SEPARATION OF DISPERSED PHASES FROM LIQUIDS IN ACOUSTICALLY DRIVEN CHAMBERS

A separation process for the collection and processing of fine secondary phases in liquids has been developed, in which a stationary acoustic field at ultrasonic frequencies is superimposed on suspensions flowing through filled chambers. The acoustic radiation force acts to drive second-phase inclusions to either the nodes or antinodes of the stationary field, and acts to hold the secondary-phase material in position relative to the bulk liquid flow. Separation devices consisting of broadband lead-zirconate-lead-titanate transducers, bonded to the ends of glass and aluminium tubes (up to 4 cm in diameter and 25 cm in length, and ranging from 0.9 to 2 mm in wall thickness). were constructed. These were driven with continuous a.c. voltages at specific frequencies in the range 0.35 1.4 MHz to obtain a forced coincidence response in which the flexural vibrations of the tube wall are matched in phase and geometry with higher-order acoustic duct modes in the liquid. This excitation produced stationary fields with peak acoustic pressures of the order of 1 MPa in water. Particles ranging in size from 0.1 to 100 mum have been concentrated in aqueous media into nodal zones with characteristic times of less than 1 s. Chambers with 0.11 volume, driven at frequencies near 625 kHz, use approximately 25 W of electrical power. Collection efficiencies of up to 97.5% have been measured. The trapped particles can be moved to either end of the cell and removed through an exit port by repeatedly sweeping the driving frequency over a known range and period. Applications of these principles include separations from fluid mixtures of fine solids, immiscible liquids, or biological cells.