NZ227342A - Protective coating for carrier bars and/or emergent part of anodes used in electrolytic production of aluminium - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £27342 22 7 3 A 2 Priority Oct *,o;■. XX .Q*£>^j..

Cotnp'.«*« Socciffcat.on Filed: ..i.S.r./Dr.'S'.S] Ciass: . '.

D"*": 2 3 DEC I99I KC . '35/ No.: Date: PATENTS ACT, 1953 COMPLETE SPECIFICATION IMPROVEMENT IN THE PROTECTIVE COATINGS FOR THE CARRIER BARS OF PRE-BAKED ANODES AND THE EMERGING PART OF SAID ANODES -U/We, SOCIETE DES ELECTRODES & REFRACTAIRES SAVOIE (SERS) Tour Manhattan -La Defense 2, 6, place de TIris 92400 COURBEVOIE, France, a French Company, hereby declare the invention for which ■£-/ we pray that a patent may be granted to roe-/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - (followed by page la) o 227349 1 a IMPROVEMENT IN THE PROTECTIVE COATINGS FOR THE CARRIER BARS OF PRE-BAKED ANODES AND THE EMERGING PART OF SAID ANODES TECHNICAL FIELD OF THE INVENTION The invention concerns a coating for affording protection from corrosion and hot oxidation, which is intended for the carrier bars of pre-baked anodes and for the emerging carbonaceous part of such anodes 5 which are used in tanks for the production of aluminium by the electrolysis of alumina dissolved in molten cryolite, using the Hall-Heroult process. It constitutes an addition to patent specification No. 222473.

STATE OF THE ART Most modern tanks for electrolytic production using the Hall- Heroult process use carbonaceous anodes which are referred to as being 'pre-baked1, being produced by shaping at a tenperature of about 120 to 160#C of a carbonaceous paste essentially formed by coke (and/or anthracite) and pitch, and then baking for around one hundred hours at 15 1150/12008C.

In the operation of shaping the paste, a certain number of cavities which are often referred to as 'anode plugs' are provided in the upper part of the anode. Carrier bars (or steel plates) will be introduced into and sealed in the cavities, with the carrier bars or 20 plates serving both to suspend the anode frcm the anodic frame structure and to supply it with current. The sealing effect is produced by casting cast iron or more rarely by means of a carbonaceous paste of special composition.

The steel carrier bars are subjected both to an elevated 25 tenperature and to the corrosive action of the fluorine-bearing effluents which are emitted by the tank in operation thereof. Furthermore, in certain operations, for example upon a downward movement of anodes which is intended to bring an end to an 'anodic Jii ft '% & •V effect' or when there are 'waves' in the electrolyte, molten cryolite | 2 bath may come into contact with the base of the steel carrier bars.

Those various phenomena cause the proportion of iron in the aluminium produced in the tank to be substantially increased by virtue of base corrosion of the carrier bars. That corrosion also has the harmful effect of reducing the service life of the anodes suspension assembly (carrier bars + connecting bar members between the carrier bars and the anode stem), which are normally recovered and re-used after extraction of the worn-out anodes frcm the tank.

Likewise, the emerging part of the anodes, that is to say the upper part which, in normal operation of the electrolysis tank, is not inmersed in the molten electrolyte, suffers frcm degradation due to combustion, which is sought to be prevented by a covering of solidified and crushed electrolysis bath which is optionally mixed with alumina, or else by metallisation produced by spraying liquid aluminium.

The attempt has also been made to protect the base of the carrier bars by means of different processes such as metallisation by spraying liquid aluminium or by fitting around the carrier bars a collar portion of aluminium which leaves a space of from 10 to 30 mm relative to the carrier bar (or the plate), the space being filled with a hot-cast carbonaceous paste. It has been proposed for that purpose to use conventional carbonaceous pastes (coke and/or anthracite and/or graphite + pitch) or more ccrnplex compositions comprising polymerisable and cokable organic substances such as epoxy resins, furfuryl resins, etc... (German patent application DE-AS No 25 47 061 - available on request).

However, those carbonaceous compositions suffer from the major disadvantage that, as soon as the anode is brought into operation, with the anode progressively reaching its equilibrium temperature, such compositions cause the production of hydrocarbon vapours originating from cracking of the carbonaceous paste. Seme aromatic compounds which are formed in that way are suspected of having harmful physiological effects and in addition they cause pollution of and promote obstruction JUL 199] £: o y/ 7 n c (L I jl\ c. 3 of the circuits for sucking off and washing effluents on the electrolysis tanks. For all those reasons, the 'collar portion pastes' which are made up solely of carbonaceous substances no longer satisfy the operators of electrolysis tanks using pre-baked anodes. Now, the presentday trend is precisely to produce a primary aluminium which is in as high a state of purity as possible in the tanks. Corrosion of the base of the carrier bars being one of the sources of iron in the aluminium produced, it was therefore important to find a way of affording effective protection and which preferably also applies to the emerging carbonaceous part of the anodes.

