Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to the beneficiation and
dewatering of slurries. The invention is particularly
applicable to the recovery of coal fines from aqueous
slurries but could also be used in the recovery of other
particulate minerals.
Conventional technology has dictated that slurries
of fine coal or other minerals are beneficiated by froth
flotation and that the concentrate is dewatered by vacuum
filters (drum, disc or belt) or to a lesser extent by
centrifuges. In some cases a thickener is required before
the concentrate is finally dewatered. Very large capital
costs are associated with the installation of appropriate
dewatering equipment and very often the resulting product
has a high water content.
The present invention provides an improved froth
flotation/dewatering process which enables an increased
degree of benefication to be achieved in the dewatering
step and/or enables the capital investment in dewatering
equipment to be substantially reduced.
The invention applies to the froth flotation/
dewatering process the technique of selective agglomeration.
This technique relies on the fact that certain rninerals
such as coal are hydrophobic or can be rendered hydrophobic.
Consequently, particles of such minerals suspended in an
aqueous slurry can be caused to agglomerate by the addition
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of an agglomeration agent, usually a hydrocarbon oil or
other organic liquids, so that the particulate mineral
will eollect preferentially with the agglomeration agent
leaving the non-hydrophobic materials in aqueous suspension.
One application of this techni~ue to the extraction of coal
fines from aqueous slurries is described in our copending
Canadian Application No. 302,545.
However, the process of the presen-t invention has wider
application and can be used to economically treat larger
quantities than would be possible with the process
described in the earlier application.
SUMMARY OF THE INVENTION
Aecording to the invention there is provided a
process for beneficiating and dewatering an aqueous slurry
of particulate material eomprising the steps of
subjecting the slurry to froth flotation;
obtaining a concentrate from the froth resulting
from the froth flotation;
adding to the concentrate an agglomeration agent
capable of causing selective agglomeration of the
particulate material in the concentrate, and
dewatering the concentrate.
Preferably, the concentrate is obtained by taking
said froth and either causing or allowing it to break to
produce a concentrate slurry and the agglomeration agent
is added to the coneentrate slurry.
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Preferably, the agglomeration agent is a liquid
hydrocarbon and is added to the concentrate slurry in
an amount falling in the range 0.5~ to 10% by weight of
said particulate matter in the concentrate.
The concentrate with added hydrocarbon may be
subjected to a shear regime prior to dewatering. In this
case the shear regime is preferably defined by a Froude
Number in the range 2 to 600 and the hydrocarbon is added
in an amount in the range 5% to 10% by weight of said
particulate matter in the concentrate. In this case too,
the hydrocarbon preferably has a viscosity in the range
1 to 10 centipoise and the dewatering step may comprise
applying the concentrate, after subjection to the shear
regime, to a screen having apertures in the size range
0.15 to 1, 00 mm.
Alternatively, the concentrate with added hydro-
carbon may be passed to dewatering equipment without being
subjected to a shear regime and in this case the amount of
hydrocarbon added may be in the range of 0.5% to 5% by
weight of particulate matter and the viscosity of the
hydrocarbon may be in the range 1 to 3 centipoise.
sRIEF DESCRIPTION OE THE DRAWINGS
In order that the invention may be more fully
explained one particular process involving application of
a shear regime will be described with reference to the
accompanying drawings, in which:-
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Figure 1 is a diagrammatic flow sheet of the
process; and
~ 'igures 2 to 4 are graphs illustrating the
improved performance which can be achieved by the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The illustrated process is particularly suitable
for recovery of coal fines of less than 0.5 mm particle
size but it may also be applied to the recovery of
similarly sized particles of other hydrophobic minerals.
In the illustrated process a slurry input is
delivered via a line 10 to froth flotation cells 12.
The slurry input may come from direct wet grinding, from
wet screening to remove coarse particles for density
separations or from a transportation slurry pipeline.
Before the slurry enters the flotation cells 12, reagents
that are needed for the flotation to occur (collector and
frother) are added through an input pipe lln The froth
flotation step is entirely conventional and will be well
known to experts in the mineral recovery field. Further
details may be obtained by reference to the text
"Handbook of Mineral Dressing" by A.F. Taggart, John Wiley
& Sons, 1954, Section 12
In the froth flotation cells 12, air bubbles attach
themselves to the hydrophobic coal particles which then
report to the foam. The foam overflows as a concentrate
product into the product line 14 and a tailing containing
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unwanted contaminants is withdrawn through line 13.
