US3025229A - Improvements in the method of making carbon anodes - Google Patents

Description

3,025,229 MPROVEMENTS EN THE METHOD F MAKENG CARBQN ANODES Clinton B. Folkrod, Spokane, Wash, assignor to Kaiser Aluminum 8; Chemical Corporaticn, Uaitland, Calif a corporation of Delaware No Drawing. Filed .Iune 3, 1959, Ser. No. 817,710 6 Claims. (Cl. 204-294) This invention relates generally to carbon electrodes. More particularly, it relates to a method of preparing carbon anodes for utilization in the electrolysis of an aluminum containing material.

The production of aluminum by electrolysis of an aluminum containing compound such as alumina dissolved in a molten electrolyte and deposition at the cathode is a very old process. The alumina is broken down into its components; the oxygen is liberated at the surface of the carbon anode where it reacts with the carbon to form a mixture of carbon dioxide and Carbon monoxide, and the aluminum is deposited at the cathode, which forms the bottom of the cell. conventionally, use has been made of two types of electrolytic cells, namely, that commonly referred to as a pre-bake cell and that commonly referred to as a Soderberg or self-baking cell. With either cell, thereduction process involves precisely the same chemical reactions. The principal difference between the two cells is one of structure. In the prebakc cell the carbon anodes are prebaked before being installed in the cell, while in the Soderberg cell the anode is baked in situ, that is, it is baked during operation of the electrolytic cell thereby utilizing part of the heat generated by the reduction process.

In the aluminum industry, particularly in the United States, the carbon anodes, either prebaked or self-baking, have been made from petroleum coke. It has now been found that good carbon anodes can be produced wherein at least a portion of the coke aggregate is comprised of gilsonite coke. The resultant anodes are resistant to aerial oxidation and are substantially free from dusting or cracking in the molten electrolyte or bath. In addition, such anodes contain less sulfur.

The instant invention comprises generally a method of making a carbon electrode wherein a coke charge is mixed with a carbonaceous binder, e.g., coal tar pitch, petroleum pitch, or mixtures thereof, and baked at an elevated temperature in suitable apparatus, at least a portion of said coke charge being comprised of calcined gilsonite coke. Gilsonite is a naturally occurring solid asphaltic material found in the Unitah Mountains of Utah and Colorado. Gilsonite can be processed in a delayed coker and catalytic reformer to give high octane gasoline and coke of very low sulfur content.

In the manufacture of prebake anodes for use in the production of aluminum the coke-binder mixture can be pressed and thereafter baked in a conventional ring furnace at a temperature of about 1100 C. (2012 F.). It will be understood that in such a furnace, where multiple layers of green anodes are baked, the maximum temperature will vary somewhat from the top to bottom layers. In the case of a self-baking anode the baking temperature is primarily controlled by the temperature of the reduction cell electrolyte which may be, for example, on the order of 970 C.-985 C. (l780 F.1799 F.).

rag smarts In the manufacture of carbon electrodes sufficient carbonaceous binder, e.g., a coal tar pitch, is required to coat each particle of carbon with a thin film, and the relative amount of binder required to produce satisfactory carbon electrodes is influenced by a number of factors, including the mesh size of the particles. In the usual practice of making carbon electrodes in the aluminum industry, on the order of about 15 to 18% by weight of binder is used for prebake anodes While on the order of about 27 to 33% by Weight of binder is used for selfbaking or Soderberg electrodes. The above percentages are based on the weight of the coke aggregate plus binder.

For the purpose of illustrating the invention and its advantages the following examples of electrodes prepared according to the invention are set forth.

In the examples screen sizes are with reference to the Tyler standard screen scale.

