U.S. patent application number 13/996686 was filed with the patent office on 2013-11-07 for electrolytic process.
This patent application is currently assigned to Akzo Nobel Chemicals International B.V.. The applicant listed for this patent is Rolf Edvinsson Albers, Kristoffer Hedenstedt. Invention is credited to Rolf Edvinsson Albers, Kristoffer Hedenstedt.
Application Number | 20130292261 13/996686 |
Document ID | / |
Family ID | 43498587 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130292261 |
Kind Code |
A1 |
Hedenstedt; Kristoffer ; et
al. |
November 7, 2013 |
Electrolytic Process
Abstract
The invention relates to a process of producing alkali metal
chlorate in an electrolytic cell comprising an anode and a cathode,
wherein at least one chromium compound having a valence lower than
+6 is added to the process, wherein said at least one chromium
compound is oxidized to hexavalent chromium within said process,
wherein substantially no hexavalent chromium is added to the
process from an external source. The invention also relates to the
use of an aqueous solution of chromium compounds as an additive to
a chlorate process.
Inventors: |
Hedenstedt; Kristoffer;
(Goteborg, SE) ; Edvinsson Albers; Rolf;
(Partille, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hedenstedt; Kristoffer
Edvinsson Albers; Rolf |
Goteborg
Partille |
|
SE
SE |
|
|
Assignee: |
Akzo Nobel Chemicals International
B.V.
Amersfoort
NL
|
Family ID: |
43498587 |
Appl. No.: |
13/996686 |
Filed: |
December 19, 2011 |
PCT Filed: |
December 19, 2011 |
PCT NO: |
PCT/EP2011/073167 |
371 Date: |
June 21, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61425927 |
Dec 22, 2010 |
|
|
|
Current U.S.
Class: |
205/503 |
Current CPC
Class: |
C25B 1/265 20130101;
C25B 1/14 20130101 |
Class at
Publication: |
205/503 |
International
Class: |
C25B 1/14 20060101
C25B001/14 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2010 |
EP |
10196408.8 |
Claims
1. Process of producing alkali metal chlorate in an electrolytic
cell comprising an anode and a cathode, wherein at least one
chromium compound having a valence lower than +6 is added to the
process in an amount from 1 to 200 g chromium/ton produced
chlorate, wherein said at least one chromium compound is oxidized
to hexavalent chromium within said process, wherein substantially
no hexavalent chromium is added to the process from an external
source.
2. The process according to claim 1, wherein the amount of
hexavalent chromium added is less than about 30 molar percent based
on the total amount of chromium added from an external source.
3. The process according to claim 1, wherein no hexavalent chromium
is added.
4. The process according to claim 1, wherein a chromium(III)
compound is added to the process.
5. The process according to claim 1, wherein said at least one
chromium compound having a valence lower than +6 is added to the
scrubber liquid, to the cell line loop after the cells, or to the
electrolyte going in to the cell.
6. The process according to claim 1, wherein said at least one
chromium compound having a valence lower than +6 is added to the
electrolyzed solution prior to the reactor; to the process stream
from the mother liquor scrubber; and/or to the reactor gas
scrubber.
7. The process according to claim 1, wherein said at least one
chromium compound having a valence lower than +6 is added upstream
of an electrolyte filter.
8. The process according to claim 1, wherein said at least one
chromium compound having a valence lower than +6 is added in an
amount resulting in a chromium content from about 0.1 to about 20 g
(calculated as sodium dichromate equivalents)/l electrolyte
solution.
9. The process according to claim 1, wherein the weight ratio of
chromium derived from chromium compounds having a valence lower
than +6 to hypochlorite ranges from about 1:30 to about 3:1.
10. The process according to claim 1, wherein the amount of
chromium compound(s) with a valence lower than +6 is added in an
amount from about 1 to about 60 g chromium/ton produced
chlorate.
11. The process according to claim 1, wherein hexavalent chromium
is formed from at least one chromium compound by means of oxidation
in an aqueous solution which hexavalent chromium is subsequently
transferred to the electrolytic cell.
12. The process according to claim 11, wherein hexavalent chromium
is formed in an aqueous solution in a medium separated from the
process prior to transfer of said aqueous solution of hexavalent
chromium to the process.
13. The process according to claim 1, wherein substantially all
hexavalent chromium within the process has been formed in-situ.
14. A process for producing alkali metal chlorate in an
electrolytic cell comprising an anode and a cathode, the process
comprising adding an aqueous solution of chromium compounds to the
chlorate process, said solution comprising chromium compounds,
wherein the molar ratio of hexavalent chromium to chromium having a
valence lower than +6 ranges from about 0:10000 to about
1:10000.
