Critical parameters and limiting conditions in binder granulation of fine powders

Abstract

The goal of this work is to present a general theoretical and practical framework of binder granulation that takes an agglomeration process from binder selection and testing to granule formation, growth and consolidation and finally to granule deformation and breakup. For agglomeration and granule growth to commence at all, a certain minimum amount of binder has to be introduced in the granulator and this has to be determined carefully. This paper presents special instrumentation and procedures for binder selection. In granulation processes such as detergents and pharmaceutical products, both the powders to be agglomerated and the binders are defined by the formulation and, usually, little liberty is given to alter the chemistry. Binder ‘selection’ in this case is practically reduced to adjusting the properties of the binder using small amounts of additives such as surfactants, polymeric compounds and small amounts of liquids and tailoring the binder to exhibit specific behavior. This allows fine-tuning of binder properties that include surface wetting, spreading, adsorption, binder strengthening and solid bridge strength. The bulk of the present work is dedicated to the theory of growth kinetics during granulation and the prediction of critical sizes which delimit different regimes of granulation. Several dimensionless parameters based on energy dissipation principles are presented and examples given about how these parameters and the critical sizes they define can be used to predict the outcome of granulation and the scale-up of the process. The above theoretical framework is then tested with experimental data from the literature and with granulation results obtained by the present authors using a specially constructed constant-shear granulator based on the principles of a Couette viscometer. A theory of consolidation of formed granules is also given and additional experimental support from the literature is presented.