Attrition of Spray-Dried Powders

Researcher: Tina Bonakdar
Principal Investigator: Prof. Mojtaba Ghadiri
Industrial Collaborators: Hossein Ahmadian, David Smith, Luis Martin de Juan, Hossam Tantawy
Sponsor: Procter and Gamble
Status: Dec 2012 - Mar 2016ImageAbstractParticle attrition in manufacturing plants handling particulate solids could cause processing as well as environmental problems, and lead to the degradation of product quality.  Spray-dried powders are particularly prone to attrition because of their porous and often weak structure.

In this thesis the mechanisms by which attrition takes place were addressed, taking account of particle size and structure and the portfolio of mechanical stresses.  The evolution of structure during the spray drying process cannot be easily controlled but plays a pivotal role in defining particle strength.  Spray-dried burkeite particles are a good example and have been used as a model porous powder to investigate the effect of structure on their strength and breakage propensity.  The dominant prevailing stresses for such particles in a process plant are caused by collision. Therefore, the particles were subjected to well-defined stresses due to impact, and the change in the particle size distribution is determined by particle size analysis based on sieving.  It was found that impact breakage of burkeite is affected by the structure, and some unexpected trends for breakage were observed; for a given impact velocity some smaller particle sizes break to a greater extent as compared with larger particles.  This was attributed to uncontrollable variations of porosity (and hence particle envelope density) as a function of size.  To study the effect of porosity on attrition of such particles, structure visualisation and analysis were carried out by Scanning Electron Microscopy (SEM) and X-ray microtomography (XRT).   SEM images showed three levels of structure in a single particle of spray-dried burkeite.  Based on the XRT results, the particle envelope density increases as particle size increases, and the variation of envelope density influences the impact breakage.  Once the relevant values of the envelope density were taken into account, then the trend of impact breakage data became unified, and the material mechanical properties of the particles were inferred from the breakage results.  The breakage of these particles were also studied using the Scirocco disperser of Malvern Mastersizer 2000, as it is widely available, in contrast to the single particle impact rig.  In this device, particles of different sizes accelerate to different velocities, and break to different extents.  It was found that the shift in the specific surface area of different sizes of spray-dried burkeite particles can be related to the dimensionless group representing the breakage propensity, using the estimated impact velocity by CFD simulation.  Following the successful outcome of this work, the approach was extended to determination of breakability of three other crystalline materials, aspirin, sucrose and –lactose monohydrate.  The outcome indicates that the Scirocco disperser can be used as an assessment method for grindability testing of materials.  The slope of the line obtained from the graph relating the shift in the specific surface area of the particles, reflecting breakage, to the particle size and density and impact velocity correlated well with that obtained from single particle impact testing.  Therefore this method can be used to assess the grindability of the powders provided the impact velocity is first determined.

To elucidate the role of structure and interparticle bond strength, DEM simulation was used to explore the effect of structure (porosity) on impact breakage of agglomerates, and to provide a better understanding of the effects of various parameters on agglomerate breakage.  The effects of impact velocity, surface energy, impact angle and porosity on agglomerate breakage were analysed.  It was found that depending on the level of surface energy, there are different patterns of impact breakage. More porous agglomerates undergo a higher extent of breakage compared to the low porosity agglomerates.  This is also the case when the breakage of agglomerates with low surface energy is compared to those with high surface energy. The agglomerate impact at normal target also results in higher breakage level compared to the impacting at inclined surfaces.  This approach has great potential to be predictive of the effect of structure on the impact strength of agglomerates.

Publications

Bonakdar, T., Ghadiri, M., Ahmadian, H., Martin de Juan, L., Xu, D., Tantawy, H. and Smith, D. (2016). Impact attrition of spray-dried burkeite particles. Powder Technology. 304: 2-7

Bonakdar, T., Ali, M., Dogbe, S., Ghadiri, M., and Tinke, A. (2016). A method for grindability testing using the scirocco disperser. International Journal of Pharmaceutics, 501: 65-74

Ali, M., Bonakdar, T., Ghadiri, M., and Tinke, A. (2015). Particle breakage in a scirocco disperser. Powder Technology. 285: 138-145