15. A process for producing alkali metal chlorate in an
electrolytic cell comprising an anode and a cathode, the process
comprising adding an aqueous solution of chromium compounds to the
chlorate process, said solution comprising at least one hexavalent
chromium compound and at least one chromium compound having a
valence lower than +6, wherein the molar ratio of hexavalent
chromium to chromium having a valence lower than +6 ranges from
about 1:10000 to about 3:10.
16. The process according to claim 2, wherein a chromium(III)
compound is added to the process.
17. The process according to claim 3, wherein a chromium(III)
compound is added to the process.
18. The process according to claim 14, wherein the at least one
chromium compound having a valence lower than +6 is a chromium(III)
compound.
19. The process according to claim 15, wherein the at least one
chromium compound having a valence lower than +6 is a chromium(III)
compound.
Description
[0001] The present invention relates to a process of producing
alkali metal chlorate in an electrolytic cell comprising an anode
and a cathode, wherein substantially no hexavalent chromium is
added to the process from an external source.
BACKGROUND OF THE INVENTION
[0002] In the electrolysis of sodium chloride to form sodium
chlorate, hexavalent chromium, usually sodium dichromate, is
conventionally added to the electrolyte introduced into the cell to
improve the current efficiency of the cell in the conversion of
sodium chloride to sodium chlorate. This is partly obtained by
suppressing the reduction of hypochlorite and chlorate at the
cathode. EP 266 128 discloses a process of producing chlorate by
diaphragmaless electrolysis. In this process, hexavalent chromium
is added to the process from an external source.
[0003] Part of the hexavalent chromate is reduced on the cathode to
Cr(III)-containing compounds such as Cr(OH).sub.3 and forms a very
thin film which suppresses the reduction of chlorate and
hypochlorite on the cathode. However, hexavalent chromium is
mutagenic, reprotoxic, and carcinogenic and thus highly poisonous.
There is thus a problem involved in the handling of hexavalent
chromium including introduction thereof to the electrolytic cell
from an external source. Therefore, strict safety precautions are a
prerequisite. Hexavalent chromium also functions as a buffering
solution in the chlorate electrolyte. The pH of the chlorate
electrolyte may thus be liable to considerable variation in the
absence of hexavalent chromium. Unfavorable pH also contributes to
lower current efficiency. This may also result in undesired
precipitation of compounds in the electrolyte. Hexavalent chromium
thus provides for high current efficiencies in the process in
several ways. The cell gas produced under optimal conditions
contains a few percent of oxygen while the remaining portion of the
cell gas is made up of hydrogen, but still with an oxygen to
hydrogen weight ratio below the lower explosion limit. Lower
current efficiency increases the oxygen/hydrogen ratio and may
necessitate actions, such as dilution, to avoid explosive cell gas
mixtures.
[0004] One object of the present invention is to provide an
environmentally adapted process which obviates handling problems
involved when introducing substantial amounts of toxic chromium(VI)
compounds into an electrolytic cell from an external source while
safeguarding appropriate process conditions are maintained. In
particular, one object is to obviate transportation of highly toxic
hexavalent chromium. A further object of the invention is to
provide an alternative compound entirely substituting or to a large
extent substituting toxic chromium(VI) compounds as raw material.
Also, the present invention intends to provide a process which
safeguards a controlled supply of hexavalent chromium in the cell
electrolyte which is independent on the amount of for example
hypochlorite in condensate streams. A further intention is to
provide a process which facilitates production of alkali metal
chlorate wherein hexavalent chromium can be provided at an acidic
pH whereby necessary pH adjustment can be reduced or eliminated. A
further intention of the instant invention is to provide a process
which rapidly provides hexavalent chromium.
THE INVENTION
[0005] The present invention relates to a process of producing
alkali metal chlorate in an electrolytic cell comprising an anode
and a cathode, wherein at least one chromium compound having a
valence lower than +6 is added to the process, wherein said at
least one chromium compound is oxidized to hexavalent chromium
within said process, and wherein substantially no hexavalent
chromium is added to the process from an external source.
[0006] According to one embodiment, the process of producing alkali
metal chlorate comprises introducing an electrolyte solution
containing alkali metal chloride and alkali metal chlorate to an
electrolytic cell, electrolyzing the electrolyte solution to
produce an electrolyzed chlorate solution, transferring the
electrolysed chlorate solution to a chlorate reactor to react the
electrolysed chlorate solution further to produce a more
concentrated alkali metal chlorate electrolyte. As the electrolysis
occurs, the main anode reaction is chlorine formation and the main
cathode reaction is hydrogen gas evolution and formation of
hydroxide. In subsequent reactions, chlorine reacts with water and
hydroxide to form a hypochlorite-hypochlorous acid mixture that in
turn results in formation of alkali metal chlorate and alkali metal
chloride. These reactions will start in the electrolysis cell and
will continue in the downstream holding vessels. The current
efficiency is less than 100% owing to a number of parasitic
reactions such as anodic water oxidation leading to oxygen
evolution, homogeneous hypochlorite decomposition also resulting in
oxygen formation as well as cathodic reduction of chlorate and
hypochlorite leading to reduced hydrogen formation. All these side
reactions contribute to increasing the O.sub.2/H.sub.2 ratio in the
cell gas.
