Note: Descriptions are shown in the official language in which they were submitted.
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UNDIVIDED ELECTROLYTIC CHLORATE CELLS WITH COATED
CATHODES
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional Patent
Application No.
60/617,762, filed October 12, 2004, which is incorporated herein by reference
in its entirety
for all purposes.
BACKGROUND OF THE INVENTION
[0002] Sodium chlorate is commonly used as a bleaching chemical in the pulp
and paper
industry. Sodium chlorate is produced by electrolyzing sodium chloride in an
electrolytic
cell using a direct current. The electrolytic cell typically includes
dimensionally stable
anodes ("DSAs"), and cathodes constructed from uncoated steel or uncoated
titanium. This
process is energy intensive, requiring approximately 5000 KWhr of electricity
to produce 1
metric ton of sodium chlorate. During the production of sodium chlorate,
sodium chloride
is oxidized to form chlorine on the positive electrode (called the anode). The
chlorine then
chemically transforms to sodium chlorate under controlled chemical conditions.
On the
negative electrode (called the cathode), water is reduced to form hydrogen gas
as a by-
product of the electrochemical reaction. However, a certain amount of
electrical energy is
wasted in producing hydrogen gas on the cathode. The wasted electrical energy
is
commonly referred to as hydrogen overvoltage. The present invention solves
these and
other problems in the art of chlorate production.
BRIEF SUMMARY OF THE INVENTION
[0003] The present invention provides novel undivided electrolytic chlorate
cells and
methods of making the cells thereof. It has been discovered that undivided
electrolytic
chlorate cells including a dimensionally stable anode and a cathode coated
with a catalytic
metal cathode coating may be used to produce chlorate in an efficient and cost
effective
manner. It has also been discovered that cathodes forming part of an undivided
electrolytic
chlorate cell may be coated, in the presence of the anode, by
electrodeposition of a catalytic
metal from a catalytic metal cathode electrodepositing solution without the
use of a physical
barrier separating the anode and cathode.
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[0001] In one aspect, an undivided electrolytic chlorate cell is provided. The
undivided
electrolytic cell includes an anode and a cathode. The cathode is in fluid
communication
with a catalytic metal cathode electrodepositing solution.
[0002] In another aspect, a method of coating a cathode is provided. The
cathode forms
part of an undivided electrolytic chlorate cell. The method includes the step
of contacting
the cathode with a catalytic metal cathode electrodepositing solution. The
method also
includes the step of electrodepositing a catalytic metal from the catalytic
metal cathode
electrodepositing solution onto the cathode thereby forming a catalytic metal
cathode
coating on the cathode.
[0003] In another aspect, the present invention provides an undivided
electrolytic chlorate
cell. The cell includes a dimensionally stable anode and a cathode. The
cathode is coated
with a catalytic metal cathode coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 illustrates an exemplary monopolar SEC. A is the electrical
connection of
the negative terminal of the DC power supply; B is the electrical connection
to the positive
terminal of the DC Power Supply; C is the anode; and D is the cathode.
[0005] FIG. 2 illustrates an exemplary bipolar SEC. A is the electrical
connection of the
negative terminal of the DC power supply; B is the electrical connection to
the positive
terminal of the DC Power Supply; C is the anode; D is the cathode; and E is
the bipolar late
where the (+) side is anodic and the (-) side is cathodic.
[0006] FIG. 3 illustrates an exemplary monopolar MEC. A is the electrical
connection
between each single electrolytic cell (SEC); B is the respective SEC.
[0007] FIG. 4 illustrates an exemplary bipolar MEC. A is the electrical
connection
between each SEC; B is the bipolar SEC.
[0008] FIG. 5 are graphs corresponding to Table 3, consisting of sodium
chloride (NaC1)
concentration versus days of operation, sodium chlorate (NaC103) concentration
versus days
of operation, cell voltage (Voltage) versus days of operation, k factor versus
days of
operation, molybdenum (Mo) and calcium (Ca) concentrations versus days of
operation.
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DETAILED DESCRIPTION OF THE INVENTION
Definitions
[0009] As used herein, a "single electrolytic cell" ("SEC") means an apparatus
having 1 or
more anode plates in combination with corresponding cathode plates (e.g. 1
anode plate
with lor 2 cathode plates or n_2 anode plates in combination with n-1, n or
n+1 cathode
plates). _
[0010] A "multiple electrolytic cell" ("MEC") is an apparatus having more than
1 SEC
assembled as part of an electrical circuit in which the electrolytic cell
configuration is
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monopolar or bipolar. Figures 1, 2, 3 and 4 illustrate monopolar SEC, bipolar
SEC,
monopolar MEC and bipolar MEC examples respectively.
[0014] The term "in-situ," refers to a process (e.g. coating,
electrodepositing) which is
performed in an intact (e.g. a pre-assembled) electrolytic cell, such as an
electrolytic
chlorate cell. Thus, in-situ processes do not require mechanical disassembly
of an
electrolytic cell (e.g. a SEC or MEC) to separate one or more anode plates
from cathode
plates, for example between electrodeposition and chlorate production, or
between chlorate
production and electrodeposition.
DESCRIPTION
[0015] The present invention provides novel undivided electrolytic chlorate
cells having
catalytic metal coated cathodes. The electrolytic cells are cost-efficient,
energy-efficient,
and allow for convenient in-situ coating of cathodes in the undivided
electrolytic chlorate
cells thereby avoiding time consuming and costly disassembly of the cells.
Undivided Electrolytic Chlorate Cells
[0016] An "electrolytic chlorate cell" is an apparatus containing an anode and
a cathode in
which chemical reactions are caused by applying an external potential
difference, typically
greater than, and opposite to, the galvanic electromotive force of the cell.
Electrolytic
chlorate cells generally convert electrical energy into chemical energy. The
chemical
reactions usually do not occur spontaneously at the electrodes when they are
connected
through an external circuit. The reaction is typically forced by applying an
external
electrical current. Thus, an electrolytic chlorate cell is an assembled
electrolytic cell
apparatus. A wide variety of electrolytic chlorate cell configurations are
useful in the art of
chlorate production. For a detailed discussion of electrolytic chlorate cell
configurations
and the chemistry of chlorate production, see Colman, "Electrolytic Production
of Sodium
Chlorate," no. 204, vol. 77, the American Institute of Chemical Engineers
(1981), which is
herein incorporated by reference in -its entirety for all purposes. See also,
"Sodium
Chlorate," in the Encyclopedia of Chemical Processing and Design, Ed. McKetta,
J., vol.
51, pp. 126-18 8 (New York), Marcel Dekker, Inc.
[0017] An "undivided electrolytic chlorate cell" is an electrolytic chlorate
cell that has no
physical barrier (e.g. a membrane or diaphragm) between the anode and the
cathode that
functions to separate the cell liquor. Thus, the cathode and anode are present
in a single
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chamber. In some embodiments, the electrolytic chlorate cell forms part of a
multiple
electrolytic cell. Thus, undivided electrolytic chlorate cells may be in the
form of a single
cell or form part of a multiple electrolytic cell, such as a multiple
electrolytic chlorate cell.
