Title of Invention	AN IMPROVED PROCESS FOR THE PRODUCTION OF MODIFIED GREEN COKE USEFUL FOR MAKING HIGH DENSITY MONOLITHIC GRAPHITE
Abstract	This invention relates to an improved process for the production of modified green coke useful for making high density monolithic graphite products. The high density monolithic graphite prepared by the improved process of the present invention, possesses a bulk density of more than 1.8 g/cm3 along with a homogeneous and fine isotropic microstructure. Process steps are: heating coal tar pitch in the presence of an inert gas, at a temperature in the range of 350-550°C to obtain green coke possessing quinoline insoluble (QI) in the range of 80-95% and toluene insoluble (TI) in the range of 85-97%, grinding the said green coke to a powder with a particle size of below 75 urn, subjecting the powdered green coke to oxidation in air or oxygen at a temperature in the range of 150-200°C, grinding the oxidized green coke to a size in the range of 1-30 µm, to obtain the modified green coke having enhanced QI in the range of (95-99%) and TI in the range of (96-99.5%).

Title of Invention

AN IMPROVED PROCESS FOR THE PRODUCTION OF MODIFIED GREEN COKE USEFUL FOR MAKING HIGH DENSITY MONOLITHIC GRAPHITE

Abstract

Full Text	This invention relates to an improved process for the production of modified green coke useful for making high density monolithic graphite products. The high density monolithic graphite prepared by the improved process of the present invention, possesses a bulk density of more than 1.8 g/cm3 along with a homogeneous and fine isotropic micro structure. The main uses of this monolithic graphite prepared by the process of the present invention are: i) As electrodes for electric discharge machines, and as electrical brushes, electrical contacts and trolley wheels, ii) As heaters and crucibles for silicon production, iii) As seals, bearings, packings, jigs, hot-pressing dies, iv) As moulds for continuous casting of metals and alloys. In the prior art, there are three methods known for the production of high density graphite (T. Ishikawa and T. Nagaoki, "Recent Carbon Technology", JEC Press Inc., 1983; A. Oya, in Pro- ceedings of the Indo - Japanese Workshop on Pitch and Pitch-Based Products", edited by O. P. Bahl and L. M. Manocha, Indian Carbon Society, NPL, New Delhi, 1989, p.58). One method involves the use of two raw materials, i.e., a coke filler and a pitch binder, whereas the other two methods involve the use of single raw materials, namely, mesocarbon microbeads or green (raw) coke. In the first method (A. Oya, in Proceedings of the Indo-Japanese Workshop on Pitch and Pitch-Based Products", edited by O. P. Bahl and L. M. Manocha, Indian Carbon Society, National Physical laboratory, New Delhi, 1989, p.58), 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 carbonised to 1000°C, followed by repeated cycles of impregnation and recarbonisation to 1000°C, and finally graphitisation to around 2700°C. Here, the impregnation and recarbonisation of the carbon product is a very cumbersome, time consuming and expensive process. Further, even inspite of several cycles of impregnation and recarbonisation of the carbon product, the bulk density of the final graphite seldom touches a value of 1.8 g cm-3 and the bending strength a value of 40 MPa. This is because large amounts of volatiles are evolved from the binder and impregnating pitches during the carbonisation / recarbonisation operations creating porosity in the product. Thus, this method suffers from its own limitations. In the second method (H. Honda and Y. YamadaJ. Jpn. Petrol. Inst., 16(1973)392; K. Hagiwara, N. Fukuda and k. Nagayama, Jpn. Patent, No. 14,173(1989); G. Bhatia , R. K. Aggarwal, N. Punjabi and 0. P. Bahl, J. Mater, sci., 29(1994), 4757; G. Bhatia , R. K. Aggarwal, N. Punjabi and 0. P. Bahl, "An improved process for the production of high density monolithic graphite from mesocarbon microbeads" - co-pending Indian Patent (Application, no.757/DEL/94), a quinoline insolubles (QI) - free or low-QI coal tar pitch is heat treated at about 400-450°C in an inert atmosphere to generate the mesophase Sphemles with a size of around 10 microns. 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 and then carbonised to 1000°C, and finally graphitised to 2700°C to obtain the high density graphite. During the heat-treatment at 400-450 °C, 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 sphemles because of the van der Waal's interactions and surface tension. These mesophase sphemles 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 15-40 % of such mesophase spherules. Thus, in this method again, there is encountered 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 40 % of the microbeads by weight of the precursor coal tar pitch upon heat treatment. Therefore this method also is not cost-effective. In the third method as described by I. Ogawa, H. Yoshida and K. Kobayashi,in J. Mater. Sci., 16(1981), 2181; by I. Ogawa, K. Kobayashi, in J. Mater. Sci., 27(1992),1161; by G. Bhatia, R. K. Agarwal and 0. P. Bahl, in Proceedings National Conference on Carbon, Kanpur, Dec. 19-20, 1991, Indian Carbon society, National Physical Laboratory New Delhi, 1996, p. 182; by G. Bhatia and R. K. Aggarwal, a green (raw) coke prepared from a coal tar pitch by heat treatment to around 500°C is pressed, carbonised to 1000°C and finally graphitized to 2700°C to get the high density graphite. 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 panicles leading to a relatively weak and low density product. In view of this .generally , the starting coal tar pitch is first subjected to a mild heat treatment such that the greeen coke obtained contains slightly excess amount of the binding components . Such a green coke is then modified so as to contain an optimum amount of binding components by way of solvent extraction as described in an earlier patent by G. Bhatia, and R K aggarwal (Pat Application No. 395/DEL/96 )or oxidation/chlorination as described by A. Oya in the Proceedings of the Indo Japanese Workshop on Pitch and Pitch Based Products Ed O P Bahl and L .M.Manocha, Indian carbon Society, National Physical Laboratory New Delhi, (1989) page 58. Here the solvent extraction is quite cumbersome and expensive as it involves the use of solvents in large quantities (in wt./vol. ratio ofl:5 or more)followed by their recovery by way of distillation in an inert atmosphere and/or under reduced pressure at a temperature of 