Refractory gold evaluation

Refractory gold evaluation testwork services

Refractory gold ores generally contain sulphide minerals, and as such, a primary flotation stage is the obvious choice to concentrate the sulphides into a smaller mass for downstream pre-treatment prior to cyanidation rather than whole of ore processing. Gold sulphide flotation is relatively straightforward and ALS can seamlessly marry this into the testing program via its fully equipped flotation laboratory in conjunction with other process flowsheets.

Once recovered into a smaller mass, the sulphides can be pre-treated via the following processes:

• Ultra-Fine Grinding: In cases such as ore containing pyritic gold, the gold is often finely disseminated in the mineral matrix at sub 10 µm particle size and consequently can be fully encapsulated (i.e. unliberated) at typical free-milling ore grind targets between P₈₀ 75-150 µm. These ore types can benefit from ultra-fine grinding to liberate the gold prior to cyanidation, which is effectively still within the so called “physical liberation” realm. Several technologies including IsaMillTM, Metso Verti-mill and Stirred Media Detritor (SMD), Deswik, and Metprotech are available to grind ores or concentrates as fine as 2-3μm to liberate fine particulate gold.
• Roasting: This traditional refractory gold pre-treatment option uses superheated air to fluidize the ore, thereby causing the sulphides to oxidize, separate from the gold, and volatilize into sulphur dioxide (SO₂). This method is sometimes referred to as a “Chemical Liberation” process and results in a calcine product that is amenable to cyanidation. In recent decades, environmental restrictions on SO₂ emissions have increasingly precluded roasting as a pre-treatment option, but it may hold a place in selected programs. ALS has available batch muffle furnaces, Midrex rotary kiln reactors, and a small-scale fluid bed roasting to investigate the amenability of roasting the ore at bench scale. A Dorr-Oliver continuous 100 mm continuous fluid bed roaster unit is also available for pilot testwork to establish the optimum roasting scheme in terms of temperature and gas composition.
• Low Pressure Cyanidation: Tests are performed at slightly elevated pressures and temperatures with oxygen sparging. This can include the Activox^® process.
• High Pressure Oxidation (HPOx): Refractory ores where gold is contained within sulphides, such as pyrite or arsenopyrite, can be chemically oxidised to enhance gold recovery. Oxidation is carried out in an autoclave at temperatures of 180 – 240° C and at oxygen pressures up to 3000kPa. Batch oxidation tests are conducted in a 3.8 litre Parr autoclave. The pressure leach residue is neutralised by washing with lime and then leached using conventional cyanidation.
• Bacterial Oxidation: Several strains of bacterial have been identified and successfully cultured for contact and oxidation of refractory gold ores. Bacterial Oxidation is a slow and involved process but has been implemented successfully in a number of industrial sized projects. Batch and pilot scale facilities are available.
• Ammoniacal Cyanidation for Gold bearing, Copper-rich Sulphide ores: Ores containing soluble copper can have very high cyanide consumption. Ammoniacal cyanidation can dramatically reduce the overall level of cyanide consumption and ALS can carry out leach tests to optimise reagent conditions. Our expertise covers all aspects of processing copper-rich sulphide ores, from flotation to selective leaching, CIP, elution and fully integrated electrowinning circuits.

Associated testwork services

Related testwork

The above refractory gold metallurgical testwork is usually conducted together with other testwork, such as:

• Comminution
• Gravity recovery
• Free milling investigations
• Heap leaching
• and more

Click on the above links or contact us for further information. We can integrate a full program for you if required.