US3504065A - Method for suppressing the escape of volatiles while pressure baking carbon articles - Google Patents

Description

* 1'. znsfom I 3,504,065

WHILE March31, 19,70

METHOD FOR SUPPRESSING- THE ESCAPE- OF-VOLA'IILES PRESSURE BAKING .CARBON ARTICLES Original Filed Aug. 15, 1966 w. w M\ INVENTOR- THEODORE EDSTROM BY ATTORNEY United States Patent 3,504,065 METHOD FOR SUPPRESSING THE ESCAPE 0F VOLATILES WHILE PRESSURE BAKING CARBON ARTICLES Theodore Edstrom, Parkview, Ohio, assignor to Union Carbide Corporation, a corporation of New York Continuation of application Ser. No. 572,573, Aug. 15, 1966. This application May 1, 1969, Ser. No. 824,361 Int. Cl. B29c 25/00; C01b 31/00 US. Cl. 26429 3 Claims ABSTRACT OF THE DISCLOSURE An improved method for making a formed carbon article is provided. The method includes the simultaneous application of a mechanical and pneumatic pressure while heating a carbonaceous charge to a carbonization temperature. The charge is initially provided with a high binder content which together with the use of a pneumatic pressure which suppresses the escape of volatiles from the charge enables a strong, highly dense article to be produced.

This application is a continuation of Ser. No. 573,573 filed Aug. 15, 1966, now abandoned.

This invention relates to the production of carbon and graphite masses and more particularly to an improved process of pressure baking carbon articles.

A recently developed pressure baking method for producing carbon articles comprises confining, in a refractory lined mold, a mixture of finely divided carbonaceous particles with a fusible and carbonizable carbonaceous binder susceptible of thermal decomposition, subjecting the mixture within the mold to a high mechanical pressure of the order of 4500 pounds per square inch to compress the same, and passing an electrical current of about 500 amperes per square inch of compressed mixture in the mold to heat the mixture to a temperature at which substantially complete carbonizing of the binder constituent occurs. By means of this process, it is possible to produce in about eight minutes carbon articles which heretofore required a production time of eight weeks.

In the above-described process, pitch and certain other binder materials produce volatile substances which condense on the press, punches, and cold sections of the mold. Accumulation of such substances prevents or interferes with proper operation of the equipment, and in an extreme case can occasion minor explosions.

A recent improvement in the above described process which overcomes this particular disadvantage and which enables the use of higher binder concentrations is disclosed in US. Patent 2,965,931 wherein it is shown that the evolution of condensible hydrocarbons can be substantially eliminated by adding finely divided sulfur to the mix blend to react with the pitch component, thereby causing the evolved gas to become non-condensible and yielding a higher percentage deposition of bonding coke. However, in addition to the fact that sulfur in the pitch bonded mix creates gases which are toxic and which are corrosive, the use of sulfur in certain carbon products is sometimes undesirable because the operating properties of the product are affected. For example, in the manufacture of carbon brushes the presence of a bond resulting from coke which has been deposited in the presence of sulfur causes relatively high friction during commutation.

The principal object of this invention therefore, is to provide a method of pressure baking carbon articles whereby the evolution of condensible hydrocarbons is substantially eliminated without the use of a sulfur additive.

Another primary object of this invention is to provide a method of pressure baking carbon articles whereby higher binder concentrations may be employed without the use of a sulfur additive.

These and other objects of this invention will become apparent from the following description, taken in conjunction with the accompanying drawing wherein FIGURE 1 is an apparatus which may be suitably employed in the process of the invention; and

FIGURE 2 is a schematic of an auxiliary pressure supply and control system.

Broadly stated, the objects of the invention are accomplished by the application of a pneumatic pressure to a carbonaceous charge while the charge is being subjected to heat and mechanical pressure. The pneumatic pressure is preferably applied by contacting the charge with a gas in a sealed chamber. The gas suppresses the escape of hydrocarbon volatiles from the charge with the result that the hydrocarbons are retained in a liquid phase and are forced to coke during the carbonization process. Thus the presence of condensates on the pressing ram and other equipment is eliminated. Furthermore, a greater carbon yield is achieved and higher binder concentrations may be employed.

Any gas may be employed in the instant process which does not oxidize the carbonaceous mix. Such gases include nitrogen, argon, helium, carbon monoxide and the like.

