Factors influencing operational efficiency of an electrostatic solvent extraction system

A continuous electrostatic liquid-liquid solvent extraction process is being studied for the concentration of metals from dilute solution, as required for the recovery of toxic or valuable metals. Complementary extraction and stripping operations occur simultaneously in cells separated by a baffle. Aqueous crossover from the extraction cell to the stripping cell dilutes the product and must be minimised whilst maintaining high mass transfer of ions. Crossover increases with the electric field intensity, partly due to an increased number of smaller, penetrative droplets. It is evident that inlet droplets have net charges as they settle under gravity with rapid transverse oscillatory motion. Oscillation amplitude depends on both field strength and frequency, although oscillation frequency is lower than the applied frequency. At low frequencies, droplet-electrode contact commonly occurs, increasing the oscillation speed and encouraging crossover. This implies that charge has been exchanged at an insulated electrode. Oscillation is greatly reduced above 100 Hz, and crossover is decreased. Above 220 Hz, fine dispersions with small oscillation amplitudes result in lower aqueous crossover. The electric field across the baffle also influences aqueous crossover. The presence of a baffle field (above the critical field strength) causes aqueous crossover into the stripping side to increase with the stripping-side electric field.