US2919234A - Electrolytic production of aluminum - Google Patents

Description

Dec. 29, 1959 H. SLATIN 2,919,234

' ELECTROLYTIC PRODUCTION OF ALUMINUM Filed Oct. s, 1956 INVENT OR BY MM/ ATTORNEY United states r" 2,919,234 Patented Dec. 2 1959 ELECTROLYTIC PnonUcrroN or ALUMINUM Harvey L. Slatin, New York, N.Y., assignor, by mesne assignments, to Timax Associates, New York, N.Y., a partnership of New York Application October 3, 1956, Serial No. 613,718

8 Claims. (Cl. 204-- 67) This invention relates to the continuous electrolytic production of high purity aluminum.

The electrolytic processes in present day use seldom produce aluminum of even 99.7% purity. In addition, the current efiiciencies obtained are in the range of 75 to less than 90%. The consumption of carbon in the production of aluminum varies from 0.6 to 1.0 lb./lb. of aluminum produced. The normal high temperatures needed in the usual aluminum electrolysis cause severe corrosion and shorten cell life. These factors add greatly to the cost of aluminum production. Also, the noxious gases discharged from these cells must be kept from contaminating the atmosphere. Such practice is both difficult and expensive to control, and present aluminum production fumes are a constant source of property damage.

It is a primary object of this invention to overcome the shortcomings of currently used processes and to provide a process which will continuously produce aluminum metal in excess of 99.8% purity at a current efiiciency in excess of 90%, with substantially no consumption of carbon or production of atmospheric contaminants, and at a temperature just above the melting point of aluminum These and other advantages may be achieved in a cell divided into two or more compartments by a dependent battle and sharing a common liquid alloy electrode containing aluminum and metal less electropositive than aluminum with respect to halogen, by continuously electrolyzing in at least one compartment of said electrolytic cell a fused salt composed of alkali and/or alkaline earth halides and at least one chloride of the said group whose cations are more electropositivethan aluminum with respect to chlorine, said electrolysis continuously occurring between a solid insoluble anode and said liquid aluminum alloy electrode acting as a cathode; simultaneously feeding aluminum trichloride below the surface of said liquid alloy electrode, thereby discharging chlorine at the insoluble anode and depositing, dissolving, and dispersing aluminum in the liquid alloy aluminum cathode. At the same time, in another compartment of said electrolytic cell, electrolyzing between said liquid aluminum alloy electrode acting as an anode and a pure aluminum cathode floating on top of a fused salt bath consisting of at least one solvent salt of the group of alkali and/or alkaline earth halides and a solute of aluminum fluoride, said electrolyte being formulated to have a greater density at the temperature of electrolysis than the liquid pure aluminum cathode and a lesser density than the liquid aluminum alloy anode, thereby selectively and preferentially dissolving aluminum from said liquid alloy anode and continuously depositing high purity aluminum at said liquid aluminum cathode.

The term high purity aluminum as used herein refers to aluminum metal whose purity lies in the range of at least 99.8,+% to over 99.99%.

