Experiments were performed in mechanically agitated contactors with 6 (flat) blade turbine impellers, to study the effect of various operating parameters on minimum/critical speed required for complete suspension in two-phase (liquid-solid) as well as three-phase (liquid-solid-gas) systems. Based on 1212 experimental measurements, a unified empirical correlation is proposed, for the estimation of minimum impeller speed for solid suspension in both two-phase and three-phase systems in terms of fundamental, operational and geometrical variables.