CA2631817A1 - System for the electrolytic production of sodium chlorate - Google Patents
System for the electrolytic production of sodium chlorate Download PDFInfo
- Publication number
- CA2631817A1 CA2631817A1 CA002631817A CA2631817A CA2631817A1 CA 2631817 A1 CA2631817 A1 CA 2631817A1 CA 002631817 A CA002631817 A CA 002631817A CA 2631817 A CA2631817 A CA 2631817A CA 2631817 A1 CA2631817 A1 CA 2631817A1
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- Canada
- Prior art keywords
- cathodes
- chlorate
- anodes
- coating
- electrolytic cell
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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- 238000004519 manufacturing process Methods 0.000 title claims abstract description 19
- BZSXEZOLBIJVQK-UHFFFAOYSA-N 2-methylsulfonylbenzoic acid Chemical compound CS(=O)(=O)C1=CC=CC=C1C(O)=O BZSXEZOLBIJVQK-UHFFFAOYSA-N 0.000 title claims abstract description 11
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 claims abstract description 20
- 239000012267 brine Substances 0.000 claims abstract description 15
- HPALAKNZSZLMCH-UHFFFAOYSA-M sodium;chloride;hydrate Chemical compound O.[Na+].[Cl-] HPALAKNZSZLMCH-UHFFFAOYSA-M 0.000 claims abstract description 15
- 238000000576 coating method Methods 0.000 claims abstract description 13
- 239000011780 sodium chloride Substances 0.000 claims abstract description 10
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims abstract description 9
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 9
- 239000011651 chromium Substances 0.000 claims abstract description 9
- 239000011248 coating agent Substances 0.000 claims abstract description 9
- NBIIXXVUZAFLBC-UHFFFAOYSA-K phosphate Chemical compound [O-]P([O-])([O-])=O NBIIXXVUZAFLBC-UHFFFAOYSA-K 0.000 claims abstract description 6
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 9
- CWYNVVGOOAEACU-UHFFFAOYSA-N Fe2+ Chemical compound [Fe+2] CWYNVVGOOAEACU-UHFFFAOYSA-N 0.000 claims description 7
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims description 6
- 229910000975 Carbon steel Inorganic materials 0.000 claims description 5
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 claims description 5
- 229910052742 iron Inorganic materials 0.000 claims description 5
- 239000011733 molybdenum Substances 0.000 claims description 5
- 229910001182 Mo alloy Inorganic materials 0.000 claims description 4
- 239000010962 carbon steel Substances 0.000 claims description 4
- ZCDOYSPFYFSLEW-UHFFFAOYSA-N chromate(2-) Chemical compound [O-][Cr]([O-])(=O)=O ZCDOYSPFYFSLEW-UHFFFAOYSA-N 0.000 claims description 4
- 239000011159 matrix material Substances 0.000 claims description 4
- 229910001080 W alloy Inorganic materials 0.000 claims description 3
- 239000006172 buffering agent Substances 0.000 claims description 3
- 229910017052 cobalt Inorganic materials 0.000 claims description 3
- 239000010941 cobalt Substances 0.000 claims description 3
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 claims description 3
- 229910052759 nickel Inorganic materials 0.000 claims description 3
- -1 dichromate ions Chemical class 0.000 claims description 2
- 229910052751 metal Inorganic materials 0.000 claims description 2
- 239000002184 metal Substances 0.000 claims description 2
- 229910052750 molybdenum Inorganic materials 0.000 claims description 2
- 229920000136 polysorbate Polymers 0.000 claims 1
- 229910045601 alloy Inorganic materials 0.000 abstract description 7
- 239000000956 alloy Substances 0.000 abstract description 7
- 229910017116 Fe—Mo Inorganic materials 0.000 abstract description 5
- 229910019142 PO4 Inorganic materials 0.000 abstract description 2
- 229910000831 Steel Inorganic materials 0.000 abstract description 2
- 239000010452 phosphate Substances 0.000 abstract description 2
- 239000010959 steel Substances 0.000 abstract description 2
- XTEGARKTQYYJKE-UHFFFAOYSA-M Chlorate Chemical class [O-]Cl(=O)=O XTEGARKTQYYJKE-UHFFFAOYSA-M 0.000 description 27
- WQYVRQLZKVEZGA-UHFFFAOYSA-N hypochlorite Chemical compound Cl[O-] WQYVRQLZKVEZGA-UHFFFAOYSA-N 0.000 description 12
- 238000000034 method Methods 0.000 description 10
- 230000003197 catalytic effect Effects 0.000 description 5
- OSVXSBDYLRYLIG-UHFFFAOYSA-N dioxidochlorine(.) Chemical compound O=Cl=O OSVXSBDYLRYLIG-UHFFFAOYSA-N 0.000 description 4
