Abstract: A helical blade is provided for a mixing or conveying apparatus which can be used for heating or cooling or in general effecting a thermal treatment such as drying, evaporating calcining or crystallizing a product fed to the apparatus. The blade comprises a pair of pressed ring shaped discs welded together. One disc is shaped with its inner margin curved into or nearly into the axial direction, and the other disc is welded to it at its inner and outer margins. The blade may be welded directly to a shaft, mounted freely thereon or welded to blocks on the shaft to provided spaces between the flights of the blade and the shaft.