EP0356802B1 - Procédé électrochimique de production d'acide chromique - Google Patents

Procédé électrochimique de production d'acide chromique Download PDF

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EP0356802B1
EP0356802B1 EP89115030A EP89115030A EP0356802B1 EP 0356802 B1 EP0356802 B1 EP 0356802B1 EP 89115030 A EP89115030 A EP 89115030A EP 89115030 A EP89115030 A EP 89115030A EP 0356802 B1 EP0356802 B1 EP 0356802B1
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Prior art keywords
solution
chromic acid
electrolysis
sodium
dichromate
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German (de)
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EP0356802A3 (en
EP0356802A2 (fr
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Hans-Dieter Dr. Block
Norbert Dr. Lönhoff
Bernd Dr. Makowka
Helmut Dr. Klotz
Rainer Dr. Weber
Bernhard Dr. Spreckelmeyer
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Bayer AG
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Bayer AG
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    • 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
    • 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
    • C25B15/00—Operating or servicing cells
    • C25B15/02—Process control or regulation
    • 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
    • C25B15/00—Operating or servicing cells
    • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
    • C25B15/085—Removing impurities

Definitions

  • Chromic acid CrO3 is technically produced using three different processes:
  • the third method membrane electrolysis, avoids this disadvantage and the associated losses of chromium of sodium dichromate in aqueous solution.
  • the electrochemical process as described, for example, in Canadian patent specification CA-A-739 447, is based on the principle that is common to membrane electrolysis with a cation-selective membrane, namely the migration of the cations located in an anode chamber through the one that forms the partition to the cathode chamber cation-selective membrane in the cathode chamber under the influence of the electric field.
  • Embodiments of the electrochemical process for the production of chromic acid are described in the patent specification CA-A-739 447.
  • the sodium ions migrate in an electric field through the membrane into the cathode compartment filled with water or aqueous solution and form the hydroxide ions formed on the cathode under hydrogen evolution, an aqueous solution containing sodium ions, while in the anode chamber the remaining dichromate ions are electrically neutralized by the hydrogen cations formed on the anode with simultaneous oxygen evolution.
  • this process amounts to a substitution of the sodium ions in the sodium dichromate with hydrogen ions, that is to say the formation of chromic acid.
  • the migration of the sodium ions through the Membrane increasingly accompanied by the migration of the hydrogen ions formed in the anode compartment, so that the use of electrical current for the desired process of sodium removal from the anode part, also called current efficiency, is steadily decreasing.
  • the chromic acid must then be separated from these solutions by fractional crystallization.
  • a mother liquor remains which contains the sodium dichromate which has not yet been converted electrochemically and residues of chromic acid which have not crystallized out.
  • This solution is usefully used again for further conversion into chromic acid in the electrolysis process.
  • the following problems result from these process principles: On the one hand, the mother liquor adhering to the chromic acid crystals and consisting of almost concentrated sodium dichromate solution must be carefully washed off in order to obtain a pure product; on the other hand, all impurities which are introduced with the sodium dichromate solution are accumulated in the system and ultimately discharged with and in the chromic acid crystals, since only the electrolysis gases hydrogen and oxygen leave the process and the membrane separating the anode compartment for anions and also for polyvalent cations is largely impermeable.
  • DE-A-3 020 261 describes a process for the electrochemical production of chromic acid from dichromate, the aim of which is to operate the production of chromic acid with a high current efficiency and to remove the impurities introduced with the dichromate.
  • the process of DE-A-3 020 261 is essentially characterized by the use of a three-room cell, the dichromate solution entering the middle room and exiting from there, poor in dichromate, and sodium ions at the cathode chamber separated by a cation-selective membrane during the flow and Gives off dichromations to the anode space separated by a diaphragm or an anion-selective membrane.
  • DE-A-3 020 260 describes the cleaning of sodium chromate solution for the electrochemical production of chromic acid by subjecting the sodium chromate solution to electrolysis in the anode compartment of a two-room cell with a cation-selective partition and the cationic impurities in the membrane precipitate with simultaneous formation of sodium dichromate in the anode compartment and of an alkaline solution containing sodium ions in the cathode compartment, as is known per se from US Pat. No. 3,305,463.
