U.S. patent application number 14/762992 was filed with the patent office on 2015-12-10 for techniques for production of chlorated products and prefabricated cathode structures.
The applicant listed for this patent is HYDRO-QUEBEC. Invention is credited to Robert SCHULZ.
Application Number | 20150354073 14/762992 |
Document ID | / |
Family ID | 51390426 |
Filed Date | 2015-12-10 |
United States Patent
Application |
20150354073 |
Kind Code |
A1 |
SCHULZ; Robert |
December 10, 2015 |
TECHNIQUES FOR PRODUCTION OF CHLORATED PRODUCTS AND PREFABRICATED
CATHODE STRUCTURES
Abstract
Techniques for producing chlorinated products include an
electrochemical process that includes the steps of providing an
anode and a cathode in an electrolyte comprising impurities such as
calcium ions, applying a voltage between the anode and the cathode
under conditions to form an electrolysis product such as sodium
chlorate in the electrolyte, and providing sufficient phosphate
ions to form with at least a portion of the calcium ions a
protective external layer including a calcium phosphate compound
such as hydroxyapatite on the cathode, while preferably avoiding
other phosphate precipitations. A pre-determined amount of
phosphate ions may be added, for example, based on the surface area
of the cathode in order to form the protective layer. Related uses
and systems are also described. Prefabricated cathodes may include
a substrate, a catalytic intermediate layer and a calcium phosphate
protective layer.
Inventors: |
SCHULZ; Robert; (Ste-Julie,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HYDRO-QUEBEC |
Montreal |
|
CA |
|
|
Family ID: |
51390426 |
Appl. No.: |
14/762992 |
Filed: |
February 22, 2013 |
PCT Filed: |
February 22, 2013 |
PCT NO: |
PCT/CA2013/050136 |
371 Date: |
July 23, 2015 |
Current U.S.
Class: |
205/350 ;
204/196.02; 204/290.03; 205/505 |
Current CPC
Class: |
C25B 15/02 20130101;
C25B 11/0405 20130101; C25B 1/265 20130101; C25B 11/0431 20130101;
C25B 11/0447 20130101; C25D 9/10 20130101; C25B 9/06 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 15/02 20060101 C25B015/02; C25B 9/06 20060101
C25B009/06; C25B 1/26 20060101 C25B001/26 |
Claims
1. An electrochemical process comprising: providing an anode and a
cathode in an electrolyte comprising impurities comprising calcium
ions; applying a voltage between the anode and the cathode under
conditions to form an electrolysis product in the electrolyte; and
providing phosphate ions in the electrolyte in an amount sufficient
to form with at least a portion of the calcium ions a protective
external layer on the cathode, the protective external layer
comprising a calcium phosphate compound, and to substantially avoid
precipitation of calcium phosphate compounds in the
electrolyte.
2. The electrochemical process of claim 1, wherein the phosphate
ions are added in an amount based on a surface area of the cathode
that is in contact with the electrolyte.
3. The electrochemical process of claim 1, further comprising
applying the protective external layer on the catalytic
intermediate layer prior to immersing the cathode into the
electrolyte.
4. The electrochemical process of claim 3, wherein the step of
applying the protective external layer comprises sputter coating,
dip coating, sol-gel, electrochemical deposition, biomimetic
coating, hot isostatic coating, or plasma spraying.
5. The electrochemical process of claim 1, further comprising
forming the protective external layer on the catalytic intermediate
layer after immersing the cathode into the electrolyte.
6. (canceled)
7. The electrochemical process of claim 1, wherein formation of the
protective external layer comprises: reacting Ca(OH).sub.2 with
H.sub.3PO.sub.4 to produce Ca.sub.3(PO.sub.4).sub.2 and water; and
reacting Ca(OH).sub.2 with Ca.sub.3(PO.sub.4).sub.2 to produce
Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, wherein the
Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 forms at least part of the
protective external layer.
8. The electrochemical process of claim 1, wherein the phosphate
ions are provided in the electrolyte in a phosphate concentration
up to about 75 ppm.
9. (canceled)
10. The electrochemical process of claim 1, wherein the phosphate
concentration is sufficiently low to prevent formation of an iron
phosphate compound or deposit on the anode.
11. The electrochemical process of claim 1, wherein the phosphate
concentration is sufficiently low to prevent an increase in O.sub.2
levels in the electrolyte.
12. The electrochemical process of claim 1, wherein the phosphate
concentration is sufficiently low to prevent an increase in voltage
requirements.
13. The electrochemical process of claim 1, wherein the phosphate
ions are at least partially provided by addition of
H.sub.3PO.sub.4.
14. The electrochemical process of claim 1, wherein the phosphate
ions are at least partially provided by inherent presence in the
electrolyte.
15. The electrochemical process of claim 1, wherein the calcium
ions are at least partially provided by inherent presence in the
electrolyte.
16. The electrochemical process of claim 1, wherein the electrolyte
comprises a first portion of calcium ions for reacting with the
phosphate ions to form the calcium phosphate compound on the
cathode, and a second portion of calcium remaining unreacted in the
electrolyte.
17-21. (canceled)
22. An electrochemical system comprising: an electrolysis chamber
for containing an electrolyte, wherein the electrolyte comprises
calcium ions and phosphate ions; an anode located in the
electrolysis chamber; a cathode located in the electrolysis
chamber; and an ion adjuster configured to adjust ion levels in the
electrolyte such that the electrolyte comprises an amount of
phosphate ions sufficient to form with at least a portion of the
calcium ions a protective external layer comprising a calcium
phosphate compound on the cathode and to avoid precipitation of
calcium phosphate compounds in the electrolyte.
23-27. (canceled)
28. An electrochemical process comprising: providing an anode and a
cathode in an electrolyte impurities comprising calcium ions and
ferric ions; applying a voltage between the anode and the cathode
under conditions to form electrolysis product in the electrolyte;
and providing phosphate ions in the electrolyte in an amount
sufficient to form with at least a portion of the calcium ions a
protective external layer on the cathode, the protective external
layer comprising a calcium phosphate compound, and to substantially
avoid precipitation of iron phosphate compounds in the
electrolyte.
29-36. (canceled)
37. The electrochemical process of claim 28, wherein the phosphate
concentration is sufficiently low to prevent formation of an iron
phosphate compound or deposit on the anode, to prevent an increase
in O.sub.2 levels in the electrolyte, or to prevent an increase in
voltage requirements.
38-53. (canceled)
54. The electrochemical process of claim 1, wherein a
pre-determined amount of phosphate ions is between about 0.025 mg
per cm.sup.2 of the cathode and about 0.2 mg per cm.sup.2 of the
cathode.
55-73. (canceled)
74. The electrochemical process of claim 54, wherein the
pre-determined amount of phosphate ions are calculated based on a
target thickness of the protective external layer to obtain.
75-80. (canceled)
81. A prefabricated cathode comprising: a substrate; a catalytic
intermediate layer; and an protective external layer comprising a
calcium phosphate compound.
82-84. (canceled)
85. The prefabricated cathode of claim 81, wherein the catalytic
intermediate layer is contiguous with an outer surface of the
substrate, and the catalytic intermediate layer comprises a metal
matrix doped with a catalytic compound.
86-88. (canceled)
89. The prefabricated cathode of claim 81, wherein the calcium
phosphate compound comprises a hydroxy calcium phosphate
compound.
90-91. (canceled)
92. The prefabricated cathode of claim 81, wherein the protective
external layer has a thickness between about 0.25 micron and about
1.5 microns.
93. (canceled)
94. The prefabricated cathode of claim 81, wherein the protective
external layer covers an entire outer surface of the catalytic
intermediate layer to prevent direct contact of the catalytic
intermediate layer with calcium impurities in the electrolyte.
95. (canceled)
96. The prefabricated cathode of claim 81, wherein the protective
external layer has a reticulum structure, honeycomb structure, a
structure enabling the hydrogen evolution reaction to take place
there-under while preventing calcium impurities from poisoning the
intermediate catalytic layer, or a structure enabling blocking of
chlorate and hypochlorite ions from reaching a surface of the
intermediate catalytic layer to reduce or avoid the following
reactions: ClO.sup.-+H.sub.2O+2e=>Cl.sup.-+2OH.sup.-; or
ClO.sub.3.sup.-+3H.sub.2O+6e.sup.-=>Cl.sup.-+6OH.sup.-.
97-112. (canceled)
113. The electrochemical process of claim 1, wherein: wherein the
cathode comprises: a substrate composed of a corrosion resistant
material; and a catalytic intermediate layer; and the process
further comprises: performing electrolysis for electrolysis
periods, comprising applying a voltage between the anode and the
cathode under conditions to form an electrolysis product in the
electrolyte; and periodically shutting down the electrolysis for
shutdown periods, comprising terminating the voltage, wherein the
corrosion resistant material of the substrate prevents release of
ferric ions into the electrolyte during each of the shutdown
periods; wherein the providing the phosphate ions in the
electrolyte is sufficient such that, during each electrolysis
period, the phosphate ions form or re-form with at least a portion
of the calcium ions the protective external layer on the catalytic
intermediate layer of the cathode, the protective external layer
comprising the calcium phosphate compound.
114. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to the field of
producing chlorinated products and, more particularly, to
techniques for cathode protection and prefabricated cathode
structures in the production of sodium chlorate.
BACKGROUND
[0002] Sodium chlorate (NaClO.sub.3) can be commercially produced
by an electrochemical process according to the following overall
reaction:
NaCl+3H.sub.2O=>NaClO.sub.3+3H.sub.2 (1)
[0003] Hydrogen discharge (H.sub.2) takes place on the cathodic
side as:
2H.sub.2O+2e.sup.-=>2OH.sup.-+H.sub.2 (2)
[0004] and chlorate (ClO.sub.3.sup.-) is formed on the anodic side
through a series of reactions:
2Cl.sup.-=>Cl.sub.2+2e.sup.-
Cl.sub.2+H.sub.2O=>HClO+Cl.sup.-+H.sup.+
HClO=>ClO.sup.-+H.sup.+
2HClO+ClO.sup.-=>ClO.sub.3.sup.-+2Cl.sup.-+2H.sup.+ (3)
[0005] On the cathodic side, the hydrogen current efficiency (CE)
is defined as the ratio between the hydrogen flow rate (J.sub.H2)
and the total applied current to the electrochemical cell:
CE=J.sub.H2/(I/2F) (4)
[0006] where I is the applied current and F is the Faraday
number.
[0007] Mild steels with low carbon content are usually used for the
cathode and dimensionally stable anodes (DSA) are used as the
anode. In practice, the electrochemical reaction often takes place
in undivided cells using mono-polar or bi-polar configurations. In
the bi-polar configuration, the anodic part is in physical and
electrical contact with the cathodic part and, as a result, severe
galvanic corrosion problems can occur during shutdowns when
cathodic protection is no longer in place.
[0008] A typical electrolytic solution includes about 550 g/l of
NaClO.sub.3, 110 g/l of NaCl and 1-3 g/l of NaClO. The process
typically takes place at pH around 6.5, temperatures between
60.degree. C. and 85.degree. C. and current densities between 2 and
4 kA/m.sup.2.
[0009] Several side reactions can lead to a reduced CE, such as the
reduction of hypochlorite and chlorate on the cathodic side:
ClO.sup.-+H.sub.2O+2e.sup.-=>Cl.sup.-+2OH.sup.- (5)
ClO.sub.3.sup.-+3H.sub.2O+6e.sup.-=>Cl.sup.-+6OH.sup.- (6)
[0010] Another parasitic reaction that may be important, especially
during start-up, is the reduction of iron oxides on steel cathodes,
which leads to an oxygen burst release from the cell:
MO+2e.sup.-=>M+O.sup.2-(cathode)
O.sup.2-=>1/2O+2e.sup.-(anode) (7)
[0011] Oxygen can also be produced through the anodic oxidation of
hypochlorite according to:
OCl.sup.-+H.sub.2O=>O.sub.2+2H++Cl.sup.-+2e.sup.- (8)
[0012] and it can also form from the decomposition of hypochlorite
as:
2OCl.sup.-=>2Cl.sup.-+O.sub.2 (9)
2HClO=>O.sub.2+2HCl (10)
[0013] The decomposition of hypochlorite can be accelerated by the
presence of Ni, Co or Cu ion impurities in the electrolyte. At one
ppm level content, Ni, Co, and Cu can lead to an increase of
O.sub.2 in the cell gas by 2.0%, 1.0%, and 0.7% respectively at
70.degree. C. and 3 kA/m.sup.2 current density. Therefore, amongst
the various elements which can be part of an electrochemical cell
or an electrode, Ni is often particularly avoided.
[0014] To reduce the cathodic parasitic reactions mentioned
previously and increase hydrogen CE, industries add dichromate
(Na.sub.2Cr.sub.2O.sub.7) to the electrolyte generally at a
concentration of 3-8 g/l. During cathodic polarisation, chromium
(VI) is reduced to chromium (III) and a thin film of chromium (III)
hydroxide forms on the cathode surface. This porous film helps to
protect the cathode against corrosion, impede the transport of the
anions to the electrode surface and hinder the unwanted side
reactions while still allowing the hydrogen evolution reaction to
take place.
Cr.sub.2O.sub.7.sup.-2+8H.sup.++6e.sup.-=>2Cr(OH).sub.3+H.sub.2O
(11)
[0015] Chromate also acts as pH buffer and it reduces the
production of the oxygen by-product. Due to the toxicity of
hexavalent chromium which increases process treatment costs, one
would wish to find a substitute or an alternative to this practice.
Therefore, it would be desirable to find a non-toxic coating
material which, once deposited on the surface of the cathode, would
inhibit the reduction of hypochlorite and chlorate and improve
CE.
[0016] The electrolyte is highly corrosive and iron cathodes
corrode readily in such environment when there is no cathodic
protection. The corrosion reduces the lifetime of cathodes and also
contaminates the electrolyte with iron impurities. Depending on the
operating conditions and the frequency of shutdowns, cathode
lifetime can be as short as five years. For this reason, some
industries use thicker cathodes and operate at larger
inter-electrode spacing to avoid short circuits by corrosion
products. Moreover, iron oxides have been found to catalyze
chlorate reduction and reduce CE. Nevertheless, iron cathodes are
cheap, they have relatively low cathodic overpotential and their
surfaces are renewed with the removal of the corrosion layer each
time power interruption for an extended period of time occurs.
Therefore, after a shutdown event, cell voltages are usually lower
but CE efficient also until the chromium hydroxide film forms again
on the surface.
[0017] Another challenge in the field of sodium chlorate production
is corrosion on cathodes particularly when iron cathodes are
used.
[0018] To minimize the effect of corrosion on cathodes, one may
think of using stainless steel cathodes instead of iron, but
stainless steels usually have much higher cathodic overpotentials
because of chromium in the alloys. The overpotential of pure
chromium is at least 100 mV higher than that of pure iron.
Moreover, the austenitic stainless steel of the 300 series also
contains Ni, which may affect the production of the oxygen
by-product as mentioned previously. In the absence of dichromate,
the CE and the rate of chlorate reduction depends strongly on the
nature and properties of the electrode material. Research to find a
substitute to iron cathodes has been ongoing for the last twenty to
thirty years, but iron cathodes are still being used. Therefore, a
new cathodic material which would be electro-active, highly
resistant to corrosion and almost as noble as the DSA cathodes to
minimize galvanic corrosion in bipolar configuration would be
desirable.
[0019] Another challenge in the field of sodium chlorate production
relates to electrolyte impurities. During electrolysis, one
observes a gradual increase of the cell voltage when electrolyte
impurities are plated or deposited on electrodes and blind or
poison the electrocatalytic activity. Positively charged impurities
such as Ca.sup.2+ and Mg.sup.2+ are attracted toward the cathode
while negatively charge impurities such as sulphate ions move
toward the anode under the effect of the electric field. The paper
entitled "Electrolytic sodium chlorate technology: current status"
by B. V. Tilak et al. published in the Electrochemical Society
Proceedings, vol. 99-21 in 1999 mentions that the deposits usually
found on cathodes are calcium and magnesium hydroxides. B. V. Tilak
et al. indicate that Ca.sup.+2 impurities at the level of 1 ppm can
lead to voltage increases of about 50 to 75 mV per month and about
100 mV per month at 1.5 ppm level. At the 20 ppb level, the voltage
increase is of only 50 mV in two years of operation. Since most
production plants operate at the ppm calcium level, they usually
clean their cells several times a year using acid wash to remove
these blind deposits.
[0020] Recently, novel cathodes presenting overpotentials about 200
mV lower than that of iron have been reported. Canadian patent No.
2687129 and Canadian patent application No. 2778865 of Schulz et
al. describe new cathodic materials of the type Fe.sub.3Al(Ru) and
Fe.sub.3AlTa(Ru), which can be used for producing sodium chlorate
with improvements over iron. These materials have a catalytic
species (Ru) within an iron aluminide metallic matrix. In spite of
their efficiency for the hydrogen evolution reaction, these new
cathodic materials are, unfortunately, also affected by calcium
impurities. In some cases, calcium impurities above a fraction of
ppm can have a negative impact on electrode performance.
[0021] Calcium impurities can also have a negative impact in the
case of the chlor-alkali technology where highly porous membranes
or diaphragms are used to separate the anolyte and catholyte
compartments. Calcium impurities at the ppm level can block these
membranes quite readily and for this reason, methods have been
developed to reduce calcium impurities to the ppb level. U.S. Pat.
