Experimental Study and Model Refinement of Minimum Fluidization Velocity in Binary Medium Systems for Gas–Solid Separation Fluidized Beds

In gas–solid separation fluidized beds, magnetite powder is widely used as the dominant dense medium, owing to its ease of recovery and preparation. The minimum fluidization velocity (Umf) of binary medium systems significantly affects bed stability and separation efficiency, yet conventional correlations, mostly derived for monodisperse systems, fail to accurately predict Umf in binary medium systems. In this work, systematic experiments were conducted on binary systems dominated by magnetite powders, including Geldart B–B, B–A, and B–C systems. Measurements of the bed pressure drop and superficial velocity showed that adding finer or lighter particles reduced Umf, with the largest reduction in B–C mixtures, where even a few percent of fines decreased Umf by over 10%. B–B mixtures exhibited smooth transitions without significant segregation, while B–A and B–C systems displayed stratification when the fine fraction exceeded 15–20%. To improve prediction, the critical bed voidage was expressed as a function of magnetite sphericity, and effective density and diameter were introduced for binary mixtures. By introducing the correction factor α, the base correlation for the Umf was refined, yielding average relative errors of 20% for B–B mixtures and 10% for B–A and B–C systems. These results demonstrate the combined influence of magnetite sphericity and mixture composition on incipient fluidization, providing a reliable approach for predicting and controlling Umf in air dense-medium fluidized beds. These results demonstrate the combined influence of magnetite sphericity and mixture composition on incipient fluidization, providing a reliable approach for predicting and controlling Umf in air dense-medium fluidized beds, with direct relevance to industrial mineral separation applications.