Note: Descriptions are shown in the official language in which they were submitted.
JGC-7750
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HIGH CONCENTRATION COAL AOUEOUS SLURRY AND
PROCESS FOR PRODUCING SAME
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a high
concentration coal aqueous slurry and a process for
producing same. More particularly, the present
invention relates to a high concentration coal aqueous
slurry having an excellent stability during storage and
transportation, and a process for producing same.
2) Description of the Related Arts
Recently, the importance of coal as an energy
~ource ha~ been rediscovered, but coal is disadvanta-
geous in that it is a solid material and thus is diffi-
cult to transport, store and handle, in comparison with
oil.
Accordingly, many attempts have been made to
eliminate the above-mentioned disadvantages by finely
pulverizing a coal material, dispersing the resultant
fine coal particle~ in an aqueous medium to provide a
coal aqueous slurry which can be stored, transported and
handled as a liquid material, and supplying the coal
aqueous slurry as a fuel for boilers for power plants
and other industrial uses. For example, such attempts
are disclosed in Japanese Unexamined Patent Publication
(Kokai) Nos. 58-38791, 60-18585, 61-57689 and 62-116691.
For the coal aqueous slurry, it is necessary
to increase the concentration of coal particles and to
enhance the stability in transportation and storage. A
high concentration coal aqueous slurry should comprise
60% to 75% by weight of finely pulverized coal particles
and 25% to 40% of an aqueous medium containing a small
amount of an additive, for example, a dispersing agent,
and this high concentration of coal particles can be
obtained by adjusting the size of the coal particles in
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the aqueous slurry to a suitable level and distribution
thereof, and by adding an appropriate additive to the
aqueous slurry.
Therefore, attempts have been made to provide
a method of controlling the distribution of the size of
the coal particles in the aqueous slurry in accordance
with an optimum particle size distribution formula.
Those attempts, however, are not always satisfactory.
Also, the conventional methods are disadvantageous in
that the high concentration of coal particles in the
aqueous slurry can be realized only by adding a certain
amount of a dispersing agent to the aqueous medium.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a
high concentration coal aqueous slurry having an excel-
lent stability during transportation and storage, and a
process for producing same.
The above-mentioned object can be attained by the
high concentration coal aqueous slurry of the present
invention comprising coal particles having a size of
500 ~m or less and dispersed in an aqueous medium
containing a dispersing additive at a particle size
distribution in which a coefficient of variation in size
of the coal particles is 0.3 or more determined in
accordance with the equation (I):
C = ~/M (I)
wherein C represents the coefficient of variation in the
size of the coal particles, M represents an average
general logarithmic size of the coal particles
calculated in accordance with the equation (II):
E( gl0S ) S (II)
in which M is as defined above, Si represents an average
size in ~m of the coal particles in fraction No. i which
is one of a plurality of fractions provided by dividing
the entire amount of the coal particles in accordance
with the order of the particle size, each fraction
consisting of coal particles having a size in a
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predetermined range, and Vi represents a ratio of the
amount in weight or volume of the portion of the coal
particles in fraction No. i to the entire amount of the
coal particles, and o represents a standard deviation of
the size of the coal particles calculated in accordance
with the equation (III):
o = (~((loglOSi-M~ x Vi) )0-5 (III)
in which Si, M and Vi are as defined above.
~he above-mentioned high concentration coal aqueous
slurry can be produced by the process of the present
invention comprising the steps of:
preparing dehydrated coal cakes comprising 60%
by weight or more of coal particles having a size of
500 ~m or less, by pulverizing a coal material in a wet
pulverizing manner and dehydrating the resultant coal
particle aqueous slurry;
mixing the dehydrated coal cakes with water
and a dispersing additive in a kneader to provide a coal
aqueous slurry (A) containing 60% to 80% by weight of
the coal particles;
subjecting a portion (B) of the coal aqueous
slurry (A) to a further fine pulverizing procedure to
further finely pulverize the coal particles in the
portion (B); and
mixing the resultant further finely pulverized
portion (B) with the remaining portion (D) of the coal
aqueous B lurry (A).
