GB2045798A - Process for preparing a pitch from a tar - Google Patents

Abstract

Pitch is prepared by mixing a tar, which can be a coal or petroleum derived tar with a nitrating agent selected from nitric acid, an organic nitrate and nitrogen pentoxide and heating the resulting mixture to and at a temperature of 150 to 400 DEG C. By this process a pitch is obtained in high yield and which upon carbonization provides carbon materials, such as fibers or filaments, having a good balance of physical properties.

Description

SPECIFICATION Process for preparing a pitch from a tar The present invention relates to a process for preparing a pitch from a tar for use in carbon materials such as carbon fibers, high-grade activated carbons, easily graphitizable needle-shaped coke used in manufacturing UHP electrodes and high-softening binders used in manufacturing graphitic electrodes.

The methods hitherto for preparing a pitch for use in carbon materials from a tar can be divided roughly into the following three catagories: (I) a method wherein low-molecular weight fractions are distilled off from a tar under a reduced pressure; (II) a method wherein the low-molecular weight fractions are extracted out from a tar with a solvent; and (III) a method wherein the low-molecular weight fractions in a tar are subjected to polycondensation after addition of an accelerator for the polycondensation.

Concerning the above-mentioned methods, there are examples of industrialization and they are excellent in principle, however, according to the following reasons, they are not always profitable from the view point of efficiency: That is, in method (I), it is necessary to operate at a high-vacuum state in the case of preparing a pitch of softening point of higher than 1 00 C, and accordingly it is economically unprofitable because of a high cost of installation of distilling apparatus. In addition, since it is very difficult to prepare the pitch of a softening point of higher than 1 50 C by the distillation method, it is difficult to prepare a pitch of softening point of 150" to 2500C which is preferably for manufacturing carbon fibers.

By the way, the above-mentioned softening point of a pitch is determined by the same method as in the Japanese Industrial Standard K-2425 (Standard Methods for Testing Tar and its Products: STPTC PT 3-62).

In the next place, in the method (II), an amount of the solvent as much as 10 to 100 times of the amount of the tar is necessary, and accordingly, an apparatus of a large scale is necessary even in the case of treating a small amount of the raw material and in addition, the solvent-insoluble residue after extracting the low-molecular weight fractions with a solvent becomes gummous. This gummous substance is hardly separable and removable.

Moreover, the accelerator of polycondensation in the method (III), for instance, dinitrobenzene and dinitronaphthalene, is expensive and there are problems of deterioration of graphitizability of the pitch due to the effect of a certain kind of the accelerator, and of residual existence of the degradation product of a catalyst as ash, the catalyst being added together with te above-mentioned accelerator and being, for instance, aluminum chloride, molybdenum chloride and potassium metal.

Furthermore, as a common defect in the above-mentioned methods (I) and (II), the reduction of the yield of pitch is pointed out, because of the low-molecularweightfractions in the tar is recovered without being subjected to polycondensation in the methods (I) and (II).

Originally, as a pitch for use in the preparation of the above-mentioed carbonaceous material, those having high-softening point and giving high yield of carbonization are required from the following reasons.

In the case of preparing carbon fibers by using a pitch as the raw material, an oxidation temperature is set up in the neighbourhood of the softening point of the pitch in the step of infusibilization of the melt-spun pitch by oxidation. And since the rate of oxidation of pitch is fasters as the softening point of the pitch is higher, the utilization of a pitch having a higher softening point is extremely advantageous from economic viewpoint.

Also in the case of preparation of activated carbon by using pitch as the raw material, after forming the pitch particles, they are subject to infusibilization by oxidation in advance of the activation by steam. Also in this step of infusibilization, the time necessary for infusibilization is more shortened as the softening point of the pitch is higher.

Moreover, as a binder for use in preparing graphitic electrodes, a hard pitch containing smaller amount of volatile fractions and giving a high carbonization yield is necessary for avoiding the onset of cracks during the step of carbonizing the electrodes.

As has been mentioned above, as a pitch for use in preparing carbonaceous materials, those of a high softening point and of giving a high yield of carbonization are preferable.

Any of the above-mentioned methods hitherto utilized has not always been an advantageous one for preparing a pitch as the raw material for preparing high-grade carbon material.

As a result of studing the kinds of the polycondensation agents and the reaction conditions of the polycondensation in order to solve the problems in the above-mentioned methods hitherto utilized, the inventors of the present invention have developed a process for preparing pitch from tar which could be supplied as the raw material for preparing carbon materials higher in quality than those prepared by the methods hitherto utilized.

