GB2261674A

GB2261674A - Pitch binder for carbonaceous refractories and process for producing a carbonaceous refractory

Info

Publication number: GB2261674A
Application number: GB9221737A
Authority: GB
Inventors: Masakazu Hashiguchi; Hiroichi Miyasaka
Original assignee: Mitsubishi Kasei Corp
Current assignee: Mitsubishi Kasei Corp
Priority date: 1991-10-18
Filing date: 1992-10-16
Publication date: 1993-05-26
Anticipated expiration: 2012-10-16
Also published as: JPH05270892A; GB2261674B; KR930007851A; GB9221737D0

Abstract

A binder for carbonaceous refractories consists essentially of a pitch containing not more than 10% by weight of a fraction of not higher than 300 DEG C, and an organic liquid having a boiling point or 50% distillation temperature of not higher than 350 DEG C. Examples of the organic liquid are toluene, xylene, hexane, kerosine, cyclohexane, ethanol, glycol, propanol, acetone and methyl ethyl ketone.

Description

BINDER FOR CARBONACEOUS REFRACTORIES AND PROCESS FOR PRODUCING A CARBONACEOUS REFRACTORY The present invention relates to a method for producing non-calcined or calcined ref ractories useful for e.g. electric furnaces or converters and a binder to be used for such carbonaceous refractories.

Heretofore, refractories useful for furnace walls of e.g. converters have been prepared by using dolomite as raw material. Namely, dolomite is molded and calcined, followed by impregnating pitch and further by calcination to obtain a refractory as a product.

However, there has been a demand for saving energy in view of an increase of costs for petroleum, and an attempt has been made to calcine refractories by utilizing the heat of e.g. converters.

Further, a refractory having a long useful life has been required to minimize the frequency of replacement of the refractory. Accordingly, dolomite as aggregate has been changed to magnesia. Further, as the aggregate, natural graphite has been used in combination.

It has been proposed that a refractory is subjected to drying treatment at a temperature of about 3000C to form a non-calcined refractory, which is then calcined by utilizing the heat of e.g. a converter.

However, pitch has a problem such that when heated, it generates a fume and off-odor, whereby an environmental problem will be created when it is used for e.g. a converter. Further, when heated, pitch will be softened and melted. Accordingly, a non-calcined refractory made thereof tends to be poor in the strength and has a problem such that when such a non-calcined refractory is used for e.g. a converter, it tends to undergo a deformation by the self gravity.

Under these circumstances,-for the production of a non-calcined carbonaceous refractory, it has been common to incorporate as a binder a thermosetting resin such as a phenol resin which is curable by drying treatment at a temperature of about 3000C at the time of mixing inorganic aggregate such as magnesia and carbonaceous aggregate such as natural graphite and which is capable of maintaining the strength of a shaped product and generates little fume during its use in e.g. a converter.

On the other hand, in the field of steel making, there has been an increasing demand for energy saving, man power saving and improvement in the efficiency of process steps in recent years, and simplification of the process for e.g. continuous casting has been attempted.

Accordingly, sliding valves, dipping nozzles and long nozzles to be used for such a continuous casting process are required to have high performance. Particularly, calcined carbonaceous refractories having high spalling resistance have been used in many cases. Such calcined carbonaceous refractories used to be prepared by mixing inorganic aggregate such as alumina and carbonaceous aggregate such as natural graphite under heating with use of pitch as a binder, followed by molding and calcining.

However, when heated, such pitch generates a fume and off-odor, and it has a problem that when mixed at a high temperature, it creates an environmental problem.

Therefore, it has been common to employ as a binder a thermosetting resin such as a phenol resin which can be mixed at room temperature and which generates little fume during the mixing.

However, such a thermosetting resin has such problems that it generates a condensate and expands at the time of curing, and the carbonization yield during the calcination is low whereby the properties, particularly the bulk density and the compression strength of the refractory tend to be low. Therefore, it has been desired to develop a binder which generates little fume and off-odor and whereby the carbonization yield during the calcination is high and the performance of the refractory will be high.

It is an object of the present invention to reduce generation of off-odor, etc. which are likely to be generated during the heating of pitch and to provide at a low cost a binder for calcined carbonaceous refractories, which has a performance equal to the performance of a conventional thermosetting resin such as phenol resin and which also provides an excellent operation efficiency.

