U.S. patent application number 12/016291 was filed with the patent office on 2008-10-23 for method for improving the performance of nickel electrodes.
This patent application is currently assigned to Bayer Material Science AG. Invention is credited to Andreas Bulan, Richard Malchow, Rolf Spatz, Rainer Weber, Hermann-Jens Womelsdorf.
Application Number | 20080257749 12/016291 |
Document ID | / |
Family ID | 39322798 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080257749 |
Kind Code |
A1 |
Bulan; Andreas ; et
al. |
October 23, 2008 |
Method For Improving The Performance of Nickel Electrodes
Abstract
The invention relates to a method for improving the performance
of nickel electrodes in alkali chloride electrolysis by adding
water-soluble platinum compounds to the catolyte.
Inventors: |
Bulan; Andreas; (Langenfeld,
DE) ; Weber; Rainer; (Odenthal, DE) ; Malchow;
Richard; (Koln, DE) ; Spatz; Rolf; (Bergisch
Gladbach, DE) ; Womelsdorf; Hermann-Jens; (Neuss,
DE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ, LLP
P O BOX 2207
WILMINGTON
DE
19899
US
|
Assignee: |
Bayer Material Science AG
Leverkusen
DE
|
Family ID: |
39322798 |
Appl. No.: |
12/016291 |
Filed: |
January 18, 2008 |
Current U.S.
Class: |
205/350 |
Current CPC
Class: |
C25D 3/567 20130101;
C25D 21/18 20130101; C25D 3/50 20130101; C25D 21/14 20130101; C25B
15/08 20130101; C25D 5/18 20130101; C25B 1/46 20130101; C25B 11/051
20210101; C25B 11/075 20210101 |
Class at
Publication: |
205/350 |
International
Class: |
C25C 7/00 20060101
C25C007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2007 |
DE |
102007003554.5 |
Claims
1. A method for improving the performance of nickel electrodes that
are used in a membrane sodium chloride electrolytic process
comprising: (a) preparing a water-soluble or alkali-soluble
platinum solution comprising: (i) a solvent and (ii) a soluble
platinum compound and (b) adding the solution to the catolyte and
thereby forming a coating on the nickel electrode.
2. The method according to claim 1, wherein the platinum compound
is a water-soluble salt or a complex acid.
3. The method according to claim 1, wherein the platinum solution
is hexachloroplatinic acid, or an alkali platinate, or a mixture
thereof.
4. The method according to claim 1, wherein the soluble platinum
compound is Na.sub.2PtCl.sub.6, or Na.sub.2Pt(OH).sub.6, or a
mixture thereof.
5. The method according to claim 1, wherein, after the addition of
the platinum solution, the electrolytic voltage is varied in the
range from 0V to 5V.
6. The method according to claim 4, wherein, after the addition of
the platinum solution, the electrolytic voltage is varied in the
range from 0V to 5V.
7. The method according to claim 5, wherein, after the addition of
the platinum solution, the difference in the electrolytic voltage
is from 0.5 to 500 mV.
8. The method according to claim 6, wherein, after the addition of
the platinum solution, the difference in the electrolytic voltage
is from 0.5 to 500 mV.
9. The method according to claim 5, wherein the voltage is varied
by pulsing the voltage or by superimposing an alternating voltage
on the electrolytic voltage, in the range from 0 to 5 V, and by a
difference of from 0.5 to 500 mV.
10. The method according to claim 8, wherein the voltage is varied
by pulsing the voltage or by superimposing an alternating voltage
on the electrolytic voltage, in the range from 0 to 5 V, and by a
difference of from 0.5 to 500 mV.
11. The method according to claim 10, which further comprises at
least one additional water-soluble compound from Group VIII of the
Periodic Table are added to the platinum solution.
12. The method according to claim 1, which further comprises at
least one additional water-soluble compound from selected from the
group consisting of palladium, iridium, platinum, rhodium, osmium
and ruthenium.
13. The method according to claim 11, wherein, the additional
water-soluble compound, based on the amount of platinum in the
platinum compound, are present in a concentration of 1 wt. % to 50
wt. %.
