CA1218958A - Molten salt electrolysis cell having a cathode sheath to retain liquid aluminum - Google Patents

Abstract

DIMENSIONALLY STABLE DRAINED ALUMINUM
ELECTROWINNING CATHODE METHOD AND APPARATUS
ABSTRACT
A method and apparatus for making a drained aluminum electrowinning cathode dimensionally stable. A
thin, ? to 10 millimeter coating of substantially stagnant molten aluminum is maintained upon the cathode surface by an openly porous sheath or membrane closely conforming to contours of the electrowinning cathode. The sheath or membrane is made from a material substantially resistant to corrosives present in the aluminum electrowinning; it may be only slightly aluminum wettable, but should be relatively electrically nonconductive.

Description

4/ :;~9/ ~33 TECHNICAL FIEI.D

This..invention relates to electrowinning of aluminum, particularly from cryolite and specifically to dimensionally stable electrodes for electrowinning aluminum and methods for their making.

BACKGROU?~D 0~ THE TNVENTION
- _ _ ~ Aluminum is ~ommonly oroduced by the electrolysis of A1203 at about 900C to 1000~. AluQinum oxid2 being electrolyzed is g~nerally dissolvea in molten Na3AlF6 (cryolite) that generally contains additives helpful to the electrolytic process such as CaF~, AlF3 and In the electrolytic cell, reduction of the aluminum oxide o~curs a~ a cathode qenerally positi~ned upon the bottom or floor of the Plectrolytic c.ell. Oxygen i5 liberated from electrochemically disassociating A12O3, and in commercial cells, generally combines with c~r~onaceous material comprising the cell anode ~nd s evolved from the cell as CO and CO2O
In many commercial cells, the ~athode is comprised of a material relatively resistant to corrosive ~ffects of contents of ~he cell such as cryolite. This cathode often covers su~stantially the @ntire floor of the 1~
",'';r

