CA2113696C - Non-consumable anode and lining for aluminum electrolytic reduction cell - Google Patents

Abstract

An oxidation resistant, non-consumable anode, for use in the electrolytic reduction of alumina to aluminum, has a composition comprising copper, nickel and iron.
The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode. The electrolyte is preferably composed of a eutectic of AlF3 and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF
and LiF.

Description

NOZJ-CONSUMABLE ANODE AND LINING
FOR ALUMINUM ELECTROLYTIC REDUCTION CELL
Background Of The Invention The subjf~ct matter described herein is related in a general ;sense to that described in Beck, et al.
U.S. Patent No. 5,006,209 ('209) issued April 9, 1991 and entil=led "ELECTROLYTIC REDUCTION OF
ALUMINA".
The presE~nt invention relates generally to the electrolytic reduction of alumina to aluminum and more particularly to an anode and to a lining for the cell used in the electrolytic reduction process.
The aforementioned Beck, et al. '209 patent is directed to a method and apparatus for the electrolytic reduction of alumina to aluminum. The electrolytic reduction is performed in an electrolytic reduction vessel having a plurality of vertically di:~posed, non-consumable anodes and a plurality of ~rertically disposed, dimensionally stable cathodes in closely spaced, alternating arrangement with the anodes. The vessel contains a molten electrolyte bath composed of (1) NaF+A1F3 eutectic, (2) KF+A1F3 eutectic and (3) LiF. In one embodiment, a horizontally disposed, gas bubble generator is 7_ocated at the vessel bottom, underlying the cathodes and the spaces between each pair of adjacent electrodes.
Finely divided particles of alumina are introduced into the bath where they are maintained in suspension in the molten electrolyte by rising gas bubbles generated at the anodes and at the gas bubble generator, during the electrolytic reduction process. The horizontally disposed, gas bubble generator may be an auxiliary anode or anode part 21 13fi96 located at substantially the bottom of the electrolytic :reduction vessel, in contact with the molten electrolyte bath, or it may be in the form of a gas sparger for bubbling air or nitrogen upwardly from the vessel bottom.
The molten electrolyte bath has a density less than the density of molten aluminum and less than the density o:. alumina. Metallic aluminum forms at each of the c<~thodes, during performance of the electrolytic reduction process, and the metallic aluminum flows downwardly as molten aluminum along each cathode t=oward the bottom of the vessel where the molten aluminum accumulates. The molten electrolyte bath is maintained at a relatively low temperature in the range of about 660oC to about 800oC (1220°-_L472oF). The molten electrolyte has a composition which provides a relatively low anode resistance, a~roids excessive corrosion of the anode and avoids deposition of bath components on the cathodes.
The anodes disclosed in the aforementioned Beck, et al. '209 patent are composed of copper or of nickel ferx-ite-copper cermet. The electrolyte bath disclosed in the Beck, et al. '209 patent produced reduced corrosion on copper anodes, compared to tree corrosion produced by other electrolyte bath compositions. However, the corrosion rate for the copper anodes was still subject to improvement.
Attempts have also been made to employ, as a non-consumable anode composition, a nickel ferrite-copper cermet. In this connection, see U.S. Patent Nos. 4,399,00~~ and 4,620,905, for example. However, a nickel ferrite-copper cermet anode has also proved to have significant drawbacks, and it has not proven to be feasible for the electrolytic reduction of alumina to aluminum on a commercial scale. U.S.
Patent No. 4,~a99,097 discloses an electrolyte cell for the electrolytic reduction of alumina to aluminum, and this cell employs an anode composed of a foundation metal which can be, among others, copper, nicke:L, steel or combinations thereof.
The cell employed in conventional processes for the electrolyt=ic reduction of alumina to aluminum comprises a vessel for containing a molten electrolyte usually composed of halides. The vessel has an external shell and has an interior lined with various mater=Lals. The bottom of the vessel has a layer of refractory material, e.g. alumina, adjacent the external ;hell, and the interior is lined at the bottom with carbon or graphite blocks. The walls of the cell also are lined with carbon or graphite blocks, but unlike the bottom, the walls are not insulated with a refractory material.
The seam: between the blocks are filled with carbon paste. During operation of the cell, the molten electrolyte penetrates into any unfilled seams or void: or cracks in the interior lining.
Penetration of: the electrolyte into the lining causes the lining to deteriorate. Penetration occurs up to ~i level called the freeze line, which is the level on the uninsulated walls where enough heat is lost from the molten electrolyte to cause it to freeze. Generally, there is a frozen ledge at this level and above, composed of solidified electrolyte and alumina.
After 1,000 to 3,000 hours of operation, the interior lining of the vessel deteriorates to the point where it. must be replaced. Disposal of spent lining removed from the vessel is a problem, with 21 13695 -' piles of spent lining accumulating around aluminum reduction pla::-its .
In a cel:L of the type disclosed in the aforementioned Beck, et al. '209 patent, an excess of alumina is introduced into the molten electrolyte, and the resulting bath composition allows the usE~ of alumina refractory brick to line the interior walls of the vessel. Because the walls are thus thermally insulated, the frozen ledge is eliminated, which is desirable. However, the alumina brick: which line the walls on the interior of the vessel are subject to the same penetration problems as c<~rbon blocks, even though the alumina blocks will 1<~st longer.
It would be desirable to have an interior lining for the vessel which is not subject to electrolyte penetration, which is easy to replace, which can be readily recycled and which allows the entire vessel to be thermally insulated.
Summary Of The Invention The present invention relates to a composition for a non-con:~umable anode to be used in conjunction with an electrolytic reduction cell, preferably a cell of the t~~pe described herein. An anode having a composition in accordance with the present invention, when used in conjunction with the electrolytic x-eduction cell described herein, at the very least regains all the features and advantages enjoyed as a x-esult of employing the cell and electrolyte bath composition of the Beck, et al.
'209 patent. In addition, the anode has improved resistance to corrosion by oxidation in the molten electrolyte bath, compared to other anode compositions i-n the same bath.

