Gravity_Concentration-Ch_21.4[1] - 21.4 GRAVITY...

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21.4 GRAVITY CONCENTRATION AND HEAVY MEDIUM SEPARATION A/Professor Nimal Subasinghe (Chapter written for SME Mining Engineering Handbook- 2009 Edition) Gravity concentration is one of the oldest processes of separating minerals and is aimed at separating heavy minerals from lighter gangue. Gravity concentrators are simple devices that are cheap to operate and cause comparatively much less environmental pollution as they do not use harmful chemical reagents. Gravity separation may be achieved in either of two ways: Absolute and Relative Separation. In relative gravity separation , all particles settle in the same direction within the device and large differences in specific gravity (density) between the minerals are needed for effective separation. As settling velocity of particles depend not only on density but also on size, it is important to prepare narrowly sized feeds to these devices. However, as particles of finer sizes settle under gravity at much slower rates the effectiveness of these devices decrease considerably below about 50 microns. The heavy and light minerals which settle at different rates are made to report to different exit streams by applying a transverse force. There are many devices of this nature, such as , jigs, spiral concentrators, wilfley tables (shaking tables), air tables, gemini tables, pinched sluices, riffled sluices, reichert cones , etc. To improve the efficiency of separation of finer material centrifugal concentrating devices have been introduced within the last two decades, such as, Knelson concentrators, Falcon concentrators, Kelsey jigs and water-only cyclones . These devices use centrifugal acceleration to enhance the movement of finer particles and have been extensively used in the gold processing plants to recover fine gold. In a bsolute gravity separation, a fluid or a non-settling slurry comprising fine solid particles with a specific gravity intermediate to those of the two minerals to be separated is used as the medium of separation. Particles of higher and lower specific gravities than the medium move in opposite directions . This technique requires only a small difference in specific gravities of the minerals for effective separation. Heavy or Dense medium separation methods are absolute separation methods. Devices of this sort include dense medium vessels and dense medium cyclones. Gravity concentrators have wide applications in the heavy mineral industry for the recovery of heavy minerals such as ilmenite, rutile, leucoxene, zircon from the lighter gangue. They have been used in the past to exploit placer gold deposits over many centuries. When the feed material comprise of particles with a wider size distribution, gravity concentrators do not produce a clean concentrate and have only been used mainly for pre-concentration or as a rougher stage . The
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Gravity_Concentration-Ch_21.4[1] - 21.4 GRAVITY...

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