Optimization of Firing Temperature for Hematite Pellets

1673 © 2013 ISIJ

ISIJ International, Vol. 53 (2013), No. 9, pp. 1673–1682

Optimization of Firing Temperature for Hematite Pellets

Tekkalakote UMADEVI,* Naveen Frank LOBO, Sangamesh DESAI, Pradipta Chandra MAHAPATRA,Rameshwar SAH and Manjunath PRABHU

JSW Steel Limited, Bellary (Dist), India.

(Received on January 28, 2013; accepted on March 29, 2013)

The heat hardening by oxidation is a process commonly used in iron ore pelletization process. Thegreen pellets are fired in induration machine using Corex gas at JSW Steel Limited Pellet Plant. The firingtemperature of induration machine strongly influences the physical and metallurgical properties of the pel-let. Due to absence of exothermic reaction and poor roasting property of hematite pellet, the energy con-sumption of hematite pellet production is at higher side and requires higher roasting temperature. Inpresent pelletization process, carbon burdened method is found to be more favorable technique in prac-tice to enhance the induration of hematite pellets. Coke breeze is added in the pellet mix at JSW pelletplants to get the inherent fuel value of a hematite green ball equal to that of a magnetite pelletizing feed.The firing temperature (from corex gas) of the induration machine and carbon addition in the pellet mix isinterrelated and decides physical and metallurgical properties of the pellets. At JSW Steel Pellet Plant thecarbon addition varies from 0.90 to 1.50% and firing temperature varies from 1 230 to 1 320°C. Fluctua-tions in physical and metallurgical properties were observed due to deviation in carbon addition and firingtemperature. Optimization of external firing temperature and coke breeze addition in the green pellet mixis necessary to get the desired properties of the pellet for iron making units. Basket trials were carried outat pellet plant induration machine by varying the external firing temperature from 1 220 to 1 330°C andcoke breeze addition from 0.7 to 1.4%. At firing temperature of 1 220, 1 250, 1 280, 1 310 and 1 330°C theoptimum carbon addition 1.30, 1.20, 1.10, 0.90 and 0.70% achieved the optimum physical and metallurgi-cal properties of the pellet for iron making units respectively.

KEY WORDS: iron ore pellet; induration; firing temperature; carbon; microstructure; physical and metallur-gical properties.

1. Introduction

The physical and metallurgical properties like tumblerindex (TI), cold crushing strength (CCS), reduction degrada-tion index (RDI) and reducibility of iron ore pellet influencethe performance of corex and blast furnace iron makingunits. The heat hardening by oxidation is a process com-monly used in iron ore pelletization process. The pellets arehardened due to re-crystallization of iron oxides, formationof slag phase and secondary components. These processestake place at higher temperature. The firing temperature ofthe induration machine depends on chemical composition ofthe raw material, and flux addition. The optimum firing tem-perature of pellets within the induration furnace decides thepellet mineralogy, and pellet mineralogy in turn decides thephysical and metallurgical properties of fired pellets. As forthe firing of hematite pellets, more heat need to be suppliedfrom external sources due to the absence of the exothermicreaction like that of magnetite oxidation. So the energy con-sumption of hematite pellet production is greater than thatof magnetite pellets. In pelletization process, certain mini-mum temperature has to be attained in order to enable the

necessary crystal transformations, and the reaction of oxidegangue constituents to generate the sufficient quantity ofsilicate/slag phase in hematite pellets to get optimum firedpellet properties. Moreover, the hematite pellet has poorroasting properties and do not achieve adequate strengthuntil the roasting temperature is higher than 1 300°C.1) Thus,it is necessary to maintain higher roasting temperature forhematite pellet as well as a narrower firing temperaturerange to get adequate bonding between the phases. In pres-ent pelletization process, to enhance the induration of hema-tite pellets, carbon burdened methods are found to be morefavourable technique in practice. The JSW Steel Limited isa 10 mtpa integrated steel plant having 2×4.2 mtpa pelletplants and corex gas is used for firing of green pellets. Toget the inherent fuel value of a hematite green ball equal tothat of a magnetite pelletizing feed started to incorporatecoke breeze in the pellet mix at JSW pellet plants. The firingtemperature (from corex gas) of the induration machine andcarbon addition in the pellet mix is interrelated to each otherand decides physical and metallurgical properties of the pel-lets. At JSW Steel pellet plant the carbon addition variesfrom 0.90 to 1.50% and firing temperature varies from 1 230to 1 320°C. Fluctuations in physical and metallurgical prop-erties were observed due to deviation in carbon addition andfiring temperature (TI – 90 to 95%, CCS – 180 to 230 kg/p,

