US3761387A - Process for producing an electrode pitch which can be easily graphitized - Google Patents

Abstract

DISSOLVING A COAL TAR PITCH IN AN AROMATIC RICH OIL FRACTION CONTAINING NAPHTHALENE AND ITS HOMOLOGS AS ITS MAIN INGREDIENTS AND HAVING A BOILING RNAGE OF 200 TO 300*C., IN A WEIGHT RATIO PITCH TO OIL FRACTION OF 1:1 TO 1:4 TO. PRODUCE A SUSPENSION; SEPARATING THE SOLID CONTENT FROM THE SUSPENSION; DISTILLING OFF SAID OIL FRACTION FROM THE FILTRATE AT SUCH A SUMP TEMPERATURE AND WITH SUCH TIME OF STAY FOR THE PITCH FORMING THE SUMP PRODUCT OF THE DISTILLATION COLUMN THAT THE CONTENT OF B-RESINS IS INCREASED WITHOUT FORMING OF ADDITIONAL A-RESINS.

Description

United States Patent 3,761,387 PROCESS FOR PRODUCING AN ELECTRODE PITCH WHICH CAN BE EASILY GRAPHITIZED Otto Wegener, Mulheim-Speldorf, Heinrich Louis, Duisburg-Hamborn, and Rudolf Oberkobusch and Gerd Collin, Duisburg-Meiderich, Germany, assignors to Rutgerswerke Aktiengesellschaft, Frankfurt am Main, Germany No Drawing. Filed Nov. 17, 1971, Ser. No. 199,774 Claims priority, application Germany, Nov. 30, 1970, P 20 58 751.3 Int. Cl. Cc 3/02 U.S. Cl. 208-45 7 Claims ABSTRACT OF THE DISCLOSURE Dissolving a coal tar pitch in an aromatic rich oil fraction containing naphthalene and its homologs as its main ingredients and having a boiling range of 200 to 300 C., in a weight ratio pitch to oil fraction of 1:1 to 1:4 to produce a suspension; separating the solid content from the suspension; distilling 01f said oil fraction from the filtrate at such a sump temperature and with such time of stay for the pitch forming the sump product of the distillation column that the content of fi-resins is increased without forming of additional a-resins.

BACKGROUND OF THE INVENTION Processes have been known to produce anisotropic,

easily graphitizable cokes which can be used for the production of graphite electrodes. However, according to the present state of the art, binding agents which are suitable for these cokes have not been known; following the production methods of easily graphitizable cokes, it could be concluded that, fundamentally, coal tar pitches having a low content of highest molecular, sootlike substances which are insoluble in quinoline and also insoluble in anthracene-oil, the so-called oz-resins can be easily graphitized. However, pitches of this type yield, according to the state of the art, in mixture with easily graphitizable cokes, graphite electrodes of low mechanical strength and high consumption. For this reason, the specifications of the graphite electrode producers require in the pitch used as binding agent, normally a relatively high minimum-content of or-resins. These or-resins cause namely during coking and graphitizing, the formation of a very strong framework of carbon and this means electrodes with satisfactory mechanical strength and low consumption.

The object of the present invention is to provide a process for producing a pitch for electrodes which can be easily graphitized, as well as yield graphite electrodes having satisfactory mechanical strength.

SUMMARY OF THE INVENTION The solution of this problem is based in part on the perception that pitches having a high amount of a constituent of p-resins (defined as substances which are insoluble in benzene but are soluble in anthracene oil) form a strong framework of carbon during coking and-in addition to this-the B-resins-like the rat-resins, yield a high share to the coking residue of the pitch. According to this perception it had to be the aim to remove the oc-l'BSil'lS from a pitch as far as possible quantitatively, and simultaneously to increase the amount of fi-resins, if possible.

By dissolving the pitch in quinoline or anthracene-oil, and filtering off the insoluble ingredients, the rot-resins will of course be quantitatively removed, but the amount of B-resins will not be increased. Filtration of a pitch suspension in these solvents is rather diflicult due to the high viscosity of the solvents. This is true particularly in the ice case of quinoline. Practically the solvents must be used in a large excess, in order to guarantee a technically good separation of the soluble and insoluble material at all. In addition anthracene-oil has the disadvantage that its boiling range is extremely close to that of the pitch so that the exact separation of the solvent from the pitch solution is difficult; on the one hand, the solvent becomes pitchlike to a certain extent and on the other hand certain parts of it distill already off so that the pitch characteristics have changed after distilling off of the anthracene-oil in a uncontrolled manner.

