US1888465A - Operation of by-product coke oven plants - Google Patents

Description

No'v. 22, 1932. s, R MILLERv 1,888,465

OPERATION 0F BY-PRODUCT COKE OVEN PLANTS ATTORNEYS NOV. 22, 1932. s, P, MILLER 1,888,465

OPERATION OF BY-PRODGT COKE OVEN PLANTS Filed Oct. 2l. 1927 2 Sheets-Sheet 2 INVENTOR ATTORNEYS Patented Nov. 22, 1932 UNITED .STATES PATENT OFFICE STUART PARMELEE MILLER, OF ENGLEWOOD, NEW JERSEY, ASSIGNOR TO THE BARRETT COMPANY, OF NEW YORK, N. Y., A CORPORATION OF NEW JERSEY OPERATION OF BY-PRODUCT COKE OVEN PLANTS Application led October 21, 1927. Serial No. 227,634.

This invention relates to improvements in the operation of coal carbonization plants such as by-product coke oven plants. More particularly, the invention relates to an inmproved method of operation, whereby, 1nstead of producing tar as a by-product, the by-product operation is so conducted as to produce a high yield of merchantable oils and an improved quality of coke.

According to the present invention the hot coal distillation gases coming from the individual coke ovens or retorts at a high temperature, around 600 to 700 C. or higher, are cooled to a regulated lower temperature, but a temperature which is nevertheless suliiciently high to retain a large part of the oil constituents in the vapor form, While particles of pitch are carried in suspension, the gases are then subjected to a cleaning treatment to separate suspended pitch particles therefrom While leaving a large part of the oils in the form of vapors, the resulting cleaned gases are then subjected to cooling and condensation to recover therefrom clean, or 'substantially clean merchantable oils, and the pitch residue is returned to the coke ovens or retorts in admiXture with coal, with resulting improvement in the coke produced. The distillation of the pitch in the coke oven or retort results in some increase in the oil vapor content of the resulting gases, and the increased oil content is largely recovered in the form of clean directly merchantable oils in the further carrying out of the process.

The practice of the process of the present invention enables substantially the maximum yield-of oils to be recovered as merchantable oils directly at a coal distillation plant.

In the customary method of operation of a by-product coal distillation plant, the gases are rapidly cooled inthe hydraulic or collector main by the introduction of ammonia liquor, or ammonia liquor and tar, and the 7 greater part of the tar content of the gases is thrown down and recovered from the collector main or hydraulic main as a heavy tar. In the operation of a coke oven, for example, the gases pass from the collector main through a cross-over main to condensers, Where a light tar or tarry oil .is condensed from the gases by further cooling. The heavy tar from the collector main and the light tar or tarry oils from the condcnsers are commonly combined and shipped to tar distillation plants and there distilled.

The distillation of tar in ordinary externally heated tar stills results in a very considcrable decomposition of oil constituents contained in the tar, with the result that the oil yield obtained as a result of the distillation is considerably less than the oil content of the tar distilled. For example, in simple stills of 10,000 gallons capacity Where successive charges of tar are distilled by the application of external heat to the still, the distillation of the tar to produce a pitch of a melting point around 170 F. gives an oil yield of only about 33 per cent of the tar distilled; While if the distillation is carried further to produce a pitch having a melting point around 300 F., the oil yield is increased only to about 44 per cent, and coking of the tar may begin before a pitch of a melting point around 400 F. is reached.

The present invention provides for the production, directly from coal distillation gases, of either a pitch of medium melting point around 200 F., or the production of a relatively hard or high melting point. pitch from which most of the oils are excluded, and the return of such pitch to the coke oven or retort. `Such pitch, in admiXt-ure with the coal, gives a high percentage of coke and a low yield of tar constituents. The tar constituents can be largely recovered as oil constituents, and only a limited amount of the heavier pitch constituents returned to the coke oven or retort, in admixture with coal during further operation. As a result, the pitch contributes to the coke produced a high percentage yield of pitch coke, and only a small increase in tar content of the resulting gases.

