US1534318A - Electrolytic refining of aluminum - Google Patents

Description

April 21, 1925.

w. HOOPES ET AL ELECTROLYTIC REFINING OF ALUMINUM Dec. 21, 19.22

anveutow In" #001 55, EC. 1 7%? v and JD Fawn/#0 5 351 their 6141mm MG ,a f-f M v Patented Apr. 21, 1925.

UNITED STATES PAITE-KNIT OFFICE.

WILLIAM HOOPES, OF PITTSBURGH, AND FRANCIS FRARY JUNIUS D. v1 1D- WARDS, OF OAKMONT, PENNSYLVANIA, ASSIGNORSTO ALUMINUM COMPANY OF AMERICA, 01 PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA.

ELECTROLYTIC REFINING OF ALUMINUM.

Application filed December 21, 1922, Serial No. 608,285.

To all whom, it may concern:

Be it known that we, WILLIAM Hoorns, FnANois C. FRARY, and JUNrUs D. En- WAnos, all citizens of the United States of 5 America, the said WILLIAM Hoorns residing at Pittsburgh, and the said FRAiNoIsC. FRARY and J UNIUS D. EDWARDS residing at Oakmont, all' in the county of Allegheny and State 'of Pennsylvania, have. invented certain new and useful Improvements in Electrolytic Refining of Aluminum, of which the following is a full, clear, and. exact description. 1 t

' This invention relates generally to the refining of aluminum by electrolytic removal or separation of themetalrfrom an alloy or a mixture thereof with other substances, and relates more particularly to a refining process in which the electrolyte floats, in a molten state, upon-the molten alloy, the latter being used as anode and the aluminum removed being deposited on a layer of molten aluminum as cathode floating on the electrolyte."

In an extensively used aluminum reduction process, wherein aluminumis produced by reduction of its oxid, a cryolite bath or electrolyte .is used, but in the herein described process, in which it is desired to have the refined metal float on the electrolyte, such a. bath, though otherwise operable, cannot be used alone for the purpose, since in the molten state it is lighter than aluminum and hence would allow the latter to sink. While other salts may be'added to give adequate density, such addition easily leads to difficulties of various sorts, resulting in failure to produce pure metal, and no prior inventor, so far as we are aware, has attained commercial success in this way or in any other. \Ve have found that success may be realized and a bath produced combining the desired characteristics of density, fluidity, stability, capacity for dissolving alumina, conductivity, and selective solution and deposition of aluminum during electrolysis, by adding to cryolite (or preferably to a mixture of aluminum and sodium fluorids richerin aluminum fluorid'than is cryolite) salts of the alkali earth metals. These metals are less easily precipitated from the bath than aluminum and we find certain of their salts to be the only generally available material suitable for the purpose of increasing the density of the bath. We find that the only materials giving in high degree the other necessary or desirable characteristics are the fiuorids of these metals (barium, strontium, calcium and magnesium). These form with cryolite, mixtures which are readily fusible, but our experience indicates that magnesium fiuorid has less effect than any of the others in increasing thedensity of the bath. Barium fluorid has been suggested as an ingredient of electrolytes for other purposes, but so far as weare aware no one has heretofore suggested their use for refining aluminum or hasgiven, with respect to the properties of barium fluorid and cryolite mixtures, the information needed for their successful'use. We-have found, however, that within certain limits, mixturesof the substances mentioned furnish excellent electrolytes, and that such a bath does not lose aluminum fluorid by volatilization at the working temperature to a serious extent. Its electrical conductivity and power of dissolving alumina are also goo Moreover, the use, in an electrolyte, of a halogen salt other than a salt of fluorin, is in general disadvantageous and in some cases impractical. This is especially true in processes designed to produce substantially pure aluminum, for the reason that chlorids inthe bath cause solution and deposition of other substances than aluminum, as for example zinc, iron, titanium, copper, and silicon, with the result that if the anode contains any of these substances the cathode metal may be contaminated therewith to an intolerable degree. Stated otherwise, if chlorin anions are present in the bath, it is difficult and in some cases impossible to prevent the solution of other substances than aluminum from the anode and their deposition on the cathode. On the other hand the presence of oxygen anions is permissible. Accordingly an advantageous feature of the present process is the use of a bath or electrolyte which may contain fluorin or oxygen anions or both but is substantially devoid of chlorids, and which therefore, as our experience shows, exerts a truly selective action upon the anode alloy in dissolving aluminum therefrom substantially to the exclusion of other metals under conditions where it would be expected that considerable amounts of other metals would be dissolved, and where, if substantial amounts of chlorids are present, they do actually dissolve.

