1. APPARATUS FOR HANDLING AND CONVEYING SOLID PARTICLE MATERIAL BY ENTRAINMENT IN FLUID, COMPRISING A CONDUIT VESSEL FORMING A FLUID-FLOW SPACE AND HAVING AN INLET AND AN OUTLET FOR THE PASSAGE OF FLUID THROUGH SAID SPACE TO DEFINE THEREIN A PRIMARY FLOW AND A PRIMARY-FLOW AXIS FOR THE FLUID, MEANS FOR PRODUCING IN SAID FLUID WITHIN SAID VESSEL A PRESSURE DIFFERENCE BETWEEN SAID INLET AND OUTLET, AGITATING MEANS FOR IMPARTING TO THE PRIMARY FLOW IN SAID SPACE A CIRCULATORY MOTION ABOUT SAID AXIS, MEANS FOR IMPARTING TO SAID AGITATING MEANS A DIRECTION OF AGITATING ACTION SUBSTANTIALLY TANGENTIAL TO THE PRIMARY FLOW AND INCLINED TOWARD SAID PRIMARY-FLOW AXIS SO THAT SAID AGITATING MEANS SUPERIMPOSE UPON SAID PRIMARY FLOW IN SAID SPACE A ROTATIONAL SECONDARY FLOW FORMING IN SAID SPACE A SINK AND A SOURCE SPACED FROM EACH OTHER ALONG SAID PRIMARY FLOW AXIS, WHEREBY FLUID-ENTRAINED SOLID PARTICLES ARE CAUSED TO CONCENTRATE ALONG THE PRIMARY FLOW DIRECTION ON HELICAL TRAVEL PATHS, DUE TO FLUID-INTERNAL RELATIVE FORCES, AND COLLECTOR MEANS SPACED FROM AND CONNECTED TO SAID OUTLET FOR ACCUMULATION OF THE SOLID PARTICLE MATERIAL CONVEYED.