chapter-5: results & discussion: quality and
TRANSCRIPT
CHAPTER-5:
RESULTS & DISCUSSION:
QUALITY AND MICROSTRUCTURAL STUDIES OF
FIRED PELLETS WITH DIFFERENT FLUXES
99
5. RESULTS & DISCUSSION: QUALITY AND MICROSTRUCTURAL
STUDIES OF FIRED PELLETS WITH DIFFERENT FLUXES
Quality of the pellets is influenced by the nature of the ore or
concentrate, associated gangue, type and amount of fluxes added and their
subsequent treatment to produce pellets. These factors in turn result in the
variation of physicochemical properties of the coexisting phases and their
distribution during the pellet induration. Hence properties of the pellets are
largely governed by the form and degree of bonding achieved between the ore
particles and the stability of these bonding phases during reduction of iron
oxides.
As the formation of phases and microstructure during induration depends
on the type and amount of fluxes added, there is a need to study the effect of
different fluxing agents, in terms of CaO/SiO2 ratio and MgO content, on pellet
quality
5.1 Pellet firing studies
Figure 44 shows the effect of firing temperature on the cold crushing
strength (CCS) of pellets at varying fineness. CCS found to increase with
increasing firing temperature. But increasing fineness (i.e. decreasing MPS) did
not result in increased pellet strength, as expected. Pellets made of 55 micron
MPS feed showed better strength as compared to other samples. It could be
attributed to the fact that with increasing fineness, the porosity of the pellet
decreases thereby resulting in the poor penetration of oxidizing gases to the
core of the pellet. In the absence of oxidizing atmosphere, the admixed coal in
the green pellets mildly reduces the hematite to magnetite. This resultant
duplex microstructure drastically reduces the strength of the pellet. Figure 45
shows the microstructure of the fired pellets made from different fineness pellet
feed and fired at 1300oC. More amount of magnetite could be observed in the
pellets prepared from 26 and 38 micron MPS pellet feed.
100
Fig. 44 Effect of the firing temperature on the cold crushing strength (CCS) of
pellets at varying fineness
0
50
100
150
200
250
300
350
1250 1260 1270 1280 1290 1300 1310
Cold
cru
shin
g s
trength
, kg/p
ellet
Firing temperature, oC
70microns
55microns
38microns
26microns
101
Fig. 45 Microstructure of fired pellets made from different fineness pellet feed
102
From the green pelletizing and firing studies, it was concluded that
pelletizing feed fineness of 55 MPS results in optimum green and fired pellet
properties. Firing temperature of 1300oC is desired to obtain required CCS of
pellets. Accordingly all the subsequent pelletizing experiments with different
fluxes were carried out at 55 MPS feed fineness and 1300oC firing temperature.
103
5.2 Effect of pellet basicity (CaO/SiO2) and MgO content on the quality and
microstructure of fired pellets using limestone and dolomite flux
Iron ore agglomerate quality plays a vital role in decreasing the reducing
agent consumption and increasing the productivity of the blast furnace. In most
of the integrated steel works, the burden mix for the blast furnace is decided as
per the availability of the iron ore agglomerates like sinter and pellets. More
attention has been given in recent years to the use of fluxed pellets in the blast
furnace due to their good strength and improved reducibility, swelling and
softening melting characteristics.
In the fluxed pellets, bonding is achieved through silicate melt formation
during induration. The amount of gangue in the concentrate, CaO & MgO in the
fluxes and binder influence the amount and chemistry of silicate melt. CaO
fluxes silicate melt as well as reacts with iron oxide to form different calcium
ferrites. MgO either enters the magnetite lattice to form magnesioferrite or
dissolves in the slag phase. These melting phases interact with each other and
dissolve a variable amount of iron oxides. As the formation of phases and
microstructure during the induration depends on the type and amount of fluxes
added, there is a need to study the effect of these fluxing agents, in terms of
CaO/SiO2 ratio and MgO content, on pellet quality by using limestone and
dolomite.
In this study, pellets with varying basicity and MgO content were
prepared and tested for cold crushing strength, reduction degradation,
reducibility, swelling and softening- melting characteristics. Optical microscope
studies with image analysis were carried out to estimate the amount of different
phases. SEM-EDS analysis was done to record the chemical analysis of the
oxide and slag phases. X-ray mapping was also carried out to understand the
distribution of CaO, MgO, SiO2 and Al2O3 in different phases. It was attempted
to establish correlation between the pellet chemistry (in terms of basicity &
MgO) and quality.
104
The amount of ingredients added for preparing green pellets with
varying basicity and MgO (Pellet A,A1, B,B1, C,C1, D,D1, E &E1) and their
quality parameters are shown in Table 18. Table 19 shows the chemical
analysis of fired pellets with varying basicity and MgO content.
Ta
ble
-18
In
gre
die
nts
of
gre
en
pelle
ts w
ith
vary
ing
am
ou
nt
of
flu
xe
s a
nd
th
eir
qu
alit
y
Pe
llet
A
Pe
llet
A1
Pe
llet
B
Pe
llet
B1
Pe
llet
C
Pe
llet
C1
Pe
llet
D
Pe
llet
D1
Pe
llet
E
Pe
llet
E1
Iro
n o
re,
wt.
%
97
.8
93
.3
97
.3
92
.8
96
.6
92
.5
95
.9
92
.1
95
.1
91
.7
Be
nto
nite
, w
t.%
0
.8
0.7
0
.8
0.7
0
.8
0.7
0
.8
0.7
0
.8
0.7
Lim
esto
ne
, w
t.%
0
.0
0.0
0
.5
0.0
1
.3
0.0
2
.0
0.0
2
.8
0.0
Dolo
mite
, w
t.%
0
.0
0.0
0
.0
2.0
0
.0
3.5
0
.0
4.7
0
.0
5.6
Pyro
xe
nite
, w
t.%
0
.0
4.7
0
.0
3.2
0
.0
1.9
0
.0
1.2
0
.0
0.6
Coa
l, w
t.%
1
.4
1.3
1
.4
1.3
1
.4
1.3
1
.3
1.3
1
.3
1.3
Gre
en
pe
llet
qu
alit
y
Dro
p n
um
be
r 4
.6
4.3
3
.9
2.7
3
.7
2.8
4
.3
3.7
4
.4
4.5
Gre
en
cru
shin
g s
tre
ng
th,
kg
/pelle
t 1
.6
1.7
1
.8
1.9
1
.8
1.9
1
.9
1.9
1
.8
2.0
Gre
en
pe
llet
mo
istu
re,%
7
.9
7.9
7
.6
7.1
7
.4
7.3
6
.9
7.2
7
.6
7.1
Ta
ble
-19
Che
mic
al a
naly
sis
of p
elle
ts w
ith
vary
ing
ba
sic
ity a
nd M
gO
co
nte
nt
Wt.%
Pe
llet
A
Pe
llet
A1
Pe
llet
B
Pe
llet
B1
Pe
llet
C
Pe
llet
C1
Pe
llet
D
Pe
llet
D1
Pe
llet
E
Pe
llet
E1
Fe
(t)
66
.0
63
.6
65
.8
63
.2
65
.4
64
.0
65
.0
63
.7
64
.8
63
.2
SiO
2
1.9
4
.2
2.0
3
.6
1.9
2
.9
2.2
2
.7
1.9
2
.5
Al 2
O3
2.2
2
.0
2.1
2
.1
2.2
2
.1
2.1
2
.2
2.2
2
.1
CaO
0
.1
0.1
0
.5
0.9
0
.8
1.3
1
.4
1.7
1
.6
2.0
Mg
O
0.1
1
.5
0.2
1
.6
0.2
1
.5
0.3
1
.7
0.2
1
.7
CaO
/SiO
2
0.0
0
.0
0.2
0
.3
0.4
0
.4
0.6
0
.6
0.8
0
.8
107
5.2.1 Microstructural analysis of the pellets with varying basicity
(CaO/SiO2) and MgO content
5.2.1.1 Optical microstructure & image analysis of the pellets with varying
basicity (referred as MgO-free pellets)
Figure 46 shows the optical microstructures of the fired pellets with
varying basicity. Image analysis studies of these pellets revealed that hematite,
magnetite and silicate melt are the major phases in the pellets. Amount of
silicate melt, which acts as the bonding phase, was found to increase with
increasing basicity, as shown in Fig. 47(a).
Distribution of silicate melt was measured in terms of silicate melt density
(number of silicate melt phases per unit area) using image analysis technique
as shown in Fig. 47(b). If the silicate melt is more distributed, there will be more
number of phases/grains per unit area, i.e. high silicate melt density. The
distribution of the silicate melt phase is more scattered in 0.4 and 0.6 basicity
pellets, as indicated by high silicate melt density. This could be attributed to the
increased mobility of the melt phase due to the formation of low melting point
olivines in this basicity range [30]. Porosity was found to decrease with
increasing basicity due to impregnation of pores with the melt phase.
