Effect of Gas Oversaturation Degree on Flotation Separation Performance of Electrode Materials from Spent Lithium-Ion Batteries

Abstract

The electrode materials from spent lithium-ion batteries consist of graphite and lithium cobalt oxides (LCO), which cannot be efficiently separated by the conventional flotation technique due to the fine size distributions of graphite and LCO. In this work, nanobubbles were introduced to the flotation system of electrode materials. Nanobubbles were produced with the method of temperature difference. Different degrees of gas oversaturation in the water/slurry were achieved by raising the temperature of cold water (stored at 4 °C for at least 72 h) to target values of 20 °C, 25 °C, and 30 °C. It was found that the height and lateral distance of nanobubbles increased with the degree of gas oversaturation of water. In addition, the larger graphite agglomerations were observed to form in the presence of nanobubbles. The D₅₀ (chord length) of graphite agglomerations increased by 8 μm, 11 μm, and 21 μm, respectively, compared with the D₅₀ of graphite in natural water. More graphite agglomerations adhered to a captive bubble with the aid of nanobubbles than in the case of no nanobubbles, which was indicated by increased wrapping angles of graphite (agglomerations) adhering to a captive bubble. Furthermore, the maximum adhesion force between a captive bubble and substrate increases to 220, 270, and 300 μN as cold water temperature increases to 20, 25, and 30 °C, respectively. The frost of nanobubbles on a graphite surface and the resulting graphite agglomerations through the bridging effect of nanobubbles are thought to be responsible for the improved flotation performance of electrode materials. The present results indicate that the flotation performance of fine minerals can be regulated by regulating the gas oversaturation degree of the slurry.

1. Introduction

Since they were successfully commercialized in the last century, lithium-ion batteries (LIBs) have been widely applied as a power source in laptops, personal mobile phones, electric vehicles, etc. due to their high energy density, small volume, and light weight [1]. In China, a large number of electric vehicles have been being produced by automobile manufacturers such as Tesla, BYD, Geely, etc. It was reported that China’s new electric vehicle sales increased by 82% in 2022, accounting for nearly 60% of global electric vehicle purchases. As the power source of electric vehicles, LIBs usually have a lifetime of 3–12 years; therefore, China will soon have to face a surge in lithium-ion battery scrapping in the future. Therefore, necessary measures will have to be taken to recycle these batteries after reaching their end of life. LIBs normally consist of an outer shell, copper/aluminum current collectors, a separator, a negative electrode made of graphite, and a positive electrode based on layered transition metal oxide such as LiCoO₂ and LiNiO₂ or mixtures thereof and so on [2]. The first step in the recycling chain of spent LIBs is the discharging and crushing operation. It was reported that several components such as the metallic outer shell and copper/aluminum current collectors were, respectively, enriched in specific coarse size fractions and therefore were separated by sieving with a screen. However, a huge challenge was posed by the separation of anode and cathode powders due to their similar fine size distribution.

Froth flotation is an effective method to separate fine minerals based on the differences in surface hydrophobicity [3,4]. During a flotation process, hydrophobic particles tend to adhere to rising bubbles and finally end up in the froth layer while hydrophilic particles are left as tailings in a slurry. Graphite shows a natural hydrophobicity while lithium cobalt oxide shows a natural hydrophilicity, and these provide the possibility of separating electrode materials with froth flotation. Aliza et al. [5] reported flotation experiments of electrode materials after pyrolysis pretreatment, and the recovery of graphite in the overflow product reached 91%. It was reported that polyvinylidene fluoride (PVDF) polymers are widely used to stick electrode materials to the current collector of batteries, which results in a decreased difference in surface hydrophobicity between cathode and anode materials. For example, the grade of cobalt increased from 24.21% to only 28.08% in flotation products as the electrode materials were subjected to flotation without any pretreatment [6].