* Corresponding author: E-mail: [email protected]: http://dx.doi.org/10.2355/isijinternational.53.1673

© 2013 ISIJ 1674

ISIJ International, Vol. 53 (2013), No. 9

RDI – 10 to 18%, and reducibility – 0.56 to 0.60%). To getthe desired physical and metallurgical properties of the firedpellets for iron making units the optimization of carbonaddition with respect to firing temperature is required. Lab-oratory experiments have been conducted varying the car-bon addition from 0.7 to 1.4% and firing profile from 1 220to 1 330°C.

2. Pelletization Process at JSW Steel Limited

The production of iron ore pellets at JSW Steel Limitedinvolves the drying of iron ore fines to get the moisture lessthan 1% and grinding the dried material to get the requiredfineness –45 μm size ≥62.0%. Prior to the formation of greenpellets, the ground ore is mixed with small amounts of bind-ing agents such as bentonite (0.7 to 0.9%), fluxes such aslimestone to get the pellet basicity (CaO/SiO2) 0.40 to 0.50and coke breeze as fuel (0.7 to 1.5%). The water is addedto the mixed ore in the mixer to adjust the moisture content(0.8 to 0.9%). These give the pellets the proper physical andmetallurgical properties needed in further processing.

The green pellets are formed in pelletizing discs and thesepelletizing discs are connected to roller screens used for sep-arating undersized (–6 mm) and oversized (+16 mm) pelletswhich are returned to the balling disc. The green pelletsfrom the roller conveyor are evenly distributed (0.50 mheight) across the width of the travelling grate indurationfurnace on hearth layer (0.05 m) for hardening of the pellets.The grate carries the green pellets through a furnace whichconsist of seven different zones i.e., updraft drying, down-draft drying, preheating, firing, after firing, cooling and aftercooling zones. The induration furnace consists of three mainsteps 1. Drying of green pellets (200 to 350°C) 2 Firing ofthe pellets at 1 250 to 1 320°C to sinter the mix 3. Coolingof the hot pellets to (900 to 80°C) before discharging themon to the conveyors. In the induration furnace corex gas isused as fuel for firing purpose. Process air is circulatedthrough these different zones with the help of five intercon-nected process fans. Figure 1 shows the process flow sheetof induration machine.

The burners in the preheat and firing zones are groupedinto seven control zones. Each burner control zone has athermocouple in the furnace hood which is connected via atemperature transmitter to the burner zone temperature con-troller. The temperature controller compares the temperaturesignal with the temperature set point and, accordingly regu-lates Corex gas flow control valve by means of current topneumatic converter and a pneumatic operator. The burners

operate as excess air burners with the furnace temperaturecontrolled by adjusting the Corex gas flow to the burnersand therefore the firing rate to maintain set point tempera-ture.

3. Experimental

Laboratory basket tests were carried out by varying thecoke breeze addition from 0.70 to 1.40% in the green mix.The raw materials used for the preparation of the green pel-lets are iron ore, limestone, bentonite and coke breeze. Thechemical analysis of the raw material is shown in the Table1. Iron ore fines and limestone of –10 mm size and cokebreeze of –6 mm size were ground separately in laboratoryball mill. Green pellet mixes for different level coke breezewere prepared by mixing the iron ore fines, limestone, andbentonite and coke breeze as per Table 2. Green pelletswere prepared using laboratory scale balling disc. The greenpellets were prepared in such a way that it should consist of~97% –16+8 mm size range pellets. The details of ballingdisc are as follows:

Disc diameter: 450 mmDisc operating angle: 45°Disc speed: 38 rpmEach set of experiment comprises firing of 20 kg green

pellets (40 numbers of experiments). The green pellets werekept in rectangular stainless steel baskets (500 mm long)and fired in pellet plant induration machine. The stainlesssteel basket was kept in the centre of the pellet bed on thehearth layer and it covered entire bed height of the indura-tion machine. The corresponding operating parameters ofthe plant are shown in Table 3. In induration machine thefiring temperature was varied from 1 220 to 1 340°C foreach mix with different carbon content (0.70 to 1.40%). Allexperiments were carried out at the same machine parame-ters (Table 3). The firing temperature set points at differentzones are shown in Fig. 2. Fired pellets were subjected toevaluations of chemical, physical and metallurgical properties.

