Most methods of water-based gold extraction are dependent on gold’s lack of buoyancy to work. However, froth flotation uses a different property of gold to separate it from ore.
Froth flotation is a method for separating hydrophobic materials (which will not mix with water) from hydrophilic materials (which readily mix with water). Since its discovery, froth flotation has been a major component for the processing of sulfide-rich ores, including copper, coal, and refractory gold. In gold mining, froth flotation is used to separate gold from gangue by utilizing gold’s hydrophobicity.
First, refractory gold ore is ground as finely as possible – usually so the individual grains are much less that 100 micrometers. The ground up ore is mixed with water to create a slurry, which is mixed with surfactants to increase the gold’s hydrophobicity. This mixture is placed into a tank or flotation cell filled with distilled water. Air bubbles are pumped into the flotation cell and the water is agitated.
Because of the surfactant, the gold is repelled by the water into the air bubbles, which rise to the top – creating a bubbly gold foam. The froth is removed from the top of the cell creating concentrated gold, while the flotation tailings at the bottom of the tank are disposed of. Thanks to froth flotation, miners and refiners are able to extract quality minerals from ore at lower grades than has been previously possible.