US1534319A

US1534319A - Refining aluminum electrolytically with fused electrolytes

Info

Publication number: US1534319A
Application number: US608286A
Authority: US
Inventors: Hoopes William; Francis C Frary
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1922-12-21
Filing date: 1922-12-21
Publication date: 1925-04-21
Anticipated expiration: 1942-04-21

Description

Api'if 21, I 925.

W. HOOPES ET AL REFINING ALUMINUM ELECTROLYTICALLY WITH FUSBD ELECTROLYTES Filed Dec. 21, 1922 anwmtozg Mum/100F155 5 Hum/s 625mm "1 Patented Apr. 21, 1925.

UNITED STATES 1,534,319 PATENT OFFICE.

WILLIAM HO'OPES, OF PITTSBURGH, AND FRANCIS C. FRABY, O1 OAKIONT, PENNSYL- VANIA, ASSIGNOBB TO ALUMINUM COMPANY OF AMERICA, PITTSBURGH,

PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA.

FIRING ALUMINUM ELECTROLYTICALLY WITH I'UBED ELEOI'BOLYTR.

Application Med December 21, 1922. Serial No. 808,888.

To all whom it comm:

Be it known t l iat we, WILLIAM Hooras and FRANCIS C. Fnaar, both citizens of the United States of America, the said WILLIAM Hooves residing .at Pittsburgh, and the said FRANCIS C. Fnan'r residing at Oalcmont, both in the count of Allegheny and State of Pennsylvania, have invented certain new and useful Im )rovements in Refining Aluminum Electro ytically with Fused Electrolytes, of which the following is a full, clear, and exact description.

This invention relates generally to electrolytic refining, particularly of aluminum, by removal or separation of the metal from an alloy or a mixture thereof with'othcr substances, and relates more particularly to a refining process in which the electrolyte floats, in a moltenstate, upon the molten alloy; the latterbeing used as anode and the aluminum removed being deposited on a layer of molten aluminum as cathode floating on the electrol lnan extensive used aluminum reduction process, wherein aluminum is produced by reduction of its oxid, a c olite bath or electrolyte is used, but in t e herein described process, in which it is desired to have the refined metal, limit on the electrolyte, such a bath. though otherwise operable, can not 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 adeguate densit such addition easily leadsto iiliculties 6 various sorts, resulting in fail-, ure 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. We have found that success may be realized and a bath produced combining the desired 7 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 find that the only materials giving in high degree the other necessary or desirable characteristics are the fluoride of these metals (barium, strontium, calcium and magnesium These form with cryolite, mixtures w ich are readily fusible, but experience indicates that ma esium fluorid as less effect than any of t e others in increasing the density of the bath. Barium fluorid has been suggested as an ingredient of electrolytes for other urposes, but so far as we are aware no one as heretofore suggested their use for aluminum refining, or has given, with res act to the properties of barium fluorid an cryolite mixtures, the information needed for their successful use. It has been found, however, that within certain limits, mixtures of 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 ower of dissolving alumina are also As stated in the copending ap lication of William \Hoopes, Francis 0. and Junius D. Edwards, Serial No. 608.285, filed Dec. 21, 1922, the density of aluminum of 99.75 per cent purity is approximately 2.29 grams per cc. at 1000 C. At this temperature, the density of molten cryolite is approximately 2.10 grams or cc. In order to increase the density of t e cryolite, so that this aluminum will float pcrature mentioned, we have found that there must be added about 20 parts of barium fluorid to each 80 arts of cryolite. If calcium fluorid were to he used, about 40 parts would be required to parts of cryoite. While the above mentioned mixture of cryolite and barium fluorid is completely fluid at temperatures above 965 0., 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 fluid. This would make the working temperature of the bath undesirably and in some cases impracthe other hand, mixtures I radium isto be excepted, for obvious reasons. Thus mixtures of cryolite and bariumon it at the temfluorid containing between about and 00 per cent of the latter constituent by wei ht are readily fusible at temperatures be ow 1000" 0., 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 suilicientl light to float on any of a number of mo ten aluminum alloys suitable for 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 lies within the proper range of densities for our purpose,

utsuch mixtures containing more than about 40 per cent of strontium fluorid are in general not so readil fusible as mixtures containing correspon ing amounts of barium fluorid.

I tiiperating with a bath of the type describe 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 11 through the molten aluminum cathode, an accumulates in the crust which forms above, where .it 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 b the sodium, but we have found that this di ficulty may be decreased, without interfering seriously with the density of 'the molten bath, by increasing the proportion of aluminum iiuorid as compared to sodium fluorid,,although such an increase does 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 we recommend the one described and claimed in the above mentioned application of ourselves and Junius D. Edwards, Serial No. 608,285. This bath has approximately the following composition:

Per cent. Barium fluorid 30 to 38 Sodium fiuorid; to Aluminum fluorid 30 to 38 Alumina -Q 0. 5 to 3 Calcium and magnesium fluorid present as unavoidable impurities) about Such a bath is completely molten at all temratures above 900 C. and enables the re ing to be carried on at about950 C. At

this temperature the bath is satisfactorily stable, has good electrical conductivity and satisfactory density, and ma dissolve as much as 7 per cent of its wei t of alumina.

