The ultrasonic treatment unit consists of an inclined, flat bottomed metal trough, usually made of stainless steel or aluminum, with the ultrasonic transducer attached near the upper end. A typical unit would be 91.5 cm (36 inches) wide and 244 cm (96 inches) long. Figure 1 is a photograph of an ultrasonic treatment unit being water tested. The formation of nodes and antinodes can be noted as can the water mist produced at the surface of the water.
The width of the unit determines the capacity. Each 30 centimeters (one foot) of trough width will treat approximately three tons of fine solids per hour, using a 25% by weight of solids, in a water slurry. The length of the trough is determined by the residence time required for efficient treatment of the solid material in the water slurry as it flows over the vibrating metal surface of the trough.
For efficient operation of the ultrasonic treatment unit, the trough is suspended with sonic insulating flexible straps. A flexible hose brings the slurry into a header box, which is separately supported in such a way that a falling sheet of slurry enters the upper end of the ultrasonic treatment trough.
The treated slurry flows off the lower end of the trough into a separation unit which recovers the wanted mineral. Figure 2 is a schematic sketch showing the incorporation of the ultrasonic treatment unit in a recovery system. The separation units may consist of a magnetic separator, a screening device, a hydrocyclone, or a leaching system.
Ultrasonic Fundamentals
The primary results of the ultrasonic treatment upon the particles in a water slurry are as follows:
- (1) the separation of particle agglomerates into independent particles, and
- (2) the production of cleaned particle surfaces.
These results are brought about by the production and collapse of many small cavitation bubbles at the liquid-solid interfaces. Cavitation bubbles are produced when sufficient ultrasonic energy is introduced into a liquid.
Due to the density change at the surfaces of solids submerged in water, attenuation the sound energy occurs at these liquid-solid interfaces promoting cavitation to take place or the surfaces of the solid particles. Also, particles which are loosely stuck together may promote cavitation near their points of contact, thereby aiding the separation of the particles from each other.
Precious Metal Recovery by Ultrasound
A field test was run using sedimentary clays from a lake bottom containing very small amounts of precious metals. A slurry was made of these clays with water which flowed through the ultrasonic treatment unit. After treatment, the slurry was pumped into a hydrocyclone which discharged into a spiral classifier for further separation and recovery of the precious metals.
Results indicated that ultrasonic treatment doubled the recovery of the precious metals from the raw ore and enhanced the concentration of the precious metals by a factor of ten.
