Dry cleaning of fine lignite in a vibrated gas-fluidized bed: Segregation characteristics

The segregation characteristics of fine lignite (−3 + 1 mm) in a vibrated gas-fluidized bed were examined. The pressure drop, ash contents and mass of a certain size fraction in lignite particles as a function of height through the bed, ash separation index, corresponding bed snapshots, and bubble size were investigated by varying superficial gas velocity. The effects of vibration parameters (frequency, amplitude, and vibration intensity) on segregation performance were also determined. Both density segregation and size segregation occurred as a function of superficial gas velocity. With increasing superficial gas velocity, the particle segregation changed to mixing state. An optimum air velocity existed to maximize segregation when fluidization velocities were between the minimum fluidization velocities of flotsam (clean coal) and jetsam (gangue). Bubble-driven flotsam-jetsam mechanism was responsible for the transition from segregation to mixing. The comparative ash segregation index and pressure drop characteristics between the vibro-fluidized bed and fluidized bed also confirmed this mechanism and suggested that application vibration achieved a high segregation performance at a low fluidization velocity because of reduced bubble sizes. The use of dimensionless vibration number (vibration intensity) alone to characterize segregation behavior could lead to misleading conclusions. Frequency and amplitude had strong effects on segregation performance.