US3855086A - Carbon anode protection in aluminum smelting cells - Google Patents

Abstract

In the operation of a cell for the electrolytic reduction of Al2O3 dissolved in a cryolite bath of aluminum metal utilizing a carbon anode, the improvement wherein there is provided at the anode an atmosphere containing water in amounts effective for preventing anode dusting.

Description

United States Patent. [191 .Sleppy et al.

[451 Dec. 17, 1974 CARBON ANODE PROTECTION IN ALUMINUM SMELTING CELLS Inventors: William C. Sleppy, Belleville, 0].;

Ronald J. Campbell, Apollo, Pa.

[73] Assignee: Aluminum Company of America,

Pittsburgh, Pa.

[22] Filed: June 28,1973

[21] Appl. No.: 374,803

[52] US. Cl. 204/67 [51] Int. Cl. C22d 3/12 [58] Field of Search 204/67 [56] References Cited UNITED STATES PATENTS 2,947,673 8/1960 Sem et all 204/67 2,464,267 3/1949 Short 204/67 3,509,030 4/ l 970 Gooding 204/67 3,696,008 l()/l972 Lcvitan 204/67 Primary Examiner.lohn H. Mack Assistant Examiner-D. R. Valentine Attorney, Agent, or Firm-Daniel A. Sullivan, Jr.

[5 7] ABSTRACT In the operation of a cell for the electrolytic reduction of A1 0 dissolved in a cryolite bath of aluminum metal utilizing a carbon anode, the improvement wherein there is provided at the anode an atmosphere containing water in amounts effective for preventing anode dusting.

4 Claims, 2 Drawing Figures PATENTEI] DEE] U974 SHEET 2 0F 2 Hili 'l ARGON TANK CARBON ANODE PROTECTION IN ALUMINUM SMELTING CELLS BACKGROUND OF THE INVENTION The present invention relates to the operation of aluminum reduction cells utilizing carbon anodes.

Depending upon operating conditions, consumption of carbon anodes in Hall-Heroult process cells ranges from one-third to three-quarters of a pound of carbon per pound of aluminum produced. The preferred conditions are those leading to the stoichiometric minimum consumption, 0.33 lbs. C/lb. Al, predicted by the net cell reaction:

In experiments on closing off the space above the electrolyte bath of a Hall-Heroult cell from the air, there has arisen an accumulation of carbon scum on the bath surface and a distribution of carbon dust throughout the electrolyte. This carbon scum and dust is caused by a deterioration of the carbon anodes. The phenomenon is referred to as anode dusting.

Carbon scum causes alumina feeding problems. The scum has made it impossible to replenish alumina consumed during electrolysis. As the dissolved alumina content of the bath decreases, scum formation accelerates. Carbon dust and scum increases the bath viscosity and hinders diffusion of oxygen-bearing ions to the anode, thus limiting anode current densities and affecting the heat balance of the cell. Increases in the viscosity and density of the bath lower the current efficiency and contribute to poor metal coalescence. The carbon in the scum and dust is not available for reaction with oxygen at the anode and so the gross consumption of carbon is increased by dusting. Because of carbon scum,

the bath agitation supplied by anode bubble evolution is reduced and the tendency for electrolyte to solidify at the metal pad-bath interface increases. Ultimately, enough carbon dust can be distributed throughout the bath in closed cells to cause electronic conduction and complete loss of metal production. These conditions must be avoided for successful operation of an enclosed cell.

Prolonging the life of anodes will not only decrease carbon consumption, but in the case of pre-baked anodes will decrease the amount of anode butts to be recycled to the production of additional anodes and thereby decreases problems attendant upon evolution of fluorides during baking of anodes.

SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a method for preventing the phenomenon of anode dusting, particularly in closed cells.

This, as well as other objects which will become apparent in the discussion that follows, are achieved, according to the present invention, by a process for producing aluminum wherein a carbon anode of a cell for the electrolytic reduction of Al O dissolved in a cryolite bath to aluminum metal is provided with an atmosphere containing water in amounts effective for preventing anode dusting.

GENERAL ASPECTS OF THE INVENTION I It is believed that anode dusting is caused by atmolite Attack by atmolite on exposed carbon anode surfaces is particularly a problem when using closed cells, e.g. cells which are closed at the top by a plate.

The atmosphere containing water can be provided by surrounding anode surfaces located above the electrolytic bath with a skirt spaced from such surfaces, and injecting water vapor into the space between the skirt and the anode surfaces. Alternatively the water vapor can be provided by closing off a space above the electrolyte bath and dissolving in the electrolyte bath alumina containing enough water that sufficient water vapor is evolved and rises into the closed space to afford the desired protection. Such alumina can be produced by heating alumina trihydrate to the desired water content by well-known techniques; eg using rotary kilns. It is desirable to feed the alumina into the bath close to the anode or anodes.