108
Fig.46 Optical microstructures of the fired basic pellets with varying basicity
109
Fig.47 Image analysis of the MgO-free pellets (a) Distribution of different
phases and (b) silicate melt density
110
5.2.1.2 Optical microstructure & image analysis of the pellets with varying
basicity with 1.5% MgO (referred as MgO pellets)
Figure 48 shows the optical microstructures of the fired MgO pellets
(with varying basicity at 1.5% MgO).
Image analysis studies, as shown in Fig. 49 (a) of these pellets revealed
that hematite, magnetite and silicate melt are the major phases, while some
amount of magnesioferrite was observed at low basicity levels.
The mean size of the pores was found to increase with increasing
basicity as indicated by low pore density, as can be seen in Fig. 49 (b). Pore
density (no of pores/mm2) is an indication of pore size. Higher the pore density
more the number of pores in a given area with small pore size and vice versa.
111
Fig.48 Optical microstructures of the fired MgO pellets with varying basicity
112
Fig.49 Image analysis of the fired MgO pellets with varying basicity (a)
Distribution of different phases and (b) pore density
113
5.2.1.3 Scanning Electron Microscopy (SEM) with Energy Dispersive
Spectroscopy (EDS) analysis of the MgO-free pellets
Figure 50 shows the SEM image of Pellet A, C & E with EDS analysis of
all pellets (A, B C, D & E). From the results it was evident that the chemistry of
iron oxides is uniform in all the pellets irrespective of basicity. But chemistry of
the slag phase found to be varying with increasing basicity. FeO content of the
slag phase decreased considerably with increased basicity as shown in the
EDS analysis of Fig.50.
X-ray mapping studies of the fired pellet samples, as shown in Fig.51,
revealed that CaO from the limestone was distributed only in silicate melt.
5.2.1.4 SEM with EDS analysis of the MgO pellets
Figure 52 shows the SEM image of Pellet A1, C1 & E1 with EDS
analysis of all the pellets (A1, B1 C1, D1 & E1). Addition of MgO to the varying
basicity pellets increased the FeO content of the slag phase as shown in the
EDS analysis.
X-ray mapping studies of the fired MgO pellet samples (Fig.53) revealed
that MgO was distributed both in silicate melt and oxide phase.
114
Pellet A Pellet B Pellet C Pellet D Pellet E
Iron Oxide
Al2O3, wt% 0.9 1.8 1.4 1.5 1.5
SiO2, wt% 0.9 0.8 - 1.9 -
Fe2O3, wt% 98.2 96.3 98.6 96.2 98.1
Slag
MgO, wt% - - 1.1 1.2 1.1
Al2O3, wt% 2.5 11.2 9.7 9.3 11.3
SiO2, wt% 67.3 55.6 50.3 42.6 41.4
CaO, wt% 0.1 9.3 20.7 27.6 30.8
FeO, wt% 30.2 18.1 14.2 16.0 10.5
Fig.50 SEM image of Pellet A C & E with EDS analysis of all the pellets
(A,B,C,D &E)
Fig
.51
Dis
trib
utio
n o
f F
e, S
i, C
a a
nd
Mg
in
fir
ed
Mg
O-f
ree
pelle
t w
ith
0.8
ba
sic
ity (
Pelle
t E
)
116
Pellet A1 Pellet B1 Pellet C1 Pellet D1 Pellet E1
Iron Oxide
Al2O3, wt% 1.5 1.6 2.9 1.4 2.0
Fe2O3, wt% 98.5 97.8 97.1 98.6 95.6
Slag
MgO, wt% - 1.5 0.6 3.5 0.8
Al2O3, wt% 0.2 14.1 12.1 8.3 10.7
SiO2, wt% 96.2 62.6 38.9 36.6 40.5
CaO, wt% 0.0 12.3 18.6 28.7 29.5
FeO, wt% 3.6 7.6 27.3 20.1 14.7
Fig.52 SEM image of Pellet A1 C1 & E1 with EDS analysis of all the pellets
(A1,B1,C1,D1 &E1)
Fig
.53
Dis
trib
utio
n o
f F
e, S
i, C
a a
nd
Mg
in
fir
ed
Mg
O p
elle
t w
ith
0.4
ba
sic
ity (
Pelle
t D
1)
118
5.2.2 Metallurgical properties of the pellets with varying basicity
(CaO/SiO2) and MgO content
5.2.2.1 Cold crushing strength (CCS)
Cold crushing strength indicates the ability of the pellets to withstand the
load during their storage & handling and the load of burden material in the
reduction furnace. Blast furnace needs pellets with CCS values in the range of
200-230 kg/pellet.
Pellet strength was found to increase up to 0.4 basicity (CaO/SiO2) in
MgO-free pellets and decreased thereafter (Fig.54). The same trend was
observed in MgO pellets also. But MgO pellets exhibited slightly lower strength
compared to the basic pellets. Both the pellets (basic and MgO), exhibited
required strength values as desired by the blast furnace.
Highest strength of MgO-free pellets at 0.4 basicity could be attributed to
decreased porosity with increased basicity. Addition of basic flux resulted in the
formation of more amount of low strength silicate melt phase (Fig. 47(a)).
Silicate melt fills up the pores between solid particles and exerts pressure to
pull them together due to interfacial forces thereby reducing the porosity. But
beyond 0.4 basicity, the positive effect of low porosity is counteracted by the
increased amount of low strength silicate melt, thereby resulting in lower
strength.
Strength of the MgO pellets found to be lower as compared to the MgO-
free pellets irrespective of basicity. This could be attributed to high amount of
silicate melt, Fig. 49(a), which is low in strength, in MgO pellets compared to
the MgO-free pellets.
119
Fig.54 Effect of pellet basicity on the cold strength of fired pellets
120
5.2.2.2 Swelling
Swelling indicates volume change of pellets during reduction. Higher
swelling reduces the strength of the pellets after their reduction thereby
resulting in high resistance to gas flow, burden hanging and slipping inside the
blast furnace. Maximum allowable swelling of pellets for the blast furnace
ranges from 16-18%.
Figure 55 shows the swelling index of MgO-free and MgO pellets with
varying basicity. Error bars are shown in the figure with 90% confidence level of
the test results.
From the results it is evident that acid pellets (zero basicity and no MgO
content) exhibited highest swelling among all the pellets. In case of MgO-free
pellets, high swelling was observed at 0.6 basicity and decreased thereafter.
MgO pellets demonstrated considerably lower swelling tendency compared to
the basic pellets at all basicity levels.
121
Fig.55 Effect of pellet basicity on the swelling of fired pellets
122
Volumetric expansion of iron ore pellets takes place during their
reduction from hematite to magnetite and wüstite. It can be mainly attributed to
the increased volume requirements for the anisotropic growth of magnetite
(111) planes parallel to the hematite (0001) planes [30]. Swelling is related to
the ability of gangue or slag phase to withstand the reduction stresses of
independent oxide particles. High melting point slag would produce sufficient
bonding strength to limit swelling and low melting point slag enhances swelling.
As shown in Fig.55, acid pellets (0 basicity and 0% MgO content)
exhibited highest swelling and MgO-free pellets exhibited higher swelling at 0.6
basicity and decreased thereafter. In acid pellets reduction is accompanied by
the reaction between Fe2+ and SiO2 to form low melting point phase, fayalite
(Fe2SiO4) that melts at 1175oC [45]. High swelling index of these pellets can be
attributed to the plastic or mobile nature of low melting point fayalitic slag that
provides a medium for absorption of the reduction stresses by increased
distances between the particles. In MgO-free pellets, high swelling values at 0.6
basicity can be compared to the other reported studies on different iron ore
fines. They reported that maximum swelling on reduction occurs in the basicity
range of 0.2-0.8.
In the present study, maximum swelling of the pellets at 0.6 basicity can
be attributed to the formation of low melting point calcium olivines between
Fe2SiO2 and Ca2SiO4, with lowest melting point of 1115oC [30]. High silicate
melt density of 0.4-0.6 basicity pellets as shown in Fig. 47(b), also confirms the
plastic or mobile nature of the low melting point slag.
Addition of MgO to the pellets increases the melting point of the slag or
silicate melt formed between the oxide particles [33]. Low swelling of MgO
pellets, as shown in Fig. 55, can be attributed to high melting point slag that
contributes sufficient bond strength to withstand the reduction stresses.
