Process for producing mesocarbon microbeads of uniform particle-size distribution

Mesocarbon microbeads of narrow particle-size distribution are produced by: subjecting a heavy oil to a primary heat treatment at a temperature T.sub.1 to prepare a pitch containing mesophase microspheres; once cooling this pitch to a temperature lower than its softening point; thereafter subjecting the pitch to a secondary heat treatment at a temperature T.sub.2, which is higher than 350.degree. C. and lower than (T.sub.1 -40.degree. C.); cooling the pitch at a cool rate lower than 200.degree. C./hour; separating from the pitch mesophase microspheres which precipitated in the secondary heat-treatment step; and thereafter obtaining by solvent extraction mesophase microspheres of substantially uniform particle size formed in the residual pitch. The mesocarbon beads of narrow particle-size distribution thus obtained are particularly suitable for use as chromatograph packing material, catalyst support, and other uses.

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing mesocarbon microbeads of 
uniform particle size distribution by using as a starting material a heavy 
oil, that is, a heavy hydrocarbon oil originated from petroleum, coal, oil 
sand, oil shale or the like. 
It is known in the prior art that mesocarbon microbeads (hereinafter 
referred to by the abbreviation "MC") can be obtained by heat treating a 
heavy oil at a temperature of 350.degree. to 500.degree. C. to obtain a 
heat-treated pitch and separating optically anisotropic microspheres 
(mesophase microspheres) formed within the pitch from the pitch matrix by 
solvent extraction. MC obtained in this manner are carbon precursors of 
spherical shapes close to perfect spheres of diameters of 1 to 100 microns 
and are constituted by condensed polycyclic aromatics in laminated 
alignment in a specific direction. Because of their unique form and 
crystalline structure, these MC have high electrical, magnetic, and 
chemical activities, and extensive utilization thereof in various 
diversified fields is expected. 
More specifically, there are great expectations for the utilization of 
these MC for production of various industrial materials, examples of which 
are: special carbon materials such as high-density isotropic carbon 
materials and electrical resistance carbons prepared by carbonization 
after molding thereof; composite materials such as electroconductive 
ceramics, dispersion-reinforced metals, and electroconductive plastics 
prepared by carbonizing the MC as they are and thereafter blending the 
resulting material with other materials; and chemical materials such as 
catalyst supports and packing material for chromatography prepared by 
forming the MC into particles as they are or after carbonization. 
For certain applications such as those for chromatography packing material 
and catalyst support among the above enumerated utilizations, the particle 
size of the MC is required to uniformly conform to specific sizes. 
However, the particle size of MC produced by a process depending on 
ordinary heat treatment of a heavy oil is distributed over a broad range 
(which may be as broad as 1 to 100 microns in most cases). Accordingly, 
the production of MC of narrow particle-size distribution by some method 
is desired in many fields. For fulfilling this need, some methods as 
described below are thought of or have been proposed. 
(a) A method wherein a portion of specific particle sizes is separated out 
by sieving or by mechanical dispersion from MC produced by an ordinary 
process. 
(b) A method wherein, by blowing superheated steam into a heavy oil thereby 
to agitate and heat the oil, thereby carrying out uniform heat treatment 
of the heavy oil, MC of narrow particle-size distribution are obtained 
(Japanese Pat. Publn. No.9599/1978). 
(c) A method wherein the growth of the mesophase microspheres is suppressed 
by using one or more additives (as disclosed, for example, in "Tanso" 
("Carbon"), No.77, P.61 (1974)). 
However, none of these methods can be said to be completely satisfactory. 
More specifically, for example, it is difficult in the above method (a) to 
classify efficiently on an industrial scale the MC which are microspheres 
of micron size. In the methods (b) and (c), it becomes difficult to obtain 
MC of perfect spherical shape, and the effectiveness in uniformization of 
the particle size is still inadequate. 
SUMMARY OF THE INVENTION 
In view of the above described state of the known methods, it is an object 
of this invention to provide a new process for producing MC of narrow 
particle-size distribution. 
As a result of intense research carried out, with the above object, with 
respect to the mechanism of formation and growth of mesophase microspheres 
in heat treatment of heavy oil or pitch, we have made the following 
discoveries. The first is that, when pitch containing mesophase 
microspheres obtained by heating treatment of a heavy oil is subjected to 
the steps of once cooling, reheating, and recooling, a remarkable 
uniformizing of the MC particle size is attained. The second discovery is 
that, by controlling the rate of the final cooling, it is possible to 
regulate the particle size of the MC. The process of producing MC of 
narrow particle-size distribution of this invention is based on and has 
been developed from these findings. 