Many pretreatment measures were proposed to remove PVDF from electrode materials including pyrolysis [7,8,9], dissolution with chemicals [10], grinding [11,12], and so on. Pretreatment with pyrolysis and chemicals usually causes the release of poisonous gas or the residual of the chemical in liquid, posing a huge challenge to environment protection. Grinding is a green and eco-friendly pretreatment technique and has been widely applied in minerals processing. However, attrition operations will inevitably lower the size distribution of electrode materials, which poses a huge challenge to conventional flotation technology for the separation of these materials. Rihan Efendi et al. reported the key factors affecting the flotation recovery of LIB electrodes, with SEM-EDS results indicating that some pretreatment methods, especially heat treatment, are insufficient to remove the entire binder and may change the surface of the anodic active material at higher temperatures (above 500 °C), reducing its hydrophobicity. Such results are detrimental to flotation recovery [13]. Sabereh Nazari et al. conducted a systematic evaluation of the potential applications of froth flotation in the sustainable recycling of used lithium-ion batteries, highlighting the critical role of pretreatment methods (such as thermal treatment, mechanical grinding, and Fenton oxidation) in removing binders and restoring the wettability of electrode materials. Under laboratory conditions, flotation after pretreatment can achieve over a 90% purity of cathode materials and over 85% recovery rate of graphite, demonstrating significant potential. However, future efforts should focus on optimizing the cleanliness of the pretreatment process and facilitating industrial applications [14]. Zhang et al. explored the application of pyrolysis technology in enhancing the dissociation efficiency of spent lithium-ion battery electrode materials. The results showed that pyrolysis improved the dissociation efficiency of the cathode from 82.88% to 99.78%, and the dissociation efficiency of the anode increased from 88.08% to 99.60%. This is a highly effective pretreatment method, and it was noted that the electrode materials after pyrolysis were primarily concentrated in the 0.045 mm particle size range [8].

Nanobubbles are tiny bulk bubbles with a diameter of less than 1 μm [15] or spherical-cap-shaped nanostructures present at solid–liquid interfaces with one to tens of nanometers in maximum height and tens to hundreds of nanometers or even several microns in lateral diameter [16]. Nanobubbles were firstly proposed to explain discontinuities or steps present in the force curves [17] and have attracted much attention since they were imaged successfully by Lou et al. [18] with an atomic force microscope (AFM) at the beginning of this century. Now, the existence of surface nanobubbles at the solid–liquid interface has been widely recognized and a number of techniques have been developed to generate nanobubbles including alcohol–water exchange [19,20], the cold-water warming method [21,22,23], depressurization [24], electrolysis [25,26], and cavitation [27,28]. Nanobubbles show potential for application in minerals flotation. Based on fundamental flotation theory, the flotation performance of fine minerals can be improved by the formation of agglomerations. A number of studies have revealed that the nucleation or adhesion of nanobubbles on the particle surface significantly influences the flotation behavior of fine particles, which is due to the ability of nanobubbles to influence the sub-processes (collision and adhesion) of flotation. Liu et al. [29] reported the agglomeration behavior of ultrafine coal particles with different hydrophobicity in the presence of nanobubbles, which was one of the reasons for improving the flotation performance of fine coal in the presence of nanobubbles. Knüpfer [20] reported that nanobubbles enhanced the agglomeration of hydrophobic alumina powders. Li et al. [30,31] reported an improved flotation performance of fine coal in the presence of surface nanobubbles. In our previous studies, we systematically studied nanobubble nucleation behavior [32], and the removal of ultrafine graphite from graphite slime slurry by a nanobubble-assisted flotation technique [33], where the method of gas oversaturation by raising the temperature of cold water was frequently adopted. This method is expected to be popularized for industrial-scale application, although necessary information on the effect of gas oversaturation degree on the flotation behavior of fine particles is still lacking. Tang et al. studied the mechanism by which nanobubbles enhance flotation performance and proposed that nanobubbles can cover the polar hydrophilic groups on the surface of graphite, enhancing the adsorption of collectors. The treatment with nanobubbles increased the contact angle of the graphite surface by 11.93°, which also confirmed the ability of nanobubbles to enhance the hydrophobicity of the graphite surface [34]. Sabereh Nazari et al. explored the impact of operational factors on the process of the nanobubble-assisted flotation of graphite. The experimental results indicated that the flotation recovery of graphite particles by nanobubbles increased as the particle size, stirring speed, and air flow rate increased, reaching up to 15%. Additionally, using nanobubbles (NBs) as a secondary collector reduced the amount of collector and frothing agent [35].

