DESCRIPTION
TITLE OF INVENTION
METHOD FOR GRANULATION OF SINTER MIXTURE
TECHNICAL FIELD
[OOO 11
The present invention relates to a method for granulating a sinter mixture for
production of sintered ore to be used in a blast furnace, and in particular to a method for
granulating a sinter mixture which is capable of maintaining the capability to produce
sintered ore at low cost even when the relative amount of iron ore smaller fines, which
are less capable of being granulated, is increased in the sinter mixture.
BACKGROUND ART
[0002]
Typically, sintered ore for use in a blast fiunace as an iron source is produced in
the following manner. A sinter mixture is prepared by mixing the following materials
in predetennined propotiions: various brands of iron ore; auxiliary materials such as
limestone; carbonaceous materials such as coke breeze; and return fines (undersize
particles resulting fiom crushing of a sinter cake, which is a sintered form of sinter
mixture, and screening of the crushed sinter cake). The thus formed sinter mixture, to
which water is added, is then subjected to blending, moisture conditioning and
granulation. With this, the sinter mixture is granulated into pseudo-form particles
(hereinafter also referred to as "pseudo particles") which are formed of a relatively large
seed particle having a size of about 3 to 5 rnrn surrounded by "powdery particles" which
are finer than the seed particle and has a size of 1 mrn or less.
[0003]
The granulated sinter mixture is charged onto a pallet of a sintering machine,
and forms a sinter mixture bed (hereinafter also referred to simply as "sinter bed") on
the pallet. The sinter bed is ignited at the top surface thereof by an ignition hood of the
sintering machine, which causes combustion of the carbonaceous material in the sinter
bed, and the burnt portion of the carbonaceous material forms a burnt zone. The burnt
zone gradually migrates from upper to lower regions of the sinter bed as air is drawn
downward through the sinter bed fiom below. Pseudo particles around the burnt zone
are heated by the heat of combustion and partially fused, so that bridges are formed
between the pseudo particles by the melt to thereby provide a sintered body. In this
manner, the sinter bed finally becomes a sinter cake. The sinter cake is discharged
from the sintering machine and crushed by a crusher into pieces, which are then
screened by a screen. The oversize pieces become sintered ore products while the
undersize pieces are returned to the sinter mixture as return h e s .
[0004]
The iron grade of sintered ore greatly affects the operation perfcrmance of a
blast h a c e . In addition, it is strongly desired to reduce carbon dioxide emission
fiom blast furnaces from a standpoint of environmental protection. In view of these
circumstances, it is important to produce high grade sintered ore. In recent years,
however, high-grade iron ore fines for use in a sinter mixture have been depleting. In
order to ensure a sufficient iron grade under these circumstances, it is expected that, in
the future, high-grade iron ore smaller fines will be used in larger amounts in place of
high-grade iron ore fines in the production of sintered ore.
[OOOS]
The "high-grade iron ore smaller fines" as mentioned herein refers to iron ore
in which undersize particles (having a size of 250 p or less) account for 80 mass % or
more and having a total iron content of 60 mass % or more, wherein the undersize
particles are defined as particles which pass through a screen with 250 pm openings
when screened based on the method for measuring the size distribution of iron ore
particles specified in JIS (Japanese Industrial Standards) M 8716. Specific examples
thereof include pellet feed iron ore having an improved iron grade due to ore dressing.
Hereinafter, such high-grade iron ore smaller fines are also referred to simply as "pellet
feed. "
[0006]
However, in the production of sintered ore, simply using larger quantities of
pellet feed causes a problem in that: the percentage of smaller fines in the sinter mixture
is increased, and part of them are not granulated and remain as they are. Because of
this, voids, formed in the sinter bed when pseudo particles are charged onto the pallet of
a sintering machine, are clogged with the non-granulated smaller fines, which results in
impairing the air permeability of the sinter bed. This in turn causes a decrease in the
burning velocity, which is a velocity at which the burnt zone migrates fiom upper to
lower regions, thereby resulting in a reduction in the capability to produce sintered ore.
[0007]
Conventionally, the reduction in the production capability associated with the
increased percentage of smaller fines in a sinter mixture is addressed by addition of
quick lime as described, for example, in Non-Patent Literature 1 listed below.
However, as described in the literature, improvement in the adhesion percentage of
powdery particles, which is achieved by quick lime, is noticeable with the use of quick
lime of up to 2 mass %, but use of quick lime in an amount greater than 2 mass %
provides almost no M e r improvement. In addition, quick lime is expensive because
it is industrially produced by heating limestone to 900°C or higher and therefore
production requires high energy. Because of this, it is desirable to reduce, to the extent
possible, the use of quick lime.
[OOOS]
Accordingly, in order to address the increased percentage of smaller fines in a
sinter mixture, techniques without resorting to the use of quick lime or with a limited
use of quick lime are being developed. Specific techniques therefor are proposed, for
example, in Patent Literatures 1 to 5 listed below.
[0009]
Patent Literature 1 discloses a method for producing sintered ore, the method
including: preparing a sinter mixture that contains, as iron material less capable of being
granulated, at least one of: collected dust; iron sand; Marra Mamba iron ore; and pellet
feed, in an amount of 30 mass % or more based on the total mass of the sinter mixture;
and adding water containing a surfactant to the iron material and granulating the iron
material with other sinter mixture materials. It is stated that, with this method, the
wettability of the above-mentioned iron material less capable of being granulated is
improved and, as a result, the capability for granulation and production of sintered ore is
improved.
[OO 1 01
However, surfactants are expensive as with quick lime mentioned above. If
large quantities of pellet feed are used, the amount of surfactant to be used is necessarily
increased. Because of this, with the method disclosed in Patent Literature 1, it is
difficult to overcome, in an inexpensive manner, the disadvantage caused by the
increased percentage of smaller fines when pellet feed is used in large quantities.
