City Research Online

Abstract

Electrochemical oxidation is one of the most efficient ways of disposing of considerable quantities of waste cyanides. The efficiency of this method seems to be limited to high concentrations, a drop in efficiency with decreasing cyanide content is observed. The high cost of this method due to the diversion of some current to the secondary processes limits its application.

Pulsed-current electrolysis is used in metal plating. When applied to cyanide oxidation, a large increase in efficiency for the same average current density as d.c. electrolysis was observed. The efficiency of p.c. electrolysis was found to depend on current density, pulse profile and concentration. Optimisation of these factors in relation to one another would further improve the efficiency.

Using Ferrocyanide as a pilot system, no increase in the average limiting current was observed with p.c. and an increase in efficiency was only encountered when currents higher than the d.c. limiting currents were used. From the Sand's equation and the Flux, the optimum pulse profile could be calculated theoretically as a function of concentration and current density. These theoretical values displayed good agreement with those measured from the chronopotentiograms, and a net increase in efficiency was observed on using the theoretical pulse profile for electrolysis.

Higher efficiencies were encountered on using p.c. for cyanide oxidation. However, it was not possible to optimise the pulse profile in a similar fashion to Ferrocyanide. The results obtained from the chronopotentiograms seemed to indicate adsorption of cyanide on the electrode. Optimisation of t_ON based on the amount of adsorbed species proved successful. These observations seem to stress that pulse does not act solely as a mass transfer relaxation method but affects the surface oxides and hence, the nature and amount of the products.

The high efficiencies obtained with p.c. electrolysis make it a very attractive proposition economically for treating dilute cyanide wastes. By careful optimisation and monitoring of either the current density or the pulse profile as a function of concentration, treatment can be carried out to very low levels of cyanide concentration without any loss in efficiency.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry T Technology > TD Environmental technology. Sanitary engineering T Technology > TP Chemical technology
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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