Theoretical Study of the Reorganization of Fractal Aggregates by Diffusion

Precipitation processes are commonly used in industry to efficiently produce large amounts of solids. During these processes a fast formation of colloidal structures is often followed by an aggregation process if the systems are unstable. In many cases these aggregates are highly porous networks that may subsequently reorganize to form a more compact particulate material. In this article, the reorganization process is theoretically examined for fractal aggregates. Two different mechanisms are proposed. For the first mechanism, particles are selected for diffusion moves inside the cluster based on their distance to the center of mass. In a second mechanism, the detachment of primary particles is described by an Arrhenius type expression. The detachment probability is assumed to be linearly dependent on the number of nearest neighbors of the primary particle. This last mechanism is demonstrated to be valid for the restructuring of silica aggregates. The obtained activation energy of 4.87 × 10^?21 J for breaking a bond between neighboring particles is similar to values found in secondary minimum coagulation.