[0018] In undivided electrolytic chlorate cells, an external potential
difference is applied
sufficient for electrolysis of an aqueous solution comprising sodium chloride
(also referred
to herein as "brine" or "chlorate liquor"). This electrolysis produces
chlorine gas at the
anode and hydrogen gas at the cathode. Since the hydrogen is produced by
breaking up
water molecules, the solution becomes basic near the cathode and a solution of
sodium
hydroxide (also called "caustic" or "alkali") is produced. The production of
chlorate in an
undivided electrolytic chlorate cell may be summarized by the follows
reactions:
20- Clz + 2e-
2H20 + 2e o- H2 + 20H"
C12 + H20 HOC1 + H} + Cl"
HOCI H+ + C1O-
2HC1O + C10- C103- + 20- +2H+
[0019] The anode and cathode may be any appropriate shape and composed of any
suitable material. For example, the cathode may be composed of any appropriate
conductive material suitable for conditions of chlorate electrolysis. Useful
metals include
those comprising iron, titanium, and/or steel. The cathode may be in any
appropriate shape,
such as a solid sheet, bar, or other solid metal configuration, or a metal
mesh or screen of
high surface area.
[0020] Anodes useful in the present invention include those comprising an
electrically
conductive anode substrate, such as titanium, tantalum, niobium and zirconium.
Typically,
the anode includes one or more anode coating(s) on the surface of an anode
substrate.
Useful anode coatings include those comprising ruthenium, titanium, tantalum,
niobium,
zirconium, platinum, palladium, iridium, tin, rhodium, antimony, and
appropriate alloys,
combinations, and/or oxides thereof. In some embodiments, the anode substrate
is a
titanium anode substrate. In some embodiments, the anode coating is a
ruthenium-antimony
oxide anode coating (i.e. a coating comprising ruthenium and antimony, e.g. a
ruthenium-
antimony mixed oxide) or derivative thereof. In other embodiments, the anode
coating is a
ruthenium-titanium oxide anode coating or derivative thereof. In other
embodiments, the
anode coating is a ruthenium-titanium-antimony anode oxide coating or
derivative thereof.
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[0021] In some embodiments, the anode is a dimensionally stable anode (DSA).
Dimensionally stable anodes are well known in the art of electrolytic cells.
See, for
example, WO 4101852, WO 4094698, US 6071570, US 5672394, US 4233340, US
5679225, US 5593556, US 5989396, US 5419824, US 4528084, and US 6572758, each
of
which are herein incorporated by reference in their entirety for all purposes.
DSAs are
highly corrosion resistant electrodes that have electrochemically active
surface coatings.
DSAs were developed to overcome the limitations of carbon and graphite
electrodes, which
are gradually eroded or decomposed during electrolytic cell operation.
Dimensionally
stable anodes are typically comprised of a titanium or similar valve metal
substrate coated
with a platinum metal or ruthenium oxide alone or in combination with other
oxides and/or
compounds.
[0022] Thus, in one aspect, the present invention provides an undivided
electrolytic
chlorate cell with a dimensionally stable anode and a cathode. The cathode is
coated with a
catalytic metal cathode coating.
Catalytic Metal Cathode Electrodepositing solution and CoatinLys
[0023] A "catalytic metal cathode electrodepositing solution" is a solution
from which a
catalytic metal or metals are electroplated onto a cathode to form a catalytic
metal cathode
coating. The catalytic metal cathode coating includes a catalytic metal that
catalyzes the
chlorate cell hydrolysis reaction that forms hydrogen and hydroxide at the
cathode, thereby
reducing hydrogen overvoltage. Where the anode includes a coating, the
catalytic metal
cathode electrodepositing solution typically does not degrade the anode
coating (e.g. a
titanium-ruthenium containing anode coating) before, during, and/or after
electrodeposition.
In some embodiments, the catalytic metal cathode coating is a metal alloy
(i.e. a catalytic
metal alloy cathode coating). The term "coating," when used in reference to a
cathode
coating, refers to at least a partial covering of the cathode. Therefore, a
cathode coating
may cover a portion or all of the cathode in order to decrease hydrogen
overvoltage.
[0024] A catalytic metal includes metal alloys, such as iron-molybdenum alloys
and
derivatives thereof. Other catalytic metals include platinum, iron-oxide, iron-
tungsten
alloys, combinations, and derivatives thereof.
[0025] In some embodiments, the catalytic metal cathode electrodepositing
solution is an
iron-molybdenum cathode electrodepositing solution. Iron-molybdenum cathode
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electrodepositing solutions include a molybdenum component (molybdenum in a
form
capable of being electroplated onto a cathode, e.g. Na2MoO4) and an iron
component (iron
in a form capable of being electroplated onto a cathode, e.g. FeC13 or FeSO4).
Thus, iron-
molybdenum cathode electrodepositing solutions include iron-molybdate cathode
electrodepositing solutions. The electrodepositing solution may further
comprise an iron
chelating agent (e.g. Na4P2O7 or C6H5Na3O7). The electrodepositing solution
may further
comprise a buffering agent, such as bicarbonate (e.g. NaHCO3).
[0026] Useful iron-molybdenum cathode coatings include those having from 5-95%
molybdenum by weight. In some embodiments, the iron-molybdenum cathode coating
contains from 5-50% molybdenum by weight. In other embodiments, the iron-
molybdenum
cathode coating contains from 10-50% molybdenum by weight. In other
embodiments, the
iron-molybdenum cathode coating contains from 15-55% molybdenum by weight. In
another embodiment, the iron-molybdenum cathode coating contains from 8-56%
molybdenum by weight. In another embodiment, the iron-molybdenum cathode
coating
contains from 8-49% molybdenum by weight. In another einbodiment, the iron-
molybdenum cathode coating contains from 10-40% molybdenum by weight. In
another
embodiment, the iron-molybdenum cathode coating contains from 25-35%
molybdenum by
weight. In another embodiment, the iron-molybdenum catllode coating contains
from 25-
30% molybdenum by weight. In some embodiments, the iron-molybdenum cathode
coating
comprises 10-20% molybdenum by weight. In some embodiments, the iron-
molybdenum
cathode coating coinprises 15-18% molybdenum by weight.
[0027] Useful iron-molybdenum cathode electrodepositing solutions include
those having
from 5-95% molybdenum by weight. In some embodiments, the iron-molybdenum
cathode
electrodepositing solution includes molybdenum in a concentration from 0.2 g/L
to 25 g/L.
In other embodiments, the iron-molybdenum cathode electrodepositing solution
includes
molybdenum in a concentration from 0.3 g/L to 20 g/L. In other embodiments,
the iron-
molybdenum cathode electrodepositing solution includes molybdenum in a
concentration
from 0.4 g/L to 16g/L. In other embodiments, the iron-molybdenum cathode
electrodepositing solution includes molybdenum in a concentration from 0.476
g/L to 15.86
g/L.
[0028] In some embodiments, the iron-molybdenum cathode electrodepositing
solution
includes iron in a concentration from 0.5 g/L to 50 g/L. In other embodiments,
the iron-
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molybdenum cathode electrodepositing solution includes iron in a concentration
from 0.7
g/L to 40 g/L. In other embodiments, the iron-molybdenum cathode
electrodepositing
solution includes iron in a concentration from 0.5 g/L to 5 g/L when used with
a
pyrophosphate chelating agent. In other embodiments, the iron-molybdenum
cathode
electrodepositing solution includes iron in a concentration from 0.9 g/L to
3.5 g/L when
used witll a pyrophosphate chelating agent. In other embodiments, the iron-
molybdenum
cathode electrodepositing solution includes iron in a concentration from 0.93
g/L to 3.1 g/L
when used with a pyrophosphate chelating agent. In other embodiments, the iron-
molybdenum cathode electrodepositing solution includes iron in a concentration
fiom 35
g/L to 45 g/L when used with a citrate chelating agent. In other embodiments,
the iron-
molybdenum cathode electrodepositing solution includes iron in a concentration
of from
39g/L to 40 g/L when used with a citrate chelating agent. In other
embodiments, the iron-
molybdenum cathode electrodepositing solution includes iron in a concentration
of about 39
g/L when used with a citrate chelating agent.
