Abstract:

The weak permeability of fine-grained tailings can cause the leaching line of the tailings dam to rise and reduce the safety factor of the tailings dam, an ultrasonic cavitation approach was proposed to enhance the permeability of the tailings. Firstly, computational fluid dynamics (CFD) software was used to simulate the cavitation bubble collapse process. Then, the transducer used for the experiments was selected based on the cavitation threshold and the simulated sound pressure, with a frequency range of 20-40 kHz and a power of 60 W. Moreover, the presence of ultrasonic cavitation effects in the tailings samples was confirmed by staining tests. Finally, ultrasound-enhanced permeability tests were performed on tailings samples with different fine particle contents using a self-made variable head permeameter and the selected transducer. A nuclear magnetic resonance instrument was used to determine the pore structure changes before and after penetration enhancement. The results showed that the obtained cavitation bubble radius change curve was consistent well with the Rayleigh-Plesset(R-P) equation fitting curve, proving the validations of the simulations. Low-frequency ultrasound had a better cavitation effect when the bubble diameter was larger than 50 μm, whereas high-frequency ultrasound was more effective when the bubble diameter was less than 25 μm. When the fine particle content was kept constant, the permeability coefficient growth rate of the tailings samples increased as the ultrasound frequency increased. When the ultrasonic frequency was kept constant, the tailings samples' permeability coefficient growth rate with higher fine particles was higher. After the ultrasound treatment, the proportion of 0-10 μm pores in the tailings sample decreased, and the decrease became larger with the increase in frequency. There was no clear trend for the proportion of pores between 10-20 μm, while the proportion between 20-40 μm increased, and the increase became larger with the increase of frequency. The proportion of pores larger than 40 μm increased relatively small. For tailings with different pore proportions, appropriate ultrasound signals of corresponding frequencies can significantly enhance the permeability enhancement effect.

Key words: ultrasonic cavitation, fine tailings, permeability enhancement, permeability coefficient, numerical simulation, pore space, nuclear magnetic resonance