Alternatively, the above-mentioned high concentra-
tion coal aqueous slurry can be produced by another
process of the present invention comprising the
steps of:
pulverizing a coal material at a high concen-
tration of 60% to 80% by weight in an aqueous medium
containing a dispersing additive, to provide a moiety
slurry (E) consisting of a high concentration coal
aqueous slurry containing coal particles having a size
of lO0 ~m or less; and
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separately providing a moiety cake (F)
consisting of dehydrated coal cakes containing coal
particles having a size of 500 ~m or less by pulverizing
a coal material in a low concentration in an aqueous
medium free from the dispersing additive and dehydrating
the resultant low concentration coal aqueous slurry; and
mixing the moiety slurry (E) with the
moiety cake (F),
B~IEF DESCRIPTION OF THE DRAWINGS
Figure l shows a relationship between the coeffi-
cient C of variation in the size of coal particles and
the concentration of the coal particles in a coal
aqueous slurry of the present invention, having a
viscosity of l000 cp;
Fig. 2 shows a relationship between the coeffi-
cient C of variation in the size of coal particles and
the precipitation percentage of the coal particles in
the coal aqueous slurry when the coal aqueous slurry is
subjected to a dispersion stability test by a
centrifugal acceleration tester;
Fig. 3 shows a relationship between the coeffi-
cient C of variation in the size of the coal particles
in the coal aqueous slurry and the ratio of the minimum
amount of a dispersing agent necessary to ad~ust the
viscosity of the coal aqueous slurry to a predetermined
level at a predetermined concentration of the coal
particles to the entire amount of the coal particles;
Fig. 4 shows the relationships between the average
general logarithmic size in ~m of the coal particles in
3G three different types of coal aqueous slurries ~, ~
and ~ shown in Figs. l to 3 and the cumulative weight
percent of the coal particles having a respective size
or less;
Fig. 5 is a flow sheet showing the process for
producing the high concentration coal aqueous slurry of
the present invention; and,
Fig. 6 is a flow sheet showing another process for
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producing the high concentration coal aqueous slurry of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors of the present invention made various
attempts to eliminate the disadvantages of the conven-
tional coal aqueous slurries, and during those attempts,
the inventors carried out research into the relationship
between the distribution of sizes of the coal particles
in the coal aqueous slurry and the stability of the coal
particles dispersed in the slurry, and discovered that
the stability of a coal aqueous slurry having a wide
distribution of size of coal particles and containing
very fine coal particles in a large enough amount is
higher than that of another coal aqueous slurry having a
narrow distribution of size of coal particles having one
single peak.
Namely, a coal powder consisting essentially of
three types of fine coal particles, each type having a
different size, and having particle size distribution
peaks at sizes of about 10 ~, about 20 ~, and about
40 ~m, was mixed with another coal powder consisting
essentially of three types of coarse coal particles,
each type having a different size, and having particle
size distribution peaks at sizes of about 80 ~m, about
160 um, and about 300 ~, at a mixing ratio of 10:90 to
90:10 by weight, and each mixture was dispersed and
kneaded in an aqueous medium containing an anionic
surface active agent, by a kneader. From research into
the physical properties of the resultant coal aqueous
slurries, it was concluded that a coal aqueous slurry
having a coefficient C of variation in size of the coal
particles dispersed therein, of 0.3 or more, determined
from the equation (I):
C = o/M (I) r
exhibits a satisfactory concentration and stability
during storage and transportation over a long period.
