Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FROTH FLOTATION OF FINE COAL OR MINERAL PARTICLES
This invention relates to the froth flotation of fine particles
and in particular to the recovery of coal from an aqueous slurry
of coal fines also containing associated impurities as suspended
fine solids by froth flotation, and to the beneficiation of fine
mineral particles by froth flotation.
Coal as mined (run-of-mine-coal) contains a proportion of shale
which when treated in water in a coal washery tends to revert to
the original clay from which it was compressed in geological
times. Separation of the coal from the fine shale and clay
particles presents considerable problems. In the case of mines
where modern, mechanical extraction techniques are used,
typically a proportion as high as about 20% of the run-of-mine-
coal consists of particles smaller than 0.5 mm. This fine "coal"
typically has a substantial coal content but also a substantial
shale content so it is important to make use of the coal content
but also to remove shale from it. Modern coal preparation
processes result in the fines (separated from coarser material)
being in the form of aqueous slurries.
In the United Kingdom the usual way of separating coal fines from
shale fines in agueous slurries is by means of froth flotation
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followed by filtration.
In the froth flotation process the partially hydrophilic coal
particles are treated with a surface active chemical compound,
known as a "collector" , so as to render the surface of the
particles hydrophobic, so that they are attracted to air rather
than water, and adding a so-called "frother", either mixed with
the collector to form a so-called "froth flotation oil" or
independently, to enable a froth of the required stability to be
produced, and then aerating the aqueous suspension so that the
coal is recovered in the froth so formed. The efficiency of this
process is seriously affected by the presence of ultra-fine (of
less than about 50 microns) matter (both coal and shale~, often
present in significant proportions in the material requiring
treatment.
The beneficiation of fine mineral particles by froth flotation is
carried out in a similar manner, the particles of the mineral
which it is desired to recover being rendered hydrophobic by the
addition of a "collector" to an agueous slurry of the mineral
particles and particles of other minerals which it is not desired
to recover, followed by the addition of a frother, and flotation
of the particles of the desired mineral.
The largest particle present within a mass of mineral particles
which are to be separated by froth flotation must be of size such
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that the desired mineral particles will ~e physically released
from the unwanted mineral particles and that the mass of each of
the desired mineral particles does not exceed its force of
attraction to an air bubble under the conditions of turbulence
occurring in the aqueous suspension of mineral particles. It is
therefore generally necessary to grind minerals so that the
particles are sufficiently small for separation by an industrial
froth flotation process. ~uring the grinding process it is
inevitable that some of the particles produced will be finer than
intended and particles of the desired mineral which are too fine
are generally difficult to recover by froth flotation. The size
at which the difficulty is met will depend on a number of
factors, including the specific gravity of the mineral which it
is desired to recover, the degree of turbulence within the
aqueous suspension of mineral particles and the size range of the
air bubbles in the suspension. Commonly, recovery of the desired
mineral and rejection of unwanted minerals starts to deteriorate
when the mineral particles are finer than about 10 microns,
becoming very poor when the particles are finer than about 1
micron. These difficulties are commonly referred to as sliming
problems.
G~ patent application 2190310A describes a process in which coal
flnes are separated from shale fines by means of a froth
flotation process and in which the particles of coal are treated
with a hydrophobic polymer, such as a hydrophobic polyvinylalkyl
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ether which is highly selective as a flocculant for the coal
fines in preference to the shale fines. GB patent publication
2212418A describes a similar process in which particles of a
desired mineral are recovered from particles of an unwanted
mineral in an aqueous slurry by means of froth flotation, after
treatment with a predominantly hydrophobic polymeric flocculating
agent which selectively flocculates the desired mineral
particles.
It has now been found that alkyl acrylate polymers are
particularly useful as selective flocculants in the froth
flotation processes of the types described in GB patent
puhlications 2190310A and GB 2212418A.
According to the invention in one aspect there is provided a
process for separating particles of a desired material from
particles of an undesired material in which a collector for the
desired material and a frother and a polymer selected from alkyl
acrylate polymers and alkyl methacrylate polymers are added to an
aqueous slurry containing the particles of the desired material
and the particles of the undesired material, and the particles of
the desired material are floated in a froth flotation cell.
