Centrifugal concentrator

Centrifugal concentrators were originally developed to improve gold recovery from alluvial sands. Since about 1990, they have been used increasingly for hard rock mines, which is now the largest area of application. Centrifugal concentrators consist of a vertical rotation bowl with a series of concentric rings that trap the gold. A centrifugal force is applied on the ore particles, in such a way that this force is 60 (in the case of Knelson) to 300 (in the case of Falcon) times higher than the gravitational force. The rotor is accelerated and feed slurry is introduced to the concentrating cone through a stationary feed tube. Upon reaching the deflector pad at the bottom of the cone, the slurry is driven outward to the cone wall by the centrifugal acceleration. As slurry flows up the cone wall, the solids fill each ring to capacity creating the concentrating bed. The tailings product overflows the bowl and the gold becomes trapped in the rings. Some centrifuges have a smooth wall at the bottom of the bowl where stratification takes place. The high density gold is concentrated at the wall forcing and displacing lower density particles away from the wall. Compaction of the bed in the rings can be prevented by introducing pressurized fluidization water from behind the rings. This helps the high-density gold particles displace the lower density gangue particles causing the gold grade to increase in the concentrating rings with time. After a period of time, the feed is stopped and the rotor is shut off. The concentrate is flushed from the cone into the concentrate launder and can be upgraded further by panning.
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Gold Ore Gravity Concentration by sluice

Gravity concentration introduction

Gravity concentration is a process to concentrate the gold mineral of interest using the difference of specific gravity of gold and gangue minerals. The specific gravity of gold is 19.5 and the specific gravity of quartz (the common gangue mineral associated with gold) is 2.65 (i.e., gravity concentration works because gold is heavy, and quartz is light). Often gravity separation methods are confused with size classification because large particles of light minerals can behave like a small particle of a heavy mineral. The most effective gravity separation processes occur when applied to ore particles of about the same size. The most important factor for a successful gravity separation is liberation of the gold particles from the gangue minerals. It is not easy to establish the degree of liberation of low-grade minerals such as gold. Classical microscopy of screened fractions to establish mineral liberation is unreliable with gold ores. The most recommended method to establish the optimum gold liberation size is grinding for different times (or grain size distributions) and applying gravity concentration to the ground products. This is a classical and important procedure to recommend any type of gravity concentration process. Because most artisanal miners do not classify (screen) the crushed/ground material (i.e. they work in open circuit), their chances to improve gold recovery are very limited.
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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.