A Novel Method for the Analysis of Particle Coating Behaviour Via Contact Spreading in a Tumbling Drum

Spray coating of particles and powders is a commonly used process in many industrial applications, e.g. food, detergents and pharmaceuticals. It involves complex interactions which include three phases; solid, liquid and gas. Consequently, the design and scale up of the process are still based on trial and error. Tumbling drums are typically used for particle coating processes, and the uniformity of the coating layer formed is very important when considering the quality of the final product.

There are two proposed mechanisms included in the spray coating of particles: (I) droplet deposition and (II) contact spreading (liquid exchange between particles). However, research to date has tended to focus on the droplet deposition mechanism rather than the latter. Moreover, in tumbling drums, contact spreading is thought to be very important, and understanding this mechanism is the focus of this study, with the aim of incorporating knowledge of this mechanism into the whole process.

In order to investigate the contact spreading between particles, a new image analysis system, based on colorimetric measurement, has been developed to quantitatively determine the colour uniformity of particles coated with dyed solutions. Here, it is demonstrated that the novel method can analyse a large number of particles in a relatively small time period, and gives data with which to assess inter-particle coating variation and uniformity of particles obtained from tumbling drum coating processes.

In this presentation, we describe results from experiments performed in a tumbling drum using a model system; alumina particles coated with dyed polyethylene glycol solutions of varying viscosities. It is observed that the uniformity of the coating increases with time. Furthermore, the viscosity of the coating solution also has an effect on the contact spreading process; the lower the viscosity, the faster the contact spreading process. This effect is attributed to difference in the formation and stability of liquid bridges between particles which influence the mechanism of liquid transfer via contact spreading.