200° C to 400 °C . The recovery of solvent involves losses also. The oxidation/chlorianation , in rum is a very sensitive process and difficult to be controlled, to an optimal level. • Thus, we find that all the methods explained above for the production of high density I monolithic graphite are associated with their own typical problems. The main objective of the present invention is to provide an improved process for the production of modified green coke useful for making high density monolithic graphite products, obviating the above mentioned drawbacks. Another objective of the invention is to provide a process for making products from modified green coke based high density monolithic graphite. Still another objective of the present invention is to provide an improved process for the (production of high density monolithic graphite having a bulk density of more than 1.8 gm/cc and having a homogeneous and fine microstructure. Yet another objective of the present invention is to provide a simple and economical process for the production of high density monolithic graphite products. Yet another objective of the present invention is to provide high density monolithic graphite with potential applications such as electrodes for electric discharge machines, and as material for hot-pressing dies, casting moulds, mechanical seals, resistance heaters, crucibles. The present invention is based on the third method of the above mentioned prior art, wherein the green coke with a slight excessive amount of binding components is subjected to oxidation, and on our finding that the sensitivity of the oxidation of the green coke can be reduced by carrying out the oxidation of green coke on a coarser size of less than 75 urn. The control over the oxidation treatment is achieved by characterizing the modified green coke with respect to quinoline and toluene insoluble contents and volatile matter cone tent. The resulting green coke is then found to lead to a crack free high density monolithic graphite. Accordingly, the present invention provides an improved process for the production of modified green coke useful for making high density monolithic graphite, which comprises; heating coal tar pitch having characteristics as herein described in the presence of an inert gas, such as herein described a temperature in the range of 350-550°C to obtain green coke possessing quinoline insoluble (QI) in the range of 80-95% and toluene insoluble (TI) in the range of 85-97%, grinding the said green coke to a powder with a particle size of below 75 µm, subjecting the powdered green coke to oxidation in air or oxygen at a temperature in the range of 150-200°C, grinding the oxidized green coke to a size in the range of 1-30 µm, to obtain the modified green coke having enhanced QI in the range of (95-99%) and TI in the range of (96-99.5%). In an embodiment of the present invention the coal tar pitch used may be such as having softening point in the range of 70-90 deg.C. In an embodiment of the present invention the inert gas used for the treatment of the coal tar pitch may be selected from nitrogen, helium or argon. In another embodiment of the present invention heat treatment of the above coal tar pitch may be for a period of 1 to 7 hrs. In still another embodiment of the present invention the oxidation may be carried out for a period of 10-100 hours. Accordingly the present invention provides a process for making high density monolithic graphite products using the modified green coke prepared by the process of the present invention, which comprises, moulding modified green coke powder into a product using conventional methods, carbonising the product by heating to a temperature of around 1000°C in an inert atmosphere, followed by graphitising the product at a temperature of around 2700°C in an inert atmosphere to get the high density monolithic graphite product. In an embodiment of the present invention, moulding of the modified green coke may be done by compression techniques. In yet another embodiment of the present invention the pressure used for compression of the modified green coke may be in the range of 1000-2500Kg/cm In another embodiment of the present invention the inert gas used may be selected from helium, nitrogen or argon. The following examples are given by way of illustration which should, however, not be construed to limit the scope of the present invention. EXAMPLE 1 A coal tar pitch having a softening point of 82°C, quinoline insoluble (QI) contents of 0.2 % and a coking value of 48.6 % was heat-treated at 520 °C for 1.5 hours to get the green coke. This green coke was ground to pass through 200 B.S. mesh and was found to possess a quinoline insoluble content of 91.8 %,toluene insoluble content of 95.2 % and a volatile matter content of 9.2 % . This coke was then subjected to an oxidation treatment in air at 180 °C for 40 hours followed by fine grinding to obtain the modified green coke powder. The resulting material i.e. modified green coke, was found to have a quinoline insoluble content of 96.1 % ,toluene insoluble content of 99.7 % and a volatile matter content of 10.3 % . The as-such green coke and the modified green coke powder were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000 °C. It was found that the plates made out of the as-such green coke (without the oxidation treatment) got swollen on this heat-treatment to 1000°C, whereas those made out of the modified green coke remained completely in- tact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.92 g/cm3 along with a bending strength of 74 MPa, Shore hardness of 70 and an electrical resistivity of 1.5 m Ohm. EXAMPLE 2 A coal tar pitch having a softening point of 74°C, quinoline insoluble (QI) content of 0.1 % and a coking value of 47.3 % was heat-treated at 530 °C for 1.0 hour to get the green coke. This green coke was ground to pass through 200 B.S.mesh and was found to possess a quinoline insoluble content of 92.5 %, toluene insoluble content of 94.4 % and a volatile matter content of 8.8 %. This green coke was then subjected to an oxidation-treatment at 170 °C for 60 hours in air followed by fine grinding to obtain the modified green coke powder. The resulting material, i.e. modified green coke, was found to have a quinoline insoluble content of 95.8 %, toluene insolubles content of 98.3 % and a volatile matter content of 9.8 %. The as such green coke and the modified green coke powders were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000°C. It