Fig. 1. Schematic diagram of induration machine.

Table 1. Raw material chemical analysis.

Description%

Iron ore Limestone Bentonite Coke breeze

Fe (Tot) 63.9 1.1 17.7 3.1

SiO2 3.6 2.2 45.9 9.0

Al2O3 2.3 0.5 16.1 5.0

CaO 0.1 51.8 2.6 2.0

MgO 0.0 1.4 2.3 0.3

LOI 2.2 42.0 8.7 5.0

C 79.7

Table 2. Raw material mix proportion.

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Mix 7 Mix 8

Iron ore fines 95.0 94.9 94.8 94.7 94.6 94.5 94.4 94.3

Limestone 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5

Bentonite 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8

Coke breeze 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4


1675 © 2013 ISIJ

3.1. Mineralogical StudiesPellet samples of all experiments were collected for

microstructural studies. For each study, the pellet samplewas divided in to two pieces at the centre using the samplecutter. The one portion of the pellet sample was mounted ina sample holder using epoxy resin. These sections were pol-ished using silicon carbide paper up to 1 000 grit using wateras a lubricant. A final polish was made by using diamondpaste. For each experiment 2 samples were prepared formicrostructural analysis.

Mineralogical characterization studies were carried outusing optical microscope. The visible microstructural phasesidentified as hematite, magnetite, silicate/slag and poresbased on the color appearance and shape.

Leica Q-win Image analyser software was used to providean objective measurement of different phases in microstruc-ture (hematite, magnetite, silicate and pore). The polishedsample was placed under a microscope for examination. Acamera is mounted behind the lens to capture the image. Theeye piece of 50X objective lens was selected for the currentstudy. Images of each sample were saved in the computerafter capturing the image at different places through out thesample. For each sample 24 images were captured for phaseanalysis. To measure the area fraction of different phases inthe pellet various tools of image analyser was used.

4. Results and Discussion

4.1. Influence of Carbon Addition on Green Pellet Car-bon Content

Figure 3 shows the influence of coke breeze addition ongreen pellet carbon content. With increase in coke breezeaddition the over all green pellet carbon increased.

4.2. Microstructural InvestigationsMicrostructural investigation was carried out by dividing

the pellets into four segments as shown in Fig. 4. They aredefined as:

(i) Shell: it extends from surface to 200 micron below(ii) Outer mantle: just below the shell (2 mm thick)(iii) Inner mantle: just below outer mantle (4 mm thick)(iv) Core: innermost part of the pellet (4–5 mm in diam-

eter).Primary phases present in iron ore pellets are hematite

(grey white) magnetite (grey white with pinkish), silicate/slag (dark grey), and pores (black).

The micrographs of pellet at optimum green pellet carboncontent and firing temperature are shown in Figs. 5(a)–5(e).The micrographs from Figs. 5(a) to 5(e) at optimum greenpellet carbon addition and firing temperature shows phaseanalysis with hematite, silicate and pore phase with minorquantity of magnetite phase. These micrographs consist ofmaximum hematite phase. We can observe re-crystallizedhematite particles well bounded with silicate/slag phase. Inhematite pellets balancing the slag formation and oxidationof magnetite phase is very important. These micrographs areassociated with balanced slag phase with re-oxidised hema-tite crystals.

The micrographs of pellets at higher green pellet carbonaddition i.e. 1.52% at different firing temperatures areshown in Figs. 6(a)–6(e). At higher green pellet carbonaddition at 1.52% the re-oxidation of magnetite phase wasnot completed. With increase in firing temperature withhigher green pellet carbon content the magnetite phase

Table 3. Induration machine parameters.

Description

Feed rate, tph 466

Machine speed, m/min 2.3

bed height, mm 500

Hearth layer, mm 50

Drying, min 15.2

Preheating, firing and after firing, min 18.7

Cooling and after cooling, min 15.8

Fig. 2. Firing profile at different temperature range.

650

750

850

950

1050

1150

1250

1350

Z1 Z2 Z3 Z4 Z5 Z6 Z7Zones

Tem

pera

ture

,

Set point 1220Set point 1250Set point 1280Set point 1310Set point 1330

C

Fig. 3. Influence of carbon coke breeze addition on total green pel-let carbon.

