Representation of mixed reactive gases on free energy (Ellingharn-Richardson) diagrams
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INTRODUCTION
IN 1944, Ellinghamm demonstrated the utility of showing, on a single diagram, the free energies of formation of a whole class of compounds. In the original work, EIlingham m plotted the available free energy data for both oxides and sulfides as a function of temperature and discussed the numerous ways such diagrams could be used. At the most basic level, these diagrams illustrate the relative stability o f each oxide and sulfide. Apparently, however, these diagrams did not receive much attention until Richardson e t al.t2-5] and Kelloggt6.n made additional constructions for a wide range of compounds and solutions. Of particular interest in the work of Richardson and co-workerst2-5] was the addition of a nomographic scale to the oxide diagram that provided a means of determining the oxygen pressure in equilibrium with any particular metal-metal oxide system at any temperature. Further modifications based on this concept were the additions of nomographic scales which permit determination of the ratio of carbon monoxide to carbon dioxide or the ratio of hydrogen to water vapor in equilibrium with any of the metal-metal oxide systems. Free energy diagrams of this type have come to be known as Ellingham or Ellingham-Richardson diagrams, and detailed discussion of the construction and use of these diagrams can be found in standard thermodynamic textbooks t8.91 or the comprehensive review of metallurgical thermochemistry by Richardson.t~o] The utility of Ellingham-Richardson diagrams is that for any set of compounds, such as oxides or sulfides, the thermodynamics of all possible reactions can be rapidly appreciated. From this, important reactions and equilibria can easily be separated from those which are not important. Further, these diagrams can be used to quantitatively esti-
mate how changes in temperature, pressure, and composition affect the relevant chemical equilibria. Since the introduction of these diagrams, computing power and methodologies have been significantly advanced so that equilibria in very complex systems can be readily calculated, and to some degree, these computer-based techniques have supplanted free energy diagrams. Nevertheless, these and other similar diagrams have retained their usefulness as a means for quickly assessing equilibria in a global way and thereby provide a means for determining which equilibria are suitable for more detailed consideration. Monatomic hydrogen provides a potentially useful means for reduction of extremely stable oxides.tin Relative to H2, gaseous H is unstable and pure H atmospheres that have a usable lifetime cannot be produced. However, equipment that can produce mixtures of H and H2 with varying fractions of H is available. These metastable mixtures have lifetimes that may be applicable to a variety of metallurgical processing schemes. In order to assess the utility of such mixtures for metal oxide reduction, it is useful to develop a means for determining the equilibria between mixtures of H and H 2 and the various metal-metal oxide systems. I
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