It will be understood that e sodium and aluminum fluoride of the electrol ma be iupphed, in parzzi at least, b cryo 'te, J1 ich as a com 1 on ere sees as 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 advantafizous to provide a distinct margin of safety tween the two densities so t at if the cell should become overheated the bath will not become so light as to permit the aluminum top layer to sink to the bottom. The density of a 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.

An electrolyte composed of cryolite saturated with alumina, and another composed of cryolite and barium chlorid have been suggested heretofore, but we have found that. the addition of alumina to cryolite fll? tualiy decreases rather than'increases the density. The addition of barium chlorid does increase the density of the bath but not so rapidly as does the addition of fiuorid of calcium, strontium or. barium. However, a bath composed of 40 per cent barium chlorid and 00 per cent cryolite has a density such that at 1000 0., aluminum of a purity of 99.75 per cent will barely float on it, but if the temperature be raised to 1050 C. its density decreases to such an extent that it is then lighter than aluminum and the latter will therefore sink. In order to obtain a density of about 2.48 at 1000 (1., which would give a density-difference of about 0.19 between the bath and the aluminum at 1000 C., and a difference of about 0.17 at 1050 C., it would be necessary to use a mixture of about parts barium chlorid and 40 parts cryolite.

While the freezing points of such mixtures are advantageously low, they labor under the disadvantages of producing an excessive amount of fumes at temperatures in the neighborhood of 1000 0., and of being poor solvents of alumina. Thus the above mentioned bath containing 60 per cent barium chlorid can dissolve only about 1 per cent of alumina, On the other hand a ath containing cryolite and 60 per cent barium fluorid (instead of thechlorid) will dissolve between 4 and 5 per cent of alumina: and a similar bath containin 40 per cent barium fluorid, which would ave a specific gravity of about 2.7 3 will dissolve between 8 and 9 r cent of alumina at 1000" C. Electra ytes of such type are therefore advantageous for the fol owing reasons.

Alumina becomes more soluble in any of the cryolite baths as their tem ture is raised, but if alumina be add until the bath is saturated '12 will be found that a Lbsaaze small drop in temperature will cause some of the alumina to precipitate out as corundum 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 at a temperature distinctly lower than that of the main body of the bath, so that if sufiicicnt 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 the form 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 difficult to re-dissolve it in the bath. A certain amount of such crust on the interior of the cell is desirable for its thermal and electrical insulating properties (as described more fully in the copendin a lication of William Hoopes, Junius dwards and Basil T. Horsfield)5 but to prevent the formation of an undesired amount of this deposit, which would otherwise radually fill up the cell and interfere witfithe operation, it is important to keep the alumina content below the saturation oint. In order to maintain this condition it is therefore desirable, in practice, to have the bath capable of dissolving considerable alumina, so as to allow for unavoidable variations incident to operating conditions; for the reason that any of the following causes may 0 rate to add alumina to the baths: (A) e hydrolysis of aluminum fluorid by moisture. Capillary action continually brings some'of the bath 11 between the top metal layer and the side ozt the call, so that it forms a crust on the top of the metal, where it is maintained high temperature and exposed to the The excess of bath above that which can solidify to form this crust drips back through the metal 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 'um is always liberated at the cathode, and some of it rises throu h the metal layer, probably in the form of vapor, and reaches the top crust where it is oxidized by contact with the air. (G) The 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 1m rtant is it to have the bath unsaturated' lhat in actual commercial operation it is generally necessary to remove at a alumina. from time to time. This may be conveniently done in various ways, as by one 0; another of the methods described hereina ter.

The cell preferred for use in the present process of refining aluminum is of the type described and claimed in copending app ication of William Hoopes, Serial No. 608,287 filed December 21, 1922. One form of this cell, with the addition of means for reducing the alumina content of the bath, is illustrated in the drawings annexed hereto, in wliich- 1 is a (plan view of the cell. Figs. 2 an 3 are cross sections on lines 2-2 and 3-3, respectively, of Fig. 1.

Figs. 4 and 5 are detail cross sections on lines Hand 5-5,.respectively, of Fig. 1, illustratin the water connections to and from and tween the water jackets.

Fig. 6 is a detail cross section on line 66 of Fig. 1, showin the method of connecting the upper electrodes to the negative busbars.

Fig. 7 is a detail 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 electrical y.

Fig. 8 is a ing a suitable the electrolyte.

Fig. 9 is a detail sectional view on the same plane as Fig. 2, illustrating the refractory electrically and thermally insulatin crust which is produced and maintains above the molten cathode layer.

The lower shell or shell section 10 is preferabl made of steel in the form of a cylindrica vessel of considerably greater diameter than hei ht, and at or near its to it is provided wit a. water jacket 11 which is most conveniently the upper edge of the shell section an outward y extending flange 12 of suitable width, and a flaring or conical ring 12 welded or otherwise ermetically joined to the underside of the flange and to the body of the shell below. a

Above the lower shell section 10 is an upper shell section 13 which may also be of steel and formed with hollow walls to provide an lpppr water jacket 14. The inner surface 0 t e upper shell section is preferably flaring, as indicated. To keep the sections electrlcalflly insulated or separated from each other a at ring or gasket 15, of asbestos .or other suitable material, may be used between the two.

