Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Activation of a cathode
The present invention relates to a method for activating a cathode suitable
for
activation on site at a production plant. The invention also relates to the
use of the
activated cathode in an electrolytic cell producing chlorine and alkali metal
hydroxide.
Background of the invention
Electrodes are commonly, when in operation, immersed in an electrolyte in an
electrolytic cell where chemical products are produced by way of oxidation and
reduction
reactions of reactants present in the electrolyte. The reduction reactions
take place at the
cathode where reduction products are obtained. The oxidation reactions take
place at the
anode where oxidation products are obtained.
Over time, the electrodes will become exhausted and deactivated due to various
deactivation processes taking place while the electrolytic cells are in
operation. In most
electrolytic processes, the electric energy is the most expensive "raw
material" in the
electrolytic reactions.
In the chlorine and alkali metal hydroxide production, it has been found that
the
cathodes are liable to progressive deactivation over time. The cathodes are
subjected to
deposition and precipitation of materials present in the electrolyte and to
other
deteriorating processes deactivating the cathode. The decrease in activity
leads to a
higher power consumption due to an increased overvoltage.
It is thus a big concern in electrolysis processes to provide active cathodes
throughout the whole electrolysis cycle.
Earlier attempts to solve this problem have involved transportation of the
deactivated cathodes to the electrode manufacturer for reactivation. However,
the
transportation of cathodes is a very expensive and time-consuming alternative
to carry
out. Another approach of providing active cathodes has involved replacement of
the
exhausted cathodes with new ones.
US 5 164 062 describes a method for preparing a new cathode comprising
coating a cathode substrate of e.g. Ni with palladium and another
electrocatalytic metal.
The pH of the coating solution may be adjusted by an organic acid, e.g. acetic
acid, oxalic
acid and formic acid, or inorganic acids to maintain the pH below 2.8.
However, the
activation by this method is not always satisfactorily increased. Furthermore,
a portion of
the active coating solution is wasted in the method described above, because
some of
the acidic electrocatalytic coating solution is rinsed away from the cathode
substrate in
order to avoid corrosion of the cathode. The rinsing solution that has taken
up remaining
electrocatalytic material must then be decontaminated from substrate ions,
e.g. nickel or
other contaminating ions, which also are present on the cathode before the
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electrocatalytic material can be reused as coating material in an
electrocatalytic solution
again. Such decontamination procedure may involve several cleaning steps
before the
electrocatalytic material has been satisfactorily cleaned.
The present invention intends to solve the above problems.
The invention
The present invention relates to a method of activating a cathode suitable for
the
production of e.g. chlorine and alkali metal hydroxide. The term "activate" or
"activation"
etc. as used herein encompass both activation of a new electrode, which is to
be
prepared, and activation of an electrode, which has already been in operation
in an
electrolytic cell and which may have lost at least some of its initial
activity.
It has been surprisingly found that the activation of a cathode comprising at
least
a cathode substrate, which may have some remains of an electrocatalytic
coating on the
substrate, can easily can be performed on site at the production site. The
method
comprises at least the following steps:
~ cleaning the cathode by means of an acid
~ coating the cleaned cathode with at least one electrocatalytic coating
solution
~ drying the coated cathode until it is at least substantially dry, and
thereafter contacting
the cathode with a solvent redissolving precipitated electrocatalytic salts or
acids
formed on the cathode, originating from the electrocatalytic coating solution,
to form
dissolved electrocatalytic metal ions on the cathode surface, so that the
electrocatalytic metal ions can precipitate as metals on the cathode.
The solvent must be able to redissolve any possible precipitated
electrocatalytic
salts or acids on the cathode originating from the electrocatalytic coating
solution. The
solvent may contain a small amount of electrocatalytic metals dissolved
therein, which
may originate from a rinsing solution containing remains of an
electrocatalytic solution.
The contacting of the solvent with the cathode is suitably performed by
spraying or in any
other way putting solvent on the cathode in a suitable amount.
By the term "substantially dry" is meant a coated cathode which contains only
a
small quantity of solution on its surface such that the solution does not
substantially flow
away from the cathode. Suitably, such quantity ranges from about 0 to about 10
ml/m2,,
preferably from about 0 to about 5 ml/m2 solution.
The cathode comprises a substrate of e.g. nickel, cobalt, copper, iron, steel,
particularly stainless steel, or alloys or mixtures thereof, preferably
nickel. The cathode
may also comprise remains of an electrocatalytic coating deposited on the
substrate,
and/or contaminants from an electrolytical process.
Used cathodes are preferably disassembled from the cells before activation.
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According to one embodiment, the cathode is welded to a pan. The used
cathode pan structure, i.e. the cathode and the pan, is preferably
disassembled and
removed from the cell before activation. For simplicity, the term "cathode",
where
otherwise not stated, will henceforth also signify the cathode pan structure.
The cathode is cleaned with a cleaning solution comprising at least one acid.
The pH of the cleaning solution is suitably adjusted by addition of an
inorganic acid, e.g.
NCI, H2S04, HN03, or an organic acid, e.g. oxalic acid or other organic acids,
or mixtures
thereof, suitably to a pH from about -1 to about 6, preferably from about -1
to about 3.
