Silicon is produced on a commercial scale in three submerged arc furnaces by the carbothermic reduction of silica (presented as either quartz or quartzite). The process is highly endothermic requiring approx 10.5MWH/tonne of silicon produced.
The furnace consists of a carbon lined crucible into which is suspended three pre-baked carbon electrodes. Very high electrical current is passed down the electrodes and a regulated arc is formed between the tip of the electrode and the carbon base of the crucible (hearth).
A steep temperature gradient extends from the zone of the arc (>3000°C) below the electrode to the top surface of the charge mix (approx 700°C).
The furnace charge is a mixture of quartz, carbonaceous reducing agents (charcoal, coal, petroleum coke) and wood chips.
The reduction of quartz by carbon can be summarized by reaction 1:
SiO2 + 2C => Si + 2CO - (1)
This reaction does not occur directly but proceeds via a series of intermediate reactions which form solid silicon carbide (SiC) and gaseous silicon monoxide (SiO).
At the bottom of the furnace is the main reaction zone (metal production zone), at temperatures exceeding 1820°C.
The first reaction to take place is the reduction of molten silica (SiO2) with carbon from the reductants to produce two gaseous products.
SiO2 + C => SiO + CO - (2)
The gaseous silicon monoxide (SiO) further reacts with carbon to form solid silicon carbide (SiC) in the furnace.
SiO + 2C => SiC + CO - (3)
The silicon carbide can then react with molten silica to form both silicon and silicon monoxide.
SiC + SiO2 = Si + SiO +CO - (4)
This silicon is drained from the furnace via a taphole into ladles which refine the silicon and transfer the silicon to the casting area.
Silicon monoxide which fails to react within the furnace oxidises in the atmosphere to form SiO2 (a dust-like material called amorphous silica fume). The silica fume is vented away for collection in a large filtration facility (baghouse) as a by-product of the silicon production.
2SiO + O2 => 2SiO2 - (5)