No. 4,176,022 of Darlington described in 1979 such a method. After
removing the particulates, one usually starts by treating the brine
with soda ash to precipitate a major portion of the calcium in the
form of calcium carbonate which is separated from the electrolyte
by filtration or other physical separation methods. This usually
brings the calcium to 2-3 ppm levels. Thereafter, one may pass the
electrolyte through an ion exchange column to obtain brine
containing less than about 0.5 ppm (500 ppb) of calcium. Finally,
U.S. Pat. No. 4,176,022 proposes the addition a phosphate to the
alkaline brine to form a calcium phosphate compound believed to be
calcium apatites substantially insoluble in brine and thereafter,
separating the compound from the electrolyte. The pH of the brine
is maintained above 10 and the temperature above 40.degree. C.
during the formation of the compound to further decrease its
solubility in the brine. They also propose to add seeds to the
electrolyte such as calcium phosphate (Ca.sub.3(PO.sub.4).sub.2) or
calcium hydroxide to ease the precipitation reaction. The final
calcium impurity level is around 20 parts per billion. Typically,
the concentration of phosphate added to the brine is from about 0.1
to about 1 wt %. As an example, they add 0.44 g and 2.24 g of
phosphoric acid (85 percent H.sub.3PO.sub.4) to one liter of brine
to reduce the calcium to 200 ppb and 20 ppb levels respectively.
More recently, in 2008, Canadian patent application No. 2 655 726
proposed a similar method to remove calcium from brine. This
application teaches the addition of 2 g of Na.sub.2HPO.sub.4 to 60
ml of electrolyte to reduce the calcium impurity level to below the
ppm level. Such references disclose the addition of phosphate in
relatively high amounts with the purpose of precipitating calcium
ions out of the bulk solution in order to obtain a targeted final
calcium impurity level.
[0022] In U.S. Pat. No. 4,004,988, Mollard et al. propose to use a
method of adding phosphoric acids or an alkali-metal salt of these
acids to the electrolyte for complexing calcium in undivided cells
used for the production of sodium chlorate. Mollard et al. disclose
that the complexing agent removes a large fraction of the calcium
during the course of electrolysis in the form of an easily
filterable precipitate. In an example, Mollard et al. add 0.5 to 2
g of sodium tripolyphosphate (Na.sub.5P.sub.3O.sub.10) to one kg of
brine containing 60 ppm to 100 ppm of calcium which is equivalent
to about 0.7 g to 2.6 g per liter. After passage through an
electrolytic cell, the solution is filtered and found to contain 5
ppm to 10 ppm of calcium. As in the previously described
references, the concentration of phosphate additive is in the range
of gram per liter of brine which means in the 1000 ppm range.
[0023] UK patent application No. 2039959 discloses a similar method
but instead of adding the phosphoric acid directly to the brine,
the phosphoric acid is mixed with the hydrochloric acid that is
regularly added to the brine for balancing the pH. In an example,
for brine containing about 35 ppm of calcium and 2 ppm of Fe, they
add 1 g to 2 g of phosphoric acid (85 percent H.sub.3PO.sub.4) per
kg of sodium chlorate produced in one case and 0.15 g of acid per
liter of electrolyte in a second case. These quantities are
relatively high and are in a similar range as the ones previously
disclosed in other references.
[0024] US patent application No. 2008/0230381A1 of N. Krstajic et
al. also discloses adding at least 1 g/l of phosphate ions to
sodium chloride brines to act as a buffering agent. As a cathode,
Krstajic et al. propose a low carbon steel substrate coated with an
electrodeposited layer of a Fe--Mo alloy whose thickness is between
10 .mu.m and 50 .mu.m. With the addition of phosphate, Krstajic et
al. mention that the observed voltage decrease with this type of
activated coating can reach values as high as 500 mV at the usual
current densities of 2.5-3 kA/m.sup.2 while the decrease is limited
to 100-150 mV with known brines. Krstajic et al. also suggest that
the dichromate concentration in the electrolyte can be reduced to
0.1 g/l or even be eliminated completely without greatly affecting
the CE.
[0025] A problem with known methods concerns the fact that elevated
amounts of phosphate additive are added with the goal of removing a
significant amount of calcium impurities in the electrolyte and to
bring the calcium impurities to levels where they are no longer
affecting the cathodes. The addition of large amounts of anions
such as PO.sub.4.sup.-3 can lead to additional problems when iron
impurities are also present in the electrolyte. In the paper
entitled "Effects of Electrolyte Impurities in Chlorate Cells" R.
A. Kus mentions that voltage increases are observed when iron and
phosphate impurities are present together at a significant
concentration level due to a synergetic effect between these
impurities which affects anode performance. For this reason,
industry currently tends not to use phosphoric acid additives since
almost all of them uses iron cathodes and therefore have iron
impurities in their electrolyte in addition to calcium
impurities.
[0026] In view of the various challenges in the field of sodium
chlorate production, there is a need for a technology that provides
at least some solutions.
SUMMARY OF INVENTION
[0027] The present invention responds to above mentioned need by
providing enhanced techniques for the production of chlorinated
products, such as sodium chlorate.
[0028] In some implementations, there is provided an
electrochemical process including providing an anode and a cathode
in an electrolyte comprising impurities comprising calcium ions;
applying a voltage between the anode and the cathode under
conditions to form an electrolysis product in the electrolyte; and
providing phosphate ions in the electrolyte in an amount sufficient
to form with at least a portion of the calcium ions a protective
external layer on the cathode, the protective external layer
comprising a calcium phosphate compound, and to substantially avoid
precipitation of calcium phosphate compounds in the electrolyte. It
should be noted that the step of providing phosphate ions may be
done before, after or during the step of applying the voltage.
[0029] In some implementations, the phosphate ions are added in an
amount based on a surface area of the cathode that is in contact
with the electrolyte.
[0030] In some implementations, the process further includes
applying the protective external layer on the catalytic
intermediate layer prior to immersing the cathode into the
electrolyte.
[0031] In some implementations, the step of applying the protective
external layer comprises sputter coating, dip coating, sol-gel,
electrochemical deposition, biomimetic coating, hot isostatic
coating, or plasma spraying.
[0032] In some implementations, the process further includes
forming the protective external layer on the catalytic intermediate
layer after immersing the cathode into the electrolyte.
[0033] In some implementations, the step of forming the protective
external layer comprises providing phosphate ions in the
electrolyte; ensuring sufficient calcium ions are present in the
electrolyte; and providing electrolytic conditions sufficient to
induce formation of the protective external layer.
[0034] In some implementations, formation of the protective
external layer comprises reacting Ca(OH).sub.2 with H.sub.3PO.sub.4
to produce Ca.sub.3(PO.sub.4).sub.2 and water; and reacting
Ca(OH).sub.2 with Ca.sub.3(PO.sub.4).sub.2 to produce
Ca.sub.10(PO.sub.4).sub.6(OH).sub.2, wherein the
Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 forms at least part of the
protective external layer.
[0035] In some implementations, the phosphate ions are provided in
the electrolyte in a phosphate concentration up to about 75 ppm, or
between about 5 ppm and about 50 ppm.
[0036] In some implementations, the phosphate concentration is
sufficiently low to prevent formation of an iron phosphate compound
or deposit on the anode, an increase in O.sub.2 levels in the
electrolyte, and/or an increase in voltage requirements.
[0037] In some implementations, the phosphate ions are at least
partially provided by addition of H.sub.3PO.sub.4. The phosphate
ions may also be at least partially provided by inherent presence
in the electrolyte. The calcium ions may also be at least partially
provided by inherent presence in the electrolyte.
[0038] In some implementations, the electrolyte comprises a first
portion of calcium ions for reacting with the phosphate ions to
form the calcium phosphate compound on the cathode, and a second
portion of calcium remaining unreacted in the electrolyte.
[0039] In some implementations, the process is for producing
chlorinated products. The chlorinated products may include sodium
chlorate and/or sodium hypochlorite.
[0040] In some implementations, there is provided a use of
phosphate ions in an electrochemical process for producing
chlorinated products in an electrolyte comprising calcium ions,
wherein the phosphate ions are provided in an amount sufficient to
form with at least a portion of the calcium ions a protective
external layer comprising a calcium phosphate compound on a cathode
and to avoid precipitation of calcium phosphate compounds in the
electrolyte.
[0041] In some implementations, there is provided an
electrochemical system comprising an electrolysis chamber for
containing an electrolyte, wherein the electrolyte comprises
calcium ions and phosphate ions; an anode located in the
electrolysis chamber; a cathode located in the electrolysis
chamber; and an ion adjuster configured to adjust ion levels in the
electrolyte such that the electrolyte comprises an amount of
phosphate ions sufficient to form with at least at least a portion
of the calcium ions a protective external layer comprising a
calcium phosphate compound on the cathode and to avoid
precipitation of calcium phosphate compounds in the
electrolyte.