was found that the plates made out of the as-such green coke (without oxidation treatment) got swollen on this heat-treatment to 1000oC, whereas those made out of the modified green coke remained completely intact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.94 g/cm3 along with a bending strength of 76 MPa, Shore hardness of 72 and an electrical resistivity of 1.4 m Ohm cm. EXAMPLE 3 A coal tar pitch having a softening point of 77°C, quinoline insoluble (QI) content of 0.1 % and a coking value of 49.3 % was heat-treated at 530 °C for 1.75 hours to get the green coke. This green coke was ground to pass through 200 B.S.mesh and was found to possess a quinoline insoluble content of 94.5 %, toluene insoluble content of 96.0 % and a volatile matter content of 8.2 %. This green coke was then subjected to an oxidation treatment at 170 °C for 20 hours in air followed by fine grinding to obtain the modified green coke powder. The resulting material, i.e. modified green coke, was found to have a quinoline insoluble content of 96.2 %, toluene insolubles content of 97.8 % and a volatile matter content of 9.0 %. The as-such green coke and the modified green coke powders were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000°C. It was found that the plates made out of the as-such green coke (without oxidation treatment) got swollen on this heat-treatment to 1000°C. whereas those made out of the modified raw coke remained completely intact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.91 g/cm3 along with a bending strength of 68 MPa, Shore hardness of 66 and an electrical resistivity of 1.6 m Ohm cm. EXAMPLE 4 A coal tar pitch having a softening point of 87°C, quinoline insoluble (QI) content of 0.2 % and a coking value of 52.3 % was heat-treated at 515 °C for 2.5 hours to get the green coke. This green coke was ground to pass through 200 B.S.mesh and was found to possess a quinoline insoluble content of 93.2 %, toluene insoluble content of 94.6 % and a volatile matter content of 9.5 %. This green coke was then subjected to an oxidation treatment at 170 °C for 10 hours in oxygen followed by fine grinding to obtain the modified green ;coke powder. The resulting material, i.e. modified green coke, was found to have a quinoline insoluble content of 97.6 %, toluene insoluble content of 98.8 % and a volatile matter content of 10.3 %. The as-such green coke and the modified green coke powders were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000°C. It was found that the plates made out of the as-such green coke (without oxidation treatment) got swollen on this heat-treatment to 1000°C, whereas those made out of the modified raw coke remained completely intact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.90 g/cm3 along with a bending strength of 76 MPa, Shore hardness of 73 and an electrical resistivity of 1.6 m Ohm cm. A summary of the above three examples is given below in Table-1. TABLE-1 SUMMARY OF PROCESSING DETAILS AND CHARACTERISTICS OF THE RESULTING HIGH DENSITY MONOLITHIC GRAPHITES IN THE ABOVE FOUR EXAMPLES (Table Removed) NOTE: The figures in parentheses refer to the values of the as-such (without oxidation treatment) green coke powders. The novelty of the present invention is in eliminating the swelling and/or crack formation in the product during carbonisation. The inventive step of the present invention is in the grinding and oxidation of the green coke. The first grinding step is to get a powder of size less than 75 µm followed by an oxidation step. The second step is in grinding the oxidised ground coke to a particle size in the range of l-30µm. The main advantages of the present invention are: 1. The sensitivity of the oxidation treatment in the process of the present invention is reduced by way of fine grinding of the green coke after the oxidation treatment and the control over the oxidation treatment is achieved by characterising the modified green coke with respect to quinoline and toluene insoluble contents and volatile matter content. 2. The high density monolithic graphite produced by the process of the present invention has a bulk density of 1.80 - 1.95 g/cm3, 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 g/cm" for the conventional high density graphite. 3. The process of the present invention does not involve repeated cycles of impregnation and recarboaisation which are employed in the process of the conventional high density graphite. Thus, the present process is not cumbersome and time consuming, and is hence economical. 4. The process of the present invention does not involve the solvent extraction operation which is employed in the process of mesocarbon microbeads based high density graphice. Besides this, the yield of the mesocarbon microbeads from the precursor is low (15-40%) whereas the process of the present invention gives a much higher yield of (65 -70 %). Thus, the present process is simple and cost effective. We Claim: 1. An improved process for the production of modified green coke useful for making high density monolithic graphite, which comprises; heating coal tar pitch having characteristics as herein described in the presence of an inert gas such as herein described at a temperature in the range of 350-550°C to obtain green coke possessing quinoline insoluble (QI) in the range of 80-95% and toluene insoluble (TI) in the range of 85-97%, grinding the said green coke to a powder with a particle size of below 75 µm, subjecting the powdered green coke to oxidation in air or oxygen at a temperature in the range of 150-200°C, grinding the oxidized green coke to a size in the range of 1-30 µm, to obtain the modified green coke having enhanced QI in the range of (95-99%) and TI in the range of (96-99.5%). 2. An improved process as claimed in claim 1 wherein the coal tar pitch used is such as having softening point in the range of 70-90 C. 3. An improved process as claimed in claims 1 and 2 wherein the inert gas used for the heat treatment of the coal tar pitch is selected from nitrogen, helium or argon. 4. An improved process as claimed in claims 1-3 wherein the heat-treatment of the coal tar pitch is done for a period of 1 to 7 hours. 5. An improved process as claimed in claims 1-4 wherein the oxidation is carried out for a period in the range of 10-100 hours. 6. An improved process for the production of modified green coke useful for making high density monolithic graphite substantially as described herein with reference to the examples.

