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3SiO(g) + CO(g) = 2SiO
2
(s,l)+ SiC(s)
∆
H
o
1500
o
C
= -1380 kJ/mol
(4)
SiC (s) + SiO (g) = 2 Si(l) + CO (g)
∆
H
o
2000
o
C
= 167 kJ/mol (5)
SiO
2
(l) + Si(l) = 2SiO (g)
∆
H
o
2000
o
C
= 599 kJ/mol (6)
2SiO
2
(s,l) + SiC (s) = 3SiO (g) + CO (g)
∆
H
o
2000
o
C
= 1364 kJ/mol (7)
The final product: Silicon is produced by reaction (5) at temperatures above 1811
0
C.
The gas in the Si-furnace is a mixture of CO and SiO at 1 atm. pressure. To produce
Si at 1811
0
C by reaction (5), partial pressure of SiO must be higher than 0.67 [2]. To
achieve a high Silicon yield, SiO-gas should preferably be produced at temperatures
above 1811
o
C. For reactions with quartz, reaction (6) and (7) a slow reaction rate at
low temperatures is thus preferable.
SiO produced at lower temperatures than 1811
o
C, e.g. higher up in the furnace may
either react with carbon to SiC by reaction (2), be captured in the furnace by reaction
(3) and (4) or leave the furnace with the off-gas. Reaction (3) and (4) are highly
exothermic. A surplus of SiO produced in the lower part of the furnace that condenses
in the upper part will thus transport energy to increase the temperature in the upper
part of the furnace.
To achieve a high silicon yield, the SiO gas must not leave the furnace with the off-
gas, but react according to reactions (2), (3) or (4). The reaction products from (3) and
(4), the "condensates" are sticky, will glue the charge materials together and hinder
material and gas flow in the furnace. Gas flow and permeability in Si-furnaces are
believed to be more affected by condensates than by amount of fines and size
distribution. At 1726
o
C. SiO
2
melts to viscous fluid that will also affect material and
gas flow as well as reaction rates. Softening of SiO
2
particles giving a semi--molten
surface that start before they reach the melting temperature will also reduce gas
permeability and reaction rates.
Phase transformation in quartz
When quartz is heated, its structure will change to different SiO
2
polymorphs as
shown in the phase diagram in Figure 2 [6].
Figure 2. SiO
2
phase diagram [6]
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