The role of bulk micro-nanobubbles in reagent desorption and potential implication in flotation separation of highly hydrophobized minerals

Ultrasonics - Sonochemistry 64 (2020) 104996

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The role of bulk micro-nanobubbles in reagent desorption and potential implication in flotation separation of highly hydrophobized minerals Weiguang Zhoua,b, Ke Liub, Long Wangc, Baonan Zhoub, Jiaojiao Niud, Leming Oua,

T

⁎

a

School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China c College of Mining Engineering, North China University of Science and Technology, Tangshan, China d Simon F.S. Li Marine Science Laboratory, School of Life Science, Chinese University of Hong Kong, Hong Kong, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrodynamic cavitation Micro-nanobubbles Reagent desorption Flotation separation Sodium oleate

Micro-nanobubbles (MNBs) generated during hydrodynamic cavitation (HC) have been extensively studied in mineral processing field in the past two decades. Many researchers have claimed that MNBs can effectively promote the collection of fine particles in flotation, while studies on MNBs assisted mineral separation are rare. In this study, the role of bulk MNBs in desorbing flotation reagent was investigated, with the aim of illustrating the potential effects of MNBs on minerals separation. The results showed that bulk MNBs could efficiently remove the sodium oleate (NaOl) from diaspore surfaces, reducing the residual concentration of NaOl on solids, which was more significant when the amount of NaOl pre-adsorbed was relatively small. Furthermore, lower residual concentration of NaOl on solids caused by MNBs cleaning made the particles less hydrophobic and flocs more friable. Given that gangue entrapment in flocs was one of the main limits for high-selective flotation, the roles of MNBs in enhancing reagent desorption and associated flocs breakup and reorganization probably contribute to higher separation efficiency of different minerals, which was confirmed by the flotation results of diaspore/kaolinite mixture.

1. Introduction Micro-nanobubbles (MNBs), referred as tiny bubbles with sizes ranging from hundred nanometers to sub-microns, are formed in considerable amounts during hydrodynamic cavitation (HC) [1,2]. Due to the extraordinary properties, like long lifetime [3,4], high surface stiffness [5–7], high gas solubility [8,9], and incredibly large micro contact angle [10], MNBs have drawn increasing attention since the 1980s [11,12]. For instance, MNBs enhanced decontamination is a hot topic in the field of surface cleaning recently. It has been found that more efficient pollutants removal can be achieved by introducing MNBs into the aqueous system [13–15]. Combination use of chemical agents and MNBs can effectively remove those stubborn stains, showing a synergistic effect between the detergents and MNBs [16]. To reveal the underlying mechanism, some high-precision measurements, like AFM [17], quartz crystal microbalance [18], high performance liquid chromatography [19,20], thermogravimetry [21], have been applied. To date, it has been generally accepted that the mechanism involves the nucleation growth/collapse of bubbles, and complex interaction of MNBs, detergents, substrates and impurities at the multiphase

⁎

interfaces [22]. Takahashi et al. [23] have discovered that the collapse of MNBs is accompanied with numerous OH% generation. These highly active free radicals boost the oxidation and decomposition of organic contaminants, thus improving the surface cleanliness. For contaminated surfaces, MNBs can promote the migration of pollutants from solid surfaces to gas films due to their great specific surface area, oil-affinity and hydrophobicity [24,25]. For clean surfaces, MNBs adsorb the pollutants preferentially in the bulk solutions, thus achieving fouling prevention [14]. In mineral processing field, MNBs-flotation technology based on HC has been applied for more than two decades [26,27]. Successful industrial practices have proven that MNBs can significantly improve minerals flotation performances [28,29]. Specifically, a higher recovery of target minerals and lower gangue minerals capture/entrainment can be obtained in the presence of MNBs, which is largely attributed to the enhanced particles mineralization, improved particle-bubble attachment, and optimized aggregate/foam structure [30,31]. However, up to date, most of the researches on MNBs assisted flotation are mainly aimed at improving the flotation enrichment of minerals. Few literatures have been published on separation of minerals in a MNBs system,

Corresponding author. E-mail address: [email protected] (L. Ou).

https://doi.org/10.1016/j.ultsonch.2020.104996 Received 28 August 2019; Received in revised form 13 December 2019; Accepted 3 February 2020 Available online 04 February 2020 1350-4177/ © 2020 Published by Elsevier B.V.