Fig. 4. Segments of pellets.

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Coke breeze,%

Gre

en p

elle

t car

bon,

%

© 2013 ISIJ 1676


increased from core to shell and we can observe the cracksat inner and outer mantle of the fired pellets. These pelletsconsist of large quantity of slag phase with magnetite phase.With increase in firing temperature with higher carbon addi-tion the formation of long stretched pores are also increased.With increase firing temperature at higher green pellet car-bon addition, the re-crystallization of hematite crystals werenot observed in core and inner mantle.

Phase analysis was carried out on each experimental pel-let to know the quantitative analysis of the fired pellets. Theinfluence of green pellet carbon content on hematite, mag-netite, silicate/slag and pore phase of the pellets at differentfiring temperatures are shown in Figs. 7–10 respectively.

The hematite phase decreased and magnetite phaseincreased with increase in green pellet carbon addition atdifferent firing temperatures. As temperature increases from1 220 to 1 330°C the hematite content decreased and mag-netite content increased. Lower hematite and higher magne-tite phase at the firing temperature 1 330°C was observed.The silicate phase increased and pore phase decreased withincrease in green pellet carbon addition. As temperatureincreases from 1 220 to 1 330°C the silicate phase increasedand pore phase decreased. We observed higher silicate phaseand lower pore phase at firing temperature 1 330°C.

(a) (b)

(c) (d)

(e)

Fig. 5. (a)–(e) Micrographs of pellet at optimum carbon and firing temperature.


1677 © 2013 ISIJ

4.3. Influence of Carbon Addition and Firing Temper-ature on FeO Content of the Pellets

In pellet plant, to get the required chemistry, physical andmetallurgical properties of the pellets for iron making units,a proper firing profile must be established to balance theinternal heat by solid fuel and external heat by gas or oil.At JSW Steel Limited Corex gas is used as external fuel tofire the iron ore green pellets. As per Fig. 3 the total greenpellets carbon content of the pellets increased with increasein coke breeze addition. Figure 11 shows the influence ofgreen pellet carbon content on FeO content of the fired pel-let. The FeO content of the fired pellet increased withincrease in green pellet carbon addition at different firingtemperature from 1 220 to 1 330°C. The Fig. 12 shows the

(a) (b)

(c) (d)

(e)

Fig. 6. (a)–(e) Micrographs of pellet at higher carbon rate with different firing temperature.

Fig. 7. Influence of carbon addition on hematite phase at differenttemperature.

50

52

54

56

58

60

62

64

1.0 1.0 1.1 1.2 1.3 1.4 1.4 1.5Green pellet carbon,%

Hem

atite

, %

Temp 1220Temp 1250Temp 1280Temp 1310Temp 1330

© 2013 ISIJ 1678


Fig. 8. Influence of carbon addition on magnetite phase at differenttemperature.

Fig. 9. Influence of carbon addition on silicate phase at differenttemperature.

2

4

6

8

10

12

14

16

18


Mag

netit

e, %

Temp 1220Temp 1250Temp 1280Temp 1310Temp 1330

6

8

10

12

14

16


Silic

ate,

%

Temp 1220 Temp 1250Temp 1280 Temp 1310Temp 1330

Fig. 12. Influence of magnetite phase percentage on FeO content of fired pellets.

Firing Temp 1220deg C

0.20

0.30

0.40

0.50

0.60

0.70

3.0 3.4 3.6 3.6 3.8 4.0 5.0 7.5Magnetite phase,%

FeO

,%


0.20

0.35

0.50

0.65

0.80

0.95

3.4 3.5 3.6 3.8 4.0 4.6 7.4 8.5Magnetite phase,%

FeO

,%


0.20

0.35

0.50

0.65

0.80

0.95

3.5 3.6 3.7 4.0 4.1 6.8 8.5 10.4Magnetite phase,%

FeO

,%


0.20

0.40

0.60

0.80

1.00

1.20

3.7 4.0 4.2 4.6 5.8 8.0 12.0 15.0Magnetite phase,%

FeO

,%


0.20

0.40

0.60

0.80

1.00

1.20

1.40

4.2 5.0 5.8 8.0 11.0 12.5 15.0 17.0Magnetite phase,%

FeO

,%

Fig. 10. Influence of carbon addition on pore phase at differenttemperature.