More particularly, the present invention provides a corrosion-resistant, non-consumable anode having a comp~~sition consisting essentially of, in wt.o, about 2.~-70 copper, about 15-60 nickel and about 1-30 iron. Preferably, the anode composition consists essentially of, in wt.o, about 45-70 copper, about 25-48 nickel and about 2-17 iron.
Most preferab:Ly the anode composition consists essentially o:E, in wt. o, about 45-70 copper, about 28-42 nickel <~nd about 13-17 iron.
Another :Feature of the present invention is a cell vessel interior lining which is impervious to penetration b~~ molten electrolyte, which can be readily replaced and which may be readily recycled.
The lining co~~ers the bottom and walls of the vessel interior and is composed of metal having the same composition as the anode composition described in the preceding paragraph. Located between the external shel_L and the interior metal lining of the vessel is refractory material, such as alumina or insulating fi~_.e brick, which thermally insulates the bottom and wa=Lls of the vessel. The interior metal lining is electrically connected to the anodes, and the lining then constitutes part of the anode arrangement. During operation of the cell, oxygen bubbles are generated at the bottom and elsewhere on the interior rletal lining when the latter is part of the anode arrangement, and these bubbles help to maintain in suspension in the molten electrolyte the finely divided alumina particles introduced into the cell.
The anodes of the present invention may be fabricated from sintered metal powders to produce an anode having ~i porous surface and a density substantially less than the theoretical density for a given composition (e. g. 60-700 of theoretical density). These less dense anodes have a resistance to corrosion by oxidation, when immersed in the cell's electr~~lyte, which is greater than that of anodes having a substantially higher density, e.g.
above 900 of ~~heoretical density; this effect is probably due 1~o a lower actual current density at the surface o:E the less dense anodes. However, the denser anodes have a greater resistance to oxidation in air.
Preferab:Ly, a cell in accordance with the present inveni~ion employs, as an electrolyte, a eutectic or near-eutectic composition consisting essentially o:E 42-46 molo A1F3 (preferably 43-45 molo A1F3) and 54-58 molo of either (a) all NaF or (b) primarily NaF with equivalent molar amounts of KF or KF plus LiF replacing some of the NaF.
Other features and advantages are inherent in the subject matter claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompany=Lng diagrammatic drawings.
Brief Descript=ion Of The Drawings FIG. 1 is a vertical sectional view of a test cell employed for determining the corrosion-resistance of a non-consumable anode having a composition in accordance with the present invention;
FIG. 2 i~~ a vertical sectional view of a test cell employed for determining the performance of a non-consumable anode lining;
FIG. 3 i:~ a triangular compositional diagram for copper-nickel-iron, showing isooxidation lines, for sintered ~~nodes .

2~ ~3s9s 7 _ FIG. 4 is a triangular compositional diagram for copper-nickel-iron, showing isooxidation lines, a region of blister corrosion and a region of high electrical re~~istance, for sintered anodes;
FIG. 5 i~~ a triangular compositional diagram for copper-ni~~kel-iron, showing isooxidation lines arising from oxidation in air, for induction melted anodes; and FIG. 6 i;~ a vertical sectional view of an electrolytic :reduction cell in accordance with an embodiment of the present invention.
Detailed Description Anode oxidation tests of various alloys were performed in ~~ test apparatus or cell indicated generally at :~0 in Fig. 1. Apparatus 10 is a laboratory ce=L1 comprising a fused alumina crucible 11 having a volume of 500 cm3 and containing an anode 12, a cathode 13, and a molten electrolyte bath 14.
Alumina crucible 11 is positioned within a stainless steel retaining can 15. Cathode 13 is a 4 mm-thick slab of TiB2 with an immersed area of about 20 cm2 or a TiBz rod having a diameter of 23 mm and a length of 100 mm with an immersed area of 23 cm2. Anode 12 is in the form oi_ a metal disc overlying and substantially covering the bottom 16 of crucible 11.
A vertical copper conductor 17 has a lower end connected to disc 12 and an upper end connected to a source of electric current (not shown). Vertical conductor 17 ~_s insulated with an alumina tube 18 so as to confine the anodic current to test disc 12.
The apparatus of Fig. 1 was placed in a furnace and held at a temperature of about 750oC. The temperature of. bath 14 was measured continuously ..