' \Ve have determined the density of aluminum of 99.7 5 per cent purity to be approximately 2.29 grams per cc. at 1000 C. At this temperature, the density of molten cryolite is approximately 2.10 grams per cc. In order to increase the density of the cryolite, so that this aluminum will float on it at the temperature mentioned, we have found that there must be added about 20 parts of barium fluorid to each 80 parts of cryolite. If calcium fluorid were to be used, about 40 parts would be required to 60 parts of cryolite. While the above mentioned mixture of cryolite and barium fluorid is completely molten at temperatures above 965 C., mixtures of calcium fluorid and cryolite containing about 40 per cent or more of the former by weight require a temperature above 1000 C. to keep them molten. This would make the working temperature of the bath undesirably and in some cases impracticably high. On the other hand, mixtures of cryolite and one or more fluorids of alkali earth metals having atomic weights above 80 are in general suitable; though of courseradium is to be excepted, for obvious reasons. Thus mixtures of cryolite and barium fluorid containing between about 20 and 60 per cent of the latter constituent by weight are readily fusible at temperatures below 1000 C., and have densities in the molten state ranging between about 2.38 and 3.15 grams per cc. at 1000 C. Even the heaviest of these mixtures is sufficiently light to float on any of a number of .molten aluminum alloys suitablefor use as anode in the electrolytic refining of aluminum.

A fused mixture of cryolite and strontium fluorid containing between about 20 per cent and 60 per cent of the latter also lies within the proper range of densities for our purpose, but such mixtures containing more than about 40 per cent of strontium fluorid are in general not so readily fusible as mixtures containing corresponding amounts of barium fluorid.

In operating with a bath of the type described above, it has been observed that there is a considerable deposition of metallic sodium at the cathode, and that this sodium, being in vapor form, passes up through the molten aluminum cathode and causes difliculties in connection with the leading out of the current from the cathode metal. Carbonaceous rods used to carry the current out of the floating layer of aluminum are attacked and eventually disintegrated by the sodium, but it has been found that this difficulty may be decreased, without interfering seriously with the density of the molten bath, by increasing the proportion of aluminum fluorid as compared to sodium fluorid, although such an increasedoes perceptibly increase the electrical resistance of the bath.

As an example of baths or electrolytes which have been found by actual use to be suitable for our purpose, We recommend one having approximately the following composition:

Barium fluorids. 30 to 38 per cent Sodium fluorid 25 to 30 per cent Aluminum fluorid 30 to 38 per cent Alumina 0.5 to 7 per cent Calcium and magnesium fluorids (present as unavoidable impurities)--- about2per cent Such a bath is completely molten at all temperatures above about 900 C. and enables the refining to be carried on at about 950 C. At this temperature the bath is satisfactorily stable, has good electrical conductivity and satisfactory density, and is capable of dissolving a satisfactory amount of alumina.

It will be understood that'the sodium and aluminum fluorids of the electrolyte may be supplied, in part at least, by cryolite, which has a composition generally accepted as 3NaF.AlF

It has been observed that the density of these molten baths decreases more rapidly with rising temperature than does the density of the molten aluminum, and hence it is advantageous to provide a distinct margin of safety between the two densities, so that if the cell should become overheated the bath will notbecome so light as to permit the aluminum top layer to sink to the bottom. The density of bath of the above mentioned composition lies between about 2.5 and 2.7 at 950 C., and between about 2.4 and 2.6 at 1100 (1., and accordingly pure aluminum will float on the bath at these temperatures, since the density of aluminum at 950 C. is about 2.30 and at 1100 C. is about 2.26 grams per cc.

A bath containing cryolite and 00 per cent of barium fluorid (instead of the chlorid) will dissolve between 4 and 5 per cent of alumina; and a similar bath containing 40 per cent barium fluorid, which would have a specific gravity of about 2.7 3, will dissolve between 8 and 9 per cent of alumina at 1000 C. Electrolytes of such type are therefore advantageous for the following reasons.

