U.S. patent application number 13/027672 was filed with the patent office on 2011-06-09 for method for producing peroxodisulfates in aqueous solution.
Invention is credited to Hans-Jurgen Forster, Hans-Jurgen Kramer, Wolfgang Thiele.
Application Number | 20110132771 13/027672 |
Document ID | / |
Family ID | 35429257 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110132771 |
Kind Code |
A1 |
Thiele; Wolfgang ; et
al. |
June 9, 2011 |
METHOD FOR PRODUCING PEROXODISULFATES IN AQUEOUS SOLUTION
Abstract
A process for preparing or regenerating peroxodisulfuric acid
and its salts by electrolysis of an aqueous solution containing
sulfuric acid and/or metal sulfates at diamond-coated electrodes
without addition of promoters is described, with bipolar silicon
electrodes which are coated with diamond on one side and whose
uncoated silicon rear side serves as cathode being used.
Inventors: |
Thiele; Wolfgang;
(Eilenburg, DE) ; Kramer; Hans-Jurgen; (Dessau,
DE) ; Forster; Hans-Jurgen; (Bitterfeld, DE) |
Family ID: |
35429257 |
Appl. No.: |
13/027672 |
Filed: |
February 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11569464 |
Nov 30, 2006 |
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PCT/EP2005/006008 |
Jun 3, 2005 |
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13027672 |
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Current U.S.
Class: |
205/472 ;
204/242; 204/252 |
Current CPC
Class: |
C25B 1/29 20210101; C25B
1/22 20130101; C25B 11/075 20210101; C25B 11/036 20210101; C25B
11/051 20210101; C25B 9/75 20210101; C25B 1/30 20130101; C25B
11/059 20210101 |
Class at
Publication: |
205/472 ;
204/242; 204/252 |
International
Class: |
C25B 1/30 20060101
C25B001/30; C25B 9/00 20060101 C25B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 2004 |
DE |
10 2004 027 623.4 |
Claims
1-7. (canceled)
8. A process for preparing peroxodisulfuric acid or a salt thereof
by performing electrolysis of an aqueous solutions of at least one
of sulfuric acid or a metal sulfate at diamond-coated electrodes
without addition of a promoter, wherein bipolar silicon electrodes
which are coated on one side only with doped diamond and whose
uncoated silicon rear side serves as a cathode.
9. The process of claim 8, wherein the electrolysis is carried out
in undivided electrolysis cells.
10. The process of claim 8, wherein the electrolysis is carried out
in electrolysis cells which are divided by at least one of an
ion-exchange membrane or a porous diaphragm.
11. The process of claim 8, wherein a diamond-coated anode composed
of a valve metal and provided with a power supply lead is used as a
boundary anode.
12. The process of claim 9, wherein a diamond-coated anode composed
of a valve metal and provided with a power supply lead is used as a
boundary anode.
13. The process of claim 10, wherein a diamond-coated anode
composed of a valve metal and provided with a power supply lead is
used as a boundary anode.
14. The process of claim 8, wherein stainless steel, Hastelloy,
platinum, impregnated graphite or silicon which has been metallized
on one side is used for the boundary cathode provided with a power
supply lead.
15. The process of claim 9, wherein stainless steel. Hastelloy,
platinum, impregnated graphite or silicon which has been metallized
on one side is used for the boundary cathode provided with a power
supply lead.
16. The process of claim 10, wherein stainless steel, Hastelloy,
platinum, impregnated graphite or silicon which has been metallized
on one side is used for the boundary cathode provided with a power
supply lead.
17. The process of claim 8, wherein a plurality of electrode stacks
comprising bipolar electrodes and boundary electrodes with power
supply lead are connected electrically in parallel within an
electrolysis cell.
18. A bipolar undivided or divided electrolysis cell comprising
bipolar electrodes coated with diamond on one side.
19. The process of claim 11, wherein the valve metal is
niobium.
20. The process of claim 12, wherein the valve metal is
niobium.
21. The process of claim 13, wherein the valve metal is niobium.
Description
[0001] The invention relates to a process for preparing or
regenerating peroxodisulfuric acid and its salts by electrolysis of
an aqueous solution containing sulfuric acid and/or metal sulfates.