EXAMPLE A Plant size, prebake anodes were fabricated employing calcined gilsonite coke for the coke aggregate. The gilsonite coke was in two portionsa coarse portion and a finer portion. The coarse portion had screen analysis as follows: plus 4 mesh, 13.8%; minus 4 plus 8 mesh, 21.2%; minus 8 plus 14 mesh, 32.4%; minus 14 plus 28 mesh, 22.4%; minus 28 plus 48 mesh, 6.8%; minus 48 plus mesh, 2.3%; minus 100 plus 200 mesh, 0.4%; minus 200 mesh, 0.7%. The finer portion had a screen analysis as follows: plus 4 mesh, 0.06%; minus 4 plus 8 mesh, 0.12%; minus 8 plus 14 mesh, 0.24%; minus 14 plus 28 mesh, 0.59%; minus 28 plus 48 mesh, 3.35%; minus 48 plus 100 mesh, 13.83%; minus 100 plus 200 mesh, 27.37%; minus 200 mesh, 54.44%. The average real density of the calcined gilsonite coke was 2.0 g./cc.

A chemical analysis of the calcined gilsonite coke is given in Table 1.

The gilsonite coke aggregate was mixed with carbonaceous binder and then pressed into anodes. One batch was mixed to the proportions 4268 lbs. coarse, 1980 lbs. fines and 1080 lbs. of carbonaceous binder. This amounts to 14.7% by weight of binder. Subsequent batches were mixed to the proportions of 4270 coarse, 1980 lbs. fines, and 1050 lbs. of carbonaceous binder (14.4% by Weight of binder). The carbonaceous binder was comprised of a mixture of coal tar pitch and petroleum pitch. From the batches, 188 plant size green anodes Were pressed.

The green anodes were baked in a conventional ring furnace using an average preheating temperature of 960.7 C. (1761.3 F.) and an average finishing baking temperature of 1113.1 C. (2035.6 F.).

Samples were taken from several anodes of each batch to obtain values of specific resistivity, real density, apparent density and chemical composition. The average results of these tests and comparative and typical values for regular production anodes appear in Table 2.

1 Relative grind-ability indicating that gilsonite coke is more diificult to grind than delayed petroleum coke.

From Table 2 it is seen that the gilsonite coke anodes compare extremely well with the regular production anodes.

One hundred and fifty-nine of the anodes were stubbed and rodded and then used in the operation of plant reduction cells. It was noted during the operation of the cells that the gilsonite coke anodes maintained their shape and area very well and that air burn was less than with regular production anodes. Also, it was observed that dusting was less prevalent with the gilsonite coke anodes than with regular production anodes.

EXAMPLE B A production run of 15,430 anodes containing calcined gilsonite coke was made. These anodes contained gilsonite coke in the amount of approximately 30 weight percent of the new coke charge, that is, excluding the butt fraction. The balance of the new coke charge was calcined petroleum coke. (It is usual practice with prebake anodes to work back into the electrode mix the butt ends of anodes used up in the reduction cells.) The gilsonite coke constituted, on the basis of total coke charge (ineluding the butt fraction), about 21 weight percent of the anodes.

The chemical analysis and the physical properties, such as density and specific resistivity, of the calcined gilsonite coke are compared in Table 3 with these properties of a typical calcined delayed petroleum coke.

eluding calcined delayed coke, calcined gilsonite coke, and ground butts) is given in Table 4.

Table 4 AVERAGE SCREEN ANALYSIS OF COKE AGGREGATE Screen size: Percent Minus 1.05 plus .742" 0.1 Minus .742" plus .525" 1.1 Minus .525" plus .371" 2.3 Minus .371" plus 3 mesh 2.4 Minus 3 mesh plus 4 mesh 2.5 Minus 4 mesh plus 8 mesh 13.4 Minus 8 mesh plus 14 mesh 20.5 Minus 14 plus 28 mesh 10.8 Minus 28 plus 48 mesh u 4.0 Minus 48 plus mesh 8.9 Minus 100 mesh plus 200 mesh 12.3 Minus 200 mesh 21.7

Table 5 CHARACTERISTICS OF GREEN GILSONlTE COKE ANODES COMPARED TO GREEN REGULAR ANODES Gilsonitc Regular Coke Anodes Auodes Hcight-inches 12.8 12.7-12.8 Pressing lemp., C 122125 Wt. in Air 210.6 207. 7-211. 9 Green Apparent Density 1. 003 1. 590-11310 From Table 5 it is seen that the anodes containing gilsonite coke and regular anodes have similar characteristics.

From the visual aspect it was noted that the baked anodes containing gilsonite coke had a considerably rougher surface appearance than the regular anodes.