Under these circumstances, the present inventors have conducted extensive studies to solve the above problems and as a result, have found that the above problems can be solved by using as a binder a pitch and an organic liquid having certain specific properties. The present invention has been accomplished on the basis of this discovery.

Thus, the present invention provides a binder for carbonaceous refractories, which consists essentially of a pitch containing not more than 10% by weight of a fraction of not higher than 3000C, and an organic liquid having a boiling point or 50% distillation temperature of not higher than 3500C.

The present invention also provides a method for preparing a carbonaceous refractory using inorganic aggregate and/or carbonaceous aggregate, which comprises incorporating to such an aggregate a binder consisting essentially of a pitch containing not more than 10% by weight of a fraction of not higher than 3000C, and an organic liquid having a boiling point or 50% distillation temperature of not higher than 3500C.

In the accompanying drawings: Figure 1 is a graph showing the relation between the heat treating temperature and the compression strength with respect to the refractory (A) of the present invention and the refractory (B) obtained by a conventional method.

Figure 2 is a graph showing the relation between the heat treating temperature and the carbonization yield with respect to the refractory (A) of the present invention and the refractory (B) obtained by a conventional method.

Now, the present invention will be described in detail with reference to the preferred embodiments.

Firstly, inorganic aggregate to be used in the present invention includes, for example, magnesia, dolomite, alumina, chromium oxide and zirconia, for noncalcined refractories. However, it is common to use magnesia as aggregate, and a powder of e.g. silicon carbide may be added thereto.

On the other hand, for the preparation of calcined carbonaceous refractories, alumina, zirconia, dolomite, magnesia or chromium oxide may, for example, be used.

However, alumina is preferably used as inorganic aggregate, and a powder of e.g. silicon carbide may further be added in an amount of from 0 to 50 parts by weight per 100 parts by weight of the inorganic aggregate.

Further, as carbonaceous aggregate, natural graphite, artificial graphite or baked coke may, for example, be mentioned. However, natural graphite is usually employed as aggregate.

The pitch to be used for the binder of the present invention contains not more than 10% by weight of a fraction of not higher than 3000C, preferably not more than 10% of a fraction of not higher than 3600C, most preferably not more than 5% by weight of a fraction of not higher than 3600C. Such a pitch may be a coal tar pitch formed by distillation and heat treatment of coal tar, a petroleum-based heavy oil or a heat treated product thereof 1 or a hydrogenated product thereof or a heat treated product thereof, or a pitch obtained by polycondensation of a low molecular weight compound such as naphthalene or a heat treated product thereof, or a pitch obtained by treating it with a solvent, or a pitch obtained from a heat treated product thereof by a suitable purification and separation. Such a pitch will be carbonized during the calcination of the refractory, but a low boiling fraction in such a pitch will evaporate without being carbonated during the calcination. The trace of such evaporation tends to remain as a defect in a shaped product, which causes a deterioration of the performance. Therefore, the pitch should preferably have the above mentioned properties.

The non-calcined carbonaceous refractory is subjected to drying treatment at a temperature of from 250 to 3000C. In order to control the generation of a fume and off-odor, it is preferred to minimize the low boiling fraction in such a pitch.

Further, the pitch is preferably the one which can be carbonized in the carbonaceous refractory during calcination, and its fixed carbon content is preferably higher than a thermosetting resin such as a phenol resin.

Particularly preferred is the one having a high carbonization yield in a shaped product prepared by mixing it with aggregate, followed by molding, since the properties of the refractory can thereby be improved.

The fixed carbon content is preferably at least 40% by weight, more preferably at least 45% by weight, most preferably at least 50% by weight.

The softening point of such a pitch is determined by the amount of the low boiling point fraction, and it is usually within a range of from 50 to 2500C, preferably from 60 to 2000C, more preferably from 70 to 1500C.

The quinoline-insoluble content (QI) and the tolueneinsoluble content (TI) in the pitch are not particularly restricted. However, the smaller the amounts of QI and TI in the pitch, the better the properties of the refractory. Namely, QI and TI are high molecular weight substances, and they do not melt and do not function as a binder component. Therefore, the smaller the amounts of QI and TI, the better. QI is particularly large molecules, and the amount of QI is preferably less than 20% by weight, more preferably less than 10% by weight, most preferably less than 5% by weight i.e. substantially 0%. Likewise, the amount of TI is preferably less than 40% by weight, more preferably less than 30% by weight, most preferably less than 20% by weight.

