The dryer comprises a casing having a longitudinal axis, a mixing shaft, a circular plate that seals an opening made in one of the end walls of the casing, and a frame supporting the casing and the shaft. In the edge portion of the opening closest to the chamber a first circumferential groove is made which is formed by a first radial cavity and by a second radial cavity deeper than the first, joined by an axial portion. An annular sealing element made of a non-stick material, having a cross-section comprising at least one axial strip and one radial strip, is inserted into the first groove in such a way that the radial strip is inserted into the second cavity and the axial strip occupies the axial portion and the mouth of the first radial cavity. A first sealing ring comprising at least one core of elastomeric material is inserted into the first cavity so as to radially compress the axial strip against the plate and a second sealing ring comprising at least one core of elastomeric material is inserted into the second cavity so as axially to compress the radial strip.