FIELD OF THE INVENTION
The present invention relates to a method of purifying a solution
containing hydrofluoric acid, nitric acid and at least silicon impurity. Further,
the present invention relates to a process of manufacturing silicon based solar
cell or silicon based electrode for battery.
BACKGROUND OF THE INVENTION
During the course of surface processing or surface cleaning of silicon
wafer in the photovoltaic manufacturing application, a mixture containing
mainly hydrofluoric acid (HF) and nitric acid (HN0 3) is used. For example, in a
process of surface texturing for crystalline silicon solar cell, such acid mixture
solution is used. However, when a fluoro-nitric acid mixture is applied as the
treating solution, silicon impurities, such as fluorosilicic acid (hexafluorosilicic
acid or hydrofluoro silicic acid ; H2SiF6), remain in the solution after etching the
surface of silicon wafer. The spent solution containing silicon impurities has less
etching performance, and thus, the spent solution is sent to waste when the
silicon concentration reaches a critical level and a new solution is introduced for
further etching process (so called "batch process").
The disposal of the spent HF/HNO3 mixture solution containing high level
of silicon impurity is difficult in an industrial size due to high load of fluorine
and nitrogen. Also, such disposal leads to high amount of consumption of the
mixture solution of hydrofluoric acid and nitric acid, which is not economically
favored. As such, an efficient purification and recycling of the spent HF/HNO3
mixture solution containing silicon impurity could be advantageous.
Japanese patent Kokai No. 2012-125723 discloses an apparatus for
removing hydrofluoro silicic acid contained in the waste liquid of a
nitrohydrofluoric acid, and recovering the refined liquid of the nitrohydrofluoric
acid by an electrodialysis having a specific configuration. However, the
electrodialysis described in said patent application requires continuous use of
further reagents, which may cause another economically unfavorable issue, and
in addition, the reduction degree of silicon impurity by the above-mentioned
method was not sufficient. For example, the concentration of H2SiF6 in feed
solution which is 2 % is reduced to 0.5 % at outlet of the purification step
in Example 1 of the Japanese patent application.
The invention now makes available an efficient method of purifying the
solution which does not show the above-mentioned problems.
DESCRIPTION OF THE INVENTION
Accordingly, it is an object of the present invention to provide an efficient
method of purifying a solution containing hydrofluoric acid, nitric acid and at
least one silicon impurity.
The present invention therefore relates to a method of purifying a solution
containing hydrofluoric acid, nitric acid and at least one silicon impurity by
treating the solution with at least one reverse osmosis (RO) membrane. The
method of purification according to the present invention can be advantageously
used for a process of treating silicon containing surface, in particular,
manufacturing silicon based solar cell or silicon based electrode for battery,
wherein the solution containing hydrofluoric acid and nitric acid is used for
etching the silicon surface of the cell or device.
It has been surprisingly found by the present inventor that the method of
the present invention enables very effective and selective removal or reduction of
silicon impurity contained in the solution containing hydrofluoric acid and nitric
acid, without substantial reduction of the contents of the two acids, thus making
the solution available to be advantageously recycled.
In the present invention, "silicon impurity" is understood to denote, in
particular, an impurity comprising at least silicon atom and preferably at least
one fluorine atom. More particularly, the silicon impurity can be produced by
the contact of the solution containing hydrofluoric acid and nitric acid with
silicon surface of the object structure. Preferably, in the present invention,
silicon impurity includes fluorosilicic acid such as hexafluorosilicic acid H2SiF6
produced by a contact of said solution with silicon surface of solar cell or of
electrode for battery wherein the solution containing hydrofluoric acid and nitric
acid is used to treat, process and/or clean such surface. The fluorosilicic acid can
be dissociated in the solution, and as such, corresponding anions, for instance
HSiF 2- 6 and SiF6 , can also exist as the silicon impurity in the solution.
All weight percentages disclosed herein are expressed relative to the total
weight of the respective solution.
Further essential feature in the method of the present invention resides on
use of at least one reverse osmosis (RO) membrane. The inventor of the present
invention has found that RO membrane can reject most of the silicon impurity
very effectively while hydrofluoric acid and nitric acid in the solution are not
significantly rejected by it.
