Note: Descriptions are shown in the official language in which they were submitted.
CA 02378956 2002-O1-10
Method for the preparation of fine grain ores
The present invention relates to a process for the
beneficiation of fine-grained ore, they ore being
subjected to gas classification by means of a gas, in
which classification two fractions are formed, namely a
coarse fraction and a first fine fraction, the coarse
fraction being separated out and the first fine
fraction being entrained with the gas, then separated
out of the gas and at least for the most pert being fed
for processing together with the coarse fraction, and
to an installation for carrying out the process.
In fluidized-bed processes, for example those used for
the direct reduction of fine-grained ore, cyclones are
used to separate out and recycle the material which is
discharged with the fluidizing gas. ThES separation
capacity, i.e. the efficiency and the separation grain
size of the cyclones, is adversely affected by the
formation of caking and adhesion in the cyclone, with
the result that more material is discharc3ed from the
fluidized-bed reactor together with the fluidizing gas.
In the case of direct reduction of fine-grained ore,
for example iron ore, it is primarily at least
partially reduced fine ore dust which causes this
caking and adhesion. The resulting increased discharge
of material either has to be recorded as a loss or, in
the case of multistage processes carried out in
downstream fluidized-bed reactors, causes problems by
blocking the holes in the distributor ba;~es of these
fluidized-bed reactors.
The fine dust which causes these problems is formed
firstly through mechanical abrasion of i=he material
which is to be reduced in the fluidized-bec: process and
secondly by being introduced with the charge material.
Particularly when using fine iron ores, the proportion
of so-called adhesion and fine grains presents a
problem. On the one hand, this material is involved in
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the formation of caking and adhesion, anc3 secondly it
is discharged from the system and lost.
To dry fine iron ores, it is the state of the art to
use fluidized-bed dryers by means of which the charge
material is simultaneously separated into a coarse
fraction and a fine fraction. These material flows are
used to set the charge grain size in a controlled way.
A process of this type is known, for Example, from
AT-B-400 578. In this known process, fine ore is dried
with the aid of a hot drying gas which flows around the
ore particles of the fine ore, and the drying gas,
after it has flowed around the ore particles, is
cleaned, with entrained ore dust pari~icles being
removed. The ore dust particles are collected and are
admixed with the dried fine ore. The drying takes place
at the same time as gas classification of the fine ore
in the fluidized-bed process, the drying gas being
passed through the fine ore to form a fl.uidized bed,
and the velocity of the drying gas being seat at a level
at which ore dust particles which are smaller than a
preselected size are entrained. The ore dust particles
entrained by the drying gas are separated out,
collected and admixed to the dried fine ore in metered
quantities. When using the ore which has been treated
in this way in a fluidized-bed direct reduction
process, the fine ore may under certain circumstances
cause problems by, in an at least partially reduced
state, leading to caking and adhesion in trLe fluidized-
bed reactor.
US-A-3 917 480 has disclosed a process for the
beneficiation of particulate material for use in
fluidized-bed reactors, in which, in a first process
step, 20-70% of the fine fraction of the material is
separated out and the remaining fraction is introduced
into a fluidized bed, a further fine fr~~ction being
separated out in the fluidized bed, di;~charged and
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admixed with the fine fraction obtained in the first
process step. The fine fractions are pelletized and
added to the fluidized bed. A drawback of this process
is that at least a not inconsiderable fine fraction
enters the fluidized bed, where it can caL~.se the above-
mentioned problems, such as adhesion and c;~king.
DE-A-197 11 629 shows a process for the pretreatment of
fine ores having a wide grain size range for direct
reduction as known, in which the fine ores belonging to
the grain size fraction of less than 6.3 mm and
preferably less than 3 mm are dried by means of a hot-
air or flue-gas flow during a pneumatic conveying
operation, if appropriate with the moist material being
recycled, the 6.3 mm or 3 mm down to approx. 0.04 mm
fraction being screened out of the dried fine ore and
being fed for direct reduction. The fine ore fraction
of a size which is smaller than approx. 0.04 mm and is
discharged from the cyclone connected down:~tream of the
dryer together with the drying air or t:he flue gas,
passes for ultrafine separation into a multicyclone.
