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Dense medium separation relies not only the difference between the specific gravity of the particles but also exploits the variation in the effective SG of a fluid medium. Particles denser than the specific gravity of the medium sink and are collected as a separate product from the ones that float. For example, a dense medium can be a chemical such as tetrabromoethane which has a specific gravity of 2.96 or a dense medium can be made using very fine particles of magnetite or ferrosilicon suspended in water as a slurry. The SG values of the slurries made from magnetite and ferrosilicon can be as high as 2.5 and 3.2 respectively. Chemicals are often used for lab scale separations while dense medium slurries are used more on an industrial scale. Dense medium separators will not be discussed herein. Sluices or sluice boxes are commonly found at alluvial operations for the recovery of liberated placer gold. Sluice boxes with riffles are one of the oldest forms of gravity separation devices used today. The size of sluices range from small, portable aluminum models used for prospecting to large units hundreds of feet long. Sluice boxes can be made out of wood, aluminum, plastic or steel. The riffles in a sluice slow and retard heavy material flowing in the slurry, which forms a material bed that traps heavy particles and creates turbulence. An overhanging lip, known as a Hungarian riffle, increases the turbulence behind the riffle, is commonly used in these units. Other configurations of sluices may use astro-turf, screens or rubber material with ridges. Pinched sluices have also been used for heavy-mineral separations for centuries. In its most basic form, the pinched sluice is an inclined trough 60 to 90 cm long, narrowing from about 24 cm in width at the feed end to 3 cm at the discharge end. Feed consisting of 50-65% solids enters the sluice and stratifies as the particles flow through the sluice and crowds into the narrow discharge area. The crowding causes the bed to dilate allowing heavy minerals to migrate and move along the bottom, while lighter particles are forced to the top. The resulting mineral bands are separated by a splitter at the discharge end. Pinched sluices are simple devices and are inexpensive to build and run. Pinched sluices are mainly used for separation of heavy mineral sands. A large number of pinched sluices are required for a high capacity operation, and a large amount of recirculation pumping is required for proper feed delivery. These drawbacks led to the development and adoption of the Reichert cone in many plants.
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