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Malachite flotation using carbon black nanoparticles as collectors: Negative impact of suspended nanoparticle aggregates
Minerals Engineering 137 (2019) 19–26
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Malachite flotation using carbon black nanoparticles as collectors: Negative impact of suspended nanoparticle aggregates
T
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Hyunjung Kima,1, , Junhyuk Youa,1, Allan Gomez-Floresa, Stephen Kayombo Solongoa, Gukhwa Hwanga, Hongbo Zhaob, Byoung-cheun Leec, Junhyun Choia a
Department of Mineral Resources and Energy Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeonbuk 54896, Republic of Korea School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan, China c Risk Assessment Division, National Institute of Environmental Research, Hwangyeong-ro 42, Seo-gu, Incheon 22689, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon black nanoparticles Collector Malachite Floatability
In this study, the flotation behavior of malachite was investigated using carboxyl-functionalized carbon black nanoparticles (CB-NPs) as collectors in a hydrodynamically simplified Hallimond tube. We found that malachite floatability increased with increasing CB-NP concentration in the low range (0.5–2 kg/ton). To understand the mechanism of this higher floatability for low CB-NP concentrations, we examined the amounts of deposited CBNPs on the malachite surface and hydrophobicity of the surface with deposited CB-NPs according to changes in the initial CB-NP concentration. The increase in malachite floatability was likely associated with increases in the amounts of deposited CB-NPs on the malachite surface, which enhanced malachite floatability by altering the hydrophobicity of its surface. Meanwhile, malachite floatability sharply decreased at CB-NP concentrations exceeding 2 kg/ton. To understand this unusual flotation behavior for high CB-NP concentrations, we also examined the amounts of CB-NPs deposited on the malachite surface and contact angles. However, the decreased malachite floatability could not be fully explained by these experiments, implying that at least one additional mechanism is involved. Therefore, additional flotation experiments including filtration processes and different flotation times were conducted, revealing that the unusual flotation behavior was likely due to the presence of suspended CB-NP aggregates, which reduced the kinetic rates of attachment between malachite and bubbles.
1. Introduction Flotation is a physicochemical technique that separates hydrophobic particles from a mixture of hydrophobic and hydrophilic particles (Choi et al., 2016a; Choi et al., 2016b; Fuerstenau et al., 2006; Gaudin, 1957). During the flotation process, the important interaction step is the attachment of minerals to the bubble surface, as the interaction between mineral particles and bubbles determines the overall efficiency of the process (Choi et al., 2018; Kim et al., 2018; Nguyen et al., 2001; Pan et al., 2012; Yoon and Ravishankar, 1994). To improve the attachment of particles to bubbles, mineral particles are selectively treated with a short-chain, water-soluble surfactant to selectively render their surfaces hydrophobic (Choi et al., 2012; Choi et al., 2014). These surfactants are usually called collectors in the flotation field (Ejtemaei et al., 2011; Jiang et al., 2012; Park et al., 2016). Copper oxide is an important copper source. It is found in the weathered regions of most Cu sulfide ore bodies (Hope et al., 2012).
The conventional method of Cu oxide flotation involves sulfidization of the mineral surface prior to selective flotation using sulfhydryl collectors such as xanthate (Cao et al., 2009; Kongolo et al., 2003). This process is effective with respect to selectivity for Cu oxide minerals. Unfortunately, this method has critical disadvantages; specifically, sulfidizing and sulfhydryl reagents are rather toxic and not environmental friendly (Min et al., 2012; Okibe and Johnson, 2002; Wang et al., 2003; Webb et al., 1976), and determining the optimum concentrations of sulfidizing and sulfhydryl reagents is extremely difficult (Feng et al., 2016; Shen et al., 2018). As an alternative to sulfidization, a variety of oxhydryl collectors have been evaluated for direct flotation tests of Cu oxide-containing substances such as fatty acids (Choi et al., 2016b; Li et al., 2018), hydroxamic compounds (Lee et al., 1998; Lee et al., 2009; Marion et al., 2017), and phosphinic/phosphonic compounds (Li et al., 2015). Although flotation tests of Cu oxide minerals were performed using fatty acids, they have affinities for most cations, and, therefore, they have inherently low selectivity in mineral flotation
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Corresponding author. E-mail address: [email protected] (H. Kim). 1 These authors equally contributed to this article. https://doi.org/10.1016/j.mineng.2019.03.025 Received 20 November 2018; Received in revised form 19 March 2019; Accepted 21 March 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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(CB-NPs) as collectors. Malachite is a representative copper oxide-based mineral (Choi et al., 2016b; Lee et al., 1998; Li et al., 2018). CB-NPs are extremely hydrophobic (Han et al., 2017; Hwang et al., 2018; Mauter and Elimelech, 2008). Through this research, we found a new malachite flotation behavior that had not been reported previously for nanoparticle flotation. Malachite floatability increases with increasing CBNP concentrations in the low range because of increased hydrophobicity on the surface of malachite caused by deposited CB-NPs. Meanwhile, for high CB-NP concentrations, malachite floatability decreased as the CB-NP concentration increased. Our experimental evidence suggested that excessive levels of suspended CB-NPs reduced the kinetic rate of malachite attachment to bubbles.
(Li et al., 2018; Lee et al., 1998). Unlike fatty acids, hydroxamic compounds, which are similar to phosphonic and phosphinic compounds, have been demonstrated to effectively and selectively recover Cu oxide minerals (Lee et al., 1998; Lee et al., 2009; Li et al., 2015; Li et al., 2018; Marion et al., 2017). Hydroxamic compounds, which are O-O type chelating reagents, were founded to chemically adsorb onto the mineral surface, and thus, they offer many advantages over alternative technologies for Cu oxide minerals (Lee et al., 1998; Lee et al., 2009). Nevertheless, the large-scale usage of hydroxamic compounds is limited owing to their practical guidelines, efficacy, and cost in plants (Li et al., 2018; Phetla and Muzenda, 2010). Because of these factors, new alternative flotation methods are required to expand knowledge and develop flotation technology for Cu oxide minerals. More recently, there have been attempts to investigate hydrophobic nanoparticles as a new class of flotation collectors (Abarca et al., 2015; Dong et al., 2017; Hajati et al., 2016; Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013a; Yang et al., 2013b). Nanoparticle flotation studies demonstrated that hydrophobic nanoparticles may offer advantages as collectors in flotation over conventional collectors (Abarca et al., 2015; Yang et al., 2011, Yang et al., 2012; Yang et al., 2013a). They suggested that as little as 10% coverage by nanoparticles on the glass bead surfaces could promote high flotation efficiency, whereas conventional molecular collector requires 25% or greater coverage for good recovery (Abarca et al., 2015; Yang et al., 2011, Yang et al., 2012; Yang et al., 2013a). Considering these advantages of nanoparticles as collectors in flotation, researchers have exerted substantial efforts to clarify the feasibility of nanoparticles as collectors in flotation via experimental and theoretical approaches (Abarca et al., 2015; Dong et al., 2017; Hajati et al., 2016; Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013a; Yang et al., 2013b). For example, hydrophobic polystyrene nanoparticles can function as flotation collectors (Abarca et al., 2015; Dong et al., 2017; Yang and Pelton, 2011; Yang et al., 2011; Yang et al., 2012; Yang et al., 2013a; Yang et al., 2013b). Polystyrene nanoparticles were deposited onto glass beads via electrostatic attraction, improving the contact angle of glass beads (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2013b). In addition, the application of polymeric soft and hard polystyrene nanoparticles as collectors in flotation of glass beads and pentlandite (nickel sulfide mineral) has been reported (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2013a; Yang et al., 2013b). The softer nanoparticles were more firmly deposited onto glass beads or pentlandite surfaces because they had a greater contact area (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2013a; Yang et al., 2013b). For pentlandite flotation, polystyrene nanoparticles bearing surface imidazole groups specifically bound nickel ions and appeared to improve flotation performance (Yang et al., 2013a). In addition, smaller, more hydrophobic polystyrene nanoparticles are more efficient flotation collectors, and as little as 10% nanoparticle coverage offers high flotation performance (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013b). Furthermore, glass bead floatability decreased with increasing conditioning time using hydrophobic polystyrene nanoparticles as collectors because collision between glass beads detaches nanoparticles deposited on their surfaces (Dong et al., 2017). Last, only one study examined silica floatability using hydrophobic talc nanoparticles as collectors (Hajati et al., 2016). By adjusting the solution pH, talc and silica can become oppositely charged, and thus, electrostatic attraction is possible (Hajati et al., 2016). Likewise, although researchers made substantial efforts to identify the feasibility of nanoparticles as collectors in flotation, relevant studies regarding various mineral and nanoparticle species are extremely limited. In particular, there has been no attempt to investigate the effect of hydrophobic nanoparticles as collectors for Cu oxide minerals. In this study, to identify the feasibility of nanoparticle flotation using Cu oxide minerals and further advance the understanding of the roles of nanoparticles as collectors in flotation, malachite flotation was performed using carboxyl-functionalized carbon black nanoparticles
