US2593741A - Process for the electrolytic production of aluminum - Google Patents

Description

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM Filed July 16, 1946 L. FERRAND 2 SI-IEETS-SHEETI April 22, 1952 Wma lvlglllzll lg T b m 3 I 3 8 E F f i T le]...

April 22, 1952 L, FERRAND 2,593,741

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM Filed July 16, 1946 2 SHEETSSHEET2 Patented Apr. 22, 1952 PROCESS .FOR THE ELECTROLYTIC PRQDUCTION OF ALUMINUM Louis Ferraml, Paris, France Application July '16, 1946, Serial No. "683,902 In .France July 17, .1943

Section '1, Public Law 690, Augus't'8j1946 Patent expires July 17, 1963 .4 Claims.

i The French patent, Ser. No. 962,781 filed by the applicant on the 730th of .June 1942 for "A Method :for the Automatic Operation of Ovens :for Melt-Electrolysis andArrangements thereof A first-feature of the inventionconsistsiin corriinuously distributing the alumina into the anodic dissolution chambers and to convey it into the bath. with the aid of an auxiliary reducing gas fed at regulataole rate and pressure;

preferably, such distribution is effected through 4 calibrated apertures; the leak flow of conveyor "gas-moreover serves as means to reduce-the lumps of alumina.

.A characteristic modification of 'the invention consists indistributing the alumina continuously we :an intimate mixture with the auxiliary re .ducing gas; said mixture may advantageously be distributed through calibrated tubes and oifices.

A-characteristic of theinvention that is a ,con-

sequence of the preceding ones consists in using the pressure of the reducing gas (methane or any other gaseous hydrocarbon) in the dissolution'chambers, or the pressure of the-mixture-of dissolved material with reducing gas, as a means to regulate the interpolar distance.

These various improvements will now be described in detail hereinafter, reference being had to the appended drawing in which:

Figures 1 and 2 are diagrammatic sectional 4 views, drawn to different scal s, ofa first embodiment. v Y

FiguresB and 4 are secticnalviews of a modi fication.

-'anode m, on which the anode-carrying shaft and the electric leads are indicated in dotted lines. Secured in fluid-tight manner on top of said chamber g by a nipple f '(Fig. 2) with :the interposition-of an asbestos gasket 7' is a reservoir T, which is also circular in sectionandinto which the alumina to be dissolved in the bath is fed at regular intervals through a tube t1 branched oil from a 'manifold T1 through which the alumina is conveyed pneumaticallyin Well known manner.

The upper end of nipple f carries a removable steel or gunmetal plate n drilled with one or more calibrated orifices such as 0 designed to 'let alumina flow therethrough at the desired rate. Of course, the diameter of the hole or holes 0 should be ascertained experimentally with great care in order that with the mean head corresponding to the level difierential it between two successive loadings the rate of flow of the alumina shall be obtained in accordance with the load of theelectrolytic oven and the number of dissolution chambers available.

Said chambers-might have an oblong section or even'be replaced, in one'and thesame anode, by one single elongated and narrow chamber become too narrow A complementary arrangement is thus necessary.

Thesaidadditionalerrangement of the invention consists in the use of a tube t2 connected with a manifold T2 amid a narrow central tube throug'h all of which aneutral or preferably a reducing gas may be flowed into the chamberq, e. :g.':methane or "some other hydrocarbon gas.

Another embodiment of the invention whereby still :more important additional technical efiects ,Figure 5 illustrates applications to vScderberg :50 rcanbe secured is 'illustratcdrin Figs. 3 and 4.

anodes.

The arrangement illustrated in .Fig. 1 ,is .designedmore especially for aluminum production taken as an example. It comprises a dissolution chamber q, circular in section, bored in the;

In the said embodiment, :the :alumina fwhich ,first tis de gassed in 5a vacuum to strip it "from the air therein, is mixed intimately, 'prior'tozits being introduced into the oven with the. aid of an injector or ta :small .gas turbine orjsome other suitable apparatus that is not comprehended in the invention, with a reducing gas flowed at such rate that the atomic carbon resulting from its dissociation at the temperature of the electrolysis will be suiiicient instead of carbon from the anode to sustain the secondary reactions of anodic oxidization. The very principle of this aspect of the invention, together with the selection of the reducing gases to be employed (as a rule readily dissociable hydrocarbons) are justified by established thermodynamic considerations and by the latest theories about aluminum electrolysis in the melt, according to which the primary electrolysis does not take place at the expense of the alumina itself but does it at the cost of the components of the fiuorinated electrolyte to which said alumina is admixed. The anodic oxygen itself thus would not be concerned in the conveyance of the current and would merely be a product of secondary reactions.

