Axial mixing in a ploughshare mixer

Motion is studied within a bladed 5 l ploughshare mixer for granular solids at two rotor frequencies, 2 Hz and 4 Hz, for fill levels between 12.5% and 70%. Positron emission particle tracking is used to follow the motion of a single radioactive tracer. Particles circulate around the bed in the transaxial plane in the direction of blade rotation. Axial motion perpendicular to this plane is significantly less and is largely contained in the region between the planes of rotation of adjacent blades. Axial mixing between these inter-plough regions is determined by calculating the transport rate across these planes. Transport is greater at 4 Hz than at 2 Hz, but decreases as fill level increases. Axial transport is further studied by recording the location of the tracer as it crosses each plane of blade rotation. These passages can be described as occurring either; (i), in defined regions of the bed; or (ii), by leading or lagging the blades by a constant angle. A transition from (i) to (ii) occurs as fill level increases. At 4 Hz or with oily rice, the transition occurs at lower fill levels.