Thermodynamics of the zinc-sulfur dioxide-water system
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I.
INTRODUCTION
THE interpretation of reaction phenomena occurring in any field involving aqueous systems requires an understanding of the heterogenous equilibria involved. Equilibrium studies are useful in the areas of analytical chemistry, corrosion science, geology, and extractive metallurgy, where thermodynamic analysis yields the conditions under which various species are stable. Knowledge of the chemistry of aqueous solutions has advanced rapidly, with the appearance of vast quantities of theoretical and experimental thermodynamic data. The assimilation of the voluminous available data into a very concise, convenient form has been accomplished by the use of diagranunatic or graphical techniques. In particular, the potential - p H diagram developed by Pourbaix m has gained wide acceptance in studies involving aqueous solutions. The Pourbaix atlas m contains diagrams for the zincwater system at 25 ~ Recently, the application of sulrite chemistry in the hydrometallurgical processing of zinc has been studied. [2.3] Although the formation of solid zinc sulfite has been known for a century, t4,5~very little quantitative information is available. Furthermore, there is a dearth of thermodynamic data for the behavior of ionic species at temperatures other than 25 ~ Consequently, the construction of potential - p H diagrams, with a few exceptions, has been limited to this temperature. It is desirable to have diagrams available which depict the equilibria which exist at elevated temperatures, where many of the applications, such as those mentioned above, 12'31and the corrosion behavior of zinc in aqueous sulfite media, are likely to be of interest. The purpose of this article is to assemble, in the form of potential - p H diagrams, the available information on the thermodynamic properties of the species relevant to the ZnH20, Zn-SO2-H20, and Zn-S-SO2-H20 systems, for which there are some data available at 25 ~ and to derive such diagrams for elevated temperatures up to 200 ~ The diagrams presented in this article were constructed using the entropy correspondence principle of Criss and Cobble t6] to obtain the high-temperature data for ionic species, as well as the thermodynamic data at D.M. LARSEN, Chief Metallurgist, is with Asamera Minerals U.S. Inc., Wentachee, WA 98807. P.B. LINKSON, Professor, is with the Department of Chemical Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia. Manuscript submitted July 21, 1986. METALLURGICAL TRANSACTIONS B
25 ~ from Wagman et al., tTl Naumov et al., t81 and various other sources shown in the tables. Experimental resuits t91 for the solid sulfite of zinc (ZnSO3" 2.5H2 O) were also incorporated. II. THERMODYNAMIC DATA AND C A L C U L A T I O N S Table I summarizes the thermodynamic data for the various species that were used in the derivation of the potential - p H diagrams at 25 ~ including the free energy of formation (AG~, molal entropy (S~ and heat capacity (C~). Some species have not been considered because of lack of reliable data consistent wi
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