Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
105361Z
THIS invention relates to dense medium separation wherein
magnetic particles are used to form the dense medium.
- Dense medium separation is a well known technique -for
separating particulate solids of different densities in
a mixture. The dense medium is a suspension of dense
particles in a liquid. The mixture of particles for
separation is mixed with the suspension, and the
different particles allowed to sink or float. The
operation may for example be effected in a cyclone.
When coarse particles are involved there is little
problem in separating the particles in the underflow
and overflow from the dense medium by simple screening.
When dense medium separation is used for particles over
1000~, the greater part of the medium may be recovered
for immediate re-use by screening alone. The magnetite
particles adhering to the washed products may be rinsed
- off and the diluted magnetite suspension so created
cleaned and concentrated in magnetic separators.
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, When attempting to wash particulate material nominally
less than 1000~, efficient separation of the cleaned
particles from the medium is not so readily effected
by simple screening. Moreover, because of the large
surface area of the cleaned particles, adherence of
magnetite is a severe problem. Consequently, in
~` 25 washin~ plants attempting to clean -1000~ particles,
1 it has been proposed that the entire separation of the
,` cleaned products from the medium be carried out by
magnetic separation.
I For acceptable recovery of the dense medium however
¦ 30 large separation areas and cons;derable dilution of
~ the suspensions is required. This is a costly step.
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According to the present invention there is provided a process
wherein a particulate material is separated by means of a dense medium
suspension made up of magnetic particles into high and low density fractions
and the dense medium is recovered from the fractions by screening and washing
to yield a first product recovered from the high density fraction and a
second product recovered from the low density fraction, with the improvement
that at least one of the products is subjected to a hydrocyclone separation
to yield a dense fraction containing substantially only dense medium particles
and a light fraction containing substantially all of the other particles and
some dense medium particles and recovering the dense medium particles from
the light fraction by means of a magnetic separation, the dense fraction and
the recovered dense medium particles being utilized to make up the dense
medium suspension.
According to the present invention there is also provided a process
wherein raw coal fines is separated by means of a dense medium suspension
made up of magnetic particles into high and low density fractions, and the
dense medium is recovered from the fractions by screening and washing to
yield a first product recovered from the high density fraction and a second
product recovered from the low density fraction, with the improvement that
at least one of the products is subjected to a hydrocyclone separation to
yield a dense fraction containing substantially only dense medium particles
and a light fraction containing substantially all of the coal derived
particles and some dense medium particles and eecovering the dense medium
particles from the light fraction by means of a magnetic separation, the
dense fraction and the reoovered dense medium particles being utilized to
make up the dense medium suspension.
~ The density separation may be effected in one, two or even more
b~ stages.
- In the case of more than one stage, the feed to each subsequent
stage is preferably dewatered.
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Each stage of the high relative density separation may be effected
in a cyclone.
According to this embodiment a wide angle cyclone, i.e. a cyclone
the cone angle of which is in the range 60 to 180 is preferred.
The method of the invention may be used in the separation of fine
coal particles from a dense medium wherein the dense particles are magnetite.
For example, it may be used to treat the overflow from a dense medium process
for the beneficiation of coal fines.
In this case the method may also be used to treat the underflow,
10 i.c. to separate thc s~gnotite particles frols the discard particles.
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The invention is discussed further with reference
to the attache~ flow sheet of a plant intended to
beneficiate coal fines.
Referring to the flow sheet, raw coal fines for
S beneficiation are mixed with an aqueous suspension of
magnetite in a tank 2. The mixture is fed to a
cyclone 4 for a conventional dense medium beneficiation
of the coal fines. The underflow from the cyclone 4
comprises a suspension of discard and magnetite particles,
and the overflow a mixture of washed coal fines and
magnetite particles. Both the underflow and overflow
are subjected to further similar treatment for
separation of the magnetite particles from the other
less dense particles (coal or discard) present therein
as follows:
The underflow from the cyclone 4 is fed to a screening
' stage 6 for removal of any coarse discard fractions
,~ (i.e. exceeding a predetermined size,say 1000~). The
j overflow from the screening stage 6 comprising the
coarse discard fraction is led away, and the underflow
is fed to a first cyclone 8.1. The overflow from the
cyclone 8.1 is fed to a dewatering stage 10 and then to
~ a second cyclGne 8.2. The screening stage 6 includes
i a rinsing portion 6.1 which is fed with water from the
I 25 dewatering stage 10. The underflow from the rinsing
¦ portion 6.1 is added to the overflow from the cyclone
~ 8.1 before ;t enters the dewatering stage 10. The under-
¦ flow from each cyclone 8.1,8.2 is adjusted to form a
i first fraction containing essentially only magnetite
particles. This fraction is recycled to the tank 2
via an overdense tank 14 and load box 16. The overflow
~ from the second cyclone 8.2 forms a second fraction
¦ containing the bulk of the discard particles and some
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dence medium particles. This second fraction is fed
to a magnetic separator 12 for recovery of the remaining
magnetite particles. The magretite recovered is
recycled from the magnetic separator 12 to the tank 2
via the overdense tank 14 and load box 16. The fine
discard is led away.
The overflow from the cyclone 4 is ~reated similarly.
It is fed to a screening stage 18 for removal of any coarse
coal fractions (i.e. exceeding a predetermined size,
say 1000~). The overflow from the screening stage 18
comprising the coarse coal fractions is led away, and
the underflow is fed to a first cyclone 20.1. The
overflow from this cyclone 20.1 is fed to a dewatering
stage 22 and then to a second cyclone 20.2. The
screening stage 18 includes a rinsing portion 18.1 which
is fed with water from the dewatering stage 22. The
underflow from the rin~ing portion 18.1 is added to the
overflow from the cyclone 20.1 before it enters the
dewatering stage 22. The underflows from the cyclones
20.1 and 20.2 containing substantially magnetite particles
~, only, are recycled to the tank 2 via the overdense tank14 and load box 16. The overflow from the cyclone
20.2 contains the bulk of the coal particles and some
l magnetite particles. It is fed to a magnetic separator
¦ 25 12 for recovery of the magnetite particles. The
3 recovered magnetite is recycled to the tank 2 via the
overdense tank 14 and load box 16, and the cleaned coal
fines are led away.
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The cyclones 8.1, 8.2, 20.1 and 20.2 are all wide angle
cyclones, i.e. having a cone angle in the range 60-
, 180.
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The magnetite particles used for forming the dense
medium are of conventional size for such processes, not
exceeding 100 microns.
The coal fines to which the process is particularly
applicable are those having a particle size not exceeding
of the order of 1000 microns, in particular those in
which the majority have a particle size less than 300
microns. Particles of this-order of size being recalcitrant
to separation techniques such as froth flotation,
require dense medium separation, and accordingly
: magnetic separation of the magnetite particles from the
coal and discard particles in the overflow and underflow
respectively.
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In a number of tests a substantially pure underflow
(i.e. containing little contaminating coal) was obtained
- when aqueous suspensions of coal particles and magnetite
particles of the order of the above sizes were fed to
cyclones having cone angles of 160 and 180.
Preceding magnetic separation treatment of the underflow
and overflow from the dense medium beneficiation with
high specific gravity separations has the advantage that
the loads on the magnetic separators are significantly
reduced. Thus smaller capacity, and accordingly less
expensive, magnetic separators may be used.
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