2010 – ALTA, Perth WA
Gold is showing quite a surge in price and interest of late. While everyone is trying to cash in with
the current high price, there are some issues to be aware of when evaluating a gold deposit.
Mineralogy is the most important characteristic as it drives the whole process route. Free, coarse
metallic gold is easily treated as opposed to complex sulphides. Gold tellurides or gold locked in
arsenopyrite these are commonly known as refractory deposits, require fine grinding and oxidative
techniques to expose the gold.
Head assays are important in terms of ore value (gold and silver), however trace elements like
copper, nickel, zinc and mercury are also important to track and be aware of. High levels of coarse,
free gold are particularly difficult to sample and can create sampling and representivity issues. Bulk
leaches or screen fire assays technique may be of consideration.
The ore characterisation is very important. Ores may range from highly weathered oxides through to
fresh primary sulphides, with a transition material in between. Each ore type may require a
significantly different process to treat efficiently.
Comminution deals with crushing and grinding to the target size to liberate or expose the gold so it
can be extracted. Selecting the correct crushing and milling circuit for a particular ore, or range of
ores, is critical for the success of the project.
Leach extraction will determine the overall recovery and success of the project. The use of leaching
enhancers (PbNO3, O2, peroxide) may be required in the presence of some reactive sulphide ore
types to promote the leaching kinetics. Reactive sulphides (arsenopyrite, pyrrhotite, marcasite) can
consume oxygen from the slurry, which is essential for gold leaching reaction.
Once gold has been dissolved it needs to be adsorbed onto carbon before it can be electro won and
smelted into doré bullion. Cyclic or Sequential Carbon Loading is required to determine the loading
capacity and rate. This test can also highlight potential issues with competing ions (Cu, Ni, and Zn)
or fouling potential (organic oxalate and humates) which may impact on the amount of carbon
required to recover that amount of gold leached.
Tailings characterisation is required to maximise the amount of tailings that can be contained in a
tails dam. Sulphide ore types may require acid generation and acid neutralising capacity tests to
ensure acid mine drainage issues do not occur within the tailings dam.
Water source is important, especially when using hyper-saline water. The water will buffer at a
lower pH than is optimum for cyanide leaching (typically from 8.5 – 9.0). As well as resulting in high
lime consumption, the formation of HCN in the leaching process requires careful management from
an operational OHS. The high lime usage may also create scale (CaCO3 and CaSO4) formation
which will need to be managed in the circuit.
These important issues, if not addressed at an early stage, may cause critical problems during the
processing of the gold ores being evaluated. Some examples will be presented where issues were
not addressed and problems occurred.