The present invention includes improvements in the production and handling of the pitch which is produced and employed vin the process, as Well as improvements in the coking operation and in the production of an improved coke. The invention also includes the direct production of merchantable oil products, particularly clean oils, which may be directly sold and employed, for example, as creosote oils, or which may be employed,

with addition of tai-,a as an improved coal tarA `0 a relatively high melting point. For eX- ample, by cooling hot coke oven gases to such a temperature that the pitch particles separated therefrom form/a pitch product having a meltingpoint around 300 F., the pitch produced represents only around one-third or less of the total tar which would be recovered from the gases, while the remaining twothirds of the normal tar content is left in the form of vapors, and can be subsequently directly recovered as clean or relatively clean oils, suitable for use, for example, for creosoting purposes. By subjecting the gases to a cleaning treatment at a still iiglier temperature, so as to separate therefrom a pitch having a melting point around 400 F., the pitch represents only around one-fourth or oneifth of the normal tar content of the gases, and the remaining three-quarters or fourfifths of the normal tar content is left in the form of vapors or clean oils which can be `subsequently directly condensed and recovered as merchantable oil products, requiring no further distillation.

Where the maximum oil yield is not desired, the pitch produced may have a lower melting point, e. g., around 200 or 250 F. If the temperature is too high, the pitch may not be sulliciently fiuid to flow from the electrical precipitator employed for cleaning the gases and separating the pitch therefrom, so that, in general, the gases should be cooled to a temperature below that at which pitch will not flow from the apparatus, so as to recover a pitch product in a sufficiently Huid state. Pitch of a melting point around 300 or 400 F., or even higher, can be directly obtained,

and the proper temperature of the gases can be readily determined and controlled by observing the melting point and character of the pitch separated from the gases at the temperature of cleaning.

Pitch of such high melting point, e. g.,

` around 400 F., gives a high yield of coke,

amounting to around three-quarters or more of the total Weight of the pitch, while it gives only a limited distillation of the pitch constituents. As a result, it is possible to, add a relatively large amount of the high melting l point directly recovered pitch to the colte oven charge, and thereby produce a new and superior coke product containing a relatively large proportion of pitch coke. Pitch of lower melting point can readily be produced and employed in the process.

In the operation of the process, the gases coming from the individual coke ovens or r'etoi-ts may be collected in a common collector or hydraulic main such as is now commonly used in by-product coal distillation plants, but instead of cooling the gases rapidly to separate tar therefrom, the gases should be kept hot and the cooling shouid be regulated. This regulation can be accomplished by indirect air cooling of the gases, or by the introduction of a very small amount of cooling liquid into the gases, which, by evaporation,

will cool the gases to the desired temperature. Ammonia liquor, in very small amount, may be employed, in which case some local overcooling of the gases may take place with separation of some heavy tar therefrom, but the average temperature of the gases can be kept suliciently high by limiting and regulating the amount of ammonia liquor to allow collection of the desired high melting point pitch.

The cleaning of the gases at a high temperature, to separate suspended pitch particles therefrom. can be effected by scrubbing of the gases with pitch of a temperature and composition the same as or similar to that cariied in suspension in the gases, but it is more advantageously effected with an electrical precipitator which is maintained at the proper temperature, for example, by heavy insulation or even by heating to prevent cooling of the gases passing through the precipitator.

The temperature at which the gases are subjected to the cleaning treatment will vary with the melting point of the pitch to be produced, and may vary with different gases for production of pitch of the same melting point, depending upon the composition of the gases, which, in turn. will vary somewhat with the character of the coal coked. the manner of carrying out the coking operation. whether rapidlypor slowly. and upon the type of oven or retort, etc. For example, with certain coke oven gases. a pitch of melting point around 290 F. (cube-in-air method) observing the melting point of the pitch separated from them, and by increasing or decreasing the temperature of the gases to obtain a higher or a lower meltin point pitch.

The pitch will be separated From the coke oven gases in a continuous manner. The pitch, while still hot and thinly fluid, may be run on to coal on a conveyor and caused to solidify in contact therewith, but this method is not ordinarily one to be recommended. The pitch can be run to a storage place and permitted to solidify and subsequently dug or mined therefrom after soliditication, but this operation involves unnecessary handling of the material.

The pitch which is continuously produced in the practice of the invention is advantageously continuously treated for the production therefrom in a continuous manner of pitch adapted for mixing directly with the coal. A particularly advantageous method of accomplishing this is to run the pitch continuously into`a trough into which a. stream of water is also directed. The pitch coming into contact with the water is suddenly chilled and broken up into a pulverulent conf dition, which is readily flushed away with the water to some convenient place of settling where the water can be drained from the pulverulent pitch. The pitch so pulverized is in a condition which enables it to be handled in the same manner that coal is handled.