That is, the present invention relates in principle to a process of preparing a pitch by admixing a nitrating agent with a tar and further heating the mixture. According to the process of the present invention, the yield of pitch is as high as 60 to 90% by weight of the raw material, a tar, and by changing the mixing ratio of the nitrating agent to the tar, pitches of a softening point in a wide range are available without reducing the yield.

Moreover, the yield of carbonization of the thus obtained pitch is also high without any occurrence of quinoline-insoluble fraction and there occurs an extremely small number of problems due to residual ash.

The following are the more detailed explanation of the present invention: The present invention relates to a process in which at least one nitrating agent is mixed with coal tar or petroleum tar, and the mixture is heated to and at a temperature of 150 to 4000C to obtain a polymer la pitch) of the tar.

As the raw material, tar, of the process of the present invention, coal tar by-produced by destructive distillation of coal, a heavy fraction remaining after distilling off the low-boiling fractions of the coal tar, soft pitch, purified tar obtained by removing the quinoline-insoluble component from tar, tar formed in petroleum industry, for instance, ethylene bottom oil; is utilized. Naturally, tars obtained by mixing one or more than two kinds of the above-mentioned tars may be utilized.

As the nitrating agent, dilute nitric acid, concentrated nitric acid or an organic nitrate such as acetyl nitrate is utilized.

The softening point of the prepared pitch can be determined at will in a broad range by adjusting the combining ratio of the nitrating agent to the tar, and usually a preparation of a pitch of a high softening point at a high yield is possible by combining 3 to 20% by weight of a nitrating agent to the tar. In addition, at a combining ratio of the nitrating agent of less than 3% by weight, the softening point of the formed pitch is lower than 1 00"C and accordingly, it is impossible to obtain a pitch of high softening point. However, when the low-molecular weight fraction of the thus formed pitch may be removed by distilltion, it is possible to obtain a pitch of softening point of 150 to 250"C although the pitch yield is reduced to about 50%.On the other hand, at a combining ratio of the nitrating agent more than 20% by weight to the tar, a pitch not preferable as the raw material for preparing high-grade carbonaceous material is obtained because of the formation and separation of quinoline-insoluble fractions.

The conditions of the above-mentioned nitration of the mixture are as follows.

Hitherto, nitration of an aromatic hydrocarbon of low-molecular weight such as benzene and naphthalene has been carried out using a mixed acid of nitric acid and sulfuric acid. On the other hand, aromatic hydrocarbon-derivatives building up the tar and having more than three condensed benzene rings structure is rich in reactivity to a nitrating agent, and in a mixture of such aromatic hydrocarbon derivatives and a nitating agent, the nitration spontaneously proceeds exothermally to form nitro-aromatics and nitrate esters of the above-mentioned derivative.

Accordingly, the reaction of the above-mentioned tar and the nitrating agent is extremely easy without gecessity of particular controlling the reaction conditions such as pressure, temperature, etc. In order to make a small amount of a nitrating agent to react effectively, the mixture may be heated.

In the next place, the above-mentioned is heated to and at a temperature in the range or 150 to 400"C to polymerize. In the polymerization and/or polycondensation of the above-mentioned mixture, the mixture is kept at the above-mentioned temperature range, preferably in a range of 250 to 3500C for a few minutes to 2 hours to complete the reaction. Since the unreacted low-molecular weight component included in the reacting tar evaporates by heating, the evaporating lower-molecular weight component may be recovered by distillation while carrying out the polymerization and/or polycondensation. Naturally, in the case of using a soft pitch from which low-molecular weight component has been removed by distillation in advance of the reaction with the nitrating agent, it is not necesary to carry out distillation during the reaction.

In the process of the present invention, it is presumed that the tar of the raw material polymerizes to form a pitch as is shown as follows.

By mixing the tar and a nirating agent (the mixing may be optionally carried out under heating), nitro-functional groups and/or nitric ester-functional groups are formed on the organic compounds constituting the tar, and such functional groups cleave homolytically from the organic compound to leave a radical by heating and then the radicals of the organic compounds mutually reactto form a polymeric substance, together with the polycondensation of the organic compounds by heating. However, the present invention is not restricted by the above-mentioned presumption.

In the pitch obtained by the process of the present invention, nitro-functional groups and/or nitric ester-functional groups are almost undetected or cannot be detected. Accordingly, the elementary analytical values of the pitch obtained by the present invention are the same as those of the pitch obtained by the method (I) hitherto utilized.

Moreover, in the case of adding an amount of nitrating agent within the above-mentioned range, no bad influence of the nitrating agent appears on the quality of the thus obtained pitch.

Furthermore, in the case where the heating temperature during pitch formation is lower than 150"C,the cleavage of nitro groups and the nitric ester groups from organic compound constituting the tar is not sufficient to accelerate the polymerization and/or polycondensation, and accordingly, it is impossible to obtain a pitch of high softening point.