In the equation (I), C represents the coefficient
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of variation in the size of the coal particles, M
represents an average general logarithmic size of the
coal particles calculated from the equation (II):
M = ~(lglOSi) x Vi (II)
in which M is as defined above, Si represents an average
size in ~m of a portion of the coal particles in
fraction No. i which is one of a plurality of fractions
provided by dividing the entire amount of the coal
particles in accordance with the order of the particle
size, each fraction consisting of coal particles having
a size in a predetermined range, and Vi represent a
ratio of the amount in weight or volume of the portion
of the coal particles in fraction No. i to the entire
amount of the coal particles, and ~ in the equation (I)
represents a standard deviation of the size of the coal
particles calculated from the equation (III):
~ ((loglOSi - M) x Vi)2)0 5 (III)
in which Si, M and Vi are as defined above.
As stated above, the variation coefficient C in the
size of the coal particles should be 0.3 or more, and
preferably as large as possible. Usually, the variation
coefficient C of the coal particle size is preferably
0.5 or more, and practically, upto 0.7 or 0.8.
When the variation coefficient C of the coal
particle size is less than 0.3, the resultant coal
aqueous slurry has an unsatisfactorily low concentration
and exhibits a poor stability.
In the coal aqueous slurry of the present inven-
tion, the coal particles must have a size of 500 ~m or
less, preferably 300 ~m or less, more preferably 200 ~m
or less.
When the maximum size of the coal particles is more
than 500 ~m, the resultant coal aqueous slurry has a
disadvantage in that, when the coal aqueous slurry is
subjected to combustion, the amount of unburnt carbon
becomes undesirably large.
Figure l indicates a relationship between the
2~ 2~ ~7
-- 7
variation coefficient C in size of coal particles in a
coal aqueous slurry and the concentration of coal
particles in a coal aqueous slurry having a viscosity of
1000 cp.
The distribution of the coal particle size can be
determined by an apparatus for the measurement of
particle size distribution available under the trademark
of Microtrac Model SRA 7995-10, from Leeds &
Northrup Co.
The concentration of coal particles in an aqueous
slurry thereof can be determined in accordance with the
heat-drying method of Japanese Industrial Standard (JIS)
M 8812.
In view of Fig. 1, it is understood that where the
variation coefficient C of coal particle sizes is in the
range of from 0.3 to about 0.5, the concentration of the
coal particles in the coal aqueous slurry having a
viscosity of 1000 cp is in the range of from about 65%
to about 72% and increases with the increase in the
variation coefficient C of the coal particle sizes, and
where the variation coefficient C of the coal particle
sizes is more than 0.5, the concentration of the coal
particles in the coal aqueous slurry having a viscosity
of 1000 cp becomes substantially constant at a level of
about 71% to about 73~.
Figure 2 shows the relationship between the
variation coefficient C of the coal particle sizes in a
coal aqueous slurry and the precipitation percentage of
the coal particles when a dispersion stability test is
applied to the coal aqueous slurry in the following
manner.
In the dispersion stability test, a coal aqueous
slurry is subjected to a 20G centrifugal acceleration
test for 8 hours, and the percentage of the precipitated
portion of the coal particles based on the entire weight
of the coal particles in the coal aqueous slurry is
calculated.
~ ~ 4~ 9 7
Figure 2 indicates that the precipitation
percentage of the coal particles at a satisfactory level
of about 60~ or less can be obtained at a variation
coefficient of the coal particle sizes of the coal
aqueous slurry of 0.3 or more. In particular, where the
variation coefficient C of the coal particle sizes is
about 0.5 or more, the precipitation percentage of the
coal particles is substantially constant at a level of
10% or less.
Figure 3 shows the relationship between the
variation coefficient C of the coal particle sizes of a
coal aqueous slurry and the ratio (~) of the minimum
amount of a dispersing agent needed to adjust the
viscosity of the coal aqueous slurry to a standard level
of lO00 cp. at the concentration as indicated in Fig. l,
to the entire amount of the coal particles.