[0029] In some embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a sodium molybdate dehydrate molybdenum component in a concentration
from
0.5 g/L to 60 g/L. In other embodiments, the iron-molybdenum cathode
electrodepositing
solution includes a sodium molybdate dehydrate molybdenum component in a
concentration
from 1.0 g/L to 45 g/L. In other embodiments, the iron-molybdenum cathode
electrodepositing solution includes a sodium molybdate dehydrate molybdenum
component
in a concentration from 1.2 g/L to 40 g/L.
[0030] In some embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a ferric chloride hexahydrate iron component in a concentration from
3.5 g/L to
35g/L. In other embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a ferric chloride hexahydrate iron component in a concentration from
4.0 g/L to
30g/L. In other embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a ferric chloride hexahydrate iron component in a concentration from
4.5 g/L to
27g/L. In some related embodiments, the ferric chloride hexahydrate iron
component is
used with a pyrophosphate chelating agent.
[0031] In some embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a ferrous sulphate heptahydrate iron component in a concentration
from 180 g/L to
220g/L. In other embodiments, the iron-molybdenum cathode electrodepositing
solution
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includes a ferrous sulphate heptahydrate iron component in a concentration
from 190 gIL to
200g/L. In other embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a ferrous sulphate heptahydrate iron component in a concentration of
about 195
g/L. In some related embodiments, the ferrous sulphate heptahydrate iron
component is
used with a citrate chelating agent.
[0032] The catalytic metal cathode coating preferably does not interfere with
the chlorate
producing chemistry. Essential intermediates, such as hypochlorite, are
preferably not
degraded by the catalytic metal cathode coating or the products of catalytic
metal cathode
coating degradation (e.g. nickel, copper, and/or cobalt metal ions) during
normal operation
of the cell. Thus, in some embodiments, power failures (depolarizations,
including random
power failures) during use of the electrolytic cell will not cause
contamination of the
chlorate liquor with ions known to catalyze hypochlorite degradation, such as
nickel, cobalt,
and/or copper. In some embodiments, the catalytic metal cathode coating is
capable of
resisting single or multiple power failures.
[0033] In some embodiments, the coating and/or electrodepositing solution does
not
contain significant amounts of nickel, cobalt, and/or copper. In other
embodiments, the
coating and/or electrodepositing solution does not contain nickel, cobalt,
and/or copper. A
significant amount of nickel, cobalt, and/or copper is an amount of nickel,
cobalt, and/or
copper that, when incorporated into the catalytic metal cathode coating of the
present
invention, releases into the chlorate liquor during operation of the cell in
amounts that
degrade hypochlorite to an extent that renders the cell economically
inefficient and/or
inoperational for large scale commercial purposes in undivided cells.
[0034] Where the catalytic metal cathode electrodepositing solution is an iron-
molybdenum cathode electrodepositing solution, the solution may include an
iron chelating
agent. A number of appropriate iron chelating agents are useful in the
electrodepositing
solution of the present invention, including pyrophosphate and citrate (e.g.
Na4P2O7 or
C6H5Na3O7 and hydrates thereof).
[0035] The catalytic metal cathode electrodepositing solution may also include
a
buffering agent, such as bicarbonate (e.g. NaHCO3). Buffering agents useful in
the present
invention include those that are capable of maintaining the pH below 10. In
some
embodiments, the electrodepositing solution is maintained at a pH below 9. In
some
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embodiments, the electrodepositing solution is maintained at a pH below 8. In
some
embodiments, the electrodepositing solution is maintained at a pH below 7. In
some
embodiments, the electrodepositing solution is maintained at a pH below 6.5.
In some
embodiments, the electrodepositing solution is maintained at a pH from 5-10.
In some
embodiments, the electrodepositing solution is maintained at a pH from 6-10.
In some
embodiments, the electrodepositing solution is maintained at a pH from 6-9. In
some
embodiments, the electrodepositing solution is maintained at a pH from 6-8.
[0036] In otller embodiments, the electrodepositing solution is a non-tartrate
ion solution.
[0037] In some embodiments, the undivided electrolytic chlorate cells having
cathodes
coated with catalytic metal cathode coating provides substantial energy
savings during
operation of the cell relative to cells having uncoated cathodes (e.g. under
the conditions of
Example 5 below). In some embodiments, the voltage saving is greater than or
equal to 350
millivolts. In another embodiment, the voltage saving is greater than or equal
to 200
millivolts. In another embodiment, the voltage saving is greater than or equal
to 100
millivolts. In another einbodiment, the voltage saving is from 350 millivolts
to 400
millivolts. In another embodiment, the voltage saving is from 300 millivolts
to 400
millivolts. In another embodiment, the voltage saving is from 200 millivolts
to 400
millivolts. In another embodiment, the voltage saving is from 100 millivolts
to 400
millivolts.
[0038] In some embodiments, the iron-molybdenum cathode electrodepositing
solution
includes a molar ratio of iron chelator to iron of from about 1:1 to about
8:1. In other
embodiments, the iron-molybdenum cathode electrodepositing solution includes a
molar
ratio of iron chelator to iron of from about 2.5:1 to about 3.5:1. In other
embodiments, the
iron-molybdenum cathode electrodepositing solution includes a molar ratio of
iron chelator
to iron of about 3:1. In some embodiments, the iron chelator is sodium
pyrophosphate
(including hydrates thereof). In some embodiments, the iron is present as
ferric chloride
(including hydrates thereof). In other embodiments, the iron-molybdenum
cathode
electrodepositing solution includes a molar ratio of iron chelator to iron of
about 1:1. In
some embodiments, the iron chelator is sodium citrate (including hydrates
thereof). In some
embodiments, the iron is present as ferrous sulphate (including hydrates
thereof). In some
embodiments the iron chelator as pyrophosphate is present in a mole ratio to
iron of 3:1. In
some embodiments the iron chelator as citrate is present in a mole ratio to
iron of 1:1.
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Methods of Coating Cathodes In-Situ
[0039] In another aspect, the present invention provides a method of coating a
cathode.
The cathode forms a portion of an undivided electrolytic chlorate cell. The
method includes
contacting the cathode with a catalytic metal cathode electrodepositing
solution, and
electrodepositing a catalytic metal (e.g. iron-molybdenuin) from the catalytic
metal cathode
electrodepositing solution onto the cathode thereby forming a catalytic metal
cathode
coating on the cathode. Thus, the electrodeposition is performed in-situ in
the presence of
the anode (i.e. the anode is in fluid communication with the electrodepositing
solution).
[0040] Where a catalytic metal is electrodeposited from the catalytic metal
cathode
electrodepositing solution, one skilled in the art will immediately understand
that not all of
the components of the catalytic metal cathode electrodepositing solution is
necessarily
electrodeposited (e.g. the buffering agent and/or the chelating agent). The
process of
electrodepositing catalytic metals onto a substrate from an electrodepositing
solution is well
known in the art. Using the teachings disclosed herein, it within the
abilities of one of skill
in the art to determine the appropriate electrodepositing conditions.
