Abstract: Particulate material such as sand is classified in two or more classifying devices (13 and 15) which operate on the principle of "hindered settling" each to provide a coarse underflow fraction and a fine overflow fraction. A chosen blend 12 of the underflow fractions is provided by pneumatic control circuitry (FIGS. 2,5,6) which actuates flow control valves (14 and 16) to open, by amounts in accordance with a chosen blend ratio, only when there is sufficient material available for blending.

Abstract: A centrifuge for separating a solids-liquid mixture comprises a rotatable drum (10) having an inner surface of revolution about the rotary axis of the drum, a solids discharge outlet (23) at one end of the drum and a liquid discharge outlet (26) at the other end of the drum. Means (32,37) are provided for feeding the solids-liquid mixture to a region adjacent the inner surface of the drum and a solids material conveyor (28) is arranged within the drum for rotation about the rotary axis of the drum at a speed slightly different from the speed of rotation of the drum. The conveyor (28) has a distal working surface formed about the rotary axis adjacent the inner surface of the drum and adapted to engage liquid-reduced solids material during rotation of the drum and the conveyor to convey it to the solids discharge outlet, while solids-reduced liquid discharges at the liquid discharge outlet at the other end of the drum.