14. The method according to claim 12, wherein, the additional
water-soluble compound, based on the amount of platinum in the
platinum compound, are present in a concentration of 1 wt. % to 50
wt. %.
15. The method according to claim 1, wherein the water soluble or
alkali soluble platinum compound solution is metered at a rate of
0.001 g Pt/(h*m.sup.2) to 1 g Pt/(h*m.sup.2).
16. The method according to claim 14, wherein the water soluble or
alkali soluble platinum compound solution is metered at a rate of
0.001 g Pt/(h*m.sup.2) to 1 g Pt/(h*m.sup.2).
17. The method according to claim 15, wherein the temperature at
which the metering of the platinum solution is from 70.degree. C.
to 90.degree. C.
18. The method according to claim 16, wherein the temperature at
which the metering of the platinum solution is from 70.degree. C.
to 90.degree. C.
19. The method according to claim 15, wherein the metering of the
platinum solution occurs during electrolysis and under a current
density between 0.1 to 10 kA/m.sup.2.
20. The method according to claim 18, wherein the metering of the
platinum solution occurs during electrolysis and under a current
density between 0.1 to 10 kA/m.sup.2.
Description
RELATED APPLICATIONS
[0001] This application claims benefit to German Patent Application
No. 10 2007 003 554.5, filed Jan. 24, 2007 which is incorporated by
reference in its entirety for all useful purposes.
1. FIELD OF THE INVENTION
[0002] The invention relates to a method for improving the
performance of nickel electrodes in alkali chloride
electrolysis.
2. BACKGROUND OF THE INVENTION
[0003] In sodium chloride electrolysis, hydrogen is evolved from an
alkaline solution. Conventionally, the cathodes in the process are
made of iron, copper, steel, or nickel. Nickel electrodes can be
either solid nickel or nickel plated.
[0004] As mentioned in Offenlegungsschrift EP 298 055 A1, nickel
electrodes can be coated with a metal from sub-group VIII,
especially the platinum metals (inter alia Pt, Ru, Rh, Os, Ir, or
Pd), of the periodic system of the elements or with an oxide of
such a metal or with mixtures thereof. After a calcination process,
the corresponding noble metal oxides are then usually present on
the surface.
[0005] The electrode so produced can be used, for example, in
sodium chloride electrolysis as the cathode for hydrogen
development. Many coating variants are known, because the coating
of metal oxides can be modified in very different ways so that
different compositions form on the surface of the nickel electrode.
According to U.S. Pat. No. 5,035,789, the cathode used is, for
example, a ruthenium-oxide-based coating on nickel substrates.
[0006] Once in operation, the plating on the nickel electrode
degrades and causes the cell voltage to increase, making necessary
to re-coat the electrode. This is technically complex, because the
electrolysis must be stopped and the electrodes must be removed
from the electrolytic cells. An object of the invention is,
therefore, to find a simpler method for increasing or restoring
performance.
[0007] ELTECH has published and offered a technique with which a
voltage reduction of from 200 to 300 mV as compared with untreated
nickel electrodes can be achieved. In this technique, a
noble-metal-containing solution of unnamed composition and
constituents is applied in situ, i.e. during operation of the
electrolysis, to the cathode side of the sodium chloride
electrolysis in membrane cells. The solution is to be added during
operation of the cell and is to lower the cell voltage.
[0008] According to the teaching of patent specification U.S. Pat.
No. 4,555,317, iron compounds or finely divided iron is added to
the catolyte in order to lower the cell voltage during sodium
chloride electrolysis. The ELTECH publication contradicts this
teaching, however, because, according to the information from
ELTECH, coating the cathodes with iron is said to interfere with
the electrolysis and to increase the cell voltage.
[0009] According to the further known Offenlegungsschrift EP 1 487
747 A1, a 0.1 to 10 wt. % platinum-containing compound is added to
sodium chloride electrolysis. The solution of the
platinum-containing compound is added to the water that forms the
catolyte, from 0.1 to 2 litres of the aqueous solution of the
platinum-compound-containing solution being added per litre of
water.
