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Characteristics of extruded carbon mixes at high pressures of extrusion
Co&n,
1976.Vol. 14. pp. 131-132. PcrgamooPress. Printed in Great Britain
LETTERS TO THE EDITOR Characteristics of extruded carbon mixes at high pressures of extrusion (Received 6 January 1976)
Extrusion and moulding are the most common methods of manufacturing carbon products. A number of studies were reported by many workers[ld] in the field of moulded carbons but very little can be found regarding the extruded carbon mixes[4,5]. Search of the literature revealed that a striking phenomenon occurs in the flow of amorphous or molten polymers through capillaries [6,7J. Namely at and above some critical flow rate, the surface and cross-section of the extruded polymer become irregular. So far there is only one paper[4] in which some increase in the viscosity of carbon mixes at high speeds was reported, but without giving an account of observed irregularities in the extruded carbon mixes. Special experiments were performed with carbon mixes so as to detect the presence of such extrudate irregularities and their results are reported below. The carbon mixes in the present investigation consisted of calcined ~troleum coke as filler and coal tar pitch (R and B softening point 78°C)as binder. The petroleum coke powder was obtained from the Raymond Mill and put to use as such. This step was considered essential due to the difficulties involved in separating the petroleum coke powder into various fractions. The check on the graded powder was done by a sieve analysis and by the bulk density determination described elsewhere[S]. Three kg of such petroleum coke powder were mixed each time with various amounts of coal tar pitch binder. The above mixtures were roll mixed at 120°Cand then ground to pass through -60 mesh. Slugs of 12.5cm dia. were made from these mixtures by extruding at two speeds of 2-4 and 25-40 m/mitt respectively from a 200 ton extrusion press. The extruded rods were checked for any extrudate irregularities. The green rods obtained above were subjected to apparent density determinations from their physical parameters. These rods
were baked to 950°Cin an electric mulllefurnace. These were then measured for their baked density, electrical resistivity and transverse strength and their results are summarised in Table I. It is seen from the results of these ex~~ments in Table 1that properties of carbon mixes are better at high speeds than those at low speeds. This is to say that carbon mixes when extruded at high speeds have higher apparent density, greater strength and lower electrical resistivity than the same at low speeds. The reason for these improved properties seems to go to the pressure alone which is effective in making the binder to penetrate to those places where it could not do so in the mixing or moulding operations. Such a product after baking will obviously have high density, more strength and less resistivity. There are cases where speeds of extrusion can be increased by putting a hot flame over the extruding die but such cases have been seen to spoil the green as well as baked carbons. This also suggests the predomjn~ce of the pressure over the speed in extrusion of the carbon mixes. The characteristics of the carbon mixes at low and high pressures follow the analogy of the product obtained from a dough mixer of sigma blade type to that obtained from a dough mixer and roll mixer both respectively. This is because of a more uniform dist~bution of the binder in the latter case. The high speed extrusion was not possible at a binder level of 42% or more due to snaky type of extrusion. This type of extrusion will not be called as extrusion irregularity since it is due to the excessive binder in the carbon mixes. The paper [4] reports an unexplained increase in the viscosity of the green mix at a shear rate of about IOUsee-‘. But in that case, the particular author has not observed any extrudate irregularities. There seems to be two reasons possible for such an increase in the viscosity of the green mix. One being due to the extrusion of
Table 1. Properties of extruded carbon mixes at low and high speeds
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Letters to the Editor
132
carbon mix in the vicinity of its exhaustion. Since the speed of extrusion gradually lowers down in a stroke with time, in order to maintain the speed one is led to increase the pressure of extrusion. The result is an increase in the viscosity of the mix. The second may be due to the removal of binder from the given mixture at such high pressures. This would leave behind a mix with compa~tively less binder which can give rise to an increased viscosity. Since the extrudate irregularities]& 71are observed with decrease in the viscosity above the critical flow rate, it seems that the increase in viscosity [4]is not due to extrudate irregularities. It is known from common experience with plastics that extrudate irregularities lead to swelling of the extruded rods. But in our ex~~ments at shear rates of as high as 650 set-’ we had smooth rods of almost the same diameters. It can therefore be safely concluded that extrudate irregularities do not occur at least up to these shear rates. The possibility of extrudate irregularities over shear rates of 650 see-’ cannot be ruled out but as a matter of fact these would be too high shear rates to be realized in usual ~nufacturi~ practice of carbon products. Acknowledgements-The author is grateful to Dr. S. S. Chari for his guidance and helpful discussions, to Dr. G. C. Jain for his keen
Clulron. 1976. Vol. 14, pp. 132433.
interest and en~uragement in the study. Thanks are due to the Director, National Physical Laboratory for permission to publish this note. Carbon Technology Unit
G.
