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Process for the production of Si by redaction of SiCl4 with liquid metal
The invention relates to the manufacture of solar grade silicon (Si) as a feedstock material for
the manufacture of crystalline silicon solar cells. The Si metal is obtained by direct reduction
of SiCl4), a precursor that is commonly available in high purity grades.
Silicon suitable for application in solar cells is commonly manufactured by the thermal
decomposition of SiHCl3. The process delivers very pure silicon, but it is slow, highly energy
consuming, and requires large investments.
An alternative route towards the formation of Si for solar cells is the reduction of SiCl4) with
metals such as Zn, Na and Mg. This process has the potential for significant cost reduction
because of lower investment costs and reduced energy consumption.
The direct reduction of SiCl4 by Zn in the vapour phase is described in US 2,773,745, US
2,804,377, US 2,909,411 or US 3,041,145. When Zn vapour is used, a granular silicon product
is formed in a fluidised bed type of reactor, enabling easier Si separation. However, an
industrial process based on this principle is technologically complex.
The direct reduction of SiCl4 with liquid Zn is described in JP 11-092130 and JP 11-011925. Si
is formed as a fine powder and separated from the liquid Zn by entraining it with the gaseous
ZnCl2 by-product This process is however not practical because the separation of the fine Si
powder from the ZnCl2 is problematic as well as the handling and melting of the fine Si
powder.
it is an object of the present invention to provide a solution for the problems in the prior art
To this end, according to this invention, high purity Si metal is obtained by a process for
converting SiCl4, into Si metal, comprising the steps of:
- contacting gaseous SiCls) with a liquid metal phase containing a metal M, M being either one
of Zn, Na and Mg, thereby obtaining a Si-bearing metal phase and M-chloride;
- separating the M-chloride from the Si-bearing metal phase; and

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- purifying the Si-bearing metal phase at a temperature above the boiling points of both M and
M-cbloride, thereby vaporising M and obtaining Si metal.
The use of Zn as metal M is preferred. It is moreover useful to combine the contacting and the
separating steps, by operating the contacting step at a temperature above the boiling point of
Zn-chloride, which evaporates.
The Si content of the Si-bearing metal phase, as obtained in the contacting step, can
advantageously contain, besides Si as solute, also at least some Si as suspended particles. It is
then advantageous to perform the contacting step, by blowing SiCl4 into a bath comprising
molten Zn, at a flow rate adapted to limit the loss of Si by entrainment with evaporating Zn-
chloride, to less than 15% (weight). To this end, a flow rate of SiCl4 lower than 0.8 kg/min per
m2 of bath surface can be used.
A Si crystallisation step can be inserted before the purification step, comprising the cooling the
Si-bearing metal phase to a temperature where the Si is barely soluble in molten zinc, such as
between 420 °C and 600 °C, thereby forming a lower liquid fraction depleted in Si and an
upper solid fraction enriched in Si, which is separated and further processed in the purification
step.
It is useful to operate the purification step at a temperature above the melting point of Si, and,
in particular, at reduced pressure or under vacuum.
It is also advantageous to recycle one or more of the different streams which arc nol
considered as end-products:
- the obtained M-chloride can be subjected to molten salt electrolysis, thereby recovering metal
M, which can be recycled to the SiCl4 reduction step, and chlorine, which can be recycled to a
Si chlorination process for the production of SiCl4;
- metal M that is vaporised in the purification step can be condensed and recycled to the SiCl2
converting process; and/or- the fraction of SiCl2 that exits the contacting step un-reacted can
be recycled to the SiCl4 converting process.