- 230000000694 effects Effects 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 4
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 3
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- SOCTUWSJJQCPFX-UHFFFAOYSA-N dichromate(2-) Chemical compound [O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O SOCTUWSJJQCPFX-UHFFFAOYSA-N 0.000 description 3
- 239000001257 hydrogen Substances 0.000 description 3
- 229910052739 hydrogen Inorganic materials 0.000 description 3
- 239000000758 substrate Substances 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 2
- 239000004155 Chlorine dioxide Substances 0.000 description 2
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 2
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 2
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 2
- UIIMBOGNXHQVGW-UHFFFAOYSA-M Sodium bicarbonate Chemical compound [Na+].OC([O-])=O UIIMBOGNXHQVGW-UHFFFAOYSA-M 0.000 description 2
- 230000004913 activation Effects 0.000 description 2
- 230000003139 buffering effect Effects 0.000 description 2
- 239000000460 chlorine Substances 0.000 description 2
- 229910052801 chlorine Inorganic materials 0.000 description 2
- 235000019398 chlorine dioxide Nutrition 0.000 description 2
- 238000000151 deposition Methods 0.000 description 2
- 230000008021 deposition Effects 0.000 description 2
- BNIILDVGGAEEIG-UHFFFAOYSA-L disodium hydrogen phosphate Chemical compound [Na+].[Na+].OP([O-])([O-])=O BNIILDVGGAEEIG-UHFFFAOYSA-L 0.000 description 2
- 229910000397 disodium phosphate Inorganic materials 0.000 description 2
- 235000019800 disodium phosphate Nutrition 0.000 description 2
- 239000012153 distilled water Substances 0.000 description 2
- 239000003792 electrolyte Substances 0.000 description 2
- 238000005265 energy consumption Methods 0.000 description 2
- 235000019799 monosodium phosphate Nutrition 0.000 description 2
- 229910052697 platinum Inorganic materials 0.000 description 2
- 238000000926 separation method Methods 0.000 description 2
- 235000019980 sodium acid phosphate Nutrition 0.000 description 2
- KIEOKOFEPABQKJ-UHFFFAOYSA-N sodium dichromate Chemical compound [Na+].[Na+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O KIEOKOFEPABQKJ-UHFFFAOYSA-N 0.000 description 2
- AJPJDKMHJJGVTQ-UHFFFAOYSA-M sodium dihydrogen phosphate Chemical compound [Na+].OP(O)([O-])=O AJPJDKMHJJGVTQ-UHFFFAOYSA-M 0.000 description 2
- 229910000162 sodium phosphate Inorganic materials 0.000 description 2
- 238000001179 sorption measurement Methods 0.000 description 2
- 239000010936 titanium Substances 0.000 description 2
- 229910052719 titanium Inorganic materials 0.000 description 2
- 229910000640 Fe alloy Inorganic materials 0.000 description 1
- 229910004619 Na2MoO4 Inorganic materials 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- WGLPBDUCMAPZCE-UHFFFAOYSA-N Trioxochromium Chemical compound O=[Cr](=O)=O WGLPBDUCMAPZCE-UHFFFAOYSA-N 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 229910000147 aluminium phosphate Inorganic materials 0.000 description 1
- 150000001450 anions Chemical class 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000004061 bleaching Methods 0.000 description 1
- 239000000872 buffer Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 229910001919 chlorite Inorganic materials 0.000 description 1
- 229910052619 chlorite group Inorganic materials 0.000 description 1
- QBWCMBCROVPCKQ-UHFFFAOYSA-N chlorous acid Chemical compound OCl=O QBWCMBCROVPCKQ-UHFFFAOYSA-N 0.000 description 1
- 150000001845 chromium compounds Chemical class 0.000 description 1
- 229910000423 chromium oxide Inorganic materials 0.000 description 1
- JOPOVCBBYLSVDA-UHFFFAOYSA-N chromium(6+) Chemical compound [Cr+6] JOPOVCBBYLSVDA-UHFFFAOYSA-N 0.000 description 1
- 239000004035 construction material Substances 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 229940077449 dichromate ion Drugs 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 238000002848 electrochemical method Methods 0.000 description 1
- 230000005518 electrochemistry Effects 0.000 description 1
- 238000004070 electrodeposition Methods 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 239000012527 feed solution Substances 0.000 description 1
- 238000009472 formulation Methods 0.000 description 1
