Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LAYERED AGGLOMERATED IRON ORE PELLETS
AND BALLS
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to agglomerated ores and, more particularly, to
layered agglomerated iron ore pellets and balls. The present invention also
relates to a method of producing same.
2) Description of the Prior Art
An important proportion of iron oxides used for reduction (ironmaking) are
used in
the shape of a pellet. The pellets are manufactured by mixing iron oxide
concentrates, additives required by the client, and one or several binders.
The
iron oxide concentrates typically contain goethite (Fe0(OH)), hematite
(Fe203),
and/or magnetite (Fe304) and usually a small portion of silica (Si02) as an
impurity. Additives such as fluxes, binders and internal fuel are typically
added to
the iron-oxide concentrate. Fluxes, such as CaO and MgO, are usually added to
obtain the desired slag during reduction. The binders, which can either be
mineral or organic, improve the adhesion of the pellet mixture. It is now
frequent
to add carbon as an internal fuel to facilitate pellet induration (or cooking)
by
improving the heat transfer towards the pellet core.
The agglomerated pellets are fired in order to obtain the necessary mechanical
properties for their handling and transportation to the oxide reduction and
iron or
steel making sites. The mechanical properties of the fired pellets are
evaluated,
among others, by their compression strength which is expressed in kilogram per
pellet (kg/pellet). An efficient pellet firing is targeted at this step.
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However, the gas diffusion towards the pellet core is a slow kinetic and
produces an
oxygen debt therein. Therefore, the carbon dioxide, which is the result of the
coke
oxidation, oxidizes the coke in the pellet core into carbon monoxide. The
carbon
monoxide reduces the hematite (Fe203) into secondary magnetite (Fe304) that is
later re-oxidized into secondary hematite. These unnecessary reactions reduce
the
process efficiency and increase the energy cost to indurate the pellets.
Therefore,
there is an economic incentive to optimize the pellet composition.
US patent No. 4,851,038 discloses a method to manufacture agglomerated and
fired
pellets. The pellets produced have a core including the iron ore and lime and
are
coated with coke powder as a solid fuel. However, coke powder is easily
removed
from the pellet surface when they enter into the furnace. In important
quantity, free
coke powder generates high risks of explosion.
US patent No. 4,504,306 discloses a method to produce iron oxide pellets
having a
two-layered structure with a core portion and a shell portion covering the
core
portion. The core portion contains between 0.3 to 1.0% by weight of carbon
while the
shell portion contains between 1.0 and 4.5% by weight of carbon.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide improved ore balls that
reduce the
induration energy costs while obtaining good quality fired pellets.
One object of the invention provides a method for producing layered iron ore
balls.
The method comprises: providing a first feed material containing a first iron-
oxide
concentrate, the first feed material being internal fuel additive free;
primarily
pelletizing, during a first residence time, the first feed material to form a
core portion;
providing a second feed material containing a second iron-oxide concentrate
and at
least one internal fuel additive; and secondary pelletizing, during a second
residence
time, the second feed material with the core portion to form a first
superficial layer
over the core portion.
The method can optionally further comprise at least one additional step
selected
amongst the group of steps comprising: screening the core portion before
secondary
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pelletizing the second feed material with the core portion to form the first
superficial
layer over the core portion and withdrawing at least one of the iron ore balls
coarser
than a first predetermined ball size and smaller than a second predetermined
ball
size; grinding the withdrawn iron-ore balls coarser that the first
predetermined
particle size to obtain a grinded recycled feed material and mixing the
grinded
recycled feed material with the first feed material; pelletizing the withdrawn
core
portions smaller than the second predetermined particle size with the first
feed
material; mixing at least one binder with at least one of the first feed
material and the
second feed material; mixing at least one fluxing agent with at least one of
the first
feed material and the second feed material; firing the layered iron ore balls
to
obtained fired pellets; and providing a third feed material and tertiary
pelletizing the
third feed material with one of the core portion and the core portion covered
with the
first superficial layer.
The at least one internal fuel additive can be added to the second feed
material can
be in an amount ranging 1.5 and 15 wt%.
