US2563285A - Manufacture of carbon electrodes - Google Patents

Description

Patented Aug. 7, 1951 UNIT D STATES PATENT OFFlCE MANUFACTURE OF CARBON ELECTRODES Frederick L. Shea, In, Chicago, and Leslie H. J uel, Morton Grove, Ill., assignors to Great Lakes Carbon Corporation, New York, N. Y., a corporation of Delaware No Drawing; Application September 9, 1948, Serial No. 48,541.

' s Qlaims. (01. 18 54.7)

pi make clear what is meant in regard to the sizesoi thecarbon solids employed, particles,

. therein. Also, when the proportion of particles in the mix is relatively large, another efiec'tjmay take place wherein the resulting product, al-

are definedas those sizes which are retained on a zol t o .3 cmeshcsiejve, and may be as'large. as 0.75 inch indiamete r. Flour is, defined as the ,solids composed oi sizes which pass such a sieve, and generally haveAQ-GOW of 200 meshor finer. The cokes preferably used for electrodes and anodes are generally substantially volatile-free petroleum cokes, coal'tar pitch cokes, anthracitecokes, etc. which are comparatively porous in the calcined state. These have generally been calcined'at temperatures'of 900-1l00 0., Sizes of inch diameter 'to 50 mesh are employed, especially'in the manufacture of larger electrodes of 1'5 40 inch diameter. V A

{The usual procedure inmaking carbon electrodes has been to mixa suitable amount 'of binder such as pitch with the entire carbon solids ag g fegate usually at an elevated temperatureto insure the binder being 'fiuid. A molding lubricantsuch as black oil maybe added in small proportions just before mixing is completed. {I -he mixture isusually cooled somewhat below the mixing temperature, but onlyto a point where it is still plastic and then molded or extruded' to thedesired size andshape. The molded articles are baked at a slowly rising temperature toabout 1000 'C. to carbonize the binder, and ma then be subjected to further heating at -2499-3000" C. for graphitization depending upon use to which theyare put. In the above practice, the use of any particular binder has its limitations because ofdifiiculties which may arise during the baking of, the molded article, especially those containing the 'particle-sized solids. For instance, swelling. of the molded mass may occur in baking which often results in loss of useiulproduct. Swelling appears to result, from an improperpenetration of 'thebinder into the particles, particularly with fhignmening point binders. These are not sufficiently fiuid'at the mixing temperature to penetrate"the' particle"po'res and displace the air An h r ,obiecti to. en b use. f,

though satisfactoryas to shape and, freedom from cracks or other defects, does not have the desired electrical conductivity and/or mechan- "ical strength. The reason for the latter-,efiect may be that the large particlestend to selectively absorb the more fluid components of. thebinder 10' leaving an interstitial residue which does not adequately bond the carbonaggregatewhen car- 'bo'nized. This is not overcome either by increasing the quantity of binder, nor adding a more liquid component, to the mix. I

We have found that by impreg nating the larger particles firstwith a suitably fluid heavy hydrocarbon material before the addition of flour solids and binderawei areable to overcome the detrimental behavior'in baking and also secure a better quality carbon article. x

Y It is anobject of this invention to provide methods of compounding carbonaceous mixtures for eliminating swellingl duiring baking of the molded article therefrom. j

Another object is to enable the use of more than one binder material in a mixture containing particles and flour carbon solids in such a way as to secure the best advantage of each forimproving the manufacture v and quality of carbon articles. ,L

desirably high coking hydrocarbonaceous materials fOIthe binder. Coal tar pitches with a softening point above about may be used Heretofore these have not been regarded generally as satisfactory because of .too great a viscosity even at high mixing temperatures, for proper mixing and penetration of the porous carbon solids.

Certain other materials which increase coke residue of pitches, e. g., phenol-aldehydecondensation products, pitch materialawith added reactive nitr c-aromatics, aldehydic material, etc.,

and which may have suitable viscosity at mixing temperatures are so heat sensitive as to setup or become too viscous before. proper penetration of porous solids in mixing has takenplace. These materials can be. employed satisfactorily according to the practice of this invention.

In one embodiment the invention comprises the general procedure of making a carbon solidsbinder mixture ready for molding comprising the following steps: First, the calcined coke particle fraction is mixed at a suitable elevated temperature with ,at least a sufficient proportion of a heavy hydrocarbon material, such as the so-called coal tar pitch resins, to thoroughly impregnate the particles and fill the maximum number of the available pores thereof, then mixing the impregnated particles with coke flour and pitch binder at a temperature above the melting point of the latter, forming the mixture into the desired shape and baking it.

