US3725222A

US3725222A - Production of aluminum

Info

Publication number: US3725222A
Application number: US00192653A
Authority: US
Inventors: A Russell; L Knapp; W Haupin
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1971-10-26
Filing date: 1971-10-26
Publication date: 1973-04-03
Anticipated expiration: 1990-04-03
Also published as: JPS4850910A; SE396776B; AT327578B; DE2251262C2; BR7207305D0; CH555410A; AU453929B2; IT966362B; NL7213843A; AU4623272A; PL82400B1; PH9821A; CA981209A; ZA727061B; FR2158238B1; DD99610A5; RO60672A; FR2158238A1; YU249272A; NL155891B

Abstract

Production of aluminum by continuous electrolysis of aluminum chloride in molten electrolyte of controlled oxide content.

Description

United States Patent 1191 1111 3,725,222 Russell et al. 1451 Apr. 3, 1973 541 PRODUCTION OF ALUMINUM 1,942,522 1/1934 Weber et al ..204/67 2,919,234 12/1959 Slatin l ..204/67 Inventors: Allen Russell, New Kensmgmn; 3,103,472 9/1963 Slatin ..204/67 Lester PP, Maryville; Warren 3,518,172 6/1970 Layne etal ..204/67 E. Haupin, New Kensington, all of Primary Examiner-John H. Mack [73] Assignee: Aluminum Company of America, Assistant Examiner D' valemme Pittsburgh, Attorney-Edward B. F oote [22] Filed: Oct. 26, 1971 57 ABSTRACT 1 1 pp N03 1921653 Production of aluminum by continuous electrolysis of aluminum chloride in molten electrolyte of controlled 52 us. 01 ..204/67 OXIde [Sl] Int. Cl. ..C22d 3/12, [58] Field of Search ..204/67 [56] References Cited 17 Claims, No Drawings UNITED STATES PATENTS 12/1916 McAfee ..204/67 PRODUCTION OF ALUMINUM This invention relates to the production of aluminum by electrolysis of aluminum chloride dissolved in molten salt.

For over 80 years the customary method of producing aluminum commercially has been the well-known t Hall-Heroult process in which alumina dissolved in a fluoride bath (principally cryolite) is reduced electrolytically. Production of aluminum by electrolysis of aluminum chloride dissolved in a molten electrolyte composed of one or more halides having higher electrodecomposition potential than aluminum chloride (e.g., alkali metal halide or alkaline earth metal halide) has been described in the literature; for example, Z. fur Elektrochemie, Vol. 54, pp. 210-215, U.S. Pat. Nos. 1,296,575, 1,854,684, 2,919,234, 3,103,472 and Canadian Pat. No. 502,977.

Although production of aluminum by electrolysis of aluminum chloride offers certain potential advantages over the Hall-Heroult process, such as operation at lower temperature and avoidance of consumption of carbon electrodes through oxidation by oxygen evolved in electrolysis of alumina, disadvantages have outweighed such advantages and production of aluminum by electrolysis of aluminum chloride has not been commercially adopted.

Major problems which have effectively precluded commercially economical continuous electrolysis of aluminum chloride dissolved in molten salts at above the melting point of aluminum stem from the presence of metal oxides such as alumina, silica, titania, and the like in the electrolytic bath. Metal oxides in the bath, and particularly undissolved metal oxides, are a primary factor in causing a gradual accumulation on cell cathodes of a viscous layer of finely divided solids, liquid components of the bath, and droplets of molten aluminum. The above-mentioned layer herein referred to as sludge inhibits or interferes with continued access of electrolytic bath to the cathodes, with the result that upon depletion of the aluminum chloride in the sludge layer by electrolysis, bath solvent in the sludge electrolyzes, with attendantloss of current efficiency in the production of aluminum. The sludge layer also interferes with circulation of the electrolytic bath, with further resultant impairment of electrical efficiency. Moreover, when the bath contains alkali metal halide or alkaline earth metal halide as the solvent for the aluminum chloride, carbonaceous cathodes of the cell are attacked by alkali metal or alkaline earth metal produced by electrolysis of such salts, causing spalling and disintegration of the cathodes, with attendant change in the anode-cathode distance and increase in maintenance expense, as well as introducing into the electrolyte particles of carbon which contribute to the formation of sludge at the cathode.

