US 3284334 A
Description (OCR text may contain errors)
United States Patent Ofifice 3,284,334 Patented Nov. 8, 1966 3,284,334 MOLDEDCARBON BODIES William Joseph Metrailer and Wayne Hoover, Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Dec. 4, 1963, Ser. No. 328,103 Claims. (Cl. 204-294) This invention relates to carbon electrodes and a process for making them from high temperature fluid coke. The present invention relates to a method of making improved carbon electrodes and to novel electrode formulations. More particularly, it relates to an all high temperature fluid coke electrode which is made without separately calcining the coke aggregate, without the need for large, coarse aggregate particles in the electrode formulation, and with a minimum amount of binder material.
Specifically, the present invention relates to a high temperature fluid coke which is made by the pyrolysis of a hydrocarbon feed at temperatures of 1800-2500 F. to produce hydrogen and coke. The coke aggregate from which the improved electrode is formulated comprises coke as produced in the fluid coking process and ground fine coke prepared by grinding a portion of the coke from the coking process. The coke electrodes prepared from this formulation use a minimum amount of binder which results in superior coke electrodes. More specifically, the present invention relates to a method of preparing improved coke electrodes which can be used for obtaining aluminum from aluminum ores.
Heretofore, coke electrodes were formed by mixing a calcined coke aggregate with a binder material, molding the mixture under pressure and baking it at elevated emperatures for several'hours. The baking or heating of the electrode at elevated temperatures carbonizes and volatilizes the volatile constituents of the binder material leaving about 50 wt. percent of the binder material as a binder coke. The two principal types of petroleum coke materials which have been used in formulating coke electrodes for reduction of aluminum are delayed coke formed by batch destructive distillation of hydrocarbons and fluid coke formed by surface deposition of coke on coke particles at relatively low temperatures. In the low temperature fluid coking processes, aromatic hydrocarbons and olefinic hydrocarbons are also produced.
The delayed coke is crushed or ground to obtain a particle size distribution of particles of less than about 200 mesh up to about inch in diameter and then calcined. The calcined delayed coke has high true particle density. Since delayed coke aggregate formulations require particles of up to about inch, it had been thought that it was necessary to have the coarse aggregate particles of up to about inch in all coke aggregate formulations in order to obtain electrodes having high bulk density and good resistivities. It was necessary with delayed coke to have the large coke particles to prevent cracking of the electrodes during the baking step with prebake electrodes where shrinking occurs due to the carbonization and volatilization of the binder material, or in baking Soder'berg electrodes in the reduction cells due to volatilization of the binder material due to the heat from the reduction cell.
In producing a low temperature fluid coke which would be satisfactory for electrode formulation, it was found necessary to produce agglomerates from a portion of the fluid coke first, which agglomerates were calcined and crushed to produce coarse coke aggregates of the same particle size used in a delayed coke electrode aggregate, i.e., to produce crushed coke agglomerates of up to /2 to %1 inch in diameter to provide the coarse coke aggregate for the electrode formulation. In utilizing low temperature fluid coke to make an all fluid coke electrode, a portion of the fl'uid coke as received from the reactor would be ground and mixed with a suitable bidder to form agglomerates, and the agglomerates would be calcined and then crushed to form the coarse coke aggregate. This was necessary because of the small size of the fluid coke particles as produced in the conventional fluid coking operation. The low temperature fluid coking process is carried out at temperatures between 900 and about 1400 F. to produce coke and light hydrocarbon byproducts. The production of substantially larger particles which would be suitable in the formulation of coke electrodes in the fluid coking process was inconsistent with satisfactory operation of the fluid bed.
Another step involved in using either delayed or low temperature fluid coke to make coke electrodes was that the coke as received from the reactor had to be first calcined at temperatures of 18002800 F. The raw or green fluid coke received from the reactor is variable in chemical and physical properties and has to be calcined. This was necessary because the coke, when used in electrode manufacture, would cause excessive shrinking and cracking of the coke electrodes during baking due to the volatilization of the volatile materials in the green coke. The coke agglomerates also had to be calcined to remove volatile binder materials.
