Abstract
An emulsion liquid membrane (ELM) extraction rotating disc contactor (RDC) column with auxiliary equipment has been designed and tested. An ELM spray column was designed as well. ELM is a suitable technique for the removal of heavy metal ions from waste or process streams. However, no design rules are available yet. This paper proposes and verifies a design procedure based on solvent extraction methods. The equipment was designed to reduce the 100 ppm cadmium concentration in a 90 L/h feed stream to 5% of its initial value, and to achieve a concentration factor of at least 12.5 in the strip phase. Trilaurylamine (1%) in kerosene was used as the extractant solution. The RDC column diameter was 70 mm and its designed height was 2.2 m. The spray column had a diameter of 50 mm and a designed height of 4.2 m. Both columns were constructed from five separate parts of 1 m height each to obtain the necessary flexibility. Experiments showed a reduction of the cadmium content down to 1% of the initial value and a concentration factor of 14 with the RDC. The efficiency of the spray column experiment was 50%. The rotor speed and the hold up in the RDC had to be kept lower (0.75 and 0.55 times, respectively) than the design values to avoid entrainment of the smallest drops with the feed phase. The validity of the models used in the design was assessed by inserting the actual experimental conditions in the design equations. This resulted in a good coincidence of the actual and calculated characteristic drop velocity, a good coincidence of most mass transfer coefficients, and a reasonable estimation of the number of equilibrium stages. The auxiliary equipment, comprising among others an electrostatic emulsion splitter, operated as designed at optimal conditions. The splitter proved to be critical at smaller emulsion droplet sizes. It can be concluded that design methods for a solvent extraction column are suitable for ELM.