Full Text

This invention relates to an improved process for the production of modified green coke useful for making high density monolithic graphite products.
The high density monolithic graphite prepared by the improved process of the present invention, possesses a bulk density of more than 1.8 g/cm3 along with a homogeneous and fine isotropic micro structure. The main uses of this monolithic graphite prepared by the process of the present invention are:
i) As electrodes for electric discharge machines, and as electrical brushes, electrical contacts
and trolley wheels,
ii) As heaters and crucibles for silicon production,
iii) As seals, bearings, packings, jigs, hot-pressing dies,
iv) As moulds for continuous casting of metals and alloys.
In the prior art, there are three methods known for the production of high density graphite (T. Ishikawa and T. Nagaoki, "Recent Carbon Technology", JEC Press Inc., 1983; A. Oya, in Pro- ceedings of the Indo - Japanese Workshop on Pitch and Pitch-Based Products", edited by O. P. Bahl and L. M. Manocha, Indian Carbon Society, NPL, New Delhi, 1989, p.58). One method involves the use of two raw materials, i.e., a coke filler and a pitch binder, whereas the other two methods involve the use of single raw materials, namely, mesocarbon microbeads or green (raw) coke.
In the first method (A. Oya, in Proceedings of the Indo-Japanese Workshop on Pitch and Pitch-Based Products", edited by O. P. Bahl and L. M. Manocha, Indian Carbon Society, National Physical laboratory, New Delhi, 1989, p.58), 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 carbonised to 1000°C, followed by repeated cycles of
impregnation and recarbonisation to 1000°C, and finally graphitisation to around 2700°C. Here, the impregnation and recarbonisation of the carbon product is a very cumbersome, time consuming and expensive process. Further, even inspite of several cycles of impregnation and recarbonisation of the carbon product, the bulk density of the final graphite seldom touches a value of 1.8 g cm-3 and the bending strength a value of 40 MPa. This is because large amounts of volatiles are evolved from the binder and impregnating pitches during the carbonisation / recarbonisation operations creating porosity in the product. Thus, this method suffers from its own limitations.
In the second method (H. Honda and Y. YamadaJ. Jpn. Petrol. Inst., 16(1973)392; K.
Hagiwara, N. Fukuda and k. Nagayama, Jpn. Patent, No. 14,173(1989); G. Bhatia , R. K.
Aggarwal, N. Punjabi and 0. P. Bahl, J. Mater, sci., 29(1994), 4757; G. Bhatia , R. K.
Aggarwal, N. Punjabi and 0. P. Bahl, "An improved process for the production of high
density monolithic graphite from mesocarbon microbeads" - co-pending Indian Patent
(Application, no.757/DEL/94), a quinoline insolubles (QI) - free or low-QI coal tar pitch
is heat treated at about 400-450°C in an inert atmosphere to generate the mesophase
Sphemles with a size of around 10 microns. 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 and then carbonised
to 1000°C, and finally graphitised to 2700°C to obtain the high density graphite. During the
heat-treatment at 400-450 °C, 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
sphemles because of the van der Waal's interactions and surface tension. These mesophase
sphemles 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 15-40 % of such mesophase spherules. Thus, in this method again, there is encountered 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 40 % of the microbeads by weight of the precursor coal tar pitch upon heat treatment. Therefore this method also is not cost-effective. In the third method as described by I. Ogawa, H. Yoshida and K. Kobayashi,in J. Mater. Sci., 16(1981), 2181; by I. Ogawa, K. Kobayashi, in J. Mater. Sci., 27(1992),1161; by G. Bhatia, R. K. Agarwal and 0. P. Bahl, in Proceedings National Conference on Carbon, Kanpur, Dec. 19-20, 1991, Indian Carbon society, National Physical Laboratory New Delhi, 1996, p. 182; by G. Bhatia and R. K. Aggarwal, a green (raw) coke prepared from a coal tar pitch by heat treatment to around 500°C is pressed, carbonised to 1000°C and finally graphitized to 2700°C to get the high density graphite. 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 panicles leading to a relatively weak and low density product. In view of this .generally , the starting coal tar pitch is first subjected to a mild heat treatment such that the greeen coke obtained contains slightly excess amount of the binding components . Such a green coke is then modified so as to contain an optimum amount of binding components by way of solvent extraction as described in an earlier patent by G. Bhatia, and R K aggarwal (Pat Application No. 395/DEL/96 )or oxidation/chlorination as described by A. Oya in the Proceedings of the Indo Japanese Workshop on Pitch and Pitch
Based Products Ed O P Bahl and L .M.Manocha, Indian carbon Society, National Physical Laboratory New Delhi, (1989) page 58. Here the solvent extraction is quite cumbersome and expensive as it involves the use of solvents in large quantities (in wt./vol. ratio ofl:5 or more)followed by their recovery by way of distillation in an inert atmosphere and/or under reduced pressure at a temperature of 200° C to 400 °C . The recovery of solvent involves losses also. The oxidation/chlorianation , in rum is a very sensitive process and difficult to be controlled, to an optimal level.
• Thus, we find that all the methods explained above for the production of high density
I monolithic graphite are associated with their own typical problems.
The main objective of the present invention is to provide an improved process for the