[0007] According to one embodiment, by the wording "substantially
no hexavalent chromium is added" is meant less than about 30 molar
%, for example less than about 20 molar % or less than about 10
molar % or less than about 3 molar %, for example less than about 1
molar % or less than about 0.1 molar % of hexavalent chromium is
added based on the total amount of chromium added to the process
from an external source.
[0008] According to one embodiment, no hexavalent chromium is added
to the process from an external source.
[0009] For reasons of simplicity, by the term "electrolyte
solution" is meant to include the volume of all streams or
solutions circulated to the electrochemical cell(s) or that will be
introduced into the cell electrolyte. Examples of such liquids
include, but are not limited to, alkaline scrubber solutions, brine
solutions, make-up streams, process water and condensate recycled
solutions. These streams include solutions of chromium compounds
which do not contain any alkali chloride and/or chlorate or are
alkali chloride and/or chlorate depleted electrolyte solutions.
[0010] Throughout the invention, if not otherwise stated,
concentrations of anions, e.g. chloride, chlorate, hypochlorite,
chromate, dichromate, sulphate, perchlorate etc, are defined as
equivalent contents of their respective anhydrous sodium salts, for
example NaCl, NaClO.sub.3, NaClO, Na.sub.2CrO.sub.4,
Na.sub.2Cr.sub.2O.sub.7, Na.sub.2SO.sub.4 and NaClO.sub.4.
[0011] In solution, several of these compounds participate in
equilibrium reactions in ways that affect the analysis. The
dichromate concentration is based on the total chromium
concentration and calculated as if all chromium was in the form of
sodium dichromate. Due to the equilibrium, 2
CrO.sub.4.sup.2-+2H.sup.+Cr.sub.2O.sub.7.sup.2-+H.sub.2O, part of
the chromium(VI) may be present as chromate. Under certain
operating conditions CrO.sub.4.sup.2- is the dominant form. [0012]
In the same way, for the equilibria
[0012] HClOH.sup.++ClO.sup.-
Cl.sub.2+H.sub.2OH.sup.30+Cl.sup.-+HClO
the hypochlorite level stated corresponds to the hypochlorite level
after transferring both Cl.sub.2 and HClO to hypochlorite in
alkaline solution by the equilibrium reactions above.
[0013] The present invention facilitates the process of providing
alkali metal chlorate by excluding or minimizing the addition of
highly toxic hexavalent chromium which is usually added to the
aqueous chloride electrolyte solution in the form of sodium
dichromate dihydrate (Na.sub.2Cr.sub.2O.sub.7. 2H.sub.2O),
potassium chromate (K.sub.2CrO.sub.4) or mixtures thereof from an
external source.
[0014] According to one embodiment, at least one chromium compound
having a valence lower than +6 is added to at least one process
stream containing either alkali metal chloride or alkali metal
chlorate or to at least one process stream containing both alkali
metal chlorate and alkali metal chlorate.
[0015] According to one embodiment, hexavalent chromium, for
example in the form of sodium dichromate, can, even if added in
inconsiderable amounts, be added to the process from an external
source, in an aqueous solution, either separately or in combination
with a chromium compound with a valence lower than +6, for example
a chromium(III) compound.
[0016] In view of addition of chromium compounds, by the wording
"added to the process from an external source", as opposed to
hexavalent chromium formed in the process, i.e. in-situ formation,
is meant added to any process stream, for example an electrolyte
stream or other process streams or to any tank, container,
scrubber, reactor connected to the cell or directly to the cell.
According to one embodiment, "added to the process" includes any
addition point to the process from which chromium with a valence
lower than +6 can be added.
[0017] According to one embodiment, a chromium compound with a
valence lower than +6 is transferred from one cell line to another
cell line with a compatible electrolyte composition although these
may be disconnected during normal operation. An example of transfer
between compatible cell lines is the transfer of electrolyte from a
unit for making potassium chlorate to a unit for making sodium
chlorate.