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which seems to be more urgent in some non-ferrous minerals flotation. Fatty acids (salts) collectors are widely used in the flotation of various types of oxidized ores [32]. Apart from surface hydrophobization that makes minerals more floatable, hydrophobic aggregation of particles induced by fatty acids adsorption has also shown great impacts on minerals flotation, especially on those fine and ultrafine mineral particles [33,34]. However, when using fatty acids as collectors, poor selectivity of collectors often results in poor flotation performance [35–37]. Gangue entrapment caused by fatty acids is normally considered as one of the main contributors to nonselective hydrophobic aggregation, which is more serious in the presence of high consumption of fatty acids (e.g., roughing operations) [38]. To address this problem, increasing the (blank) cleaning operations is always applied in industrial production. In the cleaning operations, the flotation pulp is diluted. Surface cleaning of mineral particles/aggregates leads to the desorption of flotation reagents, thus promoting the breakup of aggregates and the removal of gangue minerals entrapped [39,40]. Ideally, we hope that the target minerals will not loss while gangue minerals are effectively removed during cleaning operations. In fact, the improvement of concentrate grade is often at the expense of the recovery decrease (e.g., bauxite flotation [41]). Efficiently separating target minerals from gangues while maintaining the high recoveries is really a worth-considering issue for researchers. Here, inspired by the function of MNBs in surface cleaning and mineral flotation, we surmise that MNBs water may be more conductive for reagent removal from mineral surfaces than ordinary water, which may further benefit the minerals’ cleaning. To verify the hypothesis, a diaspore sample was chosen to explore the role of MNBs on reagent desorption from sodium oleate (NaOl) hydrophobized surfaces in this study. An open-circuit flotation tests of binary mixed minerals (diaspore/kaolinite) was then conducted to compare the flotation responses under MNBs water addition and ordinary water addition. The study is an attempt in the MNBs assisted minerals separation field. It is expected that our findings can provide some supports for a better understanding of the multiple roles of MNBs in mineral flotation.

Fig. 1. XRD of diaspore (a) and kaolinite (b) minerals. Table 1 Chemical composition analysis results.

2. Experimental

Composition

Al2O3

SiO2

Fe2O3

TiO2

CaO

MgO

K2O

Na2O

Others

Diaspore Kaolinite

70.4 39.61

1.21 43.55

0.84 0.34

8.71 1.86

0.02 0.02

0.07 0.07

0.05 0.01

0.04 0.03

18.66 14.51

2.1. Materials and reagents The diaspore and kaolinite samples were obtained from Xiaoguan mine located in Henan Province, China. The purity of these two mineral samples is above 90%, based on the XRD and chemical analyses (Fig. 1 and Table 1, respectively). Before crushing, the well crystallized diaspore samples were collected for contact angle measurements. Then, other samples were dry ground to collect the −38 μm size fraction, which was used for sorption measurements, microscope tests, and flotation tests. NaOl (AR grade) purchased from Beijing Dingguo Biotechnology Co. Ltd., was used for the chemical hydrophobization of diaspore and the final flotation. HCl and NaOH acted as pH modifier. Ultrapure water with a resistivity of 18.2MΩ was used in all the experiments.

instruments Ltd., UK) [43,44]. Each size measurement was conducted at 25 °C for 5 times, with the average recorded as the final reported data. 2.3. Contact angle measurements Contact angle measurements were carried out on the surfaces of polished diaspore by the sessile drop method. First, the well selected samples were embedded in resin. The specimens were then sequentially ground by diamond grinding wheels of roughness 100, 40 and 9 μm to obtain a flat surface. The specimens were successively polished on a microcloth using 1.0, 0.3 and 0.05 μm alumina powder solution. Then, the specimens were washed thoroughly with ultrapure water and cleaned by a microwave bath to remove any adhering alumina. Afterwards, the clean polished diaspore samples were soaked in the NaOl solution with different concentrations for 30 min (pH = 10). Then the specimens were gently washed using ultrapure water to remove the residual NaOl from the diaspore surfaces. After being dried by nitrogen gas, the specimens were then transferred to a contact angle goniometer (JY-82, Chengde Dingsheng Co. Ltd., China) for the final contact angle measurements. During each measurement, 2 μl water droplet was placed onto the NaOl-modified diaspore surfaces by a calibrated syringe. The equilibrium contact angle on both sides of the water droplet

