Ultrasonic Enhancement for Mineral Flotation: Technology, Device, and Engineering Applications

Abstract

In the past five years, the number of articles related to ultrasonic mineral flotation has increased by about 50 per year, and the overall trend is on the rise. The most recent developments in ultrasonics for flotation process intensification are reviewed herein, including effects of ultrasound treatment on an aqueous slurry, improvement in flotation methods and technological processes, device development tracking, and application effects in mineral process engineering. At this point in time, there are pilot-scale flotation tests to evaluate the feasibility of ultrasonic pretreatment technology for industrial use to enhance residue flotation separation, and the results showed that the recovery rate of concentrate is increased by about 10%. Four aspects of ultrasonic flotation process improvement are summarized, namely, changing the ultrasonic parameters, the synergistic effect of ultrasound and reagents, the ultrasonic effect of particles with different-sized fractions, and application to new systems. In addition, the effect of ultrasonic flotation mechanisms is explored through a quadratic model and numerical simulation. The combination of ultrasonic flotation with other fields, such as magnetic fields, to enhance the separation efficiency and recovery of minerals is also a future trend. It is also proposed that ultrasonic flotation technology will be used with big data, industrial Internet of Things, and automatic control technology to achieve deep bundling, optimizing the flotation process by implementing remote monitoring and control of the flotation process.

1. Introduction

In the mineral processing field, flotation has been widely applied as an efficient separation method for the purification and recovery of minerals such as coal gasification fine slag, apatite, dolomite from chalcopyrite, galena, copper oxide, quartz, and carbide slag [1]. However, with the rising prominence of resource depletion and environmental issues, challenges such as low efficiency, environmental pollution, and high energy consumption in conventional flotation have necessitated the development of ultrasonic flotation. Ultrasonic flotation is an innovative physical treatment method that introduces ultrasonic waves into the flotation system, generating a large number of microbubbles and localized high-temperature, high-pressure environments, thereby altering the physicochemical properties of the mineral surfaces. Leveraging the cavitation and acoustic radiation force effects produced by ultrasonic waves promotes the adsorption of particles in the pulp with flotation agents, enhancing flotation efficiency and recovery rates and reducing the use of chemicals and energy consumption.

In the last 20 years, with the continuous development of ultrasonic treatment, ultrasonic flotation devices have also been upgraded. New devices, such as ultrasonic generators and ultrasonic flotation cells, have been gradually applied in industrial production, achieving high efficiency, energy savings, and environmental protection. With continuous exploration and optimization in engineering practice, ultrasonic flotation shows broad application prospects in the treatment of low-grade beneficiation, improving resource utilization and reducing production costs.

Ultrasonic flotation has been reviewed [2,3,4,5], and the application of ultrasonic in flotation, leaching, and gravity has also been systematically discussed in the literature [2], with the interaction between ultrasound and minerals and agents focused on in the flotation part. The application of ultrasound in flotation was reviewed in Ref. [3], including sludge removal, oxide film removal, desulfurization, microbubble formation, flotation agent dispersion, and aggregation. The application principle of ultrasonic technology in mineral flotation is discussed in Ref. [4], including the effect of ultrasound on minerals, flotation reagents, and slurries, as well as their application status. The mechanism of ultrasonic treatment in mineral flotation is discussed in detail in Ref. [5], including the transient cavitation effect, stable cavitation effect, and acoustic radiative force effect, as well as the influence of the main parameters of ultrasonic treatment on mineral flotation and the application of ultrasonic treatment in minerals (e.g., cleaning effect, ultrasonic corrosion, and desulfurization), flotation agents (e.g., dispersion and emulsification, and changes in the properties and microstructure of the agent solution), and slurry (e.g., microbubble formation and aggregation).

Effects of ultrasound treatment on an aqueous slurry (the fundamentals of ultrasonic flotation), improvement of flotation methods and processes, development of devices, and application effects in mineral processing engineering are systematically discussed in this paper. According to the different shapes of transducers, the ultrasound devices discussed in Ref. [3] are divided into three types: horn type, plate type, and focusing type. The ultrasonic flotation equipment in Ref. [4] is intended for trough, probe, and ultrasonic flotation columns. Different from the aforementioned literature, this review mainly focuses on the development trajectory of ultrasonic flotation equipment, focusing on the development of ultrasonic flotation equipment in the last five years. At present, the technology of ultrasonic mineral flotation is still in its early stages. By analyzing the development of ultrasonic flotation and exploring its potential applications in different mineral processing methods, this study provides a theoretical basis and practical guidance for further optimization and promotion of the technology. At the same time, future research directions and development trends are proposed to address the problems and challenges of the existing technology.

2. Methodology

2.1. Innovation of This Review

This paper is divided into four parts: effects of ultrasound treatment on an aqueous slurry, improvement of flotation methods and technological processes, device development tracking, and application effects in mineral process engineering. The most notable innovations are primarily concentrated in the second and third parts. The innovation of the second part lies in providing readers or enterprises with ways to obtain better flotation effects by adjusting the ultrasonic flotation parameters, adding some reagents to achieve the synergistic effect of ultrasonic and reagents, changing the particle size of flotation minerals, and adding ultrasound to the flotation system to obtain improved flotation effects. The innovation of the third part lies in systematically summarizing and discussing the development process of mineral ultrasonic flotation device.

2.2. Literature Research

For this review, Web of Science, Elsevier and x-mol were used to screen publications, primarily based on the topics of “ultrasonic flotation”, “ultrasonic flotation & mineral”, and “ultrasonic flotation device”. The references for ultrasonic flotation devices are mostly from patents, and few existing journal articles are dedicated to ultrasonic flotation devices. Figure 1 shows the published literature on these topics in Web of Science. It can be seen from the figure that the current ultrasonic flotation of minerals is still in the early stage, and the quantity of related literature is relatively small. The publications were manually filtered by checking the titles and reading the abstracts. The text on ultrasonic flotation devices is primarily based on patents from the last 20 years, while the remaining content on ultrasonic flotation mainly discusses journal articles from the last five years. Ultimately, 84 articles matching the topics were selected for thorough analysis and summarization.

