Abstract: A large-volume reactor (10) for treating a free-flowing material, having a housing (12) which surrounds an interior space (14), a material feed (11) at one end of the housing (12) and a material discharge (13) at the other end of the housing (12). A rotor shaft (22), which is driven in rotation about its axis and conveys the material in the direction from the material feed (11) to the material discharge (13), is arranged in the interior space (14). A partition (16) in the interior space (14) defines two separate process spaces (18′, 18″) from one another, and the partition (16) includes a build-up system (24) having a build-up system passage (26) which leads from an inlet opening (40) which opens out into the first process space (18′), to an outlet opening (42) which opens out into the second process space (18″).