U.S. patent application number 12/023170 was filed with the patent office on 2012-05-31 for methods for electrochemical dechlorination of anolyte brine from nacl electrolysis.
This patent application is currently assigned to Bayer MaterialScience AG. Invention is credited to Andreas Bulan, Rainer Weber.
Application Number | 20120132539 12/023170 |
Document ID | / |
Family ID | 39327083 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120132539 |
Kind Code |
A1 |
Bulan; Andreas ; et
al. |
May 31, 2012 |
METHODS FOR ELECTROCHEMICAL DECHLORINATION OF ANOLYTE BRINE FROM
NaCl ELECTROLYSIS
Abstract
Methods for the reductive post-treatment of NaCl-containing
solutions, wherein such methods comprise: providing a
NaCl-containing solution obtained from an anode side of an NaCl
electrolysis cell, the solution comprising reducible components;
and subjecting the solution to cathodic electrochemical
reduction.
Inventors: |
Bulan; Andreas; (Langenfeld,
DE) ; Weber; Rainer; (Odenthal, DE) |
Assignee: |
Bayer MaterialScience AG
Leverkusen
DE
|
Family ID: |
39327083 |
Appl. No.: |
12/023170 |
Filed: |
January 31, 2008 |
Current U.S.
Class: |
205/625 ;
205/618 |
Current CPC
Class: |
C02F 2103/34 20130101;
C02F 1/722 20130101; C02F 1/4676 20130101; C25B 1/26 20130101; C25B
15/08 20130101; C25B 1/46 20130101 |
Class at
Publication: |
205/625 ;
205/618 |
International
Class: |
C25B 1/26 20060101
C25B001/26; C25B 1/02 20060101 C25B001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 3, 2007 |
DE |
102007005541.4 |
Claims
1. A method comprising: providing a NaCl-containing solution
obtained from an anode side of an NaCI electrolysis cell, the
solution comprising reducible components; and subjecting the
solution to cathodic electrochemical reduction.
2. The method according to claim 1, further comprising treating the
NaCl-containing solution with hydrogen peroxide.
3. The method according to claim 2, wherein the NaCl-containing
solution is treated with hydrogen peroxide before the cathodic
electrochemical reduction.
4. The method according to claim 2, wherein the NaCl-containing
solution is treated with hydrogen peroxide after the cathodic
electrochemical reduction.
5. The method according to claim 1, further comprising treating the
NaCl-containing solution with an aqueous hydrogen peroxide
solution.
6. The method according to claim 5, wherein the NaCl-containing
solution is treated with the aqueous hydrogen peroxide solution
before the cathodic electrochemical reduction.
7. The method according to claim 5, wherein the NaCl-containing
solution is treated with the aqueous hydrogen peroxide solution
after the cathodic electrochemical reduction.
8. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out in a cell having an anode
compartment and a cathode department separated by an ion-exchange
membrane or diaphragm.
9. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out galvanostatically.
10. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out potentiostatically.
11. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out with an anode and a
cathode each independently comprising a material selected from the
group consisting of carbon, graphitized carbon, graphite, and
titanium coated with a noble metal or a noble metal oxide.
12. The method according to claim 1, wherein during the cathodic
electrochemical reduction of the reducible components, which is
carried out in an electrolysis cell having an anode compartment and
a cathode compartment, an anodic reaction occurs in the anode
compartment, the anodic reaction selected from the group consisting
of chlorine production, oxygen production, oxidation of iron(II)
chloride to iron(III) chloride, and combinations thereof.
13. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out using a cathode having an
internal surface area and an external, geometric surface area,
wherein the internal surface area is greater than the external,
geometric surface area.
14. The method according to claim 13, wherein the internal surface
area is at least twice the external, geometric surface area.
15. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out in an electrolysis cell
having a cathode compartment, and wherein the NaCl-containing
solution has a residence time in the cathode compartment of 1 to 30
minutes.
16. The method according to claim 15, wherein the NaCl-containing
solution has a residence time in the cathode compartment of 1 to 10
minutes.
17. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out at a current density of 5
to 100 A/m.sup.2.
18. The method according to claim 1, wherein the cathodic
electrochemical reduction is carried out at a charge introduction
amount of 0.05 to 0.5 Ah/l where the NaCl-containing solution has a
chlorine content of about 100 mg/l.
19. The method according to claim 1, wherein at least 80% of the
reducible components are reduced by the cathodic electrochemical
reduction.
20. The method according to claim 19, wherein remaining residues of
compounds to be reduced are treated by addition of one or more
sulfur-containing reducing agents.