3. Effects of Ultrasound Treatment on an Aqueous Slurry

The possible effects of ultrasound flotation mainly include the transient cavitation effect, stable cavitation effect, and acoustic radiative force effect. Cavitation and acoustic radiative force are two common effects present in water during ultrasonic treatment, as shown in Figure 2. Transient cavitation requires high sound intensity (greater than 10 W/cm²) to trigger and undergo a rapid growth and violent collapse process, resulting in the formation of so-called hot spots: local high temperatures and pressures. It has a strong impact and peeling effect on the impurities on the mineral surface and can dissolve some components on the mineral surface. Stable cavitation occurs at low sound intensity, and the bubbles produce stable small-amplitude pulsations, improving the collision efficiency with fine particles, helping to remove soluble impurities from the mineral surface, and dispersing and emulsifying the flotation agent [5]. Siderite is soluble, and ts dissolution process will produce CO₃²⁻. CaCO₃ is generated by CO₃²⁻ and activator Ca²⁺ to occupy the active sites on the surface of quartz, attaching to the quartz surface such that the quartz shows the properties of calcite and reduces its hydrophobicity. Mechanical effects induced by ultrasonic cavitation can clean the surface of minerals and increase their active sites to a certain extent, thus improving their floatability [6]. By way of illustration, regarding the outcome of ultrasonic enhancement of coal gasification fine slag, ultrasonic treatment can enhance the hydrophobicity of the surface of coal gasification fine slag and increase the number of active sites on the surface, thus treating coal gasification fine slag flotation tailings more effectively [7]. In addition, microbubbles generated by ultrasonic cavitation can act as a bridge to promote the adsorption of collecting agent on the mineral surface and enhance mineral flocculation by increasing the contact angle and decreasing the zeta potential [8]. Acoustic radiation force is the movement of particles, oil, or bubbles in the ultrasonic field. Particle surface properties are altered by acoustic radiation forces in ultrasonic flotation to promote coal particle aggregation and flotation [9]. Ultrasonic flotation, under the synergistic effect of cavitation and acoustic radiation force, realizes the improvement of flotation efficiency compared with traditional flotation.

4. Improvement of Flotation Method and Technological Process

4.1. Ultrasonic Parameters

Selecting the appropriate ultrasonic parameters for a specified flotation system is the main consideration in the ultrasonic flotation process. The setting of ultrasonic parameters and their types will have a greater or lesser impact on cavitation and acoustic radiation forces, thus affecting the action of particles and bubbles in ultrasonic flotation, as well as the outcome of ultrasonic flotation.

Significant progress has also been made in recent years regarding the search for the optimal ultrasonic frequency for different minerals and flotation conditions. Previous studies have mostly used low-frequency ultrasound, in the range of approximately 20–60 kHz, and most of the ultrasound frequencies used in the references included in this review are in this range [10]. Specifically, innovation in the literature [11] lies in the study of the effects of ultrasonic standing waves at three different frequencies, 80 kHz, 100 kHz and 120 kHz, on the behavior of flotation bubbles and flotation effects, and 100 kHz was identified as the ideal frequency for ultrasonic flotation. Under the action of the 100 kHz ultrasonic standing waves, the bubble aggregates and coal–bubble aggregates were formed within 450 ms and 20 ms, respectively, which significantly improved the flotation efficiency of fine coal. This process is mainly attributed to the high-frequency pressure change generated by the ultrasonic standing wave, which promotes the rapid aggregation of bubbles and coal particles and thus enhances the flotation effect. In addition, the authors also mention that a study shows that 200 kHz ultrasonic standing wave can significantly improve the flotation rate of fine coal particles without the addition of chemicals or collectors. A key innovation of Chen et al.’s study is that they studied bubble–coal particle interactions in a 600 kHz ultrasonic standing wave field (USW), a significant increase compared to the frequency used in conventional ultrasonic flotation techniques (usually in the range of 20–50 kHz). Under the action of 600 kHz USW, stable tiny cavitation bubbles can be created and maintained on the surface of coal particles. The bubbles, with the action of the initial Bjerknes force, migrate to the low-pressure area (the minimum point of pressure), causing coal particles to converge at these low-pressure points. In addition, the secondary Bjerknes force establishes an attraction between small bubbles and more atmospheric bubbles, which enhances the adhesion between coal particles and bubbles. In contrast to conventional interactions, particles can be attracted and collected by the bubbles without having to undergo collision and attachment processes. This efficient bubble–particle interaction mechanism significantly improves flotation recovery [12]. The innovation of the above two papers lies in the use of high-frequency ultrasonic standing waves to enhance the collision and aggregation efficiency of mineral particles.

Regarding the effect of ultrasonic power on ultrasonic flotation, Kang et al. studied the effect of ultrasonic pretreatment on graphite flotation effect and explored the effects of three different ultrasonic powers of 0.8 kw, 1.2 kw, and 1.6 kw on the concentrate mineral yield, and the results showed that the higher the ultrasonic power, the higher the concentrate yield [13]. Esmeli et al. found that in the lignite flotation process, the power of ultrasound as a pretreatment is in the range of 30–60 W, and the recovery rate of combustible components increases with the increase in power [14]. Deng et al. studied the effects of ultrasonic pretreatment at 120 W, 180 W, 240 W, and 300 W on the quartz flotation effect and found that with an increase in ultrasonic power, the recovery rate of quartz sand concentrate was the highest at 240 W and the impurity removal effect was good, but the quartz flotation was not further improved at 300 W [15]. High power and prolonged use of ultrasound may adversely affect the conditioning process by creating turbulent conditions and/or increasing the temperature in the conditioning medium [16]. For potassium salts in materials with different particle sizes, with the increase in ultrasonic power in the range of 15–70 W, the recovery rate of coarse materials decreases and the recovery of intermediate materials first increases and then decreases, while the selectivity of flotation can be improved for fine-grade materials [17].

The ultrasound type contains a standing wave field in addition to the traditional diffuse field. In an ultrasonic field, both particles and bubbles are subjected to the force of acoustic radiation. The acoustic radiation force effect is related to the movement of bubbles or particles. In addition to Jin et al. and Chen et al. [11,12], Chen et al. [3] also used ultrasonic standing waves to study the effects of acoustic radiation forces at 50 kHz, where particles and oil droplets can be precisely manipulated [18]. Conversely, the acoustic radiation force on the bubble is sensitive to low-frequency ultrasound, and it decreases dramatically when the bubble size differs significantly from the resonance radius.

To sum up, the development of ultrasonic flotation frequencies in recent years has primarily focused on selecting different ultrasonic frequencies for the flotation system to determine the optimal ultrasonic flotation frequency, with the frequencies tested in the above cases exceeding the typical ultrasonic frequency range of 20–60 kHz. Future development trends may involve experimenting with high-frequency ultrasonic flotation systems to improve the efficiency of ultrasonic selection. Regarding the development of ultrasonic flotation power, it has been found that the flotation effect is not always positively correlated with the ultrasonic power, and the same power may also produce different effects on the same ore sample of different particle sizes. Regarding the development of ultrasonic types, in recent years, ultrasonic flotation has begun to gradually adopt the standing wave field, which is no longer limited to the traditional diffusion field, and the application range of the ultrasonic standing wave field in ultrasonic flotation may gradually expand in the future.

4.2. The Synergistic Effect of Ultrasound and Reagents

Ultrasound can produce a synergistic effect with flotation reagents (such as collecting agents, frothing agents, etc.) to improve the flotation efficiency. Ultrasound can enhance the adsorption of the collecting agent on the surface of minerals, as well as improving the stability and selectivity of bubbles. The research on the synergistic effect between ultrasound and reagents has also made significant progress in recent years.

