Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~7~22
SYS~ OR E~ECTRO~YTI~T-T-Y GEN~RATING
S~RONG SOLUTIONS OF HALO OE ~ O~YAC DS
sackqround o~ th~ In~ention
Teahnical Fi ld
The present invention relates to a proces~ and
apparakus for electrolytically generating ~trong solutions
of halogen oxyacids from the corresponding alkali metal
0alt. The present invention will be particularly
described with reference to generating a chloric acid
~HClO3) solution o high normality from sodium chlorate
(NaC103). However, it will be apparent to those skilled in
the axt that the present invention i8 al80 applicable to
the generation of other oxyacid~, for instance perchloric
acid (HC104) from sodium perchlorate (NaCl04).
Description of the Prior Art
One problem with halogen oxyacids such a~ chloric
acid i~ that they are unstable and sub~ect to
decomposition, particularly at elevated temperatures.
This prevents them from being easily stored and shipped
re~uiring that they be made at a point of use ra~her than
,
- .
2 ~ 2 2
--2--
on a large industrial scale. Present commercial methods
~or generating chloric acid on site acidify sodium
chlorate with ~ulfuric acid. This produces an Lmpure
product stream containing ~odium sulfate which has to be
removed, and which is of littla value as a by-product.
U.S. Pa~ent ~o. 4,798,715, assigned to the a~ignee
of the pre~ent application discloses the production of
chloric acid from sodium chlorate using an ion exchange
resin. Chlorine dioxide i8 then manu~actured by reducing
the chloric acid in an electrochemical cell. It is
disclosed in the patent thPt the chloric acid feed to the
electrochemical cell can ha~e a normality of about O.S up
to about 4.5. However, it is de~irable to feed chloric
acid to the electrochemical cell at a relatively high
normality, ~or in~tance, above about 1.5, in order to
obtain reduction of the chloric acid to chlorine dioxide
at optimum efficiency.
U.S. Pa~ent No 3,810,969 also discloses the
manufacture of chloric acid by reacting an alkali metal
chlorate with a stoichiometric exces~ of a cation exchange
re~in. One problem with the use of a cation exchange
resi.n ~8 that such re~ins have a relatively short
lifetime, incrQasing ths cost of manufacture of chloric
acid.
~t i~ known to produce acids using an electroly~ic
cell. U.S. Patent No. 4,115,217 disclose~ the u~e of a
three-compartment electrolytic cell for the preparation of
~odium chlorita (NaC102) from sodium chlorate (NaC103),
, . . : . . - . '-
., : . .
.. . .
.
., ' ' ' '` , . ` ' ' . ' ` ' , .
-3- 2~3~
sulfuric acid, and sulfur dioxide. A product of the
proces~ of this patent is enriched sulfuric acid (H2S04)
instead of chloric acid. ThP process compri~es reacting
the sodium chlorate in a reactor with the æulfur dioxide
to produce a residual solution of sodium ~ulfate and
sulfuric acid. Chlorine dioxide tC102) i~ also formed in
the reactor and is removed in an inert gas stream. The
re~idual selution containing sodium sulfate and sulfuric
acid is fed into the middle compar ment of the
electrolytic cell. The middle compartment is defined on
one side by an anion selective membrane and on the
oppo~ite Ride by a ca~ion selective membrane. The end
compartment~ of the cell are an anode compartment
separated from the middle compar~ment by the anian
selective membrane and a cathode compartment ~eparated
~rom the middle compartment by ~he cation æelective
membrane. When a voltage is applied to the cell, ~ulfate
ions migrate from the middle compartment through the anion
selective membrane into the anode compartment. At the
anode, water is decomposed with the evolution of oxygen
and generation of hydrogen ion~ which react with the
migrated sulfate ions to form sulfuric acid. The chlorine
dioxide formed in the reactor is fed into the catholyte of
the three-compartment cell and is reduced to chlorite ions
(C102-) at the cathode. The cation~ in the middle
cempartment, mainly sodium and hydrogen ions, migrate
through the cation selective membrane. The sodium ion~
react with the chlorite ions formed at the cathode to form
3 2 2
sodium chlorite which is precipita~ed in the cathode
compartment when saturation is reached.
Problem~ arise, however, when an electrolytic cell
such as that di~closed in U.~. Patent No. 4,115,217 is
attempted ~o be used in the manufacture of a ~trong
halogen nxyacid such as chloric acid. For one, such cells
are known to generate substantial amounts of heat because
of solution and separator resistance, which can lead to
acid decomposition. In addition, the halogen oxyacids at
high temperature are highly corro4ive preventing many
m~terials conventionally employed in electrolytic cells
~rom being u~ed in association with the oxyacid~.
