FIELD OF INVENTION
The present invention generally relates to the field of sintering of iron ore fines.
Specifically, it relates to the technique of improving the reduction degradation
properties of the sinter.
BACKGROUND OF THE INVENTION
Blast furnace iron making is a popular route for production of metallic iron from iron
ore. In India, mechanized mining of iron ore generates ~70% fines (< 10 mm in size)
that cannot be directly charged into the blast furnace as they adversely affect the
permeability of the bed and therefore the productivity of the furnace. For this reason,
ore fines are typically agglomerated to form bigger particles before being charged into
the furnace. Sintering is one of the most commonly used methods of agglomerating
iron ore fines. In the sintering process, agglomeration of ore fines takes place due to
partial melting and re-solidification of ore particles.
In this process, a mixture of ore fines (typically in the size range 0.15-10 mm), solid
fuel and fluxes are granulated in a rotating drum using water to form 'green mix'. Solid
fuel (typically coke breeze) is used to generate the heat required for melting of the ore
particles. Fluxes such as limestone, pyroxenite, olivine, dolomite, etc that contain CaO
and MgO are added in order to adjust the chemistry of sinter required to maintain a
specific slag chemistry in the blast furnace. The green mix that in formed after
granulation is then charged onto a sinter strand and the top surface is ignited while
applying a negative pressure across the height of the bed. Air is drawn through the bed
due to the negative pressure and results in combustion of solid fuel that generates the
heat required for melting and agglomeration of particles. At the end of the process, a
hot cake is formed that is convectively cooled to room temperature and reduced in size
to less than 40 mm. Crushed sinter is then screened to remove particles less than 5 mm
in size. Sinter in the size range 5-40 mm is sent to the blast furnaces for iron making.

In order to be a preferred blast furnace burden, sinter is required to possess certain
room temperature as well as high temperature properties. Properties such as Tumbler
Index (TI), Abrasion Index (AI) and Shatter Index (SI) are indicative of sinter strength
in the cold condition. While TI and SI should be as high as possible, AI should be as low
as possible. High temperature properties such as Reduction Degradation Index (RDI)
and Reducibility Index (RI) are indicative of sinter quality in various zones in the blast
furnace. Sinter should possess low RDI and high RI. Typical values of sinter properties
required by the blast furnaces are TI > 72, RDI < 32 and RI > 68.
RDI of sinter is relevant in the 500-600 °C range in the blast furnace. In this
temperature range, hematite is reduced by CO gas to magnetite. This reaction is
associated with change in the crystal structure (hexagonal hematite to cubic magnetite)
and increase in volume of the unit cell that result in generation of micro-cracks and
subsequently fines from sinter. RDI is used to quantify the amount of fines (< 3.15
mm) that are generated from sinter when it is reduced using CO gas at 550 °C. RDI is
known to be particularly sensitive to secondary hematite that is formed during the
cooling stage in sintering. It is desirable to have as low RDI as possible because
generation of fines in the blast furnace lowers the bed voidage and restricts flow of gas.
High RDI and lower TI is a problem that is typically associated with sinter made using
Indian iron ores as they have a high percentage of alumina (~2.3-2.5% compared to 1-
1.4% elsewhere in the world). The adverse effect of high alumina on the sintering
process and sinter quality is well documented in Lu et al1). Higher alumina decreases
the amount and increases the viscosity of melt that is formed during the sintering
process resulting in weaker bonds and therefore lower strength. Further, alumina
containing hematite formed during the sintering process is known to generate
magnetite with a distorted structure during low temperature degradation which
together with the structural deformation associated with the volumetric change of the

hematite crystal can promote crack initiation and propagation leading to a higher RDI15.
Several techniques have been proposed in the literature to control of sinter RDI:
a) Sintering at higher solid fuel is known to decrease hematite in sinter and
increase magnetite and wustite (FeO). Lower hematite leads to reduced degradation
and hence lower RDI2). While this technique can be easily adopted, the TI of sinter
is known to drop after a certain level of solid fuel. Increasing solid fuel in sintering
without augmenting the cooler capacity would lead to discharge of hot sinter leading
to operational problems. Further, there are environmental concerns associated with
increased CO2 emissions at higher solid fuel.
b) Blending low alumina ore fines and selective granulation of high alumina
fines are techniques which are based on decreasing the amount of alumina in the
adhering particles (<0.25 mm in size) in order to increase the amount of melt
formed during sintering1,3). While blending of low alumina ore fines is a favorable
technique, its applicability is limited due to limited availability of low alumina fines in
India. On the other hand, selective granulation in which adhering particles
containing high alumina are pre-granulated is not widely practiced on an industrial
scale probably due to the additional complexity and cost associated with a pre-
granulator and the binder that is required to maintain sufficient strength of the
granules.
c) Adjusting the size of solid fuel and fluxes is also known to improve RDI of
sinter. Increasing the fineness (< 3.15 mm) of coke breeze3) or eliminating the <0.5
mm fraction from limestone45 was found to decrease sinter RDI. However, this
technique also has limited practical applicability because in most cases increasing
the desired size fraction also increases the undesired size fraction leading to dilution
of the expected benefit. For example, when the percentage of < 3.15 mm of coke
breeze is reduced in a rod mill, the percentage of < 0.25 mm also increases. This is
undesirable as the < 0.25 mm fraction of coke breeze burns too quickly leading to
lower sinter strength.

