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
  • C25B1/22—Inorganic acids

Definitions

• the invention relates to a process for the preparation of alkali dichromates and chromic acid by electrolysis of alkali monochromate or alkali dichromate solutions.
• alkali dichromates alkali monochromate solutions or suspensions are introduced into the anode compartment of the cell and converted into an alkali dichromate solution by selectively transferring alkali ions through the membrane into the cathode compartment.
• alkali dichromate or alkali monochromate solutions or a mixture of alkali dichromate and alkali monochromate solutions are introduced into the anode compartment and converted into solutions containing chromic acid.
• an alkaline solution containing alkali ions is obtained in the cathode compartment, which can consist, for example, of an aqueous sodium hydroxide solution or, as described in CA-A-739 447, of an aqueous solution containing sodium carbonate.
• the solutions formed in the anode compartments of the cells are concentrated, it being possible for crystallization of sodium dichromate to take place, for example, at 80 ° C. and that of chromic acid at 60-100 ° C.
• the crystallized products are separated off, optionally washed and dried.
• anodes made of lead and lead alloys are suitable as anode materials. These have the disadvantage that lead ions pass into the solution in the anode compartment as a result of corrosion, which leads to impurities in the alkali metal dichromates and chromic acid produced.
• further suitable anodes are so-called dimensionally stable anodes, which consist of a valve metal such as titanium or tantalum with an electrocatalytically active layer of noble metal or a noble metal oxide.
• dimensionally stable anodes consist of a valve metal such as titanium or tantalum with an electrocatalytically active layer of noble metal or a noble metal oxide.
• such anodes only have a limited life span of less than 100 days, in particular at electrolysis temperatures above 60 ° C. and anodic current densities of 2-5 kA / m2. This lifespan is not sufficient for economical operation of an electrolytic alkali dichromate and chromic acid production.
• the object of the invention was to provide economical processes for the production of alkali dichromates and chromic acid. It has now been found that anodes made of valve metals, which are activated by electrodeposition of precious metals and / or precious metal compounds from melts containing precious metal salts, are outstandingly suitable for producing alkali metal dichromates and chromic acid.
• the invention thus relates to a process for the preparation of alkali dichromates and chromic acid by electrolysis of alkali monochromate or alkali dichromate solutions, which is characterized in that dimensionally stable anodes of valve metals are used which are activated by electrodeposition of noble metals and / or noble metal compounds from melts containing noble metal salts are.
• Anodes which are coated with platinum, iridium, with platinum and iridium compounds or alloys of the elements mentioned, produced by electrolysis of molten salts of the corresponding elements, are particularly preferred.
• the alloys can also contain proportions of platinum and iridium compounds, in particular oxides.
• Suitable anodes which have the properties mentioned are described, for example, in the magazine METALL, 34, volume, issue 11, November 1980, pages 1016 to 1018 and in the magazine Galvanotechnik, volume 70, 1979, pages 420 to 428.
• Anodes of this type are characterized in particular at electrolysis temperatures of 70 to 90 ° C and at current densities of 2 to 5 kA / m2 by a lifespan of well over 100 days without any significant change in the initial cell voltage.
• the use of these anodes allows a particularly economical operation of alkali dichromate and chromic acid production. For example, there is no need to change the anodes frequently, combined with a loss of production.
• due to the very good resistance of these anodes at temperatures above 70 ° C the specific energy consumption of the electrolysis is consistently low.
• the electrolytic cells used in the examples consisted of anode compartments made of pure titanium and cathode compartments made of stainless steel. Cation exchange membranes from Du Pont with the designation Nafion® 324 were used as the membrane.
• the cathodes were made of stainless steel and the anodes were made of titanium with the electrocatalytically active coatings described in the individual examples. The distance between the electrodes and the membrane was 1.5 mm in all cases. Sodium dichromate solutions of different concentrations were introduced into the anode compartments. Water was supplied to the cathode compartments at such a rate that 20% sodium hydroxide solution left the cells.
• the electrolysis temperature was 80 ° C in all cases and the current density was 3 kA / m2 projected front surface of the anodes and cathodes.
• a sodium dichromate solution with 900 g / l Na2Cr2O7 ⁇ 2 H2O was electrolytically converted into a solution containing chromic acid in the electrolysis cell described.
• the rate of introduction of the sodium dichromate solution was chosen so that a molar ratio of sodium ions to chromium (VI) of 0.32 was established in the anolyte leaving the cell.
• a titanium expanded metal anode with a platinum / iridium layer was used, which was produced by the so-called baking process as follows: A titanium expanded metal anode with a front projected area of 10 cm ⁇ 3.6 cm was removed after removal of the oxide layer and etching with oxalic acid a HCl-containing solution of platinum tetrachloride and iridium tetrachloride in 1-butanol wetted with a hair brush. The weight ratio of platinum to iridium of this solution was 3.6 to 1. The wetted anode was dried at 250 ° C. for 15 minutes and then tempered in an oven at 450 ° C. for 20 to 30 minutes. This measure was repeated six times, the heat treatment being carried out only after every second step after wetting and drying. The finished anode had a layer with approx. 18 mg platinum and 5 mg iridium.
• a sodium dichromate solution with 900 g / l Na2Cr2O7 ⁇ 2 H2O was converted electrolytically into a solution containing chromic acid.
• the rate of introduction of the sodium dichromate solution was chosen so that in the anolyte leaving the cell molar ratios of sodium ions to chromium (VI) set from 0.30 to 0.73, the molar ratios being changed gradually.
• the cell voltage rose from 4.7 V to 7.8 V within 18 days.
• the cause of this voltage rise was an almost complete destruction of the electrocatalytically active layer.
• a titanium expanded metal anode with a front projected area of 11.4 cm ⁇ 6.7 cm was used with a platinum layer produced by means of hot-dip galvanizing, as described in the magazine METALL, 34, volume, issue 11, November 1980, pages 1016 to 1018 is described.
• the platinum layer thickness of this anode was 2.5 ⁇ m.
• a solution with 800 g / l Na2Cr2O7 ⁇ 2 H2O was converted into a solution containing chromic acid.
• the rate of introduction of the sodium dichromate solution was chosen so that a molar ratio of sodium ions to chromium (VI) of 0.61 was established in the anolyte leaving the cell.

