Title of Invention

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.
Full Text Method for producing peroxodisulfates in aqueous
solution
Description
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.
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.
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.
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.
A further disadvantage of the diamond-coated silicon
electrodes of the prior art is their limited dimensions
of at present not more than 200 x 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.
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.
DE 200 05 681 describes the use of bipolar electrodes
coated on both sides with diamond layers.
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.

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.
According to the invention, the coating on the silicon
electrode has a thickness of from about 1 to about
2 0 µm, preferably about 5 µm.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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/1 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 7 5
to 50% and specific electric energy consumptions of
from 1.3 to 1.9 kWh/kg.
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/1 with justifiable current yields of about 50%
in undivided cells, albeit at relatively high cell

voltages.
If higher final concentrations of peroxodisulfates,
e.g. in the range from 200 to 400 g/1 of sodium
peroxodi sulfate, 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.
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 µm were
found in an undivided cell at a steady-state sodium
peroxodisulfate content of about 150 g/1 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:
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 µm of boron-doped diamond (average
about 3 000 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 x 33 mm (33 cm2) . 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 0.5 A/cm2. The total current capacity of the
electrolysis cell was thus 10 x 16.5 = 165 A. 2 1 of an
aqueous solution containing 300 g/1 of sodium sulfate
and 200 g/1 of sulfuric acid served as electrolyte. It
was circulated at a rate of about 600 1/h from a
circulation reservoir via a heat exchanger and through
the cell by pumping (batch operation). Electrolysis
operation was maintained for 5 000 hours, with only the
water which had evaporated or been decomposed being
replaced. In steady-state operation, a concentration of
170-190 g/1 of sodium peroxodisulfate was established
at a steady-state temperature of about 35°C. The total
voltage on start-up was 50 V. The mean cell voltage
changed as follows over the course of continuous
operation:

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 µm. The
thickness of the silicon cathode thus decreases by only
about 10 µm per year.
Example 2:

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:

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/1; for a final
concentration of 100 g/1 of NaPS, it was still
1.89 kWh/kg despite the fact that the current yield had
dropped to 50%.
Example 3:
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.

Even in the concentration range from 100 to 200 g/1,

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/1.
Example 4:
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/1 of sulfuric acid
served as catholyte. The anolyte once again consisted
of an aqueous solution containing 200 g/1 of sulfuric
acid and 300 g/1 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+ ions through
the cation-exchange membrane into the catholyte at the
desired high final NaPS concentrations, a further
100 g/1 of sodium sulfate were dissolved in the anolyte
during the electrolysis (i.e. a total of 400 g/1 of
sodium sulfate). The anodic and cathodic current
densities were each set to 0.5 A/cm2.
Under otherwise comparable electrolysis conditions, the
following current yields were obtained for various
final NaPS concentrations:
at a final NaPS concentration of 2 00 g/1, a current

yield of 86%
at a final NaPS concentration of 300 g/1, a current
yield of 82%
at a final NaPS concentration of 400 g/1, a current
yield of 74%
The mean cell voltages were in the range from 5.5 to
6 V. At the final concentration of 400 g/1, a still
very low specific electric energy consumption of about
1.8 kWh/kg could thus be achieved.

WE CLAIM
1. A process for preparing peroxodisulfuric acid and/or its salts by
electrolysis of aqueous solutions of sulfuric acid and or metal sulfates at
diamond-coated electrodes without addition of promoters; characterized
in that, the said process being carried out in undivided or divided
electrolysis cells, which are divided by means of ion exchange
membranes or porous diaphragms, to lower reduction loses and to
increase in the peroxodisulphate concentration wherein the bipolar
silicon electrodes are coated on one side with doped diamond and whose
uncoated silicon rear side serves as cathode in the said process.
2. The process as claimed in claim 1, wherein a diamond coated anode
composed of a valve metal, e.g. niobium, and provided with a power
supply lead is used as boundary anode.
3. The process as claimed in claims 1 and 2, 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.