Description
BACKGROUND OF THE INVENTION
[0001] Conventionally, membrane electrolysis methods are used, for
example, for electrolyzing sodium chloride-containing solutions
(see, e.g., Peter Schmittinger, CHLORINE, Wiley-VCH Verlag, 2000).
A divided electrolysis cell can be used in this case, which divided
cell consists of an anode compartment with an anode and a cathode
compartment with a cathode. The anode and cathode compartments can
be separated by an ion-exchange membrane. A sodium
chloride-containing solution, also referred to hereinafter as
brine, having a sodium chloride concentration of conventionally
approx. 300 g/l, is introduced into the anode compartment of such a
cell. The chloride ion in the brine is oxidized to yield chlorine
on the anode, and the chlorine can then be conveyed out of the cell
with the depleted sodium chloride-containing solution, also
referred to herein as the anolyte brine, which can have a remaining
sodiumchloride concentration of approx. 200 g/l.
[0002] So that the sodium chloride, which passes out of the cell in
the depleted NaCl-containing solution, does not have to be
discarded, this solution can be concentrated again with solid
sodium chloride. In so doing, impurities, such as calcium, iron,
aluminium or magnesium compounds or sulfates pass from the added
sodium chloride into the brine, such that purification has to be
performed. It is conventionally attempted, for example, to remove
the iron and aluminium impurities by precipitation and subsequent
filtration. The calcium and magnesium ions are conventionally
removed by ion exchange resins. To protect the
precipitation/filtration apparatus and ion exchange resins from
chlorine and chlorine compounds, hypochlorites and chlorates, these
strong oxidizing agents generally have to be removed. To accomplish
such removal, the chlorine/hypochlorite concentration of the
chlorine-containing anolyte brine is first lowered by lowering the
pH, and then, for example, by stripping with steam. In this way, a
chlorine content of less than 100 ppm may be achieved in the
NaCl-containing solution. Moreover, in order to remove the residual
remaining chlorine, which is in hydrolysis equilibrium with
hypochlorite, chemical reduction is conventionally performed, e.g.
with sodium bisulfite. The NaCl solution is then concentrated with
solid sodium chloride and fed to the precipitation/filtration
process, calcium and magnesium ions are removed by ion exchange
resins and the solution is fed once again to the anode part of the
electrolysis cell.
[0003] One disadvantage of using sodium bisulfite and similar
sulfur-containing compounds for chemical dechlorination is that
sulfate arises in the NaCl solution as a reaction product of the
chemical reduction with bisulfite or sulfur-containing compounds.
However, the sulfate cannot be electrochemically degraded, such
that it gradually accumulates in the NaCl-containing solution.
Modern high performance ion exchange membranes are damaged by
relatively high concentrations of sulfates, however. An excessively
high concentration of sulfate in the brine brings about the
formation of oxygen on the anode and reduces current efficiency, so
impairing the economic viability of the electrolysis method. Damage
to the coating of the anode is likewise possible. Manufacturers
therefore state maximum limit values for sulfate concentration in
the NaCl solution. As a result of recirculating the brine, some of
the NaCl solution must be removed and discarded so as to prevent
damage to the anode coating and membrane etc. In this way, large
amounts of sodium chloride are lost, which has a negative effect on
the economic viability and environmental compatibility of the
electrolysis method.
[0004] To prevent or reduce brine removal due to sulfate build-up,
it is possible, for example, to use the nanofiltration method or
the DSR method, the latter being a chromatographic method using
amphoteric resins, with which methods sulfate may be removed from
the brine (see, e.g., WINNACKER KUCHLER, Chemische Technik Prozesse
and Produkte, 5th edition, Vol. 3 Anorganische Grundstoffe,
Zwischenprodukte (inorganic primary materials, intermediates),
2005, page 438, et seq.). One disadvantage of such methods is that
sulfate formation is not prevented, but rather an additional method
step is necessary in order to remove sulfate from the brine.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention relates, in general, to methods of
removing chlorine from a solution containing NaCl, which solution
can originate from the anode half-cell of an NaCl electrolysis
cell. The chlorine contained in the NaCl-containing solution is
subjected to electrochemical treatment on a negatively polarized
electrode.
[0006] The various embodiments of the present invention provide
post-treatment methods for NaCl solutions which can achieve
dechlorination with a markedly smaller addition of
sulfur-containing reducing agents or even without such addition as
compared with known methods. The various embodiments of the present
invention provide dechlorination methods which exhibit a
significant improvement over methods known in the art.
[0007] The present inventors have found that it is possible to
dispense completely, or for the most part, with the removal, of
chlorine by acidification and/or stripping with steam, and
subsequent chemical reduction of the chlorine in the anolyte brine
with sulfur-containing compounds, if the anolyte brine is subjected
to electrochemical dechlorination.
