The mechanical flotation cell has two operation zones that depend on each other: the collection zone, where mixing and gas dispersion take place, and the froth zone, where mineral transportation takes place (Grau et al., 2019). Hydrodynamic turbulence from the impeller-stator system controls both micro processes like bubble breakup and macro processes like suspension of solids. Effective collection is based on an agitation system, which maintains balance between shear and pumping processes. The main operation parameters, such as bubble surface area flux (Sb) and Sauter mean diameter (d32), affect the flotation rate kinetics and metallurgical recovery.
Intrusion of air between the blades causes cavitation pockets which form very fine bubbles in the highly turbulent flow of the impeller (Darabi & Soltani, 2025). Higher speeds of the impeller (N) result in higher shear rates and lower Sauter mean diameter (d32), causing the size distribution towards smaller fraction sizes (Mazahernasab et al., 2021). Various geometries are used in industrial processes to maximize the degree of dispersion. The recessed disk impeller geometry allows for higher speeds and creation of small bubbles without increasing the motor power requirements (911Metallurgist). The Deep Vane (DV) geometries can produce high radial pump rates to cell walls, possessing vertical diffusers that reduce pulp swirl of tanks (Metso). The presence of stators plays a major role in bubble disintegration, resulting in significant reductions in size due to stator effects (Mesa, 2020).
The bubble surface area flux (Sb), where Sb = 6Jg / d32 and Jg is the superficial gas velocity, is regarded as the key performance indicator for determining gas dispersion efficiency (Shahbazi et al., 2012). There is a strong linear relationship between the flotation rate constant (k) and Sb based on experimental studies for different impeller designs. It should be noted that the slope of the relationship is unaffected by the particular impeller design used (Gorain et al., 1997). Complete gas dispersion can only be obtained by exceeding a critical impeller speed (NCD) to avoid impeller flooding and ensure gas dispersion throughout the reactor. Below-critical impeller speeds (less than NCD) result in insufficient bubble dispersion and consequently low k because of fewer bubble-particle collisions (Darabi & Soltani, 2025).
Optimum mineral recovery can be achieved by maintaining a balance between lift and turbulence control. Critical impeller speed for solid suspension (Njs) needs to be attained alongside NCD to ensure complete off-bottom suspension (Darabi & Soltani, 2025). Flotation of fine and medium sized quartz particles (<106 µm) increases with an increase in impeller speed from 850 rpm to 1000 rpm since collision frequency is high. On the other hand, coarse-sized particles (>106 µm) have lower recovery rates because of higher shear that leads to bubble-particle separation. Modern-day cell improvements, such as adjustable froth crowders and better launders, reduce transportation distances and minimize froth dead zones, which stabilize the recovery process and lower air volume requirement (Grau et al., 2019).
The design of the impeller stator system is important to the hydrodynamics of flotation machines because it converts mechanical energy to turbulence and radial movement of the fluid. The proper dispersion of air leads to smaller bubbles that increase the surface area flux (Sb) and bubble particle collisions. Successful operation depends on metallurgists choosing impeller designs and speeds that fulfill the requirements of NCD and Njs without agitating the slurry too much. The hydrodynamic balance maintains the attachment of coarse particles while maximizing the recovery of fine particles.
References
911Metallurgist Flotation Cell Impeller Types: Technical Manual for Sub-A Flotation Machines. 911Metallurgist Technical Library.
Darabi, H., & Soltani, F. (2025) On the Importance of Critical Impeller Speed and Complete Gas Dispersion for Enhancing Flotation Kinetics. Journal of Mining and Environment, 16(6), 1–18.
Gorain, B. K., Franzidis, J. P., & Manlapig, E. V. (1997) Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 4: Effect of bubble surface area flux on flotation performance. Minerals Engineering, 10(4), 367–379.
Grau, R., Davoise, D., Yáñez, A., & López, Á. (2019) Optimizing the froth area of large mechanical flotation cells. In Procemin-GEOMET 2019: 15th International Mineral Processing Conference.
Mazahernasab, R., Ahmadi, R., et al. (2021) Direct Bubble Size Measurement in a Mechanical Flotation Cell by Image Analysis and Laser Diffraction Technique – A Comparative Study. Iranian Journal of Chemistry and Chemical Engineering, 40(5), 1653–1664.
Mesa, D. (2020) The effect of impeller-stator design on froth stability, flotation performance and flotation hydrodynamics (Doctoral dissertation, Imperial College London).
Metso Corporation RCS™ Flotation Cell Technical Overview: Deep Vane (DV) Flotation Mechanism. Metso Mineral Processing Solutions.
Shahbazi, B., Rezai, B., et al. (2012) The Empirical Prediction of Gas Dispersion Parameters on Mechanical Flotation Cells. Engineering, 4, 11–19.