2. Materials and methods 2.1. Materials and reagents Malachite (CuCO3·Cu(OH)2), which was used as a raw material, was obtained from Junsei Co., Japan. The manufacturer reported that it has 55% Cu, and it is spherical in shape. Two sieves with −100 and +200 mesh (Tyler Standard) were used to separate particles to obtain only malachite particles with diameters ranging from 74 to 149 μm, which were used in this study. CB-NPs were supplied by Birla Carbon (Columbian Chemicals Korea, Yeosu, South Korea). To evaluate the primary size of CB-NPs, transmission electron microscope (TEM, H7650, Hitachi, Japan) observations were conducted at a voltage of 100 kV. In addition, DowFroth-250 (DF-250, CH3(OC3H6)4OH) supplied by American Cyanamid, USA was used as a frother, and analyticalgrade HCl (≥99%, Fisher Scientific) and NaOH (≥99%, Fisher Scientific) were used as pH modifiers. 2.2. Preparation of CB-NP suspensions An ultrasonic wave setup was used to disperse CB-NPs into suspensions. In detail, the setup consisted of an ultrasonic homogenizer with a horn tip of 10 mm in diameter (KNSN-RAB, KOEN, South Korea) and a reactor to accommodate the CB-NP suspension. The ultrasonic wave frequency was fixed at 20 kHz (intensity of 11 W/mL) for consistent irradiation. The cylinder-shaped reactor was in acrylic with a diameter and height of 3 and 10 cm, respectively. The reactor had double jackets for controlling the temperature. The 40 mg/L CB-NP suspension was prepared in Milli-Q water (Milli-Q Plus, Millipore Ltd., UK) and then dispersed for 10 min; this suspension was used as stock suspension. The suspension with desired CB-NP concentration was prepared by diluting the stock suspension for subsequent characterization, deposition, and flotation tests. 2.3. Zeta potential measurements To characterize the electrokinetic properties of malachite and CBNPs, their electrophoretic mobilities were measured using ELS-Z equipment (Otsuka, Hirakata, Japan); the Smoluchowski equation was used to convert electrophoretic mobility to zeta potential (Ohshima, 2007). Because the sizes of malachite exceeded the measurable range of the ELS-Z, malachite was ground further using a mortar to obtain a smaller size (< 5 µm). For electrophoretic mobility measurements, aliquots of malachite and CB-NP suspensions were re-suspended in Deionized (DI) water with a desired pH. The pH of all suspensions was adjusted using 0.1 M HCl or 0.1 M NaOH. Zeta potential measurements were performed in at least duplicate for each condition. 2.4. CB-NP deposition onto malachite and CB-NP aggregation measurements during deposition To identify the concentration of CB-NPs deposited onto the surface of malachite and evaluate the change of the particle sizes of CB-NPs 20
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with different CB-NP concentrations during deposition, absorbance and hydrodynamic diameter measurements were performed. To prepare sample suspensions, 1 g of malachite with a desired CB-NP concentration was distributed in 50-mL plastic conical tubes. Then, the suspensions were rotated at 40 rpm for 30 min. After settling the suspension of deposited CB-NPs–malachite via gravity for 1 min, the remaining amounts of CB-NPs in the supernatant were obtained. First, the extent of CB-NP deposition on the malachite surface was determined by measuring the absorbance of the supernatant CB-NP dispersion at 600 nm before and after deposition (30 min) using a UV-spectrometer (HS-3300, Humas, South Korea). The quantity of deposited CB-NPs was calculated using a calibration curve of absorbance versus CB-NP concentration. These tests were performed in at least duplicate for each condition. Second, the change of the particle sizes of CB-NPs during deposition was determined by measuring the hydrodynamic diameter of the supernatant using the dynamic light scattering (DLS) method (ELS-Z). In addition, to visually identify CB-NP deposition onto the malachite surface, a field emission scanning electron microscope (FESEM) with an energy dispersive X-ray spectrometer (EDS) (SUPRA 40VP, Carl Zeiss, Germany) was also employed. 2.5. Microflotation Flotation experiments were conducted using a Hallimond tube with 140 rpm (PC-410D, Corning Life Sciences, Mexico) at pH 7. Nitrogen gas (99.999% purity) was injected with a fixed flow rate of 30 mL/min. DF-250 was used as a frother at a concentration of 100 mL/ton. The conditioning time was 1 min for each test. To identify the flotation behavior of malachite with different CB-NP concentrations and flotation times, the flotation experiments were performed with 0.5–12 kg/ ton CB-NPs and a flotation time of 5–60 min. Note that all flotation tests were conducted using samples with residual CB-NPs in suspension after CB-NP deposition onto the malachite surface. Additional flotation experiments with filtration were conducted to identify the effects of the suspended CB-NP concentration on the flotation behavior of malachite. To eliminate residual CB-NPs after CB-NP deposition onto the malachite surface, filtration was conducted using cellulose acetate membrane filter (ADVANTEC, Toyo Roshi Kaisha, Japan) with a pore size of 5 μm and a small vacuum pump. Malachite remaining on the filter was collected and used for flotation tests. Both the floated and unfloated samples resulting from flotation were dried at 45 °C in a dryer (J-IB2, JICICO Co., Ltd., South Korea) for 24 h. The flotation experiments were performed in at least duplicate for each condition.
Fig. 1. (a) Transmission electron microscope image of carboxyl-functionalized carbon black nanoparticles (CB-NPs). The average particle size was approximately 30–40 nm. (b) The hydrophobicity of CB-NPs was determined by measuring the contact angle via the sessile drop method. The contact angle of CBNPs was approximately 108°.
2.6. Contact angle measurements 3. Results and discussion To investigate their hydrophobicity, CB-NPs were collected via suction filtration onto a 0.22-μm pore size cellulose acetate membrane filter. After filtration, the contact angle of the collected CB-NPs was measured via the sessile drop method (Hong et al., 2017; Kim et al., 2015). In addition, to identify the hydrophobicity of the malachite surface containing deposited CB-NPs, surface-flat malachite tablets were prepared using a compressor (DSP-5, Yujin Co., South Korea). First, the surface-flat malachite was mixed with 0.5, 2, 6, or 12 kg/ton CB-NPs for 30 min at room temperature. Second, surface-flat malachite with deposited CB-NPs was obtained, and then contact angle measurements were performed via the bubble captive method (Choi et al., 2016b; Kim et al., 2017; Yu et al., 2013). Note that measurement of the contact angle of malachite via the sessile drop method was not possible because malachite is highly porous, which causes water to be adsorbed by capillaries inside the pores of malachite. These samples were used to conduct the contact angle measurements (DSA100S, KRÜSS, Germany). The drop volume was fixed at 0.4 μL using a syringe with a needle with a diameter of 0.508 mm (NE, KRÜSS, Germany). The entire experimental process was automatically recorded with a camera and saved on a computer.
3.1. Characterization of CB-NPs The physicochemical characteristics of the CB-NPs used in this study were examined. TEM analyses were conducted to confirm the shape and size of the CB-NPs. Fig. 1 (a) shows the results. Their sizes ranged from 10 to 70 nm, and the average primary particle size was approximately 30 nm. In addition, the hydrophobicity of CB-NPs was determined to be approximately 108° (Fig. 1 (b)), indicating that the CB-NPs were highly hydrophobic.
3.2. Electrokinetic properties of CB-NPs and malachite Zeta potential was measured as a function of pH to examine the electrokinetic behavior of malachite and CB-NPs, and the results are presented in Fig. 2. Based on the results, the isoelectric points (IEPs) of malachite and CB-NPs were approximately pH 8–9 and pH < 6, respectively, indicating that the surfaces of malachite and CB-NPs are positively and negatively charged at pH 7, respectively. The IEPs were 21
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Fig. 2. Zeta potential measurements of malachite and carboxyl-functionalized carbon black nanoparticles (CB-NPs) as a function of the solution pH. All the experiments were performed in NaCl solution with an ionic strength of 1 mM.
Fig. 4. Microflotation measurements of malachite as a function of the carboxylfunctionalized carbon black nanoparticle (CB-NP) concentration (0.5–12 kg/ ton) using a Hallimond tube. The flotation time was 20 min. All the experiments were performed at pH 7.
similar with those reported in previous studies (Choi et al., 2016b; Han et al., 2017). The results suggest that the reason for nanoparticle deposition on the mineral surface was primarily electrostatic attraction between nanoparticles and mineral surfaces (Hajati et al., 2016; Yang et al., 2011). Based on this finding, we selected pH 7 for CB-NP deposition because malachite and CB-NPs exhibit opposite charges of almost maximum values, implying that a highly favorable interaction would occur between CB-NPs and malachite. In addition, to visually identify the deposition of CB-NPs onto the malachite surface, we performed FE-SEM with EDS measurements with 12 kg/ton CB-NPs at pH 7, and the results are shown in Fig. 3. Based on the results, CB-NP deposition on the malachite surface was confirmed. In addition, the deposition patterns of CB-NPs on the malachite surface likely involved a partial patch as a multi-layer rather than a uniform monolayer on the malachite surface.
