The segregation of ultrafine particles in a centrifugal fluidized bed and the effect of a ragging ring and pulsed fluidized water on the segregation efficiency have been investigated. A mixture of ultrafine clean coal and silica sand used as feed materials and a pilot-scale continuous Knelson centrifugal fluidized bed separator were used to evaluate the effects of four main variables, i.e. centrifugal force, fluidized water flow rate, and discharge valve opening time and closing time, on the overflow product grade and recovery. A new design of fluidization distributor, 'the ragging ring', was used to generate homogeneous fluidized beds. An orthogonal statistical test programme was conducted on the new distributor and it was found that the ragging ring could not significantly improve the overall performance of the unit. The ragging ring could not increase the superficial water velocity. This eventually resulted in similar performance as the original superficial velocity ring achieved. Although different size glass beads were used, there were not significant differences in the separation performance because of their similar porosities. The optimum parameters were identified by using different ragging materials, pulsed water flow rates and pulsation frequencies. It was noted that the ragging ring with 5-mm glass beads generated the best results with a pulsed water flow rate of 18 1/min and 1-Hz pulsation. This was probably due to achieving the appropriate porosity as well as the packing of the bed inside the ring. (C) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008.