Investigation of the Reflux Classifier for Density and Size Separations from Pilot to Full-Scale

The Reflux Classifier is a new device for separating particles on the basis of either density or size. Water flows up through a distributor plate at the base, suspending particles within the vessel. A set of parallel inclined plates amplifies the segregation rates of the particles, permitting slower settling particles to pass through the zone of inclined plates while causing the faster settling particles to return to the zone below. The inclined plates effectively increase the sedimentation area of the vessel and permit higher throughputs. Gravity separation is achieved by generating a high suspension concentration of high-density particles in the lowest zone, whereas hydrosizing occurs when the device is operated at lower concentrations and higher fluidisation rates.

This report summarises the findings of both a pilot-scale and full-scale trial of the Reflux Classifier, the latter conducted at a Lower Hunter Valley Coal Preparation Plant using coal nominally less than 2mm. The pilot-scale device had a square cross section of 0.6 x 0.6m and a height of 3.5m, and the full-scale device had a cross-section of 1.8 x 1.9m and a similar height of about 3.5m.

Gravity Separation
The objective was to recover the combustible material at the lowest possible ash content. Washability analyses were conducted to examine the variation in the D50 with the particle size. The separation performance of the pilot and full-scale devices proved to be excellent, with throughputs to the full-scale Reflux Classifier in the range from 14 to 20 t/m2-h, which is significant compared to more conventional fluidized bed devices. The low product ash of 6% and high reject ash of 79% achieved by the full-scale device for one coal seam, and similar results for other seams, indicates that the Reflux Classifier was operating well within its "comfort zone" during the trial.

During continuous sampling campaigns, the product and reject ash levels remained relatively constant despite variation in the feed ash and solids throughput, demonstrating robust operation. Additional experiments were conducted to establish the potential for lower product ash by moving the separation condition down the yield-ash curve.

Changes to the fluidisation rate were found to have minimal effect on the combustible recovery in both studies. In the pilot-scale study, increasing the fluidisation rate increased the ash content of the product stream (-2+0.25 mm) due to fine mineral matter particles being carried up through the system. In the full-scale study, however, the increase in the fluidisation rate did not increase the ash content of the product stream (-2+0.25 mm), mainly because of the high hydraulic capacity of the device resulting from the use of the inclined plates. Other possible factors include the PID control used to maintain a fixed bed density, and the absence of -0.150 mm particles in the feed.

Summary of Gravity Separation work based on Run G7 for the Full-Scale Reflux Classifier run at 54 t/h (16 t/m²-h) of solids. Data is consistent with other separations at pilot scale where the throughput was 45 t/m²-h.

Hydrosizing
In hydrosizing it is necessary to operate at significantly lower system concentrations and allow particles to settle freely. This mode necessitates the use of a lower feed solids loading than is possible in gravity separation. The Reflux Classifier provides a method of hydrosizing at solids throughputs of up to about 20 t/m2-h. We believe this throughput to be significantly higher than could be achieved if a teetered bed separator was to be used for hydrosizing.

In general, a similar level of performance was observed at the pilot and full-scales. It must be appreciated that in hydrosizing the separation depends on both the particle size and the density. Thus, a relatively narrow range of particle densities is needed before the true separation efficiency can be judged.

In general, a similar level of performance was observed at the pilot and full-scales. It must be appreciated that in hydrosizing the separation depends on both the particle size and the density. Thus, a relatively narrow range of particle densities is needed before the true separation efficiency can be judged.