The yield of pitch is, even in the case where the low-molecular weight component is recovered by distillation simultaneously with the polymerization and/or polycondensation, 60 to 90%, and in the case where the recovery is not carried out, over 90%, and the yield of carbonization (the amount of fixed carbon is higher than the yield of carbonization of the conventional methods.

Accordingly, the process for preparation of pitch according to the present invention is extremely advantageous in industrialization.

The following are the concrete explanation of the present invention while referring to Examples. By the way, softening point, the method of determination of fixed carbon and of solvent-insoluble component are based on the Japanese Industrial Standards (JIS)-K 2425 Example 1 Concentrated acid (specific gravity: 1.42, concentration: 70%) was added to a heavy fraction of coal tar obtained by recovering the fraction distilling at a temperature of lower than 230"C, at a weight ratio of 8:92, and the mixture was heated from 25"C to 250"C at a rate of 5 C/min. under continued stirring. After maintaining the mixture at 250"C for 2 hours, the mixture was left to cool to a room temperature to be a pitch with a yield of 93%.The softening point of the pitch was 1 20"C. The content of solvents-insoluble fractions of the pitch is shown in Table 1.

In the next place, the pitch was heated in an autoclave from a room temperature to 500"C at a rate of 3"C/min. under a pressure of 5 kg/cm2 and maintained for 20 hours at the temperature of 500"C to obtain a raw coke with a yield of 83% to the pitch. In the thus obtained raw coke, a developed flow texture expected in the raw materials for carbon material could be observed in naked eyes. The graphitized product obtained by graphitizing the raw coke in an electric furnace at a temperature of 2,000"C in an atmosphere of an inert gas showed the properties summarized in Table 2.

Comparative Example 1: The same heavy fraction of coal tar as that used Example 1 was further subjected to distillation under reduced pressure to recover the low-molecular weight fractions distilling at a temperature of lower than 350"C to obtain a pitch having properties shown in Table 1 with a yield of 52%. The pitch showed a softening point of 72"C.

Raw coke was obtained by coking the pitch in an autoclave under the same conditions as in Example 1 with a yield of 56%. The flow texture above-mentioned in Example 1 was not so clearly observed in the thus obtained coke. The properties of the graphitized coke are shown in Table 2.

TABLE 1 Properties of Raw Material and Products Specimen Example 1 Comparative Example 1 Properties Heavy fraction Pitch Pitch Yield of pitch (%) - 93 52 Softening point ("C) - 120 72 Fixed carbon (%) 12.7 38.9 21.6 Heptane insoluble (%) 27.6 72.3 67.2 Benzene insoluble (%) 4.5 24.5 18.3 Quinoline insoluble (%) 0.9 1.0 1.7 TABLE 2 Properties of Raw Cokes Specimen Comparative Properties Example 1 Example 1 Specific gravity 2.13 2.09 Coefficient of thermal expansion (x10-6/ C) 0.47 1.05 Diffraction of X-ray, Cho(002) A 6.721 6.833 Diffraction of X-ray, Lc(002) 370 280 As are clearly seen in Tables 1 and 2, according to the process of the invention, a superior coke of needle-like appearance with a larger specific gravity and a smaller coefficient of thermal expansion being more excellent in crystalline development is obtained as compared to the coke obtained by the method of Comparative Example 1.

Example 2: Concentrated nitric acid was added to a purified tar shown in Table 3, obtained by removing the quinoline-insoluble fraction from a coal tar according to the method of determination of quinoline insoluble matter of Japanese Industrial Standards K 2425, at a weight ratio of 12:88, and the mixture was heated at a temperature of 60"C for 30 minutes under agitation, and then it was heated to 300"C at a rate of 5 C/min. After maintaining the heated mixture at 300"C for 30 minutes and recovering the tar oil distilling during the period from the mixture, the mixture was left to cool to a room temperature to obtain a pitch with a yield of 72%.

The softening point of the thus obtained pitch was 203"C and the properties of the pitch are shown in Table3.

As is seen in Table 3, no quinoline-insoluble fraction was formed during the procedure of the present invention, and a favorable purified state was maintained.

In the next place, the thus obtained pitch was subjected to melt spinning at a temperature of 220"C to be a filamentous shape and the filaments or fibers were infusibilized by heating from 1 50"C to 240 C at a rate of 15"C/min. and keeping at the temperature of240"Cfor6 hours in an oxidizing atmosphere. During the melt spinning, the spinning property of the pitch was excellent without breaking of the filaments into pieces. The thus infusibilized filaments were carbonized at a temperature of 1,100"C in an inert atmosphere to be carbon filaments with a yield of 85% to the pitch filament. The properties of the thus obtained carbon filament are shown in Table 4. Table 4 includes also the properties of the carbon filament prepared from a raw material of lignin derivative for the purpose of comparison.