It is clear from the above discussion that the various techniques available for RDI
control have limitations and there exists need for a new technique of controlling RDI.
Therefore, a method for controlling RDI using a combination of sinter CaO and the
suction pressure employed during sintering has been proposed. Apart from improving
RDI, this method has the added advantage of improving the productivity of the
sintering process.
OBJECTS OF THE INVENTION
a) It is an object of the present invention to propose a technique for producing sinter
having improved reduction degradation index
b) Another object of the invention is to improve the productivity of sinter by employing
the above technique
SUMMARY OF THE INVENTION
The chemical constituents of sinter (CaO, MgO, SiO2 and Al2O3) and the suction
pressure applied during sinter are important parameters that control the formation and
stabilization of sinter phases and influence the sinter quality and productivity. In
general, increasing sinter CaO leads to the formation of more calcium ferrites and less
magnetite and secondary hematite; hence is favorable in terms of RDI. The amounts of
phases are, however, influenced by the temperature profile that controlled by the
suction pressure. On the other hand, increasing suction leads to higher flow rate of air
through the bed and is known to enhance re-oxidation of magnetite to secondary
hematite that has a deleterious effect on RDI.
In this invention, a combination of high sinter CaO (>12%) and high suction pressure (
> 9.5 kPa) is proposed in order to increase the amount of unassimilated hematite and
decrease the amount of magnetite precipitated during sintering and therefore, decrease
the extent of re-oxidation to secondary hematite leading to a lower RDI. Here, high CaO

increases the volume of calcium ferrite melt formed during sintering and high suction
decreases the time spent by the material above 1200°C and therefore the time available
for dissolution of primary hematite in the melt and precipitation of magnetite. During
cooling, the extent of re-oxidation is low due to the lower amount of magnetite
available after precipitation. The gangue content of magnetite and secondary hematite
is also lower at higher suction due to lower time available for dissolution of gangue in
the melt. This combination of CaO and suction, therefore, leads to the formation of
sinter that contains more primary hematite and lesser magnetite and secondary
hematite that leads to lower RDI. This invention also improves the productivity of the
sintering process as higher suction lowers the sintering time.
This invention does not suffer from the limitations associated with other techniques
available for improving sinter RDI. As it does not involve increasing the solid fuel during
sintering, concerns of increased CO2 emissions do not arise. This technique does not
demand blending of low alumina ore fines; it can be applied to high alumina iron ore
fines. Unlike the case of selective granulation, there is no need to modify the flow sheet
of the sintering process while using this invention. Further, this invention does not
necessitate modifications in the sizes of solid fuel and fluxes used in sintering. This
invention, therefore, solves the problems associated with the current processes of
improving RDI. It is easier to implement on an industrial scale and offers the additional
advantage of improving sinter productivity.
Brief Description of the Drawings
Fig. 1 Effect of suction pressure on RDI of sinter of different CaO contents

DETAILED DESCRIPTION OF THE INVENTION
Iron ore sinter should possess a high value of cold strength and low RDI for stable blast
furnace operation. Sinter made using Indian iron ore fines typically has high values of
RDI (>30) as the alumina content of ore fines is quite high (2.3-2.5% compared to 1-
1.4% elsewhere in the world). Several techniques have been proposed in the literature
to dilute the effect of alumina and improve sinter RDI. However, most of them have
practical limitations. The present invention proposes an effective process for controlling
RDI through a combination of higher CaO and higher suction pressure employed during
sintering. The current method improves reduction degradation index of iron ore sinter
and involves the steps of dry mixing raw material containing iron oxides, fluxes
containing MgO and Cao, and solid fuel, where concentration of Cao is maintained in
the range of 12 to 19 weight% of the total dry mixture, adding water to the dry mixture
in the range of 5-8 weight percent of the dry mixture and granulating in a granulating
drum to achieve granulation index of 85% (<0.25 mm), charging the wet mixture in a
grated container, igniting top surface of the wet mixture, and applying negative
pressure at the bottom of the grated container. The negative pressure at the bottom of
grated container is increased to at least 9.5 KPa after ignition of the wet mixture is
complete. Preferably, the negative pressure at the bottom of grated container is
increased in the range of 9.5 KPa to 17 KPa after ignition of the wet mixture is
completed.
The raw material as per the current invention includes iron oxide from iron ore, steel
plant reverts or a combination thereof. The iron ore fines as per the current invention
should be of hematitic in nature with alumina and silica contents less than 3% and 5%
respectively. The invention makes use of carbonate fluxes and limestone or dolomite
can be used for the purpose. The size range of carbonate fluxes is less than 3.15 mm.
Further, the grated container is a sinter pot or a sinter strand.