1001 11
Patent Literature 2 discloses a method for granulating a sinter mixture, the
method including adding a hydrated lime sluny, prepared by wet grinding of quick lime,
to a sinter mixture in a granulator by spraying. It is stated that, with this method, the
capability for production of sintered ore is increased compared to the case of adding dry
ground quick lime.
[OO 121
However, if the method disclosed in Patent Literature 2 is employed to use
large quantities of pellet feed, cost effectiveness inevitably decreases because large
quantities of hydrated lime slurry must be used, which means large quantities of quick
lime, from which hydrated lime slurry is made, must be used.
[OO 1 31
Patent Literature 3 discloses a method for granulating a sinter mixture
including a selective granulation step in which part of the sinter mixture is
pre-granulated, the method including: pre-granulating a material less capable of being
granulated such as Mana Mamba iron ore or pellet feed by adding at least one of: a
polymeric compound such as polyacrylic acid and a fine powder having an average
particle size of 200 p or less, e.g., calcium carbonate; and, thereafter mixing the
pre-granulated material with the remainder of the sinter mixture.
[00 141
However, polymeric compounds and calcium carbonate are expensive as with
quick lime mentioned above. Therefore, if the method disclosed in Patent Literature 3
is employed to use large quantities of pellet feed, cost effectiveness inevitably
decreases.
[OO 1 51
Patent Literature 4 discloses a method for granulating sinter mixture particles,
the method including: adding and mixing, into the sinter mixture particles, iron ore
super fines having an average particle size of 10 pm or less as a binder in an amount of
2 to 15 mass %. With this method, tailings, which are ore dressing residue produced at
mine mouth, are used as the iron ore super fines having an average particle size of 10
pn or less.
[00 1 61
However, since tailings are produced at mine mouth, it is unclear how to enable
stable supply of the necessary amount as a binder. In addition, it is also unclear
specifically how the iron ore super fines are obtained, aside fiom the use of tailings. In
view of these facts, the method disclosed in Patent Literature 4 is less feasible.
[0017]
Patent Literature 5 discloses a method for granulating a sinter mixture, the
method including: compressing and grinding a sinter mixture containing iron ore by a
roller press grinding machine; and then adding a polyacrylate dispersant and granulating
the sinter mixture. It is stated that, with this method, large quantities of super small
particles having a size of 45 pm or less that serve as a binder can be obtained by
processing the sinter mixture by the roller press grinding machine, and that it is
therefore possible to efficiently produce pseudo particles during granulation and
enhance the strength of the resulting granules.
[00 1 81
However, with the method disclosed in Patent Literature 5, the super small
particles obtained by dry grinding by the roller press grinding machine are dispersed in
water by the addition of the polyacrylate dispersant. Thus, cost effectiveness
inevitably decreases due to the use of the dispersant, and therefore there is room for
further improvement.
CITATION LIST
PATENT LITERATURE
[00 1 91
PATENT LITERATURE 1 : Japanese Patent Application Publication No. 2004- 18303 1
PATENT LITERATURE 2: Japanese Patent Application Publication No. S62-56533
PATENT LITERATURE 3: Japanese Patent Application Publication No. 2005-097686
PATENT LITERATURE 4: Japanese Patent Application Publication No. 2009-144240
PATENT LITERATURE 5: Japanese Patent Application Publication No. 2007- 162 127
NON-PATENT LITERATURE
[0020]
NON- PATENT LITERATURE 1 : Takeo SAT0 and four others, "Fusen" ("Flotation"),
1985, vol. 32, No. 2, pp. 84 to 90.
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[002 11
The present invention has been made in view of the foregoing problems.
Accordingly, an object of the present invention is to provide a method for granulating a
sinter mixture by which the air permeability of the sinter bed is ensured and thus the
capability to produce sintered ore is maintained at low cost even when the percentage of
smaller fines in the sinter mixture is increased by using a particular brand of high-grade
iron ore smaller fines such as pellet feed in large quantities.
SOLUTION TO PROBLEM
[0022]
In order to accomplish the above object, the present invention provides a
method for granulating a sinter mixture, the method including: forming the sinter
mixture by mixing iron ore with carbonaceous material, auxiliary material and return
fines; and subjecting the sinter mixture to blending, moisture conditioning, and
granulating, characterized in that the method includes: wet grinding the iron ore using a
vertical grinding mill, the vertical grinding mill including: a grinding unit formed of a
cylindrical vessel provided with a vertical central shaft configured to be rotationally
driven and a screw blade mounted to the vertical central shaft; a classification unit
configured to perform classification by gravity and centrifugal force; and a recirculating
unit configured to recirculate underflow from the classification unit into the cylindrical
vessel of the grinding unit, and adding an iron ore slurry obtained by the wet grinding to
the whole or part of the sinter mixture and granulating the thus prepared sinter mixture.
[0023]
In this method for granulation, it is preferred that: the sinter mixture contain a
particular brand of iron ore smaller fines in an amount of greater than 13.20 mass % to
20.00 mass % based on the total mass of the sinter mixture, the particular brand of iron
ore smaller fines comprising particles in which particles having a size of 250 pm or less
account for 80 mass % or more, the particular brand of iron ore smaller fines having a
total iron content of 60 mass % or more; and that: the iron ore slurry be added to the
whole of the sinter mixture or part of the sinter mixture in which the iron ore smaller
fines are contained in an amount of 50 mass % or more, the iron ore slurry being added
in such a manner that very small particles having a size of 10 pm or less among iron ore
particles in the iron ore slurry are present in an amount of 0.01 mass % or more relative
to 1.0 mass % of the iron ore'smaller fines.
[0024]
Examples of the particular brand of iron ore smaller fines as referred to herein
include high-grade pellet feed produced in South America.
ADVANTAGEOUS EFFECTS OF INVENTION
[0025]
With the method for granulating a sinter mixture according to the present
invention, it is possible to maintain the capability to produce sinter even when using
large quantities of pellet feed as high-quality iron ore smaller fines. This is made
possible by wet grinding iron ore using a vertical grinding mill, and adding, as a binder
for granulation, the thus obtained sluny having very small iron ore particles of 10 pm or
less suspended therein, to the sinter mixture.