[0042] In some implementations, the ion adjuster comprises an inlet
in fluid communication with the electrolysis chamber for providing
the amount of phosphate ions into the electrolysis chamber. The ion
adjuster may further include at least one measurement device for
measuring the concentration of phosphate ions, ferric ions and/or
calcium ions in the electrolyte. The ion adjuster may also include
a controller coupled to the measurement device and the inlet for
controlling an input amount of phosphate ions in response to
readings from the measurement device. The system may be configured
for producing chlorinated products, such as sodium chlorate and/or
sodium hypochlorite.
[0043] In some implementations, there is provided an
electrochemical process comprising providing an anode and a cathode
in an electrolyte impurities comprising calcium ions and ferric
ions; applying a voltage between the anode and the cathode under
conditions to form electrolysis product in the electrolyte; and
providing phosphate ions in the electrolyte in an amount sufficient
to form with at least a portion of the calcium ions a protective
external layer on the cathode, the protective external layer
comprising a calcium phosphate compound, and to substantially avoid
precipitation of iron phosphate compounds in the electrolyte.
[0044] The above process may also have one or more features as
described in other implementations described herein.
[0045] In some implementations, there is provided a use of
phosphate ions in an electrochemical process for producing
chlorinated products in an electrolyte comprising calcium ions and
ferric ions, wherein the phosphate ions are provided in an amount
sufficient to form with at least a portion of the calcium ions a
protective external layer comprising a calcium phosphate compound
on a cathode and to avoid precipitation of iron phosphate compounds
in the electrolyte.
[0046] In some implementations, there is provided an
electrochemical system comprising an electrolysis chamber for
containing an electrolyte, wherein the electrolyte comprises
calcium ions, ferric ions and phosphate ions; an anode located in
the electrolysis chamber; a cathode located in the electrolysis
chamber; and an ion adjuster configured to adjust ion levels in the
electrolyte such that the electrolyte comprises an amount of
phosphate ions sufficient to form with at least a portion of the
calcium ions a protective external layer comprising a calcium
phosphate compound on the cathode and to avoid precipitation of
iron phosphate compounds in the electrolyte.
[0047] In some implementations, there is provided an
electrochemical process comprising providing an anode and a cathode
in an electrolyte comprising impurities comprising calcium ions,
the cathode having a surface area in contact with the electrolyte;
applying a voltage between the anode and the cathode under
conditions to form an electrolysis product in the electrolyte; and
providing phosphate ions in the electrolyte in a pre-determined
amount based on the surface area of the cathode such that the
phosphate ions and at least a portion of the calcium ions are
consumed in the formation of a protective external layer covering
the surface area of the cathode in contact with the
electrolyte.
[0048] In some implementations, the pre-determined amount of
phosphate ions is between about 0.025 mg per cm.sup.2 of the
cathode and about 0.2 mg per cm.sup.2 of the cathode, or between
about 0.05 mg per cm.sup.2 of the cathode and about 0.15 mg per
cm.sup.2 of the cathode.
[0049] In some implementations, the electrolyte further comprises
ferric ions and the phosphate ions are further added in an amount
to avoid precipitation of iron phosphate compounds in the
electrolyte. The phosphate ions may be further added in an amount
to avoid precipitation of additional calcium phosphate compounds in
the electrolyte.
[0050] In some implementations, the pre-determined amount of
phosphate ions is calculated based on a target thickness of the
protective external layer to obtain.
[0051] The above process may also have one or more features as
described in other implementations described herein.
[0052] In some implementations, there is provided a use of
phosphate ions in an electrochemical process for producing
chlorinated products in an electrolyte comprising calcium ions,
wherein the phosphate ions are provided in a pre-determined amount
based on the surface area of the cathode such that the phosphate
ions and at least a portion of the calcium ions are consumed in the
formation of a protective external layer covering the surface area
of the cathode in contact with the electrolyte.
[0053] In some implementations, there is provided an
electrochemical system comprising an electrolysis chamber for
containing an electrolyte, wherein the electrolyte comprises
calcium ions and phosphate ions; an anode located in the
electrolysis chamber; a cathode located in the electrolysis
chamber; and an ion adjuster configured to adjust ion levels in the
electrolyte such that the electrolyte comprises a pre-determined
amount of phosphate based on the surface area of the cathode such
that the phosphate ions and at least a portion of the calcium ions
are consumed in the formation of a protective external layer
covering the surface area of the cathode in contact with the
electrolyte.
[0054] In some implementations, there is provided a prefabricated
cathode comprising a substrate; a catalytic intermediate layer; and
an protective external layer comprising a calcium phosphate
compound.
[0055] In some implementations, the substrate comprises stainless
steel. The stainless steel may be a 400 series stainless steel. The
substrate may include a material with sufficient corrosion
resistance to prevent ferric ions from entering an electrolyte
during shutdown periods of an electrolytic cell.
[0056] In some implementations, the catalytic intermediate layer is
contiguous with an outer surface of the substrate.
[0057] In some implementations, the catalytic intermediate layer
comprises a metal matrix doped with a catalytic compound. The metal
matrix may be an iron aluminide. The catalytic compound comprises
Ru.
[0058] In some implementations, the calcium phosphate compound
comprises a hydroxy calcium phosphate compound, such as
hydroxyapatite. The protective external layer may consist
essentially of hydroxyapatite.
[0059] In some implementations, the protective external layer has a
thickness between about 0.25 micron and about 1.5 microns. The
protective external layer may have a thickness between about 0.5
micron and about 1 micron.
[0060] In some implementations, the protective external layer is
provided in order to cover an entire outer surface of the catalytic
intermediate layer to prevent direct contact of the catalytic
intermediate layer with calcium impurities in the electrolyte. The
protective external layer may be sputter coated, dip coated,
sol-gel applied, electrochemically deposited, biomimetically
coated, hot isostatically coated, or plasma sprayed onto the
catalytic intermediate layer.
[0061] In some implementations, the protective external layer has a
reticulum structure. The protective external layer may have a
honeycomb structure.
[0062] In some implementations, the protective external layer has a
structure enabling the hydrogen evolution reaction to take place
there-under while preventing calcium impurities from poisoning the
intermediate catalytic layer.
[0063] In some implementations, the protective external layer has a
structure enabling blocking of chlorate and hypochlorite ions from
reaching a surface of the intermediate catalytic layer to reduce or
avoid the following reactions:
ClO.sup.-+H.sub.2O+2e.sup.-=>Cl.sup.-+2OH.sup.-; and/or
ClO.sub.3+3H.sub.2O+6e.sup.-=>Cl.sup.-+6 OH.sup.-.
[0064] In some implementations, there is provided a use of the
prefabricated cathode as define herein, in an electrolytic cell for
producing a chlorinated product, such as sodium chlorate and/or
sodium hypochlorite.
[0065] In some implementations, there is provided an
electrochemical process comprising providing an anode and the
prefabricated cathode as defined herein in an electrolyte
comprising impurities comprising calcium ions; and applying a
voltage between the anode and the prefabricated cathode under
conditions to form an electrolysis product in the electrolyte. This
process may also include one or more of the features as described
herein.
[0066] In some implementations, there is provided a method for
making a prefabricated cathode for use in the production of a
chlorinated product, including providing a substrate; providing a
catalytic intermediate layer on top of the substrate; and applying
an protective external layer onto the catalytic intermediate layer,
wherein protective external layer comprises a calcium phosphate
compound.
[0067] The method may be performed to produce the prefabricated
cathode having one or more features as described herein.
[0068] In some implementations, the step of applying the protective
external layer comprises sputter coating, dip coating, sol-gel
methods, electrochemical deposition, biomimetic coating methods,
hot isostatic coating and/or plasma spraying.
[0069] In some implementations, there is provided an
electrochemical process comprising providing an anode and a cathode
in an electrolyte comprising impurities comprising calcium ions,
wherein the cathode comprises a substrate composed of a corrosion
resistant material, and a catalytic intermediate layer; performing
electrolysis for electrolysis periods, comprising applying a
voltage between the anode and the cathode under conditions to form
an electrolysis product in the electrolyte; periodically shutting
down the electrolysis for shutdown periods, comprising terminating
the voltage, wherein the corrosion resistant material of the
substrate prevents release of ferric ions into the electrolyte
during each of the shutdown periods; and providing phosphate ions
in the electrolyte in an amount sufficient such that, during each
electrolysis period, the phosphate ions form or re-form with at
least a portion of the calcium ions a protective external layer on
the catalytic intermediate layer of the cathode, the protective
external layer comprising a calcium phosphate compound.
[0070] The above process may also have one or more features as
described in other implementations described herein.
[0071] In some implementations, there is provided an
electrochemical process comprising providing an anode and a cathode
in an electrolyte comprising impurities comprising alkaline earth
metal ions; applying a voltage between the anode and the cathode
under conditions to form an electrolysis product in the
electrolyte; and providing phosphate ions in the electrolyte in an
amount sufficient to form with at least a portion of the alkaline
earth metal ions a protective external layer on the cathode, the
protective external layer comprising an alkaline earth metal
phosphate compound, and to substantially avoid precipitation of
alkaline earth metal phosphate compounds in the electrolyte. The
alkaline earth metal may be calcium. It is also noted that various
implementations of the processes described herein may be performed
in relation to an alkaline earth metal in general rather than
calcium in particular.