  • the sodium chromate / sodium dichromate solution purified in this way is converted electrochemically into chromic acid in the aforementioned manner.
  • the object of the present invention is to provide a process with which a very pure, crystalline chromic acid is produced under economic conditions while maintaining the advantages of the electrochemical process for the production of chromic acid.
  • the present invention thus relates to a process for the preparation of chromic acid by multi-stage electrolysis of dichromate and / or monochromate solutions in two-room electrolysis cells, their anode and cathode compartments are separated by cation exchange membranes, at temperatures from 50 to 90 ° C., the dichromate and / or monochromate solutions being obtained by digestion of chromium ores and leaching, characterized in that the monochromate solution obtained after leaching, if appropriate after the removal of aluminum, Vanadium and other impurities at 20 to 110 ° C to a pH of 8 to 12 by adding and / or in situ production of carbonate in an amount of 0.01 to 0.18 mol / l (at 300 to 500 g / l Na2CrO4) sets, the precipitated carbonates or hydroxides are separated, the solution is concentrated to a content of 750 to 1000 g / l Na2CrO4, converted under pressure with CO2 into a dichromate-containing solution, which introduces dichromate
  • alkali metal dichromates and therefrom the chromic acid technically only chromium iron is used, which is mixed first with sodium carbonate or sodium carbonate / sodium hydroxide solution or sodium hydroxide solution, occasionally with the addition of alkaline earth metal oxides and / or carbonates, in particular calcium oxide and / or calcium carbonate, as an alkaline melting medium and secondly in a mixture with a lean agent consisting essentially of iron (III) oxide or hydroxide, preferably so-called re-ore from the leaching stage described below, is exposed at temperatures above 1000 ° C. to the action of oxygen-containing gases, preferably air.
  • the comminuted material is leached out, usually in countercurrent, over several stages in order to obtain sodium chromate as a solution with a Na2CrO4 content of approx. 300 to 500 g / l.
  • a pH value of 7.0 to 9.5 is necessary to keep the sodium chromate solution as low as possible. This pH adjustment can take place during leaching or in the solution after separation from the leached solid.
  • the required pH adjustment is carried out with dichromate or with chromic acid or with chromic acid / sodium dichromate mixtures or with sodium chromate / sodium dichromate solutions, preferably with those which are used later in the process after acidification Carbon dioxide occur under pressure or with mixtures of said, preferably used sodium chromate / sodium dichromate solutions with sodium dichromate-chromic acid solutions, the latter being removed from the chromic acid electrolysis crystallization cycle for the purpose of discharging impurities.
  • the sodium chromate solution freed from the impurities which can be precipitated at pH 7.0 to 9.5, unless the digestion of the chromium ore has been carried out in such a way that vanadium cannot dissolve during leaching, is now in the form of calcium in the form known per se of calcium oxide or calcium hydroxide in aqueous solution or slurry added to precipitate the vanadium as calcium vanadate.
  • the calcium is used in a stoichiometric excess, taking into account the calcium dissolved in the leaching of the kiln clinker.
  • the remaining sodium chromate solution is brought to 50 to 100.degree. C., preferably 70 to 85.degree. C., to precipitate the polyvalent ions remaining in solution despite the pH adjustment, in particular the excess calcium ions used, and with sodium hydroxide solution and carbon dioxide and / or sodium carbonate and / or sodium bicarbonate adjusted to pH 8 to 12, preferably 9.0 to 11.0.
  • the addition of carbon dioxide and / or sodium bicarbonate and / or sodium carbonate is carried out in an amount which has a concentration of carbonate ions in the amount of 0.01 to 0.18 mol / l, preferably 0.03 to 0.1 mol / l in the solution generated.
  • the precipitation can also take place in several stages increasing amounts of sodium chromate are carried out.
  • the precipitation of the calcium, the strontium and other polyvalent ions and surprisingly also the fluoride takes place, so that after the precipitation has been separated off, a sodium chromate solution with extremely low residual contents is present Contamination results.
  • the sodium chromate solution produced in this way residual calcium and strontium contents of less than 5 mg / l are present, while other polyvalent cations such as barium, magnesium, iron, zinc and others as well as fluoride ions are no longer present or only in an amount of the respective detection limit, the detection limits being between 0.5 and 1 mg / l.
  • the exchanger is to be regenerated by treatment with acid and is to be freed from the residues of the foreign anions introduced with the regenerating acid by washing with pure water and then to be converted into the sodium form with sodium hydroxide solution, so that the selective cation exchanger is then ready for use again.