Heavy hydrocarbon materials which may be used as the impregnant include the pitch resins obtained as distillates of coal tars and pitches; the so-called single phase extracts of carbonaceous materials, for example, a benzol-soluble fraction of pitch; and the like. In any event such impregnant preferably melts below about 80 C. and should be of a relatively low viscosity when fluid. It should be substantially all distillable and/or contain no appreciable proportion of residues insoluble in benzol or carbon disulfide. The impregnant should be used in volumetric proportions to fill any desired percentage of the available pore'voidsof the coarse particles. Depending upon the particle size, the per cent of the total voids present in the particles which are available for purposes of impregnation will vary from about 50 to 90%. In general a slight excess of the impregnating agent over and above the total which is able to penetrate the aforesaid voids is employed, said excess being preferably about 5 to over and abovethe total impregnant. The proportion of impregnant, based on the weight of the particles, is generally about 12 to 16% by weight.

The impregnated particles may then be mixed with a binder material at a suitable temperature above its melting point, in an amount necessary for the final moldingmixture generally in the range of 17% to 28% by weight, based on the total weight of particlesplus flour present in the final product. The flour carbon solids may then be mixed with the, binder and impregnated particles,

' While still in heated condition. A small amount of the black oil or other suitable lubricant is added shortly before the mixing is completed, if the article is to be extruded. This is not necessary when molding under pressure.

An alternative procedure comprises mixing the coke or carbon flour with the impregnated particles and then adding the binder. Lubricant may be added.

This latter procedure is advantageous especially wherean excess of impregnant is employed with the coke particles and it is desired to impregnate the flour to some extent before adding the binder. There is another advantage in the case of heat sensitive binders such as the phenol-aldehyde condensation products; or tars and pitch in which reactive additives such as aldehydes and nitroaromatics, and the like have been incorporated. This permits shorter time of exposure of the binder to heat during mixing. Mixing may be carried out at a lower temperature than in the usual practice.

It is essential that the coke particles be properly combined with the suitable heavy hydrocarbon impregnant in order to secure the improvements of the finished carbon product.

For example, the porous carbon particles may be impregnated by immersing them at about 125-200 C. and preferably at about 160 C. in a pitch resin which melts at about 60 C. and is substantially all distillable at 280-360 C. .at mm. of mercury pressure. The mixture is mildly agitated for a short time with continued heating 4 and thereafter excess impregnant may be drained off.

Alternatively, the impregnant may be added as increments to the agitated particles in a mixer at 160 C until sufficient material has been added to fill the available pores and Wet the particles.

The impregnation of the particle coke may be conveniently accomplished at atmospheric pressure, although in some instances the penetration thereof by the impregnant may be facilitated by applying a vacuum to the particles in a closed mixing vessel in the presence of the impregnant, and restoring the pressure to normal or even slightly super atmospheric, say to about 50 pounds gage. This aids in displacing air and filling the pores of the calcined coke or other particles with the impregnant.

.The temperature for the impregnation of the particle coke is above the melting point of the heavyhydrocarbon material. used, butbelow its cokingorvolatilization temperature.

A wider varietyof binder materials than generally. employed heretoforemay be used advantageously in the productionof carbon electrodes and other similarly produced molded or extruded carbon articles when practicing this invention.

1 Examples of the binder materials, in addition to thecommonly used soft and medium hard pitches which have softening points below about 120 C.,

are the pitches softening between about 120 and 150 C.; the liquifiable heat sensitive phenolic resins; the reactive mixtures oftars and pitches with aldehydes, nitro-aromatic compounds, and

the like, such as. are disclosed in our copending ,applicationsSerial No. 681,594, filed July5, 1946,

' now U. S. Patent No. 2,500,208,'and Serial No. i 682,924, flled July 11, 1946, now U. S. Patent No.

Example I According to this invention electrodes were prepared as follows: To 60 parts by weight of calcined petroleum coke of 8 to 20 mesh particle size, was slowly added during 15 minutes,-7.2 parts by weight of a tar distillate resin having a melting point of 60 C. The mixturewas agitated during mixing and was held at 160 C.

I The temperature was then raised to 2009C. and

19.2 parts by weight of a. coal tar pitch distillation residue having a softening point of 180 C. was blended with the impregnated coke particles with agitation during 15 minutes to form a homogenous mixture. To this hot mixture was added 40 parts by weight of calcined coke flour which was mulled into it during 30 minutes to produce uniform distribution. Finally 2 parts b weight of an oil lubricant was worked in to assist in extrusion of the green electrodes which was carried out at about C. and 3,500 pounds per square inch pressure. The extruded pieces were baked in the usual way to a temperature'of 1000 C. The apparent density of the baked electrode was .1.55, and a resistivity of 0.00196 ohm per inch considerably inferior, apparent density being 1.53, and resistivity being 0.00214 ohm per inch Cube,

Upon graphitizing the two baked electrodes just described, the results were even more favorable to electrodes of this invention. The electrode of this invention had an apparent density of 1.56 and a resistivity of 0.00033 ohm per inch cube. The values respectively for the non-impregnated electrodes were, 1.53 and .00035 ohms.