2.2. MNBs generation and characterization The MNBs generation/cavitation-flotation system used in this study was the same as that shown in our previous study [42]. First, pure water with pH10 was treated through the system with the liquid velocity of the tube throat maintained at 17.3 m/s (a velocity enough for triggering cavitation inception). The cavitation time and aeration rate were kept at 5 min and 0.15 L/min, respectively. Once the “milky” gas-liquid mixture formed, the size distribution of the cavitation bubbles in the bulk solution was measured through a Mastersizer 2000 instrument (Malvern instruments Ltd., UK) in real-time [2,27]. Changes of the size distribution of cavitation bubbles as a function of depositing time were then recorded by a Zetasizer NanoZS 90 instrument (Malvern 2

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was measured in five different positions, and the average was reported as the final value. All measurements were conducted at 25 ± 2 °C.

those reported by Couto et al. [47] and Rahman et al. [2]. On the other hand, due to the shielding effect caused by those larger micro-sized bubbles, the size distribution of nanoscale bubbles (i.e., bulk nanobubbles; BNBs) can hardly be well profiled. Given that nanoscale bubbles are of incredible stability in the aqueous solution while microscale bubbles disappear within seconds [48], the solution stagnation method and DLS technique were adopt here to detect the BNBs exclusively (Fig. 3(e)) [42,49]. It is clear that the size distribution of cavitation bubbles changes to be mainly concentrated in two size intervals (i.e., 100–300 nm, 1–3 μm) after 5 min deposition, reconfirming that the stability of larger microbubbles is rather poor. Once produced, the size of cavitation bubbles changes constantly due to the shrinking, merging, bursting/collapsing. In this study, the size distribution curve of measured bubbles shifts to the left in general, indicating the MNBs’ average size reduces gradually as deposition time prolongs. In contrast, even after two-hour stagnation, ultrafine bubbles with size of several hundred nanometers still can be detected in the bulk solution, verifying the incredible stability of BNBs. Overall, numerous micro and nano scale bubbles can be produced during HC process. Most large microscale bubbles disappear within one minute after generation, while other ultra-fine bubbles of several hundred nanometers or even several microns will survive for ten minutes or even hours in the aqueous solution.

2.4. Sorption experiments The amount of the NaOl adsorbed on diaspore surfaces was monitored using a high-precision total organic carbon analyzer (multi N/C 3100, Germany) [45]. (1) Adsorption measurement. 5 g fresh diaspore samples and 95 ml NaOl solution were mixed to make the pulp (pH = 10). After 30-min stirring, the suspension was transferred to centrifugate for 15 min. The supernatant was then collected for adsorption amount measurement. (2) Desorption measurements. First, the diaspore sediments after centrifugation in the “adsorption measurements” were taken for lowtemperature vacuum drying. Then, using a magnetic stirrer (RHBWS025, IKA Co. Ltd., Germany), 5 g NaOl modified diaspore particles were mixed with 95 ml water/MNBs water for 10 min. Afterwards, the suspension was transferred to centrifugate for 15 min, with the supernatant collected for desorption measurements. The desorption operations were carried out for 4 times in sequence, with each mixing process lasting for 10 min.

3.2. Role of MNBs in the NaOl desorption from hydrophobized diaspore aggregates

The concentration of the NaOl in the liquid was determined from the standard curve, and the amount of NaOl adsorbed/remained on the diaspore surfaces was calculated using the residual concentration. The amount of adsorption/desorption was measured three times for each sample, the average of which was recorded as the final value. The standard deviation was acquired previously.

