US 2657118 A
Description (OCR text may contain errors)
Oct. 27, 1953 N. w. F.1PH1LLlPs ET AL 2,657,118
METHOD OF' PURIF'YING CARBONACEOUS MATERIAL Filed Sept. 2l, 1950 ,f, /Vbrman [ViP/T1015 E11/tart /Yz/l/rgread .Rimas S. huid/nar JNVENTOR.
BY Wsw 47m/wr Patented Oct. 27, Q
UNITED STATES PATENT OFFICE METHOD OF PURIFYING CARBONACEOUS MATERIAL Application September 21, 1950, Serial No. 185,947
14 Claims. 1
This invention relates to the purication of coke and coal, viz. carbonaceous material of coal origin, and is particularly directed to the manufacture, from such material, of a puried carbon having a greatly reduced content of so-called ash, i. e. inorganic impurities of which important examples are iron and silicon. In a more specic sense, the invention is directed to the treatment of coke or anthracite coal for conversion into a carbon product having such purity and other characteristics as to be suitable for making carbon electrodes which can be used in electrolytic procedures of the molten electrolyte type.
For example, in the manufacture of aluminum by electrolytic reduction of alumina in a suitable fused bath, the necessary carbon electrodes have usually been manufactured from so-called petroleum coke, of relatively high purity. In order to obtain metallic aluminum having a very low content, if any, of elements such as iron, silicon, titanium and the like (e. g. aluminum with at most a iew tenths of a percent total impurity), it is essential that the electrodes be relatively free of such elements; that is to say, the electrodes are continuously consumed in the operation and -any impurities of this character pass into the bath and thence go to contaminate the deposited aluminum. Although petroleum coke may sometimes be unavailable or unavoidably costly, ordinary coke, anthracite coal and the like have not heretofore been economical substitutes ior carbon electrode manufacture, because of their high content of impurities of the character stated. lt may be noted, in passing, that references herein to colse mean ordinary coke derived from coal (usually bituminous coal) unless otherwise specifically designated.
Accordingly, chief objects of the invention are to afford improved procedure for purifying materials of this class (coke and coal) notably procedure which is of a simple, eflicient and economical character and which yields a greatly puriiied carbon product, especially one that is suitable for making electrodes of the sort mentioned above. A specific object of the invention is to afford new, improved and more effective methods of purifying such material, particularly coke, by chlorination.
To these and other ends, it has now been discovered that a fully sufficient and indeed relatively complete separation of the impurities from coke or like material may be achieved by treating it with chlorine gas at the critically elevated temperature of about l400 C. or higher, for a reasonable period of time. By such treatment the total ash content of coke can be reduced from a value of 10% or more to 2% or less, the most diilicultly separable contaminants, such as iron and silicon being removed to the point where only a minor fraction of a percent of each remains (all such percentages being given by weight, herein). The process is advantageously carried out by owing chlorine gas through the coke or coal while heating the latter to keep it in the stated, extremely high temperature range. Good results can be obtained in even a very few hours, and in all cases within a period that permits economical and efficient operation. Aocording to present understanding, the chlorine reacts with materials such as iron oxide, silica land titanium oxide to yield volatile chlorides of the metals. The chlorides are in fact formed or immediately released in a volatilized. or gaseous state and are swept out of the treatment zone in the current of gas flow. The following equations are understood to represent the type of reaction which thus occurs between the contaminating metal compounds and chlorine in the presence of carbon, all of the reaction products being established and separated in the gaseous state:
While treatment of coke with chlorine at moderately high temperatures, e. s. up to 1100 C. or so, has been heretofore proposed with the View of purifying the material by reactions of this sort, experience has indicated that such treatments are ineiective to produce a satisfactorily pure carbon, and specifically are incapable of yielding a reduction oi silica content suitable for highly pure electrode carbon or of yielding such reduction in any reasonably rapid `and efcient manner. It has neither been apparent nor recognized, however, that operation at substantially greater temperatures would afford signicant improvement in such results or would aid them in any workable manner or within an economically feasible time of treatment. indeed there seemed no practical way of obtaining a suitably pure carbon from coke by the previously suggested chlorination methods (even up to 1000"-1200o C.) except perhaps by subjecting the coke, after such treatment, to the action of hydrofluoric acid. For instance, by applying gaseous hydrogen iluoride at a suitably high temperature, much of the silicon content can be converted to a volatile fluoride, but such operation is costly, and very inconvenient because of the corrosive or reactive nature of hydroiuoric acid. With the process of the present discovery, there is ordinarily no need for a supplemental uoride treatment; on the contrary, an unusually complete separation of silicon, as well as improved purification with respect to other mineral impurities (e. g. iron, titanium and the like) can be achieved by a single, economical operation.
