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Effect of ultrasound on bubble-particle interaction in quartz-amine flotation system
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Effect of ultrasound on bubble-particle interaction in quartz-amine flotation system ⁎
C. Gungorena, O. Ozdemira, X. Wangb, S.G. Ozkana, , J.D. Millerb a b
Istanbul University-Cerrahpasa Engineering Faculty Mining Engineering Department, 34320, Avcilar, Istanbul, Turkey Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, Salt Lake City, UT 84112, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasound Flotation Quartz Amine (DAH) Bubble-particle interactions
In this study, the effect of ultrasound (US) on the quartz-amine flotation system was investigated in detail by considering various surface chemistry techniques. The effect of ultrasound on particle size, shape factor, and surface roughness were characterized by using Brunauer-Emmett-Teller (BET) surface area measurements and scanning electron microscopy (SEM) analyses. The contact angle and bubble-particle attachment time, as well as adsorption density measurements was carried out to evaluate the effect of ultrasound on quartz surface wetting ability. In addition, atomic force microscopy (AFM) analyses were conducted, and finally micro-flotation studies were performed. As a result, it was found that the micro-flotation recovery at 2 × 10−5 M dodecyl amine hydrochloride (DAH) concentration increased from 45.45% to 63.64% with 30 W ultrasonic application at conditioning step. However, the micro-flotation recovery decreased to 37.50% when the ultrasonic power increased to 150 W. The results showed some effect of ultrasound on particle size, particle shape, and surface roughness in some extent. The increase in the contact angle and the decrease in the bubble-particle attachment time were observed. A slightly high adsorption density was measured. All these show a positive effect of ultrasound on quartz flotation with amine as a collector.
1. Introduction Ultrasound is a sound wave above the human perception frequency limits, and propagates in a medium by wave series consisted of loosening and compressing phases. When a sufficient negative pressure is created in a loosening phase, cavitation bubbles occur. The size of the nascent cavitation bubbles is around 100 µm, and they are grown during several wave circles to a resonant size by taking gases and/or vapors inside. When a bubble grows too much, it cannot adsorb energy any more. Moreover, the surrounding liquid penetrates into the bubble, and the bubble collapses during the compression phase [46,31]. Compressed gases and vapors in a collapsing bubble generate heat, which is much faster than heat transportation during cavitation. Therefore, temperature of the vicinity of the collapsed bubble significantly increases up to 5000 °C; however it lasts only for a short time [48]. If a cavitation bubble collapses in a homogenous liquid, the bubble remains spherical during the collapsing due to the symmetrical pressure distribution around the bubble. On the other hand, the existence of a solid near a collapsing cavitation bubble changes the pressure distribution around the bubble, and therefore, the bubble collapses
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asymmetrically which causes a liquid jet (micro-jet) with 400–500 km/ h velocity towards the solid [46,7]. To benefit these extreme features of acoustic cavitation, ultrasound has been used for improving the flotation recovery by various researchers [12,17,42,3,13,21,58,39,41,16,40]. Although these studies have generally been focused on the practical results of ultrasound on flotation, the effect of ultrasound on mineral surface, mineral-reagent, and mineral-bubble interaction have not been studied in detail so far. Therefore, there is a need more fundamental studies to investigate the effect of ultrasound on the flotation process. In this context, this study was aimed to investigate the effect of ultrasound on quartz-amine flotation by considering characterization and surface chemistry aspects of minerals and bubbles in the presence of ultrasound. Within this scope, first, particle size and shape factor, surface roughness, Brunauer-Emmett-Teller (BET) surface area measurements, and scanning electron microscopy (SEM) analyses were carried out with and without ultrasonic treatment. Then, the contact angle, bubble-particle attachment time, adsorption density measurements, atomic force microscopy (AFM) analyses were carried out, and finally micro-flotation studies were conducted under various ultrasonic conditions.
Corresponding author. E-mail address: [email protected] (S.G. Ozkan).
https://doi.org/10.1016/j.ultsonch.2018.12.023 Received 23 July 2018; Received in revised form 12 December 2018; Accepted 13 December 2018 1350-4177/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Gungoren, C., Ultrasonics - Sonochemistry, https://doi.org/10.1016/j.ultsonch.2018.12.023
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Fig. 1. Results of the XRD analysis of the analytical grade quartz.
2. Materials and methods
amine hydrochloride (DAH) from Arcos Organics, USA was used as a collector in the micro-flotation experiments.
2.1. Materials 2.2. Methods
In the experimental studies, three different quartz samples were used according to their suitability for the experimental conditions. The first and main sample of the study was analytical grade quartz at 1.2 × 0.6 mm particle size (Carl Roth, Germany). The chemical analyses of the sample showed that the sample contained ∼98% SiO2, ∼1.1% Al2O3, ∼0.04% TiO2, and ∼0.02% Fe2O3. As seen in Fig. 1, the mineralogical analysis of the sample by XRD indicated that quartz mineral was only the main mineral. This sample was first ground using a close circuit automatic agate mortar, and wet sieved to obtain the required particle size fractions (−500, 212 × 150, 150 × 75, and −38 µm) for each experiment. The fractions were then cleaned with analytical grade 2.5% H2SO4 by volume and 2.5% NaOH by weight, and rinsed with de-ionized (DI) water (18 MΩ cm) (Milli-Q plus Millipore Ultra Pure Water system, Millipore Ltd, Molshem, France) to remove possible surface contaminants [23]. Additionally, the surface tension of the DI water was measured to be around 72.0 mN/m at 23 °C. The ground and cleaned analytical grade quartz was used for the micro-flotation tests, particle size and shape, surface roughness, BET surface area, bubble-particle attachment time measurements as well as adsorption density and SEM analyses. The second sample used in the study was a quartz mineral sample with ∼10 cm feret’s diameter, which was used for the contact angle measurements after cutting and polishing. The flat surfaced samples were cleaned with acetone, ethanol, DI water, and finally by a plasma cleaner. The third sample was a flat surfaced glass material for laboratory use (Baxter Scientific, USA). These SiO2 based flat surfaced glass samples were used in the AFM analyses in the place of quartz mineral particles to obtain clear AFM images. The glass samples were cut and cleaned with ammonium peroxide mixture (APM) which is a combination of NH4OH, H2O2, and H2O in the ratios of 1:1:5, and then rinsed with DI water [44]. The schematic structure of (a) crystalline SiO2 (quartz) and (b) amorphous SiO2 (quartz glass) is shown in Fig. 2. Moreover, analytical grade (99% of CH3(CH2)11NH2·HCl) dodecyl
2.2.1. Micro-flotation experiments The effect of ultrasound on the floatability of quartz was investigated via micro-flotation studies. The micro-flotation tests were carried out by a glass micro-flotation cell (30 × 210 mm) with 200 mL volume and a frit with 10–16 µm pore size. First, 2 g of 212 × 150 µm quartz samples were conditioned at various concentrations of DAH (1 × 10−6, 1 × 10−5, 2 × 10−5, 3 × 10−5, 5 × 10−5, 1 × 10−4, 5 × 10−4, and 1 × 10−3 M) at 1% solid ratio and at 500 rpm stirring speed in the absence of ultrasound to find the optimum DAH concentration for the further micro-flotation experiments. The micro-flotation studies were conducted using N2 gas with 50 mL/min flow rate for 2 min. All experiments were performed at room temperature (23 °C) and natural pH (6.0–6.5) to eliminate the effect of pH and observe the effect of ultrasound. The flotation products were dewatered with a black ribbon filter paper, and dried at 105 °C in a drying oven. The micro-flotation recoveries were calculated using the product weights. 2.2.2. Conditioning and ultrasonic treatment Within the scope of this study, conditioning of quartz in DAH solution was conducted by magnetic stirring. Meanwhile, the ultrasonic treatment was carried out by using an ultrasonic probe in addition to magnetic stirring. An ultrasonic probe (Bandelin HD 3200, Germany) at 20 kHz constant frequency, of which power level arrangeable from 30 to 150 W, was employed as an ultrasound source. The use of 30 W ultrasonic power was preferred during the surface chemistry studies because of obtaining satisfactory micro-flotation results at this ultrasonic power, and the well-known fact that ultrasound increases the ambient temperature according to the ultrasonic power and application time [47]. Therefore, ultrasound was used at minimum available ultrasonic power (30 W) to avoid over-heating the suspension. In the conditions used in this study, the ambient temperature increased from 23 °C to 31.9 °C at 10 min. Also, the pH of the suspension did not fluctuate from natural pH significantly at 30 W ultrasonic power [15,16]. 2
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Fig. 2. Schematic structure of (a) crystalline SiO2 (quartz) and (b) amorphous SiO2 (quartz glass) [4].