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• Chemical & Material Sciences (AREA)
• Inorganic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Inorganic Compounds Of Heavy Metals (AREA)

Applications Claiming Priority (2)

Publications (3)

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• 1988
  • 1988-08-27 DE DE3829119A patent/DE3829119A1/de not_active Withdrawn
• 1989
  • 1989-08-07 MX MX017093A patent/MX169889B/es unknown
  • 1989-08-10 RO RO141222A patent/RO107135B1/ro unknown
  • 1989-08-11 US US07/392,873 patent/US4981573A/en not_active Expired - Lifetime
  • 1989-08-15 DE DE8989115032T patent/DE58901476D1/de not_active Expired - Lifetime
  • 1989-08-15 ES ES198989115032T patent/ES2031323T3/es not_active Expired - Lifetime
  • 1989-08-15 EP EP89115032A patent/EP0356804B1/de not_active Expired - Lifetime
  • 1989-08-22 TR TR89/0675A patent/TR24791A/xx unknown
  • 1989-08-23 SU SU894614833A patent/SU1741612A3/ru active
  • 1989-08-23 JP JP1215129A patent/JP2839153B2/ja not_active Expired - Lifetime
  • 1989-08-24 BR BR898904255A patent/BR8904255A/pt not_active Application Discontinuation
  • 1989-08-25 AR AR89314749A patent/AR242995A1/es active
  • 1989-08-25 ZA ZA896497A patent/ZA896497B/xx unknown
  • 1989-08-25 CA CA000609437A patent/CA1337806C/en not_active Expired - Fee Related
  • 1989-08-25 DD DD89332097A patent/DD284059A5/de not_active IP Right Cessation
  • 1989-08-26 KR KR1019890012189A patent/KR970003073B1/ko not_active IP Right Cessation

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