[0041] Thus, in one aspect, the present invention provides an undivided
electrolytic
chlorate cell with an anode and a cathode. The cathode is in fluid
communication with a
catalytic metal cathode electrodepositing solution. Typically, the anode is
also in fluid
communication with the catalytic metal cathode electrodepositing solution.
[0042] The metllod may further include, prior to the step of contacting the
cathode with a
catalytic metal cathode electrodepositing solution, the step of washing the
cathode with an
acidic solution. The washing may further include washing the anode with the
acidic
solution. In some einbodiments, after step washing the cathode with an acidic
solution and
before contacting the cathode with a catalytic metal catliode
electrodepositing solution, the
method includes the step of washing the cathode with water. In some
embodiments, the
method includes the step of washing the cathode and anode with water.
[0043] The properties of catalytic metal cathode electrodepositing solutions
and catalytic
metal cathode coatings are discussed in detail above and are equally
applicable to the
methods of the present invention. Thus, in some embodiments, the catalytic
metal cathode
electrodepositing solution is an iron-molybdenum cathode electrodepositing
solution, as
described above. Therefore, the catalytic metal cathode coating may an iron-
molybdenum
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cathode coating and the catalytic metal may be iron-molybdenum. In some
embodiments,
the catalytic metal cathode coating is capable of resisting single or multiple
power failures.
In some embodiments, the catalytic metal cathode coating provides a voltage
saving of at
least 200 millivolts as describes above.
[0044] The properties of suitable anodes and cathodes are also described above
and are
equally applicable to the methods of the present invention. For exainple, in
some
embodiments, the anode is a dimensionally stable anode (e.g. comprising a
ruthenium
dioxide coating). In some einbodiments, the cathode comprises steel or
titanium.
[0045] In some embodiments, the electrolytic chlorate cell forms part of a
multiple
electrolytic cell.
[0046] Contacting the cathode with a catalytic metal cathode electrodepositing
solution
may be accomplished by any appropriate means. Typically, the catalytic metal
cathode
electrodepositing solution is allowed to flow into the undivided electrolytic
chlorate cell at
an appropriate flow rate thereby contacting the cathode. The appropriate flow
rate is
selected to allow electrodeposition of one or inore catalytic metals from the
solution to the
cathode while replenishing reagents consumed in the electrodepositing process.
[0047] Electrodeposition may be performed at any appropriate temperature of
the
catalytic metal cathode electrodepositing solution. Where it is desired to
decrease
electrodeposition time and/or increase current efficiencies, the temperature
of the catalytic
metal cathode electrodepositing solution may be increased above ambient
temperature (i.e.
room temperature). In some embodiments, the temperature of the catalytic metal
cathode
electrodepositing solution is from 40 C to 100 C during electrodeposition. In
other
embodiments, the temperature of the catalytic metal cathode electrodepositing
solution is
from 50 C to 80 C. In other embodiments, the teinperature of the catalytic
metal cathode
electrodepositing solution is from 50 C to 70 C. In other embodiments, the
temperature of
the catalytic metal cathode electrodepositing solution is about 70 C. In some
embodiments,
the electrodepositing solution is electrodeposited from 40 to 100 minutes. In
other
embodiments, the electrodepositing solution is electrodeposited from 50 to 90
minutes. In
other embodiments, the electrodepositing solution is electrodeposited for
about 75 minutes.
In other embodiments, the electrodeposition current efficiency (i.e. the
portion of current
used to electrodeposit the electrodepositing solution relative to the total
amount of current
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applied) is from about 40% to about 80%. In other embodiments, the
electrodeposition
current efficiency is from about 55% to about 65%.
[0048] The tenns and expressions which have been employed herein are used as
terms of
description and not of limitation, and there is no intention in the use of
such terms and
expressions of excluding equivalents of the features shown and described, or
portions
thereof, it being recognized that various modifications are possible within
the scope of the
invention claimed. Moreover, any one or more features of any embodiment of the
invention
may be combined with any one or more other features of any other embodiment of
the
invention, without departing from the scope of the invention. For example, any
feature of
the methods of coating cathodes described above can be incorporated into any
of the
asseinblies, apparatuses or systems without departing from the scope of the
invention.
EXAMPLES
[0049] The following examples are provided for the purpose of describing and
illustrating
a few exemplary embodiments of the invention only. Other einbodiments of the
invention
are possible, but are not described in detail here. Thus, these examples are
not intended to
limit the scope of the invention in any way.
Example 1
[0050] Iron-molybdenum was electrodeposited in-situ to chlorate cell cathodes
(20 cmZ)
using a 333 ml plating solution at a flow rate of 1.7 L/min, and 0.43 m/s bulk
electrolyte
flow and bulk electrolyte velocity, respectively.
[0051] The plating conditions are shown below in Table 1. Although not shown
in Table
1, NaHCO3 was included in the plating solution at a concentration of 75 g/L.
Table 1
Na2MoO4 Na4P2O7
2H20 FeCI3 6H20 10H20 P207:Fe Temp. CD Plating l*t pH pH
Mole
Ratio Time Final
/L (M) (g/L) M /L (M) M/M C A/dma min C=A*s
0.00
1.2 5 4.5 0.017 22.5 0.05 3.0 25 3.5 86 3600 7.6 8.4
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Abbreviations: M = molar, A =amperes, C = coulombs, dm2 = decimeter squared,
min. _
minutes, C = degrees Celsius, EDX = energy dispersed X-ray analysis, wt. =
weight,
C.C.E. = cathodic current efficiency, dep. = deposition, XRF = X-ray
fluorescence, ID =
identification, I = current in amperes, t=time in seconds, s= seconds,
L=litres, g =grams,
S2mz = micro-ohms meters squared, mg = milligrams, ml = millilitres.
[0052] Characterization data for the plated cathodes are shown in Table 2
below.
Table 2
% EDX
Blade Dep. XRF Mo C.C.E. Analysis Cross Section
EDX wt.
Weight Mo/mg % wt. % %
ID (mg) dep. Mo Fe Mo Fe
49 139 0.24 23.3 20.8 22.6 77.4 27.5 72.5
[0053] Chlorate cells containing iron-molybdenum plated cathodes as described
above
were tested for their ability to produce chlorate. The results are shown in
Table 3 below and
in graph form in FIG. 5. K factor is determined by the linear slope of cell
voltage (Voltage)
versus current density between the current densities of 2000 to 4000
Amperes/meter2.
EC 120 refers to an anode model of particular supplier of anodes. Stahrmet
Steel refers to a
cathode model of particular supplier of cathodes. The cathode was coated in-
situ at day 16.
Notice the voltage difference between day 15 and days 17 to 101 of >_ 277
millivolts.