BHATIA
National Physical Laboratory New Delhi-11~12, India REFERENCES
1. Seldin E. J., Proc. 1st and 2nd Carbon Conf., p. 217. University of Buffalo, New York (1956). 2. Seldin E. J., Proc. 3rd Carbon Co& p. 675. Pergamon Press, Oxford (1959). 3. Okada J. and Takeuchie Y., Proc. 4th Carbon Co& p. 54. Pergamon Press, Oxford (l%l). 4. Fincle R. L., 7th Conf. on Carbon, Abstract No. 153,Carbon 3, 370 (1965). 5. Bhatia G., Carbon 11, 437 (1973). 6. Tordella J. P., J. Appt. Phys. 27,454 (19.56). 7. Tordella J. P., Trans. Sot. Rheof. 1,203 (1957). 8. Bohra J. N. and Bhatia G., Pm. 12th Biennial Carbon Conj., in
press.
Pergamon Press. Printed in Great Briti
Changes in pore structure of a devolatilized coal char upon further heating at lower temperature (Received 6 January 1976)
It is well-known that carbonizing a coal can yield chars of quite different characteristics depending on the heating schedule and the atmosphere of the carbonization process [l]. These different characteristics might well affect the suitability of the char for various uses, e.g. as a starting material for coal gasifi~tion processes or as a starting material for producing an activated adsorbent. As a result, the effect of various carbonization processes upon the characteristics of the resulting char has been of interest and is the subject of continuing investigation (e.g. [2]). This letter reports some experimental observations on a somewhat different aspect of producing a coal char. The observations concern some alterations in the internal structure of a coal char which occurred when the char was reheated to a temperature well below the final carbonization temperature of the original char. In other words, the char was thought to be stabilized at a fairly high temperature. After cooling the char, though, reheating it to a lower tem~rature resulted in changes in the internal structure of the char. In addition, the particular non-reactive atmosphere in which the char was reheated appeared to make a significant difference in the changes observed. The raw material for this study was the calcinate of a sub-bituminous coal supplied by the FMC Corporation from the Elkol Mine, Kemmerer, Wyom~g. The as-mined coal was ground, charred by rapid heating to about 480°C then calcined by further rapid heating to about 790°C. The compositions of this calcinate have been reported before[3]. The calcinate, as shipped, was screened and the less-than-16 but greater-than-20 USS mesh fraction (nominal particle size 1.0mm) was used. In the present short study, this calcinate was heated to a temperature of 1200°Cover a period of roughiy 8 hr, and then kept at this temperature for 16hr. All this was carried out in an atmosphere of flowing nitrogen. Following this, the sample was allowed to cool overnight to room temperature under flowing nitrogen. A portion of the sample was removed for pore structure and density determinations. Part of the remaining sample was then heated within a period of minutes to a temperature of 900°C in flowing nitrogen, and held there for 10hr. After this period, it was allowed to cool overnight to room temperature under flowing nitrogen, Another portion was subjected to the same treatment, except that an atmosphere of helium was used instead of nitrogen.
