Reduction of solid wustite in H 2 /H 2 O/CO/CO 2 gas mixtures
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I.
INTRODUCTION
IN previous
studies [1-41 of wustite reduction in H2/H20 and CO/CO2 gas mixtures, three main product microstructures were identified. These structures are porous iron (type A), porous wustite covered with dense iron (type B), and dense wustite covered with dense iron (type C), Type B structures have been shown to result from the decomposition of the dense wustite prior to iron nucleation. The decomposition reaction results in the formation of relatively coarse tunnels throughout the material, and the surfaces of these tunnels are subsequently covered with a dense iron layer. In industrial reduction processes, all of the gas species H2/H20/CO/CO 2 are present in the systems, in various proportions. For example, hydrogen and steam are to be found in iron blast furnace gases due to the humidity of the air blast and the addition of hydrocarbon injectants.tS] Direct reduction processes utilize hydrocarbon sources as reductants again producing complex H z / H 2 0 / C O / C O 2 gas mixtures/6] The aim of the present study then is to determine the gas and temperature conditions necessary to produce porous iron product morphologies in these complex gas mixtures. II.
EXPERIMENTAL
A detailed description of the apparatus and experimental procedure used in the present investigation has been previously published. E71Pure dense l-ram 3 wustite samples ( y = 0.10) were reduced in a novel design reaction furnace which enables fast quenching of the samples after a given reaction time. The Hz/H20 and CO/CO2 ratios in each reduction gas mixture were chosen so as to produce a chosen oxygen partial pressure in the gas mixture from both sets of components at the reaction temperature under investigation; tSl i.e., the reaction H2 + CO2--> H20 + CO
gas flows are then combined prior to introduction into the reaction chamber. The flows in the gas streams were chosen such that the volumetric ratios (H2 + H 2 0 ) / (CO + CO2) = 1/9, 1/1, 4 / 1 , 9/1, and 19/1 were produced in the final gas mixtures, having a total flow rate of 1 L min -~. Following reduction, the partially reduced samples were mounted in ARALDITE* containing approximately *ARALDITE NY.
III.
R E S U L T S AND D I S C U S S I O N
A. Porous~Dense Iron Transition The critical gas compositions for porous iron formation on reduction of wustite in H 2 / H 2 0 / C O / C O 2 gas mixtures at temperatures between 1073 and 1373 K have been determined by the examination of partially reduced oxide samples. These critical conditions are presented in Table I and also as a function of carbon activity (Figure 1) and oxygen potential (Figure 2) in the gas mixtures at the various reaction temperatures. The activities of carbon, given relative to solid graphite, are calculated t8] assuming equilibrium for the reaction CO2 (g) + C (s) --0 2CO (g)
[21
i.e.,
1 P~o a C --
K2 Pco2
The oxygen potentials of the gas mixtures are determined through the equilibria
is at equilibrium. These individual Hz/H20 and CO/CO2
METALLURGICAL TRANSACTIONS B
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