Characteristics of combustion zone and evolution of mineral phases along bed height in ore sintering
Int. J. Miner. Metall. Mater.
Characteristics of combustion zone and evolution of mineral phases along bed height in ore sintering
Quantitative parameters of bed combustion, including the thickness of the combustion zone (TCZ), the maximum temperature of the combustion zone (MTCZ), and the bed shrinkage, were characterized through a series of sinter pot tests in transparent quartz pots. The results showed that TCZ first ascended and then descended as the sintering process proceeded. The sintering process was divided into four stages according to the variation rate of the TCZ. A “relative-coordinate” method was developed to obtain the actual reaction temperature of sinter along the height direction. With increasing the sintering temperature, the reactants transformed and entered into liquid phases. The mineral composition and microstructure of the sinter were characterized through X-ray diffraction and scanning electron microscopy-energy-dispersive X-ray spectroscopy. Liquid phases with greater Fe and Al contents were more likely to form acicular-like silico-ferrite of calcium and aluminum after crystallization because of the outward spread of Al, which led to a better fluidity of the liquid. An evolution mechanism of “solid-state reaction-liquid phases formation-crystallization” of the mineral phases is proposed.
flame front; mineral phases; sintering; evolution mechanism; transparent quartz pots
Sinter, as an important iron-containing raw material for
blast furnaces, accounts for more than 70% of iron-containing
raw materials. The output of sinter in China is on the order
of hundreds of millions of tons per year [
]. Sinter is
obtained by mixing together appropriate amounts of iron ore
fines, fuel (coke breeze), flux (lime, limestone), and water.
The raw material is ignited under negative pressure,
resulting in a series of physical and chemical changes [
combustion zone (CZ) of sinter is the region where the
solid-fuel combustion occurs, which results in an increase of
the bed material temperature and the formation of liquid
phases. In addition, the thermal decomposition of carbonates
and sulfates, as well as the reduction of hematite and other
complex oxides, also occurs in CZ [
]. The constant
negative pressure causes movement of the flame front from top
to bottom. As a result, the heat injected from the hot gas
preheated by the upper sinter, passes down the bed, raising
the ignition temperature of regions ahead of the front. This
process is called the self-regenerative function .
Furthermore, different reaction temperatures along the bed height
modify the liquid volume, the mineral phases, and the
metallurgical properties of the sinter, making the sintering an
inhomogeneous process [
]. Therefore, studies of the
characteristics of CZ, including the thickness of the
combustion zone (TCZ), the maximum temperature of the
combustion zone (MTCZ), the properties of the liquid and solid
phases, and the evolution of the sinter phases are important
for the controllable preparation of sinter.
Researchers who have previously investigated CZ of
sintering have focused mainly on numerical modeling and
sinter pot experiments. The results of numerical simulations of
the sintering process have already aroused extensive interest.
Parker and Hottel [
] proposed a combustion equation for a
single carbon particle. Combining Parker and Hottel’s equation
and the coke combustion rate equation for a quasi-particle,
some scholars developed numerical simulation models of the
temperature distribution in a sintering layer [
quantitative parameters of bed combustion, including the flame front
speed, duration time in CZ, and the maximum temperature,
were simulated through numerical modeling [
also used the discrete element method to construct a
comprehensive numerical simulation model to describe the
structural changes in an iron ore sintering bed [
Numerical simulations of combustion and heat transfer
simulate the homogeneous sintering process well and provide
solutions for the quantitative combustion parameters.
However, these mathematical models fail to consider the
high-temperature behavior of the raw material, the effect of
gravity on CZ, and the shrinkage of the bed material,
resulting in large discrepancies between the models and the
behaviors in real sintering beds.
Sinter pot tests are an indispensable means of reliably
studying actual sintering processes. They have been adopted
by many investigators. Choudhary and Nandy [
the sinter structure with high-alumina ores at different flame
front speeds. Loo and Dukino [
] obtained the maximum
temperature, flame front thickness, and residence time using
embedded thermocouples in a sintering bed. Loo and
] also explored and reported the effect of airflow rate
on the flame front parameters and the influence of the average
flame front area on the sinter yield and sinter tumble strength.