The fine ore which is separated out there is fed to a
pelletizing device and is shaped into pellets, with
water and binder being supplied, these pellets
ultimately being added to the dryer via. a delivery
device. The multicyclone which is provided for the
ultrafine separation is an extremely complex apparatus.
The present invention is based on the object of
providing a process for the beneficiation of fine-
grained ore and an installation for carrying out the
process, according to which the ore is ber..eficiated in
such a manner that, when the ore is subsequently used
in a fluidized-bed reactor, the abovementioned
problems, such as caking and adhesion in a cyclone and
a fluidized-bed reactor, are avoided. The process
should in particular be easy to carry out u~~ithout major
outlay on equipment.
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According to the invention, this object is achieved, in
a process of the type described in the introduction, by
the fact that the first fine fraction is subjected to a
further gas classification by means of a c_~as, in which
a second fine fraction and an ultrafine fraction are
formed, and the second fine fraction is fed for
processing together with the coarse fraction and the
ultrafine fraction is entrained with the gas and is
then separated out of the gas.
The invention is based on the discovery that the
ultrafine fraction of the ore, in tr.e at least
partially reduced state, is principally re;~ponsible for
caking and adhesion in fluidized-bed reactors.
Furthermore, it has been discovered that triis ultrafine
fraction adheres to the first fine fracti~an, which is
separated from the charge material by means of the
first gas classification. After reduction, i.e. in the
at least partially reduced state, during the processing
of the second fine fraction together with the coarse
fraction, the ultrafine fraction, as is known, for
example, from the abovementioned AT-B-400 578, causes
the abovementioned problems in the fluidized-bed
reactor. According to the invention, this is avoided by
separating the ultrafine fraction out of the first fine
fraction.
Preferably, the ultrafine fraction which has been
separated out of the gas is admixed with a binder,
granulated and fed for processing or discharged. In the
case of granulation and further procESSSing, the
ultrafine fraction is advantageously not lost to
further processes.
Expediently, granules which are formed from the
ultrafine fraction are processed further together with
the coarse fraction and the second fine fraction. By
way of example, the further processing involves direct
reduction downstream of the ore beneficiaticn.
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However, according to a further preferred embodiment,
granules which are formed from the ultra.fine fraction
can also be admixed with the fine-grained ore which is
to be subjected to the first gas classification.
Expediently, gas from which the entrained ultrafine
fraction has been removed is subjected to further
cleaning, in which an extremely ultrafins: fraction is
separated out, and this fraction, preferably together
with the ultrafine fraction, is granulated and fed for
processing or discharged. As a result, it is also
possible to substantially utilize the dust fractions
which are still present in the gas after the ultrafine
fraction has been separated out.
According to a preferred embodiment, at least a partial
quantity of the second fine fraction is likewise
granulated, preferably together with the ultrafine
fraction and/or extremely ultrafine fraction. In this
way, problems with caking and adhesion of ore in dust
form in subsequent processes are particul;~rly reliably
avoided.
In the process according to the invention, the first
fine fraction is separated out of the fin.=-grained ore
by means of the first gas classification preferably
with a particle size of up to 150 ~zm, and the ultrafine
fraction is separated out of the first fine fraction
with a particle size of up to 20 um by means of the
further gas classification.
According to a preferred embodiment of the process
according to the invention, a drying gay is used at
least in the first gas classification. In this case,
drying takes place at the same time as the gas
classification of the charge material.
The binder used for the granulation is advantageously
calcined lime or bentonite.
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Preferably, gas which is used in th~~ first gas
classification and from which the first Fine fraction
has been removed is used for the further gas
classification.
