Stirred Gravity Flow Extractors

Provision of a shaft through the extraction column allows for repeated redispersion of the drops via various impellers located along the shaft. A variety of industrial equipment is available, with the differences being in the design of the impellers on the shaft for dispersion, and stators in the column for baffling and coalescence. Stirred columns offer the operator increased flexibility in operation by independent control over the dispersion process. 

RDC Column. The rotating disc contactor (Fig. 19) provides for redispersion by a series of discs along the shaft, combined with a series of fixed stators. Vortices are formed in each compartment, and the shear of the fluid against the rotor or stator causes the drop breakup. In many instances. 

Oldshue-Rushton Column. This column is similar to the RDC, except that the flat rotor discs have been replaced with turbine type agitators (Fig. 20). As with the RDC, the diameter of the agitators can be varied along the shaft to compensate for the progressive change in the physical properties of the system as extraction occurs. 

Other variations of stirred columns which are available include the asymmetric rotating disc (ARD) contactor, the Kuhni column, and two types of Scheibel columns. The rotor of the ARD is located off center, which permits more elaborate baffling for the necessary transport of flows with less backmixing. 

The Kiihni column employs radial flow impellers located between perforated plates for compartmentalization. The first Scheibel column used wire mesh zones to promote coalescence and limit backmixing between turbine-agitated mixing zones. A later Scheibel column used a shrouded radial impeller and multiple ring baffles to direct most of the rotor s energy towards dispersion and away from axial mixing. 

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