In order to give the shell structuresufficient mechanical strength the sections may be secured together by means of machine studs 16 passing upwardly through the flan e 12 and threaded into pads 17 welded on t e bottom of the upper shell inside the detail sectional view illustratanode for use in deoxidizing formed by providing at be subjected to a high they can be made of 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 satisfactory for the pur ose.

Suitable water connections lbr the water jackets are provided, and for the sake of simplicity and conveniencethese connections may be so constructed and arran d that the water flows through the two ja'c etc in succession, preferably through the lower jacket first. For this purpose the jacket 11 may be provided at the bottom with an inlet nipple 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 isled 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 sufficient 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 auxite, 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 material, preferably 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 ma e by tamping into the shell a mixture of tar, pitc and granular or powedered 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 urpose of baking and solidifying the carnaceous mass.

Good electrical connection may be provided between the shell and its bottom lining by means of metal collector lates 31, welded to the inner surface otthe ell so as to be electrically and mechamcally continuous therewithwill be, the bushings and These plates extend mwardly into the bottom lining which is molded around them. At the plane of the collector lates the shell may be provided on the outsi e 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, of copper, may be bolted tightly in place. The busbars may be in the form of long fiat plates 33 embracing the lower shell section, with their ends brought out at one side of the cell for convenient connection to one terminal of a suitable source not shown) of continuous or unidirection current. During the refining operation thesebusses 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 11 per electrode may be multiple, as indicat composed preferably of a suitable number of short thick :rods 34 of graphite, arranged vertically and having co per or other metal rods 35 threaded or 0t erwise suitabl secured to the tops of the electrodes. ese metal rods serve to support the u per electrodes and convey current to or m the same, and for this purpose they may be releasably and adjust ably secured as by means of clamps 86, to metal busbars 37 extending horizontally across the cell. For convenience of access to the electrodes, 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 framework. The latter may rest on the upper shell section, in which case they are insulated from the shell section, as b any convenient and suitable means, not own.

It is recognized that, strictly speaking, the aluminum layer floatin on t e bath and the layer of alloy under yin the bath, are the upper and lower elect es respectively, but these layers are term herein the cathode and the anode, and hence it is deemed permissible as well as convenient to refer to the phite cylinders and the carbon bottomor their equivalents, as the upper and lower electrodes.

Metal or other molten material ma be withdrawn from the upper portion 0 t1 cell through a tapping notch 39, which may be closed by means of any suitable refracto material. Molten metal or other ma terial ma. be withdriiwn from the lower art of t 0 cell through a port or tapping hole 40, which may be closed by means 0 aplugofdensech On the inside giglhefcrzll itsha sitdghnggtg extending upw' m e ca 11 tom 29, over the joiiit between the shell aluminum or other suitable metal sections and well up toward or even over the top of the u per shell section. This side lining shoul be thermally and electrically insulatin to decrease or minimize the conduction 0 heat to the water jackets as well as to prevent by-passing of current around any part of the cell contents undergoing electrolytic treatment in the refining operation. The lining should also be refractory enough to remain solid at the tem ratures to which it is subjected in the e cotrolytic 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 above mentioned application of Hoopes, Edwards and Horsfield, has been found highly satisb in practice.

In t e 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 mo ten 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, however, may be put in operation in the following manner.

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 or granulated bath material placed around them. The upper electrodes are raised as the melting proceeds, and additional bath material is sup lied, until a suflicient bed of fused electro yte 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. It should be supplied in suflicient amount so that it will remain in an electrically continuous layer on the bottom of the cell throughout the refining operation. A bath layer 'of sulficient de th should be used so that the top metal (are pure aluminum) will in no case come into contact with any portion of the factory1 side crust which has previously been covered by the anode alloy. It is to be noted in this connection that the of the anode alloy, incident to the refining operation, cause correspondin changes in its volume and in the position 0 the upfir and lower surfaces of the bath layer. olten aluminum, preferably the urest obtainable, is laced on the molten ath, to serve as This is continued until changes in composition pf the crust The refining process can now be begun with the alloy as anode and the top metal as cathode, the current being led from the top metal by means of graphite electrodes dipping into it. Under t ese conditions aluminum is dissolved out of the anode alloy and deposited in molten form on the cathode. the desired amount of aluminum has been removed from the anode and added to the cathode. A portion of the to metal is then removed and the impoveris ed anode allo is withdrawn through the tap hole 40, fiesh anode alloy in the molten state bein supplied in any convenient wa prefers lyesuch that the oatin on t bath will not e contaminated. is operation may be conveniently performed by means of a carbon funnel, which, after bei preheated, is let down until it nearly reac es the bottom of the cell, which has preferably been cut out of the circuit. The refined metal entrapped in the funnel may be di ped out with a hand ladle, after which a fresh anode alloy is poured in. The funnel is then lifted out and the refining process resumed. The fresh anode alloy introduced is preferabl suflicient in amount to raise the bath an top metal until the surface of the latter is at the same level as before the withdrawal. These operations may be re ated from time to time as necessary or esirable without seriously interrupting the refining p which otherwise can 0 on continuously.