Figure 3 shows that, at the variation coefficient C
of the coal particle sizes of 0.3 or more, the necessary
minimum amount of the dispersing agent is at a
satisfactory level of 0.8% or less. In particular,
where the variation coefficience is about 0.4 or more,
the necessary minimum amount of the dispersing agent is
reduced to a low level of 0.4~ or less.
~he coal particle size distributions of the coal
aqueous slurries ~, ~ and ~ indicated in Figs. l to 3
are indicated in Fig. 4.
From the coal particle size distribution curves,
the variation coefficient C was calculated as follows.
Coal aqueous slurry ~: C = 0.722
" ~: C = 0.473
~ ~: C = 0.344
The processes of the present invention for
producing the above-mentioned high concentration coal
aqueous slurry will be explained in detail below.
In one process of the present invention, a coal
material consists of a mixture of two types of
portions PC and PF each having a different particle size
2~2~97
g
of the coal particles therein.
The coal particles in portion FC preferably have a
size of from l00 ~m to 500 ~m and are in a content of
30% by weight or more.
Also, the coal particles in portion PF pr~ferably
have a size of l00 ~m or less, and a content of coal
particles having a size of l0 ~m or less is 40% by
weight or more.
Preferably, the mixing ratio in weight of the
portion PF to the portion PC is in the range of 8:3 to
5:5.
Also, in the above-mentioned process, the aqueous
medium preferably contains a dispersing additive
consisting of, for example, at least one dispersing
agent for preventing the agglomeration of the coal
particles, in an amount needed to ad~ust the viscosity
of the resultant coal aqueous slurry to a standard level
of l000 cp or less. The amount of the additive is
preferably in the range of from 0.3~ to 0.8~ based on
the entire weight of the coal particles in the coal
aqueous slurry.
The dispersing agent preferably comprises at least
one member selected from non-ion and anion ~urface
active agents usually employed for the conventional coal
aqueous slurry.
Optionally, a stabilizer for preventing the
deposition of the coal particles is added to the coal
particle aqueous slurry. The stabilizer preferably
comprises at least one member selected from cellulose
compounds, for example, alkali metal salts of
carboxymethyl cellulose and xanthan gum materials,
usually utilized for the conventional coal aqueous
slurry.
In this process of the present invention, a coal
material is pulverized in a wet pulverizing manner and
the resultant coal particles in an aqueous medium
(water) are dehydrated to provide dehydrated coal cakes
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comprising 60% by weight or more of coal particles
having a size of 500 ~m or less; the resultant de-
hydrated coal cakes are mixed with an aqueous medium
containing a dispersing additive in a kneader to provide
a coal aqueous slurry (A) containing 60~ to 80% by
weight of the coal particles; a portion (B) of the coal
aqueous slurry (A) is subjected to a further fine
pulverizing procedure to further finely pulverize the
coal particles in the portion (B); and the resultant
further finely pulverized portion (B) is mixed with the
remaining portion (D) of the coal aqueous slurry (A), to
provide a high concentration coal aqueous slurry having
a coal particle size variation coefficient C of 0.3 or
more.
In the above-mentioned process, preferably the
portion (B) of the coal aqueous slurry (A) is in an
amount of 50% to 80~, based on the entire weight of the
coal aqueous slurry (A). Also, the fine pulverizing
procedure is preferably carried out to an extent such
that the resultant further finely pulverized coal
particles having a size of 5 ~m or less in the
portion (B) are in a content of 15% by weight or more.
The above-mentioned process will be further
explained with reference to Fig. 5.
In Fig. S, a pulverizer or mill 1 is charged with a
coal material and an aqueous medium through an inlet 2,
and the coal material is pulverized at a low
concentration of 50% by weight or less. The resultant
coal aqueous slurry is fed into a classifying device 4
through a conduit 3, and a classified portion of the
coal aqueous slurry containing coal particles having a
particle size of 500 ~ or less, preferably 200 ~m or
less, is fed into a dehydrating device 5 and dehydrated
to provide dehydrated coal particle cakes. The
remaining portion of the coal aqueous slurry containing
coarse coal particles is returned to the pulverizer 1
through the inlet 2 and re-pulverized.
i 9 7
The coal particle cakes formed in the dehydrating
device S are introduced into a kneader 6 and mixed in
the kneader 6 with an aqueous medium in an amount
necessary to adjust the concentration of the coal
particles to a level of 60% to 80%, and preferably,
containing an additive in an amount of 0.3% to 0.8%
based on the entire weight of the coal particles in the
cakes.