In carrying out the method in a manner that now seems most practical, the coke, in a suitably granular state, is charged into a furnace having appropriate means for heating the charge and for the continuous traversal of gas. Chlorine gas is continuously introducedJ say at the bottom of the furnace, and the spent gas containing the reaction products is continuously Withdrawn at the top, so that a constant flow of chlorine permeates the coke which is kept at the selected high temperature in the reaction zone. One effective expedient is to heat the charge electrically by its own resistance, e. g. by passing a current through the body of coke between electrodes which project into it. The temperature `of at least the main body or major part of the coke in the reaction zone is kept at 1400" C. or more, the hottest parts of the charge probably reaching 1500 to 1700 C. or so. Present evidence does not indicate that there is any specific upper limit to the temperature, beyond such as may be imposed by economy and convenience. Under conditions appropriate to commercial operation and with cokes of average character, amply satisfactory results have been obtained where the temperature is in the range stated; at an extremely high heat (say, if the bulk of the charge is heated to above 1600 C.), energy -requirements and adverse effects on the furnace Vstructure are apt to be objectionable.
While considerable conversion of metallic impurities to volatile chlorides can be achieved by treatments not greatly exceeding one hour, a chlorination period of several hours is ordinarily desirable, to decrease the silica and other contamination of the coke to usefully low values. In the case of large quantities of coke, the treatment at 1400o to 1500 C. should preferably have a duration of the order of 6 to 10 hours; for instance, with large furnaces operated on a sem"- continuous basis, the total passage time for any specific quantity of coke through the pre-heating, reaction and discharge Zones may be as much as l2 hours or more, yet without detriment to the efficiency of the operation. It Will be understood that such a furnace is advantageously arranged to permit some pre-heating of the coke by conduction and by the effluent gas, before it reaches the region of direct heating. As explained, chlorine gas flows continuously into the furnace, very preferably in a direction countercurrent to the travel of coke although concurrent operation can be employed in some cases.
Upon completion of the desired period of treatment with a given batch or portion of coke, it may be discharged from the furnace and appropriately cooled or quenched to prevent its burning and to permit its being handled in conventional conveying equipment. For example, dry cooling, as by combustion or other inert gases (if available) can be employed, or more conveniently in most cases, a quenching operation can be performed with a water spray and preferably in such fashion that the product is not left in a wet condition. While there is usually some chlorinein the product, such contamination is significantly less in the present process than has been found in prior chlorination treatment at temperatures of 1000 to 1100 C. This difference in chlorine contamination has been particularly noted in the case of anthracite coal; while treatment of such material at 1000 C. leaves a relatively considerable chlorine content, treatment in accordance with the present invention leaves only a much smaller amount of chlorine in the coal.
In any case, it has been found that if the chlorine content of thercoke or coal is too high, it may be simply and effectively reduced by treatment with hydrogen. For instance by treating the chlorinated coke or anthracite in a furnace at 1 i00o C. with a moderate flow of hydrogen gas for one to three hours, chlorine concentrations of 0.8 to 1.1% in such products have been lowered to 0.3 to 01.5%.
By way of illustration of the results of the present process and also to show its advantage in comparison with chlorination treatment effected at a lower temperature, the following are representative examples of many tests which were conducted with various kinds of coke and anthracite coal. The reaction vessels were vertically disposed, porcelain tubes surrounded by electrical resistance furnaces to heat the contained charge, i. e. granular coke or anthracite which was supported on a perforated graphite plate in the lower end of the tube. A metered stream of chlorine was introduced at the bottom of each tube, While the volatile chlorides and carbon monoxide Were withdrawn from top. In the case of such treatments of two cokes of ordinary but separate commercial origin (respectively designated as A and B, and each derived from bituminous coal) and like treatment of samples of a Welsh anthracite coal, the following table shows the pertinent impurity concentrations (total ash, and iron, silicon and titanium separately) of the untreated material and of materials treated in tests at the several temperatures respectively and for the times of treatment stated. Each operation at a specified temperature represented the separate treatment of a fresh 200 gram sample of material. In each case, the coke or anthracite had a particle size such that the grains would pass a screen with a 0.3 inch opening and remain on a screen with a 0.15 inch opening, and the chlorine was supplied at the rate of one liter per minute throughout the time specified.
