Liquid-fluidized bed classification (LFBC) is being investigated as a potential process for the separation of complex waste plastic mixtures for recycling. Experiments were performed in a 10 cm diameter, fluidized bed apparatus that was designed to operate in both upflow and downflow modes, for the separation of "heavier-than-water" (HTW) and "lighter-than-water" (LTW) plastic particle species, respectively, using water as the fluidization medium. The investigations were primarily focused on binary plastic particle mixtures of the typical industrial plastics, PVC, PET, PS PC, HDPE, LDPE, and PP. Particle segregation was obtained for all the binary mixtures studied. The effects of particle size and superficial liquid velocity were examined. It was found that low fluidization velocities favored segregation performance by minimizing particle dispersion. A limited number of experiments with ternary plastic particle mixtures demonstrated that the behavior of more complex mixtures appears to be an extension of binary mixture behavior, and that multiple plastic particle mixtures can be separated in the same column. The novel technique of selective thermal modification of particle size/density by application of a step change in temperature of the fluidizing medium was demonstrated to improve separation performance for both HTW and LTW plastic sample mixtures. This behavior was attributed primarily to differences in the thermal diffusivities of plastic particle pairs. The experimental data and conclusions reported in the current work can serve as the basis for the development of LFBC systems for waste plastics separation. (c) 2005 American Institute of Chemical Engineers