US3470275A - Process for making carbon agglomerates - Google Patents

Description

Sept. 30, 1969 T. E. BAN 3,470,275

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United States Patent O U.S. Cl. 264-29 5 Claims ABSTRACT F THE DISCLOSURE This invention is in a process for producing a carbon agglomerate from raw finely divided coke material which comprises carbonizing a burden of raw extrudate or compacted bodies of a mixture of such coke with a tarry binder at elevated temperatures which reach into the range of from 1600 F. to 2100 F. by passing hot gases transversely to the direction of movement of the burden, and recycling the gases through the trevelling bed to cool the burden to an exit temperature of less than about 1000 F. and to reheat the gasses for repassage through fresh burden to carbonize 0r garphitize the coke and yield a hard, baked, product which is suitable for use in making numerous carbon products, for example, electrodes.

This invention relates, as indicated, to a process for carbonizing and agglomerating relatively finely divided coke to yield a carbonaceous product which is more readily adapted for certain uses such as, for example, the formation of electrodes, and particularly electrodes useful in the production of aluminum metal. While this process will be illustrated with respect to lluid coke, it will be understood that other cokes, or mixtures thereof, such as petroleum coke or delayed coke, wood coke, or coke derived from coal may be used as wel as a substitute for part or all of the fluid coke.

The preparation 0f electrodes, for example, from fluid coke is described in Patent 2,805,199, dated Sept. 3, 1957, which sets forth the method of obtaining fluid coke and the properties of such a material. This type of coke is a specific example of a material which is useful in accordance with the present invention.

For use in the manufacture of anodes for aluminum metal production, the density of the coke should be adjusted to be in the range of from about 1.8 to about 1.95, preferably 1.87 to 1.92, and have a resistivity of 20--30 103 ohm-inches. In general, this is accomplished by calcining the coke, particularly fluid coke, at a tern perature in the range of from 2000 to 2800 F. or higher. After the coke has been calcined to adjust its density to the desired range and its resistivity to within the desired range, a portion of the coke is then ground so that from 20 to 50 weight percent of the coke utilized in manufacturing the electrodes has an average diameter of less than about 75 microns. As indicated in Patent 2,805,199, the grinding of the coke particles can be accomplished in any conventional manner, and grindingmay precede the calcining step but preferably follows the calcining step.

It was found that although electrodes made in accordance with the teachings of Patent 2,805,199 were useful, it was desirable to improve the properties of the electrodes by including in admixture with the fluid coke, a proportion of larger diameter coke which had also been calcined, but had a particle size covering a very broad range of from about one inch in diameter to about 200 mesh, the proportion having particle sizes in this range being about 80 weight percent, with the balance finer than 200 mesh. Carbon electrodes manufactured from such a 3,470,275 Patented Sept. 30, 1969 ICC mixture of calcined delayed coke and calcined uid coke are described in Patent 2,835,605, dated May 20, 1958. Principal problems with electrodes made solely from uid coke include dusting, premature breakdown or shredding of the electrode in the aluminum bath with attendent increase in the electrode requirements, and the experiencinghof diiiiculty with short-circuiting in the electrolyte bat The present process represents an improvement on prior methods of making electrodes from relatively finely divided coke, such as fluid coke, whereby it is possible to convert relatively nely divided coke material into an agglomerate wherein the coke has the desired density and resistivity characteristics as above stated, and which is amendable to crushing and size reduction to yield particles having a broad spectrum of particle sizes such that the diiculties of dusting, premature breakdown or shredding, electrode economics, and processing diiculties can be greatly minimized.

It has been found that relatively finely divided cokes may be admixed with a carbonaceous binder in an amount conveniently ranging from about 18 to 45 parts by weight per hundred parts coke. To determine the minimum amount of coal tar pitch or other binder required to adequately bond the nely divided coke, a procedure such as that described in U.S. Patent No. 2,751,782, dated lune 26, 1956, may be used. By process the minimum pigment-volume concentration, or critical pigment-volume concentration may be accurately determined, the coke being considered as a solid particulate material or pigment dispersed in the pitch. Such a determination will, of course, be made of an elevated temperature where the pitch is a liquid. The binders utilized for this purpose are conventional and include materials such as aromatic coal tar pitch binders, for example, see U.S. Patent No, 2,683,107. These binders generally have melting points within the range of from about 70 to 120 C. and contain relatively small amounts of hydrogen, usually on the order of about 5% or less. The concentration of benzene and nitrobenezene insoluble proportions represents preferably from about 20%-35% and 5%-15%, respectively, of the binder. The coke is admixed with the coal tar pitch at an elevated temperature to provide a uid mass, and this uid mass then normally extruded to produce cylindrical compacted bodies or slugs about 1.5 inches long and having a diameter of about .75 inch. When the slugs are cooled to normal ambient temperatures, they set up to a normally solid material and are then easily handled. The coke which is utilized in forming these slugs is, as indicated above, usually calcined and desirably ground. In certain instances, raw coke which is unground may be used and calcined at the time it is carbonized. Instead of extruded slugs, other compacted bodies of coke-binder mixture may be used, e.g., pellets, briquettes, pills, etc.

