The most common process factors that affect Dense Medium Cyclone (DMC) performance can be mitigated by understanding the operation of a hydrocyclone, as well as the overall Dense Medium Separation (DMS) process. “Early identification of problem areas means that appropriate solutions can be formulated, which is where the experience and expertise of Multotec comes to bear,” says Ernst Bekker, Cyclone Product Specialist.
Dense medium separation takes place where particles of differing density fractions are separated via a medium that has a density between that of the two fractions. Particles with a lower density will float, while particles with a higher density will sink. The aim is to minimise the quantity of non-valuable material in the targeted fraction while maximising the recovery of the valuable fraction.
The feed in a hydrocyclone consists of water and solids, which enters the cyclone tangentially to produce a spiralling motion. Due to centrifugal forces, the coarser particles migrate to the outside wall of the cyclone and move downwards towards the spigot or apex. The finer particles move to the centre of the cyclone and are removed through the vortex finder into the overflow.
Due to the inverse ratio of solids and water in the outgoing streams, a high slurry density is produced in the underflow and a low slurry density in the overflow stream. The same separation process applies to the medium used inside the DMC, with the medium split mainly being influenced by the ratio of spigot to vortex finder diameter, the operating head and medium characteristics.
As a result, density gradients form throughout the length of the DMC. The differences in medium density values between the feed, overflow and underflow streams are used to calculate the differentials inside the DMC. These are important parameters in defining the medium behaviour inside the cyclone and can be used for early identification of possible process related problems.
Cyclone suppliers have incorporated larger inlet designs in their product portfolio to allow for higher capacities and larger particle top sizes to be accommodated. “The drawback of these designs is that the retention time of particles inside the cyclone is reduced, which can have a negative impact on separation efficiency if it is a difficult separation. Therefore particle top size, which impacts on the cyclone inlet opening, plays an important role in determining the correct cyclone diameter, in addition to the throughput that must be accommodated,” Bekker says. The inclusion of a barrel section in the DMC depends mainly on the difficulty of separation. This allows for more residence time inside the cyclone due to its larger internal volume.
However, correct selection of the DMC does not mean that the unit will perform correctly. “OEMs constantly look at ways to improve their designs, but these improvements are only effective if the process itself is optimised,” Bekker says. The dense medium process consists of various process streams and pieces of process equipment, which can influence the DMC performance if not optimised, for example magnetic separators. “Operational personnel sometimes do not realise or understand these dynamics.”
A decrease in top size results in a finer mean particle size fed to the DMC, which can impact the separation efficiency if the cyclone selection was originally not designed for that type of material, while an increase in top size can increase the risk of blockages at the cyclone inlet. “The effect of decreasing or increasing the top size needs to be verified with the cyclone OEM,” Bekker cautions.
“Proper DMC maintenance, in addition to metallurgical audits, may be one of the most important factors to help ensure optimal DMC performance. This is often neglected due to inaccessibility or time constraints,” Bekker says. Excessive internal wear creates grooves inside the DMC, leading to turbulent flow patterns, which can cause misplacement of particles.
It is recommended that high wear areas inside the DMC be inspected on a weekly basis or as often as possible, while inspection of the rest of the DMC internals can be carried out on a monthly basis. However, inspecting the spigot timeously also gives the operator or metallurgist access to the rest of the DMC internals.