production of modified green coke useful for making high density monolithic graphite
products, obviating the above mentioned drawbacks.
Another objective of the invention is to provide a process for making products from
modified green coke based high density monolithic graphite.
Still another objective of the present invention is to provide an improved process for the
(production of high density monolithic graphite having a bulk density of more than 1.8 gm/cc
and having a homogeneous and fine microstructure.
Yet another objective of the present invention is to provide a simple and economical process
for the production of high density monolithic graphite products.
Yet another objective of the present invention is to provide high density monolithic graphite
with potential applications such as electrodes for electric discharge machines, and as
material for hot-pressing dies, casting moulds, mechanical seals, resistance heaters,
crucibles.
The present invention is based on the third method of the above mentioned prior art,
wherein the green coke with a slight excessive amount of binding components is subjected to oxidation, and on our finding that the sensitivity of the oxidation of the green coke can be reduced by carrying out the oxidation of green coke on a coarser size of less than 75 urn. The control over the oxidation treatment is achieved by characterizing the modified green coke with respect to quinoline and toluene insoluble contents and volatile matter cone tent. The resulting green coke is then found to lead to a crack free high density monolithic graphite.
Accordingly, the present invention provides an improved process for the production of modified green coke useful for making high density monolithic graphite, which comprises; heating coal tar pitch having characteristics as herein described in the presence of an inert gas, such as herein described a temperature in the range of 350-550°C to obtain green coke possessing quinoline insoluble (QI) in the range of 80-95% and toluene insoluble (TI) in the range of 85-97%, grinding the said green coke to a powder with a particle size of below 75 µm, subjecting the powdered green coke to oxidation in air or oxygen at a temperature in the range of 150-200°C, grinding the oxidized green coke to a size in the range of 1-30 µm, to obtain the modified green coke having enhanced QI in the range of (95-99%) and TI in the range of (96-99.5%).
In an embodiment of the present invention the coal tar pitch used may be such as having softening point in the range of 70-90 deg.C.
In an embodiment of the present invention the inert gas used for the treatment of the coal tar pitch may be selected from nitrogen, helium or argon.
In another embodiment of the present invention heat treatment of the above coal tar pitch may be for a period of 1 to 7 hrs.
In still another embodiment of the present invention the oxidation may be carried out for a period of 10-100 hours.
Accordingly the present invention provides a process for making high density monolithic graphite products using the modified green coke prepared by the process of the present invention, which comprises, moulding modified green coke powder into a product using conventional methods, carbonising the product by heating to a temperature of around 1000°C in an inert atmosphere, followed by graphitising the product at a temperature of around 2700°C in an inert atmosphere to get the high density monolithic graphite product. In an embodiment of the present invention, moulding of the modified green coke may be done by compression techniques.
In yet another embodiment of the present invention the pressure used for compression of the modified green coke may be in the range of 1000-2500Kg/cm
In another embodiment of the present invention the inert gas used may be selected from helium, nitrogen or argon.
The following examples are given by way of illustration which should, however, not be construed to limit the scope of the present invention.
EXAMPLE 1
A coal tar pitch having a softening point of 82°C, quinoline insoluble (QI) contents of 0.2 % and a coking value of 48.6 % was heat-treated at 520 °C for 1.5 hours to get the green coke. This green coke was ground to pass through 200 B.S. mesh and was found to possess a quinoline insoluble content of 91.8 %,toluene insoluble content of 95.2 % and a volatile matter content of 9.2 % . This coke was then subjected to an oxidation treatment in air at 180 °C for 40 hours followed by fine grinding to obtain the modified green coke powder. The resulting material i.e. modified green coke, was found to have a quinoline insoluble content of 96.1 % ,toluene insoluble content of 99.7 % and a volatile matter content of 10.3 % . The as-such green coke and the modified green coke powder were moulded into
rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000 °C. It was found that the plates made out of the as-such green coke (without the oxidation treatment) got swollen on this heat-treatment to 1000°C, whereas those made out of the modified green coke remained completely in- tact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.92 g/cm3 along with a bending strength of 74 MPa, Shore hardness of 70 and an electrical resistivity of 1.5 m Ohm.
EXAMPLE 2