123
5.2.2.3 Reduction degradation
Reduction degradation of the pellets indicates their tendency to generate
fines during reduction. It is an undesired phenomenon that occurs at low
temperatures in the upper part of the blast furnace or reduction shaft of any
direct reduction unit.
The primary cause of low temperature disintegration is due to crystalline
transformation from hexagonal hematite to cubic magnetite accompanied by
lattice distortion and volume expansion to an extent of 25% [34]. The
anisotropic dimensional change due to the transformation leads to severe
stresses in certain planes, resulting in cracks in the brittle matrix. The effect is
particularly severe in the grain boundaries. It is very clear that iron oxide in the
indurated pellets is mainly in the form of hematite; therefore, generation of
internal stress, in principle, is unavoidable. The disintegration can be reduced
by increasing the amount of stable bonding phases, which are less brittle at
lower temperatures, with homogeneous distribution. Bonding which forms
during induration can be divided into three main groups: Iron oxides bonds
(hematite, magnetite), silicate bonds and local bonds (calcium ferrite,
magnesioferrite) that are close to some particular mineral phases. Iron oxide
bonds are common and strong, but they are not stable during reduction due
their phase change. Unlike iron oxide bonds, silicate bonds remain unaltered
during reduction and they soften and melt later [7].
From the results it is evident that acid pellets exhibited highest RDI
whereas MgO-free pellets in the basicity range of 0.2-0.8 showed low RDI as
shown in Fig.56. MgO pellets demonstrated lower RDI compared to basic
pellets in the basicity range of 0 to 0.4, but high RDI in 0.6-0.8 basicity range.
124
Fig.56 Effect of pellet basicity on the RDI of fired pellets
125
Acid pellets showed high reduction degradation due to the presence of
more hematite bonds and less silicate bonds. MgO-free pellets exhibited
considerably less reduction degradation due to the presence of silicate melt, as
shown in Fig. 47(a), which is more stable compared to hematite. In the earlier
studies by the author, it was observed that uniformly distributed silicate melt
improves the RDI of the iron ore pellets [45].
MgO pellets exhibited less degradation compared to basic pellets up to
0.4 basicity. This could be attributed to the comparatively high amount of
silicate melt as shown in Fig.49 (a).
But the poor degradation of MgO pellets in the basicity range of 0.6 to
0.8 could be attributed to the increased pore size, as indicated by low pore
density in Fig.49(b), which can result in poor strength of reduced pellet matrix
and hence more degradation. Pore density (no of pores/mm2) is an indication of
pore size. Higher the pore density more the number of pores of contact in a
given area with small pore size and vice versa.
126
5.2.2.4 Reducibility
Reducibility of the pellets may be defined as the ease with which the
oxygen combined with the iron oxide can be removed. A higher reducibility
indicates more indirect reduction in the blast furnace, resulting in lower coke
rate and high productivity.
Results indicated that acid pellets reduced more compared to the MgO-
free pellets whereas MgO pellets exhibited higher reducibility compared to acid
and MgO-free pellets irrespective of their basicity as shown in Fig.57.
Reducibility of MgO-free pellets is lower than acid pellets due to the
presence of more amount of low melting point silicate melt between the iron
oxide grains in the former. During reduction at high temperature, the slag
softens and impedes the flow of reducing gas within the pellet thereby retarding
the reduction. In case of MgO pellets, silicate melt formed between the iron
oxide grains is high in melting point [33] due to presence of MgO. Relatively
high reducibility of these pellets at all the basicity levels can be attributed to
high melting point slag which does not soften at reduction temperatures and
keeps the pores open for reducing gas thereby enhancing reduction.
127
Fig.57 Effect of pellet basicity on the reducibility of fired pellets
128
5.2.2.5 Softening-Melting characteristics
Study of softening-melting characteristics of pellets helps in
understanding the formation of cohesive zone in the lower portion of blast
furnace. If the pellets soften at lower temperature and the temperature range
between softening and melting is wider, then the resistance to the gas flow will
be more in the cohesive zone.
Results indicated that softening temperature of MgO-free pellets
increased with increasing basicity and the softening-melting range decreased
considerably (Fig.58). MgO pellets showed increased softening temperature
and decreased softening-melting range at 0.4 basicity only as shown in Fig.59.
129
Fig.58 Effect of pellet basicity on the softening melting characteristics of mixed
burden
Fig.59 Effect of pellet basicity at 1.5% MgO on the softening melting
characteristics of mixed burden
130
Softening melting properties of the pellets are affected by the liquidus
phase with low melting point that is formed between wüstite and slag phase
during reduction [33]. Inferior softening melting characteristics of acid pellets
can be attributed to the FeO rich low melting fayalitic liquidus slag, whereas
MgO-free pellets exhibited superior properties due to the fact that increase in
pellet basicity increases the basicity of burden (55% Sinter + 35% pellets+
10%lump ore) slag thereby increasing its liquidus temperature as given in
Table 20. Burden slag consisting of slag formed from the entire iron burden
(sinter, pellets and lump ore). Increased basicity of burden slag facilitates the
formation of di-calcium silicate of narrow melting range, thereby decreasing the
softening-melting range as shown in Fig.58.
MgO pellets exhibited high softening temperature and low softening-
melting (S-M) range at 0.4 basicity, as shown in Fig.59. This could be due to
the formation of optimum slag similar to the slag formed by MgO-free pellets at
0.8 basicity. Four component basicity (CaO+MgO)/(SiO2+Al2O3) and viscosity of
both the slags are similar as shown in Table.20, which means that slag with
optimum liquidus temperature and viscosity is required for optimum softening-
melting characteristics. Calculation method of viscosity is mentioned elsewhere
[46]. Low amount of non-drip material in case of MgO-free pellets at 0.8 basicity
and MgO pellet at 0.4 basicity also indicates that burden slag formed is easily
flowable without impeding the burden permeability.
Ta
ble
-20
Deta
ils o
f b
urd
en
sa
mp
le a
nd
sla
g c
he
mis
try f
rom
so
fte
nin
g m
eltin
g t
est
P
elle
t
A
Pe
llet
B
Pe
llet
C
Pe
llet
D
Pe
llet
E
Pe
llet
A1
Pe
llet
B1
Pe
llet
C1
Pe
llet
D1
Pe
llet
E1
Mix
ed
bu
rde
n s
am
ple
use
d f
or
S-M
te
st
Wt. o
f sin
ter,
Gm
s
15
4.0
1
54
.0
15
4.0
1
54
.0
15
4.0
1
54
.0
15
4.0
1
54
.0
15
4.0
1
54
.0
Wt. o
f p
elle
ts,
Gm
s
98
.1
98
.1
98
.1
98
.1
98
.1
98
.1
98
.1
98
.1
98
.1
98
.1
Wt. o
f o
re, G
ms
28
.0
28
.0
28
.0
28
.0
28
.0
28
.0
28
.0
28
.0
28
.0
28
.0
Bu
rde
n s
lag
ch
em
istr
y a
fter
so
fte
nin
g m
eltin
g te
st
CaO
, W
t.%
4
4.2
4
4.5
4
5.2
4
5.4
4
6.5
4
0.3
4
0.9
4
3.3
4
3.7
4
3.7
SiO
2,W
t.%
2
8.6
2
8.6
2
7.7
2
8.2
2
7.1
3
1.9
2
9.6
2
8.4
2
7.7
2
5.8
Mg
O, W
t.%
7
.3
7.9
7
.7
8.1
7
.6
10
.8
12
.8
10
.9
11
.4
13
.6
Al 2
O3, W
t.%
1
9.9
1
9.1
1
9.2
1
8.2
1
8.6
1
6.9
1
6.6
1
7.5
1
7.3
1
6.8
Sla
g w
eig
ht,
gm
s
31
.6
32
.4
32
.7
33
.9
33
.2
34
.9
36
.6
35
.3
36
.0
36
.5
CaO
/SiO
2
1.5
1
.6
1.6
1
.6
1.7
1
.3
1.4
1
.5
1.6
1
.7
(CaO
+M
gO
)/ (
SiO
2+
Al 2
O3)
1.0
1
1.1
1
.1
1.2
1
.2
1.1
1
.2
1.2
1
.2
1.3
Calc
ula
ted
liq
uid
us te
mp
era
ture
of
sla
g,
oC
1
42
6
14
36
14
66
14
66
14
96
14
36
14
61
14
66
14
76
14
76
Non
-dri
p m
ate
rial,%
1
7.3
2
1.5
1
1.9
1
7.3
8
1
4.2
1
9.4
2
.8
11
.3
15
.4
Calc
ula
ted
sla
g v
isco
sity (
pois
e)
2.0
1
.8
1.7
1
.5
1.4
2
.0
1.5
1
.4
1.2
0
.9
132
5.2.3 Composite Quality Index (CQI) or p-Index
After evaluating the pellets for different metallurgical properties, it is often
difficult to directly ascertain the optimum pellet chemistry suitable for blast
furnace because some quality parameters like reducibility, degree of reduction
need to be maximized whereas other parameters like swelling and softening-
melting range need to be minimized. To calculate numerically the optimum
pellet chemistry, a new dimensionless index called “composite quality index”
(CQI), also called ‘p-index’, has been formulated. Similar attempts were made
earlier by other workers to formulate integral index for green pellets [47] and
integral indices for metallurgical conversions [48].
