Note: Descriptions are shown in the official language in which they were submitted.
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Process-for producing alkali metal chlorate
The present invention relates to a process for producing alkali metal
chlorate, as
well as an electrolytic cell and a plant for carrying out the process. The
invention further
relates to the use of the electrolytic cell and the plant for the production
of alkali metal
chlorate and/or chlorine dioxide.
Background of the invention
Alkali metal chlorate, and especially sodium chlorate, is an important
chemical in
the cellulose industry where it is used as a raw material in the production of
chlorine
dioxide, which is an important bleaching chemical for cellulose fibres. Alkali
metal
chlorate is conventionally produced by electrolysis of alkali metal chlorides
in open non-
divided electrolytic cells provided with hydrogen evolving cathodes. The
overall chemical
reaction taking place iri such cells is MeCI + 3Hz0 -~ MeCl03 + 3H2, where Me
is an
alkali metal. This reaction has a cell voltage of 3 V.
In the past, also electrolytic cells provided with oxygen consuming gas
diffusion
electrodes have been attempted for the production of chlorate. The English
language
Chemical Abstract (AN 1994:421025) of Chinese Patent Application No.1076226
discloses such cells for the preparation of sodium chlorate. The gas diffusion
electrode
reduces oxygen supplied to the gas chamber adjacent to the gas diffusion
electrode. The
reduction reaction taking place at the gas diffusion electrode (gas diffusion
cathode) is
2H20 + Oz + 4e ~ 40H-. The oxidation reaction taking place at the anode is
2C1~ ~ CIz
+2e . The cell voltage for the overall chemical reaction in the gas diffusion
electrode cell is
about 2 V, which implies that considerable operation costs can be saved by
replacing the
above described hydrogen evolving cathode with a gas diffusion electrode
acting as
cathode.
Operation of a cell as disclosed in English language abstract of Chinese
Patent
Application No.1076226, however, will instantly lead to poisoning of the gas
diffusion
electrode since the reaction products HCIO , CIO', and C103 formed at the
anode will
diffuse freely in the electrolyte and undesired side reactions will inevitably
take place at
the gas diffusion electrode according to the formulas below:
HCIO + 2e ~ CI- + OH~ (1)
CIO-+ Hz0 + 2e ~ CI- + 20H- (2)
CI03 + 3Hz0 + 6e -~ CI- + 60H- (3)
In many alkali metal chlorate processes, alkali metal chromates are employed
to
suppress reactions 1-3. However, alkali metal chromates can also have a
negative impact
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on the gas diffusion electrode which quickly will deactivate upon contact with
the
chromate ions.
Production of chlorate may require considerable amounts of hydrochloric acid
and alkali metal hydroxide, which also implies a considerable cost.
Furthermore, the
handling of these chemicals is complicated because of the rigorous safety
requirements
involved in transportation, storage and dosage.
An object of the present invention is to overcome the problems referred to
above
and at the same time provide an energy-efficient electrolytic process for the
production of
alkali metal chlorate. A further object of the invention is to provide a
process which makes
a large portion of~externally added pH-adjusting chemicals superfluous to the
process.
The invention
The invention relates to a process for producing alkali metal chlorate in an
electrolytic cell divided by a cation selective separator into an anode
compartment in
which an anode is arranged and a cathode compartment in which a gas diffusion
electrode is arranged. The process comprises introducing an electrolyte
solution
containing alkali metal chloride into the anode compartment and an oxygen-
containing
gas into the cathode compartment; electrolysing the electrolyte solution to
produce an
electrolysed solution in the anode compartment, electrolysing oxygen
introduced into the
cathode compartment resulting in the formation of alkali metal hydroxide in
the cathode
compartment; transferring the electrolysed solution from the anode compartment
to a
chlorate reactor to react the electrolysed solution further to form a
concentrated alkali
metal chlorate electrolyte.
In this process, the same space of the cathode compartment functions both as a
gas chamber for oxygen-containing gas and a chamber for alkali metal hydroxide
production.
According to a preferred embodiment, the gas diffusion electrode is arranged
on
the cation selective separator to minimise the ohmic resistance.