Alumina becomes more soluble in any of the cryolite baths as their temperature 'is raised, but if alumina be added until the bath is saturated it will be found that a small drop in temperature will cause some of the alumina to precipitate out as coron-- dum or in corundum-like form, with which more or less bath will be mechanically associated. In an operating cell the portion of the bath adjacent to the walls, together with. that portion forming the top crust or coming in contact with it, is usually at a temperature distinctly lower than that of the main body of the bath, so that if suflicient alumina enters to saturate this main body the natural circulation will cause a deposition of part of the alumina on the walls of the cell in theform of a thickened crust. Practical operation of such a bath has shown that when the alumina has once crystallized in this form, it is extremely diflicult to re-dissolve it in the bath. A certain amount of such crust on the interiorof the cell is desirable for its thermal and electrical insulating properties (as described more fully in the copending application of \Villiam ll'oopes, Junius 1). Edwards and Basil T. Horstield, Serial No. 608,289, filed December 21, 1922) 'but to prevent the formation of an undesired amount of this deposit, which would otherwise gradually fill up the cell and interfere with the operation, it is important to keep the alumina content be- 1 low the saturation point. In order to maintain this condition it is therefore desirable, in practice, to have the bath capable of dis solving considerable alumina, so as to allow for unavoidable variations incident to operating conditions; for the reason that any of the following causes may operate to add alumina to the bath: (A) The hydrolysis of aluminum fiuorid by moisture. Capillary action continually brings some of the bath up between the top metal layer and the side of the cell, so that it forms a; crust on the top of the metal, where it is maintained at a high temperature and exposed to the air. The excess of bath above that which can solidify to form this crust drips back through theinetal from time to time, and when the crust is disturbed or broken, parts of it sink through the metal, and return to the main body -of the bath. (B) The reaction of sodium oxid (or hydroxid) with aluminum fluorid. More or less sodium is always liberated at the cathode, and. some of it rises through the metal .layer', probably in the form of vapor, and reaches the top crust where it is oxidized by contact with the air. (C) Direct oxidation of the floating aluminum layer by air penetrating through cracks in the top crust. (D) Alumina dust, which is always present in a plant in which the Hall process of producing aluminum is operated, will settle on the crust of the refining cell if it is operated in the same plant.

So important is it to have the bath unsaturated that in actual commercial operation it is generally necessary to remove alumina from time to time. This may be conveniently done in various ways, as by one or another of the methods described and claimed in a copending application of \Villiam Hoopes and FrancisC. Frary.

, The cell preferred for use in our aluminum refining process is of the type described and claimed in the copending application of \Villiam Hoopes, Serial No. 608,287, filed December 21, 1922- One form of this cell is illustrated in the drawings annexed hereto, in which Fig. 1 is a plan View of the cell.

Figs. 2 and 3 are cross sections on lines 22 and 8-3,-respectively, of Fig. 1.

' Figs. 4; and 5 are'detail cross sections on lines t4i and 5- 5, respectively, of Fig. 1,

illustrating the water connections to and from and between the'water jackets.

Fig. 6' is a detail cross section on the line 6-6 of Fig? 1, showing the method of con-. nectingthe upper electrodes to the negative busbars.

Fig. 7 is adetail cross section on the same plane as Fig. 2, illustrating the method of securing the upper and lower shell sections together to give adequate mechanical strength without connecting the two electrically. I

The lower shell or shell section 10 is preferably made of steel in the form of a cylindrical vessel of considerably greater diameter than height, and at or near its top it is provided with a water jacket 11 which is most conveniently formed by providing at the upper edge of the shell section an outwardly extending flange 12 of suitable width, and a flaring or conical ring 12 welded or otherwise hermetically joined to the underside of the flange and to the body of the shell below.

Above the lower shell section 10 is an I upper shell section 13 which may also be of 7 steel and formed with hollow walls to provide an upper water jacketl l. The inner surface of the upper shell section is preferably flaring, as indicated. To keep the sections electrically insulated or separated from each other a fiat ring or gasket 15, of asbe used. If the water jackets are used, asin most cases they will be, the bushings and washers will not be subjected to a high temperature and hence they can be made of practically any insulating material which will not soften at temperatures below 100 C. and which can withstand the crushing stress exerted by the studs. Mica has been found to answer the purpose very satisfactorily.

