A study of the mechanism of reduction with hydrogen of pure wustite single crystals
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I.
INTRODUCTION
" D I R E C T Reduction" of iron ores is a rather quickly growing industrial process, especially where natural gas is available.1 This gas is first converted into CO + H~ which are then used for reduction, generally in shaft furnaces (Hyl or Midrex processes mainly) and possibly in fluidized beds (Fior process). But pure hydrogen could be used as well, with an advantage: it is easy to recycle after water condensation, and also a small drawback: the reduction is nearly athermic (instead of exothermic with CO). As a matter of fact, the Fior process has been operated with pure hydrogen, and the Hyl one is claimed to be suitable for hydrogen. 2 In this prospect, one phenomenon appears unfortunate: the so-called "Rate Minimum" which relates to the fact that, for many iron ores, it has been established that in the range 700 to 900 ~ the reduction rate with hydrogen does not increase regularly with temperature, as is the general rule for any solid-gas reaction, but displays a minimum. As many direct reduction processes are carried out in the range 700 to 1000 ~ the problem is of interest. But, as the literature survey below will show, there is not yet a definitive explanation of the rate minimum. So we have carried out additional experiments designed so as to obtain a better insight into the phenomenon and to find out how to avoid it. Moreover, such behavior, although rare, has also been reported in the reduction of cobalt monoxide with hydrogen, 3 and of cobalt ferrite with hydrogen-water vapor. 4 II.
LITERATURE SURVEY
Previous publications dealing with the rate minimum phenomenon have put forward interpretations which may be classified as follows: 1. The progressive recrystallization of iron, and its sintering, provides a barrier to gas diffusion. 5-1~ M. MOUKASSI, Docteur de SpOcialit6, P. STEINMETZ, MaRreA s s i s t a n t , B. D U P R E , C h a r g 6 de R e c h e r c h e s C. N. R. S., a n d C. GLEITZER, Directeur de Recherches C. N. R. S., are all with Laboratoire de Chimie du Solide Min6ral, Associ6 au C. N. R. S. no. 158, Service de Chimie Min6rale, Universit6 de Nancy I, B. P. 239, 54506 Vandoeuvre les Nancy Cedex, France. Manuscript submitted April l, 1982. METALLURGICAL TRANSACTIONS B
2. The wustite properties evolve toward a denser and more perfect crystalline state in which iron diffusion is slowed down. 12,13,14 3. Unreacted wustite islands form behind the reaction interface .15.16 4. The roles of the magnetic transformation of iron, or of its polymorphic transition, have been invoked. 13'17'18 5. Differences in the adsorption mechanism below and above the minimum have been admitted, x9 Besides the various interpretations when the rate minimum occurs, it must be stressed that for many authors such a slowing down has not been observed 2~ or has been observed only in some experimental conditions. 8'9'12'16But no correlation seems to be available between the occurrence and following parameters: nature of the starting material (ore, hematite, magnetite, wustite), porosity, grain size, thermal
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