or semicontinuously. In industrial production, they are preferably carried out continuously.
The continuous direct synthesis is carried out in fluidized-bed reactors in which chloromethane or hydrogen chloride and gaseous reaction products act as the fluidizing medium. The silicon required is milled to a powder having a particular particle size before the synthesis.
In the methylchlorosilane synthesis, the silicon powder is mixed with copper catalysts and promoters to form a catalyst-containing composition. This catalyst-containing composition is subsequently introduced into the fluidized-bed reactor and reacted therein. Unreacted chloromethane, gaseous methylchlorosilanes, gaseous by-products, catalyst constituents, and finely divided dusts leave the reactor.
In U.S. Pat. No. 4,281,149, FIG. 1 shows a system comprising a reactor, a main cyclone with recirculation and an after-cyclone with a dust collection vessel. In this process, part of the particulate material precipitated in the after-cyclone are subjected to a surface treatment and subsequently fed back into the reactor as solids. This is said to achieve a high silicon conversion. The gas stream leaving the cyclone(s) always still contains residual dust which has to be separated prior to the distillation of the methylchlorosilanes. U.S. Pat. No. 4,328,353 proposes that the gas stream which has passed through the cyclone be subjected to a hot gas filtration and the pulverulent material obtained here be recirculated to the reactor or discharged from the process. The possibility of passing these fine dusts without further treatment to a reaction with HCl (chlorosilane synthesis) is also disclosed. EP-A-900802 likewise describes a hot gas filtration with subsequent recirculation of the particles.
When very fine dusts are introduced directly into fluidized-bed reactors of the chlorosilane synthesis or methylchlorosilane synthesis, these particles are very quickly swept out again, and are thus not available to the reaction. At the same time, the recirculation of fine dusts in this manner leads to a considerably greater load on the entire dust precipitation system.
Furthermore, these very fine silicon dusts in dry form are extremely reactive toward air and atmospheric moisture, i.e. when this product stream accidentally comes into contact with air, as may occur in the case of a malfunction in the plant, spontaneous ignition has to be reckoned with.
It is an object of the present invention to provide a continuous process for the direct synthesis of chlorosilanes and methylchlorosilanes in a fluidized bed, in which process the finely divided dusts which are formed and are entrained within the reaction product can be reused effectively.