Suitable water connections for the water jackets are provided, and for the sake of simplicity and convenience these connections may be so constructed and arranged that the water flows through the'two jackets 1n succession, preferably through the lower jacket first. For this purpose the jacket 11 may be provided at the bottom with an inlet nipple 20 connected by a pipe 21 to any convenient source of water, not shown, and at the top (to prevent pocketing of air) with an outlet nipple 22 connected by a pipe 23 to the inlet nipple 24 by which water from the lower jacket is led into the bottom of the upper. The latter is equipped with an outlet nipple 25 (at the top to prevent air pocketing) which may be connected to a waste pipe 26 by means of a pipe 27. To avoid electrical grounding the pipes 21 and 27 may consist of rubber hose, as may also the pipe 23 to keep the two shell sections electrically separate. The water used when the jackets are connected should be of suflicient purity to prevent material flow of current from one shell section to the other at the voltage employed in operation.

In the bottom of the lower shell section a layer 28 of heat-insulating material may be provided, as powdered bauxite, alumina, magnesia, or refractory bricks, to decrease or minimize loss of heat through the bottom of the cell, and above this layer is a bottom lining 29 of refractory electrically conducting materiaLpreferably carbon, and preferably having a cavity or depression in its upper portion to receive the alloy or other material to be refined. The bottom lining can be conveniently and satisfactorily made by tamping into the shell a mixture of tar, pitch and granular or'powdered coke, at a temperature high enough to make the mass plastic, and placing the shell and contents in an oven in which the temperature is gradually raised, say to about 600 0., for the purpose of baking and solidifying the can bonaceous mass.

Good electrical conne:tion may be provided between the shell and its bottom lining by means of metal collector plates 31, welded to the, inner surface of the shell so as to be electrically and mechanically continuous therewith. These plates extend inwardly into the bottom lining, which is molded around them. At the'plane of the collector plates the shell may be provided on the outside with metal contact pads 32, preferably welded to.the shell so as to be mechanically and electrically continuous therewith, to which pads busses or busbars,

fining operation these busses are connected to the positive terminal or pole of the source, so that the current enters the cell at the bottom. The carbon bottom or bottom-lining, 29, constitutes What may for convenience be termed the lower electrode of the cell.

The upper electrode may be multiple, as indicated, composed preferably of a suitable number of short thick rods 34 of graphite, arranged vertically and having copper or other metal. rods 35 threaded or otherwise suitably secured to the tops of the electrodes. These metal rods serve to support the upper electrodes and convey current to and from the same, and for this purpose they may be releasably and adjustably secured, as by means of clamps 36, to metal busbars 37 extending horizontally across the cell.- For convenience of access to' the graphite cylinders, for adjustment, replacement, etc., the busbars may be arranged at two or more different levels, as indicated, and may be supported on and secured to a plurality of legs 38 to form a rigid frame work. The latter may rest on the upper shell section, in which case they are insulated from the shell section, as by any convenient and suitable means, not shown.

It is recognized that, strictly speaking, the aluminum layer floating on the bath and the layer of alloy underlying the bath, are the upper and lower electrodes, respectively, but these layers are termed herein the cathode and the anode, and hence it is deemed permissible as well as convenient to refer to the graphic cylinders and the carbon bottom- .lining, or their equivalents, as the upper and lower electrodes.

Metal or other molten material ma be withdrawn from the upper portion 0 the cell through a tapping notch 39, which may be closed by means of any suitable refractory material. Molten'metal or other material may be withdrawn from the lower port of the cell through a port or tapping hole 40, normally closed by means of a plug of dense charcoal or other suitable material.

On the inside of the cell is a side-lining 45 extending upwardly from the carbon bottom 29, over the joint between-the shell sections and well up toward or even over the top of the upper shell section. In practically all cases this side-lining should be thermally and electrically insulating, to decrease or minimize the conduction of heat to the water jackets as well as to prevent bypassing of current around any art of the cell contents undergoing electro ytic treatment in the refining operation. The linin should be refractory'enough to remain soli at the temperatures to which it is exposed in the electrolytic refining operation. Under these conditions a lining composed of or formed from a mixture of metal fluorids and alumina, as more fully explained in the copending application of \Villiam Hoopes, J unius D. Edwards, and Basil T. Horsfield, Serial No. 608,289, filed December 21, 1922, has been found highly satisfactory in practice.

