Abstract: A system for mixing and refining pulp in which the raw material is conveyed into a cylindrical drum and propelled therethrough in linear/spiral direction by a co-axial rotor comprising a plurality of mixing/refining elements. The raw material is mixed and refined during the relative rotation between the rotor and the drum in a plurality of co-axially extending grinding areas defined between rotatable grinding elements on the rotor and a facing grinding surface on the interior of the drum while being simultaneously compressed therein and alternately decompressed in areas between the grinding areas. The rotatable elements are adjustably mounted to the rotor so as to vary the spacing between the grinding surface on the interior of the drum and a rotatable grinding surface on the grinding elements, while simultaneously adjusting the material feeding angle of the grinding areas.