Speciation and reduction potentials of metal ions in concentrated chloride and sulfate solutions relevant to processing
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I.
INTRODUCTION
A knowledge of reduction potential (Eh) and speciation of a metal ion is essential in the understanding of hydrometallurgical processes and in the equilibrium modeling of dump leach reactions.~-6 Recently, species distribution diagrams for a number of base metal chloro-complexes have been reviewed ~'7's and Pourbaix diagrams have been extended to consider the Eh-pH and Eh-log acl- dependence and stability regions of a number of complexed and/or hydrolyzed metal ion species in chloride and sulfate media. 2'4'5'9-11 However, the reported association constants (K,) or stability constants (/3,) used for the construction of these diagrams usually relate to ideal solutions with unit ionic activity coefficients (yi = 1), or sometimes to solutions of particular ionic strength. Such values are questionable in solutions of high ionic strength where the ionic activities are far from ideality. ~'2'6'7''2 In a previous paper it was shown that ionic activities and reduction potentials can be reliably and rapidly estimated by emf measurements using appropriate cells and liquid-junction potential corrections (Ej) for up to 6 M CaC12 or MgCI2 solution or up to 4 M HC1.12Values of log 3'/for nonassociated ions rise rapidly in concentrated solutions according to the hydration number of the ion, due to a decrease in water activity (aw). Conversely, values of log 3/,. for associated ions decrease rapidly according to the activity of the counter-ion and its asse.ziation or stability constant. In general, the stability constant/3, for the formation of a complex ion MX~-" (Eq. [1]) is related to the activity ratio of the species involved according to Eq. [2]. M ~+ + n X /3.(a) =
~MX~,-"
[1]
aMxg-,, a~:
[2]
thermodynamic stability constant. To allow for changes in activity in practical solutions, some authors have used/3~~ and calculated values of Yi based upon the Debye-HiJckel, Davies, Hamed, or Pitzer empirical relationships, t3'~4 but in concentrated solutions, no allowance has been made for the decrease in water activity aw, and the hydration of the ions. 15 Except for the recently reported work by Majima and Awakura on water and solute activities of H 2 5 0 4 Fe2(SOn)3-H20 and HC1-FeC13-H20 solution systems,16 most researchers in fact ignore the hydration effects on the uncomplexed or complexed metal ions. Thus the accuracy of the calculated standard potential of the M C I ~ - " / M ~ couple according to Eqs. [3] and [4] is questionable. MCln~-" + ze ,-~ M ~ + nC1E~
2.303 RT
= E~
zF
(log/3 ~ - n log acl-)
[3] [4]
where E ~ represents the value in pure water. This paper therefore compares the calculated and measured reduction potentials, in terms of the Eh-pH and Ehlog acl- relationships, in order to examine the nature of chloro- or sulfato-complex speciation, and the effect of changes in ionic activities on the validity of the stability or association constants used. The systems of interest in this work are the ions associated with the processing of base metal sulfides in concentrated chloride or sulfate
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