In the refining process the aluminum alloy or mixture of aluminum and other substances lies in molten form in the bottom of the cell as indicated at 46. Floating on this is a layer 47 of fused bath or electrolyte, and on the latter is a layer 48 of molten aluminum, with the upper electrodes extending into it far enough to insure good electrical contact. The molten layers are preferably established in the cell by successively pouring the previously fused materials into place, using for the original aluminum layer the purest metal conveniently available. The cell may also be putin operation in the following manner. I

The upper electrodes are lowered into contact with the carbon bottom and current is sent through them to the latter, thereby generating heat and fusing a small quantity of powdered o'r granulated bath material placed aroundthem. The upper electrodes are raised as the melting proceeds, and additional bath material is supplied, until a sufficient body of fused electrolyte has been produced. The molten anode alloy or mixture is then poured in. Almost any aluminum alloy can be used which is denser than the molten bath and which will remain mobile during the refining. operation. Preferably. however, we use an alloy of which the principal components are aluminum and copper. The alloy should be supplied insufficient amount so that "it will remain in an electrically continuous layer'on the bottom of the ell throughout the refining operation. A bath layer of sufficient depth should be used so that the top metal (the pure aluminun1)'will in no case come into contact with any portion of the side crust which has pre- Yiously been covered by the anode alloy. It is to be noted in this connection that the changes in composition of the anode alloy, incident to the refining operation, cause corresponding changes in its volume and in the position of the'upper and lower surfaces of the bath layer. Molten aluminum, preferably the purest obtainable, is placed on the molten bath, to serve as cathode.

The refining process can now be begun, with the alloy as anode and the top metal as cathode, the current being led from the before the withdrawal.

top metal by means of gra hite electrodes dipping into it. Under t ese conditions aluminum is dissolved out of the anode alloy and deposited in molten form on the cathode. This is continued until the desired amount of aluminum has been re moved from the anode and. added to the cathode. A portion of" the top metal is then removed and the impoverished anode alloy is withdrawn through the tap hole 40, fresh anode alloy in the molten state being supplied in any convenient way, preferably such that the refined metal floating on'the bath will not be contaminated. This opera; tion .may be conveniently performed by means of acarbon funnel, which after being preheated, is let down .until it early reaches the bottom of the cell, whidlhhhas preferably been cut out of the circuit. The refined metal entrapped in the funnel ay be dipped out with a hand ladle, after which the fresh anode alloy is poured in. The funnel is then lifted out and the refining process resumed. The fresh anodealloy introduced is preferably suflicient in amount to raise the bath and top metal until the surface of the latter is at the same 1 el as These operations may be repeated from time to time as necessary orv desirable without seriously interrupting the refining process, which otherwise can go on continuously.

The energy-efficiency in electrolytic. processes of refining aluminum is dependent largely upon the perfection of the measures taken for preventing escape of heat. Theoretically almost no energy is required for the refining; but practically, in the absence of some other adequate source of heat, sufficient electrical energy must be expended to maintain the anode, the bath, and the cathode in a fused condition, and consequently the amount of electrical energy whichmust be supplied is almost exactly the equivalent of the heat permitted to escape. After the heat insulation of the cell has been perfected to the maximum practicable extent, nothing furtheFan be accomplished in limitation of the amount of heat escaping from a heated body of given dimensions, and with the minimum heatloss the energy input required by the cell will also be a minimum. In the interests of power economy the cell should be operated at the lowest practicable voltage. Accordingly the electrolyte, which furnishes the major portion of the resistance, should be in as thin a layer asis permissible, and it has been found that a layer from 2 to 1 inches thick is in general satisfactory. Vith a bath or electrolyte of any predetermined workable depth, the current density permissible varies between a lower limit which is sutlicient to maintain the anode, the bath and the cathode in a molten state.

- per square foot.

and an upper limit at which volatilization of the bath is excessive or at which too large a proportion of anode impurities go into solution. These limits, with the various bath-compositions which we have found satisfactory, are approximately 800 C. and 1100 0., respectively, with a preferable working temperature of about 950 C. Other miscible lower limit of current density also depends to a considerable extent upon the size of the cell, since the ratio of minimum of about 780 amperes and a per-' missible maximum of about 1250 amperes,

With the preferred current density mentioned, the total voltage between the terminals of the cell may be about 6 volts. Larger cells may be operated with lower current densities and at lower voltages, and by varying the size of the cell, the composition of the bath, the conductivity of the bath, and the effectiveness of the heat insulation, the present electrolytic refining process is workable commercially with current densitiesbetween about 500 and about 2500 amperes per square foot ofcross section of the bath. In general the lower practicable limit of voltage is about 3.5 volts and the upper limit is of course indefinite.