According to another preferred embodiment, the process is run in a cell in
which
the gas diffusion electrode divides the cathode compartment into a gas chamber
on one
side of the gas diffusion electrode and an alkali metal hydroxide chamber on
the other
side thereof confined between the gas diffusion electrode and the cation
selective
separator. The process comprises introducing an electrolyte solution
containing alkali
metal chloride into the anode compartment, introducing an alkali metal
hydroxide solution
into the alkali metal hydroxide chamber and an oxygen-containing gas into the
gas
chamber; thereby electrolysing the electrolyte solution to produce an
electrolysed solution
in the . anode compartment, electrolysing oxygen ~ introduced into the gas
chamber
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resulting in the formation of additional alkali metal hydroxide in the alkali
metal hydroxide
chamber; transferring the electrolysed solution from the anode compartment to
the
chlorate reactor to react the electrolysed solution further to form a
concentrated alkali
metal chlorate electrolyte.
Preferably, the electrolytic cells may be pressurised up to about 10 bar,
preferably up to about 5 bar. This may be achieved by applying an appropriate
overpressure of an oxygen-containing gas in the gas chamber and an inert gas
in the
anode compartment.
The cation selective separator, which preferably is substantially resistant to
chlorine and alkali metal hydroxide, enables efficient production of an
electrolysed
solution and concentrated alkali metal hydroxide with a low content of
chlorate ions and
chloride ions in the alkali metal hydroxide chamber. The cation selective
separator
preferably is a cation selective membrane. Suitably, the cation selective
membrane is
made from an organic material such as fluorine-containing polymer of e.g.
perfluorinated
polymers. Other suitable membranes may be made of polyethylene, polypropylene
and .
polyvinyl chloride sulphonated, polystyrene or teflon-based polymers or
ceramics. There
are further commercially available membranes suitable for use such as
NafionT"" 324,
NafionT"" 550 and NafionT"' 961 available from Du Pont, and FlemionT""
available from
Asahi Glass.
Suitably, a support is arranged on the anode and/or cathode side to support
the
cation selective separator.
The anode can be made of any suitable material, e.g. titanium. The anode
suitably is coated with e.g. RuOZ/Ti02 or Pt/Ir. Preferably the anode is a
DSAT"'
(dimension stable anode) which may have an expanded mesh substrate.
The gas diffusion electrode may be a weeping gas diffusion electrode, a
semihydrophobic gas diffusion electrode or any other gas diffusion electrodes
such as
those described in European patent applications No. 01850109.8, No.00850191.8,
No.00850219.7 and US patents US 5,938,901 and US 5,766,429. There is no
particular
restriction on the gas diffusion electrode. For example, a gas diffusion
electrode
comprising only a reaction layer and a gas diffusion layer may be used. The
gas diffusion
layer may be made from a mixture of carbon and a PTFE resin. The reaction
layer
suitably has a content of a hydrophobic material such as fluorocarbon
compounds in
order to retain proper water repellency and a hydrophilic property. In
addition, a protective
layer for more effectively preventing the gas diffusion layer from becoming
hydrophilic
may be formed on the surface of the gas diffusion layer.
The process of the invention can be described as being cyclic since in a first
step, an electrolyte solution comprising alkali metal chloride solution is
passed to an
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electrolytic cell wherein at least a part of the chloride is electrolysed to
form inter alia
hypochlorite and chlorate. The electrolysed solution is suitably withdrawn to
a
conventional chlorate reactor, e.g. such as described in US 5,419,818 for
further reaction
to produce chlorate. The chlorate reactor may comprise several chlorate
vessels.
Chlorate electrolyte can then be transferred to a crystalliser, where solid
alkali metal
chlorate may be separated by means of crystallisation while the mother liquor
containing
inter alia unreacted chloride ions, hypochlorite, chlorate may be recirculated
to the
electrolytic cell for further electrolysis. Also an alkali metal hydroxide
scrubber may be
used as a chlorate reactor in which chlorate can be formed by reacting alkali
metal
hydroxide supplied thereto from e.g. the alkali metal hydroxide chamber and
formed
chlorine gas withdrawn from the anode compartment. According to one preferred
embodiment, both an alkali metal hydroxide scrubber to which chlorine gas is
supplied
and a chlorate reactor supplied with electrolysed solution are used
simultaneously in the
process.
A concentrated chlorate electrolyte may contain from about 200 to about 1200
g/I, preferably from about 650 to about 1200 g/I.
The electrolyte solution introduced into the anode compartment suitably
contains
at least some chlorate, suitably in the range of from about 1 to about 1000,
preferably
from about 300 to about 650, and most preferably from about 500 to about 650
g/litre
calculated as sodium chlorate. Suitably, the electrolyte solution has a
concentration of
chloride ions in the range from about 30 to about 300 g/I, preferably from
about 50 to
about 250 g/I, and most preferably from about 80 to about 200 g/l, calculated
as sodium
chloride.