U.S. Patent No. 3,222,267 also describes a three-
compartment cell. An electrolytic ~olution is
electrolyzed in such a manner a~ to produce salt-free
prodllct hydroxide and the corre~ponding acid salt of
sodium bi~ul~ate. By way of example, a 10% sodium sulfate
solution was introduced into a center feed compartment.
The ~low rate and pressure of the solution is sufficient
for the solution to percolate through a Rorous diaphra~m
into an anode compartment. The flow rate and pressure
al~o prevents back migration or diffusion of protons
toward the cell cathode. Water is introduced into the
cathode compartment. Elec~rolysis in the cell produces a
2N sodium hydroxide catholyte effluent and a .075N acid
anode effluent. AB in Patent No. 4,115,217, the
compo~itions o~ the solutions, were not such that
-5~
decomposition o~ th~ effluents, or corrosion of materials
co~ventionally used in such a cell, were a problem.
A disclosure sLmilar to that of Patent NoO 3,222,267
is contained in U.S. Patent No. 3,523,7S5.
U.S. Patent No. 2,82g,095 discloses a process for the
pxoduction of acidic solutions in an electrolytic cell
using a plurality of anion exchange and cation exchange
membranes. There is no disclosure concerning the
produckion of halogen oxyacids.
U.S. Patent No. 4,504,373 discloses a three-
compartment electrodialytic cell and feeding alkali metal
sul~ate values to the cell.
a.s. Patent No. 4,740,281 discloses ~upplying a salt
and acid to one compartment of an electrodialysis
apparatus and a liquid containing water to a ~econd
compartment of the apparatus. The process is for
regenerating acids from stainle3s steel pickling baths.
Summary of the Invention
The present invention re~ides in a process and
apparakus for the electrolytic production of strong
solutions of halogen oxyacids. The pre~ent invention
comprises e~tablishing a solution of the corresponding
alkali metal salt. An electrolytic cell is provided
comprising an anode compartment containing an anode, a
cathode compartment containing a cathode, and a middle
feed cQmpartment in~ermediate the anode compartment and
cathode compar~ment. The middle feed compartment is
separated ~rom the anode compartment by a separator which
.
.~ ' ' ' .
2Q3~,2
is either a porous diaphragm or a membr~ne selective to
the migration of oxyhalogen ions, and from the cathode
compartment by a cation selective membrane. Means are
providPd for introducing said alkali metal salt solution
into said feed compartment and for applying a voltage
between the anode and the cathode. Under the influence of
the applied voltage, the alkali metal ion~ formed by the
di~a~sociation o~ the salt migrate through the cation
Relective membrane to the cathode, reacting with
electxochemically produced hydroxyl ions to form alkali
metal hydroxide. The oxyhalogen ions foxmed by the
di~as~ociation of the salt migrate through the porou~
diaphragm or membrane separator between the middle
compartment and the anode compartment, to the anode,
reacting with electrochemically formed proton~ to form
halogen oxyacid. Mean~ are provided for cooling the
electrolytic cell. It wa~ found that reducing the
temperature~ in the electrolyklc cell allowed the use of
membxanes, diaphragm~ and other materials of constxuction
that would not otherwise be allowed, and also inhibit3
decompo~ition of the halogen oxyacid, that might otherwi~e
occur. ~he electrolytic cell and acid product preferably
are cooled to a tempexature in the range of about 10C to
about 40C.
_7~ 2 ~.
Brief De~cription of the Drawin~
Further feature~ of the pres2nt invention will bacome
apparent to tho~e skilled in the ar~ to which the present
invention relates from readiny the following ~pecification
with reference to the accompanyiny drawing in which the
~igure is a flow diagram illustrating the process Qf the
presQnt invention.
De~cri~tion of Preferred ~mbodiment
In tha ~ollowing description, the principals of the
pre~ent invention will be disclosed by reference to a
~peci~ic halogen oxyacid product and ~ ~pecific alkali
metal salt feed. The halogen oxyacid hereinafter
disclosed iQ chloric acid and the alkali metal salt feed
i~ ~odium chlorate. It will be apparent to those skilled
in the art that the principal3 of the pre~ent invention,
a~ herelnafter disclosed, are applicable to the generation
of other halogen oxyacid~ u~ing as feed other alkali metal
salt~.
Referring to the Figure, a chloric acid generator is
disclosed. A ~odium chlorate solukion i~ contained in
~eed tank 12. This ~olution can be obtained by dissolving
~odium chlorata crystal3 in water, or by diluting a
concentrated solution of soclium chlorate, by way of
example. The ~odium chlorate solution is pumpad at a
controlled feed rate, by a pump 14, through line 16 into
ths middle feed compartment 18 of a thre2-compartment
electrolytic cell 20. One type of pump 14 that can be
used i~ a positive displacement pump.
