PURPOSE: To have high separation efficiency even for particles of a submicron size by separating a solid particulate-laden stream to a clean stream and the concn. solid particulate-laden stream, by a potential flow separator, then extracting solid particulates from this particulate-laden stream and subjecting the stream contg. the decreased solid particulate to a circulation treatment.

CONSTITUTION: The output stream from a centrifugal recirculating device 13 enters an inlet 23 of the potential flow centrifugal separator 15 through a pipeline 2 and is regulated to the potential flow free of stationary vortexes or secondary flow in the portion 15-1 in the separator 15 and is divided to the clean fluid stream and the concn. solid particulate-laden stream, i.e., polluted stream, in the portion 15-1. This clean stream emerges from an axial outlet 25 and the polluted stream emerges from an outlet 27. The polluted stream is sent from an inlet 29 by a pipeline 28 into a cyclone 17 where solids are collected. The solids are accumulated in a hopper 30 and are removed. Next, the outlet stream of the cyclone 17 of the outlet 21 is sent through a recirculating pipeline 32 from an inlet 33 into an ejector 13 and is mixed with the solid-laden stream inlet flow of the main process for the purpose of recirculation.