The acid reacts with the cathode substrate and is also believed to react with
precipitated
substances on the substrate and the electrocatalytic coating. The cleaning
time is not
critical and may range from about a few minutes to about 30 minutes or more.
The
temperature during the cleaning is not critical and may be at e.g. room
temperature,
suitably the temperature ranges from about 0 to about 100 °C,
preferably from about 0 to
about 35 °C.
According to one preferred embodiment of the invention, also a reducing agent
is
comprised in the cleaning solution which is believed to prevent corrosion of
the cathode
and facilitate removal of deactivating precipitates on remaining
electrocatalytic coating.
The reducing agent is also believed to stabilise activated areas of the
cathode. The
reducing agent may be present in the cleaning solution at a concentration of
from about
0.5 to about 50 wt%, preferably from about 0.5 to about 10 wt%. The reducing
agent is
suitably selected from alcohols such as isopropyl alcohol or n-pentanol, HCI,,
H3P02,
H3P03, N2H4, NH20H, NH3, Na2S, NaBH4, sodium hypophosphite (NaH~P02),
dimethylamine borane (CH3)2NHBH3, or mixtures thereof. Preferred reducing
agents are
selected from HCI, H3PO2, H3P03, N2Ha, NHZOH, and NH3, and most preferably
from HCI.
After the cleaning; the cathode is suitably rinsed and dried. The cathode is
then
contacted with at least one electrocatalytic coating solution, comprising an
electrocatalytic
metal and preferably a complexing agent.
According to one embodiment of the invention, several electrocatalytic coating
solutions, e.g. two or more coating solutions, may be contacted with the
cathode. The
coating solutions are suitably contacted with the cathode one after the other,
preferably
when the previously applied coating solution has dried on the surface of the
cathode.
The electrocatalytic coating solution or solutions are suitably applied by
means
of painting, rolling or any other plausible method suitable for on-site
coating. The
electrocatalytic coating solution suitably comprises one or several noble
metals in the
form of salts or acids or the like, selected from the platinum group, e.g. Ru,
Rh, Os, Ir, Pd,
Pt, Au, Ag, or alloys or mixtures thereof. The noble metals can suitably be
present in the
coating solution at a concentration from about 25 to about 200, preferably
from about 50
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to about 150 g metal/litre coating solution. The electrocatalytic metals are
suitably derived
from salts or acids of e.g. platinum metals such as hexa chloro platinum acid,
platinum
metal alcoxi complexing materials, chlorides or the like. The coating time of
the cathode is
not critical and may be for about one hour or more. The temperature of the
coating
solution suitably is room temperature, but may range from about 0 to about 100
°C. The
coating procedure is suitably carried out within the same temperature range,
i.e. 0-100
°C, preferably between 0 and 35 °C. Also a complexing agent may
be added to the
coating solution preferably in a concentration of from about 100 to about 500,
and most
preferably from about 350 to about 450 g /litre coating solution. The
optionally added
complexing agent facilitates the oxidation and reduction reactions taking
place when the
coating solution is contacted with the substrate. The substrate metal of the
cathode is
spontaneously oxidised to its corresponding ionic form whereas the
electrocatalytic metal
or metals in the coating solution is reduced from its ionic form to its
metallic form thereby
forming an electrocatalytic coating on the substrate. It has been found that
the
complexing agent supports the reduction/oxidation reaction taking place so as
to improve
the precipitation reaction and the adherence of the electrocatalytic metal to
the substrate.
Suitable complexing agents comprise hypophosphorous acid, sulphurous acid,
nitrous
acid, alcohols such as glycol, glycerine, acetate, propionate, succinate,
hydroxyacetate,
a-hydroxypropionate, aminoacetate, ethylenediamine, (3-aminopropionate,
malonate,
pyrophosphate, malate, citrate, ammonium salts, EDTA, or mixtures thereof.
The coated cathode is then allowed to dry so it becomes at least substantially
dried, suitably from about 0 to about 10 ml/m2, preferably from about 0 to
about 5 ml/m2.
Preferably, the coated cathode is completely dried before,it is contacted with
the solvent.
The dried cathode is then contacted with a solvent suitably comprising a
reducing agent.
It has been surprisingly found that the contacting of,the solvent with the
cathode results in
a lower overpotential, often 10-30 mV lower or more, than a cathode not
treated in this
manner. The solvent may comprise water, suitably in combination with HCI,
H3PO2,
H3PO3, H202, N2H4, NH20H, NH3, Na2S, Na2SO~,K2S03, alcohols such as isopropyl
alcohol, n-pentanol, or mixtures thereof. The lower overpotential is
considered to be
principally due to a higher deposit level of electrocatalytic metals on the
activated
cathode. The concentration of a possible reducing agent in the solvent
suitably ranges
from about 10 to about 70 wt%, preferably from about 40 to about 50 wt%. The
temperature during the contacting of the cathode with a solvent suitably
ranges from
about 8 to about 60 °C, preferably from about 15 to about 35 °C.