The four materials-the raw calcinate, the calcinate treated at 1200”for 16hr under nitrogen, the sample further treated at 900” for 10hr in nitrogen and the sample further treated at 900”for 1Ohr in helium--were tested to determine their internal pore structures by measuring the mercury densities, the helium densities, and the pore-size distributions down to a pore radius of 6.0 nm by mercury porosimetry. The instruments for carrying out these measurements have been described in an earlier paperl31. The results of these measurements are presented in Table 1 and Fig. 1. The table includes the densities, the total measured internal pore volume and the split of this volume between the macropores and the sum of the intermediate and micropores. (For this study, macropores are defined as all pores whose radius exceeds 10nm; intermediate pores are those with radii between 1 and 10nm; micropores possess radii below 1 nm.) The figure presents the pore-size distributions for the four samples from mercury ~netration measmements. The table and the figure show that heating the raw calcinate in nitrogen at 1200”for 16hr resulted in a significant increase in total pore volume. This was caused by a large increase in the macropore volume combined with marked decrease in the Table 1. Helium densities and pore volumes” of coal char as heat treatments progress
1976.Vol. 14. pp. 131-132. PcrgamooPress. Printed in Great Britain
LETTERS TO THE EDITOR Characteristics of extruded carbon mixes at high pressures of extrusion (Received 6 January 1976)
Extrusion and moulding are the most common methods of manufacturing carbon products. A number of studies were reported by many workers[ld] in the field of moulded carbons but very little can be found regarding the extruded carbon mixes[4,5]. Search of the literature revealed that a striking phenomenon occurs in the flow of amorphous or molten polymers through capillaries [6,7J. Namely at and above some critical flow rate, the surface and cross-section of the extruded polymer become irregular. So far there is only one paper[4] in which some increase in the viscosity of carbon mixes at high speeds was reported, but without giving an account of observed irregularities in the extruded carbon mixes. Special experiments were performed with carbon mixes so as to detect the presence of such extrudate irregularities and their results are reported below. The carbon mixes in the present investigation consisted of calcined ~troleum coke as filler and coal tar pitch (R and B softening point 78°C)as binder. The petroleum coke powder was obtained from the Raymond Mill and put to use as such. This step was considered essential due to the difficulties involved in separating the petroleum coke powder into various fractions. The check on the graded powder was done by a sieve analysis and by the bulk density determination described elsewhere[S]. Three kg of such petroleum coke powder were mixed each time with various amounts of coal tar pitch binder. The above mixtures were roll mixed at 120°Cand then ground to pass through -60 mesh. Slugs of 12.5cm dia. were made from these mixtures by extruding at two speeds of 2-4 and 25-40 m/mitt respectively from a 200 ton extrusion press. The extruded rods were checked for any extrudate irregularities. The green rods obtained above were subjected to apparent density determinations from their physical parameters. These rods
were baked to 950°Cin an electric mulllefurnace. These were then measured for their baked density, electrical resistivity and transverse strength and their results are summarised in Table I. It is seen from the results of these ex~~ments in Table 1that properties of carbon mixes are better at high speeds than those at low speeds. This is to say that carbon mixes when extruded at high speeds have higher apparent density, greater strength and lower electrical resistivity than the same at low speeds. The reason for these improved properties seems to go to the pressure alone which is effective in making the binder to penetrate to those places where it could not do so in the mixing or moulding operations. Such a product after baking will obviously have high density, more strength and less resistivity. There are cases where speeds of extrusion can be increased by putting a hot flame over the extruding die but such cases have been seen to spoil the green as well as baked carbons. This also suggests the predomjn~ce of the pressure over the speed in extrusion of the carbon mixes. The characteristics of the carbon mixes at low and high pressures follow the analogy of the product obtained from a dough mixer of sigma blade type to that obtained from a dough mixer and roll mixer both respectively. This is because of a more uniform dist~bution of the binder in the latter case. The high speed extrusion was not possible at a binder level of 42% or more due to snaky type of extrusion. This type of extrusion will not be called as extrusion irregularity since it is due to the excessive binder in the carbon mixes. The paper [4] reports an unexplained increase in the viscosity of the green mix at a shear rate of about IOUsee-‘. But in that case, the particular author has not observed any extrudate irregularities. There seems to be two reasons possible for such an increase in the viscosity of the green mix. One being due to the extrusion of
Table 1. Properties of extruded carbon mixes at low and high speeds
I
I
I
t
I
I
Letters to the Editor
132
carbon mix in the vicinity of its exhaustion. Since the speed of extrusion gradually lowers down in a stroke with time, in order to maintain the speed one is led to increase the pressure of extrusion. The result is an increase in the viscosity of the mix. The second may be due to the removal of binder from the given mixture at such high pressures. This would leave behind a mix with compa~tively less binder which can give rise to an increased viscosity. Since the extrudate irregularities]& 71are observed with decrease in the viscosity above the critical flow rate, it seems that the increase in viscosity [4]is not due to extrudate irregularities. It is known from common experience with plastics that extrudate irregularities lead to swelling of the extruded rods. But in our ex~~ments at shear rates of as high as 650 set-’ we had smooth rods of almost the same diameters. It can therefore be safely concluded that extrudate irregularities do not occur at least up to these shear rates. The possibility of extrudate irregularities over shear rates of 650 see-’ cannot be ruled out but as a matter of fact these would be too high shear rates to be realized in usual ~nufacturi~ practice of carbon products. Acknowledgements-The author is grateful to Dr. S. S. Chari for his guidance and helpful discussions, to Dr. G. C. Jain for his keen
Clulron. 1976. Vol. 14, pp. 132433.
interest and en~uragement in the study. Thanks are due to the Director, National Physical Laboratory for permission to publish this note. Carbon Technology Unit
G.