3.3. Flotation behavior of malachite according to changes in the CB-NP concentration The floatability of malachite was measured as a function of the CBNP concentration (0.5–12 kg/ton) for a flotation time of 20 min at pH 7, and the results are presented in Fig. 4. Overall, the CB-NP concentration considerably affected the floatability of malachite. Specifically, malachite floatability monotonically increased as the CB-NP concentration increased in the low concentration range (0.5–2 kg/ton). Malachite floatability increased from approximately 40% to approximately 80% over this range. The difference in malachite floatability was likely associated with differences in the deposited CB-NP concentration, which altered the extent of the hydrophobic force that causes interactions with bubbles. To support this hypothesis, the amounts of CB-NPs deposited
Fig. 3. FE-SEM images and EDS analysis results of the malachite surface (a and b) and the carboxyl-functionalized carbon black nanoparticle (CB-NP)–deposited malachite surface (c and d), respectively. The white and black arrows in Fig. 3d confirm the deposited CB-NPs. The deposition tests were performed at pH 7 using a CB-NP concentration of 12 kg/ton. 22
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flotation behavior, we measured the deposited CB-NP concentration on the malachite surface over this range (Table 1). The results revealed that the amounts of CB-NPs deposited onto the malachite surface did not significantly decrease; contrarily, at high CB-NP concentrations (3–12 kg/ton), the amounts of CB-NPs deposited onto the malachite surface were comparable to those at 2 kg/ton CB-NPs (approximately 1.5 mg/g). This observation was further supported by the contact angles for malachite after CB-NP deposition (Fig. 5). The contact angle was approximately 65° for CB-NP concentrations of both 6 and 12 kg/ton. We conclude from our experiments that CB-NP deposition on the malachite surface and the hydrophobicity peaked at a CB-NP concentration of 2 kg/ton. Furthermore, at higher CB-NP concentrations, CB-NPs no longer deposited on the malachite surface, and there were negligible changes in the contact angle. Nevertheless, malachite floatability sharply decreased at CB-NP concentrations exceeding 2 kg/ton, implying that another mechanism should be assessed. Accordingly, we investigated other possible mechanisms to fully understand this unusual flotation behavior.
Table 1 Concentration of carboxyl-functionalized carbon black nanoparticles (CB-NPs) deposited onto the surface of malachite and the ratio of the deposited CB-NP concentration to the initially CB-NP concentration. Initial CB-NP concentration (kg/ ton)
Deposited CB-NP concentration on the malachite surface (mg/g)
The ratio of the deposited CBNP concentration to the initial CB-NP concentration (%)a
0.5 1 2 3 4 6 8 12
0.494 0.995 1.495 1.501 1.537 1.510 1.516 1.484
99.00 99.50 74.75 50.03 38.43 25.17 18.95 12.37
± ± ± ± ± ± ± ±
0.09 0.10 0.11 0.08 0.06 0.12 0.07 0.19
a Determined as (deposited CB-NP concentration/initial CB-NP concentration) × 100.
3.4. The influence of CB-NP aggregation during deposition process Previous studies found that the aggregation of hydrophobic nanoparticles had a negative impact on flotation performance (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012). To understand the reduction in malachite floatability in the presence of high CB-NP concentrations (3–12 kg/ton), assuming that the aggregation of CB-NPs occurs at higher concentrations, we performed DLS analysis to determine the aggregated sizes of CB-NPs at CB-NP concentrations of 0.5–12 kg/ton, and the results are presented in Fig. 6. Overall, the trends illustrated that CB-NP aggregation does not occur at higher CBNP concentrations. Therefore, in our system, we can eliminate CB-NP aggregation during deposition process as a cause of the decreased malachite floatability in the presence of high CB-NP concentrations. Instead, we focused on the suspended CB-NP concentration in our flotation system. 3.5. Effects of the suspended CB-NP concentration on malachite floatability As Table 1 indicates, the amounts of CB-NPs deposited onto the malachite surface increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). In addition, at CB-NP concentrations exceeding 2 kg/ton, CB-NPs no longer deposited onto the malachite surface, indicating that approximately 1.5 mg/g is the level of
Fig. 5. Contact angles for malachite after carboxyl-functionalized carbon black nanoparticle (CB-NP) deposition at initial concentrations of 0.5 (a), 2 (b), 6 (c), and 12 kg/ton (d) at pH 7. The contact angle was measured via the bubble captive method.
on the malachite surface and the contact angle of CB-NP–deposited malachite were measured, and the results are presented in Table 1 and Fig. 5, respectively. In agreement with the hypothesis, the deposited amounts of CB-NPs on the malachite surface increased with increasing CB-NP concentration in the low range (0.5–2 kg/ton) (Table 1). This is also well supported by previous studies that reported that the deposited amounts of nanoparticles on the mineral surface increased with increasing nanoparticle concentrations (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013b). This observation was supported further by the contact angles for the malachite surface after CB-NP deposition (Fig. 5). The contact angles were approximately 42.6° and 65.2° for 0.5 and 2 kg/ton CB-NPs, respectively. The higher contact angle for 2 kg/ton CB-NPs indicated that higher amounts of CBNPs were deposited on the malachite surface, and thus, they rendered the malachite surface more hydrophobic. Accordingly, malachite floatability increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). Meanwhile, malachite floatability decreased at CB-NP concentrations exceeding 2 kg/ton (3–12 kg/ton). To understand this interesting
Fig. 6. Change in the dynamic light scattering diameter of carboxyl-functionalized carbon black nanoparticles (CB-NPs) while being deposited to malachite with initial CB-NP concentrations of 0.5–12 kg/ton. All the experiments were performed at pH 7. 23
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Fig. 7. Microflotation measurements of malachite as a function of the carboxylfunctionalized carbon black nanoparticle (CB-NP) concentration (0.5–12 kg/ ton) using a Hallimond with (circle) and without (square) filtration process. The flotation time was 20 min. All the experiments were performed at pH 7. The data obtained without filtration was obtained from those in Fig. 4.
saturation for 2 kg/ton CB-NPs. More noteworthy is that the ratios of the deposited CB-NP concentration to the initially added CB-NP concentration were 99, 99.5, 74.75, 50.03, 38.43, 25.17, 18.95, and 12.37% for 0.5, 1, 2, 3, 4, 6, 8, and 12 kg/ton CB-NPs, respectively (Table 1). These trends indicate that most of the initially added CB-NPs are deposited on the malachite surface at lower concentrations, resulting in a low suspended CB-NP concentration at initial levels of less than 2 kg/ton. On the contrary, the suspended CB-NP levels continuously increased with increasing CB-NP concentrations exceeding 2 kg/ton. Based on these trends, we questioned whether the presence of suspended CB-NPs reduces malachite floatability. Interestingly, the suspended CB-NP concentration strongly affected the floatability of malachite (Fig. 7). Quantitatively, after the filtration process, the malachite floatability values were approximately 43.85, 56.11, 81.65, 84.55, 81.55, 80.36, 81.85, and 85.53% in the presence of 0.5, 1, 2, 3, 4, 6, 8, and 12 kg/ton CB-NPs, respectively. For low CB-NP concentrations (0.5–2 kg/ton), the difference in malachite floatability was negligible regardless of the use of filtration because of almost nonexistent suspended CB-NPs or increased amount of CB-NPs deposited. Meanwhile, malachite floatability was relatively higher after filtration compared with the findings without filtration for high CB-NP concentrations (3–12 kg/ton), indicating that suspended CB-NPs negatively affected malachite floatability and their levels continuously increased with increasing CB-NP concentrations. Although the experimental evidence suggests that suspended CB-NPs are an obvious cause of the lower malachite floatability, the exact mechanism is not clear. Hence, to investigate the distinct cause of the different malachite floatability according to the presence of suspended CB-NPs, additional flotation experiments were performed.
Fig. 8. Malachite floatability as a function of the carboxyl-functionalized carbon black nanoparticle (CB-NP) concentration over the ranges of 0.5–2 (a) and 4–12 kg/ton (b) with a flotation time of 5–60 min.