Composite quality index is composed of different indices related to high
temperature metallurgical properties of pellets. Indices that need to be
increased viz., reducibility index and degree of reduction are placed in the
numerator whereas indices that need to be decreased, viz., reduction
degradation index, swelling index and softening-melting range are placed in the
denominator. Higher CQI indicates the improved pellet quality and vice versa.
Where RI is Reducibility Index; DOR is Degree of Reduction; RDI is
Reduction degradation index; SI is Swelling Index and SM is Softening-Melting
range
Figure 60 shows the CQI of basic and MgO pellets. In MgO-free pellets
highest CQI value (0.74) is observed at 0.8 basicity. Pyroxenite fluxed pellets
(zero basicity and 1.5% MgO) and dolomite fluxed pellets (0.4 basicity and
1.5% MgO) also exhibited high CQI values, 0.58 and 0.59 respectively. The
CQI (‘p-index’), which gives weightage to vital quality parameters, can be used
as a tool to assess the pellet quality rather than relying on any single
parameter.
133
Fig.60 Composite quality index of varying basicity pellets with and without MgO
134
5.3 Effect of MgO content of pellets on quality and microstructure of fired
pellets using magnesite flux
Quality of pellets, generally, is influenced by the nature of ore or
concentrate, associated gangue, type and amount of fluxes added and their
subsequent treatment to produce the pellets. These factors in turn result in the
variation of physicochemical properties of the coexisting phases and their
distribution during pellet induration. Hence properties of the pellets are largely
governed by the form and degree of bonding achieved between ore particles
and the stability of these bonding phases during reduction of iron oxides [34].
More attention has been given in recent years to the use of fluxed pellets in the
blast furnace due to their good strength and improved reducibility, swelling and
softening melting characteristics [49, 50].
For blast furnaces, where super fluxed sinter is available with high CaO
contents (~9-10%), pellets need to be acidic in nature, free from CaO, to
maintain the blast furnace slag chemistry. But acid pellets are known for their
poor high temperature properties like softening-melting characteristics and
reducibility [33].
Earlier studies by the sinter and pellet makers made it clear that MgO
addition helps in improving the high temperature properties. In case of acid
pellets, dolomite cannot be used as the source of MgO, because it contains
substantial amount of CaO.
In this study magnesite (MgCO3), was used as source of MgO.
Magnesite is a naturally occurring magnesium carbonate mineral found in two
different forms, crystalline and cryptocrystalline. The magnesite used in this
work is of cryptocrystalline form with off-white colour due to the presence of
silica. Pellets with varying MgO content were prepared and tested for cold
strength, reduction degradation index, reducibility and swelling characteristics.
Optical microscope studies with image analysis were carried out to estimate the
amount of different phases. SEM-EDS analysis was done to record the
135
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. It was attempted to establish correlation between the pellet chemistry
(in terms of MgO) and quality.
The amount of ingredients added for preparing green pellets with varying
MgO (Pellet A, B, C, D, E, F & G) and their quality parameters are shown in
Table 21. To adjust the MgO content of pellets from 0.5 to 3.0%, the amount of
magnesite was varied from 1 to 7% in the green pellets. Table 22 shows the
chemical analysis of the fired pellets with varying MgO content.
Ta
ble
-21
In
gre
die
nts
of
gre
en
pelle
ts w
ith
vary
ing
am
ou
nt
of
ma
gn
esite
P
elle
t A
P
elle
t B
P
elle
t C
P
elle
t D
P
elle
t E
P
elle
t F
P
elle
t G
Iro
n o
re,
wt.
%
97
.8
96
.9
95
.8
94
.7
93
.5
92
.4
91
.3
Be
nto
nite
, w
t.%
0
.8
0.8
0
.8
0.8
0
.7
0.7
0
.7
Ma
gn
esite
, w
t.%
0
1
.0
2.1
3
.2
4.4
5
.5
6.6
Coa
l, w
t.%
1
.4
1.4
1
.3
1.3
1
.4
1.4
1
.4
Gre
en
pe
llet
qu
alit
y
Dro
p n
um
be
r, [
-]
4.6
3
.8
4.4
4
.4
4.0
2
.8
4.6
Gre
en
cru
shin
g s
tre
ng
th,
kg
/pelle
t 1
.6
1.8
1
.9
1.7
1
.5
1.9
1
.9
Gre
en
pe
llet
mo
istu
re,%
7
.9
7.4
7
.0
8.2
7
.6
7.2
7
.2
Ta
ble
-22
Ch
em
ica
l a
naly
sis
of
ma
gn
esite
pelle
ts w
ith
va
ryin
g M
gO
co
nte
nt
Wt.%
P
elle
t A
P
elle
t B
P
elle
t C
P
elle
t D
P
elle
t E
P
elle
t F
P
elle
t G
Fe
(t)
66
.0
65
.8
65
.8
65
.4
65
.6
65
.2
64
.4
SiO
2
1.9
1
.8
1.8
1
.7
1.7
1
.7
1.9
Al 2
O3
2.2
2
.2
1.9
1
.9
1.8
1
.9
2.1
CaO
0
.1
0.5
0
.4
0.4
0
.4
0.3
0
.4
Mg
O
0.1
0
.5
0.8
1
.3
1.9
2
.3
2.9
137
5.3.1 Microstructural analysis of the pellets with varying MgO content
5.3.1.1 Optical microstructure & image analysis of pellets with varying MgO
content
Figure 61 shows the optical microstructures of the fired pellets with
varying MgO content. Image analysis studies of these pellets revealed that
hematite, magnetite, silicate melt and magnesioferrite are the major phases in
the pellets.
Amount of magnesioferrite and silicate melt, which acts as the bonding
phase, was found to increase with increasing MgO content, as shown in Fig.62.
Porosity was found to increase with increasing MgO content, especially at 1.5
to 2.5%, in the pellets. This could be attributed to the calcination of magnesite
that releases more amount of CO2, thereby increasing porosity.
138
Fig. 61 Optical microstructures of magnesite fluxed pellets with varying MgO
139
Fig. 62 Image analysis of magnesite fluxed fired pellets with varying MgO
140
5.3.1.2 SEM with EDS analysis of the pellets with varying MgO content
Figure 63 shows the SEM image of Pellet A, C, E & G with EDS analysis
of all the pellets (A, B C, D, E, F &G). From the results it was evident that
chemistry of iron oxides is uniform in all the pellets irrespective of MgO content.
But chemistry of the slag phase was found to be varying with increasing MgO
content. FeO content of the slag phase decreased considerably with increased
MgO as shown in the EDS analysis (Fig.63).
X-ray mapping studies of the fired pellet samples, as shown in Fig.64,
revealed that MgO from the magnesite was distributed primarily in
magnesioferrite phase.
141
Pellet
A Pellet
B Pellet
C Pellet
D Pellet
E Pellet
F Pellet
G
Iron Oxide
Fe2O3, wt% 98.2 98.5 96.5 97.9 97.8 97.5 98.2
Al2O3, wt% 0.9 1.5 2.2 1.2 1.5 1.7 0.9
SiO2, wt% 0.9 0.0 1.3 0.5 0.0 0.4 0.5
Slag
MgO, wt% 0.0 2.7 0.8 0.5 0.7 1.7 2.0
Al2O3, wt% 2.5 5.8 5.7 5.0 6.2 7.4 9.3
SiO2, wt% 67.4 77.3 89.6 91.0 87.0 83.6 81.6
CaO, wt% 0 1.0 0.8 1.0 1.1 1.8 2.3
FeO, wt% 30.2 13.2 3.1 2.5 4.9 5.4 4.8
Mg-Ferrite
MgO, wt% 0.0 5.3 22.5 21.3 19.6 19.6 20.7
Al2O3, wt% 0.0 3.2 6.2 3.7 3.7 4.8 4.1
SiO2, wt% 0.0 3.4 0.0 0.6 1.5 0.7 0.0
CaO, wt% 0.0 1.6 0.0 0.0 0.7 0.4 0.0
Fe2O3, wt% 0.0 85.8 71.4 74.4 73.4 73.6 73.7
Fig. 63 SEM image of Pellet A, C, E & G with EDS analysis of all pellets (A, B,
C, D, E, F & G)
Fig
. 6
4 D
istr
ibu
tio
n o
f F
e,
Si, A
l, C
a a
nd
Mg
in f
ire
d m
ag
ne
site
flu
xe
d p
elle
ts w
ith
3%
Mg
O (
Pe
llet
G)
143
5.3.2 Metallurgical properties of the pellets with varying MgO content
5.3.2.1 Cold crushing strength (CCS)
Pellet strength was found to decrease with increasing MgO content as
shown in Fig.65. Pellets up to 2.0% MgO exhibited strength values as desired
in the blast furnace.