[0010] According to JP 1011988 A, the activity of a deactivated
cathode based on a Raney nickel structure with low hydrogen
overvoltage is restored by adding, into the catolyte, a soluble
compound of a metal of the platinum group to the sodium hydroxide
solution during operation of the sodium chloride electrolysis. For
example, a sodium chloride electrolytic cell with 32 wt. % sodium
hydroxide solution, a salt concentration of 200 g/l of sodium
chloride is operated at 90.degree. C. and with a current density of
2.35 kA/m.sup.2. The cathode is subjected to currentless nickelling
for pretreatment and then nickel-plated in a nickel bath. Platinum
chlorate, for example, was metered into the catolyte as the active
compound, which resulted in a reduction in the cell voltage by 100
mV.
[0011] According to U.S. Pat. No. 4,105,516, metal compounds which
are to lower the hydrogen overvoltage and accordingly reduce the
cell voltage are added to the catolyte during the electrolysis of
alkali metal chlorides. The examples given in U.S. Pat. No.
4,105,516 in turn describe the metering and effects that arise by
addition of an iron compound added to the catolyte of a sodium
chloride diaphragm laboratory cell. The cell has an anode,
consisting of expanded titanium metal, which is coated with
ruthenium oxide and titanium oxide. The cathode consists of iron in
the form of extended metal. The examples show the use of cobalt
solution or iron solution at the iron cathode. Reference has
already been made above to the disadvantages of iron compounds in
the treatment of coated nickel electrodes.
[0012] According to the further known patent specification U.S.
Pat. No. 4,555,317, it is known that sodium chloride electrolysis
can be started with a nickel-coated copper cathode. An initial
metering under electrolysis conditions of the cell was carried out
with hexachloroplatinic acid in three steps. In the first step, 2
mg of platinum were metered in per 102 cm.sup.2, i.e. 0.02
mg/cm.sup.2, in the second step about 0.03 mg/cm.sup.2 and in the
third step about 0.2 mg/cm.sup.2. The cell voltage was lowered by a
total of about 157 mV.
[0013] According to U.S. Pat. No. 4,160,704, metal ions having a
low hydrogen overvoltage can be added to catolytes of a membrane
electrolytic cell for sodium chloride electrolysis in order to coat
the cathode. The addition takes place during the electrolysis.
However, the only example given is the addition of platinum oxide
in order to improve an iron or copper cathode.
[0014] Sodium chloride electrolysis according to the membrane
process is known in the prior art. The process is carried out as
follows: a sodium-chloride-containing solution is fed to an anode
chamber having an anode, and a sodium hydroxide solution is fed to
a cathode chamber having a cathode. The two chambers are separated
by an ion-exchange membrane. Joining multiple anode and cathode
chambers forms an electrolyser. The product streams from the anode
chamber include chlorine and a less concentrated
sodium-chloride-containing solution. The product stream from the
cathode chamber includes hydrogen, and a more highly concentrated
sodium hydroxide solution than was fed thereto. The volume flow of
sodium hydroxide solution fed to the cathode chamber is dependent
on the current density and the cell design. At a current density
of, for example, 4 kA/m.sup.2 and with the cell design of UHDE,
Version BM 3.0, the volume flow of lye to the cathode chamber is,
for example, between from 100 to 300 l/h, with a concentration of
the sodium hydroxide solution that comes off of from 30 to 33 wt.
%. The geometrically projected cathode area is 2.71 m.sup.2, this
corresponds to the membrane area. The cathode is made of specially
coated extended nickel metal provided with a special coating
(manufacturer e.g. DENORA) in order to lower the hydrogen
overvoltage.
[0015] The cathode coatings in sodium chloride electrolysis
conventionally consist of platinum metals, platinum metal oxides or
mixtures thereof, such as, for example, a ruthenium/ruthenium oxide
mixture. As is described in EP 129 374, the platinum metals that
can be used include ruthenium, iridium, platinum, palladium and
rhodium. The cathode coating does not have long-term stability, in
particular not under conditions in which electrolysis does not
occur or during interruptions in the electrolysis, during which
pole reversal processes, for example, can occur. Accordingly, more
or less pronounced damage occurs to the coating over the operating
time of the electrolyser. Likewise, impurities which pass, for
example, from the brine into the lye, such as, for example, iron
ions, can become deposited on the cathode or especially on the
active centres of the noble-metal-containing coating and as a
result can deactivate the coating. The consequence is that the cell
voltage rises, with the result that the energy consumption for the
production of chlorine, hydrogen and sodium hydroxide solution
increases and the economy of the process is markedly impaired.
