Ultrasound technology is widely applied in the flotation process. From the perspective of the theory of ultrasound, this article explains the effects and applications of ultrasound in the flotation process. To obtain a clear understanding of ultrasonic effects, we observe the phenomena of ultrasound using a high-speed camera and a CCD camera, and investigate potential applications in flotation. From these different phenomena, the ultrasonic effects are classified into three types of effect: the transient cavitation effect, stable cavitation effect, and acoustic radiation force effect. Based on these effects, the applications of ultrasound to mineral flotation are reviewed, including slime coating removal, oxidation film removal, desulfuration, tiny bubble generation, flotation reagent dispersion, and aggregation. In addition, the ultrasonic equipment and treatment methods applied in flotation are classified and compared based on their characteristics. Finally, we propose some potential directions in the study of the stable cavitation effect and acoustic radiation force effect, which are important, but are seldom mentioned in previous reports.

The generation, and stability of nanobubbles are of particular interest for fundamental research and have potential application in numerous fields. Several attempts were made in the literature to produce nanobubbles through acoustic cavitation. However, the generation and stability mechanisms of nanobubbles in the acoustic field are unclear. Here, we review the effect of ultrasound parameters on bulk nanobubbles and surface nanobubbles. On this basis, we discuss the proposed generation and stability mechanisms of nanobubbles from the perspective of transient and stable acoustic cavitation. Moreover, we propose some future research directions for a deeper understanding of the role of ultrasound in the generation and stability of nanobubbles.

In this paper, a novel flotation technique that combines nano-scale bubbles generated by hydrodynamic cavitation (HC) and carrier flotation is proposed to promote the flotation efficiency of a high-ash (43%) ultra-fine coal sample (<45 µm). We investigated the mechanism by which cavitation bubbles enhance the separation efficiency of carrier flotation using focused beam reflectance measurements, polarizing microscopy, and extended Derjaguin-Landau-Verwey-Overbeek theory. The carrier particles (polystyrene (PS)) and fine coal were pre-treated in a venturi tube and then floated in a laboratory mechanical flotation cell. The flotation results indicate that the presence of cavitation bubbles significantly improved the carrier flotation performance of high-ash ultra-fine coal. This improvement was attributed to the presence of highly hydrophobic PS, which creates additional gas nuclei in the flotation system. The nano-bubbles, which were produced by the venturi tube and adhered to the fine coal particle surfaces, were conducive to the agglomeration of fine coal particles into large aggregates. Moreover, the nano-bubbles functioned as "bridges" of interaction between the carrier particles and large aggregates of fine coal particles. This paper mainly focused on the effect of carrier (PS) and HC on high-ash fine coal. The influence of different HC intensities on carrier (PS) flotation was discussed. Two models for the interactions between the coal particles, nano-bubbles, and PS during cavitation were proposed and were proved using the E-DLVO theory.

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.