Fig. 11. Influence of green pellet carbon addition on fired pelletsFeO content.

15

17

19

21

23

25

27

29

31


Pore

, %

Temp 1220 Temp 1250Temp 1280 Temp 1310Temp 1330

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6Green Pellet Carbon,%

Fire

d pe

llet F

eO,%

Temp 1220 Temp1250

Temp 1280 Temp 1310

Temp 1330


1679 © 2013 ISIJ

influence of magnetite phase on FeO content of the pelletsat different temperature from 1 220 to 1 330°C. The FeOcontent of the pellet increased with increase in magnetitephase. In general as higher the amount of solid fuel higheris the formation of magnetite phase. Depending on the pelletcarbon content and firing conditions, secondary magnetitemay or may not be completely re-oxidised to hematite aftercooling.2) The FeO content of the pellets are directly propor-tional to magnetite content. Higher the amount of magnetitephase in the fired pellets the higher amount of FeO content.The formation amount of magnetite phase increases withincreasing the firing temperature and/or with increasing thecarbon content. The oxidation of magnetite phase to hema-tite phase is very difficult to occur under the conditionwhere the formation of magnetite phase proceeds. Highercoke addition at higher firing temperature may even resultsin a further reduction towards the formation of wustite,which, within the time available, does not re-oxidise andform fayalite with the presence of silica.

4.4. Influence of Carbon Addition and Firing Temper-ature on Physical Properties of the Pellet

Tumbler index (T.I), is the measure of resistance to gen-erate fines during handling and transportation. Figure 13shows the influence of green pellet carbon content at differ-ent temperature on tumbler index of the pellets. Withincrease in green pellet carbon content of the pellet at thedifferent temperature 1 220, 1 250, 1 280, 1 310, and1 330°C the tumbler index of the pellet increased up to opti-mum carbon addition 1.3, 1.2, 1.0, 0.9, and 0.7% respective-ly and after that decreased with increase in carbon addition.

CCS of the pellets plays a significant role in the perfor-mance of Corex and Blast furnace processes. Pellets withlow strength cannot withstand the handling loads duringtheir shipping and load of burden in the reduction furnace.Figure 14 shows the influence of green pellet carbon con-tent at different temperature on cold crushing strength of thepellets. With increase in green pellet carbon content of thepellet at the different temperature 1 220, 1 250, 1 280, 1 310,and 1 330°C the CCS of the pellet increased up to optimumcarbon addition 1.3, 1.2, 1.0, 0.9, and 0.7% respectively andafter that decreased with increase carbon addition. The tum-bler index and CCS of the pellets are influenced by FeOcontent of the pellet. Figure 15 shows the influence of FeOcontent of the pellet on T.I and CCS of the pellet. Withincrease in FeO content of the pellet at different firing tem-

peratures T.I and CCS of the pellet increased up to optimumFeO content 0.40 to 0.50% and after wards decreased withincrease in FeO content of the pellet. At lower carbon addi-tion, the FeO content may be lower than 0.5% and it is evengood for pellet strength but at lower FeO content of firedpellet below 0.40% pellet strength was poor may be due toinsufficient bonding of the hematite particles due to poor fir-ing. For FeO content of the fired pellet above 0.50% the pel-let T.I and CCS was poor due to magnetite content, moreporosity, long stretched pores and generation of stress insidethe pellet. Conversion of magnetite to hematite is a stronglyexothermic reaction and favours the grain growth and sin-tering of the particles of iron ore concentrate to form hard,strong pellets.3) As the coke breeze increases this effectbecomes more and more severe. During firing the carbonmonoxide is generated in pellet due to incomplete combus-tion of coke breeze. This carbon monoxide reduces thehematite to magnetite. As a result, a duplex structure isformed across the cross section of the pellet with hematitepredominantly in the outer mantle and shell and magnetitein the inner mantle and core (Figs. 6(d) and 6(e)). Bond andlattice strains caused due to the differential contraction/shrinkage of these phases during cooling causes cracks andthese cracks reduce the pellet strength. The higher FeO pel-lets are associated with duplex structure i.e. magnetite coreand hematite shell attributed to poor strength (Figs. 6(d) and6(e)). The optimum FeO content of the pellet should be 0.40to 0.50% to achieve better physical properties of the pellets.To avoid the formation of magnetite at higher coke rate &higher temperature and to avoid poor firing of the pellets atlower temperature & lower carbon addition the addition ofcoke breeze as carbon source and firing temperature of thepellet should be properly controlled.

