Title of Invention	"AN IMPROVED PROCESS FOR THE MODIFICATION OF SEMI-COKE SUITABLE FOR THE PRODUCTION OF HIGH DENSITY HIGH STRENGTH GRAPHITE PRODUCT"
Abstract	The present invention provides an improved process for the modification of semi-coke suitable for the production of high density-high strength monolithic graphite. The properties such as density, strength and electrical resistivity are found to be superior from the pitch coated GC based graphite in comparison to graphite products without pitch coating. The bulk density of the high density product so produced by the process of the present invention is found to be in the range of 1.80-1.90 gcm-3, bending strength in the range of 600-1200 Kgcm-2 and electrical resistivity in the range of 1.0- 2.0 mOcm.

Title of Invention

"AN IMPROVED PROCESS FOR THE MODIFICATION OF SEMI-COKE SUITABLE FOR THE PRODUCTION OF HIGH DENSITY HIGH STRENGTH GRAPHITE PRODUCT"

Abstract

The present invention provides an improved process for the modification of semi-coke suitable for the production of high density-high strength monolithic graphite. The properties such as density, strength and electrical resistivity are found to be superior from the pitch coated GC based graphite in comparison to graphite products without pitch coating. The bulk density of the high density product so produced by the process of the present invention is found to be in the range of 1.80-1.90 gcm-3, bending strength in the range of 600-1200 Kgcm-2 and electrical resistivity in the range of 1.0- 2.0 mOcm.