It has now been found advantageous to use instead of quinoline or anthracene-oil as solvents for the pitch an aromatic-rich oil fraction with a boiling range of 200 to 300 C., preferably 220 to 250 C. containing naphthalene and its homologs as its main ingredients for separating the u-resins. In view of their low viscosity and good coagulating action, these oil fractions may be used in comparatively low amounts, generally in a small excess and preferably in a ratio (pitchzoil fraction) of from 1:1 to 1:4, more preferably about 1:2. The suspension can easily be filtered and about of the undesired a-resins are separated off by the filtration. Particularly preferred oil fractions are those obtained from the residual oils of benzine pyrolysis to ethylene; such oil fractions have a lower viscosity than corresponding fractions from coal tar.

It has been futher found when using coal tar pitches produced by continuous flash distillation, that the desired fl-resins can be increased still further without forming of any additional a-resins if, during distilling oif again the solvent from the filtrate of the pitch suspension such a pressure is maintained that the pitch forming the sump product of the distillation column is maintained for a controlled period of time at a controlled sump temperature. The two controlled values--the sump temperature and the dwell timevary somewhat for pitches of different sources but they can be easily determined by simple laboratory tests. The sump temperature, therefore, must be high enough that tit-resins may be formed but it must not be so high that additional u-resins will be formed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Thus the subject matter of this invention is a process for producing an electrode pitch which can be easily graphitized and is characterized by the following steps: A coal tar pitch is dissolved in an aromatic rich oil fraction of a boiling range of 200 to 300 C., preferably 220 to 250 C. which contain as main ingredients naphthalene and its homologs to produce a suspension, the weight ratio of pitch to said oil fraction being from 1:1 to 1:4 preferably about 1:2; the resulting suspension is filtered in the heat or separated; theadded oil fraction is distilled off again at such a sump temperature and with such a time of stay for the pitch forming the sump product of the distillation column that the B-resin content in said pitch is increased without forming additional a-resins.

When the coal tar pitch used is a pitch obtained by continuous flash distillation of coking tars from the Ruhr-area the sump temperature at which the formation of [Si-resins occurs is 300 to 400 C. preferably 320 to 360 C., the time of stay is 0.25 to 5 hours preferably 1 to 2 hours. By the process of this invention the B-resins of the charged pitch the portion of which lies generally between 15 and 20% may be increased by 25-50% or more to produce an easily graphitizable electrode pitch having a fl-resin content of more than 24%, e.g. about 26%. The coking residues of the obtained electrode pitches are higher than 50% (according to Conradson). On coking a firm carbon framework is obtained. Graphite electrodes manufactured from these pitches have in addition to good electrical and thermal properties a high mechanical strength.

The present process can be applied also to thermally after-treated coal tar pitches which are used as electrode pitches for non-graphitized carbon electrodes e.g. for aluminiu m smelting. When these pitches are treated according to the process of invention also up to 90% of the undesired a-resins are removed. However the additional formation of fl-resins takes place in small amount only since in the case of these thermally treated electrode pitches the condensation process has already partially taken place. However as these pitches have before being subjected to the process of invention, already got a fi-resins content of more than 20% it is easy to obtain at the end electrode pitches containing more than 24% of Q-resins. It is true, of course, that a small decrease of the coking residue occurs.

The invention is further explained by the following nonlimiting examples:

Example I 5000 parts by weight coal tar pitch obtained by continuous flash distillation, were dissolved in 10,000 parts by weight of a residual oil fraction, rich in aromatics of the benzine pyrolyse to ethylene and the suspension thus obtained at 80 C. to 100 C. sucked off over a vacuum filter. The coal tar pitch had the following characteristics:

Softening point Kramer-Sarnow C '70 Benzol insoluble percent 27.3 Anthracene oil insoluble do 7.4 fl-resins do 19.9 Coking residue (Conradson) do 60.0

The oil fraction rich in aromatic had the following properties:

Density at 20 C. 1.008 Analysis by fractional distillation:

Start of boiling: C 226 10% C 228 50% C..- 234 90% C 247 Gaschromatogramm: Naphthalene percent 31 Methylnaphthalene do 41 Dimethylennaphthalene do 13 Diphenyl do 5 Other aromatic hydrocarbons do In the filtration step 1050 parts by weight filter-residue and 13,950 parts by weight filtrate were obtained. From the filtrate by distillation under a pressure of 100 torr, 9,650 parts by weight of oil were distilled off so, that in the sump of the distillation apparatus a final temperature of 350 C. was obtained. This temperature was more than 1 hour maintained in order to maintain a thermal aftercondensation. The special electrode-pitch (4,250 parts by weight corresponding 85% yield) had the following properties:

Softening point Kramer-Sarnow C 86 Benzol-insoluble percent 25.4 Anthracene oil-insoluble do 0.9 St-resins do 24.5 Coking residue according Conradson do 50.1

The comparison with the pitch used shows that due to the treatment according to the invention not only almost 90% of the anthracene-insoluble material-which hinders the graphitizingis removed, but at the same time the 8- resins content being essential for the solidity of the coke residue has been increased by almost 25%. The use of the special electrode-pitch made it possible to manufacture a graphite electrode having good electric and thermal properties and high mechanical strength.

