Pourable particulate material is accommodated in a vessel. A stream of gaseous fluid is directed into the lower end of the vessel and is split up into a high-speed first flow and a second flow. The material surrounding the bottom end of a central upright unobstructed passage in the vessel is entrained by the first flow and conveyed upwardly to be deposited on top of the body of material in the vessel; since new material from the body slides down about the lower end to be entrained in this manner, the body of material is gradually turned over in the vessel. The lower-speed second flow is directed through the body of material outside the passage to trickle through the material and subject the same to a treatment.