In the present invention, the RO membranes are preferably selected from
the group consisting of flat sheet membrane, spiral wound membrane, and
combination thereof.
The reverse osmosis membrane in the present invention can preferably be
thin film composite type. As the material of the membrane, polyamides,
polypiperazine amides, polyacrylonitriles, polysulfones, cellulose acetates,
polybenzimidazolines, polyoxadiazoles, polyfuranes, polyether-polyfuranes,
polyvinylamines, polypyrrolidines, carboxylated polysulfones or sulfonated
polysulfones can be preferably used. More preferably, the reverse osmosis
membrane is made of polyamides, polypiperazine amides, polyacrylonitriles,
polysulfones or sulfonated polysulfones, still more preferably polyamides.
In the present invention, the reverse osmosis membrane can be used in a
form of membrane module (spiral wound or flat sheet). One or more membrane
module(s) can be used in any configuration. Examples of the configuration
include series, parallel and combination of series and parallel.
Unlike the solution containing only hydrofluoric acid and silicon
impurities often used in semiconductor industry for the purpose of etching, the
solution intended to be subject to the purification system of the present invention
further comprises nitric acid which may bring the pH of the solution to a very
low value, for instance, by 3 to 4 orders of magnitude lower than the
hydrofluoric acid etching solution. This in turn increases the ionic strength of
the solution, and thus, may cause a substantial impact on the ionic form of the
silicon impurities. As such, proper technical operations need to be sought for the
efficient purification by the method according to the present invention.
In the present invention, the method can be performed at the temperature
from - 20°C to + 20°C. Also, the temperature in the method according to the
present invention can be no more than + 40°C, particularly no more than + 30°C.
In the present invention, the temperature is preferably around room temperature.
Flow can be varied and co-relates to the temperature condition. The pressure for
operating RO membrane can be determined according to the exact composition
of the solution. According to preferred embodiment, as the purification method
primarily targets the solution containing hydrofluoric acid, nitric acid, and at
least one silicon impurity, particularly hexafluoro silicic acid, the pressure for
operating the method according to the present invention is at least 30 bars.
Upper limit of the pressure is usually 100 bars, preferably 70 bars. Excellent
results can be attained in accordance with the pressure being in said range.
Various types of the solution containing hydrofluoric acid, nitric acid, and
at least one silicon impurity can be the subject of the purification method
according to the present invention. For example, the solution containing
hydrofluoric acid, nitric acid and at least one silicon impurity can be preferably a
solution spent for surface processing or surface cleaning for silicon wafer in
photovoltaic field. In addition, the solution can be a spent solution used for
etching the surface of silicon electrode for battery, in particular, secondary
lithium battery.
Preferably, the solution which can be used for etching the surface of silicon
wafer or silicon electrode mainly contains hydrofluoric acid and nitric acid and
further contains silicon compound as an impurity.
In the present invention, the content of nitric acid in the solution is
preferably from 0.1 % w/w to 10 % w/w, more preferably from 2 % w/w
to 10 % w/w, most preferably from 4 % w/w to 9 % w/w, before the purification
by RO membrane.
In the present invention, the content of hydrofluoric acid in the solution to
be treated can be variable depending on the application wherein the solution was
used. The content of hydrofluoric acid in the solution is preferably
from 0.001 % w/w to 50 % w/w, more preferably 0.5 % w/w to 30 % w/w, most
preferably from 1 % w/w to 20 % w/w, before the purification by RO membrane.
In the present invention, the content of silicon impurity in the fresh
solution would preferably be substantially 0 % w/w. However, once the solution
has been contacted with a silicon containing surface, it will contain certain
silicon impurities. The content of silicon impurity in such solution can be
variable depending on the application wherein the solution was used. The
content of silicon impurity in the solution is preferably from 0.1 % w/w
to 30 % w/w, more preferably from 0.5 % w/w to 20 % w/w, most preferably
from 1 % w/w to 10 % w/w, before the purification by RO membrane.
In the present invention, at least 90 , in particular at least 95 ,
especially at least 99 % of the silicon impurity in the solution is preferably
rejected by a treatment with the RO membrane.