[0072] In some implementations, there is provided an
electrochemical process comprising: providing an anode and a
cathode in an electrolyte comprising impurities; applying a voltage
between the anode and the cathode under conditions to form an
electrolysis product in the electrolyte; and ensuring that
sufficient phosphate ions and calcium ions are present in the
electrolyte such that the phosphate ions form with at least a
portion of the calcium ions a protective external layer on the
cathode, the protective external layer comprising a calcium
phosphate compound, and to substantially avoid precipitation of
calcium phosphate compounds in the electrolyte, to substantially
avoid precipitation of iron phosphate compounds in the electrolyte
when ferric ions are present in the electrolyte, and/or to consume
substantially all of the phosphate ions in the formation of the
protective external layer.
[0073] It is also noted that one or more further optional features,
aspects and implementations of processes, systems, uses, cathodes
and methods of making cathodes, which will be described in further
details in the below description, may be combined with various
implementations described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0074] FIG. 1 is a schematic diagram of an electrolytic system for
the production of sodium chlorate.
[0075] FIG. 2 is a graph of O.sub.2 content, voltage and efficiency
versus time. The efficiency curve is the top curve; the voltage
curve is the middle curve; and the O.sub.2 content is the bottom
curve.
[0076] FIG. 3 is a graph of counts (a.u.) versus two theta
(degrees), from an x-ray scan of the surface of a cathode.
[0077] FIG. 4 is a graph of voltage versus time for a chlorate cell
with calcium addition.
[0078] FIG. 5 is a graph of counts (a.u.) versus two theta
(degrees), from an x-ray diffraction spectrum of the surface of a
cathode, after electrolysis and addition of calcium and phosphoric
acid to the electrolyte, the insert being an EDX spectrum showing
the elements present on the surface of the cathode.
[0079] FIG. 6 is another graph of O.sub.2 content, voltage and
efficiency versus time. The efficiency curve is the top curve; the
voltage curve is the middle curve; and the O.sub.2 content is the
bottom curve.
[0080] FIGS. 7a and 7b are scanning electron microscope pictures of
the cross-sections of cathodic electrode structures.
[0081] FIG. 8 is a pair of scanning electron microscope pictures of
the cross-sections of cathodic electrode structures.
[0082] FIG. 9 is a pair of microscopic photographic chemical maps
of an protective external layer.
[0083] FIG. 10 is a graph of voltage versus time with different
quantities of calcium added to the electrolyte.
[0084] FIG. 11 is a graph of voltage versus time with addition of
calcium ions followed by addition of phosphoric acid.
[0085] FIG. 12 is a graph of voltage versus time with addition of
phosphoric acid followed by addition of calcium ions.
[0086] FIG. 13 is a graph of voltage versus time with additions of
phosphoric acid.
[0087] FIG. 14 is a block diagram.
DETAILED DESCRIPTION
[0088] Various techniques that are described herein for
electrolysis leverage the use of a relatively small amount of
phosphate in an electrolytic system to form a protective layer on
the cathode. Further techniques provide a prefabricated cathode
that has a protective layer including a calcium phosphate compound
for use in the electrolytic production of chlorinated products,
such as sodium chlorate (NaClO.sub.3 or ClO.sub.3) that is a
compound used in paper bleaching applications or sodium
hypochlorite (NaClO or ClO.sup.-) that is a water treatment agent.
It should be understood that while various implementations and
aspects will be discussed herein in relation to producing sodium
chlorate, such techniques may be adapted to the production of other
chlorinated products such as sodium hypochlorite and the like.
[0089] In some implementations, the production of sodium chlorate
by electrolysis is enhanced by providing the cathode with an
protective external layer including a calcium phosphate compound,
which is formed by the addition of phosphate ions in an amount to
induce formation of the calcium phosphate compound at the surface
of the cathode while preventing precipitation of calcium phosphate
compounds and/or iron phosphate compounds that can have deleterious
effects on the electrolysis. The phosphate may be provided in an
amount based on the surface area of the cathode in order to form
the protective external layer.
[0090] While other methods have emphasized the addition of higher
levels of phosphate ions with the aim of precipitating calcium ions
out of the electrolyte, various techniques described herein provide
for the advantageous addition of a reduced amount of phosphate ions
sufficient to form the protective layer on the surface of the
cathode. In some scenarios, the phosphate ions can be provided in
notably low levels to form the protective layer, which not only
enables improved functioning of the electrolysis despite remaining
calcium ions in the electrolyte, but also reduces or prevents the
formation of iron phosphate compounds that can have a negative
impact on the electrolysis. Thus, while the electrolyte may have
various impurities, such as calcium ions and ferric ions, the
protective external layer can provide protection against
electrolyte impurities and maintain efficient electrolytic
operations.
[0091] In some instances, beneficial aspects of phosphate additives
can be achieved at phosphate concentrations that are one or more
orders of magnitude lower than the concentrations previously used.
Instead of adding phosphates in accordance with the calcium ion
concentration of the electrolyte with the goal of precipitating
calcium impurities, the phosphates can be provided with the purpose
of forming the calcium phosphate protective layer in order to
counteract negative effects of calcium on cathode surfaces in
undivided sodium chlorate cells.
[0092] Providing such a protective layer may be particularly
advantageous when used in combination with cathodes that include a
substrate, such as stainless steel, and a catalytically active
coating, such as a metallic matrix doped with a catalytic species
like Ru. Calcium impurities can interfere with such catalytically
active coatings in the process of the hydrogen evolution reaction
(Equation 2) and form calcium hydroxides, resulting in cell voltage
increases gradually with the built-up of this blinding deposit. The
conditions to induce deposition of insoluble calcium phosphate
compounds, such as hydroxyapatite, can be provided close to the
surface of cathodes. Even if the average pH in the bulk electrolyte
is maintained around 6.5, the pH near the surface of the cathode is
much higher, for instance well over 10, because of the formation of
hydroxyl groups according to Equation 2. The local elevated pH
decreases the local solubility of calcium phosphate compounds and
results in the formation of a deposited layer. In addition, calcium
hydroxide forms naturally on the surface of cathodes and these
clusters of calcium hydroxide can act as seeds for the formation of
calcium phosphate (Ca.sub.3(PO.sub.4).sub.2). When small amounts of
phosphoric acid are present near the surface of the cathode, the
following reaction takes place:
3Ca(OH).sub.2+2H.sub.3PO.sub.4=>Ca.sub.3(PO.sub.4).sub.2+6H.sub.2O
(12)
[0093] These calcium phosphate molecules further react with calcium
hydroxide to form hydroxyapatite on the surface of the cathode
according to:
Ca(OH).sub.2+3Ca.sub.3(PO.sub.4).sub.2=>Ca.sub.10(PO.sub.4).sub.6(OH)-
.sub.2 (13)
[0094] The high temperatures typical of industrial chlorate
operations (about 60.degree. C. to about 85.degree. C.) are also
beneficial for the deposition because the higher the temperature,
the lower is the solubility of the calcium phosphate compounds.
Indeed, the conditions for the heterogeneous formation of
hydroxyapatite on the surface of the cathodes are provided even
though they may not be adequate for the precipitation of phosphate
compounds in the bulk of the electrolyte. Various techniques
described herein leverage such conditions for the pre-determined
addition of phosphate to be substantially fully consumed in the
formation of the calcium phosphate protective layer on the cathode
surface. It is also noted that one or more other calcium phosphate
compounds, such as Ca.sub.3(PO.sub.4).sub.2, may also be present
and have beneficial protective properties.
[0095] Various aspects of the invention will be described below,
including systems and processes for the production of sodium
chlorate, prefabricated cathodes as well as uses and methods of
making prefabricated cathodes.
[0096] Electrolytic Systems for Sodium Chlorate Production
[0097] FIG. 1 schematically illustrates a system 10 for the
production of sodium chlorate. The system 10 includes an
electrolytic cell 12 having an electrolysis chamber 14 that is
filled with an electrolytic solution 16, also referred to herein as
the electrolyte. The system 10 also includes an anode 18 and a
cathode 20, possible structures and compositions of which will be
further discussed below. The cathode 20 may have a structure
including a substrate 22, an intermediate catalytic layer 24 and a
protective external layer 26.
[0098] Still referring to FIG. 1, the electrolytic cell 12 produces
a sodium chlorate rich solution 28 that is withdrawn from the
electrolysis chamber 14 and may be supplied to downstream units 30
such as settlers, filters, evaporators, crystallizers and dryers
for further processing in order to produce sodium chlorate 32 in
solid and/or concentrated form.