3.2.1. Preparation of hydrophobized diaspore minerals using NaOl Fig. 4 presented the adsorption isotherms of NaOl on the diaspore at pH 10. The amount of adsorbed NaOl increases with the NaOl concentration in the solution in general. The curve shows that the adsorption of NaOl on diaspore fits well with the multiple-layer mode. When the NaOl concentration increases to 10−3 M, the monolayer adsorption of NaOl on diaspore starts to reach equilibrium. However, when the concentration of NaOl is beyond 1.5 × 10−3 M, the adsorption capacity of NaOl on diaspore surfaces increases sharply again, indicating that the multilayer adsorption of NaOl on diaspore surfaces occurs. Adsorption of NaOl on diaspore surfaces inevitably changes minerals surface morphology, which therefore significantly alters the physicochemical properties of diaspore surfaces [50]. Here, the variations of minerals wettability caused by NaOl adsorption were characterized by contact angle measurements. It can be seen from Fig. 5 that the initial contact angle of diaspore is around 27°, suggesting that fresh diaspore is rather hydrophilic. After interacting with NaOl, the measured contact angles of diaspore increase noticeably. The maximum contact angle (around 77°) value is obtained in the presence of 10−3 M NaOl, beyond which concentration the measured contact angle values decrease. Our results confirm that the hydrophobicity of diaspore reaches to the maximal when the mineral surface is fully singlelayer NaOl-covered. Excessive NaOl usage results in multilayer adsorption and is not conductive to the further improvement of the hydrophobicity of diaspore surface.

2.5. Microscope tests Microscope tests were performed to unveil the morphologies of hydrophobized diaspore flocs before and after being cleaned by ordinary water/MNBs water. First, 0.5 g of NaOl-modified diaspore particles was added into 100 ml ordinary water/MNBs water to prepare the mineral suspension. Then, the mineral suspension was gently mixed in a beaker for 3 min so that no extra bubble formed in this period. Afterwards, a drop of pulp was sampled for image analysis using an optical microscope (Leica DM, Germany). 2.6. Flotation tests Flotation tests were conducted in a XFG(II) flotation machine at 1400 rpm with a 160 ml cell. Mineral pulp was prepared by adding 40 g NaOl-modified artificial mixed ore (the mass ratio of diaspore and kaolinite was 2.452:1, with the original A/S about 4.56) into 120 ml water and adjusted the pH as 10. After 5-min roughing, the concentrate was subject for four cleaning operations, using the same amount of ordinary water and MNBs water as the supplementary water, respectively. Each flotation test was implemented three times, with the average regarded as the final values. Fig. 2 presents the flotation flow chart of binary mixed diaspore/kaolinite ores.

3.2.2. Effect of MNBs cleaning on the NaOl desorption from diaspore surfaces To investigate the role of MNBs in reagent removing from chemically hydrophobized surfaces, NaOl-modified diaspore particles were cleaned by two different kinds of “detergent” (i.e., ordinary water and MNBs water) and the residual amount of NaOl on diaspore surfaces was measured (Fig. 6). Obviously, with the increase of cleaning times, the content of NaOl remained on diaspore surfaces decreases gradually regardless of the cleaning agents. The higher the initial NaOl concentration on the mineral surfaces, the greater the decrease degree of NaOl residual with the increase of cleaning times. Moreover, compared with ordinary water, the residual NaOl concentration is evidently lower when MNBs water was used, which confirms that MNBs can indeed

3. Results and discussion 3.1. Preparation and characterization of MNBs water It is well known that “milky” gas-liquid mixture appears during HC, which is consist of innumerable tiny bubbles (Fig. 3(a)–(c)) [46]. To explore the properties of these fine/ultrafine bubbles, in-situ measurement of the size of cavitation bubbles in a full-scale range was firstly carried out using a laser particle size analyzer. Obviously, the size of generated bubble is mainly in the range of tens to hundreds of microns (with an average size about 50 μm) (Fig. 3(d)), which is consistent with 3

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Fig. 2. Flow of the flotation tests of diaspore/kaolinite mixtures.