53.04, and 81.93% in the presence of 0.5, 1, and 2 kg/ton CB-NPs, respectively, and at a flotation time of 60 min, malachite floatability was approximately 42.94, 57.4, and 84.38% in the presence of 0.5, 1, and 2 kg/ton CB-NPs, respectively. Eventually, it was possible to directly confirm that the interaction between bubbles and CB-NP–deposited malachite was mostly completed after 20 min of flotation in the presence of 0.5–2 kg/ton CB-NPs. Meanwhile, Fig. 8 (b) shows differences in the experimental trends of malachite floatability according to the flotation time in the presence of 4 and 12 kg/ton CB-NPs. Specifically, after 20 min of flotation, the malachite floatability values were 45.42 and 29.43% in the presence of 4 and 12 kg/ton CB-NPs, respectively. After 60 min of flotation, the values were approximately 69.82 and 57.36% in the presence of 4 and 12 kg/ton CB-NPs, respectively. The experimental kinetic results suggested that the kinetic rate of malachite floatability was relatively slower in the presence of 4–12 kg/ton CB-NPs compared with those in the presence of 0.5–2 kg/ton CB-NPs, and malachite floatability increased with increasing flotation time. As mentioned in Table 1, at CB-NP concentrations exceeding 2 kg/ton, the amounts of suspended CB-NPs continuously increased, suggesting that suspended CB-NPs inhibit the attachment of malachite to bubbles. We propose that the increase in the concentration of suspended CB-NPs likely induced to form CB-NP aggregates under severe agitation (∼140 rpm), which more easily allow them to attach onto the bubble surface. In order to confirm this hypothesis, we measured the size of suspended CB-NPs using a single particle optical sizing system
3.6. Effect of the flotation time To further clarify the cause of the different malachite floatability based on the presence of suspended CB-NPs, flotation experiments were performed using different flotation time and the results are presented in Fig. 8. Overall, Fig. 8 (a) shows that malachite floatability rapidly increased with increasing flotation times up to 20 min, whereas further increases of the flotation time had no significant effect on the kinetic rate of malachite floatability in the presence of 0.5–2 kg/ton CB-NPs. Maximum malachite floatability (approximately 80%) was observed in the presence of 2 kg/ton CB-NPs. Specifically, at a flotation time of 20 min, the malachite floatability values were calculated as 39.13, 24
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(AccuSizerTM 780/SIS, PSS-NICOMP, USA) after 60 min agitation of CBNPs at 140 rpm in the Hallimond tube without nitrogen injection. The test was conducted with the sample having the concentration of 10 kg/ ton, which corresponds to initial CB-NP concentration of 12 kg/ton (Table 1). The volume-based median diameter (d50) was determined to be about 27.3 μm, which clearly supports the formation of CB-NP aggregates. In addition, it is well known that more hydrophobic materials have faster kinetic rates with bubbles than less hydrophobic materials (Somasundaran and Lin, 1973; Xia et al., 2015). Hence, the presence of suspended CB-NPs is likely to negatively affect malachite floatability because suspended CB-NP aggregates, which have greater surface hydrophobicity, had a relatively faster kinetic rate with bubbles than CBNP–deposited malachite, which is less hydrophobic.
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Sci. Nano 4, 800–810. Hong, G., Choi, J., Han, Y., Yoo, K.-S., Kim, K., Kim, S.B., Kim, H., 2017. Relationship between surface characteristics and floatability in representative sulfide minerals: role of surface oxidation. Mater. Trans. 58, 1069–1075. Hope, G.A., Buckley, A.N., Parker, G.K., Numprasanthai, A., Woods, R., McLean, J., 2012. The interaction of n-octanoydroxamate with chrysocolla and oxide copper surfaces. Miner. Eng. 36–38, 2–11. Hwang, G., Gomez-Flores, A., Bradford, S.A., Choi, S., Jo, E., Kim, S.B., Tong, M., Kim, H., 2018. Analysis of stability of behavior of carbon black nanoparticles in ecotoxicological media: hydrophobic and steric effect. Colloid Surface A: Physicochem. Eng. Aspects 554, 306–316. Jiang, H., Sun, Z., Xu, L., Hu, Y., Huang, K., Zhu, S., 2012. A comparison study of the flotation and adsorption behaviors of diaspore and kaolinite with quaternary ammonium collectors. Miner. Eng. 65, 124–129. Kim, G., Park, K., Choi, J., Gomez-Flores, A., Han, Y., 2015. Bioflotation of malachite using different growth phases of Rhodococcus opacus: effect of bacterial shape on detachment by shear flow. Int. J. Miner. Process. 143, 98–104. Kim, G., Choi, J., Silva, R.A., Song, Y., Kim, H., 2017. Feasibility of bench-scale selective bioflotation of copper oxide minerals using rhodococcus opacus. Hydrometallurgy 168, 94–102. Kim, G., Choi, J., Choi, S., Kim, K.H., Han, Y., Bradford, S., Choi, S., Kim, H., 2018. Application of depletion attraction in mineral flotation: II. Effects of depletant concentration. Minerals 8, 450. Kongolo, K.P., Kipoka, M., Minanga, K., Mpoyo, M., 2003. Improving the efficiency of oxide copper-cobalt ores flotation by combination of sulphidisers. Miner. Eng. 16, 1023–1026. Li, Z., Rao, F., Garcia, R.E., Li, H., Song, S., 2018. Partial replacement of sodium oleate using alcohols with different chain structures in malachite flotation. Miner. Eng. 127, 185–190. Lee, J.S., Nagaraj, D.R., Coe, J.E., 1998. Practical aspects of oxide copper recovery with alkyl hydroxamates. Miner. Eng. 11, 929–939. Lee, K., Archibald, D., Mclean, J., Reuter, M.A., 2009. Flotation of mixed copper oxide and sulphide minerals with xanthate and hydroxamate collectors. Miner. Eng. 22, 395–401. Li, F., Zhong, H., Xu, H., Jia, H., Liu, G., 2015. Flotation behavior and adsorption mechanism of α-hydroxyoctyl phosphinic acid to malachite. Miner. Eng. 71, 188–193. Marion, C., Jordens, A., Li, R., Rudolph, M., Waters, K.E., 2017. An evaluation of hydroxamate collectors for malachite flotation. Sep. Purif. Technol. 183, 258–269. Mauter, M.S., Elimelech, M., 2008. Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol. 42, 5843–5859. Min, X., Yuan, C., Liang, Y., Chai, L., Ke, Y., 2012. Metal recovery from sludge through the combination of hydrothermal sulfidation and flotation. Procedia Environ. Sci. 16, 401–408. Nguyen, A., Evans, G., Schulze, H., 2001. Prediction of van der Waals interaction in bubble-particle attachment in flotation. Int. J. Miner. Process. 61, 155–169. Ohshima, H., 2007. Electrokinetics of soft particles. Colloid Polym. Sci. 285, 1141–1421. Okibe, N., Johnson, D.B., 2002. Toxicity of flotation reagents to moderately thermophilic bioleaching microorganisms. Biotechnol. Lett. 24, 2011–2016. Pan, L., Jung, S., Yoon, R.-H., 2012. A fundamental study on the role of collector in the kinetics of bubble-particle interaction. Int. J. Miner. Process. 106, 37–41. Park, K., Park, S., Choi, J., Kim, G., Tong, M., Kim, H., 2016. Influence of excess sulfide ions on the malachite-bubble interaction in the presence of thiol-collector. Sep. Purif. Technol. 168, 1–7. Phetla, T.P., Muzenda, E., 2010. A multistage sulphidisation flotation procedure for a low grade malachite copper ore. World Acad. Sci., Eng. Technol. Int. J. Chem. Molecul. Eng. 4, 580–586. Shen, P., Liu, D., Xu, X., Jia, X., Zhang, X., Liu, D., Liu, R., 2018. Effect of ethylene diamine phosphate on the sulfidization flotation of chrysocolla. Minerals 8, 216–230. Somasundaran, P., Lin, I.J., 1973. Method for evaluating flotation kinetic parameters. Trans. Metall. Soc. AIME 254, 181–184. Wang, J., Lu, J., Zhang, Q., Saito, F., 2003. Mechanochemical sulfidization of nonferrous metal oxides by grinding with sulfur and iron. Ind. Eng. Chem. Res. 42, 5813–5818. Webb, M., Ruber, H., Leduc, G., 1976. The toxicity of various mining flotation reagents to rainbow trout (Salmo gairdneri). Water Res. 10, 303–306. Yang, S., Pelton, R., Raegen, A., Montgomery, M., Dalnoki-Veress, K., 2011. Nanoparticle flotation collectors: mechanisms behind a new technology. Langmuir 27, 10438–10446. Yang, S., Pelton, R., 2011. Nanoparticle flotation collectors II: the role of nanoparticle hydrophobicity. Langmuir 27, 11409–11415. Yang, S., Pelton, R., Montgomery, M., Cui, Y., 2012. Nanoparticle flotation collectors III: the role of nanoparticle diameter. ACS Appl. Mater. Interfaces 4, 4882–4890. Yang, S., Pelton, R., Abarca, Z., Dai, Z., Montgomery, M., Xu, M., Bos, J.-A., 2013a. Towards nanoparticle flotation collectors for pentlandite separation. Int. J. Miner. Process. 123, 137–144.
