Role of mixing conditions on coagulation performance and flocs breakage formed by magnesium hydroxide

• Magnesium hydroxide was used as coagulant.
• Optimum magnesium coagulant dose was obtained.
• Floc breakage and re-growth were discussed under different mixing modes.
• Shorten ultrasonic time gave good coagulation performance.

The breakage and re-growth of flocs formed by magnesium hydroxide were evaluated for coagulation of kaolin suspension under high pH. A series of jar experiments were conducted to investigate the impacts of mixing conditions on coagulation performance and magnesium hydroxide-kaolin floc properties. Flocculation index (FI) value was then discussed with controlled experiments using on-line intelligent particle dispersion analyzer (iPDA) and optimum coagulant dose of 14.4 mg/L (magnesium ion) was obtained. The results showed that one time addition feeding mode lead to relatively large floc size at first stage compared with dropwise addition mode. Ultrasonic disruption led to small flocs and prolonged ultrasonic time gave poorer coagulation performance. Increasing rapid mixing speed led to a decrease in the final floc size. Breakage and recovery factors decreased with the increase of rapid stirring speed. The experiments also showed that magnesium hydroxide coagulation process including fast floc formation and growth of flocs and then larger flocs break into relative small particles. All of the flocs under investigation showed a limited capacity to re-growth when they had been previously broken.