Effect of physical properties of fine powders on their mixing in a rotating drum

Abstract

Mixing of powders is an essential process used in the manufacturing of many industrial products such as pharmaceutical tablets, fertilizer blends, food products, animal/ fish feeds, metal-matrix composites, plastic parts, ceramic components, etc. The quality of these products depends on the mechanism of mixing of the ingredient powders. The uniform mixing is crucial to ensure the desired biochemical or mechanical properties in the products. In this work five samples of fine powders of food items have been experimented to investigate their flow and mixing behavior in a rotating drum. The five powders are rice, corn, flour, mustard and pulse – listed in descending order of their bulk densities. The characterization of the powder particles was done by measuring their physical properties, such as, particle size, particle shape, bulk density, static angle of repose and dynamic angle of repose. It was found that the powder particles with rough surface have strong tendency to agglomerate. Examples are rice and mustard powders. They have low static angle of repose. That means they have more tendency to flow. However, upper dynamic angle of repose for these powders are higher than others. This may be linked to their agglomeration tendencies. Due to strong agglomeration tendency among these particles they take longer time to avalanche. In other words, upper dynamic angle is increased by the longer time the rice and mustard powders need to overcome the higher force due to agglomeration. Mixing behavior was investigated by mixing two powders in the rotary drum in horizontal layers. Powder loading order was varied: once the mustard is put on the top and the next time it was put in the bottom of the drum. In addition, experiments were done at three fill level of the drum: 25%, 50% and 75%. The performance of mixing of two powders in the rotary drum was evaluated by determining the mixing time and the mixing efficiency. It was found that both the mixing time and mixing efficiency are affected by the bulk density and the angle of repose of the individual powders. It was found that quick mixing is achieved if the heavier powder is loaded on top of the lighter powder. In such case of loading mixing quality is also better. It has been found that if the fill level is less than 50% the required mixing time is small and the mixing efficiency is better. At 75% fill level, the mixing time is high as mixing efficiency is low. This is because of the availability of less space for movement of the powders and formation of stagnant core along the axis of the cylinder.