Full Text	FIELD OF THE INVENTION The present invention relates to an improved process for the modification of semi coke useful for the production of high density, high strength graphite product. The said graphite product possesses potential applications such as electrodes for electric discharge machines, electrical brushes, electrical contacts and trolley wheels, heaters and crucibles for silicon production, seals, bearings, packings, jigs, hot pressing dies and as moulds for continuous casting of metals and alloys. BACKGROUND OF THE INVENTION & DESCRIPTION OF PRIOR ART High density - high strength graphite is a recent addition to the family of carbon products. It is, in fact, a high purity graphite possessing a bulk density of more than 1.8 gcm-3, bending strength of more than 600 Kgcm-2, degree of anisotropy of 0.9 - 1.1, along with a homogeneous and fine microstructure. In the prior art, there are three methods known for the production of high density, high strength graphite (Ishikawa T. and Nagaoki, T. "Recent Carbon Technology", JEC Press Inc. 1983; Oya A. "High Density - Isotropic Graphite - An Overview", Pitch and Pitch - based Products - Proc. Indo -Japanese Workshop on Pitch and Pitch-based Products, New Delhi, Nov. 24 - 25,1989, p58). The first method involves the use of two raw materials, i.e., coke filler and a pitch binder, whereas the second one by Oya, A. involves the use of single raw material, namely, mesocarbon microbeads or green (semi) coke. In the first method, a coke filler such as petroleum coke or needle coke is kneaded with a pitch binder, such as coal tar pitch to obtain a mixture, which is isostatically pressed and then carbonized to 1000°C followed by repeated cycles of impregnation and recarbonisation to1000°C and finally, graphitization to around 2700°C. Here, the impregnation and recarbonisation of the carbon product is a very cumbersome, time-consuming and expensive process. Further, inspite of several cycles of impregnation and recarbonisation of the carbon product, the bulk density of the final graphite seldom touches a value of 1.80 gcm-3 and the bending strength of 400Kgcm-2. This is because large amounts of volatiles are evolved from the binder and impregnating pitches during the carbonisation/ recarbonisation operations thereby creating porosity in the product. Thus, this method suffers from its own limitations. Reference may also be drawn to Honda H. and Yamada Y. J Jpn Petrol Inst. 1973.16 : 392; JP 14,173; Bhatia G., Aggarwal R. K., Punjabi N., Bahl O.P., J. Mater. Sci. 1994. 29 : 4757-4763; IN 189155, wherein a quinoline insoluble (Ql) free or low Ql coal tar pitch is heat treated at about 400 - 4503C in an inert atmosphere to generate the mesophase spherules with a size of around lOmicrons. These spherules also known as mesocarbon microbeads (MCMB), are separated out of the heat treated pitch by solvent extraction using a suitable solvent such as tar oil or quinoline, pressed without any external binder and then carbonised to 1000°C, and finally graphitized to 2500°C or higher to obtain the high-density graphite. During heat-treatment at 400-4509C, polymerisation and condensation reactions take place among the various constituent molecules of the precursor coal tar pitch, leading to the formation of large planar polyaromatic molecules, which aggregate together to form the mesophase spherules because of the Vanderwaal's interactions and surface tension. These mesophase spherules grow in size with the severity of heat-treatment. Therefore, to restrict their size to the desired value of around 10 microns, one has to lose in terms of their yield by weight of the precursor pitch. Generally, one is able to achieve a yield of 5 to 35% of such mesophase spherules besides problems of fabrication associated during solvent extraction. Thus, in this method also, a serious techno-economic problem of getting high yield of the mesophase spherules of the requisite size of around 10 microns, i.e., a yield of more than 35% of the microbeads by weight of the precursor coal tar pitch upon heat treatment is encountered. Therefore, this method is tedious and not cost effective. In a method disclosed by Ogawa, I., Kobayashi, K. J. Mater. Sci. 1992. 27 : 1161-1165, a green raw coke also called semi-coke prepared from a coal tar pitch by heat treatment to around 500°C is pressed without any external binder, carbonized to 1000°C and finally graphitized to 2500°C or higher to get the high density monolithic graphite. However, in this method the preparation of the green coke having an optimum amount of the binding components ( volatile matter around 10%) is a highly critical step, since a slight excess of the binding components in the green coke will result in the swelling and/or cracks in the product during the baking operation and slight deficiency of the binding components will result in an improper binding of the green coke particles leading to a relatively weak and low density product. In view of this, generally, the starting coal tar pitch is often subjected to a mild heat-treatment such that the green coke obtained contains slightly excess amount of the binding components. Such a green coke is then extracted with a suitable solvent so as to remove excess amount of the binding components or given an oxidation/ chlorination treatment. (Ref : IN 194694, IN215804). Here, the solvent extraction quite often removes the binding components which are necessary for getting good electrical, thermal and mechanical strength of the final graphite product. The oxidation /chlorination, in turn, is a very sensitive process and difficult to be controlled to an optimum level. Besides, it gives lower values of electrical and thermal conductivity in the final product. Thus, we find that all the methods described above for the production of high-density monolithic graphite are associated with their own typical problems. Thus, keeping in view the hitherto known prior art, the inventors of the present invention realized that there exists a dire need to provide a process for the modification of semi-coke so as to produce a final graphite product with considerably improved properties such as density, mechanical strength, electrical and thermal conductivity. OBJECTIVES OF THE INVENTION The main objective of the present invention is thus to provide an improved process for the modification of semi coke suitable for the production of high-density high strength graphite product, which overcomes the above-mentioned drawbacks. Another objective of the present invention is to provide a process wherein coating of the semi-coke powder is done before moulding into plates or blocks so as to get the resulting graphite of superior characteristics i.e., high density, strength and electrical or thermal conductivities compared to uncoated semi coke powder. Still another objective of the present invention is to provide a process which does not involve repeated cycles of impregnation and recarbonisation which are employed in the process of producing the conventional high density graphite. Thus, the present process is not cumbersome and time consuming, but economical. Yet another objective of the present invention is to provide a process which produces high-density, monolithic graphite having a bulk density of more than 1.80gcm-3 and bending strength of more than 600 Kgcm-2 along with a homogeneous and fine microstructure. Another objective of the present invention is to provide high density, high strength graphite, which finds application as electrodes for electric discharge machines, material for hot pressing dies, casting moulds, mechanical seals, resistance heaters and crucibles. SUMMARY OF THE INVENTION The present invention provides an improved