The process of the invention may be better understood by reference to the accompanying drawing depicting somewhat diagrammatically in vertical section one type of cell generally useful in the practice of my invention. An electrolytic cell indicated at 10 consists of an outer steel shell 11 which may be suitably insulated thermally and electrically from an inner graphite and/ or carbon lining 12. The interior of the cell is divided into two or more compartments by a dependent graphite baffle 14 which isolates the compartments both above and partially below the electrolytes. The wall linings 12 and the bathe faces 1.4 are electrically insulated in part from the electrolyte by high fired ceramic linings made of magnesia or alumina 16. The cell is provided with one or more inlet tubes 17 through which AlCl is pumped indicated by arrow 18 so that the aluminum chloride feed material enters the cell below the surface of the common liquid alloy electrode 19. The liquid alloy electrode 19 is composed of aluminum dissolved in metallic constituents more noble than aluminum with respect to chlorine. These alloys are denser than the electrolytes used in this invention. In the compartment where the raw material, AlCl is fed, known as the anolyte compartment, the anolyte electrolyte 21 is lighter than the liquid alloy electrode 19 and floats thereon. The anolyte 21 is composed of alkali. and/or alkaline earth halides, and with aluminum chloride feed, the electrolyte has sufiicient chlorides of the alkali and/or alkaline earth metals to preclude the deposition of any other halogen than chlorine at anodes 22. The cations of this electrolyte are more electropositive than aluminum with respect to chlorine. The dependent anodes 22 are electrically insulated from the cover 23 and the cell it) by spacers 24. The cell is a closed cell except for a means for introducing salts (not shown) and vent pipes indicated by arrow 25 used to withdraw the gases discharged in the anolyte compartment. The chlorine gas discharged, for example, is readily collected by known means and may be used in the regeneration of the aluminum chloride feed by reaction with bauxite or alumina in the known fashion. The gases may be purged from the cell by an inert gas. In another compartment of the cell, known as the catholyte compartment, a catholyte electrolyte 2'7 floats between the heavier liquid alloy electrode 19 and the lighter pure aluminum liquid cathode 29. The catholyte is composed of alkali and/ or alkaline earth halides and contains a sufficient concentration of aluminum fluoride to preclude the codeposition of the more electropositive cations of the solvent salt. Current is brought to the liquid cathode 29 by insoluble cathode rods 30 which dip into the liquid cathode and are electrically insulated from the cover 23 and cell ldby'insulators 31. The atmosphere above the cathode 29 may be purged by an inert gas. withdrawn via weir 33 or in any desired manner.

Direct current (or if desired auxiliary alternating current) may be imposed across the cell in any of the following three ways. (A) A source of direct current may be placed across anodes 22 and cathodes 30 directly, thereby making common liquid electrode 19 bipolar, viz., a cathode in the anolyte compartment and an anode in the catholyte compartment. (B) In addition to the direct current connections as set forth in (A) above, a source of direct current is also placed across anodes 22 and cathode leads '35 in an auxiliary circuit. (C) Finally, a source of direct current may be impressed across The cathode product may be continuously of aluminum in the liquid alloy electrode, and in order to minimize the Lorenz pinch effect, and for other reasons, the (C) circuit is preferred.

As illustrative of the process the cell 10 is filled with molten liquid alloy electrode 19 by pouring the alloy into the cell via salt feed ports in cover 23, not shown in the drawing. Suflicient metal is poured. to fill the cavity in the bottom and complete the isolation of the compartments. Care is taken to prevent the alloy from contacting the electrical insulators 16. Under the influence of the electromagnetic fields set up in the liquid alloy by the current flow, the alloy tends to be agitated and sloshes about in the bottom of the cell. Consequently, it is advisable, under the circumstances to have the level of the alloy sufliciently above the bottom of the bafie '14 to prevent accidental mixing of the anolyte 21 and catholyte 27. The respective electrolytes are poured into each compartment via its feed ports in the cover. Finally a layer of pure aluminum (29) is poured on top of catholyte 27. The sources of DC. power are connected across the electrodes and simultaneously aluminum chloride vapors are fed to the cell via feed tubes 17. The AlCl may be diluted with an inert gas if desired to facilitate feed rates and to prevent clogging. The AlCl gas feed lines are suitably lagged to prevent condensation of the halide salt.

The rate of feed is synchronized with the rate of deposition of aluminum in the alloy so that virtually all the aluminum chloride fed to the cell is completely consumed. Three faradays of electricity are theoretically required to decompose the aluminum chloride into its elements. In actual practice, somewhat more than three faradays are required. The metal product, aluminum, is deposited in the liquid cathode 29 and is collected. The purity of the metal thus obtained will be greater than 99.8% even with impure raw material feed provided that the conditions set forth herein are maintained.