In this work, the effects of gas supersaturation degree on the nucleation behavior of nanobubbles, particle agglomeration behavior, and particle–bubble interactions were studied by combining atomic force microscopy, colloidal probe technology and focused beam reflectance measurement (FBRM) particle size measurement technology. Furthermore, interaction force measurement and flotation performance were explored with different gas oversaturation degrees. The present results are expected to provide a technical reference for the separation of other fine particles with flotation.

2. Experimental

2.1. Materials

Considering the varied surface hydrophobicity and surface roughness of real graphite, alumina wafer and silicon wafer were used to replace graphite as the standard model substrate for AFM measurement. The alumina wafer shows a rather rough surface while the silicon wafer has a relatively smooth surface. In a practical flotation feed, mineral particles exhibit heterogeneous hydrophobicity and surface roughness. Therefore, investigating the effects of mineral surface hydrophobicity and roughness on nanobubble nucleation is critical for advancing flotation science and optimizing industrial flotation processes. Before imaging, the two substrates were hydrophobized by silanization treatment; photos of silicon wafer (left) and alumina (right) hydrophobic modification are shown in Figure 1; the detailed procedure was described in detail [33]. The substrate was fixed in a liquid cell with a piece of double-sided tape for AFM imaging. Ultrapure water prepared with a Milli-Q unit (18.2 MΩ·cm, TOC less than 5 ppm) was transferred into glass bottles, respectively, and stored at 4 °C in a refrigerator for at least three days to reach an equilibrium state with air before use. New and commercial graphite powders and LCO powders were used as representative electrode materials replacing those from real spent lithium-ion batteries. Both materials were purchased from Cyber Electrochemical Materials Network, with a purity of 99.95%, and are used by manufacturers as raw materials for battery production. The size distributions of the two kinds of powders measured with a laser size analyzer are shown in Figure 2.

2.2. The Production and Characterization of Nanobubbles

Nanobubbles were produced, respectively, on a silanized alumina and a silicon wafer substrate, which are two representative materials for smooth and rough surfaces, by depositing about 1.5 mL of cold water into the self-made liquid cell with a glass injector. Gas oversaturation was created as the temperature of cold water increased to the target temperature, and hence, nanobubbles were produced on the substrate surface. The characterization of surface nanobubbles was done by using an AFM (XE-100, Park Systems Corp, Suwon, South Korea) in intermittent mode. A cantilever (ContGD-G, Budget Sensors) with a nominal spring constant of 0.2 N/m and a nominal resonant frequency of 13 kHz in air was used. The real resonant frequency and spring constant were calibrated with the method described in ref. [36]. After the cantilever was immersed in the liquid, about 20 min were given to reach an equilibrium state for the system, which was indicated by a stable position of a laser dot on the position sensitive detector (PSD). The values of scan rate and amplitude ratio were set to be 0.5 Hz and no less than 80%, respectively, prior to imaging. To reduce the possible pollution from experimental tools and glass vessels to AFM imaging, substrates, tweezers and injector, and other items were cleaned thoroughly in an ultrasonic bath in the order of ultrapure water, ethanol, and ultrapure water, successively, before measurement.

2.3. Morphological Observation of Graphite Aggregates

In order to study the morphological differences of hydrophobic graphite aggregates in nanobubble systems and evaluate their degree of agglomeration, the samples were observed using optical microscopy. A graphite slurry solution was prepared with cold water, then placed on a magnetic stirrer, connected to a heating device, and the final temperatures were set to 20 °C, 25 °C, and 30 °C, respectively, with a stirring speed of 800 r/min for 15 min. After stirring, a small amount of the solution was taken with a pipette and dropped onto a glass slide, with the magnification set to 40 times, to observe the dispersion state of graphite in different solutions.