[0018] According to one embodiment, formation of hexavalent
chromium is made by addition of a chromium compound with a valence
lower than +6 to for example a separate medium, for example in a
separate vessel, from which medium transfer of chromium compounds
takes place via a process stream to the process prior to,
simultaneously or subsequently to formation of hexavalent chromium.
According to one embodiment, all or substantially all hexavalent
chromium is formed in-situ.
[0019] According to one embodiment, the addition of chromium
compounds having a valence lower than +6 may take place either
during electrolysis or when the electrolysis is stopped. In
particular it can take place when the starting electrolyte is
prepared prior to the first start-up of a new production unit.
[0020] According to one embodiment, chromium compounds, for example
dissolved in an aqueous solution, having a valence lower than +6,
for example trivalent chromium can be added in a separate vessel,
optionally a temporarily disconnected vessel, for example a tank,
and oxidized in such vessel to hexavalent chromium (in-situ
generation thereof), for example by means of hypochlorite,
chlorine, chlorite, chlorate, perchlorate, chlorine dioxide,
hydrogen peroxide, sodium peroxide, sodium peroxysulfate, ozone,
oxygen, air or other oxidizing agent or by electrochemical anodic
oxidation. Such chromium compounds can subsequently be transferred
to the electrolytic cell via a process stream by pumping the
chromium compound solution towards the cell.
[0021] According to one embodiment, hexavalent chromium is formed
from at least one chromium compound having a valence lower than +6
by means of oxidation in an aqueous solution which hexavalent
chromium is subsequently transferred to the electrolytic cell.
[0022] According to one embodiment, said at least one chromium
compound with a valence lower than +6 is added in an amount
resulting in a chromium content ranging from about 0.1 to about 20
g/l, for example from about 1 to about 10 or from about 2 to about
6 g (calculated as sodium dichromate equivalents)/1 electrolyte
solution.
[0023] According to one embodiment, chromium compound(s) with a
valence lower than +6 is/are added in an amount of from about 0.1
to about 200, for example from about 0.1 to about 100, or from
about 0.1 to about 80 or from about 1 to about 60 or from about 2
to about 20 g chromium/ton produced chlorate.
[0024] According to one embodiment, addition of at least one
chromium compound having a valence lower than +6, for example
chromium (III), to the process may be made to the alkaline scrubber
liquid and/or to the cell line loop after the cells. However,
according to one embodiment, addition of at least one chromium
compound having a valence lower than +6 may also be made to the
electrolyte solution introduced into the cell which electrolyte
solution is to be electrolyzed. According to one embodiment, the
chromium compound can also be added to the electrolyzed solution
prior to the reactor; to the process stream from the mother liquor
scrubber; and/or to the reactor gas scrubber. According to one
embodiment, the chromium compound may also be added upstream of an
electrolyte filter to prevent product contamination with small
amounts of possibly strongly colored insoluble chromium compounds,
initially present in the chromium source or formed in the
process.
[0025] According to one embodiment, chromium compounds having a
valence lower than +6 may be for example chromium halides such as
chromium(II)chloride, chromium(III)chloride, chromium(III)chloride
hexahydrate, chromium oxide such as chromium(II)oxide (CrO),
chromium(III)oxide (Cr.sub.2O.sub.3), chromic hydroxide,
chromium(IV)oxide, chromic nitrate (Cr(NO.sub.3).sub.2.9H.sub.2O),
ammonium chromate, chromic hydroxyl dichloride (Cr(OH)Cl.sub.2),
chromium sulfate pentadecahydrate, chromium sulfate, chromium
hydroxide sulfate, chromium phosphate, chromite
(FeCr.sub.2O.sub.4), or any mixtures thereof. Other suitable
examples of chromium compounds are those listed in Kirk-Othmer
Encyclopedia of Chemical Technology, John Wiley & Sons, Inc.,
Vol.6, p.526-570, 2001.
[0026] According to one embodiment, the chromium compounds can for
example be added as salts, aqueous solutions or as melts if the
melting point is sufficiently low, for example chromium trichloride
hexahydrate having a melting point of 83.degree. C. Solid compounds
containing leachable chromium can also be used as chromium
source.
[0027] According to one embodiment, chromium compounds for use may
be Cr(0), for example elemental chromium, Cr(I), Cr(II), Cr(III),
Cr(IV), Cr(V) or any combinations thereof. According to one
embodiment, at least one Cr(III) compound is used.
[0028] According to one embodiment, the extent of electrolysis is
controlled to produce an effluent from the cell in which the
desired weight ratio of alkali metal chlorate to alkali metal
chloride usually ranges from (expressed as a weight ratio) about
1:1 to about 20:1, for example from about 1:1 to about 15:1 or from
about 2:1 to about 10:1.