The amount of the pitch is required to be determined so that the aggregate will sufficiently be bonded to itself to maintain the refractory after drying or after calcination, and it is preferably at least 1% by weight, more preferably at least 3% by weight, relative to the aggregate.

If the amount of the pitch is too large, a trace of gas discharge tends to remain as a defect, and it is likely that the strength of the shaped product is thereby impaired. Therefore, the amount is preferably not higher than 20% by weight, more preferably not higher than 15% by weight, more preferably not higher than 10% by weight.

A calcined carbonaceous refractory is usually calcined to a temperature of about 1,0000C to form a final product. During the calcination, the pitch is carbonated and solidified, whereby the properties of the refractory will be maintained. It is generally accepted that pitches undergo decomposition and polycondensation at a temperature of at least 3000C and generate gases, and the reaction is most vigorous at a temperature of from about 400 to 5000C, and they solidify at a temperature of at least 5000C at the time of solidification. The pitches repeat polycondensation reactions under a fluidized condition, and they are gradually polymerized and solidified. However, if gases or low boiling point fractions evaporate during the carbonization reaction or after solidification, such traces tend to remain as pores or defects, whereby the properties of the refractory will be impaired.

Accordingly, the organic liquid to be mixed to the pitch in the present invention is preferably the one which does not leave such pores or defects in the refractory, and it is preferably the one which evaporates before the carbonization reaction or solidification, so that it does not exist during the carbonization reaction or after solidification. Namely, the boiling point or 50% distillation temperature (hereinafter referred to as a "average boiling point") of the organic liquid to be added in the present invention is preferably lower than the carbonization reaction temperature or the solidification temperature, and its average boiling point is preferably not higher than 3500C, more preferably not higher than 3000C, most preferably not higher than 2800C.

For the production of a non-calcined refractory, the organic liquid may be the one having an average boiling point of higher than 2500C. However, if the organic liquid incorporated has an average boiling point of higher than 2500C, even when the refractory is dried, the organic liquid will remain, and the remaining organic liquid will be evaporated during the calcination in e.g.

a converter, and the traces of such evaporation will remain as defects in the shaped product, whereby the properties of the refractory will be impaired.

Therefore, the lower the average boiling point of the organic liquid, the better. Namely, the organic liquid to be added is preferably one which is readily evaporated during the drying.

The drying is usually conducted at a temperature of from 250 to 3000C. Therefore, the average boiling point is preferably not higher than 2500C, more preferably not higher than 2000C.

Otherwise, the average boiling point may be so high that the organic liquid will be entirely carbonized and solidified without leaving pores or defects. However, such an organic liquid usually has a high melting point and is solid at room temperature, and it is hardly mixed with aggregate, such being undesirable. Thus, the lower the boiling point, the better.

On the other hand, if the average boiling point is too low, there will be problems such that it evaporates at the time of mixing it with the pitch, and it evaporates during the molding of the refractory, whereby the viscosity of the paste will increase and molding will be difficult.

When an organic liquid having a low boiling point is to be used, it is necessary to close the system to prevent the evaporation of the organic liquid and to suppress the change in the physical properties.

Otherwise, when used under a condition open to the atmosphere, the organic liquid is required to have such a boiling point that it is hardly evaporated. The average boiling point of the organic liquid is preferably at least room temperature, more preferably at least 500C, more preferably at least 800C.

The organic liquid to be used here may, for example, be an aromatic hydrocarbon such as toluene, xylene or a low boiling point fraction obtainable by the distillation of a heavy oil such as coal tar, an aliphatic hydrocarbon such as hexane or kerosine, an alicyclic hydrocarbon such as cyclohexane, an alcohol such as ethanol, glycol or propanol, an ester, or a ketone such as acetone or methyl ethyl ketone. These organic liquids may be used alone or in combination as a mixture of two or more of them. The organic liquid to be used is most preferably the one which is capable of dissolving the pitch entirely.

However, it may be the one which is capable of dissolving the pitch only partially or incapable of dissolving the pitch at all. Namely, the organic liquid is not particularly limited so long as it can be uniformly dispersed when mixed with aggregate. The organic liquid which is capable of dissolving the pitch and capable of being uniformly dispersed, is preferred. However, even an organic liquid which is incapable of dissolving the pitch may be used in such a manner that after pulverizing the pitch, the pitch is uniformly dispersed in the organic liquid.

