oxidising roasting behaviours of anthracite containing haematite pellets
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Oxidising roasting behaviours of anthracitecontaining haematite pellets
Z. K. Tang, G. H. Li, H. L. Zhang, Y. B. Zhang * and X. Q. Li
In this investigation, the roasting features of haematite pellets to which a small amount of
anthracite was added and the effects of carbon on the induration behaviours of the pellets were
studied. The results indicate that the suitable dosage of pulverised anthracite is 1?0–1?5%. During
the roasting, part of the original haematite grains in the pellets are reduced to magnetite, and
some original haematite grains are decomposed into magnetite. Then, the newborn magnetite is
oxidised to secondary haematite, which is beneficial to the recrystallisation of Fe2O3 in the fired
pellets. Therefore, adding a certain proportion of anthracite is an effective way to improve the
roasting strength of haematite pellets, as well as to reduce the roasting temperature and the total
energy consumption of the pellet production.
Keywords: Haematite, Pellet, Anthracite, Oxidising roasting
Introduction
Oxidised pellets, possessing good mechanical and
metallurgical properties, are high quality burdens for
blast furnace ironmaking.1,2 However, with the rapid
development of the iron and steel industries, magnetite
resources are becoming scarce. Thus, it is imperative
to make good use of haematite resources to produce
pellets.3 Imported haematite concentrate is of high
iron grade, low gangue content and fine granularity.4
However, the high quality finished pellets from haema-
tite concentrates require a higher roasting temperature
and narrower firing temperature range (1300–1350uC),
and the fired strength is relatively lower than that of
magnetite pellets.5,6 Hence, how to solve the problems
of oxidised pellets prepared from haematite concen-
trates becomes very important. Much research has been
conducted on oxidised pellets prepared from mixed
haematite–magnetite concentrates,4,6,7 where it has been
shown that roasting temperature can be reduced and
fired strength improved. However, to ensure adequate
strength for blast furnace use, magnetite needs to exceed
20%.7,8
Practical results for adding a certain proportion of
solid fuel to haematite concentrates show that the pellet
strength is increased, energy consumption is reduced and
the pellet metallurgical properties are improved.9,10
However, before the oil crisis of 1973, because of their
low price and easy use, oil and natural gas were widely
used as fuels, and the practice of adding solid fuel in
haematite pellets had not been well developed.9 Since the
oil crisis, the price of oil has soared and far exceeds that
of solid fuel, and much research on carbon containing
pellet preparation has been conducted.9,11,12 However,most of the research has focused on energy saving, and
few studies have been designed to reveal the indurationmechanisms of carbon containing haematite pellets.
In the last 20 years, some investigations on carbon
containing haematite pellets have been carried out inChina;13–17 however, most of the research was directedtowards the preparation of reduced pellets for non-blast
furnace ironmaking.
18
With the increasing requirementfor Blast Furnace (BF) fuel economy, adding solid fuelto pellets as an energy saving measure could be greatlydeveloped in the oxidised pellets.
In this investigation, oxidised pellets were made fromhaematite concentrate with a certain percentage of pulverised anthracite. The behaviour of carbon duringoxidising roasting was studied to reveal the indurationmechanisms of anthracite containing haematite pellets.
Experimental
Raw materialsThe haematite concentrate used in this investigation was
from Brazil and is characterised by high total iron(67?22%Fetotal) and low impurities (Table 1), and thespecific surface area measured by a permeability methodwas 1630 cm2 g –1.
Pulverised anthracite was used as the carbon contain-ing material, of which the BET surface area was6599 cm2 g –1, and the analysis tested according to GB/T 212-2008 is shown in Table 2.
MethodsThe experimental procedure included ball preparationand drying, roasting, strength measurement, FeO testand microscopic analysis.
Ball preparation and dryingFor each trial, 5 kg haematite concentrate was blendedwith a given proportion of pulverised anthracite using
School of Minerals Processing & Bioengineering, Central South University,Changsha 410083, China
*Corresponding author, email [email protected]
2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the InstituteReceived 1 January 2012; accepted 7 March 2012DOI 10.1179/1743281212Y.0000000024 Ironmaking and Steelmaking 2013 VOL 40 NO 1 69
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1?25% bentonite as binder. The green balls (9–15 mm indiameter) were prepared in a disc pelletiser with a
diameter of 1000 mm and then statically dried at 105u
Cin an oven for 4 h.
Roasting tests and strength measurement
Firing tests were carried out in an electrically heatedshaft furnace. To simulate the firing atmosphere, mixedgas of N2/O2 at given oxygen content (volume fraction)was pumped into the shaft furnace at a certain flowrate.The dry balls were charged into a heat resistant pot,which was then pushed downwards into the hightemperature zone in the furnace every minute in fivesteps. The pellets were fired at the stated temperature for
a certain period and then naturally cooled to ambienttemperature. The compressive strength of the pellets was
measured with an LJ-1000 experimental machine. Anaverage value of 10 pellets was used as the compressivestrength for each test.