  • the various techniques for loading cation exchangers with the cations to be removed from solutions, for connecting and operating different exchange units in series or in parallel, and preferably for alternating regeneration, are known from the literature.
  • the working temperature for the removal of the multivalent cations from the sodium chromate solution is 20 to 90 ° C., preferably 60 ° C to 85 ° C, the contact time of solution and exchanger is at least 2 min, preferably 6 min and more.
  • the conversion of sodium chromate into sodium dichromate takes place in the process according to the invention with carbon dioxide.
  • This so-called acidification of the sodium chromate can be carried out in one or more stages, and the first stage / the first stages can be operated without pressure;
  • a carbon dioxide pressure of 4 to 15 bar, preferably 8 to 15 bar, at a final temperature below 50 ° C. is required in the last stage or in the last stages , preferably below 30 ° C.
  • An at least 90% conversion of the sodium chromate at a pressure of more than 8 bar is preferred.
  • the sodium bicarbonate obtained here can be converted into sodium carbonate, which is useful for chromium ore digestion, by thermal treatment and / or reaction with sodium hydroxide solution.
  • a partial stream for the electrochemical generation of chromic acid is removed from the solution now present, which contains at least 80%, preferably at least 90%, of the chromium (VI) as dichromate and practically no longer contains polyvalent cations in detectable amounts.
  • Another partial flow is used for the previously described pH adjustment during / after the leaching of the kiln goods.
  • further parts of the solution of the preparation of sodium dichromate by adding sulfuric acid or by adding chromic acid or by adding chromic acid-sodium dichromate or by electrochemical acidification, as described, for example, in US Pat. No. 3,305,463 or as below for described the partial stream used for the production of chromic acid, which measures can also be taken simultaneously.
  • the combination of electrochemical acidification with a simultaneous addition of dichromate-chromic acid solution in batches or continuously is a suitable method for complete conversion of the remaining sodium chromate to sodium dichromate in the non-chromic acid partial stream.
  • the partial stream intended for this purpose is introduced into the anode chamber of a two-chamber electrolysis cell, the partition between the anode and cathode spaces of which is a cation-selective membrane, and therein into a solution which essentially contains sodium dichromate and only minor amounts of sodium chromate and / or chromic acid , electrolytically converted.
  • a larger number of such electrolysis cells e.g. can be summarized in the manner of filter presses, operated in parallel.
  • the voltage required to achieve a current density of 1 to 5 kA / m2, preferably 2.5 to 3.0 kA / m2, can either be applied individually to each cell which is electrically insulated from the other or, if the cells are conductively connected to one another, in a so-called bipolar circuit at the ends of such an electrically connected arrangement.
  • the voltage to be applied is a function of the electrode spacing and the electrode construction, the solution temperature, the solution concentration and the current strength and is 3.8 to 6.0 V per electrolysis cell.
  • Each electrolytic cell has in the anode compartment a feed line for the sodium chromate / sodium dichromate solution to be used and a drain for the electrolyzed solution essentially containing sodium dichromate.
  • Supply and drain are usually against set ends of the respective electrolysis cell, the feed advantageously being in the lower part of the electrolysis cell and the outflow in the upper part of the electrolysis cell.
  • the cathode chambers are equipped with inlet and outlet. Liquid is pumped both from the anode chamber and from the cathode chamber via external heat exchangers via separate openings in the frame of the cell or, as preferred, via the same openings as for the inlet and outlet.
  • the streams to be pumped from the entirety of the anode chambers and the cathode chambers are advantageously combined to form an anolyte stream and a catholyte stream and are conducted via an anolyte cooler and a catholyte cooler. From these coolers, the cooled anolyte and catholyte liquids are again distributed to the individual anode and cathode chambers. By means of this cooling, the temperature in the anode compartment and cathode compartment is kept at 50 ° C. to 90 ° C., preferably 70 to 80 ° C.
  • the electrolysis products oxygen and hydrogen are led out of anode spaces or cathode spaces via their own openings in the frame in the upper part of the cell and at the same time or exclusively via the same opening as the processes.
  • the gas streams are advantageously brought together separately after the gases and, if appropriate, freed of entrained solutions and then used, for example, as heating and fuel material in the chrome ore digestion furnace.