Notwithstanding e greater density of the bath, a ortion of it is carried up by capillary action at the area of contact betweeen the liquid aluminum and the solid boundary crust and rises to the surface of the former, where it spreads in a thin layer, the weight of which is insuflicient to overcome the surface tension of the liquid aluminum. Consequently it spreads over the entire surface of the latter, and by reason of the escape of heat into the air, solidifies there, forming a top-crust such as indicated, for example at 58 in Fig. 9. This process oes on until the crust thickens so much that ?the escape of heat being thus retarded) the temperature of its under surface can rise to the melting point of the bath. When this thickness 1s attained, quantities of unsaturated bath subsequently carried up By capillary action can accumulate in liqui form under the crust and finally gow to a mass of sufiicient dimensions to able to sink through the aluminum. Hence, if the bath is kept unsaturated with alumina, the top crust forms up to a certain thickness, after which its growth On the other hand if the freezing point of the bath is by allowing it to become saturated, In aid bath finding its we to the under sur artia 1y solidifies there and thickness. This action would,

refined metal in the if unchecked, result eventualli in bringing up a large portion of the bat from below the aluminum and causing it to attach itself to the top crust. At the same time the boundary crust at the sides of the cell thickens as previously described. and the net result would ultimately be more or less complete solidification of the bath. Moreover, i the alumina content of the bath is too hi h a crust of greater or less extent is apt to orm between the bath and the cathode, or between the bath and the anode, or in both these places. In most cases the crust is attached to the cell lining, and extends inwardly somewhat in the manner of a shelf. In general, such a crust is objectionable, as will be pointed out more fully hereinafter. For the above reasons it is desirable to keep the bath unsaturated in the normal operation of refining.

The bath crust formed on the aluminum layer as above described serves as a convenient and 00d heat insulating medium to minimize oss of heat from the top of the cell, so that a cell cover or lid of metal, refractory brick, or the like, is in general unnecessa and hence can be omitted, thus eliminatm the difliculties and disadvantages inci ant to the use of a cover. The crust described entraps sodium as already explained with consequent increase of alumina in the bath, but the amount of sodium which thus esca s from the bath can be minimized by using in the latter the highest rmissible amount of aluminum fluorid.

Instead of forming the heat-insulating to crust in the manner hereinbefore spec' call described, such a crust may be produced y dusting over the u per surface of the aluminum la or, soon a r it is put in lace, a layer of iinely divided alumina, carn, magnesia, or other suitable powdered material. This layer of finely divided material is rapidly cemented together b the liquid bath coming up from below an wetting it. The heat insulating property of the to crust ma be increased by dusting an suitable pow ered material over it after it as been formed, so that it is covered by a layer of such material, which is an excellent insulator by reason of its porous condition. Bein sup lied to the surface of the to crust a ter t e latter has solidified, the additional heat-insulating material is not cemented together and therefore retains its porosity. In general, the best material for the purpose is owdered bath, since if any of it accidenta y or incidentally finds its way below the top metal it does not contaminate the electrolyte.

Mention has been made herein of a crust that may form between the bath and the cathode metal, and between the bath and a the anode alloy. In the latter case the area of contact between bath and anode is diminass-gait ished, with the result that the current density at the active anode surface is increased. Too high a current density at the anode surface causes the aluminum thereat to be dissolved in the electrolyte faster than it can be brought to the surface from the body of the alloy, thus producing a surface im overishment of the latter, as a result of which the tendency of more electronegative elements to go into solution (as for exam le iron and silicon) is increased. On the ot er hand, if an adequate area of contact is maintained between the bath and the anode alloy there is not only less likelihood of such elements going into solution but also greater likelihood of their being reprecipitated by contact with the anode, where the aluminum reprecipitates them and thus prevents them from contaminating the bath. Similarly, the presence of a crust between the bath and the cathode metal diminishes the area of contact between these bodies and thus increases the cathode current density. Unduly high current density here tends to impoverish of aluminum the bath surface in contact with the cathode, with the result that metals other than aluminum (as for example, in the resent instance, alkali earth metal and so ium) ma be (permanently deposited. Under norma con itions, however, such metals when deposited on the cathode are largely re-dissolved at once by secondary reaction with the bath and are replaced by their equivalent of aluminum, except in so far as sodium escapes from the cathode in the form of vapor. The described decrease in the area of conducting contact surface between the bath and the two metallic layers, increases the electrical resistance of the cell; and consequently, sin:e in practice the cells are 0 erated in series on a circuit with substantiall constant current, the formation of this she f crust results in a rise of voltage across the cell terminals and therefore an increase in power consumption in the cell, without increasing the metal output. As previously stated, the formation of the shelf crust is largely due to too high a content of alumina in the bath.

Several methods are available for keeping the alumina content of the bath below the saturation point. For example the top metal (aluminum) can be ladled or tapped off and a portion of the saturated or nearly saturated bath dip ed out, liquid or solid alumina-free or dleoxidized bath being added to take the place of that which was removed. The resulting mixture will then be well below the saturation point. Or a portion of the top crust can be broken away and re moved, whereupon the top crust will reform at the expense of the saturated bath within the cell. New alumina-free or deoxidized bath can be added either in solid or liquid form to take the place of that which lll inseam went to form the new to crust. In the first method the saturated ath removed from the cell can be regenerated and prepared for re-ent into the process by crushing it and electro yzing it in a separate pot for the reduction of the alumina, the de-oxidized bath thus obtained being stored for use when required.

