Microstructural changes occurring during the gaseous reduction of magnetite
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I.
INTRODUCTION
R E L A T I V E L Y few studies have been devoted to the detailed microstructural changes occurring on reduction of magnetite. Earlier work f~-5] clearly indicated the complexity of the reactions, but the system still remains largely uncharacterized. Most investigations on the kinetics of reduction have been carried out on large porous pellets. While these studies are necessary to compare the reducibility of commercial feedstocks, they provide little basic data on the phase transformations. In addition, in large porous samples, significant variations in the gas compositions can occur within the samples during the course of the reaction, so that it is not possible to directly associate the microstructural changes with the composition of the bulk gas mixture. In situ studies of magnetite reduction t6,15J have demonstrated some of the different microstructures which can form on magnetite reduction. It is the aim of the present paper to characterize the conditions under which those structures form and the mechanisms of the phase transformations which occur. II.
EXPERIMENTAL
A. Sample Preparation Sample preparation and experimental procedures used in the present study have been described in detail in a previous publication, t71 Dense magnetite samples were made from the oxidation of pressed and sintered spectrographically pure iron pellets. Following final oxidation in 5 pct CO, 95 pet CO2 at 1523 K, the pellets were quenched, and samples --0.5 mm 3 were cut from the pellet using a scalpel. The samples were reduced in controlled gas mixtures using a novel design reaction furnace which allows fast quenching of the samples into liquid nitrogen following reduction. Cross sections of the partially reduced oxide specimens were examined either following polishing using
conventional metallographic techniques or after fracture under liquid nitrogen for scanning electron microscope examination.
B. Reduction Conditions The net effect of the reduction process is the removal of oxygen from the oxide by the gas; hence, the driving force for this chemical reaction is of prime consideration. This chemical driving force is governed by the difference in free energy between the gas phase and the reduction interface; thus, 1
METALLURGICALTRANSACTIONS B
[1]
where Po ..... is the oxygen partial pressure in equilibrium with the solid at the reaction interface. In the H z / H 2 0 system, the free energy difference is given by AG = - 2 R T I n [ ( P H 2 )
(Pn2~ l
[2]
L \ P H 2 0 / g a s \ PH2 / int-I
In the present investigation, rather than using specific H2/H20 ratios in the reduction gas at all reaction temperatures, thereby leading to additional complicating effects relating to a variable chemical driving force, an attempt was made to study (1) the effects of varying the temperature at specific chemical driving forces and (2) the effects of varying the chemical driving force at specific temperatures. From Eq. [2], to provide the same chemical driving force or reduction potential at different temperatures, e.g.,
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