- 2 -cell which typically can be 6 feet wide by 18 or more feet in length.
Molten aluminum is a su~stance relatively resistant to corrosive and 501vating effec~s in ~n aluminum electrowinnins cell. In utilizing aluminum for cathode purposes in a cell, ty~ically the cathode is an assembly including a cathodi~ current feedex covered by a pool of aluminum ranging in depth, depending upon the cell, from a few inches to in excess of a foot. The aluminum pool functions effectively as a cathode and also serves to protect curren~ feeders made from materials less than fully resistant to cell contents. For examplP, unpro'ected yraphite used as a cathode can generate aluminum carbide an undesirable contaminant,whereas no such ~ 15 contamination occurs w~en it is used as a covered curren~
~eeder. .
These pool type cell c~thode assemblies contain conductive current collectors. Where these conductive current collectors are utilized in some cell configurations, these collectors contribute to an electrlcal current flow wi~hin the cell that i5 not perpendicular to the cell bottom These nonp~pendicul~r electrical currents can interact with strong electro-magnetic fields established around cells by current flow through busses and the like contributing to strong electromagnetic fluxes within the cell.
In cells employing a pool of aluminum covering the cathode floor of the cell, the crvolite, containing the Al2O3 to be eilectrolyzed, floats atop this aluminum pool. The cell anodes are immersed in this cryolite layer.
It is important that these anodes do not contact the aluminum pool, for such contact would resul~ in a somewhat dysfunc~i~nal short circuit within the c~ll. The electromagnetic flux within the cell contributes to the formation of wave motion within the alu~inum pool contained in the cell, making prediction of the exact . 3 ~2~ 3 -depth of the aluminum pool, and therefore the minimum necessary spacing between the anode and cathode current - collector and between the anode and the in~erface between aluminum and cryolite at any particular cell location somewhat imprecise. Therefore, cell anodes are positioned - within the cryolite to be substantially above the nor~al or expected level of the interface between cryolite and aluminum within the cell.
The combination of a substantial aluminum pool depth and a positioning of the anodes above the cryolite-aluminum normal interface position to forestall short circuits triggered, for example, by wave motion in the aluminum that would locally alter the aluminum pool depth, establishes a substantial gap betw~en the anode an~
cathode in most conventional cells. A portion of the - electrical power consumed in operation of the cell is somewhat proportional to the ma~nitude of this gap.
Substantial reductions in the magnitude of this gap would result in considerable cost savings via reduced cell electrical power consumption during operation.
In one proposal, a packing or filler material is introduced into the cell, generally to a depth nornally occupied by the aluminum pool. The packing tends to brea~
up wave motion within the cell making prediction of the position of the interface between the aluminum pool and the cryolite more predictable. Where the interface position is more reliable, the anodes can be positioned somewhat closer to the interface, promoting incrementallv reduced power consumption.
In such packed cells, however, the anode and cathode remain separated by a depth of cryolite, sufficient to forestall short circuits caused by localized disruptions in the aluminum pool depth existing notwithstanding the packing~ This separation can lead ~o a large electrical power inefficiency in operating the - aluminum elec~rowinning cellO Further, materials used for packing the cell must be substan~ially resistant to ; corrosive effects of cell contents. Such materials often i are costly, and therefore packing the lzrge numbers of - these spacious electrolytic cells necessary for producing aluminum can be economically burdensome.
Ano her pro~osed sollltion has been to employ so-called drained cathodes in constructing aluminum electrolysis cells. In such cells, no pool of aluminum is maintained upon a cathode current feeder to function as a ca~hode; aluminum drains from the cathode as i~ forms to ,~ 10 be recovered from a c~llection area~ In drained cathode cells, without wave action problems attendant to the aluminum pool, the anode and the cathode may be quite closely arranged, realizing significant electrical power savin~s.
In these drained cathode cells, however r the cathode or vulnerable cathodic current feeder often is in generallv continuous contact with molten cryolite. This agSressive material, in contact with a gra~hite or carbon : cathode, contributes to material losses from the cathode as we~l as the formation of aluminum , carbides, a dysfunctional impurity. Carbon or graphite for use as a drained cathoae ~aterial of construction is there~ore of quite limited utility due to service li~e constraints.
Other longer lived materials are, in theorv, available for use in a drained cathode, Generally these materials are both conductive and aluminum wettable re,fraotory materials such as TiB2. It has been found that unless TiB2 and similar materials ~re in essentially pure form, they too lose material or corrode at unacceptable rates in khe ag~ressive cell environment~ It is believed that the molten cryolite contributes to TiB2 corrosion by ~luxing reaction products of TiB2 and aluminum generated near grain boundaries of the material. While it is known that essentially pure TiB2 does not exhibit in aluminum elec~rowinning cells as substantial a corrosion susceptibility as does lower purity TiB2~ cost and availability factors seriously limi~ the use of TiB2 sufficiently pur~ to withstand the aggressive cell environment.
-DISCLOSUR OF THE INVE~NTION

Now, therefore, it is an obj~ct of the presentS invention to provide an ec~no~ical i~proved drained ca~hode for aluminum electrolysis, substantially dimensional~y stable, when used in an aluminum electrolysis cell.
It is a further object of the present invention to provide a method for making an aluminum electrolysis cell drained cathode to be di~ensionallv stable du~-ing aluminum elPctrolysis.
It is a still further object of the present invention to provide a cathode configu~ati~n permittin~
relatively close anode and cathode spacing, thereby permitting realization of substantial electrical power savings.
The improved cathode of the present invention presents an electrically conductive surface' to aluminum being electrowon thereon from molten cry~ e ~ontaine~
within the cell. The improvement comprises a sheath or membrane conforming closelY to the presented electrowinning surface~ The sheath or membrane at least covers those portions of the electrowinning surface upon which aluminum is being electrowon. The sheath or membrane is porous or apertured. This porosity is open, that is the apertures extend from one sheath or membrane surface through a thickness of the sheath to the other so as to form continuous fluid pathways between the surfaces.
Th'ese pores or apertures are of a size and configuration whereby aluminum is retained therein during eiectrolysis, in contact wi~,h the cathode presented surface but substantially stagnant within the p~res or apertures.
The shea~h or membrane is ormed from a material substantially resistant to corroslon by contents of ~he _ 6 ~