Acid pellets exhibited highest strength compared to magnesite pellets in
spite of having high porosity comparable to the latter. This could be attributed to
the low amount of low strength gangue or slag phase, as shown in the image
analysis (Fig.62) and more recrystallization and sintering between the hematite
grains in the acid pellets. Addition of magnesite resulted in the formation of
magnesioferrite and low strength silicate melt phase, thereby reducing the
strength.
144
Fig.65 Effect of pellet MgO on the cold compression strength of fired pellets
145
5.3.2.2 Swelling
Maximum allowable swelling of pellets for the blast furnace ranges from
16-18%. Figure 66 shows the swelling index of magnesite fluxed pellets with
varying MgO. Error bars are shown in the figure with 90% confidence level of
the test results. There are three different regions in the swelling curve; region A
&C where drop in swelling is very high and region B, where it is negligible. From
the results it is evident that acid pellets (without any MgO) exhibited highest
swelling among all the pellets. Swelling reduced drastically with increasing the
MgO up to 1.0% (region A) and stabilized thereafter till 2.5% MgO (region B).
Pellets with MgO more than 1.0% demonstrated considerably lower swelling
tendency.
Addition of MgO to the pellets increases the melting point of the slag or
silicate melt formed between the oxide particles [33]. Considerable drop in the
swelling of magnesite pellets was noted up to 1.0% MgO content (region A) in
Fig.66, is because of the formation of high melting point slag, indicated by its
low FeO content as shown in Fig.63, which contributes sufficient bond strength
to withstand the reduction stresses. Melting point of slag in region B is stable,
indicated by its uniform FeO, leading to uniform swelling tendency. In region C,
further drop in swelling could be because of the presence of more amounts of
stable silicate melt ad magnesioferrite phases.
146
Fig.66 Effect of pellet MgO on the swelling of the fired pellets
147
5.3.2.3 Reduction degradation
The results of reduction degradation Index (RDI) test of pellets as a
function of MgO content has been presented in Fig.67. It is evident that acid
pellets exhibited highest RDI and the addition of MgO in the form of magnesite
decreased the RDI.
Acid pellets showed high reduction degradation due to the presence of
more hematite bonds and less silicate bonds. During reduction, these hematite
bonds diminish due to their conversion to magnetite resulting in higher
degradation.
Magnesite pellets, with MgO, exhibited considerably less reduction
degradation due to the presence of silicate melt and magnesioferrite, which are
more stable compared to hematite. In the earlier studies by the author, it was
observed that uniformly distributed silicate melt improves the RDI of iron ore
pellets [51]. In addition to silicate melt, magnesioferrite formed between the iron
oxide grains also acts as a strong bonding phase that counteracts the reduction
degradation [52].
148
Fig.67 Effect of pellet MgO on the RDI of the fired pellets
149
5.3.2.4 Reducibility
A higher reducibility indicates more indirect reduction in the blast furnace
resulting in lower coke rate and high productivity.
As shown in Fig.68, MgO addition to pellets in the form of magnesite,
improved their reducibility considerably. Up to 1.0% MgO, reducibility increased
to as high as 80% and slightly decreased thereafter. Formation of less amount
of liquid slag due to the presence of MgO and uniform porosity could be
attributed to this improved reducibility of magnesite pellets [43]. MgO addition
increases the melting point of slag which does not soften at reduction
temperatures and keeps the pores open for reducing gas thereby enhancing
reduction.
With increasing magnesite addition beyond 1.5% MgO, the amount of
silicate melt increases, as shown Fig.62, hindering the flow of reducing gases
within the pellet matrix, thereby lowering the reducibility.
After considering all the quality characteristics of magnesite pellets, viz.,
CCS, swelling, RDI & RI, the optimum magnesite dosage, to get desired
metallurgical properties, was found to be 2 to 3% to get 1.0 to 1.5% MgO
content in the fired pellets.
150
Fig.68 Effect of pellet MgO on the reducibility of the fired pellets
151
5.4 Effect of pellet MgO content on quality and microstructure of the fired
pellets using pyroxenite flux
For the blast furnace operating with mixed burden materials like sinter,
pellets and lump ore, where super fluxed sinter is available with high CaO
contents (~9-10%), pellets should be acidic in nature, free from CaO, to
maintain the blast furnace slag chemistry. But acid pellets are known for their
poor high temperature properties like softening-melting characteristics and
reducibility [33].
In this study “pyroxenite” was used as source of MgO. Pyroxenite is a
magnesium silicate rock composed largely of pyroxene with small amounts of
olivine and serpentine. Table 23 shows the chemical formula and theoretical
MgO content of these minerals [53].
Table-23 Chemical formula and theoretical MgO content of magnesium silicate
minerals
Mineral Chemical
composition Theoretical
values of MgO%
Pyroxene MgSiO3 40
Olivine Mg2SiO4 57
Serpentine 3MgO.2SiO2.2H2O 43
Pyroxenite usage as flux in the pelletizing has the following advantages;
In addition to MgO content, pyroxenite addition also increases
silica content of the pellets. This decreases the external quartz
addition to the blast furnace that is required to control its slag
basicity.
Unlike carbonate fluxes like limestone or dolomite, pyroxenite
does not undergo any endothermic reaction for dissociation;
hence energy requirement during pellet induration is
comparatively low.
152
Pyroxenite does not release any CO2 during pellet induration.
Pellets with varying MgO content were prepared and tested for cold
strength, swelling, reduction degradation, reducibility and softening-melting
characteristics. Optical microscope studies with image analysis were carried out
to estimate the amount of different phases. SEM-EDS analysis was done to
record the chemical analysis of the oxide and slag phases. X-ray mapping was
also carried out to understand the distribution of CaO, MgO, SiO2 and Al2O3 in
different phases. It was attempted to establish correlation between the pellet
chemistry (in terms of MgO) and quality.
The amount of ingredients added for preparing varying MgO green
pellets (Pellet A, B, C, D, E, F & G) and their quality parameters are shown in
Table 24. Table 25 shows the chemical analysis of the fired pellets with the
pyroxenite addition varying from 0% to 10%. Pyroxenite increased the MgO
content of the pellets from 0% to 3% and the MgO/SiO2 ratio from 0 to 0.45%.
Ta
ble
-24
In
gre
die
nts
of
gre
en
pelle
ts w
ith
vary
ing
pyro
xe
nite
co
nte
nt
& t
heir
qu
alit
y
P
elle
t A
P
elle
t B
P
elle
t C
P
elle
t D
P
elle
t E
P
elle
t F
P
elle
t G
Iro
n o
re,
wt.
%
97
.8
96
.7
95
.1
93
.3
91
.6
89
.9
88
.3
Be
nto
nite
, w
t.%
0
.8
0.8
0
.8
0.7
0
.7
0.7
0
.7
Pyro
xe
nite
, w
t.%
0
.0
1.2
2
.9
4.7
6
.3
8.0
9
.7
Coa
l, w
t.%
1
.4
1.4
1
.3
1.3
1
.4
1.3
1
.3
Gre
en
pe
llet
qu
alit
y
Dro
p n
um
be
r, [
-]
4.6
4
.3
4.4
4
.3
4.4
5
.1
4.9
Gre
en
cru
shin
g s
tre
ng
th,
kg
/pelle
t 1
.6
1.7
1
.6
1.7
1
.6
1.6
1
.6
Gre
en
pe
llet
mo
istu
re,%
7
.9
7.1
6
.7
7.9
7
.5
7.9
7
.8
Ta
ble
-25
Ch
em
ica
l a
naly
sis
of th
e p
elle
ts w
ith v
ary
ing
am
ou
nts
of
pyro
xe
nite
Wt.%
P
elle
t A
P
elle
t B
P
elle
t C
P
elle
t D
P
elle
t E
P
elle
t F
P
elle
t G
Fe
(t)
65
.4
65
.6
64
.4
63
.8
63
.4
60
.8
60
.0
SiO
2
1.9
2
.7
3.8
4
.0
5.0
5
.9
6.7
Al 2
O3
2.2
2
.0
2.0
2
.0
1.7
1
.7
1.8
CaO
0
.1
0.1
0
.1
0.2
0
.1
0.2
0
.2
Mg
O
0.0
0
.5
1.1
1
.5
2.0
2
.6
3.0
P
0.1
0
.1
0.1
0
.1
0.1
0
.1
0.1
Mn
O
0.1
0
.1
0.1
0
.1
0.1
0
.1
0.1
TiO
2
0.2
0
.2
0.2
0
.2
0.2
0
.2
0.2
Mg
O/S
iO2
0.0
0
.2
0.3
0
.4
0.4
0
.4
0.4
155
5.4.1 Microstructural analysis of the pellets with varying MgO content
5.4.1.1 Optical microstructure & image analysis of the pellets with varying MgO
content
Figure 69 shows the optical microstructures of acid and pyroxenite
pellets with varying MgO content.