For instance, with methane gas whose available energy is already low at C. (1-=l2,800 calories) and should be still lower if not null at 950 C., it can be estimated that the reactions at the expense of the anodic oxygen, separately or simultaneously, are as follows:

ZC-l-Oz-e 2CO-1=105,260 calories (1) C-{-O2 C Oz--1=93,190 calories (2) 2H2+o2+ 2H2o-T=ss',24o calories (a) or else,

81,815 1- 88,2l5 calories (6) for that portion of the methane gas that escaped dissociation. One is aware that reactions (4) and (5) take place secondarily, as the methane gas is being cracked, at 850 C.

Of course, such reactions, which all take place between gases or with highly divided carbon, are likely to happen rather than such as would involve the much more compact carbon of the anode. An inoxidizable anode is thus obtained without sacrificing the recovery of electromotive force arising from the heat liberated by these exothermic reactions.

As to the excess gas (Hz or CH4) that would escape oxidization within the electrolyte, it will be burned as it comes into contact with the surrounding air, which is to be entered upon. the credit-side of the thermo-chemical balance-sheet for the electrolytic reaction as a whole, this allowing to cut down power consumption materially provided the current density employed is properly adjusted.

It should be mentioned that said methane gas, of which natural sources are to be found only in Rumania and in the United States, can be produced most easily by availing ones self of the method for obtaining alumina from bauxites which consists in reducing the metal impurities therein with coal and then carburizing the alumina to obtain aluminum carbide according to a known method. Said carbide stripped from the metal impurities thus reduced, by its combination with water or steam, will give methane gas and alumina according to the known reaction:

Methane gas is thus obtained as a paying by-.

product which in turn will furnish the reducing carbon necessary for the anodic reactions, and this, in chemically pure state although generated from ordinary coal, without the necessity of resorting, as usual for the obtainment of pure aluminum, to high-purity and consequently costly carbon varieties such as petroleum coke.

The methane gas is preparatorily re-heated if necessary by passing the same through sole or Wall tubes such as described in the French addition No. 46,398, dated Feb. 2 1935, to Patent No.

782,136 in order to facilitate its dissociation,

whereafter it is mixed with the alumina in the proportion of about 400 liters per kilogram of the latter, after which the mixture is distributed through the manifolds T3 and the tubes ta to the various anodes through vertical orifices S, which are larger in number and smaller in section than the dissolution chambers in order that a more satisfactory dispersion of the gases below the anode surface may be obtained. Fluidtightness in the fit of said tubes is in the corresponding orifices is secured similarly with the aid of a screw-threaded nipple f and an abbestos gasket.

It will be appreciated that such method of constant distribution of dissolved material is independent of the shape of the anode system and is applicable notably to continuous so-called Soderberg anodes. Figure 5 is a sectional view of a Soderberg anode m provided with vertical aluminum tubes s' connected by screw-thread engagement at f with the tubes ii to be screwed in proportion as the anode is consumed and which are surrounded with carbon c maintained by the aluminum sheath of the anode. Said tubes ii are connected with the manifolds T4 through which the mixture of alumina with reducing gas is led down to the bath. It will be observed that with the anode equipment described hereinbefore and the cathode equipment described in the French patent filed by the applicant on the 11th of August 1943 under the proc. No. 482,247, for

A Melt-Electrolysis Oven Designed for Very Small Voltage Drop and Continuous Running- Off it becomes possible to shut off the oven in fluid-tight manner with the aid of an arch V since the bath has no longer to be taken care of.