The pneumatic pressure is preferably applied prior to the application of the mechanical pressure and then maintained thereafter. Furthermore, it may be increased or decreased during operation as will be hereinafter further described. For best results, a pneumatic pressure of at least 10 p.s.i. and as high as 2000 p.s.i. or greater is applied while a mechanical pressure of about 500 to 4000 p.s.i. compresses the carbonaceous charge. A pneumatic pressure of between 50 p.s.i. and 1000 p.s.i. is preferred.

Referring to FIGURE 1, an apparatus 10 suitable for use in the process of the invention comprises a steel cylinder 12 which encloses a high temperature ceramic liner 14 and a carbon liner 16. Adjacent the carbon liner 16 is a graphite mold 18. A carbonaceous charge 20 having been surrounded by coke packing 22, is situated within the area defined by the graphite mold 18. Mechanical pressure is applied through a graphite ram 24 which may also serve as an electrical conductor to supply current to the charge 20. The apparatus is sealed by means of Teflon 1 gasket 26 and asbestos seal ring 28 thereby preventing the escape of volatiles or gas. A gas inlet conduit 30 passes through a steel cover 32 and contacts the chamber 34 by means of a passage 35 around the ram extension 36 and the graphite ram 24.

In operation, a non-oxidizing gas is fed into the chamber 34 through the gas inlet conduit 30 by placing a gas feeding means at the opening 31 of the conduit. A pneumatic pressure is thereby applied to the carbonaceous charge 20 and escape of hydrocarbon volatiles from the charge is prevented. Mechanical pressure is applied to charge through the ram 24 while the charge is baked. Upon completion-of the process, the graphite ram is withdrawn and the nonoxidizing gas is permitted to escape through inlet conduit 30 by means of valve 46 as illustrated in FIGURE 2.

It will be appreciated that the pneumatic pressure which is applied to the charge will be dependent on the volume of the chamber 34, the quantity of gas which is fed into the chamber, and the temperature to which the gas is subjected. Normal expansion of the gas during heating will, of course, increase the pneumatic pressure since the apparatus is completely sealed. However, the pressure may be maintained at a desired value by releasing gas from the system during the process. This may be readily accomplished by one skilled in the art through the use of Tefionregistered trademark of Du Pont Corporatiou a. plastic consisting of a tetrafiuoroethylene polymer.

3 4 valves and the like which could conveniently be attached M.P.). One group of samples contained parts of binder to the system as illustrated in FIGURE 2. per hundred parts of coke flour and another group con- Referring now to FIGURE 2, a typical pressure supply tained parts per hundred. The accompanying Table I and control system comprises a gas containing means'38, shows the baked density, strength, and resistivity of these samples compared to control samples (Tests No. 1 and a compressor 40 which places the gas under pressure, a

No. 4) which were not prepared under pneumatic prespressure regulated by-pass valve 42 connected across the compressor 40, a pressure inlet valve 44, a pressure resure.

TABLE I I Applied Efiective Time Estimated Gas Pressure (p.s.i.) Apparent Flexural meeh. meeh, on fire, fin density, Resistivity, strength, pressure pressure 2 sec. temp., C. Inltial Maximum gmJce. ohm-inch p.s.i. p.s.i. p.s.1

45 l, 200 1 0 0 1. 565 0. 00299 1, 210 4, 000 4, 000 52 l, 400 0 500 l. 569 0. 00242 1, 650 4, 000 4, 000-3, 500 46 1, 000 600 1, 000 1. 576 0. 00224 1, 880 4, 000 3, 400-3, 000

1 0 denotes atmospheric pressure. 2 Applied mechanical pressure less pneumatic pressure. lease valve 46 to insure safety and to control the pressure 0 Table I indicates that at the lower pitch concentration,

