Process of separating using a rotating screen

Details Process of separating using a rotating screen Process of separating using a rotating screen

1992-07-14

1994-11-22

Dawson, Robert A.

Liquid purification or separation

Processes

Separating

2103801, 210382388, 209 18, 209270, 209273, 209683, B01D 2126

Patent

active

053666399

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF INVENTION

This invention concerns an improved screening process and apparatus for separating coarse particles from a suspension of a particulate solid material in a liquid.
Coarse particles are conveniently separated from a suspension of a particulate solid material in a liquid by means of a screen or sieve which generally consists of a perforated plate, a grid or grating or a mesh material. The mesh may be of metallic or plastics material. When the coarse particles to be separated are relatively small, i.e. in the size range from about 0.040 mm to 1 mm, a wire mesh material is the preferred separating medium.
Especially when the suspension contains a relatively large proportion of solid particles of relatively coarse size it is generally necessary to agitate the screen, for example by applying vibration thereto by means of an electric motor connected to the screen through a suitable reciprocating linkage. Such means increase the cost of the screening operation and lead to more rapid wear and consequent tearing of the screen or sieve. The screening apparatus is also noisy in operation.
Screening processes generally also suffer from the disadvantage that constant supervision is necessary because there is a tendency for the screens to blind or become blocked with solid particles which in turn causes unscreened suspension to overflow from the screens by the route intended for the oversize particles with consequent waste of fine particles.
It is known to provide screening apparatus in the form of a trommel of screening material rotating about substantially horizontal axis, which was intended to overcome the disadvantages associated with a vibrating screening device. Such an apparatus is describd in British Patent No. 2053736. However the apparatus, although quieter and less susceptible to wear and fracture of the separating medium, suffered from blinding of the screen by coarse particles.
A screening apparatus having coaxial conical or frusto-conical screens which are rotatable at different angular velocities and in different directions about a vertical axis is known from European Patent Specification No. 0278124. According to one embodiment described in that Patent specification a mixture of fine and coarse particles is dumped near the apex of an upper conical screen rotating with its apex upward. Particles retained on the screen move to the periphery of this screen and are transferred to a lower frusto-conical screen rotating with its imaginary apex downwards and in the opposite direction. Fine particles pass through the two rotating screening surfaces while the coarse particles are eventually discharged over the peripheral rim of the lower frusto-conical screen. The apparatus is designed for dry separations, for example for separating dust or ultrafine particles from cattle feed pellets [column 1, lines 46-49].
A screening apparatus having at least one circular screening member rotating in its own plane about a central axis is known from British Patent specification No. 2088750. However in that case the screening member consists of an array of radially extending bars or rods which together make up a grating and the apparatus is for dry separation of relatively coarse solid material such as coal.
Rotary screening apparatus is also known from U.S. Pat. No. 3,061,095, which shows a screen formed as a shallow cone having a vertical rim around its periphery. German Patent 41604 has rotary screens which are planar and provided with an annular rim. In each of the above the peripheral speed of the screens needs to be such as to give a centrifugal force capable of driving the coarse particles of the suspension upwardly and over the edge of the rim of the screen.
German 0LS2018677 discloses a rotary screen whose surface extends upwardly towards its periphery. Similarly, the peripheral speed of the screen needs to be sufficiently high to drive the coarse particles upwardly across the surface of the screen and over its edge.
In the above three patents the rotational speed necessary to drive the coarse pa

REFERENCES:
patent: 931280 (1909-08-01), Cunningham
patent: 1146932 (1915-07-01), Fowler
patent: 3061095 (1962-10-01), O'Malley

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