An installation for carrying out the process according
to the invention, having a first gas classifier,
provided with a feed for fine-grained ore, a gas feed
line, an outlet line for a coarse fraction and an
outlet line for gas and a first fine fraction entrained
with the gas, and having a first gas-cleaning device
which is connected downstream of the first gas
classifier and separates the first fine fr<~ction out of
the gas, is characterized in that an outlet line for
the f first f fine fraction leading out of the f first gas-
cleaning device has a line connection to a further gas
classifier, which has a gas feed line, an outlet line
for a second fine fraction and an outlet line for gas
and an ultrafine fraction entrained with the gas, and
in that a second gas-cleaning device, which separates
the ultrafine fraction out of the gas, is connected
downstream of the further gas classifier.
Preferably, an outlet line for the ultrafine fraction
leading out of the second gas-cleaning device has a
line connection to a granulation apparatus, a feed line
for a binder opening into the granulation a;oparatus.
According to a further preferred embodiment, the
granulation apparatus has a line connection to the feed
for feeding fine-grained ore to the first gas
classifier.
Advantageously, a common discharge apparatus is
provided for the coarse fraction, the second fine
fraction and the ultrafine fraction which is subjected
to granulation.
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The gas-cleaning devices are expediently designed as
cyclones.
Preferably, a further gas-cleaning device for
separating out an extremely ultrafine frG.ction, which
is entrained with the gas, is connected downstream of
the second gas-cleaning device, an outlet line for the
extremely ultrafine fraction leading out of. the further
gas-cleaning device having a line connection to a
granulation apparatus.
According to a further preferred embodiment of the
installation according to the invention, the outlet
line for the second fine fraction leading out of the
further gas classifier has a line connection to a
granulation apparatus.
Expediently, at least the first gas c:_assifier is
designed as a dryer, a feed line for drying gas opening
into the gas classifier.
Advantageously, a gas outlet line leading out of the
first gas-cleaning device has a line connection to the
gas feed line leading to the further gas cl;~ssifier.
The invention is explained in more detai7_ below with
reference to the drawing and on the basis of an
exemplary embodiment. The figure diagrammatically
depicts a flow diagram for a preferred e~:nbodiment of
the invention.
The figure illustrates a gas classifier 1, which is
designed as a fluidized-bed unit and into which fine-
grained ore 2 is introduced via a feed 3. The fine-
grained ore 2, which forms a bed 4 in the gas
classifier 1, is fluidized and subjected to gas
classification by means of a gas supplied via a feed
line 5. In the process, it is separated into a coarse
fraction 6, which is discharged via an outlet line 7,
CA 02378956 2002-O1-10
and a first fine fraction 8, which is entrained with
the gas.
In the exemplary embodiment shown, the fine-grained
ore 2 used has a grain size of from 0 to 12 mm. It is
separated into the coarse fraction 6, with a grain size
of from 0.15 to 12 mm, and the first fine fraction 8,
with a grain size of from 0 to 0.15 mm, by gas
classification. The gas used is a drying gas, the fine-
grained ore 2 being subjected to drying as well as gas
classification.
The coarse fraction 6 which is discharged via the
outlet line 7 is applied to a discharge: apparatus,
which is designed as a conveyor belt 9, and is fed for
further processing, for example direct reduction. The
first fine fraction 8 is discharged from the gas
classifier 1 together with the gas via an outlet
line 10 and is separated out of the gas. An impingement
separator 11 and a cyclone 12 are used for this
purpose. The fine fraction 8 which has bean separated
out passes into a storage hopper 13 and, from there,
via a line 14, into a further gas classifier 15, which
is likewise designed as a fluidized-bed unit. In the
further gas classifier 15, the first fine' fraction 8
forms a bed 16. The gas used is gas from which the
first fine fraction 8 has been removed by means of the
cyclone 12 and which is supplied via a feed line 17.
In the further gas classifier 15, the first fine
fraction 8, with a grain size of from 0 to 0.15 mm, is
separated into a second fine fraction 18, with a grain
size of from 20 to 150 um, and an ultrafine
fraction 19, with a grain size of from 0 to 20 um. The
second fine fraction 18 is discharged from the further
gas classifier 15 via an outlet line 20, i;~ applied to
the conveyor belt 9 and is fed for further processing
together with the coarse fraction 6.