A coal tar pitch having a softening point of 74°C, quinoline insoluble (QI) content of 0.1 % and a coking value of 47.3 % was heat-treated at 530 °C for 1.0 hour to get the green coke. This green coke was ground to pass through 200 B.S.mesh and was found to possess a quinoline insoluble content of 92.5 %, toluene insoluble content of 94.4 % and a volatile matter content of 8.8 %. This green coke was then subjected to an oxidation-treatment at 170 °C for 60 hours in air followed by fine grinding to obtain the modified green coke powder. The resulting material, i.e. modified green coke, was found to have a quinoline insoluble content of 95.8 %, toluene insolubles content of 98.3 % and a volatile matter content of 9.8 %. The as such green coke and the modified green coke powders were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000°C. It was found that the plates made out of the as-such green coke (without oxidation treatment) got swollen on this heat-treatment to 1000oC, whereas those made out of the modified green coke remained completely intact. The carbonised plates based on the modified green coke were then further heat-treated
(graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.94 g/cm3 along with a bending strength of 76 MPa, Shore hardness of 72 and an electrical resistivity of 1.4 m Ohm cm.
EXAMPLE 3
A coal tar pitch having a softening point of 77°C, quinoline insoluble (QI) content of 0.1 % and a coking value of 49.3 % was heat-treated at 530 °C for 1.75 hours to get the green coke. This green coke was ground to pass through 200 B.S.mesh and was found to possess a quinoline insoluble content of 94.5 %, toluene insoluble content of 96.0 % and a volatile matter content of 8.2 %. This green coke was then subjected to an oxidation treatment at 170 °C for 20 hours in air followed by fine grinding to obtain the modified green coke powder. The resulting material, i.e. modified green coke, was found to have a quinoline insoluble content of 96.2 %, toluene insolubles content of 97.8 % and a volatile matter content of 9.0 %. The as-such green coke and the modified green coke powders were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000°C. It was found that the plates made out of the as-such green coke (without oxidation treatment) got swollen on this heat-treatment to 1000°C. whereas those made out of the modified raw coke remained completely intact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.91 g/cm3 along with a bending strength of 68 MPa, Shore hardness of 66 and an electrical resistivity of 1.6 m Ohm cm.
EXAMPLE 4
A coal tar pitch having a softening point of 87°C, quinoline insoluble (QI) content of 0.2 % and a coking value of 52.3 % was heat-treated at 515 °C for 2.5 hours to get the green coke. This green coke was ground to pass through 200 B.S.mesh and was found to possess a quinoline insoluble content of 93.2 %, toluene insoluble content of 94.6 % and a volatile matter content of 9.5 %. This green coke was then subjected to an oxidation treatment at 170 °C for 10 hours in oxygen followed by fine grinding to obtain the modified green ;coke powder. The resulting material, i.e. modified green coke, was found to have a quinoline insoluble content of 97.6 %, toluene insoluble content of 98.8 % and a volatile matter content of 10.3 %. The as-such green coke and the modified green coke powders were moulded into rectangular plates using a conventional hydraulic press, and the plates so obtained were carbonised to a temperature of 1000°C. It was found that the plates made out of the as-such green coke (without oxidation treatment) got swollen on this heat-treatment to 1000°C, whereas those made out of the modified raw coke remained completely intact. The carbonised plates based on the modified green coke were then further heat-treated (graphitised) to 2700°C in an inert atmosphere of argon to obtain the final monolithic graphite plates, which were found to possess a bulk density of 1.90 g/cm3 along with a bending strength of 76 MPa, Shore hardness of 73 and an electrical resistivity of 1.6 m Ohm cm. A summary of the above three examples is given below in Table-1.
TABLE-1
SUMMARY OF PROCESSING DETAILS AND CHARACTERISTICS OF THE RESULTING HIGH DENSITY MONOLITHIC GRAPHITES IN THE ABOVE FOUR EXAMPLES
(Table Removed)
NOTE: The figures in parentheses refer to the values of the as-such (without oxidation treatment) green coke powders.
The novelty of the present invention is in eliminating the swelling and/or crack formation in
the product during carbonisation.
The inventive step of the present invention is in the grinding and oxidation of the green
coke. The first grinding step is to get a powder of size less than 75 µm followed by an
oxidation step. The second step is in grinding the oxidised ground coke to a particle size in
the range of l-30µm.
The main advantages of the present invention are:
1. The sensitivity of the oxidation treatment in the process of the present invention is
reduced by way of fine grinding of the green coke after the oxidation treatment and the
control over the oxidation treatment is achieved by characterising the modified green coke
with respect to quinoline and toluene insoluble contents and volatile matter content.
2. The high density monolithic graphite produced by the process of the present invention has
a bulk density of 1.80 - 1.95 g/cm3, 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 g/cm" for the conventional high density graphite.
3. The process of the present invention does not involve repeated cycles of
impregnation and recarboaisation which are employed in the process of the conventional
high density graphite. Thus, the present process is not cumbersome and time consuming,
and is hence economical.
4. The process of the present invention does not involve the solvent extraction operation
which is employed in the process of mesocarbon microbeads based high density graphice.
Besides this, the yield of the mesocarbon microbeads from the precursor is low (15-40%)
whereas the process of the present invention gives a much higher yield of (65 -70 %). Thus,
the present process is simple and cost effective.

