The electro thermal production of silicon metal is carried out by means of high energy concentration in submerged arc furnaces. The reduction of quartz as the silicon-bearing material with carbon can be represented in a simplified form by reaction (1).
Si02 + 2C = Si + 2C 0 (1)
If the operations of several furnaces for the smelting of silicon are compared, particularly the carbon balances, the direct reduction of quartz as given in this simplified reaction is possible from about 1,660°C. Kinetic considerations together with layer profiles from cooled-down furnaces indicate process reactions of a much more complex nature.
In order to be better able to follow the process reactions, a model concept should be used that follows the changes in the charged material right through to the final liquid silicon.
Under good operating conditions only preheating of the material takes place in the upper part of the furnace.
Quartz and the reduction material do not yet react with one another. It is in this area, however, that the disproportionation of SiO commences predominantly according to reaction (2).
2SiO= Si + SiO2 (2)
This reaction is essential for the recovery of the silicon and thereby the economic efficiency of the process. Further SiO reactions will be discussed later.
The disproportionation products Si and SiO2 should deposit largely on the feed material, principally on the reduction component (including wood chips) due to the more porous outer surface. Some of the products will escape into the furnace atmosphere. The objective is to keep the amount escaping as limited as possible. For this reason it must be ensured that the upper layer of the feed material has a high permeability and is cool. If temperatures in this region are too high, this will hinder disproportionation of the SiO within the material. The SiO will then leave the feed bed and disproportionate in the off gas system. This leads to increased reduction material requirements and a higher energy consumption per tonne of silicon produced. How bed temperatures can be maintained low enough will be discussed in section 3.
As the preheated material from the upper zone of the furnace descends to the middle temperature region (over 1,500°C), the formation of silicon carbide, which is important for the further process reactions, commences. It is produced according to the following reactions:
SiO + 2C = SiC + CO (3) And Si02 + 3C = SiC + 2CO (4)
Thermodynamically, reaction (3) begins at 1,520°C and reaction (4) at 1,537°C. In the former, the carbon reacts with the rising SiO and in the latter with the quartz moving downwards. Reaction (3) will quantitatively decrease as reaction (4) increases. It can be taken from this that in this temperature zone almost all the reduction material will be consumed.
This middle temperature zone also has the right conditions for the formation of the first liquid silicon metal (melting temperature 1,410°C), as the hot rising SiO reacts with the carbon according to reaction (5).
SiO+ C = Si + CO (5)
Direct reduction of quartz by the carbon according to reaction (1) is also thermodynamically possible from 1,660°C. As most of the carbon will have already been used for silicon carbide formation (reactions (3) and (4)), the amount still available for direct reduction is very limited.
As temperatures increase further, the first stage of the dissociation of the silicon carbide will begin according to reaction (6).
2Si02 + SiC = 3SiO + CO (6)
This increases the SiO content of the system.
At temperatures over 1,800°C in the lower zones of the furnace and around the electrodes, reaction (7) commences (thermodynamically possible from 1,827°C)
Si02 + 2SiC = 3 Si + 2CO (7)
the complete dissociation of the silicon carbide and the actual production of silicon metal respectively.
Most likely reaction (7) will not occur alone but rather a certain amount of the silicon carbide will react according to reaction (8).
3SiO2 + 2SiC = Si + 4SiO + 2CO (8)
The reactions of the gases generated in the process can be briefly described as follows. The unstable SiO forces its way, together with the CO formed, upwards. It is consumed to a limited extent in the medium temperature zone according to reaction (3).
SiO+ 2C = SiC + CO (3)
The remaining larger part of the SiO condenses (disproportionates) either according to reaction (2)
SiO = Si + SiO2 (2)
Or reacts with the CO rising together with it according to reaction (9)
SiO + CO = SiO2 + C (9)
or reaction (10)
3SiO + CO =2SiO2 + SiC (10)
As reactions (2), (10) and (9) are strongly exothermic they will occur in preference to reaction (3).
The principal reactions from the foregoing discussion give the following picture of the overall process:
Upper reaction zone: 2 SiO = SiO2+ Si
(SiO dissociation)
Middle reaction zone:
2 SiO + 4C = 2SiC + 2 CO
(SiC formation)
Upper reaction zone:
2 SiC + 3SiO2 = Si + 4SiO + 2CO (SiO dissociation)
Overall reaction: 2 SiO2 + 4C = 2Si + 4CO
Reaction (1), i.e., the direct reduction of quartz with carbon, can be taken as a general description of the process in that it is the same as the sum of the individual reactions. More detailed studies of the process, however, require that one considers the actual individual reactions occurring.