THE ANOLYTE COMPARTMENT The electrolyte in the anolyte compartment is composed of cations more electropositive than aluminum with respect to halogens. This restricts the salts to the alkali and alkaline earth metal halides. It is desirable, but not mandatory, that the density of the melt be less than that of liquid aluminum so as to facilitate the solu tion and absorption of aluminum at the liquid alloy cathode. Where the feed material is the chloride of aluminum, the electrolyte must contain chlorides and the chlorides Should be of sufiicient concentration to prevent the codeposition of other halogens with chlorine at the anode at reasonably high current densities. I have also found that the presence of fluorides is very beneficial in the process. Accordingly, the electrolytes in this compartment, when using an aluminum chloride feed, are preferably limited to fluorides and chlorides of the alkali and alkaline earth metals only. Accordingly, the preferred baths are composed of the alkali chlorides with the addition of fluorides. Eutectic mixtures which give fluid fused baths below 700 C. are particularly preferred. For instance, the following baths are recommended: (l) NaCl-KCI eutectic, (2) NaClCaCl eutectic, (3) NaClBaCl -KCl eutectic, (4) NaClNaF eutectic, (5) the above baths with to 25% of fluorides such as KF, NaF, BaF CaF etc. CaF is a preferred addition, and (6) NaCl+CaF In the process, substantially no salts in the electrolyte solvent are consumed and the anode is substantially an insoluble one. The preferred anode is graphite, but carbon or other insoluble conductors may be used.

The temperature in the anolyte section must be greater than the liquidus temperature of the electrolyte salts and above the liquidus temperature of the liquid alloy electrode. Usually 50 to 100 C. above these temperatures is adequate for mobility of the alloy. The temperature may be varied over a wide range from the melting point of aluminum to over 1000 C., but in order to reduce corrosion, prolong cell life, and for other reasons the preferred temperature of operation lies between about 700 C. and 750 C. The electrolytes can be compounded to melt at about 650 C.

The raw material feed may be any of the halides of aluminum, but the chloride is preferred as it is the easiest and cheapest to produce from available ores and simplest to handle. However, if the bromide, for example, were used, it would be necessary to have an alkali and/or alkaline earth bromide predominantly in the anolyte electrolyte.

Aluminum chloride is fed to the cell preferably as a gas or a vapor. It is important to insure that the lines leading from the boiler or sublimer to the cell are suitably heated to prevent condensation and plugging of the lines. It may be desired that an inert gas be passed through with the AlCl vapors to keep the lines open, or to stir the melts, or afford another means for controlling the feed rate in small cells. The aluminum chloride may be prepurified in any known fashion before introduction to the cell. Such prepurification will increase the useful life of the liquid alloy. However, even with impure aluminum chloride feed having iron, silicon, impurities and the like, metal of 99.99% may be continuously made as these more noble metals are retained in the liquid alloy and not transferred to the aluminum product as in prior processes.

The rate of feed should be controlled so that all the AlCl raw material is consumed and so that substantially no AlCl breaks through the surface of the alloy or distils into the chlorine phase. In the event that the rate of feed is greatly in excess of the rate of aluminum depo sition so that some chloride is discharged into the anolyte, no irreparable harm is done, but it is advisable to keep this action from going excessively and persistently in such direction since it is wasteful and bothersome to separate such AlCl; that finds its way into the chlorine gas stream discharging from the cell. In actual practice, the rate of feed is simply regulated automatically. Inasmuch as the efficiency in this compartment is greater than a few routine tests will quickly determine the correct feed rate coincident with the rate of deposition of aluminum in the alloy and modified by cell design and current density. The current efficiency in this compartment has been found to be greater than 96% in new cells properly conditioned.

