Apparatus for treating solids, in particular mixing, pelletizing or drying, includes a vessel receiving a solid product, and a driveshaft which is arranged in the vessel. Disposed in fixed rotative engagement with the driveshaft is at least one mixing member which rotates in a rotation direction and has two mixing arms. Each mixing arm includes a main vane of generally triangular cross section to define an apex which is directed in the rotation direction of the mixing member.