U.S. patent application number 12/128815 was filed with the patent office on 2008-09-25 for system for the electrolytic production of sodium chlorate.
This patent application is currently assigned to Industrie De Nora S/p.A.. Invention is credited to Vladimir Jovic, Nedeljko Krstajic, Gian Nicola Martelli.
Application Number | 20080230381 12/128815 |
Document ID | / |
Family ID | 38001688 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080230381 |
Kind Code |
A1 |
Krstajic; Nedeljko ; et
al. |
September 25, 2008 |
SYSTEM FOR THE ELECTROLYTIC PRODUCTION OF SODIUM CHLORATE
Abstract
A system for the electrolytic production of sodium chlorate
having a sodium chloride brine buffered with phosphate and having a
reduced or zero chromium content is disclosed. The system comprises
electrolytic cells of the undivided type with intercalated cathodes
and anodes. The cathodes can comprise steel perforated sheets
activated with a Fe--Mo alloy coating.
Inventors: |
Krstajic; Nedeljko;
(Belgrade, YU) ; Jovic; Vladimir; (Belgrade,
YU) ; Martelli; Gian Nicola; (Vimodrone (MI),
IT) |
Correspondence
Address: |
ESCHWEILER & ASSOCIATES, LLC;NATIONAL CITY BANK BUILDING
629 EUCLID AVE., SUITE 1000
CLEVELAND
OH
44114
US
|
Assignee: |
Industrie De Nora S/p.A.
Milan
IT
|
Family ID: |
38001688 |
Appl. No.: |
12/128815 |
Filed: |
May 29, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2006/069079 |
Nov 29, 2006 |
|
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|
12128815 |
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Current U.S.
Class: |
204/290.12 |
Current CPC
Class: |
C25B 1/265 20130101;
C25B 11/091 20210101 |
Class at
Publication: |
204/290.12 |
International
Class: |
C25C 7/02 20060101
C25C007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2005 |
IT |
MI2005A002298 |
Claims
1. System for sodium chlorate production comprising at least one
electrolytic cell equipped with a multiplicity of cathodes and a
multiplicity of anodes, fed with a sodium chloride brine added with
a buffering agent, wherein the buffering agent comprises at least 1
g/l of phosphate ions.
2. The system of claim 1, the cathodes of the electrolytic cell
comprising a ferrous matrix provided with a coating comprising a
molybdenum and/or tungsten alloy with at least one metal selected
from the group of iron, cobalt and nickel.
3. The system of claim 1, wherein the feed contains chromate and
dichromate ions at a concentration not exceeding 0.1 g/l.
4. The system of claim 1, the feed being free of chromium.
5. The system of claim 2, the ferrous matrix of the cathodes
comprising a carbon steel.
6. The system of claim 5, the coating comprising a galvanic coating
containing 30 to 70% Fe and 30 to 70% Mo expressed as weight
percentage.
7. The system of claim 6, the galvanic coating having a thickness
between 10 and 50 micrometres.
8. The system of claim 1, the electrolytic cell comprising a cell
of the undivided type and the cathodes are disposed in a comb-like
fashion intercalated to the anodes.
9. The system of claim 8, the cathodes comprising a ferrous matrix
comprising a perforated sheet.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of PCT/EP2006/069079,
filed Nov. 29, 2006, that claims the benefit of the priority date
of Italian Patent Application No. MI2005A002298, filed on Nov. 30,
2005, the contents of which are herein incorporated by reference in
their entirety.
FIELD
[0002] The invention relates to a process for the industrial
electrolytic production of sodium chlorate having a high yield and
high electrical efficiency.
BACKGROUND
[0003] The production of chlorates ranks among the most important
processes of industrial electrochemistry, since sodium chlorate is
the raw matter for the production of sodium perchlorate and
chlorite and more importantly of chlorine dioxide, employed for
water treatment and for bleaching in the paper industry, as a
replacement for chlorine. Sodium chlorate is commonly produced in
electrolytic cells of the undivided type starting from a sodium
chloride brine at controlled pH, with anodic production of
hypochlorite and hypochlorous acid, which quickly disproportionate
at the process temperatures (60-90.degree. C.) generating chlorate,
while hydrogen evolution takes place at the cathode side.
[0004] The electrolytic cells for chlorate production can be of the
monopolar or of the bipolar type. In the most common case, they
consist of a multiplicity of cathodes and a multiplicity of anodes
disposed in a comb-like structure and mutually intercalated.
[0005] As regards the construction materials, the anodes generally
consist of a titanium substrate activated with suitable catalytic
coatings for chlorine evolution, comprising noble metals such as
platinum, ruthenium, palladium, iridium, or oxides thereof, as such
or in admixture with other stabilising oxides. The cathodes are
generally made of a ferrous material, such as, for example, low
carbon steels, and are normally not activated. Some catalytic
coatings for hydrogen evolution suitable for ferrous cathodic
substrates are in fact known in the art, for instance comprising
molybdenum and/or tungsten alloys with iron, cobalt or nickel, in
the attempt of improving the process voltage and thus decreasing
the rather high energy costs; the voltage gain obtained with these
types of activation is nevertheless considered too small for
justifying the adoption thereof in industrial manufacturing
processes.