is reported in the literature that colloidal precipitation and micelle formation is not expected in these conditions [29]. Although the contact angle has an important role on hydrophobicity, it is not adequate for explaining the bubble-particle attachment in flotation, alone. Because, while contact angle is a static measurement method, mineral particles and bubbles are always in motion and contact for a limited time during a flotation process. In some cases, the flotation recovery can increase while the contact angle of the particles is constant. Therefore, the attachment possibility in a limited time has to be determined. For this purpose, bubble-particle attachment time measurement devices are used [34,2,11,37,52]. In our study, the bubble-particle attachment time was measured using MCT-100 electronic bubble-particle attachment time device. The bubble-particle attachment time experiments were carried out in 5 mL suspension with quartz samples at 150 × 75 µm particle size and with a bubble at 3 mm diameter, approximately. The attachment time adjusted via electronic control unit between 1 and 200 ms. The possible bubbleparticle attachments were observed through a lens at the bubble surface for a particular contact time. The experiments were repeated 20 times at different positions on the particle bed. The tests resulted with “attachment” and “no attachment” were recorded to calculate the attachment percentage for a particular contact time. The attachment time was determined by the contact time at which 50% of the observations results in attachment [38].
2.2.3. Sample characterization In the concept of the particle size analyses, the particle size of the quartz samples under 500 µm was measured using CILAS 1090 laser particle size analyzer in liquid mode. Furthermore, to determine the shape properties of quartz samples, particle shape analyses of the 150 × 75 µm quartz samples were performed using microscope photographs with “×3” magnification. The shape factors (roundness, flatness, elongation ratio, and relative width) were calculated by Leica QWin image analyzing software using the equations previously given by several researchers [14,20,27,19]. The surface roughness of the 150 × 75 µm quartz samples was measured using Zeiss Axio CSM 700 optical profilometer. The surface area of the samples was determined by multipoint BET method using Quantachrome Quadrasorb SI BET surface area measurement device. To observe the effect of ultrasound more effectively, under 38 µm sized quartz particles were used owing to having larger surface area than larger particles. SEM surface images of the samples were also taken using JEOL SEM7100 electron microscope at “×1200” magnification rate. A covering process with gold (Au) and palladium (Pd) was carried out to quartz particles to obtain a conductive surface before the SEM analyses.
2.2.4. Contact angle and bubble-particle attachment time measurements The contact angle of the quartz surface was measured with a goniometer (Ramé-hart, USA) by “captive bubble” method using flat surfaced quartz samples. First, a rectangular glass container was filled with DI water or DAH solution. Then, the quartz sample was placed on two glass stands facing the polished surface downwards. An air bubble was formed using a micro-syringe with a hook-shaped needle to attach on the surface of the quartz. Finally, the angle between the attached bubble and the quartz surface was measured via the lens with a goniometer. The bubbles could not attach to the quartz surface in 1 × 10−5 and 1 × 10−4 M DAH concentrations. Hence, the contact angle experiments were carried out at 1 × 10−3 M DAH concentration. 1 × 10−3 M DAH concentration at natural pH of the quartz (pH 6) is under the critic micelle concentration (CMC), which is 1.3 × 10−2 M. It
2.2.5. Adsorption experiments To determine the adsorption density of DAH on quartz surface, adsorption density analyses were performed using depletion method with Shimadzu TOC-VCPH/CPN total carbon (TC) analyzing device. The adsorption density was calculated using the concentration of the residual DAH solution and BET surface area of the quartz samples. Various concentration of DAH solutions (1 × 10−5, 5 × 10−5, and 1 × 10−4 M DAH) were prepared using high performance liquid chromatography (HPLC) grade water. First, the initial TC values of DAH solutions were measured. Then, the adsorption process was carried out in the absence and presence of the ultrasound using −38 µm quartz. 3
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Fig. 3. Experimental flowsheet.
The overall experimental flowsheet of this study is shown in Fig. 3.
3. Results and discussion 3.1. Micro-flotation Long chain alkyl amines, especially dodecylamine, are widely used in quartz flotation as a collector [45,8]. Even short chain amines can act both as a frother and as a collector [25]. Ren et al. [43] used alkyl ether amine (Flotigam EDA-C from Clariant) was used as a collector for the flotation of quartz. They obtained a flotation recovery above 80% with 20 mg/L EDA-C concentration. Li et al. [30] reported that the flotation recovery of quartz at natural pH reached to 90% in the presence of 300 mg/L amine. Kowalczuk [25] reported that quartz did not float without collector neither with Hallimond tube nor with mechanical flotation machine. A low flotation recovery (∼4%) was result from the entrainment [24,55]. However, it started to float in the presence of short chain alkyl amine (hexylamine) at pH 10–11. Additionally, since micro or nano bubbles can be used for enhancing the quartz flotation [9], ultrasonic treatment can improve the flotation by these smaller bubbles due to cavitation effect. The micro-flotation recoveries at different DAH concentrations without ultrasonic treatment are given in Fig. 4. As shown in Fig. 4, a 45.45% micro-flotation recovery was obtained at 2 × 10−5 M DAH concentration, and this concentration was chosen for further ultrasonic experiments to observe the possible positive or negative effects of the ultrasound. Fig. 5 shows the comparison of the micro-flotation test results with and without ultrasound at various ultrasonic powers. As seen in Fig. 5, the micro-flotation recovery increased to 63.64% and 65.57% with the use of ultrasound at 30 W and 90 W, respectively. However, it decreased to 37.50% at 150 W ultrasonic power. Similarly, in literature, there are also several studies which the negative effect of the ultrasound was observed on flotation. For instance, Videla et al. [53] reported that although the flotation recovery increased with the use of ultrasound in both of the conditioning and flotation phases, it decreased when ultrasound was applied only in the conditioning and flotation phases. Cao et al. [10] reported that ultrasonic power and application time are important parameters for the de-oxidation of the surfaces. However, its influence is related to the application power and time. Vargas-
Fig. 4. Quartz flotation results as a function of DAH concentration without ultrasound (212 × 150 µm particle size, pH 6.0–6.5, 1% solid ratio).