The resultant coal aqueous slurry (A) is withdrawn
from the kneader 6 through a conduit 7. A portion (B)
of the coal aqueous slurry (A), preferably in an amount
of about 50% to about 80% by weight, is fed into a
pulverizer 9 to further finely pulverize the coal
particles in the portion (B) and the resultant portion
(B) containing the further finely pulverized coal
particles is fed into a kneader 10 and mixed therein
with the remaining portion (D) of the coal aqueous
slurry (A) in an amount of from 20% to 50% by weight,
which is directly introduced from the kneader 6 into the
kneader 10 through a conduit 8.
The further fine pulverizing procedure is carried
out preferably to an extent such that, after the
portion (B) is mixed with the remaining portion (D), the
resultant coal aqueous slurry contains coal particles
having a size of 5 ~ or less in a content of 15~ by
weight or more, more preferably 20% by weight or more,
based on the total weight of the coal particles in the
resultant high concentration coal aqueous slurry, and
the resultant high concentration coal aqueous slurry is
recovered from the kneader 10 through a conduit 11.
In another process for producing the high concen-
tration coal aqueous slurry of the present invention, a
coal material is pulverized in a high concentration of
60% to 80~ by weight in an aqueous medium containing a
dispersing additive to provide a moiety slurry (E)
consisting of a high concentration coal aqueous slurry
containing coal particles having a size of 100 ~n or
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less; separately, a moiety cake (F) consisting of
dehydrated coal cakes containing coal particles having a
size of 500 ~ or less, preferably 100 ~m or more, is
prepared by pulverizing a coal material in a low
concentration of, for example, 30 to 50% by weight, in
an aqueous medium free from the additive and by
dehydrating the resultant low concentration coal aqueous
slurry; and the moiety slurry (E) is mixed with the
moiety cake (F).
Before the step of mixing the moiety slurry (E)
with the moiety cake (F) in the above-mentioned process,
the moiety cake (F) may be mixed and kneaded with one or
both of an aqueous medium and a portion of the final
high concentration coal aqueous slurry prepared in the
foregoing procedure, to provide a moiety slurry (G)
consisting of an aqueous slurry of coal particles in a
concentration of 60% to 80~ by weight, and the resultant
moiety slurry (G) then mixed with the moiety slurry (E).
In the above-mentioned process, the moiety cake (F)
may be further mixed with a dispersing additive in an
amount of 0.3 to 0.8~ based on the weight of the coal in
the moiety slurry (G).
In the moiety slurry (E), the coal particles
contained preferably have a size of from 100 ~ to
500 ~m and are in a content of 30% by weight or more.
Also, in the moiety cake ( F ) or the moiety slurry
(G), the coal particles preferably have a size of 100
or less, and contain particles having a size of 10 um or
less in a content of 40~ by weight or more.
Preferably, the moiety slurry (E) is mixed with the
moiety cake (F) or the moiety slurry (G) in a mixing
ratio such that the ratio in weight of the coal
particles in the moiety slurry (E) to the coal particles
in the moiety cake (F) or the moiety slurry (G) is in
the range of from 8:2 to 5:5.
After mixing the moiety slurry (E) with the moiety
cake ( F ) or the moiety slurry (G) in the above-mentioned
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process, the resultant high concentration coal aqueous
slurry preferably contains coal particles having a size
of 5 ~ or less and in a content of 15% by weight or
more, more preferably 20% by weight or more, based on
the total weight of the coal particles in the resultant
high concentration coal aqueous slurry.
