U.S. patent application number 10/601602 was filed with the patent office on 2005-01-20 for low energy chlorate electrolytic cell and process.
Invention is credited to Jackson, John R., Zhao, Mingchuan.
Application Number | 20050011753 10/601602 |
Document ID | / |
Family ID | 34062222 |
Filed Date | 2005-01-20 |
United States Patent
Application |
20050011753 |
Kind Code |
A1 |
Jackson, John R. ; et
al. |
January 20, 2005 |
Low energy chlorate electrolytic cell and process
Abstract
Alkali metal chlorates are produced by electrolyzing an anolyte
contained in an anode compartment of an electrolytic cell, the
anode and cathode compartments separated by means of a
permselective membrane having low alkali metal ion transport
efficiency. The final chlorate product can be directly crystallized
from the electrolyzed anolyte or fed directly to a chlorine dioxide
generator. Alternatively, a microporous, hydrophilic diaphragm can
be substituted for the permselective membrane provided that the
catholyte compartment is maintained at a higher hydraulic pressure
than the hydraulic pressure in the anolyte compartment.
Inventors: |
Jackson, John R.;
(Lexington, SC) ; Zhao, Mingchuan; (Columbia,
SC) |
Correspondence
Address: |
Andrew E. Pierce
161 McCracken Drive
Seneca
SC
29678
US
|
Family ID: |
34062222 |
Appl. No.: |
10/601602 |
Filed: |
June 23, 2003 |
Current U.S.
Class: |
204/252 |
Current CPC
Class: |
C25B 13/08 20130101;
C25B 1/265 20130101; C25B 13/04 20130101 |
Class at
Publication: |
204/252 |
International
Class: |
C25C 007/00; C25B
009/00 |
Claims
What is claimed is:
1. A low transport efficiency alkali metal ion permselective
membrane characterized by less than about 60% efficiency for the
transport of alkali metal ions.
2. The membrane of claim 1, wherein said membrane has less than
about 50% transport efficiency for alkali metal ions.
3. The membrane of claim 2, wherein said membrane has less than
about 20% transport efficiency for alkali metal ions.
4. The membrane of claim 1, comprising a polymer having cation
exchange groups.
5. The membrane of claim 4, characterized by high hydrogen ion
transport efficiency.
6. The membrane of claim 5, wherein said cation exchange groups are
selected from the group consisting of carboxylic acid groups and
sulfonic acid groups and said polymer is a copolymer of
tetrafluoroethylene and chlorotrifluoroethylene.
7. The low transport efficiency alkali metal ion permselective
membrane of claim 3, comprising said membrane having high hydrogen
ion transport efficiency and said membrane having cation exchange
groups selected from the group consisting of carboxylic acid and
sulfonic acid groups.
8. An electrolytic cell for the production of an alkali metal
halate, said cell comprising a low alkali metal ion transport
efficiency permselective polymer membrane and a catalytic, metal
anode and a catalytic, metal cathode or a catalytic, metal anode
and a gas-diffusion cathode.
9. The electrolytic cell of claim 8, wherein said permselective
polymer membrane has less than about 80% alkali metal ion transport
efficiency.
10. The electrolytic cell of claim 9, wherein said permselective
polymer membrane has less than about 50% alkali metal ion transport
efficiency.
11. The electrolytic cell of claim 10, wherein said permselective
polymer membrane has less than about 20% alkali metal ion transport
efficiency.
12. The electrolytic cell of claim 9, wherein said catalytic, metal
anode comprises a precious metal oxide deposited on a nickel or
titanium substrate.
13. The electrolytic cell of claim 12, wherein said catalytic,
metal cathode comprises a precious metal oxide deposited on a
nickel or titanium substrate.
14. The electrolytic cell of claim 9, wherein said cathode is a
gas-diffusion cathode.
15. The electrolytic cell of claim 9, wherein said catalytic, metal
cathode is selected from the group consisting of alloy mixtures of
nickel-molybdenum, cobalt molybdenum, nickel-tungsten,
cobalt-tungsten, nickel-iron, and nickel-cobalt on a nickel or
steel substrate.
16. The electrolytic cell of claim 9, wherein said catalytic, metal
cathode comprises an alloy coating of molybdenum, vanadium, and
nickel on a copper substrate.
17. The electrolytic cell of claim 8, wherein said polymer membrane
is characterized by less than about 20% alkali metal ion transport
efficiency, high hydrogen ion transport efficiency, and cation
exchange groups selected from the group consisting of carboxylic
acid and sulfonic acid groups.
18. An electrolytic cell for the production of an alkali metal
halate, said cell comprising an hydrophilic, microporous diaphragm
and a catalytic, metal anode and a catalytic, metal cathode or a
catalytic, metal anode and a gas-diffusion cathode.
19. The cell of claim 18, wherein said hydrophilic, microporous
diaphragm comprises a polymeric or ceramic material.
20. The cell of claim 19, wherein said microporous diaphragm
comprises a polymeric material selected from the group consisting
of a fluorinated polymer and a fluorinated copolymer.
21. The cell of claim 19, wherein said microporous diaphragm
comprises a ceramic material selected from the group consisting of
microporous titanium oxide and microporous zirconia.
22. The cell of claim 19, wherein said hydrophilic, microporous
diaphragm is characterized by a pore size of about 0.005 micron to
about 1 micron.
23. The cell of claim 22, wherein said hydrophilic, microporous
diaphragm is characterized by a porosity of about 50% to about
85%.
24. The cell of claim 19, wherein said catalytic, metal anode
comprises a precious metal oxide deposited on a nickel or titanium
substrate.
25. The cell of claim 24, wherein said catalytic, metal cathode
comprises a precious metal oxide deposited on a nickel or titanium
substrate.