with a chrome-alumel thermocouple contained in a closed-end, Based alumina tube.
The electrolyte composition generally consisted essentially o:E, in parts by weight, 66 A1F3, 26 NaF, 8 KF, and 3-~~ LiF. Corresponding mol percents are 46.7 A1F3, 36.7 NaF, 8.3 KF and about 8.3 LiF. About parts by wE~ight of alumina, having a mean particle size of about two to ten microns, were added to the electrolyte bath. The total bath 10 weight, including added alumina particles, was about 350 grams.
When cur:=ent of about 20 amperes was supplied, aluminum meta_L was produced at cathode 13 by electrolysis. Molten aluminum dripped off of cathode 13 and formed an irregularly shaped ball 19 which rested on anode 12 and was levitated by oxygen bubbles issuing from anode 12. There was no evidence of reaction of anode 12 with the metal of ball 19.
Test run; were performed typically for 6-7 hours.
A more detailed description of an electrolytic reduction process of the type involved in these tests is contained in the aforementioned Beck, et al. '209 patent.
The anodes were composed of various commercial alloys and special alloys prepared for testing.
Using a sintering procedure, copper-nickel-iron anodes were m~ide by premixing metal powders in the desired ratio and then heating, in a boron nitride-coated graphite die, to 1180oC in an argon atmosphere fox- at least one hour. The powders had a particulate size of 4 to 60 microns, but particle size is not important if the alloy is melted.
Pressure may or may not be applied to assure gas displacement from the powder mixture and to increase density. Depending upon the melting temperature of the compositi~~n, a temperature of 1180oC will either sinter the powder mixture to form a disc or cause the powder mi:Kture to melt into a disc.
It had previously been determined that several cycles of (a) current-on (e. g. for 2-5 minutes) followed by (h) current-off (e.g. at least one minute), at the beginning of a run, gave lower cell voltage and a lower rate of anode oxidation, compared to runs without such an on-off procedure.
In this connection, see the Beck et al. '209 patent at col. 11, lane 65 to col. 12, line 5. When testing the anodes here, this on-off procedure was utilized on same of the test runs employing the electrolyte described above.
At the end of each test run, there was oxide adhering to the anode, reflecting oxidation during the test run. This oxide was hammered off the anode, and the resulting anode weight loss was determined. ~f'lhe weight loss is expressed as an oxidation rate: g/cm2 h or mg/cm2 h.
Tabulated below are the results of the anode oxidation testis on anodes produced by the sintering procedure. Some of the anode compositions were tested more than once, and in such instances the oxidation weicfiht loss indicated in the table is the average for those tests. In all instances, the numbers have been rounded off to the nearest whole number. An anode composed of 1000 copper was used as a comparison base. As noted above, the bath employed in tree tests which produced the results tabulated below contained a LiF addition of 3-4 wt. o. A bath with a LiF addition substantially 21 1369fi higher than 4 wt.o will produce increased corrosion weight loss.

An ode Composition Oxidation Weight Wt . o Loss mg/cm2 h Cu 100 20-40 Cu 90:Ni 2.5:Fe 7.5 40 Cu 90:Ni 5.O:Fe 5.0 39 Cu 90:Ni 7.5:Fe 2.5 40 Cu 80:Ni 5.O:Fe 15.0 83 Cu 80:Ni lO:Fe 10 11 Cu 80:Ni l5:Fe 5 6 Cu 80:Ni 20 14 Cu 70:Ni 7.5:Fe 12.5 97 Cu 70:Ni l5:Fe 15 3 Cu 70:Ni 22.5:Fe 7.5 8 Cu 60:Ni lO:Fe 30 77 Cu 60:Ni 20:Fe 20 3 Cu 60:Ni 30:Fe 10 1 Cu 60:Ni35:Fe 5 1 Cu 60:Ni 40 9 Cu 50:Ni 25:Fe 25 3 Cu 50:Ni 37.5:Fe 12.5 1 Cu 50:Ni~~5:Fe 5 1 Cu 50:Ni 50 5 Cu 40:Ni 25:Fe 35 12 Cu 40:Ni 35:Fe 25 13 Cu 40 : Ni~~5 : Fe 15 3 Cu 40:Ni 55:Fe5 4 Cu 30:Ni 35:Fe 35 12 Cu 30:Ni 52.5:Fe 17.5 4 Fig. plotting, on the 3 was obtained by cross Cu-Ni-Fe results tabulated composition diagram, the above. The figure shows isooxidation lines, and the numbers on the isooxidation lines are mg/cm2 h. As reflected by fig. 3, the center of the area of minimum corrosion weight loss occurs at an anode having a comp~~sition of, in wt. o, about Cu 55:Ni 35:Fe 10.
Fig. 4 shows some regions of alloy composition which produce undesirable results other than mere oxidation. T:~e low-nickel alloys in region 1 suffer a catastrophi~~ blister corrosion producing blisters of metal oxide filled with a mixture of metal oxide and electrolyte. The low-iron alloys in region 2 produce a higu-resistance, oxide surface layer on the anode. The high-resistance of alloys in region 2 in Fig. 4 m;~y exclude alloys in that region from use with the :rest of the low oxidation-rate alloys reflected by :E'ig. 3 and Fig. 4, or one may be required to operate at a lower actual current density for an anode composed of a low-oxidation rate alloy in region 2.
Some unc~=rtainty in the oxidation rate occurs for alloys wit=h less than 50a copper because of increasing porosity observed in the test anodes sintered at 1:L80~C. At 50o copper, the density was about 600 of theoretical, and at 40o copper the density was about 50% of theoretical. Low density means high porosity. Nevertheless, despite the high porosity of the sintered 40-50o copper test anodes, oxidation was limited to the anode surface because the electrolyi~e bath penetrated the anode and filled the pores. In contrast, oxidation rates for the same porous tE~st pieces tested in air at similar temperatures, without employing a bath, were extremely high because of internal oxidation.
Further nests were conducted with the apparatus of Fig. 1, under conditions similar to those used in the initial tests described above, but employing a different electrolyte. The anode in these further tests had a c~~mposition consisting essentially of, in wt.o, Cu 50: Ni 37.5: Fe 12.5. In one test, the electrolyte c~~nsisted essentially of a eutectic composition o~ 44 mole A1F3 (61.1 wt. o) and 56 mole NaF (38.9 wt.s), and the oxidation weight loss of the anode was 3 mg/cm2 h. In two other tests, the electrolyte c~~nsisted essentially of a near-eutectic composition of 45 mole A1F3 (62.1 wt. o) and 55 mole NaF (37.9 wt.'o), and the oxidation weight loss was 2 mg/cm2 h and = mg/cmz h, respectively. An advantage of employing 'the two electrolytes used in these further tests is that it was unnecessary to use the on-off start-up procedure required when using the electrolyte employed in the earlier tests described above.
In another series of tests, conducted without electrolyte, high-density, alloy buttons or discs having compositions covering essentially the whole Cu:Ni:Fe diagram were prepared by melting the alloys at about 1400~~C in an induction furnace and then solidifying the molten alloys into buttons. The alloys were mE~lted in graphite crucibles, some internally uncoated and some internally coated with boron nitride. The button dimensions were about 12 mm in diameter and about 7 mm thick. Densities of the buttons were greater than 950 of theoretical.
Air oxidation tests (without employing a bath) were performed by ;subjecting the buttons to a temperature of 800oC for ~~ period typically in the range 8 hours to 280 hours. Air oxidation tests of one such button were pE~rformed for a period of over 5 months.
weight loss of alloy due to oxidation in air was measured and converted to equivalent weight loss for a time pe:=iod of 7 hours, to properly compare a.