BRIEF DESCRIPTION OF DRAWINGS
[0026]
[FIG. 11 FIG. 1 is a graph illustrating particle size distributions of pellet feed
and iron ore fines (sinter feed).
[FIG. 21 FIG. 2 shows photographs of iron ore ground with a grinding mill,
taken with a scanning electron microscope (SEM).
[FIG. 31 FIG. 3 is a schematic diagram of the overall configuration of a tower
mill employed for production of an iron ore sluny in accordance with the method for
granulating a sinter mixture of the present invention.
FIG. 41 FIG. 4 is a graph illustrating results of a compressive strength test
performed on coarse pseudo particles produced with varied perceqtages of very small
particles to be added.
[FIG. 51 FIG. 5 is a diagram illustrating a configuration of the granulation
systems in examples of the present invention.
[FIG. 61 FIG. 6 is a graph illustrating evaluation results of production rates of
sintered ore obtained fiom a test using a laboratory sintering pan.
DESCRIPTION OF EMBODIMENTS
[0027]
The method for granulating a sinter mixture of the present invention is
described below in connection with the development made for the completion of the
present invention and preferred embodiments of the present invention.
[0028]
First, the present inventors focused on the finding that very small iron ore
particles having a size of 10 pm or less (hereinafter also referred to simply as "very
small particles") can serve as a binder in granulation. The detailed mechanism by
which the very small particles act as a binder in granulation is set forth below.
[0029]
In granulation of a sinter mixture, pseudo particles are prepared which are
formed of a relatively large seed particle having a size of about 3 to 5 mm surrounded
by powdery particles which are finer than the seed particle and has a size of 1 mm or
less. Water that is added (hereinafter also referred to as "added water") serves as a
binder that bonds the seed particle and the powdery particles together. The added
water enters between the seed particle and the powdery particles and bonds them
together during the process of blending, stirring, and granulation.
[003 01
In the meantime, the very small particles having a size of 10 pm contained in
the sinter mixture, when suspended in the added water, can move freely with the water,
and therefore enter between the seed particle and the powdery particles together with
the added water. In this manner, the very small particles fill the gap between the seed
particle and the powdery particles to form bridges therebetween. thus increasing the
strength of the pseudo particles.
[003 11
It is assumed that the strength increase achieved by this mechanism occurs not
only in the pseudo particles formed of a seed particle and powdery particles, but also in
pseudo particles formed entirely of particles having a size of 1 mm or less (hereinafter
referred to as "p-type pseudo particles") with no seed particle. However, in production
of the p-type pseudo particles, large quantities of very small particles which serve as a
binder will be necessary, and moreover, p-type pseudo particles will easily disintegrate
due to the impact imposed when charged onto the pallet of a sintering machine
compared to the typical pseudo particles. In view of these facts, addition of very small
particles more desirably should be applied to granulation of pseudo particles formed of
a seed particle and powdery particles.
[0032]
The present inventors investigated the properties of pellet feed and have made
the following finding: pellet feed is smaller fines material in which particles having a
size of 250 pn or less account for 80 mass % or more, whereas very small particles
having a size of 10 pm or less are substantially not present therein. This is because
pellet feed undergoes ore dressing at mine mouth, specifically, crushing, grinding,
washing, flotation, and the like, for improvement of the iron grade, and during the
process, very small particles are removed.
[0033]
Therefore, use of pellet feed can cause deterioration of granulability.
Specifically, voids formed in the sinter bed when pseudo particles are charged onto the
pallet of a sintering machine are clogged with the smaller fines that were not granulated
and remain as they are. Because of this, the air permeability of the sinter bed is
impaired. The major cause of this problem is that pellet feed does not contain a
sufficient amount of very small particles, which can serve as a binder.
[0034]
FIG. 1 is a graph illustrating the particle size distributions of pellet feed (PF. A)
and three brands of iron ore fines (sinter feed) (SF. A, SF. B, and SF. C). With regard
to the particle size distributions, particles having a size exceeding 250 pm were
identified by measurement with the wet screening method for iron ore specified in JIS
M 8716, and particles having a size of 250 pm or less were identified by measurement
with the following method: collecting the filtrate containing particles having a size of
250 pm or less resulting from the wet screening, and analyzing the filtrate by the laser
dXhction light scattering method specified in JIS R 1629.
100351
As shown in FIG. 1, PF. A contains particles having a sue of from greater than
10 pm to 250 pn in an amount of 88.2 %, whereas it contains only 0.1 % very small
particles having a sue of 10 pn or less, which is an extremely low amount. On the
other hand, all brands of sinter feed contain very small particles having a size of 10 pn
or less in an amount of about 10% to about 15%, although their content of particles
having a size of from greater than 10 jun to 250 jun is about 10% to about 20%, which
is lower than that of PF. A.
100361
Table 1 below shows the components of PF. A. It contains 60 % or more total
iron, and thus is a high-grade pellet feed.
[003 71
[Table 11
From the findings above, the present inventors have reached the idea that: by
adding very small particles of 10 pn or less to have them act as a binder, it is possible to
ensure the air permeability of the sinter bed while maintaining the capability to produce
sintered ore even when using large quantities of pellet feed which is a high-grade iron
ore smaller fines product.
(00391
TABLE 1
Composition [mass %I
T.Fe
PF. A
(Pellet Feed) 67.59
FeO
0.3
CaO
0.05
Si02
1.53
A 1 2 0 3
0.43
MgO
0.02
CW
0.93
In addition, in light of the above-described mechanism that provides the
advantage of very small particles acting as a binder, it is important that the very small
particles be adequately blended with added water.
