Solution chemistry of tungsten leaching systems
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I.
INTRODUCTION
T H E R E is now a growing interest in the application of stability diagrams such as Eh-pH, log(metal)-pH, and log(anion)-pH diagrams 1'2 as predictive and analytical tools in hydrometallurgical processing. Peters has pioneered the use of Eh-pH diagrams in the investigation of the dissolution chemistries of sulfide minerals. 3'4 Robins, 5 Manning, 6 Ferreira, 7 and Macdonald and Bartletts among others, have applied elevated temperature Eh-pH diagrams to high temperature hydrometallurgical processes such as dissolution and precipitation. Osseo-Asare et al 9-13 have analyzed the chemistry of ammonia hydrometallurgy in terms of a variety of log (activity) v s log (activity) diagrams. Recently, Dyke et a l 1. have examined solvent extraction equilibria by means of Eh-pH diagrams. The object of this paper is to extend the application of the stability diagram concept to tungsten hydrometaUurgy.
II.
T H E STABILITY DIAGRAM CONCEPT
Consider the aqueous decomposition of tungsten dioxide WO2 + 2H20 = WO42- + 4H ยง + 2e
[1]
In terms of activities, the reaction quotient, Q, for reaction 1 is given by log O = log{WO42-} - 4pH - 2pe
[2]
may be represented graphically as shown in Figures l(a) to (c). In multi-species systems, where the stability regions of interest involve more than two compounds, it is necessary to follow the above analysis with line elimination, a process which becomes extremely tedius as the number of species increases. Several computer programs have, therefore, been offered in the literature to facilitate the stability calculations. 14-2~ The stability diagrams presented in this paper are based on calculations performed with the DIAGRAM program. 19
III.
The determination of stability regions is dependent on the availability of the relevant equilibrium constants or the availability of some reasonable means of estimating such constants. A variety of soluble polytungstates is known to exist; 2~'22 however, there are insufficient data on their free energies of formation or stability constants. Data on temperature effects are also incomplete. The approach that is taken in this paper is to limit the analysis to 25 ~ and dilute solutions. While these conditions are impractical as far as commercial practice is concerned, they nevertheless do show trends in stability relations which help to highlight the chemical principles which underly the well-known tungsten leaching schemes. Tables I to V present a summary of the
where pe = -log{e} and Eh = 0.059 pe at 25 ~ If K is the equilibrium constant for Eq. [1], then this reaction proceeds from left to right ( i . e . , WO 2- is more stable than WO2; henceforth abbreviated as WO]- >- WO2) if log K -> log Q
[3]
T H E R M O D Y N A M I C DATA
(o) (WO,~') const:
(b) pc c o n , s t /
*~
IX
WOZ4-
that is, if log{WO4:-} - 4pH - 2pe -< log K
[4]
pH
The equality in Eqs. [3] and [4] represents the limiting condition when WO]- is in equilibrium with WO2. Rearranging Eq. [4], it can be seen that WO42- --> WO2 if
(c] pH const.
pe -> [-1/2 log
K
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