Froth flotation is an essential mineral processing operation that relies heavily on proper fluid mechanics. Short-circuiting is a severe hydrodynamic problem characterized by rapid travel of the feed through the feed inlet to the tailings outlet without engaging the dispersed air bubbles. The effective residence time of the pulp becomes significantly lower as the mineral feed does not participate in the main processes of frothing and mixing. As a result, the mineral recovery is poor and the mechanical energy is spent wastefully.
The scientific method for identifying this issue is based on analyzing the macro-hydrodynamics of the reactor using the Residence Time Distribution (RTD) method. This is a probability function representing the actual time of fluid or solid staying inside the flotation cell before being discharged. If there is a significant difference between the theoretical nominal residence time calculated according to the reactor volume and the flow rate and the measured mean residence time, it may indicate an internal problem, like dead zone or short-circuiting (Couto et al., 2021).
To perform an RTD test in a flotation bank, one should use the pulse-tracer technique that is a standard practice in the industry. An identifiable pulse of a tracer is added to the feed flow. Although chemical salts are a possibility, radioactive substances enable very precise and non-invasive tests without changing the composition of the pulp. The sensors installed at the discharge outlets of the cells register the concentration of the tracer in time, resulting in experimental curves showing the speed and direction of the flow.
Further, the collected data should be analyzed quantitatively using hydrodynamic mathematical modeling in order to prove the diagnosis. There are several typical structural models, such as the tanks-in-series model and the axial dispersion model (Couto et al., 2021). On the graph of the concentration of the tracer versus time, a sharp premature peak at the outlet of the cell proves short-circuiting mathematically. On the other hand, a long tail means the presence of stagnant material in dead zones.
In modern processing plants, apart from the empirical tracer test, computational methods are commonly used for finding the causes of short-circuiting. The Computational Fluid Dynamics (CFD) is applied to simulate the multiphase flow of liquid and particles. With the help of CFD, the hydrodynamic behavior of the cell may be simulated and specific physical factors causing short-circuiting, such as inadequate feed inlet location or high gas flux, may be found (Mwandawande et al., 2022).
To sum up, short-circuiting diagnosis in the flotation bank is based on the RTD testing. Combining field tests with pulse-tracer method, mathematical modeling, and CFD simulations enables a detailed analysis of the internal flow regime. Preventing these undesirable bypassing flow lines ensures that the ore spends enough time in the cell increasing mineral recovery and concentrating grade.
References
Couto, H. J. B., Marques, C. H. d. F., & Braga, P. F. A. (2021). Liquid-phase RTD studies in a flotation column: an analysis between tanks-in-series and axial dispersion models. REM – International Engineering Journal, 74, 93–105. https://doi.org/10.1590/0370-44672019740107
Mwandawande, I., Bradshaw, S. M., Karimi, M., & Akdogan, G. (2022). Investigation of the mixing characteristics of industrial flotation columns using computational fluid dynamics. Journal of the Southern African Institute of Mining and Metallurgy, 122, 1–13. https://doi.org/10.17159/2411-9717/1589/2022