Table 3
Days NaCI NaCIO3 Voltage % O2 k factor Mo Ca
/L /L (V) SZmZ m /L m /L
1 111 490 3.364 2.3 188
2 97 3.358 2.25 187
3 97 501 3.339 2.115 186
6 99 3.327 1.95 185
7 101 3.318 1.91 187
8 103 3.308 1.905 186
9 108 3.31 1.91 185
10 109 467 3.305 1.89 185
13 111 462 3.307 1.91 182
14 112 3.303 1.9 182
15 111 469 3.313 1.835 183
17 105 2.87 1.865 187 0.69 0.11
116 449 2.919 1.865 195 0.23 0.1
22 105 2.93 1.795 193
23 108 2.931 1.875 196
24 108 465 2.93 1.88 197
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Days NaCI NaC103 Voltage % O2 k factor Mo Ca
/L /L (V) nma m /L m /L
28 98 467 2.941 1.87 198 0.09 0.15
29 110 2.935 1.925 198
30 111 458 2.935 1.895 197
31 114 2.925 1.83 195
34 107 463 2.937 1.895 199
35 106 2.936 1.87 200
36 105 472 2.93 1.905 198
37 104 2.94 1.87 200
38 115 460 2.927 1.88 198 0.19 0.23
41 101 2.948 1.865 203
42 113 2.931 1.855 199
43 116 447 2.933 1.84 198
44 104 2.946 1.835 199
45 102 467 2.947 1.875 202
48 98 472 2.956 1.81 200
49 106 2.949 1.85 202
50 111 456 2.953 1.83 204 0.67 0.48
51 107 2.95 1.85 203
52 110 2.936 1.815 198
55 112 449 2.967 1.835
56 109 2.973 1.87 206
57 106 472 2.973 1.82
58 102 2.985 1.865 2.27 0.49
59 101 2.985
62 102 485 3.014 1.82 2.5
63 102 3.026 1.88
69 104 465 3.038 1.87
70 101 3.034 1.89 208 0.9
71 105 3.038
76 99 3.032 1.88
77 107 3.032
78 111 3.034 1.89 205
79 112 3.029 1.91
80 110 3.035 1.92 202 0.38
83 103 479 3.038
84 106 3.034 1.94 205
85 104 472 3.034 1.87
86 104 3.032 1.94 202
87 109 3.022 1.95 0.25
90 110 469 3.026 1.95
91 110 3.029 1.95 199
92 111 3.029 1.93
93 113 3.034 1.94 200
94 106 458 3.033 1.91
97 96 465 3.038 1.94
98 105 3.034 1.92 200
99 105 3.042 1.97
100 102 3.039 1.96 201
101 106 3.036 1.96
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Example 2
[0054] Cathode "AW26-IS 1" (Example 2A) was plated using a new steel substrate
and
cathode "AW26-IS3" (Example 2B) was plated using a previously used steel
substrate.
AW2 6-IS 1 and AW26-IS3 were in-situ plated using identical plating solutions
without
opening the cells. The in-situ coatings were analyzed via the plating solution
before and
after plating by ICP analysis for the estimation of the deposited Fe-Mo amount
and current
efficiency for electrodeposition. The plating solution (1.43 litres) contained
1.8 g/L
Na2MoO4.2H20 (sodium molybdate dihydrate), 9 g/L FeC13.6HZ0 (ferric chloride
hexahydrate), 45 g/L Na4P2O7 10H20 (sodium pyrophosphate decahydrate), 75 g/L
NaHCO3 (sodium bicarbonate).
Example 2A
[0055] Cathode AW26-IS 1 was prepared as follows: A new steel cathode
substrate was
immersed in 8 weight % hydrochloric acid that had been heated to 40 C and
allowed to
naturally cool for 1 hour to remove mill scale and rust. Residual acid on the
cathode
substrate was then removed by rinsing the cathode under flowing tap water for
20 seconds
followed by deionised water rinsing from a wash bottle. The cathode was then
dried with
compressed air. A test cell was assembled with the acid cleaned new cathode
and a new
EC120 anode. The test cell was used for chlorate electrolysis for 1 week using
the
following conditions: Electrode gap 2.5 mm; Temperature 80 C; Electrolyte bulk
pH 6.0-
6.1; NaCI concentration 100 grams/litre; Sodium dichromate concentration 4
grams/litre;
Sodium sulphate concentration 14 grams/litre; 2 mg/litre calcium and 0.4 mg/L
magnesium
ions added to the NaCI input; Anode and cathode areas were 100 cm2 (height
32.5 cm);
Electrolyte flow rate through the test cell: 0.51itres/ampere-hour; Electrode
current density
3000 amperes/metre2. After 1 week of chlorate electrolysis the test cell was
drained while
maintaining a polarization potential on the cell until the cell was completely
drained to
prevent cathode corrosion.
[0056] The cell was rinsed by pumping through approximately 1 litre of
deionised water.
The cell was acid cleaned by filling the cell with 8 weight% hydrochloric acid
preheated to
C and allowing the acid to soak for 1 hour without disassembly of the cell.
The spent
acid was drained and the acid cleaned cell was rinsed by pumping through
approximately 1
litre of deionised water without disassembly of the cell. The plating solution
described
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above was pumped through the cell via recycle, from a 1.43 litre tank
solution, at a bulk
electrolyte velocity of 0.1 metres/second. Electrodeposition was carried out
at 70 C for 75
minutes (9000 coulombs) at a current density of 2.0 amperes/decimetre2. ICP
analysis
indicated that 1.08 g of coating was deposited at 61.5% current efficiency
with 18 weight %
molybdenum and 82% iron in the coating.
[0057] The electroplated cell was rinsed of spent plating solution by pumping
through 1
litre of deionised water. The electroplated test cell was used for chlorate
electrolysis using
the following conditions: Electrode gap 2.5 mm; Temperature: 80 C; Electrolyte
bulk pH:
6.0-6.1; NaCI concentration 100 grams/litre; Sodium dichromate concentration 4
grams/litre; Sodium sulphate concentration 14 grams/litre; 2 mg/litre calcium
and 0.4 mg/L
magnesium ions added to the NaCI input; Electrolyte flow rate through the test
cell 0.5
litres/ampere-hour; Electrode current density 3000 amperes/metre2.
[0058] Cell voltage and oxygen evolution (an indirect method of measuring
chlorate
production efficiency) was recorded after allowing one day for stabilization.
Starting on the
3rd day with coated cathode, 6 minute long cell depolarizations (power-
interruptions) were
carried out daily to determine the robustness of cathode deposits under
unforeseen power
failures during chlorate electrolysis. On day 27 a 2.5 hour rather than 6
minute power
stoppage occurred.
[0059] The following Table 4 indicates that in-situ plating of Fe-Mo alloy on
the test cell
cathode substrate resulted in a cell voltage saving of 330 millivolts even
after 13 cumulative
power outages including a very long power outage with no sacrifice in chlorate
production
efficiency as measured by % oxygen in the electrolysis cell exhaust.
Table 4
% Oxygen in
Electrolysis Cell
Days Cumulative Number Voltage Exhaust Notes
of Power Outages
1 3.205 1.72
2 3.232 1.685
3 3.267 1.73
6 3.288 1.77
7 3.281 1.76
8 2.845 1.575 Plated
9 2.894 1.59
10 1 2.914 1.62 1 st power off
13 2 2.907 1.645
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% Oxygen in
Electrolysis Cell
Days Cumulative Number Voltage Exhaust Notes
of Power Outages
14 3 2.917 1.67
15 4 2.92 1.66
17 5 2.943 1.7
20 6 2.944 1.775
21 7 2.952 1.72
22 8 2.94 1.73
23 9 2.941 1.73
24 10 2.951 1.715
27 11 2.967 1.81 2 1/2 hour power off
28 12 2.963 1.74
29 13 2.958 1.72
Example 2B
[0060] Cathode AW26-IS3 was prepared as follows. A used steel cathode that was
substantially used for prior chlorate production was immersed in 8 weight %
hydrochloric
acid that had been heated to 40 C and allowed to naturally cool for 1 hour.
