Numerical investigation of the separation behaviours of fine particles in large dense medium cyclones

• The behaviours of fine particles in an extra-large DMC are numerically studied.
• The poor separation of fines in the DMC gets worse than the widely used smaller one.
• The deficiency is explained by the asymmetrical separation zone and strong eddies.
• Several modifications with regard to DMC operation and design are introduced.
• Deteriorated separation of fines is overcome by an increased Head or cone length.

Use of large dense medium cyclones (DMCs) is a potential trend in the coal industry but its usage is limited by the poor separation efficiency of fine particles. This paper presents a numerical study of the multiphase flows and performance of DMCs used for coal preparation by means of the two-fluid model. The validity of the model has been verified by various applications. It is used here to study the behaviours of fine particles in a 2-m DMC that is larger than the biggest DMCs reported thus far in the coal industry. The numerical results show that in the extra-large DMC, the poor separation efficiency of fine particles gets worse compared to that of a widely used 1-m DMC. This deficiency is found to be attributed to the strong vortexes developed and the asymmetrical separation zone that can be characterised by the correlation between pressure gradient and tangential velocity. Several modifications with respect to mounting degree, operational Head, conical section length, and inlet number are introduced to improve the performance of the 2-m DMC. It is shown that the separation efficiency of fine particles in the 2-m DMC can be better compared to the 1-m DMC by increasing the Head or conical section length, because such modifications reduce the asymmetrical separation zone and/or amount of vortexes.