26. The cell of claim 19, wherein said cathode is a gas-diffusion
cathode.
27. The cell of claim 24, wherein said catalytic, metal cathode is
selected from the group consisting of alloys of nickel-molybdenum,
nickel-tungsten, nickel-iron, nickel-cobalt, cobalt-molybdenum,
nickel-iron, nickel-cobalt, and cobalt-tungsten on a nickel or
steel substrate.
28. The cell of claim 24, wherein said catalytic metal cathode
comprises an alloy of molybdenum, vanadium, and nickel coated on a
copper substrate.
29. The electrolytic cell of claim 18, comprising an hydrophilic,
microporous diaphragm comprising a polymeric or ceramic material
having a pore size of about 0.005 micron and a porosity of about
50% to about 85%, a catalytic, metal anode, and a catalytic, metal
cathode or a gas-diff-usion cathode.
30. A continuous, cyclic, electrolysis process for the production
of an alkali metal halate in which the addition of chromium ions
and externally supplied acids or alkalies are eliminated, said
process comprising: A. electrolyzing in an anolyte compartment of
an electrolytic cell an anolyte comprising an alkali metal halide
and halate, said anolyte separated from a catholyte in a catholyte
compartment by a low alkali metal ion transport efficiency
permselective membrane characterized by less than 60% transport
efficiency for alkali metal ions, B. maintaining the pH in said
anolyte compartment at about 6 to about 7 by adding a sufficient
amount of said catholyte to said anolyte compartment, and C.
electrolyzing said anolyte to a desired aqueous solution of an
alkali metal halate from which said alkali metal halate can be
directly crystallized.
31. The process of claim 30, wherein said catholyte comprises an
alkali metal chloride and an alkali metal chlorate which are added
to said anolyte compartment of said cell by addition of said
catholyte to an anolyte recycle process stream from which chlorine
is absorbed and oxygen is separated and said stream is recycled to
said anolyte compartment.
32. A continuous, cyclic, electrolysis process for the production
of an alkali metal halate in which the addition of chromium ions
and externally supplied acids or alkalies are eliminated, said
process comprising A. electrolyzing in an anolyte compartment of an
electrolytic cell an anolyte comprising an alkali metal halide and
halate, said anolyte separated from a catholyte in a catholyte
compartment by a microporous, hydrophilic diaphragm characterized
by a pore diameter of about 0.005 micron to about 1 micron, wherein
said catholyte compartment is maintained at a differential pressure
of about 1 inch to about 48 inches of water over the pressure
maintained in said anolyte compartment, B. maintaining the pH in
said anolyte compartment at about 6 to about 7 by adding a
sufficient amount of said catholyte to said anolyte compartment,
and C. electrolyzing said anolyte to a desired aqueous
concentration of alkali metal halate from which said alkali metal
halate can be directly crystallized.
33. The process of claim 32 wherein said catholyte comprises an
alkali metal chloride and an alkali metal chlorate which are added
to said anolyte compartment of said cell by addition of said
catholyte to an anolyte recycle process stream from which chlorine
is absorbed and oxygen is separated and said stream is recycled to
said anolyte compartment.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] This invention relates to an electrolytic cell, a low
transport efficiency alkali metal ion permselective membrane, and a
cyclic electrolysis process for the preparation of an alkali metal
chlorate in an aqueous medium from the corresponding alkali metal
chloride.
[0003] (2) Description of Related Art
[0004] It is known in this art to form sodium chlorate by the
electrolysis of sodium chloride using a sodium chlorate electrolyte
which is a mixture of sodium chloride and sodium chlorate at
concentrations close to the saturation point. In the prior art
commercial process, an electrolytic cell is used in which the anode
and cathode are exposed to the same electrolyte; the cell not being
divided into cathode and anode compartments by a porous barrier,
diaphragm, or membrane. At the anode in such a cell, chlorine is
evolved and at the cathode, hydrogen is evolved. Hydrogen produced
at the cathode and chlorine produced at the anode combine in the
prior art cell with M the hydroxyl ions also produced at the
cathode to form hypochlorite. As the electrolysis proceeds, the
concentration of hypochlorite increases to a level at which some of
the hypochlorite converts to chlorate. In order to limit the
cathodic reduction of hypochlorite and/or chlorate ions, it is the
practice in the prior art to carry out the electrolysis in the
presence of hexavalent chromium values. The use of hexavalent
chromium in the electrolyte is disadvantageous economically as well
as environmentally, as set forth in U.S. Pat. No. 5,104,499 to
Millet and U.S. Pat. No. 4,295,951 to Bommaraju et al. The anodes
of the prior art chlorate cell are made from precious metal oxides,
such as ruthenium, platinum, and iridium oxides which are deposited
on a titanium substrate. The cathodes of the prior art chlorate
cell are made either from mild steel or titanium. The following is
the overall reaction in which Me is an alkali metal:
MeCl+3H.sub.2O.fwdarw.MeClO.sub.3+3H.sub.2
[0005] Unlike electrolysis in a chlor-alkali electrolytic cell in
which the anode and cathode are separated by a diaphragm or a
membrane allowing chlorine and hydrogen gas produced in the
reaction to be evolved and recovered separately, the prior art
electrolytic cell for the production of an alkali metal chlorate is
not divided into cathode and anode compartments in order to
facilitate the reaction of hypochlorous acid and alkali metal
hypochlorite to form alkali metal chlorate.