4. Conclusion In this study, the influence of the concentration of CB-NPs as collectors in malachite flotation was systematically investigated. The key experimental findings from our study were as follows: (1) Electrophoretic mobility measurements illustrated that malachite is positively charged at certain pH values, whereas the nanoparticles are negatively charged at these values, indicating that the electrostatic interaction between malachite and CB-NPs was favorable, resulting in CB-NP deposition onto the malachite surface. (2) The amount of CB-NPs deposited on malachite surface increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). The maximum amount of deposited CB-NPs was approximately 1.5 mg/g at 2 kg/ton CB-NPs, and at higher concentrations, CB-NPs no longer deposited onto the malachite surface. The trends for the amounts of deposited CB-NPs on the malachite surface were highly consistent with the contact angle measurements. These two experimental results indicate that the suspended CB-NP concentration continuously increased with increasing CB-NP concentrations exceeding 2 kg/ton. (3) We found that the malachite floatability increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). The amounts of CB-NPs deposited onto malachite surface increased as the CB-NP concentration increased, resulting in increased hydrophobicity of the malachite surface and enhanced malachite floatability. Meanwhile, malachite floatability sharply decreased at CBNP concentrations exceeding 2 kg/ton. Our various experimental results including a filtration process and different flotation times suggested that this unusual flotation behavior of malachite can be explained by the presence of suspended CB-NP aggregates, which reduced the kinetic rates of attachment between CB-NP–deposited malachite and bubbles. References Abarca, C., Yang, S., Pelton, R.H., 2015. Towards high throughput screening of nanoparticle flotation collectors. J. Colloid Interface Sci. 460, 97–104. Cao, Z., Zhong, H., Liu, G., Zhao, S., 2009. Techniques of copper recovery from Mexican copper oxide ore. Mining Sci. Technol. (China) 19, 45–48. Choi, J., Kim, W., Chae, W., Kim, S.B., Kim, H., 2012. Electrostatically controlled enrichment of lepidolite via flotation. Mater. Trans. 53, 2191–2194. Choi, J., Hong, J., Park, K., Kim, G., Han, Y., Kim, S., Kim, H., 2014. Role of chain length and type on the adsorption behavior of cationic surfactants and the silica floatability. Mater. Trans. 55, 1344–1349. Choi, J., Lee, E., Choi, S.Q., Lee, S., Han, Y., Kim, H., 2016a. Arsenic removal from contaminated soils for recycling via oil agglomerate flotation. Chem. Eng. J. 285, 207–217. Choi, J., Choi, S.Q., Park, K., Han, Y., Kim, H., 2016b. Flotation behavior of malachite in mono- and di-valent salt solutions using sodium oleate as a collector. Int. J. Miner. Process. 146, 38–45. Choi, J., Kim, G., Choi, S., Kim, K.H., Han, Y., Bradford, S., Choi, S., Kim, H., 2018. Application of depletion attraction in mineral flotation: I. Theory. Minerals 8, 451. Dong, X., Price, M., Dai, Z., Xu, M., Pelton, R., 2017. Mineral-mineral particle collisions during flotation remove adsorbed nanoparticle flotation collectors. J. Colloid Interface Sci. 504, 178–185.
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H. Kim, et al. Yang, S., Razavizadeh, B.B.M., Pelton, R., Bruin, G., 2013b. Nanoparticle flotation collectors–the influence of particle softness. ACS Appl. Mater. Interfaces 5, 4836–4842. Yoon, R.-H., Ravishankar, S., 1994. Application of extended DLVO theory: III. Effect of octanol on the long-range hydrophobic forces between dodecylamine-coated mica surfaces. J. Colloids Interface Sci. 166, 215–224. Yu, Q., Cho, J., Shivapooja, P., Ista, L.K., Lopez, G.P., 2013. Nanopatterned smart polymer
surfaces for controlled attachment, killing, and release of bacteria. ACS Appl. Mater. Interfaces 5, 9295–9304. Xia, W., Peng, Y., Ren, C., Xie, G., Liang, C., 2015. Changes in the flotation kinetics of bituminous coal before and after natural weathering processes. Physicochem. Problems Miner. Process. 51, 401–410.
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Malachite flotation using carbon black nanoparticles as collectors: Negative impact of suspended nanoparticle aggregates
T
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Hyunjung Kima,1, , Junhyuk Youa,1, Allan Gomez-Floresa, Stephen Kayombo Solongoa, Gukhwa Hwanga, Hongbo Zhaob, Byoung-cheun Leec, Junhyun Choia a
Department of Mineral Resources and Energy Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeonbuk 54896, Republic of Korea School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan, China c Risk Assessment Division, National Institute of Environmental Research, Hwangyeong-ro 42, Seo-gu, Incheon 22689, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon black nanoparticles Collector Malachite Floatability
In this study, the flotation behavior of malachite was investigated using carboxyl-functionalized carbon black nanoparticles (CB-NPs) as collectors in a hydrodynamically simplified Hallimond tube. We found that malachite floatability increased with increasing CB-NP concentration in the low range (0.5–2 kg/ton). To understand the mechanism of this higher floatability for low CB-NP concentrations, we examined the amounts of deposited CBNPs on the malachite surface and hydrophobicity of the surface with deposited CB-NPs according to changes in the initial CB-NP concentration. The increase in malachite floatability was likely associated with increases in the amounts of deposited CB-NPs on the malachite surface, which enhanced malachite floatability by altering the hydrophobicity of its surface. Meanwhile, malachite floatability sharply decreased at CB-NP concentrations exceeding 2 kg/ton. To understand this unusual flotation behavior for high CB-NP concentrations, we also examined the amounts of CB-NPs deposited on the malachite surface and contact angles. However, the decreased malachite floatability could not be fully explained by these experiments, implying that at least one additional mechanism is involved. Therefore, additional flotation experiments including filtration processes and different flotation times were conducted, revealing that the unusual flotation behavior was likely due to the presence of suspended CB-NP aggregates, which reduced the kinetic rates of attachment between malachite and bubbles.
1. Introduction Flotation is a physicochemical technique that separates hydrophobic particles from a mixture of hydrophobic and hydrophilic particles (Choi et al., 2016a; Choi et al., 2016b; Fuerstenau et al., 2006; Gaudin, 1957). During the flotation process, the important interaction step is the attachment of minerals to the bubble surface, as the interaction between mineral particles and bubbles determines the overall efficiency of the process (Choi et al., 2018; Kim et al., 2018; Nguyen et al., 2001; Pan et al., 2012; Yoon and Ravishankar, 1994). To improve the attachment of particles to bubbles, mineral particles are selectively treated with a short-chain, water-soluble surfactant to selectively render their surfaces hydrophobic (Choi et al., 2012; Choi et al., 2014). These surfactants are usually called collectors in the flotation field (Ejtemaei et al., 2011; Jiang et al., 2012; Park et al., 2016). Copper oxide is an important copper source. It is found in the weathered regions of most Cu sulfide ore bodies (Hope et al., 2012).
The conventional method of Cu oxide flotation involves sulfidization of the mineral surface prior to selective flotation using sulfhydryl collectors such as xanthate (Cao et al., 2009; Kongolo et al., 2003). This process is effective with respect to selectivity for Cu oxide minerals. Unfortunately, this method has critical disadvantages; specifically, sulfidizing and sulfhydryl reagents are rather toxic and not environmental friendly (Min et al., 2012; Okibe and Johnson, 2002; Wang et al., 2003; Webb et al., 1976), and determining the optimum concentrations of sulfidizing and sulfhydryl reagents is extremely difficult (Feng et al., 2016; Shen et al., 2018). As an alternative to sulfidization, a variety of oxhydryl collectors have been evaluated for direct flotation tests of Cu oxide-containing substances such as fatty acids (Choi et al., 2016b; Li et al., 2018), hydroxamic compounds (Lee et al., 1998; Lee et al., 2009; Marion et al., 2017), and phosphinic/phosphonic compounds (Li et al., 2015). Although flotation tests of Cu oxide minerals were performed using fatty acids, they have affinities for most cations, and, therefore, they have inherently low selectivity in mineral flotation
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Corresponding author. E-mail address: [email protected] (H. Kim). 1 These authors equally contributed to this article. https://doi.org/10.1016/j.mineng.2019.03.025 Received 20 November 2018; Received in revised form 19 March 2019; Accepted 21 March 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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(CB-NPs) as collectors. Malachite is a representative copper oxide-based mineral (Choi et al., 2016b; Lee et al., 1998; Li et al., 2018). CB-NPs are extremely hydrophobic (Han et al., 2017; Hwang et al., 2018; Mauter and Elimelech, 2008). Through this research, we found a new malachite flotation behavior that had not been reported previously for nanoparticle flotation. Malachite floatability increases with increasing CBNP concentrations in the low range because of increased hydrophobicity on the surface of malachite caused by deposited CB-NPs. Meanwhile, for high CB-NP concentrations, malachite floatability decreased as the CB-NP concentration increased. Our experimental evidence suggested that excessive levels of suspended CB-NPs reduced the kinetic rate of malachite attachment to bubbles.