2.4. Formation and Measurement of Graphite Agglomerations with Focused Beam Reflectance Measurement (FBRM)

To quantify the influence of nanobubbles as a result of gas oversaturation degree on the interaction between graphite particles, the distributions of the chord length of graphite agglomerations acquired at different gas oversaturation degrees were measured with focused beam reflectance measurement (FBRM, Figure 3). The working principle of FBRM can be found in ref. [37]. Briefly, a laser beam launched by a laser diode embedded in FBRM’s probe is projected into a particle suspension with a high rotation speed (2–6 m/s). When the rotated laser beam encounters a particle, the laser will be reflected by the particle and the reflected laser signal will be received and recorded by the optical detector. The chord length of a particle is calculated by the product of the time in which the laser travels the particle surface and the scan speed of the focused beam. The advantages of the measurement of graphite agglomerations in situ are to protect agglomerations from potential destruction as much as possible. For specific procedures, firstly, circulation water with an expected temperature (20, 25, and 30°, respectively) was pumped into the circulation system. Then, a graphite slurry was transferred into the inner liquid cell and was subjected to conditioning treatment after the temperature of circulation water was constant. After the temperature of circulation water was constant again, the measurement controller was turned on and graphite size distribution information was recorded and analyzed.

2.5. Particle–Bubble Collision and Adhesion Experiment

The behavior of particle–bubble collision and adhesion is partly determined by particle size distribution and surface hydrophobicity. To explore and compare qualitatively the effect of gas oversaturation degree on particle–bubble collision and adhesion performance, nanobubbles were induced to nucleate on graphite particle surfaces in a slurry with different gas oversaturation degrees and graphite particles frosted with surface nanobubbles were forced to collide with each other under agitation. The specific procedures are as follows (Figure 4 and Figure 5): 1 g of graphite was dispersed in 100 mL of ultrapure water and the slurry was stirred at an agitation speed of 400 r/min for 30 min. Then, the agitation was stopped and a captive bubble with a fixed volume was produced at the terminal of the needle. The slurry was agitated again for 20 s to motivate graphite particles (agglomerations) to collide with the captive bubble. The agitation speed was set as 300 r/min. Too high of an agitation speed will result in the detachment of captive bubbles from the needle, while too low of an agitation speed is not enough for graphite particles to collide with the captive bubble. After 20 s, the agitation was stopped again and the final attachment state was recorded and analyzed.

2.6. Interaction Force Measurement

Adhesive strength is measured using a visual multifunctional tensiometer (JQ03E PRO, Shanghai Zhongchen Digital Technology Equipment Co., Ltd., Shanghai, China), with a measurement accuracy of ±0.5% FS, as shown in Figure 6. The measurement device consists of a high-sensitivity micro-balance, an automatic lifting platform, a high-speed digital camera, and a monitor. A capillary connected to a micro-injection pump was embedded in the high-sensitivity microbalance module. A captive bubble was generated at the terminal of the capillary by a micro-injection pump and then the substrate was lifted to approach the captive bubble. The separation distance between the bubble and the substrate was monitored by the high-speed camera. The interaction force information was recorded as a function of separation distance. Compared with AFM, the present device has a higher force measurement upper limit but lower force accuracy and resolution.

