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.

Graphite ore collected from Hunan province, south China was characterized by chemical analysis, X-ray diffraction, and optical microscopy. Rougher and multi-stage flotation tests using a mechanical flotation cell and a flotation column containing an additional centrifugal force field were carried out to promote its grade and economic value. In rougher flotation, both the mechanical flotation cell and flotation column reduced the ash content of the graphite ore from 15.43% to 10.8%, while the yield of the flotation column (91.41%) was much higher than that of the mechanical flotation cell (50%). In the presence of hydrophobic graphite, the seriously entrained gangue restricted further improvement in the quality and economic value of the graphite ore. Therefore, multi-stage flotation circuits were employed to diminish this entrainment. Multi-stage flotation circuits using the two flotation devices further decreased the ash content of the graphite ore to ~8%, while the yield when using the flotation column was much higher than that obtained from the mechanical flotation cell employed. On the other hand, the ash removal efficiency of the flotation column was 3.82-fold higher than that observed for the mechanical flotation cell. The Cleaner 3 flotation circuit using the flotation column decreased the ash content in graphite from 15.43% to 7.97% with a yield of 77.53%.

Bulk micro-nano-bubbles (BMNBs) have been proven to be effective at improving the flotation recovery and kinetics of fine-grained minerals. However, there is currently no research reported on the correlation between the properties of BMNBs and flotation performance. For this purpose, aqueous dispersions with diverse properties were created by altering preparation time (0, 1, 2, 3, 5, and 7 min), aeration rate (0, 0.5, 1, 1.5, and 2 L/min) and aging time (0, 0.5, 1, and >3 min). Micro- and nano-bubbles were characterized using focused beam reflection measurements (FBRM) and nanoparticle tracking analysis (NTA), respectively. The micro-flotation of quartz particles was performed using an XFG-cell in the presence and absence of BMNBs with Cetyltrimethylammonium bromide (CTAB) as a collector. The characterization of bubble sizes showed that the bulk micro-bubble (BMB) and bulk nanobubble (BNB) diameters ranged from 1–10 μm and 50–400 nm, respectively. It was found that the preparation parameters and aging time considerably affected the number of generated bubbles. When BNBs and BMBs coexisted, the recovery of fine quartz particles significantly improved (about 7%), while in the presence of only BNBs the promotion of flotation recovery was not significant (2%). This was mainly related to the aggregate via bridging, which was an advantage for quartz flotation. In comparison, no aggregates were detected when only nano-bubbles were present in the bulk solution.

An investigation was conducted in terms of the variation of particle size distribution, removing hydrophilic groups from coal surface, reagent dispersion, and formation of particles-bubbles aggregates resulted from the HIC to elucidate the mechanisms responsible for improvement in flotation performance. Results showed that the amount of <0.074 mm size fraction increased after HIC. It would have a negative effect on flotation performance, although the variation was not significant. Reagant dispersion had a marginal effect on flotation performance. The contact angle measurements indicated that HIC improved particle surface hydrophobicity, and the effect was most significant when agitation speed of the HIC was 2000 rpm, but the flotation performance was not as significant as expected. When HIC agitation speed was 1500 rpm, particles-bubbles aggregates were formed; however, the aggregates were destroyed as agitation speed increased to 2000 rpm. This fact was consistent with the experimental results that the flotation performance was optimal with the HIC speed of 1500 rpm and it deteriorated as agitation speed increased to 2000 rpm. It was deduced that the size and structure stability of particles-bubbles aggregates played a dominant role in the improvement in flotation performance.

Inherent limitation of the mechanical flotation cell is the entrainment of hydrophilic particles (gangue) in the water reporting to the froth. In this study, a combine column and cell flotation process was proposed to reduce the entrainment of gangue particles and, ultimately, ash content of coal flotation concentrate in Datun coal preparation plant, Peixian County, Jiangsu, China. The froth collected from the third and fourth cells of the 4-cell flotation machines in this plant was further re-cleaned to obtain qualified clean coal. Laboratory-scale results show that both the mechanical flotation machine and flotation column as re-cleaning flotation devices could produce the clean coal with qualified ash content. However, the yield obtained by the flotation column (84.49%) is significantly higher than that obtained by the mechanical flotation cell (68.21%). Industry-scale tests indicate the combined column and cell flotation process could produce qualified clean coal and run stably.