The Macraes Gold Project,
operated by GRD Macraes Limited, engaged GRD Minproc Limited to assist with the
evaluation of a pressure oxidation circuit, this being the key component of a
recovery enhancement project. The initial study gave sufficient confidence in
the process to enter into project execution. GRD Macraes Limited was the first
company to license Newmont Mining Corporation's pressure oxidation technology,
which aims to improve gold recovery from the carbonaceous sulfidic flotation
concentrate. This paper outlines the fundamental chemistry of the process
followed by a description of the plant that was ultimately installed. A summary
of the commissioning is given together with an overview of the production
results to date. The project ramp-up was rapid, with design temperature and
throughput being achieved within days of hot commissioning. Gold recoveries have
exceeded expectations, improving from around 70% prior to the installation of
pressure oxidation to above 84%. The project has increased the nominal gold
output at Macraes from 120,000 oz/y to 170,000 oz/y. On-going plant optimisation
for process parameters is discussed, giving rise to flexibility in gold
production, with key controlling parameters exceeding design.