PROCESS FOR TREATING COAL TAR OR COAL TAR PITCH

Abstract: A process for obtaining pitch containing a substantially pure secondary QI component alone, which comprises centrifuging coal tar or coal tar pitch at high temperatures, and heat-treating the obtained supernatant. This heat-treated supernatant may be again centrifuged at high temperatures to obtain pitch containing substantially no primary nor secondary QI components.

Description: Technical Field This invention relates to novel processes for treating coal tar and coal tar pitch (hereinafter the both are represented by coal tar). Background Art In producing high-quality carbon materials such as needle coke, carbon fiber or the like from coal tar, quinoline insoluble components (hereinafter referred to as "QI components") contained as impurities in the coal tar must be removed to utmost extent. The QI components in coal tar are carbonaceous materials in the form of fine particle 0.3 µm or less in particle size (such QI components are generally called "primary QI components"). When coal tar contains a large amount of primary QI components, the primary QI components tend to adhere to the surface of mesophase bodies (spherulites) generally called "secondary QI components" during the heat treatment of the coal tar. The adhesion of primary QI components is presumed to inhibit the coalescence of the spherulites and to hinder their normal growth. For this reason, the removal of primary QI components from coal tar has been recognized as important. For example, a method has been practiced in which oil of such property that the oil and coal tar are hardly miscible each other, e.g. petroleum-type light oil, is added to coal tar to aggregate the primary QI components of the coal tar into particles of increased particle size and the mixture is left to stand to separate the enlarged primary QI components by sedimentation (Japanese Unexamined Patent Publication No.28501/1977). This method requires the distillation of the supernatant liquid after removing the primary QI components in order to get tar and/or pitch as useful component and must recover the oil initially added by distillation. The distillation of the liquid requires a great amount of thermal energy and also results in low yields of the useful component. Further, the oil recovered by the distillation is a mixture of the petroleum-type oil added and coal-type oil derived from coal tar and thus has a limited value in use unless further treated. Since the precipitate phase separated by standing contains a large amount of petroleum-type oil, the oil as added would be recovered at a low ratio if the precipitate is not subjected to a treatment for the recovery of oil. And the recovery treatment requires equipments such as a distillation column, tanks, etc. Moreover, this method involves the use of a large-size tank for storing petroleum-type light or middle oil to be used and related installations, consequently demanding a wide space for arrangement of the equipments. On the other hand, DE-B-1189517 discloses a process of producing a special coke from mineral coal tar products comprising subjecting coal tar or coal tar pitch to centrifugation at a temperature of 100 to 400°C, heat-treating the separated supernatant in a temperature range of more than 300°C, the temperature being rised progressively by 200 to 300°C during the heat-treating step, and calcining the heat-treated product. Further, EP-A-0072242 discloses a process wherein a heat soaked material is converted into a fluid pitch using an organic fluxing liquid and the insoluble solids (QI) are removed from the fluid pitch by filtration, whereafter a specific antisolvent is added to the fluid pitch free of the insoluble solid and the mixture is separated by sedimentation, filtration, centrifugation or the like. Finally, JP-57 209 989 discloses a method of producing a coal tar pitch, wherein a coal tar pitch or a coal tar containing free carbon is subjected to centrifugation without indicating the centrifuction temperature, and wherein the coal tar pitch or coal tar thus treated is subjected to heat-treatment at ambient pressure and at a temperature of 350 to 450 °C to produce not more than 1 % of mesophase, whereafter the resultant coal tar pitch or coal tar is adjusted to form a pitch having a desired softening point. Disclosure of Invention It is the object of the present invention to develop a process for treating coal tar or coal tar pitch in order to obtain a high-quality pitch useful as material for needle coke and carbon fiber, which process needs less equipements than the process of Japanese Unexamined Patent Publication No. 28501/1977 mentioned above. We have conducted extensive research to solve or moderate the foregoing problems encountered in carrying out the