It is believed that the amount of water vapor to be provided in an atmosphere around an anode surface depends to some extent on the amount of atmolite to be neutralized. In general, it is desirable to provide at least enough water vapor to react the atmolite stoichiometrically with the water vapor in accordance with the equation NaAlF, 3/2 H O NaF /z A1 0 3HF BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational, cross-sectional, broken-away view of a Soderberg anode type cell for use in the present invention.

FIG. 2 is a schematic representation, in cross-section, of bench-scale equipment for illustrating a part of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably in the practice of the present invention, A1 0 is electrolytically decomposed to aluminum metal in an electrolyte bath between an anode and a cathodic interface fonned between aluminum metal and the electrolyte bath, the bath consisting essentially of Al O NaF. and AIR; and having a weight ratio NaF to AlF up to 1.1:]. The bath is maintained at an operating temperature greater than 40C above the cryolite liquidus temperature of the bath and effective for preventing bath crusting in interfacial areas between the bath and aluminum metal. The cryolite liquidus temperature is that temperature at which cryolite first begins to crystallize on cooling the bath.

While the bath may consist only of Al O NaF, AlF it is possible to provide in the bath at least one halide compound of the alkali and alkaline earth metals other than sodium in an amount effective for reducing the liquidus temperature of the bath below that which it would have if only Al O NaF, and AlF were present. Suitable alkali and alkaline earth metal halides are LiF, CaF and MgF In a preferred embodiment, the bath contains lithium fluoride in an amount between 1 and A preferred practice for maintaining the alumina concentration as electrolytic reduction proceeds is to add, substantially continuously, directly to the molten vbath, an alumina which has a total water of 8 to 20 percent, more preferably 10 to 18 percent. The surface area may preferably be in the range 135 to 180 m /g. The total water is a measure of the water in the alumina and is defined herein as follows:

Expose a sample of alumina to 100 percent humidity for several hours, then equilibrate the sample at 44 percent relative humidity, 25C, for 18 hours, then accurately weigh the sample, then ignite it to l,lC, then weigh again. The loss in sample weight on going from the equilibrated state at 44 percent relative humidity to the ignited state after heating at 1,100C, divided by the sample weight at l,l00C, and multiplied by 100 is the percent total water.

Surface area is measured by the Brunauer-Emmett- Teller method. See Stephen Brunauer, P. H. Emmett, Edward Teller, J. of Am. Chem. Soc, V. 60, Pgs. 309-l9, 1938.

The use of alumina of the high water content of the present invention is contrary to the commonly-held view set forth at p. 34 of The Chemical Background of the Aluminum Industry by Pearson, published by The Royal Institute of Chemistry in 1955, that alumina used in electrolytic production of aluminum should be moisture-free.

Further illustrative of the present invention are the following examples:

EXAMPLE I With reference to FIG. 2, there is shown a graphite crucible 51 having a non-conducting, refractory lining 52 with a hole 53 at its lower end. A molten aluminum metal pad 54 sits in the bottom of the alumina crucible and contacts the graphite crucible 52 to be in electrical contact with cathode lead 55. Resting on pad 54 is electrolyte bath 56 containing-4 wt. 70 Al O and NaF AlF at a bath weight ratio NaF/AlF 0.8. The electrolyte bath is at 900C. Carbon (prebaked, petroleum coke) anode 57 is immersed in the electrolyte bath to provide an electrical current density of amperes per inch on the anode. Aluminum anode skirt 58 surrounds the anode 57 as shown and is sealed at its top by plug 59 provided with orifices for the passage of anode lead 60 and gas flow pipe 61. Appropriate piping is provided for allowing varied amounts of argon gas to flow from tank 62 through impinger bottle 63 containing water 64 surrounded by an ice water bath 65. Thus, the argon gas fed into the space between the anode 57 and the skirt 58 contained water vapor picked up by -the argon from the water in the bottle 63. In operation of the cell to produce aluminum, carbon consumption was 0.33 to 0.38 pounds per pound of aluminum produced at a current efficiency of lOO percent in 29 to 4l ampere-hour tests using water vapor shielding for preventing anode dusting. With 4 to 22 torr water partial pressure in the argon, no carbon froth or scum was detected. When the impinger bottle 63 was bypassed so that only argon moved down around anode 57, a carbon scum formed on the surface of the bath 56, and electronic conduction through the carbon scum occurred.

EXAMPLES H AND III Aluminum was produced in the cell of FIG. 1. The maximum dimensions of the steel shell 20 in the horizontal were 18' 6'' X 10' 2". Its maximum height was 3 9". The maximum dimensions of the molten aluminum metal pad 21 in the horizontal were 17' 8" X 9' 4". The electrolyte bath had the same maximum dimensions as the metal pad.