Firstly, ultrasound can enhance the flotation efficiency by directly affecting the physical properties of the flotation reagents. For instance, what is innovative about Zhang et al.’s study [19] is that they used ultrasound to modify starch, introducing ultrasonic treatment to enhance the inhibition of starch in hematite reverse flotation, where starch acted as an inhibitor to make the surface of iron oxides hydrophilic, and as a flocculating agent to make the fine iron oxides form larger flocs. The ultrasonic treatment carried out prior to flotation enhanced the inhibition of hematite and selective flocculation while improving starch (MS) solubility and increasing the content of straight chain starch. This helped to reduce hematite entrainment losses and increase the iron grade of the concentrate.

Secondly, ultrasound can influence the effect of flotation reagents on minerals. For example, the novelty of [20] lies in the study of the inhibition effect on galena when ultrasonic treatment is used in combination with sulfuric acid, and the results indicate that ultrasound can significantly improve the inhibition ability of sulfuric acid on galena. The ultrasonic-induced cavitation effect creates localized hot spots on the mineral surface. They deliver an intensified energy input, which expedites the chemical interaction with sulfuric acid on the galena surface. This accelerated reaction fosters the precipitation of PbSO₄, thereby enhancing the selective separation process between galena and chalcopyrite. In addition, the separation of galena and chalcopyrite using sulfuric acid requires high temperatures (60–80 °C) and long processing times (more than 25 min), whereas ultrasound can be used to reduce the reaction time to 6 min at room temperature, significantly reducing energy consumption and improving separation efficiency.

Thirdly, ultrasound is able to affect the surface adsorption of flotation reagents on minerals, enhance the adsorption capacity of surfactants, and promote the desorption of surfactants from the surface of mineral residues [7]. A particularly innovative aspect of the research by Huang et al. is that they study the effects of ultrasonic pretreatment on the surface dissolution of scheelite as well as the physicochemical properties and microstructure of sodium oxalate solution. They found that ultrasonic pretreatment of sodium oleate or scheelite promoted the adsorption of sodium oleate on the surface of scheelite, which increased the heat of adsorption and thermal reaction rate of sodium oleate and scheelite, as well as the hydrophilicity of the surface, and led to an improvement in the flotation rate and recovery of scheelite. Nevertheless, ultrasonic treatment is more suitable for sodium oleate and less effective for scheelite [21]. Moreover, regarding coal flotation, one of the innovations of Sun et al. lay in the use of ultrasonic desorption to remove surfactant from the surface of gangue, which reduced the hydrophobicity of gangue and enhanced the wettability difference between coal and gangue. The second innovation lay in the discovery that the addition of Ca²⁺ promotes the adsorption of surfactant on the organic matter; however, the effect on the gangue was not significant, which helped to increase the flotation selectivity. Through the synergistic effect of ultrasonic treatment and Ca²⁺, the flotation response of low-rank coal was improved, and better flotation performance was obtained [22].

To sum up, development in the synergistic effect of ultrasound and reagents in recent years mainly includes the modification of flotation reagents by ultrasound, the combined benefits of flotation agents and ultrasonic processing, and the realization of surface modification of minerals through ultrasonic flotation. Applications include galena, scheelite, and coal. The foci of research include exploring the optimal combination of different types and concentrations of reagents and ultrasound and how they work together to modify mineral surfaces.

4.3. Particle Size Fractions

With regard to the influence of particle size fractions on the effect of ultrasonic flotation, some progress has been made in recent years. For instance, Chu et al. [23] innovatively applied ultrasonic pretreatment technology to improve the flotation performance of spodumene, conducted a systematic study on the effect of ultrasonic pretreatment on spodumene with different particle sizes, and revealed the effect of particle size on the effect of ultrasonic pretreatment, which is less often involved in traditional flotation studies. The quantity of NaOL adsorbed on the mineral surface was noticeably increased by ultrasonic pretreatment; thus, the floatability of spodumene was further enhanced. For coarse particles, ultrasonic pretreatment was more effective compared to conventional methods in optimizing the surface physicochemical properties. Conversely, for fine particles, ultrasonic pretreatment increased the amount of dissolution of the mineral, but did not show a significant advantage over the conventional method. Filippov et al. investigated the role of ultrasonic treatment in improving the flotation selectivity of specific minerals (sylvite KCl and halite NaCl), which had not been extensively explored in previous studies. The combined effect of ultrasonication on the physicochemical parameters of the slurry related to the flotation behavior of particles of different sizes was investigated for a potash ore flotation system, and it was found that the use of ultrasonic treatment can be beneficial for the treatment of fine-grained ore and reduce the loss of potash in the process of desliming [17]. Mao et al. innovated by investigating the effect of ultrasonic pretreatment technology on the improvement of the flotation performance of lignite particles of different size fractions, and found that the coarse particles of lignite showed a significant increase in the recovery of combustible matter after ultrasonic pretreatment, while the recovery of fine particles decreased. In this process, ultrasonic treatment mainly improved the flotation efficiency by crushing the minerals, and particles that were too fine were not conducive to lignite flotation [24]. Chen et al. determined that the sulfur content of a 0.019 mm residue increased after sonication by analyzing the residue before and after sonication [25]. Wang et al. also determined through flotation that ultrasonic flotation has a more significant crushing effect on fine slag particles of entrained-flow gasification coal fine slag than traditional flotation, and that the flotation effect is better [26].

In recent years, in order to improve the effect of mineral flotation, the influence of particle size on the effect of ultrasonic flotation has been increasingly considered. Regarding the impact of particle size fractions on research progress in ultrasonic flotation, the flotation objects have expanded to include spodumene, potash ore, lignite, and gasification coal fine slag flotation systems. For different flotation systems, the effects of coarse and fine particle size effects vary. The impact of ultrasonic flotation on specific particle sizes needs to be specifically analyzed, with universal conclusions remaining lacking. This analysis may be conducted by considering the mineral composition of each particle size and determining the particle size for flotation accordingly. Alternatively, flotation could be carried out separately for each particle size, with the most suitable flotation particle size being judged according to the flotation effect. Currently, the flotation fractions mainly comprise 60 mesh, 100 mesh, 200 mesh, 400 mesh, and 900 mesh.

4.4. Application System

With a better understanding of the mechanisms of ultrasonic flotation, new flotation systems are being developed to accommodate different types of minerals and more complex ores.