In the manufacture of carbon electrodes, sufficient binder is required to coat each particle of carbon with a thin film of binder and the relative amount of binder required to produce a satisfactory carbon electrode is influenced by a number of factors, including the mesh size and the porosity of the particles. Generally, the delayed coke aggregate has greater porosity and more binder material is required for electrode production. The porosity is due to larger pore openings in the coke which is due primarily to the manner in which the delayed coke is made. The low temperature fluid coke which has not been crushed or ground is relatively less porous and will generally require less binder. However, the pores present are internal pores and cannot be readily filled with binder. This generally results in an inferior electrode unless aggregates are included in the electrode. Grinding of the low temperature fluid coke and crushing agglomerates exposes the internal voids. In the usual practice of making carbon electrodes in the aluminum industry, 1-418% by weight of binder is used for prebaked electrodes while 26-34 by weight of binder is used for self-baking Soderberg electrodes. These percentages are based on the weight of the coke aggregate plus binder.
The principal criteria of molded electrodes for alumi num reduction are a minimum compression strength of 4000 p.s.i., a minimum real density of about 1.45 g./cc., and a maximum resistivity of 3 X10 ohm/inch.
In accordance with the conventional processes for making coke electrodes for the aluminum industry, the coke is obtained either from the delayed coking process or a conventional low temperature fluid coking process. In the delayed coking process, the coke obtained is calcined and crushed to form a suitable particle size distribution, including particles up to inch in diameter, binder is added and it is molded into an electrode and baked. Utilization of a low temperature fluid coke in an electrode requires grinding of a portion of the fluid coke, mixing part of the ground portion with a binder and forming coke agglomerates, calcining the coke agglomerates, crushing the agglomerates to form coarse particles of up to /8 to inch in diameter for use as the coarse aggregate, mixing the coarse aggregate with as produced calcined fluid coke and a portion of the ground fluid coke, adding a suitable binder, molding and baking to form the electrode.
It had therefore been thought, in order to obtain a suitable electrode which would not crack or shrink excessively on baking and which would hold up in use, that it was necessary to have coarse aggregates of up to /8 to about inch in the coke formulation with both low temperature fluid coke and delayed coke.
Heretofore, electrodes have required relatively large amounts of binder to obtain good cohesion which, on baking, frequently cause shrinking and cracking as the volatile materials in the binder cracked and baked off. Also, in using crushed delayed coke and crushed agglomerates of low temperature fluid coke, there were presented in the coarse aggregate relatively large voids which, when filled with binder, increased the over-all need of binder thereby aggravating shrinking and cracking of the electrodes on baking.
In accordance with the present invention, high tempera ture fluid coke, prepared by feeding a hydrocarbon into a fluid bed of coke maintained at a temperature of about 18002500 F. whereby the hydrocarbon is cracked to essentially coke and hydrogen, is used to formulate a molded coke electrode having improved properties. The high temperature coke obtained from this process is separated into two portions, one portion of which is ground to obtain a fine coke. The coke as produced in the reactor is mixed with the ground coke to make a coke aggregate which is mixed with a minimum amount of binder to obtain electrodes of improved properties. The high temperature coke used in accordance with the present invention can be used to form coke electrodes without separately calcining the coke obtained from the reactor, without the formation of coke agglomerates, calcining the agglomerates and crushing the agglomerates to form coarse aggregates, and with a minimum amount of binder. Coke electrodes prepared in the manner described in the present invention have low resistivity, high crushing strength, and do not crack on baking the electrodes.
The recently developed high temperature fluid coking process is used for the production of high temperature fluid coke by the thermal conversion of hydrocarbons to coke and hydrogen. The fluid coking reactor can consist basically of a reaction vessel or coker and a heater or burner vessel. In some schemes the heating can be done in the coking vessel. In a typical operation, the hydrocarbon to be processed is injected into a fluidized bed of coke in the reactor, which is maintained at a temperature of 18002500 F. Uniform mixing in the bed re sults in essentially isothermal conditions and effects instantaneous distribution of the feed stock. Reaction time is about 1-10 seconds. The high temperature fluid coking process provides a method of obtaining a high quality hard coke of relatively large particle size and hydrogen.
The high temperature fluid coker is operated with coke particles which have a particle size ranging between about 44-2500 microns in diameter. The hydrocarbon feed to the fluid bed of coke is cracked to form essentially hydrogen and coke. The coke as formed is deposited on the hot coke particles in the fluid bed in layers forminga relatively large homogeneous coke particle. In order to maintain a relatively constant average size of solids in the fluid bed, a portion of the solids is withdrawn and subjected to size reduction by conventional means and smaller seed coke is continuously fed back to the reactor and a portion of the larger coke particles is removed as product. The eflluent gas can contain uncracked methane, ethane, and up to 98% hydrogen. These gases are generally sent to a cyclone separator where any entrained coke solids are returned to the reactor.