with the increase of initial NaOl concentration. Similar findings have also been reported by Meng et al. [51]. As the amount of NaOl adsorbed on diaspore surfaces increases, the NaOl concentration gap between mineral surfaces and the bulk enlarges, thus benefiting the mass transfer of reagent (i.e., the reagent desorption process). Conversely, the enhancement of MNBs on reagent desorption is weakened in the presence of high concentration of NaOl. Nevertheless, MNBs do accelerate the migration of the adsorbed NaOl from mineral surfaces to aqueous phase. It is generally believed that bulk MNBs can promote the transfer of the adsorbed contaminants from the solid–liquid interface to liquid–vapor/liquid–liquid interface [17,22]. First, the introduction of MNBs into the bulk phase remarkably enlarges the liquid–gas interface and then promotes the accumulation of organic molecules [14]. The concentration of reagents in the bulk solutions decreases, so more organic molecules desorb into the bulk solutions to equilibrate the reagent concentration at different

effectively desorb the pre-adsorbed NaOl from diaspore mineral surfaces. More quantitative details about MNBs assisted NaOl desorption were displayed in Fig. 7. It can be surmised that the cumulative removal efficiency is mainly correlated with the initial concentration of NaOl used for hydrophobization, and the cleaning times. In the cases of ordinary water cleaning, with the initial NaOl concentration increasing from 5 × 10−4 M to 3 × 10−3 M, the reagent removal ratio increases from less than 10% to about 46% after 1st cleaning. Also, the cumulative NaOl removal ratios arise as the cleaning time increases. After 3rd cleaning, the cumulative NaOl removal ratio increases to almost 20% under 5 × 10−4 M NaOl, while it increases to around 68% under 3 × 10−3 M NaOl. The presence of MNBs results in larger NaOl removal ratios, especially ae lower initial NaOl concentrations, as the ratios of desorbed NaOl amounts with MNBs water cleaning (RB) to desorbed NaOl amounts with ordinary water cleaning (Rw) decrease

Fig. 3. Imaging and size characterization of MNBs water under different conditions (a: MNBs water after cavitation; b: MNBs water with 5 min-depositing after cavitation; c: MNBs water with 30 min-depositing after cavitation; d: size distribution of cavitation bubbles during cavitation; e: size distribution of MNBs bubbles as a function of deposition time after cavitation).

4

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which can be mainly attributed to the hydrophobic aggregation induced by NaOl [56,57]. After 1st cleaning, the size of the diaspore flocs decreases significantly, suggesting that NaOl desorption contributes to the breakup of flocs. Meanwhile, compared with ordinary water cleaning, MNBs water cleaning leads to relatively smaller flocs (i.e., smaller average area and diameter), which verifies that MNBs enhanced reagent desorption benefits the flocs breakup in suspensions [40]. Plus, more net/chain-like flocs appear in the pulp in the presence of MNBs (e.g., Fig. 8(c) and (f)), which maybe results from the MNBs capillary bridging [31,58]. In flotation pulp, the presence of MNBs enhances the desorption of NaOl from diaspore surfaces, which further reduces the mineral surface hydrophobicity and the flocs’ strength [59]. Therefore, less hydrophobic mineral flocs are more likely to rupture under certain stirring conditions [60]. A possible reagent (NaOl) desorption and diaspore flocs breakup model induced by MNBs is present in Fig. 9. Fig. 4. Adsorption capacity of NaOl on diaspore surfaces as a function of the original NaOl concentration.

3.3. Effect of MNBs surface-cleaning on the separation of hydrophobized diaspore/kaolinite mixtures

interfaces. Second, when approaching to organic-coated particles, fine and high-density MNBs gradually surround the cleaning materials. These ultrafine bubbles can readily adhere to the contaminants due to their highly surface charged and hydrophobicity, which therefore forcing the reagents detach from the solid–liquid interface by the advancing three-phase line [22]. Besides, the high temperature/pressure environment caused by compression and collapse of MNBs helps to strengthen the peeling of contaminants from solid surfaces [52]. It should also be noted that these desorbed organics can hardly re-adsorb onto the solid surfaces stably again after they attach to MNBs, probably due to the electrostatic repulsion of bubbles and the mechanical force induced by agitation or sonication [53–55]. Although NaOl adsorbed on diaspore surfaces cannot be fully removed after several cleanings (due to the strong chemisorption [35]), the introduction of MNBs into the clean system do improve the desorption efficiency of NaOl. As we mentioned above, the surface hydrophobicity and associated aggregation behavior of diaspore particles were then inevitably modified. To clarify this, the aggregation/dispersion states of hydrophobized diaspore particles before and after one cleaning was compared, and the floc morphology changes were presented by representative microphotographs under two NaOl concentrations (Fig. 8). Table 2 presented the statistics of aggregates’ parameters of diaspore flocs in Fig. 8. It is manifest that large flocs appear after fine diaspore particles being hydrophobized by NaOl,