[0029] According to one embodiment, the electrolyte solution may be
further processed to crystallize the alkali metal chlorate such as
sodium chlorate for a variety of purposes, for example for the
production of chlorine dioxide for use in the bleaching of chemical
cellulosic pulps, by reduction in the presence of a strong mineral
acid, usually sulphuric or hydrochloric acid. Chlorine dioxide may
also be generated directly from the electrolyte without prior
isolation of the chlorate, typically by adding hydrochloric acid
which acts both as an acid and a reducing agent.
[0030] According to one embodiment, the electrolysis produces a
gaseous by-product, mainly consisting of hydrogen but also some
oxygen, chlorine, hypochlorous acid, carbon dioxide and water
vapour. The by-product gas stream is passed through a water
condenser scrubber wherein part of the stream is condensed to form
an aqueous solution of hypochlorous acid, typically about 2 to 25
g/l HOCl, which aqueous solution also contains small amounts of
dissolved chlorine, which can be recirculated to the cell.
[0031] According to another embodiment, the by-product gas effluent
from the water gas scrubber is optionally passed through one or
several alkaline scrubbers in which chlorine and hypochlorous acid
are reactively absorbed to form hypochlorite. One example is a
mother liquid scrubber using the alkaline effluent from the
crystallizer. Another example is a caustic scrubber using for
example a NaOH solution.
[0032] According to one embodiment, the present invention is
particularly directed to in-situ formation of hexavalent chromium
for use in the electrolytic production of aqueous sodium chlorate
from aqueous sodium chloride. However, the present invention may
also be used in the electrolytic production of any aqueous alkali
metal chlorate solution by the electrolysis of the corresponding
chloride in which the hexavalent chromium is useful. Such aqueous
alkali chlorate solutions include besides sodium chlorate also
potassium chlorate, lithium chlorate, rubidium chlorate and cesium
chlorate; alkaline earth metal chlorates, such as beryllium
chlorate, magnesium chlorate, calcium chlorate, strontium chlorate,
barium chlorate and radium chlorate, and mixtures of two or more
such chlorates, which may also contain dissolved quantities of
alkali metal chlorides, alkaline earth metal chlorides and mixtures
thereof.
[0033] When a different alkali metal chlorate than sodium chlorate
is produced the electrolyte composition and operating conditions
may have to be adapted to account for differences in physical
properties like solubility.
[0034] According to one embodiment, the electrolytic cell is a
non-divided cell, e.g. a monopolar cell. This enables a variety of
cell configurations. At least one electrode pair of anode and
cathode may form a unit containing an electrolyte solution between
the anode and cathode which unit may have the shape of plates or
tubes. A plurality of electrode pairs may also be immersed in a
cell box. According to one embodiment, the cell is a bipolar cell.
A similar variety of bipolar cell configurations are also
possible.
[0035] According to one embodiment, the cell is a hybrid cell, i.e.
a combined monopolar and bipolar cell. This type of cells enables
upgrading of monopolar technology by combining monopolar and
bipolar sections in a cell-box. Such combination may be set up by
arranging e.g. two or three electrodes herein as a bipolar section
among a plurality of monopolar electrodes. The monopolar electrodes
of the hybrid cell may be of any type including e.g. conventional
electrodes known per se.
[0036] According to one embodiment, separate monopolar anodes and
cathodes are mounted in an electrolytic cell at the ends, whereas
bipolar electrodes are mounted in between thereby forming a hybrid
electrolytic cell. According to one embodiment, the current density
of the electrolytic process ranges from about 0.6 to about 4.5, for
example from about 1 to about 3, or from about 1.3 to about 2.9
kA/m.sup.2.
[0037] According to one embodiment, the pH is adjusted at several
positions within the range from about 4 to about 12 to optimize the
process conditions for the respective unit operation. Thus, a
weakly acidic or neutral pH is used in the electrolyzer and in the
reaction vessels to promote the reaction from hypochlorite to
chlorate, while the pH in the crystallizer is alkaline to prevent
gaseous hypochlorite and chlorine from being formed and released to
reduce the risk of corrosion. According to one embodiment, the pH
of the cell electrolyte solution, i.e. the solution comprising
alkali metal chloride undergoing electrolysis in the
electrochemical cell ranges from about 4 to about 7.5, for example,
from about 4 to about 6.5 or from about 4 to 6 or from about 4 to
5.75 or from about 4 to 5.5. According to one embodiment, the pH of
the cell electrolyte solution ranges from about 5.0 to about 7.5,
such as from about 6.5 to about 7.0. According to one embodiment,
the pH at the point of addition of a chromium compound having a
valence lower than +6 also may range from about 4 to about 7.5, for
example from about 4 to about 6.5 or from about 4 to 6 or from
about 4 to 5.75 or from about 4 to 5.5. According to one
embodiment, the pH of the cell electrolyte solution ranges from
about 5.0 to about 7.5, such as from about 6.5 to about 7.0.