Chlorination of coke and anthracite batches Per- Pcr- Per- Per- Percent cent cent cent cent Ash Fe Si Ti C1 Untreated coke A 11.7 0. 73 2. 13 0.08 After 3 hr. at l200 C 3.9 0. 5'4 0. 69 0.05 After 3 hr. at 1400 C 2.1 0. 30 0.19 0.05 Untreated coke B 13.0 0.87 3.02 0.10 After 3 hr. at 1200 C. 6. 3 0. 38 1. 50 0.06 After 3 hr. at 1400c C. 1. 4 0.26 0. 22 0.05 Untreated Welsh anthra 2. 4 0. 24 0. 46 0. 01 0. 01 After 12 hr. at 1000o C. 1.0 0.05 0.16 0.01 5. 5 After 12 hr. at 1-200 C 1.2 0. 05 0.26 0.02 2.8 After 12 hr. at 1400o C i 0.7 0.05 0. 04 0. 01 1. 9
The last operation in each set of the above, i. e. at 1400o C., represented the process of the present invention, which in the case of the cokes afforded a great improvement over lower temperature treatment, in reducing the silicon contamination, as Well as in providing a product of very low total ash content and appropriately low concentration of iron and titanium. Likewise, in the case of anthracite coal, a very marked reduction in silicon content wasW obtained, it being also noted that the chlorine content remaining after treatment by the present process was much less than after the operations at 1000 or 1200* C. It should be remarked that this specific type of anthracite had an unusually low total ash content, and that the advantages of the invention are even more fully realized, for treatment of coal, in the case of Pennsylvania or other anthracite having` a relatively high ash, including correspondingly high content of the various impurities specifically noted above.
The accompanying drawing is a diagrammatic View of a furnace employed in carrying out the process, shown in central, vertical section, the drawing also serving to illustrate the process itself. While other types of furnace may be used, the arrangement shown has been found very effective. Essentially, the apparatus comprises a vertical shaft furnace housed in a gastight steel shell I and lined with refractory material I2, preferably dense, high quality fireclay or like refractory (in brick, tile or other form), usually or preponderantly having a composition of aluminum silicate or silicates. Portions oi coke or anthracite to be treated are added at the top I 3 and portions of purified coke product are withdrawn from the bottom I4, from time to time, so that the cylindrical furnace chamber I5 is always substantially filled with coke Il, intermittently moving downward. Chlorine gas is introduced continuously through a lower inlet I8 and spent gases removed from an upper outlet I9. Although other heating means can be used, the coke charge is preferably heated by internal means, and very advantageously by the resistance heating effect of electric current traveling through the charge, as between one or more upper carbon electrodes 20 and one or more lower carbon electrodes 2I, the illustrated furnace having three such electrodes (of which two are shown) at each locality, all extending well into the furnace. Thus the electric current or currents are conducted through the coke, along paths essentially parallel to the furnace wall I2 and to thedirections of countercurrent travel of the coke and chlorine gas, the great preponderance of the current and hence of the heating eifect being disposed inn wardly of the furnace Wall.
As another example of the invention, coke was treated in a vertical shaft furnace of the character shown and having an internal diameter of 4.5 feet, lined with refractory material and provided with carbon electrodes 20, 2I arranged for passage of current through the coke charge in a vertical direction, e. g. over a distance of the order of 20 feet. The coke was in the form of large grains, having rather uniformly a size of about inch diameter. With the heated zone capacity of the furnace being about 6 tons, and with chlorine passed through the heated charge at rates varied from about 100 to about 300 pounds per hour, purified coke was produced at the rate of 250 to 500 pounds or more per hour. Quantities of raw coke were added at the top of the furnace and quantities of puried product withdrawn from the bottom at regular intervals, the production rate of suitably pure carbon being found to vary roughly with the rate of chlorine supply, within the ranges mentioned. The average analysis of the supplied coke indicated a total ash content of 10%, specific impurity concentrations (measured in each case as the designated element) being 0.8% iron, 2.2% silicon and 0.2% titanium. Approximately 50 tons of suitably purified carbon product, made in a number of runs as just described, contained on the average and with reasonable uniformity: 1.2% total ash; 0.10% iron, 0.10% silicon and 0.03% titanium.
