Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
• • 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

Definitions

• the invention relates to the removal of manganese dioxide electrodeposited on anodes during operation in an electrolytic cell.
• Oxygen anodes are extensively employed in metal electrowinning from sulphate electrolytes, with periodic stripping of the electrowon metal from the cathodes.
• Manganese present in the electrolytes used for metal electrowinning is electrodeposited anodically as manganese dioxide which has to be removed in order to avoid an excessive rise of the anode potential.
• Anodes of lead alloy are extensively used as oxygen anodes in industrial zinc electrowinning cells and the anodic manganese dioxide deposit is periodically removed by mechanical means.
• Such conventional lead alloy anodes evolve oxygen at a high potential and a further significant rise of their operating potential would entail excessive energy losses.
• catalytic anodes to reduce the anode potential and thereby achieve energy savings.
• Such anodes must provide stable long-term performance in order to be able to achieve sufficient energy savings to justify the anode costs.
• the catalytic activity of such anodes may nevertheless diminish progressively with a corresponding rise of the anode potential, so that they would no longer provide the required energy savings.
• catalytic oxygen anodes are of special interest to achieve energy savings in electrowinning, since energy consumption is a major factor in metal electrowinning.
• catalytic anodes are nevertheless particularly problematic when manganese dioxide is electrodeposited and progressively masks their active surface in the course of operation in an electrolytic cell.
• the manganese dioxide deposit might be dissolved by means of a strong reducing agent, this was found to be unsuitable for various reasons : an excessively long time would be required to dissolve and thereby remove the manganese dioxide deposit, while the electrocatalytst would likewise be subject to attack by the reducing agent.
• the electrodeposition of manganese dioxide on catalytic anodes thus constitutes a major obstacle to their efficient use, and more particularly to the use of catalytic oxygen anodes in metal electrowinning cells.
• Electrolytic cells for the production of manganese dioxide also require periodic removal and recovery of the manganese dioxide which is electrodeposited on the anodes during operation of the cells.
• An object of the invention is to provide a simple method of removing manganese dioxide electrodeposited on anodes during operation in electrolytic cells.
• a further object of the invention is to allow the reactivation of catalytic anodes by removing manganese dioxide electrodeposited on their active surface, so as to maintain their catalytic effect during prolonged operation in an industrial electrolytic cell.
• a more particular object of the invention is to provide a method of reactivating catalytic anodes, which allows electrodeposited manganese dioxide to be readily removed, so as to avoid an unacceptable rise of the anode potential during long-term industrial operation.
• a further object of the invention is to provide such a method which allows catalytic anodes to be reactivated repeatedly in an economical manner and only requires brief interruptions of anode operation for reactivation.
• the invention is based on the discovery that manganese dioxide electrodeposited on an anode can be made to rapidly flake off and thereby be readily removed by treatment with an aqueous sodium sulphite solution, and that this treatment can be carried out repeatedly without notably affecting the performance of the anode.
• the sulphite solution used for the reactivating treatment according to the invention is not acidified and may advantageously have a pH from 8 to 12.
• the duration required for the reactivating treatment according to the invention will essentially depend on the amount of manganese dioxide to be removed, and hence on the duration of anode operation and the rate of manganese dioxide deposition between successive reactivating treatments.
• the anodes to be reactivated are taken out of operation and subjected to this treatment with the sulphite solution, which can be achieved within a few minutes although the duration of this treatment may if necessary be increased up to about 1 hour during successive reactivating treatments.
• the method according to the invention thus requires only brief interruptions in operation of the anodes, and that at relatively large intervals of about 3-4 weeks or more.
• the sulphite solution used to -carry out the reactivating treatment is preferably brought into intimate contact with the anode to be reactivated, so that the manganese dioxide previously electrodeposited during operation of the anode in an electrolytic cell is penetrated by the sulphite solution, is thereby caused to flake off and is rapidly removed by the solution.
• the anode to be reactivated can be advantageously treated by immersion in a bath of the sulphite solution contained in a vessel and agitated by means of a mechanical stirrer which ensures continuous intimate contact and hence treatment of the anode with the whole solution in the bath.
• the manganese dioxide deposit can thus be flaked off and thereby be removed as rapidly as possible from the catalytic surface of the anode.
• the solid manganese dioxide flakes thus removed from the anode are relatively large and will accumulate at the bottom of the bath of sulphite solution, from which these flakes can be physically separated by decantation or filtration whenever this may become necessary.
• the manganese dioxide flakes may nevertheless be allowed to accumulate and remain at the bottom of the sulphite bath during several successive reactivating treatments since, as already indicated, no notable dissolution of the manganese dioxide or modification of the sulphite solution can be observed after repeated successive treatments over several months.
• Intimate contact with the anode to be reactivated can likewise be achieved by different means such as, for example, spraying the sulphite solution against the manganese dioxide deposit to be removed, while care must of course be taken in all cases -to avoid damaging the catalytic surface by erosion or otherwise during the reactivating treatment.