[0006] As regards the details of the chlorate manufacturing
process, the electrolyte initially consisting of a sodium chloride
brine is either progressively enriched in chlorate until reaching
the required concentration in a batch cycle, or it is at least
partially withdrawn at the cell outlet and subjected to a chlorate
separation process, while a restoration of sodium chloride
concentration is simultaneously carried out in the cell. In both
cases, the control of pH is an essential factor to keep the
efficiency high, due to the competition between the chlorate
generation reaction and the anodic oxygen evolution, and even more
between the cathodic hydrogen evolution and the undesirable
hypochlorite reduction; the optimum pH interval to maximise the
efficiency ranges between 6 and 7, and even more preferably between
6.3 and 6.6. To keep these optimum pH values, it is necessary to
buffer the process electrolyte, as will be evident to those skilled
in the art; for this purpose, the currently existing plants of
chlorate electrolytic industrial production resort to the addition
of sensible quantities of dichromate ion (3 to 5 g/l), implying a
series of annoying secondary problems. The presence of dichromate
(and of chromate in equilibrium therewith) is for instance
undesirable in the subsequent chlorine dioxide manufacturing
process, and its separation from chlorate by crystallisation is
hindered by the very similar solubility. Furthermore, the toxicity
of hexavalent chromium increases the treatment cost of process
exhausts.
[0007] It would be desirable to provide a system for sodium
chlorate production with low energy consumption making use of a nil
or extremely limited amount of chromium compounds.
DETAILED DESCRIPTION
[0008] The invention comprises a system for sodium chlorate
production having electrolytic cells fed with a buffered sodium
chloride brine, wherein the buffering agent comprises phosphate
ions at a concentration not lower than 1 g/l. Phosphate ion
concentration, as defined herein, refers to the sum of the
concentrations of all the ionic species derived from phosphoric
acid according to their mutual equilibrium in aqueous solutions,
for instance comprising H.sub.2PO.sub.4.sup.-, HPO.sub.4.sup.2-,
PO.sub.4.sup.3- anions and, optionally, the oligomers derived
therefrom. In one embodiment, the sodium chloride brine of the
system contains chromate and/or dichromate ions at a concentration
not higher than 0.1 g/l. In a further embodiment, the sodium
chloride brine is free of chromium in any form.
[0009] The electrolytic cells of the system of the invention are
equipped with cathodes consisting of a ferrous matrix, for instance
carbon steel, activated with a coating consisting of a molybdenum
or tungsten alloy with a metal comprising iron, cobalt or nickel.
The inventors have in fact surprisingly noticed that the voltage
decrease observed with this type of alloys, which is limited to
100-150 mV with the brines of the prior art at the usual current
densities of industrial processes (2.5-3 kA/m.sup.2) reaches
450-500 mV with the sodium chloride brine added with phosphate ions
in accordance with the invention. The gain in terms of energy
efficiency is therefore so high that it largely justifies resorting
to cathodes activated with this type of coating, notwithstanding
the higher manufacturing costs. The inventors noticed the
surprising efficiency of Fe--Mo alloy coatings in a weight ratio
comprised between 30:70 and 70:30, but this kind of effect can be
observed also with other formulations. Without wishing the
invention to be bound to any particular theory, it might be assumed
that the effect of ionic species added to the brine is not only
buffering the pH, but also adsorbing to the cathode surfaces,
creating films which inhibit the decomposition of the generated
chlorate or the undesirable cathodic reduction of hypochlorite. The
catalytic effect of coatings such as Fe--Mo alloy can be
attributable in part to the higher ionic adsorption and to the
formation of inhibiting films of higher efficacy, most likely due
to their reduced thickness. Such an effect is already sensible with
the chromium oxide polymer films generated under the effect of
chromate or dichromate adsorption, but it is much more evident in
presence of films containing phosphoric species. The cathodic
catalytic coating as herein described can be applied galvanically,
with a thickness comprised between 10 and 50 micrometres.
EXAMPLE 1
[0010] A series of cathodes for an electrolytic cell was prepared
starting from 0.5 mm thick carbon steel perforated sheets. The
sheets were degreased in a saturated solution of caustic soda in
ethanol for 5 minutes and then etched in 25% by weight HCl for 2
minutes. The samples were then rinsed with distilled water, dried,
weighed and immersed in a bath for Fe--Mo alloy electrodeposition.
The bath was prepared by dissolution of 9 g/l FeCl.sub.3, 40 g/l
Na.sub.2MoO.sub.4, 75 g/l NaHCO.sub.3 and 45 g/l
Na.sub.2P.sub.2O.sub.7 in distilled water, and the deposition was
carried out at a constant current density of 100 mA/cm.sup.2 at a
temperature of 60.degree. C., making use of a platinum fine mesh as
the counterelectrode, under stirring. The deposition was protracted
until obtaining a 20 micrometre thick alloy comprised of 47% by
weight molybdenum and 53% by weight iron, as detected by a
subsequent EDS test (X-ray energy dispersion spectroscopy).
