Predicting granule behaviour through micro-mechanistic investigations

Abstract

A novel micro-manipulator device has been developed to observe and measure, directly, the behaviour of binder liquid bridges between pairs of solid particles. The objective has been to develop the fundamental understanding of the role of the liquid and solid properties in the growth and consolidation of granules, from the initial contact between the liquid and particles to the resultant multi-particle bodies. On the particle level, it is the liquid bridges that are responsible for the strength of “wet” agglomerates, since they hold the particles together. In this paper, results of experiments will be reported that identify the role of liquid surface tension, bridge Laplace pressure, liquid viscosity and, hence, wetting behaviour, in the axial strength of the bridges. In particular, the differences in bridge shape when particles of different surface properties come together (i.e. in mineral mixtures) provides a crucial insight into whether granules will grow successfully or not. A parabolic approach to describing the shapes adopted by the liquid bridges, from which parameters such as resistance to deformation can be calculated, will be shown. From the theoretical behaviour of individual bridges, determined through the direct experimental observations, a simple model has been constructed, which relates granule porosity, liquid content and the physicochemical properties of the materials to the agglomerate hardness. Some experimental measurements using spherical particles and powders commonly granulated in the pharmaceutical industry, such as lactose, will be compared to the model predictions and the role of interparticle friction and liquid surface tension and viscosity will be shown quantitatively.