Note: Descriptions are shown in the official language in which they were submitted.
2 894 CA
10801S7
The present invention relates to the field of particle separation and,
more specifically, to recovery of a fine grade coal by means of heavy media
suspens ions .
As a result of the increasing scarcity of raw materials and fuels,
S It Is desirable to recover as much uS~ble coal as is economically and
physlcally posslble.
As is well known, one of the chief diffLculties in using coal as
a fuel is the pollution that results as a consequence of the high sulfur
content in the coal. It has been found, however, that very fine coal is
relatively free of sulfur thus rendering the recovery of fine coal fractions
hlghly deslrable.
A number of processes have been developed for recoverlng flne
coal in the order of 0. 5 mm or smaller. A suchllke process Is froth
flotatlon, whlch Is, however, not or less sultable for some types of coal.
For Instance, oxldlzed coal cannot efflclently be recovered by froth
flotatlon. A more sultable process Is then a separatlon accordlng to
speclflc gravlty uslng llquld separatlng medla.
Apart from the Inltlal capital investment in constructlng a
separatlng process plant, one of the major cost factors in running such
plants resides In the consumptlon of fresh water needed to efficiently
carry out the process. Accordingly, the prlor art has endeavored to
reduce the consumption of fresh water required to successfully effect
coal washing and particle
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10~30157
separation. However, such processes have, in general, resulted
in an increase in equipment cost per unit quantity of coal pro- -
cessed per unit of time as a consequence of the necessity of
clarifying or purifying the water used in the process.
Disclosures representative of the prior art in this
field are Canadian Patents Nos. 506,145 of September 28, 1954
(Fontein), 551,737 of ~anuary 1, 1958 (Marot), 606,220 of
October 4, 1960 (Leeman) and 652,359 of November 13, 1962
(Leeman).
It is an object of the present invention to provide
a coal cleaning and recovery process which effects a substantial
saving in the quantity of water required in the process. Further,
it is an object of the present invention to provide a separating
process capable of recovering extremely fine particles of coal ;
as well as for providing a process that has an inherent flexibil-
ity in selecting the level of discrimination among the particles
of coal and refuse to be recovered.
In the following "raw fine coal" denotes raw coal having
a particle size of no more than about 12.5 mm. Raw coal is a
mixture of clean coal and refuse (shale). Raw fine coal usually
is a minus 12.5 mm size fraction obtained by screening from raw
run-of-mine-coal. This raw fine coal and the remaining coarser
raw coal are usually cleaned separately to obtain coal of higher
purity.
In summary, the invention provides a continuous process
for recovering clean coal from raw fine coal having a particle
size of no more than about 12.5 mm and comprising moderately fine -
coal particles having a particle size between about 0.5 mm and
about 12.5 mm, moderately fine refuse particles having a particle
size between about 0.5 mm and about 12.5 mm, finer coal particles
having a particle size between about 0.1 mm and about 0.5 mm,
finer refuse particles having a particle size between about
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~)81)~S~
0.1 mm and about 0.5 mm and finest particles having particle
size beIow about 0.1 mm with the aid of a heavy medium comprising
magnetizable particles as weighting material comprising the
steps of (a) mixing in a mixing tank said raw fine coal with
said heavy medium and feeding the resulting mixture to a first
hydrocyclone separator means to divide said mixture into a speci-
fically heavy fraction containing the bulk of the said moderately
fine refuse particles and a specifically light fraction contain-
ing the bulk of the said moderately fine coal particles, (b)
feeding said specifically light fraction over a first draining
screen means and a first washing screen means and said specifi-
cally heavy fraction over a second draining screen means and a
second washing screen means while spraying rinsing water onto
said fractions on each said washing screen means and recovering
a washed moderately fine coal fraction from said specifically
light fraction and a washed moderately fine refuse fraction from
said specifically heavy fraction and undiluted suspensions con-
taining finer and finest particles from said first and second
draining screen means, and dilute suspensions containing finer
and finest particles from said first and second washing screen
means, (c) feeding said undiluted suspensions and heavy medium
to a first collecting tank, splitting the mixture collected in
said collecting tank into two streams, recycling one of said
streams to said mixing tank and feeding the other stream to a
secondary hydrocyclone separator means to separate said other
stream into a secondary specifically heavy fraction containing
the bulk of the said finer refuse particles and a secondary speci-
fically light fraction containing the bulk of the said finer coal
particles, (d) diluting said secondary specifically light fraction
with said fine dilute suspension derived from said first washing
screen means and diluting said secondary specifically heavy
fraction with said dilute suspension derived from said second
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108V157
washing screen means, (e~ recovering said magnetic particles
from said diluted secondary specifically light fraction in
first recovering means and recovering said magnetic particles
from said diluted secondary specifically heavy fraction in second
recovering means.