applied, pressure gauge 48 which measures the pneumatic used in tests 1, 2, and 3, which is representative of a pressure at all times and gauges 50, 52 which measure level near the maximum effectively employable without container pressure and line pressure: In operation this the application of pneumatic pressure, an increase in system is able to closely regulate the pneumatic pressure fiexural strength and reduction in electrical resistivity to be applied to the charge by controlling the quantity of resulted with an application of a pneumatic pressure. At gas, such as nitrogen which is supplied to the chamber 34. the higher binder level, the end product made without Although a pressure supply and control system such as pneumatic pressure (test 4) displays an even lower apthat illustrated in FIGURE 2 is the preferred equipment parent density, and the specific electrical resistance is for applyingapneumatic pressure, any manner of applicaeven higher, than the product made at 10 p.p.h. The tion is Within the concept of the invention. For example, marked improvement in density and flexural strength a pneumatic pressure may be applied to the charge Withas well as the lower resistivity obtained with the appliout the use of an external gas by simply sealing the apcation of pneumatic pressure at the higher binder conparatus as hereinbefore illustrated. During heating, volacentration is evident from a comparison of test 4 results tiles will escape from the charge but not from the chamwith tho e obtained from tests 5, 6, and 7.

ber 34. Since the escape of the volatiles from the apparatus is prevented, a pneumatic pressure is formed in EXAMPLE II the chamber as the.vol.afiles Continue to "2 from? the In another test, a number of 2" diameter carbon plugs charge. At some point 1n the process, equtllbrium Wlll he were made at 10 and 20 p.ph. Pitch levels from com agwmplbhed and no further ascape volanles from the ductive blend charges containing a somewhat coarser c .arge W111 Occur due.to the presure the Chamber In petroleum coke flour than that described in Example I. i manner pneumatlc pressure pP l to the charge In this series of experiments all of the plugs were initially wlthou? the use of an external nonoxldlzlng It be pressed at temperatures ranging from 700 C. to 900 C. appreciated that control of the pneumatic pressure is less and after removal from the pressure baking m old, were preclse .that achieved f i i of an external gas protectively packed in granulated coke flour and rebaked and m addition, some volatllrzation Wlll occur. However, in an electrically heated furnace to 10000 C. to ensure by (lecrqasing Size of the.chambr equilibliumfis a uniform final temperature, after which the permeability f F a relgnvely gf g g g g i gfg zfig g to gas flow of the internal carbon structure of the plug 6 ecnve smce e escape o V a g was measured. A set of permeability measurements was still sup stantially supprelssed. 31mm:1 i illiuzfrit g f gg made with the direction of gas flow oriented with the connec mg an escap" Va ve su as a y grain of the carbon structure and another set with the numeral 46 in FIGURE dir tion f fl a ainst th rain i a ainst A number of tests evaluating the effectiveness of the in- 2 gas g e g and with the direction of application pressure by the vention were performed and are set forth below. Each of the tests was performed with apparatus and equipment El i i -t the fqltlowmg rlrable m t g' sim'lar to that illustrated in the drawin en 5 15 6 PP e S mp e num ers earrng e 1 g subscripts A and W. The table also l1sts the apparent EXAMPLE I denslty and reststlvlty values obtained as well as the A series of 2" diameter by 2 /2" long cylindrical processing conditions.

TABLE II.GAS PERMEABILITY OF VARIOUS PRESSURE MOLDED CARBON SAMPLES Properties 1 Effective Processing, Binder Gas pressure, mech. press. Density, Admittance Sample No. level, p.p.h. start-end in mix, p.s.i. gm./ec. Res, ohm-cm. F (ml./sec.-em.)

