US2062340A - Means for and method of producing aluminum - Google Patents

Description

Dec. 1, 1936. v M WEAVER 2,062,340

MEANS FOR AND METHOD OF PRODUCING ALUMINUM y Filed April 26, 1952 lll/ll INVENTOR www Patented Dec. ,1, 1936 UNITED STATES aosasto n MEANs Fon AND Ms'rnon oF rnonvcmo. ALUMINUM :victor M. weaver, Niagara nu, N. Y.; Ida Shur Weaver administratrix of said Victor M.

Weaver, deceased Application April 26. 1932, Serial No. 607,608

19 Claims.

The present invention relates to means for and a method of producing aluminum and more particularly to an improved electrolytic cell, a substantially indestructible anode for use in the production of aluminum.- an improved electrolytic bath for aluminum oxide and a method of producing aluminum therefrom.

Heretofore in the production of aluminum a molten electrolyte has been used consisting of a solution of oxide of aluminum, alumina, in cryolite. The composition of the bath has been so regulated that in the molten state the specific gravity thereof is somewhat less than the specific gravity of molten aluminum and consequently the molten i5 metal remains below the fused electrolyte at the bottom of the container and remains as molten aluminum which serves as Vthe cathode of the electrolytic cell. The temperature of the electrolytic bath has been kept as low as possible, and in normal operation is about l000 C. During electrolysis of the alumina. dissolved in the fusedl cryollte, aluminum is liberated in the molten state at the cathode and oxygen is liberated at thel anode which comprise carbon eleci trodes immersed in the molten electrolyte. 'I'he electrolytic bath is covered with a coating which is usually iinely divided carbon in order to decrease the heat loss. However, a substantial por- .tion of the carbon electrodes is consumed by the oxygen liberated at their surface whereby the electrodes are caused to waste away quite rapidyly. This wasting away disarranges the electrical circuit and requires hand manipulation of electrodes to compensate for shortening thereof.

85 Other disadvantages'uch as contamination of aluminum by ash of the electrodes are also encountered.

Due to the quantity of carbon in the bath and below the aluminum cathode as lining of the cell.

40 quantities of aluminum carbide form and oat up to the top of the bath and are changed back to aluminum oxide and an oxide of carbon thus necessitating further electrolytic reduction of the aluminum oxide so formed. 'I'he reduction 5 of this additional quantity of oxide, without the creates a 'greater resistance opposed to the pasand the non-uniformity indistance therebetween (Cl. ZIM- 20) i sage of the current and therefore increases the cost per unit of metal produced.

'I'he present invention provides an electrolytic bath, the melting point of which is substantially less than the melting point of baths now in use whereby the loss of electrolyte by volatiliz'ation is substantially reduced. 'I'he invention also provides an electrolytic bath which is water thin at temperatures in the neighborhood of r160 C.-

and the specific gravity of which is about 2.11 and thereforeless than the specific gravity of molten aluminum at the same temperature. This bath mixture contains sumcient alumina from which the metallic aluminum can be obtained by electrolysis.

An improved container or cell is provided for the electrolytic operation and the cell is provided with heat insulating material provided upon all sides of the cell to prevent loss of heat through the cell walls and thereby decrease the consumption of electricl energy in the production oi.' a unit of metal which material brings no carbon into contact with aluminum and thus produces carbon free aluminum.

The invention further provides amethodof and apparatus for the production of metallic aluminum in which the molten electrolytic bath is not contaminated with carbonaceous materials and in which the surface ofthe bath is not covered with a layer of carbon and consequently oxygen liberated at the anode may be recovered in pure and uncontaminated form and not accompanied by quantities of oxides of carbon. 'I'he anode, provided .by my invention. comprises electrodes which are resistant to the attack of oxygen or iluorlne at the temperatures obtaining in'thebath and/which are also resistant to the action of the materials of the bath.-

My improved anode comprises a hollow rotatable electrode which is adapted to transmit heat to the electrolytic bath to aid in maintaining the bath in' a molten condition, andto this end a substantially large surfaceivof the anode has contact with the bath. n