[0006] In addition to the principal reaction shown in the equation
above, hypochlorite and chlorate are at produced the cathode of the
electrolysis cell in an unwanted side reaction. Because of the
presence of the highly oxidizing hypochlorite ions in the
electrolyte solution, activated cathodes such as the precious metal
oxide coated cathodes disclosed in the prior art for use in the
production of alkali metal halatas cannot be used. Instead,
titanium or mild steel cathodes are used in the commercial I / 3
production of alkali metal chlorate. In chlor-alkali cells, these
cathodes require about 450 millivolts and 200 millivolts,
respectively, more than precious metal oxide coated cathodes or
other activated cathodes. Oxygen-reduction cathodes (also referred
to as gas-diffusion or air depolarized cathodes) provide even lower
electrical consumption in chlor-alkali electrolytic cells. However,
such cathodes have not been useful in an electrolytic cell for the
production of alkali metal chlorate because of the presence of
hypochlorite ions in the electrolyte.
[0007] Millet in '499 discloses conducting an electrolysis process
for the production of an alkali metal chlorate without hexavalent
chromium values being present in the electrolyte by the
electrolysis in an electrolytic cell wherein the anode and cathode
are separated by a selectively permeable cationic membrane. Millet
describes a process for the preparation of an alkali metal chlorate
in a single stage electrolysis reaction in the anode compartment of
a "chlorine-soda" cell in which the anode of the cell can be a
precious metal oxide coated onto a titanium support and the cathode
can be of steel or a precious metal coated onto steel. Since the
electrolysis cell is described by Millet as a "chlorine-soda" cell,
inherent in such a description is the fact that the membrane
selected for use in the cell is necessarily one having 90% to 95%
alkali metal ion transport efficiency.
[0008] Bommaraju et al. in '951 describe the use of film-coated
steel cathodes in an electrolytic cell for the production of an
alkali metal halate in order to substantially avoid the presence of
hexavalent chromium ions in an alkali metal halite solution. A
film-coated, electrically conductive cathode is disclosed to
enhance the current efficiency of a cell in which the electrodes
are exposed to the same electrolyte. Precious metal oxide coated
cathodes for use in the production of alkali metal halates by
electrolysis are disclosed by Bommaraju and in U.S. Pat. No.
4,377,454 and in U.S. Pat. No. 4,530,742 to Carlin et al.
Gas-diffusion cathodes for use in an electrolytic cell for the
electrolysis of brine are disclosed in U.S. Pat. No. 5,879,521 to
Shimamune et al. and U.S. Pat. No. 6,080,298 to Andolfatto. In the
electrolysis of brine, the gas-diffusion cathodes are disposed in
contact with an ion-exchange membrane which partitions the
electrolytic cell into anode and cathode compartments.
[0009] Wanngard in U.S. Pat. No. 5,419,818 discloses an
electrolysis process for the production of an alkali metal chlorate
in a first conventional alkali metal chlorate cell without a
separator. The demand for pH-adjusting chemicals is largely avoided
by utilization of the acid and alkali metal hydroxide produced in a
second electrolytic cell having a cell separator. Preferably, the
separator is a cationic-selective membrane which allows the
production of concentrated alkali metal hydroxide.
BRIEF SUMMARY OF THE INVENTION
[0010] In the electrolysis process of the invention, electrolytic
cells having as a cell separator a low transport efficiency alkali
metal ion permselective membrane or an hydrophilic, microporous
diaphragm are used for the production of an alkali metal chlorate.
In order to provide a process with greater electrical efficiency,
the uncatalyzed mild steel or titanium cathodes utilized in
commercial, prior art cells for the production of an alkali metal
chlorate are replaced in the cells of the invention with either
catalytic, metal cathodes or gas-diffusion cathodes, which are
depolarized by feeding air or oxygen to the cathode. To utilize
such cathodes without cathode corrosion, the anolyte and catholyte
compartments must be separated by a microporous diaphragm or a low
transport efficiency alkali metal ion permselective cationic
membrane. The electrolysis process of the invention can be carried
out with essentially no losses of chlorine, produced in the
process, and, in the absence of hexavalent chromium values, derived
from the addition of sodium chromate or sodium bichromate to the
electrolyte, thus providing economic as well as environmental
advantages. The use of a hydrophilic, microporous diaphragm in the
electrolysis process of the invention requires that an higher
hydraulic pressure be maintained in the catholyte compartment of
the cell than in the anolyte compartment of the cell.
[0011] The electrolysis process of the invention is advantageous in
providing energy savings by (1) reducing the voltage consumption in
the electrolysis process, (2) eliminating the addition of chromium
ions in the form of hexavalent chromium in the electrolyte with
attendant economical and environmental advantages, and (3)
eliminating the need for external sources of hydrochloric acid and
sodium hydroxide. Added advantages of the process of the invention
include (1) eliminating the possibility of explosive mixtures of
oxygen and hydrogen gases and (2) providing an electrolysis process
for the production of an alkali metal chlorate in which there are
essentially no losses of chlorine. The hydrogen produced in the
cathode compartment of the cell is confined therein by use of a low
transport efficiency alkali metal ion permselective membrane or a
hydrophilic, microporous diaphragm thus eliminating contact of
hydrogen with the cell anolyte where it can strip away the chlorine
produced therein. The anolyte is electrolyzed to a desired solution
of an alkali metal halate from which said halate can be directly
crystallized.