Previous research on CZ has been limited, and the use of steel
pot tests with thermocouples to obtain experimental results is
less intuitive and is imperfect for investigating CZ. Some
sintering studies were creatively carried out in transparent quartz
pots, which inspired the present study [
Here, we establish a research route to characterize the
evolution of the flame front and the phase chemistry along
the bed height of sintering. Quantitative parameters of bed
combustion, including TCZ, MTCZ, and the bed shrinkage,
were measured through a series of sinter pot tests in
transparent quartz pots with the assistance of an infrared
thermometer. A relative-coordinate method was proposed to
obtain the actual reaction temperature of sintering in any
height direction. The Factsage 6.4 software was used to
calculate the amounts and constituents of liquids in different
regions of the sinter cake. The phases and microstructure of
the sinter were characterized through X-ray diffraction
(XRD) and scanning electron microscopy–energy-dispersive
X-ray spectroscopy (SEM–EDS); a mechanism for the
evolution of the phases was proposed.
2.1. Properties of raw materials
Raw materials such as iron ores, fluxes (quick-lime and
limestone), sinter return fines, steelmaking slag, and coke
breeze were supplied by a Chinese integrated steelwork. The
ore blending scheme for laboratory-scale sintering pot tests
was similar to that of the sinter plant, with the contents of Fe,
SiO2, and MgO in the sinter maintained at 57.50wt%,
4.94wt%, and 1.11wt%, respectively. The basicity (CaO/SiO2)
of the sinter was maintained at 2.00, which was controlled
through adjusting the addition of flux. The chemical
composition and proportions of the raw materials are listed in Table
1. Table 2 and Table 3 give the ash composition and the
proximate analysis results for the coke breeze as well as the
constant-volume low calorific value.
2.2. Description of transparent sintering trials
Because the temperature of CZ can (and does) exceed
1300°C , sintering trials were typically conducted in a steel
sinter pot, which rendered the sintering process invisible.
Consequently, a substantial amount of important
experimental data, such as TCZ, and the movement or migration
of the flame front, cannot be directly or accurately measured,
which hinders further research into the sintering process.
Here sintering trials were conducted in a high-purity quartz
tube with a thickness of 5 mm and a diameter of 150 mm; its
schematic is presented in Fig. 1. The height of the sintering
Sinter return fines
material layer was 800 mm. The sintering temperature was
measured using an infrared thermometer (DT-8869h) with a
measurement range from 800 to 2200°C and an accuracy of
1°C. A fixed ruler was used to measure the height of the soft
melting layer, and the whole process of the experiments was
recorded with a high-definition video camera.
In a typical procedure, raw materials were thoroughly
blended manually according to the blending scheme and
some water was added. After a quick-aging, the mixture was
transferred into a drum of 500 mm long and 200 mm in
diameter for granulation for 3 min at a rotation speed of 17
r/min. 2 kg of sinter with the particle size range between 10
and 12.5 mm was introduced into the sinter pot as the
bedding material. When the granulation was finished, the mix
was charged and ignited at a temperature of (1050 ± 50)°C
for 1.5 min with a suction pressure of 6 kPa. A suction
pressure of 10 kPa was maintained during the sintering
process. As shown in Fig. 2, the sinter cake was equally
divided into six parts for studying the evolution of the
mineragraphy of the sinter cake in the height direction. The samples
were crushed and sieved to grain sizes from 10 to 12.5 mm
before subsequent experiments. The average of three groups of
repeated trials was reported as the results of the experiment.
2.3. Characterization of sintered samples
The microstructures of samples were observed using a
Quanta 250 environmental scanning electron microscope. In
addition, the mineral compositions of the obtained sinter
were characterized by EDS. The powdered samples were
characterized by XRD using Cu Kα radiation (M21XVHF22,
MAC Science Co., Ltd., Japan).