2.7. Flotation Test

Graphite flotation under different gas oversaturation degrees was performed in a self-made Hallimond tube. Studies have demonstrated that more than half of the cathode material market was still dominated by LiCoO₂ and LiNi_0.33Co_0.33Mn_0.33O₂. The anode materials are primarily composed of graphite, which typically constitutes 12% to 21% (wt%) of the lithium-ion battery, while lithium comprises 2% to 7% (wt%). The specific proportions may vary depending on the manufacturer. Therefore, we have chosen a ratio of 4:1 for graphite to lithium cobalt oxide for our research [39,40]. Graphite and LCO were mixed at a mass ration of 0.8 g:0.2 g to model mixed electrode materials. The model materials were pre-wetted with 100 mL of room temperature water or cold water in a beaker with a magnetic stirrer at a stirring speed of 500 r/min. Then, the slurry was carefully transferred into a Hallimond tube with a self-made funnel, stirred at a speed of 500 r/min for 15 min, and then aerated at a rate of 26 mL/min. Extra room temperature water or cold water was replenished to 180 mL to keep the volume of the slurry constant for each flotation. During flotation, the flotation process was divided into four time periods: the duration of each period from the start to the end of flotation was 2 min, 4 min, 6 min, and 8 min, respectively. The obtained concentrates were filtered, dried, sampled, and analyzed for cumulative combustible recovery. A sleeve structure, as part of this tube, was specially designed so that the temperature of the slurry could be kept near the target value with the aid of a water bath, as shown in Figure 7. The circulated water that was heated to the target temperature in a large tank prior to use was injected into the sleeve with a peristaltic pump from the inlet at the bottom of the tube body and then was discharged from the outlet of the tube to the tank.

3. Results and Discussion

3.1. Imaging

Nanobubbles nucleated on smooth silicon wafer and rough alumina surfaces at different terminal temperatures were imaged with an intermittent AFM. The height and lateral size of surface nanobubbles were counted with a MATLAB R2023b (MathWorks Inc., Natick, MA, USA) program, as shown in Figure 8 and Figure 9. On both the smooth silicon wafer surface and the rough alumina surface, the height and lateral size of nanobubbles acquired at the higher gas oversaturation degree are always higher than those acquired at the lower gas oversaturation degree. For example, the height of 50% of nanobubbles acquired on smooth silicon wafer surface at a terminal temperature of 20 degrees is about 7 nm, while the value is increased to 10 nm at 30 degrees. Similarly, the lateral size of 80% of the nanobubbles acquired on the smooth silicon wafer surface at a terminal temperature of 20 degrees is about 70 nm, while the value is increased to 90 nm at 30 degrees. A similar conclusion can also be drawn as alumina acted as a substrate for AFM imaging. The size distribution of nanobubbles nucleated both on the smooth silicon wafer surface and on the rough alumina surface increased with the gas oversaturation degree at the presently researched temperature range. It is speculated that nanobubbles could merge with their neighboring ones to be the larger nanobubbles, even microbubbles, as the degree of gas oversaturation is increased further from 30 degrees. Large nanobubbles or microbubbles could detach from the substrate surface, resulting in the appearance of nonlinear correlativity between the size of nanobubbles and gas oversaturation degree. It is worth noting that nanobubbles nucleated on a rough alumina surface at 20 degrees are larger both in height and in lateral size than those on the smooth silicon wafer surface at the same terminal temperature in the case of a similar surface hydrophobicity, which means that high surface roughness is more beneficial for nanobubble nucleation than the relatively smooth silicon wafer surface. This observation is consistent with our previous research conclusions [9].

In order to further quantitatively compare the effect of gas supersaturation on the nucleation behavior of nanobubbles, we compared the nanobubbles in these four images using a MATLAB R2023b program, as shown in Figure 10. The comparison reveals that whether on the rough alumina surface or the smooth silicon wafer surface, the higher the degree of gas supersaturation, the larger the nanobubbles. Additionally, the height of the bubbles on the alumina surface is greater than that on the silicon wafer surface, indicating that the rougher the substrate, the larger the bubble size.

3.2. The Effect of Gas Supersaturation on the Morphology of Graphite Aggregates

As shown in Figure 11, it can be observed that graphite is very dispersed in the solution at room temperature and does not form aggregates. However, as the temperature of the solution increases, the graphite particles begin to aggregate with each other, and the morphology and size of the graphite aggregates correspond to the final temperature. At the final temperature of 30 °C, the graphite aggregates are the largest and most numerous, followed by those at 25 °C, while the aggregates at 20 °C are the smallest among the three. The higher the final temperature, the more aggregates are formed, and the more graphite particles are present in each aggregate. This indicates that fine particles will agglomerate into aggregates under the influence of nanobubbles, and this phenomenon becomes more pronounced with increasing gas supersaturation.

