ISSN 0967!0912, Steel in Translation, 2011, Vol. 41, No. 5, pp. 400403. Allerton Press, Inc., 2011.Original Russian Text B.P. Yurev, N.A. Spirin, 2011, published in Stal, 2011, No. 5, pp. 912.

400

The oxidation of magnetite is one of the primaryphysicochemical processes associated with the heattreatment of magnetite pellets. The hematite formedon roasting pellets is a very active chemical compo!nent. Sintering of the hematite grains increases thepellet strength and significantly intensifies the solid!phase reactions with lime. The end of oxidation isassociated with uniform pellet structure. As a rule, thepresence of an unoxidized core is associated with theappearance of concentric cracks between the hematiteperiphery and the magnetite core, which are oftenresponsible for pellet disintegration on transportationand reduction in the blast furnace [1, 2]. The temper!ature range of oxidation has a great influence on theproperties of the pellets and especially their behavioron reduction, since the oxidation of magnetite in thepellets determines the formation of solid!phase fer!rites [3].

The oxidation of magnetite is accompanied byconsiderable heat liberation [4]. Knowing the temper!ature range of magnetite oxidation, we may distributethe heat sources rationally over the conveyer roastingmachine. Therefore, it is of interest to study the oxida!tion of magnetite and the influence of various factorson this process.

Knowing the oxidation kinetics of iron!ore pelletsat high temperatures is also useful in assessing thecompletion of roasting at any time and in optimizingthe thermal conditions within the zones of the roastingmachine.

In our experiments, fluxed pellets obtained fromSokolovsko!Sarbaisk concentrate are used in series I(Table 1). The pellets are roasted in a tubular electro!furnace with a device for air heating (at air flow ratesof 55 l/min). A flow rate of 0.200.25 m/s is sufficientto allow the external diffusional drag to be ignored [5].The roasting conditions are as follows. First, the pel!lets are dried for 1 min at a gas temperature of 300C;then they are heated at 800C for 5 min. The heatingrate is 180200C/min and corresponds to the actual

rates in conveyer machines. The pellets are thenroasted at 9001350C and held at these temperaturesfor 5, 10, and 15 min.

In series II, we use a Tamman furnace. The pelletsare placed in a quartz tube (diameter 40 mm), throughwhich air is passed at 40 l/min. The roasting condi!tions are as follows:roasting temperature: 400, 600, 800, 1000, 1100,

1200, 1250, 1300, and 1350C;roasting time: 2, 5, 10, 15, 20, and 30 min;initial furnace temperature: 300C (or the roast!

ing temperature);cooling conditions: to 1000C in the furnace;

then in air; in air again; and in water.The roasted pellets are subjected to mineralogical,

chemical, and phasechemical analysis. The oxida!tion is extremal, according to the results of chemicalanalysis for ferrous oxide (Fig. 1). Beginning at 600C,the FeO content in the pellets sharply falls, with a min!imum at 10501100C. Further temperature increaseproduces more melt in the pellets and accelerates liq!uid!phase sintering. The degree of isothermal oxida!tion is reduced, and the FeO content in the pelletsrises. With increase in the roasting time, the FeO con!tent in the pellets declines at all roasting temperatures;at 13001350C, it is the same for any roasting time.

Oxidation of Iron!Ore PelletsB. P. Yureva and N. A. Spirinb

aPervouralsk Branch, Ural Federal University, Pervouralsk, RussiabUral Federal University, Yekaterinburg, Russia

AbstractThe oxidation of magnetite in the roasting of iron!ore pellets is considered, with particular atten!tion to the influence of various technological factors on the process. Laboratory data are used in optimizingthe design and operational parameters of individual zones in conveyer roasting machines for the heat treat!ment of magnetite pellets.

