Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1998-11-25
2001-02-27
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C427S113000, C427S126100, C502S101000, C204S290010
Reexamination Certificate
active
06194096
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to the treatment of carbon bodies which are exposed to high temperature oxidizing gases, in order to improve the resistance of the bodies to deterioration by these oxidizing gases.
The invention is concerned in particular with carbon bodies which are used as components of electrolytic cells for the production of aluminum, for example by the electrolysis of alumina in a molten fluoride electrolyte such as cryolite, wherein in use of the cell the carbon components are exposed to air and anodically-generated oxidizing gases.
One aspect of the invention is the method of treating such cell components or other carbon bodies to improve their resistance to deterioration by oxidizing gases at high temperatures. Further aspects of the invention concern the treated cell components and other carbon bodies, aluminum production cells including these components, an improved, preferably less odoriferous treating liquid, preferably with a reduced susceptibility to foaming, and use of this treating liquid to improve the oxidation resistance of carbon bodies.
BACKGROUND ART
Aluminum is produced conventionally by the Hall-Héroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950° C. In Hall-Héroult cells, the anodes are usually pre-baked carbon blocks that are consumed by the electrochemical reaction, corroded by contact with the electrolyte and disintegrated by the air and/or oxidizing gases present. Söderberg anodes made of a coherent carbon mass which solidifies in situ are also used.
Pre-baked anodes for aluminum production are made of a matrix of petroleum coke with pitch as binder. Their production involves various phases including preparing and treating the starting materials, mixing, forming and calcining at high temperature, followed by securing the current supply member by rodding.
The resistance of that part of the anode which remains outside the bath during cell operation is of paramount importance, not only to decrease the amount of anode consumption above the theoretical requirement but also to reduce the formation of carbon dust which is a cause of a reduction in current efficiency and an increase in cell temperature, and which must be eliminated when it collects on bath surface.
Of the several attempts to protect the anode, none has so far been satisfactory. The normal protection by aluminum spraying is costly and not always impervious. The oxidation of the carbon anodes, in the Hall-Héroult cell, outside the bath leads to a loss for the aluminum producer. Typically, instead of the theoretical consumption of 0.33 kg of carbon per ton of aluminium, often more than 0.43 kg is lost, the difference being caused mainly by air and CO
2
burn.
Many elements or compounds catalyze the oxidation reaction of carbons but the inhibition of the same reaction is more difficult to achieve. In general, the oxidation reactivity of carbon is reduced with absorbers, or with ceramic protection layers. Several absorber additives have been reported, such as metal, halogen compounds, and incorporated nitrogen. Ceramic protecting layers have been proposed, formed by low melting liquid glass, such as B
2
O
3
, Cr
2
O
3
, silica, etc.
The oxidation prevention treatment processes contemplated for the anode can be divided into two different groups, one is an additive added after the anode baking, the other is an additive added into the carbon paste. To date, only an aluminium coating protection treatment, or a thick layer of alumina and cryolite, has worked reasonably well for oxidation protection of commercial pre-baked anodes, but has several drawbacks, such as cost and difficulties in the cell operation. No oxidation protection has so far been suggested for Söderberg continuous anodes. Several other oxidation prevention treatments have worked well in the laboratory but have fallen short of the expected performance when the same treatments have been applied to the anodes tested in commercial cells. No apparent reason has been forthcoming and the discussion of such an effect has invariably been directed towards the possibility of the composition of the anode gases being the reason for such a difference.
When boron has been added to the anode paste in the form of elemental boron or boron compound, the oxidation rate of the carbon has been reduced but the contamination of aluminum is unacceptable.
Recently, U.S. Pat. No. 5,486,278 (Mangienello et al.) has disclosed a treatment process which has been shown to significantly reduce the oxidation of the anode in the laboratory as well in commercial cell tests in the pre-baked carbon anodes. This method comprises treating the anode or other component in a boron-containing liquid to intake the boron-containing liquid to a selected depth over parts of the surface to be protected, this selected depth being in the range 1-10 cm, preferably at least 1.5 cm and at most about 5 cm, preferably still at least about 2 cm and at most about 4 cm. This method was found to significantly reduce the oxidation of pre-baked anodes in laboratory tests and in commercial test cells. However, as discussed in detail below, it has been found unexpectedly that the greatly improved oxidation resistance obtained with this treatment is partly offset by a strength loss which could lead to burn-off after a critical weight loss when the anode is subjected to stress. This could lead to problems when the components are scaled up to industrial size.
Problems like those described above for pre-baked carbon anodes apply also to the carbon cell sidewalls including a lower part submerged in the electrolyte and an upper part which is exposed to CO
2
-enriched air, and which disintegrate and wear away as a result of attack by oxidizing gases.
SUMMARY OF THE INVENTION
An object of the invention is to improve the resistance to oxidation of carbon bodies in particular carbon anodes or cell sidewalls of aluminum production cells by the incorporation of boron without the inherent drawbacks of the known proposals.
The invention is based on the insight that boron impregnated anodes and other carbon cell components made usually from petroleum coke and pitch suffer the drawback that the oxidation occurs particularly at the pitch location (bonding phase). With increasing time, the excess weight loss and disintegration of pitch (which is the bonding phase) reduces the overall mechanical strength so that after a certain time and above a certain temperature, when the weight loss reaches a critical amount (which may typically be about 8-10 wt %) the material reaches it brittleness limit and starts to collapse. In large components, this brittleness limit may be encountered in the cell before the main part of the anode has been consumed, and could lead to massive failure.
The invention provides a composition for the boron-containing impregnating liquid that minimizes attack in the pitch phase and hence improves the strength of the carbon while improving the oxidation resistance or at least maintaining it at the same level. The invention therefore aims to reduce the anode consumption by reducing anode brittleness and further improving the oxidation resistance. Similar advantages can be obtained with cell sidewalls or other carbon bodies such as Söderberg anodes, subjected to attack by oxidizing gases at high temperatures. A further improvement may be provided by making the impregnating liquid less odoriferous and/or less susceptible to foaming.
THE METHOD OF THE INVENTION
The invention provides a method of treating a carbon body, in particular a carbon-based anode or sidewall of an electrolytic cell for the production of aluminum in particular by the electrolysis of alumina in a molten flouride electrolyte such as cryolite, to improve the resistance thereof to deterioration by the attack of oxidizing gases, using boron in acceptable amounts in the surface parts exposed in use to oxidizing gases, in combination with selected agents and additives.
According to the invention, the boron-containing
De Nora Vittorio
Duruz Jean-Jacques
Sekhar Jainagesh A.
Deshmukh Jayadeep R.
Kalafut Stephen
Moltech Invent S.A
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