-8- 2~7~
The electrolytic cell 20 comprises an anode
compartment 22, containing an oxygen or chlorine evol~ing
anode 24, and a ca~hode compartment 26 containing a
cathode 28. The anode compar~ment 22 is separated from
the middle compar~nent 18 by a separator 30. The
separator 30 can be either a porous diaphra~m or a
membrane which is selective to the migration of oxyhalogen
ions. The cathode compartment 26 is separated from the
middle compartment 18 ~y a cation-selective membrane 32,
which is selective to the migration of cations.
In operation, the sodium chlorate in solution
disassociates into positively charged sodium ions and
negatively chaxged chlorate ions per the following
equa~ion:
NaCl03 ~ Na~ + Cl03 ~l)
Upon the influence of an impressed direct electric
current in the cell 20, the cation constituents of the
soditum chlorate ~olution, namely, positive sodium ions,
pas~ through the cation-selective membrane 32 into the
cathode compartment 26. Hydroxyl ions produced at the
cathode 28 by the electrolysls of water react with the
~odium ions to produce sodium hydroxide, per the following
reactions:
2~0 ~ 2e- ~ ~2 ~ 20H- (2)
_9_ ~37~22
20~I- + 2Na~ ~ 2NaO~I (3)
Overall: 2H20 + 2e~ + 2Na+ - H2 + 2NaOH (4)
Under the influence of an Lmpressed direct electric
current in the cell 20, some water is carried into the
cathode compartment 26 with the sodium ions. This dilutes
the sodium hydroxide, the diluted sodium hydroxide being
withdrawn from the cathode compartment 26 through a
catholyte ef~luent line 34. In the embodiment illustrated
in the Figure, the catholyte effluent line 34 lead~ by way
of example, to a sodium hydroxide storage tank 36.
Hydrogen i~ also produced in the cathode compartment 26 by
the electroly~i~ of water, and is vented from the
compaxtment by means of a hydrogen vent 38.
The negatively charged chlorate ions in the middle
~eed compartment 18 mlgrate through the separator 30 into
the anode compartment 22. In the anode compartment, the
negatively charged chlorate ions combine with protons
produced at the anode 24 by the electroly~is of water to
produce chloric acid. The following reac~ion~ take place:
H20 ~ 2H~ ~ 2 2 ~ 2e~ (5
2 ~
Cl03 ~ H+ ~ HCl03 (
Oxygen evolved in the anode compartment 22 can be vented
to a~mo~phere in oxygen vent line 40. Chloric acid (~Cl03)
is withdra~m from the anode compar~nent 22 in anolyte
effluQnt line 42 to acid storage tank 44. Acid pxoduct is
withdrawn from the acid ~torage tank in acid product line
46.
The overall reaction for the production of chloric
acid is 5
3H20 ~ 2NaCl03 - 2HCl03 + H2 + 2NaO~I t 2 2 (7
C~loric acid i8 a ~trong oxidizing acid, and the
separator 30, broadly, has to be resi~tant to this acid.
Further, the separator 30 should have such properties that
it c~use~ a relatively low voltage drop in the acid
generator, and allows anions to pas~ easily ~o that a high
current density can be achieved. Broadly, the separator
30 can be either of a hydraulically porous nature, such as
a diaphragm, or can ~e essentially hydraulically
:~npervious to the bulk transport of electrolyte, such as
an anion-~elective membrane.
In the generator 20 r proton~ are generated at the
anode 24. Thera i8 a tendency, under the influence of an
impre~ed direct electric current, for these proton~ to
migrate to the cathode, which reduces the g~nerator
efficiency by direct reaction with OH- ions generated at
the cathode. Preferably, the diaphragm ~where the
separator 30 is a diaphragm) has a pore ~ize and pore
den~ity that creates a high fluid v~locity such as to
re~ist migration of the protons through the diaphragm,
while at the s~me time sufficient to allow the transport
of chlorate ions from the middle compartment 18 into the
anode compartment 22.
Where the ~eparator 30 is a porous diaphragm, a
number of well known diaphragm materials which have
resistance to oxidiz~ng acids and have good electrical
properties can be employed. A preferred porous diaphragm
is one made of polyvinylidens fluoride (PVDF).
Polyvinylidene fluoride ha~ good resistance to chemical
attack by chloric aci~. The polyvinylidene fluoride
diaphragms have recommended maximum service temperatures
up to expected temperatures for the generator. The
electrical and wetting properties of polyvinylidene
fluoride are suitable for the process and apparatus of the
pre~ent invention.