The heat for carrying out the endothermic coking reaction can be generated in a burner vessel, in which case a stream of coke is transferred from the reactor to the burner vessel employing a standpipe and riser system, gas being supplied to the riser for conveying the solids to the burner. Sufficient fuel is burned in the burner vessel to bring the solids therein to a temperature sufliciently above the temperature in the reactor to maintain the system in heat balance. In a transfer line heater, the temperature of the coke is raised to about 200 to 600 F. above the temperature in the reactor to supply heat to the reaction.
Coke produced in the above manner has unique physical properties which permit it to be used directly in the manufacture of carbon electrodes without further thermal treatment. Furthermore, these properties permit the use of the coke in new formulations to prepare carbon elec trodes and carbon bodies. A particular property of the coke which permits unique carbon electrode formulations is the extreme dense nature of the high temperature coke particles relative to calcined delayed coke and calcined low temperature fluid coke. This difference is readily apparent upon examination of photomicrographs of crosssections of the three types of cokes. It is also evident when one compares (see Table I below) the densities of the three types of coke as determined by hydrocarbon displacement and the packed or settled density of a narrow particle size fraction of these three cokes:
The above packed densities represent the highest values obtained on several calcined delayed and calcined low temperature fluid cokes. Therefore, the void volumes shown are minimums for these types of cokes. The low void volume of the high temperature coke is due to the absence of large internal voids which can be penetrated by liquid hydrocarbons, e.g., binders. This permits the formulation of unexpectedly good carbon electrodes with considerably less binder. Also, the lower binder content of these formulations results in less volatiles released during the baking step and allows the use of smaller particle size coke (i.e., does not require large aggregate) in electrodes formulation without cracking of the electrode in the baking operation. An example of the as produced coke and the ground coke particle size distribution is given below in Table II.
TAB LE II Coke Used, Cumulative As Produced Ground Wt. percent on- 28 Mesh (589 microns) 35 Mesh (417 microns)- 48 Mesh (295 microns).
65 Mesh (208 m1crons) 84 6 0. 1 Mesh (147 microns) 3 0. 8 150 Mesh (104 microns). 99 8 10. 9 200 Mesh (74 microns) 99. 9 28. 5 325 Mesh (44 microns) 0. 1 58. 3 Pan 41. 7 Average Particle Diameter, microns 280 45 high temperatures. The cross-section of the coke presents a tightly packed onion skin appearance. Close examination of the photomicrograph shows almost complete absence of internal voids in the coke particle. This allows the use of substantially less binder in cementing the particles together and also provides materials which have a very high packing density.
It is desirable to operate to obtain the largest particle size and average particle size from the fluid coke operation so that when grinding a portion of the as produced fluid coke the fines obtained, when mixed with the as produced large coke particles, result in a maximum density aggregate formulation.
Several unique characteristics of the high temperature coke have been discovered by applicants. High temperature coke does not require an additional calcining step which is required with both the delayed coke and the low temperature fluid coke. This eliminates one of the costly steps of using petroleum coke for making coke electrodes. Also, applicants unexpectedly found that the high temperature fluid coke did not require the addition of coarse aggregate of up to A3 to inch in size to make good coke electrodes. Applicants found that the separate agglomeration and crushing step used with low temperature fluid coke to make coarse aggregates was not required when utilizing the high temperature coke to make coke electrodes.
In accordance with the present invention, applicants found that the high temperature coke as obtained from the reactor having particles between about 150 microns and 600 microns with an average particle size of 200 to 350 microns when mixed with a portion of high temperature coke ground to a particle size of 25 to 75 microns produced electrodes with unusually high crushing strength and low resistivity. The coke as produced is mixed with the ground coke at a ratio of 1:1 to 5: 1. The ratio of average size of the as produced coke to the ground coke is between about 4:1 to about 20: 1.
In general, two types of electrodes are employed by the industry. They are (a) prebaked and (b) Soderberg self-baking electrodes. In the former, a mixture comprising 78-82% of calcined coke aggregate and 18- 22% of coal tar pitch is molded at pressures of about 3000-5000 p.s.i. or extruded and then baked for periods up to 30 days at 1800-2400 F. These preformed electrodes are then used in electrolytic cells being slowly lowered in the molten alumina as they are consumed. Usually, butt-s of the unconsumed electrodes are reground and used in subsequent electrode formulations.