Image analysis studies, as shown in Fig.70, of these pellets revealed
that hematite and silicate melt are the major phases in the acid pellets and
pyroxenite pellets consist of magnesioferrite and relict or partially assimilated
magnesium silicate phase in addition to hematite and silicate melt. The amount
of relict magnesium silicate phase was found to increase considerably beyond
1.5% MgO content in the pellets.
Porosity of the pellets decreased with increasing amount of pyroxenite
due to the formation of more silicate melt from the silica in pyroxenite.
156
Fig.69 Optical microstructures of fired pellets with varying MgO
157
Fig.70 Amount of different phases formed in the fired pellets with varying MgO
content
158
5.4.1.2 SEM and EDS analysis of the pellets with varying MgO content
Figure 71 shows the SEM image of Pellet A & D with EDS analysis of all
the pellets (A,B,C,D,E, F,& G). From the results it was noted that the chemistry
of iron oxide and magnesioferrite phases is uniform in all the pellets irrespective
of MgO content. But chemistry of the slag phase was found to be varying with
the addition of pyroxenite.
FeO content of the slag phase decreased considerably with increased
MgO content up to 1.5% (Pellet D) and thereafter no change was observed as
shown in Fig.72.
X-ray mapping studies of fired pellet samples, as shown in Fig.73,
revealed that the MgO from the pyroxenite was distributed only in
magnesioferrite phase and no presence in the slag phase.
FeO-MgO phase diagram [54] as shown in Fig.74 indicates that FeO
and MgO have complete miscibility and form sold solution. Decreased FeO
content of the slag phase by increasing the MgO content (up to 1.5% MgO or
5% pyroxenite) can be attributed to the formation of magnesio-wüstite, which
upon cooling leads to the formation of magnesioferrite.
Beyond 5% pyroxenite addition, assimilation of pyroxenite into the pellet
matrix is poor, as indicated by the increasing amount of relict magnesium
silicate phase shown in Fig.72, resulting in no further drop in FeO.
159
Pellet A Pellet B Pellet C Pellet D Pellet E Pellet F Pellet G
Iron Oxide e.g.: Point
1,2,4,6
e.g.: Point 3,4
Al2O3, wt% 0.9 1.9 1.6 1.5 1.3 1.3 0.8
SiO2, wt% 0.9 0.8 0.2 0.0 2.8 0.8 0.1
Fe2O3, wt% 98.2 97.3 98.2 98.5 95.7 97.9 98.8
Mg-Ferrite e.g.: Point 1,2
MgO, wt% - 15.1 16.3 15.6 16.3 15.3 15.7
Al2O3, wt% - 5.2 5.1 5.4 4.6 3.7 4.0
SiO2, wt% - 1.1 1.2 0.5 0.7 0.5 1.5
Fe2O3, wt% - 78.6 77.4 78.4 78.4 79.9 78.9
Slag e.g.: Point 9
e.g.: Point
5,6,9
MgO, wt% 0.0 0.3 0.1 0.0 0.2 0.2 0.1
Al2O3, wt% 2.5 0.4 0.4 0.2 0.2 0.3 0.1
SiO2, wt% 67.4 85.8 92.2 96.2 96.1 97.2 97.2
FeO, wt% 30.2 13.4 7.3 3.6 3.4 2.1 2.4
Fig. 71 SEM image of Pellet A & D with EDS analysis of all the pellets (A, B, C,
D, E, F & G)
Pellet A Pellet D
160
Fig. 72 Effect of MgO on the amount of relict Mg-silicate and FeO content of
slag phase in the fired pellets
161
(a) Acid pellet (pellet A)
(b) Pyroxenite pellet with 1.5% MgO (pellet D)
Fig.73 Distribution of Fe, Si and Mg in the fired pellets (a) Acid pellet (pellet A)
and (b) Pyroxenite pellet with 1.5% MgO (pellet D)
162
Fig.74 FeO-MgO phase diagram [54]
163
5.4.2 Metallurgical properties of the pellets with varying MgO content
5.4.2.1 Cold crushing strength (CCS)
Results, as shown in Fig.75 indicated that CCS of the pyroxenite pellets
is comparable to the acid pellets. Porosity of the acid pellets is considerably
high and decreased with increasing amount of pyroxenite, as shown in Fig.
70(a). In spite of high porosity in the acid pellets, adequate strength was
obtained due to low amount of low strength gangue or slag phase and more
recrystallization & sintering between the hematite grains, resulting in oxide-to-
oxide bond or hematite bond.
Degree of sintering between hematite grains was measured in terms of
hematite density using image analysis technique as shown in Fig.76. Density
was measured as number of hematite grains per unit area. If the sintering is not
adequate, there will be more number of grains per unit area, i.e. high hematite
density. Acid pellets showed low density, i.e. more sintering occurred between
the hematite grains. Hematite bonds are strong but they are not stable during
their reduction in blast furnace [7].
Addition of pyroxenite resulted in the formation of magnesioferrite and
more amount of low strength silicate melt phase, Fig. 70(b). Silicate melt fills up
the pores between solid particles and exerts pressure to pull them together due
to interfacial forces thereby reducing the porosity. The negative effect of silicate
melt on pellet strength is counteracted by the reduced porosity of the pyroxenite
pellets.
164
Fig.75 Effect of MgO content on cold strength of the fired pellets
Fig. 76 Effect of MgO on the density of hematite phase in the fired pellets
165
5.4.2.2 Swelling
From the results, as shown in Fig.77, it is evident that swelling index was
found to decrease with increase in pellet MgO content and a minimum of 1.5%
MgO is required to curtail the swelling as desired by the blast furnace. These
findings are also in agreement with the earlier studies on effect of gangue on
the swelling behaviour by Sharma et.al. [55,56].
Swelling is related to the ability of gangue or slag phase to withstand the
reduction stresses of independent oxide particles. High melting point slag would
produce sufficient bonding strength to limit swelling and low melting point slag
enhances swelling. In acid pellets reduction is accompanied by the reaction
between Fe2+ and SiO2 to form low melting point phase, fayalite (Fe2SiO4) that
melts at 1175oC as shown in FeO-SiO2 phase diagram [57] as in Fig.78.
High swelling index of acid pellets could be attributed to the plastic or
mobile nature of low melting point fayalitic slag that provides a medium for
absorption of the reduction stresses by increased distances between the
particles. To confirm whether this high swelling was due to presence of alkali
compounds [58, 59], the fired pellets were analysed for Na2O and K2O
contents. It was concluded that alkalies were not responsible for the observed
swelling as the Na2O and K2O content of pellets was very low, 0.02 and 0.03%
respectively.
In case of pyroxenite pellets, MgO diffuses into the wüstite phase and
increases its melting point and also increases the melting point of the slag [33].
FeO-MgO phase diagram, (Fig.74) indicates that FeO and MgO have complete
miscibility and forms sold solution. Increasing the MgO content increases the
melting point of wüstite. Low swelling of pyroxenite pellets could be attributed to
the high melting point slag that gives sufficient bond strength to withstand the
reduction stresses [30].
166
Fig.77 Effect of MgO content on swelling index of the fired pellets
Fig.78 FeO-SiO2 phase diagram [57]
167
5.4.2.3 Reduction degradation
From the results, as shown in Fig.79, it was evident that acid pellets
exhibited highest degradation and the same was found to decrease with
increasing MgO content in the pellets.
Acid pellets showed high reduction degradation due to the presence of
more hematite bonds and less silicate bonds. Pyroxenite pellets exhibited less
reduction degradation due to the presence of magnesioferrite and silicate melt,
as shown in Fig. 70(b) that are more stable compared to hematite.
In the earlier studies by the author, it was observed that uniformly
distributed silicate melt improves the RDI of iron ore pellets [51]. In addition to
silicate melt, magnesioferrite formed between the iron oxide grains also acts as
a strong bonding phase that counteracts the reduction degradation [52].
