Page 47 - Handbook of Gold Exploration and Evaluation

28 Handbook of gold exploration and evaluation

good recoveries of gold are reported down to 150 m (Nio, 1988), and some jig
manufacturers suggest 100 m as a realistic cut-off point for fine gold from a
well-classified feed. Early bowl type centrifugal separators used forces up to
60 g to enhance gravity differences between particles. Encouraging gold
separations were made at sizes down to about 38 m under laboratory
conditions and to 100 m from high slime feed materials in processing plants.
However, the finely divided state of the gold is only one consideration
influencing its hydraulic behaviour, account must also be taken of factors other
than that of particle size in gravity gold mineral processing. As discussed in
Chapter 4, hydrophobicity, shape, particle density and texture and the slimes
content of the feed all affect particles settling rates and transportability.

Influence of hydrophobicity
Hydrophobic surfaces have the property of repelling water, i.e., they do not
easily become wetted in contact with water. The phenomenon is due to
unbalanced molecular forces at the water/solids interface causing surface
tension. In the case of gold, the surfaces are variably hydrophobic depending
upon the purity of gold in the surface layer and whether the surface is clean or
coated. Any process such as leaching or plating that increases the purity of the
outer skin increases its hydrophobicity.
Finely divided gold particles, which have high surface area/mass ratios, tend
to float on the surface of water because of surface tension and be lost where
larger particles of gold with smaller surface area/mass ratios sink. The
phenomenon thus poses considerable difficulties in sample dressing unless
reagents are introduced into the water to lower its surface tension. Household
detergents are in common use in most countries for this purpose. In Kalimantan,
Indonesia, the Dyak miners use an extract from the leaves of chilli bushes to
settle the gold. The leaves are pulped in water and after a short time, the panner
steeps his fingers in the solution and then into the water in the panning dish. Any
gold floating on the surface of the water immediately sinks to the bottom.
The problem of fine hydrophobic gold, as dealt with in bench-scale
investigations, may not be as readily solved in a prototype treatment plant where
it is neither practicable nor desirable to add reagents to the process water. No
economic method has yet been devised to scavenge the hydrophobic gold
content of slime material discharged from primary roughing circuits. The
quantities of slimes are very large and desliming is necessary both to reduce the
volume of flow and to provide a classified feed for primary concentration.

1.1.6 Classification of gold ore deposits
Figure 1.10 is a simplified genetic classification scheme for ore deposits
showing the broad categories of processes and some of the common associated