The Soderberg process involves the continuous or intermittent addition of the coke and coal tar pitch mix to the top of the cell as the electrode components in the lower part of the cell are consumed. In this operation, the paste represents a blend of about 66-72% coke aggregate and 28-34% of pitch. The cell operates usually at temperatures of 1700-1900 F. The mix is baked into an electrode by the hot cell gases and heat conducted by the electrode in the period between the time it is added at the top and the time it is used. This process does not produce butts which could be later ground to provide coarse aggregate.
In accordance with applicants process, high temperature coke is obtained by coking hydrocarbon feed in a fluid bed at temperatures of about 1800-2500 F. to produce essentially coke and hydrogen. This coke material when formulated into electrodes produces electrodes of unexpectedly superior properties. These electrodes can be made without separate calcining steps for the coke, without the need for large coke coarse aggregate of A; to 4 inch size, and utilizes a minimum amount of coke binder. The use of minimum coke binder provides a molded coke electrode which, on baking, has present a minimum amount of volatile materials which crack, vaporize, and are lost from the baked electrode. The bulk density of the electrodes formed in accordance with applicants invention is very high and the electrodes have unusually high crushing strength and low resistivity.
Applicants process substantially eliminates cracking and substantially reduces shrinking of the electrodes during baking and provides an improved electrode of substantially increased crushing strength and lower resistivity. The hydrocarbon feed to the process can be any gaseous, liquid, or heavy residual hydrocarbon which can be solid at ambient temperatures. Generally, however, heavy hydrocarbon oil feeds that are suitable for the coking process are heavy or residual crudes, vacuum bottoms, pitch, asphalt, and other heavy hydrocarbon petroleum residua, or mixtures thereof. Typically, such feeds can have an initial boiling point of about 700 F. or higher, an API gravity of about 0-20, and a Conradson carbon residue content of about 5-40 wt. percent.
In the manufacture of the electrodes, the coke is admixed with a carbonaceous binder. The binder materials used are the known ones conventionally used to make electrodes. Such binders generally have melting points lying within the range of 70-120 C. They contain small amounts of hydrogen (about 5% or less). The concentration of benzene and nitrobenzene insoluble portions represent preferably about 20-35% and 5l5%, respectively, of the binder. The binder used in accordance with the present invention with the high temperature coke is utilized in an amount of 9-13 wt. percent for prebaked electrodes and 18-24 wt. percent for Soderberg electrodes. This represents a substantial decrease in the amount of binder required to obtain suitable electrodes with either calcined delayed or calcined fluid coke and results in better electrode formulations than obtained when using conventional amounts of binder even with high temperature coke.
Applicants found that one of the primary reasons why success has been obtained with this particular coke material and with a minimum amount of binder material has been the very nature and characteristics of the coke used. The high temperature coke is relatively non-porous, extremely hard, and very dense. Applicants found that less binder was needed to coat the particles and obtam good cohesion between the particles because there are very few pores an-d/ or voids for the binder to penetrate nto the c-oke. This is true even of the ground coke which s used to make the binder filler material. Since the coke 1s extremely hard and the coke layers extremely tightly packed, even the ground coke with jagged edges has substantially fewer openings or voids for the binder material to go into. This was readily shown by comparing a cross sectional piece of low temperature fluid coke with the high temperature fluid coke used in accordance with this invention. The low temperature fluid coke has substantially more internal voids than the high temperature coke.
In accordance with the present invention, binder is used in the amount of 9-13 wt. percent, preferably 10-12 wt. percent, with prebaked electrodes, and wit-h Soderberg electrodes the binder is used in the amount of 18-24 wt. percent, preferably 20-22 Wt. percent based on total aggregate and binder.
In accordance with the present invention, an all high temperature fluid coke electrode can be made without calcining the coke, without separately agglomerating, calcining and crushing coke agglomerates to make a large coke aggregate material, and can be made with a minimum of binder which minimizes the amount of shrinking and tendency to crack of the molded coke electrode during the baking step.
The high temperature fluid coker is operated to obtain a particular size product having a particle size range of minus 5000 microns to plus 50 microns, and preferably about 600 microns to microns. The average particle size of the coke as obtained from the reactor will be 200 to 500 microns, and preferably about 250 to 350 microns.
