Density-based separation in a vibrated Reflux Classifier with an air-sand dense-medium: Tracer studies with simultaneous underflow and overflow removal

The continuous separation of tracer particles in the air-sand dense-medium Reflux Classifier was investigated. The Reflux Classifier consisted of a 1 m long vertical fluidized bed section with a 2 m long channel inclined at 70° to the horizontal mounted above, both with a 20 × 100 mm cross section. Silica sand of 220 μm average diameter (−355 + 125 μm) was used as the dense medium. The Reflux Classifier produced good density separations for tracer particles ranging in size from 6.35 down to 1.0 mm. The density cut-point could be varied from 1418 to 2130 kg/m3 by varying the underflow rate and the Ep was within the range of 0.06–0.46 × 103 kg/m3 depending on particle size and gas rate. At certain gas flowrate and underflow conditions the density cut point ranged between 1534 and 1619 kg/m3 across six particle sizes, suppressing the effects of particle size on the density cut point. As air rates increased from 4.03 to 5.64 × 10−4 m3/s the density cut-point increased, as did the Ep. The results were compared with separations in a vertical fluidized bed of the same total length. Ep values in the vertical fluidized bed ranged between 0.07 and 1.49 × 103 kg/m3 over the same experimental conditions as the inclined bed and the density cut point showed more variability with the conditions. The addition of an incline above the fluidized bed provides a more stable system allowing for greater separation efficiency and minimizing the effect of changing conditions. Increasing the flow of sand medium to the underflow decreased the density cut point while raising the gas rate increased the density cut point. Raising the gas rate also increased the variability of the system which resulted in a lower separation efficiency.

► The continuous separation of tracer particles in a pneumatic Reflux Classifier was investigated.
► An air-sand dense-medium was used with vibration.
► Reflux Classifier performs better than a vertical fluidized bed.
► Ep and ρ50 compare favorably with water based separations for particles larger than 4 mm.
► Best separation conditions for coal are low gas fluidization rate and high frequency vibration.