Another advantageous method of handling the pitch is to run it ont-o a traveling belt of metal, which is cooled by water sprayed on the underside to rapidly cool and solidify the pitch. When such a belt passes yover a pulley, the solidified pitch breaks off and is thereby obtained-in a broken up condition well adapted for handling in admixture with the coal.

Another advantageous method of handling the pitch is to Hake it on a rotating roll. This is accomplished by running the pitch into a pan having a rotating` hollow metal roll dipping slightly into the pitch in the pan. The roll is cooled internally. e. g.. by means of Water. The surface of the roll as it touches the molten pitch must be warm in order that the pitch may adhere to it and be carried up by the roll. This is accomplished by spraying the cooling water onto the top inner surface of the roll at such va rate that the pitch will be suiiciently chilled to be solid so that it may be scraped off inst before the roll surface re-enters the molten pitch. The water warmed by the heat in the pitch runs to the lower surface of the roll. The Water is withdrawn continuously, but a depth of Warm water equal to l to 1/2 the diameter of the roll is allowed to remain atall times in the roll. This maintains the lower roll surface at the proper temperature and prevents local over-cooling due to cold water. The pitch scraped from the roll is well adapted for handling and use.

Other methods of converting the hot pitch into a granular or subdivided condition can be employed, although those above mentioned are particularly advantageous. The pitch can, for example, be run into a tank of water where it will be rapidly cooled and collected in lump and powder form. It can also be cooled by atomizing or spraying it through air which may if desired he cooled by the spraying of water therein.

Pitch of high melting point, e. g., around 300 or 4000 F., has many advantages when used as part of the charge of the coke ovens. Its melting point is so high that it does not soften at any temperature met with in the handling of the coal preparatory to charging the coke ovens. It can be handled'and stored the same as coal is handled and stored, and broken up and pulverized in the same way. It can be admiXed in regulated amounts with the coal before the crushing operation, or it can be separately crushed and stored in a separate bin.

The proportions in which the hard pitch is admixed with the coal can be varied. If the pitch is to be mixed with all of the coal charged to all of the coke ovens, and if the pitch available is only that produced at the coke oven plant, the percentage of pitch admixed with the coal will be small, amounting usually to less than one per cent of the coal charged. In such case, the pitch can be admixed with the coal either before crushing or after crushing, and uniformly mixed with all of the coal either before it reaches the storage bins from which the individual coke ovens are charged, or it may be stored in a separate bin and added to the coal 'in regulated amounts as the coal goes to the pulveri-zer.

Instead of adding the pitch uniformly to all of the ovens, it vcan advantageously be added in much larger amounts to a limited number of ovens; for example, in amounts representing about ten per cent of the charge to one-tenth of the ovens of the battery. If a larger amount of pitch is available from other batteries, the entire battery may be operated with a larger proportion of the hard pitch added to the coal charged. With pitch of high melting point, around 300o or 400 F., a very considerable amount of pitch can advantageously be mixed with the coal, for example, around 15 per cent or more of the total charge. I/Vhen pitch of such high melting point is mixed with the coal, the coke produced will contain a high percentage of'pitch coke. A new coke product can thus be produced with a pitch colte content of around ten or fifteen per cent or more, and such a new product has advantages for certain purposes, being a superior metallurgical coke and having a lower ash content than the coke would otherwise have.

The new coke will be lower in ash and lower in sulfur. It will be stronger, will give less braize in handling, will carry a larger burden in the blast furnace, will have a higher B. t. u. value than coal coke, and will yield less slag. When pitch is used a poorer quality of coal can be used since the pitch serves as coking constituent, i. e., the binding agent of coal.

Simultaneously with the production of the hard pitch and its/'return to the coal charge of the coke ovens, there will be produced cleaned gases from which the clean oils can be directly recovered. For example, when pitch of melting point around 300 F. is produced, an oil yield around 65 to 70 per cent of the tar which would ordinarily be recovered from the gases can be ldirectly obtained by cooling the cleaned gases; while with the production of pitch having a melting point around 400 F., the oil yield will be around per cent, although the percentage will vary somewhat with diierent coke oven gases and coking practice.

