Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1996-04-24
1998-03-10
Valentine, Donald R.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204245, 204247, C25C 308, C25C 314
Patent
active
057257442
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a cell for producing aluminum by electrolysis of alumina dissolved in a molten halide electrolyte particularly at temperatures between 680.degree.-880.degree. C.
BACKGROUND OF THE INVENTION
Aluminium is produced by the Hall-Heroult process which involves the electrolysis of alumina dissolved in molten cryolite (Na.sub.3 AlF.sub.6) at about 960.degree. C. using carbon anodes which are consumed with the evolution of CO.sub.2. However, the process suffers from major disadvantages. The high cell temperature is necessary to increase the solubility of alumina and its rate of dissolution so that sufficient alumina can be maintained in solution, but requires heavy expenditure of energy. At the high cell temperature, the electrolyte and the molten aluminium aggressively react with most materials including ceramic and carbonaceous materials, and this creates problems of containment and cell design. The anode-cathode distance is critical and has to be maintained high due to the irregular movement of the molten aluminium cathode pool, and this leads to loss of energy. Since the anodes are continually being consumed, this creates problems of process control. Further, the back oxidation of Al to Al.sup.3+ decreases the current efficiency.
Potentially, the electrolysis of alumina at low temperatures (below 880.degree. C.) in halide melts has several distinct advantages over the conventional Hall-Heroult process operating at about 960.degree. C. As shown by bench-scale tests, electrolysis at reduced current densities in low temperature melts potentially offers a significant advantage in increasing the stability of electrode materials, but it has not yet proven possible to implement the process in a way where this advantage could be realized in larger scale cells and in commercial cells. Other potential advantages are higher current and energy efficiencies and the possibility of designing a completely enclosed electrolytic cell.
Problems which hindered the practicability of low temperature electrolysis are the low alumina solubility in low temperature electrolytes, as well as low alumina solution rates. Under these conditions, a sufficiently high transport rate of oxide ion species from the bulk of the electrolyte to the anode surface cannot be maintained at the anode current densities normally used in conventional Hall-Heroult cells. The configuration of cells presently used does not permit a substantial increase of the relative surface area of anode to cathode. This means that a reduction of the current density would lead directly to a reduction of the cell productivity. Moreover, the design of presently used cells does not enable an increase of the electrolyte circulation to increase the transport rate of oxygen ions to the anode active surface area and to increase the dissolution rate of alumina in the electrolyte.
Low temperature alumina electrolysis has been described in U.S. Pat. No. 3,951,763 and requires numerous expedients such as the use of a special grade of water-containing alumina to protect the carbon anodes, and the bath temperature had to be 40.degree. C. or more above the liquidus temperature of the Na.sub.3 AlF.sub.6 /AlF.sub.3 system in an attempt to avoid crust formation on the cathode. In practice, however, the carbon anodes were severely attacked during anode effects accompanied by excessive CF.sub.4 emissions. Crusts also formed on the cathode up to electrolyte temperatures of 930.degree. C.
Because of the difficulties encountered with fluoride-based melts, major efforts to secure the advantages of low temperature electrolysis were devoted to different electrolytes, notably chloride based electrolytes where AlCl.sub.3 is used as a feed; the anode reaction being chlorine evolution. See e.g. K. Grjotheim, C. Krohn and H. .phi.ye, Aluminium 51, No 11, 1975, pages 697-699, and U.S. Pat. No. 3,893,899. However, problems related to the production of pure AlCl.sub.3 have hitherto eliminated this process from commercial application.
Another proposal to produce al
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De Nora Vittorio
Duruz Jean-Jacques
Moltech Invent S.A.
Valentine Donald R.
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