BEHAVIOR OF MULTISTAGE MIXER-SETTLER EXTRACTION COLUMN
Katsuroku TAKAHASHI and Susumu NII
Department of ChemicalEngineering
A counter-current multistage extraction column having high performance (i.e., highextraction efficiency and large maximum throughput) has been developed. The hydrodynamic behavior and the mass transfercharacteristics of the column were ana-lyzed experimentally. Each stage of the column consists of lower mixer part, upper settler partand a drop coalescer between them. Both of the continuous and the dis-persed phases rise from the mixer into the settler and areseperated into two phases within the settler. The continuous phase goes down to the lower stage mixer through the downspout pipes and the dispersed phase rises to the upper stage mixer through the riser pipes. The maximum throughput in the column is independent of the drop size because the counter-current flow in the dispersion situation is avoided. Then the column can be operated at a strongagitation where the extraction efficiency is high due to large interfacial area with small drops. As the hydrodynamic behavior, the maximum throughput, the holdup of dispersed phase and the drop size were ex-amined. The maximum throughput can be determined from the balance between the pressure drop within the downspout and the sum of the suction pressure induced by the impeller and the buoyant force of dispersed phase. The holdup of the dis-persed phase in the mixer is given by a model of dispersed phase leaving the mixer, including the diffusional flow of dispersed phase. The drop size in the mixer de-pends on the residence time of the dispersed phase as well as the Weber number. The mass transfer characteristics can be expressed by a rigid sphere model of drop because the drop diameter is small at a strong agitation in the mixer. A theoretical model of diffusion within a rigid sphere gives the mass transfer coefficient in the dispersed phase, and the correlation by Ranz-Marshall for the mass transfer around a rigid sphere gives the mass
transfer coefficient in the continuous phase. The ex-traction of copper with the five-stage mixer settler extraction column was represented by the calculation with the above hydrodynamic and mass transfer characteristics as well as the extraction reaction rate at the interface.
Keywords: extraction, extraction column, maximum throughput, holdup, drop size, mass transfer coefficients