As used herein, the term "metal sulfates" encompasses both sulfates
of metals such as zinc, nickel or iron and sulfates of alkali
metals and alkaline earth metals and also ammonium sulfate. Thus,
it is possible to use, for example, alkali metal sulfates or
alkaline earth metal sulfates, preferably alkali metal sulfates or
ammonium sulfate, as metal sulfates. It is also possible to use
mixtures of various metal sulfates, for example magnesium sulfate,
zinc sulfate or else nickel or iron sulfate, preferably in the
regeneration of etching and pickling solutions.
[0002] It is known from the prior art that diamond-coated
electrodes composed of valve metals, preferably niobium, or ceramic
materials, preferably silicon, can be used for the preparation of
peroxodisulfates of the alkali metals and of ammonium [DE 199 48
184.9, DE 100 19 683]. The diamond layer is made conductive by
doping with a trivalent or pentavalent element, preferably boron.
These have advantages over the smooth platinum anodes which have
hitherto been exclusively used in peroxodisulfate production in
that, as a result of the high potential which can be achieved on
the diamond surface, it is not necessary to add potential
increasing additives to the electrolyte in order to achieve
sufficiently high current yields, as is unavoidable in the case of
platinum anodes. The preferred use of thiocyanates as polarizers
results in anode gases which contain cyanide and make complicated
gas purification measures necessary. When diamond-coated anodes are
used, these can be dispensed with.
[0003] A further advantage of diamond-coated anodes in
peroxodisulfate production is that, even at a low sulfate content
in the anolyte, significantly higher current yields can be achieved
than when using platinum anodes.
[0004] However, despite the good stability of, in particular,
diamond-coated silicon electrodes, their use is associated with a
number of disadvantages. Thus, there is the problem of suitable
supply of electric current. Owing to the relatively low electrical
conductivity of the silicon base body, a contact has to be provided
over the entire area of the reverse side of the electrode, so that
current needs to flow only from the contacted rear side through the
small thickness of the silicon electrode of about 1-2 mm to the
diamond coating. Although this problem could in principle be solved
by adhesive bonding of the preferably metallized rear sides of the
silicon plates to a metallic substrate having a good conductivity
by means of an electrically conductive adhesive, this is relatively
complicated.
[0005] A further disadvantage of the diamond-coated silicon
electrodes of the prior art is their limited dimensions of at
present not more than 200.times.250 mm. In order to nevertheless be
able to provide large-area anodes for use in industrial
electrolysis cells, EP 1 229 149 proposed adhesively bonding a
relatively large number of such silicon-diamond electrodes by means
of an electrically conductive adhesive to a metal base plate, e.g.
composed of a valve metal, and sealing the edges by means of a
corrosion-resistant resin, e.g. epoxy resin. However, the
difficulties involved, for example in the provision of the
conductive adhesive, e.g. an adhesive composed of epoxy resin
containing silver particles, and in the complete elimination of the
oxide layers on the areas to be joined, are very great. In
addition, such an electrode construction has been found to be
insufficiently corrosion resistant for the preparation of
peroxodisulfate, so that only short operation lives of usually less
than one year can be achieved in this way.
[0006] Another possible way disclosed in the prior art for
constructing electrolysis cells having a sufficiently large current
capacity is to connect a relatively large number of bipolar
silicon-diamond electrodes in series. FR 2790268 B1 discloses such
a bipolar electrolysis cell in which the bipolar electrodes
comprise a ceramic substrate which is completely enveloped by a
diamond film. However, this cell is not proposed specifically for
the preparation of peroxodisulfates but for uses in the degradation
of pollutants or for disinfection of water.
[0007] DE 200 05 681 describes the use of bipolar electrodes coated
on both sides with diamond layers.
[0008] EP 1 254 972 proposes an electrolysis cell construction
which is suitable for various applications and can be configured as
a monopolar or bipolar, undivided or divided cell. In the bipolar
design, silicon disk electrodes coated on both sides with a diamond
layer are once again exclusively used. In the preparation of
peroxodisulfates, these cells having silicon electrodes coated on
both sides with a diamond layer and the relatively complicated cell
construction can be used effectively only for small persulfate
throughputs. If an attempt is made to increase the throughput to
industrially relevant ranges by means of a relatively large number
of individual bipolar cells, this construction results in reduced
yields due to the loss currents in the power supply leads and power
outlet leads which increase greatly with the total voltage.
