Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
  • C25C3/12—Anodes
  • C25C3/125—Anodes based on carbon
• • 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
  • C25C7/02—Electrodes; Connections thereof
  • C25C7/025—Electrodes; Connections thereof used in cells for the electrolysis of melts

Definitions

• the invention relates to a coating for protection against corrosion and hot oxidation for pre-baked anode logs and the emerging carbonaceous part of these anodes used in aluminum production tanks by electrolysis of alumina dissolved in the cryolite melted using the Hall-Héroult process. It constitutes an addition to patent application 86-16285 filed on November 14, 1986.
• anode plugs are introduced into the upper part of the anode, into which logs (or steel plates) will be introduced and sealed. ) which will serve both to suspend the anode from the anode frame and to supply it with current.
• the sealing is carried out by casting iron or more rarely by means of a carbonaceous paste of special composition.
• the steel logs are subjected both to a high temperature and to the corrosive effect of the fluorinated effluents emitted by the tank in operation.
• the molten cryolithic bath can come into contact with the base. steel logs.
• the emerging part of the anodes that is to say the upper part 9 which, in normal operation of the electrolytic cell, is not immersed in the molten electrolyte, undergoes degradation by combustion, that it is sought to avoid by covering a solidified and ground electrolysis bath, possibly mixed with alumina, or else by a metallization obtained by spraying liquid aluminum.
• the subject of the main patent is a flange paste composition which does not have the drawbacks of carbonaceous pastes, and which provides effective protection of the base of the steel logs and possibly of the carbonaceous part emerging from the anodes during the entire life of the anodes (about twenty days on average).
• This composition is essentially characterized by a carbon aggregate such as coke, artificial graphite, waste anodes or semi-graphite electrodes with a low ash content (preferably ⁇ 2%) bound by a lime aluminate cement, to low content of annoying impurities (SiO2, Fe2O3, TiO2, K2O, Cr2O3, total content preferably less than 1%) having an alumina content at least equal to 70% and, preferably, at least equal to 80%, the together being linked by addition of water.
• a carbon aggregate such as coke, artificial graphite, waste anodes or semi-graphite electrodes with a low ash content (preferably ⁇ 2%) bound by a lime aluminate cement, to low content of annoying impurities (SiO2, Fe2O3, TiO2, K2O, Cr2O3, total content preferably less than 1%) having an alumina content at least equal to 70% and, preferably, at least equal to 80%, the together being linked by addition of water.
• a complementary addition of magnesium spinel (Al2O3, MgO), whose thermal conductivity is high, makes it possible to lower the temperature of the surface of the anode, thus delaying oxidation.
• alumina for example alumina for electrolysis or globular alumina, at a content which can constitute up to 50% of the total weight of the aggregate.
• the cement content by weight, expressed in weight percent of the aggregate (carbon product + additives) is between 10 and 60% and, preferably, from 15 to 50%. It is appropriate to denote by "dry matter” the whole of the aggregate (carbonaceous product, alumina, additives) and of the cement.
• the same compositions are suitable, but it is then preferable to apply them by spraying with a gun.
• a gun it is necessary, depending on the type of gun used, to control the particle size of the aggregate and the cement (less than 1 mm and, preferably, less than 0.5 mm) and significantly increase the quantity of water.
• This amount of water will be fixed, for example, between 10 and 70% by weight relative to the dry matter (as defined above) for use in the form of collar paste and, up to 60% by weight for use in the form of a fluid paste to be sprayed on with a spray gun.
• the object of the present invention is a protective composition which does not have the drawbacks of carbonaceous pastes or comprising a carbonaceous aggregate, and which provides effective protection of the base of the steel logs and possibly of the carbonaceous part emerging from the anodes throughout the lifespan of the anodes (about twenty days on average).
