Effect of gangue content on the swelling behaviour of iron ore pellets

Minerals Engineering. VoL 3, No. 5, pp. 509-516, 1990 Printed in Great Britain

0892-6875/90 $3.00+00 Pergamon Press plc

EFFECT OF GANGUE CONTENT ON THE SWELLING BEHAVIOUR OF IRON ORE PELLETS

T. SHARMAt, R.C. GUPTA§ and B. PRAKASH§ t Dept. of Fuel & Mineral Engng., Indian School of Mines, Dhanbad-826004, India Dept. of Met. Engng., Institute of Technology, Banaras Hindu University, Varanasi-221005, India (Received 9 October 1989; accepted 11 January 1990)

ABSTRACT In the present investigation an attempt has been made to determine whether the gangue content o f the ore fines has any significant effect on the swelling behaviour o f their pellets. For this purpose two Indian iron ore fines from Bailadila and Noamundi deposits were used and chemically pure iron oxide was used to prepare synthetic ore pellets. To study the effect o f varying gangue content o f the ore fines, pure alumina and silica powders were added. The results obtained indicate that swelling of iron ore pellets is due to growth o f iron whiskers and the gangue content o f the ore fines has considerable effect on the nature o f whisker growth.

Keywords Pelletisation; iron ores INTRODUCTION Swelling and disintegration of iron ore pellets have been two major drawbacks in their acceptance as feed for blast furnaces and direct reduction furnaces. Swelling up to 20 percent has been accepted as 'Normal Swelling' and beyond this, it is called 'Abnormal' or 'Catastrophic Swelling. Catastrophic swelling of pellets is accompanied by loss of strength of the pellets and their ultimate disintegration during reduction. Numerous investigations [1-12] on several iron ores have been conducted to study the swelling behaviour of pellets and the growth of iron whiskers during reduction has been cited as the major reason for swelling and loss of strength of the pellets. Chemical composition of the pellet and pelletization and reduction parameters have been found to affect swelling of iron ore pellets. In the present investigation an attempt has been made to study the effect of gangue content on the swelling behaviour of iron ore pellets made from two different natural iron ores and synthetic ore pellets prepared from a mixture of chemically pure materials.

EXPERIMENTAL PROCEDURE Natural iron ores from Bailadila and Noamundi ore deposits and chemically pure iron oxide were used for the present study. Pure silica and alumina powders were mixed to synthesize the chemical composition corresponding to naturally occurring iron ores. The chemical analysis of the natural ores and the pure iron oxide are given in Table 1. M~ 3/S--a

509

T. SHARMA et al.

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TABLE 1 Chemical analysis of iron oxides

Sl. Nol.

I. 2. 3.

Source

Fe

Pure Iron Oxide B a i l a d i l a Ore N o a m u n d i Ore

69.9 68.0 64.0

SiO 2 AI203 (Wt. percent)

L.O.I.

........ trace ............ 0.70 1.26 1.0 2.0 3.60 2.90

The three iron oxide fines were rolled to give 12 to 13 mm diameter pellets using 10= 11 percent moisture. For preparation of synthetic pellets, chemically pure silica and alumina powders were thoroughly mixed with pure iron oxide, the mixture was pre-fired, crushed and reground to the same mesh size as that of iron ores. All pellets were fired at 1250°C for 60 minutes. A 45 mm inner diameter and 1000 mm long stainless steel tube having water-cooled quartz windows at both ends was employed as the reaction chamber in which the swelling behaviour of the pellet was studied. A schematic diagram of the apparatus is shown in Figure 1. An alumina base having a small cavity was used to keep the pellets stationary in the uniform temperature zone of the furnace. The temperature of the furnace was maintained constant in the range of + 5°(] during the experiment and pure and dry carbon monoxide gas was used for reduction of the pellet. Carbon monoxide gas was generated by dropping formic acid on sulphuric acid kept at 250°C and the gas was purified.

+ll-

llhl

U

FLOW

.... . . . . . . . .