BRIEF DESCRIPTION OF THE DRAWING
[0012] In the drawing, there is shown a flow sheet for a
continuous, cyclic process for the electrolytic production of an
alkali metal halate, such as a chlorate, in accordance with one
embodiment of the process of the invention. In this process,
purified brine enters through line 38 a catholyte recycle stream
40, which exits gas and liquid disengager 19. A catholyte stream
comprising alkali metal hydroxide, chlorate, and chloride is
removed from catholyte compartment 36 through line 24. Hydrogen is
removed from catholyte process stream 24 in gas and liquid
disengager 19 and exits through line 17. A catholyte stream
overflow from gas and liquid disengager 19 is removed through line
15 and enters anolyte recycle process stream 22 which feeds gas and
liquid disengager 18. Chlorine is absorbed from anolyte process
stream 22 in gas and liquid disengager 18 and oxygen is removed
through line 20. A final chlorate solution is removed through line
14 from anolyte recycle stream 16 which exits gas and liquid
disengager 18. Cell 26 contains an anode 30, a cathode 32,
conductive means 31 and 33, and a microporous diaphragm or a low
transport efficiency alkali metal ion permselective cationic
membrane 28 separating anolyte compartment 34 from catholyte
compartment 36. In the preferred process, the pH is maintained in
the anolyte compartment 34 at a pH of about 6 to about 7 by
measuring and controlling the pH in line 16 at a pH of about 6.8 to
about 7.2. Anode 30 is a precious metal oxide coated titanium anode
and cathode 32 is either a catalyzed, metal cathode or a
gas-diffusion cathode. Where cathode 32 is a gas-diffusion cathode,
air or oxygen is fed to one side thereof.
DETAILED DESCRIPTION OF THE INVENTION
[0013] In the process of the invention, an electrolytic cell,
preferably, a plate and frame type, containing a low transport
efficiency alkali metal ion permselective membrane or an
hydrophilic, microporous diaphragm separating the anode and cathode
compartments of the cell is used for the preparation of an alkali
metal chlorate such as sodium chlorate. The anodes used in the cell
of the invention can comprise catalytic, metal anodes such as
precious metal oxides deposited on a metal substrate such as nickel
or titanium. Examples of precious metals used to form the oxides
are ruthenium, platinum, and iridium. These anodes have been found
to have a high current efficiency in the commercial sodium chlorate
electrolytic cell environment at a very low overvoltage for
chlorine evolution and operate at close to theoretical current
efficiency values. As a result, there is little room for
improvement of these prior art catalytic, metal anodes for use in a
sodium chlorate electrolytic cell. The cathodes used in the cell of
the invention can be gas-diffusion cathodes or catalytic, metal
cathodes. For example, precious metal oxides, such as platinum and
ruthenium oxides, deposited on a nickel or titanium substrate.
[0014] In one embodiment of the process of the invention, low
overvoltage precious metal oxide coated cathodes can be used
because the cathodes are not exposed to the damaging effect of the
hypochlorite ion produced in the anolyte compartment of the cell.
In the commercial electrolysis process for producing sodium
chlorate, the cathodes of the electrolytic cells are made either
from mild steel or titanium. The advantage of mild steel over
titanium in the prior art chlorate process is that of lower
electrochemical overvoltage and, accordingly, lower electrical
power consumption and cost per ton of the sodium chlorate produced.
However, mild steel cathodes corrode when the power to the
electrolytic cell is shut off for repairs while titanium, which
does not corrode when the power is shut off, has a higher
overvoltage, thus consuming more power per ton of chlorate
produced. Titanium also forms hydrides during operation of the
prior art, commercial electrolytic cells because of the reaction
with hydrogen produced at the surface of the cathode. Therefore, in
use, the electrode slowly erodes with the formation and subsequent
spalling of these titanium hydrides from the surface of the cathode
resulting in the necessity for replacement of titanium cathodes
periodically at a substantial cost.
[0015] In terms of the actual voltages needed for the electrolytic
reaction in the process of the invention, the normal reversible
potential for the reaction is increased by the values of the
electrode potentials and ohmic drops. The increase in the value of
the electrode potential over the normal reversible potential for
the reaction is termed overvoltage. That is, the difference between
the electrode potential necessary for the flow of current and the
equilibrium value of the electrode with no current flowing is the
overvoltage of the electrode. The overvoltage is related to such
factors as the nature of the ion being discharged, the current
density, the nature and surface structure of the electrode, the
temperature, and the composition of the electrolyte.
[0016] A noble metal coated metal substrate as a low overvoltage
electrode has been disclosed in U.S. Pat. No. 4,377,454 to
Bommaraju and in U.S. Pat. No. 3,974,058 to Gokhale. Hydride
formation on a titanium cathode has been disclosed as being
effectively prevented by a catalyst coating which is a barrier to
the migration of nascent hydrogen. Such cathodes are also disclosed
in U.S. Pat. No. 4,075,070 to DuBois et al. and U.S. Pat. No.
4,530,742 to Carlin et al. The foregoing patents cited in this
paragraph are incorporated herein by reference.
[0017] In the process of the invention, non-precious metal oxide
coatings on nickel or steel substrates such as mixed oxide
electrocatalysts of nickel-molybdenum, nickel-tungsten,
cobalt-molybdenum or cobalt-tungsten can be used. These are
disclosed in U.S. Pat. No. 4,426,269 to Brown et al. Also useful as
a catalyzed, metal cathode is an alloy of molybdenum, vanadium, and
nickel coating on copper, as disclosed in U.S. Pat. No. 4,105,531
to Kuo et al. Also useful as cathodes of the cell and process of
the invention are metal alloys such as nickel-iron and
nickel-cobalt also containing other transition metals coated onto a
nickel or a steel electrode substrate, as disclosed in U.S. Pat.
No. 4,410,413 to Hall. Also useful as a cathode is a catalyst
coating of a mixture of ruthenium metal and ruthenium oxide on a
nickel substrate, as disclosed in U.S. Pat. No. 5,227,030 to Beaver
et al.. The foregoing patents cited in this paragraph are
incorporated herein by reference.