Residual acid
on the cathode substrate was then removed by rinsing the cathode under flowing
tap water
for 20 seconds followed by deionised water rinsing from a wash bottle. The
cathode was
then dried with compressed air. A test cell was asseinbled with the acid
cleaned new
cathode and a new ruthenium dioxide containing DSA. The test cell was used for
chlorate
electrolysis for 1 week using the following conditions: Electrode gap 2.5 inm;
Temperature
80 C; Electrolyte bulk pH 6.0-6.1; NaCI concentration 100 grams/litre; Sodium
dichromate
concentration 4 grams/litre; Sodium sulphate concentration 14 grams/litre; 2
mg/litre
calcium and 0.4 mg/L magnesium ions added to the NaCI input; Anode and cathode
areas
were 100 cm2 (height 32.5 cm); Electrolyte flow rate through the test cell:
0.51itres/ainpere-
hour; Electrode current density 3000 amperes/metre2.
[0061] After 1 week of chlorate electrolysis the test cell was drained while
maintaining a
polarization potential on the cell until the cell was completely drained to
prevent cathode
corrosion. The cell was rinsed by pumping through approximately 1 litre of
deionised
water. The cell was acid cleaned by filling the cell with 8 weight%
hydrochloric acid
preheated to 40 C and allowing the acid to soak for 1 hour without disassembly
of the cell.
The spent acid was drained and the acid cleaned cell was rinsed by pumping
through
approximately 1 litre of deionised water without disassembly of the cell. The
plating
solution described previously was pumped through the cell via recycle, from a
1.43 litre
tank solution, at a bulk electrolyte velocity of 0.1 metres/second.
Electrodeposition was
17
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carried out at 70 C for 75 minutes (9000 coulombs) at a current density of 2.0
amperes/decimetre2. ICP analysis indicated that 0.99 g of coating was
deposited at 59.1%
current efficiency with 21 weight % molybdenum and 79% iron in the coating.
The
electroplated cell was rinsed of spent plating solution by pumping through 1
litre of
deionised water.
[0062] The electroplated test cell was used for chlorate electrolysis using
the following
conditions: Electrode gap 2.5 mm; Temperature 80 C; Electrolyte bulk pH 6.0-
6.1; NaC1
concentration 100 grams/litre; Sodium dichromate concentration 4 grams/litre;
Sodium
sulphate concentration 14 grams/litre; 2 mg/litre calcium and 0.4 mg/L
magnesium ions
added to the NaC1 input; Electrolyte flow rate through the test ce110.5
litres/ampere-hour;
Electrode current density 3000 amperes/metreZ. The first cell voltage was
recorded after
allowing one day for stabilization.
[0063] Starting the 4th day with the coated cathode, 6 minute long cell
depolarizations
(power-interruptions) were carried out daily to determine the robustness of
cathode deposits
under unforeseen power failures during chlorate electrolysis.
[0064] The following Table 5 indicates that in-situ plating of Fe-Mo alloy on
the test cell
cathode substrate resulted in a cell voltage saving of 362 millivolts even
after 18 cumulative
power outages with no sacrifice in chlorate production efficiency as measured
by % oxygen
in the electrolysis cell exhaust.
Table 5
% Oxygen in
Electrolysis Cell
Days Cumulative Number Voltage Exhaust Notes
of Power Outages
1 3.381 2.29
2 3.375 2.06
5 3.35 1.975
6 3.347 2
7 3.357 1.99
8 3.345 1.97 Plated
9 2.955 1.66
12 1 2.955 1.815 1 st power off
13 2 2.988 1.88
14 3 2.978 1.85
15 4 2.979 1.84
16 5 2.974 1.83
19 6 2.972 1.78
18
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% Oxygen in
Electrolysis Cell
Days Cumulative Number Voltage Exhaust Notes
of Power Outages
20 7 2.973 1.80
21 8 2.972 1.78
22 9 2.976 1.83
23 10 2.974 1.87
26 11 2.973 1.84
27 12 2.976 1.82
28 13 2.985 1.83
29 14 2.975 1.82
30 15 2.981 1.83
35 16 2.995 1.84
36 17 2.995 1.85
37 18 2.995 1.88
Example 3
[0065] In order to show that electroplating on a Fe-Mo previously coated
substrate is
possible, Cathode "AW26-IS5" was prepared by in-situ acid stripping of Cathode
"AW26-
IS2", followed by in-situ plating (i.e. acid stripping and plating conducted
without
disassembling the cell). Cathode "AW26-IS2" was previously plated in an
identical manner
to Cathode "AW26-IS 1" (Example 2A) and had been used continuously for 164
days in
sodium chlorate production while being exposed to 114 controlled power outages
lasting 6
minutes each (with no more than one power outage conducted per day).
[0066] The plating solution (1.43 litres) used to prepare Cathode "AW26-IS5"
contained
1.8 g/L NaZMoO4.2H20 (sodium molybdate dihydrate), 9 g/L FeC13.6H20 (ferric
chloride
hexahydrate), 45 g/L Na4P2O7.10H20 (sodium pyrophosphate decahydrate), 75 g/L
NaHCO3 (sodium bicarbonate). The in-situ coating was analyzed via the plating
solution
before and after plating by ICP analysis for the estimation of the deposited
Fe-Mo amount
and current efficiency for electrodeposition.
[0067] Cathode "AW26-IS5" was prepared as follows. A test cell was assembled
with
Cathode "AW26-IS2" previously used for 164 days in sodium chlorate production
and a
new ruthenium dioxide containing DSA. The test cell was used for chlorate
electrolysis for
3 days using the following conditions: Electrode gap 2.5 mm; Temperature 80 C;
Electrolyte bulk pH 6.0-6.1; NaCl concentration 100 grams/litre; Sodium
dichromate
concentration 4 grams/litre; Sodium sulphate concentration 14 grams/litre; 2
mg/litre
calcium and 0.4 mg/L magnesium ions added to the NaCl input; Anode and cathode
areas
19
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WO 2006/039804 PCT/CA2005/001567
were 100 cm2 (height 32.5 cm); Electrolyte flow rate through the test cell 0.5
litres/ampere-
hour; Electrode current density 3000 amperes/metre2.
[0068] After 3 days of chlorate electrolysis the test cell was drained without
disassembly
of the cell. The cell was rinsed by pumping through approximately 0.5 litre of
deionised
water without disassembly of the cell. The cell was acid cleaned to strip the
previous
coating by filling the cell with 8 weight percent hydrochloric acid preheated
to 40 C and
allowing the acid to soak for 1 hour without disassembly of the cell. The
spent acid was
drained and the cell was re-filled with as second batch of fresh 8 weight
percent
hydrochloric acid pre-heated to 40 C and soaked for 2 hours without
disassembly of the
cell.
[0069] The spent acid was drained and the cell was re-filled with a third
batch of fresh 8
weight percent hydrochloric acid pre-heated to 40 C and soaked for 2 hours
without
disassembly of the cell. The test cell was drained of spent acid and rinsed by
pumping
through 0.5 litre of deionised water without disasseinbly of the cell. The
plating solution
described above was pumped through the cell via recycle, from a 1.43 litre
tank solution, at
a bulk electrolyte velocity of 0.1 metres/second. Electrodeposition was
carried out at 70 C
for 75 minutes (9000 couloinbs) at a current density of 2.0 amperes/decimetre2
without
disassembly of the cell. ICP analysis indicated that 1.06 g of coating was
deposited at 62.7
% current efficiency with 17.4 weight % molybdenum and 82.6 % iron in the
coating.