- QWPPOHNGKGFGJK-UHFFFAOYSA-N hypochlorous acid Chemical compound ClO QWPPOHNGKGFGJK-UHFFFAOYSA-N 0.000 description 1
- 238000009776 industrial production Methods 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 229910000510 noble metal Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- SOQBVABWOPYFQZ-UHFFFAOYSA-N oxygen(2-);titanium(4+) Chemical class [O-2].[O-2].[Ti+4] SOQBVABWOPYFQZ-UHFFFAOYSA-N 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 235000021317 phosphate Nutrition 0.000 description 1
- 229940085991 phosphate ion Drugs 0.000 description 1
- 229920006254 polymer film Polymers 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
- 229910001925 ruthenium oxide Inorganic materials 0.000 description 1
- 239000012047 saturated solution Substances 0.000 description 1
- 235000017557 sodium bicarbonate Nutrition 0.000 description 1
- 229910000030 sodium bicarbonate Inorganic materials 0.000 description 1
- 235000011121 sodium hydroxide Nutrition 0.000 description 1
- 235000015393 sodium molybdate Nutrition 0.000 description 1
- 239000011684 sodium molybdate Substances 0.000 description 1
- TVXXNOYZHKPKGW-UHFFFAOYSA-N sodium molybdate (anhydrous) Chemical compound [Na+].[Na+].[O-][Mo]([O-])(=O)=O TVXXNOYZHKPKGW-UHFFFAOYSA-N 0.000 description 1
- BAZAXWOYCMUHIX-UHFFFAOYSA-M sodium perchlorate Chemical compound [Na+].[O-]Cl(=O)(=O)=O BAZAXWOYCMUHIX-UHFFFAOYSA-M 0.000 description 1
- 229910001488 sodium perchlorate Inorganic materials 0.000 description 1
- 238000004611 spectroscopical analysis Methods 0.000 description 1
- 230000003019 stabilising effect Effects 0.000 description 1
- 238000003756 stirring Methods 0.000 description 1
- 150000003608 titanium Chemical class 0.000 description 1
- OGIDPMRJRNCKJF-UHFFFAOYSA-N titanium oxide Inorganic materials [Ti]=O OGIDPMRJRNCKJF-UHFFFAOYSA-N 0.000 description 1
- 230000001988 toxicity Effects 0.000 description 1
- 231100000419 toxicity Toxicity 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/24—Halogens or compounds thereof
- C25B1/26—Chlorine; Compounds thereof
- C25B1/265—Chlorates
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
- C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
- C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
- C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
- C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
- Electrodes For Compound Or Non-Metal Manufacture (AREA)
- Electrolytic Production Of Metals (AREA)
- Electroplating And Plating Baths Therefor (AREA)
Abstract
The invention relates to a system for the electrolytic production of sodium chlorate starting from a sodium chloride brine buffered with phosphate and having a reduced or zero chromium content. The system comprises electrolytic cells of the undivided type with intercalated cathodes and anodes; in one preferred embodiment, the cathodes consist of steel perforated sheets activated with a Fe-Mo alloy coating.
Description
SYSTEM FOR THE ELECTROLYTIC PRODUCTION OF SODIUM CHLORATE
DESCRIPTION OF THE INVENTION
The present invention relates to a process for the industrial electrolytic production of sodium chlorate, characterised by a high yield and a high electrical efficiency.
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 C) generating chlorate, while hydrogen evolution takes place at the cathode side.
The electrolytic cells for chlorate production can be of the monopolar or of the bipolar type: in the most common case, they consist however of a multiplicity of cathodes and a multiplicity of anodes disposed in a comb-like structure and mutually intercalated.
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 for instance as disclosed in US 3,632,498; 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, as disclosed in GB 992,350 and GB
1,004,380, 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.
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/1), 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.
It is an object of the present invention to provide a system for sodium chlorate production with low energy consumption making use of a nil or extremely limited amount of chromium compounds.
This and other objects will be made clear by the following description and examples, which are by no means intended to limit the extent of the invention.