with the data for anode weight loss in electrolyte reflected in Fig. 3. Isooxidation lines derived from this test are shown in Fig. 5. The region of low oxidation rate in Fig. 5 is generally similar to that shown in Fig. 3, but the region extends further to lower copp~=r concentrations. The lowest oxidation rates are along a line that is approximately three parts nickel to one part iron, which is generally consistent with Fig. 3. In the low nickel re~3ion, the air oxidation rates shown in Fig. 5 are not as high as the oxidation rates shown in Figs. 3 and 4 which reflect the oxidation of anodes in ele~~trolyte, producing blister corrosion in the low nickel region (region 1 in Fig. 4).
On the b~~sis of the foregoing considerations, a desirable anode composition, resistant to oxidation weight loss, comprises, in wt. o, about:
Cu 25-70 Ni 15-60 Fe 1-30 This composition is located within the area defined by isooxidation ring B in Fig. 4 and has an oxidation weight loss no greater than about 5 mg/cm2 h after 6-7 hours.
For a gi~,ren Cu content in the range 25-70 wt. o, an alloy also having about three parts of Ni to one part Fe generally appears to produce better oxidation resistance than an alloy having other ratios of Ni <~nd Fe ( Fig . 5 ) .
Preferab=Ly, the proportions for the anode composition are, in wt. o, about:
Cu 45-70 Ni 25-48 Iron 2-17 2~ ~3s9s This composition is located mostly within the area defined by is~~oxidation ring A in Fig. 4 which has an oxidation weight loss no greater than about 1 mg/cm2 h.
Most preferably, the proportions for the anode composition a:re, in wt. o, about:
Cu 45-70 Ni 28-42 Fe 13-17 This composition is mostly within that part of ring A, in Fig. 4, which excludes higher resistance area 2.
In addition to the anode compositions tabulated above, other compositions were tested, but the oxidation wei<~ht loss for each of these other compositions Haas extremely high, in comparison, and rendered these other compositions unusable. These other composii~ions include 304 stainless steel, 93 Cu:7 A1 aluminum bronze and Hastelloy X* (22 Cr:9 Mo:20 Fe:0.15 C:bal. Ni).
In the o:~idation tests in electrolyte, it was found that thE~ thickness of the alloy layer which oxidized after 6-7 hour runs, and which was removed from the meta=L anodes as oxide, was in general agreement with the weight loss of the anode after removal of the oxide. This finding indicates that there was no :significant dissolution of oxide from the anode into the bath in a 6 to 7 hour test run, and therefore dissolution of oxide is not a significant factor in determining oxidation rate by measuring weicfiht loss after a seven hour run.
With respect to metal buttons having compositions un the above-described, desirable *trade-mark weight proportion of Cu 25-70:Ni 15-60:Fe 1-30, it was determined that the weight loss of such a button due to oxidation in air at 800oC for 6-7 hours, was comparable to the oxidation weight loss of the above-describ~=d anode due to oxidation in the above-described ele~~trolyte bath at a temperature of 750oC
for 6-7 hours, namely a weight loss of not substantially greater than about 5 mg/cm2 h, or less.
For other compositions, having low nickel contents and exhibitin~~ blister corrosion (region 1 in Fig.
4), there was no such correlation between oxidation weight loss in the electrolyte bath and in air.
However, as to compositions of the type described two sentences above, because of the aforementioned correlation it is possible to obtain a reasonable approximation of the oxidation weight loss over an extended period (e.g. months), due to oxidation in the electrolyte bath, by determining the weight loss, for such a period, due to oxidation in air.
More part=icularly, air oxidation tests were performed, ovf~r various time periods, on buttons having a 70 Cu: l5 Ni:l5 Fe composition. One such test was conducted for over five months on a button formed by melt=ing. These tests produced data which, when plotted <~s oxidation weight loss versus the square root o:E time, produced a substantially straight line for times greater than about one day, from which onE~ could extrapolate oxidation weight loss for a year.
For a densified composition which was obtained by melting (950 of theoretical density), the air oxidation los:~ for one year, by extrapolation, would correspond to the amount of oxide produced by the corrosion of ~~ metal layer 1 mm thick, and this is an acceptable amount. For a less dense composition (910 of theoretical density), air oxidation tests were conducted over a time period of about one week, and the air o:Kidation loss was substantially greater, by a factor of ten, than that of the densified com~~osition. The increased oxidation in air of the less dense composition is attributed to internal oxid;~tion. This emphasizes the importance of providing ;~ densified composition when the anode is composed o:E a mixture of metal powders and a high resistance to oxidation in air is the desired characteristi~~. The desired high densification may be obtained either by melting the powders or, when sintering, by applying to the powders pressure sufficient to produce a density corresponding substantially to that obtained by melting. As stated earlier, though, bath penetration protects porous anodes against oxidation in the electrolyte bath.