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According to the current invention, SiCl4 is reduced with a liquid metal M. The technology for
this process is therefore much more straightforward than that required for the gaseous
reduction process. A Si-bearing alloy containing both dissolved and solid Si can be obtained,
while the chlorinated metal M either forms a separate liquid phase, or is formed as a vapour.
Metal M can be retrieved from its chloride, e.g. by molten salt electrolysis, and reused for
SiCl4 reduction. The Si-bearing alloy can be purified at high temperatures, above the boiling
points of both metal M and M-chbride, but below the boiling point of Si itself (2355 °C). The
evaporated metal M can be retrieved and reused for SiCl; reduction. Any other volatile
element is also removed in this step. It is thus possible to close the loop on metal M, thereby
avoiding the introduction of impurities into the system through fresh additions.
Jt should be noted that besides Zn, Na or Mg, metal M conld also be any element that forms
chlorides more stable than SiCl4, that can be separated from Si easily and that can be recovered
from their metal chlorides without difficulty.
In a preferred embodiment according to the invention, gaseous SiCl4 is contacted with liquid
Zn at atmospheric pressure, at a temperature above the boiling point of ZnCl2 (732 °C) and
below the boiling point of Zn (907 °C). The preferred operating temperature is 750 to 880 °C,
a range ensuring sufficiently high reaction kinetics, while the evaporation of metallic Zn
remains limited.
In a typical embodiment, the molten Zn is placed in a reactor, preferably made of quartz or of
another high purity material such as graphite. The SiCl4 which is liquid at room temperature,
is injected in the zinc via a submerged tube. The injection is performed at the bottom of the
Zn-containing vessel. The SiCl4, which is heated hi the tube, is actually injected as a gas. The
end of the injection tube is provided with a dispersion device such as a porous plug or fritted
glass. It is indeed important to have a good contact between the SiCl4, and the Zn to get a high
reduction yield. If this is not the case, partial reduction to SiCl2 could occur, or SiCl4 could
leave the zinc un-reacted. With an adequate SiCl4 - Zn contact; close to 100% conversion is
observed.
The reduction process produces ZnCl2- Tt has a boiling point of 732 °C, and is gaseous at the
preferred operating temperature. It leaves the vessel via the top. The vapours are condensed
and collected in a separate crucible.

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The process also produces reduced Si. The Si dissolves in the molten Zn up to its solubility
limit The Si solubility in the Zn increases with temperature and is limited to about 4% at
907 "C, the atmospheric boiling point of pure Zn.
In a first advantageous embodiment of the invention, the amount of SiCl4 injected is such that
the solubility limit of Si in Zn is exceeded. Solid, participate Si is produced, which remains in
suspension in the molten Zn bath. This results in a Zn metal phase with a total Si concentration
(dissolved and suspended) of preferably more man 10%, i.e. considerably higher than the
solubility limit, and thus in a more efficient and economic Si purification step. The particulate
Si is however subject to losses by entrainment with the ZnCl2 gaseous stream. This risk can be
minimised by using a sufficiently low SiCl4 flow. A Si loss by entrainment of less than 15% of
the total Si input to the process is considered as acceptable.
In a second advantageous embodiment according to the invention, the Si-bearing alloy is
allowed to cool down to a temperature somewhat above the melting point of the Zn, e.g.
600 °C. A major part of the initially dissolved Si crystallizes upon cooling, and accumulates
together with any solid Si that was already present in die bath, in an upper solid Jraction. The
lower liquid fraction of the metal phase is Si-depleted, and can be separated by any suitable
means, e.g. by tapping. This metal can be directly re-used for further S1CI4 reduction. The
upper Si-rich fraction is then subjected to the purification as mentioned above, with the
advantage that the amount of metal to be evaporated is considerably reduced.
Both, of the above first and second advantageous embodiments can of course be combined.
The Zn, together with typical trace impurities such as T1, Cd and Pb can be separated from the
Si-bearing alloy by vaporisation. Si with a purity of 5N to 6N is then obtained. For this
operation, the temperature is increased above the boiling point of Zn (907 °C), and preferably
above the melting point (1414 9C) but below the boiling point of Si (2355 °C). It is useful to
work at reduced pressure or vacuum. The Zn and its volatile impurities are hereby eliminated
from the alloy, leaving molten Si. Only the non-volatile impurities present in the Zn remain in
the Si. Examples of such impurities are Fe and Cu. Their concentration can be minimised,
either by pre-distiliing the Zn, by repeatedly recycling the Zn to the SiCl4 reduction step after
electrolysis of the formed ZnCl2, or by minimising the amount of Zn that needs to be vaporised

