A thickener is continuous settling machinery that separates solid particles from a liquid stream by letting solids settle down and creating a clear overflow and a concentrated underflow. Sizing is the process through which the area of the tank and its height are calculated according to certain objectives, such as target underflow density and overflow clarity. For example, in case of high-clay gold recovery plants, clays, usually less than 2 microns in size, will have a large specific surface area and increased slurry viscosity, reducing the settling rate and yield stress.
The most important sizing parameter for such installations is the thickener area, determined by sedimentation rate of the mineral expressed in terms of m² per day per ton (m²/t/day) or specific feed rate in tons per hour per square meter (t/h/m²). As clays settle down very slowly, the rise rate (upflow velocity in the clear-water zone) is about one-third to one-half of the free settling rate. The design safety factor equals 1.15-1.30.
The torque drive is equally important, since it needs to break through the mud yield stress (compressive rheology). There is an exponential correlation between solids percentage and shear stress, meaning that any slight changes in solids/clay percentage will result in drastic torque increases. Gold tailings with high clay percentage need high torque drives and rake lifting. Such slurries may require higher rate or high density thickeners with elevated side wall heights to reach underflow solids concentration.
Variability parameters, including throughput, particle size distribution, and clay/ultrafine content, play a crucial role in the performance of the thickener. As such, these parameters should be determined for thickener sizing. Increasing clay percentage by more than about 20% from the design parameters leads to dramatic growth of viscosity and shear stress, which results in torque overload, poor clarification ability, and lower density of underflow. Particle size analysis (full distribution, not only D80), static and dynamic settling tests, and yield stress testing are necessary.
The choice of flocculants and its dosage is important for sizing since clay requires much more flocculant and sometimes coagulant for the formation of a settleable flock. Optimal solids flux and dosage rate are found through cylinder testing and chemical screening, and this sets the necessary area. Under-dosing causes slow settling and requires larger areas, while overdosing increases fluid viscosity and torque. Therefore, flocculation sensitivity and torque/area need to be accounted for in design.
At last, geometric and hydraulic parameters should be considered in regard to area and torque: the feedwell diameter should be sufficient to break energy and avoid short-circuiting, side wall height should offer necessary time to produce certain underflow solids, and underflow pipe diameter should avoid settling and ensure pumpability. For the processing of gold with high content of clay, one often specifies large feedwell, large side walls, high-torque drive, and safety factors for area and torque.
