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The simplest form of size classification is the hand-held test sieve, fitted with a piece of precise wire mesh, and used to classify a sample of solids into two ranges of particle size – above and below the aperture of the mesh. These are used in the dry and shaken carefully and sufficiently to achieve the required size separation.

At the other extreme are the huge stationary screens (grizzlies), mounted horizontally or with a slight incline, and used in mineral processing, or coal or aggregate grading plants. These are made from sets of thick metal bars, usually fixed parallel to one another, onto which the solids to be sized will flow, either in the dry state or carried in a stream of liquid, usually water.

An important stationary, wet classifier, which can achieve a very sharp classification, is the sieve bend (originally known as the DSM screen). This, like many other screens, is made from wedge-wire bars, welded onto support bars, so that the
wider flat face of each bar adjoins the corresponding faces of the bars on either side of it, with a small aperture left between the bars. If a structure like this were to be rolled up into a cylinder, with the wedge-wire bars parallel to the axis, and the
controlled apertures on the inside of the curve, an excellent screen is formed (and is so used, for example, in the Contra-shear and well screens described above, and in centrifugal filters). The sieve bend takes an arc of this drum, usually less than a
quarter of the circumference, and mounts it with the bars horizontal, and the upper part vertical. The screen accordingly bends away from the vertical the further down its structure. A slurry is caused to flow downwards on the inside of the top of the screen and thus tangential to it. As the slurry flows down, the liquid and the finer solids flow out through the apertures, and the oversize solids flow over the end of the screen.

There is no doubt that some kind of movement of the solids being separated, in a direction parallel to the screen surface, is beneficial to the screening process. This is partly so that a sharp contact with other particles will break up any loose agglomerates among the feed, and partly so that a specific undersize particle is given every screens by vibration (movement back and forth in one direction) and gyration (a two dimensional movement, basically circular).