After adsorption process, a solid-liquid separation by centrifugation at 3300 rpm for 2 min was carried out. The TC values of the residual solutions were measured. The adsorption density was calculated using Eq. (1) [36].
Γ=
(Ci − Cr )·V m·S
(1)
where, Ci and Cr represent the initial and residual concentrations in mol/L, V is the volume of the solution in L, m is the amount of solid in g, S is the BET surface area of quartz in m2/g, and Γ is the adsorption density in mol/m2. 2.2.6. AFM microscope analysis To observe the effect of the ultrasound on the material surface in nanometer scale the topographical map of the flat surfaced glass particles was obtained using an AFM microscope (Multimode, Digital Instruments) without and with ultrasound in the absence and presence of DAH [6,33]. In the AFM experiments, the glass particles were cut from a bigger sample and attached to the magnetic disk of AFM. The measurements were performed via the standard cantilever using the soft-contact imaging technique. 4
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et al. [16] reported that the higher ultrasonic power caused a significant increase in the pulp temperature. In their study, the pulp temperature increased from 23 °C to 31.9, 60.5, and 75.1 °C at 30, 90, and 150 W, respectively. For this reason, 30 W ultrasonic power was chosen for the further studies in terms of increasing the micro-flotation recovery and preventing overheating of the pulp. The micro-flotation results clearly showed that the flotation recovery significantly increased with ultrasound during the conditioning. In order to understand the ultrasonic effect on the flotation of quartz, the following sections ultrasonic effect on the particle size, particle shape, surface morphology, and amine adsorption at the quartz surface will be discussed.
3.2. Effect of conditioning with ultrasound Particle size has a significant importance in flotation. Coarse particles of different minerals is more sensitive to the chemical medium, in comparison with finer particles. On the other hand, finer particles consume more collector due to their large surface areas [54]. The essential morphological characteristics of the particles are size, shape, and surface roughness. These parameters strongly affect the bubbleparticle interactions. Roughness and surface heterogeneity can either increase or decrease the work of bubble-particle adhesion and the probability of particle detachment from the air bubble [35]. Miller et al. [32] investigated the influence of film roughness on the wetting properties of vacuum-deposited polytetrafluroethylene (PTFE) thin films and a contact angle correlation with surface roughness was determined. It is also reported that high contact angles were observed at the surface because of surface roughness. Yekeler et al. [56] reported some correlations between the shape properties, surface roughness values, and the wettability characteristics of talc mineral. The hydrophobicity increases with the increasing surface roughness of talc particles. On the other hand, Ulusoy and Yekeler [49] investigated the relationship between the surface roughness and wettability of calcite, barite, talc, and quartz. Their results indicated that the degree of hydrophobicity of these minerals increased with the decreasing surface roughness. Krasowska and Malysa [26] stated that roughness of a hydrophobic solid surface had a crucial importance for kinetics of the bubble attachment. Vaziri Hassas et al. [51] reported that the increase in surface roughness improved the flotation recovery, contact angle, and bubble-particle attachment. Guven et al. [19] studied the morphology and floatability of quartz particles, and their results indicated that the blasted quartz particles with more angular and rougher surfaces gave better floatability compared to the un-blasted quartz particles. Ahmed [1] indicated that the surface roughness was responsible for the detachment process of particle and the final flotation recovery more than its shape. Furthermore, Yekeler et al. [56] reported that the reactivity of a solidwas clearly affected by its surface roughness. Ren et al. [43] investigated the quartz surface by AFM, and reported that the bare surface of quartz had very
Fig. 5. Micro-flotation test results with ultrasound in respect to ultrasonic power (212 × 150 µm particle size, pH 6.0–6.5, 1% solid ratio).
Hernandez et al. [50] showed that the highest cumulative Cu recovery was obtained with the minimum ultrasound power (5 W). Cilek and Ozgen [13] reported a small decrease in pyrite flotation recovery with the use of ultrasound. However, the information on the reasons of the negative effect of ultrasound on flotation is quite limited. Ultrasound can damage the physical bonds between the collector and mineral, and therefore it has a detrimental effect on flotation [18]. In addition, ultrasound increases the temperature of the medium [46,47]. Since, the ambient temperature is a dominant factor for reagent adsorption, it significantly affects the flotation process [16]. Furthermore, the gases and vapor inside a cavitation bubble can produce %H, %OH free radicals, through thermal decomposition, which can undergo secondary reactions resulting the formation of H2O2 and %HO2. Both %OH and H2O2 are quite reactive, and have a strong oxidation ability. Moreover, H2O2 is very unstable, and discharges nascent oxygen quickly. Therefore, the oxygen content of the medium increases after ultrasonic treatment. The nascent oxygen is also very active, and react with the mineral surface, and thus increasing its hydrophilicity [21,5]. In our case, it is thought that the negative effect on flotation is dependent upon the chaotic medium in the conditioning medium created by high-power ultrasound. Ultrasound creates too many turbulent acoustic flows and micro-jets in a liquid media. The increase in the number and magnitude of these factors might create a chaotic medium in conditioning which had a detrimental effect on the adsorption of collector molecules on mineral surfaces. Therefore, it is important to note that the application time and power of ultrasound have a crucial effect on the flotation. Gungoren
Fig. 6. Particle shape factor (150 × 75 µm), roughness (150 × 75 µm), BET surface area (−38 µm), d50, and d80 particle size (−500 µm) results with and without ultrasound (30 W). 5
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Fig. 7. SEM images of quartz samples (a) without and (b) with ultrasound (30 W) at ×1200 magnification rate (150 × 75 µm).
Fig. 8. Bubble-particle attachment time experiment results in the absence of presence of DAH with and without ultrasound (30 W).
Fig. 9. Contact angle, bubble-particle attachment time (150 × 75 µm), and adsorption density (−38 µm) results in the presence of DAH with and without ultrasound (30 W).
the particle size decreased a little. Additionally, the roundness of the particles was determined as 0.63 and 0.64 with and without the ultrasound, while the flatness decreased from 1.62 to 1.59, respectively. At the same time, Fig. 6 also indicated that ultrasound slightly increased the roughness of quartz from 3.70 to 4.24 µm, and increased the BET surface area from 1.020 to 1.075, which was not a remarkable change. Finally, the SEM images of the quartz samples with and without ultrasound indicated that there was no significant change on the particle surfaces as seen in Fig. 7.
low roughness (2.1 nm) similar to the glass surface used in this study. Fig. 6 shows the results of determination of the particle shape factor, roughness, BET surface area, d50 and d80 particle size distribution with and without ultrasound. Fig. 6 indicates that the ultrasound slightly changed the particle size and shape factors. The d80 size of the quartz particles decreased from 255 µm to 200 µm, while the d50 size decreased from 126 µm to 118 µm, respectively. These results indicated that the ultrasound somehow caused a good dispersion of the suspension as well as removing the fine particles on coarse particles; hence, 6
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Fig. 10. Two-dimensional dry AFM images of the flat surfaced glass samples. (a) Without ultrasound and DAH, (b) with ultrasound (30 W), without DAH, (c) without ultrasound with DAH, and (d) with ultrasound (30 W) and DAH (1 × 10−5 M DAH).
