Much aggregate is won from alluvial deposits laid down by rivers or glaciers. It is possible, although uncommon, that these deposits contain dense minerals of value e.g. ironstone, magnetite, chromite and even precious metals and diamonds--the 'placer deposits'. More usually, the objective is to reject weak porous rock types of typically low density such as chalk and lignite from the gravel. Two density separation processes that treat the relevant range of particle sizes (200-0.Smm) have been applied: jigging and dense medium separation.
Jigging has widespread application to iron ores and coal processing. The mineral particles are transported by water along a trough which is provided with a perforated base often fabricated from screening media . Repeated vertical pulsations of water cause the mineral particles to stratify according to density almost independently of particle size. The upper layer of low density porous mineral or lower layer of denser valuable mineral may be removed as appropriate. This process has been applied to beneficiate sand and gravels.
Dense medium separation employs a fluid of density intermediate between those of the two minerals it is required to separate. It is sometimes referred to as 'float-sink' separation. In place of a true liquid a suspension of a finely powdered dense substance, the 'media', is employed. In the popular processes a magnetic substance is used to permit its ready recovery from water used to wash the 'float' and 'sink' products. For suspension densities less than 2700 kg/m 3 the natural mineral magnetite (Fe304) is used owing to its low cost. Such suspensions will usually be adequate to remove weak, porous rocks as the float product from siliceous and carbonate aggregates. In order to remove ironstones as the 'sink' product a suspension of higher density containing powdered ferrosilicon alloy (FeSi) is necessary. The use of dense media or heavy media separation (HMS) upon aggregates was first reported in Canada in the 1950s during the period of extensive infrastructure construction and has since been applied in the USA, U K and Denmark. DMS has also been applied in Northamptonshire for the recovery of dense ironstone from gravel.