Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1999-10-13
2001-05-15
Phasge, Arun S (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S380000, C204S243100
Reexamination Certificate
active
06231745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to electrolytic cells. In one aspect, this invention relates to cathode collector bars of electrolytic reduction smelting cells used in the production of aluminum.
2. Background
Aluminum is produced by an electrolytic reduction of alumina in an electrolyte. The aluminum produced commercially by the electrolytic reduction of alumina is referred to as primary aluminum.
Electrolysis involves an electrochemical oxidation-reduction associated with the decomposition of a compound. An electrical current passes between two electrodes and through molten Na
3
AlF
6
cryolite bath containing dissolved alumina. Cryolite electrolyte is composed of a molten Na
3
AlF
6
cryolite bath containing alumina and other materials, e.g., such as fluorspar, dissolved in the electrolyte. A metallic constituent of the compound is reduced together with a correspondent oxidation reaction.
Electrical current is passed between the electrodes from an anode to a cathode to provide electrons at a requisite electromotive force to reduce the metallic constituent which usually is the desired electrolytic product, such as in the electrolytic smelting of aluminum. The electrical energy expended to produce the desired reaction depends on the nature of the compound and the composition of the electrolyte.
Hall-Heroult aluminum reduction cells are operated at low voltages (e.g. 4-5 volts) and high electrical currents (e.g. 70,000-325,000 amps). The high electrical current enters the reduction cell through the anode structure and then passes through the cryolite bath, through a molten aluminum metal pad, and then enters a carbon cathode block. The electrical current is carried out of the cell by multiple cathode collector bars.
As the electrolyte bath is traversed by electric current, alumina is reduced electrolytically to aluminum at the cathode, and carbon is oxidized to carbon dioxide at the anode. The aluminum, thus produced, accumulates at the molten aluminum pad and is tapped off periodically. Commercial aluminum reduction cells are operated by maintaining a minimum depth of liquid aluminum in the cell, the surface of which serves as the actual cathode. The minimum aluminum depth is about 2 inches and may be 20 inches.
The alumina-cryolite bath is maintained on top of the molten aluminum metal pad at a set depth. The current passes through the cryolite bath at a voltage loss directly proportional to the length of the current path, i.e., the interpolar distance gap between the anode and molten aluminum pad. A typical voltage loss is about 1 volt per inch. Any increase of the anode to cathode spacing restricts the maximum power efficiency and limits the efficiency of the electrolytic cell operation.
Much of the voltage drop through an electrolytic cell occurs in the electrolyte and is attributable to electrical resistance of the electrolyte, or electrolytic bath, across the anode-cathode distance. The bath electrical resistance or voltage drop in conventional Hall-Heroult cells for the electrolytic reduction of alumina dissolved in a molten cryolite bath includes a decomposition potential, i.e., energy used in producing aluminum, and an additional voltage attributable to heat energy generated in the inter-electrode spacing by the bath resistance. This latter heat energy makes up 35 to 45 percent of the total voltage drop across the cell, and in comparative measure, as much as twice the voltage drop attributable to decomposition potential.
An adverse result from reducing anode-cathode distance is a significant reduction in current efficiency of the cell when the metal produced by electrolysis at the cathode is oxidized by contact with the anode product. For example, in the electrolysis of alumina dissolved in cryolite, aluminum metal produced at the cathode can be oxidized readily back to alumina or aluminum salt by a close proximity to the anodically produced carbon oxide. A reduction in the anode-cathode separation distance provides more contact between anode product and cathode product and significantly accelerates the reoxidation or “back reaction” of reduced metal, thereby decreasing current efficiency.
The high amperage electrical current passing through the electrolytic cell produces powerful magnetic fields that induce circulation in the molten aluminum pad leading to problems such as reduced electrical efficiency and “back reaction” of the molten aluminum with the electrolyte. The magnetic fields also lead to the unequal depths in the molten aluminum pad and the cryolite bath. The motion of the metal pad increases, sometimes violently stirring the molten pad and generating vortices, and causing localized electrical shorting.
Metal pad depth variations restrict the reduction of the anode to cathode gap and produce a loss in current efficiency. Power is lost to the electrolyte interposed between the anode and cathode blocks. Movement of the molten aluminum metal pad also causes uneven wear on the carbon cathode blocks and may result in early cell failure.
Metal pad turbulence also increases the “back reaction,” or reoxidation, of cathodic products, thereby lowering cell efficiency. Metal pad turbulence accelerates distortion and degradation of the cathode bottom liner through attrition and penetration of the cryolite.
Molten aluminum metal pad stirring can be reduced by modifying the bus bar on an existing cell line to reduce the overall magnetic effects.
Whenever the anode-cathode distance is reduced, short circuiting of the anode and cathode must be prevented. In a conventional Hall-Heroult cell using carbon anodes held close to, but separated from, the molten aluminum metal pad, the shorting is caused by an induced displacement of the metal in the pad. Such displacement is caused in large part by the considerable magnetic forces associated with the electrical currents employed in the Hall-Heroult cell electrolysis. For example, magnetic field strengths of 150 gauss can be present in modern Hall-Heroult cells. This metal displacement can take the form of (1) a vertical, static displacement in the pad, resulting in an uneven pad surface such that the pad has a greater depth in the center of the cell by as much as 5 cm; (2) a wave-like change in metal depth, circling the cell with a frequency of 1 cycle/30 seconds; and (3) a metal flow with flow rates of 10-20 cm/second being common. To prevent shorting, the anode-cathode separation must be slightly greater than the peak height of the displaced molten product in the cell. In the case of aluminum production from alumina dissolved in cryolite in a conventional Hall-Heroult cell, such anode-cathode separation is held to a minimum distance, e.g., 4.0-4.5 cm.
Conventional electrolytic reduction smelting cells for the production of aluminum from alumina incorporate a pre-baked carbon anode structure suspended in the molten cryolite bath and an opposite molten aluminum metal pad cathode adjacent the cryolite bath. The molten aluminum metal pad collects on carbon blocks in the bottom of the cell and forms the liquid metal cathode adjacent the cryolite bath. The electrical current is conducted from the anode through the cryolite bath, then through the molten aluminum metal pad, and through the cathode blocks to the external electric bus bar of the cell.
In the conventional cathode today, multiple steel cathode collector bars extend from the external bus bars through each side of the electrolytic cell into the carbon cathode blocks. The steel cathode collector bars are attached to the cathode blocks with cast iron, carbon glue, or rammed carbonaceous paste to facilitate electrical contact between the carbon cathode blocks and the steel cathode collector bars.
The flow of electrical current through the aluminum pad and the carbon cathode follows the path of least resistance. The electrical resistance in a conventional cathode collector bar is proportional to the length of the current path from the point the electric current enters the cathode collector bar to the nearest external bus. The lower resist
Homley Graham E.
Ziegler Donald P.
Alcoa Inc.
Glantz Douglas G.
Klepac Glenn E.
Phasge Arun S
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