[0016] It is likewise possible for only individual elements to
exhibit damage to the cathode coating, and it is not always
economical to stop the entire electrolyser therefor and remove the
element with the damaged coating, because this is associated with
considerable production losses and costs.
[0017] Methods for improving nickel electrodes for sodium chloride
electrolysis which are coated with elements of the platinum metals
(sub-group VIII of the periodic system), referred to hereinbelow as
platinum metals, their oxides or mixtures thereof, have not
hitherto been directly known from the prior art.
SUMMARY OF THE INVENTION
[0018] The object of the invention is, therefore, to develop a
specific method for improving nickel electrodes coated with
platinum metals, platinum metal oxides or mixtures thereof, for use
as cathodes in the electrolysis of sodium chloride, which process
can be used while electrolysis operation continues and avoids a
prolonged interruption in electrode operation to restore cathode
activity.
[0019] The invention relates to a method for improving the
performance of nickel electrodes that are used in a membrane sodium
chloride electrolytic process comprising:
[0020] (a) preparing a water-soluble or alkali-soluble platinum
solution comprising: [0021] (i) a solvent and [0022] (ii) a soluble
platinum compound
[0023] and
[0024] (b) adding the solution to the catolyte.
[0025] The invention provides a method for improving the
performance of nickel electrodes having a coating based on platinum
metals, platinum metal oxides or mixtures of platinum metals and
platinum metal oxides, for sodium chloride electrolysis according
to the membrane process, characterised in that, in the electrolysis
of sodium chloride, a water-soluble or alkali-soluble platinum
compound, in particular hexachloroplatinic acid or especially
preferably an alkali platinate, particularly preferably sodium
hexachloroplatinate (Na.sub.2PtCl.sub.6) and/or sodium
hexahydroxyplatinate (Na.sub.2Pt(OH).sub.6), is added to the
catolyte.
[0026] For purposes of the specification, the term "Group VIII
metals" includes all metals listed in sub-Group VIII of the
Periodic Table, their metal oxides, and any mixtures of the metals
and metal oxides.
[0027] The term "nickel cathode" includes electrodes used as
cathodes that are solid nickel or nickel plated, regardless of any
additional metal coatings on the electrode.
[0028] The term "platinum solution" includes an alkali or water
based solution containing at least platinum and the solvent.
DETAILED DESCRIPTION OF THE INVENTION
[0029] In this method it is possible in particular either to meter
in the sodium hexachloroplatinate in the form of an aqueous
solution or in alkaline solution, or the hexachloroplatinic acid is
metered directly into the catolyte, in particular the sodium
hydroxide solution, a reaction then taking place with the lye to
form the chloroplatinate.
[0030] The addition of the platinum compound is effected in
particular while the electrolysis is taking place, under normal
electrolysis conditions, at a current density of from 0.1 to 10
kA/m.sup.2, particularly preferably at a current density of from
0.5 to 8 kA/m.sup.2.
[0031] In a further preferred form of the platinum addition, the
electrolytic voltage is varied, after the addition of the platinum
compound, in particular in a pulsed manner, in the range from 0 to
5 V in order to deposit platinum in a more finely divided form on
the cathode. The voltage here describes the voltage between the
anode and the cathode.
[0032] To that end it can be sufficient, depending on the rectifier
used to produce the electrolytic direct voltage, to lower the cell
voltage in order to use the residual ripple of the rectifier
therefor. In an alternating voltage in the mentioned voltage range,
the residual ripple of the rectifier can result with an amplitude
of from 0.5 to 500 mV. Modern rectifiers scarcely possess any
residual ripple, but it is possible to produce a residual ripple
artificially. The residual ripple is between 20 and 100 Hz, for
example.