According to another preferred embodiment, the chlorate concentration in the
electrolyte solution introduced into the anode compartment is from about 1 to
about 50
g/l, preferably from about 1 to about 30 g/I.
Suitably, most of the chlorine gas generated in the anode compartment is
dissolved in the electrolysed solution. Dissolved chlorine spontaneously
undergoes partial
hydrolysis to form hypochlorous acid according to the formula:
CIZ +H20-~ HCIO+HCI
The hypochlorous acid is dissociated in the presence of a buffer or hydroxide
ions (B-) to
hypochlorite according to the formula
HCIO+B- ~ HB+CIO-
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The pH of the electrolysed solution in the anode compartment preferably is
above 4 in
order to stimulate the dissolution of chlorine. The electrolysed solution
containing chlorine
and/or hypochlorous acid can be transferred to the chlorate reactor. The pH of
the
electrolyte solution supplied to the anode compartment suitably ranges from
about 2 to
5 about 10, preferably from about 5.5 to about 8. The concentration of alkali
metal
hydroxide in the alkali metal hydroxide chamber suitably ranges from about 10
to about
500, preferably from about 10 to about 400, more preferably from about 20 to
about 400,
and most preferably from about 40 to about 160 g/I calculated as sodium
hydroxide. The
alkali metal hydroxide produced can be directly withdrawn or recirculated to
the alkali
metal hydroxide chamber for further electrolysis until the desired
concentration has been
achieved. The produced alkali metal hydroxide can be used for alkalisation of
the chlorate
electrolyte in the chlorate reactor and before crystallisation of chlorate.
The alkali metal
hydroxide can also be used for precipitating hydroxides of alkaline earth
metals, iron and
aluminium for purification of fresh alkali metal chloride used in the
electrolyte solution.
Alkali metal hydroxide can also be used for chlorine absorption from process
vent from
the chlorate reactor and as earlier stated, for absorption of chlorine gas
withdrawn from
the anode compartment for direct production of alkali metal chlorate in an
alkali metal
hydroxide scrubber.
According to one preferred embodiment, alkali metal chromate is added to the
electrolyte solution as pH buffering and to suppress undesired reactions.
Chromate may
be added in an amount from about 0.01 to about 10 g/I, preferably up to about
6 g/I.
According to another preferred embodiment, no chromate is added to the
electrolyte
solution.
The temperature in the electrolytic cell suitably ranges from about 20 to
about
105 °C, preferably from about 40 to about 100 °C.
The chlorate is preferably produced by a continuous process, but a batchwise
process can also be used. The process of the present invention may
advantageously be
integrated in the production of chlorine dioxide using either chlorate
electrolyte or alkali
metal chlorate salt as raw material.
The invention also relates to an electrolytic cell for producing alkali metal
chlorate comprising a cation selective separator dividing the cell into an
anode
compartment in which an anode is arranged and a cathode compartment in which a
gas
diffusion electrode is arranged. An inlet for electrolyte solution and an
outlet for
electrolysed solution are provided in the anode compartment, and an inlet for
introducing
oxygen-containing gas is provided in the cathode compartment.
According to one preferred embodiment, the gas diffusion electrode is arranged
on the separator to minimise the ohmic resistance.
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According to another preferred embodiment, the gas diffusion electrode divides
the cathode compartment into a gas chamber on one side of the gas diffusion
electrode
and an alkali metal hydroxide chamber on the other side thereof confined
between the
gas diffusion electrode and the cation selective separator. An inlet and an
outlet for alkali
metal hydroxide solution is provided in the alkali metal hydroxide chamber.
Preferably, the cation selective separator may be any of the cation selective
membrane described above. Preferably, also an outlet for oxygen-containing gas
is
provided in the gas chamber. Preferably, a separate outlet for chlorine gas is
provided in
the anode compartment and/or in the chlorate reactor. Chlorine gas may also
leave the
anode compartment via the outlet for electrolysed solution. According to one
embodiment
of the invention, the anode compartment is not provided with a separate outlet
for
chlorine gas.
The construction of the above described embodiments of electrolytic cells
preferably is so robust that the cells can withstand electrolyte flows and
other physical
conditions that are conventional in the art of chlorate production.
