Process for the preparation of silicon or ferrosilicon

Claim 1

A process for the preparation of silicon or ferrosilicon by reduction of silicon oxide in the form of silicas or cation containing silicates and oxides, optionally in the presence of iron or iron oxide, using a carbonaceous reducing agent in a reaction zone which reaction zone comprises an energy supply zone and, a subsequent reaction zone, whereby at least heat energy and condensed phases from the subsequent reaction zone are brought to the reentry supply zone, while elementary silicon and a gas phase are leaving the energy supply zone, and whereby at least the carbonaceous reducing agent and the gas phase leaving the energy supply zone are brought to the zone of subsequent reaction, while a gas leaves the zone of subsequent reaction, and condensed phases enter the energy supply zone, whereby at least 33% of the silicon oxide material is fed directly into the energy supply zone, so that the supply rate of the silicon oxide to the energy supply zone can be controlled independent of the charging of energy to the energy supply zone, so that the temperature in the energy supply zone can be influenced, and regulating the supply rates of silicon oxide and energy to give a low content of silicon monoxide in the gas leaving the zone of subsequent reaction, the improvement comprising (a) awaiting and securing a carbon reduction mode in the reaction zone to obtain a greater difference in temperature between the gases leaving the zone of subsequent reaction and the silicon product than at carbide reduction, by filling reducing agent in the energy supply zone before start up and directly applying an energy to silicon oxide charging ratio within the interval stated in (b), or by altering a carbide reduction mode into a carbon reduction mode by use of an energy to silicon oxide charging ratio above the interval stated in (b), and (b) after securing carbon reduction according to step (a) supplying the energy and the silicon oxide in a ratio within an interval, the lower limit of said interval being defined by the lowest value permitting a constant carbon reduction, and the upper limit of said interval being defined by the highest value permitting a constant carbide reduction, whereby the gas leaving the zone of subsequent reaction has a silicon monoxide content below 1.5 mole percent.