The invention consists of a system for sodium chlorate production comprising electrolytic cells fed with a buffered sodium chloride brine, wherein the buffering agent comprises phosphate ions at a concentration not lower than 1 g/I. By phosphate ion concentration it is hereby intended 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 H2PO4 , HP042 , PO43 anions and optionally the oligomers derived therefrom. According to a preferred embodiment of the invention, the sodium chloride brine of the system of the invention contains chromate and/or dichromate ions at a concentration not higher than 0.1 g/I; in an even more preferred embodiment, the sodium chloride brine is free of chromium in any form.
In a largely preferred embodiment, 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 selected between iron, cobalt and 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/m2) reaches 450-500 mV
with the sodium chloride brine added with phosphate ions in accordance with the present 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 present 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 for instance 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 are preferably applied galvanically, with a thickness preferably comprised between 10 and 50 micrometres.
A series of cathodes for 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, then etched in 25% by weight HCI for 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/I FeCI3, 40 g/I Na2MoO4, 75 g/I NaHCO3 and 45 g/I Na2P2O7 in distilled water, and the deposition was carried out at a constant current density of 100 mA/cm2 at a temperature of 60 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).
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.
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 g/l NaCI added with 3 g/l of Na2Cr2O7, 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/m2 at a temperature of 61 C, and the test was protracted for 8 hours, until obtaining a chlorate concentration of about 0.8 mol/I. 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/I.
The test of example 2 was repeated with a feed brine having a starting composition of 300 g/I NaCI added with 3 g/I of sodium acid phosphates (as the sum of Na2HPO4 and NaH2PO4) and 0.1 g/I Na2Cr2O7, 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/m2 at a temperature comprised between 60 and 61 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 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/I for the cell with activated cathodes, and of 0.07 mol/I for the cell with non activated cathodes.
The test of example 2 was repeated with a feed brine having a starting composition of 300 g/l NaCI added with 3 g/l of sodium acid phosphates (as the sum of Na2HPO4 and NaH2PO4) and free of chromium, in accordance with the invention. The initial feed pH was 6.41. ach of the two cells, equipped with new cathodes, was operated at a current density of 2.5 kA/m2 at a temperature of 61 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 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/I
for the cell with activated cathodes, and of 0.076 mol/I for the cell with non activated cathodes.
The examples demonstrate that a downright 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, meanwhile reducing or eliminating the content of chromium used for buffering the feed solution.
Example 2 shows, 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.
Example 3 shows that the brine in accordance with the invention, with a significant phosphate content, allows reducing the addition of chromium to minimum levels, maintaining in any case the process efficiency at acceptable levels also making use of non activated cathodes. Moreover, the energy saving obtainable through the use of non activated cathodes is more than interesting, and the efficiency in this case is substantially preserved.
Example 4 shows that the brine totally free of chromium according to a preferred embodiment of the invention, coupled to 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.
The foregoing description is not intended to limit the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is univocally 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.
DESCRIPTION OF THE INVENTION
The present invention relates to a process for the industrial electrolytic production of sodium chlorate, characterised by a high yield and a high electrical efficiency.
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 C) generating chlorate, while hydrogen evolution takes place at the cathode side.
The electrolytic cells for chlorate production can be of the monopolar or of the bipolar type: in the most common case, they consist however of a multiplicity of cathodes and a multiplicity of anodes disposed in a comb-like structure and mutually intercalated.
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 for instance as disclosed in US 3,632,498; 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, as disclosed in GB 992,350 and GB
1,004,380, 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.
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/1), 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.
It is an object of the present invention to provide a system for sodium chlorate production with low energy consumption making use of a nil or extremely limited amount of chromium compounds.
This and other objects will be made clear by the following description and examples, which are by no means intended to limit the extent of the invention.
The invention consists of a system for sodium chlorate production comprising electrolytic cells fed with a buffered sodium chloride brine, wherein the buffering agent comprises phosphate ions at a concentration not lower than 1 g/I. By phosphate ion concentration it is hereby intended 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 H2PO4 , HP042 , PO43 anions and optionally the oligomers derived therefrom. According to a preferred embodiment of the invention, the sodium chloride brine of the system of the invention contains chromate and/or dichromate ions at a concentration not higher than 0.1 g/I; in an even more preferred embodiment, the sodium chloride brine is free of chromium in any form.
In a largely preferred embodiment, 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 selected between iron, cobalt and 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/m2) reaches 450-500 mV
with the sodium chloride brine added with phosphate ions in accordance with the present 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 present 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 for instance 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 are preferably applied galvanically, with a thickness preferably comprised between 10 and 50 micrometres.