DOI: 10.3103/S0967091211050202

Table 1. Characteristics of pellet and concentrate samples

MaterialContent, wt % Moisture

content, %Fe S FeO SiO2 CaO

Concentrate 66.2 0.46 28.3 4.3 1.24 10.0

Pellets of basicity:

0.8 62.4 0.46 26.0 4.5 3.60 9.3

1.2 60.8 0.45 25.6 4.5 5.40 9.1

1.4 60.0 0.45 24.7 4.5 6.30 8.8

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OXIDATION OF IRON!ORE PELLETS 401

Table 2 illustrates the influence of pellet heatingand cooling on the FeO content. It is evident that, withincrease in roasting temperature, the FeO content inthe pellets rises in any heating and cooling conditions.With decrease in the heating and cooling rates, theFeO content declines.

To study the oxidation of pellets roasted in thegiven conditions, we create polished diametric sec!tions. Their inspection under a microscope reveals dif!ferent patterns of magnetite replacement by hematite,corresponding to the degree of magnetite oxidation.

The deposition of secondary hematite depends onthe roasting temperature and time. Low!temperaturehematite in the form of relatively small acicular depos!its at the edges of the magnetite grains is observed inthe outer region of the pellet at 400, 600, 800, and100C, with holding for 20, 10, 5, and 2 min, respec!tively. In the central zone, such hematite is observedafter longer roasting.

Complete oxidation of the magnetite at the pelletsurface is observed at 800, 1000, and 1100C, withdecrease in holding time from 20 to 2 min. In the cen!ter, the magnetite grains are completely oxidized at1000C (15!min holding) and 1100C (10!min hold!ing). With increase in temperature to 1200C (5!minholding), recrystallization of the secondary hematitebegins; its grains take on microcrystalline isometricform.

At 1250C (5!min holding), the hematite dissolvesin the melt that forms. With increase in the holdingtime, solution begins at 1200C. Above 1300C, thehematite dissociates and is replaced by magnetite inthe maghemite stage. Research shows that, on high!temperature roasting, considerable quantities ofmaghemite are formed.

In the peripheral zone of pellets heated with thefurnace to 1350C and then quenched in water, relicsof molten hematite grains may be seen in the field ofwell!crystallized secondary magnetite grains. Hema!tite formed below 1200C dissociates, with the forma!tion of secondary magnetite. Fast cooling of pelletsroasted at high temperature is associated with the for!mation of maghemite or a solid solution of maghemitewith magnetite.

Chemical analysis indicates the presence ofmaghemite in the pellets. The excess FeO in relationto the hematite observed under a microscope may beattributed to maghemite. For comparison, we considerthe quantity of ferrous oxide in the magnetic fractionof the pellets and the elementary!cell dimension of themagnetite. The presence of Fe2O3 in the solid solutionreduces the cell dimension.

At 1100C, the oxidation of the pellets is a maxi!mum; the magnetite grains are completely oxidizedover the whole pellet. With increase in temperature inthe center of the pellet, no hematite is formed. At1250C, hematite begins to dissolve in the slag melt.

In high!temperature roasting, the hematite disso!ciates and secondary magnetite with a high Fe2O3 con!tent is formed in the maghemite stage. Thanks to thiscomponent and the maghemite in the pellets roastedat high temperatures, they are satisfactorily hardenedon reduction.

On roasting, the hardening of pellets obtained fromrich ore or concentrate with a low content of barrenrock is predominantly due to recrystallization of theore grains, which are bonded by crosslinks or formaggregations in the course of diffusion in the solid orplastic state. If the unroasted concentrate has an ele!vated content of barren rock, with slag!forming orbinding agents, slag binding predominates in theroasted pellets. The presence of about 10% of suchagents is sufficient to form such bonds. In these condi!tions, isolated ore grains appear in the slag mass.

24

20

16

12

8

4

01200400 800

FeO

, %

Roasting temperature, C

1

2

3

4

Fig. 1. Ferrous!oxide content in pellets as a function ofthe temperature, in roasting for 5 (1), 10 (2), 15 (3), and20 (4) min.