The amount of the organic liquid to be incorporated varies depending upon the viscosity and the softening point of the pitch, the viscosity of the organic liquid to be added and the mixing temperature with the aggregate. Namely, the amount of the organic liquid can be determined so that the binder after mixing can be kneaded with inorganic aggregate such as alumina or magnesia, and carbonaceous aggregate such as natural graphite which is optionally added, at room temperature or under heating. The amount is required to be increased when the viscosity or the softening point of the pitch is high. Likewise, the amount is required to be increased when the viscosity of the organic liquid to be added is high. The amount of the organic liquid may be determined so that the binder would be in a liquid state at the kneading temperature so that it can be kneaded.However, mixing is preferably conducted at a low temperature so that generation of a fume is as little as possible to avoid the off-odor and the environmental problem. More preferably, mixing is conducted at room temperature. The amount of the organic liquid i.e. the the viscosity of the binder is determined to meet such desirability.

When the viscosity of the binder is low, it can be mixed in a liquid state with aggregate. When the viscosity of the binder is high, it may be kneaded with aggregate by means of a kneader. The viscosity of the binder is preferably at a level where uniform kneading can be conducted by a kneader, preferably not higher than 100 poise, more preferably not higher than 80 poise, most preferably not higher than 50 poise.

If the viscosity of the binder is too low i.e. if the amount of the organic liquid is too much, there will be problems such that the volume at the time of mixing it with aggregate will be large and a large apparatus will be required, it takes a long time for drying a shaped product, and a large quantity of energy will be required.

Therefore, the viscosity is preferably at least 1 centi poise, more preferably at least 5 centi poise, more preferably at least 10 centi poise. As described above, the amount of the organic liquid to be mixed varies depending upon the conditions. However, it is usually in an amount of from 0.01 to 10 parts by weight, preferably from 0.1 to 5 parts by weight, most preferably from 0.3 to 3 parts by weight, per part by weight of the pitch.

Mixing of the pitch with the organic liquid may be conducted in such a manner that the pitch is pulverized and then mixed, or the pitch is melted and then the organic liquid is mixed thereto, or the pitch is dissolved in the organic liquid, or after such dissolving operation, the organic liquid is further mixed.

The pitch may be the one dissolved in the organic liquid or the one mixed and dispersed in the organic liquid. In the latter case, the pitch is of such a particle size that it will not sediment in the organic liquid. Namely, the particle size is preferably not more than 200 zm, more preferably not more than 100 Fm.

The mixing temperature may be at room temperature, when the pitch is pulverized. However, the mixing may be conducted in such a state that the pitch is melted at a temperature of at least the softening point of the pitch.

Further, it is possible that the pitch is melted in the organic liquid, and then the mixture is mixed in a liquid state at room temperature or under heating, if necessary.

In the binder of the present invention, a thermosetting resin such as a phenol resin may be incorporated to the pitch to such an extent that the feature of the present invention will not thereby be impaired.

The binder obtained by the above described operation may be used without any change in a conventional method for the production of a calcined carbonaceous refractory, and it may simply be replaced by the currently employed thermosetting resin such as a phenol resin to obtain (i) a non-calcined refractory or (ii) a calcined refractory which is not different from the conventional product.

Namely, (i) the non-calcined carbonaceous refractory can be produced by molding said mixture, followed by drying. The molding can be conducted by a common method such as mold press molding or vibration molding.

The drying is conducted in an inert gas, in the atmospheric air or in a breeze. When drying is conducted in the present of oxygen, infusibilization of the pitch will be accelerated, and the properties of the noncalcined carbonaceous refractory will be improved, particularly the compression strength will be improved, whereby a deformation of the non-calcined carbonaceous refractory when piled will be minimized. Therefore, drying can be conducted while controlling the oxygen concentration.

The non-calcined carbonaceous refractory thus prepared, can be calcined during the use in e.g. a converter in the same manner as the conventional product, and the properties can be improved without necessity of modifying the conventional method of use.

(ii) A calcined carbonaceous refractory can be produced by molding said mixture followed by calcination in accordance with a conventional method. The molding can be conducted by a usual method such as mold press molding or vibration molding.

The calcination is conducted in an inert gas, in the atmospheric air or in a coke breeze. The calcination can be conducted while controlling the oxygen concentration.

