FIELD OF THE INVENTION
The present invention relates to a method for improving the reducibility and the
swelling index of iron ore pellets.
The present invention relates to a method for increasing the reducibility and
decreasing the swelling of iron ore pellets i.e., concurrently improving both
reducibility and swelling behavior of iron ore pellets, without making any change in
its chemical composition.
BACKGROUND AND PRIOR ART TO THE INVENTION
High quality pellets are required as feed for different iron making processes like
Blast Furnace, Midrex, Corex etc. Reducibility and swelling index of pellets are two
such quality parameters which decides the suitability and superiority of pellets in
terms of quality.
Reducibility of iron ore pellet is an important property since it has direct influence
on improving producivity and lowering energy consumption in iron making
process. Higher reducibility results in faster and higher degree of indirect reducion
in blast furnaces and thereby increases producivity and decreases energy
consumption. Lower reducibility on the other hand, results in slower and low
magnitude of indirect reducion. As a result of low reducibility, higher coke rate is
required to meet the thermal demand raised by increased magnitude of direct
reducion. This results in lower producion with poor hot metal quality.
During reducion of pellets inside blast furnaces, there is an effective increase in
pellet volume due to change in metallurgical phases in pellets from hematite to
magnetite, magnetite to Wustite, and finally Wustite and iron. This volume increase
result in swelling of pellets. A significant swelling of pellets results in crack
formation and disintegration of pellets Therefore, lower swelling of pellets is
required so that volume expansion of pellets is minimal possible in blast furnaces
during reducion and avoids any disintegration. Excess swelling obstructs the gas
passage through the blast furnace burden and fines generated due to excess
swelling and disintegration further decreases the voidage of bed and deteriorates
the gas acceptance of furnace. Higher swelling of pellets can also result in hanging
of burden since pellet burden does not get accommodated easily in the available
volume in furnace due to its increased bulk volume and gets stuck up in the

furnace. These factors result in lower producivity, higher coke rate, and poor hot
metal quality.
The general correlation of reducibility and swelling index of pellets is positive i.e. if
reducibility increases swelling also increases and vice versa. Therefore generally an
optimize value of reducibility and swelling are targeted so that reducibility of pellets
does not result in excess swelling and disintegration of pellets.
In order to improve the reducibility of iron ore pellets, various methods have been
tried and are mentioned henceforth.
Porous iron ore pellets and a process for manufacturing same was described by
Kazumasa Taguchi et al in Patent No US4350523. Pellets having a pore size
distribution consisting of more than 30% of pores having a diameter greater than
10 microns and a balance of pores having a diameter smaller than 10 microns, a
total porosity greater than 30%, and an FeO content less than 1% by weight was
described.
Marcus Eduardo Emrich BOTELHO et al in Patent No US 20140096650 Al
described a method of improving reducibility comprising the steps of preparing a
raw material mixture which contain metallic Ni powder, pelletizing the said mixture
obtained, burning the said raw pellets and reducing the said burnt pellets under
reducing conditions with presence of CH4.
Isao Fujita et al in Patent No CA 1099519 A described an invention related to fired
iron-ore pellets having at least two precipitated slag phases of different chemical
compositions which appear among iron-oxide grains. The pellets having two
different slag phases contain not less than 4% by weight of CaO + Si02, so that
large cracks may be produced in the course of reducion. The cracks thus formed
cause each pellet to be split into several pieces. These pellets thus exhibit excellent
and improved reducibility.
Yatsunami Kazuharu in Patent No JPS55107738 (A) described a process to raise
the reducibility of green pellets and enhance the producivity thereof by specifying
the fine ore blending rate of the pellets and carrying out pelletizing with water of a
value higher than the proper value to flatten the pellets. Fine iron ore is blended in
a rate of 20-50% in a blending process, and the water content of the blend before
pelletizing is adjusted to a value 0.5- 2.0% higher than the max. value of proper