Another method of preventing saturation of the bath with alumina is to tie-oxidise the alumina continuously, or from time to time, for example by electrolyzing the bath according to the Hall process of producing aluminum from alumina. This may be accomplished by placing a carbon electrode in contact with the bath and connecting it with the positive terminal of the cell, thus making the carbon electrode an anode. Any current leaving the carbon anode serves to electrolyze alumina in the usual manner, depositing aluminum on the cathode metal, or on the anode metal, or on both, depending upon the voltage used, the oxygen bein liberated at the anode and formin CO, wit a portion of the carbon. The car 11 dioxid gas so produced bubbles up through the top metal and may be artially reduced to monoxid but neverthe ess serves to carry ofl some of the oxygen from the bath and therefore to deprive it of alumina, thus decreasing the alumina to a point below saturation. In ractice the introduction of a carbon ano e into the bath presents some difiiculty, for the reason that carbon is lighter than the bath and hence must be forcibly held down in the latter, and for the further reason that any device used for holding the electrode down necessarily passes through the top metal. One way is illustrated in Figs. 1 and 8 of the drawings. In these fi ures 50 represents a carbon disk into whic is threaded a carbon stud 51, into the up r end of which is threaded a water-coo ed iron terminal 52. The latter is screwed into the bottom of a pi 53 which serves to support the termina and the disk and also to supply the elcctrolyzing current and the cooling water. At its top the pi e is fitted into t e underside of a closed c amber 54: through which a water supply pipev 55 proects down into the pipe and well to the ottom of the latter. Water thus introduced into contact with the iron terminal 52 rises around the ipe 55 and flows out of the chamber 54 y way of pipe 56. The pipe 53 is fastened on a support 57 in such mannor as to hold the carbon disk 50 submerged in the bath below the aluminum layer 48. The en port 57 is shown as composed of two channe irons arranged back to back. It may be insulated from the frame 88, as indicated at 57. Around the carbon stud 51, water-cooled terminal 52, and the lower end of pipe 53 is an insulating and refractory crust 58 which may consist of a mixture of .current is suiliciently1 bath and corundum previously cast in place. This crust serves to prevent the aluminum top metal from making contact with the carbon disk or with the electrically associated parts, and thereby prevents a shortcircuit between the top metal and the deoxidizing anode. The latter can be supplied with current from an independent source, not shown, or it can be electrically connected with the positive bus 33 in any convenient manner, preferably through a suitable circuit-breaker, not shown, from which current may be carried by means of a cable 59, Fig. 1, connected with the pipe 53. In practical operation it is usually sufficient to tie-oxidize the bath intermittently, depending up'on the rate (as determined by experience) at which oxygen finds its way into the bath. If the voltage of the tie-oxidizing lower than that of the main current (whic is supplied to the cal hon bottom of the cell) the former current will flow entirely between the de-oxidizing anode'and the cathode aluminum and the aluminum resulting from tie-oxidation of alumina will be deposited mostly if not entirely on the cathode. Oil the other hand, if the de-oxidizing voltage is hi her than the main voltage (between the cefi terminals) part, at least, of the de-oxidizing current may flow between the de-oxidizing anode and the metal below the electrolyte, thus de siting some of the aluminum in the atter metal.

Another method of? de-oxidizing the electrolyte is to remove the top la er of aluminum and reverse the electrica connections, making the upper electrodes the anode and the layer of metal at the bottom the cathode. Alumina is then reduced as in the Hall process, and when the operation has proceeded as far as desired normal conditions are resumed.

The energ -efiiciency in electrol tic processes of re ning aluminum is ependent largely upon the perfection of the measures taken for reventing escape of heat. Theoretically most no energy is re uired for the refinin but practically, in t e absence of some ot er adequate source of heat sufiicient. electrical ener y must be expended to maintain the ano e, the bath, and the cathode in a fused condition, and consequently the amount of electrical ener y 'which must'be sup lied is almost exactly t e equivalent of the eat permitted to escape. After the heat insulation of the cell has been perfected to the maximum practicable extent, nothing further canbe accomplished in limitation of the amount of heat escaping from a heated body of given dimensions, and with the minimum heat-loss the ener input uired by the cell will also be a minimum. n the interests of ower economy the cell should be opera at the lowest maintain the anode, the

practicable voltage. Accordingly the electrol te, which furnishes the major portion of the resistance, should be in as thin a la er as is permissible, and it has been found t lat a layer from 2 to 4 inches thick is in general satisfactory. With a bath or electrolyte of any predetermined workable depth, the current density permissible varies between a lower limit which is sufiicient to bath. and the cathode in a molten state, and an upper limit at which volatilization of the bath is excessive, orat which too large a proportion of anode impurities go into solution. These limits, with the various bath-compositions which have been found practicable, are approximately 800 (1. and 1100 C., respectively, with a preferable working temperature of about 950 C. The permissible lower limit of current density also varies inversely with the dimensions of the cell, since the heat loss per unit of volume in a large cell is less than that in a small cell on account of the smaller ratio of heat-dissipating area to the volume.

