Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
4686
Process for pre~aration of stable coal-water mistures.
The present invention relates to a process for the preparation of
stable coal_Nater mi~tur~s. Mor~ precisely it relates to the produc
tion of coal-water mistures with a high coal content, suitable for
use as a substitute for fuel oil or metallurgical coke (e.g. in
blast furnaces).
The increase in the price of oil, only slightly mitigated by the
recent small reductions, as well as the prospects of possible
scarcity of this raw material on the markets, have given a spur to
the search for oil substitutes, or at least greater o i 1 savings.
For these reasons, major fuel_oil consuming sectors such as electri
city generating authorities and the iron and steel industry are
actively engaged in seeking alternative and fuel-savine solutions.
As a result, oil-water, coal-oil, coal-water and coal-oil-water
mistures have been proposed.
In particular, coal is coming to play an ever more important role
for use as ausiliary fuel injected into the blast furnace via the
tuyeres.
Among the various ~olutions one of the most promising for replacing
fuel oil, especially for maintaining regularity of blast-furnace
operation and reducing production costs, is the use of coal-water
mi es with a high coal content.
In this regard it has been found that mixtures containing from 70 to
8 ~ coal are of special interest, both from the fuel consumption
and blast-furnace operating quality aspects. ~owever, some practical
difficulties are encountered with mistures of this kind, mainly
~Z7468~;
concerning pumping and phase separation, with settlement of the coal
on the bottom of the storage tanks.
m is invention proposes to eliminate these difficulties by
providing a simple, cheap process for preparing a coal-water mixture
with a coal content of 70 to 80% which is easy to pump and is stable
timewise.
Thus the present invention provides a process for the prepa-
ration of stable coal-water mixtures suitable for use as fuel, in
which there is between 70% and 80% coal by weight. In this process,
coal is ground together with water in the proportions desired for the
final mixture, plus a small quantity of at least one fluidizing agent
amounting to between 0.05 and 2~ by weight, the grinding being carried
to such as extent that at least 60% of the coal is finer than 74 ~m
and less than 10% is coarser than 250 ~m.
The present invention, in particular, provides a process for
the preparation of stable coal-water mixtures suitable for use as
fuel, in which there is between 70% and 80% coal by weight, in which
coal is ground together with the water in the proportions desired for
the final mixture, plus a small quantity of at least one fluidizing
agent amounting to between 0.05 and 2% by weight, the fluidizing agent
being selected from the group consisting of humic acid based
' compounds and lignin sulphonates. The grinding is carried to such an
extent that at least 70% of the coal is finer than 74 ~m and less than
10% thereof is coarser than 250 ~m. The grinding is also performed in
two stages. The first one of these stages comprises feeding the coal
together with the water and the additives in the proportions desired
for the final mixture and processing the resulting mixture to obtain a
mixture in which at least 50% of the coal is finer than 74 ~m and less
than 20% is coarser than 250 ~m. The seocnd grinding stage comprises
processing the mixture obtained at the end of the first stage to
obtain the desired final mixture in which at least 70% of the coal has
a particle size of less than 74 pm and less than 10% thereof is
coarser than 250 pm and in uhich the coal particles have a harmonic
mean diameter of around 50 ~m.
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According to this invention, coal preferably finer
than 3 mm in size, is fed into a mill together with the
quantity of water desired in the final mixture. Typically
70-80% by weight of coal is added to 20-30% of water.
At this stage between 0,05 and 2% by weight of known fluid-
i~ing agents such as humic acid based compoundsior their
derivates or lignin sulphonates are added.
Grinding must result in a coal-water mixture where
the harmonic mean diameter of the coal particles is around
50 ~m. It is also necessary that at least 60% and preferably
at least 70% of the coal should be finer than 74 ,um, while
less than 10% should be coarser than 250 ym.
In the experiments carried out, it has been
found useful to perform the grinding in two stages in a disc
mill. While the final grading must be as indicated above,
the first stage of grinding must provide a mixture in which
the harmonic mean diameter of the coal particles is around
60 ~m, at least 50% of the coal being finer than 74 ,um,
and less than 20% coarser than 250 um.