Further, an additional additive can be optionally contained in the solution
of the present invention. The examples of such additives include a surfactant, a
further acid, such as acetic acid, etc.
The remainder of the solution usually consists of water so as to make up
100 % of the solution.
The exact composition of the solution containing hydrofluoric acid and
nitric acid can be varied depending on the technology field to be applied.
In the present invention, the solution purified by the reverse osmosis
membrane is preferably recycled to a flow of treatment of a silicon containing
surface for further use. Such treatment includes a surface texturing of solar cells,
in particular silicon based solar cell, or an etching of silicon based electrode for
batteries, in particular lithium ion battery, as explained above. Through this
recycling process, the spent solution can be recovered and be sent to the
circulation of the process for further use and thus consumption of the solution
can be drastically reduced.
In the present invention, it is intended to be most preferred to reject all of
the silicon impurity in the solution containing hydrofluoric acid and nitric acid
by at least one reverse osmosis membrane while maintaining similar or slightly
reduced contents of hydrofluoric acid and nitric acid in the solution which is
available for further use. However, when the contents of the two acids in the
solution are not sufficiently high after the purification with RO membrane, these
two acids can be spiked to recover their original concentration in the solution.
Therefore, in one particular embodiment of the present invention, the method of
purification according to the present invention further comprises a step of adding
hydrofluoric acid and/or nitric acid to set the concentration of hydrofluoric acid
and nitric acid contained in the solution to a target level. When there are further
components other than hydrofluoric acid and/or nitric acid in the solution and if
the content of such components in the solution is reduced by the reverse osmosis
membrane process, such components can be also added. These optional steps
allow to recover a solution suitable for use in a further treatment step of a silicon
containing surface.
The method of purifying the solution containing hydrofluoric acid, nitric
acid and at least one silicon impurity with at least one reverse osmosis membrane
according to the present invention can be advantageously applied for a process of
manufacturing silicon (in particular crystalline polysilicon) based solar cell to
recycle the solution containing hydrofluoric acid and nitric acid used for
texturing the silicon surface. Therefore, another aspect of the present invention
is a process for manufacturing silicon based solar cell, at least comprising a step
wherein a solution containing hydrofluoric acid, nitric acid and at least one
silicon impurity is purified in accordance with the process according to the
present invention. The above-described preferences made to the details, such as
the contents of hydrofluoric acid, nitric acid and silicon impurity in the solution,
the types of RO membrane, the origin and specific compounds of the silicon
impurity, the possibility of having further steps, etc., can be similarly adopted in
this aspect of the invention.
Also, the method of purifying the solution containing hydrofluoric acid,
nitric acid and at least one silicon impurity with at least one reverse osmosis
membrane according to the present invention can be advantageously applied for
a process of manufacturing silicon based electrode for battery to recycle the
solution used for etching the silicon surface of the electrode. Therefore, further
aspect of the present invention is a process for manufacturing silicon based
electrode for battery, at least comprising a step wherein a solution containing
hydrofluoric acid, nitric acid and at least one silicon impurity is purified in
accordance with the process according to the present invention. The abovedescribed
preferences made to the details, such as the contents of hydrofluoric
acid, nitric acid and silicon impurity in the solution, the types of RO membrane,
the origin and specific compounds of the silicon impurity, the possibility of
having further steps, etc., can be similarly adopted in this aspect of the invention.
Should the disclosure of any patents, patent applications, and publications
which are incorporated herein by reference conflict with the description of the
present application to the extent that it may render a term unclear, the present
description shall take precedence.
The following example illustrates the invention in further detail without
however limiting it.
Example
Example 1
In order to confirm the performance of the method according to the present
invention, a lab scale device model for membrane system is used. A test is made
with a small flat sheet membrane unit working in full recirculation mode.
Pressure is applied using the feed/recirculation pump, and static pressure is
applied using an inert gas, if necessary. The maximum operating pressure
is ~ 30 bars.
A thin film composite polyamide membrane is used for testing.