  • Water is entered into the cathode compartments either directly via the feed pipe or by adding to the catholyte liquid in the cooling circuit e.g. after catholyte cooler.
  • the cathode liquid usually consists of 8 to 30%, preferably about 12 to 20% sodium hydroxide solution;
  • the cathode compartment liquid can be modified if desired by adding agents which dull the alkali produced, for example carbon dioxide and / or sodium dichromate solution and / or sodium dichromate / sodium chromate solution from the aforementioned acidification with carbon dioxide.
  • agents which dull the alkali produced for example carbon dioxide and / or sodium dichromate solution and / or sodium dichromate / sodium chromate solution from the aforementioned acidification with carbon dioxide.
  • the concentration of the cathode compartment liquid can be adjusted via the water supply, it is preferably chosen to be as high as possible, restrictions mainly result from the membrane material used.
  • Cation-selective membranes which can be used as partitions between the anode and cathode space in the two-room electrolysis cells in the process according to the invention, have already been described several times and have been on the market for a long time. Reinforced membranes with increased durability are preferred by incorporating fibers and fabrics. Both single-layer membranes and bimembranes, consisting of two different membrane types on top of one another, can be used, the two-layer membranes opposing the possible diffusion of hydroxide ions through the membrane with a higher resistance, thus offering the advantage of a higher current efficiency.
  • suitable membranes have a perfluorocarbon polymer structure with sulfonate exchange groups
  • suitable reinforcing materials are also fluorocarbon polymers, preferably polytetrafluoroethylene, they are commercially available, for example, under the names ®Nafion 324, Nafion 435, Nafion 430 and Nafion 423, from DuPont, UNITED STATES.
  • the electrodes to be used on the cathode side are those that have already proven themselves in chlor-alkali electrolysis in the production of sodium hydroxide solution of various concentrations. They are usually made of steel, stainless steel or nickel and can be activated to reduce the overvoltage of the hydrogen.
  • the electrodes to be used on the anode side must be resistant to the attack of the acidic and oxidizing medium and to the electrolytically generated oxygen. They consist of a titanium framework and are optionally coated after application of an intermediate oxide layer made of titanium oxide or tantalum oxide or tin oxide either wet-galvanized or melt-galvanized EP-A-0 356 804 or after the stoving process with platinum or with platinum / iridium with a predominant iridium content.
  • the types of anodes that can be used are those that have proven themselves in other gas-generating processes, for example anodes in perforated plate shapes, expanded metal anodes, knife anodes, spaghetti anodes and blind anodes.
  • the distance between the electrodes is chosen to be as small as possible, preferably less than 10 mm.
  • Materials that are resistant to sodium dichromate in particular titanium and post-chlorinated PVC, are suitable as construction material for the electrolysis cells.
  • the degrees of conversion of sodium dichromate to chromic acid in the individual stages are such that in the last stage a conversion to 55 to 70%, preferably 59 to 65%, has taken place, so that a Sodium ion: chromic acid ratio of 0.45: 0.55 to 0.30: 0.70, preferably from 0.41: 0.59 to 0.35: 0.65, particularly preferably 0.4: 0.6 .
  • the electrolysis cells used for this conversion in all stages are similar to those described in the last section for converting the sodium chromate / sodium dichromate solution into a solution essentially containing sodium dichromate, and they are preferably set up and operated together with them , so that their current and voltage supply as well as their hydrogen and oxygen cleaning and disposal as well as their cathode room liquid treatment, their cooling and concentration and disposal can be summarized.
  • the same monopolar or bipolar current and voltage supply is selected, the current density here too is 1 to 5 kA / m2, preferably 2.5 to 3.0 kA / m2, and the voltage to be applied per electrolysis cell is 3.8 to 6.0 Volt; higher voltages are possible, but are avoided for economic as well as technical reasons.
  • the product of the respective previous stage is fed to the electrolytic cells via the feed line of the anode chambers, and the product is fed to the next stage in each case via the outlet.
  • the anolytes are collected for each stage and passed over a heat exchanger for the purpose of heat dissipation and fed back cooled on the opposite side of the anode chamber in the lower part.
  • the total number of heat exchangers for anolytes is therefore equal to the number of electrolysis stages.
  • the catholytes can be combined for all stages and are then preferred together wise combined with the cathode liquid from the step described above of converting sodium chromate / sodium dichromate into sodium dichromate solution, cooled and then redistributed to the individual cathode compartments.