According to this invention, briefly summarized, there is provided a 
process for producing mesocarbon microbeads of narrow particle-size 
distribution which comprises: preparing a primary heat-treated pitch 
containing mesophase microspheres by subjecting a heavy oil to a primary 
heat treatment; once cooling the pitch thus prepared to a temperature 
equal to or lower than the softening point thereof; thereafter subjecting 
the pitch to a secondary heat treatment at a temperature which is equal to 
or higher than 300.degree. C. and, moreover, is equal to or lower than the 
temperature which is 20.degree. C. lower than the primary heat-treatment 
temperature; cooling the pitch at a cooling rate equal to or lower than 
200.degree. C./hour; separating, from the pitch thus heat treated, 
mesophase microspheres which precipitated in the secondary heat-treatment 
step; and thereafter obtaining by solvent extraction mesophase 
microspheres of substantially uniform particle size formed in the residual 
pitch. 
The nature, utility, and further features of this invention will be more 
clearly apparent from the following detailed description, beginning with a 
consideration of general and fundamental aspects of the invention and 
concluding with a specific example of practice illustrating a preferred 
embodiment thereof and comparison examples, when read in conjunction with 
the accompanying illustrations comprising drawings and photomicrographs as 
briefly described below.

DETAILED DESCRIPTION 
The reason why uniformization of particle size is attainable by the process 
of this invention is not fully clear, but it may be considered to be as 
follows. 
In a pitch in a state wherein it has been subjected to a primary heat 
treatment and then once cooled, mesophase microspheres of diverse sizes as 
indicated in FIG. 1(a) are dispersed, similarly as in pitch which has been 
heat treated by an ordinary process as described hereinbefore. When this 
pitch is reheated, of the mesophase microspheres, those of high solubility 
(considered to be principally those formed in the cooling step after the 
primary heat treatment) are dissolved again, while those of low solubility 
(considered to be principally those of high degree of heat treatment 
formed in the heating step) do not dissolve but settle on the bottom of 
the vessel as indicated in FIG. 1(b). When the pitch after reheating is 
cooled, the mesophase component which has dissolved is again separated out 
as microspheres of uniform particle size determined by the rate of 
cooling, as indicated in FIG. 1(c). 
The mesophase microspheres which are insoluble and have settled on the 
bottom accumulate, as they are, on the bottom while coalescing during the 
above described steps. Accordingly, by separating the upper phase and the 
lower phase, e.g., by decantation at a stage where the matrix pitch 
retains its liquid form in the state of FIG. 1(b) or 1(c), for example, at 
a temperature of the order of approximately 200.degree. C., mesophase 
microspheres of uniform particle size are obtained in the cooled 
substances of the upper phase. Then, by subjecting these mesophase 
microspheres to solvent extraction, MC of uniform particle size are 
obtained. 
In the process of this invention, firstly, a heavy oil such as 
atmospheric-pressure residue oil, reduced-pressure residue oil, decant oil 
from the catalytic cracking, thermal cracking tar, or coal tar is heated 
at 350.degree. to 500.degree. C. and thus subjected to a primary heat 
treatment. While specific temperature and times of this primary heat 
treatment differ with the kind of the starting material heavy oils 
(inclusive of materials ordinarily called pitches), it is preferable to so 
select these conditions that the quantity of the quinoline insoluble 
component (i.e., mesophase) of the pitch after the primary heat treatment 
will be 5 to 15 percent by weight. 
Then, the pitch after the primary heat treatment is once cooled to a 
temperature equal to or below the softening point thereof. The lower limit 
of the cooling temperature is not critical and may be room temperature. 
However, unless the cooling is carried out to a temperature equal to or 
below the softening point, the separation by settling described above with 
reference to FIGS. 1(b) and 1(c) will not occur to an ample degree. The 
reason for this may be considered to be contributive effects such as an 
increase due to the cooling in the difference between the specific 
gravities of the mesophase and the matrix pitch and the removal due to the 
cooling of .beta.-resin existing on the surface of the mesophase at a high 
temperature and contributing to the formation of micell structure between 
the mesophase and the matrix pitch. The cooling rate is not particularly 
critical and may be any value below 400.degree. C./hour, for example. 
The pitch thus cooled is further subjected to a secondary heat treatment at 
a temperature which is equal to or higher than 300.degree. C. and is equal 
to or lower than the temperature resulting as a difference when 20.degree. 
C. is subtracted from the primary heat-treatment temperature. 
We have found that when this secondary heat-treatment temperature is less 
than 300.degree. C., the particle size of the MC becomes ununiform. The 
reason for this may be considered to be as follows. The secondary heat 
treatment has the function of again dissolving in the matrix pitch the 
mesophase microspheres formed in the primary heat treatment and the 
function of causing the mesophase microspheres which do not dissolve to 
settle onto the bottom of the vessel thereby to be separated. At a low 
temperature, however, the solubility does not become sufficiently great, 
and, also, the viscosity of the matrix pitch does not decrease to a degree 
sufficient to give rise to settling. 