By cooling the cleaned gases there can be directly condensed therefrom a total oil condensate suitable for use for creosoting purposes. It will contain a considerable amount of heavy oil constituents, including constituents which are grcaselike or resinous or solid in an isolated state at ordinary temperature,

but which in admixture with lighter oil constituents, blend therewith to form a homogeneous product. Instead of condensing a total oil from the gases. the gases can be fractionally cooled and different oils separated therefrom, particularly a creosote oil which will be a heavy oil, and a lighter carbolic or tar acid oil, from which tar acids can be directly extracted.

The cooling of gases and the recovery of clean oils therefrom can be carried out in condensers of either the indirect or direct type, such as are commonly employed at coke oven bv-product plants. In case heavy tar is separated from the gases in the collector main before they go to the electrical precipitator, this heavv tar may require further handling.

Where the precipitator is arranged on top of the collector main and the pitch is refiuxed back into the main, no heavy tar should be produced if the main is at a suiiiciently high temperature. rI`he temperature should be held up to the point at which hi h melting point pitch results. This can e accomplished by allowing accumulation of tar, or pitch, atomizing it and thus keeping main surfaces clean, or by supplying sutiicient pitch to the main to prevent accumulation of too hard pitch in the main. If the collector main is operated with circulation therethrough of pitch, some pitch may be separated from the gases and added to the circulating pitch. while some of the oils in the circulating pitch may be vaporized and increase the vapor content of the gases.

While the cleaning of the coke oven gases at a suiiiciently high temperature to give a hard pitch of high meltin point presents advantages in oil yield an in providing a pitch of low oil content for return to the coke ovens, the process can nevertheless be carried out with many of the advantages hereinbefore described when the gases are cleaned at a lower temperature and when pitch of medium melting point is obtained; for example, pitch of melting point 'around 200 F. or somewhat higher or lower, or pitch of melting point around 225 F. or 250 F. When pitches of such melting point are produced from the gases, the oil yield will be somewhat lower, but the oil yield will, nevertheless, be much greater than the oil yield obtained by thedistillation of tar in ordinary externally heated tar stills to produce pitch of similar melting point.

When such pitch of medium melting point is returned to the coke oven in admixture with coal, it will be subjected to further distillation during the coldng operation. It will,

for the most part be converted into coke, giving, for example, iaround two-thirds of its weight of coke, but it will also'give a considerable amount of oils and tar constituents, which will escape from the coke oven with the gases produced by the coking of the coal. Some of the constituents produced from the pitch in the coke oven will be heavy constituents, but the greater part of the pitch is left behind in the coke ovens as coke, and since the pitch is subjected to destructive distillation in the coke oven, the distillates from the pitch will be made up largely of oils which will be carried in the gases in vapor form. When the coke oven gases from suchoovens are subjected to cleaningwith an electrical precipitator, a considerable part of the oil vapors produced from the pitch in the coke ovens will be left in vapor form and collected with the clean oils from the gases subsequent to the cleaning in the electrical precipitator. The pitch thrown out of the gases in the electrical precipitator will contain only the heaviest of the constituents produced by the distillationof the pitch in the coke ovens, and these will be returned to the coke ovens in the further carrying out of vthe process.

Accordingly, even with pitches of medium melting point, a high oil yield is obtained from the cleaned gases, while the pitch is returned to the process for the production of coke, additional oils and permanent gases.

The gases from the coke ovens or retorts to which pitch is returned in relatively large amount may be kept separate from the gases from the other ovens or retorts and separately cleaned with an electrical precipitator at a high temperature and clean oils subsequently recovered therefrom. "In this way oils can be directly produced of somewhat different composition from those obtained from normal coal distillation gases from ovens or retorts to which no pitch is charged with the coal. The cleaning of the gases at a high temperature with an electrical precipitator gives directly a pitch product which can be readily handled in a hot fluid state and clean oils, and eliminates troubles resulting from the handling of tar separated from coal distillation gases Where pitch in considerable amount is added to the coal charged to the ovens or retorts.

This invention will be further described in connection with the accompanying drawings, Which illustrate in a somewhat conventional aud diagrammatic manner,`parts of a by-product coke oven plant arranged for the practice of the invention.

In the accompanying drawings Fig. 1 is a diagrammatic view in elevation With parts in section, and

Fig. 2 is an enlarged view of part of the apparatus.

