Old foundry sand in a container (1) is transformed into a fluid bed (4) by means of air blown through the bottom plate (3). Air from a nozzle (5) below this plate accelerates sand into a blow pipe (6) and further against the inner surface of an impact dome (7). The local fluidisation degree is adjusted so that a region (A) of sand with reduced fluidisation (almost in its natural state as a runny granular material) is formed around the blow pipe. The appts. has a means for allowing the geometrical dimensions of the region (A) (and consequently the loading of the blow pipe) to be adjusted.