For better recovering of the magnetic material, in the
process according to the invention said recovering of said
magnetic particles from said diluted secondary specifically
light fraction in first recovering means is preferably carried
out by separating said fraction into a tertiary relatively coarse
and a tertiary relatively fine light fraction and feeding said
tertiary relatively coarse and relatively fine light fractions
to first and second magnetic separators respectively, and said
recovering of said magnetic particles from said diluted secondary
specifically heavy fraction in second recovering means is
carried out by separating said fraction into a tertiary relative-
ly coarse and a tertiary relatively fine heavy fraction and
feeding said tertiary relatively coarse and relatively fine
heavy fractions to third and fourth magnetic separators respect-
ively, and a further step is included comprising feeding atleast a portion of said diluted tertiary relatively fine light
fraction from which said magnetic particles have been substan-
tially recovered to a second collecting tank for further use
in the process. Preferably liquid suspension collected in this
second collecting tank is fed upstream of said second recovering
means to further dilute said diluted secondary heavy fraction.
With this arrangement, the necessity of using large quantities
of either clarified or fresh water in the recovery of the
magnetic particles at this stage of the process can be entirely
eliminated without any significant reduction in the proportion
of magnetic particles recovered.
Advantages of the invention will become apparent as
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108V157
consideration is given to the following detailed description
of an embodiment of the invention and accompanying drawing in `~
which:
Figure lA and B is a schematic illustration of the
various stages of a process according to the present invention
wherein the lines represent conduits and the arrows the
direction of flow in each conduit.
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10801S7
Referrir.g to Figure lA, raw coal which is a mixture of
coal, shale and various refuse particles which has first been
screened to remove all particles larger than e.g. approximately
a half inch in size is introduced into a mixing tank 10 together
with a suspension of magnetizable particles which may include
recycled coal and refuse elemer.ts at 12. The mixing tank 10 is
provided with an overflow 14 to provide a constant head of pressure
of e.g. 4.5 m of liquid column, but that can be selected in the
rar,ge from 3 m to 6 m of liquid column.
From the outlet of the mixing tank 10, the mixture which
is called a heavy media mixture is fed to a first separating stage
16 which may consist of a hydrocyclone or a plurality of hydro-
cyclones in parallel which function to separate the mixture
according to the density of the particles. Thus, a relatively
ls light fraction will be introduced into the collecting tank 13,
whereas a relatively heavy fraction will be introduced into the
collecting tank 20.
The relatively heavy and light fractions are then
passed tangentially to sieve bends 22 and 24 which, preferably, -;
have a fairly large radius of curvature of e.g. about 1000 mm
and slot widths o~ approximately 1 mm so that only particles
smaller than approximately O.S mm will pass therethrough.
At this jur,cture, it should be noted that since for
practical purposes the input to the first separating stage is
' 10~0~7 ,, ~.
conventionally large in volume, the hydrocyclones used in the
first separating stage should all be of the low pressure heavy
media type which are unable to discriminate between particles of
less than about 0.5 mm in size. ~he particles that do not pass
: 5 through the slots of the sieve bends 22 and 24 are passed to
drair,ir,g screens 23 and 25 and rinsing screens 26 and 28
respectively where the fractions are first drained and then
; rinsed off with clarified water from nozzles 30 and by fresh
water from the nozzles 32. Thus, clean coarse coal is recovered at
34 and clean coarse refuse is recovered at 36. ;
The drainings from sieve bends 22 and 24 and the draining `-
screens 23 and 25 which comprise the major portion of the heavy
medium and unseparated fine particles are collected and fed to
a tank 38 via a specific gravity controller 40 which also receives
lS the suspension overflowing at 14 from the mixing tank 10 and
recovered heavy media from a subsequent stage of the process.
From tar.k 38, a portion of the heavy media containing
magnetite ar.d unseparated fine particles is fed back through
pump 42 to a collecting tank 44 whère the medium is re-introduced
for mixture with the incoming raw coal.
The remaining portion of the heavy medium from tank 38
is fed via pump 46 to a secondary separating stage which may
comprise a plurality of hydrocyclones in parallel as at 48 which
should be of the high pressure type so as to be able to separate
particles smaller thar. about 0.5 mm. The operating pressure of
these hydrocyclones is preferably selected in the range from 8 to
~()801S~7 ~
15 m of liquid column. The hydrocyclone 48 will separate the
incoming suspension of particles ir. heavy media into a relatively
light fraction which is collected in tank 50 and which consists
-~' mainly of coal and heavy media and a relatively heavy fractior,
which is collected in tank 52 which consists primarily of refuse
and heavy media.
According to the present invention, the fractions from
the secondary separating stage 48 are then diluted with the dilute
suspensionsderived at 27 and 29 from the rinsing screens 26 and
28. Thus, the dilute suspension from the coarse light fraction is
introduced at 54 and the dilute suspension from the coarse heavy
fractionlis introduced at 56. Additionally, the light f'raction is '~
further diluted with clarified water as at 58, whereas, according to a
preferred embodiment of the present invention the heavy fraction is
further diluted by liquid as at 60 which originates on the light
fraction side of the secondary separating stage, as will be ex-
plained hereinafter.
