Flotation provides a number of process alternatives for gold ores containing readily floatable minerals, summarized as follows:
- Flotation of the free gold and gold bearing sulphides to produce a gold rich concentrate, the concentrate can be treated by cyanidation, regrinding and cyanidation, intensive cyanidation, oxidative pretreatment and cyanidation or direct smelting.
- Flotation of free gold sulphides to produce a sulphide-free tail for cyanidation.
- Flotation of carbonaceous material, carbonates or other material that otherwise could interfere with the processing.
- Differential flotation, e.g. separation of gold, gold bearing pyrite and arsenopyrite and pyrite.
Free metallic gold can be recovered very effectively by flotation, although more commonly it is recovered together with sulphide minerals, where gold is intimately associated with the sulphides as fine unliberated grains (solid solutions or inclusions), or occurs with barren, hydrophobic sulphides. The most common gold bearing sulphides are pyrite, arsenopyrite and to a lesser extent pyrrhotite.
The flotation of gold from sulphide-free ores containing very low concentrations of free gold is difficult due to the low mass of material reporting to the concentrate and the high density of gold (s.g.19.3). For example 5x10-3% (50 gr/t) Au would be a very high gold ore grade, compared with grades of above 0.5% for Cu, Pb or Zn ores typically treated by flotation. This result in very poor froth stability and decrease recovery and concentrate grade. Despite these factors, close to 100% recovery of free gold has been achieved by flotation under optimized conditions in some applications, with concentration ratios of 30-300 achieved.
In placer or gravity concentrate treatment most the gangue minerals are oxides and silicates which are hydrophilic, and strong collectors can be used to maximize gold recovery with little concern for co-recovery of sulphides. This type of flotation is rare but has been proposed for low grade ores where gold is too fine to be recovered by gravity concentration. Thus, conditions can be selected solely for gold recovery and not to optimize selectivity by decreasing sulphide recovery.