3. Results and discussion
3.1. Migration behavior of CZ
The migration of CZ and bed shrinkage during the
sintering process are complicated processes that are not only
dependent on the heat generated from the combustion of the
coke particles but also on the properties of the raw material,
moisture content, airflow rate, and the thickness of the feed
layer. Loo and Dukino [
] have reported a method to
determine the bed temperature using thermocouples. To
effectively overcome the technical difficulties brought about
by thermocouples, we instead used infrared thermometry in
this study. The CZ was defined as the region where the
sintering temperature was greater than 1000°C. Images of the
actual sintering process in a transparent quartz pot with a
height of 800 mm are shown in Fig. 3. In the pattern, the
variation of TCZ and the bed shrinkage are clearly
observed; the detailed statistical results are displayed in Fig. 4.
The results show that the TCZ first ascended and then
descended as the sintering process proceeded. The sintering
process can be divided into four stages according to the
variation rate of the TCZ. Stage 1 is the ignition period,
where the combustion of natural gas and coke breeze
releases large amounts of heat, leading to an increase in bed
temperature, a rapid increase of the TCZ within 3 min, and
a sharp increase in the temperature of the exhaust gas.
Stage 2 is the post-ignition period, where the energy mainly
originates from the combustion of coke breeze and from the
self-regenerative function; the heat released in this stage
slightly exceeds that released in stage 1 [
with that of stage 1, the TCZ of stage 2 increased slowly
from 65 to 80 mm between the sintering time of 3 and 16
min. Stage 3 is the period of rapid increase of the TCZ.
Because of the energy accumulation during the early
sintering process, the self-regenerative function becomes
increasingly significant, accompanied by a rapid increase of
the TCZ. At 23 min, the TCZ and the bed shrinkage
reached their respective maxima simultaneously. At this
point, CZ migrated to the bottom of the bed (Fig. 3). In
stage 4, which occurs toward the end of the sintering
process, the bed shrinkage remains unchanged, with a final
shrinkage of 190 mm. A noticeable descent of the TCZ and
a dramatic increase of the temperature of the sintering
exhaust gas during this period (Fig. 4) indicate that the
sintering was finished.
The self-regenerative function along the bed height is
known to modify the liquid volume, mineral phases, and the
metallurgical properties of the sinter, making the sintering
an inhomogeneous process. Therefore, confirming the
maximum temperature of CZ of sinter cake is vital to
establish the corresponding relation between the sintering
temperature and the properties of the sinter. Such research is
relatively infrequent because bed shrinkage during the
sintering process makes the measurement of combustion
We developed a method to establish the sintering
temperature distribution for the sinter cake in the height direction.
The thickness of the sinter material bed was 800 mm, and
the height of the sinter cake was 604 mm. If the top of the
sinter cake is taken as the original point, the distance from
the center of CZ to the original point (DFCO) can be
recorded together with the sintering temperature as a function
of sintering time (Fig. 5). As shown in Fig. 5, the center of
CZ descended as the reaction progressed and reached the
bottom of the sinter cake at 23 min. The results indicated
that MTCZ increased from 1175 to 1409°C during sintering,
as shown in Fig. 5. Combining the variation of the MTCZ
and the DFCO with sintering time, we obtained the
sintering temperature distribution for the sinter cake in the
height direction, as shown in Fig. 6. To further study the
sinter, the sinter cake was divided into six parts (Fig. 2)
and the sintering temperature of each part was
approximated on the basis of the integral mid-value through the
T = τ 0 (1)
τ −τ 0
where T is the sintering temperature measured by the
infrared thermometer, τ is the sintering time, and the
calculated integral mid-values ( T ) is defined as the sintering
temperature of each sinter part (1–6). The red points in Fig.
6 are the calculated T .