aluminum electrolysis cell. It is preferred - that the sheath or membrane be relatlYely nonelectrically conductive. It is desirable but not essen~ial that the sheath or membrane be - somewhat ~e~able by the mol~en aluminum being retained wi~hin the pores and thereby substantially coating the cathode with a film of aluminum.
.
A drained cathode used ~or aluminum electrowinning is therefore rendered relatively dimensionally stable by providing a substantial~y stagnant coati~g of molte~
aluminum upon the surface o~ such a cathode presented for the electrowinning process. In preferred embodiments, this coati~g or film retained upo~ the cathode electrowi~ g surface ls not less than about 0.5 millimeter and not greater than about 10.0 mlllimeters. Aluminum depositing upon the cathode in a depth greater than the sheath thickness continues to drain from the cathode surface to be recovered.
A drained cathode structure results from the practice of the instant invention.
Aluminum being electrolyzed ~ills the porous shëath thereby protecting the cathode substantially from contact with cryolite ` contained within the cell by pro~iding a substantially stagnant aluminum coating upon the cathode. The cathode is reudered less subject to corrosion and there~ore substan~ially dimensionally stable. Yet a narrow separation betwee~ anode a~d cathode within the cel 1 ~.an be maintained since substant~al wave motioR within the relatively thin aluminum coating pro~ided upon the cathode by the sheath is u~ ely.

I n another aspect of the inve~tion, the drained electrowinni~g sur~ace of a refraetory hard me~al boride, nitride, carbide or mi~tures or comblnations thereof has molten aluminum retained ln substantially stagna~t contact therewith by at least one piece of a substantially non-electrically conducti~e material selected from.Si3N4, BN, AlON, Si.410N, Al~ a~li AlBl ~. This piece can either be an apertured sheath, as described previously, or could be made up of several discrete pieaes o~
any suitable shape which ~re so arranged as to lea~e spaces in which the molten aluminum is retained in stagnant contact with the ele~ctrowinning surface.

The æbove and other features and advæntages of the invention will become apparent from the following detailed description o~ the invention along with the drawings of the invention and e~amples accompanying the detailed description, all forming a part of the speci~ication.

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8 ~Z~9~

DESCRIPTION OF THE DRAWINGS

- Figure 1 is a cross-sectional view taken transversely of an aluminum electrolYsis cell embodying the invention.
Figure 2 is an expanded view of a eathode shotln in ~igure 1.
FigurP 3 is an elevational cross-section of a ; cell portion immediately adjacent the aluminum electrolysis surface of the cathode showing a sheath confi~uration.
Pigure 4 is an elevational cross-section of a cell portion immediately adjacent ~he aluminum electrolysis surface GL the cathod~ showir.g an altærr.ate sheath configuration.

B:~ST EMBODIME~T O~ THE INVENTION

: The present invention provides a drained cathode structure for use in an aluminum electrolysis cell. The drained cathode is substantially dimensionally stable.
Referring to the drawi.ngs, an alu~inum electr~ly~is cell 20 10 is shown generally in Figure 1. The cell 10 includes an anode 12 and a cathode 14 contained within a houslng 16 that includes a liner assembly 18.
The housing 16 includes a shell 25 usually made from a suitable or conventional substance like steel.
Contained within the housing 16 is a liner asse~bly 18 that includes a layer 27 that generally resists aggressive attack upon the shell 25 by contents of the cell such as cryolite. In this best embodiment, the laver 27 functions also as a current conduc~or for supplying elec~rical current to the cathode 14. In equal7y preferred embodiments, this layer 27 can include embedded current conductors (not shownj for supplying electrical current to the cathode 14. Refractorv materials and graphite are ~a2~