  • cathode compartment fluid is removed from the circuit and fed to further processing, for example by concentration.
  • a preferred form of further processing is evaporation in a vacuum in one to three evaporator stages using the heat released during the electrolysis, so that at least some of the heat exchangers with which the electrolytic heat is removed from the catholyte liquid are identical to some of the heat exchangers for the evaporation of the removed cathode compartment liquid.
  • the composition of the cathode compartment liquid is the same as that of the previous step of converting sodium chromate / sodium dichromate solution to sodium dichromate solution.
  • the temperatures of the solutions in the electrolysis cells are 50 to 90 ° C., preferably 70 to 80 ° C. in all stages.
  • Membranes to be used, anodes, cathodes and construction materials are the same as described above.
  • the function of the cells can be changed at certain time intervals that they realize another sodium dichromate-chromic acid conversion step by changing the flow direction of the anode chamber liquids.
  • the electrolysis stage with the highest degree of conversion to chromic acid can take over the function of the stage with the lowest degree of conversion and vice versa.
  • each cell arrangement can thus take over the function of each electrolysis stage in chronological order.
  • the anode chamber liquid removed from the last stage of the multi-stage electrolysis is fed to a one to three stage evaporation, the last evaporation stage being designed as an evaporation crystallizer. It is evaporated to such an extent that a crystallization of chromic acid takes place when the solubility limit is exceeded. It is preferably evaporated to a water content in the mixture of 9 to 20% by weight, particularly preferably to a water content of 12 to 15% by weight.
  • the temperature to be set in the crystallizer is 50 to 110 ° C., preferably 55 to 80 ° C., particularly preferably approx. 60 ° C.
  • Crystallizers or crystallization evaporators with an internal heating chamber or with an external heating circuit are suitable for the preferably continuous crystallization. In any case, they must be operated at reduced pressure, so that the evaporation can be carried out at the above temperatures.
  • Crystallizers made of titanium are preferably used, which make it possible to produce crystals free of fine grains, that is to say those in whose operation the crystal suspension is screened at least partially according to the crystal size; these are so-called FC (forced circulation) crystallizers and also guide tube crystallizers, for example in combination with hydrocyclones or settling tanks / tanks; Guide tube crystallizers with a clarification zone are even more suitable, e.g. double propeller (DP) crystallizers and fluidized bed crystallizers (see W. Woehlk, G. Hofmann, International Chem. Engineering 27 , 197 (1987); RC Bennett, Chemical Engineering 1988, p. 119 ff).
  • DP double propeller
  • the crystal sludge removed from the crystallizer can be further thickened via a liquid cyclone (hydrocyclone) or settling container and is either placed directly or after thickening on a centrifuge which is made of titanium in its parts in contact with liquids.
  • the liquid is spun off the crystal cake as far as possible and then the crystal cake is washed one or more times, preferably one to three times, with saturated or almost saturated chromic acid solution.
  • the saturated or almost saturated chromic acid solution can be obtained outside the centrifuge by dissolving chromic acid, preferably by dissolving a portion of the purified chromic acid in the form of the moist, washed filter cake and / or by dissolving an intended fraction of fine particles from the crystalline product dried in the last process step
  • Chromic acid can be produced, but can also be generated in the centrifuge itself by spraying or spraying water or dilute chromic acid solution onto the filter cake.
  • the total amount of water to be used for the washing process is between 3 and 25% by weight, based on the moist centrifuge cake (filter cake), preferably between 4 and 10% by weight, this amount of water is used as such or in the form of a chromic acid solution either all at once or added in portions to the filter cake to be washed out.
  • washing solution is added in several portions, the resulting solutions running out of the filter cake can be combined or collected separately; in the case of separate pick-up, it is possible to use the processes which are contaminated differently and decreasing from washing step to washing step again in the next centrifuging cycle as washing solution for the respective preceding washing stages.
  • the sequence from the first washing step after centrifuging off the mother liquor or, in the case of single-stage cake washing, the entire washing liquid running off is fed to the evaporation crystallizer, the temperature of the solution being maintained or increased along the way.
