An electrochemical study of the oxidative dissolution of synthetic copper-silver-selenide minerals in aqueous media.

The electrochemical behaviour of synthetic components of copper refinery anode slimes e.g. silver selenide (naumannite) & copper silver selenide (eucairite) in aqueous media, e.g. 1M HN03 & 1M HC104 solutions, have been investigated using a variety of techniques including linear sweep cyclic voltammetry, chronoamperometry, chronopotentiometry, intermittent galvanostatic polarisation & stripping voltammetry. The electrochemically treated mineral electrodes have been subjected to a series of solid state analysis e.g. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis & electron microprobe analysis to identify possible phase transitions on the surface, while the aqueous solutions were analysed by atomic absorption spectroscopy. Considerable attention was also paid to the synthesis of Ag2Se, Cu2Se & CuAgSe by hydrochemical & solid state (vapour transmission) methods. The characterisation of the synthetic minerals was accomplished by the above analytical methods. Under anodic conditions, silver selenide is oxidised directly to elemental selenium via the reaction Ag2Se = 2Ag+ + Se + 2e. No evidence for the formation of any intermediate phases, e.g. AgSe, was found by surface analysis, though CV & IGP experiments suggest the possible presence of an intermediate. The formation of a metastable elemental selenium product was proved by both EDXA & electrochemical techniques e.g. cathodic stripping (including cathodic excursions in CV & IGP) in which the reduction of the elemental selenium to H2Se was observed. The elemental selenium product is porous & does not impose restriction on the diffusion of product or reactant across the silver selenide/electrolyte interface. Instead, the slow heterogeneous charge transfer process plays an important role in determining the rate. Silver selenide was reduced at potentials more negative than 0.2V to elemental silver & H2Se gas. Copper was found to be preferentially "leached" into solution during the 11 anodic polarisation of copper silver selenide (Cu:Ag=30:l in the solution at initial stages), forming a silver-rich product layer which was subsequently oxidised to elemental selenium. This is probably due to the catalytic effect of silver ions released instantaneously at the imposition of polarisation. EMPA reveals positive evidence for formation of Ag2Se on the electrode surface, which is similar to a mechanism proposed to explain the catalytic effect of Ag+ (aq) on sulphides e.g. chalcopyrite, heazlewoodite & pyrite. The Fe3+/Fe2+ couple was studied with a newly-developed technique to obtain the kinetic parameters diffusion coefficient, charge transfer coefficient, half wave potential E1/2 & standard heterogeneous rate constant. These results were comparable to literature values. Furthermore, this work also included an extensive study of the theoretical aspects of fundamental electrochemistry & led to the development of a set of powerful & accurate software routines for digital simulation of electrochemical data. This included provision for a wide variety of electrochemical mechanisms & for the techniques of cyclic voltammetry, chronoamperometry & chronopotentiometry. The digitally simulated data were extensively assessed using numerical & analytical methods, including where applicable convolution & allied techniques. A new numerical technique was developed to analyse complicated electrochemical reactions e.g. simple electron transfer process coupled with a catalytic reaction. In this independent variation of homogeneous chemical kinetic & diffusional parameters was possible. The implications of the above simulation routines to the study of electrode processes involving solid phase(s) e.g. minerals were also considered. However, due to the complexities arising from the involvement of solid phase(s), more work is needed so that these systems can be theoretically treated.