Fractal Law: A New Tool for Modelling Agglomeration Process

Abstract

Wet agglomeration of granular materials is widely carried out in many industrial fields, such as the pharmaceutical and food industries. This process must be optimized in terms of yields and quality of the final product. A lot of studies have shown that agglomeration occurs under capillary, viscous and frictional forces and can be divided into four steps: nucleation, consolidation, growth and rupture. This general description of agglomeration does not take into account the simultaneous changes of agglomerates size, texture and saturation degree with respect to the increase in water content. The aim of this work is to propose a description of the agglomeration based on experimental observations and to take into account the aforementioned aspects to propose a model. In this study, experiments are carried out on kaolin in a low shear mixer for various operating conditions: three speeds of blade rotation, two wetting modes. The evolution of agglomerates granulometric and textural parameters is taken into account by image analysis and immersion in paraffin, respectively. This study makes it possible to highlight an increase in agglomerates size associated with a solid volume fraction reduction when there is an increase in water content. A power law connects solid volume fraction and median diameter of agglomerates making it possible to identify a fractal growth process between the grain and pasty-like scale. This well-know approach in colloidal aggregation is new in the powder agglomeration field. Fractal dimension defined this way is mainly affected by the drop size generated during wetting. These experiments highlight the weak influence of the other process parameters tested. For a wetting mode, an equation leading to the direct determination of the desired size of agglomerates with water content is proposed. Applied to agglomeration, fractal morphogenesis should become a potential tool in product engineering.