[0040]
As described above, calcium carbonate and the like are used in Patent
Literature 3, tailings are used in Patent Literature 4, and iron ore ground with a roller
press is used in Patent Literature 5, each as a binder containing very small particles.
However, when these methods are employed, additional processes will be necessary in
order to allow the very small particles to adequately act as a binder. For example, a
process for suEciently dispersing and suspending the very small particles in water, e.g.
by adding a polymeric dispersant, will be necessary.
[004 11
In view of this, the present inventors have determined that, as a technique for
adding very small particles, an optimal approach to produce the effect of very small
particles acting as a binder at low cost is the following: preparing, in advance, a sluny
containing very small iron ore particles suspended in water (hereinafter also referred to
as "iron ore slurry"), and adding this iron ore slurry to a sinter mixture.
[0042]
As a method for preparing iron ore slurry, wet grinding is preferred. With this,
it is possible to simultaneously carry out grinding of iron ore and suspending of the very
small iron ore particles, obtained by the grinding, in water, to thereby achieve a
condition in which the very small particles are adequately blended with water without
the need for addition of a dispersant. If dry grinding is employed, a separate process
for mixing and kneading will be necessary to suspend the very small particles obtained
by the grinding in water.
[0043]
Based on the above, it has been found that, by wet grinding iron ore fines to
obtain a slurry with very small iron ore particles having a size of 10 pn or less
suspended therein and adding the obtained slurry to a sinter mixture, the air
permeability of the sinter bed is ensured and thus the capability to produce sintered ore
is maintained at low cost even when the percentage of smaller fines in the sinter mixture
is increased by the use of large quantities of pellet feed.
100441
1. Properties of Iron Ore Particles Obtained by Grinding with a Grinding Mill
FIG. 2 shows photographs of iron ore ground with a grinding mill, taken with a
scanning electron microscope (SEM). FIGS. 2(a) and 2(b), and FIGS. 2(c) and 2(d)
are all comparative examples, with FIGS. 2(a) and 2(b) showing a result of grinding
with a ball mill as disclosed in Patent Literature 5, and FIGS. 2(c) and 2(d) showing a
result of grinding with a roller press as disclosed also in Patent Literature 5. FIGS.
2(e) and 2(f) show an example of the present invention, with a tower mill used for
grinding as described below. The photographs of FIGS. 2(a), 2(c), and 2(e) were taken
at a magnification of 5 0 0a~nd~ th e photographs of FIGS. 2(b), 2(d), and 2(f) wcre taken
at a magnification of 3000~.
[0045]
FIG. 3 is a schematic diagram of the overall configuration of a tower mill
employed for production of an iron ore slurry in accordance with the method for
granulating a sinter mixture of the present invention. As shown in FIG. 3, the tower
mill 1 is a vertical wet grinding mill generally including: a grinding unit; a classification
unit; and a recirculating unit. The grinding unit is provided in the form of a cylindrical
vessel 4 which includes a vertical central shaft 2 configured to be rotationally driven
and a double helical screw blade 3 mounted to the vertical central shaft 2. Into the
cylindrical vessel 4, steel balls are charged as a grinding medium. The classification
unit includes: an elutriation chamber 5 connected to an upper portion of the side of the
cylindrical vessel 4; and a cyclone classification system 6 connected to the elutriation
chamber 5. The recirculating unit includes: a piping 8 connected to a lower portion of
the cylindrical vessel 4 from a bottom of the elutriation chamber 5 via a recirculating
pump 7; and a piping 9 connected to a top of the elutriation chamber 5 from a bottom of
the cyclone classification system 6.
COO461
Iron ore to be ground are fed from the top of the cylindrical vessel 4 together
with water. The fed iron ore falls downwardly within the cylindrical vessel 4 and,
together with the steel balls within the cylindrical vessel 4, cyclically undergoes the
movements of: rotating in a circumferential direction by the rotation of the screw blade
3 imparted by the rotationally driven vertical central shaft 2; being hoisted and delivered
upwardly by the screw blade 3; and falling downwardly by its own weight. These
complex movements provide shear force and compressive force acting between the iron
ore particles or between the iron ore particles and the steel balls to thereby grind (crush)
the iron ore.
[0047]
Within the cylindrical vessel 4, the particle size of the iron ore particles
gradually diminishes as the grinding progresses. When the rate at which the particles
are delivered upwardly by the rotation of the screw blade 3 exceeds the rate at which
they fall in the water by their own weight, they are suspended in the water filling the
cylindrical vessel 4 to form a slurry, which flows into the elutriation chamber 5 located
on the upper portion of the side of the cylindrical vessel 4.
[0048]
The iron ore sluny that has flowed into the elutriation chamber 5 is subjected
to rough classification by the action of gravity. During this process, a portion of the
iron ore slurry that has been classified as containing coarse particles flows into the
piping 8 fiom the bottom of the elutriation chamber 5 as undefflow (see "U/F" in FIG.
3), thereby being returned to the lower portion of the cylindrical vessel 4 via the
recirculating pump 7 for regrinding. In the meantime, a portion of the iron ore slurry
containing fine particles flows into the cyclone classification system 6 as overflow (see
"OIF" in FIG. 3).
COO491
The iron ore slurry that has flowed into the cyclone classification system 6 is
subjected to W e r classification into smaller classes by the action of centrihgal force.
During this process, a portion of the iron ore slurry that has been classified as
containing coarse particles is returned to the top of the elutriation chamber 5 via the
piping 9 as underflow (see "UIF" in FIG. 3). In the meantime, a portion of the iron ore
slurry containing fine particles is discharged out of the system of the tower mill 1 as
overflow (see "O/F" in FIG. 3). In place of the iron ore slurry discharged out of the
system, a new supply of iron ore and water is fed into the cylindrical vessel 4. In
addition, the steel balls serving as a grinding medium gradually wear off, and thus a
new supply thereof together with iron ore is fed into the cylindrical vessel 4 as needed.
