Research at the Parker Centre on two novel alternatives to roasting for pre-treatment of refractory sulfide ores prior to cyanidation may produce a one-step process which avoids the use of cyanide.

Pressure and bio-oxidation are the most successful current alternatives to roasting. This is particularly true for arsenic-containing ores which cannot generally be roasted for environmental reasons. However, many ores cannot be processed using these methods due to economic and processing factors such as the presence of "preg-robbing" materials in the ore.

The first technology under investigation by the Parker Centre makes use of a previous observation that gold is leached to a certain extent when the chloride ions are present in the pressure oxidation process.

PhD student Jim Qing Liu has confirmed that gold is dissolved by chloride ions in acidic solutions at elevated temperatures in the presence of oxidants such as ferric ions and dissolved oxygen. The equilibrium shown below can be driven to the right hand side under the right conditions:

Au + 4Cl^- + 3Fe³⁺ <=> AuCl₄^- + 3Fe²⁺

Higher temperatures promote this reaction due to a decrease in the electrochemical potential for the oxidation of gold and at the same time an increase of the potential for the reduction of ferric ions. These changes in potential promote the dissolution of gold.

In addition, under conditions typical of pressure autoclave oxidation, the refractory sulfide minerals are oxidised, leading to the generation of ferric ions. Consequently, the pressure oxidation system as such is a self-sufficient system in terms of sulfide mineral oxidation and gold leaching, provided sufficient chloride is present.

The second approach utilises a recent discovery by another PhD student. Zhang Suchun found that during the oxidation by dissolved oxygen of refractory gold concentrates in alkaline solutions under carefully controlled conditions, the gold is also dissolved by a reaction which has not yet been identified.

Several local and overseas refractory gold-bearing concentrates containing arsenopyrite and pyrite have been used in the testwork. The dissolution of gold during 48 hour experimental runs varied between 20% and 80%, depending on the experimental conditions and the source of the concentrate.

This unexpected and exciting observation offers the potential of a one-step process for the recovery of gold from refractory materials without the use of pressure oxidation and, more importantly, which avoids the use of cyanide. Such a process could offer substantial capital and operating cost benefits. It might also be able to handle concentrates that are not amenable to conventional treatment such as those containing substantial amounts of copper or containing carbonaceous "preg-robbing" ore components.

Work on both these areas is continuing under the supervision of Professor Mike Nicol. This research aims to define the most appropriate conditions for effective leaching of gold and to understand the complex chemistry involved in both the behaviour of the gold and of the sulfide minerals which host the gold.

For further information, contact
Professor Mike Nicol (nicol@central.murdoch.edu.au)
or Jim Qing Liu (jliu@central.murdoch.edu.au).

First published in the Gold Mining Journal.