adsorbed layer was similar at different collector (EDA-C) concentrations. The flat quartz surface was very hydrophilic with absence of DAH in DI water. On the other hand, the contact angle of quartz was determined as 52° in the presence of 1 × 10−3 M DAH dosage without the ultrasound, and increased to 62° with the ultrasound. In addition, bubble-particle attachment time measurements and adsorption density experiments were also conducted, and the results are shown in Figs. 8 and 9. According to Fig. 8, the bubbles could not attach to the quartz surface with the absence of DAH in accordance with the contact angle measurements. These results showed that the hydrophilic character of quartz did not change with ultrasonic treatment. On the other hand, the attachment time of the quartz samples conditioned in 1 × 10−5 M DAH solution without ultrasound was 75 ms, and decreased to 50 ms with the ultrasound. Fig. 9 indicates a very slight influence of ultrasound on the adsorption density of DAH at quartz surface. The adsorption densities of DAH at the surface of quartz particles in 1 × 10−5 M DAH solution were 8.11 × 10−7 mol/m2 and 8.05 × 10−7 mol/m2 without and with ultrasound, respectively. In flotation process, when a particle encounters a bubble, the process of film thinning and liquid drainage occurs before attachment is happened. The shape, sharpness, and pointedness of the particles have
The results from the conditioning with ultrasound showed the effect on particle size, surface roughness, and particle shape in some extent. As mentioned in the introduction section, near a solid surface cavity collapses asymmetrically, and create high-speed microjets of liquid. The impact of microjets on solid surface could be very strong. It is reasonably expected such effect due to ultrasonic effect. 3.3. Contact angle, bubble-particle attachment time, and adsorption The contact angle of quartz can be either zero or greater than zero depending on the water adsorption on its surface. Zero contact angle occurs when the surface of quartz is totally hydroxylated [28]. Kowalczuk [25] reported that hexylamine did not change the hydrophobicity of quartz (between 15° and 25°). On the other hand, the flotometric contact angle of quartz increased with hexylamine concentration from 0° to 36°, and remained constant for higher concentrations. It means that the presence of hexylamine effects not only flotation of quartz but also “uncovers” its natural (flotometric) hydrophobicity. Yoon and Yordan [57] reported that the induction time for the quartz-DAH system was the lowest at pH 10.5 and decreased with amine concentration. Ren et al. [43] investigated the adsorption of Clariant EDA-C via AFM and reported that the morphology of the 7
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an important role on the rate of film thinning and rupture, and hence the overall flotation rate. It is also known from literature that the particles with sharper edges and rougher surfaces attached easily to the bubbles than smooth ones because of the increasing in the rate of film thinning and the rupture [22,19]. In summary, the ultrasonic effect on particle surface roughness might facilitate the contact angle and bubble-particle attachment. Slightly increased adsorption density of DAH at quartz particle surface with ultrasound might be due to the cavitation bubbles. As reported by Wang and Miller [59] that the adsorption of amine increased significantly in the presence of air bubbles by transferring amine molecules to the quartz/air bubble interface from air bubble/amine solution interface during bubble attachment.
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Kowalczuk, Flotation and hydrophobicity of quartz in the presence of hexylamine, Int. J. Miner. Process. 140 (2015) 66–71. [26] M. Krasowska, K. Malysa, Kinetics of bubble collision and attachment to hydrophobic solids: I. Effect of surface roughness, Int. J. Miner. Process. 81 (4) (2007) 205–216. [27] H. Kursun, U. Ulusoy, Influence of shape characteristics of talc mineral on the column flotation behavior, Int. J. Miner. Process. 78 (4) (2006) 262–268. [28] R.N. Lamb, D.N. Furlong, Controlled wettability of quartz surfaces, J. Chem. Soc.Faraday Trans. 1 78 (1982) 61–73. [29] J.S. Laskowski, Flotation of potash ores reagents for better metallurgy, Soc. Mining Metall. Explor. (1994) 225–244. [30] X. Li, Q. Zhang, B. Hou, J. Ye, S. Mao, X. Li, Flotation separation of quartz from collophane using an amine collector and its adsorption mechanisms, Powder Technol. 318 (2017) 224–229. [31] T.J. Mason, Ultrasound in synthetic organic chemistry, Chem. Soc. Rev. 26 (6) (1997) 443–451. [32] J.D. Miller, S. Veeramasuneni, J. Drelich, M.R. Yalamanchili, Effect of roughness as determined by atomic force microscopy on the wetting properties of PTFE thin, Polym. Eng. Sci. 36 (14) (1996) 1849–1855. [33] J.D. Miller, S. Veeramasuneni, J. Drelich, M.R. Yalamanchili, Effect of roughness as determined by atomic force microscopy on the wetting properties of PTFE thin films, Polym. Eng. Sci. 36 (14) (1996) 1849–1855. [34] A.V. Nguyen, J. Ralston, H.J. Schulze, On modelling of bubble–particle attachment probability in flotation, Int. J. Miner. Process. 53 (1998) 225–249. [35] A. Nikolaev, Flotation kinetic model with respect to particle heterogeneity and roughness, Int. J. Miner. Process. 155 (2016) 74–82. [36] O. Ozdemir, M. Cinar, E. Sabah, F. Arslan, M.S. Celik, Adsorption of anionic surfactants onto sepiolite, J. Hazard. Mater. 147 (1–2) (2007) 625–632. [37] O. Ozdemir, H. Du, S.I. Karakashev, A.V. Nguyen, M.S. Celik, J.D. Miller, Understanding the role of ion interactions in soluble salt flotation with
3.4. AFM microscope analysis The two-dimensional AFM images of the flat surfaced quartz glass samples without and with ultrasound and DAH were taken, and the results are given in Fig. 10. The color bar in Fig. 10 shows the height of surfaces which darkest and lightest colors indicate lower and higher parts of the surface, respectively. Fig. 10(a) and (b) show the effect of ultrasound on the glass surfaces. As seen from Fig. 10(a), (b) that the distribution of lighter and darker areas in the AFM images were similar to each other without (a) and with (b) of ultrasound in the absence of DAH. This clearly proved that the ultrasound showed no significant change on the glass surfaces. Meanwhile, in order to see the effect of ultrasound on the adsorption of DAH molecules on the surfaces, the AFM analyses were carried out in the presence of DAH without/with the ultrasound. It is seen from Fig. 10(c) that there is a light-color zone while the other parts of the image is darker. This means that DAH molecules accumulated at some parts of the surface during the adsorption. When the ultrasound was applied during the adsorption process, the DAH molecules adsorbed on the glass surface more orderly as seen from the equal distribution of lighter and darker parts in Fig. 10(d). The results imply that the ultrasound may affect on the organization of adsorbed amine molecules at the quartz surface, which also will affect the bubble-particle attachment. 