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.