[0038] The concentration of chlorate and of chloride as well as
hypochlorite in the electrolyte used in the electrochemical cell
may vary widely, depending on the extent of electrolysis of the
chloride solution. According to one embodiment, the electrolyte
solution contains alkali metal halide, e.g. sodium chloride in a
concentration from about 80 to about 180, for example from about
100 to about 140 or from about 106 to about 125 g/l electrolyte.
According to one embodiment, the electrolyte solution contains
alkali metal chlorate in a concentration from about 200 to about
700, e.g. from about 450 to about 650 or from about 550 to about
610 g/l. According to one embodiment, the concentration of
hypochlorite in the electrolyte solution ranges from about 0 to
about 6, for example from about 0.01 to about 4 or from about 0.1
to about 4 or from about 0.3 to about 3 g/l. The electrolyte may
also comprise significant amounts of inactive compounds accumulated
in the process over the course of time, for example sodium sulfate
introduced as an impurity in the sodium chloride source or sodium
perchlorate formed by a side reaction in the process.
[0039] According to one embodiment, the weight ratio of chromium
derived from chromium compound(s) having a valence lower than +6
added to the process to hypochlorite ranges from about 1:30 to
about 3:1, for example from about 1:10 to about 2:1, or from about
1:8 to about 1:1.
[0040] According to one embodiment, the amount of hexavalent
chromium formed from the chromium compound having a valence lower
than +6 is in the range from about 0.1 to about 25 grams calculated
as sodium dichromate ions/l electrolyte solution in the cells, for
example from about 0.2 to about 15 g/l electrolyte solution, for
example from about 1 to about 8 g/l electrolyte solution. In order
to arrive at such suitable amount of hexavalent chromium, a
corresponding amount of chromium compound with a valence lower than
+6 is added.
[0041] According to one embodiment, the flow to the chlorate cells
normally is from 75 to 200 m.sup.3 of electrolyte per metric ton of
alkali metal chlorate produced. According to one embodiment, the
electrolytic cell operates at a temperature ranging from about 60
to about 100.degree. C., or from about 65 to about 90.degree. C.
According to one embodiment, the temperature of the stream or
solution at the addition point of a chromium compound having a
valence lower than +6 ranges from about 60 to about 100.degree. C.,
or from about 65 to about 90.degree. C. According to one
embodiment, the temperature of the stream or solution at the
addition point of a chromium compound having a valence lower than
+6 ranges from about 15 to about 40.degree. C., for example from
about 15 to 30.degree. C. According to one embodiment, a part of
the electrolyzed solution is recycled from the reaction vessels to
a salt dissolver, and a part is recycled for alkalization and
electrolyte filtration and final pH adjustment before introduction
into the chlorate crystallizer. Water from the thus alkalized
electrolyte can be evaporated in the crystallizer. According to one
embodiment, the mother liquor, which is saturated with respect to
chlorate and contains high contents of sodium chloride, is recycled
directly to the preparation slurry via cell gas scrubbers and
reactor gas scrubbers. According to one embodiment, the pressure in
the cell is about 20 to 30 mbar above the atmospheric pressure.
[0042] According to one embodiment, the (electrical) conductivity
in the cell electrolyte ranges from about 200 to about 700, for
example from about 300 to about 600 mS/cm.
[0043] According to one embodiment, the electrolytic cell and the
electrodes arranged therein may be as further disclosed in EP
1242654 and WO 2009/063031.
[0044] The invention also relates to the use of an aqueous solution
or a melt of chromium compounds as an additive to a chlorate
process, said solution comprising at least one hexavalent chromium
compound and at least one chromium compound having a valence lower
than +6, wherein the molar ratio of hexavalent chromium to chromium
having a valence lower than +6 ranges from about 1:10000 to about
3:10, for example from about 1:10000 to about 2:10, for example
from about 1:10000 to about 1:10, for example from about 1:10000 to
about 1:100 or from about 1:10000 to about 1:1000, or from about
0:10000 to about 1:10000. According to one embodiment, the molar
ratio of hexavalent chromium to chromium having a valence lower
than +6 ranges from about 0:10000 to about 1:10000.
[0045] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the gist and scope of the present
invention, and all such modifications as would be obvious to one
skilled in the art are intended to be included within the scope of
the claims. The following examples will further illustrate how the
described invention may be performed without limiting the scope of
it. If not otherwise specified, all percentages given herein
concern percent by weight.