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per kg of Si in the purification step. In such optimised conditions, a Si purity exceeding 6N
could be achieved.
Finally, the molten Si is allowed to cool down and to solidify as a metallic block. Alternatively
the liquid metal Si can readily be cast in any suitable form, used for growing single crystals or
for casting multi-crystalline ingots in a directional solidification process.
The following example illustrates the invention. 4192 g of metallic Zn is heated to 850 °C in a
graphite reactor. The height of the bath is about 15 cm and its diameter is 7 cm. A Minipuls™
peristaltic pump is used to introduce SiCl4 in the reactor via an quartz tube. The immersed
extremity of the tube is fitted with a porous plug made of alumino-silicate. The SiCl4, which
has a boiling point of 58 °C, vaporises in the immersed section of the tube and is dispersed as a
gas in the liquid Zn. The SiCl4 flow is ca. 150 g/h, and the total amount added is 1625 g. The
flow rate corresponds to 0.65 kg/min perm2 of bath surface. The ZnCl2, which is formed
during the reaction, evaporates and is condensed in a graphite tube connected to the reactor
and is collected in a separate vessel. Any un-reacted SiCl4 is collected in a wet scrubber
connected to the ZnCl2 vessel. A 2n-Si alloy, saturated in Si at the prevailing reactor
temperature and containing additional solid particles of Si, is obtained. The total Si content of
the mixture is 9%. It is sufficient to increase the amount of SiCl4 added, at the same flow-rate
of 150 g/h, to increase the amount of solid Si in the Zn-Si alloy. This Zn-Si alloy containing
solid Si is heated to 1500 °C to evaporate the Zn, which is condensed and recovered. The Si is
then allowed to cool down to room temperature; 229 g of Si are recovered.
The Si reaction yield is thus about 85%. The Si losses can be attributed to the entrainment of
particles of Si with the escaping ZnCl2 vapours, and to the incomplete reduction of SiCl4into
Si metal. Of the remaining Si, about 40 g are found in the ZnCl2 and 3 g in the scrubber.

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Claims
1. Process for converting SiCl4 into Si metal, comprising the steps of:
- contacting gaseous SiCl4 with a liquid metal phase containing a metal M, M being either one
of Zn, Na and Mg, thereby obtaining a Si-bearing metal phase and M-chloride;
- separating the M-chloride from the Si-bearing metal phase; and
- purifying the Si-bearing metal phase at a temperature above the boiling points of both M and
M-chloride, thereby vaporising M and obtaining Si metal.

2. Process according to claim 1, wherein metal M is Zn.
3. Process according to claim 2, wherein the contacting and the separating steps are
performed simultaneously, by operating it at a temperature above the boiling point of Zn-
chloride, which evaporates.
4. Process according to claims 2 or 3, wherein the Si content of the Si-bearing metal
phase, as obtained in the contacting step, contains at least part of the Si as suspended particles.
5. Process according to claim 4, wherein the contacting step is performed by blowing
SiCl4 into a bath comprising molten Zn, at a flow rate adapted to limit the loss of Si by
entraiaoient with evaporating Zn-chloride to less than 15%.
6. Process according to claim 5, whereby the flow rate of SiCl4 is lower than 0.8 kg/min
per m2 of bath surface.
7. Process according to any one of claims 2 to 6, wherein a Si crystallisation step is
inserted before the purification step, comprising trie cooling of the Si-bearing metal phase to a
temperature of between 420 and 600 "C, thereby forming a lower liquid fraction depleted in Si
and an upper solid fraction enriched in Si, which is separated and further processed in the
purification step.
8. Process according to any one of claims 1 to 7, whereby the purification step is
performed at a temperature above the melting point of Si.

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9. Process according to claim 8, whereby the purification step is performed at reduced
pressure or loader vacuum.
10. Process according to any one of claims 1 to 9, further comprising the steps of:
- subjecting the separated M-chloride to molten salt electrolysis, thereby recovering metal M
and chlorine;
• recycling M to the SiCl4, reduction step; and
- recycling the chlorine to a Si chlorination process for the production of SiCL,.
11. Process according to any one of claims 1 to 10, wherein the metal M that is vaporised
in the purification step, is condensed and recycled to the SiCl4 converting process.
12. Process according to any one of claims 1 to 11, wherein the fraction of SiCl4 that exits
the contacting step un reacted, is recycled to the SiCl4 converting process.

The invention relates to the manufacture of high purity silicon as a base material for the production of e.g. crystalline
silicon solar cells. SiCU is converted to Si metal by contacting gaseous SiCU with liquid Zn, thereby obtaining a Si-bearing alloy and
Zn-chloride, which is separated. The Si-bearing alloy is then purified at a temperature above the boiling point of Zn. This process
does not require complicated technologies and preserves the high purity of the SiCU towards the end product, as the only reactant is
Zn, which can be obtained in very high purity grades and continuously recycled.