The current density at the insoluble anode may be varied over a wide range from a fraction of an ampere per square inch to over 60 amperes per square inch, but in order to prevent codeposition of halogens, to prolong anode life and eliminate anode effect, to reduce entrainment losses and to maintain high current efliciency, the anode current density should be kept below about 20 amperes per square inch and preferably below 10 amperes per square inch. The cathode current density can also be varied from a fraction of an ampere per square inch to over 200 amperes per square inch without affecting the operability of the process. However, in order to maintain a high efficiency, a current density in excess of 10 amperes per square inch is desirable, but because of cell geometry this is not always possible. The preferred liquid cathode alloy current density is at least 25 amperes per square inch. For this and other reasons, there are enormous advantages to designing the electrolytic cell so that the anolyte compartment is smaller than the catholyte compartment. In addition, the anolyte compartment may be made narrower at the bottom than the top to permit better current densities as indicated above.

THE BIPOLAR LIQUID ELECTRODE The liquid electrode consists of a solution of aluminum dissolved in metallic elements more noble than aluminum with respect to halogen and particularly in respect to chlorine. The alloy composition is chosen so as to have a high percentage of aluminum and suficient more elec- 30-40% +AlF tronegative metals to increase the density of the resultant alloy so that it is heavier than the electrolytes in both compartments. The alloy should be stable and have a low enough melting point to be fluid and mobile at the temperature of operation, i.e., about 700 C. In this connection, the eutectics of aluminum with these more noble elements are particularly valuable. Among the metal solvents fulfilling the conditions cited above are Sn, Zn, Pb, Cu, Bi, Sb, Ag, Au, Cd, Si, and the like. The preferred alloy compositions are (l) 77Al33Cu, (2) A1-Sn, (3) AlCu+Si, (4) AlCu+Zn, (5) Al-Cu+Sn, (6) AlAg eutectic, (7) AlZn, and (8) AlGe. The preferred bath is that consisting of aluminum and copper With or without additions of other metals, and the best results are obtained with the eutectic alloy (1) melting at 548 C. As the composition of the bipolar electrode remains substantially constant throughout the life of the cell, the copper serves as the principal solvent for the metallic impurities.

Occasionally, depending on the purity of the feed material, it may be necessary to cleanse the liquid alloy and remove the accumulated impurities. Either all or a part of the alloy may be withdrawn from the cell and replaced by fresh metal, and the copper and aluminum recovered from the siphoned metal. As the alloy may hold considerable quantities of metallic impurities without adversely aflecting the high purity of aluminum obtained at the cathode, the frequency of such purification steps is sporadic.

It has also been found that the depth of the alloy bears on the purity of the metal obtained and the efliciency of the process. If the metal is too shallow, there is a greater tendency to lose AlCl and if the metal is too deep, it tends to freeze in the cell in the bottom. The precise depth is aflected by cell design, and the thickness of the liquid alloy layer should intentionally be made greater in the anolyte compartment. The thickness is also influenced by the cell capacity. In the small capacity cells used in laboratory tests, alloy thicknesses of six to eight inches have been satisfactory.

THE CATHOLYTE COMPARTMENT The electrolyte in the catholyte chamber is composed of cations more electropositive than aluminum with respect to halogens and contains aluminum fluoride as a solute in sufficient concentration to prevent codeposition of the solvent metallic cations with aluminum in the liquid alloy cathode 29. The electrolyte must have a density at the temperature of operation, ca. 700 C., between the lighter pure aluminum and the heavier alloy electrode 19. Although these conditions restrict the composition of the electrolyte to the alkaline earth and alkali metal halides, the denser alkaline earth metal halides are particularly required. These salts, as those in the anolyte chamber, are preferably prepurified before use by fusing in a protective atmosphere to prevent hydrolysis, by preliminary electrolysis, and by ridding the baths of moisture. I have also found that the aluminum fluoride content of the catholyte should be in excess of 20 to 25% by weight in order to maintain the high standard of purity, 99.99% aluminum, at the higher current densities. Although the best results are obtainable with an all fluoride bath, the conductivity of such baths may be low and it may be necessary, therefore, to include chlorides in the formulas. In place of aluminum fluoride, cryolites or cryolites and aluminum fluoride may be used as solutes. Also, aluminum oxide may be added to the electrolyte, but better efficiencies and results are obtained with the fluorides and chlorides. The following salt baths which are fluid and mobile below about 700 C. are recommended. (1) BaF 30 40%+LiF l525%+AlF 20-35%+CaF balance. (2) BaF 30-40% +BaCl 20-30%+CaF balance. (3) BaF 20-40% +NaCl 5-l5% +AlF 2035%+CaF balance. (4) LiF 15-25% +BaCl 3045%+AlF 2035%+CaF 6 balance. (5 BaF 30-40% +NaF 10-25% +AIF 20. 35% +CaF balance. (6) Any of the above baths wherein the calcium fluoride content is below 5 percent. The addition of lithium fluoride increases the conductivity enormously, but because of cost and other reasons, sodium fluoride is substituted for the lithium fluoride. Therefore, the electrolytes listed above are preferred in the following order: (5), (1), (3), (2), and (4).