FeO test
To analyse the behaviour of carbon during the roasting
process, the FeO content of fired pellets was measuredby chemical titration.
Microscopic analysis
Microstructure features of fired pellets at various roastingconditions were studied using a Leica DMRXE micro-scope with an automatic image analyser (Germany).
Results and discussionRoasting characteristics of anthracitecontaining haematite pelletsEffects of anthracite dosage
The effects of anthracite dosage on the fired pelletcompressive strength are shown in Fig. 1. It can be seenthat the pellet strength with 0?5% anthracite is a littlelower than that with no anthracite. When the anthracite
dosage reaches 1?0–1?25%, the compressive strength ismaximum and then decreases greatly if the anthracite
dosage further increases from 1?5 to 4%. The resultsindicate that the appropriate anthracite amount of 1?0– 1?5% can improve the strength of the fired haematite
pellets.
Effects of roasting temperature
The curve of the compressive strength as a function of
roasting temperature is presented in Fig. 2.
As shown, the strength of the pellets with 1?0%
anthracite is always higher than that of the pellets withoutanthracite at the same roasting temperature; moreover, for
the anthracite containing haematite pellets, the strength
increases with increasing temperature. However, thestrength of the pellets in the absence of anthracite not
only does not increase markedly until 1200uC but also
actually decreases due to the decomposition of Fe2O3
above 1300uC. The results imply that the roasting tem-perature can be decreased, and the suitable firing tem-
perature range is enlarged by adding an appropriateamount of anthracite into haematite pellets.
Effects of roasting time
The curve of the roasting time affecting the fired pelletcompressive strength is plotted in Fig. 3. It can be seen
that the compressive strength increases gradually withthe prolonging of roasting time and reaches a maximum
at y25 min.
Effects of oxygen content
The fired pellet compressive strength is greatly affectedby the change in the roasting atmosphere in the furnace,
as shown in Fig. 4. The strength of the pellets roasted in
the inert atmosphere (0%O2) is slightly higher than thatroasted in 10%O2. The pellet strength reaches the
maximum when the oxygen content is 20% and then
decreases gradually with increasing oxygen content.
Table 2 Analysis of anthracite/wt-%
FCd V d Ad
Main chemical composition of ash
Fetotal CaO MgO Al2O3 SiO2
77.6 6.48 15.6 9.26 9.45 1.50 36.82 42.95
1 Effects of anthracite dosage on fired pellet compres-
sive strength (oxygen content: 20%; airflow: 6 L min –1;
roasting temperature: 1280 C; roasting time: 20 min)
2 Effects of roasting temperature on fired pellet compres-
sive strength (oxygen content: 20%; airflow: 6 L min –1;
roasting time: 20 min)
Table 1 Chemical composition of materials/wt-%
Materials Fetotal FeO SiO2 CaO MgO Al2O3 LOI*
Iron concentrate 67.22 0.55 2.17 0.01 0.05 0.55 0.59Bentonite 7.07 … 60.61 0.94 2.2 17.98 10.41
*LOI, loss on ignition.
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Induration mechanisms of anthracite containing haematite pelletsRoles of carbon during roasting
In the oxidative atmosphere during the oxidised pellet
production, the growth of Fe2O3 grains was not observed
until the roasting temperature reached 1300uC. However,
if the temperature is too high (.1350uC), Fe2O3 will be
decomposed as per the following reaction5,18
6Fe2O3?4Fe3O4zO2DG 8~140380{81:38T Jð Þ
lnPo2~{70649:22
T z40:96
(1)
The relative decomposition temperatures with different
oxygen contents were calculated according to equa-
tion (1) and given in Table 3.
It can be seen that the decomposition temperature of
Fe2O3 increases with increasing oxygen content. For the
anthracite containing pellets, some of the oxygen is
consumed by the burning of carbon, and the oxygen
content within the pellets is decreased, so that the de-
composition temperature of Fe2O3 is lowered.
A direct reduction reaction occurs when solid carbon
comes into contact with iron oxides in the pellets.
Therefore, Fe2O3 in anthracite containing haematite
pellets would be reduced to Fe3O4 by the solid carbon in
anthracite
6Fe2O3zC~4Fe3O4zCO2 (2)
Moreover, CO and H2 will be produced during the
heating of anthracite, which will also be reduced to
magnetite19
3Fe2O3zH2~2Fe3O4zH2O (3)
3Fe2O3zCO~2Fe3O4zCO2 (4)
To clarify the effects of anthracite on the induration of
haematite pellets, a test, as depicted in Fig. 5, was
designed to analyse the reduction and decomposition of
haematite in the anthracite containing pellets during the
roasting. In this experiment, to ensure a reducing atmo-
sphere, the proportion of anthracite is relatively high
compared with that in anthracite containing pellets.