suitable for fabricating this layer 27, as are other . suitable or conventional materials.
An insulating layer 2g is provided to resist heat flow from the cell 10. While a variety of well-known S structures are available for making this insulating s.ructure, commonly the insulating layer 29 ~nsists of crystallized contents of the electrolytic cell.
The anode 12 is fabricated from any suitable or conventional material and immersed in a cryolite phase 30 i ' 10 contained in the cell. Since oxvgen ions react at the anode, the material mus~ be either resistant to attack by oxygen or should be made of a material that can be agreeably consumed by the oYygen. Typically carbon,. or graphi_e i.s utilized. The anode 12 should be arran~ed for vertical movement within he cell 50 that a desired s~acing can be maintained between the ano~e and cathode notwithskanding the anode ~eing consumed by evolved .
oxvgen.
The cathode 14 is mounted in the cell in electrical contact with the conductive liner 27 or with conductors contained within the liner. Referring to ~igures 2, 3 and 4, i m~y be seen that the c--thode has a suxface 31 for electrolyzing aluminum. This sur~ace is covered by a sheath 33 or membrane having apertures 35 or bein~ openly porous. The porosity should communicate through the thickness of the sheath 33 so that alu~inum being formed by electrolysis fills the apertures 35 or pores. Once filled, the aluminum in the pores remains substantially stagnant with further electrolysis occurring not on the presented surface 31 bu~ upon a surface 37 defined by the filled poro~s sheath 33. Aluminu~ forming at this surface drains away to recovery areas 40,41' from which it is removed. Aluminum is ~aintained in the recovery areas 40,41 to a depth necessary to; insure immersion of edge portions 45 of the sheath 33.
By coating in this manner, the substance of the cathode is shielded from contact with cryolite. Once th2 cathode iS
.~
~. .

- 10 ~

shielded from the cryolite, a vaxiety of matexials can'be used in making the cathode that would otherwise be ~ ' unde~irable due to eleva~ed material losses in the aggressive cell environment.
Desirably~ refractory metal borides, carbides and nitrides are ,hereby rendered suitable for use in fabricating drained ca~hodes. For purposes of this invention, particularly of use are borides, carbides and ni~rides of: titanium; zirconium niobium; tungsten;
tantalum; molybdenum; silicon; as well as mixtures thereof~ Titanium boride of at least 97.5 percent purit;~
and TiB2 composited with other of the refractory metal boride carbides and nitrides are most preferred. While these materi21s can be prohibi~ively expensi~re wh~re consumed or corroded at a significant rate in an aluminum cell, once under a thin protective aluminum ~oating, theY
may be employed for electrolyzing for extended periods with little material losses. Any cathode surface selected should be both electrically conductive and at least significantly aluminum wettable.
In an equally preferred alternate' to the best em~odiment, the cathode includes a refractory met,al boride, nitride, or carbide layer 47 applied to a suitable or conventional electrically conductive substrate 49 such as graphite. Where the refractory layer 47 is TiB2 and i p~otected by maintaining an aluminum film or coating on the TiB2 surface using the sheath 33 or membrane, a particularly advantageous, substantially dimenslonally stable cathode structure results.
Since when using a dxained cathode structure~ no pool of aluminum exists in,which wave motion might cause a sXort between anode and cathode, the anode and cathode can be positioned closely opposing each other. This close positioning permi s cell operation at a reduced cell voltage, the anode being positioned in molten oryolite only a short distance from the sheathed cathode upon which molten aluminum is being electrolytically generated.
.