[0070] The electroplated cell was rinsed of spent plating solution by pumping
through 0.5
litre of deionised water without disassembly of the cell. The electroplated
test cell was used
for chlorate electrolysis using the following conditions: Electrode gap 2.5
mm; Temperature
80 C; Electrolyte bulk pH 6.0-6.1; NaCI concentration 100 grams/litre; Sodium
dichromate
concentration 4 grams/litre; Sodium sulphate concentration 14 grams/litre; 2
mg/litre
calcium and 0.4 mg/L magnesium ions added to the NaCI input; Electrolyte flow
rate
through the test cell 0.5 litres/ampere-hour; Electrode current density 3000
amperes/metre2.
[0071] Cell voltage and oxygen evolution (an indirect method of measuring
chlorate
production efficiency) was recorded after allowing one day for stabilization.
Starting on the
3rd day with coated cathode, 6 minute long cell depolarizations (power-
interruptions) were
carried out daily to determine the robustness of cathode deposits under
unforeseen power
failures during chlorate electrolysis.
CA 02583827 2007-04-11
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[00721 The following Table 6 indicates that in-situ recoated Fe-Mo alloy,
plated after acid
stripping the previous Fe-Mo coated cathode used in chlorate production,
resulted in a cell
voltage saving of 319 millivolts after 6 cumulative power outages with no
sacrifice in
chlorate production efficiency as measured by % oxygen in the electrolysis
cell exhaust.
Table 6
Oxygen in
Cumulative Number Electrolysis Cell
Days of Power Outages Voltage Exhaust Notes
(%)
0 3.308 1.815 Operation with Uncoated
Cathode
1 2.899 1.475 1st day with AW26-IS2
4 1 2.951 1.73
J, 1 1 Continuous operation
164 114 2.992 1.735 Shut Down
1 3.024 1.71 Restarted with Used AW26-
IS2
2 3.038 1.74
3 3.032 Acid Stripped and Plated as
AW26-IS5
4 2.938 1.72
5 2.982 1.63
6 1 2.980 1.635 1st power outage
9 2 2.982 1.635
3 2.982 1.64
11 4 2.980 1.67
12 5 2.985 1.635
13 6 2.989 1.66
17 7 2.989 1.68
Example 4
[0073] Another example of plating on a previously Fe-Mo coated substrate was
conducted
using a plating formulation that produced a higher molybdenum containing
coating.
10 Cathode "AW24-IS2" was prepared by in-situ acid stripping Cathode "Pilot
#3", followed
by in-situ plating (i.e. acid stripping and plating conducted without opening
the cell).
Cathode "Pilot #3" was previously plated in a pilot scale operation consisting
of a 15 metre2
commercial cell with a plating solution consisting of 65.082 Kg of NaZPzO7
(sodium
pyrophosphate anhydrous), 12.96 Kg FeC13 (ferric chloride anhydrous), 4.32 Kg
NaMoO4.2HZ0 (sodium molybdate dihydrate), 179.7 Kg NaHCO3 (sodium bicarbonate)
in
2470L of deionised water. Plating of "Pilot #3" was conducted at 3300 amperes,
solution
flow rate of 216 gallon per minute, approximately 70 to 75 C for 90 minutes.
Hydrogen gas
21
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WO 2006/039804 PCT/CA2005/001567
produced during the plating process was diluted with air to ensure safe
operation of the
process. ICP analysis indicated that a coating of 1605 g of coating was
deposited at 47.9%
current efficiency with 17 weight % molybdenum and 83 weight % iron in the
coating.
[0074] Catllode "Pilot #3" was obtained by opening the 15 m'' cell and cutting
a 100 cm'
(height 32.5 cm) cathode sample for operation in a sodium chlorate producing
test cell.
"Pilot #3" was placed into a sodium chlorate producing test cell that was
previously
operated with an uncoated mild steel cathode. "Pilot #3" was operated
continuously in
sodium chlorate production for 115 days and exposed 41 power outages of 6
minute
duration (with no more than one power outage per day). At the conclusion of
chlorate
operation with "Pilot #3", the coating was in-situ acid stripped and then in-
situ plated as
"AW24-IS2" (acid stripping and plating without disassembly of the cell). "AW24-
IS2" was
plated using 1.43 litres of plating solution having 40 g/L NaZMoO~.2H20
(sodium
molybdate dihydrate), 9 g/L FeC13.6H20 (ferric chloride hexahydrate), 45 g/L
Na4P2O7.10H20 (sodium pyrophosphate decahydrate), 75 g/L NaHCO3 (sodium
bicarbonate). Estimation of the deposited Fe-Mo amount and current efficiency
were
conducted by preparing a duplicate cathode in a separate plating cell,
determining
molybdenum and iron content by x-ray fluorescence (XRF) and current efficiency
by
weight gain.
[0075] Cathode "AW24-IS2" was prepared as follows. A sodiuin chlorate
producing test
cell was assembled with an uncoated mild steel cathode and new ruthenium
dioxide
containing DSA and operated for 4 days to obtain baseline cell voltage data.
The uncoated
mild steel cathode was replaced by Cathode "Pilot #3" prepared by conditions
described
above and operated in sodium chlorate production for 115 days and exposed 41
power
outages of 6 minute duration (with no more than one power outage per day)
using the
following conditions: Electrode gap 2.7 mm; Temperature 80 C; Electrolyte bulk
pH 6.0-
6.1; NaCI concentration 100 grams/litre; Sodium dichromate concentration 4
grams/litre;
Sodium sulphate concentration 14 grams/litre; 2 mg/litre calcium and 0.4 mg/L
magnesium
ions added to the NaCl input; Anode and cathode areas were 100 cm2 (height
32.5 cm);
Electrolyte flow rate through the test cell 0.51itres/ampere-hour; Electrode
current density:
3000 amperes/metre'.
[0076] At the conclusion of sodium chlorate operating with "Pilot #3", the
test cell was
drained without disassembly of the cell. The cell was rinsed by pumping
through
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approximately 0.5 litre of deionised water without disassembly of the cell.
The cell was
acid cleaned to strip the previous coating by filling the cell with 8 weight
percent
hydrochloric acid preheated to 40 C and soaking the cell for 1 hour without
disassembly of
the cell. The spent acid was then drained and the cell was re-filled with as
second batch of
fresh 8 weight percent hydrochloric acid preheated to 40 C and soaked for 2
hours without
disassembly of the cell.
[0077] The spent acid was drained and the cell was re-filled with a third
batch of fresh 8
weight percent hydrochloric acid pre-heated to 40 C and soaked for 2 hours
without
disassembly of the cell. The test cell was drained of spent acid and rinsed by
pumping
through 0.5 litre of deionised water without disassembly of the cell. The
plating solution
described above for "AW24-IS2" was pumped through the cell via recycle, from a
1.43 litre
tank solution, at a bulk electrolyte velocity of 0.1 metres/second.
Electrodeposition was
carried out at 70 C for 50 minutes at a current density of 3.5
amperes/decimetre2 without
disassembly of the cell. XRF analysis and weight gain of a separate duplicate
cathode
indicated that approximately 0.92 g of coating was deposited at 49.4 % current
efficiency
with 49 weight % molybdenum and 51 weight % iron in the coating. The
electroplated cell
was rinsed of spent plating solution by pumping througli 0.5 litre of
deionised water without
disassembly of the cell.