conventional methods and found the following. (i) When coal tar is centrifuged at high temperatures, the primary QI components are efficiently separated with ease. (ii) When the supernatant liquid separated by the centrifugation at high temperatures as described in (i) above is heat-treated, the secondary QI components can be easily formed without interference from the primary QI components. As a result, a high-quality pitch which substantially contains only pure secondary QI components is obtained. The resulting pitch is useful as materials for needle coke and carbon fiber. (iii) When the high-quality pitch prepared by the same procedure as described in (ii) is centrifuged again at high temperatures, a high-quality pitch substantially free from the primary and secondary QI components is prepared. The pitch thus obtained is extremely useful as materials for needle coke and carbon fiber, and pitch for impregnation. (iv) The mixture obtained by admixing solids which comprise the primary QI components as separated above in (i) with coal tar or pitch is useful as materials for producing high-quality binder pitches. (v) Meso-carbon microbeads comprising the secondary QI components obtained by washing the solids separated by the secondary centrifugation as described above in (iii) are extremely useful as starting materials for high-quality carbon materials. The present invention has been accomplished based on these novel findings and provides the following processes. (1) A process for treating coal tar or coal tar pitch comprising the steps of subjecting coal tar or coal tar pitch to centrifugation at a temperature of 100 to 400 °C and heat-treating the separated supernatant in a temperature range of 300 to 500 °C, characterized in that said heat-treating the separated supernatant is effected at a constant temperature in said temperature range of 300 to 500 °C and a pressure ranging from above ambient pressure to 2026 k Pa (20 kg/cm²·G) for 0.5 to 50 hours (2) A process for treating coal tar or coal tar pitch comprising the steps of subjecting coal tar or coal tar pitch to a primary centrifugation at a temperature of 100 to 400 °C and heat-treating the separated supernatant in a temperature range of 300 to 500 °C, characterized in that said heat-treating the separated supernatant is effected at a constant temperature in said temperature range of 300 to 500 °C and a pressure ranging from above ambient pressure to 2026 k Pa (20 kg/cm²·G) for 0.5 to 50 hours and that the heat-treated liquid is subjected to a secondary centrifugation at a temperature of 150 to 450 °C. In the present invention, it is preferable to remove the components boiling at less than a centrifugal temperature by distilling the starting coal tar before centrifugation. Coal tar is centrifuged at 100 to 400°C. The centrifugation at less than 100°C entails difficulty in removing a sufficient amount of primary QI components and that at temperatures over 400°C involves an increased tendency to generate gas due to the thermal decomposition of tar or to change the properties of tar. More preferable centrifugal temperature is from 200 to 350°C. Various types of centrifuge can be used which are operable at the temperature in the above range. The centrifugal force to be applied is usually about 500 to about 4000G, preferably about 2000 to about 3500G. The percent removal of QI components is suitably determined according to the properties of starting coal tar, kind of the end product, the desired properties of end product, etc. When the solids separated by centrifugation are added to coal tar which is not subjected to centrifugation and are mixed therewith, the properties of coal tar are improved and the mixture can be effectively used as starting materials for various types of binder pitches. The supernatant liquid separated by the centrifugation is heat-treated at a temperature of 300 to 500°C and a pressure in the range of above ambient pressure to 2026 k Pa (20 kg/cm²·G) for about 0.5 to about 50 hours. Preferably, heat treatment is carried out at a temperature of about 350 to about 450°C under the same pressure and time conditions as above. The heat treatment at less than 300°C entails difficulty in the progress of polycondensation not only of pitch contained in starting coal tar but of heavy components contained in tar oil or condensed ring compounds having relatively low boiling point, high reactivity and instability, so that high-quality pitch can not be obtained. The heat treatment at temperatures over 500°C entails difficulty in operating apparatus for heat treatment and preparing high-quality pitch because of coking trouble therein. When the heat treatment is carried out at pressures over 2026 k Pa (20 kg/cm²·G), improvements of the effects can not be attained, whereas the