[0033] If the amplitude is likewise regulated, it can be +100 or
-100 mV around the resting potential for the time of the noble
metal metering. The resting potential is the voltage at which no
further current flows. That potential is normally about 2.1 to 2.3
V, depending on the cell technology and membrane used. However, it
is also possible in particular to carry out the noble metal
metering when the cell voltage is 0 V, in which case the amplitude
must be chosen greater than the resting potential.
[0034] Higher modulated amplitudes are likewise conceivable.
[0035] Platinum metals that can be present in metal or metal oxide
form as the electrode coating on the nickel within the scope of the
invention are in particular ruthenium, iridium, palladium,
platinum, rhodium and osmium.
[0036] In a further preferred form of the novel method, in addition
to the platinum compound, at least one other further soluble
compounds of sub-group 8 of the periodic system of the elements, in
particular compounds of palladium, iridium, rhodium, osmium or
ruthenium, can additionally be added. Such compounds are used in
particular in the form of water-soluble salts or complex acids.
[0037] After deactivation has been detected, the addition in the
case of first-time metering is preferably carried out as follows: a
platinum compound is added to the catolyte, in the feed to the
cathode chamber, at a cathode area of 2.71 m.sup.2, from 0.02 to 11
g Pt per cathode element, corresponding to from 0.007 g/m.sup.2 to
4 g/m.sup.2, at a current density of from 1 to 8 kA/m.sup.2. The
area used as the basis is the geometrically projected cathode area,
which also corresponds to the membrane area. The rate of metering
can be such that the platinum-containing solution, based on the
platinum content per m.sup.2 of cathode area, is metered at a rate
of from 0.001 g Pt/(hm.sup.2) to 1 g Pt/(hm.sup.2).
[0038] The addition can take place at a current density preferably
under normal operating conditions, or alternatively at a higher or
lower current density. For example, the addition can take place at
a current density of in particular from 0.1 to 10 kA/m.sup.2.
[0039] The temperature at which the metering of the platinum
compound preferably takes place is from 70 to 90.degree. C. The
metering can also take place at a lower temperature, however.
[0040] If a further voltage increase is observed when metering is
complete, this can immediately be offset by metering again. This
metering requires a markedly smaller amount of noble metal in order
to restore the original voltage. Depending on the quality of the
brine, the lye or on stoppages, a further, but smaller addition of
platinum may be necessary within a period of from 1 to 3 weeks. The
addition of the platinum compound to the catolyte can likewise take
place in the feed to the cathodes. The required amounts of platinum
are to be calculated according to the scale of the damage. In the
case of relatively considerable damage, corresponding to a high
voltage increase, more platinum must be metered in, while
correspondingly less platinum must be metered in in the case of
slight damage, corresponding to a slight voltage increase.
Overdosing with platinum does not result in any further improvement
or lowering of the cell voltage, however.
[0041] The amount, based on the platinum, of the further soluble
compounds from sub-group 8 in the solution to be added is
particularly preferably from 1 to 50 wt %.
[0042] In a preferred embodiment, the variation in the electrolytic
voltage can be effected by superimposing an alternating voltage on
the electrolytic voltage. The frequency of the superimposed
alternating voltage is in particular from 10 to 100 Hz. The
amplitude can then be from 10 to 200 mV.
[0043] By means of the method according to the invention it is
possible for the first time to effect a voltage reduction by up to
200 mV in the case of damaged nickel electrodes coated with
ruthenium and/or ruthenium oxides or mixtures thereof.
[0044] The preparation of the alkali platinate can be carried out
by reaction of hexachloroplatinic acid with lye. This can be
carried out separately or directly in situ if, for example,
hexachloroplatinic acid is metered directly into the sodium
hydroxide supply to the elements or to the electrolyser. The
hexachloroplatinic acid is particularly preferably metered directly
into the feed to the elements.