In a cell having a cross section through the electrolyte of 9.6 square feet it has been found that the preferable current is, in general, 8500 amperes, but that the process is workable with currents between 7500 and 12000 amperes. The preferable current density in a cell having the electrolyte crosssectional area mentioned is therefore 885 amperes per uare foot, with a permissible minimum of :1 out 780 amperes and a permissible maximum of about 1250 amperes, per square foot. 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 com osition of the bath, the conductivity of the ath, and the effectiveness of the heat insulation, the present electrolytic refining rocess is workable with current densities tween about 500 and about 2500 am eres er square foot of cross section of the ath. n general the lower practical limit of Voltage is about 3.5 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 ma be protected against oxidation in the air y means of a non-oxidizable coating which may consist of molten bath material ap lied to the rods-as a. thin layer and allowe to freeze thereon.

' Practically any alloy of aluminum which has a density greater than that of the electrolyte and remains suflicientlyfiuid or mobile throughout the operation ma be refined by our process. Preferably, owever, we use an alloy of which the principal components are aluminum and copper.

The layer of aluminum floating on the molten bath or electrolyte should be of sullicient expanse to touch the side lining of the cell around the entire perimeter thereof and should be thick enough to insure firm contact with this lining, in order to prevent or minimize volatilization of the bath, which occurs to a greater or less extent at working temperatures and increases as the temperature rises. On account of the surface tension of molten aluminum the top layer should be of substantial depth, and it is therefore desirable to maintain a thickness of at least two inches at all times. I

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

We claim- 1. In the electrolytic refining of aluminum, the steps comprising electrol zing a molten body of impure metal or al oy containing aluminum, as anode, with a superimposed fused bath containing aluminum fluorid, and removing alumina from the bath as may be necessary to keep thebath unsaturated therewith.

2. In the electrolytic refining of aluminum, the method of maintaining the electrolyte fluid at the operating temperature,

taining aluminum, as anode, with a super-,

im osed fused bath containing aluminum an sodium fluorids, and removing alumina from the bath as may be necessary to keep the bath unsaturated therewith.

5. In the electrolytic refinin of aluminum, the ste s comprising e ectrolyzing with a fused uorid bath, and treating the bath from time to time to keep it unsaturated with alumina.

6. In the electrolytic refining of aluminum with a fused bath containing aluminum and barium fluoride under conditions causing accumulation of alumina in'the bath, the step com rising treating the bath electrolytically rom time to time to prevent saturation thereof with alumina.

7. In the electrol tic refining of aluminum with a fused ath containing alumi- 1mm and barium fluorids under conditions causing accumulation of alumina in the bath, the step comprising removing alumina from the bath as may be necessary to keep the bath unsaturated therewith.

8. In the electrolytic refining of aluminum with a fused bath containing aluminum and barium fluorids under conditions causing accumulation of alumina in the bath, the steps colnprisin electrolytically de-oxidizing alumina in tie bath and depositing the resulting aluminum on the cathode, to keep the bath unsaturated with alumina.

9. In the electrol tic refining of aluminum with a fused liath containing aluminum, sodium and barium fluorids under conditions causing accumulation of alumina in the bath, the steps comprising electrolytically de-oxidizing alumina in the bath and depositing the resulting aluminum on the cathode, to keep the bath unsaturated with alumina.

10. The method of combined refinin and reduction, which consists in electrolytically refining impure metal acting as anode in a fused electrolyte, and reducin material from said electrolyte by current mm a carbon electrode immersed in said electrolyte.

11. The method of simultaneous refining and reduction in a single cell, which consists in electrolytically depositing substan tially pure metal from a molten anode containmg the metal in metallic form, upon a molten cathode in a suitable molten bath, and reducing oxid of said metal from said bath by current from a deoxidizing electrode immersed in said bath, and de siting the metal so produced on said catho 12. In electrolytic refining, the steps comprising electrol zing with an im ure metal anode in a bat capable of disso vin oxid of said metal and keepin said 'bat unsaturated with said oxid y electrolyzing with a carbon anode immersed therein.

13. The method of operating an electrolytic cell which consists in electrolyzing with a soluble electrode and a fused bath to deposit metal from the latter, and also electrolyzing the bath by means of an oxidizable electrode to reduce oxid in said bath.

14. The method of refining aluminum which consists in passing current from a body of molten alloy containing aluminum, as anode, to a molten body of aluminum as cathode, throu h an intermediate bathof molten electro yte capable of dissolving alumina whereby aluminum is removed from the anode and deposited on the cathode and maintaining the fluidity of said bath at working tem rature by intermittently removin alumina therefrom,

15. the electrolytic refining of aluminum, the steps comprising assing current from a body of molten a oy containing aluminum, as anode to a molten body of aluminum as catho e, through an intermediate buth of molten electrolyte containing aluminum, sodium, and barium fluoride whereby aluminum is removed from the anode metal and deposited. on the cathode; and passin from an unfused oxidizable anode throug 1 the bath to the molten cathode, to electrolytically deoxidize the alumina in the bath and add the resulting aluminum to the molten cathode.

