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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 sintering@mail.csu.edu.cn

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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