Process for the production of Si by reduction of SiHCl3 with liquid Zn
The invention relates to the manufacture of solar grade silicon as a feedstock material
for the manufacture of crystalline silicon solar cells. The Si metal is obtained by direct
reduction of SiHCl3, 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 according to the Siemens process or its variants. 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
SiHCl3 with metals such as Zn. This process has the potential for significant cost
reduction because of lower investment costs and reduced energy consumption. It is a
variation on the reduction of SiHCl4 with Zn.
The direct reduction of SiHCl4 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 SiHCl4 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. There is however no explanation why the
entrainment of fine powder Si with SiHCl2 can take place. It proved impossible to
repeat the process as described in these patents. The essential technical features
enabling to discharge substantial amounts of the generated polycrystalline silicon
powder together with the vapour of the zinc chloride are missing.

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 SiHCl3 into Si metal, comprising the steps of:
- contacting gaseous SiHCl3 through an injection tube with a liquid metal phase containing
Zn, whereby the end of the injection tube is provided with a dispersion device, and thereby
obtaining a Si-bearing metal phase, ZnCl2 and H2;
- separating H2 and the ZnCl2 from the Si-bearing metal phase; and
- purifying the Si-bearing metal phase at a temperature above the boiling point of Zn, thereby
vaporising Zn and obtaining Si metal.
Using SiHCU instead of e.g. SiCl4 allows relying on the well proven first steps of the classic
Siemens process. The invented process could also be useful to increase the capacity of an
existing plant in an economic way.
The contacting and the separating steps are performed in a single reactor. This is rendered
possible by the fact that a major part (more than 50% by weight) of the formed Si is retained
in the liquid metal phase.
It is useful to combine the contacting and the separating steps, by operating the contacting
step at a temperature above the boiling point of ZnCl2, which evaporates. The ZnCl2 can be
permitted to escape so as to be collected for further processing.
The Si-bearing metal phase as obtained in the contacting step can advantageously contain,
besides Si as solute, also at least some Si in the solid state, e.g. as suspended particles.
Formation of particular Si may indeed occur during the contacting step, when the Zn metal
gets saturated in Si. Solid state Si can also be obtained by cooling the Si-bearing metal phase
as obtained in the contacting step, preferably to a temperature of between 420 and 600 °C.
The solid state Si can preferably be separated from the bulk of the molten phase, e.g. after
settling. This Si metal phase is however still impregnated with Zn and has to be further
processed in the purification step.

It is advantageous to perform the contacting step by injecting SiHCb into a bath
comprising molten Zn in a way enabling to limit the loss of Si by entrainment with
evaporating ZnCb, to less than 15% (weight). Flow rates of SiHCl3 up to 50 kg/min
per m of bath surface are compatible with the abovementioned low Si losses.
Preferably the gaseous SiHCl3 is adequately dispersed in the bath, e.g. by using
multiple submerged nozzles, a submerged nozzle equipped with a porous plug, a
rotating gas injector, or any other suitable mean or combination of means. The SiHCl3
can be injected along with a carrier gas such as N2. A flow rate over 10, and preferably
12 or more kg/min per m2 of bath surface is advised to perform the process in a more
economical way.
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. The purification can
advantageously be performed in again the same reactor as the first two process steps.
It is also advantageous to recycle one or more of the different streams which are not
considered as end-products.
The obtained ZnCl2 can be subjected to molten salt electrolysis, thereby recovering Zn,
which can be recycled to the SiHCl3 contacting step, and Cl2, which can be recycled as
HC1 to a process of hydrochlorination of impure Si, thereby generating SiHCl3. The H2
needed for the production of HC1 is generated in the contacting step and in the
hydrochlorination process. Said impure Si could be metallurgical grade Si or any other
suitable precursor such as ferrosilicon.
Any Zn that is vaporised in the purification step can be condensed and recycled to the
SiHCl3 contacting step. Similarly, any SiHCl3 that exits the contacting step un-reacted
can be recycled to the SiHCl3 contacting step, e.g. after condensation.