Two baked anodes containing gilsonite coke were cored and specific resistivity and density tests as well as chemical analyses were performed on the core samples. Six core samples were taken from each of the two anodes thereby resulting in twelve core samples. The same number of core samples were taken from two regular anodes for comparison. Tables 6A and 6B give the comparative results of specific resistivity and density test while Table 7 gives the data on the chemical analyses of the core samples.

CHARACTERISTICS OF BAKED ANODE CORES Sample No. Specific Resistivity Real Density Apparent Density The gilsonite coke was ground in a ball mill along with calcined petroleum coke in such manner that the gilsonite coke would comprise 30% of the dry coke aggregate (excluding butts). This amounted to about 21% by weight of the total coke charge (including butts).

The average screen analysis of the coke aggregate (in Table 6B REGULAR ANODES Sample No Specific Apparent Real Resistivity Density Density Table 7 CHEMICAL ANALYSES BAKED CARBON CORE COMPOSITES Gilsonite Coke Anodes Regular Anodes Item Cores Cores Cores Cores C1-C6 Dl-D6 Al-Afi Bl-BG Percent Percent Pet'cen Percent The anodes containing gilsonite coke were employed in reduction cells. In some of the testing, the gilsonite coke containing anodes were installed in electrolytic cells with an equal number of regular anodes. One test involved the exclusive use of gilsonite coke containing anodes in an entire pot room (70 reduction cells) for a period in excess of one month.

The operation of the cells during the testing period was closely watched. It was observed that the cells operating with gilsonite coke containing anodes required less skimming than cells employing the regular anodes. The current efficiency and carbon consumption of the cells employing gilsonite coke containing anodes were substantially the same as those values for cells employing regular anodes.

It will be understood that various changes, modifications and alterations may be made in the instant invention without departing from the spirit and scope thereof and, as such, the invention is not to be limited except by the appended claims, wherein -I claim:

1.. A method of making a carbon anode useful in aluminum reduction processes wherein a coke charge is mixed with a carbonaceous binder and baked at an elevated temperature, the improvement which comprises utilizing for the coke charge calcined gilsonite coke and mixing said coke charge with from about 14.4 to 33% by weight of a carbonaceous binder.

2. A method according to claim 1 wherein said coke charge is mixed with from about 14.4 to 18% by weight of a carbonaceous binder.

3. A method according to claim 1 wherein said coke charge is mixed with from about 27 to 33% by weight of a carbonaceous binder.

4. A method of making a carbon anode useful in aluminum reduction processes wherein a coke charge is mixed with a carbonaceous binder and baked at an elevated temperature, the improvement which comprises utilizing a substantial amount of calcined gilsonite coke in the coke charge and mixing said coke charge with from about 14.4 to 33% by weight of a carbonaceous binder.

5. A method according to claim 4 wherein said coke charge is mixed With from about 14.4 to 18% by weight of a carbonaceous binder.

6. A method according to claim 4 wherein said coke charge is mixed with from about 24 to 33% by weight of a carbonaceous binder.

References Cited in the file of this patent UNITED STATES PATENTS 2,582,764 Bailey Jan. 15, 1952 2,653,878 Sejersted et al Sept. 29, 1953 2,764,530 Klemgard Sept. 25, 1956 2,835,605 Nelson et a1. May 20, 1958 2,848,424 Stanko Aug. 19, 1958 OTHER REFERENCES Roese et al.: Condensed Chemical Dictionary, 5th ed., 1956, Reinhold Pub. Corp., New York, N.Y., page 520.

Claims (1)

1. A METHOD OF MAKING A CARBON ANODE USEFUL IN ALUMINUM REDUCTION PROCESSES WHEREIN A COKE CHARGE IS MIXED WITH A CARBONACEOUS BINDER AND BAKED AT AN ELEVATED TEMPERATURE, THE IMPROVEMENT WHICH COMPRISES UTILIZING FOR THE COKE CHARGE CALCINED GILSONITE COKE AND MIXING SAID COKE CHARGE WITH FROM ABOUT 14.4 TO 33% BY WEIGHT OF A CARBONACEOUS BINDER.