Extending The Size Range Of The Reflux Classifier

A comprehensive study examining the potential of the Reflux Classifier to be applied to the beneficiation of coarser coal up to 8 mm in size was undertaken. It was established in this study that the Reflux Classifier can be operated successfully using a feed with an upper size of 4 mm and also an upper size of 8 mm, offering high combustible recovery and control of the separation density to target low ash products. The solids throughput was examined at rates of up to 36 t/m².h, with the throughput limited by the mechanics of the feed systems. It was necessary to also limit the solids rate to this level to ensure the trial work did not impact negatively on the plant.  Thus the work was conducted below the throughput limit of the device. A major finding from the study was the critical importance of providing sufficient fluidization water, though beyond the critical level the process was largely insensitive to the fluidization rate. It was concluded the required velocity is nominally 10 m/h per mm of top-size, hence for a genuine nominal 4 mm top size the required fluidization velocity is 40 m/h, regardless of the solids throughput.

The study demonstrated that the Reflux Classifier is a valid competitor to dense medium cyclones for a feed up to 4 mm in size, but not competitive above 4 mm. In general, the separation density was found to approach a constant value as the particle size increased, but the drift was still higher than for dense medium cyclones. In using a Reflux Classifier to process a feed up to 4 mm in size there would be little negative impact on the overall plant performance.

In general, the operation of the pilot scale Reflux Classifier, the RC600, using the coarser feed, with particles up to 8 mm in size, was straightforward. Reject from the RC600 was observed within a couple of minutes of supplying feed to the unit, and steady operation was apparent within about 15 minutes. Consistent performance was observed at all times throughout the campaign, when sufficient fluidization rates were used, with rapid response to changes in set point evident. Product quality appeared to be very high despite the very low grade of the feed. And changes in product ash were secured through changes in the set point. Lower fluidization rates would be possible by reducing the cross-sectional area of the lower zone, with a transition section between the two parts of the Reflux Classifier.