Just as in the anolyte chamber, it may be necessary from time to time to add make-up salts or to correct the composition of the bath. Appropriate additions of prepurified and moisture-free salts may be added via their respective salt feed ports in the cover.

The liquid cathode is pure aluminum which is either generated in situ, or preferably poured into place at the start. In actual operation, when a new cell is started, it may take some time before the metal produced will run 99.99%. This is principally due to the failure to precondition the bath prior to use, but it is also caused by the nature of the reactions in the process. For example, as the cell design afiects such controlling factors as current density, capacity, and mobility, it is necessary to make a few routine tests in order to learn the optimum controls to produce the highest purity of metal continuously.

The current density at the liquid alloy anode may be varied from a fraction of an ampere per square inch to over 30 amperes per square inch without adversely affecting the purity of the metal. But, depending on the concentration of aluminum and other metallic impurities accumulated in the aluminum alloy, and depending on the nature of the electrolyte, for the highest purities, the liquid anode current density less than 10 amperes per square inch is preferred. The cathode current density is influenced mostly by the cell geometry and within such limitation may be varied from a few amperes to over amperes per square inch. However, in order to maintain the cell contents in a mobile liquid condition without resorting to impractical auxiliary external heating, a current density above about 5 amperes per square inch is preferred. I have found that better results are obtained if a layer of aluminum of at least 2 inches is provided or allowed to accumulate and remain in the cell at all times during collection and operation.

Theoretically three faradays of electricity are required to produce a gram mole of metallic aluminum and in actual practice, the current efliciency is usually over 98% and close to 100%. However, it may become necessary from time to time to adjust the relative quantities of current passing through each compartment of the cell in order to maintain a substantially constant composition of the alloy. It has been found that the maintenance of a substantially constant high aluminum content in the liquid alloy is one of the dominant factors in allowing the continuous production of high purity aluminum at high current efiiciencies. The efliciency in the anolyte compartments and the efficiency in the catholyte compartments may not be identical. If auxiliary DC power were not provided, it would be necessary to add aluminum metal to the liquid alloy electrode. This practiceis not advantageous. Consequently, the composition of the electrode is corrected by increasing the DC power input (and feed) on the anolyte side or as may rarely be required, by increasing or decreasing the DC input on the catholyte side.

THE REACTIONS OF THE INVENTION In the anolyte compartment, under the influence of the direct current, chlorine is discharged at the insoluble anode and is collected in any known fashion. Aluminum is set free at the liquid alloy aluminum cathode either by primary electrolysis and/ or secondary chemical reduction. In any event, the aluminum liquid is accepted by the alloy electrode, dissolved, dispersed and distributed throughout the body of the alloy by electromagnetic, thermal and other means. In this compartment, the only raw material feed is aluminum chloride. None of the salts of the electrolyte are consumed in the process. In the catholyte compartment, the same current that supplies aluminum to the alloy and/ or another direct current selectively and preferentially dissolves the aluminum from the liquid alloy and deposits it at the floating cathode. The metal that is deposited undergoes two purifications within the single cell and metal of high purity is continuously produced. It is not necessary to use a very pure raw material feed as has been required in processes in the past, as instead of transferring all the impurities to the finished metal product, in this invention none of the impurities are transferred to the cathode product, but instead are retained by the alloy liquid electrode.