RECALL OF THE INVENTION OF THE MAIN PATENT The subject-matter of specification No. 222473 is a collar portion paste composition which does not suffer frcm the disadvantages of carbonaceous pastes and which provides for effective protection for the base of the steel carrier bars and optionally the emerging carbonaceous part of the anodes throughout the service life of the anodes (that is to say around 20 days on average). That composition is essentially characterised by a combination of carbonaceous aggregate selected from coke, graphite, semi-graphite and scrap from anodes or electrodes of semi-graphite with a low ash content (preferably < 2%) which is bound by a calcium aluminate cement, with a low proportion of troublesome impurities (Si02» Fe203, TiOg, K2^» ^03 and Mn203 with the total content less than 5% by weight, preferably less than 1% by weight and more preferably being lower than 1% by weignt), having an alumina content which is at least equal to 70% by weight and preferably at least equal to 80% by weight , the whole being bound by the addition of water.

Those impurities are referred to as troublesome as, when they are introduced into the electrolysis bath, they are reduced and the corresponding metals Si, Fe, Ti, Cr, Mn and K mix with the liquid aluminium. powder in a proportion of from 0.01 to 5% by weight, for example passivated aluminium in grain form < 0.1 mm, substantially enhances the resistance to oxidation.

A complementary addition of an anti-oxidant such as a metal 2273-12 4 It is also possible to add alunina to the paste, for exarcple alunina for electrolysis or spherical alunina, in a proportion which may constitute up to 50% of the total weight of the aggregate.

The proportion by weight of cement, expressed as a percentage by weight of the aggregate (carbonaceous substance + alumina + additives) is between 10 and 60% and preferably frcm 15 to 50%. The term 'dry matter' will be used to denote the whole of the aggregate (carbonaceous substance, alunina, additives) and the cement. compositions are suitable but it is then preferable for them to be applied by gun spraying. For that purpose, depending on the type of gun used, it is necessary to control the granulometry of the aggregate and the cement (less than 1 mm and preferably less than 0.5 mm) and substantially increase the amount of water. and 40% by weight with respect to the dry matter (as defined above) for use in the form of a collar portion paste and up to 60% by weight for use in the form of a fluid paste to be sprayed with a gun.

SUBJECT-MATTER OF THE PRESENT INVENTION The subject-matter of the present invention is a protective composition which does not suffer from the disadvantages of carbonaceous pastes or pastes comprising a carbonaceous aggregate, and which provides for effective protection of the base of the steel carrier bars and possibly the emerging carbonaceous part of the anodes throughout the service life of the anodes (that is to say around twenty days on average). That composition is essentially characterised by calcium aluminate cement, with a low proportion of troublesome impurities (Si02, Fe^O^, Ti02, f^O, CE2O3 and M^O^) with the total content preferably being lower than 1%) , having an alumina content which is at least equal to 70% by weight and preferably at least equal to 80% by weight, and magnesian spinel in a proportion of from 0.1 to 10% by weight. It is also possible to add alumina in the form of fine powder To protect the emerging part of the anode, the same That amount of water will be fixed for example at between 10 1 227342 which may attain 902 by weight of the dry matter. The compositions covered by the invention therefore fall within the following ranges: £calciun aluninate cement 99.9 to 10% by weight of the dry matter alumina in fine powder form < 0.5 <m> 0 to 903! by weight of the dry matter magnesian spinal < 0.1 nm 0.1 to 10% by weight of the dry matter The water content may be between 10 and 80* of the total weight of dry matter, contents between 10 and approximately 50% corresponding to a pasty composition which can be used as a collar portion paste, while contents ranging up to 80% correspond to a fluid paste which can be used by gun spraying, in which case the granulcmetry of the dry matter is less than lmm and preferably less than 0.5nm.

DESCRIPTION OF THE INVENTION Protection for the emerging part of the anodes The protective coating for the emerging part of pre-baked anodes was employed using four formulations for application by gun spraying, which are adapted - by reducing the water content - for use as a paste for collar portions.

SECAR —2—3 Magnesian spinel H20 A 18 25 5 50 B 29.2 25.1 5 38.7 C 35.9 13.4 10 38.7 D 61.3 0.1 0 38.7 The spraying conditions were as follows: the carbonaceous 25 substrate for the tests is formed by a core portion of a diameter of 30 mm and 120 nm in height.

O The different compositions A, B, C and D are applied to a group of samples and a certain number of reference samples are kept without a coating. The array of samples is treated for one hour at 1000°C in 30 nitrogen to remove any trace of water. A carboxyreactivity test is then carried out for tvro hours at 1000°C in a flow of CO^ of^ 50 litres/hour.

At the end of the test, the samples are cooled in a stream of nitrogen © / AV -7 NOV I99Jm'j I fj L 227 6 and each core portion is weighed to determine its weight loss. The results are expressed in milligrams in relation to a surface area of 1 cm and a period of one hour. The result is therefore given in -2 v,-l mg.cm .n .

