Gold gravity concentration by Knelson Concentrator

Gold recovery by gravity concentrators is the most cost effective and environmentally friendly method available. Particles are classifi ed based on their specific density difference in gravity concentration. Gold has a specific gravity of 15.0 – 19.3, and a typical ore usually has a specific gravity of about 2.6 (e.g. quartz is 2.7, Pyrite 5.0, Magnetite 5.3 and Feldspar 2.6). Only free gold particles can be recovered by gravity concentration, particles should be liberated by crushing and grinding before gravity processing. Sluices, jigs, spirals and tables have been used for a long time for free gold recovery from placer or free milling ores in the past.

Knelson centrifugal gravity concentrator is an effective machine to obtain better metallurgical performance than other wet gravity concentrators due to its ability to recover coarse and fine gold from primary and placer deposits. Knelson separator has a concentration cone carrying the water injection holes inside the rings that rotates about a vertical axis and develops an enhanced gravitational force. Process water is fed into the cone through the water injection holes and feed slurry is then introduced into the cone(Refer attached figure). The slurry is forced outward and up the cone wall under the influence of centrifugal force. The feed slurry fills the rings and creates a concentrating bed which is fluidized by the water injected through the holes inside the each ring. Heavy particles are retained in the concentrating cone and light particles are drifted out of the cone by process water.

Concentrates are flushed from the cone at the end of a concentration period in batch type Knelson concentrators. This type of concentrator has a limited mass yield capacity. The CVD (continuous variable-discharge) type concentrator was developed recently, CVDs can deliver a continuous stream of
concentrate and this feature makes them useful in the processing of metallic ores. A method for the determination of the gravity recoverable gold content (GRG) of the ores using Knelson concentrators was developed by the researchers. The Knelson concentrators are commonly used in the gold industry due to their ability to recover gold from alluvial and primary ore deposits, old tailings or pre-concentrates. They can be employed in small scale mining operations, gravity processing plants or grinding circuits of gold recovery plants to recover gold or other heavy minerals.

Working principle of Knelson centrifugal concentrator

The application of centrifugal force has proved an effective technique for the recovery of fine heavy minerals in recent years. The centrifugal force acting on a particle can be equal to or even higher than 50 times the force of gravity, significantly increasing the settling rate of a particle. The size of particles that can be captured becomes finer as the intensity of centrifugal forces generated is increased.

The Kneson centrifugal concentrator is an enhanced gravity device that generates a high gradient centrifugal field whereby the deposition and stratification of fine particles occur inside a smooth centrifugal wall. Feed slurry is introduced into a rotating rotor bowl and accelerated by an impeller as it flows on the inner wall of the rotor. The lower part of the rotor is inclined at a slight angle to provide a migration zone, while the cylindrical upper part acts as the retention zone. The strong centrifugal forces, normal to the wall, lead to the hindered settling and stratification of particles in the migration zone. The weak driving force, parallel to the inclined rotor surface, moves the stratified particles up towards the top. The lighter particles, on the outside of the bed, migrate out of the rotor assembly due to their lower specific gravity or small size. Heavy particles are retained in the concentrating (retention) zone where the concentrate is cleaned by fluidization water.

Gold gravity process plant

Gravity separation has been an important method of gold recovery for thousands of years. There is no doubt that it is an economical, pollution-free method, especially when compared to cyanidation. It is still extensively used in the world and has experienced a resurgence in popularity because of the advent of the Knelson Concentrator. It can be expected that gold recovery by gravity separation will continue to play an important role, especially for small and middle sized gold mines.

In most of the gold gravity circuits, primary gold concentrates with 0.2 to 5% gold are produced by Knelson Concentrators or jigs. There is a trend of replacing jigs by Knelson Concentrators. The primary concentrate is accumulated and treated daily by a shaking table to produce a concentrate with a gold content of 40-80% for smelting. The tabling operation is labour intensive, a security risk, and expensive, especially when the primary concentrates are produced with the Knelson Concentrator. Exploiting the use of the Knelson Concentrator as a cleaner to replace or supplement shaking tables promises to be very attractive.

For the third stage (smelting), the high grade of the furnace feed is obtained at the cost of recovery during tabling. Smelting is expensive, cannot separate silver from gold, as well as having environment problems due to the release of sulphur dioxide. Intensive cyanidation is not suitable for many gold mines because of the small quantity of the concentrate produced, typically 1 tonne per day, making its operation difficult to justify. Therefore, chlorination, an alternative which can treat high grade gold gravity concentrates to produce very pure gold and separate silver, is a promising alternative.

In many gold mine sites, for the primary stage of upgrading, there is a trend for jigs to be replaced by the Knelson Concentrator; for the secondary stage, upgrading is likely to remain gravity, but tabling will be supplemented or replaced by Knelson Concentrators.

Main Methods of Gold Ore Processing

1.Gold Gravity
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.

2.Gold Amalgamation
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.

3.Gold flotation
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.

4.Gold Cyanidation
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