Teetered Bed Separator with Jigging Action

The phenomenon of "bed inversion" was observed for fluidised beds of plastic particles. At low fluidisation rates, small sized but high density particles form a suspended bed below larger but less dense particles of similar settling velocities. At higher fluidisation rates, the order reverses; the smaller denser particles then reside above the larger less dense particles. The observation of this phenomenon graphically demonstrates the fact that TBS units should be operated at the lowest practical fluidisation rate to achieve a high bed suspension density and hence maximise density separation, and minimise size separation. It was essential to recognise the importance of the suspension density when determining the additional benefit of jigging.

A pulsed water supply was then applied to a laboratory scale Teetered Bed Separator, (TBS). The effects of applying pulses of various frequencies and amplitudes to fluidised beds of coloured plastic tracer particles were examined. The coloured plastic particles were of various size and density combinations that were chosen so as to form a mixed bed at the specific fluidisation rate used in an earlier ACARP TBS study. We wanted to examine the benefit of jigging by commencing with a system of particles that could not be separated by the TBS. This system of particles represented, precisely, the separation limit of a TBS. If this combination of particles could be separated, we would have convincing evidence that the jigging provided a fundamental mechanism for improved separation. However, only very slight improvements in the density separation of the particles were observed when pulsed fluidisation was compared to steady fluidisation flows at an equivalent average volumetric rate. It is noted that equivalent average volumetric rates also generated equivalent bed heights and densities. This extensive preliminary work suggested that the pulsation would probably provide only little additional benefit over and above that achieved in a TBS operated at low fluidisation rates.

The laboratory scale TBS facility was then used to process a minus 2.0mm plus 0. 25mm coal feed in a continuous manner. Three such trials were conducted: The first of the jig experiments, referred to as Run 1, was conducted with the same average fluidisation rate as used in the earlier ACARP TBS work, but with a pulsed fluidisation at a frequency of 1 Hz, (which was found to be the optimum frequency in the preliminary work on the coloured particles). The second experiment, referred to as Run 2, was conducted with the same pulsed fluidisation condition, but at an average fluidisation rate 60% of that used in the first run. A lower average fluidisation rate was possible in this case because portions of the jigging cycle consisted of a fluidisation rate high enough to suspend the particles for at least part of the cycle. The third experiment, Run 3, was conducted with a steady fluidisation of the same average flow rate as Run 1. This run was effectively a repeat of one of the runs of the earlier TBS work and acts as the base case for this work.

Samples from the three coal runs were analyzed to produce partition curves for each of five narrow size ranges. Each run resulted in very similar separation performance. This indicated that significant advantages were not availed by applying cyclic pulses to the fluidisation water supply of a TBS for the processing of small coal. It is suggested that any advantages that may have arisen from the pulsed action may have been lost due to mixing of the bed on the upward portion of the water flow cycle. The effects of mixing could be expected to be significantly higher in a commercial scale system where the vessel height to diameter ratio is considerably smaller.