Example II Apparent Resistivity Crushing Density ohms/inch Strength Baked l. 57 0.0020 Graphitized l. 55 0.00036 Example III Utilization of certain binders resolves itself into the use of impregnated particles together with that amount of binder which will give an external porosit capable of permitting the escape of gases during baking without creating an internal pressure and consequent swelling of the electrode in baking. For example, a coke aggregate comprising 60 parts by weight of 4 to 20 mesh particles and 40 parts by weight of flour was mixed with 27 parts by weight of a binder consisting of fuel pitch having a cube in air melting point of 150 C., and 20% of its weight of a nitronaphthalene. This mixture was extruded into electrodes and upon baking, a volume swelling occurred amounting to 1.3%. The resistivity of the resulting baked electrode was 0.0025 ohm per in.

However, when another mixture otherwise comparable to this one was made up by first impregnating the particles with 8.3 parts of a pitch resin (melting point about 60 C.) and then combining with 17.1 parts by weight of the indicated nitro-modified binder and the coke flour, the resulting molded mixture did not swell but underwent shrinkage in baking amounting to 2.5% and an appreciable lower resistivity resulted at 0.0022 ohm per in. The total proportions of binder or binder plus impregnant in these two tests though differing on a weight basis were approximately equal in volume. Therefore, these examples show the advantages resulting from the practice of this invention.

When manufacturing carbon electrodes in excess of 20 inches in diameter according to the method described in the first paragraph of Example I, improvements of about in apparent density, about in conductivity and about in crushing strength are obtained.

We claim as our invention:

1. A process for forming a carbon electrode which comprises separating calcined coke into particles larger than about mesh and coke flour smaller than about 50 mesh, mixing the particles with a hydrocarbonaceous impregnant, said impregnant characterized by being normally solid, melting below about C. and boiling at about 280 C. to about 360 C. at 15 mm. of mercury pressure, said mixing being carried out at a temperature at which the impregnant is freely fluid, to impregnate said particles, thereafter mixing the unimpregnated coke flour and a pitch binder melting higher than said impregnant, with the impregnated particles at a temperature above the melting point of said binder, forming the mixture while hot into an electrode and baking it.

2. The process of claim 1 wherein the impregnant is in the proportions of 12-16%.

3. The process of claim 1 wherein the impregnant is in the proportions of 12-16% and the binder in proportions of 17-28% both by weight of the total coke in the mixture.

4. The process of claim 1 wherein the flour is mixed with the impregnated particles and the binder is then added.

5. The process of claim 1 wherein the flour and binder are added simultaneously.

6. The process of claim 1 wherein the impregnant is substantially completely soluble in benzene.

. 7 The process of claim 1 wherein the impregnant is a coal tar distillate.

8. A process for forming a composition suitable for making a carbon electrode which comprises separating calcined coke into particles larger than about 50 mesh and coke flour smaller than about 50 mesh, mixing the particles with a hydrocarbonaceous impregnant, said impregnant characterized by being normally solid, melting below about 80 C. and boiling at about 280 C. to about 360 C. at 15 mm. of mercury pressure, said mixing being carried out at a temperature at which the impregnant is freely fluid, to impregnate said particles, thereafter mixing the unimpregnated coke flour and a pitch binder melting higher than said impregnant, with the impregnated particles at a temperature above the melting point of said binder.

FREDERICK L. SHEA, JR. LESLIE H. JUEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 748,019 Rubitschung Dec. 29, 1903 2,270,181 Cole et al Jan. 13, 1942 2,365,055 Cole Dec. 12, 1944

Claims (1)

1. A PROCESS FOR FORMING A CARBON ELECTRODE WHICH COMPRISES SEPARATING CALCINED COKE INTO PARTICLES LARGER THAN ABOUT 50 MESH AND COKE FLOUR SMALLER THAN ABOUT 50 MESH, MIXING THE PARTICLES WITH A HYDROCARBONACEOUS IMPREGNANT, SAID IMPREGNANT CHARACTERIZED BY BEING NORMALLY SOLID, MELTING BELOW ABOUT 80* C. AND BOILING AT ABOUT 280* C. TO ABOUT 360* C. AT 15 MM. OF MERCURY PRESSURE, SAID MIXING BEING CARRIED OUT AT A TEMPERATURE AT WHICH THE IMPREGNANT IS FREELY FLUID, TO IMPREGNATE SAID PARTICLES, THEREAFTER MIXING THE UNIMPREGNATED COKE FLOUR AND A PITCH BINDER MELTING HIGHER THAN SAID IMPREGNANT, WITH THE IMPREGNATED PARTICLES AT A TEMPERATURE ABOVE THE MELTING POINT OF SAID BINDER, FORMING THE MIXTURE WHILE HOT INTO AN ELECTRODE AND BAKING IT.