The following examples are given for further illustrative purposes only and are not intended in any way to limit the scope of the invention.

Example I The anolyte reaction was tested by fusing a eutectic mixture of NaCl and KCl to which about 10% CaF by weight was added in a covered graphite lined crucible having a high fired alumina collecting dish in the bottom. Auxiliary external heating was provided. A hollow graphite cathode was lowered into the bath and the crucible itself served as the anode. The cell was vented to a chlorine absorber. With the commencement of electrolysis, gaseous aluminum chloride was fed into the cell via the hot hollow cathode. The current flow through the cell was maintained at about 200 amperes. The rate of feed was erratic, but substantially no AlCl was found in the anode chlorine gas. At the end of 90 minutes of feeding and electrolyzing, the feed was stopped and the current discontinued. The weight of total feed was about 350 grams. A metal button was separated from the frozen salts and analyzed. The results are shown in Table I.

TABLE I Temperature of electrolysis 700 C.-750 C. Purity of AlCl 97%. Cathode current density 16-32 amps./in. Anode current density 2.7 amps/ink Purity of aluminum 99.26% Al.

Example 11 that shown in the accompanying drawing was used and the cell was provided with a means for external heating. In addition, the anolyte compartment was fitted with a well making it about an inch deeper than the catholyte compartment for feeding the raw material.

The bottom of the cell was filled with a layer of 33Cu77Al alloy to complete the seal. The alloy depth in the anolyte compartment was about four inches. The anolyte chamber was filled with about six inches of BaCl- NaClKCl eutectic mix melting at about 650 C. and about 10% by weight of LiF had been added. The melt floated on the liquid alloy. The catholyte compartment was vxed with a mix containing 40BaF -17LiF 30AlF -13CaF which fused at about 650 C. and floated on the liquid alloy. At about 700 C., both electrolytes were fluid and mobile. The electrolytes were prepared separately and fused under a protective atmosphere to prevent hydrolysis. The electrolytes were also electrolyzed at low voltage to eliminate metallic impurities, moisture, etc. The AlCl feed was impure (about 97%) and contained about 0.3% Fe, 0.1% Si, and 1.8% insolubles.

into the cell below the surface of the molten alloy in the .well via a heated graphite-lined line.

The temperature was maintained at 725 C.i C. The metal deposited at the cathode during the first few hours was discarded. The cell was kept in continuous operation and no metal was taken from the cell until about one inch of aluminum floating cathode had formed. Several samples of product were taken for analysis periodically thereafter.

Aluminum chloride was fed at a rate of about 2.5 pounds per hour. The feed rate was erratic and occasionally some AlCl was found in the exhaust chlorine, but usually the feed kept pace in synchronization with the rate of aluminum deposition in the bipolar liquid alloy. The graphite anode showed substantially no deterioration due to electrolysis. Likewise no fluorine or fluorine gaseous compounds of any substantial quantity were ever detected in the anode gas. The results are shown in Table II.

TABLE II Liquid alloy cathode current density 19.5-22.3 amps/in Solid anode current density 14.6-16.6 amps/m Aluminum cathode current density 12.9-14.8 amps/ink Liquid alloy anode current density 12.9-14.8 amps/in. Purity of aluminum 99.94% Al.

