On the Production of Pure Silicon. (II) : The Semi-Industrial Research

The semi-industrial test for the production of pure silicon, which has been employed in our laboratory, is reported. The excellent electrical properties of pure silicon for the crystal rectifier, are of as good industrial use as germanium. A process employing the reduction of silicon tetrachloride gas with zinc vapor was developed by our fundamental experiments which proceeded to the semi-industrial test as described below. In this paper, the process and equipments are described. No radical changes were made in the principles of the process as mentioned in the first report. The steps of the process are outlined as follows : (1) Ferrosilicon is pulverized and stored in dryness. (2) Powdered ferrosilicon is chlorinated to produce silicon tetrachloride. Chlorine is passed through on powdered ferrosilicon at about 450-500℃, and the produced chloride is collected in a condenser. (3) The raw silicon tetrachloride is purified by distillation in a dried atmosphere to prevent the hydrolysis of chloride. In the first distillation the chlorinated impurities are eliminated and chlorine is completely removed by the second distillation. (4) The jet method is employed for the reduction of silicon tetrachloride with zinc vapor in a cylindrical reactor heated to 950～1,000℃. (5) Contaminated zinc and chloride in the acicular crystals of silicon formed in the reduction are leached out by hydrochloric acid. (6) Vacuum melting is employed to prepare the silicon ingot after purification. The purity of silicon is found to be about 99.99% by spectrographic analysis