[0011] The so obtained samples were installed in a commercial cell
for chlorate production, intercalated in a comb-like fashion with a
series of titanium anodes activated with ruthenium and titanium
oxides as known in the art, and subjected to a series of
electrochemical characterisations as disclosed hereafter. Another
cell equivalent to the former was also assembled, the only
difference being the cathodes, obtained from the same carbon steel
perforated sheet but free of catalytic coating.
EXAMPLE 2
[0012] The cells of example 1, one comprising Fe--Mo alloy-coated
steel cathodes and the other with non-activated cathodes, were
employed in a discontinuous sodium chlorate manufacturing process.
The feed brine had an initial composition of 300 g/l NaCl added
with 3 g/l of Na.sub.2Cr.sub.2O.sub.7, as known in the art. The
initial feed pH was 6.41. Each of the two cells was operated at a
current density of 2.5 kA/m.sup.2 at a temperature of 61.degree.
C., and the test was protracted for 8 hours, until obtaining a
chlorate concentration of about 0.8 mol/l. The cell with the
activated cathodes worked at a very stable voltage, comprised
between 3.01 and 3.02 V, with a 98% efficiency. The cell with the
non activated cathodes worked at a voltage comprised between 3.14
and 3.17 V with 97% efficiency. In both cases, the hypochlorite
concentration was quickly stabilised at a value of 0.06 mol/l.
EXAMPLE 3
[0013] The test of example 2 was repeated with a feed brine having
a starting composition of 300 g/l NaCl added with 3 g/l of sodium
acid phosphates (as the sum of Na.sub.2HPO.sub.4 and
NaH.sub.2PO.sub.4) and 0.1 g/l Na.sub.2Cr.sub.2O.sub.7, in
accordance with the invention. The initial feed pH was 6.40. Each
of the two cells, equipped with new cathodes, was operated at a
current density of 2.5 kA/m.sup.2 at a temperature comprised
between 60 and 61.degree. C., and the test was protracted for 8
hours, until obtaining a chlorate concentration of about 0.8
mol/l.
[0014] The cell with the activated cathodes worked at a voltage
comprised between 2.86 and 2.87 V, with a 97% efficiency. The cell
with the non-activated cathodes worked at a voltage comprised
between 3.08 and 3.12 V, with 91% efficiency. The hypochlorite
concentration was quickly stabilised at a value of 0.06 mol/l for
the cell with activated cathodes, and of 0.07 mol/l for the cell
with non-activated cathodes.
EXAMPLE 4
[0015] The test of example 2 was repeated with a feed brine having
a starting composition of 300 g/l NaCl added with 3 g/l of sodium
acid phosphates (as the sum of Na.sub.2HPO.sub.4 and
NaH.sub.2PO.sub.4) and free of chromium, in accordance with the
invention. The initial feed pH was 6.41. Each of the two cells,
equipped with new cathodes, was operated at a current density of
2.5 kA/m.sup.2 at a temperature of 61.degree. C., and the test was
protracted for 8 hours, until obtaining a chlorate concentration of
about 0.8 mol/l.
[0016] The cell with the activated cathodes worked at a voltage
comprised between 2.50 and 2.53 V, with a 94% efficiency. The cell
with the non activated cathodes worked at a voltage comprised
between 3.16 and 3.17 V with 72% efficiency. The hypochlorite
concentration was quickly stabilised at a value of 0.065 mol/l for
the cell with activated cathodes, and of 0.076 mol/l for the cell
with non activated cathodes.
[0017] The examples demonstrate that a reduction in the energy
consumption of the electrolytic manufacturing process of sodium
chlorate starting from sodium chloride is made possible by the
system of the invention, while reducing or eliminating the content
of chromium used for buffering the feed solution.
[0018] Example 2 illustrates, as known by those skilled in the art,
that the activation of cathodes consisting of a ferrous substrate
by means of a molybdenum and iron alloy in combination with a brine
of the prior art improves the electrochemical performances and the
process efficiency. The extent of such improvement is nevertheless
rather modest.
[0019] Example 3 illustrates that the brine in accordance with the
invention, with a significant phosphate content, allows reduction
of the addition of chromium to minimum levels, maintaining the
process efficiency at acceptable levels and making use of
non-activated cathodes. Moreover, the energy savings obtainable
through the use of non-activated cathodes is more than interesting,
and the efficiency in this case is substantially preserved.
[0020] Example 4 illustrates that the brine totally free of
chromium according to an embodiment of the invention, coupled with
the use of activated cathodes, allows such a high energy saving
that the small efficiency loss of the process can be considered
negligible, also in view of the lower cost for the treatment of
exhausts permitted by the absence of chromium. The total
elimination of chromium, on the other hand, does not allow the use
of non-activated cathodes any more, because the process efficiency
is lowered to non-acceptable levels.
[0021] The foregoing description is not intended to limit the
invention, which may be used according to different embodiments
without departing from the scope thereof, and whose extent is
unequivocally defined by the appended claims. Throughout the
description and claims of the present application, the term
"comprise" and variations thereof such as "comprising" and
"comprises" are not intended to exclude the presence of other
elements or additives.
* * * * *