The spray column, shown schematically in Fig. 4.29, is the simplest of the contactors. The heavy aqueous phase enters the top of the vertical cylinder through a distributor and flows downward under gravity, usually as the continuous phase. A distributor at the bottom of the column disperses the entering organic phase into small drops, which rise through the continuous heavy phase and collect in a layer at the top. Coalescence of the dispersed phase drops and axial circulation and mixing of the continuous phase result in relatively low efficiency of contacting. Very tall columns may be required to obtain only a few theoretical stages.

Because of absence of internal structure, spray columns are sometimes selected for

liquid-liquid separation when suspended solids are present, as in the extraction of zirconium from hafnium in acidic thiocyanate solutions, wherein solid thiocyanate polymers tend to form (Chap. 7).