Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a process for the
agglomeration of fines, in particular coal fines, in
slurry or powder form by means of a mineral oil product
dispersed in water.
The process can, e.g., be applied to aqueous coal
slurries or coal fines containing waste water from the
washing treatment and in that case facilitates the sub-
sequent separation of the water in a known manner, for
example by centrifugation; it can likewise be applied
to concentrated slurries or dry dust. In each case the
process yields an excellent granular, non-dusty product.
The invention therefore relates to a process for
the agglomeration of fines in slurry or powder form by
means of a mineral oil product dispersed in water,
characterized in that from 5-15% by weight, based on
anhydrous fines, of a bitumen emulsion is added to the
fines, the bitumen having a penetration at 25C below 5
and a Conradson value over 35. In the preferred em-
bodiment ~ the process a bitumen emulsion is utilized
which contains as emulsifier a minor proportion, prefer-
ably 1-10% by weight based on the proportion of bitumen,
of alkaline metal naphthenate.
Preferred fines are coal fines, although this process
can also be used for the agglomeration of other fines,
such as coke fines, iron fines, sponge iron fines from
steel works, ores, such as iron ores. If desirable the
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granules can be briquetted. Heat treatments can be
applied as well. In general 3 mineral fines are preferred.
The recovery and utilization of the portion of fines,
in particular those obtained during the production and
preparation of coal has always been a problem for the
mining industry. With the increasing mechanization in
the fields of working and transport the output of fines
is growing, as a result of which this problem is being
aggravated. The effluents from the washing treatment
entrain a lot of coal dust and have to be purified in
settling basins and/or centrifuges. The resultant slurries
are partly worked up further, at any rate dewatered by
means of centrifuges or vacuum filters and nevertheless
only yield a dusty, low-grade coal with limited applications.
The same applies to the more or less dry coal dust which
is additionally obtained in major quantities.
Several processes are known which facilitate the
separation of water from the coal slurries. For example,
in the Convertol process and in the newer Olifloc process,
mineral oil fractions, namely light or heavy fuel oil,
are admixed to the slurries. They act simultaneously as
flotation agents for the working-up treatment and as
flocculation agents for dewatering the slurries. However,
the incorporation of the mineral oil requires special
equipment, and the addition of mineral oil has no detect-
able effect on the consistency o' the resultant coal fines.
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This also applies to the Esso Research Engineering
Company's process (U.K. patent specifications 1,041,547
and 1,123,597), in which relatively low-boiling hydro-
carbons are used as pure flotation agents. In this
process the addition of liquid surfactants, such as long-
chain alcohols, is also suggested. It is then obvious
to employ the mineral oil fraction in dispersed form
from the beginning, as is in fact known from the process
according to U.K. patent specification 1,351,986. This
process, however, is not intended for coal fines but
for the sludges obtained during bi~ogical sewage
purification.
Whereas these known processes are based on liquids
which are not water-miscible and which selectively wet
coal particles, it has now been found that the corre-
sponding flocculation effect can also be achieved with
solid bitumen. An admixture of approximately 5-15% by
weight, based on anhydrous fines, of bitumen emulsion
to the coal fines-containing waste waters or slurries
produces a satisfactory flocculation, as a result of
which the subsequent removal of water, for example by
means of screen-type centrifuges, can be appreciably
facilitated.
Suitable bitumens have a Ring and Ball softening
point of at least 100C. Although straight-run bitumen
can be used, the residue from a thermal cracking treat-
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ment, brought to the stated specification by high-
vacuum distillation, is preferred.
The advantage of the process according to the
invention resides in that the dewatered fines, in
particular coal fines, agglomerate or granulate durably
and are thereby more versatile and more readily applic-
able than the coal dust obtained in the customary manner.
For example, the flotation concentrate from a hard-coal
washing treatment was, on the one hand, dewatered in the
conventional manner over vacuum filters (A), and, on the
other hand, prior to dewatering was agglomerated by the
addition of 15% by weight, based on anhydrous coal, of
a special bitumen emulsion (B). The emulsiom contained
approximately 50% by weight of a bitumen having a PEN/25
of 1-2 and a Conradson value of 40. The screen analysis
produced the following results:
A B
below 0.2 mm 69% 5%
0.2-0.6 mm 24~ 19%
0.6-2.0 mm 4% 18%
over 2.0 mm 3% 58%
It was further found that the chemical nature of the
emulsifier contained in the bitumen emulsion plays a role
in the process. Especially the agglomerate formed in
aqueous slurries can vary in its microscopic structure
from spongy to coarse- grained. This process is affected
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very considerably by the emulsifier. A granulated micro-
structure means better settling of the agglomerated
slurry, more rapid and more complete dewatering in the
centrifuge and, in particular, a considerable increase
in the filtering rate, in the event of the slurry being
dewatered over vacuum filters.