[0099] The cathode 20, which may also be referred to as the
cathodic electrode structure, may include three or more layers. In
some scenarios, the cathodic electrode structure has three layers
including the substrate 22, the intermediate catalytic layer 24
that is provided directly onto the substrate 22, and the protective
external layer 26 that is provided directly onto the intermediate
catalytic layer 24. It may also be envisioned to provide one or
more additional layers in between the other layers.
[0100] In some implementations, the substrate 22 may be composed of
a corrosion resistant material, such as stainless steel that may be
from the 400 series. More regarding the substrate will be described
further below.
[0101] In some implementations, the intermediate catalytic layer 24
may be composed of a high porosity highly active catalytic
material, such as Fe.sub.3Al(Ru) and Fe.sub.3AlTa(Ru). More
regarding the intermediate catalytic layer will be described
further below.
[0102] In some implementations, the protective external layer 26
includes a calcium phosphate compound. More regarding the formation
and properties of the protective external layer will be described
further below.
[0103] Phosphate Dosing and Formation of Protective Layer
[0104] The protective external layer may be formed in a number of
ways. In one scenario, the protective external layer is formed in
situ within the electrolytic cell. In other scenarios, the
protective external layer may be formed ex situ in order to make a
prefabricated cathode that can be used in the electrolysis system.
More regarding the prefabricated cathode and ex situ methods will
be described further below.
[0105] As noted above, the protective external layer may be formed
in situ within the electrolytic cell by the addition of phosphate
ions and, in some cases, calcium ions.
[0106] The phosphate ions may be added in a relatively low amount
that is sufficient to form the protective external layer and to
avoid one or more other reactions such as the precipitation of
calcium or ferric ions from the bulk electrolyte solution and/or
deposition of iron phosphate compounds at the anode.
[0107] In terms of the quantity of phosphate ions added to the
electrolyte, this may depend on a number of factors including the
surface area of the cathode to be protected, the thickness of the
protective layer to be formed, the composition and structure of the
electrodes, the composition and properties of the electrolyte, and
so on.
[0108] In some scenarios, the phosphate is added in an amount
sufficient to form the protective layer with a thickness between
about 0.25 micron and about 1.5 microns, between about 0.5 micron
and about 1 micron, or between about 0.6 micron and about 0.9
micron. FIG. 8 illustrates a protective layer formed on a catalytic
intermediate layer and has a thickness generally ranging between
about 0.5 micron and about 1 micron.
[0109] In addition, a general guideline may be followed whereby the
amount of phosphate added to the electrolyte is at most sufficient
to form a protective layer having a thickness of about 1 micron.
For instance, in one scenario that is discussed in the Examples
section below, about 0.1 mg of phosphate (PO.sub.4) per cm.sup.2 of
cathode may be added. It should therefore be understood that the
amount of phosphate to add may be determined based on the surface
area of the cathode rather than the volume of the electrolyte or
the quantity of calcium ions present in the electrolyte. For
instance, the amount of phosphate to add may be between about 0.025
mg per cm.sup.2 of cathode and about 0.2 mg per cm.sup.2 of
cathode, or 0.05 mg per cm.sup.2 of cathode and about 0.15 mg per
cm.sup.2 of cathode, for example.
[0110] Furthermore, the amount of phosphate to add may be
pre-determined by calculations and/or empirical tests, as will be
appreciated from the Examples. In one example, no more than 75 ppm,
50 ppm, 30 ppm, 20 ppm or 15 ppm of phosphate ions is added to the
electrolyte in order to form the protective layer on the
cathode.
[0111] In some scenarios, the electrolyte may not initially contain
sufficient calcium ions to form the protective layer with the
phosphate ions. Addition of calcium ions may be performed before,
during or after the addition of phosphate ions, in a sufficient
amount to allow formation of the protective layer. FIGS. 11 and 12
illustrate that the calcium ions and phosphate ions may be added in
various orders in order to stop voltage increases.
[0112] It should also be noted that the phosphate may be added as a
one-time dose or may be added periodically in incremental doses.
FIG. 13 illustrates a scenario where multiple phosphate doses were
added incrementally to the system over a period of time.
[0113] Referring back to FIG. 8, the protective layer can be seen
as being porous with a reticulum like or honeycomb structure having
a network of structural elements and dispersed void spaces. In some
scenarios, the porosity and permeability of the protective layer
are sufficiently low to protect the underlying catalytic layer from
calcium poisoning and sufficiently high to avoid impeding the
hydrogen evolution reaction to occur.
[0114] Referring briefly back to FIG. 1, the phosphate may be added
to the electrolyte 16 via one or more phosphate addition lines 34,
which may be a separate line or may be configured to add the
phosphate to another inlet of the electrolytic cell 12, such as the
dilute HCL inlet as illustrated. The phosphate may be added
manually or automatically. It may be added in response to a
measurement or reading that is taken regarding the electrolytic
system. In this sense, the phosphate lines 34 may be part of an ion
adjuster which adjusts the amount of phosphate ions, and possibly
calcium ions, in the electrolyte in order to ensure providing the
small amount of phosphate sufficient to form the protective layer.
The ion adjuster may have various other components such as a
measurement device and a controller.
[0115] In some implementations, the phosphate is added in the form
of phosphoric acid H.sub.3PO.sub.4. However, it should be note that
the phosphate may be added in other forms, such as acids or salts,
as monophosphate or polyphosphate compounds, or in other forms.
[0116] Furthermore, addition of the phosphate may be performed in
order to promote substantially total consumption of the phosphate
in the formation of the protective layer. For example, controlling
the conditions of the electrolytic system (e.g. temperature, pH,
composition of electrolyte, etc.) so as to favor the formation of
the calcium phosphate compounds on the surface of the cathode may
limit or prevent any other reactions involving phosphate. This
controlling step may be done at or near the start of the
electrolytic operations so that the protective layer can form as
quickly as possible. As will be described further below, the
cathode may also be coated ex situ with the protective layer using
a number of methods, such that the cathode has the protective layer
upon commencing the electrolytic operations.
[0117] Prefabricated Cathodes and Ex Situ Manufacturing
[0118] The prefabricated cathode may include the substrate, the
catalytic layer and the protective layer. The prefabricated
cathodes described herein may be used to replace iron cathodes
currently used in the industry, in order to provide corrosion
resistance, high activity toward the hydrogen evolution reaction, a
good hydrogen current efficiency and low hydrogen overpotential. In
some cases, the prefabricated cathodes do not catalyze the
reduction of hypochlorite and chlorate and do not produce
significant oxygen by-product.
[0119] The prefabricated cathodes can provide several advantages,
such as providing a protective coating of the catalytic layer at
the beginning of the electrolytic operations and also providing
tolerance to electrolyte impurities and contribute to counteracting
negative effects of calcium impurities on cathode surfaces.
[0120] The method of manufacturing the prefabricated cathode may
include the step of applying the protective external layer by
sputter coating, dip coating, sol-gel methods, electrochemical
deposition, biomimetic coating, hot isostatic coating, or plasma
spraying. The method of applying the protective layer may be chosen
and carried out in order to obtain certain properties of the
protective layer, such as a layer thickness, permeability and
porosity within certain ranges.
[0121] Referring to FIG. 14, a cathode structure 36, which may
include the substrate and catalytic layer, may be supplied to an ex
situ pre-treatment unit 38 for application of the protective layer.
The pre-treatment unit 38 may be configured to perform one or more
of the above mentioned methods for applying the protective layer,
thereby producing the prefabricated cathode 20. The prefabricated
cathode 20 is then supplied to the electrolytic cell 12 for the
production of sodium chlorate.
[0122] In some scenarios, the prefabricated cathodes may remain in
the electrolytic cell until the end of their working life. If the
pre-applied protective coating is damaged or partially removed, an
in situ method of regenerating the protective layer may be used,
such as adding a pre-determined amount of phosphate as described
herein. In addition, in some situations, after a certain period of
operation the electrolyte may be removed from the electrolytic cell
12 and the cell may be cleaned by introducing a mild acid liquor
that is usually provided in the cell for about one hour. If such
cleaning or other maintenance operations are conducted which remove
the protective external layer from the cathodes, one may reapply
the protective external layer in situ to enable continued
protection during the next phase of electrolysis.
[0123] Alternatively, after a certain period of operation, the used
cathodes 40 may be removed from the electrolytic cell 12 for
cleaning and/or maintenance. The used cathodes may be removed, for
example, during shutdown operations of the electrolytic cell 12.
The used cathode 40 may be provided to a cleaning unit 42 in order
to inspect and clean the cathode to produce a cleaned cathode 44.
The cleaning process may remove the protective layer from the
cathode structure and thus the cleaned cathode 44 may be supplied
to the pre-treatment unit 38 in order to reapply the protective
layer so that the cathode can be reused in the electrolytic
cell.
[0124] Cathode Substrates and Catalytic Layers
[0125] In some implementations, as mentioned previously, the
cathode has a structure including a substrate and a catalytic
layer.