One ~uitable polyvinylidene ~luoride (PVDF) diaphragm
i8 marketed by Porex Technologie~ Corp. under tha
trademark POREX. The polyvinylidene fluoride (PVDF) is
marketed by Pennwalt Corp. under the trademark KYNAR. The
POREX diaphragm~ typically have an average pore ~ize of
about 25 micron~, a void volume of about 40%, and a
densi~y of about 1.05 grams per cubic centimeter. Ano~her
-12~ 2
suitable polyvinylidene fluoride (PVDF) diaphragm is one
marketed by Millipore Corporation under the trademark
DURAPORE.
Examples of other material~ having re~i~tance to
chloric acid are polytetrafluoroPthylene (PTFE),
fibergla~s, polyvinyl chloride (PVC), ~tyrene-
acrylonitrile, and ceramics. Mo~t hydrocarbons, such as
rubber, are readily at~acXed by strong oxidizing agents.
One porous polyvinyl chloride (PVC) diaphragm
commercially available is marketed by Microporous Products
Division of Amerace Corporation under the trademark
AMERSI~. Porous polytetrafluoroethylene (PTFE) diaphragm~
are commercially available from Millipore Corporation
under the trademark "F~UOROGARD", and from Norton Company
under the trademark "ZITEX". The wottability of
polytetra~luoroethylene (PTFE) or other ~luorocarbon~ can
be improved by treating the polytetra~luoroethylene or
fluorocarbon with a ~urfactant such a~ ZONYL (trademark,
E. I. DuPont de Nemours & Company). Alternatively, it i-
~
possible to compound into the polytetrafluoroethylene(P~FE), in the manufacture of the diaphragm, a wettable
acid resi~tant ~iller such as ground NAFION (trademark, E.
I. DuPont de Nemours ~ Company) or a ceramic such as
boro~ilicate glass. NAFION is the trademark for a
perfluorocarbon copolymer marXeted by E. I. DuPont de
Nemours & Co. It 19 also possible to improve the
wettability of polytetrafluoroethylene or other
fluorocarbon diaphragms by treating the diaphragms with a
-13~ 2 ~ ~ 7 ~ 2 2
N~FION solution. The diaphragms can also be porous
NAFION.
Where ~he separator 30 is a membrane, the back
migration of proton~ to the cathode can be minLmized by
using a membrane which has ion-selective ~ualities
necessary to ~uppress the flow of protons. An example of
one such me~brane i~ a perfluorinated membrane marketed by
Tosoh under the trademar~ TOSFLEX. Other mem~ranes
selective to the migration of oxyhalogen ions include a
fluoroethylene polymer/tetrafluoroethylene marketed by RAI
under khe trade designation "R4030", a tetrafluoroethylene
membrane maxketed by RAI under trade de~ignation "R1030"
and a membrane marketed by Asahi glass under the trade
designation "A~V".
The cation selective membrane 32 is commonly of the
type consi~ting of a cation exchange resin prepared in the
form of thin ~heets. A preferred membrane is a
perfluorinated copolymer having pendant cation exchange
functional groups. Broadly, these perfluorocarbons are a
copolymer of at least two monomers with one monomer being
~elected from A group including vinyl fluoride,
hexafluoropropylene, vinyliclene fluoride,
trifluoroethylene, chlorotrifluoroethylene, perfluoro
(alkylvinyl ether), tetrafluoroethylene and mixtures
thereof.. The second monomer often i~ selected ~rom a
group of monomers usually containing an SO2F or sulfonyl
fluoride pendant group. One suitable membrane i9 a
-14~ 2 2
perfluorocarbon membrane marketed by E. I. DuPont de
Nemours & Company under the trademark NAFION.
Since chloric acid is a strong oxidi~ing agent, many
materials used for separator 30 will be more ~u~ceptible
to corrosion by the chloric acid at higher temperatures
than at lower temperatures. Substantial electrical
resi~tance and heat build-up, particularly in the
separator 30 and acro~ ~he middle compartmenk 18, at high
current den~ities, can occur in the cell 20. An aspect of
the pre~ent invention is cooling the cell 20 so as to
maintain the separator 30 within a temperature range of
abouk 10C to about 40C, preferably at a temperature near
to room temperature of about 20C (68F).
The Figure discloses one method for cooling the cell
20. Referring to the Figure, a portion of the chloric
acid in tank 44 is withdrawn in line 50 through
circulation pump 52 to coil 54 o~ heat axchanger 56. The
he~t exchanger 1~ maintained at a temperature effective to
cool the acid in coil 54. The cooled acid i8 then
recirculated to the anolyte compartment 22 by means of
line 58. Line 58 contain~ a rotometer 60 which measures
the c~mount of acid recirculated, and a flow control val~e
62 to control the amount recirculated.
In combina~ion with acid cooling, the chloric acid
generator can compri~e a recirculation line 70 leading
from the middle feed compartment 18 of the cell 20 to a
coil 74 of hea~ exchanger 56, via recirculation pump 7~.