water content values. The blend is then pelletized with a pelletizer. The blending
rate and the water content value enable the resulting pellets to be flattened. By this
flattening the reducibility of the pellets themselves is enhanced to assure sufficient
pellet strength.
Above cited inventions along with many available literature had associated
disadvantages with them and application of them either results in decreased
crushing strength, increased swelling, increased cost or are very complex in
naturefor implementation in actual pellet plant units. Most of the times improved
reducibility resulted in worsening of other metallurgical parameters especially
swelling. In this work a method is developed to improve the reducibility of iron ore
pellets while concurrently improving swelling index of pellets. The said method also
resulted in not having any adverse effect on cold crushing strength of pellets. Since
the said method in this work addressed the disadvantages associated with earlier
work therefore it makes the process of producing high reducibility pellets more
attractive.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a method for improving the
reducibility and the swelling index of iron ore pellets.
It is a further object of this invention to propose a method for improving the
reducibility and the swelling index of iron ore pellets whereby the reducibility and
swelling indexes of iron ore pellets are concurrently improved i.e. reducibility of
iron ore pellets is increased while swelling of iron ore pellets is decreased.
Another object of the present invention is to propose a method for improving the
reducibility and the swelling index of iron ore pellets in which cold crushing
strength of iron ore pellets either remains the same or improves while reducibility
increases and swelling decreases.
Yet another object of this invention is to propose a method for improving the
reducibility and the swelling index of iron ore pellets in which more high melting
point slag is formed to stabilize the pores in the pellets to ensure high porosity and
high reducibility.
A further object of the invention is to propose a method for improving the
reducibility and the swelling index of iron ore pellets in which metallurgical phases

with high hardness are formed to overcome the disturbing forces during reducion
and ensures lower swelling of pellets.
A still further object of the invention is to propose a method for improving the
reducibility and the swelling index of iron ore pellets, which is simple and cost
effective.
These and other objects and advantages of the invention will be apparent from the
ensuing description when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1: Schematic of conventional MgO based pellet making process
Fig 2: Schematic of proposed MgO based pellet making process for improving
reducibility and swelling
Fig 3: Figure showing CCS of pellets prepared using conventional (set 1) and
proposed method (set 2)
Fig 4: Figure showing Swelling of pellets prepared using conventional (set 1) and
proposed method (set 2)
Fig 5: Figure showing Reducibility of pellets prepared using conventional (set 1) and
proposed method (set 2)
Fig 6: Figure showing comparison of metallurgical phases in pellets prepared using
conventional (set 1) and proposed method (set 2)
Fig 7: Fig showing comparison of hardness of slag bonds in pellets prepared using
conventional method (set 1) and proposed method (set 2)
SUMMARY OF THE INVENTION
This invention relates to a method for improving the reducibility and the swelling
index of iron ore pellets by which both the reducibility and swelling index of pellets
are improved without any adverse effect on cold compressive strength of pellets and
without change in pellet chemistry.
During pellet making, various factors contribute to the development of porous
structure of pellets which results in improved porosity of pellets. Generally,
addition of very fine materials other than limestone in pellet making, results in
blockage of pores and thus decreases porosity. In this work, however, addition of
very fine MgO based flux has been used to stabilize pores, due to formation of high

melting point slag in the pellets structure. These pores in turn resulted in improved
reducibility of pellets. Additionally, for a particular chemical composition of pellets,
improved reducibility generally deteriorates the swelling behavior of pellets i.e. with
increase in reducibility for a given chemical composition pellets, swelling also
increases and deteriorates pellets quality. But in this work, addition of MgO based
flux, in very fine particle size distribution, results in increased hardness of slag
phases which absorbs the additional disturbing forces arising due to increased
reducibility during reducion swelling phenomenon. This resulted in improving the
swelling index of pellets while concurrently improving reducibility index. Formation
of high melting point slag phases also results in decreasing/avoiding any coating of
individual grains of pellets by slag which results in availability of more surface area
for reducing gases and thereby more reducibility. Addition of fine particles
supported formation of more slag phases which in turn resulted in maintaining the
cold crushing strength of pellets.
BRIEF DESCRIPTION OF THE INVENTION
This invention relates to a method for improving the reducibility and the swelling
index of iron ore pellets to concurrently improve reducibility and swelling index of
iron ore pellets without affecting other properties like cold crushing strength etc.
In iron ore pelletization, iron ore fines, fluxes and solid fuel are mixed, dried and
grounded to make powder, suitable for pellet making. This powder is then used for
making wet spherical ball after addition of bentonite and water. These wet balls are
then fired at temperature of 1200-1300 °C for achieving suitable strength.
In this work, a method has been developed to produce pellets with high reducibility
and low swelling. In this method, MgO bearing fluxes were separately grounded to
very fine particle size and added to the mixture of iron ore powder and other fluxes
etc. material for making pellets. Separate grinding of MgO bearing flux resulted in
better control over particle size distribution of MgO bearing phases so that finer
product can be achieved. In co-grinding, due to difference in grindability index of
MgO bearing flux and iron ore, control over MgO particles is very limited. Addition
of very fine MgO bearing fluxes results in easier dissociation of fluxes during pre-
heating and better assimilation of MgO bearing phases during firing.
Better assimilation of MgO bearing flux results in formation of slag bonds with
higher MgO content. With increase in slag MgO content, its melting point increases