As shown in Fig. 5, a cylinder was first made by
briquetting haematite concentrate, and then the cylinder
bottom was closed. Its inner diameter is 20 mm, whereas
the outer diameter is 30 mm. To allow the upward gas
flow into the inner cylinder, several holes of 0?1 mm in
diameter were drilled through the cylinder bottom.
Dry pellets of 2–3 mm in diameter were prepared
from haematite concentrates in the absence of anthracite
in advance and then were charged onto the surface layer
of the inner cylinder. The cylinder bottom, the pul-
verised anthracite layer and the pellets were separated by
the inert material of Al2O3 powder to avoid their contact
with each other.
4 Effects of oxygen content on fired pellet compressive
strength (airflow: 6 L min –1; roasting temperature:
1280 C; roasting time: 20 min; anthracite dosage: 1 ?0%)
Table 3 Relationship between atmospheric oxygen content and decomposition temperature of Fe2O3
Oxygen content/wt-% 0.1 1 5 10 21 30 40Decomposition temperature/ uC 1203 1278 1334 1360 1389 1403 1414
5 Schematic diagram of test on role of anthracite during
roasting
3 Effects of roasting time on fired pellet compressive
strength (oxygen content: 20%; airflow: 6 L min –1;
roasting temperature: 1280 C; anthracite dosage: 1?0%)
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At the beginning of the trial, the sample was placed in
an electrically heated shaft furnace, and 6 L min –1 N2
with 99?99% purity was pumped in from below. Thesample was taken out and immersed into water imme-
diately after roasting at 1280uC for 20 min. Subse-
quently, the FeO contents of the cylinder bottom and
the small pellets were measured.
It is shown that the FeO contents of the cylinder
bottom and the small pellets were 4?35 and 28?69%respectively, i.e. the FeO content of the small pellets is
obviously higher than that of the cylinder bottom.
In N2 atmosphere, haematite may decompose into
magnetite and release a little O2 according to equation (1),
and the FeO content increases accordingly. It is the
decomposition reaction of haematite that occurs withinthe cylinder bottom. However, because of being sepa-
rated by Al2O3 powder, the haematite within the cylinder
bottom cannot be reduced by anthracite or upward flow-
ing reducing CO/H2 gases, which are produced by the
gasification of anthracite at high temperature. Therefore,the increase in FeO content in the cylinder bottom is only
caused by the decomposition of haematite.
However, as regards the haematite in the small pellets,
on the one hand, it may be decomposed into magnetite
in N2 gas, as in the haematite within the cylinder bot-tom; on the other hand, it can be also reduced into
magnetite by upward flowing CO/H2 produced by the
gasification of anthracite according to equations (3) and(4). Therefore, the increase in FeO content of the smallpellets is caused by both the decomposition and thereduction of haematite, and the latter is predominant.
The above results show that anthracite plays the roleof reductant during the roasting, and a large number of newborn magnetite are produced due to the reductivereaction of haematite by CO/H2, the outcome of the
gasification of anthracite.
Changes in FeO content during roasting
Figure 6 illustrates the effects of roasting time on theFeO content of the fired pellets.
The reduction of haematite mainly occurs in the initialroasting stage. The FeO content first increases and thendecreases after 6 min. The reason is that the reductionrate of haematite to magnetite is faster than the oxidationrate of newborn magnetite to haematite in the initialroasting stage. However, the reduction rate decreaseswhile the oxidation rate increases, accompanied by theconsumption of carbon, and the maximum FeO contentis attained when the reduction rate is equal to the
oxidation rate. Subsequently, FeO decreases along withthe oxidation of magnetite until oxidation is complete.The results show that partial original haematite (OH)grains can be reduced first and turned into magnetitegrains by the anthracite powder dispersed in the pellets.However, the newborn magnetite can be subsequently
oxidised into secondary haematite (SH) grains.
Crystallisation of Fe2O3 in anthracite containing pellets
The microstructures of haematite pellets with no anthra-cite and with 1?0% anthracite are presented in Fig. 7. Forthe haematite pellets roasted in the absence of anthra-cite, because of the high recrystallisation temperature(.1300uC) of OH grains, most of the OH particles keep
their original shape and discernible angularities (grain 1in Fig. 7a). A few bond junctions between Fe2O3 grainsare formed by the recrystallisation of OH grains (grain 2in Fig. 7a).
