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|INTRODUCTION Dense medium processes have been used to separate coal and ores for over a hundred years. In fact Sir Henry Bessemer first patented a process in 1858. In recent years, their importance relative to other gravity dependent processes has increased. This is particularly true in the coal industry where over 60% of all preparation plants operating in 1984 in North America (1) contained either dense medium vessel or cyclone circuits. In addition, the stringent product quality demands of the consumer, together with increases in mining costs, dictate that the cleaning circuits be both predictable and efficient. Dense medium systems are the most predictable and efficient of all gravity- dependent processes. The majority of the text of this chapter will be concerned with dense medium applications as they relate to the coal industry. However, the techniques and calculation procedures can be applied, in general, to all non-coal minerals (such as diamonds, marginal iron and lead-zinc ores and gravel) which take advantage of a difference in specific gravity between the saleable product and the reject. The object in dense medium cleaning is to produce a liquid whose effective specific gravity is between that of the concentrate and the reject. In which case the lighter product will float to the surface and the heavier product will sink to the bottom. Unfortunately, this simple process becomes complicated by the following practical necessities: (a) collecting and removing the float and sink products from the medium without disturbing the free settling effects; (b) maintaining and controlling the specific gravity of the medium; (c) separating and reclaiming the medium once the products have left the separating vessel; and (d) separating finer sized particles which take longer to fall through the dense, and often viscous, medium. As a result of the last issue (item d) two main types of dense medium separators have evolved: dense medium vessels and dense medium cyclones. Dense medium vessels are essentially large baths through which the dense medium flows at a relatively slow velocity. The lighter products float to the surface and are carried out of the vessel by the medium flow. The heavier products sink to the bottom of the vessel and are physically transported out either by means of a conveyor (usually a chain conveyor) or a rotating, lifting arrangement (Figure 1). They generally|