Pressure spectral responses of liquid–solid fluidized beds for coarse-particle flotation: Effects of particle size and liquid velocity

Fluidized bed flotation (FBF) is a gas–liquid–solid separation technology increasingly applied to coarse-particle recovery, yet the pressure-fluctuation behavior of the liquid–solid fluidized bed that underpins its hydrodynamics remains insufficiently understood. In this study, dynamic pressure signals were measured in an FBF column under single-phase water-flow and liquid–solid fluidization conditions for three ore particle size fractions (−900 + 600, −600 + 450, and − 450 + 280 μm) over a range of superficial liquid velocities. The liquid–solid beds exhibited staged fluidization behavior, progressing through fixed-bed, central partial-fluidization, fluctuating partial-fluidization, and complete-fluidization regimes. Conventional time-domain statistics primarily reflected overall fluctuation intensity and showed substantial overlap among fluidization regimes and particle size fractions, limiting their capability for state discrimination. In contrast, frequency-domain analysis identified a pronounced broadband spectral enhancement within 25–75 Hz under liquid–solid conditions, whereas this response remained substantially weaker under single-phase water flow. The enhancement generally increased with liquid velocity, and the medium and coarse particles exhibited lower-frequency spectral activity than the fine particles, with the strongest activity occurring at approximately 58–63 Hz for the fine particles and predominantly within approximately 37–43 Hz for the medium and coarse particles. A band-energy metric, R mid , defined from the 25–75 Hz spectral energy ratio, exhibited a more consistent velocity-dependent trend than the time-domain standard deviation. These findings demonstrate that frequency-domain features better resolve the characteristic pressure response of liquid–solid fluidization, and that R mid can serve as an auxiliary indicator of liquid–solid fluidization activity under fixed particle-size conditions. The present findings establish the baseline pressure-fluctuation response of the liquid–solid system and provide a basis for extending the analysis to gas–liquid and gas–liquid–solid conditions.