~ 8 - The sheath 33 or membrane can be of any suitable or conventional construction having a plurality of pores .- or apertures traversing iks thickness. The precise configuration can be an openl~ porous rigid foam 51/ 2 5 single layer honevc~mb structure, an interconnected cellular structure, or a bar r'nd grid arrangement 53 to name a few, dependin~ upon the material of construction.
The pores or apertur~s form intersLices in 'he sheath that fill with molten aluminum during electrolysis to coat the cathode surface 31.
The sheath 33 or membrane may be formed from an~
suitable or convention21 m~terial substantially inert to aggressive chemical ~ttack in the cell environm~nt.
Electrical conductivit~f is nct requisite. Preferably the material used for the sheath will be at least -~lightly wettable by aluminum ~o assist in fill ing Inters~ices in the sheath with molten aluminum. Particularly useful for making the shea~h or mem~rane are. Si3M4, BN, AlON, SiAlON, AlB12, AlN, TiB2, and combinations thereof, The sizing of pores 35 or aper.ures within the sheath 33 or membrane is critical to effective imDlement~tion of the instant in-~en~ion. '~h~ sheath or membrane sho~dbe su~stantially inriltrated with molten aluminum so that the molten aluminum forms a continuous elec.rical current pathway between the surface 31 of the cathode and cryolite phase 30 surrounding the shea~h. Yet alum7 num filling the sheath or membrane inters~ices should remain substantially stagnant avoiding circulation leading to significant contact between the molten cryolite phase 30 30 and the cathode surface 31. Since areas of khe cathode 14, below the aluminum liquid and in the recovery areas 40, 91 do not contribute substantially to aluminum electrowinning, they are no~ sheathed.
The ~hickness o~ the shea~h should pre~erably be such as to hold a thickness of between about 0.5 millimeter to about 10.0 millimeters of molten aluminum substantially stagnant upon the cathode surface ~1. Most ~2~

preferably, this thic~nes~ is between 1.0 and 2.5 millime ers.
~ Desirable cross setional dimensions o individual pores or aper~ures by necessitv vary widely as a function of aluminum, cryolite a~d sheath ~aterial interfacial tensions. Generally the more aluminum wettable the sheath material, ~he smaller the pores mav be made, and the less wPttable by aluminum the sheath - material, ~he larger the pores may be in cross-section.
The wide variance in these traits from one sheath mater~al to another requires individual determination o~ acceptable pore sizes for each sheath material of construction and cryolite phase formulation~ Generally a suitable pore will be fo~dha ~ g d~si~s, ~er ~ ~pth, ~etw ~ ~ut 25 microns and 50Q0 microns. It is to ~e expected that the thickness of the sheath 33 will impact upo~..the desirable pore or aperture 35 cross-sectiQnal dimension.
~ The following examples are offered to further illustrate the features and advantages of the invention.

EX~IPLE 1 Two ~luminum electrolysis cells are a~semble~ in accordance with Figure 1 and the best embodiment of the invention. A TiB2 til o~ 99 percent purity is used to form the refractory layer 47l adhered to a graphite substrate 49, thereby forming the cell cathode 14. A
sheath of grid configuration as shown in Figure 4 is placed upon the electrolyzing surface 31 of the cathode in one o~ the cells. The sheath is a plate 34.9 x 12.4 x 2.3 millimeters drilled to include a plurali~y of 2.6 millimeter diameter apextures. The sheath or grid is formed from BN. The cells are filled with eryolite ha~ing the composition (percen~ by weigh~) Na3A1~6 79~5~
A1~03 10.0%
CaF2 6.8%
AlF~ 3~7~

5~
_ 13 -and electrolvsis is commenced using a cell voltage of between about 2.98-3.27 volts D~C. at a current density of 0.5 amperes per square cen~imeter of cathode surCace.
Anode-cathode spacing is abou~ %.5 centimeters.
After lO operating hours, the cells are shut down and the Ti~2 tiles checked for material losses. The ~ile from the cell having sheath protection providing a layer or aluminum on the refractory laver 47 surface 31 is found to have a layer of 7 mils or less in ~hickness in which grain boundry corrosion was observed, whereas the tile from the unprotec,ed cathode is found to have suffered grain boundrv ~p~ corrosion losses of be~ween 2;
and 30 microns in thickness. In the cell having a protected cathode cur ent ef~iciency during alu~inum 1~ electrolysis was found to be 66.8 percent, this efficiency custo~arily being substantially greater when applied to commercial scale cells. The aluminum produced in the cell was found to be contaminated with 65 parts per million titanium.