[0018] Another class of cathodes useful in the cells and process of
the invention, which is known to have low overvoltage
characteristics in an electrolytic cell for the production of
chlorine and caustic, is an oxygen-reduction cathode, also referred
to as a gas-diffusion or air depolarized cathode. Representative
gas-diffusion cathodes, disclosed as useful for the electrolysis of
brine, are those cathodes disclosed in U.S. Pat. No. 5,879,521 to
Shimamune et al. and U.S. Pat. No. 6,080,298 to Andolfatto, each
incorporated by reference. Where a gas-diffusion cathode is
utilized in the electrolysis process of the invention to produce an
alkali metal chlorate, the gas-diffusion cathode is placed in the
cathode compartment of the electrolysis cell so as to divide the
cathode compartment into a solution compartment, on the
permselective membrane side and a gas compartment on the opposite
side of the gas-diffusion cathode. The gas-diffusion cathode is
prepared, for instance, by molding a mixture of a hydrophobic
substance such as a polytetrafluoroethylene resin together with a
catalyst such that the hydrophobic properties of the cathode
prevent liquids from passing through the cathode. Accordingly, when
the process of the invention is performed utilizing a gas-diffusion
cathode, air or oxygen is fed to a gas compartment, formed on one
side of the cathode, which is not shown,.
[0019] In one embodiment of the electrolytic process of the
invention, every mole of oxidation and reduction that takes place
in the electrolytic cell, is accompanied by a mole of ion transport
through the cationic permselective membrane. Where sodium is
utilized as the alkali metal in the electrolytic production of
sodium chlorate, the sodium ion moves from the anolyte to the
catholyte through the periselective membrane. The hydrogen ion can
also move from the anolyte compartment to the catholyte
compartment. At the same time, water accompanies the transport of
these ions from the anolyte to the catholyte. All of the sodium
that moves from the anolyte to the catholyte in the form of an ion
has to be added back to the anolyte in the form of a sodium
hydroxide solution to maintain the pH of the anolyte in the desired
range. This constant circulation of sodium from the anolyte to the
catholyte as an ion and back to the anolyte as a solution has the
effect of keeping the pH of the anolyte at the desired level
without any transport of anolyte anions to the cathode compartment.
In order to prevent excessive dilution of the anolyte by the
addition of a portion of the catholyte, in the process of the
invention, that portion of the catholyte which is added to the
anolyte to control the pH therein may be concentrated by
evaporation of water. In the process of the invention,
permselective cell membranes having low alkali metal ion transport
efficiency, generally, less than about 60% of theoretical,
preferably, less than about 50% of theoretical, and, most
preferably, less than 20% of theoretical are used. The ion
transport efficiency of the permselective membrane is expressed as
the percentage of the amount of ion transport which actually occurs
in comparison with the theoretical amount expected to occur for the
measured current flow used. Preferably, said membrane has high
hydrogen ion transport efficiency and high water transport
efficiency.
[0020] The low transport efficiency alkali metal ion permselective
membrane must be made of a material which is stable in the cell
electrolyte mixture of alkali metal chlorate, hypochlorite and
hydroxide solution. The material of the preferred cationic ion
exchange membrane can be made of a polymer having cation exchange
groups such as carboxylic acid groups and sulfonic acid groups.
Suitable polymers include copolymers of vinyl monomers such as
tetrafluoroethylene and chlorotrifluoroethylene, and a
perfluorovinyl monomer having an ion-exchange group or a reactive
group which can be converted into an ion-exchange group.
[0021] Alternatively, the permselective membrane of said cell can
be replaced by an hydrophilic, microporous diaphragm comprising a
polymeric or ceramic material, provided the catholyte compartment
of said cell is maintained at a higher hydraulic pressure than the
pressure in the anolyte compartment of said cell. Generally, a
difference in hydraulic pressure of about 1 to about 48 inches of
water, preferably, an hydraulic pressure differential of about 2 to
about 24 inches of water, and, most preferably, a catholyte
compartment differential pressure of about 4 to about 12 inches of
water is used.
[0022] Various halogenated polymers are suitable for the formation
of the hydrophilic, microporous, diaphragm material useful in the
cell and process of the invention. Examples of useful polymeric
diaphragm materials are: fluorinated polymers and fluorinated
copolymers such as poly(vinylidene fluoride) and
poly(tetrafluoroethylene). Other classes of materials suitable for
formation of useful microporous, hydrophilic diaphragms are ceramic
materials such as microporous titanium oxide ceramics and
microporous zirconia ceramics.
[0023] The pore diameter of useful microporous diaphragms can be,
generally, about 0.005 to about 1 micron, preferably, about 0.01 to
about 0.2 micron, and, most preferably, about 0.05 to about 0.1
micron. The diaphragm thickness usefully can be about 10 microns to
about 2 millimeters, preferably, about 25 microns to about 500
microns, and, most preferably, about 75 microns to about 250
microns. The porosity can usefully be about 50% to about 85%,
preferably, about 60% to about 80% and, most preferably, about 65%
to about 75%.
[0024] In the process of the invention, caustic from the catholyte
is ultimately added to the anolyte in an amount sufficient to
control the pH at a desired range of about 6 to about 7. This
caustic is not added directly to the anolyte compartment of the
electrolytic cell but, rather, at steady state conditions, is added
to the anolyte compartment by the addition of said catholyte to an
anolyte recycle process stream, as shown in the FIGURE. This is
added to the anolyte as a mixture of caustic and cell feed brine
catholyte solution which overflows from the catholyte compartment
to the anolyte compartment. A portion of this catholyte solution
also enters the anolyte compartment through the diaphragm where
this is used as the cell separator. Control of the pH in the
anolyte compartment in the range of 6 to 7 is achieved by measuring
the pH in the anolyte recycle stream at a point downstream of the
gas and liquid disengager. In the process of the invention,
chlorine is absorbed and oxygen is separated. Said recycle process
stream enters the anolyte compartment of the cell. The addition of
hexavalent chromium to the electrolyte is unnecessary.
