A dense medium cyclone (DMC) is one of the essential gravity separators utilized in a coal preparation plant through the application of centrifugal forces using a liquid medium (normally a mixture of water and magnetite) to separate valuable coal from gangue on the basis of differences in densities (Meyer & Craig, 2015). Hydraulic performance evaluation involves assessment of how well the separation is made concerning particle sizes and density classifications of particles in an extremely complex flow regime inside the cyclone. Intermediate sized coal having dimensions of between 0.5 mm and 12 mm is mainly fed to the cyclones (Zhang et al., 2024).
Partition curve (commonly known as the Tromp curve) is the main mathematical and graphical representation that engineers use in evaluating DMCs’ hydraulic performances. The Tromp curve represents the probability of particles classified according to their specific gravity being reported in the underflow (reject) and overflow of the cyclone (Meyer & Craig, 2015). If the separation was perfectly performed mathematically, then the probability distribution curve would form a straight line vertically; however, this is not achieved due to hydraulic turbulence.
The two key parameters obtained from the partition curve are the cut-point density (d50) and the Ecart Probable (Ep). The former represents the exact density at which the particles have an equal probability of being part of the clean coal products or the rejects. On the other hand, the Ecart Probable represents the degree of sharpness of the separation. The lower the Ecart Probable is, the more efficient and sharper the separation process was (Iveson et al., 2019).
Traditionally, creating the partition curve involved conducting float-sink analysis on the feed, products, and rejects. Though very accurate in determining the density distribution of the materials under investigation, these tests are associated with several drawbacks. They depend on the use of toxic and expensive heavy liquids that are not environmentally friendly. Besides, their lengthy testing duration creates significant delays in the adjustment of hydraulics (Iveson et al., 2015).
The shortcomings associated with the use of heavy liquids have led to the application of density tracers as well as sensor technology in preparation plants today. Density tracers refer to very small artificial particles with a known specific gravity and color that are added to the cyclone feed directly to determine the partition curve quickly (Ambrós, 2023). In some cases, the use of “spy particles” with micro-sensors to monitor the acceleration and fluid dynamics in three dimensions as they traverse the swirling flow within the cyclone is done (Ambrós, 2023).
Apart from density tracers used by preparation plants, safer laboratory techniques have been introduced to examine the efficiency of partitioning. Semi-batch reflux classifiers using aqueous glycerol solutions are one such example. Such procedures have proven effective in place of sink-float methods while preserving laminar flow and suppressing particle-size effects to ensure the construction of partition curves based on densities (Iveson et al., 2015). Through the adoption of the technologies, hydraulic operation of the DMC can be maximized and minimize coal losses.
References
Ambrós, W. M. (2023). Gravity Concentration in Urban Mining Applications—A Review. Recycling, 8(6), 85. https://doi.org/10.3390/recycling8060085
Iveson, S. M., Hunter, D. M., & Galvin, K. P. (2015). A water-based method for measuring density-based partition curves of separators used in coal and mineral processing. Minerals Engineering, 79, 196–211. https://doi.org/10.1016/j.mineng.2015.06.008
Iveson, S. M., Price, A., & Galvin, K. P. (2019). Separation of coal with a top size of up to 6 mm in a full-scale REFLUX™ Classifier. International Journal of Coal Preparation and Utilization, 42, 694–714. https://doi.org/10.1080/19392699.2019.1646255
Meyer, E. J., & Craig, I. K. (2015). Dynamic model for a dense medium drum separator in coal beneficiation. Minerals Engineering, 77, 78–85. https://doi.org/10.1016/j.mineng.2015.03.002
Zhang, F., Bournival, G., & Ata, S. (2024). Overview of Fine Coal Filtration. Part I: Evaluation of Filtration Performance and Filter Cake Structure. Mineral Processing and Extractive Metallurgy Review, 1–22. https://doi.org/10.1080/08827508.2024.2334956