Although the oxidation loss for one year, extrapolated :From the air oxidation data described above, constitutes the oxide corroded from a metal layer 1 mm thick, other data suggest that oxidation loss in an electrolyte bath could be substantially less, e.g. the=_ oxide corroded from a metal layer about 0.3 mm i~hick. More particularly, the extrapolation producing the one year oxidation loss of 1 mm of mei~al was based on air oxidation data from tests conducted over a period of time in excess of five month;, on dense buttons having a density of at least 950 of theoretical density. Tests conducted on anodes of the same composition and density, in an electrolyte bath for seven hours, produced only about one-third the oxidation loss produced by tests conducted in air for the same time period. If the same differential occurs at longer time periods, from one day to in excess of one week, an extrapolation of the date which would be produced by tests in t:he electrolyte bath for that period would indicate a one year loss of about 0.3 mm of metal.
It is ex~~ected that the oxide forming on the anode will di~~solve in the electrolyte bath at a certain rate ;end maintain a steady state thickness and oxidation rate after a certain period to time.
Since the thi~~kness, on the anodes, of the metal layers which underwent oxidation, agreed with the weight loss for the anodes at a time of 6-7 hours, the dissolution rate of the oxide is assumed to be less than l00 of the relevant thickness at that time. The di;~solution rate would then be equal to the oxidation rate when the oxidation rate is ten times smaller than at 6-7 hours. Such a reduced oxidation rata would occur at a time two orders of magnitude lon<~er than 6-7 hours, or about a month.
Calculations indicate that the oxide dissolution rate at one month is only about O.llo of the aluminum ~~roduction rate when the nominal anode and cathode current density is 0.5 A/cmz. This gives a projected mE=tal contamination rate of about O.llo which would be=_ acceptable for commercial practice.
As previously noted, a low density anode having a relatively porous surface is subject to penetration bar the electrolyte bath and exhibits lower corrosion due to oxidation, when immersed in the electrolyte, than does a denser anode having a relatively imporous surface. As described above, high density anodes (e. g. 950 of theoretical density) are obtained from molten alloy or by sintering metal powders at relatively high temperatures and pressures. Low density anodes 21 13696 v (e. g. 60-700 of theoretically density) are obtained by sintering metal powders at lower temperatures and pressures (wh:ich can be determined empirically).
It is be:Lieved that the difference in oxidation rates in the f~lectrolyte, between low density and high density <~nodes, is due to differences between the anode's a~~tual current density and its superficial current density. For a given current (expressed in amperes) and a given rectangular anode, the superficial current density (amps/cm2) on an anode surface is dependent upon the straight line dimensions of the surface, from edge to edge. For a rectangular surface, the superficial surface area equals the straight line length times the straight line width of that surface, and the superficial current densit=y equals the current divided by the superficial a~~ea of all anode surfaces. Thus, for an anode having a relatively high density (e. g.
greater than X350 of theoretical density) and a relatively non-porous surface, the actual surface area and the ;superficial surface area are essentially tree same, and so are the actual and superficial current densities. However, for an anode having a relatively low density (e.g. 60-70a of theoretica7_ density) and a substantially porous surface with a multitude of depressions, the actual area of an anode surface is substantially greater than its superficial area, and therefore the actual current density for that anode is substantially smaller than its superficial current density. The data suggests that the rate of oxidation and the anode voltage drop decrease with decreasing actual current dens i t:y .
For a given superficial current density, there is a minimum bath temperature below which anode t.. n:
a ', a .