The trend of improvement in ultrasonic flotation lies primarily in its application in existing flotation systems. Kruszelnicki et al.’s innovation was the first systematic exploration of the role of ultrasonic pretreatment in the flotation of carbonaceous copper-bearing shale. The specific mechanism of the effect on the floatability of the minerals was explored in depth, specifically on the generation of ultrafine bubbles and the cleaning effect through transient bubble collapse. Ultrafine bubbles contributed to the enhancement of the adhesion efficiency of minerals to the bubbles, improving the flotation selectivity. The findings showed that the ultrasonic pretreatment improved the flotation performance of minerals by altering the charge state of the mineral surfaces and enhancing the adsorption of the collecting agent. Under the ultrasonic pretreatment condition, the amount of collecting agent to achieve the same flotation recovery was significantly reduced [27]. Liao et al. were innovative in applying ultrasonic pretreatment technology to the separation process of pyrrhotite and chlorite. It was observed that ultrasonic pretreatment could promote the dissolution of iron oxide particles on the surface of pyrrhotite and the desorption of iron ions adsorbed on the surface of chlorite in the mixed minerals, which in turn facilitated the separation between pyrrhotite and chlorite [28].

Ultrasonic treatment also involves the selective separation of minerals through the application of ultrasonic flotation. For instance, in an innovative move, Wu et al. applied ultrasonic treatment to selectively recover ilmenite from titanaugite [29]. The floatability of titanaugite was suppressed, while the floatability of ilmenite was improved. Selective separation was enhanced when the ultrasonic treatment time was 5 min and the power was 500 W. The innovation of Fang et al. was that they investigated how ultrasonic pretreatment affects the selective adsorption of ilmenite and its associated minerals into sodium oleate through surface dissolution. A new way to physically improve mineral surface properties to enhance the flotation performance of specific minerals was provided. Ultrasound promotes the oxidation of Fe²⁺ to Fe³⁺ and the formation of Fe(OH)₃ precipitates that readily adsorbs NaOL, increasing ilmenite recovery while reducing the adsorption of accompanying minerals for effective separation [30].

New advances made in recent years regarding the application system of ultrasonic flotation include the expansion of the system for the flotation of carbon-bearing copper shale, pyrrhotite, ilmenite, and so on. Ultrasonic treatment can significantly improve the flotation effect of carbon-bearing copper shale, promote the separation of pyrite and chlorite, and increase the flotation of ilmenite by inhibiting the flotation of titanaugite, realizing the selective separation of the two. It can also play a positive role in the selective flotation of ilmenite from gangue minerals of ilmenite.

Regarding improvements in the ultrasonic flotation process, the main development trends are outlined in Figure 3, in terms of ultrasonic parameters, ultrasonic frequency, ultrasonic power, type of ultrasound affect cavitation, and acoustic radiation force effects, which in turn affect the ultrasonic flotation effect. The main trends include selecting the appropriate ultrasonic frequency within higher frequencies beyond the low-frequency range, selecting the appropriate ultrasonic power, and trying to use the ultrasonic standing wave for ultrasonic flotation. These approaches aim to enhance flotation efficiency through the interaction of ultrasound and flotation reagents. Additionally, the impact on different particle sizes of mineral particles varies. With a deeper understanding of the mechanisms of ultrasonic flotation, new flotation systems are being developed to adapt to different types of minerals and more complex ores.

5. Ultrasonic Flotation Device

The design and optimization of ultrasonic flotation devices are key to achieving high-efficiency flotation. In this chapter, the development track of ultrasonic flotation devices is discussed, and the development trends of ultrasonic flotation devices are predicted.

Ordinary ultrasonic devices and ultrasonic flotation devices have some fundamental similarities in composition, but there are clear differences in their components and design features due to their different application purposes. Ordinary ultrasonic devices mainly consist of ultrasonic generator, ultrasonic transducer/probe, power amplifier, control system, and cooling system. The ultrasonic generator is responsible for generating electrical signals to control the ultrasonic waves, while the ultrasonic transducer is responsible for converting these electrical signals into mechanical vibration. The ultrasonic transducer/probe will then convert electrical energy into ultrasonic vibrations. Compared with ordinary ultrasound, the ultrasonic flotation device features more flotation columns and an ultrasonic vibration plate acting on the pulp and bubbles.

The development of ultrasonic flotation devices in the early stages primarily focused on the ultrasonic device itself, such as enhancing the ultrasonic device and its mobility [31], achieving high-precision control of the ultrasonic vibrator [32], making the ultrasonic generator probe more stable to increase the ultrasonic wave output for floating [33], and improving the rigidity of the ultrasonic flotation device [34]. With the development of ultrasonic flotation devices, research has mainly focused on enhancing the ultrasonic flotation effect in various respects, including pulp flow [35,36,37,38], cleaning the surface of mineral particles [39,40,41,42,43,44,45,46,47], ultrasonic emulsification [40,48,49,50], and flotation bubbles [27,34,41,42,43,44,45]. Some ultrasonic flotation devices for laboratory use have also been invented, allowing for better pulp dispersion [51] or for the ultrasonic intensity to be adjusted according to the experimental needs [52].

Ultrasonic devices may not act directly in the flotation process. As a case in point, the innovation of Liao et al. is the use of an ultrasonic nebulizer instead of a direct drug feeder, which improves the phosphorite flotation efficiency and concentrate indices by increasing the contact area and improving the mass transfer efficiency. The medicament disperser and phosphorus slurry disperser utilize an annular tube structure with an additional packing layer to promote more thorough mixing and contact, further improving the phosphorus ore flotation efficiency. The main device includes a raw ore pulp tank, flotation cells, a pharmaceutical dissolution tank, and an ultrasonic atomization drug feeder [53].

In recent years, there has been development in ultrasonic flotation devices that can effectively clean mineral surfaces. An innovative approach by Chen et al. introduced ultrasonic pretreatment to effectively clean the graphite surface and dissociate the graphite intergrown with impurities. The main device includes an ultrasonic device, flotation cells and agitators, which utilize the cavitation effect to produce tiny bubbles that bridge fine graphite particles to achieve effective separation [35]. The research demonstrates that the intensity of ultrasonic cavitation has a significant effect on graphite recovery: both horn-type and bath-type ultrasonic waves contribute to flotation. Horn-type ultrasonic waves show a more pronounced effect, leading to an increase of 7% in the recovery of the flotation, whereas bath-type ultrasonic waves resulted in only a 2% increase [10]. Ma et al. were innovative in inventing an ultrasonic cleaning structure that disperses and strips mineral particles through ultrasonic action to effectively reduce slurry viscosity and intercoagulation, improve the hydrophobicity and capture rate, reduce mineral encapsulation, and enhance bubble transport, thus improving the sorting efficiency and concentrate grade. This device is suitable for granular minerals with a diameter range of 100–400 μm [54]. The innovation of Long et al. lies in the coal slurry flotation pretreatment device integrating a classification device and an ultrasonic generator, which utilizes the cavitation effect of ultrasonic waves to make the fine sludge on the surface of the fine-grained coal slurry after classification come off and effectively improves the quality of the finedcoal after flotation. The main device includes a pretreatment device, dosing device, classification device. and flotation device. Another advantage of the device is its ability to process both fine- and coarse-grained slime over a wide range of sizes [43].