The invention also provides means for a'gitating the molten bathtomaintainga substantially equal distribution oi the constituents throughout the bath as the electrolysis proceeds and to incorporate added alumina therein. A hollow, rotatable, relatively indestructible anode is probided by the invention in which a combustible charge can be burned and the heat of the combustion transmitted through the walls of the electrode to the electrolytic bath to aid in maintaining the bath in a molten condition. 'o

The invention, therefore, provides an electrode the inner walls of which are made of a material to withstand the action of a combustible charge and of products of the combustion, the outer surfaces of which are formed of material to withstand the destructive action of oxygen liberated at the anode and to withstand the action of the constituents of the bath itself. The invention also provides a heat interchanger in the form of an anode for regulating the temperature of the electrolytic bath.

In the drawing:

Fig. 1 represents a longitudinal vertical section showing the heat insulated bath, the rotatable anode and the means for introducing the combustible charge into the anode.

Fig. 2 is a cross section of a bath showing a plurality of anodes and showing with more particularity thel means by which electrical contact is made with the cathode.

Fig. 3 is an enlarged fragmentary section of an anode wherein the inner and outer lining materials are plainly shown.

Referring more particularly to the drawing the' casing of an electrolytic cell or container is shown generally at 5 and may be of any suitable composition, as for instance, steel sheets. The inner surface of the sheets is provided with a heat insulating lining B preferably of powdered alumina over which is placed an additional layer of insulating materialv 1 comprising slabs of fused aluminum oxide. The cell is provided with a cover portion 8 which is also provided with heat insulation of a type similar to that used upon the bottom and side walls of the cell, whereby the completed and operating cell will retain the maximum quantity of heat contained within the walls thereof and the losses by radiation be reduced to a substantial (minimum.

'Ihe cell itself may be supported upon suitable supports such as the I beams 9 or, under some conditions of operation, may be placed directly upon the fioor of the cell room. 'I'he cell is provided with suitable insulating sleeves, as at Ill projecting through the bottom of the cell by means of which electrical contact is made with the cathode I3 of the cell which at the temperatures of operationis molten aluminum. I propose to use sleeves of alundum in which frozen aluminum metal Il acts as terminal contacts for the cathode having `electrical contact with suitable bus bars or main current lines I2.

Provision is made adjacent the bottom of th cell for drawing oil molten aluminum, preferably at a constant rate, and a rate which is substantially equivalent to the rate of deposition of the metal although the operation can be practiced intermittently. To this end, a suitable oii'take is positioned in the side wall of the cell adjacent the bot` tom thereof, as at I 5. Another exit is provided in a wall of the cell to permit the withdrawal of the other products formed by the electrolysis of the alumina, to wit, oxygen. This withdrawal port i6 is placed in the side wall of the cell adjacent the top cover thereof and as this product is liberated in substantially pure state, provision is made for collecting and storing this element, and 4in general, the exit pipe I1 leads to a convenient storage tank or compressor.

As will be made more apparent, the product" which is electrolyzed, alumina, is continuously decomposed during the process of the electrolysis and I propose to add additional quantities of alumina to the bath to maintain the composition of the bath at the desirable concentration pointed out in detail hereinafter. This gradual addition of alumina becomes important since certain constituents of the bath, for instance cryolite is, in fused condition, a solvent for the heat insulating material placed upon the inner walls of the cell or container. It, therefore, becomes of importance to maintain' a saturated condition in the bath with respect to the alumina. To this end charging ports I8 are formed in the cover 8 through which additions of alumina may be made, preferably at a constant rate, by suitable automatic mechanism of conventional type.