[0025] In accordance with the process of the invention, the pH of
the anolyte of the electrolytic cell is maintained in the range of
about 6 to about 7. The anolyte is characterized, for example, as a
mixture of sodium chloride and sodium chlorate. This mixture is
desirably maintained so as to have a sodium chloride concentration
from about 60 grams per liter to about 300 grams per liter and a
sodium chlorate concentration from about 60 grams to about 700
grams per liter of the anolyte. The flow rate of anolyte through
the cell should be high enough to keep the pH between 6 and 7 as
anolyte passes from the bottom to the top of the cell. It has been
found that alkali metal chlorate solutions can be made in
accordance with the process of the invention at chloride to
chlorate mole ratios as low as 0.20. This low ratio is suitable for
utilization of the chlorate product as feed in a chlorate
crystallizer or the chlorate solution can be fed directly to a pulp
mill chlorine dioxide generator. Further chemical treatment
necessary for commercial alkali metal chlorate solutions, such as
the removal of hexavalent chromium, is unnecessary.
[0026] In the following examples, there are illustrated the various
aspects of the invention, but these examples are not intended to
limit the scope of the invention. Where not otherwise noted in this
specification and claims, temperature is in degrees centigrade and
parts or percentages are by weight.
EXAMPLES 1-4
Forming No Part of this Invention
[0027] In order to demonstrate the reduced power usage that can be
expected where a precious metal oxide coated cathode is substituted
for the titanium or mild steel cathodes presently utilized in
commercial sodium chlorate electrolysis cells, a comparison was
made of these cathodes as well as an oxygen reduction cathode by
measuring the single electrode potential of these cathode materials
against a saturated calomel reference electrode. The conditions for
measuring the single electrode potentials of these cathode
materials were as follows: Sodium hydroxide electrolyte--one molar,
current density--one amp per square inch, temperature--30 to 40
degrees centigrade. Where an oxygen reduction cathode was used,
pure oxygen was fed to the cathode. Cathodes of titanium and mild
steel, represent commercial cathodes utilized for the production of
sodium chlorate. The precious metal oxide coated cathode was a
mixture of platinum and ruthenium oxides thermally deposited on a
nickel substrate. The oxygen reduction cathode utilized was a
nickel-cobalt-Teflon.RTM. mixture thermally deposited upon a
sintered nickel substrate.
[0028] For a titanium cathode, the measured volts against a
saturated calomel reference electrode were found to be -1.55 (-1.07
theoretical).
[0029] For a mild steel cathode, the measured volts against a
saturated calomel electrode were found to be -1.20 (-1.07
theoretical).
[0030] For a platinum and ruthenium oxide coated nickel substrate
cathode, the measured volts against a saturated calomel electrode
were -1.08 (-1.07 theoretical).
[0031] For the oxygen reduction cathode containing a
nickel-cobalt-Teflon mixture thermally deposited on a sintered
nickel substrate, the measured volts against a saturated calomel
electrode were -0.33 (+0.16 theoretical).
[0032] These measured single electrode potentials for these cathode
materials can be converted to conventional power usage units of
kilowatt hours per short ton of sodium chlorate as follows:
[0033] Titanium cathode: 4,800; mild steel cathode: 4,290; platinum
and ruthenium oxides coating on nickel: 4,120; and
nickel-cobalt-Teflon thermally deposited coating on a sintered
nickel substrate: 3,040.
[0034] The results above show that mild steel consumes
significantly less energy when used as a cathode than titanium,
however, mild steel is subject to corrosion in a commercial
electrolytic cell for the production of sodium chlorate when the
power to the cell is shut off for any reason. Titanium does not
corrode under these conditions. But the use of such cathodes in a
cell for the production of sodium chlorate results in the
consumption of more power per ton of chlorate produced and because
titanium forms hydrides during operation of the cell as a result of
reaction with hydrogen produced at the surface of the cathode.
Thus, a titanium cathode slowly erodes and must be replaced at a
substantial cost, as compared to the cost of a cathode of mild
steel.
[0035] Both precious metal oxide coated cathodes and oxygen
reduction cathodes can provide substantially lower power
consumption when used in a sodium chlorate electrolytic cell.
However, such cathodes are impractical for use in a conventional
sodium chlorate electrolytic cell in which the anode and cathode
are exposed to the same electrolyte because these cathodes corrode
when the power is shut down resulting in contamination of the
electrolyte with corrosion products. In addition, the use of an
oxygen reduction cathode requires feeding air or oxygen to the
cathode and prior art commercial electrolytic cells for the
production of sodium chlorate have no means for feeding the
required gas to the cathode.
EXAMPLE 5
[0036] A pilot plant size plate and frame type electrolytic cell
electrolysis process for the production of sodium chlorate is
described in which the anode and cathode compartments are separated
by a microporous diaphragm.
[0037] The anode used in the cell was a platinum-iridium oxide
coated titanium expanded mesh substrate. The activated cathode was
a platinum-ruthenium oxide coated nickel expanded mesh substrate.
The electrode size was 3 inches by 10 inches providing a total of
30 square inches of active surface area. The microporous diaphragm
utilized to separate the anolyte and catholyte compartments of the
cell was a hydrophilic polyvinylidene fluoride sheet sold under the
trademark Duropore.RTM. by the Millipore Corporation. This
diaphragm had a pore diameter of about 0.1 micron, a thickness of
110 plus or minus 30 microns, and a porosity of 70%.
[0038] During the operation of the cell, the current input was
measured at 35 amps or 1.17 amps per square inch current density.
The temperature of both the anolyte and catholyte solutions was
maintained at about 65 degrees centigrade by steam heat exchangers.