Table 2. Ferrous!oxide content in pellets as a function ofthe temperature (roasting time 10 min), %

Roasting temperature,

C

FeO content in pellets in different heating and cooling conditions, %

initial furnace

temperature 300C, cooling in water

initial furnace temperature equal to roasting temperature

cooling in water

cooling in air

cooling in furnace to 1000C, then in air

1100 Trace 4.61 No data 4.35

1200 No data 8.65 No data 6.80

1250 No data 14.77 13.40 14.00

1300 No data 19.00 12.38 8.23

1350 17.5 18.50 17.00 13.00

Nancy Prieto

Nancy Prieto

Nancy Prieto

Nancy Prieto

Nancy Prieto

Nancy Prieto

Nancy Prieto

402

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YUREV, SPIRIN

In the present case, porosity and clastic micro!structure is retained in the pellets up to 6001100C(with holding for 302 min). Only the removal of car!bon dioxide on carbonate decomposition produceslarger pores. With increase in temperature, the smallhematite grains at the contact point fuse together,forming crosslinks, but there are no bonds in the pel!lets; blastoclastic microstructure is formed.

At 1000C (20!min holding) and 1100C (5!minholding), hematite bonds is formed to create the pel!lets ore framework. At first, these bonds are weak, butrecrystallization of the hematite on increasing theroasting time and temperature strengthens the pellet.The microstructure becomes porphyroblastic, sincethe initial silicates retain their grain shape and formporphyric inclusions in the hematite binder; corre!spondingly, the small pores combine and take on atwisting outline. Above 1250C (10!min holding), theformation of slag binder begins. At high temperature,the liquid phase steeps all the pellets cementing themagnetite grains; in other words, slag binder is formed.The strongest pellets are those that pass through theliquid!phase stage and are characterized by uniformgranular crystalline structure. This pellet structure isobtained in high!temperature roasting (above1300C).

The oxidation kinetics of iron!ore pellets may bestudied in a vertical tube furnace. In Fig. 2, we plotkinetic oxidation curves for fluxed pellets. It is evidentthat, with increase in roasting temperature, the degreeof magnetite oxidation increases up to 1300C for pel!

lets of basicity 0.8 (whose properties resemble those ofunfluxed pellets) but only up to 1200C for pellets ofbasicity 1.2 and 1.4. With further increase in tempera!ture, the degree of oxidation sharply falls. Withincrease in basicity from 0.8 (Fig. 2a) to 1.2 (Fig. 2b)and 1.4 (Fig. 2c), the degree of magnetite oxidationbegins to fall. This is especially pronounced above1200C. Thus, the degree of oxidation is 100% for pel!lets of basicity 0.8 at 1200C, with 10!min holding(curve 4), but 92% for pellets of basicity 1.2 and 80%for pellets of basicity 1.4. For 1300C, the correspond!ing figures are 100, 85, and 75%. For pellets of basicity1.2 and 1.4, 100% magnetite oxidation is observed onroasting for more than 15 min at 10001200C. Thereis no extensive development of liquid phases in thatcase.

The dependence of the degree of oxidation on thetemperature and basicity is more clearly shown inFig. 3. It is evident that, with 7!min heat treatment, thedegree of oxidation is 70 (1), 80 (3), and 90% (5)at 1100C and 78 (2), 88 (4), and 100% (6) at 1300C.With increase in the basicity, the degree of oxidationfalls.

With different basicity, the maximum degree of oxi!dation is observed at 11001200C: 100, 90, and 85%for basicity 0.8, 1.2, and 1.4, respectively. At 13501400C, the degree of oxidation approaches the initialvalue (at 900C), on account of dissociation of thehematite. Thus, roasting with holding at 11001150C produces the maximum degree of magnetiteoxidation.

It is evident in Fig. 4 that the strength of roasted pel!lets falls markedly with increase in the basicity, exceptfor the case of pellets roasted at 1100 and 1200C, forwhich the strength begins to rise at basicity 1.2. Thedrop in pellet strength with increase in basicity at900 and 1000C is explained in that these temperaturesare not characterized by fusion of the silicate mineralsor by linking of the small hematite grains to form achain that may recrystallize and create a hematiteframework for the pellet. The growth in pellet strengthobserved at basicity 1.2 and above, at temperatures of

100806040

1460 104

20

2 8 12

(c)

Roasting time, min

51 6

243

100806040

5

1

62

43

200

(b)

100806040

5

1

6

2

43

20

(a)

Deg

ree

of o

xida

tion

, %

Fig. 2. Kinetic curves of the oxidation of iron!ore pellets at900C (1), 1000C (2), 1100C (3), 1200C (4), 1300C(5), and 1350C (6) when the basicity is 0.8 (a), 1.2 (b),and 1.4 (c).

