The decomposition of wustite under mixed chemical reaction/mass transport limitations
- PDF / 660,630 Bytes
- 7 Pages / 603.28 x 783.28 pts Page_size
- 74 Downloads / 199 Views
I.
INTRODUCTION
T H E rates of oxidation and reduction of inorganic solids by reaction with gaseous environments have been the subject of many investigations. Recently, however, there has been renewed interest in these reactions with emphasis on the morphological changes which occur during transformation. Although qualitative explanations may now be given to many of these phenomena, the time is ripe for a more rigorous approach to these problems. A general case is considered, one in which there is reaction between a gas and a solid oxide surface, prior to metal formation, under conditions where the gas velocity is sufficiently high that the gas composition at the solid surface is the same as in the bulk gas atmosphere. Two limiting conditions to the decomposition reaction can be readily identified. The first is when the chemical reaction rate at the gas/oxide interface is very slow compared with mass transport within the solid. In this situation the composition of the solid surface is that approaching equilibrium with the bulk material. The second limiting condition is when the chemical reaction rate is very fast compared with mass transport in the solid. This causes the composition at the interface to approach equilibrium with the gas phase. These situations are referred to as the chemical reaction limited and mass transport limited decomposition rates, respectively. In reality there must always be slight deviations from these respective limiting conditions to provide the free energy differences to drive the other reactions which are simultaneously taking place. In a number of reaction systems sufficient data have been accumulated to describe these two limiting cases. Accurate predictions of mixed chemical reaction and mass transport reaction rates have not yet been formulated. The present paper presents solutions to this problem for a particular case of practical importance, the decomposition of wustite (Fel-yO) by H : / H : O or CO/CO2 gas mixtures. For reduction in H:/H:O gas mixtures the reaction is described by the equation
The conditions considered are those when the applied gas mixture has a chemical potential which results in a wustite in a metastable state i . e . , in an open system this would eventually result in the formation of iron metal and gas rather than wustite. In terms of the equilibrium conditions shown in Figures 1 and 2, reducing gas mixtures are to the left of the line separating the Fe and FeO phase fields. The oxygen potentials at the Fe/FeO equilibria along with the equivalent H2/H20 and CO/CO2 gas mixtures at various reaction temperatures are given in Table I. The reactions considered here are those occurring prior to iron nucleation on the wustite. Previous attempts ~'2'3to describe mixed reactions in these systems considered the chemical reaction rate at the oxide surface to be described by the relationship, rate of oxygen removal = k ( c - Ce). k is a rate constant for the reaction temperature and gas mixture, c is the concentration of the iron ions in wustite at the reaction surf
Data Loading...