In the apparatus illustrated the coke oven 1, has the usual uptake pipes, 2, leading from the individual coke ovens to the collector main 3. From the center box 4 of the collector main, a line 5 leads to a receptacle 6 for pitch, or for heavy tar and ammonia liquor, which may collect in the centerbox during operation. From the collector main 3, the gases lead through a short pipe 7, to an electrical precipitator 8, iny which the gases are subjected to a cleaning treatment at a high tem,- perature.l The cleaned .gases then pass through the cross-over main 9, to a cooler for creosote oil 10, and then through the line 11, to the' coolers or condensers 12 and 13, shown as direct coolers with Water sprays 141 and 14. The oil condensed in these coolers is collcctedin decanters 15 and 16, or may be collected in one decanter, from which liquor is drawn off and collected in tank 19. Oil from the decanter or tank 15, or the total oil, may be returned by line 17, having pump 18 therein, to the creosote condenser 10. The creosote oil condensed and collected in 10 is run oil' to receptacle 28. From the condenser 13 the gases pass through line 20 to exhauster 21, and then to the ammonia absorber or saturator and light oil scrubber (not shown).

From the electrical precipitator 8, the separated pitch runs out through spout 22 to a trough 23 into which a stream of Water is introduced at high velocity through pipe 24, with resulting granulation of the hot pitch, which runs down with the Water and is collected in settling basin 25. A stream of Water discharged at 45 lbs. gauge pressure through a standard 11/8 inch re nozzle, and on to which the pitch falls, will readily'granulate 1000 to 2000 gallons of'pitch per hour. After drainage of Water from the pitch it can be handled like coal and conveyed to the mixing bins and Withdrawn therefrom in regulated amounts for admixture with the coal, after which the admixture, suitably disintegrated and mixed together to form a uniform mixture will be stored in the storage bins and charged to the coke ovens as required.

The collector main 3, is shown in F ig. 2 as having spray lines 26 for the introduction of a limited amount of ammonia liquor, Where this is desired for regulating the temperature of the gases. 1f a large amount of ammonia liquor is introduced, the gases Will be cooled to too low a temperature, but by using a very small amount of ammonia liquor and providing only a limited contact of the gases therewith, the average gas temperature of the gases leaving the collector main and going to the. electrical precipitator can be kept sufficiently high even though some local overcooling of part of the gases and separation of heavy tar therefrom may take place in the collector main. Only around 131/2 lbs. of Water are required per thousand cubic feet of gas to cool the gas, e. g., from 700o to 300 C. In case a large amount of hot pitch is circulated through the bottom of the collector main 27, to prevent accumulation of hard pitch therein, the pitch will be drawn oif into the receptacle 6, and can be further circulated if desired.

In the drawings no insulation is shown, but in practice the electrical precipitator will be heavily insulated to prevent loss of temperature therein, and the connecting pipe 7, leading to the collector main, as Well as the uptake` pipes and the collector main may be provided with the insulation to prevent excessive lowering of temperature by atmospheric cooling and to permit better regulation of the temperature by a limited introduction of, for example, ammonia liquor into the collector main. When a circulating stream of hot pitch is employed in the collector main atmospheric cooling may be relied up'on to a large extent, if not-entirely, for cooling of the gases to the desired lower temperature, While insulation can be employed to prevent too great cooling.

In the operation of the apparatus described, the pitch will be mixed with the coal, for example, to the extent of about one or two per cent of the charge Where it is admixed with the coal for all of the ovens, or to the extent, for example, of ten per cent 0r more where it is mixed with the coal for only a limited number of ovens. During the coking of the coal the high melting point pitch will also be coked, and there will be produced a coke product containing pitch coke which may be present to a very considerable extent, e. g., to the extent of ten per cent or more of the total coke. The coking of the pitch at the same time that the coal is coked, will result in driving oi' more or less volatile constituents and decomposition products from the pitch and these will admix with the distillation gases T e hot gases coming from the individual coke ovens are collected in the collector main,

and are cooled to a regulated lower temperature which will vary with the melting point of pitch to be produced, but which should not be too high, since the temperature of the gases must be a safe margin below that which would cause carbonizing of pitch deposits on the tubes of the electrical precipitator. The temrature ofthe gases can readily be regulated y noting the character and melting point of the pitch, and increasing or decreasing the gas temperature to give pitch of theydesired melting point, for example, pitch of around 300 F. melting point, or even pitch of around 400 F. melting point. The gases at the regulated temperature are passed through the electrical precipitator, lated or which is heated of temperature so that the gases leave the precipitator at astemperature the same as or not greatly below that at which they enter the precipitator. If the gases are cooled to :any materialextent in the precipitator, it ref stilts in further condensation of constituents which would otherwise be carried in vapor form with the gases and subsequently recovered as 'part of the clean oils. If the temperature of the gases passing thru the electricaprecipitator issuiiiciently high, the suspend articles carried by the gases and separated t erefrom in the electrical precipitator will give a pitch of regulated melting point, and will leave the oils, including a largepart of the heavy oils and constituents which in anv isolated state are semi-solid or solid at ordinary temperature as vapors to be carried along to the condenser.