and thus it does not flow out to fill inherent pores and voids in the pellets. This
results in higher porosity and high reducibility of pellets. Additionally, high melting
point slag, due to its low flowability, does not coat the surface of individual grains
of pellets which in turn results in higher surface area availability for reducing gases
and thus higher degree of reducion .The hardness of slag bonds also increases with
decrease in size of MgO bearing fluxes because of increase in the MgO content of
slag bonds. This results in decreasing the extent of distortion and deformation
arising due to increased magnitude of reducion which in turn, decreases the
tendency of swelling of iron ore pellets during reducion swelling phenomenon.
Since the chemical composition of pellets, particle size distribution of iron ore,
firing profile etc. were not altered or any other ingredient/chemical/material was
not added to improve the reducibility therefore, cold crushing strength of the pellets
was also not affected adversely in this method.
DETAILED DESCRIPTION OF THE INVENTION
Thus according to this invention is provided a method for improving the reducibility
and the swelling index of iron ore pellets
For making iron ore pellets suitable as blast furnace input, iron ore fines,
limestone, pyroxenite, olivine, dunite, anthracite coal, coke breeze etc. are loaded
into ball mill for grinding. The output of the ball mill is then mixed with water and
bentonite and conveyed to balling equipment like disc or drum for formation of wet
spherical balls. These balls are then charged into induration furnace of pellets
plants for firing and achieving suitable crushing strength.
The grindability index of MgO bearing fluxes is much higher than iron ore. For
example: Bond work index (BWI) of dunite, pyroxenite and olivine are -16.5, 17.5
and 22 kWh/t respectively which is significantly higher than iron ore -9.5-10.5
kWh/t. Since there is a significant difference between the BWI value of MgO
bearing phases and iron ore therefore co-grinding method of preparing pellet feed
as described in previous paragraph does not help in controlling particle size
distribution of MgO based fluxes. Even though the mean particle size of whole
mixture is 75-85 lira but the mean particle size of MgO based flux is significantly
higher than 75-85 lira Due to high grindability value, MgO bearing fluxes are
grounded to coarser size while iron ore gets grounded to finer particle size
distribution. The coarser particle size distribution of MgO based fluxes leads to its

poor participation in assimilation into slag bonds during firing. This process
though supports in maintaining the softening melting properties of pellets at higher
temperature but it does not help in improving the reducibility of the pellets.
Improving reducibility by any other means like decreasing firing temperature leads
to poor CCS, higher swelling etc. while addition of any other compound or
increasing particular flux changes the chemistry of pellets which is not desirable as
per blast furnace operation.
In this work, MgO bearing fluxes with mean particle size less than 65 jirn and more
preferably less than 50 pm were proposed to add into the mixture of ground iron
ore, limestone and coal/coke breeze. These MgO bearing flux needs to be ground
separately and mixed with the mixture of ground iron ore, limestone and anthracite
coal/coke breeze. Schematic of the conventional method of pellet making is
provided in Figure 1. Schematic of the proposed method in this work is provided in
Figure 2. These schematics are described hereafter.
In the conventional method as shown in schematic given in Fig 1, iron ore fines (1),
limestone fines (2), MgO based fluxes like dunite, Olivine, pyroxenite etc. (3), and
solid fuel like coal/coke (4) are charged into dryer (5) for drying. After drying the
material is conveyed into grinding mill (6). The output of grinding mill along with
bentonite and water is charged into high intensity shear type mixer (7). The wet
mixture discharged from mixing unit is conveyed to balling equipment (8) either
drum or disc. The wet spherical balls formed in balling equipment are then
conveyed and charged into induration furnace (9) wherein pellets are dried, fired
and heat hardened to achieve suitable strength for iron making furnaces.
In the proposed method (fig 2) in this work, wherein both reducibility and swelling
index of the pellets are improved, iron ore fines (1), limestone fines (2) and solid fuel
(anthracite coal/coke fines (3) are charged into the drying unit (4). Dried fines
obtained from drying unit (4) are then charged into grinding unit like ball mill etc.
(5). Simultaneously, MgO based fluxes (6) like dolomite, dunite, olivine, pyroxenite,
magnesite etc, are charged into another grinding unit (7). Grinding unit (7)
produces MgO based ultra-fines with mean particle size less than 65 urn and more
preferably less than 50 nm. Ground powder from ball mill (5) and ball mill (7) are
then charged in high intensity shear mixer (8). Bentonite and water are also added
to the mentioned mixture in the same high intensity mixer (8). Wet mixture of
powders is then discharged from mixing unit (8) and conveyed to bailing equipment

(9) where wet spherical balls are formed which are then charged into induration
furnace (10) for drying, preheating, firing and attaining heat hardened strength
suitable for iron making furnaces like blast furnaces. Ultrafine MgO based flux
added in this work supports to produce pellets with high reducibility and lower
swelling index without having any adverse effect on cold crushing strength and
without any change in pellet chemical composition. Example 1 describes this
method and basic fundamentals related to the same in detail.
Example 1:
In this work, two sets of pellets were made to simulate the conventional method of
adding MgO based flux as well as method proposed in this work. Effect of proposed
method of MgO based flux addition on reducibility and swelling index of pellets was
also established. Dunite with a chemistry given in table 1 was used as MgO based
flux to simulate both the method in this work. The bond work index of iron ore
used in this work is 9.78 kWh/t while dunite is 16.64 kWh/t. Since Co-grinding of
all the constituents mixed together in pellet making (as shown in fig 1) results in
coarser size of MgO based flux (because of its high BWI value) and finer size iron
ore (because of low BWI value) therefore dunite was grounded separately to achieve
two different size mean particle size. Dunite with 88.31 jam mean particle size was
used to simulate conventional method (fig 1) wherein MgO based flux is coarser
and dunite with 62.70 |am mean particle size was used to simulate the proposed
method wherein MgO based fluxes was grounded to much finer size than iron ore
in terms of mean particle size. First set of pellet, namely set 1, was prepared using
powders of iron ore, limestone, anthracite coal, bentonite and coarser dunite (88.31
|im MPS) while in the second set of pellets, namely set 2, coarser dunite was
replaced with finer dunite (62.7 (am MPS). The amount of MgO based flux (i.e.)
dunite was kept at 3.29% of dry mixture for both the sets of pellets .In both set 1
and set 2 pellets, powders were mixed thoroughly using turbo mixture. 6% water
was added to dry mixture by weight and thoroughly mixed manually to prepare a
uniform wet mixture. Wet mixture was added into a pelletizing disc and additional
water was added during disc rotation. Wet spherical balls were formed in the disc
and wet balls of size (10-12.5 mm) were screened out. Smaller balls were again
poured into the disc and wet mixture was added to increase the size to 10-12.5
mm. Finally all pellets with 10-12.5 size were prepared. These wet balls were then
dried, preheated and fired in pellet plant using inconel basket. Both the set 1 and
set 2 pellets were then tested for cold crushing strength, chemical composition,

reducibility, swelling index and characterized using optical microscope for analysis
of different metallurgical phases.
Table 1: Chemical composition of MgO based fluxes and pellets produced in this
work

It can be seen that the pellet produced using coarse dunite (set 1) and fine dune
(set 2) are identical in chemical composition with some slight differences which
comes due to errors in analysis methods.
Fig 3 gives the CCS of pellets prepared using coarse dunite and fine size dunite
wherein coarse dunite addition simulates conventional method of MgO addition
while fine dunite simulates proposed method in this work. It can be seen that there
is no detrimental effect on CCS rather CCS slightly increased by 1.5 kg/p.
Fig 4 gives the swelling index of pellets prepared using coarse dunite (set 1) and
fine dunite (set 2). It can be seen that there is a 0.45% point decrease in the
swelling of pellets. This amounts to a 3.05% improvement in swelling index of
pellets by proposed method.
Fig 5 gives the reducibility value of pellets for comparing the pellet quality formed
using conventional method (figl) Vis-a-vis proposed method (fig 2). It can be seen
that there is a 2.6% point increase in the reducibility of pellets prepared using
proposed method in this work. This amounts to 3.75% improvement in reducibility
as compared to base (conventional) case.
Fig 6 gives the relative change in metallurgical phases in the pellets prepared using
proposed method vis-a-vis conventional method. It can be seen that Magnesio-
ferrite, a high melting point phase, increased by 13.62% in proposed method as
compared to the conventional way of making MgO based pellets. This phase
supports in retaining the shape of the pellets and don't let pellet structure deform.
Thus it helps in improving the reducibility and swelling index of pellets. In MgO
based iron ore pellets, prepared using proposed method in this work, smaller MgO
based flux particles were used, therefore, assimilation of MgO based flux (in this
case dunite) was easier which supported in more availability of MgO for formation

of Magnesio-ferrite as well as silica for the formation of silicate melt (with high MgO
content) which increased by 24.26% in proposed method as compared to
conventional method.
Smaller MgO based flux particles results in higher silicate melt formation. Higher
silicate melt formation can result in higher CCS. Since the silicate melt contained
higher MgO also, which hinders flow of silicate melt into the pores/voids of pellets
therefore the effect of silicate melt volume on CCS was balanced out by the effect of
low flowable, high viscous silicate melt which binds the pellet grains less effectively
than low melting slag phase.
Silicate melt with high MgO content results in increasing the hardness of silicate
melt. Fig 7 shows the comparative change in hardness of silicate melt (measured
using nano-indentation technique) of pellets prepared using proposed method vis-
a-vis conventional method. It can be seen that hardness of silicate melt or slag
bonds increased significantly using proposed method (set 2) as compared to
conventional method (set 1). Silicate melt being hard, results in overcoming the
deforming and disturbing forces during reducion swelling phenomenon. This
results in lower swelling. High melting point silicate melt due to high MgO content
also supports in increasing the reducibility of pellets by avoiding coating of
individual grains of pellets by slag due its low flowable behavior. It can be seen that
the pellet porosity increased slightly in proposed method and along with
decreasing/avoiding grain coating phenomenon, the proposed method supports in
producing higher reducibility pellets.
Similar results and advantages are expected byincreasing the fineness of other
MgO based flux in pellet making. It is also to be understood that the invention is
not limited by the specific example and embodiment described hereinabove, but
includes such changes and modifications as may be apparent to one skilled in the
art.

We Claim:
1. A method for improving the reducibility and the swelling index of iron
ore pellets, comprising the steps of charging iron ore fines (1),
limestone fines (2) and solid fuel (3) into the drying unit (4)to obtain
dried fines,
charging said dried fines obtained from drying unit (4) into grinding
unit (5) to produce a ground powder,
simultaneously charging MgO based fluxes (6) into another grinding
unit (7) to produce MgO based ultra-fines ,
charging said ground powder from grinding unit (5) and MgO based
ultra-fines from grinding unit (7) in the mixing unit (8) followed by
adding bentonite and water thereto, to form a wet mixture of powders,
discharging said wet mixture of powders from mixing unit (8) and
conveying to balling equipment (9) where wet spherical balls are
formed which are then charged into induration furnace (10) for drying,
preheating and firing.
2. The method as claimed in claim 1, wherein said solid fuel is selected
from anthracite coal fines, coke fines.
3. The method as claimed in claim 1, wherein said grinding unit (5, 7) is
a ball mill etc.
4. The method as claimed in claim 1, wherein said MgO based fluxes are
selected from dolomite, dunite, olivine, pyroxenite, magnesite etc.
5. The method as claimed in claim 1, wherein said mixing unit (8) is a
high intensity shear mixer.

6. The method as claimed in claim 1, wherein said MgO based ultra-fines
have a mean particle size less than 65 µm and more preferably less
than 50 µm.
7. A method as claimed in claim 1 wherein MgO based fluxes are added
after separately grounding to achieve a mean particle size below 65
nm or more preferably 50 µm.
8. The method as claimed in claim 1 which improves the reducibility and
swelling index of iron ore pellets by more than 3%.
9. The method as claimed in claim 1 which improves the reducibility and
swelling of iron ore pellets without having any adverse effect on cold
crushing strength and without any change in chemistry of pellets.