A "refractory" gold ore is an ore that has ultra-fine gold particles disseminated throughout its gold occluded minerals. These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes.  These refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold.  A refractory ore generally contains sulphide minerals, organic carbon, or both.  Sulphide minerals are impermeable minerals that occlude gold particles, making it difficult for the leach solution to form a complex with the gold.  Organic carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon.  This so-called "preg-robbing" carbon is washed away because it is significantly finer than the carbon recovery screens typically used to recover activated carbon.
Pre-treatment options for refractory ores include:
# Roasting
# Bio-oxidation, such as bacterial oxidation
# Pressure oxidation
# Albion process
The refractory ore treatment processes may be preceded by concentration (usually sulphide flotation).  Roasting is used to oxidize both the sulphur and organic carbon at high temperatures using air and/or oxygen.  Bio-oxidation involves the use of bacteria that promote oxidation reactions in an aqueous environment.  Pressure oxidation is an aqueous process for sulphur removal carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures.  The Albion process utilises a combination of ultrafine grinding and atmospheric, auto-thermal, oxidative leaching.