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processing | Continuous and batch mixers Right: The CPeX labora-


tory continuous mixing system from Farrel


Pomini is said


to be designed for develop- ment of


challenging product


formulations


initiative into processing of fine powders. The project employs Positron Emission Particle Tracking (PEPT) techniques to provide insights into continuous mixing. The Ajax mixer uses intermeshing screws within a


profiled casing but, rather than flights or kneading elements in a compounding extruder, the screw arrangement consists of a series of paddles adjusted to resemble a discontinuous double helix. Ajax says the mixer offers a great deal of flexibility in the degree of mixing through changes to the operating parameters as well as the screw design, which can incorporate ribbons and various custom profile paddles. Changes can also be made to paddle frictional characteristics and angles. PEPT involves the use of radioactive tracers of a similar size and density to the powder particles being passed through the system. These can be detected in real time by radiation sensors. Trials were carried out using a dry powder while two variables (feed rate and screw rotation speed) were set at either high or low levels. The radioactive particles were recycled through the system multiple times to achieve between 80-100 passes per experiment in order to capture the various particle paths that may be taken during transit through the system. “The PEPT technique has given great insight into the


twin screw mixer’s operation, by showing how powder particles travel through the mixer,” says Ajax Technical Director Eddie McGee. Figure 1 shows a three dimen- sional plot of one of the tracer particles within the high feed/ low screw speed regime (the colour of the path changes with respect to time spent in the system). Analysis of hundreds of such particle paths shows, among other things, that particle pathways for all regimes flowed mainly in an axial fashion, with no backflow being observed. Particles stayed loosely bound to the motions of paddles on a particular shaft, usually


Figure 1: This 3D plot was produced using Positron Emission Particle Tracking to show particle paths in a continuous powder mixing system from Ajax Equipment


as they flowed smoothly with the bulk. When struck by the paddle, the particles were lifted across the centre of the z axis, and subsequently influenced by the other shaft’s paddle motions. In addition, McGee says, moving from shaft to shaft


“illustrates a far-reaching radial effect which differenti- ates the twin screw from single screw mixing.” He says that in processes where granular materials and liquids need to be combined, the twin screw provides both squashing and shear of the bulk, “which encourages good combining and dispersion of the ingredients.” The tests also proved that particle motion is much


slower at the walls of the mixer due to frictional forces; that there is an even spread of particle journey lengths between a minimum and maximum time (no two particle journeys were ever exactly the same); and that the average residence time of the particles could be decreased by increasing either the powder feed rate or screw speed, or both—indicating that axial flow is more efficient when there is more bulk solid for the paddles to push against.


“Mixing was also observed to be excellent under


extremely starved conditions, indicating the machine may be operated at wide degrees of channel fill, without drastically affecting product quality overall,” McGee notes.


Click on the links for more information: ❙ www.mti-mixer.de/enwww.zeppelin-systems.comwww.cacciaengineering.comwww.plasmec.itwww.promixon.comwww.battaggion.comwww.hf-mixinggroup.comwww.farrel-pomini.comwww.ajax.co.uk


62 COMPOUNDING WORLD | November 2016 www.compoundingworld.com


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