An experimental study of scale growth rate and flow velocity of a super-saturated caustic–aluminate solution

Scale growth in alumina refineries occurs where supersaturated solutions are in contact with solid surfaces. Whilst some scale formations may prove beneficial in protecting equipment and pipe surfaces from chemical attack or abrasion, more commonly its deposition results in reduced equipment performance and plant productivity. Experiments have shown that the rate of scale growth is 60% higher in concentric reducers (contractions) than it is in adjacent straight pipes. One method of reducing the rate of gibbsite scale growth on the walls of precipitator tanks is to increase the flow velocity of the supersaturated slurry near the wall. For the supersaturated caustic–aluminate slurry encountered at Queensland Alumina Limited, (QAL), the results of this research suggests that a minimum slurry flow velocity of 2.9 m/s near the walls of the tank would reduce the rate of scale growth to zero on the tank walls. The energy required to develop this wall velocity would, however, be both uneconomic and impractical.