168
Fig.79 Effect of MgO content on reduction degradation of the fired pellets
169
5.4.2.4 Reducibility
From the results shown in Fig.80, it was evident that acid pellets
exhibited better reducibility compared to the pyroxenite pellet with 1.5% MgO.
Pellets with 0.5% and 1.0% MgO exhibited lower reducibility as compared to
the acid pellets.
Pyroxenite addition results in increased silica in the pellets that forms
fayalite with FeO. Melting of the fayalite at lower temperatures blocks the pores
[60], hindering the flow of reducing gases within the pellet matrix, thereby
lowering the reducibility. But further addition of pyroxenite increases the MgO
and decreases the FeO content of the slag, as shown in Fig.72, thereby
increasing its melting point. This kind of slag does not soften at reduction
temperatures and keeps the pores open for reducing gas thereby enhancing
reduction.
170
Fig.80 Effect of MgO content on reducibility of the fired pellets
171
5.4.2.5 Softening-Melting characteristics
Softening- melting characteristics of the pellets help in understanding the
formation of cohesive zone in the lower portion of the blast furnace. If the
pellets soften at lower temperature and the temperature range between
softening and melting is wider, then the resistance to the gas flow will be more
in the cohesive zone. Results shown in Fig.81 indicated that addition of
pyroxenite to the pellets up to 1.5% MgO increased the softening temperature
of the pellets and decreased the softening-melting range.
Inferior softening melting characteristics of the acid pellets can be
attributed to the FeO rich low melting fayalitic slag, as shown in FeO-SiO2
phase diagram Fig.78, whereas the pyroxenite pellets exhibited superior
properties due to the fact that MgO increases the melting point of the slag.
172
Fig.81 Effect of MgO content on the softening-melting characteristics of the
fired pellets
173
5.5 Advanced metallurgical testing of pyroxenite fluxed pellets
During this entire study, different type of pellets prepared from different
fluxes viz., limestone, dolomite, magnesite and pyroxenite, were tested for their
metallurgical properties and microstructural characteristics. Based on the
results of this test work and availability of fluxes at Tata Steel captive mines,
limestone fluxed pellets and pyroxenite fluxed pellets were found to be suitable.
But limestone fluxed pellets increases the basicity of the blast furnace unless
the sinter CaO is reduced, which is undesirable in view of the lower strength of
sinter. Hence pyroxenite fluxed pellets (with MgO content) were recommended
as suitable burden material for the blast furnaces. Tata Steel pellet plant
management also agreed to produce the pyroxenite fluxed iron ore pellets,
instead of limestone pellets, in the recently commissioned 6 MTPA pelletizing
plant.
It was found that MgO addition in the form of pyroxenite improves the
bonding phase by forming magnesioferrite and high liquidus temperature slag in
the fired pellets. These pellets exhibit superior high temperature metallurgical
properties without using any CaO based fluxing agent. High degree of
reduction coupled with low swelling and low amount of gangue in these pellets
is estimated to improve the productivity and reduce the coke rate by 15 to
20kg/ton of hot metal.
It was decided to fine-tune the pyroxenite pellet chemistry, before
commercial production in the 6 MTPA pellet plant, to find out the minimum
amount of MgO to get the desired high temperature properties.
Accordingly, pellets with varying MgO content were tested as per
advanced test procedures viz., advanced free swelling, advanced reduction
degradation and advanced swelling and softening, developed at Ijmuiden
Technology Centre (IJTC), Tata Steel Europe, The Netherlands. These tests
simulate the conditions in the stack zone and softening zone of the blast
174
furnace. Pellets with varying MgO content from 0 to 2.0% in the intervals of
0.3% were prepared for metallurgical testing as shown in Table 26.
Table-26 Chemical analysis of varying MgO pellets using pyroxenite as flux
Batch-A Batch-B Batch-C Batch-D Batch-E Batch-F Batch-G
Fe(T) 65.8 65.76 65.59 65.25 64.94 63.67 62.09
CaO 0.06 0.23 0.27 0.32 0.44 0.51 0.7
SiO2 2.21 2.22 2.69 2.82 3.21 4.02 5.24
MgO 0.02 0.32 0.7 0.94 1.11 1.56 2.29
Al2O3 2.52 2.57 1.84 1.93 1.86 2.08 2.28
5.5.1 Advanced free swelling
Swelling index indicates volume change of the pellets during reduction.
Advanced free swelling test determines the swelling at two different reduction
time durations; for 30 minutes and for 90 minutes. Swelling after 30 minutes of
reduction indicates the highest swelling that the pellets could undergo, whereas
swelling for 90 minutes indicates the final swelling after partial repair of the
reduced matrix.
Swelling index of the pellets was found to decrease with increase in MgO
content as shown in Fig.82. High swelling index of the acid pellets could be
attributed to the plastic or mobile nature of low melting point fayalitic slag that
provides a medium for absorption of the reduction stresses by increased
distances between the particles. In case of pyroxenite pellets, MgO diffuses in
to the wustite phase and increases its melting point and also increases melting
point of the slag [33]. High melting point slag offers sufficient bond strength to
withstand the reduction stresses thereby reducing the swelling [30].
As far as the swelling behaviour, 0.6% MgO in the pyroxenite pellets was
found to be the optimum.
175
Fig. 82 Free swelling index of varying MgO pyroxenite pellets
176
5.5.2 Advanced reduction degradation
Results of the test with varying MgO pyroxenite pellets are shown
Fig.83. It was obvious from the results that with increasing MgO, disintegration
(% <3.15mm) of pellets, after reduction and tumbling, decreased substantially
as compared to acid pellets. The disintegration of pellets indicates their
tendency to generate fines during reduction in the blast furnace stack zone.
Acid pellets exhibit high reduction disintegration due to the presence of more
hematite bonds and less silicate bonds. Pyroxenite pellets exhibited less
disintegration as they contain more amounts of magnesioferrite and silicate
melt.
Figure 84 shows the reduction degree of pellets during the test. Results
indicate that acid pellets reduced quite faster as compared to MgO pellets. It
could be due to the fact that, the former, due to their excessive swelling, result
in more open and cracked structure which is favourable for the reduction,
whereas MgO pellets reduce slowly due to more amount of silicate melt that
impedes the diffusion of reducing gas inside the pellets. Figure 85 shows the
pictures of the test samples before and after tumbling.
The test results indicated that a minimum of 0.6 to 0.9% MgO is required
in the pellets to control their disintegration in the stack zone of the blast furnace.
177
Fig.83 Disintegration and reduction time of pellets as a function of MgO content
Fig.84 Reduction time as a function of pellet MgO content
178
Fig.85 Reduced pellet samples with varying MgO before and after tumbling
0% MgO Before tumbling After tumbling
0.6% MgO Before tumbling After tumbling
0.9% MgO Before tumbling After tumbling
1.2% MgO Before tumbling After tumbling
179
5.5.3 Advanced swelling and softening
This test simulates the behaviour of pellets during their high temperature
reduction in the stack zone and during softening in the cohesive zone of the
blast furnace. Results of the test with varying MgO pyroxenite pellets are given
in Table 27 and depicted in Fig.86. Test results indicated that addition of MgO
increases the softening temperature of the pellets as compared to the acid
pellets (Fig.87). PEFA in the Fig 87 denotes the reference pellet sample from
Ijmuiden pellet plant. Figure 88 shows the pellet samples after completion of
the test.
From these results, especially based on pressure drop and softening
temperature, it was concluded that a minimum of 0.3 to 0.6% MgO is desired in
the pyroxenite pellets to achieve better high temperature properties. Better
performance of the MgO pellets could be attributed to the formation of high
melting point slag.
Ta
ble
-27
Ad
va
nce
d s
we
llin
g a
nd s
oft
enin
g t
est
results f
or
va
ryin
g M
gO
pelle
ts
B
atc
h-A
B
atc
h-B
B
atc
h-C
B
atc
h-D
B
atc
h-E
B
atc
h-F
B
atc
h-G
Mg
O,%
0
.02
0.3
2
0.6
2
0.9
1
1.2
1
1.5
2
2.0
3
dR
/dt
[%/m
in]
0.6
4
0.4
0
0.4
8
0.4
0
0.4
3
0.4
3
0.5
8
Red
uctio
n tim
e [
min
] 1
02
16
1
13
5
15
6
15
2
15
2
12
6
Delta
P a
t th
e e
nd
of re
du
ction
[m
mW
K]
17
.58
2.9
3
4.1
8
2.9
7
3.0
9
3.8
3
2.8
5
Be
d t
em
pe
ratu
re a
t 10
% s
hri
nka
ge [
oC
] 1
06
0
10
86
10
82
11
34
11
04
11
08
10
97
Be
d t
em
pe
ratu
re a
t 10
% s
hri
nka
ge [
oC
] 0
1
13
8
11
31
11
79
11
56
11
57
11
51
Be
d t
em
pe
ratu
re a
t d
P 1
00
mm
WK
[oC
] 1
07
4
11
74
11
61
12
08
11
81
11
84
11
93
181
Fig.86 Effect of MgO content on reduction time, delta P and softening
temperature
Fig.87 Effect of pellet MgO content on their softening temperature
Fig
.88
Pelle
t sa
mp
les a
fte
r co
mp
letion
of
the
te
st
183
5.5.4 Critical observations from the advanced metallurgical tests
The following conclusions can be drawn from the results of the advanced
metallurgical test work;
1. 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.