A portion of the product coke is separated and ground to make the fine portion of the coke aggregate to a size which the particle size distribution will be 200 microns to minus 40 microns, and preferably about 85 microns to minus 40 microns. Of the finely ground coke, 40-60 wt. percent would be minus 40 microns. The grinding is carried out in such a manner that the average particle size of the ground portion will be 25 to 75 microns, and preferably 40 to 50 microns. The ratio of the average size of the coke obtained from the reactor to the ground coke will be 4:1 to 20:1, and preferably 6:1 to 10:1. The amount of coarse coke as produced from the reactor to ground coke will depend upon the particular particle size distribution and the average particle size of the ground and and fine coke. However, generally it can be said that the ratio of coarse to fine coke in the aggregate will be 1:1 to :1, and preferably 2:1 to 3:1.
In selecting the particle size distribution for a particular coke aggregate formulation, it is preferable to have a dumbbell type distribution of below about wt. percent overlay between the as produced and ground particles in the formulation. It is preferred that the fines material be svfliciently small so that they, in conjunction with the binder, will form a suitable filler material able to cement the as produced coarse coke together into a high density, high crush resistant molded electrode.
In the Soderberg aluminum reduction process, the entire electrode is consumed. That is, there are no butt or unusued portions of the electrodes remaining. However, in prebaked electrode reduction cells, the cells are operated in such a manner that usually -30% of the car bon electrode is not consumed. The unused butt can be crushed and blended back with additional coke and again formed into useful electrodes. The coke butts could be ground to form the fine fraction which is combined with the as produced or coarse coke obtained from the reactor but generally it is preferred to do a minimum of grinding of the butts and to form a more coarse coke fraction. This has both economic advantages as well as possible particle distribution size advantages. Applicants found that the amount of crushed butts to be added and the particle size of the crushed butts as related to the as produced coke and the fines coke for producing good electrodes were important. Electrodes with satisfactory crushing strength and resistivity can be obtained from the high temperature fluid coke used in accordance with this invention, when 15-30 wt. percent of crushed butts are blended back with the as produced and ground coke with the particle size distribution of the as produced and ground coke as previously described.
In one embodiment of the invention the butts consisting of particles of 1 inch to +4 mesh are blended back with an as produced coke-fine coke aggregate as previously described. In such a formulation, the ground butts, for example, being 4 inch to about 1 inch in size, would constitute about wt. percent of the total aggregate. Unground as produced coke of minus 800 to plus 150 microns would constitute about 55 wt. percent, and finely ground coke of about 75 to minus 40 microns would constitute about wt. percent of the aggregate. Of the finely ground coke, 40-60 wt. percent would be minus 40 microns. In the mixture, there are lean areas of the particles of the size between the crushed butts and unground fluid coke particles and finely ground coke particles. This particle size distribution, therefore, represents a triple dumbbell type distribution with a small amount of coke particles in the lean areas between the three particle sizes with the most particles. Thus, it will be seen that there is not a graduation of size in the coke particles but gaps or lean areas in order to obtain maximum packing density and to permit the use of minimum amount of carbonaceous binder in the electrode mixture.
The conditions at which the high temperature fluid coking step or process is carried out are critical. The broad temperature range of the fluid coke bed reactor can be 18002500 F., and preferably 19502200 F. The average superficial linear velocity of the fluidizing gas which consists of evolved hydrogen in the fluid bed of the reactor can be 0.3 to 5 ft./sec. and the preferred range is 0.5 to 3 ft./sec. The temperature in the transfer line burner is generally 200-500 higher than the temperature in the fluid bed reactor and can be 2100-3000 F., and more generally 2100-2500 F. The contact time in the reactor is generally 1-10 see.
The invention and its advantages will be better illustrated by the following examples of electrodes prepared in accordance with applicants invention.
Example 1 The coke electrode aggregate formulation was prepared by mixing a ratio of 65 parts of as produced coke (refer to Table II) with 35 parts of ground coke which was separated into two portions to which was added 12 wt. percent and 14 wt. percent coal tar binder (based on total mix) and two electrodes were prepared. In each case the coke and pitch were mixed at a temperature of about 50 F. above the softening point of the pitch which was about 230 F. The mix was placed in a mold and pressed at about 5000 p.s.i.g. to form molded green carbon electrodes. The green electrodes were packed in fine carbon and baked at 2000 F. The green electrodes were gradually heated from ambient temperature to the 2000 F. temperature over a period of up to about 48 hours. After baking, the electrodes were inspected and results obtained are shown below in Table III.