METER

2. STAINLESS STEEL TUBE 3. ALUMINA CRUCIBLE L,.IRON ORE PELLET 5.LIGHT SOURCE 6. WATER COOLEDWINDOWS

Fig.l Line Diagram of the Apparatus For determining the swelling index, a pellet of known diameter was kept inside the furnace and a photograph of the pellet was taken with the help of an external light source. The pellet was heated under argon atmosphere, till the desired reduction temperature (1000 o ± 5°C) was obtained. The pellet was soaked at this temperature for 20 minutes and a photograph of the pellet was taken. Carbon monoxide gas was allowed to flow through the reduction tube for a certain time after which a photograph of the pellet was taken. Flow of carbon monoxide gas was then stopped and the pellet was cooled to room temperature under argon atmosphere and weight loss of the pellet was determined. The experiment was repeated for different periods of reduction and corresponding swelling values were noted. In order to ensure high accuracy of the results, the volume of the pellet was calculated by projecting the pellet photograph on a screen and measuring the average diameter before and after the reduction for a given time.

Effect o f gangue content on iron ore pellets

511

The swelling index of the pellet during heating and at different fractions of reduction was calculated as Swelling index = 100(V a - V i ) / V i where,V i ffi V a ffi

initial volume of pellet and volume of pellet after reduction

Also the structure of the pellet was studied by scanning electron microscope. RESULTS A series o£ swelling tests at 1000°C with 100 percent carbon monoxide flowing at the rate of 1.5 l/rain was carried out. The experiments were repeated to ensure the accuracy of the results and the set of results obtained in the case of pure iron oxide is plotted in Figure 2. The percentage volume increases due to the thermal expansion of the pellet is indicated by the intercept at the Y-axis and is shown as 'Thermal Swelling' (ST). During reduction, the volume of the pellet increases with reduction time and this swelling is termed 'Reduction Swelling' (SR). The volume attains a maximum value after 40 minutes of reduction and is termed 'Maximum Swelling' (SM). After achieving maximum swelling, with further increase in reduction time, a slight decrease in volume is observed and this decrease in volume is termed 'Shrinkage, (Ss) and final value o£ swelling index towards the completion of reduction is shown as 'Final Swelling (SF). Hence: S,

= S R + Sr

(1)

SF = S, - Ss

(2)

where SM ffi Maximum swelling SR ffi Reduction swelling (Swelling due to reduction) S F = Final swelling S s ffi Shrinkage. 120 100 X LU

o

80

ii

i'

bJ

I

IF

u~ 40

I I

Z

z

6O

!

2G ST I t

0

2o

4'0 6'0 TIME(rain.}

8'0

Fig.2 Swelling vs. time plot for pure iron oxide pellet Figure 3 shows the swelling behaviour of pellets made from the three iron oxides in which percentage increases in volumes have been plotted against degree of reduction. The values of maximum swelling for pure iron oxide, Bailadila ore and Noamundi ore pellets are 119, 103 and 42 percent respectively. In all three pellets, the maximum swelling corresponds to around 70 percent reduction. The difference in maximum swelling in these pellets may be due to the variation in the nature and percentage of the gangue content in the pellet.

512

T. SI-IARMAet al. 120

~ I r o n

100 x

8o

z (.9

z 60

~

~o

20

2'0

6'o ;o

,;o

% REDUCTION Fig.3 Swelling vs. reduction plot for three oxides Synthetic pellets were made from pure iron oxide by mixing pure alumina and silica powders to develop a composition corresponding to that of natural Bailadila and Noamundi ores. These pellets were made from mixtures of pure iron oxide silica and alumina prefired at 1350°C for 6 hours and then fired at 1250°C for 60 minutes. The results obtained after the swelling tests are shown in Figure 4. It is interesting to observe that the swelling indices for synthetic pellets are within ± 2 percent of the corresponding values obtained in the case of natural ore pellets.