Further, since I desire to maintain the melting point or fusion point of the bath at a desirably low point, I find it advantageous to add alumina continuously to the bath at a rate which is substantially equal to that at' which the bath is depleted in this product. A preferable composition of the bath comprises 5.50 grams of cryolite for each 13.92 grams of potassium fluoride (dehydrated), and 2.0 grams of lithium fluoride to which 1.0 gram of aluminum oxide has been added. The melting point of this particular mixture,

that is, 24.5% cryolite, 62.1% potassium fluoride, 8.9% lithium fluoride and 4.5% alumina is 760 C., and from my experiments I believe it to be the lowest melting point mixture of cryolite, lithium fluoride, potassium fluoride and alumina and therefore represents a mixture in the nature of an eutectic mixture. In practice, the quantities may be varied over a range wherein the melting point of potassium fluoride, cryolite mixture varies from about '760 C. to about 800 C.

As mentioned hereinabove, by maintaining the bath saturated with alumina the walls of the electrolytic cell are protected from the solvent action of the bath. In order to maintain a substantially homogeneous composition throughout the. bath, to prevent depletion of alumina at the electrode surfaces and at the points of contact with the walls of the bath, means are provided vfor slowly agitating and mixing the molten materials.

In the structure illustrated in the drawing, a rotatable anode 20 is suspended in the bath and when rotated produces sufficient agitation thereof to maintain a substantially homogeneous composition therethrough. The anode also serves as a heat interchanger to aid in maintaining the temperature of the bath at a sufiicient degree andat a substantially static point, i. e. between about 760 C and 800 C. Depending upon the current density used in the electrolysis and the size of electrolytic furnace and its heat insulation, the anode may be used to either furnish heat by burning the combustible material within the anode or at greater electric intensity heat may be dissipated through the anode walls to a cooling fluid to maintain the static temperature condition in the bath. In Fig. 1 of the drawing the hollow electrode 20 is suspended horizontally in the bath and to a point as close as praticable to the cathode I3, but not so close that contact of the anode and cathode occurs, or that'short circuiting is produced,- or that the metallic portions of the cathode will alloy with the anode material. In practical operations the clearance between the anode and cathode is on the order of a half centimeter; such proximity insures a substantially low bath resistance and thus decreases the energy outlay in the production of aluminum.

The hollow electrodes 20', extending through openings in -the side walls of the cell or container, are largely iioated by the bath and supganas 2s attached to the side walls ofthe een and are normally insulated from the cell cas/ing by the insulation ring 24.

The height of the bath within the cell is so regulated that the surface i8 of the bath is lower than the bearing surfaces of the anode and the glands 22 in order toA prevent. escape of oxygen between the anode l`and. cell wallwith consequent loss of material from" the cell or fusion of the constituents of the bath between the anode and the gland. A ring gear 42o! insulating material is attached to a protrudingend Il' of anode 20 for engagement -with 'a power shaft whereby the anode'can befr rotated atf anyV desirable rate.

f' Provision is madewhereby a combustible mixture may be' led into the hollowl anode and :pipes 2l' through which combustible material v of any desired type, either powdered, solid. liquid, or 'gaseous` fuel may be led and be dis- `charged through convenient jets 25 into the interior of the anode and there come in contact with and unite with a desirable supporter of combustion such as air or oxygen. The sup- :porter ofi-combustion, which is desirably preheated, is conducted into the interior of the an.-

, ode through the elbow connection 2l and unite with the combustible substances issuing from `the jets 2B, a portion of the heat so-liberated being conducted to the bath as aforesaid.

The products of the combustion are withdrawn from the interior of the anode and discharged through the elbow 21 preferably through a conventional checker work or regenerator system before being discharged lto the stack. -It is to be noted that byl the .use of a checker work or regenerator, the air or oxygen used to support the combustion of the combustible material maybe pre-heated before entrance into the hollow anode and thus further aid in effecting a substantially complete energy recovery. Where the exhaust products of combustion are passed through a regenerator and air is passed through the pre-heater or regenerator prior to its entrance into the combustion chamber, such entering air is at a temperature-of about 400 C. and the temperature upon the interior of the anode is in the neighborhood of 800 C.

