Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-02-06
2002-11-05
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S247400, C204S294000, C204S290100, C204S290110, C204S290120, C204S290150, C205S380000, C205S384000, C205S385000, C427S113000, C427S126100, C252S387000
Reexamination Certificate
active
06475358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to the production of protective coatings for carbonaceous components of electrolytic cells used in the production of aluminum. The invention more specifically relates to coating compositions which provide carbonaceous components of electrolytic cells with protection from deterioration during electrolysis and components containing the same.
2. Description of Related Art
The manufacture of aluminum is conducted conventionally by the Hall-Heroult electrolytic reduction process, whereby alumina is dissolved in molten cryolite and electrolyzed at temperatures of about 900 to 1000° C. This process is conducted in a reduction cell typically comprising a steel shell provided with an insulating lining of suitable refractory material, which is in turn provided with a lining of carbon which contacts the molten constituents. One or more anodes, typically made of prebaked carbon blocks, are connected to the positive pole of a direct current source, and suspended within the cell. One or more conductor bars connected to the negative pole of the direct current source are embedded in the carbon cathode substrate comprising the floor of the cell, thus causing the cathode substrate to become cathodic upon application of current.
Prebaked anodes used in the production of aluminum are comprised of an aggregate of petroleum coke with pitch as a binder, while the carbon lining is typically constructed from an array of prebaked cathode blocks, rammed together with a mixture typically comprising of anthracite, tar, and coal tar pitch.
Aluminum is produced in a molten form within an electrolysis cell as a result of the following reaction:
2Al
2
O3+3C→4Al+3CO
2
In the conventional design of the Hall-Heroult cell, aluminum collects as a pool of molten aluminum along the base of the cell. In doing so, oxygen becomes liberated and reacts with the available carbon on the surface of the anodes to produce carbon dioxide gas. Theoretically, 0.334 kg of anodic carbon is consumed per kilo of aluminum produced as represented by the above reaction. In reality, however, anodic consumption is 25-35% greater.
Excess consumption of the prebaked anodes is the result of a series of secondary reactions, which can be summarized as follows:
i) Air oxidation: oxidizing reactions result from oxygen in the air contacting the upper part of the anode and, if the anode is left unprotected, reacting to produce carbon dioxide;
ii) Boudouard reaction: carbo-oxidation reactions result from CO
2
at the surface of the anode being immersed in the electrolyte and producing carbon monoxide (known as the Boudouard equilibrium); and
iii) Dusting: the selective oxidation of pitch coke with respect to petroleum coke, results in the release of carbon particles, generating dust, which has negative effects on the operation.
The loss effected by such secondary reactions within the electrolytic cell amounts to approximately 10% of the production cost of aluminum.
The economic inefficiencies of aluminum production can be further attributed to the deterioration of the carbon lining or cathodic material of the electrolytic cell as a result of erosion and penetration of electrolyte and liquid aluminum, as well as intercalation by metallic sodium.
Although the Hall-Heroult process for aluminum production is the most reliable to date, there is a continual need for improvement. In view of the economic impact of the inefficiencies of this process, considerable effort has focused on the development of improved electrolytic cell components which are capable of withstanding the harsh conditions imposed by the electrolysis of aluminum.
For instance, U.S. Pat. No. 3,852,107 to Lorkin et al. teaches of an impermeable protective coating for electrodes comprising a matrix having a melting point under 1000° C. and a refractory filler, dissolved or suspended in a liquid carrier such as water. As an example, the matrix component of this coating was described as a graphite wettable material such as boric acid and/or a glaze forming material such as sodium aluminum fluoride. Suggested refractory fillers include oxides, carbides, nitrides or borides. The use of a suitable surface tension modifying agent such as chrome ore was suggested in certain situations to improve the wetting of the graphite.
U.S. Pat. No. 4,624,766 to Boxall, et al. describes an aluminum wettable, cured, carbonized cathode material for use in aluminum electrolysis cells, comprising a hard refractory material in a carbonaceous matrix which includes a carbonaceous filler and carbon fiber bonded by a non-graphitized amorphous carbon, this matrix having a rate of ablation essentially equal to the rate of wear and dissolution of the refractory hard material in the operating environment of the cell.
Sekhar et al., WO 98/17842 published Apr. 30, 1998, describes a method for applying a refractory boride to components of an aluminum electrolysis cell by forming a slurry of particulate preformed refractory boride in at least two grades of colloidal carriers selected from the group consisting of colloidal alumina, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminum phosphate, cerium acetate and mixtures thereof, the two colloidal carriers preferably each being of the same colloid, followed by drying. The two grades of colloidal carrier have mean particle sizes which differ from one another by about 10-50 nanometers.
U.S. Pat. No. 5,486,278 to Manganiello discloses a method of impregnating a carbonaceous cell component with a boron-containing solution to improve resistance to deterioration during cell operation. When water was used as the solvent for the boron-containing solution, a surfactant was required to achieve an acceptable treatment time. Alternatively, the solvent could be chosen from methanol, ethylene glycol, glycerin and mixtures thereof. This method required the intake of the boron-containing solution to a depth of 1-10 cm into the component to be protected. This patent further disclosed that the air oxidation of carbonaceous components treated in this manner was comparable to the net consumption of similar components treated with traditional aluminum protective coatings.
Despite previous efforts, conventional techniques for performing the electrolysis of aluminum are still employed most often. This indicates that a more technically superior or economically profitable method of combating carbonaceous cell component deterioration is not known.
Lignosulfonates, such as ammonium lignosulfonate, have long been used as binders in a variety of different industries but not in aluminum electrolysis cells.
It is an object of the present invention to provide an effective and economical method of treating components of an electrolytic cell, for producing aluminum, to protect them from deterioration during operation of the cell.
BRIEF SUMMARY OF THE INVENTION
The present invention in its broadest aspect relates to a method of treating a carbonaceous cell component of an electrolytic cell for the production of aluminum, to improve the resistance of the component to deterioration during operation of the cell. The method comprises preparing a liquid suspension of refractory material dispersed in a lignosulfonate binder solution and applying the liquid as a protective coating to the carbonaceous cell component, followed by drying the coating. The refractory material may be selected from a wide variety of refractory compounds, such as boron, zirconium, vanadium, hafnium, niobium, tantalum, chromium and molybdenum compounds.
As a by-product of the pulp and paper industry, lignosulfonate is both abundant and relatively inexpensive. It has been found to be surprisingly effective as a binder in the harsh environment of an aluminum electrolysis cell.
According to one embodiment of the invention, lignosulfonate binder is used in the coating of prebaked carbon anodes. For this purpose, a liquid suspension is prepared of a boron compound, e.g. boric acid, boron oxide, hydrated boron oxide or borax, aluminum fluoride and
Bergeron Jules
Mirtchi Amir A.
Alcan International Limited
Bell Bruce F.
LandOfFree
Method for providing a protective coating for carbonaceous... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for providing a protective coating for carbonaceous..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for providing a protective coating for carbonaceous... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2929699