Most emulsifiers generally used for bitumen emulsions,
which emulsifiers essentially comprise alkaline soaps of
higher fatty acids, are only moderately suitable in this
respect. On the other hand, very good results were ob-
tained with a special emulsion which had been prepared
with, e.g., 3% by weight, based on the proportion of
bitumen, of alkaline metal naphthenate. The naphthenates
in question are the salts of the higher molecular naph-
thenic acids, which preferably have approximately thesame boiling range as the lubricating oil fractions.
Generally, in the vacuum distillation of naphthenic
mineral oil these acids are neutralized in the column
using caustic soda or caustic potash solution and with-
drawn as a side-fraction. The raw product contains, for
example, approximately 50% of unsaponifiable material, 25%
of alkaline naphthenate and 25% of water and excess alkali.
The naphthenate is suitably employed in the raw state,
being dispersed with the calculated amount of water and
the calculated amount of bitumen without further additives
being incorporated in this dispersion in a known manner.
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By way of example, filtration tests were carried
out with a coal slurry containlng 20% by weight of dry
matter having a grain composition as indicated above
(A). This was compared with the same slurry, agglomer-
ated with 15~ by weight, based on dry matter, of a specialbitumen emulsion as described above (B); this emulsion
contained a commercial-grade emulsifier consisting
essentially of alkaline soaps of higher fatty acids.
In the further comparison a corresponding emulsion was
used which had been prepared with approximately 3% by
weight of alkaline naphthenate in the described manner
(C). The following table shows the times in seconds in
which under identical conditions identical amounts of
filtrate had passed.
A B C
20 ml of filtrate 34 117 11 sec.
40 ml of filtrate 56 226 20 sec.
60 ml of filtrate 86 _ 33 sec.
The present process may be applied to more or less
aqueous coal slurries. In each case it is sufficient to
add the requisite amount of bitumen emulsion at room
temperature to the slurry and mix intensively for a
brief period, suitably in a continuous stirrer. The
process may equally well be applied to dry coal dust,
similarly by the addition of the requisite amount of
bitumen emulsion at room temperature - elevated temper-
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atures generally produce worse results - and by brief,
intensive mixing suitably in a worm-screw kneader designed
for continuous operation. This process yields a granulate
which is fully comparable to the above-described granulate
formed in the aqueous phase. In the dry process as well,
the chemical nature of the emulsifier has an effect, in
addition to the consistency and origin of the emulsified
bitumen. Under equal conditions the preferred alkaline
naphthenate emulsions yield a better granulate, i.e.,
one having more uniform grain distribution and, in
particular, less fine grains (below 0.2 mm) and coarse
grains (over 5 mm), as the following comparative test
shows.
Air-dry coal dust was intimately mixed in a kneader
designed for continuous operation with 10% by weight of
bitumen emulsion (approximately 50% by weight of bitumen).
The speed of the kneading screw was set at 105 rpm~
resulting in an average residence time in the trough of
5 minutes. The bitumen emulsion was sprayed on in the
first tenth part of the mixing section. The screen
analyses of the untreated coal dust (A) were compared
with those of the dried granulates, which had been
prepared, on the one hand, with a special bitumen
emulsion as described above containing a commercial-
grade emulsifier (B) and, on the other hand, with acorresponding emulsion which had been prepared in the
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described manner with approximately 3% by weight of
alkaline naphthenate (C).
It is evident that the granulate C contains far
less of the undesirable fine and coarse grains than the
granulate B.
A B C
below 0.2 mm 52% 4% 2%
0.2-0.5 mm 35% 21% 14%
0.5-1.0 mm 10% 43% 38%
1.0-2.0 mm 2~ 21% 28%
2.0-3.2 mm 1% 3% 9%
3.2-5.0 mm - 3% 6%
over 5.0 mm - 5~ 3%
It should be mentioned that the preferred special
emulsion, containing a bitumen having a PEN/25C below 5
and a Conradson value over 35, as well as alkaline -
naphthenate as emulsifier, is not a standard product,
does not conform to any standard and is not suitable
for most applications, for examp~e in road construction.
In the case ~ coal fines the agglomerated product
is ready for use, e.g., for burning in power station
boilers.
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