Abstract:
Although oxygen is widely employed as the oxidant of choice in gold
leaching by cyanide, its low aqueous solubility presents some drawbacks
in practical application; hydrogen peroxide has therefore been considered
as a possible alternative. The aim of this investigation was to study the
catalytic decomposition of hydrogen peroxide, which generates an
oxidizing intermediate species, and to understand its effect on cyanide
destruction. Operating conditions that facilitated the effective
decomposition of hydrogen peroxide were established by varying the pH
and catalyst type and concentration. The oxidizing intermediate, detected
using an indirect technique, was found to be the hydroxyl radical (OH˙).
OH˙ is commonly generated in acidic solutions, but this work demonstrated
that it is also produced at the alkaline pH values necessary for cyanide
gold leaching. The effects of free and complexed iron and copper catalysts
on the oxidation and consumption of hydrogen peroxide and cyanide were
also investigated. It was shown that the cyano complexes of Fe(II) and
Cu(I) are also effective as decomposition catalysts. Hydrogen peroxide
concentrations above 0.01 M decreased the free cyanide concentration,
which was attributed to the probable formation of the cyanate anion
(CNO−). Although cyanide consumption increased due to its oxidation in
the presence of OH˙, excessive cyanide consumption in the presence of
copper was attributed primarily to its complexation by the unstable
copper(I) cyanide species. Rate constants for the decompositions of H2O2
and cyanide by ferrocyanide and copper cyanide were calculated; the latter
was identified as being a better catalyst.