(Li et al., 2018; Lee et al., 1998). Unlike fatty acids, hydroxamic compounds, which are similar to phosphonic and phosphinic compounds, have been demonstrated to effectively and selectively recover Cu oxide minerals (Lee et al., 1998; Lee et al., 2009; Li et al., 2015; Li et al., 2018; Marion et al., 2017). Hydroxamic compounds, which are O-O type chelating reagents, were founded to chemically adsorb onto the mineral surface, and thus, they offer many advantages over alternative technologies for Cu oxide minerals (Lee et al., 1998; Lee et al., 2009). Nevertheless, the large-scale usage of hydroxamic compounds is limited owing to their practical guidelines, efficacy, and cost in plants (Li et al., 2018; Phetla and Muzenda, 2010). Because of these factors, new alternative flotation methods are required to expand knowledge and develop flotation technology for Cu oxide minerals. More recently, there have been attempts to investigate hydrophobic nanoparticles as a new class of flotation collectors (Abarca et al., 2015; Dong et al., 2017; Hajati et al., 2016; Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013a; Yang et al., 2013b). Nanoparticle flotation studies demonstrated that hydrophobic nanoparticles may offer advantages as collectors in flotation over conventional collectors (Abarca et al., 2015; Yang et al., 2011, Yang et al., 2012; Yang et al., 2013a). They suggested that as little as 10% coverage by nanoparticles on the glass bead surfaces could promote high flotation efficiency, whereas conventional molecular collector requires 25% or greater coverage for good recovery (Abarca et al., 2015; Yang et al., 2011, Yang et al., 2012; Yang et al., 2013a). Considering these advantages of nanoparticles as collectors in flotation, researchers have exerted substantial efforts to clarify the feasibility of nanoparticles as collectors in flotation via experimental and theoretical approaches (Abarca et al., 2015; Dong et al., 2017; Hajati et al., 2016; Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013a; Yang et al., 2013b). For example, hydrophobic polystyrene nanoparticles can function as flotation collectors (Abarca et al., 2015; Dong et al., 2017; Yang and Pelton, 2011; Yang et al., 2011; Yang et al., 2012; Yang et al., 2013a; Yang et al., 2013b). Polystyrene nanoparticles were deposited onto glass beads via electrostatic attraction, improving the contact angle of glass beads (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2013b). In addition, the application of polymeric soft and hard polystyrene nanoparticles as collectors in flotation of glass beads and pentlandite (nickel sulfide mineral) has been reported (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2013a; Yang et al., 2013b). The softer nanoparticles were more firmly deposited onto glass beads or pentlandite surfaces because they had a greater contact area (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2013a; Yang et al., 2013b). For pentlandite flotation, polystyrene nanoparticles bearing surface imidazole groups specifically bound nickel ions and appeared to improve flotation performance (Yang et al., 2013a). In addition, smaller, more hydrophobic polystyrene nanoparticles are more efficient flotation collectors, and as little as 10% nanoparticle coverage offers high flotation performance (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013b). Furthermore, glass bead floatability decreased with increasing conditioning time using hydrophobic polystyrene nanoparticles as collectors because collision between glass beads detaches nanoparticles deposited on their surfaces (Dong et al., 2017). Last, only one study examined silica floatability using hydrophobic talc nanoparticles as collectors (Hajati et al., 2016). By adjusting the solution pH, talc and silica can become oppositely charged, and thus, electrostatic attraction is possible (Hajati et al., 2016). Likewise, although researchers made substantial efforts to identify the feasibility of nanoparticles as collectors in flotation, relevant studies regarding various mineral and nanoparticle species are extremely limited. In particular, there has been no attempt to investigate the effect of hydrophobic nanoparticles as collectors for Cu oxide minerals. In this study, to identify the feasibility of nanoparticle flotation using Cu oxide minerals and further advance the understanding of the roles of nanoparticles as collectors in flotation, malachite flotation was performed using carboxyl-functionalized carbon black nanoparticles
2. Materials and methods 2.1. Materials and reagents Malachite (CuCO3·Cu(OH)2), which was used as a raw material, was obtained from Junsei Co., Japan. The manufacturer reported that it has 55% Cu, and it is spherical in shape. Two sieves with −100 and +200 mesh (Tyler Standard) were used to separate particles to obtain only malachite particles with diameters ranging from 74 to 149 μm, which were used in this study. CB-NPs were supplied by Birla Carbon (Columbian Chemicals Korea, Yeosu, South Korea). To evaluate the primary size of CB-NPs, transmission electron microscope (TEM, H7650, Hitachi, Japan) observations were conducted at a voltage of 100 kV. In addition, DowFroth-250 (DF-250, CH3(OC3H6)4OH) supplied by American Cyanamid, USA was used as a frother, and analyticalgrade HCl (≥99%, Fisher Scientific) and NaOH (≥99%, Fisher Scientific) were used as pH modifiers. 2.2. Preparation of CB-NP suspensions An ultrasonic wave setup was used to disperse CB-NPs into suspensions. In detail, the setup consisted of an ultrasonic homogenizer with a horn tip of 10 mm in diameter (KNSN-RAB, KOEN, South Korea) and a reactor to accommodate the CB-NP suspension. The ultrasonic wave frequency was fixed at 20 kHz (intensity of 11 W/mL) for consistent irradiation. The cylinder-shaped reactor was in acrylic with a diameter and height of 3 and 10 cm, respectively. The reactor had double jackets for controlling the temperature. The 40 mg/L CB-NP suspension was prepared in Milli-Q water (Milli-Q Plus, Millipore Ltd., UK) and then dispersed for 10 min; this suspension was used as stock suspension. The suspension with desired CB-NP concentration was prepared by diluting the stock suspension for subsequent characterization, deposition, and flotation tests. 2.3. Zeta potential measurements To characterize the electrokinetic properties of malachite and CBNPs, their electrophoretic mobilities were measured using ELS-Z equipment (Otsuka, Hirakata, Japan); the Smoluchowski equation was used to convert electrophoretic mobility to zeta potential (Ohshima, 2007). Because the sizes of malachite exceeded the measurable range of the ELS-Z, malachite was ground further using a mortar to obtain a smaller size (< 5 µm). For electrophoretic mobility measurements, aliquots of malachite and CB-NP suspensions were re-suspended in Deionized (DI) water with a desired pH. The pH of all suspensions was adjusted using 0.1 M HCl or 0.1 M NaOH. Zeta potential measurements were performed in at least duplicate for each condition. 2.4. CB-NP deposition onto malachite and CB-NP aggregation measurements during deposition To identify the concentration of CB-NPs deposited onto the surface of malachite and evaluate the change of the particle sizes of CB-NPs 20
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with different CB-NP concentrations during deposition, absorbance and hydrodynamic diameter measurements were performed. To prepare sample suspensions, 1 g of malachite with a desired CB-NP concentration was distributed in 50-mL plastic conical tubes. Then, the suspensions were rotated at 40 rpm for 30 min. After settling the suspension of deposited CB-NPs–malachite via gravity for 1 min, the remaining amounts of CB-NPs in the supernatant were obtained. First, the extent of CB-NP deposition on the malachite surface was determined by measuring the absorbance of the supernatant CB-NP dispersion at 600 nm before and after deposition (30 min) using a UV-spectrometer (HS-3300, Humas, South Korea). The quantity of deposited CB-NPs was calculated using a calibration curve of absorbance versus CB-NP concentration. These tests were performed in at least duplicate for each condition. Second, the change of the particle sizes of CB-NPs during deposition was determined by measuring the hydrodynamic diameter of the supernatant using the dynamic light scattering (DLS) method (ELS-Z). In addition, to visually identify CB-NP deposition onto the malachite surface, a field emission scanning electron microscope (FESEM) with an energy dispersive X-ray spectrometer (EDS) (SUPRA 40VP, Carl Zeiss, Germany) was also employed. 2.5. Microflotation Flotation experiments were conducted using a Hallimond tube with 140 rpm (PC-410D, Corning Life Sciences, Mexico) at pH 7. Nitrogen gas (99.999% purity) was injected with a fixed flow rate of 30 mL/min. DF-250 was used as a frother at a concentration of 100 mL/ton. The conditioning time was 1 min for each test. To identify the flotation behavior of malachite with different CB-NP concentrations and flotation times, the flotation experiments were performed with 0.5–12 kg/ ton CB-NPs and a flotation time of 5–60 min. Note that all flotation tests were conducted using samples with residual CB-NPs in suspension after CB-NP deposition onto the malachite surface. Additional flotation experiments with filtration were conducted to identify the effects of the suspended CB-NP concentration on the flotation behavior of malachite. To eliminate residual CB-NPs after CB-NP deposition onto the malachite surface, filtration was conducted using cellulose acetate membrane filter (ADVANTEC, Toyo Roshi Kaisha, Japan) with a pore size of 5 μm and a small vacuum pump. Malachite remaining on the filter was collected and used for flotation tests. Both the floated and unfloated samples resulting from flotation were dried at 45 °C in a dryer (J-IB2, JICICO Co., Ltd., South Korea) for 24 h. The flotation experiments were performed in at least duplicate for each condition.
Fig. 1. (a) Transmission electron microscope image of carboxyl-functionalized carbon black nanoparticles (CB-NPs). The average particle size was approximately 30–40 nm. (b) The hydrophobicity of CB-NPs was determined by measuring the contact angle via the sessile drop method. The contact angle of CBNPs was approximately 108°.