4. Conclusions The effect of ultrasound on the quartz flotation was studied. The results indicated that during the conditioning with ultrasound the quartz flotation recovery increased significantly. A decreased quartz flotation recovery was obtained at a higher ultrasonic power. The higher ultrasonic power may interrupt the adsorption between amine molecules and surface, which is due to the physical bonds. An increased contact angle and shorter bubble-particle attachment time were observed with ultrasonic treatment. The results show some effect of ultrasonic treatment on the particle size and surface roughness in some extent. These effects may facilitate the bubble-particle attachment. The cavitation bubbles may benefit the amine adsorption and amine molecular organization as evidence by adsorption density and AFM image. Acknowledgements This study was supported by the Research Fund of Istanbul University, Project No.: 31626, BYP-2018-29597, FBA-2017-25533, FAB-2017-25658, and The Scientific and Technological Research Council of Turkey (TUBITAK), “2214-A International Research Scholarship during Ph.D. studies”. References [1] M.M. Ahmed, Effect of comminution on particle shape and surface roughness and
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alkylammonium and alkylsulfate collectors, Adv. Colloid Interface Sci. 163 (1) (2011) 1–22. O. Ozdemir, C. Karaguzel, A.V. Nguyen, M.S. Celik, J.D. Miller, Contact angle and bubble attachment studies in the flotation of trona and other soluble carbonate salts, Miner. Eng. 22 (2) (2009) 168–175. S.G. Ozkan, Effects of simultaneous ultrasonic treatment on flotation of hard coal slimes, Fuel 93 (2012) 576–580. S.G. Ozkan, Further investigations on simultaneous ultrasonic coal flotation, Minerals 7 (2017) 10. S.G. Ozkan, C. Gungoren, Enhancement of colemanite flotation by ultrasonic pretreatment, Physicochem. Prob. Miner. Process. 48 (2) (2012) 455–462. S.G. Ozkan, H.Z. Kuyumcu, Design of a flotation cell equipped with ultrasound transducers to enhance coal flotation, Ultrason. Sonochem. 14 (5) (2007) 639–645. L. Ren, H. Qiu, Y. Zhang, A.V. Nguyen, M. Zhang, P. Wei, Q. Long, Effects of alkyl ether amine and calcium ions on fine quartz flotation and its guidance for upgrading vanadium from stone coal, Powder Technol. 338 (2018) 180–189. S. Siddiqui, M. Keswani, B. Brooks, A. Fuerst, S. Raghavan, A study of hydrogen peroxide decomposition in ammonia-peroxide mixtures (APM), Microelectron. Eng. 102 (2013) 68–73. R.W. Smith, J.L. Scott, Mechanisms of dodecylamine flotation of quartz, Miner. Process. Extract. Metall. Rev. 7 (2) (1990) 81–94. K.S. Suslick, The chemical effects of ultrasound, Sci. Am. (1989) 80–86. K.S. Suslick, The Chemistry of Ultrasound, The Yearbook of Science & The Future, Encyclopaedia Britannica, Chicago, 1994, pp. 138–155. K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. McNamara III, M.M. Mdleleni, M. Wong, Acoustic cavitation and its chemical consequences, Philos. Trans. R. Soc. Lond. Ser. A 357 (1999) 335–353. U. Ulusoy, M. Yekeler, Correlation of the surface roughness of some industrial
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minerals with their wettability parameters, Chem. Eng. Process. Process Intensif. 44 (5) (2005) 555–563. Y. Vargas-Hernandez, L. Geaete-Garreton, L. Magne-Ortega, High Power Ultrasound to Recover Fine Particles in Flotation Process, ULT 001, (2002). B. Vaziri Hassas, H. Caliskan, O. Guven, F. Karakas, M. Cinar, M.S. Celik, Effect of roughness and shape factor on flotation characteristics of glass beads, Colloids Surf., A 492 (2016) 88–99. D.I. Verrelli, P.T.L. Koh, W.J. Bruckard, M.P. Schwarz, Variations in the induction period for particle–bubble attachment, Miner. Eng. 36–38 (2012) 219–230. A.R. Videla, R. Morales, T. Saint-Jean, L. Gaete, Y. Vargas, J.D. Miller, Ultrasound treatment on tailings to enhance copper flotation recovery, Miner. Eng. 99 (2016) 89–95. A.M. Vieira, A.E.C. Peres, The effect of amine type, pH, and size range in the flotation of quartz, Miner. Eng. 20 (10) (2007) 1008–1013. L. Wang, Y. Peng, K. Runge, D. Bradshaw, A review of entrainment: mechanisms, contributing factors and modelling in flotation, Miner. Eng. 70 (2015) 77–91. M. Yekeler, U. Ulusoy, C. Hicyılmaz, Effect of particle shape and roughness of talc mineral ground by different mills on the wettability and floatability, Powder Technol. 140 (1–2) (2004) 68–78. R.H. Yoon, J.L. Yordan, Induction time measurements for the quartz-amine flotation system, J. Colloid Interface Sci. 141 (2) (1991) 374–383. Z.A. Zhou, Z. Xu, J.A. Finch, J.H. Masliyah, R.S. Chow, On the role of cavitation in particle collection in flotation – a critical review-II, Miner. Eng. 22 (5) (2009) 419–433. X. Wang, J.D. Miller, Dodecyl amine adsorption at different interfaces during bubble attachment/detachment at a silica surface, Physicochemical Problems of Mineral Processing 54 (1) (2018) 81–88.
Contents lists available at ScienceDirect
Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Effect of ultrasound on bubble-particle interaction in quartz-amine flotation system ⁎
C. Gungorena, O. Ozdemira, X. Wangb, S.G. Ozkana, , J.D. Millerb a b
Istanbul University-Cerrahpasa Engineering Faculty Mining Engineering Department, 34320, Avcilar, Istanbul, Turkey Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, Salt Lake City, UT 84112, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasound Flotation Quartz Amine (DAH) Bubble-particle interactions
In this study, the effect of ultrasound (US) on the quartz-amine flotation system was investigated in detail by considering various surface chemistry techniques. The effect of ultrasound on particle size, shape factor, and surface roughness were characterized by using Brunauer-Emmett-Teller (BET) surface area measurements and scanning electron microscopy (SEM) analyses. The contact angle and bubble-particle attachment time, as well as adsorption density measurements was carried out to evaluate the effect of ultrasound on quartz surface wetting ability. In addition, atomic force microscopy (AFM) analyses were conducted, and finally micro-flotation studies were performed. As a result, it was found that the micro-flotation recovery at 2 × 10−5 M dodecyl amine hydrochloride (DAH) concentration increased from 45.45% to 63.64% with 30 W ultrasonic application at conditioning step. However, the micro-flotation recovery decreased to 37.50% when the ultrasonic power increased to 150 W. The results showed some effect of ultrasound on particle size, particle shape, and surface roughness in some extent. The increase in the contact angle and the decrease in the bubble-particle attachment time were observed. A slightly high adsorption density was measured. All these show a positive effect of ultrasound on quartz flotation with amine as a collector.