Another object of the invention provides a layered iron ore ball comprising: a
core
portion containing a first iron-oxide concentrate, the core portion being
substantially
internal fuel additive free; and a shell portion covering the core portion,
the shell
portion containing a second iron-oxide concentrate and at least one internal
fuel
additive added to the second iron-oxide concentrate.
The core portion is preferably agglomerated on a first balling device and the
shell
portion is agglomerated over the core portion on a second balling device. The
at
least one internal fuel additive preferably comprises carbon and the carbon
concentration in the shell portion is preferably between 1.5 and 15 wt% and,
more
preferably, between 1.5 and 10 wt%.
The shell portion has preferably a thickness ranging between 250 and 3000 pm
and,
more preferably, ranging between 500 and 1000 pm. The volume of the core
portion
is preferably at least 60% of the volume of the iron ore ball.
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A further object of the invention provides iron ore pellets resulting from an
induration
process applied on the layered agglomerated iron ore balls as described above.
The
iron pellets thus obtained preferably have a cold compressive strength (CCS)
above
350 kg/pellet.
In the specification, the term "ball" refers to the agglomerated material
before its
induration while the term "pellet" refers to the same agglomerated material
after its
induration. The term "layered pellet" is used to designate a pellet
originating from
layered balls. The term "conventional pellet" is used to designate a pellet
originating
from a ball having the same internal fuel content in the shell and the core
portions.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will become apparent
from
the following detailed description, taken in combination with the appended
drawings,
in which:
Fig. 1. is a schematic view of a quarter of a conventional fired pellet
representing the
proportion of secondary hematite and secondary magnetite inside the fired
pellet;
Fig. 2 includes Figs. 2A, 2B, 2C and 2D, Figs. 2A and 2B are two micrographs
of a
conventional fired pellet and Figs. 20 and 2D are two schematic views of the
fired
pellet showing respectively where the micrographs of Figs. 2A and 2B were
taken;
Fig. 3 is a schematic flow sheet of a process for the production of layered
balls in
accordance with an embodiment of the invention;
Fig. 4 includes Figs. 4A, 4B, 4C and 4D, Figs. 4A and 4B are two micrographs
of a
layered fired pellet and Figs. 4C and 4D are two schematic views of the fired
pellet
showing respectively where the micrographs of Figs. 4A and 4B were taken;
Fig. 5 includes Figs. 5A, 5B, 5C and 5D, Fig. 5A is a micrograph, taken in the
shell
portion, of a conventional fired pellet having a low silica content, Fig. 5B
is a
micrograph, taken in the core portion, of the conventional fired pellet having
a low
silica content, Fig. 50 is a micrograph, taken in the shell portion, of a
layered fired
pellet having a low silica content, and Fig. 5D is a micrograph, taken in the
core
portion, of the layered fired pellet having a low silica content;
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Fig. 6 includes Figs. 6A, 6B, 6C and 6D, Fig. 6A is a micrograph, taken in the
shell
portion, of a conventional fired limestone pellet, Fig. 6B is a micrograph,
taken in the
core portion, of the conventional fired limestone pellet, Fig. 6C is a
micrograph, taken
in the shell portion, of a layered fired limestone pellet, and Fig. 6D is a
micrograph,
taken in the core portion, of the layered fired limestone pellet;
Fig. 7 is a graph representing the cold compressive strength (CCS) of low-
silica
pellets having a variable coke content; and
Fig. 8 is a graph representing the cold compressive strength of limestone
pellets
having a variable coke content.
It will be noted that throughout the appended drawings, like features are
identified by
like reference numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An important proportion of the iron oxides that are used for ironmaking are
used in a
pellet shape. To manufacture pellets, a fine iron ore concentrate is first
agglomerated
on one or several balling devices (disk, drum or any equipment allowing ball
agglomeration) and the agglomerated balls are fired in an induration furnace
to
increase their mechanical properties such as their cold compression strength
(CCS),
which is expressed in kilogram per pellet (kg/pellet).
Iron ore concentrates usually contain goethite (Fe0(OH)), hematite (Fe203),
and/or
magnetite (Fe304) and usually a small portion of silica (Si02) as an impurity.