Thereafter, the compacted bodies are charged to a horizontally moving traveling grate` Such traveling grates may be of conventional design, such as the straight line continuously traveling grate, or it may be of the circular type such as described in U.S. Patent 3,302,936, dated Feb. 7, 1967. The raw extrudate slugs are charged to the traveling grate to form a relatively shallow bed having a depth generally less than about l2 inches, and preferably in the range of from about 4 to about 8 inches in depth. The compacted bodies are baked by means of a draft of hot nonoxidizing gases passed transversely, preferably downwardly, through the burden composed of the compacted bodies or raw extrudate slugs in a first zone. For the purposes of this invention gases containing less than about 5% by volume of oxygen are considered nonoxidizing or substantially neutral. In the cases of very reactive carbon forms, oxygen-free gases are desirably used. The gases may beneficially be reducing. The retention time of each incrementalv transverse section of the burden in the zone will depend, of course, upon the temperature and rate of passage of the gases through the bed, but in general, retention times on the order from 5 to minutes are all that is necessary to develop and move downwardly through the burden, a heat front which reaches a maximum of from about 1600" F. to about 2000 F. Carbonization of the binder together with some graphitization occurs along the temperature front at about 1700 F. Volatiles are removed before these temperatures are acquired, and the binder of tar pyrolytically decomposes into a hard char matrix which bonds the uid coke particles into a coherent coked product. The gases are passed through the burden at a rate which is generally within the range of from about 80 to about 160 standard cubic feet of gas (standard conditions) per minute per square foot of grate area, hereinafter abbreviated as s.c.f.m.

The burden then passes into a cooling or recuperation zone wherein the ow of gases transversely to the movement of the burden is reversed, and the gases which have by now been stripped of condensable materials externally of the burden and cooled, are recirculated through the burden at substantially the same rate as in the previous zone to extract heat therefrom to condition the gases for repassage through the first zone to effect the hardening, agglomeration, and carbonizing of the raw extrudate slugs. Usually, the draft in the first zone is downdraft, and the draft in the recuperation zone is updraft. Thus, the temperature gradient in the tirst zone proceeds downwardly and forwardly in the direction of the continuous horizontal movement of the burden toward the grates, and in the cooling or recuperation zone, the temperature gradient proceeds upwardly and forwardly in the direction of movement of the burden until the final exit temperature of the burden is generally less than 1=0O0 F. and usually ranges from a low of 150 F. to a maximum of about 800 F. To effect further cooling, water may be sprayed onto the burden as it discharges from the traveling grate. The retention time of each incremental transverse section of the burden in the cooling zone is about equal to the retention time in the carbonizing zone.

The carbonizing and preheating draft from the cooling zone usually contains combustible matter and may be further heated through the introduction of air to the systern which causes spontaneous ignition at the temperatures involved, or it may be reheated through the introduction of heated products of combustion from a torch, such as a gas torch. Gases generated and admitted to the circuit in the course of the carbonization procedure necessitate continual venting of some of the gas stream. The vented gas may be routed through an afterburner for removing smoke, combustible gases, and other entrained combustible particulate matter.

It is believed that while the extrudate is heated to a temperature of 1700 F., the binder pyrolytically decomposes to form a high char matrix which lbonds with the uid coke particles to form a coherent coked specimen. The product when cooled is then crushed to give a structure which corresponds closely to that of delayed coke, i.e., having a particle size over a broad spectrum of particle sizes and in the range of from about 1 inch in diameter to about 200 mesh, such a spectrum constituting about 90% of the product, and the balance being finer than 200 mesh. To reduce the spectrum to 80% of the product from 90%, iiuid coke may be added. This product may then be utilized directly in the manufacture of electrodes according to known procedures, and when the starting material has been iluid coke, such as described in the aforementioned Patent 2,805,199, the electrodes so produced are particularly adapted for use in the electrolytic production of aluminum from alumina.