The problem of sludge formation caused by metal oxides in the bath is complicated by the fact that, in general, the lower the aluminum chloride concentration in the bath, the better is the electrical conductivity.

From the standpoint of optimum electrical conductivity of the bath, and attendant minimizing of power consumed, it is desirable to operate the electrolysis process at an aluminum chloride concentration in the bath of about 1 to percent by weight, whereas at such concentrations of aluminum chloride metal oxides are only very slightly soluble in the bath. Thus, operating in the cell.

desirable aluminum chloride concentration range enhances the problem of sludge formation.

A further disadvantage of metal oxides in the electrolytic bath is that dissolved metal oxides have a lower electrodecomposition potential than aluminum chloride, and upon electrolyzing release oxygen at the cell anodes. Carbon is the most practical material to use for anodes, but evolved oxygen. reacts with the carbon to form gaseous oxides. Such consumption of anode carbon affects the operating characteristics of the cell deleteriously by changing the anode-cathode distance, as well as adding to anode expense.

It is the primary object of this invention to improve the production of aluminum by electrolysis of aluminum chloride, and particularly to increase the electrical efficiency of the electrolytic cells and otherwise reduce the cost of operation.

In the production of aluminum in accordance with this invention, aluminum chloride dissolved in molten salt of higher electrodecomposition potential than aluminum chloride such as alkali metal halide or alkaline earth metal halide is electrodecomposed continuously, the concentration of aluminum chloride in the electrolytic bath of salt and aluminum chloride being in the range of l to 15 percent by weight, and preferably 3 to 10 percent by weight, and being maintained in such ranges by adding aluminum chloride continuously or intermittently to the bath to replace aluminum chloride electrodecomposed. Molten aluminum produced settles out of the bathand can be withdrawn in any suitable way, such as by tapping or siphoning it from the It has been found that in carrying out the above process continuously (i.e., for periods of over 700 hours of operation) under conditions in which metal oxides are introduced into the electrolytic bath it is highly important that the concentration of metal oxides in the bath, expressed as oxygen, be kept below 0.25 percent by weight, and preferably below 0.1 percent by weight, and more preferably below 0.05 percent. Metal oxides, such as alumina, silica, (silicon is considered herein as a metal, albeit it is a metalloid), iron oxide, titanium oxide, and lime are only slightly soluble in the electrolytic bath, and as mentioned previously, metal oxides are a primary cause of the formationof the above-mentioned undesirable sludge. Moreover, although metal oxides are only slightly soluble in the bath, electrolysis of the dissolved oxides releases oxygen at carbonaceous anodes of the cell, oxidizing the carbon with resultant increase in the anode-cathode distance in the cell and an attendant gradual increase in electrical resistance.

By maintaining a concentration of metal oxides in the bath below 0.25 percent by weight as aforesaid, the electrolysis of aluminum chloride can be carried out indefinitely without formation of sludge at the cathodes in amounts which significantly affect the process or equipment detrimentally. For example, the processcan be continued indefinitely with an anode-cathode distance of less than 1 inch, a cathode currentdensity of about 10 amperes, a voltage of less than 5 volts between anode and cathode, and a current efficiency of better than percent with respect to electrodecomposition of aluminum chloride.

Metal oxides may enter the bath in various ways; for example, as impurities in bath components (i.e., aluminum chloride or solvent) fed to the cell. Also, moisture which leaks into the cell or is present in cell walls or bath components used in the process reacts with molten aluminum in the cell to form alumina. Likewise, contact of the bath with cell linings or other structural parts of the cell which contain metal oxides, such as refractories containing alumina or silica, can introduce such oxides into the bath.