According to this invention, SiHCl3 is reduced with liquid Zn. 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 Zn either forms a separate liquid phase, containing
most of the solid Si, or is formed as a vapour. Zn can be retrieved from its chloride,
e.g. by molten salt electrolysis, and reused for SiHCl3 reduction. The Si-bearing alloy
can be purified at high temperatures, above the boiling points of both Zn and ZnCl2.
but below the boiling point of Si itself (2355 °C). The evaporated Zn can be retrieved
and reused for SiHCl3 reduction. Any other volatile element is also removed in this
step. It is thus possible to close the loop on Zn, thereby avoiding the introduction of
impurities into the system through fresh additions.
It should be noted that besides Zn, another metal could also be used that forms
chlorides more stable than SiHCl3, that can be separated from Si easily and that can be
recovered from its chloride without difficulty.
In a preferred embodiment according to the invention, gaseous SiHCl3 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 SiHCl3, which is liquid
at room temperature, is injected in the zinc via a submerged tube. The injection is
performed in the lower part of the Zn-containing vessel. The SiHCl3, which is heated
in the tube, is actually injected as a gas. It can also be vaporized in a separate
evaporator, and the obtained vapours are then injected in the melt. The end of the
injection tube can be provided with a dispersion device such as a porous plug or fritted
glass. It is indeed important to have a good contact between the SiHCl3 and the Zn to
get a high reduction yield. If this is not the case, partial reduction could occur, or
SiHCl3 could leave the zinc un-reacted. With an adequate SiHCl3 - Zn contact, close to

100% conversion is observed. Finely dispersing the SiHCl3 is an important factor in
limiting the entrainment of finely dispersed Si with the gaseous flow.
The reduction process produces H2 and ZnCl2. ZnCl2 has a boiling point of 732 °C,
and is gaseous at the preferred operating temperature. It leaves the Zn-containing
vessel via the top, together with H2 and unreacted SiHCl2. The vapours are condensed
and collected in a separate recipient.
The process also produces 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 SiHCl3 injected is
such that the solubility limit of Si in Zn is exceeded. Solid, particulate Si is produced,
which may remain in suspension in the molten Zn bath and/or aggregate so as to form
dross. This results in a Zn metal phase with a total (dissolved, suspended and drossed)
mean Si concentration of preferably more than 10%, i.e. considerably higher than the
solubility limit, and thus in a more efficient and economic Si purification step. The
particulate Si can be subject to losses by entrainment with the ZnCl2 gaseous stream,
however, in practice the Si loss by entrainment is less than 15% of the total Si input,
and this is to be 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 the bath, in an upper
solid fraction. The lower liquid fraction of the metal phase is Si-depleted, and can be
separated by any suitable means, e.g. by pouring. This metal can be directly re-used for
further SiHCl3 reduction. The upper Si-rich fraction is then subjected to the
purification as mentioned above, with the advantage that the amount of Zn to be
evaporated is considerably reduced.

Both of the above first and second advantageous embodiments can of course be
combined.
When the purification step is performed above the melting point of Si, the molten
silicon can be solidified in a single step, chosen from the methods of crystal pulling
such as the Czochralski method, directional solidification and ribbon growth. The
ribbon growth method includes its variants, such as ribbon-growth-on-substrate (RGS),
which directly yields RGS Si wafers.
Alternatively, the molten silicon can be granulated, the granules being fed to a melting
furnace, preferably in a continuous way, whereupon the molten silicon can be
solidified in a single step, chosen from the methods of crystal pulling, directional
solidification and ribbon growth.
The solid material obtained can then be further processed to solar cells, directly or
after wafering, according to the solidification method used.
The Zn, together with typical trace impurities such as Tl, 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 °C) 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-distilling the Zn,
by repeatedly recycling the Zn to the SiHCl3 reduction step after electrolysis of the
formed ZnCl2, or by minimising the amount of Zn that needs to be vaporised per kg of
Si in the purification step. In such optimised conditions, a Si purity exceeding 6N
could be achieved.