The properties of sinter depend on the amount of phases present in sinter which in turn
depend on several factors such as the chemistry of sinter, the granulometry of raw
materials, the amount of solid fuel and the suction pressure employed. Formation of
phases in sinter is a complex process that involves the following steps: Combustion of
solid fuel generates heat and the temperature of the bed increases due to which
calcination of carbonate fluxes takes place. At higher temperatures, the calcined flux
reacts with iron oxide to form calcium ferrite that melts at around 1200°C; this is known
as primary melt. Gangue minerals such as SiO2 and Al2O3 present in iron ore dissolve in
the primary melt to form secondary melt. When combustion of solid fuel is complete,
the bed temperature drops and the secondary melt solidifies. During re-solidification,
complex calcium ferrites (known as Silico-Ferrites of Calcium and Aluminium),
magnetite, hematite and calcium silicates precipitate from the melt. Re-oxidation of
magnetite to secondary hematite may take place during further cooling. Any hematite
particles that do not dissolve in the primary melt also remain in the final sinter. The
amounts of phases depend on the time spent above 1200°C during which dissolution of
original phases and precipitation of new phases takes place.
At a given solid fuel level, increasing the CaO content of sinter mix leads to an increase
in the amount of ferrites as more amount of CaO is available for forming ferrites by
reacting with iron oxide. Simultaneously, the amount of magnetite decreases and the
amount of secondary hematite formed after re-oxidation decreases. Therefore,
increasing the CaO content is known to be favorable in terms of sinter RDI. On the
other hand, it is also known that increasing suction at any CaO level enhances re-
oxidation of magnetite to secondary hematite due to increase in the flow rate of air
through the sinter bed6). This is turn has an adverse effect on sinter RDI. However, the
amount of secondary hematite formed would also depend on the amount of magnetite
formed after precipitation. For a given percentage of solid fuel, increasing the suction
during sintering typically decreases the time spent by the material at high temperature
as the flow rate of air through the bed increases and leads to faster cooling.

In the present invention, the effects that sinter CaO and suction pressure have on the
formation of phases in sinter are advantageously exploited to improve RDI of sinter.
According to the present invention, certain values of sinter CaO and suction pressure
are employed during sintering of iron ore fines in order to obtain a high amount of
unassimilated hematite and lower amounts of magnetite and secondary hematite in
sinter. Higher sinter CaO ensures that calcium ferrites precipitate predominantly during
re-solidification. Also, a high value of suction pressure decreases the time spent by the
sinter mix above 1200°C leading to a large percentage of primary hematite and less
precipitated magnetite. In contrast to earlier findings reported in the literature,
employing higher suction in the present invention does not lead to higher secondary
hematite because at higher CaO levels, formation of calcium ferrites takes preference
over formation of magnetite as a result of which the amount of magnetite and
subsequently, the amount of secondary hematite formed is lower compared to that
formed at lower CaO levels. Therefore, the combination of high CaO and high suction
pressure results in desired phase percentages which lead to a lower sinter RDI. The
present invention also has an added advantage of improving the sinter productivity
because for a given level of CaO, increasing the suction pressure decreases the time
required to produce the same amount of sinter.
Fig. 1 shows the effect of suction pressure employed during sintering on RDI of sinter
containing different levels of CaO. It can be observed from the figure that at CaO levels
less than 12.5%, RDI of sinter increased with increase in suction pressure. However,
when the sinter CaO was greater than 12.5%, RDI of sinter decreased with increase in
suction pressure.
The present invention can be applied to hematitic iron ore whose alumina content is
less than 3% and silica content is less than 5%. For any given iron ore fines, the level
of sinter CaO and the suction pressure beyond which sinter RDI is in the acceptable

range can be determined using pilot scale sintering experiments. The following example
shows the specific embodiments of the present invention:
Example
This example demonstrates reduction in sinter RDI by increasing suction pressure for a
given level of sinter CaO. Pilot scale sintering trials were conducted at three levels of
suction -9, 11 and 13 kPa to produce sinter with a CaO level of 12.5%. The chemistry
and granulometry of raw materials used in the experiments are shown in Tables 1 and
2 respectively. The target sinter chemistry was CaO - 12.5%, MgO - 1.6% and
CaO/SiO2 (basicity) - 2.8. The amounts of materials used in the sinter mix are:
Hematitic iron ore from source X - 60.4 wt%, Coke breeze - 4.5 wt%, Limestone - 16
wt%, Pyroxenite - 2.0 wt%, Calcined lime - 1 wt% and Sinter return fines - 16 wt%.


The bed height, suction during ignition, ignition temperature and ignition time were
kept constant at 600 mm, 6 kPa, 1100°C and 2 min respectively in the sintering
experiments. Pot sintering experiments was conducted in a square pot having a cross
section area of 0.16 m2. After the green mix was charged into the pot up to the height
of 600 mm, the top of the green bed was ignited using an ignition hood fuelled by coke
oven gas while simultaneously applying suction across the bed. After the ignition phase,
the suction was increased and maintained constant throughout the experiment. The
experiment was considered to be complete when the temperature of the off gas from
the bed reached a maximum known as the Burn Through Point (BTP). The hot sinter
cake from the experiment was allowed to cool to room temperature after which it was
stabilized by dropping the cake from a height of 3 m thrice onto a steel plate.
Stabilized sinter was then screened into various size fractions and tested for its Tumbler
Index (TI) and RDI. Tumbler Index test was performed according to the IS 6495:1984
standard while RDI test was performed according to the IS 10823:1994 standard. Sinter
quality and productivity from the tests is shown in Table 3. It is clear from the table
that beyond a suction pressure of 9 kPa, the RDI of sinter is < 30 and the RDI
decreased by ~5 percentage points when the suction pressure was increased from 9 to
13 kPa. It can also be observed from the table that the productivity improved steadily
with increase in suction pressure. It also be seen from the table that the cold strength
of sinter is not affected by variation in suction pressure.


REFERENCES
1) L. Lu, R.J. Holmes and J.R. Manuel, 2005, "Effects of alumina on sintering
performance of hematitic iron ores", ISIJ, 47 (3), 349
2) T. Umadevi, P. Karthick, P.C. Mahapatra, M. Prabhu and M. Ranjan, 2012,
"Optimization of FeO in iron ore sinter at JSW Steel Limited", IMSM, 39 (3), 180
3) C.E. Loo, 2005, "A perspective of geothitic ore sintering fundamentals", ISIJ, 45 (4),
436
4) T. Umadevi, A.V. Deodhar, S. Kumar, C.S. Gururaj Prasad and M. Ranjan, 2008,
"Influence of coke breeze particle size on quality of sinter", IMSM, 35 (8), 567
5) R. P. Bhagat, S.K. Sil, U.S. Chattoraj, M.C. Goswami, D.P. Singh and B.K. Das, 2006,
"Pot Sintering studies for improvement in reducibility and RDI of sinter", Tata
Search, Vol. 1, 127
6) M.K. Choudhary and B.Nandy, 2006, "Effect of flame front speed on sinter structure
of high alumina iron ores", ISIJ, 46 (4), 611

WE CLAIM:
1. A method for improving reduction degradation index of iron ore sinter, the method
comprising:
dry mixing raw material containing iron oxides, fluxes containing MgO and Cao,
and solid fuel, concentration of Cao being maintained in the range of 12 to 19
weight% of the total dry mixture;
adding water to the dry mixture in the range of 5 to 8 weight percent of the dry
mixture and granulating in a granulating drum to achieve granulation index of
85% (<0.25 mm);
charging the wet mixture in a grated container;
igniting top surface of the wet mixture; and
applying negative pressure at the bottom of the grated container, the negative
pressure at the bottom of grated container is increased to at least 9.5 KPa after
ignition of the wet mixture is complete.
2. The method according to claim 1, wherein the negative pressure at the bottom of
grated container is increased in the range of 9.5 KPa to 17 KPa after ignition of the
wet mixture is complete.
3. The method according to claim 1, wherein the raw material containing iron oxide is
selected from iron ore, steel plant reverts and a combination thereof.

4. The method according to claim 3, wherein the iron ore fines are hematitic in nature.
5. The method according to claim 4, wherein alumina and silica contents in the iron
ore fines are less than 3% and 5% respectively.
6. The method according to claim 1, wherein the source of CaO is carbonate flux such
as limestone and dolomite.
7. The method according to claim 6, wherein size range of the carbonate fluxes is less
than 3.15 mm.
8. The method according to claim 1, wherein the grated container is a sinter pot.
9. The method according to claim 1, wherein the grated container is a sinter strand.
10.The method according to claim 1, wherein the size of iron ore fines is less than 10
mm.

ABSTRACT

A low reduction degradation index is desired in Iron ore sinter. Indian iron ore fines
contain a high percentage of alumina and this has a deleterious effect of sinter quality,
particularly RDI. This invention proposes a method for improving the RDI of sinter by
increasing the CaO content and the suction pressure employed during sintering. This
combination of sinter CaO and suction pressure helps in controlling RDI by increasing
the amount of unassimilated hematite and decreasing the amount of secondary
hematite present in sinter.