process for the modification of semi-coke, wherein the semi-coke (or green coke) with a slight excessive amount of binding components is subjected to extraction treatment with a suitable solvent such as toluene, tar oil etc. which is then is given a coating with a pitch based material in presence of a suitable solvent like toluene, tar oil to obtain pitch coated semi coke, which is then calcined to distill out excess solvent. The modified semi coke is then molded into blocks using isostatic press without using extra binder and carbonized at 1000°C and higher to 2500°C in an inert atmosphere to obtain high density, high strength monolithic graphite. It has been observed that the properties such as density, mechanical strength, electrical and thermal conductivities of the final graphite product are considerably improved as compared to those obtained from the green coke without such a pitch coating. Accordingly, the present invention provides an improved process for the modification of semi coke useful for the production of high density, high strength graphite, wherein the steps comprising: [a] heating a coal tar pitch in the presence of inert gas to produce a semi coke; [b] grounding the semi coke as obtained in step [a] in a ball mill to get fine powder; [c] coating the powdered semi coke as obtained in step [b] with 1-4% by weight of a pitch material followed by calcination to produce the modified semi coke; [d] molding the uncoated semi coke of step [b] and coated semi coke of step [c] separately into plates by means of conventional press; [e] carbonizing the molded plates as obtained in step [d] by heating at a temperature in the range of 1000 to 1400 degree C in an inert atmosphere to produce carbonized plates; [f] graphitizing the carbonized plates as obtained in step [e] by heating in inert atmosphere at a temperature in the range of 2300 to 2700 degree C to produce the desired high density, high strength graphite. DETAILED DESCRIPTION OF THE INVENTION The present invention provides an improved process for the modification of semi-coke suitable for the production of high density-high strength monolithic graphite, which comprises heating a suitable coal tar pitch in the presence of an inert gas such as nitrogen, argon etc. at a temperature in the range of 450-500°C to obtain the semi-coke which is ball milled into a fine powder with a size of above 1-10µm. The coal tar pitch used for the said process is having a softening point of 80-100°C, quinoline insoluble (Ql) contents of less than or equal to 0.2 % and a coking value of 44-48% was heat-treated at 450-500°C for 2 - 5 hours in the presence of nitrogen to get the semi coke. This semi coke is ball milled to obtain a particle size of around 5u,m having a quinoline insoluble of 96-97.5%, toluene insoluble of 97-98.5% and volatile matter of around 9-11%. In case the values of toluene insoluble and quinoline insoluble contents are found to be slightly less than above mentioned values, the semi coke is extracted with a suitable solvent such as toluene or tar oil or suitable tar based oil so that the desired values of quinoline insoluble (i.e. 96-97.5%) and toluene insolubles (i.e. 97-98.5%) with a volatile mater of 9-10% are obtained. This semi coke is then given a coating of pitch based material (1-4% by wt.) by mixing a pitch dissolved in a solvent (toluene or tar based) and the resulting semi coke is calcined at temperatures up to 300°C in inert atmosphere or under partial vacuum to remove the excess solvent from the semi coke. The modified semi coke powder so obtained is moulded into plates or blocks using a conventional hydraulic press or an isostatic press at a pressure of 1400 to 2000 kgcm-2 and carbonized to around 1000°C in an inert atmosphere of nitrogen or argon. The carbonized plates based on the modified green coke were then further heat-treated (graphitized) at 2500 °C in an inert atmosphere of nitrogen or argon to obtain the final high density, high strength graphite plates, which were found to possess a bulk density of 1.80-1.90 gcm-3 along with a bending strength of 600-1200 Kgcm-2, and an electrical resistivity of 1.0-2.0 mΩcm. The novelty of the present invention resides in coating the semi-coke powder before moulding into plates or blocks so as to get the resulting graphite of superior characteristics i.e., high density, strength and electrical or thermal conductivities compared to uncoated semi coke powder. In an embodiment of the present invention, the coal tar pitch is having a softening point in the range of 88°C to100°C, quinoline insoluble content in the range of 0.0 to 0-4% and coaking value in the range of 44 to 48%. In another embodiment of the present invention, the coal tar pitch is heated at a temperature in the range of 450°C to 500°C for a time period of 2.00 to 5.00 hours in the presence of inert gas. In yet another embodiment of the present invention, the grounded semi coke has an average particle size in the range of 1-10 µm, quinoline insoluble content in the range of 96 to 97.5%, toluene insoluble content in the range of 97 to 98.5% and volatile matter in the range of 9 to 11%. In still another embodiment of the present invention, calcination of the modified semi coke is done at the temperature in the range of 200 to 300°C followed by an inert atmosphere to remove the excess solvent. In yet another embodiment of the present invention, molding is done at the pressure in the range of 1400 to 2000 kg/cm2. In a further embodiment of the present invention, carbonization of the molded plates is done at a temperature in the range of 1000 to 1400°C. In another embodiment of the present invention, graphitization of the carbonized plates is done at a temperature in the range of 2300 to 2700°C. In yet another embodiment of the present invention, the graphitized carbon material is having the bulk density in the range of 1.80x10-3kg/cm3 to 1.91x10-3kg/cm3. In still another embodiment of the present invention, the graphitized carbon material is having the compressive / bending strength in the range of 1020 kg/cm2 to 1180 kg/cm2. EXAMPLES The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. EXAMPLE 1 A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2 % and a coking value of 43.8% was heat treated at 470°C for 3.5 hours in the presence of nitrogen to obtain the semi coke. This semi coke (SC-1) was ground in a centrifugal ball to get a fine powder. The semi coke powder was found to possess a quinoline insoluble content of 96.7%, toluene insoluble content of 97.8%, volatile matter content of 9.3%. A portion of this semi coke was coated with 2% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to obtain pitch coated (modified) semi-coke particles. The raw semi-coke and the semi-coke powder modified with a pitch coating were moulded separately into blocks using a cold isostatic press. The blocks so obtained were carbonised to a temperature of 1000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite. The high density graphite made from the raw semi-coke powder i.e., without having a pitch coating was found to possess a bulk density of 1.89 gcm-3, compressive strength of 960 Kgcm-2, shore hardness of 70 and an electrical resistivity of l.lmOxm. On the other hand, the high density graphite made from the modified semi-coke powder (with a pitch coating) was found to possess superior properties i.e., a bulk density of 1.91 gcm-3, compressive strength of 1180 Kgcm2, shore hardness of 75 and electrical resistivity of 1.1 mΩcm. EXAMPLE 2 A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2% and a coking value of 43.8% was heat treated at 470°C for 3 hours in the presence of nitrogen to obtain the semi coke. This semi coke (SC-2) was ball milled in a