Cyclic production of silicon or silicon compounds and hydrogen, by pyrolyzing impure oil-containing sand or shale and forming silicon tetrafluoride for conversion into silicon by thermite process

摘要

A cyclic method for production of crystalline silicon (Si), silane, silicon nitride or carbide and hydrogen (H 2) is based on pyrolysis of oil-containing sand or shale (I) (as mixture of hydrocarbons and silicates), contaminated with potassium aluminum silicate and carbonate. Released H 2is heated with fluorine to give hydrogen fluoride, which is reacted with the Si of (I) to give silicon tetrafluoride for conversion into Si by thermite methods using aluminum. Cyclic production of crystalline silicon (Si), silane, silicon nitride or carbide and hydrogen (H 2) involves pyrolyzing oil-containing sand or shale (I) (as mixture of hydrocarbon energy source (tar) and silicates (SiO 2)), contaminated with potassium aluminum silicate and carbonate, at more than 2000[deg] C. The released H 2is fed into a gas main or heated at ca. 4000[deg] C with a specific amount of fluorine (F 2) to give hydrogen fluoride (HF). The hot HF is immediately reacted with the Si of (I) to give silicon tetrafluoride (SiF 4) gas and water vapor; and the hot SiF 4(contaminated with HF) is fed directly into a combustion chamber supplied continuously with aluminum (Al) powder, in which SiF 4is converted into high purity crystalline Si by a thermite process. The obtained Al fluoride (AlF 3) powder (stable towards aqueous base) is filtered off before electrolytic conversion (in hexafluorosilicate form) into more Al and F 2. The necessary DC current is permanently obtained using process heat; the thermite process stoichiometrically releases 1172 KJ of energy per unit time and cooling with preheated water gives steam for AC generation. Heat from SiF 4production is also used for electricity generation. In the pyrolysis stage, the SiF 4-H 2O mixture is passed into the center of an Al powder-filled rotating drum with a welding flame in the cylinder center, and heat conduction causes exponential decrease of high temperatures in the cylinder wall direction (as in the subsequent thermite process). Cooling of the double-walled drum with water allows generation of a large amount of current, and Al and F 2are recycled to the cyclic process.