A fluid-bed continuous classifier of polydisperse granular solids

The use of a tapered fluidization column to continuously classify streams of dissimilar solids has been investigated. Solids classification is based on the different propensity of dissimilar solids to segregate when operated in the transient fluidization regime. A test apparatus based on this concept was set up and operated with mixtures of solids having equal density (about 2500 kg/m3) and different diameter (flotsam = 125 mm, jetsam = 500 mm). The jetsam mass fraction of the fed mixture ranged between 0.25 and 0.75. The mass flow rate of the solids mixture ranged between 2.4 and 5.8 kg/h. Results showed that the jetsam fractional mass in the jetsam-enriched stream was nearly 100% and that in the flotsamenriched stream was as small as 2–8%, under the operating conditions investigated. The overall classification effectiveness was as high as 90%. Operation of the classifier was successfully carried out for more than 6 h, with excellent reliability and steadiness of the performance. The proposed classification technique features several advantages over other fractionation methods: absence of moving or easily wearable mechanical parts in the classifier; no need for a liquid medium; limited solids attrition; possibility to operate the classifier at high temperatures and/or under controlled atmospheres; possibility to perform fractionation at the same time as other physico-chemical processes (e.g. drying and calcination).