2.6. Contact angle measurements 3. Results and discussion To investigate their hydrophobicity, CB-NPs were collected via suction filtration onto a 0.22-μm pore size cellulose acetate membrane filter. After filtration, the contact angle of the collected CB-NPs was measured via the sessile drop method (Hong et al., 2017; Kim et al., 2015). In addition, to identify the hydrophobicity of the malachite surface containing deposited CB-NPs, surface-flat malachite tablets were prepared using a compressor (DSP-5, Yujin Co., South Korea). First, the surface-flat malachite was mixed with 0.5, 2, 6, or 12 kg/ton CB-NPs for 30 min at room temperature. Second, surface-flat malachite with deposited CB-NPs was obtained, and then contact angle measurements were performed via the bubble captive method (Choi et al., 2016b; Kim et al., 2017; Yu et al., 2013). Note that measurement of the contact angle of malachite via the sessile drop method was not possible because malachite is highly porous, which causes water to be adsorbed by capillaries inside the pores of malachite. These samples were used to conduct the contact angle measurements (DSA100S, KRÜSS, Germany). The drop volume was fixed at 0.4 μL using a syringe with a needle with a diameter of 0.508 mm (NE, KRÜSS, Germany). The entire experimental process was automatically recorded with a camera and saved on a computer.
3.1. Characterization of CB-NPs The physicochemical characteristics of the CB-NPs used in this study were examined. TEM analyses were conducted to confirm the shape and size of the CB-NPs. Fig. 1 (a) shows the results. Their sizes ranged from 10 to 70 nm, and the average primary particle size was approximately 30 nm. In addition, the hydrophobicity of CB-NPs was determined to be approximately 108° (Fig. 1 (b)), indicating that the CB-NPs were highly hydrophobic.
3.2. Electrokinetic properties of CB-NPs and malachite Zeta potential was measured as a function of pH to examine the electrokinetic behavior of malachite and CB-NPs, and the results are presented in Fig. 2. Based on the results, the isoelectric points (IEPs) of malachite and CB-NPs were approximately pH 8–9 and pH < 6, respectively, indicating that the surfaces of malachite and CB-NPs are positively and negatively charged at pH 7, respectively. The IEPs were 21
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Fig. 2. Zeta potential measurements of malachite and carboxyl-functionalized carbon black nanoparticles (CB-NPs) as a function of the solution pH. All the experiments were performed in NaCl solution with an ionic strength of 1 mM.
Fig. 4. Microflotation measurements of malachite as a function of the carboxylfunctionalized carbon black nanoparticle (CB-NP) concentration (0.5–12 kg/ ton) using a Hallimond tube. The flotation time was 20 min. All the experiments were performed at pH 7.
similar with those reported in previous studies (Choi et al., 2016b; Han et al., 2017). The results suggest that the reason for nanoparticle deposition on the mineral surface was primarily electrostatic attraction between nanoparticles and mineral surfaces (Hajati et al., 2016; Yang et al., 2011). Based on this finding, we selected pH 7 for CB-NP deposition because malachite and CB-NPs exhibit opposite charges of almost maximum values, implying that a highly favorable interaction would occur between CB-NPs and malachite. In addition, to visually identify the deposition of CB-NPs onto the malachite surface, we performed FE-SEM with EDS measurements with 12 kg/ton CB-NPs at pH 7, and the results are shown in Fig. 3. Based on the results, CB-NP deposition on the malachite surface was confirmed. In addition, the deposition patterns of CB-NPs on the malachite surface likely involved a partial patch as a multi-layer rather than a uniform monolayer on the malachite surface.
3.3. Flotation behavior of malachite according to changes in the CB-NP concentration The floatability of malachite was measured as a function of the CBNP concentration (0.5–12 kg/ton) for a flotation time of 20 min at pH 7, and the results are presented in Fig. 4. Overall, the CB-NP concentration considerably affected the floatability of malachite. Specifically, malachite floatability monotonically increased as the CB-NP concentration increased in the low concentration range (0.5–2 kg/ton). Malachite floatability increased from approximately 40% to approximately 80% over this range. The difference in malachite floatability was likely associated with differences in the deposited CB-NP concentration, which altered the extent of the hydrophobic force that causes interactions with bubbles. To support this hypothesis, the amounts of CB-NPs deposited
Fig. 3. FE-SEM images and EDS analysis results of the malachite surface (a and b) and the carboxyl-functionalized carbon black nanoparticle (CB-NP)–deposited malachite surface (c and d), respectively. The white and black arrows in Fig. 3d confirm the deposited CB-NPs. The deposition tests were performed at pH 7 using a CB-NP concentration of 12 kg/ton. 22
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flotation behavior, we measured the deposited CB-NP concentration on the malachite surface over this range (Table 1). The results revealed that the amounts of CB-NPs deposited onto the malachite surface did not significantly decrease; contrarily, at high CB-NP concentrations (3–12 kg/ton), the amounts of CB-NPs deposited onto the malachite surface were comparable to those at 2 kg/ton CB-NPs (approximately 1.5 mg/g). This observation was further supported by the contact angles for malachite after CB-NP deposition (Fig. 5). The contact angle was approximately 65° for CB-NP concentrations of both 6 and 12 kg/ton. We conclude from our experiments that CB-NP deposition on the malachite surface and the hydrophobicity peaked at a CB-NP concentration of 2 kg/ton. Furthermore, at higher CB-NP concentrations, CB-NPs no longer deposited on the malachite surface, and there were negligible changes in the contact angle. Nevertheless, malachite floatability sharply decreased at CB-NP concentrations exceeding 2 kg/ton, implying that another mechanism should be assessed. Accordingly, we investigated other possible mechanisms to fully understand this unusual flotation behavior.
Table 1 Concentration of carboxyl-functionalized carbon black nanoparticles (CB-NPs) deposited onto the surface of malachite and the ratio of the deposited CB-NP concentration to the initially CB-NP concentration. Initial CB-NP concentration (kg/ ton)
Deposited CB-NP concentration on the malachite surface (mg/g)
The ratio of the deposited CBNP concentration to the initial CB-NP concentration (%)a
0.5 1 2 3 4 6 8 12
0.494 0.995 1.495 1.501 1.537 1.510 1.516 1.484
99.00 99.50 74.75 50.03 38.43 25.17 18.95 12.37
± ± ± ± ± ± ± ±
0.09 0.10 0.11 0.08 0.06 0.12 0.07 0.19
a Determined as (deposited CB-NP concentration/initial CB-NP concentration) × 100.
3.4. The influence of CB-NP aggregation during deposition process Previous studies found that the aggregation of hydrophobic nanoparticles had a negative impact on flotation performance (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012). To understand the reduction in malachite floatability in the presence of high CB-NP concentrations (3–12 kg/ton), assuming that the aggregation of CB-NPs occurs at higher concentrations, we performed DLS analysis to determine the aggregated sizes of CB-NPs at CB-NP concentrations of 0.5–12 kg/ton, and the results are presented in Fig. 6. Overall, the trends illustrated that CB-NP aggregation does not occur at higher CBNP concentrations. Therefore, in our system, we can eliminate CB-NP aggregation during deposition process as a cause of the decreased malachite floatability in the presence of high CB-NP concentrations. Instead, we focused on the suspended CB-NP concentration in our flotation system. 3.5. Effects of the suspended CB-NP concentration on malachite floatability As Table 1 indicates, the amounts of CB-NPs deposited onto the malachite surface increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). In addition, at CB-NP concentrations exceeding 2 kg/ton, CB-NPs no longer deposited onto the malachite surface, indicating that approximately 1.5 mg/g is the level of
Fig. 5. Contact angles for malachite after carboxyl-functionalized carbon black nanoparticle (CB-NP) deposition at initial concentrations of 0.5 (a), 2 (b), 6 (c), and 12 kg/ton (d) at pH 7. The contact angle was measured via the bubble captive method.
on the malachite surface and the contact angle of CB-NP–deposited malachite were measured, and the results are presented in Table 1 and Fig. 5, respectively. In agreement with the hypothesis, the deposited amounts of CB-NPs on the malachite surface increased with increasing CB-NP concentration in the low range (0.5–2 kg/ton) (Table 1). This is also well supported by previous studies that reported that the deposited amounts of nanoparticles on the mineral surface increased with increasing nanoparticle concentrations (Yang et al., 2011; Yang and Pelton, 2011; Yang et al., 2012; Yang et al., 2013b). This observation was supported further by the contact angles for the malachite surface after CB-NP deposition (Fig. 5). The contact angles were approximately 42.6° and 65.2° for 0.5 and 2 kg/ton CB-NPs, respectively. The higher contact angle for 2 kg/ton CB-NPs indicated that higher amounts of CBNPs were deposited on the malachite surface, and thus, they rendered the malachite surface more hydrophobic. Accordingly, malachite floatability increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). Meanwhile, malachite floatability decreased at CB-NP concentrations exceeding 2 kg/ton (3–12 kg/ton). To understand this interesting
Fig. 6. Change in the dynamic light scattering diameter of carboxyl-functionalized carbon black nanoparticles (CB-NPs) while being deposited to malachite with initial CB-NP concentrations of 0.5–12 kg/ton. All the experiments were performed at pH 7. 23
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Fig. 7. Microflotation measurements of malachite as a function of the carboxylfunctionalized carbon black nanoparticle (CB-NP) concentration (0.5–12 kg/ ton) using a Hallimond with (circle) and without (square) filtration process. The flotation time was 20 min. All the experiments were performed at pH 7. The data obtained without filtration was obtained from those in Fig. 4.