The process of the invention may be used to recover coal fines
from an aqueous slurry containing coal fines and shale fines, and
also to recover fine particles of a desired mineral from an
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aqueous slurry containing both the particles of the desired
mineral and fine particles of one or more undesired minerals. The
process is useful when treating a slurry arising in the treatment
of copper ores, copper nickel ores and ores containing copper and
metals of the platinum group.
Examples of suitable polymers are alkyl acrylates in which the
alkyl group may be methyl, ethyl, butyl, propyl, amyl, hexyl,
heptyl or octyl (2-ethylhexyl). The polymer may also be an alkyl
methacrylate having the same alkyl group. The polymer may be an
interpolymer in which the methacrylate has an alkyl group such as
methyl or ethyl or butyl or another group such as a fatty acid,
e.g. lauryl, cetyl or stearyl; and associated acrylic esters.
To be effective in the method of the invention the selected
polymer should have an average molecular weight in the range of
about 10,000 to about 40,000, preferably about 20,000 to about
30,000. A preferred alkyl acrylate polymer is poly (n-butyl
acrylate) which is commercially available as a highly viscous
liquid from BASF United Kingdom Ltd under the trade mark ACRONAL
LR 8820. The polymer may also be a copolymer of an alkyl
acrylate or alkyl methacrylate and associated acrylic ester with
a monomer such as maleic anhydride or vinylpyrrolidone. Mixtures
may be used.
., ~ -
, ,.
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To be useful in the process of the invention the polymer must be
dispersible in water. If the polymer i5 a liquid it can either
be dispersed directly in the aqueous slurry or predispersed or
dissolved in a carrier liquid. If the polymer is a solid it must
be predispersed or dissolved in a carrier liquid. If desired a
dispersant or emulsifying agent such as a non-ionic alkoxylated
ester of a fatty acid may be used to aid dispersion of the
polymer.
In another aspect the invention includes an additive composition
for use in separating particles of a desired material from
particles of an undesired material, the additive composition
comprising a polymer selected from alkyl acrylate polymers and
alkyl methacrylate polymers and a carrier liquid therefor.
When the process is used to recover coal fines the carrier liquid
for the polymer may be a conventional collector as used in the
froth flotation of coal, for example gas oil, Diesel oil,
kerosene or other petroleum or coal-based distillates, or
mixtures thereof. The polymer may also be dispersed or dissolved
in a conventional froth flotation oil which consists of not only
the collector but also the frother, and possibly other additives
such as an emulsifier or dispersant.
When the process is used to recover a desired mineral from an
undesired mineral the frother which is used in the conventional
,.
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froth flotation may act as the carrier liquid for the polymer.
Any of the known frothing agents used in the froth flotation of
minerals, for example methylisobutylcarbinol, a propoxylated
butanol or a polypropylene glycol, may be used as the carrier.
In the recovery of coal fines the amount of polymer used will
usually be in the range of about 0.3g to about 200g per tonne of
solids treated, and when the polymer is added as part of an
additive composition containing the collector and the frother the
additive composition will usually contain about 0.2% to about 15%
by weight of polymer, about 50% to about 90% by weight of
collector or carrier liquid and about 10% to about 30% by weight
of frother.
In the recovery of a desired mineral the amount of polymer used
will usually be in the range of about O.lg to about lOg per tonne
of total mineral solids or about 0.5g to about 50g per tonne of
the desired mineral, and when the frother acts as the carrier
liquid for the polymer the additive composition will usually
contain about 0.2% to about 25% by weight of polymer and about
75% to about 99.8% by weight of frother.
Although in the recovery of a desired mineral the polymer is
usually added to the frother as the polymer i5 soluble in the
frother, if the polymer is soluble in the collector or another
flotation reagent, such as a modifier ~which modifies the
.,
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collection or flotation), the polymer may be added to that
reagent.
Variations in the structure of the alkyl acrylate polymers or
alkyl methacrylate polymers, which vary their hydrophobicity,
will vary the characteristics of the flocculation of the desired
material and also the secondary characteristics of the flotation
of the desired material.
For instance in the recovery of coal fines a poly (n-butyl
acrylate) polymer which has a moderately high degree of
hydrophobicity will strongly but selectively flocculate the ultra
fine coal particles in a slurry, and will also yield a very dry
froth compared to that obtained in a conventional froth flotation
process.