The coil 74 functions to cool at least a portion of the
-15- 2~ 2~
sodium chlorate in the middle feed compar~ment 18. The
cooled ~odium chlorate from coil 74 is retl~rn0d to the
middle feed compartment 18 by mean~ of return line 77
which feed~ into feed line 16 of the generatox. In the
e~bodLment illustrated in the Figure, the eombined flows
of line 77 and line 16 lead to the middle compartment 18
of cell 20 by means of line 76.
It i~ po~sible that some decomposition of chloric
acid can occur at elevated temperatures. If de3ired,
additional pro~ision can be made for cooling the chloric
aeid in aeid storage tank 44. Thi~ can be aecomplished by
mean~ of reeirculation line 64 which takes part of the
~low from the acid cooling coil 54 and returns it directly
~o ~he aeid storage tanX 44. ln this way, ~he temperature
of the chloric acid is maintained in a eooled state in
both the cell 20 and in the acid ~torage tank 44.
The cooling medium in the heat exehanger 56 can be
any conventional eooling medium. A chilled bath type heat
exchanger can be employed. Preferably, the recirculation
rate~ and rates of eooling the chloric acid and
reciraulated chlorate solution are effeetive to maintain
both ~he ehlorate ~olution and anolyte (chlorie acid) at a
temperature in the range of about 10 to about 40C, more
preerably about 20C (room temperature).
~he eathode 28 can be any ~uitable material
eonventionally employed a~ a cathode. Preferred such
materials are a niekel, steel or titanium expanded metal
me~h ox sheet. ~lternatively, the eathode ean bg a gas
-16- 2 ~J~
diffusion electrode such as disclosed in prior Patent No.
4,377,496 entitled ~Ga~ Diffusion Electrode and Proces~
A gas diffusien electrode, as di~closed in these patents,
changes the cathode reaction to eliminate production of
hydrogen while continuing ~o produce hydroxyl gxoups.
Therefore, hydrogen is no longer evolved. The cell
voltage is also ubstantially reduced resulting in a lower
co~t of electrical energy, as is reported in Patent No.
4,377,496. The disclosure of this prior patent is
incorporated herein by reference.
The anode can ~e either chlorine evolving or oxygen
evolving dapending on the electrolyte com~osition. A
preferred anode is dimensionally stable. That is, th~
thickne~s of the anode does not decrease significantly
during use. Such anodes usually comprise a film-forming
valva me~al ~ubstrate, such as titanium, tantalum,
zirconium, niobium tungsten, and alloys thereof, which has
the capacity to conduct an electrolyte current in the
cathodic direction and to re~ist the passage o~ current in
the anodic direction. These metals are also resistant to
corrosion in tha electrolytes at conditions u~ed within an
electrolytic cell. A preferred valvQ metal, based on
C08t, availability, and electrical and chemical
propertiss, i~ titanlum. It i5 well known that in the
anodic direction, the valve metals passivate, that is the
resistancQ o~ the valve metals to the passage of current
goes up rapidly due to the formation of an oxide layer
thereon. It is therefore customary to apply
-17~
electrs~chemically active coatings to the valve metal
sllbstrate. The electrochemically active coatings have the
capacity to continue to conduct current to the
electrolyte, for example by the evolution of oxygen, over
5 long period~ of time without becoming pas~ivated. Such
coatings are those provided from platinum or other
platinum group metals or they can be repre~ented by active
oxide coatings such as platinum group metal oxide~,
magnetite, ferrite, ~pinels, e.g. cohalt oxide, or mixed
metal oxide coatings. The coatings also preferably
c4ntain at least one oxids o~ a valve metal with at least
one oxide of a platinum group metal including platinum,
palladium, xhodium, iridium, and ruthenium or mixtures
thereof and with other metals.
An example of one such dimensionally stable oxygen
evolving anode i8 a titanium sub5 trate which has been
coated with a precious metal oxide and valve metal oxide
coating. This anode is marketed by the assignee of $he
pre~ent application under the trade designation EC-600.
The anode 24 may be in the form of a sheet or expanded
metal mesh. Examples of suitable chlorine evolving anodes
that can be used in the present invention are di~closed in
U.S. Patents No~. 3,632,498; 3,751,296; 3,778,307;
3,840,443 and 3~,933,616.
For the production of perchloric acid, a lead oxide
or platinum anode should be used.
-78~
In operation, the flow diagram for the chloric acid
generator i5 essentially the same where the separator 30
is either a diaphxagm or a membrane.
Where the separator 30 is a diaphragm, the
concentration of the sodium chlorate solution in feed line
16 can be a function primarily of the concentration of the
acid de~ired in lines 42, 46. Chloric acid, at xoom
temperature can decompose spontaneously at a concentration
above about 3.6N. To produce chloric acid at a
concentration less than 3.6N, it is necessary to feed to
the middle compartment 18 a sodium chlorate ~olution
having a normality le~s than about 4. The concentration
of the chloric acid in lines 42, 46 is preferably above
about 0.1. This requires a sodium chlorate feed having a
normality more than about 0.1. Thus, the concentration of
the ~odium chloràte feed preferably is in the range of
about O.lN to about 4N.