A series of cathodes for 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, then etched in 25% by weight HCI for 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/I FeCI3, 40 g/I Na2MoO4, 75 g/I NaHCO3 and 45 g/I Na2P2O7 in distilled water, and the deposition was carried out at a constant current density of 100 mA/cm2 at a temperature of 60 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).
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.
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 g/l NaCI added with 3 g/l of Na2Cr2O7, 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/m2 at a temperature of 61 C, and the test was protracted for 8 hours, until obtaining a chlorate concentration of about 0.8 mol/I. 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/I.
The test of example 2 was repeated with a feed brine having a starting composition of 300 g/I NaCI added with 3 g/I of sodium acid phosphates (as the sum of Na2HPO4 and NaH2PO4) and 0.1 g/I Na2Cr2O7, 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/m2 at a temperature comprised between 60 and 61 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 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/I for the cell with activated cathodes, and of 0.07 mol/I for the cell with non activated cathodes.
The test of example 2 was repeated with a feed brine having a starting composition of 300 g/l NaCI added with 3 g/l of sodium acid phosphates (as the sum of Na2HPO4 and NaH2PO4) and free of chromium, in accordance with the invention. The initial feed pH was 6.41. ach of the two cells, equipped with new cathodes, was operated at a current density of 2.5 kA/m2 at a temperature of 61 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 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/I
for the cell with activated cathodes, and of 0.076 mol/I for the cell with non activated cathodes.
The examples demonstrate that a downright 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, meanwhile reducing or eliminating the content of chromium used for buffering the feed solution.
Example 2 shows, 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.
Example 3 shows that the brine in accordance with the invention, with a significant phosphate content, allows reducing the addition of chromium to minimum levels, maintaining in any case the process efficiency at acceptable levels also making use of non activated cathodes. Moreover, the energy saving obtainable through the use of non activated cathodes is more than interesting, and the efficiency in this case is substantially preserved.
Example 4 shows that the brine totally free of chromium according to a preferred embodiment of the invention, coupled to 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.
The foregoing description is not intended to limit the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is univocally 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.
Claims (9)
1 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 said buffering agent comprises at least 1 g/l of phosphate ions.
2. The system of claim 1, wherein said cathodes of said electrolytic cell consist of a ferrous matrix provided with a coating consisting of 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 or 2, wherein said feed contains chromate and dichromate ions at a concentration not exceeding 0.1 g/l.
4. The system of claim 1 or 2, wherein said feed is free of chromium.
5. The system of any one of claims 2 to 4, wherein said ferrous matrix of said cathodes is a carbon steel.
6. The system of claim 5, wherein said coating is a galvanic coating containing 30 to 70 %
Fe and 30 to 70 % Mo expressed as weight percentage.
Fe and 30 to 70 % Mo expressed as weight percentage.
7. The system of claim 6, wherein said galvanic coating has a thickness comprised be-tween 10 and 50 micrometres.
8. The system of any one of the previous claims, wherein said electrolytic cell is of the un-divided type and said cathodes are disposed in a comp-like fashion intercalated to said anodes.
9. The system of claim 8, wherein said cathodes comprise a ferrous matrix consisting of a perforated sheet.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT002298A ITMI20052298A1 (en) | 2005-11-30 | 2005-11-30 | SYSTEM FOR THE ELECTROLYTIC PRODUCTION OF CHLORATO SODICO |
| ITMI2005A002298 | 2005-11-30 | ||
