and metallurgical silicon are manufactured by reducing quartzites with coke in an electric-arc furnace (carbothermal reduction). The SiO₂-content of the quartzite for the manufacture of ferrosilicon must if possible be above 96%, that for the manufacture of metallurgical grade silicon should be generally as pure as possible and have as high a SiO₂ content as possible (see Table 3.4-1).

-, phosphorus- and arsenic- contents are undesirable, since they form poisonous flue gases. High Al₂O₃-contents lead to the formation of sticky slags, which can contaminate the end product.

In the production of ferrosilicon the Moeller iron in the charge is added in the form of turnings or shredded iron. For silicon-contents above 45%, a shaft electric furnace is used with a power of 8 to 40 MW, whose substructure is lined with carbon bricks. The undesirable formation of SiC is avoided by rotating or oscillating the furnace. It operates with three phase electricity, which in the case of metallurgical grade silicon is introduced by way of graphitized electrodes. Ca. 11 to 14 MWh of electricity energy is consumed in producing 1 t of silicon. The yield based on the silicon-content of the quartz is ca. 80%.

The energy costs are ca. 21 % of the raw material costs (quartz and coal) and 28% of the total manufacturing costs. The reduction proceeds in the following steps:


 

Carbothermal reaction requires temperatures in excess of 2000°C. After ca. 1 to 2 h the continuously operating furnace is tapped with an auxiliary electrode. The liquid silicon (m.p. 1413°C) is collected in pans or ingot molds. If necessary, further metallurgical processes such as slag extraction or air blasting with reactive gases can be carried out to rid the silicon melt of included metallic and non-metallic impurities, before the silicon solidifies into brittle blocks.

Latest developments concern the quenching of liquid silicon by feeding a jet of molten silicon into water (water granulation) or casting into cooled ingot molds. The material thus produced exhibits an improved reactivity in the synthesis of methylchlorosilanes (Rochow process).

Metallurgical grade silicon is marketed in a coarsely crushed form or as a finely ground powder in different particle sizes. Powders with increased purity due to acid washing, particularly for the removal of metallic impurities, are specialty products. They are utilized, for example, in the manufacture of silicon nitride powder or reaction-bonded silicon nitride ceramic components and are therefore the starting materials for engineering ceramic specialties.