It has also been found that an alkyl acrylate polymer or alkyl
me~hacrylate polymer is particularly useful when it is used in
conjunction with a hydrophobic polyvinylalkyl ether, such as
polyvinylethyl ether or polyvinylisobutyl ether, as a selective
flocculant, and that when used together the alkyl acrylate
polymer or alkyl methacrylate polymer and the polyvinyl alkyl
ether are complementary in their effect.
The following examples will serve to illustrate the invention in
which parts are percent by weight unless otherwise indicated. In
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the examples, the froth flotation oil was a hydrocarbon oil
acting as the collector and containing an emulsifier and a
frother. The poly (n-butyl acrylate) used in Example I had an
average molecular weight of 20,000 to 30,000, as did the
polyalkylmethacrylate used in Example II. ACRONOL LR 8820 is a
trade mark of BASF United Kingdom Ltd., as is LUTANOL A25.
Example I
Two additive compositions were prepared as follows:-
(1) proprietary froth flotation oil 98
poly (n-butyl acrylate) (ACRONOL LR 88203 2
(2) proprietary froth flotation oil 88
poly (n-butyl acrylate) (ACRONOL LR 8820) 2
polyvinylethyl ether (LUTANOL A25 ? lo
Froth flotation tests were carried out on a coal/shale slurry in
which the particle size of the solids was less than 700 microns
and the solids content was 5.1% by weight, using compositions (1)
and (2) and as a control the proprietary froth flotation oil
alone. In each test 0.062g. of the composition was added to 2.65
litres of the slurry in a Denver DR cell turning at 1500 rpm.
After a conditioning time of 1 minute the air supply to the cell
was turned on, and the froth which was produced was taken off for
160 seconds.
The results obtained are shown in Table I.
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These results indicate the poly (n-butyl acrylate) polymer
selectively flocculates the ultra-fine coal particles improving
their separation from the ultra-fine shale and clay. When used
on its own in the froth flotation oil the dry froth which it
produces yields the minimum of clay in the coal product. When
used in conjunction with the polyvinyl ethyl ether stronger
frothing yields a higher recovery of coal with only slight
increase in the clay content.
Example II
An additive composition was prepared as follows:
proprietary froth flotation oil go
polyalkylmethacrylate 10
The polyalkylmethacrylate is an interpolymer of a mixed
alkylmethacrylate and associated acrylic esters available from
Rhone-Poulenc.
Comparative tests were performed in a coal flotation plant and
filter using the composition and the oil alone. ~amples of the
coal ~lurry feed to the flotation were analysed together with
~amples of the froth, tailing products and filter cake. The
recovery of combustible coal in the froth and the rejection of
the ash into the tailing were calculated in the usual way. The
results are set out in Table II and show that the polymer
selectively flocculates the ultra-fine coal particles so
improving their separation from the ultra fine shale and clay.
The filtration ~f the coal products is thus improved.
LS/DAS/17.3.92
Ref: F40
Y~
TABLE I
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ _
ADDITIVE CONTROL (1) (2)
COMPOSITION
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ _
COAL 39.4 38.7 42.4
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ _ _
COAL PRODUCT ASH% 11.39.1 9.5
_______________________ _____________ __________. .__________
TAILING ASH % 75.576.3 78.8
_______________________ _____________ __________. ,__________
COMBUSTIBLES 70.270.8 75.8
RECOVERY %
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ _
FEED ASH 50.2 50.3 99.4
CALCULATED
__________________________________________________________
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TABLE II
CONTROL COMPOSITION
____________________ __________ ____________ .____________
DOSAGE RATE 130 81 97ml/minute
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _
ASH IN FEED 45.3 46.5 42.7
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _
COAL PRODUCT
FROTH (% ASH) 12.8 11.5 12.1
____________________ _________ ____________ .___________
FILTER CAKE
(% ASH) 12.9 12.4 12.1
___________________ ~_________ ____________. ___________
TAILING
(% ASH) 84.1 84.0 84.4
____________________ __________ ____________. ___________
~O SOLIDS IN
FROT~ 27.5 28.2 27.4
% MOISTURE .
IN FILTER CAKE 30.9 28.4 25.9
COMBUSTIBLE 86.7 85.6 88.5
RECOVERY %
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
ASH REJECTION (%) 84.6 87.2 83.7
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
COMBUSTIBLE
RECOVERY X 73.4 79.6 74.1
ASH REJECTION _ ___ ___
______________________________________________ _ _
L.S/DAS/21.3.92