'I'he core of the anode. Fig. 3, comprises a metal capable of withstanding the extreme temperatures of the bath and which in addition is a relatively good conductor of. heat and electricity as for instance a steel cylinder. In order to prevent excessive corrosion and decomposition of the anode, I coat the inter-loro! the steel core 20 with a deposit of nickel 2l which is on the order of si, of an inch in thickness'or bet-- ter. After the deposition of the nickel the coated steel is rolled in order to produce a substantially coherent and non-porous coating upon the inner surface. In order to prevent excessive decomposition of the outer surface, the outer surface of the steel anode is steamed at a temperature sulllcient to form magnetic oxide of iron, that is, at temperatures above about 500 C.

The magnetic oxide layer 22 is further protected by a deposit of silver I3 on the order of of aninch or more in thickness. This surface is rolled in sheets to obtain the proper coherence of the silver layer. 'I'he layer of silver is suillciently porous to permit a slow diffusion of oxygen therethrough at the temperatures ofthe bath,` that is, from about 750- C. to about 800 C.; it does not combine with oxygen at this temperature and is sufllclently dense to prevent passage of liquid constituents of the bath through it. The reduced ends or extremities 24 and-of the anode protrude ,through the side wallsijof the .electrolyticl cell. and an end makes connection with the source of current. This-end is preferably coated with an electrically conducting material, as for instance acopper .sheet 2|'. which contacts with the electric contact member 31 which leads from a suitable source of current.

'Ihe inlet and exit pipes 26 and 21 (are provided with anged portionsl 3l for labutment trolytic bath by burning combustible material within the hollow anode 20, my invention contemplates another mode of `operation underoperating conditions where excessive amounts' of heat are generated -by passage of the electric current through the bath. When it is desired to increase the output of the individual cell, the

Anumber of amperes flowing-'through the cell is increasedv thus increasing the amount of heat generated in the bath by the passage of current between the anodel 20 and cathode I3. When current on the order of 15,00020,000 amperes per anode is used, large vquantities of heat are generated and strong convection currents are set up ln the bath aiding in the mixing action whereby the addedalumina is uniformly -distributed and dissolved -in 4the electrolytic solvent of cryolite, lithium fluoride and potassium uoride.

To prevent the temperature of the bath from rising to a point where certain of the constituents thereof are appreciably volatile, I lpropose to conduct a cooling fluid through the hollow anode 20 and extract lthe excess heat from the bath and thus maintain the temperaturefthereof in the neighborhood of the range 760 C. to`

In many localities desirable working conditions ,will include the use of so-called off peak power" and my invention can be used advantageously by vemploying the anode described herein as atemperature stabilizer. Thus, wlere the electrolysis is performed` as an intermittent operation with olf peak power, in contradistinction-to a continuous operation, the anode'can be heated by burning combustible material therewithin in order to prevent the freezing of the bath to a hardstone-like substance which would cause damage to the interior of the cell, until such time as the power is again available when normal operation can be resumed.

During all conditions of operation of the cell,

I contemplate maintaining a proper temperature control within very close limits and such operation may be carried out due to the fact that the anode is substantially indestructible. It is possible to maintain such temperature control by means of a suitable thermocouple, as for instance a couple composed of a platinum and ninety percent platinum, ten percent rhodium couple immersed in the bathat some convenient portion and connected with lead wires to a suitable potentiometer. By employing the temperature stabilizing condition in conjunction with proper temperature control with an anode composed of materials which comprise the best electrical conductors known and the proper design of the electrolytic cell such as described herein, large outputs can be maintained with a consequent decrease in cost of metal produced which is -not the case where the anode is constantly wasted away and hand manipulation resorted to to compensate for this destruction. I therefore contemplate handling large currents per electrode, as for instance in the neighborhood of fifteen to twenty thousand amperes or more and the furnace can be supplied with a number of electrodes connected in parallel.

As the resistance of the bath is a large item of the electrical circuit, much heat is set up by the passage of this large current and with the cell suiciently well heat insulated as described, a rise of temperature will show itself upon the pyrometer. Conditions of operation and the design of the furnace will therefore determine what oillce the anode will serve in the matter of a stabilizer of temperature. In a small furnace, combustibles will be burned within the anode to supply heat thereto in order to maintain the correct temperature of the bath while in a furnace or cell of large capacity it will be found desirable to abstract heat from the furnace during the more vigorous and increased electrolytic action. This can be done by passing a. cooling medium through the anode when the temperature of the bath rises to maintain the temperature within a very close limit, as for instance between the '750 C. to 800 C. as mentioned above.