Regarding ultrasonic emulsification of flotation reagents, Yin et al. invented a device for emulsifying flotation reagents, which resulted in particle sizes of approximately 0.2–10 μm through ultrasonic emulsification [48]. Li et al. were innovative in developing a device for improving the flotation separation efficiency of mixed materials from the positive and negative electrodes of waste lithium batteries. Their device utilizes ultrasound to sufficiently emulsify the reagent, forming a fine homogeneous shape that improves the adsorption capacity on the surface of graphite negative electrode materials, enhancing the flotation separation efficiency. The main device includes flotation cells, ultrasonic tanks, ultrasonic components, impellers, and transfer pumps [54]. The innovative aspect of Yin et al.’s work is the achievement of synergistic coupling between ultrasonic emulsification and ultrasonic intensification, whereby ultrasonic emulsification enables the collecting agent and foaming agent to form an emulsion with the water and improves their dispersion in the water and ultrasonic intensification effectively destroys the structure of the fly ash and facilitates the exposure and separation of the carbon. The primary device comprises screening machinery, an ultrasonic generator, and flotation cells. By improving the efficiency of char removal and reducing the risk of environmental pollution, the efficient reduction and resource utilization of fly ash was achieved. The patent clarifies that the ultrasonic frequency during emulsification treatment is 32–48 kHz, the ultrasonic frequency is set to 28–48 kHz during enhanced flotation, air is introduced in the bubbling flotation process, the control ventilation is 0.05–0.10 m³/h, and the emulsion reagent is added to the mortar five times, each addition following a ratio of 1.4 L of mortar to 100 mL of emulsifying reagent [55].

The ultrasonic flotation device has seen significant progress and innovation in enhancing the ultrasonic flotation effect. Gao et al.’s innovative approach involves the use of focusing action-type ultrasonic reinforcement to enhance ultrasonic effects, facilitate the interaction between bubbles and minerals, and augment the efficiency of collisional adhesion. The flotation column is narrowed from the middle to the top and bottom, and a total of six ultrasonic transducers are set on the side walls of the flotation column to strengthen the collision and adhesion efficiency of flotation bubbles and mineral particles [56]. Sun’s team successively researched the ultrasonic frequency range and action area and developed a multi-frequency ultrasonic flotation device [57] and inclined ultrasonic flotation device [58]. The innovation of the former lies in the use of multi-frequency ultrasonic excitation mechanism, which applies multi-level ultrasonic action on the flotation bubbles to strengthen the collision–adhesion effect between bubbles and mineral particles. The innovation of the latter is that the ultrasonic transducer is arranged in a V shape with the horizontal plane to enlarge the area of ultrasonic transducer action such that the flotation bubbles in the slurry are more fully subjected to ultrasonic action.

New achievements have also been made in the development of ultrasonic bubble refining technology for ultrasonic flotation devices. Shi et al.’s invention of an ultrasonic jet coupling device refines bubbles with ultrasonic waves. This enhances the adsorption of bubbles to mineral particles in the pulp, forming a swirling pulp circulation field, which accelerates the discharge of gangue minerals and improves the efficiency of coal slurry flotation [59]. On the other hand, Wei et al. invented an ultrasonic vibration device that allows for the control of both the speed and volume of bubble generation [60]. The device facilitates the aggregation of bubbles under the action of ultrasonic waves, with the bubble size increasing with an increase in the thickness of the vibration plate of the ultrasonic transducer [61].

Regarding the combined effect of the ultrasonic flotation device and sieve plate, the innovation lies in the combination of the ultrasonic transducer and filling-type sieve plate. This combination facilitates the removal of the mineral surface cover layer and the dispersion of mineral particles through ultrasonics, while simultaneously achieving static stratification via the filled screen plate. As a result, the hydrophobic particles adhering to the bubbles are less likely to be dislodged, strengthening the sorting effect and preventing the screen plate from being clogged. The main device includes a flotation column, ultrasonic power supply, foam overflow vessel, concentrate outlet, filling type sieve plate and ultrasonic transducer [62]. Li et al. prevented the settling of coarse particles by filling the sieve plate and effectively eliminated the effect of ultrasonic damping pulsation on the stability of the bed layer, achieving effective recovery of coarse particles of 0.25–1 mm [50].

In addition, the recent development of big data, the industrial Internet of Things and automatic control technology has made significant progress and applications in the field of flotation. Rockwell Automation provides comprehensive industrial data analysis solutions to help mining companies optimize the flotation process [63]. Siemens’ MindSphere is an open Internet of Things operating system that enables remote management and optimization [64]. ABB provides automatic control systems; for example, it can be used in mineral processing equipment to improve productivity through precise control [65]. FL Smidth developed advanced control algorithms for optimizing mineral processing processes, including flotation operations [66]. Li et al. invented an ultrasonic flotation device that incorporates a PID intelligent control system. This addition enables the device to achieve intelligent fluidization, intelligent tailing, and intelligent ultrasound, effectively improving the sorting accuracy and adaptability of the device, thereby improving the flotation efficiency [50]. With the industrial application of ultrasonic flotation technology, the combined utilization of ultrasonic flotation equipment and other technologies can significantly enhance flotation efficiency. The utilization of big data, Internet of Things, and automatic control technologies represents key trends for future development.

Regarding the development of ultrasonic flotation devices, there has been a progression, as shown in Figure 4, from the early simple design of ultrasonic flotation equipment, such as a movable part consisting of a floating slide to expand the area of the floating guiding surface [31], to the ultrasonic flotation process, such as an increase in the mixing degree of the slurry, to the ultrasonic flotation of minerals, such as cleaning the surface of the mineral particles [39], and then to the ultrasonic flotation of flotation equipment to achieve an emulsifying effect on the flotation reagents [48]. At present, flotation equipment is increasingly relying on the advancements in big data, the Internet of Things, automation, and other technologies. The development trend of ultrasonic flotation equipment, along with the deep integration of these three technologies, will likely become a key trend in future development. In addition, the combination of ultrasonic fields and other physical fields such as magnetic fields [67] and electric fields in ultrasonic flotation equipment is also a future development trend aimed at achieving efficient flotation of complex minerals.

6. Application Effects in Mineral Process Engineering

This chapter summarizes the literature in terms of effects and products. There are several ways to improve the flotation effect, mainly involving altering roughness of minerals, cleaning surfaces of minerals, increasing porosity of minerals, promoting dispersion of minerals, increasing the active site of minerals, and changing the zeta potential of minerals, as shown in Figure 5. In general, ultrasonic flotation enhances mineral flotation by acting on the mineral surfaces, but the existing literature does not demonstrate that ultrasonic flotation has an effect on the crystal structure of minerals.

