The recovery of fine gold particles from a slurry of minerals using liquid carbon dioxide

A novel liquid–liquid separation for the recovery of free or liberated gold from mixtures of fine particles was evaluated. The separation, conducted at 295 K and carbon dioxide vapor pressure of 6.0 MPa, was based on the difference in wettability of precious metals and minerals. Contact angle results indicated that in the presence of water and liquid CO2, gold was slightly hydrophobic. The addition of CO2-soluble fluoroalkyl or fluoroether thiols increased the hydrophobicity of gold exposed to water and CO2 to a greater extent than alkylthiols. Minerals, including quartz, pyrite and magnetite, were hydrophilic. During the separation an aqueous slurry of fine mineral and gold particles was mixed with liquid carbon dioxide. Fine gold particles, −325 mesh, dispersed in the liquid CO2 or agglomerated at the H2O–CO2 interface. The minerals tended to remain in the aqueous phase. Poor results were obtained, however, when attempts were made to recover gold from an Australian gold-mining gravity concentrate. This was attributed to the gold particles in the concentrate being too large, agglomerated in slimes, or coated with minerals.