During flocculation, what mixing regime promotes the formation of larger, more settleable flocs?

Slow mixing, specifically a regime characterized by gentle shear, promotes the formation of larger, more settleable flocs during flocculation. Flocculation is the process of destabilizing colloidal particles – tiny particles suspended in a liquid – and causing them to aggregate into larger clumps called flocs. These flocs should ideally be large and dense enough to settle out of the water under gravity. The key to achieving this lies in the mixing regime.

Initially, rapid mixing, often referred to as dispersion or flash mixing, is used to distribute the flocculant (a chemical that promotes floc formation) evenly throughout the water. This ensures that all the colloidal particles are exposed to the flocculant. However, subsequent mixing must be significantly slower. Slow mixing reduces the shear forces acting on the forming flocs. Shear forces are forces that tend to break apart aggregates. High shear forces, like those generated by rapid stirring or high-speed impellers, disrupt the weak bonds between particles as they begin to aggregate, preventing them from growing into larger flocs. Instead, they create smaller, less settleable flocs, or even redispersion of particles.

Slow mixing allows the particles to collide and bind together more effectively. The binding forces between particles, such as van der Waals forces and electrostatic attraction (after neutralization by the flocculant), are relatively weak. Gentle collisions, facilitated by slow mixing, increase the probability of these weak forces overcoming repulsive forces (like electrostatic repulsion between similarly charged particles) and forming stable bridges between particles. As these bridges form, the flocs grow in size. The slower the mixing, the more time the particles have to find each other and bind, leading to larger flocs.

Specific mixing devices used to achieve this slow mixing regime include slow-turning impellers (like baffled turbines or paddle mixers) operating at low speeds, or long-channel mixers. These devices create a gentle, laminar flow pattern, minimizing shear. The goal is to create conditions where floc growth is favored over floc breakage. For example, a slow-turning paddle mixer in a tank allows particles to diffuse towards each other and aggregate without being sheared apart, unlike a high-speed propeller which would break them up. The optimal slow mixing regime is dependent on factors like the type of flocculant used, the characteristics of the colloidal particles (size, charge), and the water temperature, but the fundamental principle remains: gentle shear promotes larger, more settleable flocs.