[0009] It was therefore an object of the present invention to
provide a process for preparing or regenerating peroxodisulfuric
acid and/or its salts, in which the above-described disadvantages
of previous processes and electrolysis cells are at least partly
avoided. It has been found that peroxodisulfates can advantageously
be prepared in undivided or divided electrolysis cells in a simple
manner by using bipolar silicon electrodes which have been coated
on one side with doped diamond, with the uncoated silicon rear
sides acting directly as cathodes.
[0010] According to the invention, the coating on the silicon
electrode has a thickness of from about 1 to about 20 .mu.m,
preferably about 5 .mu.m.
[0011] It was highly surprising that only the coating on the anode
side of the bipolar electrode is necessary in order to achieve
satisfactory results with the uncoated silicon rear side which then
functions as cathode. In the case of an undivided bipolar cell, it
was also surprisingly found that lower persulfate losses occur as a
result of cathodic reduction when using a silicon cathode according
to the invention compared to the metal cathodes which are usually
used in the prior art in persulfate production.
[0012] Furthermore, it has been found that it is not only possible
to achieve high persulfate formation rates when using the bipolar
electrodes according to the invention but this can be achieved even
at very low cell voltages and thus low specific electric energy
consumptions. This is based firstly on the recognition that the
silicon cathode surfaces are freed of the poorly conductive oxide
layers which are initially present by means of the cathodic
reaction and are also kept completely free during the course of the
electrolysis. For example, it was found in a long-term experiment
(cf. example 1) that the cell voltage is even reduced further with
increasing time of operation, while in the case of the
diamond-coated silicon electrodes adhesively bonded to a metal
substrate according to the prior art, an opposite tendency is
observed as a result of increasing corrosion.
[0013] The process of the invention thus advantageously makes it
possible to prepare peroxodisulfuric acid and/or its salts at a
genuine bipolar electrode with a high current yield and a low
electric energy consumption even though only the slightly
conductive silicon is used as cathode. In addition, no costs for a
cathode coating are incurred.
[0014] A further advantage of the inventive bipolar silicon
electrodes coated on one side with diamond is the lower catalytic
activity of the silicon rear side compared to a metallized
electrode rear side, e.g. composed of platinum or stainless steel.
It has been found that reduction losses of peroxodisulfate are
therefore lower when electrolysis is carried out in an undivided
electrolysis cell. This leads, in the case of undivided cells, to
the increase in the peroxodisulfate concentration with electrolysis
time being somewhat steeper and the achievable final concentration
being higher than when a metallized cathode is used under otherwise
identical electrolysis conditions.
[0015] Compared to the bipolar electrodes of the prior art which
are coated with doped diamond on both sides, cost savings are
advantageously achieved both for the electrodes themselves and for
the electrolysis cells equipped therewith and also as a result of
the lower electric energy consumptions which can be achieved.
[0016] The process of the invention for preparing peroxodisulfuric
acid and/or its salts can be carried out both in undivided
electrolysis cells and in electrolysis cells which are divided, for
example by means of ion-exchange membranes or porous
diaphragms.
[0017] The bipolar silicon electrodes according to the invention
which are coated on one side with diamond are particularly useful
for undivided electrolysis cells having a relatively simple
construction, as are described, for example, in DE G 200 05 681.6
for the disinfection of water. It is advantageous in terms of the
current input for the monopolar boundary anodes to comprise a
diamond-coated valve metal. The term "valve metal" refers to a
metal which when connected as an anode becomes coated with an oxide
layer which becomes nonconductive even at high voltages. Connected
as anode, the metal blocks. Connected as cathode, the oxide layer
is dissolved and current flows in a fairly uninhibited fashion.
Thus, valve metals behave like a rectifier when different
polarities are applied. Examples of suitable valve metals are
tantalum, titanium, niobium and zirconium. For the purposes of the
present invention, preference is given to using niobium.
[0018] The monopolar boundary cathodes preferably comprise a
suitable material having a good conductivity, e.g. stainless steel,
Hastelloy, platinum and impregnated graphite. For the purposes of
the present invention, preference is given to using high-alloy
stainless steels or Hastelloy. A silicon boundary cathode having a
metallized rear side and with a current supply plate composed of a
material having a good conductivity, e.g. copper, as contact can
also be used due to the good long-term stability in undivided
cells. Particularly when using boundary electrodes composed of
metallic materials, optimal current input can be achieved in a
simple manner and without large voltage drops because of the good
conductivity.