The invention may be better understood by having reference to the annexed drawings wherein: FIG. 1 is a diagrammatic and schematic representation of a single stage indurating process embodying the principles of this invention. FIG. 2 is a diagrammatic and schematic representation of a two stage indurating process embodying the principles of the present invention.

Reference may be had to the flow diagram of FIG. 1 which illustrates a specific embodiment of the present invention utilizing a raw extrudate of a fluid petroleum coke such as set forth in the Patent 2,805,199, or a mixture of delayed coke and liuid coke as set forth in Patent 2,835,605. In the example illustrated, the basis is 2,000 pounds of raw extrudate slugs 1.5" long by 0.75 in diameter which are introduced onto a traveling grate machine of conventional structure to a bed depth of about four inches. The temperature at the point of introduction throughout the entire depth of the bed is approximately 70 F., or ambient temperature. As the burden moves into the tirst zone, the direction of travel being from left to right as shown in FIG. 1, it is contacted with heated gases having an average temperature of about 2000 F. To effect the carbonization of one ton of raw extrudate, 7300 pounds of such gas having a heat content of 4.16 million B.t.u. are required, The rate of gas ow is about s.c.f.m./sq. foot of grate area.

The temperature of the burden is rapidly increased by the movement of the gases downwardly through the burden in a direction which is transverse to the direction of movement of the burden in a horizontal plane. There develops a heat front which can easily be detected with temperature measuring devices disposed within the burden which proceeds along a gradient moving diagonally, downwardly and forwardly through the burden until the temperatures of the uppermost layers of the raw extrudate are raised to temperatures at or above 1700 F. This gradient will be identified as the carbonizing gradient and will represent an imaginary line of temperatures progressing substantially diagonally downwardly and forwardly through the burden at 1700 F. The temperatures of the burden ahead of this gradient will, therefore, be in excess of 1700 F. in the first zone, and the temperatures rearwardly of the gradient will be, therefore, less than 17.00 F. The retention time of the material within the rst zone averages about 7 minutes, and of the material within the second zone averages about 7 minutes, and the carbonizing of the raw extrudate occurs throughout the entire cross section of the burden. The grates reach a temperature desirably not in excess of about 1000 F. In those cases where temperatures are apt to exceed about 1000 F., it may be desirable to provide a hearth layer of inert material, or already carbonized slugs to serve as an insulating layer to protect the grates. Alternatively, the grates may desirably be liquid cooled. At the end of the process, the treated burden may be stripped from such a hearth layer, and the hearth layer recycled for reuse, or blended with the final product, and a portion thereof recycled as a hearth layer. The hearth layer technique in the art of conducting gas-solid phase reactions on a traveling grate with heated gases does not constitute a critical part of the present invention.

The temperatures of the gases exiting from the first zone average about 900 F., and as will be noted, the weight of the gases has increased due to the inclusion therein of volatile materials driven out of the coke-binder composition. The heat content as indicated on the drawing is about 1.7 million B.t.u. These gases are conducted conveniently through a scrubber, which may be a packed tower into which water is introduced from the upppermost portion thereof and allowed to trickle down over the packing in a countercurrent fashion to the gases. This procedure serves to remove condensible and entrained materials, and to cool the gases to minimize damage to blower equipment utilized for return of the gases in an updraft manner through the burden as it passes through the cooling zone. The cooled gases serve also to control the temperature of the grate and .prevent deleterious overheating. In the specific example illustrated, the amount of gases returned through the cooling zone amounts to 7,385 pounds having a heat content of .15 million B.t.u. The average temperature of the gases as they enter the grate area is approximately 150 F.

As indicated above, the gases traverse the horizontally moving burden in a direction normal to the movement of the burden, and because of the increase in the weight of the gases, a portion of the gases are vented through an afterburner, entering the afterburner at a temperature of about 1100" F. and amounting to 2155 pounds. The heat content of the gases at this point is .63 million B.t.u. Air is introduced into the afterburner to elect ignition of combustible components of the gases, raise the temperature thereof, and consume the exit gases being passed through a heat exchanger prior to exhausting to the atmosphere.

