In the case of production sand and gravel deposits in aggregate crushing plant, the content of silt and clay may be so high as to cause difficulty in handling, screening or crushing. Consolidated 'hard rocks' such as limestones and gritstones may become intermixed with associated clay bands during the extraction operations. An effective treatment is to disintegrate this material by agitation with water to disperse the clays which may be ultimately removed by classification. Two pieces of equipment are commonly employed for this purpose: the log-washer and the washing barrel or scrubber.
The log-washer comprises a shallowly inclined trough containing one or two slowly rotating shafts fitted with paddles, flights or tynes. Material and water is fed into the lower part of the trough and powerfully mixed. Dispersed clay and water overflows the trough whilst coarse, cleaned aggregate is transported against the incline of the trough until discharged. The shafts were originally made from logs, hence the name, and the device is generally associated with high clay contents.
The underflow of these screens passes to a classifier which will separate the clays from the fine aggregate which will then often be suitable for direct sale as fine concrete aggregate. Alternatively, a second classifier is employed to divide the sand into coarser and finer fractions often called 'sharp' and 'soft' sand respectively. This classifier achieves a separation between the coarser, denser and more rounded mineral particles directed to the underflow or sand product and the finer, less dense and flaky particles contained in the overflow. Thus, the underflow can be a finished coarse sand product comprising competent mineral particles without impurity. The overflow, however, may comprise a mixed suspension of fine, but competent, sand particles and larger particles of weak, porous and lower density minerals or those exhibiting very flaky shape. This is especially the case where the deposit contains lignite, shale and coal which will tend to contaminate the fine sand in this manner. Fortunately these larger undesirable particles can be relatively easily removed by screening the overflow through sieves of narrow aperture (less than 1ram) such as sieve bends to produce a clean fine sand.