[0019] It is also possible for a plurality of electrode stacks
comprising bipolar electrodes and boundary electrodes with power
supply lead to be connected electrically in parallel in an
electrolysis cell. If necessary, the spacing between the bipolar
electrodes can be set or fixed by means of spacers. Such electrode
stacks connected in parallel make it possible to accommodate
relatively large power capacities in an electrolysis cell without
an unjustifiably high total voltage being necessary. The voltage
can thus also be optimally matched to the available rectifier
voltage. In addition, the short circuit currents in the common feed
and discharge lines for the electrolyte solutions can be minimized
further as a result, which can additionally be aided in a known
manner by installation of additional resistance sections in these
lines.
[0020] Undivided bipolar cells having the structure provided by the
invention can be used particularly advantageously when the
peroxodisulfate concentration does not have to be very high for the
application in question, for example for the oxidative degradation
of pollutants in process solutions and wastewater. As can be seen
from example 2, sodium peroxodisulfate reaction solutions having a
content of from 50 to 100 g/l can be prepared very effectively in
batch operation in an undivided cell provided with the bipolar
electrodes according to the invention at current yields of from 75
to 50% and specific electric energy consumptions of from 1.3 to 1.9
kWh/kg.
[0021] Even better current yields or the same yields at higher
final peroxodisulfate concentrations can be achieved by shielding
of the cathode by means of suitable materials which inhibit mass
transfer to the cathode surface, as can be seen from example 3.
Materials suitable for these purposes are, for example, PVC gauzes.
The process of the invention thus makes it possible to obtain
sodium peroxodisulfate concentrations from 150 to 200 g/l with
justifiable current yields of about 50% in undivided cells, albeit
at relatively high cell voltages.
[0022] If higher final concentrations of peroxodisulfates, e.g. in
the range from 200 to 400 g/l of sodium peroxodisulfate, are
desired, the use of divided electrolysis cells provided with the
bipolar silicon electrodes according to the invention is preferred.
As can be seen from example 4, current yields of from about 75 to
85% can be achieved in this way, albeit with a more complicated
cell construction and higher cell voltages of from about 5.5 to 6
V. However, comparatively very good specific electric energy
consumptions of less than 2.0 kWh/kg can still be achieved in this
way.
[0023] A further surprising effect of the process of the invention
are the very low corrosion rates at the silicon cathodes which are
found in undivided electrolysis cells in a long-term experiment
using an acidic persulfate-containing electrolyte. Thus,
surprisingly low corrosion rates of only 2-3 .mu.m were found in an
undivided cell at a steady-state sodium peroxodisulfate content of
about 150 g/l in a long-term experiment over about 7 months (cf.
example 1). This was particularly surprising because 10-100 times
greater corrosion was observed even on platinum cathodes of the
prior art under these very highly corrosive conditions. Even
cathodes made of graphite or high-alloy stainless steels were found
to be unsuitable in such peroxodisulfate-containing sulfuric acid
electrolyte solutions because they were insufficiently
corrosion-resistant.
EXAMPLES
Example 1
[0024] An undivided bipolar electrolysis cell having a construction
analogous to that in DE G 200 05 681.6 contained 9 bipolar silicon
electrodes coated on one side with about 3 .mu.m of boron-doped
diamond (average about 3000 ppm of boron). A niobium electrode
coated on one side with diamond and provided with a power supply
lead served as boundary anode. The boundary cathode with power
supply lead comprised Hastelloy. The bipolar electrodes had a
dimension of 100.times.33 mm (33 cm.sup.2). The mean spacing of the
about 1 mm thick bipolar electrodes was set to about 2 mm by means
of spacers. The electrolysis current was regulated at a constant
16.5 A, corresponding to an anodic and cathodic current density of
6.5 A/cm.sup.2. The total current capacity of the electrolysis cell
was thus 10.times.16.5=165 A. 2 l of an aqueous solution containing
300 g/l of sodium sulfate and 200 g/l of sulfuric acid served as
electrolyte. It was circulated at a rate of about 600 l/h from a
circulation reservoir via a heat exchanger and through the cell by
pumping (batch operation). Electrolysis operation was maintained
for 5000 hours, with only the water which had evaporated or been
decomposed being replaced. In steady-state operation, a
concentration of 170-190 g/l of sodium peroxodisulfate was
established at a steady-state temperature of about 35.degree. C.
