A concentrating device that exploits differential densities of coal and its associated impurities by a combination of sluicing and centrifugal action. The material gravitates down through a stationary spiral trough with six turns (five for ore treatment) of mean radius 8 in (20.32 cm) with a fall per turn of 11 in (27.94 cm). Heavy particles stay on the inside, the lightest ones climb to the outside, and the resulting bands are separated at convenient points.