Additives such as binders, solid fuels (internal fuel or carburant), and.
fluxes are
typically added at the agglomeration step (pelletization step). The
concentration of
each additive varies according to the user's needs.
The binders, which can either be mineral or organic, improve the adhesion of
the ball
mixture. It is now frequent to add an internal fuel to facilitate pellet
induration by
improving the heat transfer towards the ball core. The internal fuel is either
added as
coke, low temperature coke, pulverized coal, petroleum coke or anthracite.
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The agglomerated balls are usually fired in a moving grate furnace or a grate
kiln
wherein they are first dried to remove their water content. The agglomerated
balls
are then indurated to create physical links between the particles and
consequently
increase their mechanical properties. Finally the fired pellets are cooled
down to
recover their energy content and to obtain pellets at a suitable temperature
for
subsequent handling. Several chemical reactions occur during the induration
process
such as the solid fuel combustion, the oxidation of magnetite, if any, and the
calcination of fluxes.
Referring to FIG. 1, it will be seen that a pellet 20 can be divided, for the
gas
behavior, into two zones: an advection zone 24 and a diffusion zone 26. The
advection zone 24 is a superficial layer of the pellet 20 wherein the air is
continuously replaced without having recourse to diffusion phenomena. The
thickness of the advection zone 24 can vary but is usually between 250 and
3000
j_tm. The diffusion zone 26 is located inside the pellet 20 and the air
circulates
through diffusion therein. The diffusion kinetic is faster proximate to the
advection
zone 24 and slower proximate to core of the pellet 20.
When the agglomerated balls have a uniform composition, FIGS. 1 and 2 show
that
the fired pellets include more secondary magnetite sites in the diffusion zone
26 than
in the advection zone 24. Secondary magnetite is formed when combustion is
incomplete due to an oxygen debt.
At the beginning of the induration process, an oxygen debt occurs in the
diffusion
zone 26 since the gas diffusion towards the core of the pellet 20 is a slow
kinetic.
Therefore, the carbon dioxide, which is the result of the coke combustion
and/or the
calcination of the flux additives, oxidizes the coke contained in the core of
the pellet
20 into carbon monoxide:
CO2 (g) (from coke combustion) + C(s) --> 2 CO(g). (1)
The carbon monoxide reduces the hematite (Fe203) into secondary magnetite
(Fe304):
3 Fe2O3(s) + CO(g) (from coke oxidation by CO2) --> 2 Fe304(5) + CO2(g).
(2)
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An important proportion of the secondary magnetite is later reoxidized into
secondary hematite:
2 Fe304(s) + 112 02(g) ----> 3 Fe203(s) (3)
These unnecessary reactions reduce the process efficiency and increase the
energy
costs to indurate the balls. Therefore, there is an economic incentive to
optimize the
ball composition.
In accordance with an embodiment, agglomerated balls are manufactured without
internal fuel in the diffusion zone 26 and, therefore, only the advection zone
24 has
internal fuel in its composition. The internal fuel in the advection zone 24
can be
coke, half-coke, pulverized coal, petroleum coke and/or anthracite.
The internal fuel is therefore rapidly consumed, solely by a complete
combustion
reaction. Substantially no residual carbon monoxide is formed and thus
substantially
no secondary magnetite. The time required to form the secondary magnetite and
to
reoxidize the later into secondary hematite is eliminated and it allows, among
others,
to increase the process productivity or modify the induration cycle to save
fossil
energy used at the burners (or a combination of both).
To manufacture layered pellets with a core portion 30 and a shell portion 32,
or a
superficial layer, (see FIG. 4), the balls are agglomerated in at least two
agglomeration steps since the core portion 30 and the shell portion 32 have a
different fuel content. A first agglomeration step produces the core portion
30 and a
second agglomeration step agglomerates the shell portion 32 over the core
portion
30.
Balls are usually agglomerated on one or several balling devices such as
balling
disks and balling drums. Since the pellet manufacturers usually have
predetermined
specifications for the pellet granulometry, largest and smallest balls are
rejected. The
largest balls are grinded and the grinded particles are returned with the
finest balls
as a feed material to a balling device which can be the same or a different
one than
the balling device(s) used for the agglomeration of the grinded feed material.