While a longer actual treatment time for a given portion of coke was employed in some of these runs and may sometimes be more economical in continuous production, it was found that a contact time of 8 to 12 hours at the specified temperature and with chlorine flow preferably at the higher end of the range last above mentioned, will usually assure production of a purified carbon having well under 0.2% iron, well under 0.2% silicon and less than 0.05% titanium. It will now be appreciated that the conditions of operation in any given case should and easily can be selected to suit the specific circumstances, including the average analysis of the coke used and the specific purity desired in the product. The process can be successfully run so that there is little or no free chlorine in the eiiluent gas; economical results of this type were obtained in the several furnace operations described herein, at least 50% of the supplied chlorine being consumed in actual reaction with various components or constituents of the ash content of the coke. Some of the chlorine is unavoidably lost in attack on the refractory lining of the furnace (which must be re-surfaced or renewed from time to time) and a small proportion may be retained in the coke, as explained above, but the operation has proved amply efcient in yielding a pure, electrode-type carbon at reasonable cost.
In a modied furnace of the same upright shaft type and electrical heating arrangement (illustrated in the drawing), but having a thicker lining of refractory brick and thus an internal diameter of 3 feet, like results were obtained. This furnace had a total holding capacity of 4.7 tons of coke, including 2.8 tons in the heated zone. The raw coke had an ash content of about 9%, including about 0.6% Fe, 2.0% Si and 0.08% Ti. With the temperature of the charge at 1400 C. (and higher at the center) and with chlorine supplied at 94 lbs. per hour, which equalled 0.36 lb. of chlorine per pound of product, a production rate of over 3 tons per day (of 24 hours) was achieved. The ash content of the product was about 2%, including 0.09% iron, `0.15% silicon, and 0.04% titanium. At a slightly lower production rate and with a little less chlorine a puriiied coke was obtained having only 0.66% total ash, 0.05% Fe, 0.07 Si and 0.04% Ti.
When anthracite coal is treated in a furnace which is to be heated by passage of electric current through the char-ge, it is usually desirable to perform first at least a partial calcination, e. g. by pre-heating the coal with the exhaust gases and by conduction in an upper part of the furnace, in order to reduce the resistance of the coal to a. value permitting suiiicient flow of current. In a continuous operation on anthracite, it will be understood that the furnace can be filled at the outset with coke or with anthracite which has been suitably calcined; thereafter, subsequently introduced quantities of the coal are sufficiently pretreatedin the upper part of the furnace before descending to the principal reaction zone.
In the examples of furnace operations, the temperature was maintained at 1400i C. or higher throughout nearly all of the charge between the electrodes, i. e. except for a small part at lower temperature immediately adjacent the refractory Wall. No very critical regulation was employed il .or'foundnecessary .'As statediabove, it-wasfapparent that the-.temperature in the center orfhot- `test partzof the charge was considerably higher, probably reaching 1600 C. or more. .ItWas found desirable,to.avoidrhydrolysis of volatile chlorides with moisture in the coke entering the top of the furnace, that the coke have a moisturecontent .of not more vthan about 0.5%, such'condition -being achieved lby vprelin'iinary drying if necessary. Direct Vcondensation of chlorides yin the'entering coke was `satisfactorilyavoided by adding small amounts `of 'coke frequently rather than slarger quantities less often,1and by keeping the'charge level'at not more than a certain distance (compatible with heat economy) above the uppermost current-supplying electrodes, e. g. 8 feet inthe larger of theabove furnaces and 3 feet in the smaller. In theseways, deposition of finely divided solids in the interstices of freshly charged coke was avoided; while such material would presumably be eliminated in the high temperature zone, it sometimes tendedl'toimpede proper downward vflow or charging ofthe coke, orto have-a vclogging-effect against a -desirably high irate -of Ygas passage. When necessary,preliminarytcleaning or liketreatmentsfof a conventional sort may be performed ori-the lcoke cron thefcoal from which it is made, such as'tabling operations or other physicalbeneiciation, to `separate particles or pieces of foreign matter'prior to the present process.