A further advantage of the invention is that the Si can be recovered in the molten state
at the end of the purification process. Indeed, in the state-of the art Siemens process
and its variants, the Si is produced as a solid that has to be re-melted to be fashioned
into wafers by any of the commonly used technologies (crystal pulling or directional
solidification). Directly obtaining the Si in the molten state allows for a better
integration of the feedstock production with the steps towards wafer production,
providing an additional reduction in the total energy consumption of the process as
well as in the cost of the wafer manufacturing. The liquid Si can indeed be fed directly
to an ingot caster or a crystal puller. Processing the Si in a ribbon growth apparatus is
also possible.
If one does not wish to produce ready-to-wafer material, but only intermediate solid
feedstock, it appears advantageous to granulate the purified Si. The obtained granules
are easier to handle and to dose than the chunks obtained in e.g. the Siemens-based
processes. This is particularly important in the case of ribbon growth technologies. The
production of free flowing granules enables the continuous feeding of a CZ furnace or
a ribbon growth apparatus.
Example 1
The following example illustrates the invention. 7180 g of metallic Zn is heated to 850
°C in a graphite reactor. The height of the bath is about 16 cm and its diameter is 9 cm.
A Minipuls™ peristaltic pump is used to introduce SiHCl3 in the reactor via a quartz
tube. The immersed extremity of the tube is fitted with a porous plug made of quartz.
The SiHCb, vaporises in the immersed section of the tube and is dispersed as a gas in
the liquid Zn. The SiHCb flow rate is ca. 250 g/h, and the total amount added is 3400
g. The flow rate corresponds to 0.66 kg/min per m2 of bath surface. The ZnCl2, which
evaporates during the reaction, condenses in a graphite tube connected to the reactor
and is collected in a separate vessel. Any un-reacted SiHCb is collected in a wet
scrubber connected to the ZnCl2 vessel. A Zn-Si alloy, saturated in Si at the prevalent
reactor temperature and containing additional solid particles of Si, is obtained. The
total Si content of the mixture is 11%. It is sufficient to increase the amount of SiHCl3
added, at the same flow rate of 250 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; 627 g of Si is recovered.
The Si reaction yield is thus about 89%. The Si losses can be attributed to the
entrainment of particles of Si with the escaping ZnCl2 vapours, and to the incomplete
reduction of SiHCl3 into Si metal. Of the remaining Si, about 60 g is found in the
ZnCl2 and 7 g in the scrubber.
Example 2
This example illustrates the granulation of the molten Si, a process which is
particularly useful when the purification step is performed above the melting point of
Si. One kg of molten Si is contained in a furnace at 1520 °C. The crucible containing
the molten metal is under inert atmosphere (Ar). The furnace allows the crucible to be
tilted. The molten Si is poured over a period of 3 minutes into a vessel containing 100 1
of ultra-pure water at room temperature, under agitation. The Si readily forms granules
of a size between 2 and 10 mm.
Example 3
165 kg of metallic Zn are heated to 850 °C in a graphite reactor placed in an induction
furnace. The height of the bath is about 45 cm and its diameter is 26 cm. A membrane
pump is used to transport SiHCl3 into an evaporator (doubled jacket heated vessel).
The gaseous SiHCl3 is then bubbled through the zinc bath via a quartz tube. The
SiHCl3 flow is ca. 10 kg/h, and the total amount added is 90 kg. The flow rate
corresponds to 3.1 kg/min per m2 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 SiHCl3 is collected in a wet
scrubber connected to the ZnCl2 vessel. A Zn-Si alloy, saturated in Si at the prevalent
reactor temperature and containing additional solid particles of Si, is obtained. The
total Si content of the mixture is about 14%. 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; 16.4 kg of Si are recovered.
The Si reaction yield is thus about 88%. The Si losses can be attributed to the entrainment of particles of Si with the escaping ZnCl2 vapours, and to the incomplete
reduction of SiHCl3 into Si metal. Of the remaining Si, about 1.6 kg are found in the
ZnCl2 and 600 g in the scrubber.

