THE CONTINUOUS MIXING OF PARTICULATE SOLIDS surface due to free-flight segregation. It is also possible that due to difference in the frictional property of salt and sand, the latter is carried by the wall of I'0 1'5 :-0 Reduced time, t R Symbol Wt. of tracer • $.5 ---0-- 7-5 • 30,0 Figure 4 Effect of amount of tracer on residence time distribution, (C•urves) the drum to a greater height than the former, giving a predominance of sand in the few top layers in the cascading surface. As a result, at low flow rates, salt particles are more likely to follow a cascading path which passes below the level of the outlet weir and consequently build up near the outlet. At higher flow rates, on the other hand, it has been observed that an appreciable depth of the bed of material in the drum takes part in the out- flow and though in this case also a similar surface separation is likely to occur this does not result in the build up of salt at the outlet. The possibility that salt is carried by the drum wall to a lower height and consequently has a lower cascading path has been examined by carrying out another set of experiments using sand and salt separately as feed material. For otherwise identical conditions the outlet as viewed from the end for the two systems

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