The current is led from the molten aluminum cathode preferably by means of the electrodes or current-carrying members described in the copending application of Francis C. Frary, Serial No. 672,867, filed November 5, 1923. These are made of graphite, in the form of short, thick rods or cylinders, and may be protected against oxidation in the air by means of a non-oxidizable coating which may consist of molten bath material applied to the rods as a thin layer and allowed to freeze thereon.

It is to be understood that the invention is not limited to the details hereinspecifically illustrated or described but can be carried out in other. ways and with other appgratus, without departure from its spirit.

the p nded claims the expression fluori of an alkali earth metal having an atomic weight greater than 80, is used in a generic sense to include either barium or strontium fluorid or both, each of these metals having an atomic weight greater than that named. Similarly, in the claims speciing barium fluorid it is to be understood at for a part of such fluorid, strontium fluorid may be substituted in suitable proportion.

We claim- A 1. In the electrolytic refining of aluminum, the steps comprising establishing between a lower layer of molten metal containing aluminum, as anode, and an upper layer of molten aluminum as cathode, an intermediate layer of molten electrolyte of greater density than the molten aluminum and composed essentially of fluorids and substantially free from chlorid, and passin current from the anode metal through the electrolyte to the aluminum cathode whereby aluminum is removed from the anode metal and deposited in the molten state on the cathode. I

2. In the electrolytic refining of aluminum, the steps comprising establishing between a lower layer of molten metal containing aluminum, as anode, and an upper layer of molten aluminum as cathode, an intermediate layer of molten electrolyte of greater density than the molten aluminum and composed essentially of fluorids and substantially free from other compounds except oxid, and passing current from the anode metal through the electrolyte to the aluminum cathode whereby aluminum is removed from the anode metal and deposited in the molten state on the cathode.

3. In the electrolytic refining of aluminum, the steps comprising establishing between a lower layer of molten metal containing-aluminum, as anode, and an upper layer of molten aluminum gas cathode, an intermediate layer of molten electrolyte containing aluminum and sodium fluorids aid between 20 and 60 per cent, approximately, of fluorid of an alkali earth metal having an atomic weight greater than and passing current from the anode metal through the electrolyte to the aluminum cathode whereby aluminum is. removed from the anode metal and deposited in the molten state on'the cathode.

4. In the electrolytic refining of aluminum, the steps comprising establishing between a lower layer of molten metal containing aluminum, as anode, and an upper layer of molten aluminum as cathode, an intermediate layer of molten electrolyte substantially free from chlorid and containing aluminum andsodium fluorids andbetween 20 and 60 per cent, approximately, of fluorid of an alkali earth metal having an atomic weight greater than 80; and passing current from the anode metal through the electroremoved from the anode removed from the anode ing aluminum as anode, and an upper layer of 'molteiLal'uminum as cathode, an intermediate layer of moltenelectrolyte containing aluminum and sodium fluorids and between 20 and 60 per cent, approximately, of barium. fiuorid; and passing current from the anode metal through the'electrolyte to the aluminum cathode whereby aluminum is metal and de posited in the molten state on the cathode.

6. In the electrolytic refining of aluminum, the steps comprising establishing between a-lower layer'of molten metal containing aluminum, as anode, and an upper layer 7 f-1nolten aluminum as cathode, an intermediate layer of molten electrolyte substantially free from chlorid and containing aluminum and sodium fluorids and between and 60 per -'eent, approximately, of barium fiuorid; and passingcurrcnt from the anode metal through the electrolyte to the aluminum cathode whereby aluminum is metal and deposited in the molten state on the cathode.

7. In the electrolytic refining of aluminum, the steps comprising establishing between 21 lower layer of molten metal containing aluminum as anode, and an upper layer of molten aluminum ascathode, an intermediate layer of molten electrolyte containing between 20 and per cent, approxi mately, of barium fluorid, -and containing sodium and aluminum fluorids in such proportions that the ratio of aluminum fiuorid to sodium liuorid is greater than cryolite; and passing current from the anode metal through the electrolyte to the aluminum cathode whereby aluminum is removed from the 'anode metal and deposited in the molten state on the cathode.

i 8.. In the electrolytic refining of alumi- Y num, the steps comprising establishing between a lower layer of molten metal containing alun'nnum, as anode, and an upper layer of molten aluminum as cathode, an lntermediate layer of molten electrolyte containing cryolite, additional aluminum fluorid, and barium fluorid, the latter fiuorid constituting between 20 and per cent, approximately. of the whole; and passing current from the anode metal through the electrolyte to the aluminum cathode whereby aluminum is removed from the anode metal and deposited in the molten state on the cathode.