  • the mother liquor of the chromic acid crystallization flowing out of the centrifuge which is saturated or slightly supersaturated in chromic acid, is mostly fed to the anode side of the multi-stage electrolysis of sodium dichromate to chromic acid without further cooling.
  • the mother liquor, the composition of sodium dichromate and chromic acid, is converted to a degree of conversion 50% of the sodium dichromate in chromic acid corresponds to the level selected among the electrolysis levels that most closely corresponds to the degree of conversion according to the incoming mother liquor.
  • the applicable electrolysis level can be determined mathematically and / or experimentally.
  • the fourth electrolysis stage is suitable, for example, for absorbing the mother liquor in an eight-stage system, whereas the fifth electrolysis stage in an eleven-stage system .
  • water can be added to the mother liquor before it enters electrolysis, or the corresponding amount of water can be fed directly into the anode chambers or the associated cooling circuit for the anode liquid. Any amount of water added is limited so that the water content of the resulting solution does not exceed 50% by weight, that is to say between 25 and 50% by weight.
  • a small part of the mother liquor flowing out of the centrifuge is fed into the upstream acidification stages for the removal of impurities which have been introduced into the electrolysis circuit, i.e. either into the partial stream removed in process step 7 for the pH adjustment in step 1 or else, as preferred, in the partial stream withdrawn in step 7 for the production of sodium dichromate.
  • the discharged solution runs through again all the cleaning steps listed to remove accumulated impurities, in the second case the discharged solution completely leaves the chromic acid production process. If one speaks of the smaller part of the mother liquor flowing out of the centrifuge, then the smaller part in the long-term average is referred to.
  • short-term is considered to be a period of time that does not exceed approximately thirty times the period in which the average volume of sodium dichromate solution flowing from step 7 of the multi-stage electrolysis is the total anode liquid volume of the multi-stage electrolysis including cooling circuits and the crystallizer and the stack container possibly installed in this anode liquid flow.
  • Preference over the unevenly timed removal of mother liquor is the timely even removal of a small part of the mother liquor into the streams of sodium dichromate solution, which are used for producing sodium dichromate or for adjusting the pH in step 1.
  • a small part of the mother liquor is to be understood as meaning a part which contains between 2% and 20%, preferably between 5% and 10%, of the molar amount of chromium (VI) which is introduced from step 7 into the multi-stage electrolysis.
  • the pure, crystalline, moisture-laden chromic acid generated in step 12 can be converted into salable goods in different ways after removal or ejection from the centrifuge. If a chromic acid solution generated outside the centrifuge is used to wash the chromic acid crystals in step 12, this moist chromic acid crystal cake is well suited for this and a corresponding proportion is removed. A commercially available, very pure chromic acid solution can also be prepared from the moist crystal cake without further treatment. In order to obtain dry, crystalline goods, the water must be removed below the chromic acid decomposition temperature, that is to say in the temperature range below 195 ° C., preferably from 165 to 185 ° C.
  • the drying can be followed by a dedusting by screening or classifying in order to remove dusty or finely crystalline fractions, wherein the fine material which has been envisaged can be used to prepare chromic acid solution for washing the thrown off chromic acid crystals in the centrifuge in step 12.
  • the gases produced during the electrolysis, oxygen in the anode chamber and hydrogen in the cathode chamber are each drawn off individually from the electrolysis chambers, normally from the upper part of the electrolysis cell and together with the respective anode chamber liquid or cathode chamber liquid.
  • the gas streams can, for example, be washed with water or passed over so-called droplet separators or mist separators.
  • contacting the oxygen stream with an absorbent reactive towards chlorine for example aqueous sodium hydroxide solution and moist activated carbon, is recommended.
  • both the oxygen and the hydrogen are fed into the chrome ore digestion furnace in separate lines as oxidizing agent or as fuel.
  • a sodium alkali product is formed in the cathode compartments in addition to hydrogen, from the hydroxide ions generated at the cathode and from Anode spaces immigrated over the cation-selective membranes, as already described above.
  • the sodium alkali product from the cathode compartments is preferably used for the production of solid sodium carbonate for chromium ore digestion and as a conditioning agent for the chromium ore residue and for sodium chromate solution.
  • Intermediate stages on the way to solid sodium carbonate can be: dilute and concentrated sodium hydroxide solution, sodium carbonate solution, sodium bicarbonate.