0---0

Noturol Ore Pellet

x~x

Synthetic Ore Pellet

120

100

X Q Z m (.9 z --3 .J UJ ,,,

/

80

60

0

x.~

f

20

0

z.~-'°~'-"o

/

2,0

'~x

i L,O %

t, BO

x .,o,~, °

Noomundl Ore

A 80

• t00

REDUCTION

Fig.4 Swelling vs. reduction plot for natural and synthetic ore pellets The effect o f gangue content was further studied using pellets prepared by adding the gangue constituents to the ore. While making such pellets, care was taken to maintain the alumina silica ratio as in the case o f naturally occurring Bailadila and Noamundi iron ores. To decrease the gangue level of Bailadila and Noamundi ores, pure iron oxide was added. It has been observed that increased gangue content resulted in a decrease in the swelling index of the pellet in all the three cases. For a given gangue content, the variation in

Effect o f gangue content on iron ore pellets

513

swelling index for pellets prepared from the three different base material was only 2 to 4 percent as shown in Figure 5 in which reduction swelling (SR) has been plotted against gangue content. Hence,it can be said that under identical pellet processing and reduction condition, the swelling index is directly affected by the total gangue content.

120

100

~ 0

Pure Iron Oxide

x--x

Bailodilo Ore

A--&

Noomundi Ore

80 Z

~ so ,.J Ixl

20 ,, I

2

I

I

I

I

4

6.

8

10

GANGUE CONTENT(Wt%)

Fig.5 Reduction swelling (SR) vs. gangue content plot Several authors [1-12] have suggested phase changes and/or growth of iron whiskers as the probable reasons for higher swelling indices in the pellets. Zhuravlev [13] and others [1415] have pointed out that the presence of gangue and its nature might be the reason for checking the growth of iron whiskers leading to decreased swelling indices. To verify this, micro-structural study of reduced pellet by Scanning Electron Microscope was carried out for the pellets with varying gangue content.

Mlcrostructural Study From Figures 2,3 and 4, it is observed that maximum swelling occurs around 70 percent reduction followed by a decrease in swelling index with further increase in degree of reduction. Figure 6 shows the morphology of the fractured surface of pure iron oxide at different degrees of reduction. It can be noticed that initially (0% reduction), there is absence of fibrous structure, but at 45 percent reduction, fibres of iron appear and their length and number increase with the reduction. After 70 percent reduction these fibres are observed to sinter or collapse which may be causing shrinkage (Ss) after maximum swelling at 70 percent reduction. The structure of the fractured surfaces of fired and reduced pellets of the three iron oxides are shown in Figure 7(a) and (b) respectively. Longer whiskers are seen in the micrographs of reduced iron oxide and Bailadila ore pellets. In case of the Noamundi ore pellet only a few whiskers of smaller size are seen. The difference in the number and size of whiskers may be due to the variation in gangue content of the pellets. Figure 8 shows the microstructures of reduced Bailadila and Noamundi ore pellets prepared from natural and synthetic ores. The morphology of whiskers in both cases is similar. Thus, it further supports the view that the gangue content is probably responsible for controlling whisker growth, and hence swelling index of the pellet. From the foregoing results, it seems that the swelling index of pellets is mostly affected by the gangue content.

514

T. SHARMAet

al.

Fig.6 SEM micrograph of fractured pellets having various degree of reduction

Fig.7 (a) S.E.M. micrograph of fired pellets (b) S.E.M. micrograph of reduced pellets