[0008] The present invention provides a method for the reductive
post-treatment of NaCl-containing solutions obtainable from the
anode side of an NaCl electrolysis, characterized in that the
reducible components in the NaCl-containing solution are reduced by
cathodic electrochemical reduction.
[0009] One embodiment of the present invention includes a method
comprising: providing a NaCl-containing solution obtained from an
anode side of an NaCl electrolysis cell, the solution comprising
reducible components; and subjecting the solution to cathodic
electrochemical reduction
[0010] The NaCl-containing solution, which can be treated according
to the various embodiments of methods of the present invention, may
originate in particular from NaCl electrolysis using membrane
methods, NaCl electrolysis using membrane methods in which a gas
diffusion electrode is used on the cathode side, or from diaphragm
methods.
[0011] The methods according to the invention may be used to remove
dissolved chlorine, hypochlorite which is still present, chlorate
and other reducible compounds such as for example nitrogen
trichloride, without reaction products passing into the
NaCl-containing solutions, whereby markedly smaller quantities of
the NaCl-containing solution have to be worked-up, or removed and
discarded. The economic viability and environmental compatibility
of NaCl electrolysis performed using the methods according to the
various embodiments of the invention are markedly improved.
DETAILED DESCRIPTION OF THE INVENTION
[0012] As used herein, the singular terms "a" and "the" are
synonymous and used interchangeably with "one or more" and "at
least one," unless the language and/or context cleary indicates
otherwise. Accordingly, for example, reference to "a material"
herein or in the appended claims can refer to a single material or
more than one material. Additionally, all numerical values, unless
otherwise specifically noted, are understood to be modified by the
word "about."
[0013] Certain preferred embodiments of the present invention
include methods wherein the NaCl-containing solution is
additionally treated before or after the electrochemical reduction
by chemical reduction via treatment with hydrogen peroxide, and
more preferably, an aqueous hydrogen peroxide solution.
[0014] Generally, hypochlorite and chlorate are, inter alia, also
present in addition to chlorine in the chlorine-containing anolyte
brine passing out of the anode chamber. These compounds or ions can
be reduced on a cathode under cathodic potential, for example,
according to the following reaction equations:
cathode: 2Cl.sub.2+2e.sup.-.fwdarw.2Cl.sup.-
2OCl.sup.-+4H.sup.++2e.sup.-.fwdarw.Cl.sub.2+2H.sub.2O
2ClO.sub.3.sup.-+6H.sup.++4e.sup.-.fwdarw.Cl.sub.2+3H.sub.2O
[0015] At the counter-electrode, the cell's anode, chlorine may for
example be produced from sodium chloride-containing solution. It is
likewise conceivable for oxygen to evolve on the anode or for
iron(II) chloride to be oxidised to yield iron(III) chloride.
[0016] The electrode compartments may for example be separated by
an ion-exchange membrane, including, e.g., conventional commercial
membranes of the type DUPONT NX 982 or 324 made by DuPont de
Nemours. In this way, mixing of anolytes and catholytes and the
components contained therein and mixing of the gases formed at the
respective electrodes can be prevented.
[0017] Diaphragms may also be used to separate the electrode
compartments. If a diaphragm, for example, is used to separate the
electrode compartments, the electrode compartments should be
rendered inert, in order to prevent the formation of explosive
mixtures such as for example mixtures of chlorine and hydrogen.
[0018] Hydrogen may be formed in membrane or diaphragm methods
during the cathodic reaction at elevated current densities or in
the case of excessive residence times of the electrolytes in the
cathode chamber in the event of galvanostatic operation of the
cell. Galvanostatic operation means that a current intensity is
established, and this is maintained by adjusting the voltage. Thus,
in the event of galvanostatic operation, electrolysis is performed
at a generally constant current intensity, the cell voltage being
adjusted accordingly. In this case, where the residence time of the
electrolytes in the cell is too long, the secondary reaction of
water electrolysis may take place, during which hydrogen is formed
at the cathode. To avoid this, the surface area of the cathode may
be enlarged, e.g., by using three-dimensional cathodes. A
three-dimensional cathode can include, for example, a graphite bed
or carbon nonwovens.
[0019] However, the electrolysis cell may also be
potentiostatically operated, i.e., at a constant potential
corresponding to a constant cell voltage. Operation at constant
potential has the advantage that, at a sufficiently low selected
potential, the above-stated compounds may be reduced without
hydrogen formation. One disadvantage is that very high current
densities cannot be selected, such that a long residence time is
necessary and/or a large anode surface area should be provided.