4.5. Influence of Carbon Addition and Firing Temper-ature on Reduction Degradation Index of the Pellet

The reduction degradation index of pellet is defined as aquantitative measure of the degree of disintegration, whichcould occur in the pellet in the upper part of the blast fur-nace after some reduction at lower temperature. The primarycause of disintegration at lower temperature is thought to bethe crystalline transformation during the formation of cubicmagnetite from hexagonal hematite generating stresses,which leads to the weakening of pellets. It is very clear thatiron oxide in the indurated pellet is mainly in the form ofhematite; therefore, generation of internal stress is unavoid-

Fig. 13. Influence of green pellet carbon content on T.I of the pel-lets.

84.0

86.0

88.0

90.0

92.0

94.0

96.0

98.0


T.I (

+63.

mm

),%

Temp 1220 Temp1250Temp 1280 Temp 1310Temp 1330

Fig. 14. Influence of green pellet carbon content on strength(CCS) of the pellet.

160

170

180

190

200

210

220

230

240


CCS,

kg/p


© 2013 ISIJ 1680


able. As the coke breeze content increases with firing tem-perature the effect becomes more and more severe. Figure16 shows the influence of coke breeze addition at differenttemperature on reduction degradation index of the pellet.With the addition of coke breeze at different temperature thereduction degradation index of the pellet decreased up tooptimum coke breeze addition and firing temperature andagain increased with increase in carbon addition. At highergreen pellet carbon addition 1.52% and at higher tempera-tures higher quantity of slag and magnetite phases wereformed. Along with the magnetite phase we can see thenumber of cracks formed due to the generation of stressinside the pellet and this leads to poor strength. Poorstrength pellets cannot sustain mechanical loading duringreduction and hence adversely affects the RDI.4) Higher themagnetite phase, higher the FeO and lower the pellet RDI.Pellets with higher carbon content are associated with high-er magnetite/FeO and silicate phase and the pellet RDI wasat higher side due to the internal stress created during thecrystal transformation. At lower carbon addition and lowerfiring temperature the RDI of the pellet was higher side dueto formation of less slag phase and poor bonding of slagphase with hematite particles. The pellet porosity alsodecides the RDI of the pellet. The volume increase at thehematite to magnetite transition generally fluctuatesbetween 10 to 15% instead of 5% excepted in literature.5)

This apparent swelling appears along with the developmentof inner grain porosity as long stretched pores (Figs. 6(a)–6(e)). The higher the quantity of green pellet carbon addition(1.52%) with increase in firing temperature formation oflong stretched pores increased and deteriorated the sinterstrength and RDI. The optimum RDI required for iron mak-ing units are RDI (–6.3 mm) should be less than 11.5%. To

get the optimum pellet mineralogy balancing of pelletporosity, slag formation and oxidation of magnetite is nec-essary. The balanced mineralogy can be achieved by balanc-ing the carbon addition with respect to firing temperature.At firing temperature 1 220, 1 250, 1 280, 1 310 and 1 330°C

Fig. 15. Influence of FeO content on pellet strength (T.I).


84

87

90

93

96

99

0.26 0.28 0.30 0.35 0.38 0.40 0.48 0.62

FeO,%

T.I (

+6.3

mm

),%170

185

200

215

230

CC

S,kg

/p

T.I CCS

Firing Temp 1250dec C

87

90

93

96

99

0.28 0.28 0.30 0.35 0.38 0.42 0.68 0.82FeO,%

T.I (

+6.3

mm

),%

175

190

205

220

235

CC

S,kg

/p

T.I CCS


87

90

93

96

99

0.30 0.32 0.35 0.38 0.44 0.54 0.84 0.92FeO,%

T.I (

+6.3

mm

),%

180

190

200

210

220

230

240

CC

S,kg

/p

T.I CCS


87

90

93

96

99

0.32 0.36 0.42 0.48 0.68 0.70 0.95 1.10FeO,%

T.I (

+6.3

mm

),%

185

195

205

215

225

235

CC

S,kg

/p

T.I CCS


84

86

88

90

92

94

96

98

0.42 0.50 0.65 0.85 0.98 1.08 1.15 1.25FeO,%

T.I (

+6.3

mm

),%

160

170

180

190

200

210

220

230

CC

S,kg

/p

T.I CCS

Fig. 16. Influence of green pellet carbon content on pellet RDI.