In producing aluminum in accordance with this invention, introduction of metal oxides into the bath is controlled, as indicated above. Moreover, in continuous operation of the electrolytic process, with attendant continuous or intermittent feeding of aluminum chloride to the bath to replace aluminum chloride which is electrolyzed, it is especially important in the interest of maintaining the above-mentioned low concentration of metal oxides in the bath that the aluminum chloride fed to the bath contain a total of less than 0.25 percent by weight of metal oxides, preferably less than 0.1 percent, and even more preferably less than 0.05 percent by weight.

References herein to metal oxides include oxygenated compounds containing additional ions besides metal and oxygen, e.g., oxyhalides and oxynitrides.

In the preferred operation of the process the electrolyte employed consists essentially of one or more alkali metal halides or alkaline earth metal halides which have a higher electrodecomposition potential than aluminum chloride, the chlorides being preferred, and the process is carried out at a temperature below 730 C. but above the melting point of aluminum (660 C.). For example, a mixture of equal parts by weight of sodium chloride and lithium chloride is particularly satisfactory as the ele ctrolytejlt will be understood that other components can also be added to the bath, if desired, to modify bath characteristics.

Electrolytic cells of known types employing spaced monopolar electrodes, or spaced bipolar electrodes between anode and cathode terminals, can be used in producing aluminum in accordance with the invention. A particularly suitable type of cell is described in U.S. Pat. application Ser. No. 178,650, of Dell, I-Iaupin and Russell, filed Sept. 8, 1971. In the interests of excluding moisture from the cell and recovering gaseous materials evolved in the electrolysis it is desirable that the cell be closed except for one or more outlets for such gaseous materials, and one or more inlets for feeding aluminum chloride into the cell. As already indicated, it is also desirable to avoid using in structural parts of the cell which come in contact with the electrolytic bath any materials which introduces metal oxides into the bath. Also, prior to starting aluminum production in a new'cell it is advisable to remove moisture which is present in the cell walls, electrodes, and other structural parts, to avoid reaction of such moisture with molten aluminum to form alumina in the bath. Such moisture can be driven off conveniently by feeding molten electrolyte into the cell and maintaining it-at elevated temperature long enough that moisture in the cell vaporizes and escapes through openings in the cell, such as outlets for vapors evolved during electrolysis.

The use of the conditions described above make possible substantial economics in the production of aluminum by electrolysis of aluminum chloride as a result of improvement in electrical efficiency through 5 avoidance of sludge formation and deterioration of the electrodes, and through reduced maintenance expense for carbonaceous anodes and cathodes.

If on occasion during operation of the process the amount of undissolved metal oxides in the bath happens to increase sufflciently that the concentration of metal oxides in the bath exceeds a predetermined desired level, that situation can be corrected by various procedures. Undissolved metal oxide can be separated from the bath, as by filtration of undissolved oxides from the bath, to reduce the concentration of metal oxide in the bath to the desired level. Another alternative is to alter the conditions of operation of the electrolysis process temporarily so that undissolved metal oxide in the bath is dissolved and electrolyzed until the concentration of metal oxide in the bath returns to the desired level. For example, undissolved oxide in the bath can be dissolved by temporarily increasing the capacity of the bath to dissolve metal oxide present by increasing the concentration of aluminum chloride in the bath, or decreasing the temperature of the bath sufficiently that enough additional metal oxide dissolves and electrolyzes for the concentration of metal oxide into the bath to return to the predetermined desired level, whereupon the prior concentration of aluminum chloride in the bath, or the prior bath temperature, can be restored.

Another way of increasing the capacity of the bath to dissolve metal oxides so that metal oxide can be removed by electrodecomposition thereof is to add to the bath a component suitable for that purpose. For example, when the baths solvent for aluminum chloride is alkali metal chloride, a small concentration of a fluoride e.g., about 1 percent by weight, expressed as fluorine can be introduced into the bath for that purpose; magnesium fluoride, aluminum fluoride, sodium fluoride, calcium fluoride, or cryolite are examples of fluorides that can be used.