Example 3: The purified coal tar prepared and used in Example 2, a dilute nitric acid (concentration of 25%) and acetyl nitrate were mixed at a weight ratio of 82:12:6 and after stirring for one hour at a temperature of 20"C, the mixture was heated to 350"C at a rate of 3 C/min. and kept at the temperature of 350"C for one hour, during which the distilling tar oil was recovered sufficiently under reduced pressure. By leaving the heated mixture to cool to a room temperature, a pitch of a softening point of 225"C was obtained wih a yield of 63%. The insoluble fractions of the pitch into solvents are shown in Table 3. During the above-mentioned procedure, no forming of the quinoline-insoluble fraction was observed supporting the favourable purification of the tar.

In the next place, the thus obtained pitch was subjected to melt spinning at a temperature of 240"C to formulate into filaments or fibers, and the filaments were treated as in Example 2 to be carbon filaments. The properties of the thus obtained carbon filament are shown in Table 4.

Comparative Example 2 The purified coal tar prepared in Example 2 was heated at a temperature of 330"C under a reduced pressure of 1mmHg until no more distillate came out to obtain a pitch of a softening point of 145"Cwith a yield of 36%.

Since the softening point of the thus obtained pitch was too low to be used as a raw material for preparing carbon filament, the pitch was subjected to heat-treatment for the improvement of the softening point.

The heat-treatment was carried out at a temperature of 400"C for 6 hours in an atmosphere of gaseous nitrogen under normal pressure to obtain a pitch with a yield of 24% to the purified tar as a raw material. The softening point of the thus obtained pitch was 182"C. The other properties of the pitch are shown in Table 3 with the formation of quinoline-insoluble fraction of 9.3%. The development of spherulites was recognized microscopically in the pitch. On melt-spinning the pitch at a temperature of 200"C, a considerable number of nozzles was clogged by the spherulites, and moreover, the thus obtained filaments were apt to be broken into chips showing the unfavourable spinning property making the fabrication of carbon filament difficult.

TABLE 3 Properties of Purified Tar and Pitches in Examples 2 and 3, and Comparative Example 2 Specimen Example 2 Example 3 Comparative Example 2 Purified Properties tar Pitch Pitch Pitch Yield of pitch (%) - 72 63 24 Softening point ("C) - 203 225 182 Fixed carbon (%) 11.2 60.1 63.3 52.8 Heptane insoluble (%) 24.3 89.4 92.3 80.8 Benzene insoluble (%) 3.2 53.1 55.5 42.1 Quinoline insoluble (%) 0 0 0 9.3 TABLE 4 Properties of Carbon Filaments Specimen Properties Example 2 Example 3 From Lignin deriv.

Diameter (micrometer) 8to 10 8 to 10 10 to 15 Tensile strength (kg/mm2) 90 85 50 to 60 Young's modulus (kg/mm2) 5,300 5,000 3,300 to 4,000 Elongation at break (%) 1.7 1.7 1.5 The pitch for use in preparing carbon fibers according to the present invention was obtained at a larger yield than the pitch obtained by a conventional method in Comparative Example 2 and has a favourable spinning property, and the carbon fiber were obtained at a high carbonization yield of 85%. The carbon fiber prepared from the pitch according to the present invention was superior to the carbon fiber obtained from a lignin derivative in tensile strength and in elastic modulus as is seen in Table 3.

As has been explained, according to the present invention, pitches for carbon materials of high quality are economically prepared.

Claims (8)

1. A process for preparing a pitch from a tarwhich comprises mixing said tar with at least one nitrating agent selected from nitric acid, an organic nitrate and nitrogen pentoxide and heating the resulting mixture to and at a temperature of 150 to 400"C.

2. A process according to claim 1, wherein the mixture is heated to and at a temperature of 250 to 350"C.

3. A process according to claim 1 or 2, wherein the tar is a coal tar and/or a petroleum tar.

4. A process according to claim 1,2 or 3 wherein the amount of nitrating agent is 3 to 20% by weight of said tar.

5. A process according to any one of claims 1 to 4 wherein the nitrating agent comprises acetyl nitrate.

6. A process according to claim 1 substantially as described in any one of the Examples.

7. A carbon material obtained by carbonizing a pitch prepared by a process as claimed in any one of the preceding claims.

8. Carbon fibers of filaments obtained by melt spinning a pitch prepared by a process as claimed in any one of the preceding claims and infusibilizing the resulting melt spun filaments or fibers.