The
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smallest ball can be either grinded or sent to a balling device. Usually an
important
proportion of the agglomerated balls are rejected.
It is preferable to use at least one balling device for each agglomeration
step since
the core portion 30 does not contain internal fuel. Moreover, the rejected
balls of
each agglomeration step can be grinded separately and sent to feed at least
one
balling device of the appropriate agglomeration step. For example, the balls
rejected
after the agglomeration of the core portion 30 are fine balls and coarse
balls. The
coarse balls are grinded and the grinded particles are fed with the fine balls
60 (FIG.
3) to at least one balling device that agglomerates the core portion 30. The
additives
(fluxes, binders, and internal fuels) contents can be adjusted in the feed
material
depending on the level and the content of the recirculated material. The same
can be
done during the second balling step where fine layered balls (the core portion
32 with
an outer layer of carbon rich material) and coarse layered balls can be
recirculated,
preferably only into the feed of the second balling or agglomeration step.
Example
Several processes can be designed to produce layered balls. An example of a
manufacturing process for two layered balls is now described referring to FIG.
3. In
the first agglomeration step, an iron oxide concentrate provided from a first
iron oxide
concentrate bin 40 is mixed in a first mixer 42 with recirculated mineral
provided from
a first recirculation bin 44, as will be explained in more details below. Iron-
oxide
concentrate from bin 40 and recirculated mineral from the bin 44 are mixed in
predetermined proportions to meet the ball specifications for the core portion
30.
Thereafter, the output of mixer 42 is mixed in a second mixer 46 with
additives, such
as binders and fluxes, provided from a first additive bin 48. Additives of the
first
additive bin 48 are substantially free of internal fuel since they are mixed
with the
iron oxide concentrate of bin 40 and the recirculated material of the bin 44
to form
the core portion 30 of the balls. The output of mixer 42 and the additives of
bin 48
are also mixed in predetermined proportions to meet the ball specifications.
One
skilled in the art will appreciate that the content of bins 40, 44 ,48 can be
mixed in a
single mixer. The output of the second mixer 46 forms a core portion feed
material
50. A first balling device 52 is fed with the feed material 50 to agglomerate
the core
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,
portions 53 of the two-layered balls 90. The core portions 53 are screened on
a first
screen 54 to withdraw coarse core portions 56 that have a diameter larger than
a
predetermined value. The remaining core portions 53 are screened on a second
screen 58 to withdraw fine core portions 60 characterized with a diameter
smaller
than a specified value. The coarse core portions 56 are recovered, grinded
(not
shown), and sent to the first recirculation bin 44 as a feed material for the
core
portion 30. One skilled in the art will appreciate that the fine core portions
60 can
also be grinded, either together or separately from the coarse core portions
56 or
sent directly to a bin or the balling device. The coarse and fine core
portions 56, 60
can be sent either to the same or a different bin. The number and the content
of the
bins 40, 44, 48 and the mixers 42, 46 can differ from the one shown in FIG. 1.
In the second agglomeration step, a shell portion 82 is added to the core
portion 30
having a diameter corresponding to the predetermined specifications. The core
portions 30 are sent to a second balling device 70. A second recirculation bin
64
containing recirculated material 68, as will be explained in more details
below, and a
second iron oxide bin 72 containing an iron oxide concentrate feed are mixed,
in
predetermined proportions in a primary mixer 74. The output of the primary
mixer 74,
the content of a second additive bin 76, and the content of a second internal
fuel bin
78 are mixed in predetermined proportions in a secondary mixer 80. As for the
first
agglomeration step, one skilled in the art will appreciate that the content of
bins 64,
72, 76 and 78 can be mixed in one or more mixers. The additive bin 76 contains
additives such as binders and fluxes and the internal fuel bin 78 contains
internal
fuels such as coke, half-coke, pulverized coal, petroleum coke, and
anthracite. The
content of the additive bin 76 and the fuel bin 78 can be contained in a
single bin as
one skilled in the art will appreciate.