9. In the electrolytic refining of aluminum, ie steps comprising establishing between a lower layer of molten metal containing aluminum, as anode, and an upper layer of molten aluminum as cathode, an intermediate layer of molten electrolyte substantially free from chlorid but containing between 20 and 60 per cent, approximately, of barium fluorid, and containing sodium and aluminum fluorids in such proportions that the ratio of aluminum fluorid to sodium anode metal and deposited in the molten state on the cathode.

. 10. In the electrolytic refining of aluminum, the steps comprising establishing between a lower layer of molten 'metal containing aluminum, as anode, and an upper layer of molten aluminum as cathode, an intermediate layer of molten electrolyte substantially devoid of chlorid and containing cryolite, additional aluminumfiuorid, and barium fluorid, the latter fluorid constituting between 20 and 60 per cent, approxin'iately, of the whole; and passing current from the anode metal through the electrolyte to the aluminum-cathode whereby aluminum is removed from the anode metal and deposited in the-molten state on the cathode.

11. In the electrolytic refining of aluminum, the steps comprising establishing a lower layer of molten metal containing aluminum, as anode, and having a materially higher density than aluminum, an upper layer of molten substantially pure aluminum as cathode, and an intermediate la er of fused electrolyte composed of a suitab e mixture molten at 1050 C., capable of dissolving a substantial amount of alumina and having a density less than that of the anode metal but greater than that of the aluminum cathode metal; and passing current between the anode and the cathode.

12. In the electrolytic refining of aluminum, the steps comprising establishing a lower layer of molten metal containing aluminum, as anode. and having a materially higher density than aluminum, an upper layer of molten substantially pure aluminum as cathode, and an intermediate layer of fused electrolyte composed of a suitable mixture molten at about 950 C., capable of dissolving a substantial amount of alumina, and having. a density less than that of the anode metal but greater than that of the aluminum cathode metal; and passing current between the anode and the cathode;-

13. In the electrolytic refining of'aluminum, the step comprising electrolyzing an impure aluminum-bearing molten metal or alloy of relatively high density as anode, with a superimposed molten bath containing cryolite and suliicient barium fluorid to We the bath a substantially higher density t an aluminum and still permit the bath to dissolve a substantial amount of alumina.

14. In the electrolytic refining of aluminum, the step comprising elec'trolyzmg an impure aluminum-bearing molten metal or alloy of relatively high density, as anode, with a superimposed molten batl1 substantially free from chlorid and containing cryolite and sufiicient barium fiuorid to give the bath a substantially higher density than aluminum and still permit the bath to dissolve a substantial amount of alumina.

15. In the electrolytic refining of aluminum, the step comprising electrolyzing an impure molten aluminum-bearing metal or alloy of relatively high density as anode, with a superimposed molten bath containing aluminum, sodium, and barium fiuorids, in proportions adapted to give the bath a greater density than that of aluminum and permit solution of a substantial amount of alumina.

16. In the electrolytic refining of aluminum, the step comprising electrolyzing an impure molten aluminum-bearing metal or alloy of relatively high density as anode, with a superimposed moltenbath containing aluminum, sodium, and barium fiuorids, in pro ortions adapted to give thebath a greater ensi than that of aluminum and permit solution of a substantial amount of alumina, the ratio of aluminum fluorid to sodium fluorid in said bath being greater than in cryolite.

17. In the electrolytic refining of aluminum, the step comprising electrolyzing an impure molten aluminum-bearing metal or alloy of relatively high density as anode, with a'su rimposed molten bath containing barium fl il brid bet-wen 30 and 38 per cent, sodium fluorid between 25 and 30 per cent, and aluminum fluorid between 30 and 38 per cent.

18. Inthe electrolytic refining of aluminum, the step comprising electrolyzing an impure -molten aluminum-bearing metal or alloy of relatively high density as anode, with a superimposed molten bath of approximately the following ualitative and quantitative composition; barium fluorid between 30 and 38 per cent, sodium fluorid between 25 and 30 per cent, aluminum fluorid between 30 'and 38 per cent, and alumina in amount less than suflieient to saturate the bath at the working temperature.