As shown in Fig. 7b, a large number of crystal bond junctions between grains and a compact microstructureare formed. The recrystallisation between grains can beenhanced because the activity of the newborn SH grainsis higher than that of OH grains, which is helpful to therecrystallisation of Fe2O3 grains.4 Therefore, the forma-tion of SH grains during the roasting of anthracitecontaining haematite pellet is able to improve the pel-let strength. It is the reason why adding a certain
6 Effects of roasting time on FeO content of anthracite con-
taining haematite pellet (oxygen content: 20%; anthracite
dosage: 1?0%; airflow: 6 L min –1; roasting temperature:
1280 C)
7 Microstructures of fired haematite pellets with anthracite dosages of a 0% and b 1?0% (oxygen content: 20%; airflow:
6 L min –1; roasting temperature: 1280 C; roasting time: 20 min)
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proportion anthracite is an effective way to improve theroasting performance of pure haematite pellets.
ConclusionsThe roasting characteristics of anthracite containing hae-matite pellets were studied. The findings indicate that theaddition of some pulverised anthracite to the haematitepellets is able to improve the compressive strength of the
finished pellets. The appropriate dosage of anthracite is1?0–1?5%.
The effects of anthracite powder on the induration of haematite pellets have also been investigated. On the onehand, part of the heat needed in the roasting process canbe supplied by the combustion of anthracite. On the otherhand, some of the haematite is reduced by carbon or CO/H2, and a part of haematite is also readily decomposed athigh temperature due to the decrease in lower oxygencontent within the pellets, which leads to the transforma-tion of haematite into magnetite during roasting.
Based on microstructure analysis, it can be found thatnewborn magnetite is oxidised into SH during roasting.Because the activity of Fe2O3 from the SH grains ishigher than that from the OH grains, Fe2O3 recrystalli-sation junction between grains can be strengthened bythe SH grains at lower roasting temperature. The SHgrains in the haematite pellets are able to improve thepellet strength.
Acknowledgement
The authors wish to express their thanks to the NationalNatural Science Foundation of China (grant nos.50604015and 50804059) for the financial support of this research.
References1. K. Ye: Sinter. Pelletiz. (China), 2003, 3, (1), 1–4.
2. G. Qiu, T. Jiang, X. Fan, D. Zhu and Z. Huang: Scand. J. Metall.,
2004, 33, 39–46.
3. G. Li, Z. Tang, Y. Zhang, Z. Cui and T. Jiang: Ironmaking
Steelmaking , 2010, 37, (6), 393–397.
4. G. Li, X. Li, Y. Zhang, G. He and T. Jiang: Ironmaking
Steelmaking , 2009, 36, (5), 393–396.
5. Y. Chen and J. Li: Cent. South Univ. Technol. (China), 2007, 38,
(1), 70–73.6. Z. Huang, Y. Zhang, Y. Chen and J. Zhuang: Cent. South Univ.
Technol. (China), 2003, 34, (6), 606–610.
7. Z. Huang, Y. Zhang, S. Zhu, J. Zhuang and X. Luo: Iron Steel
(China), 2004, 39, (4), 9–13.
8. T. Jiang, Y. Zhang, Z. Huang, G. Li and X. Fan: Ironmaking
Steelmaking , 2008, 35, (1), 21–26.
9. J. Apbill: Proc. 4th Int. Symp. on ‘Agglomeration’, Beijing, China,
June 1985, Metallurgy Industry Press, 65–77.
10. S. Dutta and A. Ghosh: Metall. Mater. Trans. B , 1994, 25B,
15.
11. M. Abraham and A. Ghosh: Ironmaking Steelmaking , 1979, 6, (1),
14–23.
12. C. Bryk and W. Lu: Ironmaking Steelmaking , 1986, 13, (2), 70–
75.
13. Q. Wang, Z. Yang and J. Sun: J. Iron. Steel Res. (China), 1998, 10,
(3), 2–4.14. X. Yang, Z. Guo and D. Wang: Eng. Chem. Metall. (China), 1995,
16, (2), 118–126.
15. Q. Wang, Z. Yang and J. Tian: Ironmaking Steelmaking , 1998, 25,
(6), 443–447.
16. X. Ma, Q. Wang and M. Jiang: Sinter. Pelletiz. (China), 1999, 24,
(3), 24–26.
17. Q. Wang, Z. Yang and J. Tian: Ironmaking Steelmaking , 1997, 24,
(6), 457–460.
18. X. Chen: ‘Pyrometallurgical process physical chemistry’ (in
Chinese); 1984, Beijing, Metallurgy Industry Press.
19. X. He: ‘Coal chemistry’ (in Chinese); 2010, Beijing, Metallurgy
Industry Press.
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