The pitch thrown down in the electrical precipitator is run into a comminuting device, which in the drawings is a trough supplied with water to cause the disintegration of the pitch. The hot high melting point lpitchwill be immediately disintegrated`by the cold water coming in contact therewith, and the water will wash the finely divided pitch with it into a ksuitable collecting receptacle. The comminuted pitch can then be handled like coal. The excess water can be drained from it and it can then be storedand handled with the same equipment with which coal is handled. Its high melting point prevents any danger of softening during handling or storage. Its brittle character makes it readily disintegrated, so 'that it can be uniformly admixed with the coal.,

The hot cleaned gases from the electrical C precipitator pass first to the creosote ccn- Which is heavily insup to prevent lowering denser, where they lare cooled in part by indirect cooling, tion of the lighter oils subsequently condensed from thegases. In this way a creosote oil can be recovered which will be a heavy creo.

sote oil and of particular value for that purpose, because of its heavy character. The gases will subsequently be further. cooled to a low temperature, e. g., around 250 to C., to condense vthe lighter oils, such as carbolic or tar acid oils, which will be clean oils and can be directly extracted for the recovery of tar acids therefrom.

In the drawings the gases from all of the ovens are shown as leading to the same collector main and to the same common system for handling the gases. In practice it will be understood that a battery of ovens will have or may have a plurality of collector mains and cross-over mains leading to the same condensing system or to individual condensing systems. When a part only of the battery is operated with the addition of the hard pitch to the coal charged to the ovens, the gases from this part of the battery may be those collected in the same collector main, in which casethe gases may be handled separately from those collected in other collector mains where no pitch is charged with the coal to the ovens, and pitch and oils of somewhat diiferent character separately recovered therefrom. Separate collector mains may it desired be installed to handle these gases.

The employment of hard pitch, containing, `for example, only around a third to a quarter of its Weight of decomposable volatile constituents, and yielding from two-thirds to threequarters or "more of pitch coke has the advantage, among others, that it does not materially increase the tar content of the coal distillation gases, nor decrease the coke yield from'the charge. The hard pitch nevertheless, give on coking oil and pitch con.

during the coking of the mixture in the individual oven or retort. In this way all or practically all of the tar content of the gases is converted into a high yield of valuable oil and coke with some production of gas, and without the production of the usual tar which requires subsequent distillation. The by-product coke oven can be operated accordinvly for the production of only clean oils, eche, gas, ammonia and light oils, which are all marketable products, and, in particular,

and in part by the introducn there isobtained a high yield of valuablel oils without the separate production and distillation of tar,;and Vwith greatly increased yields of oils-'as compared with the yields which would be obtained by separate recovery and distillation of tar according to ordinary tar distillation methods.

I claim: l

l. The method of producing coke and clean oils from coal which comprises coking coal in a coke oven, removing entrained-solid pari ticles and entrained liquid particles of high boiling range from the resulting hot fresh distillation gases at a temperature at which the constituents of the gas are separated into gases and vapors which pass oi and a pitch residue which is left behind; returning at least part of the pitch `residue to the oven in admiXture with the coal and coking the mixture therein, and recovering oils of lower boiling range from the gases and vapors.

- 2. The method of producing coke and clean oils from col which comprises coking coal in a coke oven, removing entrained solid and liquid particles from the resulting hot fresh distillation gases at a temperature above the dew point of the tar' oils of lower boilin range and at a temperature sufciently hig to form atresidue' comprising mainly pitch, returning at least a part of said pitch residue to the oven' in admixture with coal and coking the mixture therein, and condensing the gass `from which the said entrained solid andu liquid particles were separated to recover oils 'of low boiling range.

In testimony whereof I allix myv signature. STUART PARMELEE MILLER.

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