TABLE III Electrode Composition:
Pitch Content, Wt. percent From the above data it can be clearly seen that the physical properties of the electrode prepared with 12 wt. percent binder are far superior to the electrode prepared with 14 wt. percent binder. The loss on baking of the electrode with 14 wt. percent binder was substantially greater than the loss on baking of the electrode with 12 wt. percent binder due primarily to the volatilization on leaking of the excess binder added to that formulation. The bulk density and the resistivity are about the same since, after baking, the volatile materials of the binder are removed by cracking and volatilization. The completely unexpected result, however, is in the substantial increase in crushing strength of the electrode containing the 12% binder, primarily 12,400 psi. as compared with 8,420 p.s.i. for the electrode containing 14 wt. percent binder. The crushing strength of the electrode with 14 wt. percent binder, however, is still very high as compared to electrodes made with calcined low temperature fluid coke and calcined delayed coke. However, fine cracks appeared in the electrodes containing 14 wt. percent binder. Therefore, though the physical properties of the electrode prepared with the 12 wt. percent binder and 14 wt. percent binder are both good, the electrode containing 14 wt. percent pitch binder had fine cracks which cannot be tolerated in good carbon electrodes. The above example shows the criticality of the maximum amount of binder which can be used in a good electrode to be about 13 wt. percent.
In this example, the coke aggregate consisted of as produced coke having an average diameter at least 6 times as large as the average diameter of the smaller ground coke, and the weight ratio of the coarse to fine coke was about 2:1. The distinguishing feature in this formulation was the critical concentration of binder which was used with this new high temperature coke to make superior electrodes.
In order to further show the superiority of the coke electrodes of the present invention over the electrodes prepared in a conventional manner, the following electrodes were prepared: Two separate coke formulations were prepared; from calcined delayed coke and calcined low temperature fluid coke having substantially the same particle size distribution as used in the electrodes prepared above (see Table II for particle size distribution). In each case the coarse and ground coke were mixed in a ratio of about 65 to 35 parts by weight. In each case 12 wt. percent coal tar binder was used to make electrodes. The coke aggregates and pitch binders were mixed for /2 hour at a temperature of about 50 F. above the softening point of the pitch. The mix in each case was placed in a mold and pressed at 5000 p.s.i.g. to form green electrodes. The green electrodes were removed from the molds and packed in fine carbon and baked at a temperature of 2000" F. The maximum temperature was obtained after baking for a period of up to about 48 hours. The electrodes were removed from the baking oven and tested and were found to exhibit the characteristics shown below in Table IV.
TABLE IV [Comparison of electrodes prepared* from delayed coke, fluid coke, and high temperature fiuid coke using 12 Wt. percent pitch binder] *All electrodes were prepared with essentially identical particle size distribution and with the same preparation pro cedure and amount of pitch binder.
The above data clearly show that the high temperature fluid coke used in accordance with the present invention results in obtaining coke electrodes of superior quality to the electrodes prepared in the same manner from calcined delayed coke and calcined low temperature fluid coke.
Example 2 A Soderberg electrode is prepared from a coke electrode formulation having a particle size distribution about the same as that shown in Table II. In this formulation a ratio of coarse as produced coke to fine coke of about 2:1 is used having a ratio of average particle size of coarse to fine of about 6:1. A coal tar pitch is added and mixed to prepare a formulation containing the binder and the coke aggregate. The formulation contains about 82 wt. percent coke aggregate and about 18 wt. percent of pitch. The formulation is fed to an electrode cell operated at a temperature of 1750 F. As the formulation is fed to the cell, the binder is baked out and a high density, high crush strength, low resistivity coke electrode is fonmed by the time the formulation reaches the molten aluminum bath. The formulation is baked into a solid electrode by heat conduction and the hot cell gases in the period between the time it is added at the top of the cell and the time that it is consumed in the reduction of aluminum ore to aluminum. By using the high temperature fluid coke in accordance with the present invention in the electrode formulation, a substantial improvement ove other Soderberg electrode coke formulations is obtained In this process the coke is used as produced and does not undergo a separate calcining step. Large coarse coke aggregate is not used. The coke therefore does not have to be agglomerated and crushed to provide large coke aggregate.