A further alternative procedure is to reduce the current density employed in the electrolytic cell to a level at which the rate of electrolysis of dissolved metal oxide increases relative to the rate of electrolysis of aluminum chloride, and maintaining such reduced current density until sufficient metal oxide has been electrolyzed that the amount of metal oxide in the bath returns to the desired level. Thereafter the current density can be increased to return it to its original level.

As a specific example of the invention, aluminum was produced by continuous electrolysis of aluminum chloride at 695700 C. in an electrolytic cell of the type described in the aforesaid U.S. Pat. application of Dell, Haupin and Russell consisting ofa metal shell having an electrolysis chamber lined with silicon nitride-bonded fused silica, and having a graphite anode in the upper portion thereof and a graphite between each of the opposed electrodes was about 1 inch. The cell was closed except for an inlet through the top for feeding aluminum chloride into the electrolytic bath, an outlet in the top for chlorine and aluminum chloride vapors generated, and an outlet for withdrawal of molten aluminum produced.

The above-mentioned electrolysis compartments were kept immersed in electrolytic bath consisting essentially of sodium chloride and lithium chloride, plus about 6-7 percent by weight of aluminum chloride. Aluminum chloride having a total metal oxide content of less than 0.03 percent by weight, and essentially free of moisture, was fed into the bath continuously to replace aluminum chloride decomposed and maintain the aluminum chloride content of the bath at about 6-7 percent by weight.

The cell was operated continuously for 120 days at about 3.3 volts per electrolysis compartment and an average cathode current density of 8.5 amperes per square inch, without noticeable formation of sludge in the bath. The concentration of metal oxides (expressed as oxygen) in the electrolytic bath remained at less than 0.002 percent by weight of the bath throughout the operation. Molten aluminum produced collected in the lower part of the electrolytic chamber and was drawn off periodically. 5 and 2/10 kilowatt hours of electric power was consumed per pound of aluminum produced.

We claim:

1. In a process for continuous production of aluminum by electrolysis of aluminum chloride dissolved in molten solvent having a higher electrodecomposition potential than aluminum chloride, in which introduction of metal oxide into the electrolytic bath of aluminum chloride and solvent occurs, and aluminum chloride is fed into the said bath to replace the aluminum chloride decomposed, the improvement comprising limiting introduction of metal oxide into the said bath sufficiently that the percent by weight of oxide (expressed as oxygen) in the said bath does not exceed 0.25 percent.

2. A process in accordance with claim 1 wherein the percent by weight of oxide in the bath does not exceed 0.1 percent.

3. A process in accordance with claim 1 wherein the percent by weight of oxide in the bath does not exceed 0.05 percent.

4. A process in accordance with claim 1 wherein the said aluminum chloride fed into the electrolytic bath contains less than 0.25 percent by weight of metal oxide.

5. A process in accordance with claim 4 wherein the said aluminum chloride fed into the electrolytic bath contains less than 0.1 percent by weight of metal oxide.

6. A process in accordance with claim 4 wherein the said aluminum chloride fed into the electrolytic bath contains less than 0.05 percent by weight of metal oxide.

7. A process in accordance with claim 1 wherein the concentration of aluminum chloride in the said bath is maintained at l-l 5 percent by weight.

8. A process in accordance with claim 7 wherein the aluminum chloride fed into the electrolytic bath contains less than 0.25 percent by weight of metal oxide.

9. A process in accordance with claim I wherein upon increase in the concentration of metal oxide in the bath above a predetermined desired level through introduction of undissolved metal oxide into the bath, sufficient metal oxide is removed from the bath to reduce the concentration of metal oxide in the bath below the said predetermined level.

10. A process in accordance with claim 9 wherein the removal of metal oxide from the bath is effected by increasing the capacity of the bath thru effected by increasing the capacity of the to dissolve metal oxide and thereafter electrolyzing sufficient dissolved metal oxide from the bath that the concentration of metal oxide in the bath returns to. the said predetermined desired level.

11. A process in accordance with claim 10 wherein the said increasing of the capacity of the bath to dissolve metal oxide is effected by increasing the concentration of aluminum chloride in the bath. I

12. A process in accordance with claim 10 wherein the said increasing of the capacity of the bath to dissolve metal oxide is effected by decreasing the temperature of the bath.

