In the mining industry, thickener tanks are primarily used for dewatering concentrates and tailings in ore processing plants, thickening backfill tailings for underground backfill systems, and slurry thickening before and after long-distance slurry pipeline transportation systems.
It can be said that the thickener is an indispensable piece of equipment in mineral processing flowsheets.
Consequently, the selection of a thickener is a critical step. Prior to selection, it is usually necessary to determine the required settling area of the thickener, from which the thickener diameter can then be derived.
There are three commonly used methods for calculating thickener area:
(1) Calculation based on unit area capacity;
(2) Calculation based on the settling velocity of the largest particle in the overflow;
(3) Calculation using the clarification test analysis method.
(1) Calculation Based on Unit Area Capacity
Where:
A = Required thickener area (m²)
Gd = Dry solids feed rate to the thickener (t/d)
q = Unit area capacity (t/(m²·d))
Generally, the value of q is selected based on industrial or pilot-scale test data. In the absence of test data, it can also be selected by referring to actual performance indicators from similar concentrator plants.
Once the area is obtained, the diameter is calculated using the following formula:
Where:
D = Thickener diameter (m)
A = Thickener area (m²)
Advantages: Simple calculation; no complex testing required.
Disadvantages: Relies on empirical data; significant errors may occur when comparable reference conditions are lacking; does not directly reflect overflow clarity requirements.
(2) Calculation Based on Settling Velocity of the Largest Particle in the Overflow
Where:
A = Thickener area (m²)
Gd = Dry solids feed rate to the thickener (t/d)
R1 = Liquid-to-solid weight ratio of the feed slurry before thickening
R2 = Liquid-to-solid weight ratio of the underflow slurry after thickening
u0 = Free settling velocity of the largest particle in the overflow, typically obtained through testing (mm/s)
K = Effective area coefficient of the thickener, generally taken as 0.85–0.95 (the maximum value is used for thickeners with a diameter greater than 12 m)
K1 = Feed rate fluctuation coefficient, ranging from 1.05 to 1.20, related to the fluctuation range of the ore feed grade.
Advantages: Directly anchored to overflow clarity requirements; physically well-defined.
Disadvantages: Requires precise definition of the “maximum allowable particle” size; does not account for flocculation or hindered settling effects between particles; often represents a one-dimensional idealized calculation.
(3) Calculation Using the Clarification Test Analysis Method
This method is the most reliable for determining thickener area and best reflects the true settling characteristics of the material. It is commonly used for detailed design or for analyzing materials that are difficult to settle. Its core principle is based on static batch settling tests. By obtaining the curve of the mudline interface height over time, the solids flux is derived, and the limiting “minimum flux” is identified to determine the required area.
Basic procedure:
Conduct a clarification settling test: Place a representative slurry sample (usually at feed concentration or diluted to various concentrations) into a graduated cylinder. After thorough mixing, allow it to settle and record the decrease in the height of the interface between the clear liquid layer and the slurry over time.
Plot the curve: The slope of the linear segment of the curve represents the interfacial settling velocity at that concentration. At higher concentrations, the hindered settling phase will exhibit a convex curve.
Calculate solids flux: By repeating the test at different concentrations (or utilizing the Kynch theory of sedimentation from a single test), a flux versus concentration curve can be plotted. This curve exhibits a minimum point, which represents the “bottleneck” flux that limits the thickener’s handling capacity.
Determine the area.
This method requires only a single batch settling test, is convenient to operate, and is widely applied in the mineral processing field.
Advantages: Accounts for all complex factors of real slurry, including hindered settling and flocculation, providing accurate and reliable results.
Disadvantages: Requires a representative ore sample and a certain amount of testing time; demands some empirical skill in operation and data interpretation (e.g., determining critical points).
The three methods for calculating thickener area each have their own focus and complement one another:
A. The unit area capacity method is convenient for preliminary estimation and analogous design.
B. The maximum particle settling velocity method provides clear limitations for processes where overflow clarity is a priority.
C. The clarification test analysis method ensures design accuracy using direct test data and serves as the core basis for final detailed engineering design.
In practical engineering application, the empirical method is often used first to outline the selection range. Subsequently, the settling velocity is verified based on overflow requirements. Finally, the optimal area is determined through systematic settling tests. This progressive, multi-layered approach ensures that the thickener meets both the handling capacity requirements and achieves clean water recovery for reuse, laying a solid foundation for the efficient and stable operation of the entire flowsheet.
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