EX~ ~LE 2 Cells identical to those of Example l are assembled and operated for lOO hours ~efore being shut down for evaluation of tile corrosion. The protected cathode is found to have suffered between 5 and ll microns ~5 corrosion of the TiB2 refractory layer 27, the unprotected cathode between 26 and 40 microns.
While a preferred embodiment has bee~ described in detail, it will be apparent that various modifications and altexations may be made thereto without departing from the scope of the appended claims~ Particularlv a great variety of drained cathode cell configurations are conceivable deriving substantial benefit from sheathed confi~uration providing a protective layer of molten aluminum upon the electrolysis surface 31, the subject of the instant in~ention.
.

Claims (16)

WHAT IS CLAIMED IS:

1. An electrolysis cell for electrowinning aluminum having a cathode presenting a drained electrically conductive electrowinning surface to contents of the cell, the cathode comprising a sheath closely conforming to contours of the presented surface at least where the presented surface contacts aluminum being electrowon, the sheath having a plurality of apertures traversing a sheath thickness, the apertures being of a size and configuration such that molten aluminum is retained within the apertures during electrolysis remaining substantially stagnant and in contact with the presented surface; the sheath being made of a material substantialy resistant to corrosion by contents of the electrolysis cell.

2. The electrolysis cell of Claim 1, the electrically conductive electrowinning surface being made from a material selected from refractory metal borides, nitrides, carbides, carbon and mixtures thereof; and the sheath being made of an electrically non-conductive material selected from a group consisting of Si3N4, BN, AlON, AlB12, SiAlON, AlN, TiB2, and mixtures thereof.

3. The electrolysis cell of Claim 2, the electrically conductive electrowinning surface being TiB2.

4. The electrolysis cell of any of Claims 1, 2 or 3, the apertures having no dimension other than depth smaller than 25 microns nor larger than 5000 microns.

5. The electrolysis cell of any of Claims 1, 2 or 3, the sheath being between about 0.5 and 10.0 millimeters in thickness.

6. The electrolysis cell of any of Claims 1, 2 or 3, the sheath being between 1.0 and 2.5 millimeters in thickness.

7. A method for making a drained aluminum electrowinning cathode surface dimensionally stable, comprising the steps of first providing on said cathode surface in close conformance to contours thereof a sheath, capable of retaining liquid aluminum; and then electolyzing a composition for electrowinning aluminum in contact with said sheath resulting in the formation and/or maintenance of a coating upon the cathode including substantially stagnant molten aluminum.

8 The method of Claim 7 wherein the stagnant coating is between about 0.5 and 10.0 millimeters in thickness.

9. The method of Claim 8, the electowinning cathode surface being made from one of carbon, and refractory metal carbides, nitrides and borides.

10. The method of Claim 9, the electrowinning cathode surface being made from TiB2.

11. The method of any one of Claims 8, 9 and 10 wherein the coating is between about 0.5 and 2.5 millimeters in thickness.

12. An electrode presenting a drained electrically conductive electrowinning surface for use in an electrolysis cell used to electrowin aluminum, the electrode comprising an openly porous sheath closely conforming to contours of the presented surface at least where the presented surface is in contact with aluminum being electrowon, pores within the sheath being of a size and shape such that during electrolysis molten aluminum is retained therein substantially stagnant and in contact with the presented surface, the sheath being substantially resistant to corrosive effects of contents of the electrolysis cell.

13. The electrode of Claim 12, the electrically conductive electrowinning surface being of a material selected from a group consisting of carbon and refractory metal borides, carbides, and nitrides and the sheath being made of a non-conductive material selected from a group consisting of Si3N4, BN, AlON, AlB12 SiAlON, AlN, TiB2, and mixtures thereof.

14. The electrode of Claim 12, the electrowinning surface being made of TiB2.

15. The electrode of any one of Claims 12, 13 or 14, the pores having no dimension other than depth smaller than 25 microns nor larger than 5000 microns.

16. The electrode of any one of Claims 12, 13 or 14, the sheath being between about 0.5 millimeter and about 10.0 millimeters in thickness.