Howevert an alternate method of operation is to ~eed
a more concentrated chlorate solution to middle
compartment 18, but to d11ute the acid in lines 42, 46
with water 80 that the acld does not decompose. The
alternate method has ~he ad~antage that it reduces the
coll voltage re~uired, ~ince ~he conductivity of chlorate
~olu~ion is higher at higher concentrations.
In thi~ alternate method of operation, the
concentration of the sodium chlorate ~olution in feed linQ
16 ~hould be leas than that at which precipitation of
sodi~m chlorate occurs. Sodium chlorate ha~ a maximum
-19- 2~'3~
solubility of 7.4M at 0C and 10.7~ a~ 23~C. The middle
compartment 18 lo es water with the migr~tion of sodium
ion3 to the cathode 28, and due to inefficiencies. The
concen~ration of the chlorate ~olution in feed line 16
should take into account this loss of water, and thus can
be near but should be ~omewhat less than the solubility
lLmits, e.g., 10.7M, assuming the temperature in the cell
to be about room temperature. This alternate method is
partlcularly applicable where the separator 30 is a
membrane.
For the production of chlorine dioxide in an
electrolytic cell as disclosed in prior Patent No.
4,798,715, the chloric acid concentration pre~erably is
above about 1.5, more preferably above about 2. This
requires tha~ the sodiurn chlorate solution feed in line 16
pre~erably ha~e a nonnality of at least about 2.
The feed rate o~ the sodiu~l chlorate solution in line
16 is a function of the amount of chlorate ion in the
product lines 42, 46, the concentration of the sodiurn
chlorate solution, and the water balance in the generator.
The f~ed rate and concentration of the ~odi~n chlorate
~olution can both be ad~usted depending upon chlorate ion
and water balances. An aspect of the pre~ent in~ention is
that the feed of the ~odlum chlorate solution in lins 16,
when the separator 30 iB a diaphra~n, suppresses the back-
migratlon of protons through the diaphra~n 30 to the
cathode 28~ ~his is important where an acid product of
high normality is desired. The higher the norrnality o~
-20- ~J7~22
the acid, the higher the proton content in the anode
compartment 22~ and the greater the likelihood of back-
migration of protons ~o the cathode 28.
Preferably~ ~he generator of the present invention is
operated at a relatiYely high current density, to reduce
capital costs. Satisfactory results can be obtained with
current densities in the range of about 2-5 kiloamp~ per
s~uare m~tex.
The following Examples illustrate the present
invention.
Example 1
A chloric acid generator 20 a~ shown in the Figure
was operated to convert ~odium chlorate into ~odium
hydroxide and chloric acid. The generator 20 employed an
expanded mesh titanium anode 24 having a precious metal
oxi.de coating marketed by the assignee of the present
application under the trade de~ignation EC-600. The
cathode 28 wa~ titanium mesh. The diaphragm 30 was a
~heet o~ porous polytetra~luoroethylene ("Kynar") marketed
by Porex Technologies Corp. under the trademark Porex.
The diaphragm typically has an average void volume of
about 40~ and a density of about 1.05 grams per cubic
centimeter. Average pore size is about 25 microns. The
diaphragm ha~ a service temperature up to about 300F
(149C). The membrane 32 was made of NAFION 324
(trademark, B. I. DuPont de Nemours & Co.). The generator
wa~ constructed of chlorinated polyvinylchloride. The
men~rane 32 and diaphragm 30 had an active area o~ 20
-21- 2~3~2~
square centimetexs. The middle compartment 18 gap was
0.25 inches (0~64 cm).
The generator was operated under the following
conditionss
Cuxrenk Density 4 kiloamps per square meter
Volkage 8.5 volts
The following Table 1 shows measured temperatures,
concenkrations~ and flow rates at various points in the
generator. The chloric acid and sodium hydroxide product
concentrations were measured to determine current
efficiency at the anode and at the cathode.
~r r~ D
D ~ 0
o o o
~ ~ ~~ ~D 2 ~
o~D
U~ C~ o
. . .
U ~D rAI M O O
NO
~rI U
u~ ~
'~I O ~ ~ o. .