Oxidation, %100

80

60

40

60 104

20

2 8 12

5

1

62 43

Roasting time, min

Fig. 3. Kinetic curves of the oxidation of iron!ore pellets at1100C (2, 4, 6) and 1300C (1, 3, 5) when the basicity is0.8 (1, 2), 1.2 (3, 4), and 1.4 (5, 6).

3

STEEL IN TRANSLATION Vol. 41 No. 5 2011

OXIDATION OF IRON!ORE PELLETS 403

1100 and 1200C, is due to the formation of hematitebinder: an ore framework whose strength increases withrise in the treatment temperature.

In Fig. 5, we see that the pellet strength increaseswith increase in treatment temperature to 12501300C, regardless of the basicity. Above 1300C, thestrength falls for pellets of basicity 1.2 (2) and 1.4 (3),because the removal of oxygen and the restructuring ofthe crystal lattice in the pellets outer shell (associatedwith the dissociation of hematite and the formation ofsecondary magnetite) increases the microporosity andultraporosity, which weakens the bonds in the structure.

CONCLUSIONS

By mineralogical, chemical, and phasechemicalanalysis, we have studied the oxidation of iron!ore pel!lets produced from Sokolovsko!Sarbaisk concentrate.We have investigated the temperature conditions ofstructure formation and the bonds between the miner!als in the pellets. The oxidation of magnetite was con!sidered for different heat!treatment conditions. Thedynamics of magnetite oxidation, hematite dissocia!tion, mineral formation, and strength development inthe pellets was studied as a function of the temperatureand treatment time. The results permit optimization

of the heat treatment on OK!108 (OK!116) roastingmachines at Sokolovsko!Sarbaisk enrichment facility.

ACKNOWLEDGMENTS

This work was performed under state contract02.740.11.0152 of the Federal Agency on Science andInnovation.

REFERENCES

1. Melamud, S.G. and Yurev, B.P., Oxidation Kinetics ofTitaniumMagnetite Kachkanar Pellets, Metally,2000, no. 1, pp. 310.

2. Melamud, S.G., Lopatin, Yu.N., and Yurev, B.P.,Stress State in Roasted Zonal Pellets, Metally, 2002,no. 1, pp. 39.

3. Yurev, B.P., Bratchikov, S.G., Desyatnik, V.N., et al.,Temperature Intervals and Kinetics of the Oxidation ofIron!Ore Granules, Izv. Vyssh. Uchebn. Zaved., Chern.Metall., 1970, no. 10, pp. 2124.

4. Yurev, B.P. and Bratchikov, S.G., Heat of Oxidation ofNatural Magnetite, Izv. Vyssh. Uchebn. Zaved., Chern.Metall., 1970, no. 6, pp. 1619.

5. Berman, Yu.A. and Markov, A.D., Oxidation Kineticsof Magnetite!Concentrate Pellets, Izv. Vyssh. Uchebn.Zaved., Chern. Metall., 1971, no. 1, pp. 3134.

2000

1600

1200

800

400

01.41.00.8 1.2

Pellet basicity

Stre

ngth

, N/p

elle

t1

23

46 5

Fig. 4. Strength of pellets of different basicity roasted at1350C (1), 1300C (2), 1200C (3), 1100C (4), 1000C(5), and 900C (6), with 5!min holding.

1600

800

01100900

Roasting pellet, C

1

23

1300

Stre

ngth

, N/p

elle

t

Fig. 5. Temperature dependence of the pellet strength with10!min holding, when the basicity is 0.8 (1), 1.2 (2), and1.4 (3).