Direct Simulations of Flocculation In Sedimenting Solid-Liquid Suspensions

Direct simulations of flocculation in sedimenting solid-liquid suspensions

Gravity-based separation (settling) is common practice in many processes involving solids handling, specifically in mining and related (mineral processing) applications. A widely used way to increase settling rates, and to allow for gravity based separation of fine particles is to promote aggregation so that flocs are formed that fall faster through liquid than the primary particles. We study flocculation in sedimenting solid-liquid suspensions numerically by performing direct simulations with full resolution of the solid-liquid interfaces and with full (two-way) coupling between liquid flow and solid particle motion. In the simulations uniformly sized solid spheres are released. They fall under the action of (net) gravity, they collide with one another according to a hard-sphere model, they interact through the interstitial liquid, and they tend to aggregate as a result of a square-well potential surrounding each sphere. By means of the simulations we study how the settling velocity depends on the strength of the square-well potential and the other key parameters in the process: the solids volume fraction and the particle size. The observations regarding settling velocities are interpreted in terms of the micro structure of the suspension (radial and angular distribution functions), and in terms of the floc size distribution and the fractal dimension of the flocs.