centrifugal ball mill to get a fine powder. The semi coke powder was found to possess a quinoline insoluble content of 96.5%, toluene insoluble content of 97.5%, volatile matter content of 9.5 %. A portion of this semi coke was coated with 2% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to give a coating of pitch onto the semi-coke particles. The raw semi-coke and the semi-coke powder modified with pitch coating were moulded separately into blocks using a cold isostatic press. The blocks so obtained were carbonised to a temperature of 1000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite. The high density graphite made from the raw semi-coke powder i.e., without a pitch coating was found to possess a bulk density of 1.86 gcm-3, compressive strength of 960 Kgcm-2, shore hardness of 70 and an electrical resistivity of l.lmΩcm. On the other hand, the high density graphite made from the modified semi-coke powder was found to possess superior properties i.e., a bulk density of 1.87 gcm-3, compressive strength of 1110 Kgcm-2, shore hardness of 75 and electrical resistivity of 1.1 mΩcm. EXAMPLE 3 A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2% and a coking value of 43.8% was heat-treated at 470°C for 3.45 hours in the presence of nitrogen to get the semi coke. This semi coke (SC-3) was ball milled in a centrifugal ball mill to obtain a fine powder. The semi coke powder was found to possess a quinoline Insoluble content of 96.8%, toluene insoluble content of 97.9%, volatile matter content of 9.6%. A portion of this semi coke was coated with 2% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to give a coating of pitch onto the semi-coke particles. The raw semi-coke and the semi-coke powder modified with pitch coating were separately moulded into plates using a hydraulic press. The plates so obtained were carbonised to a temperature ofl000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite. The high density graphite made from the raw semi-coke powder i.e., without a pitch coating was found to possess a bulk density of 1.88 gcm-3, bending strength of 900 Kgcm-2, shore hardness of 70 and an electrical resistivity of l.lmOxm. On the other hand, the high density graphite made from the modified semi-coke powder (with a pitch coating) was found to possess superior properties i.e., a bulk density of 1.90 gcm-3, bending strength of 1020 Kgcm-2, shore hardness of 75 and electrical resistivity of 1.1 mΩcm. EXAMPLE 4 A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2% and a coking value of 43.8% was heat-treated at 480 °C for 3 hours in the presence of nitrogen to obtain the semi coke. This semi-coke (SC4) was ground in a centrifugal ball mill to obtain a fine semi coke powder. The semi coke powder was found to possess a quinoline insoluble content of 97.2%, toluene insoluble content of 98.4 %, volatile matter content of 9.5%. A portion of this semi coke was coated with 4% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to give a coating of pitch onto the semi-coke particles. The raw semi-coke and the semi-coke powder modified with pitch coating were separately moulded into plates using a hydraulic press. The plates so obtained were carbonised to a temperature of 1000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite. The high density graphite made from the raw semi-coke powder i.e., without a pitch coating was found to possess a bulk density of 1.85 gcm-3, bending strength of 960 Kgcm-2, shore hardness of 72 and an electrical resistively of 1.2mΩcm. On the other hand, the high density graphite made from the modified semi-coke powder (with a pitch coating ) was found to possess superior properties i.e., a bulk density of 1.89 g/cm3, bending strength of 1160 Kgcm-2, shore hardness of 76 and electrical resistivity of 1.2 mΩcm. ADVANTAGES The main advantages of the present invention are: • The sensitivity of the solvent extraction treatment in the process of the present invention is reduced by way of coating of the semi-coke with the pitch material. • The high density monolithic graphite produced by the process of the present invention has a bulk density of 1.80-1.90 gcm-3, which is comparable to that of the graphite made from the green coke or mesocarbon microbeads methods and significantly higher than the value of 1.70 - 1.80 gcm-3 for the conventional high density graphite. The bending strength of the graphite possesses values in the range of 600-1200 Kgcm-2 which is also considerably higher than a value of 400 Kgcm-2 for the conventional high density graphite. • The process of the present invention does not involve repeated cycles of impregnation and recarbonisation which are employed in the process of the conventional high density graphite. Thus, the present process is not cumbersome and time consuming, and hence is economical. • The process of the present invention gives a much higher yield (50-60%) of semi-coke operation, whereas in the process of MCMB-based high-density graphite, the yield of the MCMBfrom the precursor is low (15-35%). Thus, the present process is simple and cost effective. We claim: 1. An improved process for the modification of semi coke useful for the production of high density, high strength graphite, wherein the steps comprising: [a] heating a coal tar pitch in the presence of inert gas to produce a semi coke; [b] grounding the semi coke as obtained in step [a] in a ball mill to get fine powder; [c] coating the powdered semi coke as obtained in step [b] with 1-4% by weight of a pitch material followed by calcination to produce the modified semi coke; [d] molding the uncoated semi coke of step [b] and coated semi coke of step [c] separately into plates by means of conventional press; [e] carbonizing the molded plates as obtained in step [d] by heating at a temperature in the range of 1000 to 1400 degree C in an inert atmosphere to produce carbonized plates; [f] graphitizing the carbonized plates as obtained in step [e] by heating in inert atmosphere at a temperature in the range of 2300 to 2700 degree C to produce the desired high density, high strength graphite. 2. A process as claimed in claim 1, wherein the coal tar pitch is having a softening point in the range of 88°C tol00°C, quinoline insoluble content in the range of 0.0 to 0.4% and coaking value in the range of 44 to 48 %. 3. A process as claimed in claim 1, wherein the coal tar pitch is heated at a temperature in the range of 450°C to 500°C for a time period of 2.00 to 5.00 hours in the presence of an inert gas. 4. A process as claimed in claim 1, wherein the grounded semi coke has an average particle size in the range of 1-10 µm, quinoline insoluble content in the range of 96 to 97.5%, toluene insoluble content in the range of 97 to 98.5% and volatile matter in the range of 9 to 11%. 5. A process as claimed in claim 1, wherein calcination of the modified semi coke is done at the temperature in the range of 200 to 300°C followed by an inert atmosphere to remove the excess solvent. 6. A process as claimed in claim 1, wherein molding is done at a pressure in the range of 1400 to 2000 kg/cm2. 7. A process as claimed in claim 1, wherein carbonization of the molded plates is done at the temperature in the range of 1000 to 1400°C. 8. A process as claimed in claim 1, wherein graphitization of the carbonized plates is done at the temperature in the range of 2300 to 2700°C. 9. A process as claimed in claim 1, wherein the final high density, high strength graphite is having bulk density in the range of 1.80x10-3 to 1.91xl0-3kg/cm3 and compressive/ bending strength in the range of 1020 to 1180 kg/cm2. 10. An improved process for the modification of semi coke substantially as herein described with reference to the foregoing examples.