[0020] If chlorine is produced on the anode from a sodium
chloride-containing solution, the chlorine may be fed into the
already existing substance circuits of the NaCl electrolysis.
Likewise, oxygen could be evolved anodically and further
utilized.
[0021] If a brine is used as anolyte and chlorine is produced, and
if at the same time the electrolysis cell is provided with a
diaphragm, the pressure in the cathode chamber should preferably be
higher than that in the anode chamber, so that the catholyte passes
into the anode chamber and chlorine-containing anolyte does not
pass into the cathode chamber. If the reverse were the case,
chlorine-containing anolyte would be forced into the catholyte,
which would be undesirable since chlorine needs to be removed from
the catholyte.
[0022] The anode material used is, for example, a standard material
for NaCl electrolysis anodes, such as titanium provided with a
coating containing a noble metal or a noble metal oxide. This
material could likewise be used as the cathode material. Generally,
materials resistant to chlorine and sodium chloride-containing
solutions may be used. Noble metals are here understood in
particular to be metals from the series comprising osmium, iridium,
platinum, ruthenium, rhodium and palladium.
[0023] Carbon, graphitized carbon and/or graphite may likewise be
used as the cathode material. Various shapes of electrode may be
produced therefrom.
[0024] The pH value of the electrolytes may preferably be so
selected that sufficient material strength is achieved if the
chlorine is present for the most part as hypochlorite. This may be
the case preferably at a pH value greater than 7. However, it is
likewise feasible for the pH value to be lower than 7, such that
less chlorine is present in dissolved form as hypochlorite.
[0025] Since each pH adjustment constitutes additional expense, the
chlorine-containing anolyte is preferably introduced untreated from
the NaCl electrolysis anode chamber directly into the cathode
chamber for electrochemical chlorine reduction.
[0026] Since very low concentrations of compounds such as chlorine,
hypochlorite, chlorate or nitrogen trichloride have to be reduced,
it is advantageous for the local current density at the cathode to
be selected to be very low. To this end, electrodes which have a
large surface area may be used as the cathode. Cathodes with a
large surface area may be understood to include those in which the
internal surface area is larger than the external, geometric
surface area, preferably at least twice as large.
[0027] For example, use may be made of metal electrodes with a foam
structure, e.g., of titanium sponge, sintered titanium electrodes,
or spherical metals, graphite or foam-like graphite, graphite
coated with a noble metal, woven carbon fabrics, carbon cloth and
carbon nonwovens.
[0028] Metal electrodes may preferably be coated with noble
metal(s), noble metal oxide(s) or noble metal compound(s) or
mixtures thereof. Graphite electrodes may likewise preferably
contain noble metal(s), noble metal oxide(s) or noble metal
compound(s) or mixtures thereof.
[0029] The residence time of the NaCl-containing solution can
preferably be adjusted in such a way that, as far as possible, all
reducible compounds may be reduced, without there being any onset
of water reduction according to
2H.sub.2O+2e.sup.-.fwdarw.H.sub.2+2OH.sup.-
[0030] The residence time in the cathode compartment is preferably
about 1 to 30 minutes, and more preferably 1 to 10 minutes.
[0031] The current density, calculated in relation to the true
surface area, is preferably about 5 to 100 A/m.sup.2. The amount of
charge to be introduced for a chlorine content of anolyte brine of
100 mg/l amounts to 0.05 to 0.5 Ali/1 of anolyte brine. Markedly
higher amounts of charge are needed if hydrogen evolution is
permitted as a parallel reaction.
[0032] A further preferred alternative embodiment of the new method
is the additional treatment of NaCl-containing solution from an
NaCl electrolysis anode chamber by addition of hydrogen peroxide.
One advantage of the addition of hydrogen peroxide over the
addition of for example sodium sulfite or sodium bisulfite is that
no sulfate forms in the NaCl-containing solution during chemical
reduction, but instead only water. When employing the addition of
hydrogen peroxide, unreacted chlorine, chlorate or hypochlorite
and/or optionally excess hydrogen peroxide is reduced in an
electrochemical cell in the cathode chamber connected downstream of
NaCl hydrolysis. The added quantity of hydrogen peroxide should
preferably correspond as far as possible to the redox equivalent of
the compounds to be reduced. Either a deficit or an excess of, for
example, 0.95 parts or 1.2 parts, respectively, relative to the
redox equivalent, may be used. Preferably, a deficit is used.
[0033] The hydrogen peroxide may be apportioned to the
chlorine-containing anolytes for example by means of a pump, and
mixing may take place for example by means of a static mixer in a
pipe. The solution treated in this way may then be reduced
electrochemically. Excess hydrogen peroxide is then preferably
reduced, as are other reducible compounds still present.