[0126] The substrate may include a stable corrosion resistant
substrate which does not contain nickel. The substrate may be
stainless steel of the 400 series or another material having
anti-corrosion properties. Stainless steel of the 400 series may be
used, in spite of having higher cathodic overpotentials. As shown
in the below Table 1 listing commercial stainless steel available
from the AK Steel company, these alloys have very low carbon
contents similar to those of mild steels and no nickel. They are
highly corrosion resistant in media containing chloride ions. In
electrolytic operations, corrosion problems can occur during
shutdowns when there is no cathodic protection and mild steel
cathodes can suffer from degradations. The corrosion product on the
steel can remove the surface layers at each power interruption.
While calcium and phosphate still present in the electrolyte can
lead to the regeneration of the protective layer, another
advantageous feature is to provide an improved substrate compared
to mild steel.
TABLE-US-00001 TABLE 1 Commercial stainless steel available from
the AK Steel company Typical Chemical Composition % Other
Significant Stainless Types Cr Ni C Elements Stainless Steels of
the 400 series 409 11 -- .01 Ti - .20 Aluminized 409 11 -- .01 Ti -
.20 Aluminum coating 400 12 -- 0.15 Al - .15 400 Cb 11.5 -- .01 Cb
- .15, Al - .15 410S 12 -- 0.15 11 Cr-Cb 11 -- .01 Si - 1.30, Cb -
.35 430 16.5 -- .05 434 16.5 -- 0.65 Mo - 1.0 436 16.8 -- .06 Mo -
1.0, Cb - .40 439 17 -- .012 Ti - .30 Aluminized 439 17 -- .012 Ti
- .30 435 Mod. 19.5 -- .02 Cb - .70, Cu - .50 18 SR 17 -- .02 Al -
1.70, Ti - .20 18 Cr-Cb 17.5 -- .02 Ti - .25, Cb - .55 444 17.5 --
.015 Ti - .25, Cb - .15 Mo - 2.0
[0127] The catalytic layer may include a metallic matrix doped with
a catalytically active compound. For instance, the catalytic layer
may include Fe.sub.3Al(Ru) and Fe.sub.3AlTa(Ru), which can be used
for producing sodium chlorate with improvements over iron. These
materials have a catalytic species (Ru) within an iron aluminide
metallic matrix. In spite of their efficiency for the hydrogen
evolution reaction, these new cathodic materials are,
unfortunately, also affected by calcium impurities. As shown in
FIG. 4, an increase in voltage of 60 mV is observed in cells
containing these alloys after adding 2 ppm of calcium impurities to
the electrolyte. The calcium deposit on the cathode tends to poison
the electrode and reduces the activity of the catalytic species.
The increase in voltage occurs on a time scale of an hour instead
of months which is more rapid than on conventional cathodes. The
reason for this comes from the fact that these cathodes are highly
porous. They typically have an effective surface about a hundred
times higher than that of steel cathodes. Therefore, the blind
deposit on the surface can block a large number of catalytic sites
located within the pores of the catalytic structure quite rapidly.
However, the use of a protective layer which includes a calcium
phosphate compound enables the catalytic layer to provide
beneficial operation without being poisoned by calcium
impurities.
[0128] In order to have a sufficient lifetime, cathodes should be
able to sustain power interruptions. The most severe corrosion
conditions often occur during a power interruption when there is no
cathodic current protection.
[0129] In some scenarios, the prefabricated cathodes have a
structure that can sustain multiple current interruptions.
[0130] Various aspects and implementations described herein provide
advantages such as reduced phosphate demand, prolonged operating
times between cell cleaning, maintenance of low voltage levels,
enhanced protection of cathodes, reduced negative impact on anodes,
and so on.
EXAMPLES & EXPERIMENTATION
[0131] Various examples and experimental results will now be
described.
[0132] FIG. 2 shows a series of shutdowns in a chlorate cell which
operates with an iron cathode and a DSA anode at 70.degree. C. and
2.5 kA/m.sup.2. The electrolyte contains 550 g/l of NaClO.sub.3,
110 g/l of NaCl and 3 g/l of dichromate. Current interruptions in
open circuit (OC) for 2, 5, 10 and 15 min followed by an
interruption in short circuit (CC) for 15 min is observed. In open
circuit, the voltage of the cathode with respect to the DSA reaches
its corrosion potential at 1.08V quite rapidly. The longer the
interruption, the larger is the oxygen burst release and the lower
is the CE following the shutdown. When, the interruption is taking
place under a CC condition (which is equivalent to a bipolar
configuration), corrosion is so severe that a large crust of
corrosion products falls at the bottom of the cell and as a result,
the oxygen release following the event is relatively small.
[0133] FIG. 3 shows an x-ray scan taken from the surface of a
cathode after several months of electrolysis. Apart from sodium
chlorate, one observes calcium hydroxide (Portlandite) but also
some calcium sulphate hydroxide (Cesanite) deposits.
[0134] These blind deposits are responsible for cell voltage
increases observed after a long period of operation.
Example 1
[0135] Beneficial effects of the addition of very small amounts of
phosphate additives on the performance of cathodes were observed in
the following experiment.
[0136] A bath of 350 litres of sodium chlorate electrolyte
containing 550 g/l of NaClO.sub.3, 110 g/l of NaCl and 3 g/l of
dichromate was used for electrolysis. The pH and temperature were
maintained at 6.5 and 70.degree. C. respectively. A steel cathode
and a DSA anode were used in the experiment. After 74 hours of
continuous electrolysis at 2.51 kA/m.sup.2, the cell voltage was
3,056 volt. Then 4 ppm of calcium impurities was added to the
electrolyte in the form of CaCl.sub.2 and in only one hour, the
voltage raised to 3,074, which means an increase of 18 mV. A
shutdown event of 15 min without cathodic protection (i.e. open
circuit--OC) to induce some corrosion of the cathode and generate
iron impurities in the electrolyte was performed and, after 22
hours of continuous electrolysis, the voltage reached 3,060 volt.
This voltage is near the original value which suggests that part of
the surface impurities and corrosion product on the cathode surface
was removed by this sequence of events (shutdown in OC followed by
hydrogen discharge).
[0137] Thereafter, a second shutdown in open circuit of 15 min was
conducted and during this event, 10 ml of phosphoric acid
(H.sub.3PO.sub.4 --85%) was added to the 350 litres of electrolyte.
Considering a density of electrolyte of 1.3 g/l and the density of
phosphoric acid of 1.7 g/l, this addition corresponds to 0.047 g/l
or 36 ppm of phosphate ions (PO.sub.4.sup.-3) to the electrolyte.
This is also equivalent to 15.3 mg/l of phosphorus in the
electrolyte. This very small addition of phosphate compound lead to
a large voltage drop of 62 mV after 22 hours of constant
polarization as indicated in the Table 2, which summarizes the
series of experiments. Finally, a second addition of 10 ml of
phosphoric acid was performed during another shutdown event of 15
min and an additional voltage drop of 71 mV was observed after 16
hours of electrolysis. The total voltage reduction from the value
observed after adding the calcium impurities (3.074 to 2.927 volt)
was 147 mV. This is a large decrease for a very small addition of
phosphate ions which corresponds to less than 0.1 g/l or 75 ppm. At
these very low levels of phosphate in the electrolyte, there is no
precipitation of iron phosphate compounds on the anode which, as
mentioned previously, could be detrimental to the overall operation
of the cell.
TABLE-US-00002 TABLE 2 Series of events conducted as part of the
electrolysis experiment Voltage Duration End voltage variation
Event (hours) (volt) (mV) Constant polarization 74 3.056 Addition
of 4 ppm Ca.sup.++ Constant polarization 1 3.074 .uparw.18 mV
Shutdown without protection 0.25 (open circuit--OC) Constant
polarization 22 3.06 .dwnarw.14 mV Shutdown (OC) & addition 10
0.25 ml H3PO4 (85%) Constant polarization 22 2.998 .dwnarw.62 mV
Shutdown (OC) & addition 10 0.25 ml H3PO4 (85%) Constant
polarization 16 2.927 .dwnarw.71 mV Shutdown with cathodic 6
protection Constant polarization 30 2.94 .uparw.13 mV
[0138] After the experiment described above, the electrolyte was
analysed for impurity content and the results are shown in Table
3.
TABLE-US-00003 TABLE 3 Impurity content in the bath after
experiment Ca P Fe Impurity content (mg/l) 2.9 19 4.3
[0139] Since 4 ppm of calcium and a total of 30.6 mg/l (twice the
amount of 15.3 mg/l) of phosphorous were added to the electrolyte,
Table 3 shows that a significant amount of impurities is still
present in the electrolyte at the end of the experiment. The
residual calcium impurities in the bath no longer affect the cell
voltage because of a protective coating on the surface of the
cathode.
[0140] FIG. 5 shows an x-ray diffraction spectrum of the surface of
the cathode after removing the electrode from the bath at the end
of the experiment. The surface was dried in air prior to analysis.