The output of mixer 80 forms a feed material 82 that is agglomerated over the
core
portion 30 to form the two-layered balls 90. The feed material 82 contains a
mixture
of the iron oxide concentrate and the additives, including the internal fuel,
that
composes the shell portions 32. The shell portions 32 are agglomerated over
the
core portions 30 in the second balling device 70 producing two-layered balls
90 with
a size distribution. The two-layered balls 84 can be screened on a screen 88
to
withdraw the fine agglomerates 68 that have a diameter smaller than a
specified
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value. The fine agglomerates 68 are recovered and are sent to the second
recirculation bin 64. Since the core portion 30 are substantially internal
fuel free, the
recirculated agglomerates 68 are preferably returned only to the second
agglomeration step. One skilled in the art will appreciate that the two-
layered balls 84
can also be screened on a first screen (not shown) to withdraw coarse two-
layered
balls that have a diameter larger than a predetermined value.
The two-layered balls 90 having a diameter, or size, that corresponds to the
specifications are recovered and sent to a storage bin 92 until they are fired
into
pellets in an induration furnace (not shown).
One skilled in the art will understand that each feed material such as the
iron oxide
concentrate, the additives, including the internal fuel, and the recirculated
material
can be contained in more than one bin. Each additive can be contained in its
own
bin. The mixing step before each agglomeration step can be carried out in any
number of mixers and/or in any mixing order of the feed material. Furthermore,
more
than one balling device can be used for each agglomeration step.
The core portion 30 and the shell portion 32 can contain different additives
or iron
oxide concentrates. For example, the shell portion 32 can contain olivine as
fluxing
agent while the core portion 30 can contain dolomite. Alternatively for iron-
oxide
concentrates containing magnetite, the core portion 30 can contain an iron
oxide
concentrate having a low magnetite content while the iron oxide concentrate of
the
shell portion 32 can have a high magnetite content. Furthermore, any additive
can be
added in different proportions in the core portion 30 and the shell portion
32. For
example, the core portion 30 can contain a low dolomite content comparatively
to the
shell portion 32.
The internal fuel concentration in the shell portion 32 can vary depending on
several
parameters. A carbon concentration in the shell portion 32 ranging between
approximately 1.5 and 15 wt% is adequate and, more preferably, between 1.5 and
10 wt%. Above 15 wt%, it is typically difficult to uniformly disperse the
internal fuel.
The core portion 30 represents typically between 60 and 80 % of the volume of
a
ball. Therefore, the residence time of the balls on the balling devices 70 of
the
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second agglomeration step is relatively short comparatively to the residence
time of
the balls on the balling devices 52 of the first agglomeration step.
Preferably, the thickness of the shell portion 32 corresponds substantially to
the
thickness of the advection zone 26 during the induration process.
The two-layered balls 84 withdrawn from the balling device 70 and/or the core
portion 53 do not have to necessarily be submitted to screening steps if it is
presumed that the balls 84 and/or the core portions 53 already meet the
specifications. As mentioned earlier, the core portion 30 represents usually
the most
important portion of the ball volume. If the core portions 30 are screened
adequately
after the first agglomeration step, the screening after the second
agglomeration is
optional since the residence time of the balls on the balling devices 70 of
the second
- agglomeration step is relatively short. Therefore, two-layered balls are
usually
produced with a narrow distribution of the ball size or granulometry, even
without a
screening step following the agglomeration of the shell portion. Obviously, no
screening are necessary after any agglomeration if there is no ball size
specifications.
The recirculated coarse material 56 of the first agglomeration step can be
grinded
and the grinded particles can be sent as a feed material to the second
agglomeration
step. On the opposite, the recirculated material 68 of the second
agglomeration step
should not be sent as a feed material to the first agglomeration step since it
contains
internal fuel while the core portion 30 substantially does not.
As people in the art will understand, the process can produce balls that have
more
than two layers. The process can include any number of agglomeration steps.