Full Text

FIELD OF THE INVENTION
The present invention relates to an improved process for the modification of semi coke useful for the production of high density, high strength graphite product. The said graphite product possesses potential applications such as electrodes for electric discharge machines, electrical brushes, electrical contacts and trolley wheels, heaters and crucibles for silicon production, seals, bearings, packings, jigs, hot pressing dies and as moulds for continuous casting of metals and alloys.
BACKGROUND OF THE INVENTION & DESCRIPTION OF PRIOR ART
High density - high strength graphite is a recent addition to the family of carbon products. It is, in fact, a high purity graphite possessing a bulk density of more than 1.8 gcm-3, bending strength of more than 600 Kgcm-2, degree of anisotropy of 0.9 - 1.1, along with a homogeneous and fine microstructure.
In the prior art, there are three methods known for the production of high density, high strength graphite (Ishikawa T. and Nagaoki, T. "Recent Carbon Technology", JEC Press Inc. 1983; Oya A. "High Density - Isotropic Graphite - An Overview", Pitch and Pitch - based Products - Proc. Indo -Japanese Workshop on Pitch and Pitch-based Products, New Delhi, Nov. 24 - 25,1989, p58). The first method involves the use of two raw materials, i.e., coke filler and a pitch binder, whereas the second one by Oya, A. involves the use of single raw material, namely, mesocarbon microbeads or green (semi) coke.
In the first method, a coke filler such as petroleum coke or needle coke is kneaded with a pitch binder, such as coal tar pitch to obtain a mixture, which is isostatically pressed and then carbonized to 1000°C followed by repeated cycles of impregnation and recarbonisation to1000°C and finally, graphitization to around 2700°C. Here, the impregnation and recarbonisation of the carbon product is a very cumbersome, time-consuming and expensive process. Further, inspite of several cycles of impregnation and

recarbonisation of the carbon product, the bulk density of the final graphite seldom touches a value of 1.80 gcm-3 and the bending strength of 400Kgcm-2. This is because large amounts of volatiles are evolved from the binder and impregnating pitches during the carbonisation/ recarbonisation operations thereby creating porosity in the product. Thus, this method suffers from its own limitations.
Reference may also be drawn to Honda H. and Yamada Y. J Jpn Petrol Inst. 1973.16 : 392; JP 14,173; Bhatia G., Aggarwal R. K., Punjabi N., Bahl O.P., J. Mater. Sci. 1994. 29 : 4757-4763; IN 189155, wherein a quinoline insoluble (Ql) free or low Ql coal tar pitch is heat treated at about 400 - 4503C in an inert atmosphere to generate the mesophase spherules with a size of around lOmicrons. These spherules also known as mesocarbon microbeads (MCMB), are separated out of the heat treated pitch by solvent extraction using a suitable solvent such as tar oil or quinoline, pressed without any external binder and then carbonised to 1000°C, and finally graphitized to 2500°C or higher to obtain the high-density graphite. During heat-treatment at 400-4509C, polymerisation and condensation reactions take place among the various constituent molecules of the precursor coal tar pitch, leading to the formation of large planar polyaromatic molecules, which aggregate together to form the mesophase spherules because of the Vanderwaal's interactions and surface tension.
These mesophase spherules grow in size with the severity of heat-treatment. Therefore, to restrict their size to the desired value of around 10 microns, one has to lose in terms of their yield by weight of the precursor pitch. Generally, one is able to achieve a yield of 5 to 35% of such mesophase spherules besides problems of fabrication associated during solvent extraction. Thus, in this method also, a serious techno-economic problem of getting high yield of the mesophase spherules of the requisite size of around 10 microns, i.e., a yield of more than 35% of the microbeads by weight of the precursor coal tar pitch upon heat treatment is encountered. Therefore, this method is tedious and not cost effective.

In a method disclosed by Ogawa, I., Kobayashi, K. J. Mater. Sci. 1992. 27 : 1161-1165, a green raw coke also called semi-coke prepared from a coal tar pitch by heat treatment to around 500°C is pressed without any external binder, carbonized to 1000°C and finally graphitized to 2500°C or higher to get the high density monolithic graphite. However, in this method the preparation of the green coke having an optimum amount of the binding components ( volatile matter around 10%) is a highly critical step, since a slight excess of the binding components in the green coke will result in the swelling and/or cracks in the product during the baking operation and slight deficiency of the binding components will result in an improper binding of the green coke particles leading to a relatively weak and low density product.
In view of this, generally, the starting coal tar pitch is often subjected to a mild heat-treatment such that the green coke obtained contains slightly excess amount of the binding components. Such a green coke is then extracted with a suitable solvent so as to remove excess amount of the binding components or given an oxidation/ chlorination treatment. (Ref : IN 194694, IN215804). Here, the solvent extraction quite often removes the binding components which are necessary for getting good electrical, thermal and mechanical strength of the final graphite product. The oxidation /chlorination, in turn, is a very sensitive process and difficult to be controlled to an optimum level. Besides, it gives lower values of electrical and thermal conductivity in the final product. Thus, we find that all the methods described above for the production of high-density monolithic graphite are associated with their own typical problems.
Thus, keeping in view the hitherto known prior art, the inventors of the present invention realized that there exists a dire need to provide a process for the modification of semi-coke so as to produce a final graphite product with considerably improved properties such as density, mechanical strength, electrical and thermal conductivity.

OBJECTIVES OF THE INVENTION
The main objective of the present invention is thus to provide an improved process for the modification of semi coke suitable for the production of high-density high strength graphite product, which overcomes the above-mentioned drawbacks.
Another objective of the present invention is to provide a process wherein coating of the semi-coke powder is done before moulding into plates or blocks so as to get the resulting graphite of superior characteristics i.e., high density, strength and electrical or thermal conductivities compared to uncoated semi coke powder.
Still another objective of the present invention is to provide a process which does not involve repeated cycles of impregnation and recarbonisation which are employed in the process of producing the conventional high density graphite. Thus, the present process is not cumbersome and time consuming, but economical.
Yet another objective of the present invention is to provide a process which produces high-density, monolithic graphite having a bulk density of more than 1.80gcm-3 and bending strength of more than 600 Kgcm-2 along with a homogeneous and fine microstructure.
Another objective of the present invention is to provide high density, high strength graphite, which finds application as electrodes for electric discharge machines, material for hot pressing dies, casting moulds, mechanical seals, resistance heaters and crucibles.
SUMMARY OF THE INVENTION
The present invention provides an improved process for the modification of semi-coke, wherein the semi-coke (or green coke) with a slight excessive amount of binding components is subjected to extraction treatment with a suitable solvent such as toluene, tar

oil etc. which is then is given a coating with a pitch based material in presence of a suitable solvent like toluene, tar oil to obtain pitch coated semi coke, which is then calcined to distill out excess solvent. The modified semi coke is then molded into blocks using isostatic press without using extra binder and carbonized at 1000°C and higher to 2500°C in an inert atmosphere to obtain high density, high strength monolithic graphite. It has been observed that the properties such as density, mechanical strength, electrical and thermal conductivities of the final graphite product are considerably improved as compared to those obtained from the green coke without such a pitch coating.
Accordingly, the present invention provides an improved process for the modification of semi coke useful for the production of high density, high strength graphite, wherein the steps comprising:
[a] heating a coal tar pitch in the presence of inert gas to produce a semi coke;
[b] grounding the semi coke as obtained in step [a] in a ball mill to get fine powder;
[c] coating the powdered semi coke as obtained in step [b] with 1-4% by weight of a pitch material followed by calcination to produce the modified semi coke;
[d] molding the uncoated semi coke of step [b] and coated semi coke of step [c] separately into plates by means of conventional press;
[e] carbonizing the molded plates as obtained in step [d] by heating at a temperature in the range of 1000 to 1400 degree C in an inert atmosphere to produce carbonized plates;
[f] graphitizing the carbonized plates as obtained in step [e] by heating in inert atmosphere at a temperature in the range of 2300 to 2700 degree C to produce the desired high density, high strength graphite.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an improved process for the modification of semi-coke suitable for the production of high density-high strength monolithic graphite, which

comprises heating a suitable coal tar pitch in the presence of an inert gas such as nitrogen, argon etc. at a temperature in the range of 450-500°C to obtain the semi-coke which is ball milled into a fine powder with a size of above 1-10µm. The coal tar pitch used for the said process is having a softening point of 80-100°C, quinoline insoluble (Ql) contents of less than or equal to 0.2 % and a coking value of 44-48% was heat-treated at 450-500°C for 2 - 5 hours in the presence of nitrogen to get the semi coke. This semi coke is ball milled to obtain a particle size of around 5u,m having a quinoline insoluble of 96-97.5%, toluene insoluble of 97-98.5% and volatile matter of around 9-11%. In case the values of toluene insoluble and quinoline insoluble contents are found to be slightly less than above mentioned values, the semi coke is extracted with a suitable solvent such as toluene or tar oil or suitable tar based oil so that the desired values of quinoline insoluble (i.e. 96-97.5%) and toluene insolubles (i.e. 97-98.5%) with a volatile mater of 9-10% are obtained.
This semi coke is then given a coating of pitch based material (1-4% by wt.) by mixing a pitch
dissolved in a solvent (toluene or tar based) and the resulting semi coke is calcined at
temperatures up to 300°C in inert atmosphere or under partial vacuum to remove the
excess solvent from the semi coke. The modified semi coke powder so obtained is moulded
into plates or blocks using a conventional hydraulic press or an isostatic press at a pressure
of 1400 to 2000 kgcm-2 and carbonized to around 1000°C in an inert atmosphere of
nitrogen or argon. The carbonized plates based on the modified green coke were then
further heat-treated (graphitized) at 2500 °C in an inert atmosphere of nitrogen or argon to
obtain the final high density, high strength graphite plates,
which were found to possess a bulk density of 1.80-1.90 gcm-3 along with a bending strength of 600-1200 Kgcm-2, and an electrical resistivity of 1.0-2.0 mΩcm.
The novelty of the present invention resides in coating the semi-coke powder before moulding into plates or blocks so as to get the resulting graphite of superior characteristics i.e., high density, strength and electrical or thermal conductivities compared to uncoated semi coke powder.

In an embodiment of the present invention, the coal tar pitch is having a softening point in the range of 88°C to100°C, quinoline insoluble content in the range of 0.0 to 0-4% and coaking value in the range of 44 to 48%.
In another embodiment of the present invention, the coal tar pitch is heated at a temperature in the range of 450°C to 500°C for a time period of 2.00 to 5.00 hours in the presence of inert gas.
In yet another embodiment of the present invention, the grounded semi coke has an average particle size in the range of 1-10 µm, quinoline insoluble content in the range of 96 to 97.5%, toluene insoluble content in the range of 97 to 98.5% and volatile matter in the range of 9 to 11%.
In still another embodiment of the present invention, calcination of the modified semi coke is done at the temperature in the range of 200 to 300°C followed by an inert atmosphere to remove the excess solvent.
In yet another embodiment of the present invention, molding is done at the pressure in the range of 1400 to 2000 kg/cm2.
In a further embodiment of the present invention, carbonization of the molded plates is done at a temperature in the range of 1000 to 1400°C.
In another embodiment of the present invention, graphitization of the carbonized plates is done at a temperature in the range of 2300 to 2700°C.
In yet another embodiment of the present invention, the graphitized carbon material is having the bulk density in the range of 1.80x10-3kg/cm3 to 1.91x10-3kg/cm3.

In still another embodiment of the present invention, the graphitized carbon material is having the compressive / bending strength in the range of 1020 kg/cm2 to 1180 kg/cm2.
EXAMPLES
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
EXAMPLE 1
A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2 % and a coking value of 43.8% was heat treated at 470°C for 3.5 hours in the presence of nitrogen to obtain the semi coke. This semi coke (SC-1) was ground in a centrifugal ball to get a fine powder. The semi coke powder was found to possess a quinoline insoluble content of 96.7%, toluene insoluble content of 97.8%, volatile matter content of 9.3%.
A portion of this semi coke was coated with 2% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to obtain pitch coated (modified) semi-coke particles. The raw semi-coke and the semi-coke powder modified with a pitch coating were moulded separately into blocks using a cold isostatic press. The blocks so obtained were carbonised to a temperature of 1000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite.
The high density graphite made from the raw semi-coke powder i.e., without having a pitch coating was found to possess a bulk density of 1.89 gcm-3, compressive strength of 960 Kgcm-2, shore hardness of 70 and an electrical resistivity of l.lmOxm. On the other hand, the high density graphite made from the modified semi-coke powder (with a pitch coating) was found to possess superior properties i.e., a bulk density of 1.91 gcm-3,

compressive strength of 1180 Kgcm2, shore hardness of 75 and electrical resistivity of 1.1 mΩcm.
EXAMPLE 2
A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2% and a coking value of 43.8% was heat treated at 470°C for 3 hours in the presence of nitrogen to obtain the semi coke. This semi coke (SC-2) was ball milled in a centrifugal ball mill to get a fine powder. The semi coke powder was found to possess a quinoline insoluble content of 96.5%, toluene insoluble content of 97.5%, volatile matter content of 9.5 %.
A portion of this semi coke was coated with 2% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to give a coating of pitch onto the semi-coke particles. The raw semi-coke and the semi-coke powder modified with pitch coating were moulded separately into blocks using a cold isostatic press. The blocks so obtained were carbonised to a temperature of 1000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite.
The high density graphite made from the raw semi-coke powder i.e., without a pitch coating was found to possess a bulk density of 1.86 gcm-3, compressive strength of 960 Kgcm-2, shore hardness of 70 and an electrical resistivity of l.lmΩcm. On the other hand, the high density graphite made from the modified semi-coke powder was found to possess superior properties i.e., a bulk density of 1.87 gcm-3, compressive strength of 1110 Kgcm-2, shore hardness of 75 and electrical resistivity of 1.1 mΩcm.
EXAMPLE 3
A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2% and a coking value of 43.8% was heat-treated at 470°C for 3.45 hours in the presence of

nitrogen to get the semi coke. This semi coke (SC-3) was ball milled in a centrifugal ball mill to obtain a fine powder. The semi coke powder was found to possess a quinoline Insoluble content of 96.8%, toluene insoluble content of 97.9%, volatile matter content of 9.6%.
A portion of this semi coke was coated with 2% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to give a coating of pitch onto the semi-coke particles. The raw semi-coke and the semi-coke powder modified with pitch coating were separately moulded into plates using a hydraulic press. The plates so obtained were carbonised to a temperature ofl000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite.
The high density graphite made from the raw semi-coke powder i.e., without a pitch coating was found to possess a bulk density of 1.88 gcm-3, bending strength of 900 Kgcm-2, shore hardness of 70 and an electrical resistivity of l.lmOxm. On the other hand, the high density graphite made from the modified semi-coke powder (with a pitch coating) was found to possess superior properties i.e., a bulk density of 1.90 gcm-3, bending strength of 1020 Kgcm-2, shore hardness of 75 and electrical resistivity of 1.1 mΩcm.
EXAMPLE 4
A coal tar pitch having a softening point of 88°C, quinoline insoluble content of 0.2% and a coking value of 43.8% was heat-treated at 480 °C for 3 hours in the presence of nitrogen to obtain the semi coke. This semi-coke (SC4) was ground in a centrifugal ball mill to obtain a fine semi coke powder. The semi coke powder was found to possess a quinoline insoluble content of 97.2%, toluene insoluble content of 98.4 %, volatile matter content of 9.5%.