A mica mat 22 was provided between the steel shell 20 and graphite block 23 for the purpose of preventing current flow through shell 20. Mat thicknesses of from 6 to 20 mils have been used.

The pad 21 of molten aluminum was supported on carbonaceous cathode block lining 24 and carbonaceous tamped lining 25. The carbonaceous linings were supported on an alumina fill 26, there being interposed between the tamped lining and the till some quarry tile 27. A layer of red brick 28 was provided between the graphite block 23 and quarry tile 27'.

FIG. 1 is a representative vertical section through the cell and it will be realized that, for instance, similar graphite blocks 23 would appear in other elevational sections through the cell.

The anode 29 was a Soderberg-type carbon anode. The composition charged to form this self-baking .anode was 31 percent pitch of softening point equals 98lO0C (cube-in-air method) and 69 percent petroleum coke. The coke fraction was 30 percent coarse, 16 percent intermediate, and 54 percent fine, the size distributions of the coarse, intermediate, and fine coke being given in Table I.

Table l Coke Size Distribution Cumulative 7r Greater Than Sieve Size The cathode current was supplied through steel collector bars, such as bar 30, to the block lining 24. The current supply is indicated by the plus and minus signs on the anode and on collector bar 30 respectively.

The space above the bath 31 was sealed from the surrounding air by a closure 32, including a cast iron manifold 33, Ceraform Refractory board 34, which is a soft (for obtaining a good seal) fibrous electrical and heat insulating board available from the Johns-Manville Co., steel shell 35, steel plate 36, and fire clay brick, eg 50% M 0 and 50% $0,, 37. Within shell 35 there was provided a castable 38 serving a primarily insulative function and a castable 39, e.g. calcium-aluminatebonded tabular alumina, selected for its refractory properties. The particular heat transfer situation was chosen to maintain the upper surface 45 of bath 31 substantially in molten condition, i.e. free of any crusting.

Alumina is charged from hopper 40 through a fill valve and feeder assembly 41 of the type disclosed in US. Pat. No. 3,681,229 issued Aug. 1, 1971 to R. L. Lowe entitled Alumina Feeder. Measured quantities of alumina are fed onto the exposed molten bath surface through Inconel-600 pipe 42. The distance between the bottom of pipe 42 and the top of bath 31 is about 1 foot. The feeder 41 is a shot-type feeder, i.e. separate quantities of alumina are fed at timed intervals. In Examples 11 and 111, two feeders 41 were used, and these fed-in alumina approximately every 5 minutes, the quantities of alumina being adjusted to maintain the desired alumina concentration in the bath. It takes about 1 minute to discharge the alumina increments which were about 1,500 grams. Pipe 42 is directed so as to impinge alumina onto the bath 31 where gas 44 is rising alongside the anode. This assures that the water evolved from the charged alumina protects the anode against production of carbon dust therefrom. This practice also promotes dissolution because of the bath agitation caused by the gas evolution. By charging the alumina in line with a spike row (spikes 45a, b, and 0 lie in a vertical plane parallel to the plane of FIG. 1, which plane also contains pipe 42) in the Soderberg anode (cracks usually occur in the anode in line with spike rows), the dissolution rate is enhanced by the additional gas evolution occurring at the cracks. Feeders 41 were operated using air as the fluidizing medium, it being recognized that this represents a small leakage of air past cover 32 to the bath.

The particular alumina used for Examples 11 and 111 had a total water of 16.95 percent. This alumina was 98 plus 325 mesh and its water content alone was sufficient to prevent anode dusting, i.e. a decomposition of the anode such that carbon particles build up in and on the bath.

The production data for Examples 11 and 111 are presented in Tables 11 to IV.

Table 11 Pot Production Data Example No. Data Name 11 111 N.M. not measured Table 111 Pot Electrical Data Example No. 11 111 Data Name Volts/Pot 5.13 5.17 Average Ampcres 66.874 72,207 Kilowutts/Pot 343. 1' 373.3 Ohmic Voltage Drop in Bath 1.70 1.68

Table IV Pot-Bath Data Example No.