The total voltage on start-up was 50 V. The mean cell voltage
changed as follows over the course of continuous operation:
TABLE-US-00001 Operating time of 5 h 50 h 500 h 5000 h Mean cell
voltage 4.95 V 4.60 V 4.35 V 4.18 V
[0025] After 5000 hours of operation, the electrodes were removed
and the weight loss was determined. The mean decrease in the
silicon electrode thickness was calculated therefrom as an average
of 3 .mu.m. The thickness of the silicon cathode thus decreases by
only about 10 .mu.m per year.
Example 2
[0026] The dependence of the current yield on the final
concentration of sodium peroxodisulfate (NaPS) achieved was
determined by means of the undivided electrolysis cell from example
1 under the same electrolysis conditions (current density,
temperature, batch operation, electrolyte composition). The
following results were obtained:
TABLE-US-00002 Final concentration of NaPS in g/l 25 50 75 100 125
150 Current yield of NaPS 84 77 64 50 40 34 formation in %
[0027] At the favorable cell voltage of about 4.2 V established
after a prolonged period of operation, the specific electric energy
consumption was 1.23 kWh/kg for a final concentration of 50 g/l;
for a final concentration of 100 g/l of NaPS, it was still 1.89
kWh/kg despite the fact that the current yield had dropped to
50%.
Example 3
[0028] The same undivided electrolysis cell as in examples 1 and 2
was equipped with a PVC gauze resting on the cathodes of the
bipolar electrode plates and the boundary cathode; this gauze could
be pressed onto the surface by means of a plastic spacer.
Electrolysis was again carried out under the same electrolysis
conditions as in example 2. The following current yields, based on
the final NaPS concentration achieved, were obtained.
TABLE-US-00003 Final concentration of NaPS in g/l 50 75 100 125 150
175 200 Current yield of NaPS 84 77 73 68 61 54 49 formation in
%
[0029] Even in the concentration range from 100 to 200 g/l,
relatively favorable current yields were obtained and these were an
average of about 20% higher than without shielding of the cathode
surfaces. However, the cell voltages were about 0.8 V higher due to
the additional resistance of the gauze shielding. Nevertheless, a
very favorable specific electric energy consumption of about 1.85
kWh/kg was still obtained at, for example, a final NaPS
concentration of 150 g/l.
Example 4
[0030] The nine bipolar electrodes and the two monopolar boundary
electrodes of the undivided electrolysis cell used in examples 1 to
3 were used in a divided bipolar cell. Cation-exchange membranes
which were fixed on both sides by means of anode and cathode
spacers made of plastic were used for separating anolyte and
catholyte. The anode and cathode spaces bounded by sealing frames
had a thickness of 2-3 mm each. Anolyte and catholyte were
circulated in separate circuits through a heat exchanger. 500 g/l
of sulfuric acid served as catholyte. The anolyte once again
consisted of an aqueous solution containing 200 g/l of sulfuric
acid and 300 g/l of sodium sulfate. To avoid an excessively large
decrease in the sodium sulfate concentration due to both
consumption to form peroxodisulfate and the transport of Na.sup.+
ions through the cation-exchange membrane into the catholyte at the
desired high final NaPS concentrations, a further 100 g/l of sodium
sulfate were dissolved in the anolyte during the electrolysis (i.e.
a total of 400 g/l of sodium sulfate). The anodic and cathodic
current densities were each set to 0.5 A/cm.sup.2.
[0031] Under otherwise comparable electrolysis conditions, the
following current yields were obtained for various final NaPS
concentrations:
at a final NaPS concentration of 200 g/l, a current yield of 86% at
a final NaPS concentration of 300 g/l, a current yield of 82% at a
final NaPS concentration of 400 g/l, a current yield of 74%
[0032] The mean cell voltages were in the range from 5.5 to 6V. At
the final concentration of 400 g/l, a still very low specific
electric energy consumption of about 1.8 kWh/kg could thus be
achieved.
* * * * *