⎪ Minerals processing and materials handling ⎪
technology,” Enslin tells MCA . “South Africa’s coal has a notoriously high ash content, along with high percentages of near-density material in the mix. So, despite the high efficiency of DMS processes in general, the finely dispersed mineral particles in our coal require us to constantly weigh separation efficiency against the economic value of recov - ering the fines. “Coal mining regions in South Africa, India, China and certain parts of Australia tend to mine coal seams inevitably with rock mixed in. To liberate the coal particles, we crush the rock to break it apart and release the coal. At this point, we have lumps of coal and lumps of rock, and that's where the dense medium separation comes in. But with our coal, we often need to crush it finer to achieve the required liberation, typically to a size fraction +1.0 mm,” he explains. “For many years, our dense medium cyclones have been suitable for use to separate particle size fractions down to +2.0 mm. To further improve efficiency, the -2.0 mm discards would be sent to spirals, teeter-bed/fluidised-bed separators, or similar units for further beneficia - tion. But we have discovered that even this fine fraction can be processed using a dense-medium cyclone. It does present challenges on the plant design side, but we have proved that it is pos- sible,” adds Frikkie Enslin. “Multotec continues to play a leading global role in advancing DMC technology, building increasingly efficient plants that can treat fine fraction material. So instead of sending fines through an additional separation process, such as spirals, the dense medium cyclone plant can deliver higher efficiency fines separation directly,” says Bekker. “And while dense-medium cyclone plants are expensive to build and magnetite is costly, the value of the fine size fraction often makes dense-medium separation a cost- and resource- efficient way to go, particularly for the coal ore bodies we have in the southern hemisphere,” he concludes. A key takeaway, according to Bekker, is that although dense medium cyclone design has remained largely unchanged over the years, our understanding of the separating mechanism and the interaction between the cyclone and the overall process has improved. This understanding is helping provide better recommendations on select- ing cyclone units and the overall process, which benefits the clients. This understanding also helps determine where dense medium separation is the best option and where other separation process- es could be more economical, which can only be done if one understands the advantages and limitations of dense medium cyclones and the dense medium separation process. https://www.multotec.com/
Enslin points out that, despite improvements over the years, a modern DSM cyclone still resembles the original design published in the DSM bible.
A comparison between the separation efficiency of dense-medium separation and jig-washer technologies.
respect to dense medium recovery efficiency and energy consumption,” says Enslin. “Today, we still use ground magnetite to make a dense medium. For coal, because we cut at a relatively low density, magnetite is ideal. For other higher-density minerals, such as diamonds and iron ore, we use ferrosilicon (FeSi),” he notes. “Within each dense media type, there are also different grades based on their particle size distributions and shape. Magnetite can have a fine, medium, or coarse particle distribution, which can each be chosen to optimise cut points to best suit the composition of the coal being cleaned,” he explains. The dense medium cyclone (DMC) According to popular accounts, the DMC was first identified around 1939 by the Dutch State Mines (DSM) in the Netherlands, when a hydro- cyclone used to process loess in a dense medium circuit for coal cleaning became clogged. During cleaning, a significant amount of clean coal was found in the vortex finder, indicating that it was being concentrated in the cyclone's overflow, prompting further research and development that culminated in DSM's patenting of its DMC in 1942. DSM established a subsidiary, Stamicarbon, which licensed the technology along with a design manual for its licensees, the DSM bible. “Despite improvements over the years, a mod- ern DSM cyclone still looks very similar to the original design published in the DSM bible,”
says Enslin. Despite the many DMS technologies now on the market, he continues, the humble dense medium cyclone is still the technology of choice in the vast majority of coal-handling and pro- cessing plants. Enslin quotes Tim Napier-Munn, the minerals engineer, professor and author of Statistics for Mineral Engineers, who said: “It is difficult to see a serious competitor for the DMC in the foreseeable future in the processing of bulk commodities such as coal and iron ore.” Napier-Munn’s reasons include: • Efficient separations at a specific density. • The wide particle size that can be treated. • High tonnage throughput for a relatively small footprint. • A mature technology with relatively straightforward operation. With larger DMS cyclones up to 1.45 m in diam- eter now available to efficiently wash coarser coal particles, the need for DMS baths is also waning, notes Enslin.
Fines separation and SA’s contribution
“Back in 1949, under PJ van der Walt at the Fuel Research Institute of South Africa, we were one of the very first countries to investigate fines separation using DMCs, and we still have some of these original fines DMC plants in South Africa today. As a result of this work, we can get higher efficiencies from DMS separation than any other conventional fines processing
March-April 2026 • MechChem Africa ¦ 21
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