When the foam concentrate overflows into product
line 14 it breaks to produce a concentrate slurry
containing the coal fines in suspension. Line 14 may
include a foam breakiny device to accelerate the formation
of such a slurry. The concentrate slurry is delivered by
line 14 into an agglomeration tank 16 in which it is
subjected to a shear regime by rotation of an impeller 21.
The concentrate slurry being delivered to agglomer-
ation tank 16 receives an addition of a hydrocarbon
agglomeration agent. The hydrocarbon may be added through
an inlet 15 but it could alternatively be injected at the
exit of the slurry stream within tank 16. The choice of
hydrocarbon additive depends on the requirement for the
final agglomerated product. If a very strong agglomerate
is required, then the hot addition of a heavy hydrocarbon
such as tar or pitch is needed. For less stringent
requirements a cheap heavy fuel oil like "furnace oil" may
be used. Although these types of oils are poorly
selective they are relatively cheap. In a situation where
further beneficiationis possible and desired a lighter,
more selective oil such as industrial diesel fuel,
distillate or kerosene may be used. These oils are more
expensive but produce cleaner agglomerates. The
properties of these hydrocarbons are given in Table 1.
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-TABLE 1
Properties of Hydrocarbo_
Type of Viscosity Specific Gravity
Hydrocarbon (Centipoise) (at 15C)
Kerosene 1.5 0.788
Distillate 2.53 0.829
Industrial Diesel Fuel 2.64 0.841
Furnace Oil 38.0 0.931
Tar - ~25
Pitch ~350
The level of hydrocarbon addition is preferably
between 5 to 10% on a dry solids basis and the hydro-
carbon is preferably emulsified prior to its injection
into the slurry.
Impeller 21 is rotated so as to produce a shear
regime defined by a Froude Number in the range 2 to 600.
After a suitable residence time within the agglomeration
tank, preferably of the order of 5 min. to 30 min.,
agglomerates and water overflow from the tank in a stream
17 onto a dewatering screen or sieve bend 18 having
apertures in the range 0.15 to 1.00 mm depending on the
material being treated. The agglomerates are captured
by the screen and pass from the bottom of the screen at
19 to product handling whilst the water, together with any
contaminants, passes through the screen in a stream 20
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which may be directed either to a clarified water supply
or to a water clarifier system.
An indication of the effectiveness which may be
expecte~ of the above process is given in the following
working example:
WORKING EXAMPLE 1
A slurry was made by mixing 100 gm of bituminous
coal (ash content 13.4% adb) with approximately 1200 ml
of water. ~his slurry was placed in a Denver Laboratory
flotation machine. Collector (diesel oil) and frother
(MIBC) were added to the cell at the addition rates of
1 kg and 0.1 kg per tonne of solids respectively. After
mixing for one minute, air was introduced into the cell.
The frothl containing approximately 25~ by weight solids,
was collected ~or dewatering. After the foam had
collapsed, the product slurry was placed in a stirred
beaker with a shear regime corresponding to a Froude
Number of 17.3. Hot "sunker C" oil was added to the beaker,
at an addition of 10~ on a solids basis, to agglomerate
the coal particles. Dewatering was carried out on a 0.5
mm screen. Product yield was approximately 75% and the
product ash was 7.2% (adb). Product moisture was 27.8%
but it has been observed that water drains freely frorn the
product if allowed to stand for a short time. The product
was in the form of small agglomerates which were able
to be handled readily.
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Table 2 contains drainage rates for typical
agglomerates.
TABLE 2
Drainage from Agglomerates
5 Drainaqe Time Product Moisture
(minutes) ~ (Wt ~)
15.6
13.3
12.9
12.8
The process described in some detail above with
reference to the drawingsis advanced by way of example
only and could be modified considerably. For example
the dewatering screen 19 could be replaced by some other
type of dewatering device, for example by a vacuum filter
or centrifuge device.