=
Referring simultaneously to FIGS. 2 and 4, it will be seen that the core
portion of the
fired two-layered pellets (FIG. 4D) contains less secondary magnetite than the
core
portion of a conventional pellet (FIG. 2D). The terms "conventional pellets"
or
"conventional balls" refer to pellets or balls having the same internal fuel
content in
the core portion 30 and in the shell portion 32. Moreover, the core and shell
portions
30, 32 of the fired two-layered pellets (FIGS. 4C and 4D) have similar
micrographs.
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On the opposite, the micrographs of the core and shell portions 30, 32 of the
conventional pellets (FIGS. 2C and 2D) differ.
As it will be seen with the following examples, the layered balls provide
important
energy reduction combined with an increase of the productivity of the
induration
process following the ball agglomeration. Moreover, it reduces the production
of
green house gases (GHG).
As people in the art will understand, the layered balls are preferably used by
pelletizing plants that add an internal fuel to their balls and indurate them
in a moving
grate induration furnace.
Example 2
The second example concerns the cold compressive strength (CCS) of fired
layered
pellets comparatively to conventional pellets having the same internal fuel
content in
the core portion 30 and in the shell portion 32. The CCS is a normalized index
to
measure the mechanical properties of the balls or pellets respectively before
or after
induration.
The pellets were pellets for blast furnaces (acid pellets) originating from
balls
containing approximately 5 wt% of silica, between 0.75 and 1.5 wt% of coke as
an
internal fuel, 0.6 wt% of CaO, 0.25 wt% of MgO, and substantially no magnetite
in
the iron oxide concentrate. Dolomite and limestone were added as fluxes.
Table 1 shows the results obtained for pellets wherein the core portion 30
represented 78% of the volume of the ball and the shell portion 32 represented
the
remaining 22%. The first pellet batch was conventional pellets having the same
coke
content in the shell and the core portions 32, 30. A CCS of 366 kg/pellet was
obtained for pellets originating from balls containing 1.5 wt% of coke.
The second pellet batch was two-layered balls having a total coke content of
1.5'
wt%. The core portion 30 did not contain coke and the coke content of the
shell
portion 32 was 6.82 wt%. A CCS of 517 kg/pellet was obtained, which is 1.4
times
higher than for conventional pellets.
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Since the CCS obtained was higher than the usual specifications, the total
coke
content was reduced for the third and the fourth pellet batches, which also
contained
two-layered pellets. The total coke content for the third and the fourth
pellet batches
were 1.0 wt% and 0.75 wt% respectively in the balls prior to induration. For
both
batches, the core portion 30 did not contain coke and the shell portion 32
contained
4.55 wt% coke for the third batch and 3.41 wt% for the fourth batch. CCS of
532
kg/pellet and 549 kg/pellet were obtained for the third and fourth batches
respectively.
Consequently, higher CCS than for conventional pellets can be obtained with
pellets
originating from layered balls having lower coke contents.
Table 1
Core / Shell (vol%) 78/22
Total coke added (wt%) 1.5 1.0 0.75
Coke in core/in shell (wt%) 1.5 / 1.5 0 / 6.82 0 / 4.55 0 / 3.41
CCS (kg/pellet) 366 517 532 549
Example 3
The third example is similar to the second one but acid pellets were tested.
Acid
pellets originate from balls containing approximately 5 wt% of silica, between
0.75
and 1.5 wt% of coke as an internal fuel, 1 wt% of CaO, and 0.33 wt% of MgO.
The
fluxes were added as limestone.
Table 2 shows the results obtained. The first pellet batch was conventional
pellets
originating from balls having the same coke content in the core and the shell
portions
30, 32. A CCS of 373 kg/pellet was obtained for pellets containing 0.97 wt% of
coke.
The second pellet batch was two-layered pellets originating from balls having
a total
coke content of 0.97 wt%. The core portion 30 did not contain coke and the
coke
content of the shell portion 32 was 4.43 wt%. A CCS of 537 kg/pellet was
obtained,
which is 1.4 is times higher than for conventional pellets. The third and the
fourth
pellet batches were also two-layered pellets. The total coke content of the
balls for
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the third and the fourth pellet batches were 0.65 wt% and 1.3 wt%
respectively. For
both batches, the core portion 30 did not contain coke and the shell portion
32
contained 2.72 wt% coke for the third batch and 5.9 wt% for the fourth batch.