• This composition is essentially characterized by a mixture of lime aluminate cement, with a low content of annoying impurities (SiO2, Fe2O3, TiO2, K2O, Cr2O3, total content preferably less than 1%) having an alumina content at least equal at 70%, and preferably at least equal to 80% and magnesium spinel in a proportion of 0 to 10%.
• compositions covered by the invention are therefore located in the following ranges: lime aluminate cement 99.9 to 10% by weight alumina fine powder ⁇ 0.5mm 0 to 90% magnesium spinel ⁇ 0.1mm 0 to 10%
• the water content can be between 10 and 80% of the weight of the dry materials, the contents between 10 and approximately 50% corresponding to a pasty composition, usable as flange paste, and the contents going up to 80% corresponding to a fluid paste usable by spraying with a gun, with, in this case, a particle size of the dry materials less than 1 mm and preferably less than 0.5 mm.
• the protective coating of the emerging part of prebaked anodes was implemented according to four formulations for application by spraying with a spray gun, which are adapted - by reducing the water content - for the use of paste paste.
• the carbon substrate for the tests consists of a core 30 mm in diameter and 120 mm high.
• the different compositions A, B, C, D are applied to a group of samples and a certain number of controls are kept without coating. All the samples are treated for 1 hour at 1000 ° C. under nitrogen to remove all traces of water.
• a 2 hour carboxyreactivity test is then carried out at 1000 ° C. under a CO2 flow rate of 50 liters / hour.
• the samples are cooled under a stream of nitrogen and each carrot is weighed to determine its weight loss.
• the results are expressed in milligrams related to an area of 1 cm2 and for an hour duration. The result is therefore given in mg.cm ⁇ 2.h ⁇ 1.
• the following results were obtained: Carrots spray coated: AT 36 B 36 VS 38 D 38 Witnesses (without coating) 47
• the protective coating is placed in the paste state.
• the corrosion conditions during electrolysis being difficult to simulate, the tests were carried out under real conditions, by measuring the reduction in the diameter of the logs after extraction of the used anodes (ie after approximately 3 weeks of stay in the electrolysis).
• the compositions for this use are included within the following limits: Cement: 10 to 99.9% Alumina: 0 to 80% Mg Spinel 0 to 10%
• the optimal compositions expressed in Secar Cement / Secar cement + Al2O3 + Spinel ratio being between 5 and 50%.
• the percentage of water to be added depends on the cement content and the particle size of the alumina. It is between 30 and 50 and preferably between 35 and 45% by weight relative to the dry materials (that is to say 35 to 45 grams of water per 100 grams of the mixture of Secar cement + alumina + magnesium spinel ).
• the reduction in corrosion of the anode logs is evident from the first cycle and is accentuated during the following cycles. It can be estimated that the useful life of the anode logs is extended by at least 30%.
• Secar cement based on calcium aluminate having an alumina content at least equal to 70% and preferably at least equal to 80%.
• the composition of Secar 80 and 71 cements from the company LAFARGE FONDU INTERNATIONAL and Spinel of Magnesium, from PEM (PECHINEY ELECTROMETALLURGY) are recalled below:

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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)
• Prevention Of Electric Corrosion (AREA)
• Furnace Housings, Linings, Walls, And Ceilings (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

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Cited By (4)

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Citations (3)

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• 1988
  • 1988-11-21 IN IN814/MAS/88A patent/IN169360B/en unknown
  • 1988-12-19 CA CA 586322 patent/CA1340408C/fr not_active Expired - Fee Related
  • 1988-12-20 NO NO88885645A patent/NO885645L/no unknown
  • 1988-12-21 EP EP19880420430 patent/EP0322326B1/de not_active Expired - Lifetime
  • 1988-12-21 ES ES88420430T patent/ES2043877T3/es not_active Expired - Lifetime
  • 1988-12-21 DE DE19883873713 patent/DE3873713T2/de not_active Expired - Fee Related
  • 1988-12-21 IS IS3424A patent/IS1465B6/is unknown
  • 1988-12-21 BR BR8806778A patent/BR8806778A/pt not_active Application Discontinuation
  • 1988-12-21 AU AU27429/88A patent/AU610964B2/en not_active Ceased

Patent Citations (3)

Non-Patent Citations (2)

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