2. 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%
3. 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.
4. 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.
5. 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.
184
5.6 Summary of the results of effect of flux addition on the pellet quality
Results of the effect of different fluxes on the pellet quality are
summarized in Table 28. Results of the advanced metallurgical tests of
pyroxenite pellets, viz., advances swelling, advanced RDI and advanced
swelling & softening, are given in Table 29.
Based on resultant pellet quality observed during the test work and on
the availability of fluxes at the captive mines, pyroxenite was suggested as
suitable flux for pelletizing. Use of pyroxenite as flux in the pelletizing found to
yield the following advantages;
In addition to MgO content, pyroxenite addition also increases silica
content of pellets. This decreases the external quartz addition to blast
furnace that is required to control its slag basicity.
Unlike carbonate fluxes like limestone or dolomite, pyroxenite does not
undergo any endothermic reaction for dissociation; hence energy
requirement during pellet induration is comparatively low.
Pyroxenite does not release any CO2 during pellet induration.
Ta
ble
-28
Su
mm
ary
of
the
re
sults o
f e
ffe
ct
of diffe
ren
t flu
xe
s o
n th
e p
elle
t q
ualit
y
Typ
e o
f flux
use
d in
p
elle
tizin
g
Fe
(t),
W
t.%
S
iO2,
Wt.%
A
l 2O
3,
Wt.%
C
aO
, W
t.%
M
gO
, W
t.%
S
we
llin
g
Ind
ex,
%
Red
ucib
ility
In
de
x,
%
Red
uctio
n
de
gra
da
tio
n
ind
ex,
%
-3
.15
mm
CC
S,
Kg
/pe
llet
Lim
esto
ne
6
6.0
1
.9
2.2
0
.1
0.1
3
9.7
7
3.7
3
9.0
2
39
6
5.8
2
.0
2.1
0
.5
0.2
1
0.5
6
0.5
1
2.0
2
41
6
5.4
1
.9
2.2
0
.8
0.2
1
3.2
6
7.2
1
0.6
2
68
6
5.0
2
.2
2.1
1
.4
0.3
1
9.9
6
5.8
6
.7
24
6
6
4.8
1
.9
2.2
1
.6
0.2
1
0.2
6
8.0
1
1.9
2
37
Dolo
mite
63
.6
4.2
2
.0
0.1
1
.5
17
.7
76
.2
25
.2
24
3
6
3.2
3
.6
2.1
0
.9
1.6
1
1.7
6
5.5
5
.6
23
1
6
4.0
2
.9
2.1
1
.3
1.5
1
3.1
7
1.6
6
.6
25
5
6
3.7
2
.7
2.2
1
.7
1.7
1
3.9
7
3.3
1
4.6
2
41
6
3.2
2
.5
2.1
2
.0
1.7
8
.1
69
.0
24
.3
22
1
Ma
gn
esite
65
.8
1.8
2
.2
0.5
0
.5
23
.5
76
.6
35
.4
22
1
6
5.8
1
.8
1.9
0
.4
0.8
1
5.4
8
0.9
2
8.5
2
07
6
5.4
1
.7
1.9
0
.4
1.3
1
2.9
7
8.5
2
4.3
2
12
6
5.6
1
.7
1.8
0
.4
1.9
1
2.7
7
6.0
1
3.5
2
03
6
5.2
1
.7
1.9
0
.3
2.3
1
0.6
7
4.2
1
0.2
1
99
6
4.4
1
.9
2.1
0
.4
2.9
3
.5
74
.0
13
.2
19
2
Pyro
xe
nite
65
.6
2.7
2
.0
0.1
0
.5
22
.6
70
.9
22
.7
21
6
6
4.4
3
.8
2.0
0
.1
1.1
1
7.7
7
1.8
2
2.1
2
40
6
3.8
4
.0
2.0
0
.2
1.5
1
3.3
7
6.2
1
8.4
2
43
Ta
ble
-29
Su
mm
ary
of
resu
lts o
f th
e a
dva
nced m
eta
llurg
ica
l te
sts
of p
yro
xe
nite
pelle
ts
Ch
em
ica
l A
na
lysis
A
dva
nce
d s
we
llin
g
Ad
va
nce
d s
we
llin
g a
nd s
oft
en
ing
A
dv. R
DI
Fe
(t),
W
t.%
S
iO2
, W
t.%
A
l2O
3,
Wt.%
C
aO
, W
t.%
M
gO
, W
t.%
Sw
elli
ng
%
(3
0 m
in)
Sw
elli
ng
%
(9
0 m
in)
Re
duction
tim
e [
min
]
De
lta
P a
t th
e e
nd
of
red
uctio
n
[mm
WK
]
Be
d
tem
pe
ratu
re
at 1
0%
be
d
sh
rin
kag
e
[oC
]
Be
d
tem
pe
ratu
re
at 2
5%
be
d
sh
rin
kag
e
[oC
]
Be
d
tem
pe
ratu
re
at
100
mm
WK
[o
C]
% <
3.1
5
mm
N
.T
[%]
65
.8
2.2
1
2.5
2
0.0
6
0.0
2
49
.1
47
.3
102
17
.58
106
0
- 1
07
4
27
.97
65
.8
2.2
2
.6
0.2
3
0.3
2
22
.0
23
.14
161
2.9
3
108
6
113
8
117
4
-
65
.6
2.7
1
.8
0.2
7
0.7
0
16
.6
13
.54
135
4.1
8
108
2
113
1.0
1
16
1
5.7
5
65
.3
2.8
1
.9
0.3
2
0.9
4
15
.4
14
.68
156
2.9
7
113
4
117
9.0
1
20
8
4
64
.9
3.2
1
.9
0.4
4
1.1
1
16
.8
16
.48
152
3.0
9
110
4
115
6.0
1
18
1
2.3
4
63
.7
4.0
2
.1
0.5
1
1.5
6
16
.1
14
.79
152
3.8
3
110
8
115
7.0
1
18
4
2.0
2
62
.1
5.2
2
.3
0.7
0
2.2
9
15
.2
14
.47
126
2.8
5
109
7
115
1
119
3
-
187
5.7 Implementation of results at 6 MTPA iron ore pelletizing plant at Tata
Steel Jamshedpur
The following recommendations, emerged from the results of the test work,
were implemented in the recently commissioned 6 million ton capacity iron ore
pelletizing plant;
Mean particle size of the pelletizing feed was maintained at 55 microns.
As a result green pellets of desired quality were obtained in the plant.
Drop number of the pellets was achieved 12 to 15, green crushing
strength of 1.5 to 1.7 kg/pellet and green pellet moisture was around
9.2%.
Pyroxenite was added as fluxing agent to produce pellets with 0.9%
MgO in the pellets. Before this recommendation, limestone was selected
as fluxing agent.
As a result of pyroxenite addition, the swelling index of the pellets
produced during the 6 months operation was <18% (average value).
Pellets produced also exhibited excellent reducibility ~ 77% (average
value).
Pyroxenite pellets, first of their kind, are being produced on commercial
scale at the rate of 11000 tons per day at 6 million ton capacity iron ore
pelletizing plant, Tata Steel, Jamshedpur. Figure 89 shows the microstructure
of fired pellet sample from the plant. It confirms the formation of magnesioferrite
and silicate melt during induration. Microstructure of partially assimilated
pyroxenite particle is also shown in Fig.90, which was observed in under-fired
pellets. Continuous efforts are under progress to stabilize and ramp up of the
plant’s grinding, pelletizing and induration circuits to produce high quality
pyroxenite pellets at the rated capacity of 18000 tonnes per day.
Fig
.89
Mic
rostr
uctu
re o
f p
yro
xe
nite
pelle
t sa
mp
le fro
m 6
MT
PA
pe
llet
pla
nt o
f T
ata
Ste
el
Fig
.90
Mic
rostr
uctu
re o
f p
yro
xe
nite
pelle
t w
ith
re
lict p
yro
xe
nite
part
icle