BHATIA
National Physical Laboratory New Delhi-11~12, India REFERENCES
1. Seldin E. J., Proc. 1st and 2nd Carbon Conf., p. 217. University of Buffalo, New York (1956). 2. Seldin E. J., Proc. 3rd Carbon Co& p. 675. Pergamon Press, Oxford (1959). 3. Okada J. and Takeuchie Y., Proc. 4th Carbon Co& p. 54. Pergamon Press, Oxford (l%l). 4. Fincle R. L., 7th Conf. on Carbon, Abstract No. 153,Carbon 3, 370 (1965). 5. Bhatia G., Carbon 11, 437 (1973). 6. Tordella J. P., J. Appt. Phys. 27,454 (19.56). 7. Tordella J. P., Trans. Sot. Rheof. 1,203 (1957). 8. Bohra J. N. and Bhatia G., Pm. 12th Biennial Carbon Conj., in
press.
Pergamon Press. Printed in Great Briti
Changes in pore structure of a devolatilized coal char upon further heating at lower temperature (Received 6 January 1976)
It is well-known that carbonizing a coal can yield chars of quite different characteristics depending on the heating schedule and the atmosphere of the carbonization process [l]. These different characteristics might well affect the suitability of the char for various uses, e.g. as a starting material for coal gasifi~tion processes or as a starting material for producing an activated adsorbent. As a result, the effect of various carbonization processes upon the characteristics of the resulting char has been of interest and is the subject of continuing investigation (e.g. [2]). This letter reports some experimental observations on a somewhat different aspect of producing a coal char. The observations concern some alterations in the internal structure of a coal char which occurred when the char was reheated to a temperature well below the final carbonization temperature of the original char. In other words, the char was thought to be stabilized at a fairly high temperature. After cooling the char, though, reheating it to a lower tem~rature resulted in changes in the internal structure of the char. In addition, the particular non-reactive atmosphere in which the char was reheated appeared to make a significant difference in the changes observed. The raw material for this study was the calcinate of a sub-bituminous coal supplied by the FMC Corporation from the Elkol Mine, Kemmerer, Wyom~g. The as-mined coal was ground, charred by rapid heating to about 480°C then calcined by further rapid heating to about 790°C. The compositions of this calcinate have been reported before[3]. The calcinate, as shipped, was screened and the less-than-16 but greater-than-20 USS mesh fraction (nominal particle size 1.0mm) was used. In the present short study, this calcinate was heated to a temperature of 1200°Cover a period of roughiy 8 hr, and then kept at this temperature for 16hr. All this was carried out in an atmosphere of flowing nitrogen. Following this, the sample was allowed to cool overnight to room temperature under flowing nitrogen. A portion of the sample was removed for pore structure and density determinations. Part of the remaining sample was then heated within a period of minutes to a temperature of 900°C in flowing nitrogen, and held there for 10hr. After this period, it was allowed to cool overnight to room temperature under flowing nitrogen, Another portion was subjected to the same treatment, except that an atmosphere of helium was used instead of nitrogen.
The four materials-the raw calcinate, the calcinate treated at 1200”for 16hr under nitrogen, the sample further treated at 900” for 10hr in nitrogen and the sample further treated at 900”for 1Ohr in helium--were tested to determine their internal pore structures by measuring the mercury densities, the helium densities, and the pore-size distributions down to a pore radius of 6.0 nm by mercury porosimetry. The instruments for carrying out these measurements have been described in an earlier paperl31. The results of these measurements are presented in Table 1 and Fig. 1. The table includes the densities, the total measured internal pore volume and the split of this volume between the macropores and the sum of the intermediate and micropores. (For this study, macropores are defined as all pores whose radius exceeds 10nm; intermediate pores are those with radii between 1 and 10nm; micropores possess radii below 1 nm.) The figure presents the pore-size distributions for the four samples from mercury ~netration measmements. The table and the figure show that heating the raw calcinate in nitrogen at 1200”for 16hr resulted in a significant increase in total pore volume. This was caused by a large increase in the macropore volume combined with marked decrease in the Table 1. Helium densities and pore volumes” of coal char as heat treatments progress