saturation for 2 kg/ton CB-NPs. More noteworthy is that the ratios of the deposited CB-NP concentration to the initially added CB-NP concentration were 99, 99.5, 74.75, 50.03, 38.43, 25.17, 18.95, and 12.37% for 0.5, 1, 2, 3, 4, 6, 8, and 12 kg/ton CB-NPs, respectively (Table 1). These trends indicate that most of the initially added CB-NPs are deposited on the malachite surface at lower concentrations, resulting in a low suspended CB-NP concentration at initial levels of less than 2 kg/ton. On the contrary, the suspended CB-NP levels continuously increased with increasing CB-NP concentrations exceeding 2 kg/ton. Based on these trends, we questioned whether the presence of suspended CB-NPs reduces malachite floatability. Interestingly, the suspended CB-NP concentration strongly affected the floatability of malachite (Fig. 7). Quantitatively, after the filtration process, the malachite floatability values were approximately 43.85, 56.11, 81.65, 84.55, 81.55, 80.36, 81.85, and 85.53% in the presence of 0.5, 1, 2, 3, 4, 6, 8, and 12 kg/ton CB-NPs, respectively. For low CB-NP concentrations (0.5–2 kg/ton), the difference in malachite floatability was negligible regardless of the use of filtration because of almost nonexistent suspended CB-NPs or increased amount of CB-NPs deposited. Meanwhile, malachite floatability was relatively higher after filtration compared with the findings without filtration for high CB-NP concentrations (3–12 kg/ton), indicating that suspended CB-NPs negatively affected malachite floatability and their levels continuously increased with increasing CB-NP concentrations. Although the experimental evidence suggests that suspended CB-NPs are an obvious cause of the lower malachite floatability, the exact mechanism is not clear. Hence, to investigate the distinct cause of the different malachite floatability according to the presence of suspended CB-NPs, additional flotation experiments were performed.
Fig. 8. Malachite floatability as a function of the carboxyl-functionalized carbon black nanoparticle (CB-NP) concentration over the ranges of 0.5–2 (a) and 4–12 kg/ton (b) with a flotation time of 5–60 min.
53.04, and 81.93% in the presence of 0.5, 1, and 2 kg/ton CB-NPs, respectively, and at a flotation time of 60 min, malachite floatability was approximately 42.94, 57.4, and 84.38% in the presence of 0.5, 1, and 2 kg/ton CB-NPs, respectively. Eventually, it was possible to directly confirm that the interaction between bubbles and CB-NP–deposited malachite was mostly completed after 20 min of flotation in the presence of 0.5–2 kg/ton CB-NPs. Meanwhile, Fig. 8 (b) shows differences in the experimental trends of malachite floatability according to the flotation time in the presence of 4 and 12 kg/ton CB-NPs. Specifically, after 20 min of flotation, the malachite floatability values were 45.42 and 29.43% in the presence of 4 and 12 kg/ton CB-NPs, respectively. After 60 min of flotation, the values were approximately 69.82 and 57.36% in the presence of 4 and 12 kg/ton CB-NPs, respectively. The experimental kinetic results suggested that the kinetic rate of malachite floatability was relatively slower in the presence of 4–12 kg/ton CB-NPs compared with those in the presence of 0.5–2 kg/ton CB-NPs, and malachite floatability increased with increasing flotation time. As mentioned in Table 1, at CB-NP concentrations exceeding 2 kg/ton, the amounts of suspended CB-NPs continuously increased, suggesting that suspended CB-NPs inhibit the attachment of malachite to bubbles. We propose that the increase in the concentration of suspended CB-NPs likely induced to form CB-NP aggregates under severe agitation (∼140 rpm), which more easily allow them to attach onto the bubble surface. In order to confirm this hypothesis, we measured the size of suspended CB-NPs using a single particle optical sizing system
3.6. Effect of the flotation time To further clarify the cause of the different malachite floatability based on the presence of suspended CB-NPs, flotation experiments were performed using different flotation time and the results are presented in Fig. 8. Overall, Fig. 8 (a) shows that malachite floatability rapidly increased with increasing flotation times up to 20 min, whereas further increases of the flotation time had no significant effect on the kinetic rate of malachite floatability in the presence of 0.5–2 kg/ton CB-NPs. Maximum malachite floatability (approximately 80%) was observed in the presence of 2 kg/ton CB-NPs. Specifically, at a flotation time of 20 min, the malachite floatability values were calculated as 39.13, 24
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(AccuSizerTM 780/SIS, PSS-NICOMP, USA) after 60 min agitation of CBNPs at 140 rpm in the Hallimond tube without nitrogen injection. The test was conducted with the sample having the concentration of 10 kg/ ton, which corresponds to initial CB-NP concentration of 12 kg/ton (Table 1). The volume-based median diameter (d50) was determined to be about 27.3 μm, which clearly supports the formation of CB-NP aggregates. In addition, it is well known that more hydrophobic materials have faster kinetic rates with bubbles than less hydrophobic materials (Somasundaran and Lin, 1973; Xia et al., 2015). Hence, the presence of suspended CB-NPs is likely to negatively affect malachite floatability because suspended CB-NP aggregates, which have greater surface hydrophobicity, had a relatively faster kinetic rate with bubbles than CBNP–deposited malachite, which is less hydrophobic.
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4. Conclusion In this study, the influence of the concentration of CB-NPs as collectors in malachite flotation was systematically investigated. The key experimental findings from our study were as follows: (1) Electrophoretic mobility measurements illustrated that malachite is positively charged at certain pH values, whereas the nanoparticles are negatively charged at these values, indicating that the electrostatic interaction between malachite and CB-NPs was favorable, resulting in CB-NP deposition onto the malachite surface. (2) The amount of CB-NPs deposited on malachite surface increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). The maximum amount of deposited CB-NPs was approximately 1.5 mg/g at 2 kg/ton CB-NPs, and at higher concentrations, CB-NPs no longer deposited onto the malachite surface. The trends for the amounts of deposited CB-NPs on the malachite surface were highly consistent with the contact angle measurements. These two experimental results indicate that the suspended CB-NP concentration continuously increased with increasing CB-NP concentrations exceeding 2 kg/ton. (3) We found that the malachite floatability increased with increasing CB-NP concentrations over the low range (0.5–2 kg/ton). The amounts of CB-NPs deposited onto malachite surface increased as the CB-NP concentration increased, resulting in increased hydrophobicity of the malachite surface and enhanced malachite floatability. Meanwhile, malachite floatability sharply decreased at CBNP concentrations exceeding 2 kg/ton. Our various experimental results including a filtration process and different flotation times suggested that this unusual flotation behavior of malachite can be explained by the presence of suspended CB-NP aggregates, which reduced the kinetic rates of attachment between CB-NP–deposited malachite and bubbles. References Abarca, C., Yang, S., Pelton, R.H., 2015. Towards high throughput screening of nanoparticle flotation collectors. J. Colloid Interface Sci. 460, 97–104. Cao, Z., Zhong, H., Liu, G., Zhao, S., 2009. Techniques of copper recovery from Mexican copper oxide ore. Mining Sci. Technol. (China) 19, 45–48. Choi, J., Kim, W., Chae, W., Kim, S.B., Kim, H., 2012. Electrostatically controlled enrichment of lepidolite via flotation. Mater. Trans. 53, 2191–2194. Choi, J., Hong, J., Park, K., Kim, G., Han, Y., Kim, S., Kim, H., 2014. Role of chain length and type on the adsorption behavior of cationic surfactants and the silica floatability. Mater. Trans. 55, 1344–1349. Choi, J., Lee, E., Choi, S.Q., Lee, S., Han, Y., Kim, H., 2016a. Arsenic removal from contaminated soils for recycling via oil agglomerate flotation. Chem. Eng. J. 285, 207–217. Choi, J., Choi, S.Q., Park, K., Han, Y., Kim, H., 2016b. Flotation behavior of malachite in mono- and di-valent salt solutions using sodium oleate as a collector. Int. J. Miner. Process. 146, 38–45. Choi, J., Kim, G., Choi, S., Kim, K.H., Han, Y., Bradford, S., Choi, S., Kim, H., 2018. Application of depletion attraction in mineral flotation: I. Theory. Minerals 8, 451. Dong, X., Price, M., Dai, Z., Xu, M., Pelton, R., 2017. Mineral-mineral particle collisions during flotation remove adsorbed nanoparticle flotation collectors. J. Colloid Interface Sci. 504, 178–185.
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