[0078] The electroplated test cell was used for chlorate electrolysis using
the following
conditions: Electrode gap: 2.7 mm; Temperature 80 C; Electrolyte bulk pH 6.0-
6.1; NaCI
concentration: 100 grains/litre; Sodium dichromate concentration 4
grams/litre; Sodium
sulphate concentration: 14 grams/litre; 2 mg/litre calcium and 0.4 mg/L
magnesium ions
added to the NaCI input; Electrolyte flow rate through the test cell 0.5
litres/ampere-hour;
Electrode current density 3000 amperes/metre2.
[0079] Cell voltage and oxygen evolution (an indirect method of measuring
chlorate
production efficiency) was recorded after allowing one day for stabilization.
Starting on the
4th day with recoated cathode, 6 minute long cell depolarizations (power-
interruptions)
were carried out daily to determine the robustness of cathode deposits under
unforeseen
power failures during chlorate electrolysis.
[0080] The following Table 7 indicates that in-situ recoated of Fe-Mo alloy
after stripping
off of Fe-Mo coating used in the chlorate resulted in a cell voltage savings
of 304 millivolts
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even after 9 cumulative power outages with no sacrifice in chlorate production
efficiency as
measured by % oxygen in the electrolysis cell exhaust.
Table 7
Oxygen in
Cumulative Number Electrolysis Cell
Days of Power Outages Voltage Exhaust Notes
(%)
4 3.245 1.80 Mild steel cathode
2.811 1.69 Fe-Mo coated Cathode "Pilot #3"
l. 1 1
11 2.906
1 l J. Continuous operation
118 41 2.988 1.760
119 In-situ acid stripped and recoated as AW24-
IS2
122 2.936 1.695
123 1 2.938 1.705 1st Power Outage
124 2 2.942 1.67
125 3 2.944 1.645
126 4 2.937 1.61
129 5 2.934 1.645
130 6 2.936 1.645
131 7 2.943 1.62
132 8 2.943 1.615
133 9 2.941 1.635
137 10 2.959 1.670
5 Example 5
[0081] Another example of plating on a previously coated substrate was
conducted using
a plating fonnulation prepared from a citrate based plating bath instead of a
pyrophosphate
based plating bath to illustrate plating bath flexibility.
[0082] Cathode "Citrate #1" was prepared by in-situ acid stripping Cathode
"AW26-IS4",
followed by in-situ plating (i.e. acid stripping and plating conducted without
disassembly of
the cell). Cathode "AW26-IS4" was prepared in an identical manner to Cathode
"AW26-
IS1" described in Example 1 but was instead operated continuously for 119 days
in sodium
chlorate production at 4.0 kA/m2 and 85 C and exposed to 83 power outages of 6
minutes
(with no more that one power outage conducted per day).
[0083] The plating solution (1.43 litres) used to prepare Cathode "Citrate #1"
contained
2.4 g/L Na2MoO4=2H20 (sodium molybdate dihydrate), 195 g/L FeSO4=7H2O (ferrous
sulphate heptahydrate), 206 g/L C6H5Na3O7=2H20 (sodium citrate dihydrate).
NH4OH
(ammonium hydroxide) solution was added to the plating solution until pH 6.1
was reached.
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Estimation of the deposited Fe-Mo amount and current efficiency were conducted
by
preparing a duplicate cathode in a separate plating cell, determining
molybdenum and iron
content by x-ray fluorescence (XRF) and current efficiency by weight gain.
[0084] Cathode "Citrate #1" was prepared as follows. A test cell was assembled
with
cathode coated with "AW26-IS4" coating previously used for 119 days in sodium
chlorate
production and a new Eltech ruthenium dioxide containing DSA. The cell was
acid cleaned
to strip the previous coating by filling the cell with 8 weight percent
hydrochloric acid
preheated to 40 C and soaking the cell for 1 hour without disassembly of the
cell. The
spent acid was then drained and the cell was re-filled with as second batch of
fresh 8 weight
percent hydrochloric acid preheated to 40oC and soaked for 2 hours without
disassembly of
the cell. The spent acid was drained and the cell was re-filled with a third
batch of fresh 8
weight percent hydrochloric acid pre-heated to 40 C and soaked for 2 hours
without
disassembly of the cell.
[0085] The test cell was drained of spent acid and rinsed by pumping through
0.5 liter of
deionised water without disassembly of the cell. The plating solution
described above for
Catllode "Citrate #1" was pumped through the cell via recycle, from a 1.43
litre tank
solution, at a bulk electrolyte velocity of 0.1 metres/second.
Electrodeposition was carried
out at ambient temperature for 60 minutes at a current density of 3.5
amperes/decimetre2
without disassembly of the cell. XRF analysis and weight gain of a separate
duplicate
cathode indicated that approximately 1.02 g of coating was deposited at 32.1 %
current
efficiency with 19.9 weight % molybdenum and 80.1 weight % iron in the
coating. The
electroplated cell was rinsed of spent plating solution by pumping through 0.5
litre of
deionised water without disassembly of the cell.
[0086] The electroplated test cell was used for chlorate electrolysis using
the following
conditions: Electrode gap 2.7 mm; Temperature 80 C; Electrolyte bulk pH 6.0-
6.1; NaCI
concentration 100 grams/litre; Sodium dichromate concentration 4 grams/litre;
Sodium
sulphate concentration 14 grams/litre; 2 mg/litre calcium and 0.4 mg/L
magnesium ions
added to the NaCI input; Electrolyte flow rate through the test cell 0.5
litres/ampere-hour;
Electrode current density 3000 amperes/metre2.
[0087] Cell voltage and oxygen evolution (an indirect method of measuring
chlorate
production efficiency) was recorded after allowing one day for stabilization.
Starting on the
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3rd day with coated cathode, 6 minute long cell depolarizations (power-
interruptions) were
carried out daily to determine the robustness of cathode deposits under
unforeseen power
failures during chlorate electrolysis.
[0088] The following Table 8 indicates that in-situ recoated Fe-Mo alloy,
plated with a
citrate based plating solution after acid stripping the previous Fe-Mo coated
cathode used in
chlorate production, resulted in a cell voltage saving of 317 millivolts as
compared to a
typical cell voltage realized with an uncoated mild steel cathode, which is
3.254 V (average
of measured cell voltage with an uncoated mild steel cathode in Table 4).
Table 8
Oxygen in
Cumulative Number Electrolysis Cell
Days of Power Outages Voltage Exhaust Notes
1 In-situ acid stripped and plated
as "Citrate #1"
2 2.892 1.94
3 2.917 1.845
4 1 2.928 1.86 1s' power outage
8 2 2.965 1.87
Example 6
[0089] An example comparing methods of estimating the deposited Fe-Mo amount
and
current efficiency by ICP analysis of the plating solution before and after
plating versus
XRF and weight gain measurements of the cathode subsequent to plating was
conducted.
[0090] In Example 2A, ICP (inductively coupled plasma) analysis of the plating
solution
before and after plating indicated that Cathode "AW26-IS 1" contained 1.08 g
of coating,
deposited at 61.5% current efficiency with 18.4 weight % molybdenum and 81.6
weight %
iron in the coating.
[0091] Cathode "AW26-A2" was prepared in an identical manner to "AW26-IS1".
XRF
(X-ray fluorescence) analysis and weight gain measurement of "AW26-A2"
immediately
after plating indicated that "AW26-A2" contained 1.08 g of coating, deposited
at 63.9%
current efficiency with 17.8 weight % molybdenum and 82.2 weight % iron in the
coating.
26
