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Critical impeller speed for the suspension of solid particles, N,,, has been measured in 0.3, 0.4,0.57, 1 .O, and 1.5 m ID mechanically agitated contactors. Tap water and quartz particles (100, 340, 700, 850, 2,000 pm) were used as liquid and solid phase, respectively. The impeller speed was varied from 3.5 to 13.3 r / s and solid loading from 0 to 50 wt. YO.Three types of impellers were employed: disk turbine, pitched turbine downflow, and pitched turbine upflow. The impeller diameter to vessel diameter ratio was varied in the range 0.175 to 0.58 and the impeller blade width to impeller diameter ratio was varied in the range 0.25 to 0.4. The impeller clearance from the tank bottom was varied from 0.5 to 0.167 of tank diameter. The effect of impeller blade thickness was also studied.The pitched-blade impellers were found to be more efficient than a conventional disk turbine, and the pitched turbine downflow type was found to be more efficient than a pitched turbine upflow impeller. An attempt has been made to explain the mechanism of suspension on a rational basis and a correlation has been proposed for the estimation of Ncs that is expected to be useful in design.

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