Tribo-Electrification and Associated Segregation of Formulated Bulk Powders

Powder handling operations can give rise to the electrification of particles therein, causing particles to adhere to the walls of processing equipment. This can lead to a loss of powder through deposition; however the problem often extends further and affects the end products’ quality. Segregation can also occur when powders have been subjected to tribo-electrification. The existing literature reveals that the few methods that are available to predict the dynamic charging of bulk powders are unsuitable for testing/handling small quantities of powders, some of which are highly active. Furthermore, very little work has been reported on the effect of triboelectrification on the segregation of components of mixtures.

The objective of this work is to develop a methodology for investigating the triboelectrification of small quantities of bulk powders through the adaptation of a Retsch® shaking device, with the aim of characterising two common pharmaceutical excipients; namely alpha-lactose monohydrate (a-LM) and hydroxypropyl cellulose (HPC). The electric charge transferred to the particles has been quantified as a function of shaking time, frequency and container material. The temporal trend follows a first order rate process.

Using numerical simulation based on Distinct Element Modelling, the transient charge accumulation of an assemblage of alumina beads inside the shaking device was predicted based on the single particle contact charge obtained from the experiments. It is shown that the inclusion of electrostatic parameters into the DEM model leads to an acceptable prediction of the charge build-up.

Binary mixtures comprising of a-LM and HPC were firstly tribo-charged and then the wall adhered particles were separated by a selective dissolution of one component and the filtration of the non-dissolving component, followed by a
gravimetric analysis. The findings reveal that a considerable level of segregation can take place on the wall-adhered particles.

The methodology developed in this work has the potential to be used to characterise small quantities of pharmaceutical powders including active pharmaceutical ingredients (API), which are sparse in the early development stages.