We have found further that, when the secondary heat-treatment temperature 
is higher than the upper limit of the primary heat-treatment temperature 
minus 20.degree. C., also, the particle size of the MC becomes ununiform. 
The reason for this may be considered to be that, in the case where the 
secondary heat treatment is carried out at a high temperature, the result 
is not merely the dissolving again of the mesophase microspheres formed in 
the primary heat treatment but is also the formation of new mesophase 
microspheres. Therefore, it is necessary to carry out the secondary heat 
treatment at a temperature at which the matrix pitch will not, 
essentially, give rise to an additional thermal cracking and thermal 
condensation reaction. Since the upper limit temperature differs with the 
chemical properties and history of the pitch, the determination of the 
upper limit temperature on the basis of the primary heat-treatment 
temperature as described above is suitable. 
More preferably, the secondary heat-treatment temperature is a temperature 
equal to or higher than 350.degree. C. and equal to or lower than the 
temperature which is 40.degree. C. lower than the primary heat-treatment 
temperature. The time duration of the secondary heat treatment is not 
particularly critical. That is, the lower limit is a time in which 
uniformization of the MC particle size can be achieved, while the upper 
limit is a time in which new mesophase is not excessively formed. On the 
basis of actual results, however, the lower limit may be of an order such 
that cooling is started immediately after the secondary heat-treatment 
temperature has been reached. While the upper limit depends also on the 
secondary heat-treatment temperature among other factors, it may be of the 
order of 120 minutes. However, the secondary heat-treatment time is 
preferably as short as possible as long as the separation of the insoluble 
mesophase by uniform settling can be achieved. The rate of temperature 
rise to the secondary heat-treatment temperature also is not very 
critical, but a practical rate is of the order of 1.degree. to 20.degree. 
C./minute. 
The pitch after the secondary heat treatment is cooled at a cooling rate 
equal to or lower than 200.degree. C./hour. We have found that when this 
cooling rate exceeds 200.degree. C./hour, the particle size of the MC 
obtained is excessively small. Even when this cooling rate is equal to or 
below 200.degree. C./hour, the particle size of the MC is influenced by 
the cooling rate used. More specifically, a high cooling rate results in a 
small MC particle size, while a low cooling rate results in a large MC 
particle size. The reason for this is that the crystalline growth rate has 
an influencing effect on the particle size. Accordingly, it is necessary 
to select the cooling rate in accordance with the purpose of utilization 
of the MC. By thus selecting the cooling rate, it is possible to regulate 
the MC particle size to any desired size within a range of 1 to 30 
microns. 
In practicing the process of this invention, it is necessary to carry out 
the secondary heat treatment and the succeeding cooling step with 
substantially no agitation. For the primary heat treatment, the continuous 
pitch producing apparatus of multiple vessel type described in the 
specification of U.S. Pat. No. 4,080,283 (incorporated herein by 
reference), for example, can be used. 
The mesophase which has settled and coalesced or bulked in the secondary 
heat-treatment step as described hereinabove, is thereafter separated, for 
example, by decantation or tapping from the bottom of the vessel, from the 
pitch in which mesophase microspheres of uniform particle size are 
dissolved or dispersed, at any time at which the pitch retains its liquid 
form, at a temperature of approximately 200.degree. C., for example. The 
mesophase material thus separated and removed can, of course, be utilized 
as a starting material for forming carbon materials and the like. 
On the other hand, the pitch containing mesophase microspheres of uniform 
particle size after the secondary heat treatment are mixed, while being 
heated according to necessity, with an aromatic solvent of, for example, 
quinoline, pyridine, anthracene oil, or the like, and the matrix pitch is 
selectively dissolved thereby to obtain mesophase microspheres as MC by 
solid-liquid separation. In the instant specification, the series of these 
process steps is referred to as "solvent extraction". 
While the solid-liquid separation can, of course, be accomplished also by 
means of a screen sieve or a filter, the use of liquid cyclones is 
preferable for industrial production. Preferably, the obtaining of the MC 
from the pitch in this manner is carried out by a process involving the 
use of the multistage liquid cyclones of the copending U.S. patent 
application Ser. No. 222,901 (incorporated herein by reference). The 
after-stage liquid cyclones are used for washing MC and imparting a 
further classification effect, and the use therein of a non-aromatic 
solvent is also possible. 
According to this invention as described above, by once cooling a pitch 
containing mesophase microspheres obtained by heat treatment of a heavy 
oil, thereafter reheating the pitch, and then further cooling the pitch at 
a specific cooling rate, MC having a very narrow particle-size 
distribution and, moreover, a particle size regulated by control of the 
cooling rate are obtained, these MC being suitable for use as 
chromatograph filler material, catalyst support, etc. 