3.3. Effect of Gas Oversaturation Degree on Size Distribution of Graphite Agglomerations

The size distributions of graphite (agglomerations) at different gas oversaturation degrees were measured in situ with FBRM, and the results are shown in Figure 12. The horizontal coordinate corresponding to the points in each column of the figure represents the number of measurements, which increases from left to right. The D₅₀ of the chord length of graphite in room temperature water (in the absence of nanobubbles) is about 11 μm. This value is lower than that measured previously with a laser particle size analyzer, which could be ascribed to the difference in measurement principles between FBRM and the laser particle size analyzer. As graphite powders were dispersed in cold ultrapure water stored in a 4 °C environment for at least 72 h, nanobubbles were expected to nucleate on the graphite surface as a result of gas oversaturation, as observed on a highly oriented pyrolytic graphite (HOPG) surface with AFM. The D₅₀ of the chord length of graphite agglomerations increased to 19, 22, and 32 μm, respectively, as the temperature of the graphite slurry increased from 4 °C to 20 °C, 25 °C, and 30 °C, respectively. Compared to the conditions of water at room temperature, the increments of D50 (chord length) for graphite aggregates were 8, 11, and 21 μm, respectively. The degree of the gas oversaturation of the graphite slurry was expected to increase with the terminal target temperature. The larger nanobubbles were nucleated on the graphite surface at the higher gas oversaturation degree, resulting in the formation of the larger graphite agglomerations. It is possible for the nanobubble to grow by gas diffusion or coalescence behavior and eventually detach from the solid surface if the degree of the gas oversaturation of the graphite slurry is further increased, as was reported by [41]. However, this phenomenon was not observed in the present temperature range.

3.4. Effect of Nanobubbles on Collision and Adhesion Performance Between Graphite (Agglomerations) and a Captive Bubble

The adhesion conditions of graphite particles (or agglomerations) under different gas oversaturation degrees were captured by a camera and the wrapping angles were measured and recorded one by one, as shown in Figure 13. It was found that more graphite particles adhered to the captive bubble as surface nanobubbles were introduced on the graphite surface, which is indicated by both the thickness and wrapping angle of graphite adhered to the bubble. Furthermore, the wrapping angle of graphite particles increases with gas oversaturation degree. This means that more and more graphite particles aggregated to form agglomerations and adhered to the captive bubble. For the case of well-dispersed graphite particles in the absence of nanobubbles, the collision probability between graphite and the captive bubble is significantly limited by the low inertia of graphite. As surface nanobubbles were induced to nucleate on graphite surface, on one hand, fine or single graphite particle aggregated with each other to form large-sized graphite agglomerations, which should be responsible for an improved collision probability between graphite and the captive bubble. On the other hand, the adhesion behavior between a particle and a bubble is partly determined by the surface force between them. It was reported that Van der Waals forces between a particle and a bubble were changed as a result of microbubble nucleation on a solid surface. A higher adhesion efficiency between particles and bubbles in the presence of surface nanobubbles is expected. It is worth noting that only collision and adhesion behavior between graphite (agglomerations) and a captive bubble was studied, while detachment, as one of the key sub-processes of the flotation process, was not reflected; the detachment of particles with weak hydrophobicity could happen during the rising of mineralized bubbles. Therefore, the visualization results between graphite and a captive bubble could be not consistent with flotation results.

3.5. Effect of Nanobubbles on Interaction Force Between a Bubble and Substrate

Force curves between a captive bubble and a substrate at different gas oversaturation degrees are shown in Figure 14. A jump-in phenomenon in the advancing force curve was observed during the approach of the captive bubble to the substrate, which is caused by the extension deformation of a captive bubble in the presence of an attractive interaction force. With further approach, the separation distance between the captive bubble and substrate decreased, and therefore the deformation degree of the captive decreased, which is reflected by the decreased attractive force. As the substrate was lifted to the targeted position coordinates (that is, the intersection point in Figure 14), the substrate was retracted and the maximum adhesion force was observed before a thorough separation of the captive bubble from the substrate surface. It can be seen that the maximum adhesion force increases with gas oversaturation degree. Under room temperature water conditions, the maximum adhesion force between the substrate and the bubble is approximately 195 μN, increasing to 220 μN, 270 μN, and 300 μN as the water temperature rises to 20 °C, 25 °C, and 30 °C, respectively. In other words, the bigger the nanobubbles, the higher the maximum adhesion force. The present results are consistent with the aforementioned size distribution results shown in Figure 12. The bridging effect of nanobubbles is responsible for the formation of graphite agglomerations. It was reported that a weak repulsive force in the advancing force curve was observed due to nanobubble nucleation on the substrate surface [33]; this phenomenon was not observed in the present work, which is due to the low force resolution of the present measurement device.

