Apparatus (1) and process for treating particulate material or powder (33) of a size capable of being fluidized in a retort (31) mounted for rotation on a pair of end axles (18, 41). Retort (31) is mounted on a tilt frame (5) for tilting movement in a vertical plane. Gas conduits (18A, 18B) are mounted within an axle (18) for the supply and exhaust of gas for retort (31). A conduit (55) mounted within the other axle (41) permits particulate material to be passed into or out of the retort (31) as shown in FIG. 1B. A removable injection assembly (90, FIG. 10) is utilized for the injection of additional particulate material. A removable sampling assembly (95, FIG. 11) is utilized for removing a sample of the particulate material from the retort (31). As the retort (31) is rotated, particles of the particulate material are constantly intermingled with each other and the walls of the retort (31). Microwave energy as shown in FIGS. 13-14 may be utilized to heat or dry materials within the retort (31). A plenum (105H) and a permeable membrane (105R) may be provided in an end wall (105B) of a retort (105A) as shown in FIG. 17.