1322323 Separating solid particles and liquid ENVIRONMENTAL PURIFICATION SYSTEMS Inc 10 June 1970 [16 June 1969] 30312/69 Heading B1D A mixture of liquid and solids, such as white water in a paper mill, is divided into two fractions having respectively greater and lower concentrations of solids than the mixture by apparatus wherein the mixture is in contact with at least one moving surface which creates a velocity gradient therein, layers of liquid close to the surface(s) moving therewith and becoming relatively free of solids, while layers away from the surfaces move in the opposite direction and become concentrated in solids, the layers being separately discharged. In Fig. 2 mixture enters by a duct 40 into the spaces between a series of discs 20 mounted on a rotary drum 14, clean liquid being carried with the discs to flow over weir 44, generating a liquid head which causes dirty liquid layers in the middle of the spaces to flow the other way and over weir 42. Both weirs are adjustable, and scrapers 50 and troughs 52, 54 may be included to collect a very clean liquid fraction adhering to the discs, which is separately discharged or flows into trough 48. Fig. 4 (not shown) illustrates a similar construction wherein a solid shaft replaces drum 14. In Figs. 5, 6 (not shown), the discs are replaced by concentric tubes, rotating together in an arcuate vessel with adjustable weirs (118, 120) in its flat wall. In Fig. 8 (not shown), the mixture is delivered into a drum rotating about a horizontal axis, clean liquid being carried up its rising side to one outlet and dirty liquid flowing over a weir adjacent its falling side. In Figs. 9, 10, mixture flows through conduit 170 into the space between two endless moving belts 162 supported by vertical rollers 166, clean liquid passes over weir 174 and dirty liquid over weir 172.