Electrolytic cell and process for metal reduction

Details Electrolytic cell and process for metal reduction Electrolytic cell and process for metal reduction

1989-08-25

1991-05-14

Niebling, John F.

Chemistry: electrical and wave energy

Processes and products

204 60, 204 61, 204 71, C25C 306, C25B 102, C25B 100

Patent

active

050153439

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
This invention relates to an electrolytic cell and to the electrolytic formation of an electrode product using a molten salt.
2. Background of Invention
In the electrolytic decomposition of alumina in molten salts, the Hall-Heroult process is commonly employed. Examples of Hall-Heroult cells are shown in U.S. Pat. Nos. 3,839,167, 3,996,117, and 4,269,673.
It has been known that the power consumption can be decreased in a Hall-Heroult cell by placing the anode and cathode in close proximity to one another. However, to maintain typical operating temperatures and gain the energy conserved in a Hall-Heroult cell as a result of reducing anode-cathode distance an equal reduction of thermal losses to the environment is required. The degree of insulation possible to reduce thermal energy lost is limited by the need to maintain a frozen electrolyte layer on the sidewall for protecting the lining materials. An increase in current could be used to achieve normal operating temperature, but this is limited by the magnetic stability of the cell and would reduced the energy conserved.


DISCLOSURE OF INVENTION

This invention enables the use of molten salts with limited reactant, e.g. alumina, solubility to be such as aluminum. An advantage is that some of these salts have low liquidus temperatures, e.g. in the range 300.degree. to 900.degree. C., preferably in the 500.degree. to 800.degree. C., such that the molten salt operating temperature can be lower than usual Hall-Heroult cell operating temperatures, for instance from 900.degree. C. down to the melting point of aluminum (or even below the melting point of aluminum if it is desired to produce solid aluminum). These salts are not as corrosive as those conventionally used in the Hall-Heroult process, are lower in density, and have lower alkali metal activity. The reduced corrosion eliminates the need for frozen electrolyte to protect the materials used in cell construction. The lower density of the salt improves cell stability because of the greater density difference between the metal and the salt, requiring greater force differences to create the same amplitude waves at the bath-metal interface. The lower alkali metal activity improves current efficiency, and eliminates swelling of carbon materials in the cell, enhancing cell life. The invention additionally provides the potential for the use of graphitic cathodic floors, rather than the usual carbon floors; graphitic floors are currently not feasible, because of high intercalation of alkali metal species into them, leading to premature failure.
Thus, it is proposed that improved results can be achieved with the use of an electrolytic cell, e.g. a Hall-Heroult cell, comprising an electrode having an evolution of the desired electrode product, and a molten salt with limited reactant solubility. According to the invention, reactant solubility is .ltoreq.1 wt %. This allows the use of all broader ranges of molten salt, at lower operating temperatures with benefits in both physical properties and lessened chemical reactivity of the molten salt with cell components such as the materials of construction (for instance the refractories used for the sidewalls and the floor) and the electrodes.
In a preferred embodiment, the anode and cathode are in close proximity to one another, i.e. 0.25"-1.25", and the outside walls of the cell are thermally insulated sufficiently to maintain the electrolyte temperature at the decreased power levels. Operation can be without a frozen sidewall; some suspended solid reactant will usually be present.
Percentages herein are on a weight basis, unless indicated otherwise. Reactant concentrations are based on total weight of molten salt plus reactant, although such is not an essential point in view of the low reactant concentrations.
The invention comprises an improvement concerning the electrolytic decomposition of a substance in a molten salt electrolyte, e.g., a chloride and/or fluoride electrolyte, which typically has low solubility for an oxide whose decompo

REFERENCES:
patent: 4504366 (1985-03-01), Jarrett et al.
patent: 4681671 (1987-07-01), Duruz
patent: 4707239 (1987-11-01), Murphy et al.

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