Referring to Fig. 6, a coal material and an aqueous
medium free from an additive are charged into a
pulverizer or mill la through an inlet 2a, to provide a
low concentration coal aqueous slurry. The resultant
low concentration coal aqueous slurry is introduced into
a classifying device 4 through a conduit 3a and a
classified moiety slurry consisting of a portion of the
pulverized coal aqueous slurry and containing coal
particles having a size of 500 un or less, preferably
200 um or less r is introduced into a dehydrating
device 5. The remaining portion of the coal aqueous
slurry containing coarse coal particles is recycled to
the pulverizer la through the inlet 2a and
re-pulverized.
The moiety cake (F) is introduced from the
dehydrating device 5 into a kneader 6 and mixed and
kneaded with at least one member selected from an
aqueous medium which contains an additive, for example,
a dispersing agent, in an amount of 0.3% to 0.8% based
on the weight of the coal particles, and a portion of
the final high concentration coal aqueous slurry
prepared in a foregoing procedure and supplied through a
conduit 11, to convert the moiety cake (F) to a moiety
slurry moiety (G) consisting of an aqueous slurry of
coal particles in a concentration of 60% to 80% by
weight.
The moiety slurry (G) is fed from the kneader 6
into a kneader 10 through a conduit 7.
Separately, a coal material, an aqueous medium, and
a dispersing additive are fed into a pulverizer lb
through an inlet 2b and the coal material is pulverized
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therein at a high concentration to provide a moiety
slurry (E) consisting of a high concentration coal
aqueous slurry containing pulverized coal particles
having a size of 100 ~ or less.
In the pulverizer lb, the dispersing additive is
preferably in an amount of from 0.3% to 0.8% based on
the weight of the coal particles.
The moiety slurry (E) is introduced from the
pulverizer lb into the kneader 10 through a conduit 3b
and mixed with the moiety slurry (G) therein.
The moiety cake (F) may be directly introduced into
the kneader 10 together with an additional amount of an
aqueous medium and an additive.
In an example, a high concentration coal aqueous
slurry was prepared by the process as indicated in
Fig. 6.
A moiety slurry (E) was prepared from a coal
material, water, and an anionic surface active agent in
an amount of 0.4% based on the weight of the coal
material in the pulverizer lb. The resultant moiety
slurry (E) contained pulverized coal particles at a coal
particle concentration of about 68% by weight. The
pulverized coal particles had a size of 100 ~ or less
and contained particles having a size of 10 ~ or less
in a content of 40~, based on the entire weight of the
coal particles. The moiety slurry (E) was fed into the
kneader 10.
A moiety cake (F) was prepared from a coal material
and an aqueous medium free from the dispersing additive
by using the pulverizer la, the classifying device 4,
and the dehydrating device 5, and then fed together with
an aqueous medium into the kneader 6 to provide a moiety
slurry (G) containing coal particles having a size of
200 ~ or less and in a concentration of about 75~ by
weight.
The mixing ratio of the moiety slurry (E) to the
moiety slurry (G) corresponded to a mixing ratio in
.
2 ~ 9 7
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weight of the coal particles in the moiety slurry (E) to
those in the moiety slurry (G), of 70:30.
The resultant high concentration coal aqueous
slurry contained pulverized coal particles having a size
of 200 ~ or less and in a concentration of about 70%,
and had a variation coefficient C of the size of the
coal particles of 0.495.
In accordance with the present invention, a coal
aqueous slurry having a high concentration and a high
dispersion stability of coal particles during storage
and transportation, and thus utilizable for industrial
use, can be obtained by controlling only the largest
size of the coal particles and the variation coef-
ficient C of size of coal particle to specific levels,
respectively.
Also, the present invention effectively reduces the
amounts of the dispersing agent and/or stabilizer needed
to stabilize the coal particles dispersion.