A particularly important feature or attribute of the process, especially when carried-out `in .a furnace forexample of the type shown, ist-hat the carbonaceous.charge'exhibits a surprisingly I high `temperature gradient in regions close to 4and adjoining the inside .furnace wall. Thus it was discovered `that in the present procedure, 4notably kwhen performed with internal heating of a coke charge, the temperature gradient next to the re-claybrick Wall (isefthefinner `surface of the Wall `l2 in th'e drawing) is about 200"C, per inch, a condition that kwas unexpected `View of the relativelyhigh thermalconductivity of coke. Indeed, evidence r'indicates that 4where the average temperature of the vcharge'is 14'00" C. or more and the temperature at the center may be as'high as-about 1600o C., the temperature begins to fall off rapidly (when considered radially outward from the center of theffurnace'toward the wall) at say, several inches from the Wall, and reaches the stated, very high gradientat the outermost localities of 'the chargebody. A notable advantage of this condition is that the deterioration of the furnace, especially by cherriical attack, is greatly reduced; vthat is `to say, at
Vtemperatures Well below 1400o C. 'and especially at about 1200 C. (or less), the'rate of attack of furnace gas (i. e. chlorine) Aon the .refractory materialis muchv less than at140`0 ,andihighen Thus the process may-be effectivelyperformed (in a -zone advantageously lined with` refractory) with essentially all of the charge vat the desired, high .temperatures and yetl with an economically Vlow rate of rattack or disntegrationof the y.furnace lining, i. e. very considerably lower-thaniif such `refractorymaterial (aluminum silicates) or the like Were subjected to temperatures .in .the range of 1400c to 1600 or so. At the same time, the small portion or layer of coke that adjoins the wall tends to `travel downward more slowly than the mainbodyof the charge, and thus even this layer somewhattendsto .be compensated, by a klonger time of treatment, fortheslower purifiation there occurring. The advantageous and .unusually high temperature gradientat .the peripheryof lthe charge Vdoes not seem to depend 'significantly on the chemical composition .of ylining, except that it vhas been fully realizedfinfurnaces having a lining ofvconventional dense refractory, and may-be less where a large-.amount of porousor other highly insulating material is used .and provides for a lar-ge temperature drop, be- -tween the reaction chamber and Ythe youter `steel shell. As indicated above, the peripheralgradient-in the cokeor anthraciteisenhanced-or very lsignificantly -effectuated 'by `procedure invloving internal `heating of vthe charge, especially v.by .the passage lof electric vcurrent in the manner -described abovein reference to the drawing. Thus the supplied current vis easily controlled'to Yheat Hthemajor part of the coke charge to the desired ,temperature of 14:00u C. or so, i. eso that the average temperature of the -charge is at such temperature or higher, yet Withiinsufcient heating effect (due to the stated gradient) to heatthe refractory lining above about 1200 YC.
vIt will now be seen that the `described zprocedures of the invention affordveconomical :and eilicient operations `for thepurication of -coaltype `material to obtain a carbon `product suitable'for various uses, especially for .the manufacture of carbon electrodes. The method -is peculiarly effective in thecase` of coke anda'nthra'-l vcite coal. In fact it is usefully.capableofremov- ,ing inorganic ash material (e. g.`iron, silicon or Vthe like) from bituminous coal,-but the properties-of this material whenheated ordinarily render its treatment ldifficult and uneccnomical. Therefore while some bituminous coals, especially types having characteristicssomewhat lapproaching those of anthracite, might 'be successfully treated, bituminous coal is not in general VAnow considered a preferred Yraw material for the process described herein.
It'is to be understood that the invention is not limited to the specific vprocedures Vhereinabove described but ymaybe carried out inother `Ways Without departure'from its spirit.
1. `A method 'of purifying Vcarbonaceous matelterial :of coal origin which contains inorganic contaminating material, comprising treatingthe `carbonaceous material with chlorine at fa .tem- Aperature of-atleast about 1400 C. to convertinorganic .contaminating material 'to Vvolatilized form, and vlseparating the volatilized material v:from the carbonaceous material.
2. A method of purifying silicon-containing fcarbonaceous material of coal origin-:comprising treating the material `with chlorine at vaitem- Aperature of yat least about 14007 C. toconvert silicon contained -in said material to volatilized, compound form, and separating the volatilized silicon compound from the material.
`3. :A method of purifying :material of :the 'class consisting :of coke and anthracitecoaL said material containing inorganic impuritiesincluding -consisting of coke and anthracite coal to reduce thel content y. of `iron andsilicon in -said material,
Acomprising treating `the material with -chlorine at a temperatureofat least about 11400.C.for at least three hours, to convert impurities in the material, including iron and silicon, to volatilized form, and separating the volatilized impurities from the material.
5. A method of purifying coke containing impurities constituting an ash content of more than 5%, comprising treating the coke by passing chlorine gas through it While maintaining the coke at a temperature of at least about 1400 C., to reduce the ash content to not more than about 2% by converting impurities in the coke to volatilized form, said Volatilized impurities being carried away from the coke with the passing gas.