| PCT/EP2006/069079 WO2007063081A2 (en) | 2005-11-30 | 2006-11-29 | System for the electrolytic production of sodium chlorate |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2631817A1 true CA2631817A1 (en) | 2007-06-07 |
Family
ID=38001688
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002631817A Abandoned CA2631817A1 (en) | 2005-11-30 | 2006-11-29 | System for the electrolytic production of sodium chlorate |
Country Status (9)
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|---|---|
| US (1) | US20080230381A1 (en) |
| EP (1) | EP1954855A2 (en) |
| CN (1) | CN101321897A (en) |
| AR (1) | AR058261A1 (en) |
| BR (1) | BRPI0619215A2 (en) |
| CA (1) | CA2631817A1 (en) |
| IT (1) | ITMI20052298A1 (en) |
| NO (1) | NO20082616L (en) |
| WO (1) | WO2007063081A2 (en) |
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| WO2010130546A1 (en) * | 2009-05-15 | 2010-11-18 | Akzo Nobel Chemicals International B.V. | Activation of cathode |
| EP2681159A2 (en) | 2011-03-04 | 2014-01-08 | Tennant Company | Cleaning solution generator |
| CA2750881A1 (en) * | 2011-08-23 | 2013-02-23 | Hydro-Quebec | Method for reducing the impact of impurities on electrodes used for the electrosynthesis of sodium chlorate |
| BR112014026603A2 (en) * | 2012-04-23 | 2017-06-27 | Chemetics Inc | modified stainless steel cathode for electrolyser |
| US9556526B2 (en) | 2012-06-29 | 2017-01-31 | Tennant Company | Generator and method for forming hypochlorous acid |
| CA3115138C (en) | 2018-10-02 | 2023-02-28 | Nouryon Chemicals International B.V. | Selective cathode for use in electrolytic chlorate process |
| CN113718274B (en) * | 2020-09-28 | 2023-06-20 | 上海赛一水处理科技股份有限公司 | Electrolyte in solid dosage form, preparation method and application thereof |
| IT202200025551A1 (en) * | 2022-12-14 | 2024-06-14 | Soc Chimica Bussi S P A | PROCEDURE FOR THE PRODUCTION OF CHLORATES |
Family Cites Families (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2887444A (en) * | 1957-03-25 | 1959-05-19 | George H Graves | Process of and means for chlorinating swimming pools or the like |
| GB1004380A (en) * | 1962-11-21 | 1965-09-15 | Ici Australia Ltd | Electrochemical cells and manufacture of electrodes therefor |
| GB1195871A (en) * | 1967-02-10 | 1970-06-24 | Chemnor Ag | Improvements in or relating to the Manufacture of Electrodes. |
| FR2244708B1 (en) * | 1973-09-25 | 1977-08-12 | Ugine Kuhlmann | |
| FR2446869A1 (en) * | 1979-01-16 | 1980-08-14 | Ugine Kuhlmann | IMPROVED FARADAY YIELD IN THE ELECTROLYTIC PREPARATION OF SODIUM CHLORATE |
| US4194953A (en) * | 1979-02-16 | 1980-03-25 | Erco Industries Limited | Process for producing chlorate and chlorate cell construction |
| US4354915A (en) * | 1979-12-17 | 1982-10-19 | Hooker Chemicals & Plastics Corp. | Low overvoltage hydrogen cathodes |
| US4461692A (en) * | 1982-05-26 | 1984-07-24 | Ppg Industries, Inc. | Electrolytic cell |
| NO168145C (en) * | 1989-08-21 | 1992-01-15 | Forsvarets Forsknings | CELL |
| US5084149A (en) * | 1989-12-26 | 1992-01-28 | Olin Corporation | Electrolytic process for producing chlorine dioxide |
| US5294307A (en) * | 1992-07-31 | 1994-03-15 | Huron Tech Corp | Integrated process for the production of alkali and alkaline earth metal chlorates and chlorine dioxide |
| US7329388B2 (en) * | 1999-11-08 | 2008-02-12 | Princeton Biochemicals, Inc. | Electrophoresis apparatus having staggered passage configuration |
-
2005
- 2005-11-30 IT IT002298A patent/ITMI20052298A1/en unknown
-
2006
- 2006-11-29 EP EP06819847A patent/EP1954855A2/en not_active Withdrawn
- 2006-11-29 WO PCT/EP2006/069079 patent/WO2007063081A2/en not_active Ceased
- 2006-11-29 BR BRPI0619215-7A patent/BRPI0619215A2/en not_active IP Right Cessation
- 2006-11-29 CA CA002631817A patent/CA2631817A1/en not_active Abandoned
- 2006-11-29 CN CNA200680044989XA patent/CN101321897A/en active Pending
- 2006-11-30 AR ARP060105299A patent/AR058261A1/en not_active Application Discontinuation
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2008
- 2008-05-29 US US12/128,815 patent/US20080230381A1/en not_active Abandoned
- 2008-06-12 NO NO20082616A patent/NO20082616L/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| WO2007063081A3 (en) | 2007-11-08 |
| NO20082616L (en) | 2008-06-12 |
| WO2007063081A2 (en) | 2007-06-07 |
| CN101321897A (en) | 2008-12-10 |
| AR058261A1 (en) | 2008-01-30 |
| US20080230381A1 (en) | 2008-09-25 |
| BRPI0619215A2 (en) | 2011-09-20 |
| ITMI20052298A1 (en) | 2007-06-01 |
| EP1954855A2 (en) | 2008-08-13 |
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