[0034] A similarly feasible preferred embodiment of the method
according to the invention consists firstly in electrochemically
reducing only the majority, i.e. at least 80%, preferably at least
90%, particularly preferably at least 95%, of the chlorine present,
and then in treating the remainder of the chlorine, chlorate and
hypochlorite for example by means of conventional chemical
reduction by the addition of for example sodium sulfite or hydrogen
peroxide. In this way, the purge quantity of brine may likewise be
markedly reduced over the prior art.
[0035] The invention will now be described in further detail with
reference to the following non-limiting examples.
EXAMPLES
[0036] The electrolysis cell used in the Examples below for the
reduction of chlorine, chlorate and hypochlorite consists of an
anode compartment with an anode and a cathode compartment with a
cathode, which is formed of a bed of graphite and a current
distributor. The material of the anode in the anode compartment and
of the current distributor in the cathode compartment consists of a
titanium expanded metal coated with noble metal oxide, a so-called
Standard DSA.RTM. Coating made by Denora. The volume of the anode
or cathode compartment with anode or current distributor amounts to
230 ml.
[0037] The electrolyte was introduced both into the anode and into
the cathode compartment from below and removed again from
above.
[0038] Anode compartment and cathode compartment are separated by a
commercially available ion-exchange membrane from DuPont de
Nemours: DUPONT 324 or Nafion 982. The membrane area amounts to 100
cm.sup.2.
[0039] An NaCl-containing solution with an NaCl concentration of
204 g/l was introduced into the anode compartment at a volumetric
flow rate of 1.01/h.
[0040] The brine to be treated, as may conventionally be removed
from the anode compartment of an NaCl electrolysis, was introduced
into the cathode compartment. The composition was as follows: the
NaCl content was approx. 200 g/l, pH value approx. 4, the chlorine
content approx. 400-450 mg/l.
[0041] In the Examples given, no hydrogen was evolved in the
cathode compartment on the cathode, consisting of a graphite bed of
graphite balls with an average diameter of 2 mm, this being
monitored by measurement.
Example 1
[0042] In the above-described cell, provided with a Nation 982
ion-exchange membrane from DuPont de Nemours, the
chlorine-containing, NaCl-depleted anolyte brine was passed at a
volumetric flow rate of 1.0 l/h out of the NaCl electrolysis into
the cathode compartment with a chlorine content of 422 mg/l. The
cathode compartment was filled with graphite balls, the residual
volume of the cathode compartment after deduction of the volume of
graphite balls amounting to 160 ml. The residence time of the brine
to be treated in the cathode compartment was 5.6 min. The voltage
amounted to 1.72 V, and the current intensity to 0.8 A. The
concentration of chlorine in the outflow of the cathode compartment
was approx. 89 mg/l. The pH value of the anolyte brine was pH 4. A
charge of 0.48 Ah/l of brine was introduced. The current density
relative to the total surface area of the graphite balls used was
8.5 A/m.sup.2.
Example 2
[0043] The chlorine-containing anolyte brine from another NaCl
electrolysis with a corresponding chlorine content of 1522 mg/l and
a pH value of 10 was introduced at 1.0 l/h into the above-described
cell, provided with a Nafion 324 ion-exchange membrane from DuPont
de Nemours. The cathode compartment was filled with graphite balls,
the residual volume of the cathode compartment after deduction of
the volume of graphite balls amounting to just 95 ml. The residence
time of the NaCl brine to be reduced was 5.7 min. The cell voltage
amounted to 2.33 V, and the current intensity to 1.5 A. The
concentration of chlorine in the outflow of the cathode compartment
was approx. 113 mg/l. The current density relative to the total
surface area of the graphite balls used was 9.7 A/m.sup.2.
Example 3
[0044] The chlorine-containing anolyte brine from an NaCl
electrolysis with a corresponding chlorine content of 422 mg/l and
a pH value of 4 was introduced at 1.1 l/h into the above-described
cell, provided with a DUPONT Nation 982 ion-exchange membrane. The
cathode compartment was filled with graphite balls, the residual
volume of the cathode compartment after deduction of the volume of
graphite balls amounting to 95 ml. The residence time of the brine
to be reduced in the cathode compartment was 5.3 min. The cell
voltage amounted to 1.72 V, and the current intensity to 0.8 A. The
concentration of chlorine in the outflow of the cathode compartment
was less than 1 mg/l. The current density relative to the total
surface area of the graphite balls used was 5.2 A/m.sup.2.
[0045] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiments disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined, by the appended claims.
* * * * *