Although I have found that the composition of baths mentioned above seems a desirable composition for use wherein the electrolyte comprises 13.92 grams potassium fluoride, 5.50 grams cryolite, for each two grams of lithium fluoride, during the course of my experiments I have found that when 3.5 grams cryolite and 13.92 grams potassium fluoride were mixed together and heated a clear fluid was obtained at about 820 C. The addition of one gram of lithium uoride made the bath somewhat more iluid and this bath could be melted at 760 C. Upon addition of one gram of aluminum oxide, however, very little solubility was incurred at these lower temperatures and at a temperature of 850 C. the bath contained quantities of undissolved floating ma- I terial.

At higher temperatures the bath cleared up. In another experiment with lithium iluoride and cryolite 6.24 grams of lithium uoride and 5.50 grams of cryolite were heated together and the material melted at approximately 710 C. However, this bath contained very little solvent power for aluminum oxide at the lower temperatures and in order to obtain solubility the temperature had to be increased in the neighborhood of or considerably above 850 C.

When, however, 5.5 grams o f cryolite were mixed with 13.92 grams of potassium fluoride and one gram of lithium fluoride a bath was obtained which melted between '760 C. and 780 C. which had a certain amount of fluidity at that temperature and which dissolved aluminum oxide to a moderate degree. Upon increasing the amount of lithium fluoride to two grams the iluidity of the bath was found to be increased and the solvent action of the bath for aluminum oxide more rapid. This bath was found to give good results from the point of view of rapidity of solution of aluminum oxide, fluidity of bath and low fusion temperature since the bath was perfectly clear and mobile and water thin at a temperature of '760 C. The specific gravity of this bath, as determined at a temperature of about 800 C. was 2.11 and so that the bath would float upon molten aluminum at the same temperature.

It is to be particularly pointed out that the present invention provides a substantially indestructible anode and one in which loss due to the resistance oifered to the passage of the current is at a minimum. The coating of silver provides a relatively indestructible electrode, one which is a good conductor of heat and electricity, one which is unaffected by the chemical action of the constituents of the fused and molten bath, an electrode which is solid at the temperature of the bath, and an electrode unaffected by oxygen or fluorines.

Further, the temperature at which the electrolysis takes place is suiiciently low to prevent excessive volatilization of potassium fluoride in the bath and in this respect the process differs materially from the process as currently in use where bath temperatures of l,000 C. and upward are not uncommon and wherein appreciable loss of the bath constituents are suered due to vaporization.

What is claimed is:

i. .The method of depositing aluminum electro-- lytically which comprises passing electric energy through a molten electrolytic bath containing alumina at a temperature below 800 C. thereby depositing aluminum at a cathode o molten aluminum below the level of the molten bath and liberating oxygen at the surface of a rotating anode which resists the action of oxygen.

2. The method of depositing aluminum electrolytically which comprises passing electric energy through a molten electrolytic bath and adding heat energy to the bath to aid in maintaining the molten condition thereof by burning a combustible charge in the interior of e. hollow anode.

3. 'I'he method of depositing aluminum electrolytically which comprises passing electric energy through a molten electrolytic bath and adding heat energy to the bath to 'aid in maintaining the molten condition thereof by rotating a hollow anode in contact with the bath but spaced from the molten aluminum below the bath and burning a combustible charge in the interior of the anode.

4. The method of depositing aluminum electrolytically which comprises passing electric energy throughl a molten bath containing alumina, thereby depositing aluminum at the cathode, adding heat energy to the bath to aid in maintaining the molten condition thereof by burning a combustible charge in the interior of a hollow anode, and rotating the anode in the bath.