[OOSO]
With the process described above, the tower mill 1 is capable of continuously
supplying very small iron ore particles which serve as a binder in granulation, in the
form of an aqueous sluny in which they are suspended. Moreover, a slurry tank for
temporary storage of the iron ore slurry may be placed between the devices, e.g.,
between the elutriation chamber 5 and the cyclone classification system 6, or at the
outlet of the cyclone classification system 6 fiom which the overflow is discharged, in
order to ensure a balance between the amount to be fed and the amount to be
discharged.
[005 11
In comparing the photographs of FIGS. 2(a), 2(c), and 2(e), it is seen that the
iron ore ground in a tower mill has a significantly small particle size compared to the
iron ore ground in a ball mill or a roller press. As described above, when a tower mill
is used, the iron ore is stirred not only in a circumferential direction but also in a vertical
direction to be ground. Because of this, strong shear force and compressive force are
applied between particles compared to the case of grinding only by circumferential
stirring caused by the rotation of a vessel, as is the case, for example, with a ball mill,
which is a typical wet grinding mill other than a tower mill. It is therefore believed
that the iron ore ground in a tower mill has a significantly small particle size compared
with the case of using a different type of grinding mill.
[0052]
Furthermore, in light of the mechanism that provides the advantage of very
small particles acting as a binder by entering between the seed particle and the powdery
particles to form bridges therebetween, it is believed that, if there are a greater number
of irregularities on the surfaces of the very small particles, a greater advantage of them
acting as a binder is achieved when they enter between the seed particle and the
powdery particles. In comparing the photographs of FIGS. 2(b), 2(d), and 2(f) in this
regard, it is seen that the surfaces of the iron ore ground in a tower mill have more
irregularities than those of the iron ore ground in a ball mill.
[0053]
Based on the above, it was observed that the iron ore slurry produced by
grinding with a tower mill has characteristics that allow it to serve as a good binder in
granulation.
[0054]
2. Strength of Resulting Granules
Four sinter mixtures having a composition shown in Table 2 below were
prepared, with pellet feed (PF. A) as a high-grade iron ore smaller fines product mixed
therein. To the respective sinter mixtures were added very small particles in different
percentages to produce coarse pseudo particles of about 10 mm or more in diameter
(hereinafter also referred to as "green balls"). They were dried at 105°C for two or
more hours, and were subsequently subjected to a compressive strength test to find the
crushing stress on each green ball. The added very small particles were those obtained
by grinding SF. A, which is pisolite, to a condition in which very small particles of 10
pm or less was present in an amount of 60 mass %.
[OOSS]
[Table 21
TABLE 2
[0056]
FIG. 4 is a graph illustrating results of a compressive strength test performed
on the coarse pseudo particles produced with varied percentages of very small particles
to be added. As shown in FIG. 4, when 0.60 mass % or more very small particles were
added relative to 45.60 mass % pellet feed (PF. A), which is a high-grade iron ore
smaller fines product, in the sinter mixture, i.e., when 0.013 mass % or more very small
particles were added relative to 1.0 mass % pellet feed (PF. A), the strength of the
Amount of Materials [mass %]
SF. A (Pisolite)
PF. A (Pellet Feed)
Auxiliary Material A
(blast hrnace dust)
Quick Lime
Very Small Particles
[amount excluded from
total mixture amount]
>
45.60
45.60
5.90
2.90
0.00 (base)
--+
4
4
4
3.00
1
4
4
4
4
0.60
+
+
+
+
1.20
resulting granules was increased compared to those to which very small particles were
not added. This indicates that the addition of very small particles provided the
advantage of them acting as a binder.
[0057]
3. Evaluation of Sinter Production Rate by Test Using Laboratory Sintering Pan
Pseudo particles were produced by the following process: mixing iron ore of
various brands, auxiliary material, retum fines, carbonaceous material and the like in
specified proportions as shown in Table 3 below; charging the sinter mixture into
granulator equipment; and granulating the sinter mixture with addition of water and, for
the inventive example, an iron ore slurry (slurry of very small particles). In Table 3,
the amounts of the iron ore slurry and carbonaceous materials are both indicated as
amounts excluded fiom the total amount of the mixture. This is intended to be in
conformity with Table 2 for the iron ore slurry, and, for the carbonaceous materials, to
be in accordance with common practice in actual equipment operation of sintering. As
shown in FIG. 5, a granulation system A including two drum mixers and a granulation
system B including a high speed stirrer mixer and a pan pelletizer were used for
granulation. In Conventional Examples 1 and 2 and Comparative Example 1,
granulation was performed only using a granulation system A, which was followed by
burning. On the other hand, in Comparative Examples 2 and 3, and Inventive Example,
sinter mixtures having a specified composition shown in Table 3 were charged into the
respective granulation systems A and B, and granulated with addition of water and, for
the inventive example, an iron ore slurry. Thereafter, the sinter mixtures granulated in
the respective granulation systems A and B were combined and burned.
[005 81
[Table 31

TABLE 3 - Continued
t I Amount of Materials [mass %]
Classification Conventional
Example 1
Granulation System B
Conventional Example 2
SF. A (Pisolite)
PF. A (Pellet Feed)
Auxiliary Material (blast
furnace dust)
Auxiliary Material (Quick lime)
System B subtotal
Very Small Particles Slurry
[amount excluded from total
mixture amount]
Carbonaceous Material (coke
breeze)
[amount excluded from total
mixture amount]
Comparative Example 1
Note: Symbol "-" indicates the material is not included in the mixture.
-
-
-
-
0.00
-
4.80
Comparative
Exmple
-
-
-
-
0.00
-
4.80
-
-
-
-
0.00
-
4.80
-
-
-
-
0.00
-
4.80
-
-
-
-
0.00
-
4.80
-
-
-
-
0.00
-
4.80
-
-
-
-
0.00
-
4.80
9.35
9.35
0.50
0.80
20.00
-
4.80
[0059]
The iron ore slurry was obtained by wet grinding SEA, which is a pisolite
brand of ore, using a tower mill. In Inventive Example, the weight ratio of the iron ore
to the water based on the total weight of the iron ore slurry was about 5050. For the
test, a sample of iron ore slurry containing 50 mass % very small particles of 10 pm or
less, based on the amount of the iron ore contained therein, was used.
