6. Conclusion
Reductions of segregation rates by orders of magnitude bytailoring the particle size distribution do not seem to have beenreported in the granular research literature. Yet, mixing of grains andpowders of different sizes and densities is ubiquitous in industrialoperations. In the pharmaceutical industry, small drug crystals areoften blended with larger excipient (filler) particles and the resultingmixture is transported and then compacted into tablets. Poor mixingor segregation can lead to out-of-spec tablets, with too little or toomuch drug, resulting in whole batch failures. Our results suggest thatintroducing additional intermediate particle sizes in between thesmallest and the largest in the original mixture may reduce thetendency of the particulate mixture to segregate. There is evidencethat duringfluidized bed processing, which is known to lead toseparation offines and coarse particles, powders with a broadcontinuous particle size distribution do not segregate[48]. Our resultshave focused on non-cohesive particle mixtures of equal density inorder to avoid complications associated with the Reverse Brazil-nuteffect[7,21,23]and agglomeration of particles. It remains to be seen ifthese results can be applied to reduce segregation in granular andpowder systems of differing density and cohesion where a range ofother modes of segregation, beyond the Brazil-nut effect appear. Priorresearch has shown that cohesion can assist or hinder mixing ofgranular materials[49], so further work is required to determine theeffect of cohesion on segregation in a vertically vibrated bed. Inaddition, our results have focused on a vertically vibrated cylinder butthere are certainly other scenarios where one can separate particlescompletely by size and obtain all the Brazil nuts (large particles) onthe top of a mixed nut (particle) system. With implications in
situations ranging from the production of cereal[2]to pharmaceuticalmanufacture[24],understanding and reducing granular segregationhas the potential to drastically reduce costs associated with low yieldand out-of-spec products.
6. Conclusion
Reductions of segregation rates by orders of magnitude bytailoring the particle size distribution do not seem to have beenreported in the granular research literature. Yet, mixing of grains andpowders of different sizes and densities is ubiquitous in industrialoperations. In the pharmaceutical industry, small drug crystals areoften blended with larger excipient (filler) particles and the resultingmixture is transported and then compacted into tablets. Poor mixingor segregation can lead to out-of-spec tablets, with too little or toomuch drug, resulting in whole batch failures. Our results suggest thatintroducing additional intermediate particle sizes in between thesmallest and the largest in the original mixture may reduce thetendency of the particulate mixture to segregate. There is evidencethat duringfluidized bed processing, which is known to lead toseparation offines and coarse particles, powders with a broadcontinuous particle size distribution do not segregate[48]. Our resultshave focused on non-cohesive particle mixtures of equal density inorder to avoid complications associated with the Reverse Brazil-nuteffect[7,21,23]and agglomeration of particles. It remains to be seen ifthese results can be applied to reduce segregation in granular andpowder systems of differing density and cohesion where a range ofother modes of segregation, beyond the Brazil-nut effect appear. Priorresearch has shown that cohesion can assist or hinder mixing ofgranular materials[49], so further work is required to determine theeffect of cohesion on segregation in a vertically vibrated bed. Inaddition, our results have focused on a vertically vibrated cylinder butthere are certainly other scenarios where one can separate particlescompletely by size and obtain all the Brazil nuts (large particles) onthe top of a mixed nut (particle) system. With implications in
situations ranging from the production of cereal[2]to pharmaceuticalmanufacture[24],understanding and reducing granular segregationhas the potential to drastically reduce costs associated with low yieldand out-of-spec products.
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