Data Name I 111 Wt.-% CaF- 3.11 3.17 Wt A1 0,; 4.09 4.00 Wt.-% AlF 48.97 45.08 Wt.-% LiF 5.61 10.165 Wt.-% NaF 38.13 36.94 Wt.-% Mg? .38 .28 Liquidus emperaturc. "C 882 906 Calculated Wt.-Ratio NaF/AlF .78 .82 Calculated Wt.-% C olite 63.4 61.9 Calculated Excess Al 3 23.4 20.5 Bath Operating Temperature, C 898 922 "Eutectic Temperature, C 799 814 Conductivity. ohm"'inches' 4.87 5.67 Bath Depth, inches 8.26 7.62 Metal Depth, inches 6.02 6.25

With special reference to Table IV, the excess A1F indicates the quantity of AlF above that present under the heading cryolite, formula 3NaF.AlF In each of Examples I1 and 11], A1 0 would be the first substance to crystallize on going below the given liquidus temperature. The eutectic temperature provides an estimate of the cryolite liquidus temperature in this case. The eutectic temperature is determined by finding the liquidus temperature for progressively decreasing A1 0 content, correspondingly increasing NaF AlF and constant bath ratio NaF/AIF and'selecting the minimum liquidus temperature on the basis of the resulting group of liquidus temperature values. The A1 0 in solution is that at the particular bath operating temperature. Conductivity data is likewise for the given operating temperature.

Gases evolved from the Soderberg anode (e.g. hydrocarbons), fluorides from the bath, and anode reaction gas (e.g. CO were vented from cover 32 through an opening (not shown) and passed through a burner to burn the hydrocarbons. Because it is difficult to provide an absolute sealing of the bath from the air using cover 32, i.e. leaks can be present in cover 32, a pressure of 0.03 to 0.1 inch of H 0, measured negatively from atmospheric pressure, is maintained between the cover 32 and the burner, in order to prevent fume leakage from the cover 32. The burned gases were then fed to a scrubber system.

EXAMPLE IV This Example illustrates how a suitable alumina product as used in Examples 11 and 111 can be produced.

Bayer-process alumina hydrate was treated in a kiln to produce kiln activated alumina suitable for use in the process of the present invention as follows. Kiln dimensions were 360 feet length and 9 /2 feet inner-diameter. Residence time of the material in the kiln was 1 to 1V2 hours. The charged hydrate moved countercurrent to the combustion gases introduced into the lower end of the kiln. A maximum temperature of 400 to 500C was achieved 10 to 15 feet inside the lower end of the kiln. Natural gas was burned at a rate of 6,500 cubic feet (standard temperature and pressure) per hour to produce the combustion gases. This natural gas flow rate was selected by testing the product for the desired total water. The volume ratio of air to gas was 10:1. An alumina having a 12.5 percent total water was produced. Anywhere from 88 to weight of the particles had a size greater than 325 mesh.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

it will likewise be recognized that the action of water in the present invention will be subject to energyrelated laws such as rates of reaction and chemical equilibrium constants and that anode dusting need only be prevented to an extent such that there be no dustingrelated impairment of cell operation.

All percentages given herein are in percent by weight unless indicated otherwise.

What is claimed is:

1. In the operation of a cell for the electrolytic reduction of A1 dissolved in a cryolite bath to aluminum metal utilizing a carbon anode, the improvement comprising providing at the anode an atmosphere containing water in amounts effective for preventing anode dusting.

2. In the operation of a cell as claimed in claim 1, the improvement further being characterized by a closing of the top of the cell.

3. In the operation of a cell as claimed in claim 1, the improvement being further characterized by the providing of a skirt around the anode, said atmosphere being provided within said skirt.

4. In the operation of a cell as claimed in claim 1, the improvement being further characterized in that the amount of water is at least enough for reacting atmolite stoichiometrically according to the equation NaAlF, 3/2 H O NaF k A1 0 3HF.

UNITED STATES PATENT OFFICE CERTEFECATE OF CGRRECTION Patent No. 3,855,086 Dated December 17, 1974 Inventor-(5) C. et 8.1

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:'

In the Abstract, After bath" change of line 2 to --to--.

Signed and sealed this 11th day of February 1975.

(SEAL) Attest:

' C. MARSHALL DANN RUTH MASON Commissioner of Patents Attesting Officer 7 and Trademarks FORM P0405, I Y I.1sco\vm4-mc wave-Pee ".5. GOVERNMENT PRINTING OFFICE 7 I9, 0-35-3,

Claims (4)

1. IN THE OPERATION OF CELL FOR THE ELECTROLYTIC REDUCTION OF AL2O3 DISSOLED IN A CRYOLITE BATH T ALUMINUM METAL UTILIZING A CARBON ANODE, THE IMPROVEMENT COMPRISING PROVIDING AT THE ANODE AN ATMOSPHERE CONTAINING WATER IN AMOUNTS EFFECTIVE FOR PREVENTING ANODE DUSTING.

2. In the operation of a cell as claimed in claim 1, the improvement further being characterized by a closing of the top of the cell.

3. In the operation of a cell as claimed in claim 1, the improvement being further characterized by the providing of a skirt around the anode, said atmosphere being provided within said skirt.

4. In the operation of a cell as claimed in claim 1, the improvement being further characterized in that the amount of water is at least enough for reacting atmolite stoichiometrically according to the equation NaAlF4 + 3/2 H2O -> NaF + 1/2 Al2O3 + 3HF.

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