6.1. Pretreatment

Ultrasonic flotation pretreatment can improve the flotation efficiency by promoting mineral dispersion and cleaning the mineral surface through ultrasound. For example, Gong et al. removed clay from the surface of foundry dust coal powder with the help of an ultrasound-assisted flotation process, which is innovative in the recycling and utilization of foundry dust as it targets a specific type of waste, in this case foundry dust. It was found that after ultrasonic pretreatment for 30 min, the yield was 55.3%, the loss on ignition (LOI) was 19.3%, and the recycled aggregate concrete was 69.22% [68]. In a follow-up study, the recovery rate of pulverized coal in tailings was increased to 74.36% after pre-soaking-assisted mechanical stirring pretreatment was used to replace ultrasonic flotation, which shows that although ultrasonic pretreatment can improve the flotation effect, it may not be the best pretreatment method [69]. The innovation of the study by Wang et al. lies in the application of ultrasonic flotation to the recovery of graphite and cathode materials from spent lithium-ion batteries (LIBs), a relatively new field of application, where conventional flotation is seldom involved in the recovery of such materials. It was also found that ultrasonic treatment could effectively destroy the organic binder and electrolytes on the surface of electrode materials in waste LIBs, thus enhancing the flotation differentiation between graphite and cathode materials. Additionally, graphite and cathode materials in LIBs were effectively separated under a dosage of methyl isobutyl methanol (MIBC) of 400 g/ton and diesel fuel of 200 g/ton and 3-min ultrasonic treatment [70]. Liao et al. were innovative in applying ultrasonic pretreatment to the separation process of pyrite and chlorite by promoting the dissolution of iron oxide particles on the surface of pyrite and the desorption of adsorbed iron ions on the surface of chlorite through ultrasonic treatment, which in turn enhanced the separation of pyrite and chlorite by increasing the recovery of pyrite from 77.50% to 81.72% and decreasing the content of magnesium oxide from 6.57% to 5.74%, thus increasing the grade and recovery of pyrite [16]. Chen’s team were innovative in introducing ultrasound to the graphite recovery process of spent carbon cathodes from the electrolytic aluminum industry, using ultrasound to promote particle dispersion to clean the mineral surface and increase the porosity of the graphite surface, thus enhancing the flotation performance. The slurry was pretreated with ultrasound for 30 min. The slurry recovery was 96.24%, which was 10.2% higher than that of the conventional flotation, the tailings content was 9.68%, which was 17.31% lower than that of the conventional flotation, and the grade of the concentrate was 84.95% [71]. The innovation of Bagheri et al. lies in the use of an ultrasonic pretreatment technique to improve the flotation performance of sphalerite in their study, whereby ultrasonic treatment removed the impurities and oxide layer from the mineral surface and cleaned the mineral surface, improving the hydrophilicity of the sphalerite and enhancing the interaction between the collecting agent and mineral surface. The sphalerite recovery was enhanced from 70.30% to 73.79%. However, the ultrasonic treatment also increased the flotation of unwanted associated minerals (e.g., pyrite), with the flotation enhancement of tailings pyrite increasing from 31.95% to 54.38%, resulting in a significant decrease in the separation efficiency of zinc [72]. This indicates that ultrasonic pretreatment does not always have a positive effect on mineral flotation.

It is also worth noting that Dongfang et al. were innovative in investigating the effect of ultrasonic pretreatment on the flotation behavior of galena, not only for a single mineral sample but also for a mixture of galena and pyrite. It was found that the recovery of galena increased after the ultrasonic pretreatment of a single galena sample; however, the recovery of galena decreased after the ultrasonic pretreatment of a mixture of galena and pyrite samples [73]. This suggests that an interfering effect may occur between minerals when ultrasonic flotation of multiple minerals is performed.

Ultrasonic flotation pretreatment can also improve flotation efficiency by acting on the mineral surface and changing the mineral surface structure or properties. For example, Mao et al. were innovative in studying how ultrasonic pretreatment changed the surface topography of lignite, including the generation of cracks, gaps, and cavities, and how these changes affected the flotation performance of lignite. It was found that an increase in ultrasonic power and treatment time led to an increase in lignite surface roughness and a decrease in concentrate yield by step, except at 18 watts. In particular, for lignite flotation treated with 108 W ultrasonic waves for 3 min, the concentrate yield was reduced by 16.65% compared to untreated lignite flotation, while the measurements of surface roughness and induction time provided a new technical tool for evaluating the effect of ultrasonic pretreatment [74]. Xue et al. were innovative in applying ultrasonic pretreatment to the hydrophobic–hydrophilic separation (HHS) process in order to enhance residual carbon in coal gasification fine slag (CGFS) enrichment and revealed the adsorption behavior of n-heptane on the residual carbon surface by molecular dynamics simulations, which is typically a spontaneous process driven by van der Waals forces. This simulation approach provides micro-level insights for understanding the ultrasonic pretreatment affecting HHS processes. It was found that ultrasonic pretreatment plays a positive role in the effectiveness of HHS by changing the surface properties of CGFS particles and enhancing the hydrophobicity of residual carbon through mechanical action and chemical oxidation. In addition, the microbubbles generated by ultrasound helped the particles to trap and separate at the oil–water interface, which further improved the efficiency of HHS. With the continuous increase in ultrasound power, the ash content of the concentrate was reduced from 31.81% to 18.93%, while the ash content of the tailings was greater than 96.00% [75]. Chen et al. dissociated elemental sulfur and silver-bearing lead minerals in the oxidative pressure leaching residue of zinc sulfide through the use of ultrasonic waves, increasing the surface structural differences between the elemental sulfur and the silver-bearing lead minerals, which contributed to their separation. The innovation lies in the evaluation of the feasibility of ultrasonic pretreatment technology for enhancing the flotation separation of residues in industry through pilot-scale flotation tests. It was found that the sulfur grade and concentrate recovery were increased by 11.63% and 9.10%, respectively, and that the recovery of lead and silver in the tailings was increased by 8.72% and 9.50%, respectively [25]. Ann Bazar et al. found that ultrasonic pretreatment improved the dispersion of talc by altering the surface properties of talc and enhanced the adsorption of inhibitors by generating active sites on the surface of talc. This was shown to reduce the flotation recoveries of talc by approximately 15% at lower inhibitor dosages and 8% at higher inhibitor dosages [76]. Ren et al. combined ultrasonic pretreatment slurry and composite collector flotation to enrich residual carbon from coal gasification slag. The structural properties and surface chemistry of CGFS were improved, and the ultrasonic pretreatment slurry demonstrated an excellent crushing effect on tightly connected carbon-gray particles. The loss indices of three concentrates obtained by combined processes compared to traditional flotation increased by 1.52 times, 1.42 times, and 1.20 times, respectively, while the loss indices of tailings decreased by 51.48%, 86.46%, and 35.58%, respectively [77].