CCS of
498 kg/pellet and 419 kg/pellet were obtained for the third and fourth batches
respectively.
Consequently, higher CCS than for conventional pellets can be obtained with
layered
pellets having lower coke contents. High coke contents in the shell portion 32
of
layered pellets reduced the mechanical properties of the pellet. There is
probably an
optimum coke content for the shell portion 32 for each type of pellets.
Table 2
Core / Shell (vol.%) 78/22
Total coke added (wt%) 0.97 0.65 1.30
Coke in core / in shell (wt%) 0.97 / 0.97 0 / 4.43 0 / 2.72 0 / 5.9
CCS (kg/pellet) 373 537 498 419
Example 4
The fourth example concerns the mechanical properties of layered pellets
comparatively to conventional pellets having the same internal fuel content in
the
core portion 30 and in the shell portion 32. It also concerns two-layered
pellets with
different internal fuel contents that were fired under different operating
conditions.
The pellets were low silica pellets originating from balls containing
approximately 1.5
wt% of silica, between 0.75 and 2 wt% of coke as internal fuel, 0.4 wt% of
CaO, and
0.3 wt% of MgO. The fluxes were added as dolomite.
In addition to the CCS, the mechanical properties of the pellets were also
evaluated
with the ISO tumble index. The ISO tumble index is a relative measure of the
resistance of the pellets to size degradation by impact and abrasion, when
subjected
to a tumble test in a rotating drum.
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During the tests, the gas flow rate for the induration process was modified
between
two levels: a regular and a higher gas flow rates. The productivity of the
induration
process was also measured in tons of green balls per hour (TGB/h).
Table 3 shows the results obtained for the fired pellets. The first pellet
batch was
conventional pellets that were fired with a regular gas flow. A CCS of 314
kg/pellet
and a tumble index of 97.0 were obtained for pellets originating from balls
containing
1.6 wt% of coke. The productivity was 615 tGB/h.
Batches 2 to 6 relate to two-layered pellets. The productivity variations
(wt%), the
variation of the fuel (oil) burned at the burners during the induration (wt%),
the
variation of coke contained in the pellets (wt%), the variation of the energy
costs to
manufacture the fired pellets ( /0), and the variation of the GHG released per
ton of
fired pellets (tFP) (wt%) were also calculated.
The second pellet batch had a total coke content of 0.9 wt% and was fired with
a
regular gas flow. A CCS of 364 kg/pellet and a tumble index of 95.9 were
obtained.
The CCS of the two-layered pellets was better than the one of conventional
pellets
with a lower internal fuel content. The tumble index was however slightly
lower. A
productivity gain of 8 wt% combined with reductions of 19 wt% and 44 wt% of
the oil
burned and the coke added to the balls were obtained. Consequently, the
overall
energy cost was reduced by 32 wt%. The GHG released were also reduced by 36%.
Similar results are given in Table 3 for balls containing different internal
fuel contents
that were fired in different operating conditions.
In conclusion, while keeping similar mechanical properties than with
conventional
pellets, the layered pellets allow a significant increase of the productivity
combined
with reduction of the overall energy costs and the GHG released. Increasing
the gas
flow rate in the furnace allows an additional productivity gain.
FIG. 5 compares the micrographs of the core and the shell portions 30, 32 of
conventional and two-layered pellets. The core portion 30 of the two-layered
pellets
(FIG. 5D) contains less secondary magnetite than the core portion 30 of a
conventional pellet (FIG. 5B). Moreover, the core and shell portions 30, 32 of
the
two-layered pellets (FIGS. 5C and 5D) have similar micrographs. On the
opposite,
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the micrographs of the core and shell portions 30, 32 of the conventional
pellets
(FIGS. 5A and 5B) differ.