Example II In a manner analogous to Example I, another coaltar pitch also prepared by continuous flash distillation was dissolved in the ratio 1:2 in the described oil fraction rich in aromatic, the resulting suspension was filtered and the oil distilled, so that a sump temperature of 320 C. was more than 2 hours maintained. The properties of the charged and of the pitch obtained in a yield of 83% were as follows:

Special Pitch electrode charged pitch Softening point (K.-S.), C 75.5 90 Benzol-insoluble, percent 25. 1 2G. 8 Anthraceneoil-insoluble, percent 8. 1 0.8 B-Resins, percent 17.0 26. 0 Coking residue (Conradson), percen 50. 8 53. 0

Thus, due to this treatment, the amount of the anthracene-insoluble has been reduced by 90% and the amount of fi-resins more than 50% increased.

Example III Special Pitch electrode charged pitch Softening point (K.-S.), 0.... 83. 0 84. 5 Benzolinsoluble, percent 25. 4 25. 6 Anthraceneoil-insoluble, percent. 9.1 1. 3 fi-Resins, percent 16. 3 24. 3 Coking residue (Conradson), percent 53. 9 53. 2

This treatment reduced the content of substances insoluble in anthraceneoil by about 85% and increased the fi-resins content by 50%.

Example IV As described in Example I a normal electrode pitch obtained by thermal aftertreatment of a coal tar pitch was dissolved in a aromatic rich oil-fraction in the weight ratio of 1:2 and the suspension thus obtained was filtered and the oil distilled off in a manner that the sump temperature was maintained at 360 C. for 2 hours. The properties of the charged pitch and the yield of 85 produced itch were:

Charged Final pitch product Softening point (K.-S.), C 85 80 Benzol insoluble, percent 32. 7 26. 4 Anthraceneoil-insoluble, percent 10. 5 1. 2 fi-Resins, percent 22. 2 25. 2 Coking residue (Conradson), percent. 54. 1 5 1 By the treatment of the already precondensed pitch, the anthraceneoil-insoluble material was by almost reduced, but the amount of [i-resins was increased only by 14%.

What is claimed is: 1. Process for producing an electrode binder pitch in high yield from coal tar pitch starting material, which binder pitch can be easily graphitized comprising the steps of heating and dissolving the coal tar pitch in an aromatic solvent comprising as main ingredients one or more n-methylnaphthalene materials where n is from Zero to 3 and having a boiling range of 200 to 300 C.,

the pitch to solvent weight ratio being at least 1:1

and no greater than 1:4,

to produce a suspension,

hot filtering the solid content from said suspension to produce a filtrate of reduced alpha-resin content compared to the starting pitch,

distilling olf said oil fraction from the filtrate at subatmospheric pressure in a distillation column,

and treating distillation sump product of the distillation column at such a sump temperature and with such a time of stay that the amount of beta-resins in the sump product is increased without formation of additional alpha-resins therein.

2. Process according to claim in which said aromatic rich oil fraction has a boiling range of 220 to 250 C.

3. Process according to claim 1 in which the said coal tar pitch is dissolved in the said aromatic rich oil fraction in a weight ratio of 1:2.

4. Process in accordance with claim 3 wherein the oil comprises 31% naphthalene, 41% methylnaphthalene and 13% dimethylnaphthalene,

the filtration is carried out at 80-110 C.,

the distillation is carried out at 0.13 atmospheres and reaches a final sump temperature of 320-360 C., and

wherein the final sump temperature is maintained for 1-2 hours.

5. Process in accordance with claim 4 wherein the final sump temperature is 350 C. and is held for one hour.

6. Process according to claim 1 in which coal tar pitch of the coking tars from the Ruhr-area are used and the sump temperature is maintained from 300-400 C., with a time of stay for the pitch of 0.25 to 5 hours.

7. The process of claim 6 wherein the stay time is 1-2 hours and the sump temperature is maintained from 320- 60 C.

References Cited UNITED STATES PATENTS 3,558,468 1/1971 Wise 208-8 1,722,872 7/ 1929 Weindel 20845 3,490,586 1/1970 Jaisle 208-45 3,147,205 9/1964 Ohsol 208-45 FOREIGN PATENTS 1,080,866 8/ 1967 Great Britain 20844 DELBERT E. GANTZ, Primary Examiner V. OKEEFE, Assistant Examiner