The results obtained were as follows: Core portions covered by spraying A 36 B 36 C 38 D 38 Reference sarples (without coating) 47 The particular level of effectiveness of formulations A and B will be noted, reducing reactivity in relation to CC>2 by close to 25%.

Long-term tests were carried out under industrial conditions on the emerging part of the anodes of a certain number of Hall-Heroult electrolysis tanks. The results are less precise by virtue of the fact that, after the sturps of worn-out anodes are withdrawn from the tank, they are coated with electrolysis bath which can never be entirely removed therefrom, but they confirm the order of magnitude of 25% in the reduction of oxidation of the emerging part of pre-baked anodes. Protection for the carrier bars of pre-baked anodes For protecting anode carrier bars, the protective coating is set in position in the form of a paste. The conditions in respect of corrosion in the course of electrolysis are difficult to simulate so that tests were carried out under real conditions, by measuring the reduction in diameter of the carrier bars after removal of the vrorn-out anodes (that is to say after a period of about three weeks in the electrolysis tank).

The compositions for that use fall within the following ranges: Cement: Alumina: Mg spinel /'■' v o \ !i ~7 N0V I99j$ to 99.9% by weight of the dry matter 0 to 80% by weight of the-dry-matter. 0.1 to 10% by weight °f the dry 227342 7 The optimm compositions expressed in terms of the ratio of Secar cement/Secar cement + A12°3 + spi^l 31,6 between 5 and 50%. The percentage of water to be added depends on the cement content and the granulcmetry of the alumina. It is between 30 and 50 and preferably between 35 and 45% by weight with respect to the dry matter (that is to say 35 to 45 grams of water for 100 grams of the Secar cement + alumina + magnesian spinel mixture). The reduction in corrosion of the anode carrier bars is apparent as frcm the first cycle and beccmes more accentuated in the course of the following cycles. It can be estimated that the useful service life of the anode carrier bars is extended by at least 30%.

Although the invention has been described in the particular situation of using Secar cement, it applies in the same manner to any calcium aluminate-based cement with an alunina content that is at least equal to 70% and preferably at least equal to 80%. Set out below are the compositions of the Secar 80 and 71 cements from LAFARGE FONDU INTERNATIONAL and magnesian Mg spinel from FEM (PECHINEY ELECTRCMETALLURGIE): SECAR 80 SECAR 71 A1203 80.5% "1 71% 1 67 - 71% CaO 18.0% J 98-5% 27% 1 98% $0.5 Si02 0.20% ^ 0.35% i 51.0 Fe203 0.15% 0.25% 40.16 Ti02 0.03% ' 0.43% 0.05% } 0.72% 1^0 0.03% 0.05% - Cr203 0.01% 0.01% - Mn203 0.01% . 0.01% - MgO 0.1% 0.2% 28 - 31% Na20 0.25% 0.35% - so3 0.1% 0.15% - P205 traces - traces B2°3 - traces • 'J 9 ~ 4 O « i J • 1

Claims (5)

WHAT WE CLAIM IS;

1. A coating for affording protection from corrosion and oxidation in the hot condition of the carrier bars of pre-baked anodes and/or the emerging part of such anodes which are intended for electro-CD lysis tanks for the production of aluminium using the Hall-Heroult process, characterised in that the coating is formed by a mixture of calcium aluminate cement with a low proportion of troublesome impurities, and having an alumina content which is at least equal to 70% by weight, magnesian spinel and optionally alumina in fine powder form, {&*) which are bound with water, in the following proportions: 0 calcium aluminate cement alumina in fine powder form magnesian spinel water 10 - 99.9% by weight of the dry matter 0 to 90% by weight of the dry matter 0.1 to 10% by weight of the dry matter, and 10 to 80% of the total weight of the dry matter.

2 . A protective coating according to claim 1 characterised in that, for application by spraying to the emerging part of pre-baked anodes, the composition is within the following limits: dry matter (calcium aluminate cement, magnesian spinel and optionally alumina) 100 parts by weight and water 10 to 80 parts by weight.

3 . A protective coating according to claim 1 characterised in that, for application in the form of a collar portion paste to anode carrier bars, the composition is within the following limits: calcium aluninate cement: 20 - 99.9% by weight of the dry matter alunina in fine powder form: 0 - 80% by weight of the dry matter magnesian spinel: 0.1 to 10% by weight of the dry matter, and water 30 to 50% of the total weight of the dry matter.

4. A protective coating according to claim 1, substantially as hereinbefore described.

5. Use of the coating according to any one of claims 1 to 4 for the protection of carri£E-.ba$s o'$>$l&baked anodes and/or the emerging ** . i. ,\! r- V.V v'' J i part of such anodes. :■ 1 ' V . £ dated this /3 day op Oy fi\ ' A . .J. P A RK & s O N ■/* 14 NOV 1991"J PEn ^ M \ AilFMT- Ti 'I"