The calcined carbonaceous refractory thus prepared, can be used in e.g. a converter or an electric furnace in the same manner as the conventional product, and the properties can be improved without necessity of modifying the conventional method of use.

Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.

EXAMPLE 1 60% by weight of a pitch (fraction of not higher than 3600C: 3%, QI < 1%) obtained by heat treatment of coal tar and 40% by weight of toluene (boiling point: 1150C) were mixed to obtain a binder having a viscosity of 150 cp at room temperature.

77% by weight of electrofused magnesia, 14% by weight of natural graphite and 9% by weight of the binder were mixed at room temperature, followed by molding to obtain a shaped product. The shaped product was heated in a coke breeze at a rate of 140C/hr and subjected to drying treatment at 3000C.

The compression strength of the dried product was 310 kg/cm2.

The dried product was further heated in a coke breeze at a rate of 140C/hr and calcined at l,0000C to obtain a calcined product. The compression strength of the calcined product was 210 kg/cm2.

EXAMPLE 2 In Example 1, the proportions of the pitch and the toluene were changed to 70% by weight and 30% by weight, respectively, to obtain a binder having a viscosity of about 40,000 cp at room temperature, and it was mixed in the same manner as in Example 1 at about 700C (about 400 cp) to obtain a shaped product. With respect to the obtained shaped product, the relation of the heat treating temperature and the compression strength, and the relation of the heat treating temperature and the carbonization yield are shown in Figures 1 and 2, respectively (as indicated by A in the Figures).

Further, the results obtained with respect to a shaped product prepared in the same manner by using a binder comprising a phenol resin and ethyl alcohol, are also shown in Figures 1 and 2 (as indicated by B in the Figures).

COMPARATIVE EXAMPLE 1 70% by weight of a pitch (fraction of not higher than 3600C: 15%, QI: 2%) obtained by heat treatment of coal tar and 30 parts by weight of toluene (boiling point: 1150C) were mixed to obtain a binder having a viscosity of 150 cp at room temperature. A shaped product is prepared in the same manner as in Example 1. The compression strength of the dried product was 185 kg/cm2, and the compression strength of the calcined product was 135 kg/cm2.

COMPARATIVE EXAMPLE 2 60% by weight of a pitch (fraction of not higher than 360 C: 3%, QI < 1%) obtained by heat treatment of coal tar and 40% by weight of a coal tar distillation oil having a 50% distillation temperature of 3000C were mixed to obtain a binder having a viscosity of 150 cp at room temperature.

A shaped product was prepared in the same manner as in Example 1.

The compression strength of the dried product was 210 kg/cm2, and the compression strength of the calcined product was 155 kg/cm2.

COMPARATIVE EXAMPLE 3 The compression strength of a commercially available magnesia carbonaceous non-calcined refractory was 250 kg/cm2. The compression strength of a shaped product obtained by calcination in the same manner as in Example 1 was 190 kg/cm2.

EXAMPLE 3 77% by weight of alumina, 14% by weight of natural graphite and 9% by weight of the binder obtained in Example 1 were mixed, and the mixture was molded in the same manner as in Example 1 to obtain a shaped product.

The shaped product was heated in a coke breeze at a rate of 140C/hr and calcined at 1,0000C to obtain a calcined product. The compression strength of the calcined product was 215 kg/cm2.

EXAMPLE 4 70% by weight of a pitch (fraction of not higher than 3600C: 3%, QI < 1%) obtained by heat treatment of coal tar, 10% of a low boiling point fraction having a 50% distillation temperature (average boiling point) of 2350C obtained by distillation of coal tar and 20% by weight of toluene (boiling point: 1150C) were mixed to obtain a binder having a viscosity of 30 poise at room temperature.

Using the binder, a shaped product was prepared in the same manner as in Example 1.

The compression strength of the calcined product was 215 kg/cm2.

EXAMPLE 5 60% by weight of a pitch (fraction of not higher than 3600C: 3%, QI < 1%) obtained by heat treatment of coal tar, 20% by weight of a low boiling point fraction having a 50% distillation temperature (average boiling point) of 2350C obtained by distillation of coal tar and 20% by weight of toluene (boiling point: 1150C) were mixed to obtain a binder having a viscosity of 160 cp at room temperature.

The compression strength of the calcined product was 210 kg/cm2.