10 0-0 4, 200 1. 613 00829 1207 1. 597 488 1755 10 500-500 4, 300 1. 621 00940 0907 1. 611 620 1866 10 1, 000-1, 000 3, 800 1. 611 00870 1290 -1. 608 652 1286 20 500-500 4, 700 1. 706 00574 0292 l. 702 505 0359 20 Vacuum 1, 300 3, 900 1. 707 0300 1. 704 00540 0453 20 Vacuum 2, 500 5, 200-2, 700 1. 718 00545 0097 start-end l. 724 300 0139 1 After rebaking to 1,000 C. samples were molded from a conductive carbonaceous It is evident from the data that the admittance of the blend charge of petroleum base calcined coke flour (55 plugs made from 20 p.p.h. binder mix with pneumatic Texas coke flour) and a coal tar pitch binder (175 C., pressure applied during forming is lower by a factor of 6 to 20 as compared with those made at a lower binder be appreciated that many other forms of comminuted level. In addition to the lower permeability to gas flow carbon or graphite can be used in the practice of the the carbon articles have a large increase in apparent deninvention. For example, such materials as pulverized sity and somewhat lower electrical resistivity. coal, artificial and natural graphites as well as lampblack and gas black in various proportions can be used to im- EXAMPLE HI 5 part desired properties in the final carbon or graphite In another series of experiments, samples of petroleum article. In addition, coal tars as Well as still higher coking coke flour mixes bonded in one case with 105 melting value pitches may be employed as binders. For instance point pitch were heated in a mixer, extruded through a blend of graphite flour and carbon black bonded with a conventional extrusion jumbo into 1" diameter rods, high melting point pitch is found to be particularly efcut into 3" lengths and then pressure baked to about 10 fective in obtaining articles having high apparent density. 850 C. in accordance with the process of this invention. What is claimed is: In this series of pressure bakes the preformed green 1. In a process for making a formed carbon article plugs which were nonconductive were heated by passage from a charge comprising carbonaceous particles and a of current through the graphite mold liner rather than fusible carbonaceous binder, said binder tending to evolve by direct passage of current as was the case with the substances which condense on the apparatus used in said articles described in the earlier examples. The extrusion process, said process comprising providing a binder conmixes contained petroleum coke 90 flour and pitch protent of greater than about 15% by weight of said carportioned in a weight ratio of 70 parts/ 30 parts. To this bonaceous charge placing said charge in a mold, submix 4 parts of petroleum ba e ummer oil a added jecting said charge within said mold to a mechanical as an extrusion aid. pressure while simultaneously heating said charge to a After removal from the pressure-bake apparatus the earhOIllZafieH temperature, Sealing Said apparatus and carbon plugs were rebaked conventionally to 1000 C. during the application of Said mechanical Pressure P' and then to 3000 C. to convert them to graphite. Plugs p y a Pilellhlatic Piessure higher than atmospheric whi h were extruded fro th same mixes b t t pressure from an external source of a non-oxidizing gas pressure baked were also conventionally baked. A comto Said Charge whereby the escape of Volatiles from Said parison of the apparent density obtained after extrusion Charge is substantially pp ed and whereby an inand after the 1000 C. and 3000 C. baking step between crease in density and flexural strength is achieved in the the pressure-baked and conventionally baked plugs is formed carbon article.

shown in Table III. 3 2. The process of claim 1 wherein said pneumatic TABLE III 105 C. M.P. pitch binder, Pressure- 1000 C. baked A.D., 3000 C. graph A.D., 800 C. coking value 1 Carbonization Conditions 2 Green A.D., g./cc. gJcc. g./cc. binder Final Pneumatic Initial pneumatic Approx. Control, pressure, pressure, pressure, temp. Control, Pressure Control, Pressure Control, Pressure Percent percent p.s.i.g. p.s.i.g. C. no press. applied no press. applied no press. applied carbon carbon 150 0. M.P. pitch binder 1 Coking value represents the weight of carbon deposited by the pitch binder at high temperatures relative to the weight of the pitch originally used. 2 Estimated mechanical pressure was approximately 500 lbs./in.

The values shown under the 800 C. Coking value pressure is between about .20 pounds per square inch and column in the table show the marked increase in perabout 2000 pounds per square inch. centage of carbon obtained from both kinds of pitch 3. The process of claim 1 wherein said gas is nitrogen. when the plugs were baked under pneumatic pressure.

It should be pointed out that the greater convenience References Cited and more rapid processing ofiered by pre-forming by UNITED STATES PATENTS extrusion would not be possible except for the higher I binder levels made usable by the application of pneu- 3 9/ 1961 Balaquer l854.7 matic pressure during the rapid baking step. Low binder 3,246,056 966 Shea et al 264-29 levels, not in excess of about 15% by weight of the 3,249,964 5/ 966 Shaler 185 carbonaceous charge were found to be required for rapid 3,336,424 3/1957 n y 26489 baking prior to the invention. In the above described FOREIGN PATENTS experiment, charges having a binder level of 30% were successfully processed. Charges containing only 15% 759,160 10/1956 Gr at Britain. binder would not be extrudable. Another advantage for 1 9/ 1961 G at Brita nthe pre-forming technique is elimination of the need 5 for applying high mechanical pressures to the green JULIUS FROME Primary Examiner carbon during the baking stage. As indicated, the me- JOHN M L Assistant Examiner chanical pressures applied were never more than about 500 lbs./in. US. Cl. X.R.

While all of the examples described were limited to 7 2 64-93, 332 1 i the use of petroleum coke flour-containing mixes it will

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