Fresh air may be passed through the heat exchanger to preheat the same prior to introduction into the system, or the heat may be recovered from the heat exchanges for use elsewhere in the plant. Thus, this portion is shown in dotted lines. In the normal course, ambient air in the amount of 1966 pounds may be pumped into the hood through which the gases are circulating, and in the specific example shown, natural gas in the amount of 110 pounds having a heat content of 2.35 million B.t.u. may be admixed and introduced into the recirculating gases within the hood to further increase the temperature and heat content to that point which is necessary to effect the carbonization desired. The balance of the gases from the cooling zone are, of course, recirculated through the hood as illustrated in FIG. 1, these gases now having a temperature of about 1300* F. and amounting to 5280 pounds. The heat content is approximately 1.84 million B.t.u.

As indicated in FIG. 1, there results from each ton of raw extrudate applied to the traveling grate, a production amounting to about 1830 pounds of hardened, carbonized slugs which are now ready to be crushed and utilized in the manufacture of electrodes as previously indicated. It will be noted that the amount of time required for conditioning the coke has been reduced from a matter of days, in some cases, and hours in others, to a matter of a relatively few minutes. The ability to recuperate heat content from the carbonized burden improves the economics of the treating process, and the independence from predetermined mixtures of various kinds of coke in order to derive desired properties in the final electrodes effects further economies. ,Instead of utilizing coke or blends of various kinds of coke, as indicated above, there may also be used mixtures of coke with asphaltic materials such as gilsonite or tar, for example, in Patent 3,025,229.

FIG. 2 in `the annexed drawings, illustrates a modification of the basic process which, instead of a single stage heating operation, as shown in FIG. 1, there is provided a two-stage heating system.

As indicated, there is provided a preheating zone in which gases at a temperature of about 900 F. are passed downwardly through the burden where they become cooled by giving up the heat to the burden. The gases exiting at the base of the burden are propelled by means of a fan through the terminal portion of the burden to cool the burden just prior to dumping from the traveling grate, said gases then exiting at a temperature of about 400 F. In like manner to the system shown in FIG. l, these gases are passed through an afterburner and heat exchanger prior to being exhausted to the atmosphere.

The central portion of the system shown in FIG. 2 corresponds substantially to the single stage heating system shown in FIG. 1 with the exception that the temperatures of the burden more rapidly reach the desired 1700 F. gradient because of the preheating step. A portion of the gases exiting from the updraft cooling zone are also passed through afterburner means to remove combustibles and to raise the temperature of the gases for introduction into the preheating zone. Inasmuch as the temperature of the gases may be too high for use in a preheating zone, split stream cooling means are provided so that a portion of the gases exiting from the afterburner are cooled and the resultant temperature of the gases as introduced into the preheating zone averages about 900 F. The modied process, as illustrated in FIG. 2, also results in the production of a flame front or temperature gradient proceeding downwardly and forwardly through the burden and defning a line of temperatures of approximately 1700 F. The temperatures of the burden forward of this gradient line are in excess of 1700 F., and the temperatures rearward of this line are less than 1700 F. The temperatures of the burden, as in the case of the burden in the system illustrated in FIG. 1 are substantially above the volatilization temperature of any volatile components contained in the binder or in the petroleum coke so that there is little or no internal condensation of volatilized organic material within the burden itself. Any condensation that may occur, occurs externally of the burden and principally in the scrubber which is common to both modifications. The cooling zone is likewise analogous to the cooling zone in the single stage heating system of FIG. 1 and effects, through a reversal of the direction of flow of the gases transversely through the burden, a reduction in the temperature of the solid material constituting the burden, and an increase in the temperature of the gases passing therethrough. Such movement of the gases through the burden results in a reversal of the direction of the llame front or the temperature gradient of 1700 F. so that such gradient now proceeds generally upwardly and forwardly. The temperatures of the burden to the rear of the gradient in the cooling zone are, therefore, above about 1700 F., and the temperatures ahead of the 1700 F. gradient are less than 1700 F. A final cooling zone is provided for updraft cooling utilizing exhaust preheat gases which have been cooled to a relatively low temperature by the fresh burden introduced at ambient temperatures. Because some combustible material may be extracted from the burden in this zone, the gases exiting from the nal cooling zone are also passed through an afterburner and heat exchanger to exchange the heat content `thereof with incoming air or gas which is then introduced into the central carbonizing and recuperating zone in the manner as shown in FIG. 1. Ignition of the combustible components of the gas within the central section circulating above the burden enables adjustment of the heat content of the circulating gases to a proper point, i.e., about 2000 F. to promote establishment and propagation of the llame front in the carbonizing zone downwardly toward the grates, as indicated above.