4. The process as claimed in claims 1 to 3, 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.
5. A bipolar undivided or divided electrolysis cell equipped with bipolar
electrodes coated with diamond on one side for use in a process as
claimed in any of claims 1 to 4.


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.

Documents:

03823-kolnp-2006 abstract.pdf

03823-kolnp-2006 claims.pdf

03823-kolnp-2006 correspondence others.pdf

03823-kolnp-2006 description(complete).pdf

03823-kolnp-2006 form-1.pdf

03823-kolnp-2006 form-2.pdf

03823-kolnp-2006 form-3.pdf

03823-kolnp-2006 form-5.pdf

03823-kolnp-2006 international publication.pdf

03823-kolnp-2006 international search authority report.pdf

03823-kolnp-2006 priority document.pdf

03823-kolnp-2006-abstract-1.1.pdf

03823-kolnp-2006-claims-1.1.pdf

03823-kolnp-2006-correspondence others-1.1.pdf

03823-kolnp-2006-correspondence-1.2.pdf

03823-kolnp-2006-correspondence-1.3.pdf

03823-kolnp-2006-form-26.pdf

03823-kolnp-2006-others.pdf

03823-kolnp-2006-priority document-1.1.pdf

3823-KOLNP-2006-(29-02-2012)-CORRESPONDENCE.pdf

3823-KOLNP-2006-ABSTRACT.pdf

3823-KOLNP-2006-AMANDED CLAIMS.pdf

3823-KOLNP-2006-CORRESPONDENCE.2.1.pdf

3823-KOLNP-2006-CORRESPONDENCE.pdf

3823-KOLNP-2006-DESCRIPTION (COMPLETE).pdf

3823-KOLNP-2006-EXAMINATION REPORT.pdf

3823-KOLNP-2006-FORM 1.pdf

3823-KOLNP-2006-FORM 18 1.1.pdf

3823-kolnp-2006-form 18.pdf

3823-KOLNP-2006-FORM 2.pdf

3823-KOLNP-2006-FORM 26.pdf

3823-KOLNP-2006-FORM 3 1.1.pdf

3823-KOLNP-2006-FORM 3.pdf

3823-KOLNP-2006-FORM 5.pdf

3823-KOLNP-2006-GRANTED-ABSTRACT.pdf

3823-KOLNP-2006-GRANTED-CLAIMS.pdf

3823-KOLNP-2006-GRANTED-DESCRIPTION (COMPLETE).pdf

3823-KOLNP-2006-GRANTED-FORM 1.pdf

3823-KOLNP-2006-GRANTED-FORM 2.pdf

3823-KOLNP-2006-GRANTED-SPECIFICATION.pdf

3823-KOLNP-2006-OTHERS.pdf

3823-KOLNP-2006-REPLY TO EXAMINATION REPORT 1.1.pdf

3823-KOLNP-2006-REPLY TO EXAMINATION REPORT.pdf

3823-KOLNP-2006-TRANSLATED COPY OF PRIORITY DOCUMENT.pdf


Patent Number 252992
Indian Patent Application Number 3823/KOLNP/2006
PG Journal Number 24/2012
Publication Date 15-Jun-2012
Grant Date 13-Jun-2012
Date of Filing 19-Dec-2006
Name of Patentee DEGUSSA INITIATORS GMBH & CO.KG
Applicant Address GUSTAV-ADOLPH-STRASSE 3 82049 PULLACH
Inventors:
# Inventor's Name Inventor's Address
1 THIELE, WOLFGANG HAINBUCHENWEG 19, 04838 EILENBURG
2 FORSTERHANS-JURGEN BRANDENBURGER STRASSE 3, 06749 BITTERFELD
3 KARMER, HANS-JURGEN GRIESENER STRASSE 7, 06844 DESSAU
PCT International Classification Number C25B 1/28
PCT International Application Number PCT/EP2005/006008
PCT International Filing date 2005-06-03
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 10 2004 027 623.4 2004-06-05 Germany