The above described procedure results in substantial savings in preparing Soderberg electrodes.
The high temperature fluid coke used to make the improved coke electrodes in accordance with the present invention can be prepared in a fluid bed wherein the heat is provided in a transfer line burner, a moving bed or fluid bed burner by burning a suitable fuel, or where the heat is provided by resistance heating of electrodes either in a separate heater vessel or in the reactor. The heat can be provided by burning an extraneous fuel in the burner, such as a hydrocarbon or hydrogen. Any coke prepared by thermal decomposition of hydrocarbons at temperatures between l8002500 F. to produce essentially coke and hydrogen whereby a hard, dense, low porosity coke where the coke deposits to form laminar onion-skin layers on the seed coke can be used.
The coke formulations of the present invention can be used as high temperature lining, heating electrodes, elec trodes in electrolytic reduction of aluminum ore to metallic alumina, or of other metal ores to molten metal and anywhere where reactivity, crushing strength, or electrical resistivity of the carbon in a carbon body is important.
These electrodes find greatest utility in their use as anodes for the obtaining of aluminum from its ore by the electrolytic process. The principles involved can be utilized, however, in the preparation of other electrodes. It is to be understood that the invention is not limited to the specific examples which have been offered merely as illustrations and that modifications may be made without departing from the scope of the invention.
What is claimed is:
1. A molded coke body comprising a coke aggregate containing coarse coke particles having an average size between 200 to 500 microns and fine coke particles having an average size between 25 to 75 microns, and 9 to 13 wt. percent, based on total weight of said body, of a carbonaceous binded material, said coarse and said fine coke particles being produced by the thermal decomposition of a hydrocarbon feed in a fluidized bed of coke particles at a temperature of 1800 to 2500 F.
2. The molded coke body of claim 1 wherein the binder concentration is 10 to 12 wt. percent.
3. The molded coke body of claim 1 wherein the ratio of coarse to fine coke is 1:1 to 5 :1 parts by weight.
4. The molded coke body of claim 1 wherein the ratio of the average particle size of the coarse coke to the fine coke is 4:1 to 20:1.
5. A Soderberg electrode code formulation comprising a coke aggregate containing coarse coke particles having an average size between 200 to 500 microns and fine coke particles having an average size between 25 to 75 microns and 18 to 24 wt. percent, based on total weight of said formulation, of a carbonaceous binder material, said coarse and said fine coke particles being produced by the thermal decomposition of a hydrocarbon feed in a fluidized bed of coke particles at a temperature of 1800 to 2500 F.
6. The composition of claim 5 wherein the ratio of coarse to fine coke is 1:1 to 5:1 parts by weight.
7. The composition of claim 5 wherein the ratio of the average particle size of the coarse coke to the fine coke is 4:1 to 20:1.
8. A green prebake electrode formulation consisting essentially of coarse coke having an average particle size of 250 to 350 microns, fine coke having an average particle size of 40 to 50 microns, a ratio of coarse coke to fine coke of 2:1 to 3:1 and a ratio of the average size of coarse coke t-o fine coke of 6:1 to 10: 1, and containing 9 to 13 wt. percent of carbonaceous binder, said coarse coke and said fine coke being produced by the thermal decomposition of a hydrocarbon feed in a fluidized bed of coke particles at a temperature of 1800 to 2500 F.
9. The composition of claim 8 wherein 15 .to 30 wt.
1 1 percent of ground prebake electrode butts having a particle size of up to A; to inch are included in the formulation.
10. A green Soderberg electrode formulation consisting essential of coarse coke having an average particle size of 250 to 350 microns, fine coke having an average particle size of 40 to 50 microns, a ratio of coarse coke to fine coke of 2:1 to 3:1 and a ratio of the average size of coarse coke t-o fine coke of 6:1 to 10: 1, and containing 18 to less than 24 wt. percent of carbonaceous binder, said coarse coke and said fine coke being produced by the thermal decomposition of a hydrocarbon feed in a flu- 12 idized bed of coke particles at a temperature of 1800 to 2500 F.
References Cited by the Examiner UNITED STATES PATENTS 2,805,199 9/1957 Banes et a1 204294 2,881,130 4/1959 Pfeiifer et a1 208127 3,197,395 7/1965 Nelson 204-294 10 JOHN H. MACK, Primary Examiner.
H. S. WILLIAMS, Assistant Examiner.