3.2. Phases transformation and microstructure observation
3.2.1. Microstructure observation
To compare the phase transformations of the sinter cake
along the height direction, XRD patterns of sinters 1–6 are
presented in Fig. 7. The key phases of iron oxide⎯ hematite,
silico-ferrite of calcium and aluminum (SFCA) (SFCA–M14O20,
SFCA-I–M20O28, and SFCA-II–M34O48), and magnetite, are
marked in the patterns. With increasing sintering
temperature, the peaks of the SFCA phases gradually decreased in
intensity and those of hematite and magnetite increased in
intensity, which indicates that sintering at a temperature
greater than 1300°C provided no advantage for the
formation of SFCA; these results are consistent with those of
previous studies [
]. Notably, sample 1 exhibited a phase
transformation from hematite to SFCA at temperature less
than 1250°C. Furthermore, the best temperatures for the
formation of SFCA were 1250 and 1260°C.
Typical microstructures of sinter samples 1–6, as
observed by SEM, are shown in Fig. 8. Labels A, B, and C in
Fig. 8 indicate low-, medium-, and high-magnification
images of the sinter, respectively. As evident in the figure, the
porosity of the sinter was gradually reduced from sample 1
to sample 6. Pores in the sinter were mainly derived from
the gaps between the material particles, the combustion of
fixed carbon, the evaporation of moisture and crystal water,
and the contraction of the solid–melt assemblage [
]. In the
process of sintering, the mass of liquid increased at the
bottom of the sinter cake because of the high sintering
temperature, which is verified in a later section. Because of the
action of gravity of the upper sintered zone, CZ was
compressed and the liquid enhanced the densification by filling
As shown in Figs. 8(1-B), 8(2-B), and 8(2-C), a mass of
acicular-like calcium ferrite (ACF) was found. The ACF,
which possess better strength and reducibility than the
non-acicular SFCA, was the key component. Compared
with the ACF of sample 2, the ACF of sample 1 was slender,
with a diameter of 1.4 μm and a length of 60 μm, which
indicates that the crystallization was imperfect at 1216°C. In
addition, much of the skeletal hematite (SH) was found
around the pores, with a morphology of fishbone-like
features connected by parallel crystals, as shown in Fig. 8(1-C).
A number of other authors [
] have reported that
volume expansion will occur during the reduction of this kind
of SH, which would adversely affect the low-temperature
reduction disintegration of sinter. This volume expansion
is mainly due to the rapid cooling of the surface of the
sinter cake making the transition of the Fe2O3 phase
incomplete. However, at 1250°C in Figs. 8(2-B) and 8(2-C),
ACF was well crystallized, with a fascinating dendritic
The SFCA of sample 3, which was sintered at 1260°C,
shows the coexistence of ACF and columnar-like calcium
ferrite (CCF). The CCF of sample 3, with an average
diameter of 12.5 μm and an average length of 160.8 μm, is
another SFCA with a low Fe content. Some of the SFCA in
sample 3 retained the dendritic structure with a small
amount of acicular calcium ferrite, indicating that an
increase of the sintering temperature would lead to the
transformation of ACF to CCF. As shown in Figs. 8(4-B) and
8(4-C), the SFCA of sample 4 exhibited the columnar
microstructure, whereas the dendritic structure was absent. The
SFCA content of sample 4 was significantly lower than that
of samples 1–3. The CCF crystallized at the boundary of
iron oxide (hematite, magnetite) and slag, which indicates
that hematite provides Fe2O3 and silicate provides elements
such as Si, Al, and Ca in the CCF crystallization process.
Figs. 8(5-B) and 8(5-C) show the microstructure of sample
5, which was sintered at a higher temperature of 1364°C;
interconnected crystals of Fe2O3 are formed because of the
soft melting of Fe2O3. More importantly, the content of
magnetite gradually increased because of the decomposition
of Fe2O3 at temperatures above 1300°C, with a decreased
level of SFCA. Interestingly, a small quantity of ACF was
found in the gap of Fe2O3 interconnected crystals. Figs.
