Because of its high specific density and its natural occurrence in ancient rimes, gold was mainly recovered by gravity separation from placer-type deposits. Gravity recovery is still extensively used, and has in fact experienced a resurgence in popularity due to the advent of the Knelson Concentrator
What are the advantages of applying gravity concentration in gold recovery? By removing gold from the pulp circulating through the mills, the amount of gold locked up behind the mill liners is considerably reduced; selective grinding can be applied to the pyritic concentrate to ensure the release of enclosed gold particles without over-grinding; the reduced gold content of the cyanidation feed (by approximately 50%) results in a lower concentration in the cyanide solution, which has certain benefits in filtration and precipitation. The loss of gold-bearing solution as a result of imperfect washing on the filters (0.3-0.5%) is diminished. Fine particles of osmiridium are also recovered when present. There is no doubt that gravity concentration is an economical, pollution-free method compared to other methods, especially amalgamation and cyanidation.
Final gold gravity concentrates are generally smelted to remove base metals and
other impurities in order to produce a gold-silver bullion containing typically more than 95% precious metals. The smelted product is suitable for direct sale and/or for further refining.
If the surface of a gold particle is clan. it can be trapped by clean mercury. Amalgamable gold is said to be “free milling”, but this is incorrect terminology since free milling traditionally refers to cyanidation recovery, which is normally much higher than that of amalgamation. Amalgamation is a concentrating process in which metallic gold or silver, or an alloy of the two, passes preferentially across a water mercury interface, after which the metal-laden mercury (amalgam) is obtained.
Various amalgamation units can be used: plates, inclined at 8 degree, either stationary, or oscillating with a small amplitude, and coated with silver; pocket amalgamators (liquid and amalgams are held in a pocket onto which the pulp to be amalgamated flows); and grinding amalgamators, which are used for the ores in which the gold is difficult to amalgamate, because it is either very fine, or locked with other minerals. The usual apparatus is the amalgamating barrel. Gold particles in the size range of 0.1-1 mm (-0.03 mm particles are easily floated away), a pulp density of 10 to 25 % solids would make the plate amalgamation more effective.
An amalgam which contains significant concentrations of mercury must be treated prior to smelting to gold bullion. Retorts are operated under a slightly negative pressure and mercury vapour is usually fumed into a water condensation system. The vapour is cooled rapidly to below the boiling point (375°C) and the liquid mercury is collected under water to avoid re-evaporation. Retorting of amalgams yields a mercury free product in the form of “sponge’ gold, which can be treated directly by smelting for the removal of residual base metal impurities. Mercury is highly toxic and has a cumulative physiological effect; therefore,
amalgamation has not been tolerated in many countries.
Gold has a good natural floatability. In the remote past (fifth century B.C.), gold grains were separated from sands by means of goose feathers coated with grease. The gold grains attaching to the surface of the feathers.
In most base metal mineral processing plants, gold is generally floated into copper and lead concentrates, and is recovered during metallurgical processing as a by-product. This is not the case for zinc concentrates, which clearly creates an incentive for gravity recovery ahead of zinc flotation circuits (even when a copper concentrate is first produced, some GRG reports to the zinc circuit). For gold that is associated with pyrite, a gold-bearing pyrite concentrate can be produced by flotation. The concentrate is then either reground or pressure-oxidized and then cyanided. Gold flotation can also be used as a supplement of gravity for gold fines and flakes easily lost during gravity separation.
However, by far the most common interface between gold flotation and gravity is the use of gravity recovery in grinding circuits ahead of flotation in order to increase overall gold recovery and to recover it into a higher-value product (payment in excess of 99% for gold bullion, as opposed to 92-95% for gold recovered into flotation concentrates).
Flash flotation. with a retention time of 1-3 minutes, improved the recovery of 30-100 μm gold particles from cyclone underflow or the discharge of a ball mill. There was significant coarse liberated gold in the circulating load of a ball mill that might not be readily floated. Recovering gold from the cyclone underflow by the installation of the Knelson Concentrator improved the recovery of coarse gold particles ahead flotation circuits.
Gold cyanidation has been the most important method of gold extraction from gold ores since its first commercial application in 1889. The cyanidation process was responsible for doubling gold production in the world in two decades