Finally, the potential role of MNBs in enhancing minerals separation was explored by separating the binary mixed minerals (diaspore and kaolinite) using NaOl as collector. Fig. 10 shows the Al2O3 recovery and A/S ratio of the concentrate obtained after each flotation operation (the “0” on the horizontal axis referring as the roughing). After one roughing operation, a product of 76.73% Al2O3 and 7.66 A/S was obtained, indicating that NaOl is of strong collecting capability and medium flotation selectivity towards the diaspore/kaolinite mixtures. NaOl mainly interacts with diaspore and kaolinite through the active Al sites on these mineral surfaces [36]. The moderate flotation selectivity is likely attributed to the difference of the abundance of Al atoms sited on these two mineral surfaces [35,61]. To further purify the rough concentrate, blank cleanings were operated. With the increase of blank cleaning times, the A/S ratio of the concentrate increases gradually while the Al2O3 recovery decreases at the same time. Compared with the flotation under ordinary water, adding MNBs water in the cleaning operations results in higher Al2O3 recoveries constantly, which reconfirms that bulk MNBs benefit the flotation enrichment of fine minerals. In this study, it is supposed that not only those fresh MNBs but also the “newborn” NaOl-coated MNBs formed after fresh MNBs interacting with reagent are significant contributors [49]. As for the effects of MNBs on final concentrates’ grade, it seems to be complex under these two conditions (i.e., adding ordinary water for cleaning operations and adding MNBs water for cleaning operations). A

Fig. 5. Contact angles of diaspore after interacting with different concentrations of NaOl. 5

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Fig. 6. Residual NaOl content on pre-hydrophobized diaspore surfaces after cleaned by two different cleaning agents ((a): diaspore pre-hydrophobized by 10−4 M NaOl; (b): diaspore pre-hydrophobized by 5 × 10−4 M NaOl; (c): diaspore pre-hydrophobized by 10−3 M NaOl; (d): diaspore pre-hydrophobized by 3 × 10−3 M NaOl).

higher A/S ratio can be acquired by adding ordinary water in the first cleaning operation, but MNBs water shows greater advantages in lifting final concentrate’s grades when the cleaning operation exceeds 2 times. With the increase of the blank cleaning times, the concentration of the

residual NaOl in pulp decreases gradually. The reduction of particles’ hydrophobicity just promotes the flocs breakup and reorganization, thus alleviating the gangue entrapment in mineral flocs, which finally lifts the A/S ratios of the concentrates. The existence of MNBs in the

Fig. 7. Effect of MNBs cleaning on the cumulative NaOl removal ratio from diaspore surfaces ((a): diaspore pre-hydrophobized by 10−4 M NaOl; (b): diaspore prehydrophobized by 5 × 10-4M NaOl; (c): diaspore pre-hydrophobized by 10−3 M NaOl; (d): diaspore pre-hydrophobized by 3 × 10−3 M NaOl). 6

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Fig. 8. Effect of MNBs water cleaning on the morphology of diaspore flocs (a: diaspore + 10−3 M NaOl; b: diaspore + 10−3 M NaOl + ordinary water cleaning; c: diaspore + 10−3 M NaOl + MNBs water cleaning; d: diaspore + 5 × 10−4 M NaOl; e: diaspore + 5 × 10−4 M NaOl + ordinary water cleaning; f: diaspore + 5 × 10−4 M NaOl + MNBs water cleaning). Table 2 Statistics of aggregates parameters in micrographs. Series

a b c d e f

Total count

21 38 101 191 182 166

Area (μm2)

Diameter (μm)

