Incipient fluidization is a stage of operation in a fluidized bed reactor, whereby at minimum fluidization velocity, the force of a fluid on solid is sufficient to stabilise the weight of the solid material. By using the similar principle, carbon can be separated out of other residues with no danger of leaching or contamination. This separation method applies in an air-fluidized bed of high-aspect-ratio which works at incipient fluidization. In a study carried out by R.H. Hurt, K.A. Davis, N.Y.C. Yang, T.J. Headley, G.D. Mitchell (1995), they studied on eight carbon containing fly ash samples for characterization purpose. Prior to the characterization process, the samples were separated via incipient fluidization to reduce the interactions between mineral matters during the characterization test. From their study, they were able to collect unburned carbon with relatively high purity, approximately 75 wt % carbon from 90-180 µm size fraction of the fly ash sample. This air-fluidized bed works on the basis of density to segregate the carbon particles from the mineral-rich particles. Ash of specified size range is injected into a 1 cm diameter tube to form bed. It should be within height-to-diameter ratio of four. Then, nitrogen gas is initiated through a fritted disk at minimum fluidization velocity, whereby at the flow rate the solid bed is not fully fluidized (R.H. Hurt et al., 1995). The flow rate was fixed at this point in order to generate local particle motion and prevent macro mixing due to large-scale bed flows.
As a result of the incipient fluidization separation, the sample was separated in low and high density extract. The low density extract is the desired product, unburned carbon enriched extract. The fluidizing air is interrupted and desired product is removed from the fluidizing bed via capillary suction sampling system once sufficient separation is achieved.