The reaction time during
which electrocatalytic metals can precipitate as metals on the cathode
suitably is from
about 1 to about 60 minutes or until the electrode is completely dried.
Suitably, solvent
can thereafter again be deposited on the cathode to repeat the precipitation
procedure of
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electrocatalytic metals in case such remains exist on the cathode in the form
of salts or
acids. Suitably, an amount from about 10 to about 100 ml solvent/m2 cathode
area is
contacted with the cathode, preferably from about 50 to about 100 ml
solvent/m2.
The activated cathode is then preferably rinsed by a rinsing solution such as
5 water to avoid corrosion, preferably with a basic solution such as NaOH
after that the
solvent on the cathode has substantially dried. Preferably, the basic rinsing
solution has a
concentration of e.g. NaOH from about 0.0001 to about 50 wt%, and most
preferably from
about 0.0001 to about 20 wt%.
The activated cathodes are usually run in the electrolytic cells until their
activity
is found to be too low, i.e. at an uneconomically low level. This crucial
extent of
deactivation can be optimised by a person skilled in the art by estimating the
electric
energy consumed and the activation costs. When the reactivation is to be
initiated, the
used cathodes are preferably disassembled and removed from their cells.
Suitably, the
reactivation can be performed in connection with replacement of the membranes
arranged in the electrolytic cell.
The present invention also relates to a cathode obtainable by the method as
described above.
The invention further concerns the use of an activated cathode in an
electrolytic
cell for producing chlorine and alkali metal hydroxide.
The invention being thus described, it will be obvious that the same may be
varied in many ways. Such variations are not to be regarded as a departure
from the gist
and scope of the present invention, and all such modifications as would be
obvious to
one skilled in the art are intended to be included within the scope of the
claims. The
following examples will further illustrate how the described invention may be
performed
without limiting the scope of it.
Example 1: A cleaning solution was prepared from concentrated hydrochloric
acid (37 wt%) to yield a final concentration of 20 wt% hydrochloric acid. The
cathode to
be activated was contacted with the cleaning solution by means of painting. 50
ml
cleaning solution/m2 geometric cathode area was applied. The solution was then
allowed
to react during 10 minutes at room temperature (25 °C). The cathode was
thereafter
rinsed thoroughly with deionised water. Meanwhile, a coating solution of RhCl3
was
prepared by dissolving the rhodium salt in a 20 wt% hydrochloric acid
solution, resulting
in a final rhodium concentration of 50 g Rh metal/litre coating solution. Also
a coating
solution of RuCl3 was prepared by dissolving the Ru salt in another 20 wt%
hydrochloric
acid solution resulting in a concentration of 50 g Ru metal/litre coating
solution. The
rinsed cathode was allowed to dry in room temperature, whereafter the Rh
coating
solution was applied thereto in an amount of 50 ml/mz geometric cathode area
by means
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of painting. The cathode was then allowed to dry for 1 hour. The Ru coating
solution was
then applied to the Rh coated cathode in an amount of 50 ml/m2 geometric
cathode area.
The cathode was then dried whereafter an aqueous solution of 50 wt% H3P02 was
painted on the cathode. Thereafter, the cathode was allowed to dry whereupon
it was
rinsed with water. The cathode obtained showed satisfactory activation.
Example 2: Two deactivated nickel-based cathode samples P1 and P2 were
cleaned by means of painting with a 20wt% hydrochloric acid solution for 5
minutes. The
cathode samples were thereafter rinsed with water and thereafter dried. The
two samples
were both coated with a 40 ml RhCl3 coating solution having a rhodium content
of 150
g/litre /m2. The coated samples were then allowed to dry for 1 hour. Unreacted
rhodium
precipitated during the drying stage and formed rhodium chloride salt on the
cathode
substrate. The P1 sample was gently rinsed with a caustic solution having a pH
of 10,
whereupon unreacted precipitated rhodium metal salt (RhCl3) and nickel
chloride were
rinsed off the cathode sample. The remaining amount of rhodium on the P1
sample only
amounted to a small portion of the initially precipitated rhodium content.
This was judged
from the rhodium colour the rinsing solution got since the metal was partially
washed off.
The P2 cathode sample was gently sprayed with a 20 wt% hydrochloric acid after
the
RhCl3 solution had dried on the sample, whereupon precipitated RhCl3 was
redissolved.
Subsequent precipitation of metallic rhodium could then take place on the P2
sample.
The addition of hydrochloric acid to the P2 cathode was repeated once after
that the P2
cathode had dried. 15 minutes after the second addition of hydrochloric acid,
i.e. after
that the cathode was substantially dry, the cathode was rinsed with caustic
solution in the
same manner as the P1 sample. No colour shift could be observed in the rinsing
solution
due to rinsed off rhodium. It was thus shown that a much higher amount of
rhodium had
adhered to the P2 sample than to the P1 sample as a result of adding the
solvent to the
coated and dried sample. Electrolytic trials performed involving use of the
activated
cathodes showed that the cell voltage was 230 mV lower for the P2 cathode than
for the
P1 cathode when the used electrolytic cell was operated at a current density
of 4.7 kA/m2.