6. A method of purifying coke which contains impurities including more than 0.5% iron and more than 1% silicon, comprising continuously passing chlorine gas through a body of the coke having a moisture content of not more than 0.5% while maintaining the coke at a temperature of at least about 1400" C., to reduce the iron and silicon content to not more than about 0.2% each by converting impurities in the coke, including iron and silicon, into volatilized form, said volatilized impurities being carried away from the coke with the passing gas.
7. A method of purifying inorganic impuritycontaining carbonaceous material of coal origin, comprising treating the material with chlorine at a temperature of at least about 1400o C. for at least about six hours, to convert inorganic impurities in the material into Volatilized form, and separating the volatilized impurities from the material.
8. A method of purifying coke which contains impurities including iron and silicon, comprising treating the coke with chlorine at a temperature of at least about 1400 C. for at least eight hours, to convert impurities in the material, including iron and silicon, into volatilized form, separating the Volatilized impurities from the coke and recovering the purified coke.
9. A method of treating lcarbonaceous material of coal origin to purify it, comprising passing chlorine gas through a body of said material which is laterally confined at a boundary that extends around the body, and heating a major part of the body to at least about 1400c C. during said passage of chlorine, while maintaining a temperature gradient in the outermost portion of the material adjacent said boundary, and thereby keeping the temperature of the material at said boundary at a value substantially below 1400 C.
10. A method of treating coke to purify it, comprising passing coke in divided form downward through a vertically elongated reaction zone peripherally lined with dense refractory material, while passing chlorine gas upward through said zone, and heating the coke so that a major part of the coke is heated to a temperature of at least about 14.00 C. during said passage of chlorine, while maintaining a sharp temperature gradient in the coke near the periphery of the zone, decreasing toward said periphery, and thereby preventing the temperature of the coke at said periphery from rising above about 1200" C.
11. A method of treating coke to purify it, comprising passing chlorine gas through a body of coke in permeable form, while confining the body of coke Within a surrounding surface and While heating the body of coke by releasing heat internally thereof at localities which are at least nearly all spaced internally of the body from said surface, said heat being sumcient to heat a major part of said body to at least about '1400 C., while maintaining a temperature gradient in the portion of the coke immediately adjacent said surface, so as to prevent the temperature of said surface from rising higher than about l200 C.
12. A method of treating carbonaceous material of coal origin to purify it, comprising passing said material in divided form through an elongated reaction zone peripherally lined with dense refractory material, while passing chlorine gas through said zone, and while passing electric current through the material in said zone between terminal regions spaced lengthwise of the zone and lying principally in the path of the material, so that a major part of the material is heated to a temperature of at least about 1400 C., a temperature gradient decreasing toward the periphery of the zone being maintained in the material near said periphery, and the aforesaid heating of the material by the current being ef.- fected so that by said gradient the temperature of the material at said periphery is kept substantially below 1400" C.
13. A method of treating coke to purify it, comprising passing successive quantities of coke in divided 'form downward through a vertically elongated reaction zone wherein the coke is laterally conned at a predetermined boundary. while passing chlorine gas upward through said zone, and while heating a major part of the coke in said zone to at least about l400 C. by passing electric current through the coke, at least nearly all of said current being passed along paths clistributed in a region of the zone spaced centrally inward of said boundary, a temperature gradient decreasing toward said boundary being maintained in the coke near said boundary, and the heating of the coke by the current as aforesaid being controlled to prevent the temperature of the coke at said boundary from rising above about 1200" C.
14. A method of treating coke to purify it. comprising passing chlorine gas through a body of coke which is laterally confined at a boundary that extends around the body and lengthwise thereof in a predetermined direction, said coke being in divided form, and heating a major part of the body to at least about 1400 C. during said passage of chlorine, by passing electric current through said coke, at least nearly all of said current being passed along paths distributed in a region of the body spaced centrally inward of said boundary, a temperature gradient decreas- 1 ing toward said boundary being maintained in the coke near said boundary, and the heating of the coke by the current as aforesaid being controlled to prevent the temperature of the coke at said boundary from rising above about 1200 C.
NORMAN W. F. PHILLIPS. ETHAN A. HOLLINGSI-IEAD. JAMES S. MUTCI-IMOR.
References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,071,442 Lee Aug. 26, 1913 1,271,713 Hutchins July 9, 1918 1,277,707 Dyrssen Sept. 3, 1918 1,303,362 Mott 1 May 13, 1919 1,671,673 Doerscheek et al. May 29, 1928 2,260,746 Hanawalt et al. Oct. 28, 1941 2,315,346 Mitchell Mar. 30, 1943