Energy efficient and intensified leaching with ultrasound controlled cavitation

Purpose and goal
The project is a further development of an ultrasonic controlled cavitation reactor for leaching of minerals and by-products that are known to be very difficult to leach (scheelite). The purpose was to evaluate the effects of longer exposure time as well as higher temperature, static pressure and cavitation intensity through optimisation. Up-scaling is done by extending the pipe structure and adapting to a higher static pressure. The cavitation intensity is also increased by a modification of the nozzle for flow initiation of cavitation bubbles. Two industrial partners are linked to the project.

Structure and implementation
The effect of ultrasound controlled cavitation in a leaching process is evaluated with a concentrate of scheelite (CaWO4) in a basic leaching reagent (NaOH, 30 mol). Eight tests were conducted with a modified reactor, excited at 22 kHz and 39 kHz at 60 °C and 77 °C. The cavitation intensity was also varied with hydrodynamic cavitation via a newly developed nozzle. The reactor was modified to withstand overpressure as well as long exposure time (which changed the piezo material). Long-term exposure and higher process temperature were of particular interest. The samples were analysed using ICP-OES.

Results
The cavitation intensity is controlled by ultrasound, temperature and flow initiation of cavitation bubbles. Excitation signals and process parameters are optimised in relation to materials and the coating reagent. The method can double the yield compared to reference. The process is highly temperature dependent. For higher cavitation intensity, the system must be overpressured (the reactor has been modified for this). Overheating can be handled through better materials and new geometries for better cooling effect. Implementation on a pilot scale will be done in cooperation with two industrial partners.