EXAMPLE 1
[0046] Dissolution of chromium trichloride hexahydrate in acidic,
neutral and alkaline aqueous solutions, was performed.
[0047] Four beakers 1-4 with 100 ml aqueous solution each were
prepared at room temperature (295 K) according to the below: [0048]
1. 0.05 M HCl [0049] 2. Deionized (DI) water [0050] 3. 0.05 M NaOH
[0051] 4. Chromium free chlorate electrolyte (110 g/l NaCl, 550 g/l
NaClO.sub.3 in water, no pH adjustment).
[0052] To each beaker, 22 mmol of CrCl.sub.3*6H.sub.2O was added
and the opening was covered with parafilm. The solutions were left
standing for 72 hours at room temperature.
TABLE-US-00001 TABLE 1 Observations during solubility tests of
Chromium(III) chloride in different solvents Beaker/solvent 1 2 3 4
0.05M HCl DI water 0.05M NaOH Cr free electrolyte 0 h All All
Precipitation All dissolved, dissolved. dissolved. observed. but
slower to Green Dark green Grayish-green dissolve. Green 5 h
Emerald green Bluish green Bluish green Green 72 h Emerald green
Blue to purple Bluish green. Bluish green. pH 1.42 pH 3.03 Grey pH
1.92 precipitate at the bottom. pH 4.97
[0053] When the mixture in beaker 3, with the precipitate, was
acidified with 2 M HCl to a pH of 1.90 the precipitate was
dissolved within 48 h. The colors indicate that chromium remained
trivalent in all cases since trivalent chromium species varies in
color from green to violet blue (dark green
[CrCl.sub.2(H.sub.2O).sub.4]Cl, pale green
[CrCl(H.sub.2O).sub.5]Cl.sub.2, and violet
[Cr(H.sub.2O).sub.6]Cl.sub.3)) and hexavalent chromium varies from
orange (dichromate) to yellow (chromate).
EXAMPLE 2
[0054] 1.5 dm.sup.3 electrolyte containing 120 g/l NaCl and 580 g/l
NaClO.sub.3 was prepared by dissolution at 90.degree. C. in water
and dilution to 1.5 dm.sup.3. To the warm electrolyte 6 g/l of
CrCl.sub.3*6H.sub.2O was added. The color was at first bluish green
but darkened as gas started to evolve. After a few seconds a
substantial amount of yellow gas was formed. 3 minutes later the
gas evolution ceased and the color of the electrolyte became dark
orange. This demonstrated that dichromate had formed by oxidation
of Cr(III) by means of chlorate which simultaneously had been
reduced to ClO.sub.2(g) in the acidic solution.
EXAMPLE 3
[0055] 100 ml of chromium free electrolyte containing 110 g/l NaCl
and 550 g/l NaClO.sub.3 was prepared at room temperature. 22 mmol
CrCl.sub.3*6H.sub.2O was added and the solution was placed on a
heater equipped with magnetic stirrer and heated to 95.degree. C. A
glass thermometer was placed in the beaker to monitor the
temperature. The pH after Cr(III) addition was 1.92.
TABLE-US-00002 TABLE 2 Observation during heating of Cr(III)
containing electrolyte with a pH of 1.92 Cr oxidation state
Temperature Observations based on color 22.degree. C. Bluish Green
III 60.degree. C. Emerald green III 67.degree. C. Indications of
yellowing III + VI lighter green 85.degree. C. Moss green III + VI
90.degree. C. Brownish yellow. Gas Mainly VI bubbles appear
95.degree. C. Orange VI
At 95.degree. C., essentially all chromium(III) had been oxidized
to chromium(VI). The time required to raise the temperature from
22.degree. C. to 95.degree. C. was approximately 20 min.
EXAMPLE 4a
[0056] 50 ml of synthetic electrolyte (110 g/l NaCl, 550 g/l
NaClO.sub.3 and 3 g/l Na.sub.2Cr.sub.2O.sub.7) was prepared and the
pH was adjusted to 6.90 with NaOH(s). When the electrolyte had been
cooled down from 90.degree. C. to 40.degree. C., 0.22 mmol of
CrCl.sub.3*6H.sub.2O crystals was added to the electrolyte and a
brown precipitation was formed. The pH after addition of the
CrCl.sub.3*6H.sub.2O crystals was 5.80. The mixture was placed on a
heater with a magnetic stirrer and a glass thermometer was placed
in the beaker. At 88.degree. C., minor gas evolution occurred. Even
as the temperature reached 100.degree. C. the precipitation
remained. Thus, no oxidation took place due to the high pH.