treatment at pressures lower than ambient pressure results in low yield of pitch by the loss of low boiling components. In general, the higher the reaction temperature is, the shorter the time required for the completion of reaction will be. The secondary QI components are formed in the reaction product during the heat treatment. The reaction product is distilled by a usual method and its softening point is adjusted according to its use. The pitch thus obtained has high-quality, containing substantially only pure secondary QI components, and is useful as materials for producing needle coke, carbon fiber, etc. In the present invention, when the reaction product resulting from the foregoing heat treatment is further treated by secondary centrifugation at high temperatures, high-quality pitch substantially free from the primary and secondary QI components can be obtained. In this case, prior to the secondary centrifugation, components boiling at lower than the centrifugal temperature can be removed by distillation, when required. The secondary centrifugation is conducted at a temperature of 150 to 450°C. The centrifugation at lower than 150°C entails difficulty in removing a sufficient amount of secondary QI components and that at temperatures over 450°C involves a tendency to impair the properties of resulting tar. More preferable secondary centrifugal temperature is from 200 to 400°C. The centrifugal force to be applied is usually about 500 to about 5000G, preferably about 2000 to about 4000G. The reaction product with the secondary QI components removed to a desired extent is distilled in a conventional manner to adjust its softening point according to use. The pitch finally obtained can be substantially free from the primary and secondary QI components and are extremely useful as starting materials for high-quality needle coke and carbon fiber, pitch for impregnation, etc. Specially, when used as the material for needle coke, the pitch does not develop abnormal expansion (puffing) during graphitization. Therefore, there is no need of using Fe₂O₃, and thus the pitch is extremely useful. The secondary QI components (meso-carbon microbeads) obtained by washing solids separated by the secondary centrifugation with oil such as benzene, toluene, xylene, creosote oil, etc. can be effectively used as starting materials for high-density isotropic carbon articles such as device for producing semiconductor, electrode for electrical discharge machining, etc. Example 1 Dehydrated coal tar (containing 2.0% by weight of primary QI components) was centrifuged at a high temperature to give supernatant liquid (containing trace of primary QI components). The centrifuge used was of the transverse- and continuous-type having a holding volume of 40 ℓ and was operated at the revolutions of 3000 rpm, a centrifugal force of 2280G, a temperature of 200°C and a treating amount of 1 ton per hour. The supernatant liquid was heat-treated at a temperature of 380°C and a pressure of 303.9 k Pa (3 kg/cm²·G) for 14 hours to give a reaction product (containing 2.5% by weight of secondary QI components) in a yield of 75% by weight (based on dehydrated coal tar). The properties of the soft pitch thus obtained are shown in Table 1 below. Softening point (R & B method) BI (wt.%) QI (wt.%) β-resin (wt.%) 40.2°C 26.2 2.5 23.7 The soft pitch obtained as above and containing only pure secondary QI components was carbonized at a temperature of 470°C and a pressure of 618.19 k Pa (6.3 kg/cm²·G) to produce raw coke. Subsequently, the raw coke was calcined at a temperature of 1400°C to form needle coke (bulk density of 2.15 g/cc). The needle coke was molded with a binder pitch into a shaped body of 16 mm in diameter and 130 mm in length and was graphitized at 2700°C. The properties of the electrode piece as produced above and a conventional electrode piece are shown in Table 2 below. Thermal Expansion Coefficient (x 10⁻⁷/°C) Abnormal Expansion (Puffing) Example 1 Conventional 3.5 not observed electrode piece (petroleum origin) 8.5 not observed Conventional electrode piece (coal origin) 8.0 observed Example 2 Dehydrated coal tar as used in Example 1 was subjected to centrifugation and heat-treatment under the same conditions as in Example 1. The reaction product was then subjected to a secondary centrifugation at a revolution rate of 3000 rpm, a centrifugal force of 2280G, a temperature of 270°C and a treating amount of 1 ton/hour, using the same centrifuge as employed in Example 1. The product thus separated as a supernatant liquid was distilled to give a pitch having the properties as listed in Table 3 below, in a yield of 55% (based on dehydrated tar). Softening point (R & B method) QI (wt.%) TI (wt.