Moreover, the invention extends to froth flotation
processes in which the shear regime is omitted but an
agglomeration agent is added to the froth concentrate
prior to dewatering in order to improve the dewatering
efficiency obtainable with conventional dewatering
equipment such as vacuum filters. In such cases the
agglomeration agent may be a hydrocarbon oil with a
viscosity of between 1 and 3 centipoise and is added at
the rate of 0.5 to 5% relative to the weight of solids.
Sufficient turbulence is present in the pipes or launders
taking the product slurry to the dewatering device to mix
the hydrocarbon with the solid particles.
The general phenomenology associated with oil
addition to filter feed can be qualitatively interpreted
in terms of a flocculation process in which size
enlargement occurs through groups of particles being held
together by oil bridges at their points of contact.
This proposed flocculation process is believed to lead to
an increase in bed porosity and an effective increase
in the particle size of the units around which liquid
flow can occur. Both of the factors can be shown
theoretically to lead to increased filtration rates and
a reduction in the capillary forces causing liquid to be
retained by the cake.
The "cleaning" process in which the ash content of
the filter cake is reduced can be interpreted in terms
of the relative affinities (wettabilities) of the oil
phase to the coal material and mineral matter. Particles
of mineral matter are generally hydrophilic as a result
of the presence of ionisable groups on their surface or,
in the case of clay materials, through the electric
charge generation caused by isomorphous substitution within
the crystal lattice. As a consequence, oil droplets do not
adhere to clay particles in contrast to the strong
adhesion experienced in the case of hydrophobic coal
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particles. Furthermore, since the mineral matter
contained in coals is largely kaolinitic or
montmorillonitic, the particle size of these
components is often much less than that of the coal
material and is capable of passing through the filter
cloth and reporting to the filtrate. This concept
is consistent with the increase, in ash content of the
filtrate solids seen in the further Working Examples below.
WORKING EXAMPLE 2
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A sample of flotation concentrate from a coal
preparation plant cleaning bituminous coal, was taken.
This sample had a pulp density of 32% by weight. This
material was filtered using a vacuum leaf filter with a
cloth aperture of 0.4 mm at a suction of 68 kPa. Figures
2 and 3 show the improved filtration results that can be
obtained by the use of small amounts of oil.
The results show that considerable decrease in
filter cake moisture from 26.6~ to 14.1% can be obtained
for an oil addition of 1~ on a feed basis. Furthermore,
this decrease in cake moisture is accompanied by an
equally significant increase in filtration and cake
pick-up rates. Figure 3 shows that a considerable
"cleaning" of the filter cake was taking place with the
result that a lowering of the filter cake ash from 18.0%
to 10.9% was also achieved for an oil addition of 1%.
This lowering of filter cake ash was accompanied by a
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corresponding increase in the ash content of the filtrate.
WORKING EXAMPLE 3
Research into different flotation circuits (Firth,
B.A., Swanson, A.R., and Nicol, S.K. Flotation Circuits
for Poorly Floating Coals, Int. JO Mineral, Process,vol. 5 (1979)
pages 321 - 334) has shown that advantages can be
obtained by floating coarse and fine particles separately.
However, in the past the filtration of very fine material
has been extremely difficult. The data in figure 4 shows
that the addition of a small quantity of kerosene (1%)
to a minus 76 ~ m feed can improve filtration dramatically.
This oil addition enabled the cake moisture to be lowered
from 40% down to 16% at 68 kPa suction of a 38 ~ m mesh
filter cloth.
WORKING EXAMPLE 4
Flotation concentrates from a washery preparing
bituminous coal for coking purposes, was fed to a
continuous pilot scale drum filter of approximately lm2
filtration area. The applied vacuum was 67kPa and the
filter mesh was 0.4 mm. The feed had a pulp density of
25% and an ash content of 14%. Without oil the product
moisture was 26% and the product rate was 1 kg/min. The
ash of the filter cake and the solids in the filtrate
were 12.6%and 16.1% respectively. The filter cake was not
evenly spread over the drum indicating inefficient use
of the filter area.
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After the addition of 1% of kerosene the
production of filter cake jumped -to 2~ kg/min and the
cake was thick and even. The product moisture fell to
22~%. The ash of the filter cake was 11.3~whereas the
ash of the filtrate solids was 18.5%.
It is to be understood that the invention is not
limited to the specific process and conditions detailed
herein and that many modifications and variations will
fall within the scope of the appended claims.
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