Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-07-13
2002-08-06
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S392000
Reexamination Certificate
active
06428675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low temperature production of aluminum in an electrolytic cell, in particular, production of aluminum at 200° C. or less.
2. Prior Art
A conventional method for producing aluminum which has been employed for decades is the Hall-Heroult process. In a Hall-Heroult process, alumina is electrolytically reduced to aluminum. A molten electrolytic bath typically including sodium cryolite (Na
3
AlF
6
) and other additives is contained in a carbon lined cell. Anodes positioned in an upper portion of the cell extend downwardly through the top of the bath. Electric current is supplied to the anodes to provide a source of electrons for reducing the alumina to aluminum which accumulates as a molten aluminum pad. The molten aluminum pad forms a liquid metal cathode. A cathode assembly positioned in a bottom of the cell supports the molten aluminum pad and completes the cathodic portion of the cell. Upon introduction of alumina into the molten electrolyte bath, the alumina dissolves and reacts to form molten aluminum and carbon dioxide according to the following reaction:
Al
2
O
3
+3/2C→2Al+3/2CO
2
Cryolite melts at about 910° C., hence this process is performed at about 950° C. or higher. At this high temperature, the electrolyte and molten aluminum aggressively react with the materials forming the cathode assemblies and the cell itself with concomitant difficulties in containing the reactants. One solution to this problem has been to operate aluminum production cells with relatively high heat losses so that a frozen layer of the bath forms on the inner walls of the cell. However, this solution also has drawbacks in the energy losses associated therewith, the costs for disposal of spent cells and difficulties encountered in maintaining the thickness of the frozen bath layer when regulating the bath temperature. Smelting processes using non-consumable (inert) anodes which produce oxygen are likewise subject to these heat balance problems.
Attempts have been made at lowering the temperature of the Hall-Heroult process. One route to this goal is to use a solvent having a lower melting point than cryolite. U.S. Pat. No. 3,725,222 describes a method of producing aluminum from AlCl
3
with an electrolyte bath of alkali metal chloride or alkaline earth metal chlorides at a temperature below 730° C. but above the melting point for aluminum (660° C.). Although this process operates at a lower temperature than Hall-Heroult electrolytic decomposition of alumina, bath temperatures over 660° C. are still problematic. In addition, AlCl
3
electrolysis requires relatively pure AlCl
3
as a feed material which is costly and renders commercial use of this process prohibitively expensive. Another similar proposal described in “Light Metals”, Vol. 1, 1979, pp. 353-361 suggests production of aluminum from alumina in a LiCl/AlCl
3
bath to form a species of AlOCl which is then electrolyzed at 700° C.
Low temperature salt bath processes also exhibit low alumina solubility and low alumina solution rates. This can be overcome by using specialized grades of alumina to control the water content therein such as disclosed in U.S. Pat. Nos. 3,852,173 and 3,951,763. Unfortunately, these approaches leave the anodes subject to degradation and are also commercially unacceptable due to the cost of the specialized grades of alumina.
Other attempts to improve the Hall-Heroult process have focused on the use of non-consumable (inert) anodes in metal chloride baths. See U.S. Pat. No. 4,681,671. More recently, U.S. Pat. No. 5,013,343 discloses electrolysis of alumina with inert anodes in very low solubility conditions (less than 1 weight percent alumina) allowing for lower bath temperatures. However, this system is only feasible when the surface area of the anode is greatly increased such as by drilling numerous holes deep into the anode. U.S. Pat. No. 5,725,744 describes a similar system in which the physical characteristics of the anodes are altered to improve efficiency.
Another drawback to conventional Hall-Heroult processes resides in the use of cryolite. When a Hall-Heroult cell experiences a condition know as anode effect, fluoride in the cryolite is released, usually in the form of HF or CF
4
. Emissions of HF, CF
4
and other fluoride compounds are environmentally hazardous. Hence, an electrolyte bath without fluoride compounds is desirable.
Accordingly, a need remains for a non-fluoride based electrolyte system which is operable at temperatures well below those employed in conventional Hall-Heroult processes, avoids the need for specialized alumina and may be operated with conventional styled anodes.
SUMMARY OF THE INVENTION
This need is met by the process of the present invention which includes a method of producing aluminum in an electrolytic cell having the steps of (a) providing an anode and a cathode in a molten bath containing AlCl
3
and an alkali metal chloride, the bath having a temperature of less than about 300° C. and;(b) passing a current between the anode and the cathode; and (c) adding Al
2
O
3
to the bath such that solid aluminum is produced at the cathode. The alkali metal chloride is selected from the group consisting of NaCl, LiCl, KCl, MgCl
2
, CaCl
2
, BeCl
2
, BaCl
2
, and combinations thereof, and preferably is NaCl. The concentration of alkali metal chloride in the bath is about 10 to 50 mole percent The molten bath is maintained at a temperature of no more than about 200° C., preferably at about 150 to 200° C.
Prior to step (b), the cell is pre-electrolyzed by setting a voltage between the anode and the cathode at a level which causes the current to drop to zero to eliminate oxides in the bath. Preferably, the pre-electrolysis voltage level is less than about 2.0 volts, more preferably about 1.5 to 1.7 volts.
Upon addition of alumina to the cell, aluminum is produced at the cathode. When the cell is operated at sufficiently high cathode current densities, the electrolytic reaction is driven so that the concentration at the cathode of the aluminum species undergoing electrolysis is very low or zero causing the alkali metal chloride to solidify at the cathode. In this manner, solid aluminum is trapped in the frozen alkali metal chloride.
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Haupin, W.E., “Oxide Solubility in Lithium Chloride-Aluminum Chloride Melts”,Light Metals, vol. 1, 1979, pp. 353-361. (No Month).
Alcoa Inc.
Klepac Glenn E.
Meder Julie W.
Valentine Donald R.
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