A feed solution matching the typical composition of a spent acid wafer
texturing mixture (HNO 3 5 % w/w, HF 2 % w/w, and H2SiF 6 2 % w/w) is
processed through the membrane. It is demonstrated that over 95 % of silicon
impurities are rejected by the RO membrane, while approximately 2/3 of the
acids are maintained in the permeate.
This clearly demonstrates that silicon is almost completely rejected by the
membrane while HNO 3 and HF are not significantly rejected by the membrane.
Thus, advantage of using reverse osmosis membrane in recycling the acid wafer
texturing mixture used in the photovoltaic industry is proven.

C L A I M S
1. A method of purifying a solution containing hydrofluoric acid, nitric
acid and at least one silicon impurity by treating the solution with at least one
reverse osmosis membrane.
2. The method according to claim 1, wherein the solution mainly
contains hydrofluoric acid and nitric acid and comprises silicon compound as an
impurity.
3. The method according to claim 1 or 2, wherein the content of nitric
acid in the solution before the purification is from 0.1 % w/w to 10 % w/w,
preferably from 2 % w/w to 10 % w/w, more preferably from 4 % w/w
to 9 % w/w.
4. The method according to any one of claims 1 to 3, wherein the content
of hydrofluoric acid in the solution before the purification is from 0.001 % w/w
to 50 % w/w, preferably from 0.5 % w/w to 30 % w/w, more preferably
from 1% w/w to 20 % w/w.
5. The method according to any one of claims 1 to 4, wherein the content
of silicon impurity in the solution before the purification is from 0.1 % w/w
to 30 % w/w, preferably from 0.5 % w/w to 20 % w/w, more preferably
from 1% w/w to 10 % w/w.
6. The method according to any one of claims 1 to 5, wherein the reverse
osmosis membrane is made of polyamides, polypiperazine amides,
polyacrylonitriles, polysulfones, cellulose acetates, polybenzimidazolines,
polyoxadiazoles, polyfuranes, polyether-polyfuranes, polyvinylamines,
polypyrrolidines, carboxylated polysulfones or sulfonated polysulfones,
preferably polyamides, polypiperazine amides, polyacrylonitriles, polysulfones
or sulfonated polysulfones.
7. The method according to any one of claims 1 to 6, wherein one or
more reverse osmosis membranes are used, in series or in parallel, or in
combination of series and parallel.
8. The method according to any one of claims 1 to 7, wherein the silicon
impurity is generated from contacting a solution containing hydrofluoric acid
and nitric acid with a silicon containing surface.
9. The method according to claim 8, wherein the contacting a solution
containing hydrofluoric acid and nitric acid with a silicon containing surface is
selected from a surface texturing of solar cells, in particular silicon based solar
cell, or an etching of silicon based electrode for batteries, in particular lithium
ion battery, preferably is a surface texturing of silicon based solar cell.
10. The method according to claim 8 or 9, further comprising recycling
the purified solution to a flow of treatment of the silicon containing surface.
11. The method according to any one of claims 1 to 10, further comprising
a step of adding hydrofluoric acid and/or nitric acid to set the concentration of
hydrofluoric acid and nitric acid contained in the solution to a target level.
12. A process of manufacturing silicon based solar cell, at least
comprising a step of purifying a solution containing hydrofluoric acid, nitric acid
and at least one silicon impurity in accordance with the method according to any
one of claims 1 to 11.
13. A process of manufacturing silicon based electrode for battery, at least
comprising a step of purifying a solution containing hydrofluoric acid, nitric acid
and at least one silicon impurity in accordance with the method according to any
one of claims 1 to 11.
14. The process according to claim 12 or 13, further comprising recycling
the purified solution to a flow of treatment of a silicon containing surface during
the manufacturing process.
15. The process according to any one of claims 12 to 14, wherein the
reverse osmosis membrane is made of polyamides, polypiperazine amides,
polyacrylonitriles, polysulfones, cellulose acetates, polybenzimidazolines,
polyoxadiazoles, polyfuranes, polyether-polyfuranes, polyvinylamines,
polypyrrolidines, carboxylated polysulfones or sulfonated polysulfones,
preferably polyamides, polypiperazine amides, polyacrylonitriles, polysulfones
or sulfonated polysulfones.