WE CLAIM:
1. Process for converting SiHCl3 into Si metal, comprising the steps of:
- contacting gaseous SiHCI3 through an injection tube with a liquid metal phase
containing Zn, whereby the end of the injection tube is provided with a dispersion device, and
thereby obtaining a Si-bearing metal phase, ZnCl2 and H2; contacting gaseous SiCl4 with a
liquid metal phase containing Zn, thereby obtaining a Si-bearing metal phase and Zn-chloride;
- separating H2 and the ZnCl2 from the Si-bearing metal phase; and
- purifying the Si-bearing metal phase at a temperature above the boiling point of Zn,
thereby vaporising Zn and obtaining Si metal,
characterised in that the contacting and the separation steps are performed in a single
reactor.
2. Process according to claim 1, wherein the contacting and the separating steps
are performed simultaneously, by operating them at a temperature above the boiling point of
ZnCl2, which evaporates.
3. Process according to claims "1 or 2, wherein the Si-bearing metal phase that is
obtained in the contacting step, contains at least part of the Si in the solid state.
4. Process according to any of claims 1 to 3, wherein a cooling step of the Si-
bearing metal phase, preferably to a temperature of between 420 and 600 °C, is inserted
before the purification step, thereby converting at least part of the Si present as a solute in the
Si-bearing metal phase that is obtained in the contacting step, to the solid state.
5. Process according to claims 3 or 4, whereby the Si present in the solid state is
separated, forming the Si-bearing metal phase that is further processed in the purification
step.

6. Process according to any one of claims 1 to 5, whereby the purification step is
performed at a temperature above the melting point of Si, thereby forming purified liquid Si.
7. Process according to claim 6, whereby the purification step is performed at
reduced pressure or under vacuum.
8. Pi 'ocess according to any one of claims 1 to 7, further comprising the steps of:

- subjecting the separated ZnCI2 to molten salt electrolysis, thereby recovering Zn and
Cl2;
- recycling the Zn to the SiHCl3 contacting step;
- recycling the H2and the Cl2 to a reactor for the production of HCI;
- hydrochlorination of an impure silicon source with HCI for the production of SiHCl3.

9. Process according to any one of claims 1 to 8, wherein the Zn that is vaporised
in the purification step, is condensed and recycled to the SiHCK contacting step.
10. Process according to any one of claims 1 to 9, wherein the fraction of SiHCl3
that exits the contacting step un-reacted, is recycled to the SiHCl3 contacting step.
11. Process according to claims 6 or 7, comprising a single solidification step of
the purified liquid Si, using a method chosen from the group of crystal pulling, directional
solidification, and ribbon growth.
12. Process according to claims 6 or 7, comprising the granulation of the purified
liquid Si.
13. Process according to claim 12, comprising the steps of:

- feeding the granules to a melting furnace; and
- applying a single solidification step, using a method chosen from the group of crystal
pulling, directional solidification, and ribbon growth.

14. Process according to claims 11 or 13, whereby the solid material is wafered
and further processed to solar cells.

The invention relates to the manufacture of high purity silicon as a base material for the production of e.g. crystalline silicon solar cells. SiHCl3 is converted to Si metal by contacting gaseous SiHCl3 with liquid Zn, thereby obtaining a Si-bearing alloy, H2 and ZnCl2, which are 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 SiHCl3 towards the end product. The only other reactant is Zn, which can be obtained in very high purity grades, and which can be recycled after electrolysis of the Zn-chloride.