A liquid-fluidized bed of inert particles was used to separate a pure object from a mixture. One (binary solid-liquid-fluidized bed) or two (tertiary solid-liquid-fluidized bed) types of objects with relatively large-sized particles were immersed in an inert-particle bed, and the bed behavior was observed for different liquid velocities. The void fraction and apparent density of the inert-particle suspension were predicted by considering the effect of the change in object position for different liquid velocities. The prediction method, which considers the change in the minimum fluidization velocity, accurately expressed the changes in the void fraction and the apparent density of the bed with the position of the objects in the bed. Using this method, the liquid velocity required to separate a certain kind of object from a mixture can be predicted.