The above cases show that in recent years, ultrasonic flotation pretreatment has been demonstrated to improve the flotation efficiency by promoting the dispersion of minerals, cleaning the surface of minerals, acting on the surface of minerals, and changing the structure or properties of the surface of minerals. However, during the ultrasonic flotation of multiple minerals, interactions between minerals may form, resulting in poor flotation results. Ultrasonic flotation pretreatment does not always improve mineral flotation.

6.2. Simultaneous Treatment

Cleaning the mineral surface is the main method of simultaneous ultrasonic flotation to improve flotation efficiency. For example, the innovative nature of [78] lies in the application of ultrasonic treatment during different stages of flotation. The ultrasonic treatment exposed more adsorption sites by having a cleansing or stripping effect on the mineral surfaces, thereby increasing the specific surface area, and also altered the zeta potential of the mineral surfaces, making them more negatively charged, which contributed to the enhancement of the sodium oleate adsorption capacity on the mineral surface. Simultaneous ultrasonic treatment improved the flotation separation efficiency of magnesite and dolomite, resulting in a flotation recovery of 91.06% for magnesite concentrate, nearly a 20% increase compared to conventional flotation. Chen et al. innovatively enhanced the efficient recovery of valuable metals from waste printed circuit boards (WPCBs) by introducing ultrasound into the flotation process and found that ultrasonic flotation facilitated the detachment of epoxy resin and minute metal fragments from the glass fiber surfaces in WPCBs. Through the cavitation effect and surface cleaning, the hydrophobicity of the non-metallic components and particle dispersion were enhanced, which facilitated a significant increase in the total metal recovery, as well as Cu, Al, and Zn recovery from the WPCBs. They were increased by 7.20%, 4.82%, 15.41%, and 5.58%, respectively [79].

Moreover, simultaneous ultrasonic flotation treatment utilizing cavitation and acoustic radiation forces goes beyond cleaning mineral surfaces. For example, one of the innovations of Mao et al.’s study on the effect of ultrasonic flotation in the pulp–froth zone on the selectivity and kinetics of flotation of high-ash lignite was the elucidation of the mechanism of ultrasonic flotation in the froth zone. The ultrasonic treatment carried out in the froth zone produced large bubbles and a thin froth layer due to ultrasonic cavitation and oscillation, improved the water-carrying recovery of fine particles, increased the concentrate yield by 14.53% and the combustible recovery by 21.42%, and concentrate ash was reduced by only 1.56%. The ultrasonic flotation in the pulp zone provided optimal surface cleaning of the coal surface, where the ultrasonic treatment increased the concentrate yield by 9.72% and the combustible recovery by 18.54%, while the ore ash was reduced by 7.75% [80].

On the other hand, acoustic radiation force effect plays a greater role in high-frequency simultaneous ultrasonic treatment. For example, Chen et al. innovated by studying the effects of different ultrasonic standing wave (USW) frequencies on the formation of microbubbles, dispersion of conventional flotation bubbles (CFBs), and movement of particles in the fine coal flotation process. The research demonstrated that the high-frequency USW produced more microbubbles as carriers of coal particle motion and promoted the collision and adhesion between coal particles and bubbles, increasing the recovery of fine-grained materials and flotation rate through acoustic radiative force. The maximum recovery obtained by the 600 kHz USW increased from 22% in the absence of USW to 53%, while low-frequency (50 kHz) USW may reduce these metallurgical indicators [81].

Nevertheless, simultaneous ultrasonic flotation treatment does not always enhance the flotation efficiency. Take the innovative application of heterogeneous carrier flotation technology as an example, using Cu²⁺-treated coarse-grained pyrite as a carrier to enhance fine-grained chalcopyrite recovery. Although ultrasonic treatment may disrupt the Cu²⁺-treated layer on the surface of pyrite, ultrasonic treatment does not significantly promote the loss of chalcopyrite fines due to the hydrophobic forces between chalcopyrite and pyrite, whereas acidic treatment effectively separates chalcopyrite fines on coarse pyrite particles. At pH 2, approximately 96% of chalcopyrite fines were separated from Cu²⁺-treated coarse pyrite particles [82].

Ultrasonic pretreatment and ultrasonic simultaneous treatment may have a more positive effect on flotation efficiency. For example, Wang et al.’s study introduced a novel approach that comprehensively applied ultrasonic pretreatment and ultrasonic emulsification technology. They investigated the effects of ultrasonic pretreatment slurry and ultrasonic emulsification flotation agents on flotation enrichment of residual carbon in entrained-flow gasification coal fine slag. The effect of ultrasonic flotation on the crushing of entrained-flow gasification coal slag particles was more significant than that of traditional flotation. Ultrasonic treatment enhanced the dissociation of gasified coal fines through mechanical and cavitation effects, removed the high-ash fines on the surface, and reduced the mechanical entrainment of high-ash fines, thus improving the selectivity of flotation. In addition, the ultrasonic emulsification improved the dispersion of the collecting agent on the surface of the coal dust gasification fines and reduced the surface tension, which helped the collecting agent cover the particle surface more uniformly and enhanced the hydrophobicity in the flotation process, thus improving the flotation efficiency of the fines. The concentrate yield and ash content were reduced by 9.94% and 16.54%, respectively, while the quality of concentrate improved significantly, with the flotation perfection index increasing by 12.60% [26].

Ultrasonic technology not only improves the hydrophobicity of minerals but also proves its versatility in solid waste resource utilization by reducing the ash content of concentrates and increasing the recovery of tailings ash and combustibles. Ultrasonic power and treatment time are key parameters that affect the effectiveness of ultrasonic treatment. The above shows that an increase in ultrasonic power can significantly reduce the ash content of the concentrate, but care also needs to be taken to avoid over-treatment leading to reoxidation of mineral particles or other adverse effects. Moreover, the ultrasonic standing wave (USW) is used as a fine-grained flotation aid, and its frequency has a significant effect on flotation efficiency. The overall flotation effects of ultrasonic flotation are shown in

Table 1. Mineral ultrasonic flotation effects.