resistance and voltage increase substantially, due to a type of <~node effect which also occurs with graphite anodes in the same bath. Examples thereof are reflected in the following tabulation.
Current Density, amps/c~m2 Bath Temperature, oC
0.1 690 0.5 715 This increased anode resistance establishes a lower limit on the operating temperature of the bath.
Referrincfi again to Fig. 4, region 2 thereon (the region oi= high anode resistance) is for low density (i.e. 50 to 900 of theoretical density), sintered anodes immersed in electrolyte. For high density, induction melted anodes immersed in electrolyte, t;he upper boundary line 3 for region 2 swings upward and to the left of ring A so that all of ring A is within high resistance region 2. A
desirable anode composition for a high density anode which has a relatively good resistance to oxidation, and which is outside blister region l, consists essentially of., in weight percent, copper 60, nickel and iron 15 or copper 65, nickel 20 and iron 15.
25 The high anode resistance described in the preceding paragraph is attributable to the high resistance of a surface oxide which forms on an anode having a composition in region 2. It is postulated that this high resistance can be overcome by incorporating into the composition a small quantity of another metallic element which will improve the conductivity of the surface oxide which forms on the anode.
The follc>wing information relates to the effect of electrolytE~ composition on anode oxidation rate.

In Beck, et a:l. U.S. Patent No. 5,006,209 it was shown that an electrolyte consisting essentially of, in parts by eight, 66 A1F3, 26 NaF, 8 KF and 3 LiF
provided a re:Latively low level of corrosion, gave a low anode resistance and did not give cathode deposits for i~he anodes described therein. The corresponding mol percents for this composition are:
46.7 A1F3, 36.7 NaF, 8.3 KF and about 8.3 LiF. It has now been determined that, for the anode compositions of the present invention, the molar ratio of AlF i,o NaF is the important criterion. A
eutectic of A=LF3 and NaF (44 mol o Al F3 and 56 mol o NaF) is the most advantageous electrolyte composition. A range of A1F3 departing slightly from the 44 molo eutectic amount, i.e. a range of 42-46 molo A1F3 (preferably 43-45 molo A1F3), is permissible.
The alka7_ine fluoride, employed with the A1F3 in the eutectic or near-eutectic compositions described in the preced_ng two sentences, can be either all NaF or primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF plus LiF.
In an electro7_yte composed of A1F3 in the range 42-46 molo and NaF in the range 54-58 molo, the corresponding ranges in wt.o would be 59-63 wt.o AlF3 and 37-41 wt.°. NaF. The electrolyte compositions of the Beck, et al. '209 patent which conform to the ranges of mol per cents described above are quite useful in accordance with the present invention.
The other electrolyte compositions of the Beck, et al. '209 patent are useful.
Anode oxidation rate tests for Cu 70: Ni 15: Fe 15 and for Cu 50: Ni 37: Fe 13 show that oxidation is minimized when the AlF3 content of the electrolyte is around the eutectic, 44 molo AlF3, balance alkaline fluorides, as described above. At above about 46 molo A1F3 the anodes develop high resistance. ,fit below about 42 molo A1F3 the anodes suffer blister corrosion and there are cathode deposits. Th.= aforementioned on-off procedure is necessary at compositions near 46 molo A1F3 but is not necessary at 42-45 molo A1F3.
In Beck, et al. U.S. Patent No. 5,006,209 it was indicated that A1203 particles having a size in the range 2-10 ~.m are preferred. It has now been determined th;~t reduction grade A1203, which contains up to about 100 ~,m particles, works in the cell of Fig. 1 because the 100 ~.m particles are agglomerates of smaller particles that disintegrate in the electrolyte into smaller particles of the desired size.
Referrin~~ now to Fig. 2, illustrated therein is a test cell comprising a metal crucible 9 containing an electrolyte bath 14 into which extends a cathode 13. The crucible constitutes the anode of the cell and has a com~~osition consisting essentially of, in wt. o, copper '70, nickel 15, iron 15. This corresponds to an anode composition in accordance with the present invention. The crucible was cast from induction melted alloy. The electrolyte composition consists essentially of, in parts by weight, AlF3 E6, NaF 26, KF 8, LiF 4. This is the same electrol~~te composition as was used in the initial tests with the cell of Fig. 1, described above. The cE~ll of Fig. 2 was operated at a bath temperature o:° 755~C for 5.1 hours, and under those time and temperature conditions, the crucible had an oxidation rate of 6.3 mg/cmzh. The result of the test conducted on the cell of Fig. 2 suggests the usefulness of the alloy composition employed in the present invention not only as a horizontally disposed bott~~m anode in the cell (Fig. 1), but also as an interior lining for all walls of the cell, vertical as w~?11 as horizontal (see Fig. 6).
Referrin!~ now to FIG. 6, indicated generally at 20 is a vesse:L for use in the electrolytic reduction of alumina to aluminum. Vessel 20 is constructed in accordance wi~~h an embodiment of the present invention and comprises an external shell 21, an interior meta:L lining 22 and a refractory layer 23 located between external shell 21 and interior metal lining 22. RE=fractory layer 23 is typically composed of a_Lumina or insulating fire brick.
Located within refractory layer 23 are a plurality of conduit portions 24 for circulating a cooling fluid through the refractory layer.
Contained within vessel 20 is a molten electrolyte 25 having a composition typically the same as that described above for use with test cell 10. Preferab=_y, the electrolyte consists essentially oi= Al F3 + NaF eutectic in which A1 F3 is present at about 44 mol°s but part of the 56 mole NaF
may be replaced by equivalent molar amounts of KF or KF and LiF. ~~n example of an electrolyte which is essentially a eutectic composition, which includes all three alk~~line fluorides, and which also conforms to tree electrolyte of the Beck, et al. '209 patent, is set. forth below:
Compound Molo Wt.o A1F3 44.2 63.2 NaF 34.6 24.8 KF 11.6 7.7 LiF 9.6 4.3 .. .,, .
..