EXAMPLES
Example 1
[0045] A commercial electrolyser having 144 elements whose nickel
cathodes were provided with a coating based on ruthenium/ruthenium
oxide from Denora was operated at a mean voltage of 3.12 V. Of
these 144 elements, one exhibited a voltage increased by more than
100 mV as compared with the mean value. The following treatment
cycle was begun: 65.88 litres of a hexachloroplatinate solution
(1.19 g Pt/l) was metered at a rate of 10.98 l/h, during operation,
into the sodium hydroxide solution (conc. 31.5%) of a membrane
electrolyser at a current density of 4.18 kA/m.sup.2 over a period
of 6 hours. 78.25 g of platinum thus reached the surface of 144
cathodes (surface area of a cathode: 2.71 m.sup.2). This
corresponds to an amount of platinum of 0.21 g Pt/m.sup.2. The cell
voltage fell on average to 3.08 V, the current consumption rose to
4.57 kA/m.sup.2. Converted to 4 kA/m.sup.2, this corresponds to a
reduction in the voltage by 80 mV, accordingly from 3.09 to 3.01.
Elements having a markedly higher voltage were no longer present.
On the following day, a further 16.44 litres of the same solution,
corresponding to 0.05 g Pt/m.sup.2, were metered in. The cell
voltage did not improve further as a result.
[0046] After 9 days, the mean voltage rose to 3.02 V (based on 4
kA/m.sup.2), so that further metering of platinum in the form of
hexachloroplatinic acid was carried out. 4.12 litres of the
hexachloroplatinate solution (1.19 g Pt/l) were thereby metered in
uniformly in the course of 2 hours, so that 4.9 g of platinum
reached the surface of 144 cathodes (0.012 g Pt/m.sup.2). The
electrolysis was continued during the metering, the mean voltage
thereafter was 3.01 V.
[0047] The cell voltage at a current density of 4 kA/m.sup.2 was on
average 3.09 V before the metering and 3.01 V after the metering,
which corresponds to a voltage reduction of 80 mV.
Example 2
[0048] A laboratory electrolytic cell was operated as described in
Example 1 at a current density of 4 kA/m.sup.2 at a cell voltage of
3.05 V with a standard cathode coating from Denora on the nickel
cathode. After shutting down the cell without applying a protective
potential, damage to the cathode coating occurred. A protective
potential is conventionally applied during a shut-down in order to
protect the coating of the cathode from damage. After re-starting,
the cell voltage was 3.17 V.
[0049] A solution of hexachloroplatinate having a platinum content
of 1250 mg/l Pt was metered into the catolyte while the cell was
operating. After metering the solution for 2 hours with a metered
amount of 5 ml/h, the voltage fell to 3.04 V. A total of 12.5 mg of
platinum (12.5 mg/100 cm.sup.2) was added.
Example 3
[0050] The test of Example 2 was repeated, but a solution having a
platinum concentration of 250 mg/l was metered in (same metering
time and same feed capacity). Addition here 2.5 mg Pt/100 cm.sup.2.
The voltage fell from 3.16 V to 3.07 V, i.e. by 90 mV.
[0051] Further additional metering did not bring about any further
voltage reduction.
Example 4
Comparison
[0052] A laboratory electrolytic cell was operated as described in
Example 1 at a current density of 4 kA/m.sup.2 at a cell voltage of
3.08 V with a standard cathode coating from Denora on nickel
electrodes. After shutting down the cell without applying a
protective potential, damage to the cathode coating occurred. A
protective potential is conventionally applied during a shut-down
in order to protect the coating of the cathode from damage. After
re-starting, the cell voltage was 3.21 V.
[0053] A solution of rhodium(III) chloride having a rhodium content
of 125 mg/l was metered in over a period of 4 hours at 5 ml/h.
Metering was then continued for a further 2 hours with a solution
having a concentration of 1250 mg/l and at 5 ml/h, as a result of
which a further 50 mV voltage reduction was achieved. The voltage
reduction was only 60 mV.
[0054] All the references described above are incorporated by
reference in its entirety for all useful purposes.
[0055] While there is shown and described certain specific
structures embodying the invention, it will be manifest to those
skilled in the art that various modifications and rearrangements of
the parts may be made without departing from the spirit and scope
of the underlying inventive concept and that the same is not
limited to the particular forms herein shown and described.
* * * * *