Example III A laboratory cell built substantially like that used in Example 11 was used except that the compositions of the anolyte and catholyte were changed. The prepurified anolyte was a sodium chloride-potassium chloride eutectic containing about 10% C213; by weight which melts at about 650 C. and is suificiently fluid above 700" C. The electrolyte floated on the 77Al-33Cu liquid alloy. The catholyte contained melted at about 650 C. and was fluid at 700 C. The melt floated on the liquid alloy electrode. The temperature was maintained between 700 C. and 750 C. The same impure grade of AlCl was fed to the bath, but at an average rate of about 1 /2 pounds per hour. The current was maintained at 500 'amperes but varied from 400 to 500 ,amperes. The same procedure was followed for collecting aluminum samples. The results are shown in Table III. The alloy electrode was tested from time to time and showed no pronounced change in composition. Any corrections were made by supplying additional current to the anolyte section only. The catholyte was found to be quite stable and no appreciable distillation of A101 could be detected in the catholyte chamber. The rate of feed of AlCl to the anolyte compartment was found to be in reasonable harmony with the rate of aluminum deposition in the liquid alloy.

TABLE III Liquid alloy cathode current density 11.1-13.9 amps/m Solid anode current density 8.3-10 amps./in. Liquid alloy anode current density 7.9-9.2 amps/ink Aluminum cathode current density 7.9-9.2 amps./in. Purity of aluminum 99.99% Al.

The metal obtained is silvery bright in color and of high purity. The process hereindescribed will operate exclusively on the DC. input with high efficiencies provided that the cell is properly designed and of a large size. The purity of the AlCl feed will determine the length of life of the liquid alloy electrode. However, even with impure raw material feed, the impurities are not transferred to the metal product, but instead are accumulated in the liquid alloy electrode where they can cause 'no harm. The metal obtained will have a purity in excess of 99.9% and sometimes over 99.99%. There is no consumption of carbon in the electrolysis and no harmful fluorine and/or fluorine compound gases are produced. The chlorine generated in the anolyte section is of good purity and readily collected and useful.

Having described in considerable detail the method of applying the invention and having described the scope of the invention, it is apparent that the invention is not limited in any way to many of the details of equipment and procedure herein disclosed, but the invention can be modified in some ways without departing from the spirit and scope as defined by the appended claims.

I claim as my invention:

1. A process for the continuous electrolytic production of high purity aluminum metal comprising in a cell divided into at least two compartments sharing in common as cathode of one and anode of the other a liquid alloy electrode containing aluminum and at least one of the group of metals less electropositive than aluminum with respect to halogens, electrolyzing in one compartment of said cell a fused salt electrolyte located between said liquid alloy electrode acting as cathode and an immersed insoluble anode, said fused salt electrolyte being substantially free of oxide and aluminum compounds and composed of at least one of the alkali and alkaline earth halides at least one of said halides being the chloride, feeding aluminum chloride into and below the surface of said liquid alloy cathode electrode, thereby discharging chlorine at said insoluble anode and directly supplying said liquid alloy cathode with aluminum, and in another compartment of said cell electrolyzing a fused salt electrolyte whose density is less than said aluminum carrying liquid alloy electrode now acting as an anode relative to a cathode of molten metal floating on said electrolyte, said electrolyte being composed of at least one halide of the group consisting of the alkali and alkaline earth halides and containing dissolved therein 20% or over aluminum fluoride and substantially free of aluminum oxide, thereby receiving aluminum from said liquid alloy anode and depositing high purity aluminum at said liquid metal cathode.

2. A process for the continuous electrolytic production of aluminum as set forth in claim 1 wherein said liquid alloy containing aluminum contains at least one metal of the group copper, lead, zinc, tin, antimony, silicon, cadmium, and silver.

3. A process for the continuous electrolytic production of high purity aluminum as set forth in claim 1 wherein the aluminum alloying nobler metal is copper.

4. A process for the continuous electrolytic production of high purity aluminum as set forth in claim 1 wherein said first mentioned electrolyte between said shared liquid alloy electrode and said insoluble anode floats on said liquid alloy electrode acting as a cathode.

5. A process for the continuous electrolytic production of high purity aluminum as set forth in claim 1 wherein the molten metal cathode is aluminum.

6. A process for the continuous electrolytic production of high purity aluminum as set forth in claim 1 wherein the direct current supplied to said cell comprises two independent and electrically isolated circuits thereby causing current to pass from said liquid alloy cathode thru said electrolyte to said anode and causing another current to pass from said liquid aluminum cathode thru said aluminum fluoride electrolyte to said liquid alloy anode.

7. A process for the continuous electrolytic production of high purity aluminum as set forth in claim 1 wherein the direct current circuit is imposed across said insoluble anode and said bipolar liquid cathode in addition to the direct current circuit connected across the insoluble anode and the liquid aluminum cathode.

8. A process for the continuous electrolytic production of high purity aluminum as set forth in claim 1 wherein all the anions of both electrolytes are of the group of chlorides and fluorides and mixtures thereof.

References Cited in the file of this patent UNITED STATES PATENTS 387,876 Heroult Aug. 14, 1888 503,929 Hall Aug. 22, 1893 935,796 Von Kugelgen et al. Oct. 5, 1909 1,297,946 Weaver Mar. 18, 1919 1,535,458 Frary Apr. 28, 1925 1,937,509 Burgess Dec. 5, 1933 2,665,244 Menegoz Jan. 5, 1954 2,752,303 Cooper June 26, 1956 FOREIGN PATENTS 4,169 Great Britain of 1889 745,530 Great Britain Feb. 29, 1956

Claims (1)

1. A PROCESS FOR THE CONTINUOUS ELECTLYTIC PRODUCTION OF HIGH PURITY ALUNINUM METAL COMPRISING IN A CELL DIVIDED INTO AT LEAST TWO COMPARTMENTS SHARING IN COMMON AS CATHODE OF ONE AND ANODE OF THE OTHER A LIQUID ALLOY ELECTRODE CONTAINING ALUMINUM AND AT LEAST ONE OF THE GROUP OF METALS LESS ELECTROPOSITIOVE THAN ALUMINUM WITH RESPECT TO HALOGENS, ELECTROLYZING IN ONE COMPARTMENT OF SAID CELL A FUSED SALT ELECTROLYTE LOCATED BETWEEN SAID LIQUID ALLOY ELECTRODE ACATING AS CATHODAE AND AN IMMERSED INSOLUBLE ANODE, SAID FUSED SALT ELECTROLYTE BEING SUBSTANTIALLY FREE OF OXIDE AND ALUMINUM COMPOUNDS AND COMPOSED OF AT LEASTS ONE OF THE ALKALI AND ALKLALINE EARTH HALIDES AT LAEAST ONE OF SAID HALIDES BEING THE CHLORIDE, FEEDING ALUMINUM CHLORIDE INTO AND BELOW THE SURFACE OF SAID LIQUID ALLOY CATHODE ELECTRODE, THEREBY DISHARG ING CHLORINE AT SAID INSOLUBLE ANOD E AND DIRECTLY SUPPLYING SAID LIQUID ALLOY CATHODE WITH ALUMINUM, AND IN ANOTHER COMPARTMENT OF SAID CEL ELECTROLZING A FUSED SALT ELECTROLYTE WHOSE DESITY IS LESS THAN SAID ALUMINUM CARRYING LIQUID ALLOY ELECTRODE NOW ACTING AS AN ANODE RELATIVE TO A CATHODE OF MOLTEN METAL FLOATING ON SAID ELECTROLYTE, SAID ELECTROLYTE BEING COMPOSED OF AT LEAST ONE HALIDE OF THE GROUP CONSISTING OF THE ALKALI AND ALKALINE EARTH HALIDES AND CONTAINING DISSOLVED THEREIN 20% OR OVER ALUMINUM FLORIDE ANED SUBSTANTIALLY FREE OF ALUMINUM OXIDE, THEREBY RECEIVING ALUMINUM FORM SAID LIQUID ALLOY ANODE AND DEPOSITING HIGH PURITY ALUMINUM AT SAID LIQUID METAL CATHODE.

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