EXAMPLE 4b
[0057] 50 ml of synthetic electrolyte (110 g/l NaCl, 550 g/l
NaClO.sub.3 and 3 g/l Na.sub.2Cr.sub.2O.sub.7) was prepared and the
pH was adjusted to 6.40 with NaOH(s). It was then heated to
85.degree. C. while stirring. A small amount of
CrCl.sub.3*6H.sub.2O crystals (.about.0.05 g) was added and a brown
precipitation was formed. To this electrolyte 0.2 ml of sodium
hypochlorite (.about.150 g/l in 0.5 M NaOH) was added whereby the
precipitation was dissolved. The remaining electrolyte was yellow
and had a pH of 6.00. The hypochlorite had oxidized all chromium
(III) to chromium (VI).
EXAMPLE 5
[0058] Chlorate electrolyte was withdrawn from the electrolysis
cell outlet of a chlorate plant during operation. The addition of a
small amount of CrCl.sub.3*6H.sub.2O crystals (.about.0.1 g/100 ml
electrolyte) resulted in complete dissolution and oxidation of
chromium (III) to chromium (VI). A larger amount of
CrCl.sub.3*6H.sub.2O (.about.0.5 g/100 ml) resulted in formation of
a brown precipitation whereby no further oxidation to hexavalent
chromium took place.
EXAMPLE 6
[0059] A hypochlorite containing caustic scrubber solution was
withdrawn from a chlorate plant and CrCl.sub.3*6H.sub.2O crystals
were added (.about.0.1 g/100 ml electrolyte). The
CrCl.sub.3*6H.sub.2O crystals first dissolved forming a green
solution and later oxidized forming a pale yellow Cr(VI) containing
solution.
[0060] In view of the above examples, it can be concluded
CrCl.sub.3*6H.sub.2O crystals can be easily dissolved and that pH
is reduced on dissolution of acidic CrCl.sub.3*6H.sub.2O.
Precipitation occurred close to neutral pH and in weakly alkaline
solutions, presumably due to formation of Cr(OH).sub.3(s) or
CrO.sub.2(s). Sodium chlorate has been found to oxidize chromium
(III) to chromium (VI) under acidic conditions and at elevated
temperatures while chlorine dioxide is formed which can be
recovered by absorption in alkaline electrolyte or caustic, whereby
it forms chlorate, chlorite and/or chloride.
[0061] Sodium hypochlorite can oxidize chromium(III) to
chromium(VI) in strongly alkaline solutions and down to at least pH
5.8, for example to at least pH 5 or below pH 5. Hypochlorite can
even dissolve precipitations formed in neutral solutions and
oxidize chromium with a valence lower than +6 to hexavalent
state.
[0062] These examples demonstrate that trivalent chromium but also
other valencies of chromium lower than +6 are a viable alternative
to hexavalent chromium as raw material in a process for the
production of alkali metal chlorate since it is easily oxidized to
the hexavalent state; either by chlorate or hypochlorite oxidation.
Addition of a chromium compound, for example chromium(III), to the
process may be made for example in the scrubber caustic solution,
in the cell line loop after the cells, or to the inlet of the
cell.
EXAMPLE 7
[0063] A 203 ml (conventional dichromate-containing) electrolyte
composition containing 110 g/dm.sup.3 NaCl, 550 g/dm.sup.3
NaClO.sub.3, 5.0 g/dm.sup.3 Na.sub.2Cr.sub.2O.sub.7 was used in
trials conducted at a pH of 6.1 and a temperature of 25.degree.
C.
A hypochlorite solution of 2 g (NaClO content was 124 g/dm.sup.3)
was added to the electrolyte and subsequently 0.4788 g of a 50 wt %
solution of Cr(III)Cl.sub.3.times.6H.sub.2O. It could be noted a
change of colour occurred after addition of the solution of
Cr(III)Cl.sub.3.times.6H.sub.2O such that the electrolyte initially
turned darker but after a while turned yellow again as Cr(III)
oxidized to Cr(VI).
EXAMPLE 8
[0064] A 261.28 g (conventional dichromate containing electrolyte)
composition containing 110 g/dm.sup.3 NaCl, 550 g/dm.sup.3
NaClO.sub.3, 5.0 g/dm.sup.3 Na.sub.2Cr.sub.2O.sub.7 was used in
trials conducted at a pH of 6.1 and a temperature of 25.degree. C.
0.4815 g of a 50 wt % solution of Cr(III)Cl.sub.3.times.6H.sub.2O
was added thereto and subsequently a hypochlorite solution of 2 g
(NaClO content was 124 g/dm.sup.3).
[0065] It could be noted the solution started to change colour
subsequent to addition of hypochlorite solution indicating
oxidation of Cr(III) to Cr(VI).
* * * * *