' 5. The method of depositing aluminum electroy lytically which comprises passing electric energy through a molten bath containing about 62.0% potassium fluoride, 24.0% cryolite, 9% lithium fluoride, 5% alumina, adding heat energy to the stituents of the bath by a coating upon the ex- `als terior of the anode which resists the action of oxygen.

6. The method of depositing aluminum electrolytically which comprises passing electric energy through a molten bath containing about 62.0% potassium fluoride, 24.0,% cryolite, 9% lithium iluoride, and alumina, adding heat energy tothe bath to aid in maintaining `it ina molten condition by burning a combustible charge in the interior of a hollow anode, and protecting the anode from the action of theoxygen and the constituents of the bath by a coating of silver upon the exterior of the anode.

7. The method of depositing aluminum electrolytically which Icomprises passing electric energy through a molten bath containing about 62.0% potassium fluoride, 24.0% cryolite, 9% lithium uoride, and 5% alumina, adding heat energy to the bath to aid in maintaining it in a molten condition by burning a combustible charge in the interior of a hollow anode, and protecting the anode from the` action of the oxygen of the constituents of the bath, and a coating upon the interior of the anode to protect the same from the action of the combustible kcharge and the products of the combustion.

8. The method of depositing aluminum electrolytically which comprises passing electric energy through a molten bath containing about 62.0% potassium uoride, 24.0% cryolite, 9% lithium iluoride and 5% alumina, adding heat energy to the bath to aid in maintaining it in a molten condition by burning a combustible charge in the interior of a hollow anode, and protecting the anode from the action of the oxygen of the vconstituents of the bath, and a coating of nickel upon the interior of the anode to protect the same from the action of the combustible charge and the products of the combustion.

9. The method of depositing aluminum electrolytically which comprises passing electrical energy through a molten electrolytic bath containing alumina at a temperature below 800 C. thereby depositing aluminum at a cathode of molten aluminum-below the surface of the bath, maintaining the temperature of the bath below 800 C. by heat interchange through the heat comducting walls of an anode, and circulating a duid in said anode. y

10. A composition suitable for use as an electrolytic bath for the production of aluminum comprising about 62.0% potassium iuoride, 24.0% cryolite, 9.0% lithium fluoride and 5% alumina.

11. A composition suitable toruse as an electrolytic bath for the production of aluminum containing -about 62.0% potassium fluoride andV 9.0% lithium iluoride in addition to cryolite and alumina to saturation and melting in the neighborhood of 760 C. to a water thin liquid with weight,of 2.11 grams per cubic centimeter at about 800 C.

' 12. An electrolytic cell for the production of aluminum comprising an electrolytic bath containing alumina having a melting point below that of silver, ,a cathode o! aluminum beneath said bath, and a rotatable metallic anode positioned horizontally in said bath.

13. An electrolytic cell for the production oi.'A aluminum comprising an electrolytic bath containing alumina having a melting point below that of sliver, a cathode of aluminum beneath said bath, and a hollow metallic anode positioned horizontally in said bath.

14. An electrolytic cell for the production of aluminum comprising an electrolytic bath containing alumina having a melting point below that of silver, a cathode of. aluminum beneath said bath, and a hollow rotatable metallic anode positioned horizontally in saidbath.

15. An 'electrolytic cell for the production of aluminum comprising an electrolytic bath containing alumina having a melting point below that of silver, a cathode oi aluminum beneath said bath, and a hollow rotatable anode covered with a`metal substantially inert toward oxygen at the temperature of operation of the cell.

16. An electrolytic-*cell for the production of aluminum comprising an electrolytic bath containing alumina having a melting point below that of silver, a cathode of aluminum beneath said bath, and an anode coated with silver.

17. An electrolytic cell for the production of aluminum comprising an electrolytic bath conl taining alumina having` a melting point below that off silver, a cathode of aluminum beneath said bath, and a hollow metallic anode coated vwith silver upon the portions of the anode in contact with the bath and with nickel upon the inner portions of the anode.

18. The method of depositing aluminum elecand lithium fluoride,` the resultant bath having a t freezing point below 800 C.

` VICTOR M. WEAVER.

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