3.6. Effect of Nanobubbles on Flotation Performance of Electrode Materials

The cumulative combustible recovery at different flotation times and gas oversaturation degrees is shown in Figure 15. As expected, the cumulative combustible recovery related to graphite recovery increases with flotation time. For example, the cumulative recovery increases from 26.32% to 43.02% in room temperature water (that is, in the absence of nanobubbles) as flotation time increases from 2 min to 8 min. The flotation recovery of graphite at each flotation time was improved in the presence of nanobubbles, which is indicated by the upwardly shifted combustible recovery curve. The enhanced flotation performance of graphite in the presence of nanobubbles can be attributed to the following two reasons: (i) single graphite particles were induced to accumulate with each other to form graphite agglomerations, resulting in an increase in the collision probability between graphite agglomerations and flotation bubbles during the flotation process, and (ii) the properties of the solid–liquid interface were changed due to the nucleation of nanobubbles on the graphite surface. It is worth noting that the ash content of the flotation concentration is also increased with gas oversaturation degree. It is speculated that micro and/or nanobubbles were also nucleated on the LCO surface, which resulted in an increase in LCO recovery. Further technical measures should be taken in the future to guarantee a favorable flotation selectivity as much as possible.

4. Conclusions

The present work explored the influence of the degree of gas oversaturation on the flotation separation performance of electrode materials from lithium-ion batteries by combining AFM imaging, FBRM measurement, collision and adhesion interaction visualization between graphite and a captive bubble, and a flotation test. The results show that surface nanobubbles can be effectively induced on hydrophobic silicon, alumina, and graphite surfaces with the method of gas oversaturation. The height distribution and lateral size distribution of surface nanobubbles nucleated both on a smooth silicon wafer surface and on a rough alumina surface increased with gas oversaturation degree. Fine graphite particles were induced by surface nanobubble nucleation to form large agglomerations, as indicated by the increased chord length of particles in the presence of nanobubbles. More graphite agglomerations adhered to a captive bubble with the aid of nanobubbles than in the case of no nanobubbles. The frost of nanobubbles on the graphite surface and the resulting graphite agglomerations through the bridging effect of nanobubbles are thought to be responsible for the improved flotation performance of electrode materials. The present results suggest that controlling the gas oversaturation degree of the slurry by varying the temperature or compression could be an effective method to improve the flotation performance of fine mineral particles. It should be noted that, normally, there is a wider range of surface hydrophobicity in real raw minerals than that in the binary materials used in the present work. We expect that nanobubbles only nucleate on objective particle surfaces and not on gangue particles. However, this is not the case in a real nanobubble-assisted flotation. The stability of nanobubbles is also a significant factor affecting flotation efficiency. If the lifespan of the nanobubbles is short, the time they spend covering the surface of the electrode material will also be reduced. The stability of nanobubbles is influenced not only by the roughness and temperature of the substrate surface but also by factors such as the pH value of the solution, which means that the method requires specific conditions to achieve optimal results. In practical applications, it is challenging to control flotation conditions and maintain the stability of nanobubbles, which limits the industrial application of nanobubble-assisted flotation methods. More measures may be taken to promise a satisfactory flotation selectivity. In the future, we will attempt to use nanobubbles in the flotation of real spent lithium-ion batteries to compare whether the real spent lithium-ion batteries are affected by the gas oversaturation degree in the same way as simulated lithium-ion batteries.