16. In the electrolytic refinin of aluminum, the steps comprising assmg current from a body of molten a 10 containing aluminum, as anode, to a mo ten body of aluminum as cathode, through an intermediate molten bath containing aluminum, barium, and sodium fluorids'whereby aluminum is removed from the anode metal and deposited on the cathode; and from time to time passing electrol zing current throu h the bath to the oath e from a carbon ano e' in contact with the bath, to deoxidize the alumina in the bath and thereby revent too great accumulation of alumina t erein.

17. In the art of refining by electrol sis, the improvement comprising forming a vs a molten electrode floated on a molten bath a heat-insulating crust com osed at least in part of frozen bath materia and thereafter restricting the thickness of said crust by controllin the composition of said bath.

18. In t e art of refinin by electrolysis, the improvement compgising electrolyzing a molten bath lying a vs a molten anode in a cell having heat-insulating side surfaces, and de ositing the refined metal on a molten cath e floated on and covering said bath, and establishing above said cathode a heat-insulatin crust substantially contin- 105 uous with sai heat-insulating side surfaces whereby volatilization and heat losses are minimized.

19. In the electrolytic refining of aluminum, the steps com rising establishing a lower layer of mo ten alloy containing aluminum, as anode, an upper layer of molten aluminum as cathode, and an intermediate laycr of molten electrol to containr ing cryolite and a metal fluori serving to increase the density of the electrolyte; estab-- lishing above the molten aluminum layer a heat-insulating top-crust composed, at least in part, of molten electrolyte co up from below and freezing above the a minum; passing current through the three layers mentioned so as to remove aluminum from the lower and add it to the upper layer; and treating the bath to remove alumina, whereby excessive rise of the freezing point of the bath and consequent excessive thickening of the top-crust, are prevented.

20. In the electrolytic refining of aluminum, the steps comprising establishing a m lower layer of molten alloy containing aluminum, as anode, an upper layer 0 molten aluminum as cathode, and an intermediate layer of molten electrolyte containing cryolite and a metal fluori serving to increase the density of the electrolyte estab' lishing above the molten aluminum a er a intermediatelayer of molten electrolyte conv taining aluminum fluorid;

taining aluminum fluorid; establishing above the molten aluminum layer a heat-insulating top-crust composed, at least in part, of alumina and of molten electrol te coming up from below and freezing a vs the aluminum; and controlling the composition of the electrolyte to prevent excessive thickening of the top-crust by progressive freezing of electrolyte thereon.

22. In the electrolytic refining of aluminum, the steps comprising passing current from a lower layer of molten alloy containing aluminum, as anode, to an up er layer of molten aluminum as cathode, rough an intermediate layer of molten electrol conesta lishing above the molten aluminum layer a heat-insulating top-crust composed, at least in part, of molten electrolyte coming up from below and freezing above the aluminum; and controlling the composition of the electrolyte to prevent excessive thickening of the top-crust by progressive freezing of electrolyte thereon.

23. In the electrolytic refining of aluminum, in which molten cathode aluminum, electrolyte, and anode alloy containing aluminum, are arranged in downwardly successive layers, the steps comprising establishing above the top layer a heat-insulating crust of suitable thickness and composed, at least in part, of molten electrolyte coming up from below and freezing above thetop layer, and controlling the composition of the electrolyte to prevent excessive thickening of the crust.

24. In the electrolytic refining of aluminum, in which cathode aluminum, electrolyte, and anode alloy all in the molten state, are arranged in downwardly successive layers, the step com rising establishing above the top layer a eat-insulating crust composed, at least in part, of electrolyte brought up from below and frozen above the top layer.

25. n the electrolytic refining of aluminum, the ste s com rising establishing a molten lower a or o alloy containing aluminum, as ano e, an up er cathode layer of molten aluminum, an an intermediate layer of electrol containing aluminum, sodium and bar um fluorids; establishing above the aluminum la er a heat-insulating top-crust composed, at east in part, of electrolyte coming up from below and freezing above the aluminum; passing current from the anode alloy through the electrolyte to the aluminum cathode; maintaining the electrolyte unsaturated with alumina to prevent excessive thickening of the top-crust; and maintaining the aluminum layer sufiicient in expanse to cover the entire layer of electrolyte and thereby diminish volatilizetion of the latter and escape of sodium vapor to the top-crust.

26. In the electrolytic refining of aluminum, in which cathode aluminum, electrolyte, and anode alloy all in the molten state, are arranged in downwardly successive layers, the improvement comprising establishing on the aluminum layer a heat-insulating crust of suitable thickness composed, at least in part, of molten electrolyte coming up from below and freezing above the aluminum layer, andmaintaining the aluminum la er suflicint in expense to cover the entire ayer of electrolyte and thereby diminish volatilization of the latter.

27. In the electrolytic refining of aluminum, in which cathode aluminum, electrolyte, and anode alloy, all in the molten state, are arranged in downwardly successive layers, the step comprising maintaining the aluminum layer sufiicient in expanse to cover the entire layer of electrolyte and thereby diminish volatilization thereof.

28. In the electrolytic refining of aluminum, in which cathode aluminum, electrolyte, and anode alloy all in the molten state, are arranged in downwardly successive layers in a suitable cell or vessel, the step comprising maintainin the aluminum cathode layer sufiicient in e th and'expanse to make firm contact with t e sides of the cell or vessel and cover the entire layer of electrolyte.

