AN EMPIRICAL DESCRIPTION FOR THE CLASSIFICATION IN AN INCLINED COUNTER-FLOW SETTLER

The performance of an inclined counter-flow settler was examined and f ound to offer significant cut-point control in size classification, an d hence a possible alternative to using hydrocyclones. A suspension of fine bituminous coal, nominally less than 300 mu m was fed into the b ase of a 25 mm diameter tube, 1.4 m long, and inclined at an angle of 45 degrees. The slurry flowed up through the vessel, depositing relati vely coarse particles onto the lower portion of the wall of the tube, resulting in the remaining suspension becoming finer and less concentr ated. The deposited particles slid down the wall, and were pumped from the base of the tube at an elevated pulp density, while the remaining suspension flowed out through the top of the vessel. Tests were condu cted using eight different feed pulp densities, ranging from 2.4 to 21 .7% solids. The classification or partitioning of the particles betwee n the underflow (coarse) and overflow (fine) streams was determined fo r each of the feed pulp densities at six different flow rates. The max imum flow rate, relative to the minimum flow rate, was 4.5. The separa tion sizes, defined by the D-50, ranged from about 3 to 116 mu m. In o rder to reduce the degrees of freedom in the experiments, the underflo w was operated to produce the maximum possible underflow concentration . The subsequent minimization of the waterflow in the underflow also r esulted in a minimization of the fines entrainment into the underflow by the water. There was excellent agreement between the reduced partit ion curves from 38 of the 48 tests, with poor agreement for the experi ments conducted at the lowest flow rate and or feed pulp densities. Fu rthermore, all of the data were found to correlate according to phi(0) , = 0.44(V-t/V-0), where phi(0) is the volume fraction of the solids i n the overflow, V-0 is the velocity of the overflow through the tube, and V-t is the terminal velocity of the D-50 particle. Copyright (C) 1 996 Elsevier Science Ltd