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Claims (9)

1. Procédé de production d'acide chromique par électrolyse en plusieurs étapes de solutions de bichromate et/ou de monochromate dans des cellules électrolytiques à deux compartiments, dont les compartiments anodique et cathodique sont séparés par des membranes d'échange cationique, à des températures de 50 à 90°C, les solutions de bichromate et/ou de monochromate étant obtenues par attaque de minerais de chrome et épuisement, caractérisé en ce qu'on ajuste la solution de monochromate obtenue après épuisement, le cas échéant après avoir séparé l'aluminium, le vanadium et d'autres impuretés, à 20-110°C à une valeur de pH de 8 à 12, par addition et/ou formation in situ de carbonate en une quantité de 0,01 à 0,18 mole/l (pour 300 à 500 g/l de Na₂CrO₄), on sépare les carbonates ou hydroxydes précipités, on concentre la solution à une teneur de 750 à 1000 g/l de Na₂CrO₄, on la transforme avec du CO₂ sous pression en une solution contenant du bichromate, on fait arriver la solution de bichromate dans le compartiment anodique de cellule électrolytique, on produit par électrolyse une solution contenant de l'acide chromique dans laquelle le rapport molaire des ions sodium à l'acide chromique a une valeur de 0,45:0,55 à 0,30:0,70 et on traite l'acide chromique par cristallisation, lavage et séchage.
2. Procédé suivant la revendication 1, caractérisé en ce que la température de l'électrolyse s'élève à 70-80°C.
3. Procédé suivant la revendication 1 ou 2, caractérisé en ce que l'électrolyse est conduite en 6 à 15 étapes.
4. Procédé suivant l'une des revendications 1 à 3, caractérisé en ce qu'on règle le rapport des ions sodium à l'acide chromique à une valeur de 0,4:0,6.
5. Procédé suivant l'une des revendications 1 à 4, caractérisé en ce qu'on traite la solution de départ (solution de monochromate) avec un échangeur cationique.
6. Procédé suivant l'une des revendications 1 à 5, caractérisé en ce qu'on conduit l'électrolyse à une densité de courant de 1 à 5 kA/m² de surface anodique.
7. Procédé suivant l'une des revendications 1 à 6, caractérisé en ce que la liqueur-mère obtenue lors du traitement de l'acide chromique est recyclée en totalité ou en partie dans le processus électrolytique.
8. Procédé suivant l'une des revendications 1 à 7, caractérisé en ce que la cristallisation est conduite par évaporation de l'eau à des températures de 55 à 80°C.
9. Procédé suivant l'une des revendications 1 à 8, caractérisé en ce qu'une partie de la liqueur-mère obtenue dans la cristallisation de l'acide chromique est déchargée du circuit d'électrolyse.
EP89115030A 1988-08-27 1989-08-15 Procédé électrochimique de production d'acide chromique Expired - Lifetime EP0356802B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE3829121A DE3829121A1 (de) 1988-08-27 1988-08-27 Elektrochemisches verfahren zur herstellung von chromsaeure
DE3829121 1988-08-27