~a ~ o ~ OOO-DO
cn ~ P. ~
~D
~1~ ~ ~ O
o
~! ~ a~o o
~ ~ ~1
h~ UIn
O ~ ~ o
o ~ o
_lO
N
~o ~
~ ~ o~Co
O ~ ~ U7~ooo
N
C) U
I O Zæ~z
~ . .
h 1~ U 1!4
U~ o
_~3_ 2~
Table 1 shows that both cooled acid and cooled salt
were recircula~ed from the heat exchanger 56, in lines 64
and 77, re~pectively, at 16C. This maintained the
anolyte in line 42 at about 26.5C ~the cell 20 nd
diaphragm 30 being at about the same temperature), and the
chloric acid product in line 46 at about 18.6C. The
concentration of the chloric acid product obtained in line
46 was 2.06N. Cathode and anode current efficiencies were
determined by dividing the actual production rate~ by the
theoretical rates and multiplying by 100. Theoretical
rates were ~a~ed on amperage.
The following cell efficiencies were obtained:
Cathode (NaOH) CE~ 29.1
Anode lHC103) CE~ 23.6
The generator was disassembled at the end of 200
hours on line and the Porex diaphragm wa~ in excellent
condition. The generator has ~een succe~sfully operated
for longer period~.
~ample 2
The generator described in Example 1 was u~ed. It
was operated at the same current density of 4 Xilo~mps pex
squaxe meter as ~n Example 1. The voltage drop in the
call was 7.5 as compared to 8.5 in Ex~mple 1. The
following ~abla 2 gives mea~ured temperature,
concentrations and flow rate~ at various point3 in the
generator. ~he ma~or difference of operation from Example
1 was ths absence of cooled recirculated chlorate ~olution
in line 77 from heat exchanger 56. Thus, the cell was
-24- 2~ 2~5
operated at a higher temperatnre of about 59C, as
evidenced by th~ temperature in anolyte product line 42.
. .
~: o ~
" ~ a) ~ _, ,, O o
r~l O
;o
O~ o o
~.~ Ln o~
a~
r
'~ U ~ ~D
~ x ~ u
~r~ o~ ~ o . .
` o
~r
~ ~o
c~ ~ u
~ ~ ~ o oo
~ ~ a) ~ cO Oo
~ ~ o ~
~ ~ ~rl ~ _~_100
E~
r~
r~
~ o
a
o
~1 V In
~ ~ O
o a~ 0 u~ ~ooo
~:: h~ ~'1
o ~
2;V0
0
Ul ~1 O _l
El C~ ~,
In O
: ', . . .
..
. ... .
.,
.
.
.
. . . ~
2 ~ 2
-~6-
The generator functioned for only 36 hsurs before
exces~ive corro~ion of the diaphragm 30 occurred. Current
efficiencies were determined~ as followss
Cathode (NaOH) CE% 28.6
Anode (HC103) CE% 11.3
~he co~centration of the chloric acid in line 46 wa~
only 1.8N. Thus, although the c811 efficiency increased
slightly for sodium hydroxide production when compared
with Example 1, the cQll efficiency wa~ significantly less
~or acid production. The comparative data of Example 1
illu~tr~te~ the importance of maintaining the generator
20, in the production of oxyhalogen acids, at a relati~ely
low temperatura. Example 1 also demonstrate~ that by
sufficiently cooling the cell 20, the life of the
diaphra~m 30 can be 3ignificantly extended.
One principle use for chloric acid is as a precursor
in the manufacture o chlorine dioxide (Cl02). ~hlorine
dioxide is a strong oxidizing agent. Some of the market
areas or chlorine dioxide are: water treatment, pulp and
paper processing, flour processing, municipal waste
treatment, petroleum well in~ection, crop and meat
storage, and bleaching such materials as textiles, oils,
shellacs, varnishes, waxes, and straw products.
For water treatment, the use of chlorine dioxide is
particularly attractive a~ environmental regulations are
being tight~n~d concerning the production of chlorinated
organics and trihalomethanes ~TH~8). Trihalomethanes are
signi~icantly reduced with the use of chlorine dio~ide
in~tead o chlorina as a reactant.
. .
-27~
One chlorine dioxide generAting proces~, known as the
MathiQson process, reacts sulfur dioxide with ~odium
chlorate (NaCl03) and sulfuric acid to produce chlorine
dioxide. This reaction also produce~ an undesirable
5 sodium bi~ulfate ~alt cake (NaHSO4). When this process is
used in the pulp and paper industry, excess salt cake
causes an im~alance for mills ~rying to reduce sulfur
emissions.
