Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-11-06
2002-03-05
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S290130, C204S290140, C204S290110, C204S290010
Reexamination Certificate
active
06352625
ABSTRACT:
The present invention relates to a cathode, which can be used for the preparation of an alkali metal chlorate by electrolysis of the corresponding chloride, and its manufacturing process.
Although the activation of cathodes for the electrolytic synthesis of sodium chlorate has formed the subject of many papers, there have, however, been very few studies devoted to the formation of specific cathodes.
It is known that, in the electrolytic preparation of sodium chlorate, there are, in parallel with the reactions leading to the final product, many secondary reactions. Thus, at the cathode, apart from the reduction of water to hydrogen, a hypochlorite ion reduction reaction occurs.
Sodium chlorate is manufactured on an industrial scale in electrolytic cells, each of them comprising several mild-steel cathodes and several titanium anodes coated with ruthenium oxide. They are generally supplied with an electrolytic solution consisting of approximately 100 g/l of sodium chloride, of approximately 600 g/l of sodium chlorate and of sodium dichromate in an amount lying between 2 and 5 g/l. The latter is used to reduce or even eliminate the hypochlorite ion reduction reaction.
Despite the major role played by the dichromate in the reduction of hypochlorite ions and its ease of use, chromium (VI) is at the present time under threat because the alkali metal chlorate thus prepared requires a purification step, but above all because it pollutes the environment. Consequently, it is obviously important, from an ecological standpoint, to find a replacement solution.
Thus, document U.S. Pat. No. 4,295,951 proposes the use of a cathode whose substrate, made of titanium, iron or a titanium alloy, is coated with a non-conducting protective layer consisting of a film of halopolymers, such as Teflon®.
Moreover, French Patent FR 2,311,108 has disclosed a cathode in which the substrate is a plate made of titanium, zirconium, niobium or an alloy essentially consisting of a combination of these metals and applied to this substrate is a layer of a metal oxide, essentially consisting of an oxide of one or more metals chosen from ruthenium, rhodium, palladium, osmium, iridium and platinum and optionally an oxide of one or more metals chosen from calcium, magnesium, strontium, barium, zinc, chromium, molybdenum, tungsten, selenium and tellurium.
However, according to LINDBERGH and SIMONSON, Journal of the Electrochemical Society, 1990, Vol. 137, No. 10, p. 3094-3099, these cathodes only allow the kinetics of the hypochlorite ion reduction reaction to be slowed down but do not allow the reaction to be eliminated.
The Applicant has now discovered a cathode which allows the hypochlorite ion reduction reaction to be inhibited while still retaining good properties with respect to the water reduction reaction.
This specific cathode comprises a substrate made of an element chosen from the group formed by titanium, nickel, tantalum, zirconium, niobium and alloys thereof, the said substrate being coated with an interlayer of a mixed oxide based on titanium and on ruthenium and with an external layer of metal oxides comprising titanium, zirconium and ruthenium.
Advantageously, the interlayer contains a mixed oxide of titanium and ruthenium.
Preferably, the external layer of metal oxides contains titanium, zirconium and ruthenium.
Better still, the external layer mainly consists of ZrTiO
4
accompanied by RuO
2
and optionally by ZrO
2
and/or TiO
2
.
According to the invention, it is preferable to use, as substrate, titanium or nickel or alloys of titanium or nickel. Better still, it is preferred to use titanium.
The ruthenium/titanium molar ratio in the interlayer preferably lies between 0.4 and 2.4.
The zirconium/titanium molar ratio in the external layer generally lies between 0.25 and 9, preferably between 0.5 and 2.
The ruthenium in the external layer represents between 0.1 and 10 mol %, preferably between 0.1 and 5 mol % with respect to the metals in the composition of this layer.
Another subject of the invention is the process for preparing the specific cathode, comprising the following steps:
a) pretreatment of a substrate in order to give the surface roughness characteristics,
b) coating of the pretreated substrate using a solution A containing essentially titanium and ruthenium, followed by drying and then calcining of the substrate thus coated,
c) coating of the substrate obtained at b) using a solution B comprising titanium, zirconium and ruthenium, followed by drying and calcining of the substrate.
The pretreatment generally consists in subjecting the substrate either to sand blasting followed by washing in acid, or to pickling using an aqueous solution of oxalic acid, hydrofluoric acid, a mixture of hydrofluoric acid and nitric acid, a mixture of hydrofluoric acid and glycerol, a mixture of hydrofluoric acid, nitric acid and glycerol or a mixture of hydrofluoric acid, nitric acid and hydrogen peroxide, followed by washing one or more times in degasified demineralized water.
The substrate may be in the form of a solid plate, a perforated plate, expanded metal or a cathode basket made from expanded or perforated metal.
Solution A is generally prepared by making essentially an inorganic or organic salt of titanium and of ruthenium react, at room temperature and with stirring, with water or in an organic solvent, optionally in the presence of a chelating agent. The temperature may be raised slightly above room temperature in order to help to dissolve the salts.
Advantageously, an inorganic or organic salt of titanium and of ruthenium are made to react with water or in an organic solvent, optionally in the presence of a chelating agent.
The titanium and ruthenium are preferably each present in solution A with a concentration ranging from 0.5 to 10 mol/l.
Solution B is generally prepared by making an inorganic or organic salt of titanium, of zirconium, of ruthenium and optionally of other metals react, at room temperature and with stirring, with water or in an organic solvent, optionally in the presence of a chelating agent. When the reaction is exothermic, an ice bath is used to cool the reaction mixture.
Advantageously, an inorganic or organic salt of titanium, of zirconium and of ruthenium is made to react with water or in an organic solvent, optionally in the presence of a chelating agent.
The preferred salts of titanium and of ruthenium are chlorides, oxychlorides, nitrates, oxynitrates, sulphates and alkoxides. Advantageously, ruthenium chlorides, titanium chlorides and titanium oxychlorides are used.
As zirconium salts, chlorides, sulphates, zirconyl chlorides, zirconyl nitrates, and alkoxides, such as butyl zirconate, may be used.
Zirconium and zirconyl chlorides are particularly preferred.
As organic solvent, mention may be made of light alcohols, preferably isopropanol and ethanol and even more preferably absolute Isopropanol and absolute ethanol.
Although water or an organic solvent can be used, indiscriminately, to prepare solution B, it is preferred, however, to use an organic solvent when the metal salts are solid at room temperature.
Thus, when the metal salt is zirconium chloride, absolute ethanol or absolute isopropanol are used as solvent.
Titanium and zirconium are generally each present in solution B with a concentration ranging from 0.5 to 5 mol/l. The ruthenium concentration in solution B is generally between 10
−3
and 10
−1
mol/l, preferably between 10
−3
and 5×10
−2
mol/l.
Solution A may be deposited on the pretreated substrate by using various techniques, such as sol-gel, electroplating, galvanic electrodeposition, spraying or coating. Advantageously, the pretreated substrate is coated with solution A, for example using a brush. The substrate thus coated is then dried in air and/or in an oven at a temperature of less than 150° C. After drying, the substrate is calcined in air at a temperature of between 300 and 600° C. and preferably of between 450 and 550° C. for a time ranging from 10 minutes to 2 hours.
For step (c) of the process according to the p
Andolfatto Franåoise
Delmas François
Atofina
Bell Bruce F.
Pennie & Edmonds LLP
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