Process-Specific		System		Comparison of Results Compared to Conventional Flotation
Ultrasonic pretreatment		Coal	Residual carbon from CGFS	Keeping the ash fluctuations of tailings under 2%, the concentrate ash saw a drop from 31.81% to 18.93% as ultrasonic power escalated.
				The combined method increased the three concentrates’ LOIs by 1.52, 1.42, and 1.20 times, and LOIs of cut tailings by 51%, 86%, and 36%.
			Combustible components	The presence of micro–nano bubbles during 37 kHz and 80 kHz treatments raised the combustible recovery by roughly 35% and 25%, respectively.
		Sphalerite	Low ash, fine lignite	As ultrasonic power and time rose, concentrate yield generally fell, with the exception at 18 W.
		High-zinc-grade tailing	Silver-bearing and sulfur lead minerals	Sulfur grade rose 11.63% and concentrate recovery increased by 9.10%, while the lead and silver recoveries from tailings climbed to 8.72% and 9.50%, respectively.
		Chlorite	Sphalerite	Flotation recovered over 73% of Zn from tailings, yielding a rougher concentrate at 28.61% Zn grade.
		Spent carbon cathode	Pyrrhotite	Pyrrhotite recovery rose from 77.50% to 81.72%, while the MgO content dropped from 6.57% to 5.74%.
		Foundry dust	Graphite	Flotation recovery rates reached 96.24%, tailings contained 9.68%, and concentrate grades peaked at 84.95%.
		Gasification coal fine slag	Clay minerals and coal powder	Tailings yield was 55.3%, with 19.3% LOI and 69.22% RPC.
	Simultaneous ultrasonic treatment	Lignite	Residual carbon	Concentrate yield dropped by 9.94%, ash content dropped by 16.54%, and the flotation perfect index rose by 12.60%.
		Magnesite and dolomite	Concentrates and combustibles	In pulp-zone ultrasonic flotation, concentrate yield and combustible recovery rose 9.72% and 18.54%, respectively, while froth zone saw gains of 14.53% and 21.42%.
		Waste printed circuit boards	Magnesite and dolomite	Magnesite concentrate recovery hit 91%.
		Sphalerite	Pyrite particles	Metal recovery from WPCBs collectively rose by 7.20%, with specific gains of 4.82% in Cu, 15.41% in Zn, and 5.58% in Al.

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In addition, ultrasonic treatment technology as a physical treatment method has obvious advantages in reducing the use of chemical additives and lowering the environmental impact, which helps to promote the development of the mineral processing industry in a green and sustainable direction. Even so, it is not a complete replacement for traditional flotation; rather, it serves as a complementary means to improve the overall flotation performance by improving mineral surface properties and promoting particle dispersion. The joint application of ultrasonic technology with other technologies, such as synergizing with composite capture agents, provides new ideas for improving the flotation efficiency of specific minerals. This integrated and innovative approach may become a significant direction for the future development of ultrasonic flotation technology.

With the current findings, ultrasound technology shows wide applicability in different types of mineral and material recovery and performs well in improving flotation efficiency, but the differences in the effect of ultrasound treatment on flotation performance at different frequencies and treatment times indicate that there is scope for further optimization of ultrasound parameters to achieve optimum flotation results for different types of minerals. However, as mineral properties, particle size, chemical composition, and other factors increase, they may negatively affect the recovery rate and selectivity of the final destination minerals and require targeted optimization. The introduction of ultrasonic treatment technology may increase energy consumption and operating costs; therefore, evaluating its economic efficiency and cost/benefit ratio is crucial for realizing industrial applications.

7. Research Prospect and Direction

Physical and chemical reactions in the ultrasonic flotation process can be better understood and predicted using quadratic models and numerical simulations. This helps to optimize ultrasonic flotation process parameters, improve beneficiation efficiency, and reduce the energy consumption and environmental impact. Experimental data from ultrasonic flotation technology can also be used to validate and improve the quadratic model, aligning it more with practical applications. Jiang et al. developed a quadratic model to predict polyvinyl chloride purity, explored the interactions between pH, AlCl₃ concentration, and ultrasonic treatment time, and achieved microplastic recovery and purity of more than 99.65% [83]. Xue et al. verified the spontaneous adsorption process of n-heptane on the residual carbon surface in coal gasification fine slag through molecular dynamic simulation, which was driven by van der Waals forces and provided a theoretical basis for improving hydrophobic–hydrophilic separation efficiency through ultrasonic pretreatment [75].

The use of composite flotation technology combining ultrasound with other physical fields (e.g., electric fields and magnetic fields) and the development of new ultrasound-assisted chemical treatment methods also represent a development trend. Wang et al. combined ultrasonic slurry strong magnetic flotation technology to realize the separation and extraction of carbon and iron powder in the gangue while enhancing the efficiency of decarbonization and iron removal [49]. Cao et al. found that the combination of ultrasound and electrochemistry for the desulfurization of fine coal was more effective than the ultrasonic enhanced flotation method [84]. Future development trends may involve more research and practical applications that combine ultrasonic flotation with magnetic fields, electric fields, and other force fields to enhance beneficiation efficiency, reduce ore waste, and lower production costs. Additionally, it is important to explore the synergistic mechanisms of ultrasonic flotation with these force fields, optimize process parameters, and develop new equipment and devices. This will help promote the development of ore extraction and separation technology and realize more effective utilization of mineral resources.

8. Conclusions

Ultrasonic flotation technology, a new type of mineral processing technology, has played a significant role in improving the utilization rate of mineral resources and reducing environmental pollution, such as coal, tailing containing zinc mineral, pyrite, potash ore, spent lithium-ion batteries, and electronic waste.

In recent years, the development of the flotation method has focused on four main areas: changing the ultrasonic parameters, the synergistic effect of ultrasonic waves and reagents, the ultrasonic effect of different particle sizes, and the application of new systems. These foci aim to enhance the efficiency of flotation through the use of ultrasonic flotation technology. Additionally, ultrasonic flotation devices have evolved from simple designs to the integration of complex technologies. The evolution of equipment for ultrasonic flotation has progressed from early designs aimed at improving efficiency by increasing the area of the floating guide surface to more advanced technologies that enhance slurry mixing, clean the surfaces of mineral particles, and emulsify flotation reagents. Current developments rely on big data, the Internet of Things, automation, and other technologies, reflecting future trends. Additionally, the integration of ultrasonic fields with other physical fields aims to increase the flotation efficiency of complex minerals. Regarding the effectiveness of ultrasonic flotation, its applicability across different types of minerals and material recovery is broad; however, performance varies greatly with different frequencies and treatment times, influenced by factors such as mineral characteristics, particle size and chemical composition. Furthermore, ultrasonic flotation is being combined with other physical fields (magnetic, electric, etc.) to improve the flotation of complex minerals. Research employing quadratic models and numerical simulations is underway to study ultrasonic flotation mechanisms and further investigate these mechanisms.

Future research should focus on gaining an in-depth understanding of the cavitation and acoustic radiation effects generated by ultrasonic flotation. This can be achieved through the application of ultrasonic treatment in the flotation process to optimize operating parameters and integrate other technologies. The goals include facilitating the crushing or aggregation of minerals, cleaning or modifying mineral surfaces, enhancing the synergistic effects of composite reagents, aiming to improve mineral flotation effectiveness, and promoting the resourceful use of waste minerals. Ultrasonic flotation facilitates green flotation, high-efficiency flotation, and intelligent flotation.