knjiga solarna energija dobra

172 SOLAR GRADE SILICON FEEDSTOCK
• The process is operated batchwise.
• Large amount of by-products need to be handled or recycled.
More recent developed processes have attempted to overcome some of these
disadvantages.
5.4.2 The Union Carbide Process
Research on this process was initiated in 1976 after the international oil crisis. The US
Government funded several projects with the objective of finding a route to inexpensive
solar grade polysilicon. The Union Carbide process for silane production, with fluidised
bed production of polysilicon, was selected for further funding. When the funding was
not provided for political reasons, Union Carbide decided to use the silane technology
for the production of semiconductor grade polysilicon. Silane deposition technology for
polysilicon rods was licensed from Komatsu Electronic Metals, Japan. In 1990, the business
was sold to Komatsu, which became Advanced Silicon Materials (ASiMI). By 1998,
two large industrial plants were constructed in the United States with the nominal capacity
of 5500 MT polysilicon, thus ranking ASiMI as number two or three worldwide among
the polysilicon producers. A schematic overview of the process is given in Figure 5.4.
The main process steps are as follows:
The hydrogenation of tetrachlorosilane through a mass bed of silicon metal is carried out
in a fluidised bed reactor as already described by equation (5.33).
The trichlorosilane is separated by distillation while the unreacted tetrachlorosilane is
recycled back to the hydrogenation reactor.
MG-Si: Metallurgical
grade silicon
TCS: Trichlorosilane
TET: Tetrachlorosilane
DCS: Dichlorosilane
TCS synthesis
MG-Si
by
Separation
H 2
hydrogenation
TCS TCS/TET
of
TET by
TET
TET in
DCS
distillation
fluidised bed
Redistribution 2
reactor
Residues:
spent mass
TET
Low boiling
impurities
TCS
Redistribution 1
in catalytic
column
Separation
DCS/TET/TCS
distillation
in catalytic
column
Separation
DCS/TET/SiH 4
Purification
of SiH 4
distillation
Electrical
energy
Pyrolysis
on heated
silicon rods
SiH 4
H 2
Poly
silicon
TET
Figure 5.4
A schematic representation of the Union Carbide Polysilicon process