The traditional leaching process is characterized by extensive reaction time, low efficiency, and considerable leaching reagent consumption. It was well demonstrated that using ultrasound could effectively enhance the leaching reaction rate by removing the passivating layer and increasing the mass transfer rates. This improvement would result from bubble cavitation and other mechanical-chemical mechanisms that ultrasounds can generate during ultrasound-assisted leaching (UAL). Thus, these days using UAL for the recovery and recycling of various valuable metals has markedly received attention as an environmentally friendly process. However, surprisingly no comprehensive overview has been provided to focus on various reaction mechanisms through the applications of UAL and deliberate them for nearly two decades. This work has explored various applications of UAL applied for ore and waste processing by a systematic approach to fill this gap. An overview of different mechanisms (mechanical, thermal, sonochemical) based on main ultrasound operating variables (frequency, power, and time) and their level of leaching effectiveness on the leaching metallurgical responses in varied conditions was discussed in detail. It was indicated that the common approach for conducting UAL investigation mainly focuses on improving the leaching efficiency of metals by using single-frequency ultrasound. While analyzing the correlation between ultrasonic cavitation theory and assisted leaching process and exploring the systematic effect of multi-frequency ultrasonic system need to be further clarified in future research. In general, the present work is going to potentially pave the path for understanding UAL and further its development in the future.

This paper reviews recent developments in the fundamental understating of ultrafine (nano) bubbles (NBs) and presents technological advances and reagent types used for their generation in flotation. The generation of NBs using various approaches including ultrasonication, solvent exchange, temperature change, hydrodynamic cavitation, and electrolysis was assessed. Most importantly, restrictions and opportunities with respect to the detection of NBs were comprehensively reviewed, focusing on various characterization techniques such as the laser particle size analyzer (LPSA), nanoparticle tracking (NTA), dynamic light scattering (DLS), zeta-phase light scattering (ZPALS), and zeta sizer. As a key feature, types and possible mechanisms of surfactants applied to stabilize NBs were also explored. Furthermore, flotation-assisted nano-bubbles was reported as an efficient method for recovering minerals, with a special focus on flotation kinetics. It was found that most researchers reported the existence and formation of NBs by different techniques, but there is not enough information on an accurate measurement of their size distribution and their commonly used reagents. It was also recognized that a suitable method for generating NBs, at a high rate and with a low cost, remains a technical challenge in flotation. The application of hydrodynamic cavitation based on a venturi tube and using the LPSA and NTA in laboratory scales were identified as the most predominant approaches for the generation and detection of NBs, respectively. In this regard, neither pilot- nor industrial-scale case studies were found in the literature; they were only highlighted as future works. Although the NB-stabilizing effects of electrolytes have been well-explored, the mechanisms related to surfactants remain the issue of further investigation. The effectiveness of the NB-assisted flotation processes has been mostly addressed for single minerals, and only a few works have been reported for bulk materials. Finally, we believe that the current review paves the way for an appropriate selection of generating and detecting ultrafine bubbles and shines the light on a profound understanding of its effectiveness.