Effect of gangue content on iron ore pellets

515

Fig.8 S.E.M. micrograph of reduced natural and synthetic ore pellets DISCUSSION When iron ore pellets are reduced by carbon monoxide gas, the pellets undergo volume change which involve thermal expansion due to heating, phase trans.formation and growth of iron whiskers during reduction. The volume change in the system is a result of the balancing of stresses developed in the individual grains due to expansion and the strength of the bonds among the grains. When the volume change is excessive, the bonding between adjacent planes becomes weak and the pellet cracks. In order to minimise swelling the growth of iron whiskers should be restricted. The presence of gangue seems to impart strength to the pellet by developing slag bonds. This is why the swelling index of pure iron oxide pellet is highest and that of the Noamundi ore pellet (5.6% gangue) is lowest. In presence of gangue several compounds are formed during pellet firing. The amount and composition of these compounds depend upon the gangue content, firing temperature and time. Formation of compounds having low melting point result in the development of slag bonds which, are thought to have better resistance to the force exerted by the growing whiskers. When the bonding strength of the pellet is low, the growing whiskers push the adjacent ore grains, and hence result in increased volume and sometimes, cracking and disintegration of the pellet. However when the bonding strength of the pellet is high either due to stronger bridging of iron oxide or due to presence of slag bonds, these whiskers are not able to push the adjacent surfaces mechanically and in that case lower swelling is observed. Fuwa et al. [1], Zhuravlev [13] and a few other workers [14-15] have reported that, in general, swelling could be minimised by addition of silica and alumina i.e. by creating more slag bonds having better resistance to the force exerted by growing whiskers. Low melting slag gives a coating on the surface of growing whisker and restricts its growth. Decrease in volume beyond 70 percent reduction may be due to coalescence of the whiskers. In order to confirm this, experiments were carried out in which the reduction was arrested at various stages by stopping the flow of carbon monoxide and the volume of the pellets was determined as a function of time under argon atmosphere. The results plotted in Figure 9 indicate that volume of pellet decreases with increase in retention time at 1000°C. These results confirm that swelling increases with increasing rate of whisker growth and at the same time sintering and collapse of whiskers and pores may also begin due to the diffusion of iron. The rate of growth of whiskers together with simultaneous sintering ultimately result in an overall increase in shrinkage.

T. SHARMA et al.

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o----o

CO a~mosphere

o----o

Ar

atmosphere

125

100 x ~J r7 Z (.9 Z

75

~J

50 ,

"o,, b "o~ 22-_.,o

25

0

0

I

I

20

40

[

130

i

[

l

80

100

120

TIME (min.)

Fig.9 Swelling vs. time plot under CO and Ar atmosphere CONCLUSIONS From the present investigation we can conclude that: (1)

the swelling ismainly due to the growth of iron whiskers during reduction.

(2)

the growth of whiskers and sintering of reduced iron take place together and shrinkage in pellet is due to the sintering of iron whiskers.

(3)

the growth of iron whiskers is restricted due to the presence of gangue, and finally swelling decreases with increase in gangue content of the pellet. REFERENCES

.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Fuwa T. & Banya S., Trans. ISI Japan, 9, 137 (1969). Grance L., Prec. ICSTIS Tokyo Suppl. Trans. ISI Japan, 11, 45 (1971). Moon J.T. & Walker R.D., Iron making Steel making, 2, 30 (1975). Wenzel W. & Gudenau H.W., Trans. Soc. of Mining Eng. of AIME, 252, 281 (1972). Wright .K., Prec. Aust. Inst. of Mine and Met., 265, 1 (1978). Lu W.K., Iron making Proceedings, Atlantic City, 33, 61 (1974). Veto Ende H., Grebe K. & Thomalla S., Stahl u Eisen, 90, 586 (1970). Lu W.K., Scand. J of Metallurgy, 3, 49 (1974). Lu W.K., Scand. J. of Metallurgy, 2, 273 (1973). Lu W.K., Scand. J. of Metallurgy, 2, 169 (1973). Lu W.K., Scand. J. of Metallurgy, 2, 65 (1973). Taniguchi S., Ohmi M. & Fukuhara H., Trans. ISI Japan, 18, 633 (1978). Zhuravlev F.M., Metallurgia, 4, 14 (1977). Kojima K., Nagano K., Kishi T. & Koyama K., Tetsu to Hagane, 57, Paper 15 (1971). Chumak A.D., Zhuravlev A.M., Malyseva T.A., Drozhilov L.A. & Merlin A.V., Russ. Met., 3, 5 (1979).