[0060]
It is noted that, in the vertical wet grinding mill (tower mill) shown in FIG. 3,
the concentration of the iron ore particles in the slurry can be increased by increasing
the amount of iron ore to be fed or by reducing the amount of water to be fed with the
iron ore. However, an increased concentration of the iron ore particles results in
reduced efficiency of grinding because it leads to hindering the movement of the iron
ore particles in the water during the grinding process. Accordingly, in order to perform
efficient grinding, the particle concentration in the slurry needs to be adjusted to 25
volume % or less, and more preferably to 20 volume % or less.
[006 11
Meanwhile, the concentration of the iron ore particles in the slurry can be
reduced by reducing the amount of iron ore to be fed or by increasing the amount of
water to be fed with the iron ore. However, a reduced concentration of the iron ore
particles also results in reduced efficiency of grinding because it reduces the frequency
of collision between the iron ore particles or between the iron ore particles and the
grinding media (steel balls) during the grinding. Accordingly, in order to perform
efficient grinding, the particle concentration in the slurry needs to be adjusted to 15
volume % or more. However, the amount of water that can be added for granulation is
limited. Thus, if granulation is performed with an amount of water exceeding the limit,
water will be present over the entire surface of the material, and therefore the strength of
the pseudo particles can no longer be maintained. In view of this, the concentration of
the iron ore particles in the slurry should desirably be as high as possible.
[0062]
Based on the above, the suitable concentration of the iron ore particles in the
iron ore slurry is preferably in the range of 15 volume % to 25 volume %. With regard
to the density of iron ore, pisolite or Marra Mamba, which is relatively porous, has a
density of about 4 g/cm3, and hematite, which is relatively dense, has a density of about
5 g/cm3. Thus, the preferred concentration range, when expressed in mass instead of
volume, is about 41 mass % to about 63 mass %.
[0063]
In the inventive example described above, SF. A, which has a density of about
4 g/cm3, was used to obtain the iron ore slurry. Thus, when the iron ore was ground to
a level that provides a 20 volume % concentration of the iron ore particles in the slurry,
the weight ratio of the iron ore to the water was about 5050. The percentage of very
small particles of 10 pn or less in the iron ore particles in the iron ore slurry should
desirably be as high as possible because the amount of water that can be added for
granulation is limited as described above. However, an increased percentage of very
small particles in the iron ore particles in the slurry results in decreased grinding
efficiency and grinding throughput because an increased amount of the iron ore particles
are to be recirculated until they reach a predetermined particle size before being
discharged fiom the grinding mill.
[0064]
To achieve a good balance, in the inventive example, an iron ore slurry
containing 50 mass % very small particles of 10 pm or less based on the total mass of
the iron ore particles was used. However, if the ability of the grinding mill is
sufficiently high, it will be relatively easy to achieve an increased percentage of very
small particles without decreasing the grinding throughput. Conversely, even with a
low-performance grinding mill only capable of providing an iron ore slurry containing a
low percentage of very small particles, there will be no problem as long as a
predetermined amount of very small particles, in accordance with the amount of the
pellet feed used, can be added.
[0065]
A test using a laboratory sintering pan was conducted in the following manner.
The pseudo particles of Conventional Examples 1 and 2, Comparative Examples 1 to 3,
and Inventive Example produced as described above were each charged into a
laboratory sintering pan of 300 mm in inner diameter so as to provide a layer of sinter
mixture with a thickness of 500 mm and with a weight of about 600 kg. In the process,
the sinter mixture was ignited by an LPG burner for one minute while air is drawn
downward through the bed from below at a pressure of 20 kPa, and subsequently it
underwent burning at a constant pressure of 9.8 kPa. Three minutes after the exit gas
temperature reached a maximum level, the drawing of air was discontinued, thus
forming a sinter cake to complete the burning. After the completion of the sintering
test, the sinter production rate was determined by the following process for evaluation.
[0066]
The sinter cake was immediately removed fiom the laboratory sintering pan,
and allowed to cool to room temperature. After completion of cooling, the formed
sinter cake was dropped four times from a height of 2 m and then passed through a
screen with 5 mm openings to measure the mass of the oversize pieces. In this manner,
the sinter production rate was determined. The term "sinter production rate" as used
herein refers to a value determined by dividing the mass of the oversize pieces
remaining on the screen with 5 mm openings by the effective area of the sintering
machine and the sintering time. The value can be found by the following formula (1).
In this case, i.e., a test using a laboratory sintering pan, the cross-sectional area of the
laboratory sintering pan was used as the effective area of the sintering machine.
100671
Sinter production rate (ton/m2;/day) = [mass (ton) of sintered ore having a
particle size of 5 mm or more 1 {effective area (m2) of sintering machine x sintering
time (rnin))] x 60 x 24 ...( 1).
[0068]
[Conventional Example 11
In Conventional Example 1, the whole of the sinter mixture was granulated in
the granulation system A with the use of 0 mass % high-grade iron ore smaller fines
(pellet feed (PF. A)), i.e., without the use of pellet feed. Thereafter it was charged into
the sintering machine to be burned. The sinter production rate obtained in this
example is used as a reference value (1 00). The sinter production rates of the below
examples are given as a ratio relative to the reference value (hereinafter referred to as
"relative production rate") by which the sinter production rate for each example was
evaluated.
[0069]
FIG. 6 is a graph illustrating evaluation results of production rates of sintered
ore obtained h m the test using a laboratory sintering pan. As stated above, in FIG. 6,
the sinter production rate of Conventional Example 1 is a reference value (1 OO), and the
sinter production rates of Conventional Example 2, Comparative Examples 1 to 3 and
Inventive Example are relative values to the reference value.
[0070]
[Conventional Example 21
In Conventional Example 2, the whole of the sinter mixture was granulated in
the granulation system A under three conditions using the following amounts of
high-grade iron ore smaller fines (pellet feed (PF. A): 5.00 mass %, 10.00 mass %, and
20.00 mass %. Thereafter they were charged into the sintering machine to be burned.
In Conventional Example 2, the relative production rate decreased with the increase in
the amounts of pellet feed used. In particular, when the pellet feed was used in an
amount of 20.00 mass %, the relative production rate fell below 70 as shown in FIG. 6.
This confirms that the use of large quantities of pellet feed resulted in a significant
decrease in the production rate.
[0071]
[Comparative Example 11
In Comparative Example 1, the whole of the sinter mixture was granulated in
the granulation system A under three conditions using the following amounts of
high-grade iron ore smaller fines (pellet feed (PF. A)): 5.00 mass %, 10.00 mass %, and
20.00 mass %, and to the sinter mixture was added quick lime as a binder in an amount
of 3.00 mass %. Thereafter they were charged into the sintering machine to be burned.
In Comparative Example 1, the relative production rate as a whole increased by the
addition of quick lime as shown in FIG. 6. However, when the pellet feed was used in
an amount of 20.00 mass %, the relative production rate was less than 100 despite the
fact that very large quantities of quick lime, i.e., 3.00 mass %, were added.
[0072]
[Comparative Example 21
In Comparative Example 2, the sinter mixture was granulated with the use of
13.20 mass %, in total, high-grade iron ore smaller fines (pellet feed (PF. A)).
Specifically, 20.00 mass % of the total mass of the sinter mixture, containing 9.35
mass % pellet feed, was granulated in the granulation system B, and the remaining
80.00 mass % of the total mass of the sinter mixture, containing 3.85 mass % pellet feed,
was granulated in the granulation system A. In this example, quick lime was added in
an amount of 1.80 mass % in total. Thereafter the materials subjected to granulation in
the granulation system A and granulation system B were mixed together and charged
into the sintering machine to be burned. As shown in FIG. 6, the result of
Comparative Example 2 confums that the relative production rate is maintained at a
level comparable to that of Conventional Example 1 provided that pellet feed is used in
an amount of up to 13.20 mass %.
[0073]
[Comparative Example 31
In Comparative Example 3, the sinter mixture was granulated with the use of
20.00 mass %, in total, high-grade iron ore smaller fines (pellet feed (PF. A)).
Specifically, 20.00 mass % of the total mass of the sinter mixture, containing 9.35
mass % pellet feed, was granulated in the granulation system By and the remaining
80.00 mass % of the total mass of the sinter mixture, containing 10.65 mass % pellet
feed, was granulated in the granulation system A. In this example, quick lime was
added in an amount of 1.80 mass % in total. Thereafter the materials subjected to
granulation in the granulation system A and granulation system B were mixed together
and charged into the sintering macbine to be burned. As shown in FIG. 6, the result of
Comparative Example 3 confirms that when the pellet feed was used in an amount of
20.00 mass %, the relative production rate was at most about 84% even with the
combined use of the granulation system B and with the addition of 1.80 mass % quick
lime in total.
[0074]
[Inventive Example]
In Inventive Example, the sinter mixture was granulated with the use of 20.00
mass %, in total, high-grade iron ore smaller fines (pellet feed (PF. A)). Specifically,
20.00 mass % of the total mass of the sinter mixture, containing 14.00 mass % pellet
feed (70 mass % of the sinter mixture in the granulation system B), was granulated in
the granulation system B, and the remaining 80.00 mass % of the total mass of the sinter
mixture, containing 6.00 mass % pellet feed, was granulated in the granulation system A.
In this example, quick lime was added in an amount of 1.80 mass % in total.
Furthermore, an iron ore slurry was added in the granulation system B. That is, in this
example, an iron ore slurry was added to the portion that is part of the sinter mixture
and contains 50 mass % or more iron ore smaller fines. In this operation, the iron ore
slurry was added in such a manner that the iron ore in the iron ore slurry, excluding
water, was present in an amount of 0.40 mass % based on the total mass of the sinter
mixture. As stated above, the very small particles account for 50 mass % of the iron
ore in the iron ore slurry. Accordingly, the amount of very small particles of 10 pn or
less that were added translates into 0.01 mass % relative to 1.0 mass % of the pellet feed
mixed in the sinter mixture. In Inventive Example, as shown in FIG. 6, the relative
production rate was maintained at a level comparable to that of Conventional Example 1
because of the addition of the iron ore slurry, despite the use of 20.00 mass % pellet
feed.
[0075]
That is, it is seen that, in Inventive Example, a good production rate of sintered
ore is maintained provided that pellet feed is used in an amount of up to 20.00 mass %,
i.e., even when the amount of pellet feed exceeds 13.20 mass %, in which case the
relative production rate decreased in all the Comparative Examples 1 to 3. This is
achieved by adding an iron ore slurry while controlling the amount of the very small
particles in the iron ore sluny to 0.01 mass % or more relative to 1.0 mass % pellet feed.
100761
Furthermore, in practical operation of sintering, the proportions of mixture
components are often indicated based on the amount of newly supplied material as
being 100 mass %, from which the amount of return fines is excluded as with that of the
carbonaceous material. When evaluated in terms of the proportions on the newly
supplied material basis, it is determined that up to 25 mass % pellet feed can be used
without decreasing the production rate in accordance with Inventive Example.
[0077]
Based on the above findings, the present invention provides the method for
granulating a sinter mixture, the method including: wet grinding iron ore using a
vertical grinding mill, the vertical grinding mill including: a grinding unit formed of a
cylindrical vessel provided with a vertical central shaft configured to be rotationally
driven and a screw blade mounted to the vertical central shaft; a classification unit
configured to perform classification by gravity and centrifugal force; and a recirculating
unit configured to recirculate underflow from the classification unit into the cylindrical
vessel of the grinding unit; and adding an iron ore slurry obtained by the wet grinding to
the whole or part of the sinter mixture and granulating the thus prepared sinter mixture.
[0078]
In the method, it is preferred that: the sinter mixture contain a particular brand
of iron ore smaller fines (e.g., pellet feed) in an amount of greater than 13.20 mass % to
20.00 mass % based on the total mass of the sinter mixture, the particular brand of iron
ore smaller fmes comprising particles in which particles having a size of 250 pn or less
account for 80 mass % or more, the particular brand of iron ore smaller fines having a
total iron content of 60 mass % or more; and that: the iron ore slurry be added to the
whole or part of the sinter mixture, the iron ore slurry being added in such a manner that
very small particles having a size of 10 jm or less among iron ore particles in the iron
ore slurry are present in an amount of 0.01 mass % or more relative to 1.0 mass % of the
iron ore smaller fines.
[0079]
Set forth below is a summary of production of sintered ore which employs the
granulation method of the present invention. For example, the following configuration
may be adopted: the sinter mixture is formed of iron ore containing large quantities of
pellet feed as a high-quality iron ore smaller fines, auxiliary material, return fines,
carbonaceous material, and the like; and the process is implemented through three
systems: a granulation system A, a granulation system B, and a grinding system. The
materials in the granulation system A are granulated by a granulator including drum
mixers to be formed into pseudo particles. The materials in the granulation system B
are blended and moisture conditioned by a high speed stirrer mixer, and then granulated
by a pan pelletizer to be formed into coarse pseudo particles.
[OOSO]
The materials in the grinding system are ground and mixed together with water
by wet grinding using a tower mill, a type of vertical grinding mill, thereby being
formed into an iron ore slurry. The iron ore slurry produced in the grinding system is
fed to the high speed stirrer mixer together with the materials of the granulation system
B when granulation is performed in the granulation system B, so that it serves as a
binder for granulation of the materials of the granulation system B. The pseudo
particles produced by granulation by the respective granulation systems are mixed
together during the process of being charged into a surge hopper, and further mixed
during the process of being fed by a roll feeder and during the process of falling and
depositing on a pallet of a sintering machine to form a bed of sinter mixture.
[008 11
The thus formed sinter mixture bed is ignited at the top surface by an ignition
hood and burned sequentially from upper to lower regions of the sinter bed by drawing
of air from below while moving h m a feed inlet to a discharge outlet. The sinter cake
after burning is discharged fkom the discharge outlet of the sintering machine. Then it
is subjected to crushing by a crusher and cooling by a cooler, followed by screening by
a screen to be delivered to a blast furnace.
[0082]
In the present invention, wet grinding is adopted as a technique of preparing an
iron ore sluny to be added to the sinter mixture. The reason for this is set forth below.
As previously noted, the very small particles are required to be adequately blended with
added water because of the mechanism to provide the advantage of the very small
particles acting as a binder. In this regard, wet grinding is an outstanding technique
that satisfies the need by grinding iron ore and suspending the very small ground
particles in water without the use of a dispersant or the like.
[0083]
For wet grinding, a tower mill, a type of vertical grinding mill, is used. As
stated above, tower mills are capable of generating a large grinding force by complex
stimng in a circumferential direction and in a vertical direction. Thus, they are
capable of producing very small particles of good characteristics to act as a binder
which are attributable to numerous irregularities that are formed on their surfaces.
INDUSTRIAL APPLICABILITY
[0084]
With the method for granulating a sinter mixture according to the present
invention, it is possible to ensure the air permeability of a sinter bed and thus maintain
the capability to produce sintered ore at low cost even when the percentage of smaller
fines in the sinter mixture is increased by using large quantities of particular brand of
high-grade iron ore smaller fines such as pellet feed. Accordingly, the present
invention is very useful as a technique that is capable of addressing the depletion of
high-grade iron ore fines.
REFERENCE SIGNS LIST
[OOSS]
1 : tower mill (vertical grinding mill), 2: vertical central shaft,
3: screw blade, 4: cylindrical vessel, 5: elutriation chamber,
6: cyclone classification system, 7: recirculating pump, 8: piping,
9: piping
We claim:
1. A method for granulating a sinter mixture including: preparing a sinter mixture
by mixing iron ore with carbonaceous material, auxiliary material and return fines; and
subjecting the sinter mixture to blending, moisture conditioning, and granulating,
characterized in that the method comprises:
wet grinding the iron ore using a vertical grinding mill, the vertical grinding mill
including:
a grinding unit formed of a cylindrical vessel provided with a vertical
central shaft configured to be rotationally driven and a screw blade mounted
to the vertical central shaft;
a classification unit configured to perfom classification by gravity and
centrifugal force; and
a recirculating unit configured to recirculate underflow from the
classification unit into the cylindrical vessel of the grinding unit; and
adding an iron ore slurry obtained by the wet grinding to the whole or part of the
sinter mixture and granulating the sinter mixture.
2. The method for granulating a sinter mixture according to claim 1, characterized
in that:
the sinter mixture contains a particular brand of iron ore smaller fines in an
amount of greater than 13.20 mass % to 20.00 mass % based on the total mass of the
sinter mixture, the particular brand of iron ore smaller fines comprising particles in
which particles having a size of 250 p or less account for 80 mass % or more, the
particular brand of iron ore smaller fines having a total iron content of 60 mass % or
more; and
the iron ore slurry is added to the whole of the sinter mixture or part of the
sinter mixture in which the iron ore smaller fines are contained in an amount of 50
mass % or more, the iron ore slurry being added in such a manner that very small
particles having a size of 10 pm or less among iron ore particles in the iron ore sluny
are present in an amount of 0.01 mass % or more relative to 1.0 mass % of the iron ore
smaller fines.