Dense medium separation or Heavy medium separation (HMS), or the sink and float processis applied to the pre-concentration of minerals (including Coal), i.e, the rejection of gangue prior to grinding for final liberation. It is also used in coal preparation to produce a commercially gradedend-product, clean coal being separated from the heavier shale or high ash Coal.

Dense medium separation is applicable to any ore in which, after a Suitable degree of liberation by crushing, there is enough difference in specific gravity between the particles to separate those which will repay the cost of further treatment from those which will not. The process is most widely applied when the density difference occurs at a coarse paticle size, as separation efficiency decreases with size due to the slower rate of settling of the particles.


Particles should preferably be larger than about 3mm in diameter, in which case separation can be effective ona difference in specific gravity of 0.1 or less, explains Mr. Padmesh Gupta. The process offers some advantages over other gravity processes. It has the ability to make sharp Separation at any required density, with a high degree of efficiency even in the presence of high percentage of near-density material (NGM).


The density of separation can be closely controlled, within a relative density of +-0.005 Kg/ltr and can be maintained, under normal conditions, for indefinite periods. The separating density can, however, be changed at will and fairly quickly, to meet varying requirements. The process is, however, rather expensive, mainly due to the ancillary equipment needed to clean the medium, elaborates Mr. Padmesh Gupta.


Heavy liquids have wide use in the laboratory for the appraisal of gravity-separation techniques on ores. Heavy liquids testing may be performed to determine the feasi-bility of dense medium separation on a particular ore, and to determine the economic separating density, or it may be used to assess the efficiency of an existing dense medium circuit by carrying out tests on the sink and float products. The aim is to separate the ore sample into a series of fractions according to density, establishing the relationship between the high and low specific gravity materials.


Tetrabromoethane (TBE),having a specific gravity of 2.96, is commonly used and may be Diluted with white spirit or carbon tetrachloride (sp. Gr.1.58) to give a range of density Below 2.96 .Bromoform ( 2.89) may be mixed with carbon tetrachloride to give densities In the range 1.58- 2.89  The most widely used medium for metalliferous ores is now ferrosilicon, while magnetite is used in coal preparation. Recovery of medium in both cases is by magnetic separation. Magnetite (sp. Gr 5.1) is relatively cheap, and is used to maintain bath densities of up to 2.5 Kg/Ltr.




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