8(6-B) and (6-C) show the SEM images of sample 6, which
was collected from the bottom of the sinter cake; the images
show a large quantity of magnetite and a small amount of
3.2.2. Thermodynamic analysis of phase transformation
using Factsage 6.4
The mass fractions of the liquid phases of sinter under
1216–1394°C were calculated through Factsage 6.4 using
the Equilib and Phase Diagram modules. The main
databases were FToxid-SLAGA, FToxid-SPINA, and
FToxid-CORU. To reduce the complexity and improve the
accuracy of the analysis, only the major components of the sinter
were considered during the calculation. The chemical
constituents were determined by chemical analysis and X-ray
fluorescence analysis. With respect to the forms of Fe in the
raw material, all forms were converted to the form of Fe2O3
and Fe3O4 in accordance with the proportion of total Fe and
FeO. All the oxides were normalized as shown in Table 4.
The thermodynamic calculation results for the liquid phase
in the sintering process are shown in Fig. 9. The results
show that the mass fraction of liquid phase increased from
12.9% to 62.85% as the sintering temperature was increased
from 1216 to 1394°C, which indicates that more liquid
phase would be produced at the bottom of the sinter cake.
3.3. Phase evolution mechanism of sinter cake
The literature contains a large number of reports of
studies of sinter phases, especially studies of SFCA. The
formation sequence of phases was proposed by using tablet
]. The phase evolution of SFCA in sinter
mixture was characterized through a series of in situ XRD
and heat/quench experiments, improving understanding of the
effect of oxygen partial pressure that may maximize the
formation of SFCA in industrial sintering processes [
Loo and Leung [
] reported the influence of phosphorus,
basicity, silica level, maximum sintering temperature, and
gangue level on the bonding phase structure of sinters.
] indicated the reaction sequences and
formation of phases in sinter using a 400-mm test pot. In the case
of the formation mechanism of SFCA, Webster et al. [
and Ding and Guo [
] have extensively researched the
formation of calcium ferrite in sinter. On the basis of these
previous studies, an evolution mechanism involving
“solid-state reaction—liquid phase formation—crystallization”
of the mineral phase of high-hematite iron ores was
proposed through quartz sintering pot texts and thermodynamic
calculations, as shown in Fig. 10. The initial liquid phase
contained more Fe and Al. The liquid phase was more likely
to form ACF after crystallization because of the outward
spread of Al, which led to better fluidity of the liquid.
Therefore, ACF was easier to generate during low-temperature
sintering (Figs. 10(c), 10(d), and 10(e)). With an increase of
sintering temperature, the SiO2 content of the liquid phase
increased. The affinity of CaO·SiO2 was stronger than that of
CaO·Fe2O3, which led to a decrease in SFCA content. Liquid
with more Si and Al would increase the viscosity, which is
likely to be CCF after crystallization (Figs. 10(e) and 10(f)).
As shown in Figs. 10(g) and 10(h), Fe2O3 will decompose
with the further increase in sintering temperature.
A research route of “combustion zone—liquid and solid
phase of sinter—evolution of mineral phase” was
established to characterize the evolution of the flame front and
the mineral phases along the bed height of sintering.
(1) Quantitative parameters of the bed combustion,
including TCZ, MTCZ, and the bed shrinkage were measured.
The TCZ first increased and then decreased as the sintering
process proceeded. The sintering process was divided into
four stages according to the variation rate of TCZ.
(2) Notably, sample 1 exhibited a phase transformation
from hematite to SFCA at temperature less than 1250°C.
Sintering at a temperature greater than 1300°C provided no
advantage for the formation of SFCA.
(3) The initial liquid phase contained more Fe and Al.
The liquid phase was more likely to form ACF after
crystallization because of the outward spread of Al, which led to a
better fluidity of the liquid. With increasing sintering
temperature, the SiO2 content of the liquid phase increased,
which led to a decrease of SFCA content.
This work was financially supported by the National
Science Foundation for Young Scientists of China (No.
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