Fig. 17. Influence of green pellet carbon content on pellet reduc-ibility.

10

11

12

13

14

15

16

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

RDI (

-6.3

mm

),%


0.45

0.48

0.51

0.54

0.57

0.60

0.63

0.66


Red

ucib

ility

, (dR

/dT)

40,%

O2/

min Temp 1220 Temp1250

Temp 1280 Temp 1310Temp 1330


1681 © 2013 ISIJ

the optimum carbon addition of 1.30, 1.20, 1.10, 0.90 and0.70% achieved the pellet RDI ≤11.5% for iron makingunits respectively.

4.6. Influence of Carbon Addition and Firing Temper-ature on Reducibility of the Pellet

Reducibility is the ease with which the oxygen combinedwith the iron can be removed. The reducibility is generallycompared at 40% degree of reduction. The influence of

Fig. 18. Influence of magnetite phase on pellet reducibility.

Fig. 19. Influence of pore phase on pellet reducibility.


0.50

0.54

0.58

0.62

0.66

3.0 3.4 3.6 3.6 3.8 4.0 5.0 7.5Magnetite phase,%

Red

ucib

ility

, (d

R/d

T)40

,%O

2/m

in


0.50

0.54

0.58

0.62

0.66

3.4 3.5 3.6 3.8 4.0 4.6 7.4 8.5Magnetite phase,%

Red

ucib

ility

,(dR

/dT)

40,%

O2/

min


0.49

0.52

0.55

0.58

0.61

3.5 3.6 3.7 4.0 4.1 6.8 8.5 10.4Magnetite phase,%

Red

ucib

ility

,(dR

/dT)

40,

%O

2/m

in


0.46

0.5

0.54

0.58

0.62

3.7 4.0 4.2 4.6 5.8 8.0 12.0 15.0Magnetite phase,%

Red

ucib

ility

,(dR

/dT)

40,%

O2/

min


0.44

0.47

0.50

0.53

0.56

4.2 5.0 5.8 8.0 11.0 12.5 15.0 17.0Magnetite phase,%

Red

ucib

ility

,(dR

/dT)

40,%

O2/

min


0.50

0.54

0.58

0.62

0.66

21.0 23.0 25.0 27.0 29.0 31.0Pore phase,%

Red

ucib

ility

, (d

R/d

T)40

,%O

2/m

in


0.50

0.54

0.58

0.62

0.66

21.0 23.0 25.0 27.0 29.0Pore phase,%

Red

ucib

ility

,(dR

/dT)

40,%

O2/

min


0.49

0.52

0.55

0.58

0.61

22.0 23.0 24.0 25.0 26.0 27.0 28.0Pore phase,%

Red

ucib

ility

,(dR

/dT)

40,

%O

2/m

in


0.46

0.49

0.52

0.55

0.58

0.61

18.0 20.0 22.0 24.0 26.0Pore phase,%

Red

ucib

ility

,(dR

/dT)

40,%

O2/

min


0.44

0.47

0.50

0.53

0.56

14.0 16.0 18.0 20.0 22.0 24.0 26.0Pore phase,%

Red

ucib

ility

,(dR

/dT)

40,%

O2/

min

© 2013 ISIJ 1682


green pellet carbon content on pellet reducibility is shownin Fig. 17. Reducibility of iron ore pellet decreased withincrease in green pellet carbon addition at different temper-atures. The pellet fired at 1 220°C shows higher reducibilityand pellet fired at 1 330°C shows lower reducibility withincrease in carbon addition. Reducibity of the pellet mainlydepends on the magnetite content of the pellet and porosity.Figure 18 shows the influence of magnetite phase on pelletreducibility. The pellet reducibility decreased with increasein magnetite phase percentage at different firing tempera-ture. In iron ore pellet hematite is much more reducible thanmagnetite. It is highly desirable that the all the lower oxides(magnetite/FeO) of the pellet formed during heating shouldbe oxidized to hematite during cooling cycle. Higher mag-netite content in the pellet decreases the pellet reducibility.

