Observations on impact attrition of granular solids

Particle breakability assessment using an Aero S disperser

Milling is widely used in various industries to tailor the particle size distribution for desired attributes. The ability to predict milling behaviour by testing the breakability of a small quantity of material is of great interest. In this paper, a widely available aerodynamic dispersion method, i.e. the Aero S disperser of Malvern Mastersizer 3000 has been evaluated for this purpose. This device is commonly used for dispersion of fine and cohesive powders, as the particles are accelerated and impacted at a bend, but here its use for assessing particle breakability is explored. The fluid flow field is modelled using one-way coupled Computational Fluid Dynamics approach, as the particle concentration is low, following which the particle impact velocity is calculated by Lagrangian tracking and used in the analysis of particle breakage. Experimental work on the breakability is carried out using aspirin, paracetamol, sucrose and α-lactose monohydrate particles. The relative shift in the specific surface area is determined and together with the calculated particle impact velocity and physical properties, they are used to calculate the breakability index. A good agreement is obtained with the single particle impact testing and aerodynamic dispersion by Scirocco disperser, indicating the breakability could also be inferred from this method.

Quantifying Dry Milling in Pharmaceutical Processing: A Review on Experimental and Modeling Approaches

Particle size reduction by mechanical means is an important unit operation in the pharmaceutical industry, used to improve flow, solubility, and in amorphization of drugs. It is usually achieved by the fracturing of particles under the action of applied energy. Despite being pervasive in the pharmaceutical field, it is one of the least understood processes owing to the complexity of material and process variables involved during milling. To comprehend the process, efforts should be focused on techniques that measure the particle size as well as the control the process. With the ongoing initiative of US FDA to encourage design in quality, the review is focused on some process analytical tools to characterize particle size distribution as well as process modeling tools to simulate particle size reduction. Additionally, an overview of some fundamental aspects related to milling is provided. To this end, the review is limited, mainly concentrating on some of experimental and modeling approaches used to quantify and understand the physics behind the process of dry milling.

Development of a novel approach towards predicting the milling behaviour of pharmaceutical powders

A novel approach has been developed for evaluating the milling behaviour of pharmaceutical powders based on their material and mechanical properties obtained by single particle impact testing. Milling behaviour of two widely used pharmaceutical excipients, namely microcrystalline cellulose and alpha-lactose monohydrate has been analysed in an oscillatory single ball mill. It is found that the milling behaviour of these two powders can be described by analogy with a first-order rate process except for alphaLM at 18Hz of milling frequency. At the same time, single particle impact testing has been used to infer the material properties that are related to breakage. The milling rate of these powders is found to correlate well with the parameter representing the material properties including the particle size, density, hardness and critical stress intensity factor. This provides the basis for a novel approach towards analyzing the milling behaviour of a material based on a simple and reliable approach.