The oldest miners noted the importance and advantages of gold concentration by gravity. It is important to mention that the earliest type of gravity equipment was developed to at least medieval times and probably by pre-Roman times; it was some form of sluice. Nevertheless, relatively low capacities, coupled with the high water requirement imposed by relative low operation densities, have been long-standing limitations of such equipment.
Probably, the main advantage of gravity concentration is the early recovery of the gold since it is not help up in extended thickening, leaching, filtration and pregnant solution treatment processes. Basically, improved gold recovery resulting from preferential treatment of sulphides and other gold bearing minerals, removal of these minerals has the important benefit that satisfactory cyanidation recoveries can be performed on the remainder ore at a coarser grind than would otherwise be the case. Also, it is important to indicate an improved recovery resulting from the reduced head grade of the ore going to cyanidation, which reduces solution gold losses.
Essentially, there is an improved recovery resulting from the exclusion of coarse gold from the cyanidation circuit. Nevertheless, these advantages are offset by the following disadvantages: increased circuit complexity and increased security risks. Obviously, the metallurgist and plant designer must observe and study how to reduce the complexity of the recovery circuit where the circulating load is usually the critical aspect of many gold operations. It is generally accepted that gravity concentration is the most simple and economical of all methods of gold concentration, allowing the recovery of free gold and gold-bearing minerals at as coarse a size range as possible.
Basically, minerals particles are separated by virtue of the differences in density, the greater the difference in density between two minerals, the more easily they can be separated. Nevertheless, the smaller particles, the more powerful, relative to gravity, are fluid force and slurry viscosity, and the efficiency of gravity separation tends to be lower in the finer size ranges. Gravity separation methods are grouped in three categories, dense medium separation, whereby particles are immersed in a bath containing a medium of intermediate density in order to separate particles, ones sink and other float. The second group is related to separation in vertical currents, the best example is the jig, which takes advantage of different rates of settling. The last group is related to separation in streaming currents or thin film sizing, being the best example shaking tables, spirals and cones.
Many gold operations of the latter half of the nineteenth century used to crush the material in stamp mills followed by recovery of liberated gold particles by passing the slurry leaving the stamp mills over sloping amalgam plates. The gold due to its high density sank through the slurry and was absorbed by mercury. The gold could then be recovered by retorting. Then, the gravity has been used for many years to recover gold and the presence of difficult deposits has been the motivation to develop new devices.