13. A process in accordance with claim 10 wherein the said solvent consists essentially of alkali metal chloride, and the said increasing of the capacity of the bath to dissolve metal oxide is effected by adding a fluoride to the bath.

14. A process in accordance with claim 9 wherein the said removal of metal oxide from the bath is effected by temporarily decreasing the current density sufficiently to increase the rate of electrolysis of metal oxide from the solution.

15. A process in accordance with claim 9 wherein the said removal of metal oxide is effected by filtration of undissolved oxides from the bath.

16. A process in accordance with claim 1 wherein the said solvent consists essentially of at least one salt selected from the group consisting of alkali metal halides and alkaline earth metal halides.

17. A process in accordance with claim 1 wherein the said solvent consists essentially of at least one salt selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,725,222 Dated April 3, 1973 Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 2 Change "economics" to "economies".

Claim 10, lines3 and 4 After "bath" delete "thru effected by increasing the capacity of the Signed and seaiLed this 20th day of November 1973.

(SEAL) Attest:

EDWARD, M.FLETCHER,JR. RENE D. TEGTME YER v Attesting Officer I Acting Commissioner of Patents FORM PO-lOSO (10-69) USCOMM-DC 60376-P69 a 11.5, GOVERNMENT PRINTING OFFICE: (969 o-3ss-:3A

Claims

2. A process in accordance with claim 1 wherein the percent by weight of oxide in the bath does not exceed 0.1 percent.
3. A process in accordance with claim 1 wherein the percent by weight of oxide in the bath does not exceed 0.05 percent.
4. A process in accordance with claim 1 wherein the said aluminum chloride fed into the electrolytic bath contains less than 0.25 percent by weight of metal oxide.
5. A process in accordance with claim 4 wherein the said aluminum chloride fed into the electrolytic bath contains less than 0.1 percent by weight of metal oxide.
6. A process in accordance with claim 4 wherein the said aluminum chloride fed into the electrolytic bath contains less than 0.05 percent by weight of metal oxide.
7. A process in accordance with claim 1 wherein the concentration of aluminum chloride in the said bath is maintained at 1-15 percent by weight.
8. A process in accordance with claim 7 wherein the aluminum chloride fed into the electrolytic bath contains less than 0.25 percent by weight of metal oxide.
9. A process in accordance with claim 1 wherein upon increase in the concentration of metal oxide in the bath above a predetermined desired level through introduction of undissolved metal oxide into the bath, sufficient metal oxide is removed from the bath to reduce the concentration of metal oxide in the bath below the said predetermined level.
10. A process in accordance with claim 9 wherein the removal of metal oxide from the bath is effected by increasing the capacity of the bath thru effected by increasing the capacity of the to dissolve metal oxide and thereafter electrolyzing sufficient dissolved metal oxide from the bath that the concentration of metal oxide in the bath returns to the said predetermined desired level.
11. A process in accordance with claim 10 wherein the said increasing of the capacity of the bath to dissolve metal oxide is effected by increasing the concentration of aluminum chloride in the bath.
12. A process in accordance with claim 10 wherein the said increasing of the capacity of the bath to dissolve metal oxide is effected by decreasing the temperature of the bath.
13. A process in accordance with claim 10 wherein the said solvent consists essentially of alkali metal chloride, and the said increasing of the capacity of the bath to dissolve metal oxide is effected by adding a fluoride to the bath.
14. A process in accordance with claim 9 wherein the said removal of metal oxide from the bath is effected by temporarily decreasing the current density sufficiently to increase the rate of electrolysis of metal oxide from the solution.
15. A process in accordance with claim 9 wherein the said removal of metal oxide is effected by filtration of undissolved oxides from the bath.
16. A process in accordance with claim 1 wherein the said solvent consists essentially of at least one salt selected from the group consisting of alkali metal halides and alkaline earth metal halides.
17. A process in accordance with claim 1 wherein the said solvent consists essentially of at least one salt selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides.