US2539092A - Electrolytic apparatus for reduction of aluminum bromide - Google Patents

Description

Jan. 23, 1951 w. G. LOVELL ETAL 2,539,092

ELECTROLYTIC APPARATUS FOR REDUCTION OF ALUMINUM BROMIDE Original Filed Nov. 12, 1940 Jnoenkors Patented Jan. 23, 195?.

ELECTROLYTIC APPARATUS FOR REDUC- TIUN F ALUMINUM BROMIDE Wheeler G. Lovell and Nelson E. Phillips, Detroit, Mich, assignors to General Motors Corporation, Detroit, Mich, a corporation of Delaware Original application November 12, 1940, Serial No. 365,188, now Patent No. 2,373,320, dated April 10, 1945. Divided and this application December 23, 1944, Serial No. 569,486

6 Claims.

This application is a division of our copending application Serial No. 365,188, filed November 12, 1940, now Patent No. 2,373,320, dated April 10, 1945.

This invention relates generally to the electrolytic reduction of compounds of aluminum to produce metallic aluminum. More specifically the invention has to do with the electrolytic reduction of a fused electrolyte containing aluminum bromide.

Among the objects of the invention are the following: to provide an improved method and apparatus for producing metallic aluminum, to provide a convenient and practical electrolytic method and apparatus for the production of aluminum that has more advantageous energy relations than present methods, to provide a process and apparatus for producing aluminum that reduces the cost as compared with present practice, to provide a practical and economical method and apparatus for producing aluminum of a high degree of purity; to provide im rovements in a method and apparatus of producing substantially pure aluminum which makes it possible to utilize economically a wider variety of cheaper aluminum containing raw materials than present commercial processes; to provide improvements in a method and apparatus for recovering substantially pure aluminum as it is formed by the electrolytic reduction of the aluminum compound in the electrolyte; to provide improvements in anodes and cathodes, especially those for use in the electrolytic reduction of aluminum bromide. Other objects and advantages of the invention will become more apparent as the description proceeds. Reference is herewith made to the accompanying drawing forming a portion of this specification in which the figure illustrates somewhat diagrammatically one form of apparatus adapted to carry out the method in accordance with the invention.

General description The device illustrated comprises a means for heating and continuously circulating through a generally closed system a fused electrolyte of suitable boiling point containing aluminum bromide, air and water being excluded from the system. The fused, non-aqueous electrolyte passes through an anode which directs the electrolyte in proper relation with respect to a vibratory cathode. The device is so constructed and controlled as to deposit the aluminum at the cathodein a finely divided form and to remove it therefrom and cause it'to flow with the elec- 2 trolyte past the cathode to a chamber where it is removed. In order to insure that the aluminum is deposited in a form that can be readily removed by vibrating the cathode or readily carried by the flowing electrolyte, special addition agents may be provided for the electrolyte. In order to make the electrolyte sufiiciently conducting, ingredients such as sodium bromide or potassium bromide form a portion or" the electrolyte. In the device illustrated the aluminum particles are filtered from the electrolyte by means of a filter type piston, but other methods for separating the solid and liquid may be used.

Detailed description In the drawing is a reservoir in adapted to contain the electrolyte. A suitable heating means [2 provides sufiicient heat to make up for heat lost from the apparatus and to maintain the electrolyte in the chamber at the required temperature necessary to keep the electrolyte'in a liquid state when the apparatus is not being operated. Make-up electrolyte may be added to the reservoir from time to time by means of opening l4 and cover It. Electrolyte is pumped from the reservoir by any suitable pump, which in the form shown is constructed as follows. Extending into the electrolyte in the reservoir is a glass cylinder I8, the lower end of which is in flow communication with the electrolyte by means of openings 20. In the cylinder adjacent the openings is an impeller 22 formed of tantalum. The impeller is secured to a glass rod 24 which is adapted to be rapidly rotated by the electric motor 26. Rotation of the impeller during operation thus causes a flow of the electrolyte continuously through cylinder I 8 to an outlet 28, thence to the electrolytic cell proper, indicated generally by the reference numeral 3E The electrolyte enters the cell and passes through an electrically conducting cup-shaped anode 32 having a series of openings 36 in the lower end thereof. Preferably the anode is formed of carbon. Other insoluble conducting anodes may be used. The openings are so arranged as to direct the electrolyte downwardly towarda cathode 36. v

In the form shown the cathode is a rod of tantalum curved to form a ring, the ring being joined to a rod 38 mounted for oscillation within a sleeve 4i) adjustably secured to one wall 42 of the cell. The sleeve and rod pass through the wall. The rod 38 is adapted to be rapidly vibrated or ocked back and forth within the sleeve by ri'leans' of rod 44 fixed thereto at one end, connecting rod 65 pivoted at the opposite end of rod 46, eccentric t3 electric motor 50. The rapid rocking or vibration of the cathode removes the aluminum therefrom in finely divided form. The aluminum tends to'deposit in the form of feathery trees or dendrites and advantage is taken of this. Certain addition agents may be added to the electrolyte to control more fully the deposition and cause the formation of the aluminum in finely divided form. The source of electric current is connected in any desired manner to the anode and cathode. The positive side of the current source may be connected to a flange 31 of the anode, for example, while the other side of the current source may be connected to arm 44 connected to the cathode.

The aluminum particles flow with the circulating electrolyte from the electrolytic cell through the outlet 56 and through the tube 58 to a filter chamber indicated genera ly by the reference numeral 63. Within the filter chamber is a piston element 62 of carbon or other suitable. material, said element having openings 54 in the head thereof. Above the head of the piston is a filter element 55. In the device illustrated the filter element is a glass fabric. The purpose of the glass filter and the openings in the head of the piston is to separate the aluminum particles from the electrolyte and to cause the particles of aluminum to collect on the filter element. By means of a glass rod 58 connected to the head of the piston, the piston and filtering element with the particles of aluminum thereon are raised above the inlet in the filter chamber to bring the aluminum particles in contact with a second piston Hi within the filter chamber. In the form shown the piston ll! is of carbon. The aluminum particles are pressed together and form a cake or mass H which adheres to the piston 10, and thus removes the cake from the active filter area so as to permit better flow and also to keep the cake away from the electrolyte which may contain small amounts of bromine and might thus otherwise dissolve some of the metal. Mechanical means may be used for suspending the cake from piston H1. The cake of metal is gradually built up by successive additions and preferably extends above the level of the electrolyte, thus permitting the electrolyte to drain off. The piston it may be removed from time to time by means of a tubular glass rod l2 secured th reto, a cover 14 of the filter chamber being removed during the removal of the piston 70, and all or a portion of the"cake of aluminum may be removed from the piston '50.

The strained electrolyte leaves the filter chamber through an outlet 15 and returns to the heated reservoir Iii by means of tube Hi. In the form of apparatus illustrated, the pump 22, etc., moves the electrolyte to the cup-shaped anode and the flow of electrolyte from this point to the reservoir 18 is due to the force of gravity acting thereon.

As the aluminum is set free at the vibrating cathode, bromine is liberated at the anode, which is centrall arranged in the upper portioncf the cell. The free bromine boils up through the hot electrolyte. It is important to maintain high temperatures so that the bromine will not be too soluble in the electrolyte and temperatures approaching the boiling point of the bromine-free electrolyte are desirable. Under these conditions the aluminum halide from the electrolyte wi l boil ofi with the bromine, and to make a partial separation of thegtwo andreturn. the aluminum bromide-to .the cell, the vapors. pass-through a reflux column. This comprises a glass tube or receptacle 8i] nearly filled with glass rings or beads 82. The top of receptacle is closed by a glass cover 'El. Surrounding a portion of receptacle so is an air chamber 84 having an air outlet 35 and an air inlet tube 33. Most of the vaporized aluminum bromide leaving the cell is condensed by the condenser-reflux column and flows back into the cell. A small amount of the aluminum bromide and the bromine pass out of the reflux-condenser through an outlet 9% arranged near the top thereof, and by means of a passage 92 enter a glass receiver 9 A water condenser 96 condenses any uncondensed bromine and aluminum bromide vapors and returns the same to the receiver.

The design of the anode is important. The bromine must be evolved rapidly in order to avoid the formation of a gaseous film which increases the electrical resistance of the cell. The anode must be designed to be as close to the cathode as possible in order to keep the cell resistance at a minimum. Some circulation of electrolyte past the anode is necessary in order to avoid the formation of a layer of low conductivity; too much stirring is to be avoided as it promotes the solution of the bromine in the electrolyte with the consequent redissolving of the aluminum. The electrical resistance of the anode should be as low as possible. Several forms of anodes have been used in accordance with the above requirements, the form shown in the drawing being preferred. The bottom of the anode is turned to aradius so that it will fit within the cathode and it has a streamlined surface for the free removal of the'bromine.

It is important in the operation of the cell that the surface of the anode not be allowed to become a. poor conductor of electricity. Under certain conditions, particularly after moisture has had access to the electrolyte, a high resistance film may be built up on the anode. For this reason, the electrolyte must at all times be protected from the introduction of even small amounts of water, in order to assure continuous satisfactory operation of the cell for long periods of time.

The requirements for the cathode are that it should be a good conductor and so shaped that the flow of electric current will be equally distributed and so that it will not obstruct the flow of liquid or of solid metal particles. High current densities are applied so that the deposited metal will be in a form which can be readily dislodged. The upper limit of electrical current in any particular application is governed by the amount of heat generated and this is dependent upon the electrical resistance of the cell. The amount of heat generated should not be such as to cause too vigorous boiling of the electrolyte. Another factor to be considered in the determi nation of the maximum current is the energy efficiency desired. The electrical energy lost in the internal resistance of the cell as heat, varies with the square of the current, while the amount of metal produced varies directly as the current. The current used in practice in the cell depends consequently upon the economic balance between pounds of metal produced per hour, and kilowatt hours per pound required to produce it.

In one form and size of apparatus constructed in accordance with the invention from 5!) to 225 amperes of current have been passed through the electrolyte in a cell having a diameterof-six inches. Current densitiesas high as approximately 15,000 amperes per square foot of cathode area have been used. Good current efficiencies in the range of 80 to 90% have been consistently obtained.

- The composition of the electrolyte may vary considerably. It is at present preferred as a matter of experience and compromise that the sodium bromide (when this is used as the current carrier) be about 24% of the electrolyte, the balance being aluminum bromide. Lower proportions of sodium bromide increase the resistance of the electrolyte, while higher proportions reduce the resistance. Considering the electrical resistance alone, the ideal would be the largest possible amount of sodium bromide. The temperature of the electrolyte preferably should be maintained as near the boiling point of the electrolyte as can be in order to remove the bromine as rapidly as possible from the vicinity of the electrode and to keep the electrolyte as free of bromine as possible. In view of this, the lower the concentration of sodium bromide the lower the permissible operating temperature, while the higher the concentration of sodium bromide the higher the required operating temperature. With the at present preferred bath com osition (24% sodium bromide and 76% aluminum brom de) an electrolyte temperature of about 850 F. has been used with success. With an electrolyte composed of 19% sodium bromide and 90% aluminum bromide an operating tem erature of about 509 F. may be used, while with an electrolyte compo ed of 20% sodium bromide and 80% alum num bromide an operating temperature of 650 F. mav be used.

In order to ensure that the electrolyte flow through the cell is as near streamline flow as possible, the anode is so designed as to distribute the electrolyte substantially uniformly over the 'crosssection of the cell. In the form il strated the aluminum released from the cathode flows downwards with the el ctrolyte. The rate of flow of the electrolyte should be fast enough so as to continuously remove the aluminum from the cell and still not agitate the bath enough to cause the bromine to redissolve any appreciable amount of the aluminum.

It has been found desirable to add small amounts of certain substances to the bath or electrolyte in order to control the size of the particles of aluminum. Under some conditions of operatic-n, especially with very pure materias, .or on prolonged electro ysis of less pure materials, the aluminum forms mo s-like a gregations, which, when detached from the cathode, tend to agglomerate in large pieces of the mossy, p rous type up to an inch or more in diameter. Such large pieces are undesirable in that they do not flow through the cell properly since they are subjected to diverse liquid currents and to mechanical striking by the vibratory cathode. As a resut they may be subject d to the action of bromine in the cell and partially dissolved, thus resulting in a decrease in the current efficiency of the cell. The large pieces also have a tendency to clog the connecting tubes to the filter chamber. We have found that small amounts of such materials as naphthalene, phenanthrene, or anthracene when added to the electrolyte prevents the formation of the large particles of aluminum and thus obviates the above mentioned disadvantages of such large particles. One part of the addition agent in about a quarter of a million parts of electrolyte has proven sufiicient. The addition agent must be replenished from time to time as needed. Other addition agents which have been used are lubricating oil, rubber and the like. Although we do not wish to be bound by any definite theory, we believe that the mechanism of the action is about as follows. When the addition agent is added to the hot electrolyte, it decomposes-and forms insoluble colloidal carbon which is absorbed on the clean and active surfaces of the aluminum particles so as to keep the particles from sticking or welding together to form large particles. In accordance with our theory organic materials which decompose in aluminum bromide at bath temperautres to form colloidal carbon may be used efiectively; We prefer to use hydrocarbons or compounds containing little or no oxygen so as to prevent the formation of aluminum oxide as a final product to contaminate the bath.

Due to the fact that aluminum bromide solutions of bromine are very corrosive to most met: als, especially at the high temperatures used herein, it is necessary to use materials that are not attacked thereby. In the form of apparatus shown, the cell proper, the fi ter chamber, the reservoir and the passages connecting the same are formed of Pyrex glass. It is contemplated that other vitreous and ceramic materials, as we as enamel surfaces may be used, as well as carbon when its electrical properties permit. While it is at present preferred that the anode be of carbon it may be formed of tantalum. The best results have been obtained with a cathode formed of tantalum. It is contemplated that the cathode may also be formed of carbon.

The cake of aluminum after removal from the filter chamber is a coherent, spongy mass which may be readily broken up. small amounts of aluminum bromide and sodium bromide (when sodium bromide is used as the current carrier). The cake of aluminum may be heated in an electric muffle or other furnace to melt the aluminum. During the heating the small amount of aluminum bromide is distilled off and may be recovered for return to-the apparatus. The sodium bromide and the aluminum melt and the latter separates into a lower layer which may be drained off and may be cast into pigs. Remarkably pure aluminum has been produced in accordance with the invention.

We Wish it to be understood that we do not desire to be limited to the exact details of construction and operation shown and described, for obvious modifications will occur to a person skilled in the art.

We claim:

1. In a paratus for producing aluminum in solid, particulate form, a ring-shaped cathode, means for rapidly vibrating said ring-shaped cathode, a cup-shaped anode located generally above said ring-shaped cathode and having its bottom portion curved to a radius to fit within said ring-shaped cathode, said curved bottom portion of the anode having a plurality of openings therein, and pumping means for continuously pumping a fused, non-aqueous electrolyte into the upper portion of the cup-shaped anode and through said openings past the ring-shaped cathode.

2. In an apparatus for producing aluminum in solid, particulate form from a fused electrolyte containing aluminum bromide; an electrolytic cell having therein a ring-shaped cathode and a cup-shaped anode located generally above said ring-shaped cathode and having its bottom portion curved to a radius to fit within the ring- It contains shaped cathode, said curved bottom portion having a plurality of openings therein; said electrolytic cell having an opening below said ringshaped cathode; means for rapidly vibrating said ring-shaped cathode; means for continuously flowing said fused non-aqueous electrolyte into the upper portion of said cup-shaped anode and through the openings in the curved bottom of said anode past the vibrating ring-shaped cathode and out through said opening in the cell; and

means for passing electric current from said anode to said vibrating cathode to deposit aluminum at said cathode in solid, particulate form, the vibration of said cathode dislodging the aluminum particles from the cathode and the dislodged aluminum particles being carried out of the cell by the electrolyte flowing therefrom.

3. In an apparatus for producing aluminum in solid, particulate form from a fused, nonaqueous electrolyte containing aluminum bromide; a ring-shaped cathode of tantalum, means for rapidly vibrating said ring-shaped cathode, an insoluble cup-shaped anode located generally above said cathode and having its bottom portion curved on a radius to flt within the ringshaped cathode, said curved bottom portion of the anode having a plurality of openings therein, means for continuously flowing a fused, nonaqueous electrolyte containing aluminum bromide into the top of said cup-shaped anode and through the plurality of openings in the curved bottom portion of the anode to a point beyond the vibrating ring-shaped cathode, means for passing electric current through the electrolyte from said cup-shaped anode to the vibrating, ring-shaped cathode to liberate bromine at the curved lower surface of the anode and deposit aluminum at the vibrating, ring-shaped cathode in solid, particulate form, said vibration dislodging aluminum particles from said cathode so that they are carried by the flowing electrolyte to a point beyond the ring-shaped cathode, and means for removing from the electrolyte the liberated bromine.

4. In an apparatus for producing aluminum in solid, particulate form from a fused, non-aqueous electrolyte containing aluminum bromide; an electrolytic cell having an opening in the bottom thereof, a ring-shaped cathode in said cell above shaped cathode, said curved bottom portion of the anode having a plurality of openings therein, means for continuously flowing a fused, nonaqueous electrolyte containing aluminum bromide into the top of said cup-shaped anode and through the plurality of openings in the curved bottom portion of the anode past the vibrating ring-shaped cathode and out said opening in the bottom of the cell, means for passing electric current through the electrolyte from said cup-shaped anode to the vibrating ring-shaped cathode to liberate bromine at the curved lower surface of the anode and deposit alumintu'n at the vibrating, ring-shaped cathode in solid, particulate form, said vibration dislodging solid, aluminum particles from said cathode so that they are carried by the flowing electrolyte out of the cell through said opening in the bottom thereof, and means for removing from the cell the liberated bromine. said cell being closed except for the opening in the bottom thereof for flow of electrolyte therefrom, the means for entr of electrolyte to the top of the anode and the bromine removing means.

5. In apparatus for producing aluminum in solid, particulate form from a fused, non-aqueous electrolyte, a ring-shaped cathode, means for rapidly vibrating said cathode, a generally cupshaped anode located generally above said cathode and having its bottom portion curved to a radius to fit within the ring-shaped cathode, said curved bottom portion having a plurality of openings therein, means for retaining electrolyte in the space between the electrodes and means for introducing electrolyte into the cup-shaped anode.

6. An apparatus as in claim 5 in which the ring-shaped cathode is of tantalum.

WHEELER G. LOVELL. NELSON E. PHILLIPS.

REFERENCES CITED The fo lowing references are of record in the f le of this patent:

UNITED STATES PATENTS Number Name Date 189,658 Schoepflin Apr. 11, 1877 650,646 Long May 29, 1900 860,657 Hatfield July 23, 1907 995,476 McNitt June 20, 1911 1,186,306 Greenawalt June 6, 1916 1,251,302 I'ainton Dec. 25, 1917 1,801,011 Koeppen Apr. 14, 1931 1,820,844 .Steinbuch Aug. 25, 1931 1,897,308 Hunter Feb. 14, 1933 1,965,399 Wehe July 3, 1934 2,071,087 Philipp Feb. 16, 1937 2,151,599 J-aeger Mar. 21, 1939 2,218,021 Corneil Oct. 15, 1940 2,257,746 Janes Oct. 7, 1941 2,373,320 Lovell Apr. 10, 1945 2,376,535 Fisher May 22, 1945 FOREIGN PATENTS Number Country Date 16,475 Great Britain July 15, 1912 479,081 Great Britain Jan. 31, 1938 557,386 Great Britain Nov. 18, 1943 556,017 France Sept. 14, 1922

Claims (1)

1. IN APPARATUS FOR PRODUCING ALUMINUM IN SOLID, PARTICULATE FORM, A RING-SHAPED CATHODE, MEANS FOR RAPIDLY VIBRATING SAID RING-SHAPED CATHODE, A CUP-SHAPED ANODE LOCATED GENERALLY ABOVE SAID RING-SHAPED CATHODE AND HAVING ITS BOTTOM PORTION CURVED TO A RADIUS TO FIT WITHIN SAID RING-SHAPED CATHODE, SAID CURVED BOTTOM PORTION OF THE ANODE HAVING A PLURALITY OF OPENINGS THEREIN, AND PUMPING MEANS FOR CONTINUOUSLY PUMPING A FUSED, NON-AQUEOUS ELECTROLYTE INTO THE UPPER PORTION OF THE CUP-SHAPED ANODE AND THROUGH SAID OPENINGS PAST THE RING-SHAPED CATHODE.

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