The use of a neutral or a reducing gas to flush the alumina or to form an intimate mixture therewith according to one of the above-described features leads to a twofold result:

(a) With suitable adjustment of the gas pressure by means of a pressure regulator controlling the feed into each oven the level of the bath in each dissolution chamber will sink until balance is obtained between the pressure of the gas within the chamber and the height H of liquid bath that surrounds the anode above the bottom of the latter. It will be found that such balance is reached or slightly exceeded as soon as the meter arranged on each anode indicates a very low rate of flow. The occurrence of such low rate of flow only at one anode indicates that same is higher than the adjacent "ones, and once said anode has been set horizontal according to the procedure already described in the aforesaid patent it will only be necessary to sink the same until leak flow appears at some other anode, and so on. The simultaneous existence of such a leak flow at all the anodes in one and the same oven attests that they all are immersed by an equal amount in the bath, which means that the interpolar distance d between their bottoms and the metal layer d1 is the same at all the anodes (Fig. 1 and even Fig. where two or more Soderberg anodes are-provided).

A highly reliable way is thus afforded to adjust said interpolar distance instead of the rough methods usually resorted to. Incidentally, such condition is of capital importance in the efficient performance of the electrolysis.

That a preparatory adjustment of the interpolar equidistance is indispensable will be appreciated readily since, due to the fact that the pressure of the reducing gas or mixture in the supply line is constant everywhere, it is necessary that the back-pressure below each anode should be the same to ensure equality in the amounts of alumina fed to each of them.

(17) Where the alumina is fed in by gas flush, the leak flow (that can be adjusted at any desired value) will also be efiective to drive away.

alumina lumps encased in the cryolite and to bring them below the active surface of the anode into impoverished regions of the bath where their dissolution will be accelerated by the turbulent action of the gases from the electrolysis. In these conditions, with proper regulation of "the flow of alumina through the orifices 0 (Figs.

1 and 2), a practically constant percentage of alumina in the bath is obtained and the occurrence of the anode effect is avoided. Moreover, such conveyance effect cannot but be aided by the increase in weight of the superficial layers of the bath at the outlet of the dissolution chambers as soon as the percentage of alumina exceeds with an attendant drop in the melting point, and reaches 24%, which corresponds to the cryolite-alumina eutectic.

Last, the method of flushing or mixing the alumina with gas is advantageous in that light alumina varieties now become available that usually cannot be dealt with by hand-feed methods on account of the excessive amountsof dust raised.

I claim:

1. The method of producing electrolytic aluminum in an oven having a carbon anode suspended in a layer of molten alumina electrolyte comprising dispersing finely powdered alumina in an atmosphere of gaseous methane, and discharging the resulting suspension directly into the space between the anode and the molten layer, the proportion of alumina to methane in said suspension being such that the methane is suificient in amount to combine the entire oxygen content of each particle of alumina therein; whereby to obviate oxidation of the carbon anode.

2. The method in accordance with claim 1 in which the suspension is discharged into said space through a duct in the anode.

3. The method in accordance with claim 2, in which the formation of said suspension takes place prior to introduction of the materials into said anode duct.

4. The method in accordance with claim 2, in which the formation of said suspension is carried out within the anode.

LOUIS FERRAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 527,848 Gooch et a1. Oct. 23, 1894 1,837,070 Roth Dec. 15, 1931 1,851,817 Dow Mar. 29, 1932 2,231,030 Scofield et al Feb. 11, 1941 FOREIGN PATENTS Number Country Date 9,676 Great Britain of 1901 318,431 Great Britain- Sept. 5, 1929 511,076 Great Britain Aug. 14, 1939 115,749 Switzerland July 1, 1926

Claims (1)

1. THE METHOD OF PRODUCING ELECTROLYTIC ALUMINUM IN AN OVEN HAVING A CARBON ANODE SUSPENDED IN A LAYER OF MOLTEN ALUMINA ELECTROLYTE COMPRISING DISPERSING FINELY POWERDED ALUMINA IN A ATMOSPHERE OF GASEOUS METHANE, AND DISCHARGING THE RESULTING SUSPENSION DIRECTLY INTO THE SPACE BETWEEN THE ANODE AND THE MOLTEN LAYER, THE PROPORTION OF ALUMINA TO METHANE IN SAID SUSPENSION BEING SUCH THAT THE METHANE IS SUFFICIENT IN AMOUNT TO COMBINE THE ENTIRE OXYGEN CONTENT OF EACH PARTICLE OF ALUMINA THEREIN; WHEREBY TO OBVIATE OXIDATION OF THE CARBON ANODE.

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