The spectrum clearly reveals the presence of calcium phosphate
oxide Ca.sub.10(PO.sub.4).sub.6O which comes from dehydration of
hydroxyapatite Ca.sub.10(PO.sub.4).sub.6(OH).sub.2. This
demonstrates the existence of a thin layer of hydroxyapatite on the
surface of cathodes in chlorate cells when both calcium impurities
and very small amounts of phosphate are present in the electrolyte.
This thin layer of hydroxyapatite protects the surface of cathodes
from the detrimental effects of calcium impurities and leads to a
substantial decrease in cell voltages. At phosphate concentrations
lower than 0.1 g/l or 75 ppm, the conditions for promoting the
formation of hydroxyapatite (including high pH, high temperature,
and presence of Ca(OH).sub.2 seeds) at the surface of cathodes
exist even though the conditions for precipitation of calcium
phosphate compounds in the bulk of the electrolyte may not be
adequate. Therefore, calcium ions may still be present in the
electrolyte in fairly high concentrations, but their negative
impact on the cathode is reduced because of the hydroxyapatite
protecting layer. Moreover, the phosphate ions concentration is low
enough to avoid the formation of iron phosphate compounds on the
surface of anodes.
Example 2
[0141] FIG. 6 shows an electrochemical test similar to the one
shown in FIG. 2 but with a stainless steel cathode. In particular,
FIG. 6 shows a series of shutdowns in a chlorate cell which
operates with a stainless steel cathode of the 400 series and a DSA
anode at 70.degree. C. and 2.5 kA/m.sup.2. Current interruptions in
open circuit (OC) for 2, 5, 10 and 15 min followed by an
interruption in short circuit (CC) for 15 min is observed. As it
can be seen by comparing FIG. 6 with FIG. 2, the cell having a
stainless steel cathode has a potential 190 mV (4.42-4.23 volt)
higher than that of the iron cell. As mentioned before, this is due
to the presence of chromium in the stainless steel alloy. The
background O.sub.2 release is also slightly higher by 0.4% (3.1%
versus 2.7%). But surprisingly, the cathodic current efficiency of
the stainless steel cathode is much higher and stays high even
during current interruptions.
[0142] This is due to the surface of stainless steel which is
stable even under corrosion conditions. In open circuit, a very
stable passive surface layer forms on the stainless steel. This
passive surface layer is similar to the chromium hydroxide layer
which forms by the addition of dichromate in the electrolyte and
which provides high cathodic current efficiency. The presence of
chromium in the stainless steel alloys thus provide high current
efficiency at all times while in the case of the mild steel
cathode, the iron oxide scale lowers current efficiency
significantly each time there is a shutdown. Also notable on FIG. 6
is that the open circuit voltage of the stainless steel cell is
much lower than that of the iron cell (0.35 versus 1.08 volt) which
indicates that the stainless steels are much nobler than mild
steels and as a result, galvanic corrosion in bi-polar or short
circuit conditions would be reduced significantly. Moreover, the
high corrosion resistance of the stainless steels lowers the
overall amount of iron impurities in chlorate electrolytes and as a
result, reduces the possibilities of formation of iron phosphate on
anodes.
[0143] To overcome the drawback of the higher overpotential of the
stainless steels, the stainless steel substrates may be coated with
thin catalytic layers which ease the hydrogen evolution reaction
described in Equation 2. Examples of such catalytic layers are the
Fe.sub.3Al(Ru) and Fe.sub.3AlTa(Ru) alloys.
[0144] FIGS. 7a and 7b show scanning electron microscope pictures
taken from the cross-sections of such cathodic electrode structures
containing a stainless steel substrate of the 400 series and a thin
catalytic top-layer of the type described previously.
[0145] To complete the cathodic electrode structure according to
one embodiment of the present invention, on top of this bi-layer, a
coating is added which protects this electrode from impurities in
the electrolyte. This top surface layer may be the hydroxyapatite
protective layer described previously.
[0146] An example of the overall cathodic electrode structure
includes (i) a stainless-steel substrate of the 400 series
containing a small amount of carbon and no nickel; (ii) catalytic
mid-layer such as the ones based on an iron-aluminide metal matrix
doped with a catalytic element such as Ru and as described in
Canadian patent documents Nos. 2687129 and/or 2778865; and (iii) a
top-layer including or consisting essentially of hydroxyapatite on
the surface of the catalytic mid-layer to protect the cathode from
the impurities present in the electrolyte.
[0147] As discussed previously, the top-layer of hydroxyapatite on
the surface of the cathode can be formed in-situ by introducing in
the electrolyte a small amount of phosphate containing compounds,
for example in quantities less than 0.1 g/l or 75 ppm of phosphate
ions (PO.sub.4.sup.-3). This surface layer of hydroxyapatite can
also be prepared ex-situ by depositing the coating prior to use the
electrode assembly in an electrochemical chlorate cell. Indeed,
coatings of hydroxyapatite can be deposited by several methods
including thermal spray, sputter coating, dip coating, sol-gel,
electrochemical and electrophoretic deposition, biomimetic coating
and hot isostatic coating. The most commonly used method to
fabricate a hydroxyapatite coating is by plasma spray which is
classified as a thermal spray technique.
Example 3
[0148] FIG. 8 shows two scanning electron microscope pictures taken
from the cross-sections of cathodic electrode structures containing
a stainless steel substrate of the 400 series, a catalytic
intermediate layer and a thin hydroxyapatite top layer. Note that a
tungsten coating was provided on top of the sample prior to
performing the electron microscopy to protect the sample during
handling.
[0149] FIG. 9 shows chemical maps of the hydroxyapatite layer
indicating the presence of calcium and phosphorous elements.
Example 4
[0150] At room temperature, a cell containing a Fe.sub.3Al(Ru) type
cathode was tested with different quantities of calcium impurities
added to the electrolyte. One can note an increase of 100 to 150 mV
in only one or two hours when 1 to 2 ppm of calcium is added,
showing the negative impact of calcium on such electrodes. FIG. 10
shows the results of these tests.
Example 5
[0151] FIG. 11 shows the voltage over time for a cell containing
such catalytically enhanced electrodes at a temperature of
68.degree. C. After about 4 min of electrolysis, 2 ppm of Ca.sup.+2
ions were added to the electrolyte. The voltage then increased
systematically. After about 45 min of electrolysis, phosphate ions
PO.sub.4.sup.-3 were added in the form of phosphoric acid. The
effect was an immediate halt to the increase in cell voltage.
Example 6
[0152] In another trial as reported in FIG. 12, the voltage
evolution is shown for a similar cell as in Example 5 but where
phosphoric acid is added to the electrolyte before the addition of
calcium ions. After about 2 hours of electrolysis, 2 ppm of
Ca.sup.+2 was added. No significant increase in voltage was
observed after the addition of the calcium.
Example 7
[0153] FIG. 13 illustrates that low levels of phosphates that can
prevent the negative voltage increases induced by calcium
impurities. Calcium was present in an amount of about 2 ppm.
Example 8
Calculation Estimate for Phosphate Addition
[0154] As discussed above, the phosphate may be added to the
electrolyte or otherwise provided based on the surface area of the
cathode to be covered by the protective layer.
[0155] In a case where phosphate is added to the electrolyte in
order to form the protective layer, the amount of phosphate may be
determined in a number of ways. In one example, the phosphate
addition is pre-determined based on a calculation methodology such
as the one described below.
[0156] Various properties of the calcium phosphate compound may be
measured or taken from literature, for example: [0157] Molecular
formula of hydroxyapatite: Ca.sub.5(PO.sub.4).sub.3OH; [0158]
Molecular weight of hydroxyapatite: 502.31 g; [0159] Mass fraction
of phosphate (PO.sub.4) in hydroxyapatite (284.91/502.31): 56.7%;
[0160] Calculated density of hydroxyapatite (100% dense): 3.156
g/cc; and [0161] Porous hydroxyapatite can have porosity up to 90%
(0.35 g/cc=>89% from literature).
[0162] The maximum phosphate requirement on cathodes may then be
calculated as follows: [0163] Assuming that a protective layer of
about 1 micron thickness is to be obtained, as it has been found
that this thickness is sufficient to provide protective properties;
[0164] For a surface of 1 cm.sup.2=>Volume of layer: 10.sup.-4
cm.sup.3; [0165] Mass of phosphate (100% dense) per cm.sup.2 of
cathode surface=>3.156 g/cc.times.10-4 cc.times.56.7%=0.179 mg;
[0166] Therefore, less than 0.18 mg of phosphate (PO4) per cm.sup.2
of cathode surface may be provided; and [0167] If a hydroxyapatite
surface layer of one micron thickness with 45% porosity would be
sufficient to block the calcium from poisoning the catalytic
intermediate layer, then 0.18 mg.times.(1-0.45)=0.1 mg of phosphate
(PO.sub.4) per cm.sup.2 of cathode would be the requirement.
* * * * *