Preferably, the cell is
constructed to withstand a flow in the anode and/or the cathode compartment
preferably
of at least about 0.5 m3h~'m-2, more preferably at least about 1 m3h~'m-2,
even more
preferably at least about 3 m3h~'m-2, and most preferably at least about 5 m3h-
'm-z.
Preferably, also inlets and outlets are so designed to cope with these
conditions.
The invention further concerns a plant comprising an electrolytic cell as
described above in which the outlet of the anode compartment is connected to a
chlorate
reactor, suitably via the outlet for electrolysed solution. The chlorate
reactor may in turn
be connected to a crystalliser for transferring chlorate electrolyte which can
be
precipitated in the crystalliser and separated from the mother liquor. The
chlorate reactor
is suitably connected to the anode compartment such that a part of the alkali
metal
chlorate electrolyte can be recirculated to the anode compartment.
The plant suitably comprises storage vessels for alkali metal chloride and/or
electrolyte treatment agents such as alkali metal chromate.
The reactor may also be an alkali metal hydroxide scrubber to which chlorine
gas can be withdrawn from the anode compartment and reacted with alkali metal
hydroxide to produce alkali metal chlorate. Suitably, an alkali metal
hydroxide container is
connected to the alkali metal hydroxide chamber for supply and circulation of
alkali metal
hydroxide. The container, suitably a tank, can be continuously fed with water
and
recirculated alkali metal hydroxide to adjust the concentration of the alkali
metal
hydroxide concentration fed to the alkali metal hydroxide chamber. The outlet
of the alkali
metal hydroxide chamber may be connected to several units in the chlorate
plant for
alkalisation, e:g..to the inlet for alkali metal hydroxide scrubber or other-
chlorate reactor, .
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or to the crystalliser for transferral of alkali metal hydroxide. Preferably,
both an alkali
metal hydroxide scrubber and a conventional chlorate reactor receiving
electrolysed
solution are arranged in the plant.
The invention also relates to the use of the electrolytic cell and the plant
comprising the electrolytic cell for the production of alkali metal chlorate,
preferably
sodium chlorate, but also e.g. potassium chlorate. The sodium chlorate may be
produced
as solid sodium chlorate salt or sodium chlorate electrolyte for production of
chlorine
dioxide, preferably by means of an on-site chlorine dioxide generator.
Brief description of the drawings
Fig. 1 schematically illustrates an electrolytic cell according to one
embodiment
of the invention. Fig. 2 schematically illustrates a plant for producing
sodium chlorate
according to the invention.
Description of embodiments
Fig. 1 shows an electrolytic cell 1 for the production of sodium chlorate. The
cell
comprises an anode compartment 2 in which an anode 2a is arranged, a cation
selective
membrane 3, a cathode compartment 5 divided by a gas diffusion electrode 5a
into an
alkali metal hydroxide chamber 4 and a gas chamber 6. Inlets and outlets for
sodium
chloride and electrolyte in the anode compartment 2 are illustrated by arrows
7. A
separate outlet for chlorine gas may be provided in the anode compartment (not
shown).
Inlets and outlets for sodium hydroxide in the alkali metal hydroxide chamber
are
illustrated by arrows 8. Inlets and outlets for oxygen in the gas chamber 6
are illustrated
by arrows 9. A further arrangement of the electrolytic cell (not shown)
comprises a two
compartment cell, i.e. without a separate gas chamber, in which the gas
diffusion
electrode is arranged directly on the cation selective separator.
Fig. 2 schematically illustrates a plant for producing sodium chlorate. An
electrolyte solution 7 comprising sodium chloride and chlorate electrolyte
obtained from
chlorate reactor 10 is introduced into the anode compartment 2 of an
electrolytic cell 1.
The electrolyte solution is electrolysed to form an electrolysed solution
which is pumped
through the anode compartment 2 to the chlorate reactor 10 where the formation
of
chlorate continues. The chlorate electrolyte in the chlorate reactor 10 is
alkalised with
alkali_metal_k~ydroxide_pcoduced_in~he_cath~ode chamber 4 before it is
withdrawn to the
crystalliser 12, where sodium chlorate is crystallised. Chlorate electrolyte
may also be
withdrawn to a reaction vessel (not shown) for production of chlorine dioxide.