In order to indicate more fully the nature and utility of this invention, 
the following specific example of practice thereof and comparison examples 
are set forth, it being understood that these examples are presented as 
illustrative only and are not intended to limit the scope of the 
invention. 
Comparison Example 1 
Decant oil (boiling point range 440.degree. C. and higher) obtained by 
thermal cracking of petroleum was heat treated at 450.degree. C. for 75 
minutes and then cooled at a rate of approximately 400.degree. C./hour 
thereby to prepare a primary heat-treated pitch. A photomicrograph 
(magnification of 172X) taken through a polarizing microscope of this 
pitch is shown in FIG. 2(a). It is observable in this figure that a large 
number of mesophase microspheres have been formed in the pitch, but these 
microspheres are of various particle sizes. 
The above described pitch was mixed with 15 times its quantity of 
quinoline, and the matrix pitch was dissolved thereby to separate out MC 
in a yield of 5.4 percent by weight (based on the pitch). A 
photomicrograph (magnification of 1,000X) taken through a scanning 
electron microscope of the MC thus obtained is shown in FIG. 2(b), and the 
particle-size distribution thereof is indicated in FIG. 3. As is apparent 
from FIG. 3, the particle size of the MC is distributed over a wide range 
of approximately 1 micron to 20 microns or more. 
Example 1 
The primary heat-treated pitch obtained in Comparison Example 1 was 
reheated to 380.degree. C. at a temperature rise rate of 3.degree. 
C./minute and was then immediately cooled at a cooling rate of 60.degree. 
C./hour. Then, when the temperature reached 200.degree. C., the 
supernatant part of the pitch was taken out by decantation. At this time, 
a sediment was left as residue on the bottom. The supernatant part of the 
pitch was further cooled at the rate of 60.degree. C./hour. 
A photomicrograph (magnification of 172X) taken through a polarizing 
microscope of the pitch thus obtained is shown in FIG. 4(a). This pitch 
was subjected to quinoline extraction similarly as in Comparison Example 1 
thereby to obtain MC. A photomicrograph taken through a scanning electron 
microscope of the MC thus obtained is shown in FIG. 4(b), and its 
particle-size distribution is indicated in FIG. 5. 
It is observable from FIGS. 4(b) and 5 that the particle-size distribution 
of the MC thus obtained is in a range of approximately 10 to 14 microns, 
and that MC of remarkably improved particle-size distribution were 
obtained by the process of this invention. 
The yield based on the pitch of the MC thus obtained was 3.6 percent by 
weight. That is, by comparison with Comparison Example 1, of the MC of 5.4 
percent by weight formed in the primary heat treatment, 66.7 percent 
thereof was converted through the secondary heat treatment into MC of 
uniform particle size, while the remaining 33.3 percent precipitated 
without being dissolved again. In contrast, as will be apparent from FIG. 
3 corresponding to Comparison Example 1, of the MC formed in Comparison 
Example 1, the portion having particle sizes of 10 to 14 microns is only 
11 percent. 
Thus, the secondary heat treatment has not only the effectiveness of merely 
selecting a portion of a specific particle-size range from the MC formed 
in the primary heat treatment but also the astonishing effectiveness of 
recreating a desired particle size distribution. 
Comparison Example 2 
The same starting material as that of Comparison Example 1 was treated 
under the same conditions as those of the secondary heat treatment of 
Example 1, that is, heating to 380.degree. C. at a temperature rise rate 
of 3.degree. C./minute and cooling immediately thereafter to room 
temperature at a cooling rate of 60.degree. C./hour, thereby to obtain a 
primary heat-treated pitch. A photomicrograph taken through a polarizing 
microscope of this pitch is shown in FIG. 6, from which it is apparent 
that no mesophase microspheres were formed. 
Therefore, it is evident that the MC of uniform particle size obtained in 
Example 1 were not formed newly by the secondary heat treatment but were 
MC resulting from the mesophase microspheres formed in the primary heat 
treatment which were uniformized by being dissolved again in the pitch 
matrix in the secondary heat treatment and then reprecipitated. 
Comparison Example 3 
The same starting material as that of Comparison Example 1 was heat treated 
at 450.degree. C. for 75 minutes and thereafter gradually cooled at a 
cooling rate of 60.degree. C./hour to room temperature thereby to prepare 
a primary heat-treated pitch. 
A photomicrograph taken through a polarizing microscope of this pitch is 
shown in FIG. 7. The mesophase microspheres formed in this case did not 
include any very small microspheres in contrast to those of Comparison 
Example 1 but were not of uniform particle size. Thus, it is obvious that 
mere slow cooling in the cooling step is insufficient for uniformization 
of the particle size of the mesophase microspheres, and carrying out of a 
secondary heat treatment is necessary.