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The gas and the ultrafine fraction 19 which is
entrained with the gas are discharged from the further
gas classifier 15 via an outlet line 21. ,The gas is
cleaned by means of a cyclone 22, in which the
ultrafine fraction 19 is separated out.
The ultrafine fraction 19 passes into a storage
hopper 23 and, from there, to a granulation
apparatus 24. A feed line 25 for a b:_nder 26 for
granulating the ultrafine fraction 19 opens into the
granulation apparatus 24. The binder 26 used in the
exemplary embodiment shown is calcined lime or
bentonite.
The ultrafine fraction 19 is granulatE~d to form
granules 27 with a grain size of from 0.5 to 4 mm, and
the granules 27 are likewise applied to the conveyor
belt 9. They are fed for further processing together
with the coarse fraction 6 and the second fine
fraction 18.
However, it is also possible for the granules 27 to be
fed to the first gas classifier 1 via a line (not shown
in more detail in the figure) and to be introduced into
this classifier together with the fine-grained ore 2.
According to a further preferred embodiment, the
ultrafine fraction 19 is not fed to the granulation
apparatus 24 but rather, as indicated by dashed line 28
in the figure, is discharged and fed, for example, for
landfill. The advantage of this variant is that the
step of granulating the ultrafine fraction 19, which in
quantitative terms forms a relatively small proportion
of the overall charge of fine-grained ore 2, is
dispensed with, yet at the same time the ultrafine
fraction 19 is prevented from causing disruption in
subsequent processing steps, in particular from causing
caking and adhesion. The small quantity of charge
material lost is of only subordinate imporvance if ore
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prices are low.
In the exemplary embodiment shown, the gas which has
been cleaned by means of the cyclone 22 is fed to a
further gas-cleaning device 29 which may, for example,
be an electrostatic filter. By means of the further
gas-cleaning device 29, an extremel~r ultrafine
fraction 30, which is still entrainf~d following
cleaning in the cyclone 22, is separated out of the gas
and is likewise fed to the granulation apparatus 24 and
granulated together with the ultrafine fraction 19.
However, it is also equally possible for i~he extremely
ultrafine fraction 30 to be discharged and landfilled,
as indicated by dashed line 28 in the figure.
The second fine fraction 18 which is di:>charged from
the second gas classifier 15 and is not entrained with
the gas can at least in part be fed, via a line 31
which is indicated in dashed lines in the figure, to
the granulation apparatus 24, where it i.s granulated
together with the ultrafine fraction 19 and the
extremely ultrafine fraction 30 and is fed for further
processing.
The invention is to be explained in even mere detail on
the basis of the following exemplary embodiment:
Fine-grained ore 2 with a grain size of from 0 to
12 mm, which represents 100% of the charge material, is
introduced into the first gas classifier 1 and is
separated into a coarse fraction 6, with a grain size
of from 0.15 to 12 mm, and a first fine' fraction 8,
with a grain size of from 0 to 0.15 mm. The coarse
fraction 6 makes up 67% of the charge.
The first fine fraction, which makes u~ 33% of the
charge, is introduced into the further gas
classifier 15 and is separated into a second fine
fraction 18, with a grain size of from ~;0 to 150 um,
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and an ultrafine fraction 19, with a grain size of from
0 to 20 um. The second fine fraction 18 makes up 29% of
the fine-grained ore 2 which is used, and t:he ultrafine
fraction 19 makes up 4% of the charge material.
Electrostatic filters are used to separate an extremely
ultrafine fraction 30 with a grain size of up to 1 um
out of the gas from which the ultrafine fraction 19 has
been removed.
The invention is not restricted to th.e exemplary
embodiment given above. Naturally, there is
considerable scope for variations with regard to the
grain size of the material used and with regard to the
grain size of the fractions which are separated out and
their quantitative proportions in the material used.