Publications (3)

Publication Number Publication Date
EP0356802A2 EP0356802A2 (fr) 1990-03-07
EP0356802A3 EP0356802A3 (en) 1990-05-16
EP0356802B1 true EP0356802B1 (fr) 1992-07-29

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EP89115030A Expired - Lifetime EP0356802B1 (fr) 1988-08-27 1989-08-15 Procédé électrochimique de production d'acide chromique

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US (1) US5068015A (fr)
EP (1) EP0356802B1 (fr)
JP (1) JP2812730B2 (fr)
KR (1) KR0152524B1 (fr)
AR (1) AR247252A1 (fr)
BR (1) BR8904252A (fr)
CA (1) CA1338145C (fr)
DD (1) DD284060A5 (fr)
DE (2) DE3829121A1 (fr)
ES (1) ES2042904T3 (fr)
MX (1) MX170481B (fr)
PL (1) PL163883B1 (fr)
RO (1) RO107136B1 (fr)
TR (1) TR24735A (fr)
ZA (1) ZA896495B (fr)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6063252A (en) * 1997-08-08 2000-05-16 Raymond; John L. Method and apparatus for enriching the chromium in a chromium plating bath
CN103668301B (zh) * 2013-12-11 2016-08-24 中国科学院青海盐湖研究所 利用电解法制备铬酸钠溶液的装置和方法
CN108103521B (zh) * 2017-12-22 2019-10-15 四川省银河化学股份有限公司 一种提高电解法制备铬酸酐品质的方法
CN113184907A (zh) * 2021-05-31 2021-07-30 河钢承德钒钛新材料有限公司 一种铬酸钠的纯化方法

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0356803A2 (fr) * 1988-08-27 1990-03-07 Bayer Ag Procédé de préparation d'un électrolyte pour l'obtention de bichromates de métaux alcalins et d'acide chromique
EP0356804A2 (fr) * 1988-08-27 1990-03-07 Bayer Ag Procédé de production de dichromates alcalins et d'acide chromique

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA739447A (en) * 1966-07-26 W. Carlin William Electrolytic production of chromic acid
US3046091A (en) * 1958-12-19 1962-07-24 Pittsburgh Plate Glass Co Preparation of alkali metal dichromate
US3305463A (en) * 1962-03-16 1967-02-21 Pittsburgh Plate Glass Co Electrolytic production of dichromates
US3454478A (en) * 1965-06-28 1969-07-08 Ppg Industries Inc Electrolytically reducing halide impurity content of alkali metal dichromate solutions

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0356803A2 (fr) * 1988-08-27 1990-03-07 Bayer Ag Procédé de préparation d'un électrolyte pour l'obtention de bichromates de métaux alcalins et d'acide chromique
EP0356804A2 (fr) * 1988-08-27 1990-03-07 Bayer Ag Procédé de production de dichromates alcalins et d'acide chromique

Also Published As

Publication number Publication date
US5068015A (en) 1991-11-26
TR24735A (tr) 1992-03-06
DD284060A5 (de) 1990-10-31
KR0152524B1 (ko) 1998-10-15
AR247252A1 (es) 1994-11-30
JP2812730B2 (ja) 1998-10-22
CA1338145C (fr) 1996-03-12
BR8904252A (pt) 1990-04-10
EP0356802A3 (en) 1990-05-16
PL163883B1 (pl) 1994-05-31
JPH02104684A (ja) 1990-04-17
DE58901937D1 (de) 1992-09-03
EP0356802A2 (fr) 1990-03-07
KR900003418A (ko) 1990-03-26
RO107136B1 (ro) 1993-09-30
MX170481B (es) 1993-08-25
ES2042904T3 (es) 1993-12-16
ZA896495B (en) 1990-05-30
DE3829121A1 (de) 1990-03-01

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