Abstract
The removal of silver metal from used fixing baths is of interest both as means of salvaging the valuable element from environmentally toxic effluents and as means of increasing the useful life of the bath. Several methods have been proposed. They can be classified underthe headings of (i) reduction, in which silver ions are reduced to silver metal by electrolysis,
(ii) precipitation as an insoluble silver salt, or (iii) removal by ion-exchange.
The aim of this research was to investigate the use of ultrasound as a novel adjunct to the normal electrochemical technique employed for the remediation of waste water from photographic processes. This was accomplished by studying the effect of ultrasound upon limiting current densities and potentials at various ultrasonic frequencies, powers and temperatures on 'model' and 'real' systems.
The author chose as a model electrochemical system Fe(CN)63- I Fe(CN)64- redox in dilute KCI. It was found that limiting current densities for this system at a platinum electrode appeared to be little affected by the frequency of simultaneous ultrasonic irradiation in the range
20-800 kHz, and was not influenced by choice of bath or probe as sonic source provided
measurements were made at constant ultrasonic intensity. The limiting current density was found to be dependent on the ultrasonic intensity at constant frequency. Similarly, the formal potential and thefonnal rate constant of the redox couple varied with ultrasonic intensity in the same range. Application of ultrasound to a rotating disc electrode provided an increase in limiting current density across the frequency range employed. The magnitude of the increase was some 1.5-fold which corresponded to an effective rotation rate of I00,000 rpm ( 1667 Hz) for a silent solution in order to achieve the same transport rates. It was found that changes in macroscopic temperature throughout the experiment were insufficient to cause the observed enhanced diffusion.
The findings from the model system were extended to a 'real' system 1.e. silver removal. Similar results were observed with Ag+/Ag redox couple in dilute sodium thiosulphate and sodium bisulphite (i.e. "wash" solution).
The author has demonstrated during the course of this work that:
(i) the decomposition voltage of the "wash" solution decreased with increasing ultrasonic intensity at 20 and 500 kHz,
(ii) the discharge potentials of hydrogen and silver shifted anodically whereas the discharge potential of oxygen shifted cathodically with increasing ultrasonic intensity at 20. 38
and 500 kHz;
(iii) the half-wave potential of the Fe(CN)63-/Fe(CN)64- redox couple shifted cathodically with increasing ultrasonic intensity in the range 20-800 kHz;
(iv) the electrode kinetic parameters i.e. the apparent heterogeneous rate constant and the exchange-current density did vary significantly with increasing ultrasonic intensity at constant frequency;
(v) the mass-transport rate was further enhanced with ultrasonic intensity;
(vi) the cell geometry and the distance between the ultrasonic horn and the working electrode influenced the shape of the steady-state voltammogram i.e. the limiting current density.
Having established the effects of the various parameters on the discharge characteristics of the 'model' systems, the work was extended to a 'real' system i.e. the deposition of silver from "wash" solutions in which the effect of rotating the electrode at various rotation speeds in the absence and presence of ultrasound was assessed. Ultrasound was delivered by both baths, probes and an "idealised" industrial cell called "SonoEcoCell". It was found, for the optimum conditions, that the "SonoEcoCell" is capable of reducing silver ion concentration down to approximately 0.5 ppm (starting from 4000 ppm). It was also found that the author's new process is cost and time effective compared with other traditional desilvering processes.
| Date of Award | 1998 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Timothy Mason (Supervisor) & John P Lorimer (Supervisor) |