Pores also influence the properties of iron ore pellet.Pores provide diffusion path for reactant gases and surfacearea for reaction. Figure 19 shows the influence of pelletporosity on pellet reducibility. With increase in pore phasepellet reducibility increased. With increase in firing temper-ature the pellet porosity decreased and pellet reducibilityalso decreased. The decrease in pellet porosity is due toincrease in silicate/slag phase of the pellet due to increasein carbon addition and firing temperature. Many researchesfound that reducibility was increased with increasing poros-ity and strength was increased with decreasing porosity.6,7)

In a single homogeneous pellet, the microstructure afterinduration depends mainly on the bed permeability, carboncontent, temperature cycle and O2 pressure in the same area.The process of complete oxidation of reduced phases isdependent on exposure of these phases to oxygen. The over-all oxygen partial pressure was partially controlled by thecarbon rate. However, higher addition of the carbon beyonda certain limit and particular firing temperature badlyimpairs the pellet quality. The optimum coke breeze addi-tion and firing temperature is an effective measure topromote oxidation. The higher content of carbon & higherfiring temperature of the pellets decreases oxidation of mag-netite into hematite and pellet remains with magnetite, andslag phase.

5. Conclusions

Green pellet carbon addition increased with increase incoke breeze addition.

(1) The mineralogical phases like hematite & porephase decreased and magnetite and slag phase increasedwith increase in coke breeze addition. At highest firing tem-perature pellet showed lower hematite & pore phase andhigher magnetite and slag phase.

(2) The FeO content of the fired pellet increased with

increase in green pellet carbon addition at different firingtemperature from 1 220 to 1 330°C due to increase in mag-netite phase.

(3) With increase in FeO content of the pellet at differ-ent firing temperatures T.I and CCS of the pellet increasedup to optimum carbon content 0.40 to 0.50% and after wardsdecreased with increase in FeO content of the pellet.

(4) With increase in green pellet carbon content at dif-ferent temperature 1 220, 1 250, 1 280, 1 310, and 1 330°Cthe tumbler index and CCS of the pellet increased up to opti-mum carbon addition 1.3, 1.2, 1.0, 0.9, and 0.7% respective-ly and after that decreased with increase in carbon addition.The tumbler index and CCS of the pellets are influenced byFeO content of the pellet.

(5) With the addition of coke breeze at different firingtemperature (1 220, 1 250, 1 280, 1 300, and 1 330) thereduction degradation index of the pellet decreased up tooptimum coke breeze addition (1.3, 1.2, 1.1, 0.9, and 0.7)and firing temperature and again increased with increase incarbon addition. Pellets with higher carbon content and fir-ing temperature are associated with higher magnetite/FeOand silicate phase even though, the pellet RDI was at higherside due to the internal stress created during the crystaltransformation. At lower carbon addition and at lower firingtemperature the RDI of the pellet was higher side due to for-mation of less slag phase and poor bonding of slag phasewith hematite particles.

(6) Reducibility of iron ore pellet decreased withincrease in green pellet carbon addition at different temper-atures. The pellet fired at 1 220°C shows the higher reduc-ibility due to lesser magnetite, slag & higher pore phase andpellet fired at 1 330°C shows the lower reducibility due tohigher magnetite, slag phase & lower pore phase withincrease in carbon addition.

(7) At firing temperature 1 220, 1 250, 1 280, 1 310 and1 330°C the optimum carbon addition 1.30, 1.20, 1.10, 0.90and 0.70% achieved the optimum physical and metallurgicalproperties of the pellet for iron making units respectively.

REFERENCES

1) G. Li, T. Jiang, Y. Zhang and Z. Tang: Intech Open – Material Science,InTech, (2012), ISBN: 978-953-51-0122-2.

2) M. I. Pownceby and J. M. F. Clout: Trans. Inst. Min. Metall. C, 112C(2003), 44.

3) K. Kiyonaga: US patent, (1981), 4278462.4) T. Umadevi, P. Prachetan Kumar, M. G. Sampath Kumar, S. Kumar

and M. Ranjan: Ironmaking Steelmaking, 35 (2008), No. 6, 421.5) F. M. Jean, M. Tom, F. Danelle and P. Guy: Ironmaking Conf. Proc.,

Association of Iron & Steel Technology, Warrendale, PA, (2000), 429.6) N. Ponghis and A. Poos: Proc. Ironmaking Conf., Vol. 36, Iron & Steel

Society, Warrendale, PA, (1977), 91.7) K. Taguchi: Proc. Ironmaking Conf., Vol. 39, Iron Steel Society of

AIME, Washington, (1980), 363.

Documents

Optimization of Firing Temperature for Hematite Pellets