In the
anode compartment 2, some amount of chlorine gas may be produced during the
electrolysis. The formed.chlo~ine gas may be transferred to a sodium hydroxide
scrubber
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11 where the chlorine gas is absorbed in sodium hydroxide which results in the
formation
of sodium chlorate. The sodium hydroxide scrubber may thus also work as a
sodium
chlorate reactor. Sodium chloride may continuously be added to the electrolyte
solution 7
before introduction into the anode compartment. Water may be continuously
added to a
'sodium hydroxide tank 4a to maintain an appropriate concentration of the
sodium
hydroxide passing through the sodium hydroxide chamber. Sodium hydroxide may
be
used for alkalisation also in the crystalliser 12.
Example 1
The experiment was run as a batch process with a start volume in the reactor
vessel of 2 litres. The start concentration of the electrolyte in the anode
compartment was
110 g of NaCl/l, 550 g of NaCl03 and 3 g Na2Cr20, /I. This solution was pumped
through
the anode compartment of an electrolytic cell at a rate of 25 I/h
corresponding to an
approximate linear velocity across the anode of 2 cm/s. Sodium hydroxide
solution of a
concentration of 50 g/I was pumped through the cathode compartment at linear
velocity
across the cathode of 2 cm/s. An excess of oxygen gas was fed to the gas
compartment.
The cell was a laboratory cell containing an anode compartment with a
dimensionally
stable (DSA) chlorine anode and a cathode compartment with a silver plated
nickel wire
gas diffusion electrode loaded with uncatalyzed carbon (5-6 mg/cm2). The area
of each
electrode was 21.2 cmz. The anode and cathode compartments were separated by a
cation selective membrane, Nafion 450, and the distance between each electrode
and
the membrane was 8 mm.
Solid sodium chloride was added to the reactor vessel and fed to the anode
compartment, at a rate of 0.71 g~Ampere-'h-' to keep the concentration of
sodium chloride
constant in the reactor vessel. Water was added to the cathode compartment at
a rate of
0.5 ml~Ampere-'min-' to keep the concentration of sodium hydroxide constant.
Electrolysis was conducted at a temperature of 70 °C in the
electrolysis cell, a current
density of 0.2-3 kA/m2 and at a pH of 6.2. The current was varied between 0.5-
6.3 A. The
electrolysis was run for 30 h.
The current efficiency for the electrolysis was 92% calculated on the
hydroxide
ions produced in the cathode compartment. The current efficiency was
calculated as the
quotient between actual and theoretical maximum production of sodium
hydroxide. The
production of hydroxide ions was determined by analyzing the hydroxide ion
content in
the catholyte and multiplying it by the collected flow. The production of
NaCl03 was
calculated from the total amount of NaCl03 formed in the anode compartment
throughout
_ the electrolysis. The.. estimated current efficiency for chlorine formed was
close to 100%.
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The current efficiency for the chlorate production was 95% calculated as the
quotient
between actual recovered and theoretical maximal production of sodium
chlorate.
Example 2
The experiment was run as a batch process with a start volume in the reactor
vessel of 2 litres. The start concentration of the electrolyte in the anode
compartment was
110 g of NaCI/I, 550 g of NaCl03, and 3 g Na2Cr20~/l. This solution was pumped
through
the anode compartment of an electrolytic cell at a rate of 25 I/h
corresponding to an
approximate linear velocity across the anode of 2 cm/s. An excess of oxygen
gas was fed
to the gas compartment. The cell was a laboratory cell containing an anode
compartment
with a dimensionally stable (DSA) chlorine anode and a cathode compartment
with a gas
diffusion electrode made of silver, PTFE and carbon on a silver screen). The
area of each
electrode was 21.2 cm2. The anode compartment and the gas diffusion electrode
were
separated by a cation selective membrane (Nafion 450). The distance between
the anode
and the membrane was 8 mm. There was no distance between the membrane and the
gas diffusion electrode. Solid sodium chloride was added to the reactor vessel
and fed to
the anode compartment at a rate of 0.71 g*A-'h-' to keep the concentration of
the sodium
chloride constant in the reactor vessel. Electrolysis was conducted at a
temperature of 70
°C in the electrolysis cell, a current density of 0.2-3 kA/m2 and a pH
of 6.2. The current
was varied between 0.5-6.3 A. The electrolysis was run for 30 h. The
production of
NaCl03 was calculated from the total amount of NaCl03 formed in the anode
compartment throughout the electrolysis. The estimated current efficiency for
chlorine
formed was close to 100 %. The current efficiency for the chlorate production
was 97
calculated as the quotient between the actual recovered and the theoretical
maximal
production of sodium chlorate.