Metal lining 22 in Fig. 6 forms a penetration-proof barrier between the molten electrolyte and refractory la:~rer 23. Vertically disposed within vessel 20 are a plurality of nonconsumable anodes 26 each having an anode composition in accordance with the present invention. Also vertically disposed within vessel 20 are a plurality of dimensionally stable cathodf~s 27 arranged in close, alternating spaced relation with anodes 26. The cathodes may be composed of titanium diboride.
As shown in FIG. 6, vessel 20 and its principal components, namely, external shell 21, interior metal lining :?2 and refractory layer 23 all comprise a bottom and walls extending upwardly from the bottom. Refractory layer 23 thermally insulates the vessel bottom and walls.
Interior metal lining 22 has a composition essentially tree same as the composition of anodes 26, and that composition has been discussed in detail above. That part of lining 22 which is exposed to aiz- (i.e. above molten electrolyte 25) has a high density (e. g. 950 or more of theoretical density). Too low a density produces relatively rapid oxidation in air. Induction melting of the alloy from which is produced the exposed part of the lining will give the desired high density.
Vessel 20, anodes 26 and cathodes 27 constitute part of an electrolytic reduction cell. Interior lining 22 is electrically connected to anodes 26 in a conventional. manner, and this is indicated schematically at 28 in FIG. 6. The interior metal lining thus constitutes part of the anode arrangement of the cell, and during operation of the cell, fine oxygen bubbles are generated at the bottom and walls of interior lining 22. These .a.a:.:.
w bubbles help v~o maintain in suspension, in the molten electrolyte, the finely divided alumina particles which are introduced into or form within electrolyte 25 in the course of an electrolytic reduction process in accordance with the present invention.
Balls of aluminum 30 form at and drop from cathodes 27 and roll down an inclined vessel bottom 33 to a tap location 34, in this embodiment adjacent one wall of the cell, although it might alternatively be in the middle of the cell bottom, for example. The fine bubbles of oxygen formed on bottom anode 7_ining 22 levitate aluminum balls 30 and facilitate their transport to tap location 34 where the aluminum is removed by a suitable removal device. One embodiment of a removal device is a pierced, titanium diboride member 31 which is wet internally and externally by aluminum and is mounted in the lower, inlet end of a suction tube 32 disposed above tap location 34. Member 31 has a lower-most extremity at tap location 34. A sump (not shown) may be provided at tap location 34 to assist in accumulating molten aluminum there.
Titanium diboride member 31 will remove molten aluminum from the cell.
Lining 2~: is in the form of sheet material, and it may be relatively thin. In some typical embodiments, lining 22 may have a thickness of about 3.18 - 9.52 mm (1/8 - 3/8 in.).
Because lining 22 is composed of an alloy which is substantially resistant to oxidation losses in the aforementioned molten electrolyte, the lining may be used over a long period of time without the need for replacement. After an extended period of use, lining 22 may be readily removed from within vessel 20 and replaced by a similar lining. Because it is composed of copper base alloy, the spent metal lining, removf~d from vessel 20, has substantial salvage value as recyclable material. Within proper reconstitution and reworking, the spent lining can be returned to a sheet form for use again as an interior meta_L lining for vessel 20; or it can be recycled into material useful for other purposes.
The foregoing detailed description has been given for clearness of understanding only, and no unnecessary l.~mitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims (32)

1. An oxidation-resistant, non-consumable anode for use in an electrolytic reduction cell for aluminum, said anode having substantially 100% of its composition throughout the anode consisting of, in wt. %:
copper 25-70 nickel 15-60 iron 1-30.

2. A non-consumable anode as defined in claim 1 wherein substantially 100% of composition consists of, in wt.%:

copper 45-70 nickel 25-48 iron 2-17.

3. A non-consumable anode as defined in claim 2 wherein substantially 100% of said composition consists of, in wt.%:
copper 45-70 nickel 28-42 iron 13-17.

4. A non-consumable anode as defined in any one of claims 1 to 3 wherein:
the weight ratio of said nickel to said iron is about 3 to 1.

5. A non-consumable anode as defined in any one of claims 1 to 3 wherein:
said anode is composed of sintered metal powders and has a porous surface.

6. A non-consumable anode as defined in claim 5 wherein:
said anode has a density substantially less than the theoretical density for said composition.

7. A non-consumable anode as defined in claim 6 wherein:
said anode has a density of about 60-70% of said theoretical density.

8. In combination with the anode of any one of claims 1 to 3, a cell for the electrolytic reduction for alumina to aluminum, said cell comprising:
a vessel having a bottom and walls extending upwardly from said bottom;
a plurality of anodes vertically disposed within said vessel, each anode having a composition in accordance with the anode of any one of claims 1 to 3;
a plurality of cathodes vertically disposed within said vessel, said cathodes being arranged in close, alternating spaced relation with said vertically disposed anodes;
said vessel having an external shell and an interior metal lining;
said metal lining having an upper part which is normally exposed to air;
a refractory layer located between said external shell and said metal lining for thermally insulating the bottom and walls of said vessel;
said metal lining being electrically connected to said anode, having the same composition as said anodes and having a relatively high density at said part of the lining which is normally exposed to air.

9. In the combination of claim 8 wherein:
each anode is composed of sintered metal powders and has a porous surface;
each of said anodes having a density substantially less than the theoretical density for said composition.

