Granule nucleation and growth: Competing drop spreading and infiltration processes

Wet granulation is the formation of powder particle assemblies bound by a liquid binder. Granules are produced in order to prevent mixture segregation, improve flow characteristics and ease of powder handling. During this process powder is agitated inside a high shear mixer or fluidised bed and a liquid binder is added onto the powder mixture. The binder droplets then penetrate into the powder and bind them together. The kinetics of this drop-powder interaction is strongly linked to the properties of the granules produced, with shorter penetration times often leading to a more desirable narrower distribution of properties, providing the assembly formed is strong enough. Whilst there have been several separate studies into drop penetration times into porous powders and drop spreading on non-porous surfaces, relatively little work has focussed on the competitive spreading–infiltration process that would occur in reality.To investigate this, single drop penetration experiments were carried out into static dry and pre-wetted powder beds. Pre-wetted in this case means to have been previously wetted by one identical drop. Viscous binders have been shown to exhibit lesser degrees of spreading in shorter times on non-porous surfaces [1,2]. However, the current work has discovered that, on porous surfaces, the infiltration rate has a greater degree of dependence, to that of spreading, on changes in viscosity. This means that higher viscosity binders will spread comparably further but require a smaller reduction in ‘remaining drop’ volume for capillary induced suction to pin the contact line.

The simultaneous drop spreading and infiltration behaviours of a selection of liquids on both dry and pre-wetted powder beds were studied. Tanners Spreading law was applied and found to perform very well for pre-wetted substrates for the range of liquid properties studied. The relative dependencies of both spreading velocity and infiltration rate on changing liquid properties are also determined.Figure optionsDownload as PowerPoint slide