Of course, the type of mill is not binding
according to the invention. Typically a mixture produced in
this manner containing 73% by weight of coal having a mean
diameter of 54 ,um, has an apparent
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~ 3 ~
5 viscosity of about 300 cP ~Brookfield at 30 rpm).
This mi~ture i5 e~tremely stable. After 45 days the suspension is
still of e cellent quality, is readly pumpable and has a virtu~lly
negligible vertical concentration gradient.
It i8 important to note that the addition of fluidizing agerts has
a very marked effect, especially on the viscosity of the suspension.
However, depending on the particle size and the total quantity of
coal, ~a~imum efficiency is attained with a given quantity of fluid
izing agent, beyond which the viscosity may rise even markedly. In
our esperiments it has been seen that the ma~imum efficiency is
15 attained for additions of between 0,3 and 1~3~o.
The present i~vention will now be illustrated in relation to a series
of practical e~periments performed on a medium-high volatiles American
coal commonly used in iron and steel making, having the following
characteristics: volatile matter 30~8~o~ fised carbon 64~ 2~o~ ash 5%
(the percentages are calculated on a dry-weight basis).
~he minus 3 mm coal was wet ground in a disc mill, as described above,
feeding to the first mill the coal, water and additive in the
quantities desired for the final suspension.
A mi~ture of activated salts of humic acid and phosphates was added
25 at a fi~ed rate of 0, 5%.
The results obtained are reported in the following table:
12~4686
_ _ _ _ _ _ _ _
I = ,, ~ ~= ~ ~ ~o o ~o .
U O O h h O 0 æ O c ~ ~
--5 _____ -~ _ ~ (~ 0~ _
n m5 _ _~_ _ 0 _ ~ ~ ~0 0
_ _ _ _ _ U~ U~ C~J CO ~t ~o
n ~N . ~ N r~l ~ 0~ ~ ~0 u~
n ~ '~ 3 3 _ . . _
1.~.'~ _ __ ~ c ~ ~ u~ u~ ~ ~
3u~12~ o ~ u~ ~ ~ ~ c ~ ~ 1~ u~ ~ ~
n ~ m e ~c o~ o~ 2 o uo~ o 0~ u~ o uo~ ~ o
~~ r _ _l _ _ _
O CO ~ ~ O ~ O ~ ~ .
Il ~ '~ ~ ~ co ~i o ~o O~ c~i u~
'. ~ _ _ _ ~ ___
12746B~ii
-- 5 --
~he static stability of the mi~tures over the course of time is
indicated by the variation in ths number of seconds required for
a 20 g rod 3 mm in dia~eter to penetrate under its own ~eight
through a 180 mm depth of mixture stored in the undisturbed state.
In the first three tests the coal had all settled out a~ter one
week, so penetration of the rod was stopped by the coal layer.
The assessment was made as objectively as possible, beingbased on
the ratio of the penetration time a~ter five weeks compared with
that when the mi~ture had just been made up (~ero weeks). With a
ratio of less than 4 the stability of the mi~ture was considered
ve~y good, while if it was between 4 and 10 it was classed as good.
Of course, viscosity also has a bearing on the assessment, stable
mi~tures with a vi æosity of less th2n about 500 cP being classed
as good. This i8 why Test 4 was considered to be only satisfactory,
because although its stability was good its viscosity was 1400 cP.
A~ is evident from the Table, as soon as more than 6~o of the coal
is finer than 74 ~m, time-stable mi~tures with good viscosities are
obtained. Of course, if mi-tures with very high stability are not
needed, because they are to be used immediately after being prepar
ed, then it is not secessary to grind the coal any finer t~an 50~0
minus 74 ~m. In this description the average diameter of the
particles is calculated as the harmonic and not the arithmetic
mean.
The mi~tures thus prepared are suitable for substituting fuel oil
and similar petroleum derivatives in applications such as, for
instance, fuel in thermal-electric power stations or as au~iliary
74686
6 --
fuel for injection into blast furnaces via the ~uyeres. Ihe mi~tures
appear to be very i~teresting for the latter application, not only
from the economic point of view but also because they permit part-
icularly uniform, efficient blast-furnace operation.
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