We Claim:
1. An improved process for the production of modified green coke useful for making
high density monolithic graphite, which comprises; heating coal tar pitch having
characteristics as herein described in the presence of an inert gas such as herein
described at a temperature in the range of 350-550°C to obtain green coke
possessing quinoline insoluble (QI) in the range of 80-95% and toluene insoluble
(TI) in the range of 85-97%, grinding the said green coke to a powder with a
particle size of below 75 µm, subjecting the powdered green coke to oxidation in
air or oxygen at a temperature in the range of 150-200°C, grinding the oxidized
green coke to a size in the range of 1-30 µm, to obtain the modified green coke
having enhanced QI in the range of (95-99%) and TI in the range of (96-99.5%).
2. An improved process as claimed in claim 1 wherein the coal tar pitch used is such
as having softening point in the range of 70-90 C.
3. An improved process as claimed in claims 1 and 2 wherein the inert gas used for
the heat treatment of the coal tar pitch is selected from nitrogen, helium or argon.
4. An improved process as claimed in claims 1-3 wherein the heat-treatment of the
coal tar pitch is done for a period of 1 to 7 hours.
5. An improved process as claimed in claims 1-4 wherein the oxidation is carried out
for a period in the range of 10-100 hours.
6. An improved process for the production of modified green coke useful for making
high density monolithic graphite substantially as described herein with reference
to the examples.