A portion of this semi coke was coated with 4% (by wt) of a pitch material was dissolved in hot toluene used as a solvent to give a coating of pitch onto the semi-coke particles. The raw semi-coke and the semi-coke powder modified with pitch coating were separately moulded into plates using a hydraulic press. The plates so obtained were carbonised to a temperature of 1000°C in an inert atmosphere of nitrogen which were then further heat-treated (graphitized) to 2500°C in an inert atmosphere of argon/nitrogen to obtain the final monolithic high density, high strength graphite.
The high density graphite made from the raw semi-coke powder i.e., without a pitch coating was found to possess a bulk density of 1.85 gcm-3, bending strength of 960 Kgcm-2, shore hardness of 72 and an electrical resistively of 1.2mΩcm. On the other hand, the high density graphite made from the modified semi-coke powder (with a pitch coating ) was found to possess superior properties i.e., a bulk density of 1.89 g/cm3, bending strength of 1160 Kgcm-2, shore hardness of 76 and electrical resistivity of 1.2 mΩcm.
ADVANTAGES
The main advantages of the present invention are:
• The sensitivity of the solvent extraction treatment in the process of the present invention is reduced by way of coating of the semi-coke with the pitch material.
• The high density monolithic graphite produced by the process of the present invention has a bulk density of 1.80-1.90 gcm-3, which is comparable to that of the graphite made from the green coke or mesocarbon microbeads methods and significantly higher than the value of 1.70 - 1.80 gcm-3 for the conventional high density graphite. The bending strength of the graphite possesses values in the range of 600-1200 Kgcm-2 which is also considerably higher than a value of 400 Kgcm-2 for the conventional high density graphite.

• The process of the present invention does not involve repeated cycles of impregnation and recarbonisation which are employed in the process of the conventional high density graphite. Thus, the present process is not cumbersome and time consuming, and hence is economical.
• The process of the present invention gives a much higher yield (50-60%) of semi-coke operation, whereas in the process of MCMB-based high-density graphite, the yield of the MCMBfrom the precursor is low (15-35%). Thus, the present process is simple and cost effective.

We claim:
1. An improved process for the modification of semi coke useful for the production of
high density, high strength graphite, wherein the steps comprising:
[a] heating a coal tar pitch in the presence of inert gas to produce a semi coke;
[b] grounding the semi coke as obtained in step [a] in a ball mill to get fine powder;
[c] coating the powdered semi coke as obtained in step [b] with 1-4% by weight of a pitch material followed by calcination to produce the modified semi coke;
[d] molding the uncoated semi coke of step [b] and coated semi coke of step [c] separately into plates by means of conventional press;
[e] carbonizing the molded plates as obtained in step [d] by heating at a temperature in the range of 1000 to 1400 degree C in an inert atmosphere to produce carbonized plates;
[f] graphitizing the carbonized plates as obtained in step [e] by heating in inert atmosphere at a temperature in the range of 2300 to 2700 degree C to produce the desired high density, high strength graphite.

2. A process as claimed in claim 1, wherein the coal tar pitch is having a softening point in the range of 88°C tol00°C, quinoline insoluble content in the range of 0.0 to 0.4% and coaking value in the range of 44 to 48 %.
3. A process as claimed in claim 1, wherein the coal tar pitch is heated at a temperature in the range of 450°C to 500°C for a time period of 2.00 to 5.00 hours in the presence of an inert gas.
4. A process as claimed in claim 1, wherein the grounded semi coke has an average particle size in the range of 1-10 µm, quinoline insoluble content in the range of 96 to 97.5%, toluene insoluble content in the range of 97 to 98.5% and volatile matter in the range of 9 to 11%.
5. A process as claimed in claim 1, wherein calcination of the modified semi coke is done at the temperature in the range of 200 to 300°C followed by an inert atmosphere to remove the excess solvent.
6. A process as claimed in claim 1, wherein molding is done at a pressure in the range of 1400 to 2000 kg/cm2.
7. A process as claimed in claim 1, wherein carbonization of the molded plates is done at the temperature in the range of 1000 to 1400°C.
8. A process as claimed in claim 1, wherein graphitization of the carbonized plates is done at the temperature in the range of 2300 to 2700°C.
9. A process as claimed in claim 1, wherein the final high density, high strength graphite is having bulk density in the range of 1.80x10-3 to 1.91xl0-3kg/cm3 and compressive/ bending strength in the range of 1020 to 1180 kg/cm2.
10. An improved process for the modification of semi coke substantially as herein described with reference to the foregoing examples.

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Patent Number

279514

Indian Patent Application Number

456/DEL/2009

PG Journal Number

04/2017

Publication Date

27-Jan-2017

Grant Date

24-Jan-2017

Date of Filing

09-Mar-2009

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI - 110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	BHATIA GOPAL	NATIONA PHYSICAL LABORATORY, NEW DELHI, INDIA.
2	RAMAN VASANTHA	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.
3	SENGUPTA PINAKI RANJAN	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.
4	BISHT RAJENDRA SINGH	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.
5	NEGI TRILOK SINGH	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.
6	PUNDORA PREM LAL	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.
7	MANDEEP KAUR	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.
8	ANIL KUMAR	NATIONAL PHYSICAL LABORATORY, NEW DELHI, INDIA.

PCT International Classification Number

H01G

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA