MechChem Africa September-October 2023

ore prior to sorting. “Here only 1 to 2% of the inflow is diamond bearing and this passes through the spigot. The rest all goes out through the overflow. And if the underflow ratio goes up to just 4 or 5%, it becomes a problem because of the very accurate diamond sorting pro cesses that must take place downstream of the cyclone, which can easily become overloaded,” says Bekker. In a similar way, he adds that dense medium cyclones are now being looked at to pre-concentrate iron ore; manganese; and for some PGM and copper applications. “The idea is to remove waste rock before sending the product to the grinding mill, so all the milling energy is focused only on the valuable target material, which is an interesting cost saving and productivity increasing concept,” he adds. “While a small percentage of the valuable material is likely to be lost to the DMC, if

in the early years, this resulted in easy separation, typically with 70% being coal at the overflow and only 30% being rejected as waste rock to the underflow. But we are now dealing with much lower grade coal ore, so the ratio is more or less the other way around, 70% being rejected as waste material through the spigot and only 30% being separated out as coal at the vortex finder in some cases,” Bekker notes. “This causes capacity issues at the spigot in handling the larger volumes of waste materials and a decision has to be made as to whether to make the spigot bigger or to replace the dense medium cyclone with a bigger one,” he adds. He says that certain design ratios apply between the cyclone diameter, the vortex finder, the spigot and the inlet diameter. If the cyclone diameter is D, for example, then the vortex finder is normally 0.43×D and the spigot can be anything between 50% and 70% of the vortex finder diameter. The standard inlet opening of a dense medium cyclone is normally 20% of the cyclone diameter. “If the inlet opening is made bigger, more volume can be introduced into the unit, but this reduces the residence time inside the cyclone, which results in more misplace ment of waste in the overflow and/or coal in the underflow, particularly in the case where the near density material (NDM) is high. Near density material is defined as the amount of material present around the expected cut density in a band of +/- 0.1 RD units. For example, if the cut density is RD1.6, then the material present in the density range of RD1.5 to RD1.7 would be classified as near density material. If the percentage of NDMs is very high, let’s say above 65%, then separation is diffi cult. Easy separation has anything between 20 to 25% NDMs and for medium density cyclones treating very good quality coal, NDM percentages can be as little as 2% in some of Multotec’s overseas operations. “But here in South Africa, we use a lot of low-grade coal and it is quite difficult to distinguish clearly between the good coal and the waste. There are operations that can have up to 90% of the ore coming into the dense medium cyclone that is classified as near density material, making separa tion very difficult,” Bekker tells MechChem Africa, adding that cyclones for these ap plications need to be sized correctly, with very accurate feed conditions. Diamonds and pre-concentration prior to milling Alongside coal, Bekker says that dense medium cyclones are widely used as part of diamond pre-processing from run-of-mine

you can recover 90 to 95% of the valuables from 30% of the total ore mass, then the savings and productivity improvements from reduced milling can far outweigh this loss,” he points out. “At Multotec, we are process specialists who offer a variety of different minerals processing solutions and combinations. We spend time with the clients, building trust and sharing knowledge, regardless of any associated order or monetary value. And because we are an OEM supplier of several different technologies, we are able to steer our clients towards the right combination of technologies, whether those involve hydrocyclones, dense medium cyclones or combinations of several technologies. “Every system we design and build is customised for specific client needs to be fit-for-purpose – and we take responsibility for ensuring this is the case,” Ernst Bekker concludes. q

Multotec’s range of ceramic lined cyclones provides the ultimate in wear resistance with its alumina tiled cyclone engineering design.

A common application for hydrocyclone clusters is in a mill circuit to classify right-sized material and to send the oversized fraction back to the mill for further grinding.

20 ¦ MechChem Africa • September-October 2023