Mineralogical parameters affecting the recovery of gold grains by gravity are in order of decreasing importance:
-Degree of liberation
-Mineral density differential
Fine free gold, which is too small to be recovered efficiently, depends on the installed equipment: 500 μm for sluices; 200 μm for jigs, 50–100 μm for spirals, 50 μm for shaking tables and 20–40 μm for centrifugal concentrators.
Association of gold with other minerals and in particular locking of fine gold grains in quartz and other “light” gangue minerals reduces the average particle density. As an example a quartz particle with 14% (w/w) native gold has an average density close to that of pyrite (5 g/cm3). Since gold metal is very ductile and malleable, free gold grains become flattened, instead of breaking up during grinding and milling, and this will be more pronounced with larger gold grains.
The benefits of gravity recovery can be readily assessed by undertaking leach tests with and without pre-treatment coupled with mineralogical analysis and examination of gold leach tails solids. Early equipment included shaking tables, spirals, drums and other devices. The alternatives expanded to more sophisticated and efficient centrifugal separators such as Knelson and Falcon concentrators with the latter perhaps being more applicable to the treatment of finer solids. In recent times, a further range of gravity equipment has been successfully commercialized including in-line pressure jigs. equipment development has also extended to the use of high-intensity cyanidation devices to solubilize gold from the concentrates.