There has thus been provided an improved agglomerating and carbonizing process whereby relatively nely divided particulate coke material may be treated rapidly and eflciently to produce a hardened carbonized product which, after crushing, is particularly useful as a material yfrom which to produce electrodes, particularly electrodes for use in electrometallurgical processes, such as the production of aluminum. In general, two types of electrodes are employed by the aluminum industry including (a) prebaked electrodes and (b) Soderberg self-baking electrodes. In the former process, a mixture comprising about 78% to 82% of a coke aggregate produced in accordance with the present invention and from 13% to 22% of coal tar pitch is molded at pressures of about 3,000-10,000 p.s.i. and extruded and then baked for periods up to 30 days at l800 F. to 2400 F. Such preformed electrodes are then used in electrolytic cells being slowly lowered into the molten alumina as they are consumed.

The Soderberg process involves the continuous or intermittent addition of a coke-coal tar pitch paste into 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 70%-72% coke aggregate such as produced in accordance with the present invention and 25 %-'35 of pitch. The cells usually operate at a temperature of 1700 F. to 1900" F. and electrodes are consumed at the rate of about 0.5 inch to 1.0 inch per day. The paste is baked into an electrode by the hot cell gases in the period of time elapsing between when it is added at the top and the time it is used. The net consumption of coke or electrode represents 0.4 to 0.7 pound per pound of aluminum metal produced. It can be seen that the actual manner of fabricating the electrodes is not the essence of this invention. The production of an indurated coke aggregate suitable for subsequent crushing to a desired broad range of particle sizes is a principal objective of this invention.

What is claimed is:

1. A process for carbonizing .and aggregating relatively nely divided coke comprising the steps of:

(a) charging compacted bodies of a coke-binder composition to a traveling grate to form a burden thereon, said burden having a depth of from about 4" to about 12";

(b) moving the burden into a preheating zone;

(c) passing through the burden hot substantially nonoxidizing gases at a temperature between about 500 F. and 2000 F. transversely to the direction of movement of the burden to preheat the burden and cool the gases;

(d) moving the preheated burden into a carbonizing zone;

(e) passing hot substantially nonoxidizing gases transversely to the direction of movement of the burden to elevate the temperature of the adjacent burden toa carbonizing temperature of at least about 1700 F. progressively in a direction generally along and transversely to the burden to establish a carbonizing gradient through the burden;

(f) retaining each incremental transverse section of said burden in said carbonizing zone for a period of from about 5 minutes to about 15 minutes;

(g) moving the burden into a cooling and recuperating zone;

(h) passing through the burden substantially nonoxidizing gases which are cooler than the burden leaving the carbonizing zone transversely to the direction of movement `of said burden to reduce the temperature of the burden and transfer a portion of the heat content of the burden to the gases;

(i) recycling at least a portion of the gases from the recuperating zone to the carbonizing zone for repassage through the burden;

(j) increasing the heat content of th egases recycled to the carbonizing zone;

(k) moving the burden into a final cooling zone;

(l) passing through ther burden substantially nonoxidizing gases issuing from the preheating zone and having a temperature -less than the burden leaving the recuperating zone to further cool the brdenand raise the temperature of the gases issuing from the burden in the final cooling zone; and

(m) recovering the heat from the gases issuing from the burden in the final cooling zone for utilization in heating the gases entering the preheatng zone.

2. The process of claim 1 which is additionally characterized by th estep of removing condensible material from the gases issuing'from the carbonizing zone externally of the burden. i

3. The process of claim 1 wherein the gases being recycled to the carbonizing zone are heated to a temperature of at least about 2000 F.

4. The process of claim 3 wherein combustibles in the gases being recycled are contacted with air and ignited to raise the temperature thereof and consume the oxygen content of the air to maintain the recycle gas substantially non-oxidizing.

5. The process of claim 1l wherein the rate of ilow of the gases through the burden in each zone is maintained in the range of from about to about 160 standard cubic feet of gas per minute per square foot of great area.

References Cited UNITED STATES PATENTS 2,838,385 6/1958 Brown 44-23 3,009,863 11/ 1961 Angevine 202-26 3,010,882 11/1961 Barclay et al. '202-26 3,013,951 12/ 1961 Mansfield 201-27 3,077,439 2/1963 Shea et al. 202-26 3,331,754 7/ 1967 Mansfield 201-39 FOREIGN PATENTS 23,941 12/ 1929 Australia.

DONALD I. ARNOLD, Primary Examiner JOHN H. MILLER, Assistant Examiner 2 U.s. c1. x.R. zei- 21, 27, 29

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