• Catalytic anodes comprising a lead sheet base provided with uniformly distributed, catalytically activated titanium sponge particles partly embedded in the lead base surface, were operated in an electrolytic pilot test cell for zinc electrowinning.
• the cell contained an industrial zinc sulphate electrolyte circulating between an aluminium cathode spaced at 20 mm from the anode. This electrolyte was kept at a temperature of about 35°C and contained approximately 60 grams of zinc sulphate per liter, 130 to 200 grams of sulphuric acid per liter and about 3 grams of manganese sulphate per liter.
• the anode current density was maintained at 400 A/m 2 with respect to the geometric lead anode base area, while the zinc produced during operation of the pilot cell was periodically stripped off the aluminium cathode and the corresponding zinc current yield was determined.
• the anode operating potential was continuously recorded and each time a rise of the anode potentially by about 100 mV was observed, operation of the cell was interrupted and the anodes were taken out of the cell for reactivation as described below.
• This set of catalytic anodes comprised titanium sponge particles which were catalytically activated by means of ruthenium-manganese oxide formed thereon by thermal decomposition of corresponding Ru and Mn salts, were uniformly distributed and fixed by pressing onto a lead sheet base.
• the catalytically activated titanium sponge particles were thus partly embedded and applied with a loading which corresponded in this case to 700 grams of Ti sponge, 20 grams of ruthenium and 25 grams of manganese per square meter of the geometric surface area of the lead sheet base.
• a first pair of catalytic lead anodes (BM2) of this set was tested for 308 days in the zinc electrowinning pilot cell, was activated every 3 to 4 weeks and operated with a mean anode potential of about 1 .6 V vs. NHE at 400 A/m2 over this test period, while zinc was produced in the pilot cell with a high current efficiency from 80 % to 94 % during this period.
• the anode potential was 1.55 V vs. NHE at the beginning of the test period and exhibited a progressive rise during operation in the pilot cell.
• the catalytic anodes were reactivated every 3 to 4 weeks, each time the anode potential underwent a rise up to a value lying between 1.65 and 1.70 V vs. NHE.
• the test anode was taken out of operation, removed from the pilot cell and immersed in a bath of aqueous sulphite solution containing 80 grams per liter sodium sulphite in demineralized water, which- was continuously agitated by means of a stirrer.
• the manganese dioxide deposit was observed to rapidly flake off the surface of the anode and sink to the bottom of the bath during this reactivating treatment, which was generally achieved in 2 to 10 minutes, but was increased gradually up to about one hour in successive reactivating treatments in order to ensure susbstantially complete removal of the manganese dioxide deposit up to the end of the test period of about 10 months.
• a second pair of catalytic lead anodes (GE 3) of the same set was similarly tested at 400 A/m 2 in the zinc electrowinning cell and reactivated every 3 to 4 weeks in the manner described above.
• the results were substantially the same in this case as those given above for the first pair of anodes (BM2) : here 5 successive reactivating treatments allowed the anode operating potential to be maintained at an average value slightly above 1.6 V vs. NHE, with peak values of about 1.7 V vs. NHE and similar zinc current efficiencies were obtained, so that the testing of these anodes was terminated after 120 days.
• a third pair of catalytic lead anodes (GE 1) of the same set was similarly tested at 400 A/m 2 in the zinc electrowinning cell and reactivated every 3 to 4 week as in the manner described above.
• the results obtained in this case were likewise substantially the same as those given above for the first pair of anodes (BM2) and this test was terminated after 214 days of operation in the pilot cell.
• the effect of the reactivating treatment according to the invention can be achieved reproducibly so as to restore the catalytic activity of the anodes during prolonged operation of the anodes under industrial conditions.
• the reactivating treatment according to the invention was also carried out successfully using sodium sulphite solution to remove manganese dioxide electrodeposited on a titanium anode having a catalytic coating of Ti02/Ru02/Sn02.
• This is a dimensionally stable anode with a commercial coating extensively used in the chlorine industry, for example in diaphragm cells and in hypochlorite cells.
• This type of anode is described for example in U.S. Pat. No 3.776.834 which is incorporated by reference herein.
• Reactivation of catalytic anodes was also carried out according to the invention by treatment with aqueous sodium bisulphite and potassium sulphite solutions, which likewise caused the manganese dioxide to flake off.
• the method according to the invention is particularly suitable for the operation and reactivation of catalytic oxygen anodes operating in cells for metal electrowinning from acid electrolytes containing manganese which is deposited anodically as manganese dioxide and must be removed at intervals to avoid an excessive rise in the anode operating potential.
• the invention can also be applied to reactivate other types of catalytic anodes on which manganese dioxide is electrodeposited from a manganese-containing electrolyte, such as catalytic chlorine anodes operating in cells for the production of hypochlorite by electrolysis of sea-water.
• the method of treatment with sulphite solution according to the invention may be usefully applied to recover manganese dioxide anodically deposited during operation of electrolytic cells for the production of manganese dioxide.

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

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Citations (4)

• 1984
  • 1984-04-09 EP EP84200497A patent/EP0157951A1/fr not_active Withdrawn

Patent Citations (4)

Non-Patent Citations (1)

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