Ir order to recover the magnetic particles and separate
them from the fine coal and refuse fractions, the suspensions in
collecting tanks 50 and 52 are I'ed tan~entially throu~h one or
more rapped sieve bends 62 and 64 for -the ]ight fractions composed
of coal and magnetic particles and 66 and 68 for the heavy fraction
composed of refuse and magnetic particles. 'I`he drainings from
the sieve bends 62 and 64 are fed to a first series of magnetic
separators 72 ar,d 74 while the relatively coarser particles are
fed to a second series of magnetic separators 76 and 78. The
magnetic particles are collected at 80 and 82 and fed by
l~Olg7
suitable conduits back to the tank 38 via the specific gravity
controller 40 either directly or through a splitter box 84. The
splitter box 84 is lir,ked by conduits to acorltinuous cycling
storage facility consisting of a tank 86 and conduits 88 and 90
so that a sufficient quantity of magnetic particles in suspension
will be available for maintaining the specific gravity of the
heavy media at the required value. The operation of such storage
facilities is well understood in the art and will therefore not
be further described.
Returning now to the ref`use side of the secondary
separating stage 48, as previoUsly noted, the heavy fraction from
the apex outlet of the hydrocyclone 48 is collected in tank 52
where it is mixed with the dilute suspension as at 56 derived at
29 from the rinsing screen 28. I`he contents of the collecting
tank 52 are contir,uously fed to the series of rapped sieve bends
66 and 68 with the drainings from the sieve bends 66 and 68 being
fed to a third series of magnetic separators 92 and 94 while the
coarser particles are mixed ~ith clarified li4uid as clt 91 and 93
and are then fed to a fourth series of magne-tic separators 96 and
98.
In an alternative embodiment of the present invention the
plurality of magnetic separators on the coal and refuse sides,
may each be a common magnetic separator having different sections
for the different f`ractions. For example, the magnetic separators
at 72 and 76 may be replaced by a common magnetic separator having
two separating sections.
~ ~o80~7
Just as with the light fraction magnetic separators 72 through 78,
the magnetic particles are recovered from the magnetic separators 92
through 98 for the re-use in the process by being combined with the
magnetic particles recovered from the magnetic separators 72 through 78.
Preferably according to the present invention, the suspensions
recovered from the magnetic separators 72 and 74 which includes the
Elnest coal and shale fractlons below e.g. 0.1 mm is fed vLa conduit
100 directly to a collectlng tank 102. Whereas the clean coal recovered
from the magnetic separators 76 and 78 is fed first to a centrifuge 104
(Flgure lB) via conduit 106. The centrifuge 104 recovers clean coal at
108 which w ill be in the order of 0.5-0.1 mm in size . The liquLd recovered
from the centrlfuge 104 is fed back through conduit 110 to the tank 102.
From tank 102, the relatively clean liquLd which is commonly
water is pumped vla pump 112 back to collecting tank 52 to be used as
a dllutlon medlum to facllltate separatLon of the magnetlc partlcles from
the refuse coming from the apex of the hydrocyclone stage 48. Thus, the
necessity for employing eLther clarified or fresh water at this stage is
ellmlnated whlch wlll result in a large conservation of water. For example,
where the process Is designed ts accomodate approximately 300 tons per
hour of raw coal, a saving of approximately 1,300 gallons per minute
can be effected. This is achievable by virtue of the feature of the present
inventlon that the liquid collected In tank 102 will have an essentially
negligible portion of clean coal therein as a result of the preceding steps
of the process. As Indicated in broken lines, from the tank 102 the liquid
can also be pumped via pump 112 and a conduit 136 to a thickener 124 to
separate~y recover a finestfraction relatLvely rich in coal at 134; the
conservation of water is then, however, much less.
Returning now to the refuse side of the secondary separating stage
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~Q~0157
48, the suspensions recovered from the drainings of Lhe sieve bends 66 and
68 after removal of the magnetic particles in magnetic separators 92 and 94,
may be fed directly to a pump tank 114 whereas the coarse particles from
the sieve bends 66 and 68 after removal of the magnetic particles in magnetic
separators 96 and 98 are fed to a centrifuge 116 which produces clean refuse
at 118 and water containing only the fines particles at 120. If it is desired
to remove such particles, this suspension may be combined with the
suspension derived from the magnetic separators 92 and 94 and then fed to
a thickener 122.
As is conventLonal, the thickener 122 is employed to remove the
flnes partLcles Ln the order of 0.1 mm or less .
Other types of apparatus may, of course, be employed such as flotation
tanks or filters. Water is recovered at tank 126 from the thickener 122, and
then pumped via pump 128 to a tank 130 where the water is stored for use in
the process previously described. The bottom product of the thickener is
discharged vLa conduit 132.
From the foregolng, it will be seen, that the process of the present
Lnvention will enable the recovery of the finest of coal partLcles whLch, as
prevLously noted, are substantLally free of sulphur thus renderLng such a
product desLrable for use
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10801S7 -
in many industrial applications. Further, thc opcration of tlte
process can be adjusted very simply to vary the separation dis-
crimination of the process while maintaining a substantial
saving in water consumption.
It will be obvious to those skilled in the art that
various modifications may be made in the process of the present ;:
invention without departing from the spirit and scope thereof
as defined in the appended claims.
"
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