Chemistry, microstructure, and reduction characteristics of dolomite-fluxed magnetite pellets
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THE
addition of dolomite t o iron ore pellets improves t h e i r high temperature properties primarily by increasing t h e i r softening point and reducibility. These improved properties reportedly lead t o inc r e a s e d blast furnace efficiency and productivity.~'2 The present study was done t o evaluate the effect of composition, specifically MgO l e v e l and CaO/SiO2 r a t i o on high temperature properties. Hibbing concentrate, which c o m e s from a magnetic !aconite mine at Hibbing, Minnesota, was used. Dolomite was added to provide the magnesium, and limestone to adjust the CaO/SiO2 ratio. MgO enters the magnetite lattice to form magnesioferrite, while CaO r e a c t s with the silicate gangue to form a s l a g . This slag helps t o bond the pellets and acts as a m e d i u m for ion transfer. The effects of MgO content and CaO/SiO2 r a t i o were examined a l o n g with the effect of different firing conditions. This was done by interrelating microstructure, c h e m i s t r y , and reduction characteristics. An understanding of these relations is necessary not only t o d e t e r mine the optimum flux addition but also the composition r a n g e that can be tolerated. Because the pellets represent an at least five component system, the JOIIN J. FRIEL and EDWIN S. ERICKSON, JR. are Engineers, Homer Research Labs, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted May 31, 1979. METALLURGICAL TRANSACTIONS B
equilibrium a m o n g FeO-Fe~O3-CaO-SiO2-MgO must be examined. Known equilibrium relationships in ternary systems such as CaO-SiO2-FeO and CaDSiO2-Fe203 can be applied only if equilibrium exists in the pellets on some observable s c a l e . It is u n r e a sonable to a s s u m e that complete equilibrium will be attained throughout pellets that have been subjected t o peak firing temperatures for only five rain. Howe v e r , l o c a l equilibrium in accordance w i t h known phase relations can be observed on the s c a l e of several hundred micrometers. The demonstration of such l o c a l equilibrium thus permits interpretation of the observed microstructures and their relations h i p s to physical and chemical properties. The effect of MgO content has been discussed by Saeki et a l~ who found that additions of up t o 2 pc! MgO increased softening and meltdown temperatures. Beyond 2 pc!, little i n c r e a s e was achieved. They also found that 1.5 pct MgO produced the greatest reducibility at 1250°C. The effect of the CaO/SiO2 r a t i o has been discussed by Thaamirtg. 3 H e found that the most favorable r a n g e is between 1.2 and 1.8 for one of the Swedish concentrates studied. Moreover, he explained that if the CaO/SiO2 ratio is too low, a glassy silicate is formed that causes the pellet t o s w e l l excessively and contributes t o high temperature disintegration under reducing conditions. If the ratio is too high, e x c e s s CaO is available t o form calcium ferrites, which contribute to low temperature disintegration.
ISSN 0360-2141 !80/0611-0233500.75!0 © 1980 A
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