Quantitative Analysis of Powder-Binder Agglomeration

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Author Last Name: 
J.D. Litster, R.Smith, S. Iveson, N. Page, S.Forrest J. Bridgewater
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Research Area: 
Particle Formation
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This report summarises progress in IFPRI project 37 in 2001. This is the first year of the continuation project. We have established new areas of investigation with new objectives. Thus, we report here a summary of work in progress at a fairly early stage.

Iveson and Page at The University of Newcastle have continued to investigate the mechanics of partially saturated powders. For a range of powders, both spherical and non spherical, dimensionless strength is independent of strain rate for Capilliary No (Ca) less than 10-4 and strongly dependent on Ca above this value. Particle shape has a strong effect on yield stress due to its effect on particle packing, friction and particle interlocking. The failure mode also changes with strain rate. At low strain rate, brittle fracture is observed, while at high strain rate pellets deform plastically.

Forrest and Bridgewater at Cambridge University have established a methodology for characterizing powder flow during granulation using Positron Emission Particle Tracking (PEPT). The technique is being used to study powder flow in a plough share mixer granulator throughout a batch granulation. Powder flow characteristics change significantly as liquid is added to the granulation. Changes in the granule velocity and local bed density are linked to regions of granule motion that are characterised by inter-granule contact time and force. Further experiments are required to determine values for the relaxation ratio and develop the relationship between granule motion and granule properties. Extension of this analysis to a greater range of experimental conditions will develop the relationship between granule motion and granule properties for a ploughshare mixer.

A brief review of the status of wet granule breakage research is presented. Wet granule breakage is a relatively poorly studied phenomenon. Experimental evidence shows that it may play an important role in controlling granule size distribution in mixer granulators and can contribute significantly to binder distribution under some conditions. However, little attempt has been made to link breakage rates quantitatively to the mechanical properties of the granules (or even to characterise them). The theory of granule breakage and the mechanical characterisation of granules under dynamic conditions need further development in order to produce quantitative predictions of conditions for granule breakage. The work of Tardos and coworkers based on a Stokes deformation number analysis combined with the mechanical characterisation approach of Iveson and Page provide a good starting point for further development.