Abstract: Apparatus for drying a wet material such as sewage sludge comprises a drum (11) mounted for revolution about a horizontal axis and containing interconnected inner (22) and outer (21) concentric spirals containing a continuously-recirculatable charge of particulate heat exchange/pulverizing medium in each coil of the spirals. Sludge is introduced at one end and mixed with the heated charge; the charge and sludge traverse the length of the drum together through the outer spiral while drying takes place; the dried material is removed at the other end and the charge transferred to the inner spiral for transmission to the first end while re-heating takes place by ducted heated air.