In column contactors described so far, energy available for droplet dispersion (and hence, the interfacial area available for solute transfer) is derived solely from the density difference between the phases; so the physical properties of the system set a limit to achievable extraction efficiency. This limitation may be overcome by supplying additional energy to the contactor and this concept has given rise to a large variety of so-called mechanical columns. Columns with coaxial rotating members of various designs have been described in the literature and the so-called rotating disc contactor illustrated in Figure 2e is a good example. This is basically a spray column with a central rotating shaft bearing a series of flat discs that rotate between fixed annular baffles. The shear forces set up produce very small droplets of dispersed phase and a correspondingly large interfacial area for mass transfer. Whilst such a unit gives good extraction efficiencies, the rotating shaft involves seals or bearings within the column and is therefore unsuitable for processing toxic or corrosive liquids.