Abstract: The mixer includes a chamber having first and second, side-by-side tub like cavities with first and second rotatable shafts located in the first and second cavities respectively. Each shaft has two ribbon members coupled to the shaft and which extend helically around the shaft along a substantial length of the shaft with the two ribbon members located about 180 degrees apart. The ribbon members of the two shafts do not intermesh but are located close to each other. A drive mechanism is provided for rotating the two shafts in opposite directions to cause the ribbon members of the two shafts to move toward each other below the plane of the two shafts, upward, and away from each other above the plane of the two shafts for mixing the particulate material. The ribbon members of the two shafts cause the particulate material in the two cavities to move in opposite directions along their lengths.