Documents:

733-DEL-1999-Abstract.pdf

733-del-1999-claims.pdf

733-del-1999-correspondence-others.pdf

733-del-1999-correspondence-po.pdf

733-del-1999-description (complete).pdf

733-del-1999-form-1.pdf

733-del-1999-form-19.pdf

733-del-1999-form-2.pdf

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Patent Number

215804

Indian Patent Application Number

733/DEL/1999

PG Journal Number

12/2008

Publication Date

21-Mar-2008

Grant Date

03-Mar-2008

Date of Filing

14-May-1999

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	GOPAL BHATIA	NATIONAL PHYSICAL LABORATORY, DR. K. S. KRISHNAN ROAD, NEW DELHI-110012 INDIA.
2	RAJENDRA KUMAR AGGARWAL	NATIONAL PHYSICAL LABORATORY, DR. K. S. KRISHNAN ROAD, NEW DELHI-110012 INDIA.
3	JAGPAL SINGH MAHUR	NATIONAL PHYSICAL LABORATORY, DR. K. S. KRISHNAN ROAD, NEW DELHI-110012 INDIA.
4	OM PRAKASH BAHL	NATIONAL PHYSICAL LABORATORY, DR. K. S. KRISHNAN ROAD, NEW DELHI-110012 INDIA.

PCT International Classification Number

C01B 57/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA