CHAPTER-6:
SUMMARY AND CONCLUSIONS
190
6. SUMMARY AND CONCLUSIONS
6.1 Summary of laboratory test work
Based on the entire laboratory test work, findings are summarized as
following;
6.1.1 Characterization of iron ore fines and fluxes

Iron ore fines from the Noamundi captive mines of Tata Steel contain
high iron values (64 to 66% Fe). Total gangue content ranges from 3.5 to
4%. Around 25% of the fines are minus 150 microns size that can be
readily used for the pelletizing.

From the characterization studies of iron ore fines using QEMSCAN; it
was found that primary iron minerals are hematite and goethite.
Kaolinite, limonite, gibbsite and quartz are present as gangue minerals.
Hematite and goethite are present in the proportion of 68% and 30%
respectively. 90% of the alumina in the sample is associated with
goethite, gibbsite and kaolinite.

Ore is massive in nature showing no clear layering described by different
mineral phases. Goethite is mostly associated with hematite and the
latter is present as inclusion within goethite and vice versa. Size wise
mineralogical study indicated that finer fraction is rich in goethite
whereas hematite is concentrated more in coarse fractions.

Liberation analysis indicated that goethite is more liberated in the finer
fractions and hematite in the coarser fractions. Deportation analysis
showed that Al is predominantly contributed by goethite and the average
Al content in the goethite is around 3%.

XRD analysis of fluxes showed that limestone is composed of calcite,
dolomite and magnesite are composed of magnesite and small
quantities of quartz whereas pyroxenite is primarily composed of
enstatite.
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
From thermo gravimetric analysis (TGA) it was evident that magnesite
dissociation occurs at lower temperatures (around 500 oC) followed by
dolomite (around 700oC) and limestone (around 750oC). Pyroxenite,
which is a magnesium silicate, does not dissociate unlike the above
carbonate fluxes. Devolatilization of coal was found to start from around
315oC.
6.1.2 Grinding, green pelletizing and induration studies

Grinding of the iron ore fines, up to mean particle size (MPS) of
55 microns, was found to be optimum. Over grinding beyond this MPS
generated more amount of ultrafines, <25 micron particles.

Pelletizing feed with MPS of 55 microns exhibited optimum green pellet
properties viz., drop number, green compression strength and moisture
content.

Green pellets prepared from pellet feed of different fineness exhibited
self-preserving behaviour that was used for predicting the size
distribution of green pellets. D50 of green pellets was found to be high at
46 micron MPS of pelletizing feed.

Firing studies of pellets prepared from pellet feed of varying fineness
indicated that firing temperature of 1300 oC and MPS of 55 microns for
green pellet feed are required to attain desired cold crushing strength of
pellets.
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6.1.3 Effect of pellet basicity (CaO/SiO 2) and MgO content on quality and
microstructure of the fired pellets using limestone and dolomite flux
Effect of pellet basicity (CaO/SiO2) and MgO content on the melt formation
and microstructure during the induration of iron ore pellets was examined. Fired
pellets with varying basicity (0 to 0.8) and MgO (0 and 1.5%) content were
tested for cold strength, reduction degradation index, reducibility, swelling and
softening-melting characteristics. Optical microscope studies with image
analysis software were carried out to estimate the amount of different phases.
SEM-EDS analysis was done to record the chemical analysis of oxide and slag
phases. X-ray mapping was also carried out to understand the distribution of
CaO, MgO, SiO2 and Al2O3 in different phases.

With increasing basicity, the amount of silicate melt, which acts bonding
phase, was found to increase in the fired pellets. FeO content of the
silicate melt was decreased with increased basicity of the pellets.
Addition of MgO to both acid and limestone-fluxed pellets resulted in the
formation of high melting point slag during their induration.

Acid pellets exhibited highest swelling, whereas maximum swelling in the
MgO-free pellets was observed at 0.6 basicity. Addition of MgO to both
acid and limestone-fluxed pellets at all the basicity levels considerably
reduced the swelling tendency of pellets due to the formation of high
melting point slag that gives sufficient bond strength to withstand the
reduction stresses.

With
increasing
basicity,
MgO-free
and
MgO
pellets
exhibited
considerably lower reduction degradation compared to acid pellets due
to the formation of more amount of silicate melt, which is more stable
under the reducing conditions in blast furnace.

Reducibility of MgO-free pellets is slightly lower compared to acid pellet
due to the formation of silicate melt in the former, which softens and
impedes the flow of reducing gas within the pellet thereby retarding the
reduction. Addition of MgO to both acid and limestone-fluxed pellets at
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all basicity levels increased their reducibility by forming high melting
point slag which does not soften at reduction temperatures and keeps
the pores open for reducing gas thereby enhancing reduction.

Inferior softening and melting characteristics of the acid pellets could be
attributed to the formation of FeO rich low melting fayalitic liquidus slag.
MgO-free pellets with increasing basicity exhibited increased softening
temperatures and low softening-melting range due to the formation of
burden slag with high liquidus temperature.

To relatively compare pellet quality based on vital quality parameters, a
new dimensionless index called
“composite quality index’
was
formulated. Higher composite index indicates the improved pellet quality
and vice versa. Limestone fluxed MgO-free pellets at 0.8 basicity, and
dolomite fluxed pellets at 0.4 basicity & 1.5% MgO exhibited optimum
metallurgical quality parameters among all the pellets studied.
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6.1.4 Effect of pellet MgO content on quality and microstructure of fired
pellets using magnesite flux
Effect of magnesite addition, to increase the MgO content, on the melt
formation and microstructure during the induration of the iron ore pellets was
examined. Fired pellets with varying MgO content (from 0 and 3.0%) were
tested for cold compression strength, swelling, reduction degradation index and
reducibility. Optical microscope studies with image analysis, and SEM-EDS
analysis was done to record the amount of phases and their chemical analysis.

Addition on magnesite resulted in the formation of magnesioferrite in the
fired pellets. FeO content of the silicate melt /slag phase in the pellets
decreased from 30% in the acid pellets to around 5% in the magnesite
fluxed MgO pellets. Lower FeO in the melt increases its melting point.

CCS of both acid pellets and magnesite fluxed pellets was within the
acceptable limit for the blast furnace. In magnesite pellets, CCS
decreased with increasing MgO due to the formation of low strength
silicate melt phase.

Acid pellets exhibited highest swelling. Addition of MgO considerably
reduced the swelling tendency of the pellets due to the formation of high
melting point slag that results sufficient bond strength to withstand the
reduction stresses.

Reduction degradation of the pellets was found to be reduced with
increasing MgO, due to the formation of more amount of magnesioferrite
and silicate melt, which are more stable under the reducing conditions in
the blast furnace.

MgO addition considerably improved the reducibility of the pellets,
especially in the range of 0.5 to 1.5% MgO. Formation of less amount of
liquid slag due to the presence of MgO could be attributed to this
improved reducibility of magnesite pellets.

At 1.0 to 1.5% MgO content, fired pellets exhibited optimum metallurgical
properties.
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6.1.5 Effect of pellet MgO content on quality and microstructure of fired
pellets using pyroxenite flux
Effect of pyroxenite addition, to increase the MgO content, on the melt
formation and microstructure during the induration of iron ore pellets was
examined. Fired pellets with varying MgO content (from 0 and 10.0%) were
tested for cold compression strength, swelling, reduction degradation index and
reducibility. Optical microscope studies with image analysis, and SEM-EDS
analysis was done to record the amount of phases and their chemical analysis.

Addition of pyroxenite up to 5% to get 1.5% MgO in the pellets was
found to be an optimum flux dosage for blast furnace grade hematite
pellets. FeO in the slag phase reduced from 30% in acid pellets to
around 3% in the pyroxenite fluxed pellets containing 1.5% MgO.
Pyroxenite addition beyond 5% showed its poor assimilation in the pellet
matrix and increased the amount of relict magnesium silicate phase
resulting in no further drop in slag FeO.

Strength of the pyroxenite pellets is comparable to acid pellets in spite of
high amount slag phase that filled up the pores between the oxide grains
thereby reducing the porosity. The negative effect of slag phase on pellet
strength is counteracted by the reduced porosity of pyroxenite pellets.

RDI of pyroxenite pellets was found to be superior, compared to acid
pellets, due to the formation of magnesioferrite and more amount of
silicate melt, which are more stable under the reducing conditions in the
blast furnace.

Swelling index of acid pellets was poor due to the formation of low
melting point fayalitic slag whereas the pyroxenite pellets exhibited less
swelling by forming high melting slag with low FeO that gives sufficient
bond strength to withstand the reduction stresses.

Pyroxenite pellets resulted in better softening-melting characteristics
compared to the acid pellets due to the formation of high melting point
slag and magnesio-wustite phase during reduction.
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6.1.6 Advanced metallurgical testing of the pyroxenite fluxed pellets
Based on resultant pellet quality observed during the test work and availability
of fluxes at the captive mines, pyroxenite fluxed pellets were suggested as
suitable burden material for the blast furnaces at Tata Steel. It was decided to
fine-tune the pyroxenite pellet chemistry, before commercial production in the
pellet plant, to find out the minimum amount of MgO in pellets to get the desired
high temperature properties.

Addition of MgO in the form of pyroxenite improved the quality of pellets
as compared to acid pellets. Acid pellets exhibited inferior metallurgical
properties; free swelling index~ 49%, softening temperature~1074oC,
disintegration -3.15mm ~28% against the desired target of <17%,
>1150oC and <5% respectively.

It was observed that minimum of 0.6% MgO is required in the pyroxenite
pellets to control the swelling index with in the target range. Pellets with
< 0.6% MgO resulted in swelling >17%

As per the advanced swelling and softening test, at least 0.3 to 0.6%
MgO% is required in the pellets to obtain desired softening temperature
and lower pressure drop. Beyond this level, there was no appreciable
improvement in quality.

Advanced reduction degradation tests indicated that MgO>0.6% is
required to reduce the disintegration of pellets during reduction in the
stack zone of blast furnace.

Considering the target pellet quality parameters with respect to the
above test results, it was concluded that 0.6% to 0.9% MgO is desired in
the pyroxenite pellets to obtain required high temperature properties.

A patent was also filed on the use of magnesium silicate (pyroxenite) as
flux in the pelletizing for improved metallurgical properties.
197
6.2 Conclusions on green pelletizing and effect of fluxes on pellet quality
The investigations carried out during this test work and the results obtained
thereof conclude the following salient points;

In the Noamundi iron ore fines, hematite and goethite minerals were
found to be distributed in coarser and finer fractions respectively. To
produce desired quality pellets, these fines need to be blended
thoroughly to achieve homogenies mineral distribution in the pellet feed.

Grinding of the ore fines in the ball mill needs to ensure that the mean
particle size of the pelletizing feed is maintained at 55 microns. At this
fineness, both green pellet quality and the fired pellet strength found to
be optimum.

Pellets need to be fired up to 1300 oC to achieve desired cold crushing
strength to withstand handling & storing and loads inside the shaft of the
blast furnace.

Acid pellets prepared from the Noamundi iron ore fines, without any flux
addition, exhibited inferior metallurgical properties. Insufficient formation
of silicate melt phase during pellet firing and high amount of FeO in the
melt phase found to deteriorate the acid pellet quality.

Substantial improvement in the quality of pellets, as compared to acid
pellets, was observed due to the addition of fluxing agents. Selection of
fluxing agents depends on the overall burden chemistry of the blast
furnace that includes sinter, pellets and lump ore.

In the limestone fluxed pellets, (CaO/SiO 2) needs to be maintained at 0.8
to obtain desired metallurgical properties for the blast furnace. In case of
dolomite fluxed pellets, basicity of 0.4 and MgO content of 1.5% found to
be optimum for the desired pellet quality.

MgO addition to the pellets found to be more effective in improving their
metallurgical properties. It forms magnesioferrite and high melting point
bonding phase within the pellets. These phases helped in considerably
reducing the undesired swelling and reduction degradation of pellets.
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
Pellets fluxed with magnesite exhibited improved metallurgical properties
at 1.0 to 1.5% MgO content. Magnesite addition improved the reducibility
of pellets due to the formation of fewer amounts of liquid slag and
uniform porosity.

For the first time, pyroxenite (magnesium silicate mineral) was
established as suitable flux for the pelletizing. Unlike carbonate fluxes
like limestone or dolomite, pyroxenite does not undergo any endothermic
reaction for dissociation.

0.6 to 0.9% MgO was found to be optimum in the pellets when
pyroxenite is used as fluxing agent.

A new dimensionless index called “composite quality index” (also called
‘p-index’) was formulated for the pellets. This index can be used as a tool
to relatively compare quality of different pellets based on vital
metallurgical quality parameters.

Based on resultant pellet quality observed during this entire test work
and availability of fluxes at the captive mines, pyroxenite fluxed pellets
were suggested as suitable burden material for the blast furnaces at
Tata Steel.

Use of pyroxenite flux for pelletizing of the Noamundi iron ore fines, as
suggested, resulted in improved metallurgical properties of pellets,
especially swelling index and reducibility, at the 6 MTPA capacity iron
ore pelletizing plant of Tata Steel at Jamshedpur.