29. In the electrolytic refining of aluminum, in which cathode aluminum, electrolyte, and anode alloy, all in the molten state, are arranged in downwardly successive layers in a suitable cell or vessel, the step comprising maintaining the aluminum cathode aycr at least about ,2 inches thick and sufficient in expanse to make firm contact with the sides of the cell or vessel and cover the entire layer of electrolyte.

30. In the electrolytic refining of aluminum, in which cathode aluminum, electrovaluminum on a mo 31. In the art of refining by electrolysis,

the improvement comprising forming above a molten electrode floated on a molten bath a heat-insulating crust composed at least in part of frozen bath material.

In the electrolytic refining of aluminum, the step comprising electrolyzing a fused bath and de siting the separated aluminum on a mo ten aluminum cathode floating on the electrolyte and completely inclosed in fused and frozen bath-ingredients.

33. In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath and de ositing the separated Ilzen aluminum cathode floating on the bath, and maintaining around and above the cathode an inclosure composed of frozen bath-ingredients.

34; In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath in contact with molten metallic bodies as anode and cathode; and maintaining adequate area of contact between the fused bath and the molten metallic bodies, and an inclosure composed of frozen bathingredients around and above the cathode.

35. In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath in contact with molten metallic bodies as anode and cathode, and maintaining adequate area of contact between the fused bath and the molten metallic bodies, and above the cathode a heat-insulating crust com osed of frozen bath-ingredients.

36. In t e electrolytic refining of aluminum, the steps comprising clectrolyzing a fused bath in contact with a molten aluminum cathode and a molten aluminum-alloy anode, and maintaining adequate area of contact between the fused bath and the anode. i

37 In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath in contact with molten metallic bodies as anode and cathode, and regulating the composition of the bath to prevent formation of crust between the bath and said metallic bodies.

In testimony whereof we hereto afiix our signatures.

WILLIAM noorns. FRANCIS o. FRARY.

lyte, and anode alloy, all in the molten state, are arranged in downwardly successive layers, the step oomprising maintaining the aluminum layer su 'cient in expanse to cover the entire layer of electrolyte and thereby diminish volatilization thereof, while maintainirig around and above the aluminum layer aninclosure composed of frozen electrolyte.

31. In the art of refining by electrolysis, the improvement comprising forming above a molten electrode floated on a molten bath a heat-insulating crust composed at least in part of frozen bath material.

In the electrolytic refining of aluminum, the step comprising electrolyzing a fused bath and de ositing the separated aluminum on a 1110 ten aluminum cathode floating on the electrolyte and completely inclosed in fused and frozen bath-ingredients.

33. In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath and depositing the separated aluminum on a molten aluminum cathode floating on the bath, and maintaining around and above the cathode an inclosure composed of frozen bath-ingredients.

34; In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath in contact with molten metallic bodies as anode and cathode; and maintainmg adequate area of contact between the fused bath and the molten metallic bodies, and an inclosure composed of frozen bathingredients around and above the cathode. 35. In the electrolytic refining of alumi num, the steps comprising electrolyzing a fused bath in contact with molten metallic bodies as anode and cathode, and maintaining adequate area of contact between the fused bath and the molten metallic bodies, and above the cathode a heat-insulating crust com osed of frozen bath-ingredients.

36. In t e electrolytic refining of aluminum, the steps comprising olectrolyzing a fused bath in contact with a molten aluminum cathode and a molten aluminum-alloy anode, and maintaining adequate area of contact between the fused bath and the anode.

37. In the electrolytic refining of aluminum, the steps comprising electrolyzing a fused bath in contact with molten metallic bodies as anode and cathode, and regulating the composition of the bath to prevent formation of crust between the bath and said metallic bodies.

In testimony whereof we hereto affix our signatures.

WILLIAM HOOPES. FRANCIS o. FRARY.

Oortiflcate of Correction.

It is hereby certified that in Letters Patent No. 1,534 319, granted April 21, 1925,

upon the a. w Oakmont, em lvania trolyticallymith used requiring'correction as follows: insert the word a as 1 an m] a an error appe 9, line 72, c aim 15, after the word passing current; and that t e said Letters 1: 1s correction. therein that the same may conform to this IBth-day of August, A. D. v

lieot'on of William Hoopes, of Pittsburg and Francis C. Frank of for an improvement in lecfilectrolyges,

" Refining Aluminum" ars in the printed specification Patent should be read with the record of the case in the KARL FENNING, Acting C'mnmiaeiootr of Patents.

Certificate of Correction.

Itis hereby qertified that in Letters Patent No; 1,534 19, granted April 21, 1925, upon the a igljeation of William Hoopes, of Pittsburg and Francis C. Frarv, of Oakmont, lvlmia for an improvement in Refining Aluminum" Electmlyticallpmith used'illectml es, an error appears in the printed specifieation requiring'correction as follows: a 9, line 72, claim 15, after the word passin insert the word current; and that t e said Letters Patent should be read with t is correction therein that the same may cbnform to the record of the case in the Patent Ofiiee. w

SiFned and sealed this 18th'day of August, A D. 1925.

' m1 KARL FENNING,

Acting Uommiuhgcr of Patents.