A differential hydraulic pressure of about 4 inches of water from
the catholyte compartment to the anolyte compartment was maintained
across the diaphragm. During cell operation, purified brine was fed
to the cathode compartment of the cell together with a catholyte
recycle stream in accordance with the FIGURE. The brine feed rate
to the cathode compartment was 1.52 milliliters per minute. The
catholyte was a mixture of sodium hydroxide, sodium chloride, and
sodium chlorate. The catholyte was circulated from the top of the
cathode compartment, passed through a gas and liquid disengager and
returned to the bottom of the cathode compartment. A mixture of
sodium chloride and sodium chlorate as anolyte was circulated from
the top of the anolyte compartment, passed through a gas and liquid
disengager and returned to the bottom of the anode compartment.
Both catholyte and anolyte streams were circulated at a rate of
about 2 to about 4 liters per minute. No hexavalent chromium was
added to the anolyte or catholyte.
[0039] At the beginning of the operation of the cell, the pH of the
anolyte compartment was adjusted by the addition of small amounts
of hydrochloric acid and sodium hydroxide. After a steady state of
pH was achieved, inconsequential amounts of hydrochloric acid and
sodium hydroxide were needed to adjust pH over the 40 hour period
of operation of the cell. During cell operation, a catholyte
overflow stream was removed from the gas and liquid disengager and
added to the anolyte recycle stream at a rate of about 1.7
milliliters per minute. An anolyte overflow stream from the anolyte
gas and liquid disengager was obtained at a rate of about 1.4
milliliters per minute. During cell operation, at a brine feed rate
to the cathode compartment of 1.52 milliliters per minute and
utilizing a current of 35 amps, the anolyte reached a concentration
of 192 grams per liter of sodium chloride, 260 grams per liter of
sodium chlorate, and about 3 to about 5 grams per liter of sodium
hypochlorite. During cell operation, a concentration of 48 grams
per liter of sodium hydroxide, 198 grams per liter of sodium
chloride, and 212 grams per liter of sodium chlorate were obtained
in the catholyte. No hypochlorite was detected in the
catholyte.
[0040] The current efficiency of the cell was measured in two ways:
(a) the rate of oxygen evolution can be measured and the current
loss calculated as that required to produce oxygen as a by-product
and (b) the current efficiency can be measured by analyzing the
amount of sodium chlorate made and sodium chloride consumed. The
current efficiency of this electrolysis process was 93-94%. The
cell voltage averaged 2.74 volts which corresponds to 4,006
kilowatt-hours for the production of each short ton of sodium
chlorate produced.
EXAMPLE 6
[0041] Example 5 was repeated, except that in the process, using a
plate and frame type electrolytic cell, an hydrophilic,
polytetrafluoroethylene- , microporous diaphragm sold under the
trademark Advantec.RTM. by Advantec MSF, Inc. was used to separate
the anode and cathode compartments of the cell.
[0042] The same cathode and anode as those described in Example 5
were used. The diaphragm pore diameter was 0.1 micron, the
thickness was 25 plus or minus 1 microns, and the porosity was 71%.
The current input was 35 amps or 1.17 amps per square inch current
density. The temperature of both the anolyte and catholyte
solutions was maintained at about 65 degrees centigrade by steam
heat exchangers. The differential hydraulic pressure from the
catholyte to the anolyte compartment was maintained at 4 inches of
water.
[0043] In the process, purified brine was first acidified to a pH
of about 3.5 to remove carbonate ions before being fed, at a rate
of about 1.56 milliliters per minute, to the cathode compartment
together with a catholyte recycle stream in accordance with the
FIGURE. The cell was run for about 30 hours. During this period,
the anolyte concentration was measured at 188 grams per liter of
sodium chloride, 245 grams per liter of sodium chlorate, and 3 to 5
grams per liter of sodium hypochlorite. The catholyte concentration
was measured at 58 grams per liter of sodium hydroxide, 190 grams
per liter of sodium chloride, and 160 grams per liter of sodium
chlorate. No hypochlorite was detected in the catholyte. The
current efficiency was found to be 93-94%. Chlorine losses were
about 0.005% of the current used. This contrasts with commercial
electrolytic cells for the production of sodium chlorate which
typically exhibit chlorine losses of between 0.3 and 0.5% of the
current used. The cell voltage averaged 2.63 volts which
corresponds to 3,845 kilowatt-hours for the production of each
short ton of sodium chlorate.
EXAMPLES 7
(CONTROL) AND 8
[0044] In these examples, the inventive process was compared to a
prior art process using two laboratory size electrolytic cells for
the preparation of sodium chlorate. In Example 7 a titanium cathode
was used and in the second cell of Example 8 an oxygen reduction
air depolarized cathode was used. Comparison of the voltage
differences between the two cells showed anode to cathode cell
voltages of 3.1 to 3.3 volts with the titanium cathode and 1.4 to
2.1 volts with the air depolarized cathode.
[0045] Each of the electrolytic cells were plate and frame type
cells in which the anode and cathode compartments are separated by
a cation exchange membrane made of Nafion.RTM. 324, which had a
measured sodium ion transport efficiency of 79%. The electrode size
was 4 square inches of active area. The current input was 4 amps or
1.0 amp per square inch current density. The air depolarized
cathode was a nickel-cobalt sintered nickel cathode. In both cells
the anode was a ruthenium and titanium oxide coating on a titanium
substrate. The catholyte was maintained at a concentration of
sodium hydroxide of 140 to 190 grams per liter. The anolyte
concentration of sodium chloride was maintained at 280 to 310 grams
per liter. The concentration of sodium chlorate was up to 164 grams
per liter. Both anolyte and catholyte liquors were recirculated at
the rate of 640 to 660 milliliters per minute and maintained at a
temperature of 70 degrees centigrade. The pH in the anolyte
compartment was maintained at 6.2 to 6.7 by the addition of a
solution of sodium hydroxide obtained directly from the cathode
compartment. It is noted that no hexavalent chromium was added to
the anolyte or catholyte.