10. In the combination of claim 8 wherein:
said cell has a tap location;
said vessel bottom is inclined toward said tap location to accumulate molten aluminum at said tap location;
and said cell comprises a removal means at said tap location for removing the molten aluminum which accumulates there.

11. In the combination of claim 10 wherein said removal means comprises:
a suction tube having an inlet end disposed above said tap location of the cell;
a pierced, titanium diboride member mounted in said inlet end of said suction tube for removing molten aluminum from said cell;
said pierced, titanium diboride member having a lowermost extremity at said tap location.

12. In a cell for the electrolytic reduction of alumina wherein said cell comprises:
a vessel having an external shell;
an interior metal lining;
and a refractory layer located between said external shell and said internal metal lining;
said metal lining having substantially 100% of its composition consisting of, in wt.%:

copper 25-70 nickel 15-60 iron 1-30.

13. In a cell as defined in claim 12 wherein substantially 100% of said composition consists of, in wt.%:
copper 45-70 nickel 25-48 iron 2-17.

14. In a cell as defined in claim 13 wherein substantially 100% of said composition consists of, in wt. %:
copper 45-70 nickel 28-42 iron 13-17.

15. In a cell as defined in any one of claims 12-14 and comprising:
a plurality of vertically disposed anodes having the same composition as said lining;
and means electrically connecting said interior metal lining to said anodes.

16. In combination, a cell and an electrolyte for the electrolytic reduction of alumina to aluminum, said cell comprising:
a vessel having a bottom and walls extending upwardly from said bottom;
a plurality of non-consumable anodes vertically disposed within said vessel;
and a plurality of dimensionally stable cathodes vertically disposed within said vessel in close, alternating, spaced relation with said vertically disposed anodes;
said vessel comprising an external shell and an interior metal lining;
each of said anodes having substantially 100%
of its composition throughout the anode consisting of, in wt.%:
copper 25-70 nickel 15-60 iron 1-30;
said electrolyte being contained within said vessel and of a composition containing 42-46 mol% AlF3, the balance being substantially either (a) NaF or (b) NaF
and KF or (c) NaF, KF and LiF.

17. A combination as defined in claim 16 wherein each of said anodes has substantially 100% of its composition consisting of, in wt.%:
copper 45-70 nickel 25-48 iron 2-17.

18. A combination as defined in claim 16 wherein each of said anodes has substantially 100% of its composition consisting of, in wt.%:
copper 45-60 nickel 28-42 iron 13-17.

19. A combination as defined in any one of claims 16-18 wherein:
each of said anodes is composed of sintered metal powders and has a porous surface.

20. A combination as defined in claim 18 wherein:
each of said anodes has a density substantially less than the theoretical density for said composition.

21. A combination as defined in any one of claims 16-18 wherein said interior metal lining has (a) an upper part, located above said electrolyte, which is exposed to air, (b) a lining composition the same as said anodes and (c) a relatively high density at said part of the lining which is exposed to air.

22. A combination as defined in claim 21 wherein:
said interior metal lining is electrically connected to said anodes.

23. A combination as defined in claim 21 and comprising:
a refractory layer between said external shell and said interior metal lining, for thermally insulating the bottom and the walls of said vessel;
said lining forming a penetration-proof barrier between said refractory layer and said electrolyte for protecting said refractory layer from said electrolyte.

24. A combination as defined in claim 16 wherein:
said electrolyte has an AlF3 content of 43-45 mol%.

25. A non-consumable, metallic electrode which is resistant to air oxidation, said electrode comprising:
a density which is at least 95% of the theoretical density of the metallic composition of said electrode;
a relatively imporous surface compared to the surface of a less dense electrode;

and a composition throughout said electrode including:
copper 70 wt.% max.
nickel greater than 30 wt.%
the balance being substantially all iron.

26. A non-consumable electrode as defined in claim 25 wherein:
said iron content is in the range of about 13-30 wt.%.

27. A non-consumable electrode as defined in claim 25 or 26 wherein:
the weight ratio of nickel to iron is about 3 to 1.

28. In combination, the anode of any one of claims 1 to 3, and an electrolyte for use with said anode for the electrolytic reduction of alumina to aluminum, said electrolyte containing 42-46 mol% AlF3, the balance being substantially either (a) NaF or (b) NaF and KF or (c) NaF, KF and LiF.

29. In the combination of claim 28 wherein said electrolyte comprises:
AlF3 in the range 43-45 mol%.

30. In combination, a cell and an electrolyte for the electrolytic reduction of alumina to aluminum, said cell comprising:
a vessel having a bottom and walls extending upwardly from said bottom;
and a plurality of non-consumable anodes vertically disposed within said vessel;

each of said anodes having substantially 100%
of its composition throughout the anode consisting of, in wt.%:
copper 25-70 nickel 15-60 iron 1-30;
said electrolyte being contained within said vessel, said electrolyte being of a composition containing 42-46 mol% AlF3, with the balance being substantially either (a) NaF or (b) NaF and KF or (c) NaF, KF and LiF.

31. A combination as defined in claim 30 wherein substantially 100% of the composition of said non-consumable anode consists of, in wt.%:
copper 45-70 nickel 25-48 iron 2-17.

32. A combination as defined in claim 31 wherein substantially 100% of the composition of said non-consumable anode consists of, in wt. %:
copper 45-70 nickel 28-42 iron 13-17.