Table 3
Description Conven- Layered
tional
Gas flow rate Regular Higher
Batch 1 2 3 4 5 6
Coke (0/0) 1.6 0.9 1.6 2.0 1.6 2.0
ISO Tumble (c)/0 +6.3mm) 97.0 95.9 94.0 94.3 94.7
94.3
CCS
(kg/pellet) 314 364 378 415 404 396
Productivity (tGB/h) 615 665 740 745 795 815
Productivity gain (%) +8 +20 +21 +18 +21
Oil (%) -19 -27 -67 -68 -67
Coke (%) -44 0 +25 0 +25
Energy (global) ($) -32 -13 -21 -34 -21
GHG (per tFP) (%) -36 -8 -3 -20 -3
Example 5
The fifth example is similar to the fourth one but self-fluxed pellets were
manufactured. Self-fluxed pellets originate from balls containing
approximately 3.75
wt% of silica, 2 wt% of coke as an internal fuel, 3.7 wt% of CaO, and 1.3 wt%
of
MgO. Dolomite and limestone were added as fluxes. During the tests, the gas
flow
rate for the firing was kept constant.
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Table 4 shows the results obtained for the fired pellets. The first pellet
batch was
conventional pellets. CCS of 294 kg/pellet and a tumble index of 96.8 were
obtained.
The productivity of the induration process was 345 tGB/h.
Batches 2 and 3 relate to two-layered pellets. As for the fourth example,
while
keeping the mechanical properties similar to the ones of the conventional
pellets, the
layered pellets allow a productivity increase combined with reduction of the
overall
energy cost and the GHG released.
FIG. 6 compares the micrographs of the core and the shell portions 30, 32 of
conventional and two-layered pellets. Fig. 6A is a micrograph of the shell
portion 32
of a conventional pellet which is compared to a micrograph of the shell
portion 32 of
the two-layered pellet (Fig 60). Fig. 6B is a micrograph of the core portion
30 of a
conventional pellet which is compared to the micrograph of the core portion 30
of the
two-layered pellet (Fig. 6D).
Table 4
Description Conventional j
Layered
Gas flow rate Regular
Batch 1 2 3
ISO Tumble (% +6.3mm) 96.8 95.8
94.7
CCS (kg/pellet) 294
Productivity (tGB/h) 345 370
405
Productivity gain (%) +7
+17
Oil (%) -39 -
43
Coke (%) 0
+31
Energy (global) ($) -24 -
13
GHG (per tFP) (%) -14 -3
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FIG. 7 is a graph comparing the CCS of low-silica pellets for various coke
contents.
The CCS of layered pellets is higher than the one of conventional pellets. The
maximum CCS is obtained for pellets having an overal coke content proximate to
2
wt%.
FIG. 8 is similar to FIG.7. However it concerns self-fluxed pellets with
various coke
contents. As for the low-silica pellets, the CCS of self-fluxed layered
pellets is higher
than the one of conventional pellets. Higher CCS are obtained for layered
pellets
having low coke content (less than 1.5 wt%).
Even if the above examples use mainly dolomite and limestone as fluxing agents
and coke as internal fuel, one skilled in the art will understand that any
appropriate
material can be used. For example, forsterite (Mg2S104), olivine, and slaked
lime
(Ca(OH)2) can be used as fluxing agent. Similarly, low temperature coke,
pulverized
coal, petroleum coke and anthracite can be used as internal fuel.
The shell portion has preferably a thickness ranging between 250 and 3000 m,
more preferably 500 and 2000 m. The volume of the core portion is typically
above
=
60% of the ball volume, preferably above 70%.
It will be appreciated that the nature and the content of the additives added
to the
first and second feed materials can vary. Moreover, one skilled in the art
will
appreciate that a liquid, usually water, is typically added to the first and
second feed
material for their agglomeration. The moisture content of the first and second
feed
material can vary in accordance with the nature of the ball produced.
Similarly, the
nature, the content and the particle size distribution of the additives such
as the
fluxes can vary in accordance with the nature of the pellets produced.
The embodiments of the invention described above are intended to be exemplary
only. One skilled in the art will appreciate that the numerical values such as
percentages are approximations and are not exact numbers. One skilled in the
art
will also appreciate that the term "free of" means "substantially free of".
The scope of
the invention is therefore intended to be limited solely by the scope of the
appended
claims.