%) 87.5°C 0 31.5 A 80 part-by-weight portion of the above pitch was mixed with 20 part-by-weight of hydrogenated heavy anthracene oil which was obtained by hydrogenation of heavy anthracene oil (5% distillation temperature of 300°C, 50% distillation temperature of 360°C, aromaticity = 0.97) at a hydrogen pressure of 10.13 M Pa (100 kg/cm²) and a temperature of 380°C for 3 hours in the presence of 7% by weight of Ni/Mo/Al₂O₃ catalyst based on the heavy anthracene oil. The mixture was heat-treated at a temperature of 420°C for 2.5 hours while blowing N₂ gas into the mixture at a rate of 5 ℓ/min per 1 kg of the mixture. The mixture was left to stand to separate heat-treated pitch. The heat-treated pitch was spun at a temperature of 330°C and the filaments obtained were subjected to infusion treatment at a temperature of 250°C for 3 hours in an atmosphere of oxygen. The filaments were heated at a temperature of 1200°C for 3 hours in an atmosphere of nitrogen to give carbon fibers of 7 µm in diameter and having a tensile strength of 2383.8 N/mm² (243 kg/mm²) (average value of 15 samples). Example 3 Dehydrated coal tar (containing 2.0% by weight of primary QI components) was centrifuged primarily in the same manner as Example 1 to give supernatant liquid. The supernatant liquid was heat-treated at a temperature of 395°C and a pressure of 303.9 k Pa (3 kg/cm²·G) for 16 hours to obtain a reaction product. The reacted tar thus heat-treated (containing 3.4% by weight of QI components) was centrifuged at a revolution rate of 3000 rpm, a centrifugal force of 2280G, a temperature of 270°C and a treating amount of 1 ton/hr. The properties of soft pitch thus obtained is listed in Table 4 below. The yield of soft pitch was 73% based on the dehydrated tar. Softening point (R & B method) BI (wt.%) QI (wt.%) β-resin (wt.%) 39.1°C 24.2 trace 24.2 Needle coke (bulk density of 2.16 g/cc) and electrode-piece were produced from the soft pitch in the same manner as Example 1. The properties of the electrode-piece are listed in Table 5. Thermal Expansion Coefficient (x 10⁻⁷/°C) Abnormal Expansion (Puffing) Example 3 2.9 not observed

Claims: A process for treating coal tar or coal tar pitch comprising the steps of subjecting coal tar or coal tar pitch to centrifugation at a temperature of 100 to 400 °C and heat-treating the separated supernatant in a temperature range of 300 to 500 °C, characterized in that said heat-treating the separated supernatant is effected at a constant temperature in said temperature range of 300 to 500 °C and a pressure ranging from above ambient pressure to 2026 k Pa (20 kg/cm²·G) for 0.5 to 50 hours A process as defined in claim 1 in which the centrifugation is carried out at a temperature of 200 to 350 °C. A process as defined in claim 1 or 2 in which the centrifugation is performed at a centrifugal force of 500 to 4000G. A process as defined in any one of claims 1 to 3 in which the centrifugation is effected at a centrifugal force of 2000 to 3500G. A process as defined in any one of claims 1 to 4 in which the heat treatment is conducted at a temperature of 350 to 450 °C. A process for treating coal tar or coal tar pitch comprising the steps of subjecting coal tar or coal tar pitch to a primary centrifugation at a temperature of 100 to 400 °C and heat-treating the separated supernatant in a temperature range of 300 to 500 °C, characterized in that said heat-treating the separated supernatant is effected at a constant temperature in said temperature range of 300 to 500 °C and a pressure ranging from above ambient pressure to 2026 k Pa (20 kg/cm²·G) for 0.5 to 50 hours and that the heat-treated liquid is subjected to a secondary centrifugation at a temperature of 150 to 450 °C. A process as defined in claim 6 in which the primary centrifugation is carried out at a temperature of 200 to 350 °C. A process as defined in claim 6 or 7 in which the primary centrifugation is performed at a centrifugal force of 500 to 4000G. A process as defined in any one of claims 6 to 8 in which the primary centrifugation is effected at a centrifugal force of 2000 to 3500G. A process as defined in any one of claims 6 to 9 in which the heat treatment is conducted at a temperature of 350 to 450 °C. A process as defined in any one of claims 6 to 10 in which the secondary centrifugation is carried out at a temperature of 200 to 400 °C. A process as defined in any one of claims 6 to 11 in which the secondary centrifugation is performed at a centrifugal force of 500 to 5000G. A process as defined in any one of claims 6 to 12 in which the secondary centrifugation is effected at a centrifugal force of 2000 to 4000G.

Inventors: FUJITA, KATSUMI, GODA, YASUNORI, MAEDA, TOYOHIRO, MORI, MAKIHIKO, NAKAGAWA, YOSHITERU
Applicants: OSAKA GAS CO., LTD
Application year: 1984
Main classification: C10C1
IPC codes: C10B57, D01F9, C10C3, C10C1