MechChem Africa January 2019

In the context of increasing coal tonnage requirements using lower quality coal and the least water possible, Multotec’s expertise in cyclones, screens and dense medium separation technologies is being put to use to improve understanding of equipment and to enable operators to look proactively for opportunities to optimise water usage. Optimising water and coal washing efficiency

T he growing scarcity of water is placing pressure on the coal mining industry in many countries. On one hand, as reserves of better coal are depleted,minesmusttreatmorelowerquality coal and meet increasing tonnages; while on the other, they must reduce water consump- tion. The coal preparationprocess is normally water-based and uses substantial amounts of water, so optimising its performance will impact positively not only the environment but also the bottom line. One important response to this pressure has been the move from the traditional tech- nologyof bathwashers tocentrifugalwashers such as cyclones, as this allows for a higher tonnage per footprint while maintaining op- timal separation efficiency. Depending on the coal characteristics, it may be possible to re- duce themedium to ore ratiowhilemaintain- ingoptimal separationefficiencyand, bydoing

so, reduce water use in the medium circuit. These types of changes, however, can only be made if the material characteristics, the separation process, and factors affecting the separationprocess, are properly understood. To optimise the effectiveness of cy- clones, the parameters surrounding the dense media separation (DMS) cyclonemust bewell understood. The cyclone’s efficiency is determined by the nature of the feed, by the cyclone’s dimensions and mainte- nance, and by the circuit’s influence on the equipment. Thecycloneappears tobea simplepieceof equipment, but the operator needs to under- stand thephysics of how it operates, aswell as themechanismof separation. For this reason, Multotec introduced short courses for cus- tomers, addressing topics such as principles

TeePee™ panels fromMultotec provide

significantly increased drainage capacity compared to conventional flat panels. The increased exposed surface areas enable screens to operate at higher capacities and ensure a 50% increase in effective drainage area. of separation, operation and fault-finding. An additional process that is incorporated into modern designs to reduce water loss is the use of coal centrifuges, which can reduce the surfacemoisture content of thedenseme- diumcircuit product from8.0 to15%down to 5.0 to 12% depending on the size of particles being processed. A proper understanding of the design of this equipment and factors af- fecting the performance of centrifuges goes a longway in helping to optimise this process. The performance of screens is also be- coming an important point of focus in coal washing, in light of rising tonnages per footprint and media volumes. Factors such as bed depth, spray water addition and the open area on screen panels all affect the performance of desliming screens. Here, it is vital to understand screening principles so that optimal screen performance is ensured; this not only improves screening efficiency but also conserves water. Withdrain and rinse screens, it is crucial to drainhighervolumesofmediaassociatedwith ever increasing tonnages being processed, to minimise media losses. While it may not be practical to replace the screen with a larger version, alternative screen panel designsmay beanoptionforimprovingdrainagecapability. When drain sections perform optimally,

Multotec has optimised dense medium separation processes in the pre-concentration of minerals, specifically for coal and diamonds, for over 40 years.

28 ¦ MechChem Africa • January 2019