Mean

Std.Dev

Mean

Std.Dev

6115.05 1683 1545.72 693.9 511.45 405.1

10411.81 2561.3 3236.81 1149.73 768.85 617.68

55.44 34.07 31.15 24.24 21.03 19.3

59.95 27.63 26.64 16.01 14.20 11.91

pulp strengthens these sub-processes. More selective aggregation is thought to occur in the pulp under this condition, probably due to the enhanced adsorption selectivity of reagent on solids in the presence of MNBs [62]. As a result, the grades of the final concentrates are further improved. In addition, the enhancement of MNBs on NaOl desorption seems to be more significant when the initial NaOl concentration is relatively low, which may be one of the main reasons for the rapid improvement of concentrate grade after repeatedly cleanings. Based on the aforementioned, it implies that MNBs may play dual roles in affecting final concentrate’s grade. MNBs can not only enhance reagent desorption and associated flocs breakup/reorganization, but also enhance mineral particles collection. The former normally benefits the grade lifting while the latter is likely to reduce the separation

Fig. 10. Effect of black cleaning times on the flotation separation of diaspore/ kaolinite mixtures in the absence and presence of MNBs (C(NaOl) = 8.2 × 10−4 M).

efficiency. The final concentrate grade depends on which role predominates, with the primitive hydrophobicity of minerals (closely related to the reagent concentration in the pulp) one of the key influencing factors. In the early stage of blank cleaning, mineral particles are

Fig. 9. Schematic diagram of MNBs enhanced NaOl desorption and flocs breakage of diaspore aggregates. 7

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hydrophobic enough, and MNBs enhanced particles collection predominates, thus resulting in lower A/S ratio in the final concentrate. As blank cleaning continues, the hydrophobicity of mineral particles decreases gradually, and the reagent desorption enhanced by MNBs predominates gradually. As a result, the grade of the final concentrate rises quickly. The results of MNBs assisted minerals separation shown in this study inspires us that pretreating supplementary water by HC in the cleaning stage may benefit the separation of different fine minerals. In mineral flotation, MNBs seem to act not only as collectors, flocculants, but also show characters similar with dispersants in some cases. A special advantage of using MNBs for minerals cleaning is that no extra surfactant is needed, which is very environment-friendly and cost-saving. Though the conclusions are drawn based on the separation of diaspore/kaolinite in a NaOl-flotation system, we are convinced that they may also be applicable in other mineral flotation systems. All in all, this study is only a beginning, much work needs to be done before fully understanding the role of MNBs in minerals separation.

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4. Conclusions In this study, the effect of MNBs generated during HC on NaOl desorption was investigated in a diaspore flotation system. The results indicate that compared with ordinary water cleaning, MNBs water cleaning shows higher NaOl removal efficiency, which is more significant when the concentration of NaOl pre-adsorbed is relatively low. The huge specific surfaces (oil-affinity) of MNBs and the energy release when MNBs collapse are likely main contributors [22]. Moreover, reagent desorption enhanced by MNBs cleaning significantly decreases the hydrophobicity of diaspore particles/flocs. As a result, large mineral flocs are more prone to break up and selectively reorganize under stirring conditions. This is beneficial to the separation of different hydrophobized minerals since gangue entrapment in flocs is one of the most significant limiting factors toward high-selective flotation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of the Natural Science Foundation of China (No. 51674291), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N532) with Shenzhen related Supporting Fund (No. KYTDP20181011104002), R&D Fund by Shenzhen Clean Energy Research Institute (No. CERI-KY-2019003/K19405406), as well as Natural Science Foundation of Hebei Province (NO. E2019209494). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2020.104996. References [1] H. Oliveira, A. Azevedo, J. Rubio, Nanobubbles generation in a high-rate hydrodynamic cavitation tube, Miner. Eng. 116 (2018) 32–34. [2] A. Rahman, K.D. Ahmad, A. Mahmoud, M. Fan, Nano-microbubble flotation of fine and ultrafine chalcopyrite particles, Int. J. Min. Sci. Technol. 24 (2014) 559–566. [3] J.H. Weijs, D. Lohse, Why surface nanobubbles live for hours, Phys. Rev. Lett. 110 (2013) 054501. [4] Y. Sun, G. Xie, Y. Peng, W. Xia, S. Jie, Stability theories of nanobubbles at solid? Liquid interface: a review, Colloids Surf., A 495 (2016) 176–186. [5] B.M. Borkent, S.M. Dammer, H. Schönherr, G.J. Vancso, D. Lohse, Superstability of surface nanobubbles, Phys. Rev. Lett. 98 (2007) 204502.

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