19. In the refining of aluminum, the step which consists in subjecting to electrolysis a combination of aluminum with a heavier metalin the molten state as anode, and substantially'pure aluminum as cathode, said anode and cathode being gravitat-ively separated by an electrolyte containing cryolitc and a heavy fluorid of a metal not more readily deposited than aluminum in amount sufficient, to raise the specific gravity of the electrolyte without unduly decreasing its solvent power for alumina or its electrical conductivity, or increasing its freezing point.

20. In a process of electrolytically refining aluminum alloys with a fused electrolyte, compounding the electrolyte to give it a higher density than aluminum and a selective action in dissolving aluminum from the alloy.

21. In a process of electrolytically refining aluminum alloys with a fused electrolyte, compounding the electrolyte to give it a higher density than aluminum and a selective action in dissolving aluminum from the alloy, and deposit-ing from said bath upon a suitable cathode, aluminum of a high degree of purity.

22. In a process of electrolytically refining aluminum alloys with a fusedelectrolyte, compounding the electrolyte so as to limit the production of sodium at the cathode.

\ 23. In a proccss of electrolytically refining aluminum alloys with a fused eleetrol te, wherein current-carrying cans are emplbyed in contact with the mol en cathode, compounding the electrolyte to minimize the production at the cathode of substances which attack the current-carrying means, whereby the useful life of the latter is substantially prolonged.

24. In a process of electrolytically refining aluminum alloys with a fused electrolyte, wherein a molten anode, electrolyte, and cathode are arranged in gravitatively separated layers, with current-carrying means in contact with the cathode, compounding the electrolyte to limit the production at the cathode of substances which attack said ourrent-carrying means, and to maintain the gravitative separation of said layers.

25. In a process of electrolytically refining aluminum alloys with a fused electrolyte, wherein "a molten anode, electrolyte, and cathode are arranged in gravitatively separated layers, compounding the electrolyte to limit the production of sodium at the cathode and maintain the gravitative sepathe cathode and give the electrolyte a selective action in dissolving aluminum from the anode, and a density adapted to maintain the gravitative separation of said layers.

27. In a process of electrolytically refining aluminum alloys with a fused electrolyte, limiting the introduction of oxygen into the electrolyte by compounding the electrolyte so as to limit the production of metallic sodium at the cathode.

28. In a process of electrolytically refining aluminum alloys with a fused electrolyte, compounding the electrolyte to give the same, when fused, a greater density than molten aluminum at the same temperature and to remain fluid at permissible working temperatures without excessive volatilization.

29. An electrolyte composition for the purpose described containing aluminum and sodium fluoride, and .between 20 and160 per cent, approximately, of fluorid of an alkali earth metal having an atomic'weight greater than 80.

30. An electrolyte composition for the purpose described, substantially freefrom chlorid, containing aluminum and sodium fluorids, and between 20 and 60 per cent,

approximately, of fluorid of an alkali earth metal having an atomic weight greater than 80. p

31. An electrolyte composition for the purpose described composed essentially of fluorids and substantially free from other compounds except oxid, and containing aluminum and sodium fluorids, and between and 60 per cent, approximately, of fluori-d of an alkali earth metal having an atomic weight greater than 80.

32. An electrolyte composition for the purpose described containing aluminum and sodiumifluorids and between 20 and 60 per cent, approximately, of barium fluorid.

33. An electrolyte composition for the purpose described containing aluminum and sodium fiuorids and between 20 and 60 per cent, approximately, of fiuorid of an alkali earth metal having an atomic weight greater than 80, and less than about 2 per cent of fluorid of an alkali earth metal having an atomic weight below 80.

34. An electrol' composition for the purpose describe containin the followmg compounds in approximately the proportions named: alumlnum fiuorid 30 to 38 per cent, sodium fluorid to per cent, and barium fluorid 30 to 38 per cent. 35. An electrolyte composition for the purpose described containing aluminum and sodium fiuorids and between 20 and 60 per cent, approximately, of fluorid of an'alkali earth metal v havlng an atomic weight greater than 80,with less than about 7 per v cent of alumina. v A

v36. An electrolyte composition for the purpose described containin the following compounds in approximate y the propor tions named: alumlnum fluorid 30 to 38 per cent, sodium fluorid 25 to 30 per cent, and

barium fluorid 30 to 38 per cent; with less than about 7 per cent of alumina.

In testimony whereof we hereto afiix our signatures.

. WILLIAM HOOPES.

FRANCIS C. FRARY. JUNIUS D. EDWARDS.

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