An advantage of the chloric acid process of the
present in~ention is that it can be integrated into the
production of chlorine dioxide, for the pulp and paper
indu~try, without ~he production of unwanted sodium
bisulfate salt cake. When the separator 30 of the
electrolytic cell of the pre~ent invention is an anionic
membrane, a substantially pure chloric acid stre~m is
produced in lins 46. The chloric acid can then chemically
react with sulfur dioxide to produce a chlorine dioxide
product ~tream, and a by-product stream which i9 sulfuric
acid. Sulfuric acid, in contraqt with sodiwn bisulfate,
is a useful by-product. If a diaphragm is used as the
separator 30, in place of an anionic membrane, some sodium
chlorate rom the middle feed compartment 18 of the
chloric acid generator flows into the anode compar~ment 22
making a mixture, in product line 46, of chloric acid and
~odi~n chlorate. The sodium chlorate reacts with sulfur
dloxide and sulfuric acid, a~ in the Mathie~on process, to
~orm some salt cake. However, the sulfur dioxide can be
reacted preferentially with the chloric acid and sulfuric
-28- 2~3~
acid will be produced with very little salt cake. ~he
sodium bisulfate salt ~hat i5 produced can be sepàrated
from the sulfuric acid and recycled to the feed
compartment 18 of the chloric acid generator of the
present invention. In the chloric acid yenerator, the
sodium bisulfate is converted to additional sodium
hydroxide and sulfuric acid.
Processes similar to the Mathieson process, which use
sodium chlorate and sulfuric acid as feed components, are
known as the Solvay, R-2 and SVP proce~ses. In the Solvay
process, sodium chlorate i~ reacted wi-th sulfuric acid and
a reducing agent such as methanol to produce chloxine
dioxide. Thi~ proce~s produce~, as a by-product, sodium
sulfate (Na2S04) which is of little value. By the present
invention, chloric acid can be reacted directly with a
reducing a~ent, such as methanol, to produce chlorine
d~oxlde without producing the ~alt by-product. In the R-2
and SVP processes, sodium chlorate, sodium chloride, and
~ul~uric acid are reacted to produce chlorine dioxide.
The processes also produce chlorine gas (C12) and sodium
sulfate as by-products. The chlorine gas is a
particularly undesirable by-product.
The production of chlorine dioxide (C102) contaminated
with chlorin~ gas (C12) is also di~closed in two patents,
2S No. 4,806,215 "Combined Process for Production of Chloxine
Dioxide and Sodium Hydroxide", and No. 4,853,0g6
"Production of Chlorine Dioxide in an Electrolytic Cell".
~he first patent is a three compartment cell which
~37~2?.
produces NaOH, Cl2 and Cl02 from HC1 and ~aCl03. The
second paten~, in Fig. 3, de~eribes a proeess for
reducing, but not eliminating, the chlorine that
contaminates the chlorine dioxide.
In the pre~ent invention, th2 ehloric acid in line 46
can be reduced directly to chlorine dioxide by feeding the
chloric acid to an eleetrochemieal cell as disclosed in
U.S. Patent No. 4~798,715, discuRsed abo~e, assigned to
the a~signee of the present application. An advantage of
this reduction i~ that the ehlorine dio~ide is produced
completely free of chlorine by-product ga~. The
di~closure of U.S. Patent No. 4,798,715 i9 ineorporated by
referenee herein.
Other u~es ~or chloric acid are as a catalyst in the
polymerization of acrylonitrile, and in the production of
perchloric acid. In Kirk Othmer, Vol. 5, page 656, it i5
disclosed that chloric acid can be electrolytically
oxidized to perehloric aeid in an electrochemical proce~s.
Perchlorie acid uses are in medicine, in analytical
chemistry, a~ a eatalyst in the manufacture of various
esters, as an ingredient of an electrolytic bath in the
depo~itioll of lead, in electro-polishing, and in the
manufacture of Qxplosive~. The conventional mathod for
produeing ammonium perehlora~e, diselosed in Kirk Othmer,
Vol. 5, pg~ 660, is to reaet sodium perchlorate, ammonia,
and hydxoehloric acid to produce ammonium perchlorate and
by~produet sodium chloride:
-30- ~3~
NaC10~ + NH3 + HCl ~ NH4~104 ~ NaCl (8)
In accordance with the present invention, æodium
chlorate can be converted to chloric acid and odium
hydroxide with the three eompartmPnt cell as described
above in reactions 1 through 7. The chloric acid is then
eleetrolytieally oxidized to perchlorie aeid using the
electrochemical proees~ described abovs and on pg. 656,
Vol. S of Kirk Othmer. The perchloric acid ean then be
reaeted with ammonia to produee ammonium perchlorate:
NH3 J~ HCl 04 ~ N~I4 ~10~ ( 9 )
A~ an altexnative, sodium perchlorate ean be
eon~erted to perehlorie aeid and sodium hydroxide in the
three compartment aeid generator. The perchloric acid can
then ba reaeted with ammonia to produee ammonium
perehlorate a~ in reaetion (9).
From the above deseription of a preferred embodiment
o~ the invention, those skilled in the art will perceive
improvements, ehanges and modification~. Such
improvements, changes and modifieations within the skill
of the art are intended to be eovered by the appended
elalm~.