EXAMPLE 9
[0046] In this example, a pilot plant size electrochemical cell for
the production of sodium chlorate was run in order to measure the
current efficiency, the consumption of hydrochloric acid and sodium
hydroxide solution, and the overall cell performance, including the
maintenance of a pH of about 6.4 to about 6.8 in an anolyte recycle
stream entering the bottom of the anode compartment of the cell. A
gas and liquid disengager through which the anode recycle stream
passes was used to absorb chlorine to minimize chlorine losses from
the system.
[0047] The electrolytic cell was similar in components to those of
the cell described in Example 8, except that the electrode size was
3.5 inches by 10 inches for a total of 35 square inches of active
surface area. The current input was 35 amps or 1.0 amp per square
inch current density. The hydrogen evolution cathode was a platinum
and iridium oxide coating on a titanium expanded mesh substrate.
The anode was a platinum and iridium oxide coating on a titanium
expanded mesh substrate. The catholyte was maintained at a
concentration of 140 to 190 grams per liter of sodium hydroxide and
the anolyte was a mixture of sodium chloride and sodium chlorate
having a concentration of between 188 to 301 grams per liter sodium
chloride and 7 to 45 grams per liter sodium chlorate. The anolyte
was circulated from the top of the anode compartment to the bottom
of the anode compartment in the anolyte recycle stream at a rate of
2 to 4 liters per minute and the temperature of the anolyte and
catholyte compartments was maintained at 60 degrees centigrade. No
hexavalent chromium was present in the anolyte or catholyte. In the
cathode compartment, the agitation of the catholyte by the release
of hydrogen at the cathode was sufficient to provide uniform
circulation of the catholyte. The pH at the top of the anolyte
compartment was controlled by the addition of sodium hydroxide
solution, from the cathode compartment, to the anolyte recycle
stream and by the adjustment of the flow rate through the anode
compartment. The sodium hydroxide solution from the cathode
compartment was metered into the anolyte recycle stream as this
stream exited at the top of the anolyte compartment of the cell.
The addition of caustic at this point in the anolyte recycle stream
resulted in a high pH zone in the piping as well as in the
disengager located downstream of the caustic addition point.
Because of the high pH in this disengager, most of the chlorine,
which might otherwise have escaped to the atmosphere in a lower pH
anolyte environment, was absorbed from the anolyte. Subsequently,
measuring the pH of the anolyte recycle stream at a point
downstream of the disengager allows the maintenance of the anolyte
recycle stream in an acceptable pH range by feeding this recycle
stream to the bottom of the anolyte compartment of the cell.
[0048] The current efficiency of the cell was measured at 96-97% in
two ways: (a) by measuring the rate of oxygen evolution from the
anode compartment and calculating the current loss required to
produce the by-product oxygen and, (b) by analyzing the amount of
sodium chlorate made and sodium chloride consumed by the
electrolysis current over the test period. Chlorine losses were
measured at 0.02% of the current used. It is known in commercial
sodium chlorate electrolytic cells that chlorine losses are between
0.3 and 0.5 percent of the current used. The cell voltage averaged
2.86 volts. Sodium ion transport efficiency was measured at 77% and
there was a gradual dilution of anolyte salts over the course of
the test.
[0049] During operation of the cell, no outside sources of
hydrochloric acid and sodium hydroxide were utilized. The hydrogen
produced at the cathode and the oxygen produced at the anode were
not allowed to mix by the use of the permselective membrane
separating anolyte and catholyte compartments of the cell. These
gases could be discharged from the cell without mixing and,
accordingly, without the formation of explosive mixtures.
EXAMPLE 10
[0050] This example is a repeat of Example 9 except that a Nafion
551 permselective membrane was used and the anolyte was a mixture
of sodium chloride and sodium chlorate having a concentration of
between 70 to 125 grams per liter of sodium chloride and 480 to 593
grams per liter of sodium chlorate. The temperature of the anolyte
and catholyte compartments was maintained at 65 degrees centigrade,
and the pH in the anolyte was controlled with concentrated, 765
grams per liter, sodium hydroxide solution from an external
source.
[0051] Under these conditions, the current efficiency of the cell
was measured at 92-94%, chlorine losses were measured at 0.35% of
the current used, and the cell voltage averaged 2.98 volts. The pH
was not controlled as closely as in Example 9 thus resulting in
higher chlorine losses. During operation of the cell, no outside
sources of hydrochloric acid and sodium hydroxide were utilized.
The hydrogen produced at the cathode and the oxygen produced at the
anode were not allowed to mix by the use of the permselective
membrane separating anolyte and catholyte compartments of the cell.
These gases could be discharged from the cell without mixing and,
accordingly, without the formation of explosive mixtures.
[0052] Sodium ion transport efficiency of the cell membrane was
measured at 65%. By the end of the test, the sodium chloride
concentration was 70 grams per liter, the sodium chlorate
concentration was 593 grams per liter, and the mole ratio of
chloride to chlorate was 0.22. This liquor is suitable for direct
feed to a sodium chlorate crystallizer or a pulp mill chlorine
dioxide generator with no further treatment.
[0053] While this invention has been described with reference to
certain specific embodiments, it will be recognized by those
skilled in the art that many variations are possible without
departing from the spirit and scope of the invention and it will be
understood that it is intended to cover all changes and
modifications of the invention disclosed herein for the purpose of
illustration which do not constitute departure from the spirit and
scope of the invention.
* * * * *