COMPARATIVE EXAMPLE 4 70% by weight of a pitch (fraction of not higher than 3600C: 15%, QI: 2%) obtained by heat treatment of coal tar and 30% by weight of toluene (boiling point 1150C) were mixed to obtain a binder having a viscosity of 150 cp at room temperature. Using this binder, a shaped product was prepared in the same manner as in Example 1.

The compression strength of the calcined product was 150 kg/cm2.

COMPARATIVE EXAMPLE 5 60% by weight of a pitch (fraction of not higher than 360eC: 3%, QI < 1%) obtained by heat treatment of coal tar and 40% by weight of a coal tar distillation oil having a 50% distillation temperature of 3200C were mixed to obtain a binder having a viscosity of 150 cp at room temperature.

A shaped product was prepared in the same manner as in Example 1.

The compression strength of the calcined product was 145 kg/cm2.

COMPARATIVE EXAMPLE 6 The compression strength of a commercially available alumina carbonaceous calcined refractory was 180 kg/cm2.

According to the method of the present invention, a pitch binder is used instead of a phenol resin, whereby it is possible to obtain a carbonaceous refractory having improved properties (particularly improved high temperature properties).

Mixing property with aggregate: mixing and molding can be conducted at a temperature around room temperature in the same manner as in the case of a conventional phenol resin.

Thermosetting property: drying and curing treatment can be conducted in a temperature range of from 200 to 3000C.

Generation of gases: as opposed to the conventional phenol resin, there is no substantial generation of water.

Improvement in the high temperature strength: there is no substantial difference in strength as between the temperature around 2000C and the temperature around 6000C. Further, the strength at a temperature of at least 1,0000C is high as compared with a phenol resin, and the strength at a temperature of at least 1,4000C is further improved.

Spalling resistance; by virtue of soft carbonaceous nature, when added to a refractory which is expected to be used at a high temperature, it lowers the thermal expansion of the refractory and improves the heat conductivity, whereby a refractory having excellent heat shock resistance can be obtained.

Corrosion resistance: when added to a refractory which is expected to be used at a high temperature, penetration of a melt is minimum, since a heat deformation during the use if small, and the wetting resistance to the melt, corrosion resistance and oxidation resistance will be improved.

Other merits: the carbonization yield is high as compared with a phenol resin, and the bulk density of the refractory will be improved. Further, crystallinity is better than the phenol resin, whereby the amount of natural graphite required can be reduced.

Claims

CLAIMS:

1. A binder for carbonaceous refractories, which consists essentially of a pitch containing not more than 10% by weight of a fraction of not higher than 3000C, and an organic liquid having a boiling point or 50% distillation temperature of not higher than 3500C.

2. The binder according to Claim 1, wherein the pitch contains not more than 10% by weight of a fraction of not higher than 3600C.

3. The binder according to Claim 1, wherein the pitch has a softening point of from 60 to 2000C.

4. The binder according to Claim 1, wherein the pitch contains not more than 5% by weight of a quinolineinsoluble content (QI).

5. The binder according to Claim 1, wherein the pitch contains substantially no quinoline-insoluble content (QI).

6. The binder according to Claim 1, wherein the organic liquid has a boiling point or 50% distillation temperature of not higher than 3000C.

7. The binder according to Claim 1, wherein the organic liquid has a boiling point or 50% distillation temperature of not higher than 2500C.

8. The binder according to Claim 1, wherein the organic liquid dissolves at least a part of the pitch.

9. The binder according to Claim 1, wherein the organic liquid is an aromatic hydrocarbon.

10. The binder according to Claim 1, wherein the organic liquid is used in an amount of from 0.1 to 5 parts by weight per part by weight of the pitch.

11. A method for preparing a carbonaceous refractory using inorganic aggregate and/or carbonaceous aggregate, which comprises incorporating to such an aggregate a binder consisting essentially of a pitch containing not more than 10% by weight of a fraction of not higher than 3000C, and an organic liquid having a boiling point or 50% distillation temperature of not higher than 3500C.

12. The method according to Claim 11, wherein the organic aggregate is magnesia, dolomite or alumina.

13. The method according to Claim 11, wherein the carbonaceous aggregate is natural graphite.

14. The method according to Claim 11, wherein the aggregate and the binder are kneaded at a binder viscosity of not higher than 80 poise.