1. Introduction Ultrasound is a sound wave above the human perception frequency limits, and propagates in a medium by wave series consisted of loosening and compressing phases. When a sufficient negative pressure is created in a loosening phase, cavitation bubbles occur. The size of the nascent cavitation bubbles is around 100 µm, and they are grown during several wave circles to a resonant size by taking gases and/or vapors inside. When a bubble grows too much, it cannot adsorb energy any more. Moreover, the surrounding liquid penetrates into the bubble, and the bubble collapses during the compression phase [46,31]. Compressed gases and vapors in a collapsing bubble generate heat, which is much faster than heat transportation during cavitation. Therefore, temperature of the vicinity of the collapsed bubble significantly increases up to 5000 °C; however it lasts only for a short time [48]. If a cavitation bubble collapses in a homogenous liquid, the bubble remains spherical during the collapsing due to the symmetrical pressure distribution around the bubble. On the other hand, the existence of a solid near a collapsing cavitation bubble changes the pressure distribution around the bubble, and therefore, the bubble collapses
⁎
asymmetrically which causes a liquid jet (micro-jet) with 400–500 km/ h velocity towards the solid [46,7]. To benefit these extreme features of acoustic cavitation, ultrasound has been used for improving the flotation recovery by various researchers [12,17,42,3,13,21,58,39,41,16,40]. Although these studies have generally been focused on the practical results of ultrasound on flotation, the effect of ultrasound on mineral surface, mineral-reagent, and mineral-bubble interaction have not been studied in detail so far. Therefore, there is a need more fundamental studies to investigate the effect of ultrasound on the flotation process. In this context, this study was aimed to investigate the effect of ultrasound on quartz-amine flotation by considering characterization and surface chemistry aspects of minerals and bubbles in the presence of ultrasound. Within this scope, first, particle size and shape factor, surface roughness, Brunauer-Emmett-Teller (BET) surface area measurements, and scanning electron microscopy (SEM) analyses were carried out with and without ultrasonic treatment. Then, the contact angle, bubble-particle attachment time, adsorption density measurements, atomic force microscopy (AFM) analyses were carried out, and finally micro-flotation studies were conducted under various ultrasonic conditions.
Corresponding author. E-mail address: [email protected] (S.G. Ozkan).
https://doi.org/10.1016/j.ultsonch.2018.12.023 Received 23 July 2018; Received in revised form 12 December 2018; Accepted 13 December 2018 1350-4177/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Gungoren, C., Ultrasonics - Sonochemistry, https://doi.org/10.1016/j.ultsonch.2018.12.023
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Fig. 1. Results of the XRD analysis of the analytical grade quartz.
2. Materials and methods
amine hydrochloride (DAH) from Arcos Organics, USA was used as a collector in the micro-flotation experiments.
2.1. Materials 2.2. Methods
In the experimental studies, three different quartz samples were used according to their suitability for the experimental conditions. The first and main sample of the study was analytical grade quartz at 1.2 × 0.6 mm particle size (Carl Roth, Germany). The chemical analyses of the sample showed that the sample contained ∼98% SiO2, ∼1.1% Al2O3, ∼0.04% TiO2, and ∼0.02% Fe2O3. As seen in Fig. 1, the mineralogical analysis of the sample by XRD indicated that quartz mineral was only the main mineral. This sample was first ground using a close circuit automatic agate mortar, and wet sieved to obtain the required particle size fractions (−500, 212 × 150, 150 × 75, and −38 µm) for each experiment. The fractions were then cleaned with analytical grade 2.5% H2SO4 by volume and 2.5% NaOH by weight, and rinsed with de-ionized (DI) water (18 MΩ cm) (Milli-Q plus Millipore Ultra Pure Water system, Millipore Ltd, Molshem, France) to remove possible surface contaminants [23]. Additionally, the surface tension of the DI water was measured to be around 72.0 mN/m at 23 °C. The ground and cleaned analytical grade quartz was used for the micro-flotation tests, particle size and shape, surface roughness, BET surface area, bubble-particle attachment time measurements as well as adsorption density and SEM analyses. The second sample used in the study was a quartz mineral sample with ∼10 cm feret’s diameter, which was used for the contact angle measurements after cutting and polishing. The flat surfaced samples were cleaned with acetone, ethanol, DI water, and finally by a plasma cleaner. The third sample was a flat surfaced glass material for laboratory use (Baxter Scientific, USA). These SiO2 based flat surfaced glass samples were used in the AFM analyses in the place of quartz mineral particles to obtain clear AFM images. The glass samples were cut and cleaned with ammonium peroxide mixture (APM) which is a combination of NH4OH, H2O2, and H2O in the ratios of 1:1:5, and then rinsed with DI water [44]. The schematic structure of (a) crystalline SiO2 (quartz) and (b) amorphous SiO2 (quartz glass) is shown in Fig. 2. Moreover, analytical grade (99% of CH3(CH2)11NH2·HCl) dodecyl
2.2.1. Micro-flotation experiments The effect of ultrasound on the floatability of quartz was investigated via micro-flotation studies. The micro-flotation tests were carried out by a glass micro-flotation cell (30 × 210 mm) with 200 mL volume and a frit with 10–16 µm pore size. First, 2 g of 212 × 150 µm quartz samples were conditioned at various concentrations of DAH (1 × 10−6, 1 × 10−5, 2 × 10−5, 3 × 10−5, 5 × 10−5, 1 × 10−4, 5 × 10−4, and 1 × 10−3 M) at 1% solid ratio and at 500 rpm stirring speed in the absence of ultrasound to find the optimum DAH concentration for the further micro-flotation experiments. The micro-flotation studies were conducted using N2 gas with 50 mL/min flow rate for 2 min. All experiments were performed at room temperature (23 °C) and natural pH (6.0–6.5) to eliminate the effect of pH and observe the effect of ultrasound. The flotation products were dewatered with a black ribbon filter paper, and dried at 105 °C in a drying oven. The micro-flotation recoveries were calculated using the product weights. 2.2.2. Conditioning and ultrasonic treatment Within the scope of this study, conditioning of quartz in DAH solution was conducted by magnetic stirring. Meanwhile, the ultrasonic treatment was carried out by using an ultrasonic probe in addition to magnetic stirring. An ultrasonic probe (Bandelin HD 3200, Germany) at 20 kHz constant frequency, of which power level arrangeable from 30 to 150 W, was employed as an ultrasound source. The use of 30 W ultrasonic power was preferred during the surface chemistry studies because of obtaining satisfactory micro-flotation results at this ultrasonic power, and the well-known fact that ultrasound increases the ambient temperature according to the ultrasonic power and application time [47]. Therefore, ultrasound was used at minimum available ultrasonic power (30 W) to avoid over-heating the suspension. In the conditions used in this study, the ambient temperature increased from 23 °C to 31.9 °C at 10 min. Also, the pH of the suspension did not fluctuate from natural pH significantly at 30 W ultrasonic power [15,16]. 2
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Fig. 2. Schematic structure of (a) crystalline SiO2 (quartz) and (b) amorphous SiO2 (quartz glass) [4].
is reported in the literature that colloidal precipitation and micelle formation is not expected in these conditions [29]. Although the contact angle has an important role on hydrophobicity, it is not adequate for explaining the bubble-particle attachment in flotation, alone. Because, while contact angle is a static measurement method, mineral particles and bubbles are always in motion and contact for a limited time during a flotation process. In some cases, the flotation recovery can increase while the contact angle of the particles is constant. Therefore, the attachment possibility in a limited time has to be determined. For this purpose, bubble-particle attachment time measurement devices are used [34,2,11,37,52]. In our study, the bubble-particle attachment time was measured using MCT-100 electronic bubble-particle attachment time device. The bubble-particle attachment time experiments were carried out in 5 mL suspension with quartz samples at 150 × 75 µm particle size and with a bubble at 3 mm diameter, approximately. The attachment time adjusted via electronic control unit between 1 and 200 ms. The possible bubbleparticle attachments were observed through a lens at the bubble surface for a particular contact time. The experiments were repeated 20 times at different positions on the particle bed. The tests resulted with “attachment” and “no attachment” were recorded to calculate the attachment percentage for a particular contact time. The attachment time was determined by the contact time at which 50% of the observations results in attachment [38].
2.2.3. Sample characterization In the concept of the particle size analyses, the particle size of the quartz samples under 500 µm was measured using CILAS 1090 laser particle size analyzer in liquid mode. Furthermore, to determine the shape properties of quartz samples, particle shape analyses of the 150 × 75 µm quartz samples were performed using microscope photographs with “×3” magnification. The shape factors (roundness, flatness, elongation ratio, and relative width) were calculated by Leica QWin image analyzing software using the equations previously given by several researchers [14,20,27,19]. The surface roughness of the 150 × 75 µm quartz samples was measured using Zeiss Axio CSM 700 optical profilometer. The surface area of the samples was determined by multipoint BET method using Quantachrome Quadrasorb SI BET surface area measurement device. To observe the effect of ultrasound more effectively, under 38 µm sized quartz particles were used owing to having larger surface area than larger particles. SEM surface images of the samples were also taken using JEOL SEM7100 electron microscope at “×1200” magnification rate. A covering process with gold (Au) and palladium (Pd) was carried out to quartz particles to obtain a conductive surface before the SEM analyses.
2.2.4. Contact angle and bubble-particle attachment time measurements The contact angle of the quartz surface was measured with a goniometer (Ramé-hart, USA) by “captive bubble” method using flat surfaced quartz samples. First, a rectangular glass container was filled with DI water or DAH solution. Then, the quartz sample was placed on two glass stands facing the polished surface downwards. An air bubble was formed using a micro-syringe with a hook-shaped needle to attach on the surface of the quartz. Finally, the angle between the attached bubble and the quartz surface was measured via the lens with a goniometer. The bubbles could not attach to the quartz surface in 1 × 10−5 and 1 × 10−4 M DAH concentrations. Hence, the contact angle experiments were carried out at 1 × 10−3 M DAH concentration. 1 × 10−3 M DAH concentration at natural pH of the quartz (pH 6) is under the critic micelle concentration (CMC), which is 1.3 × 10−2 M. It
2.2.5. Adsorption experiments To determine the adsorption density of DAH on quartz surface, adsorption density analyses were performed using depletion method with Shimadzu TOC-VCPH/CPN total carbon (TC) analyzing device. The adsorption density was calculated using the concentration of the residual DAH solution and BET surface area of the quartz samples. Various concentration of DAH solutions (1 × 10−5, 5 × 10−5, and 1 × 10−4 M DAH) were prepared using high performance liquid chromatography (HPLC) grade water. First, the initial TC values of DAH solutions were measured. Then, the adsorption process was carried out in the absence and presence of the ultrasound using −38 µm quartz. 3
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Fig. 3. Experimental flowsheet.
The overall experimental flowsheet of this study is shown in Fig. 3.
3. Results and discussion 3.1. Micro-flotation Long chain alkyl amines, especially dodecylamine, are widely used in quartz flotation as a collector [45,8]. Even short chain amines can act both as a frother and as a collector [25]. Ren et al. [43] used alkyl ether amine (Flotigam EDA-C from Clariant) was used as a collector for the flotation of quartz. They obtained a flotation recovery above 80% with 20 mg/L EDA-C concentration. Li et al. [30] reported that the flotation recovery of quartz at natural pH reached to 90% in the presence of 300 mg/L amine. Kowalczuk [25] reported that quartz did not float without collector neither with Hallimond tube nor with mechanical flotation machine. A low flotation recovery (∼4%) was result from the entrainment [24,55]. However, it started to float in the presence of short chain alkyl amine (hexylamine) at pH 10–11. Additionally, since micro or nano bubbles can be used for enhancing the quartz flotation [9], ultrasonic treatment can improve the flotation by these smaller bubbles due to cavitation effect. The micro-flotation recoveries at different DAH concentrations without ultrasonic treatment are given in Fig. 4. As shown in Fig. 4, a 45.45% micro-flotation recovery was obtained at 2 × 10−5 M DAH concentration, and this concentration was chosen for further ultrasonic experiments to observe the possible positive or negative effects of the ultrasound. Fig. 5 shows the comparison of the micro-flotation test results with and without ultrasound at various ultrasonic powers. As seen in Fig. 5, the micro-flotation recovery increased to 63.64% and 65.57% with the use of ultrasound at 30 W and 90 W, respectively. However, it decreased to 37.50% at 150 W ultrasonic power. Similarly, in literature, there are also several studies which the negative effect of the ultrasound was observed on flotation. For instance, Videla et al. [53] reported that although the flotation recovery increased with the use of ultrasound in both of the conditioning and flotation phases, it decreased when ultrasound was applied only in the conditioning and flotation phases. Cao et al. [10] reported that ultrasonic power and application time are important parameters for the de-oxidation of the surfaces. However, its influence is related to the application power and time. Vargas-
Fig. 4. Quartz flotation results as a function of DAH concentration without ultrasound (212 × 150 µm particle size, pH 6.0–6.5, 1% solid ratio).
After adsorption process, a solid-liquid separation by centrifugation at 3300 rpm for 2 min was carried out. The TC values of the residual solutions were measured. The adsorption density was calculated using Eq. (1) [36].
Γ=
(Ci − Cr )·V m·S
(1)
where, Ci and Cr represent the initial and residual concentrations in mol/L, V is the volume of the solution in L, m is the amount of solid in g, S is the BET surface area of quartz in m2/g, and Γ is the adsorption density in mol/m2. 2.2.6. AFM microscope analysis To observe the effect of the ultrasound on the material surface in nanometer scale the topographical map of the flat surfaced glass particles was obtained using an AFM microscope (Multimode, Digital Instruments) without and with ultrasound in the absence and presence of DAH [6,33]. In the AFM experiments, the glass particles were cut from a bigger sample and attached to the magnetic disk of AFM. The measurements were performed via the standard cantilever using the soft-contact imaging technique. 4
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et al. [16] reported that the higher ultrasonic power caused a significant increase in the pulp temperature. In their study, the pulp temperature increased from 23 °C to 31.9, 60.5, and 75.1 °C at 30, 90, and 150 W, respectively. For this reason, 30 W ultrasonic power was chosen for the further studies in terms of increasing the micro-flotation recovery and preventing overheating of the pulp. The micro-flotation results clearly showed that the flotation recovery significantly increased with ultrasound during the conditioning. In order to understand the ultrasonic effect on the flotation of quartz, the following sections ultrasonic effect on the particle size, particle shape, surface morphology, and amine adsorption at the quartz surface will be discussed.
3.2. Effect of conditioning with ultrasound Particle size has a significant importance in flotation. Coarse particles of different minerals is more sensitive to the chemical medium, in comparison with finer particles. On the other hand, finer particles consume more collector due to their large surface areas [54]. The essential morphological characteristics of the particles are size, shape, and surface roughness. These parameters strongly affect the bubbleparticle interactions. Roughness and surface heterogeneity can either increase or decrease the work of bubble-particle adhesion and the probability of particle detachment from the air bubble [35]. Miller et al. [32] investigated the influence of film roughness on the wetting properties of vacuum-deposited polytetrafluroethylene (PTFE) thin films and a contact angle correlation with surface roughness was determined. It is also reported that high contact angles were observed at the surface because of surface roughness. Yekeler et al. [56] reported some correlations between the shape properties, surface roughness values, and the wettability characteristics of talc mineral. The hydrophobicity increases with the increasing surface roughness of talc particles. On the other hand, Ulusoy and Yekeler [49] investigated the relationship between the surface roughness and wettability of calcite, barite, talc, and quartz. Their results indicated that the degree of hydrophobicity of these minerals increased with the decreasing surface roughness. Krasowska and Malysa [26] stated that roughness of a hydrophobic solid surface had a crucial importance for kinetics of the bubble attachment. Vaziri Hassas et al. [51] reported that the increase in surface roughness improved the flotation recovery, contact angle, and bubble-particle attachment. Guven et al. [19] studied the morphology and floatability of quartz particles, and their results indicated that the blasted quartz particles with more angular and rougher surfaces gave better floatability compared to the un-blasted quartz particles. Ahmed [1] indicated that the surface roughness was responsible for the detachment process of particle and the final flotation recovery more than its shape. Furthermore, Yekeler et al. [56] reported that the reactivity of a solidwas clearly affected by its surface roughness. Ren et al. [43] investigated the quartz surface by AFM, and reported that the bare surface of quartz had very
Fig. 5. Micro-flotation test results with ultrasound in respect to ultrasonic power (212 × 150 µm particle size, pH 6.0–6.5, 1% solid ratio).
Hernandez et al. [50] showed that the highest cumulative Cu recovery was obtained with the minimum ultrasound power (5 W). Cilek and Ozgen [13] reported a small decrease in pyrite flotation recovery with the use of ultrasound. However, the information on the reasons of the negative effect of ultrasound on flotation is quite limited. Ultrasound can damage the physical bonds between the collector and mineral, and therefore it has a detrimental effect on flotation [18]. In addition, ultrasound increases the temperature of the medium [46,47]. Since, the ambient temperature is a dominant factor for reagent adsorption, it significantly affects the flotation process [16]. Furthermore, the gases and vapor inside a cavitation bubble can produce %H, %OH free radicals, through thermal decomposition, which can undergo secondary reactions resulting the formation of H2O2 and %HO2. Both %OH and H2O2 are quite reactive, and have a strong oxidation ability. Moreover, H2O2 is very unstable, and discharges nascent oxygen quickly. Therefore, the oxygen content of the medium increases after ultrasonic treatment. The nascent oxygen is also very active, and react with the mineral surface, and thus increasing its hydrophilicity [21,5]. In our case, it is thought that the negative effect on flotation is dependent upon the chaotic medium in the conditioning medium created by high-power ultrasound. Ultrasound creates too many turbulent acoustic flows and micro-jets in a liquid media. The increase in the number and magnitude of these factors might create a chaotic medium in conditioning which had a detrimental effect on the adsorption of collector molecules on mineral surfaces. Therefore, it is important to note that the application time and power of ultrasound have a crucial effect on the flotation. Gungoren
Fig. 6. Particle shape factor (150 × 75 µm), roughness (150 × 75 µm), BET surface area (−38 µm), d50, and d80 particle size (−500 µm) results with and without ultrasound (30 W). 5
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Fig. 7. SEM images of quartz samples (a) without and (b) with ultrasound (30 W) at ×1200 magnification rate (150 × 75 µm).
Fig. 8. Bubble-particle attachment time experiment results in the absence of presence of DAH with and without ultrasound (30 W).
Fig. 9. Contact angle, bubble-particle attachment time (150 × 75 µm), and adsorption density (−38 µm) results in the presence of DAH with and without ultrasound (30 W).
the particle size decreased a little. Additionally, the roundness of the particles was determined as 0.63 and 0.64 with and without the ultrasound, while the flatness decreased from 1.62 to 1.59, respectively. At the same time, Fig. 6 also indicated that ultrasound slightly increased the roughness of quartz from 3.70 to 4.24 µm, and increased the BET surface area from 1.020 to 1.075, which was not a remarkable change. Finally, the SEM images of the quartz samples with and without ultrasound indicated that there was no significant change on the particle surfaces as seen in Fig. 7.
low roughness (2.1 nm) similar to the glass surface used in this study. Fig. 6 shows the results of determination of the particle shape factor, roughness, BET surface area, d50 and d80 particle size distribution with and without ultrasound. Fig. 6 indicates that the ultrasound slightly changed the particle size and shape factors. The d80 size of the quartz particles decreased from 255 µm to 200 µm, while the d50 size decreased from 126 µm to 118 µm, respectively. These results indicated that the ultrasound somehow caused a good dispersion of the suspension as well as removing the fine particles on coarse particles; hence, 6
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Fig. 10. Two-dimensional dry AFM images of the flat surfaced glass samples. (a) Without ultrasound and DAH, (b) with ultrasound (30 W), without DAH, (c) without ultrasound with DAH, and (d) with ultrasound (30 W) and DAH (1 × 10−5 M DAH).
adsorbed layer was similar at different collector (EDA-C) concentrations. The flat quartz surface was very hydrophilic with absence of DAH in DI water. On the other hand, the contact angle of quartz was determined as 52° in the presence of 1 × 10−3 M DAH dosage without the ultrasound, and increased to 62° with the ultrasound. In addition, bubble-particle attachment time measurements and adsorption density experiments were also conducted, and the results are shown in Figs. 8 and 9. According to Fig. 8, the bubbles could not attach to the quartz surface with the absence of DAH in accordance with the contact angle measurements. These results showed that the hydrophilic character of quartz did not change with ultrasonic treatment. On the other hand, the attachment time of the quartz samples conditioned in 1 × 10−5 M DAH solution without ultrasound was 75 ms, and decreased to 50 ms with the ultrasound. Fig. 9 indicates a very slight influence of ultrasound on the adsorption density of DAH at quartz surface. The adsorption densities of DAH at the surface of quartz particles in 1 × 10−5 M DAH solution were 8.11 × 10−7 mol/m2 and 8.05 × 10−7 mol/m2 without and with ultrasound, respectively. In flotation process, when a particle encounters a bubble, the process of film thinning and liquid drainage occurs before attachment is happened. The shape, sharpness, and pointedness of the particles have
The results from the conditioning with ultrasound showed the effect on particle size, surface roughness, and particle shape in some extent. As mentioned in the introduction section, near a solid surface cavity collapses asymmetrically, and create high-speed microjets of liquid. The impact of microjets on solid surface could be very strong. It is reasonably expected such effect due to ultrasonic effect. 3.3. Contact angle, bubble-particle attachment time, and adsorption The contact angle of quartz can be either zero or greater than zero depending on the water adsorption on its surface. Zero contact angle occurs when the surface of quartz is totally hydroxylated [28]. Kowalczuk [25] reported that hexylamine did not change the hydrophobicity of quartz (between 15° and 25°). On the other hand, the flotometric contact angle of quartz increased with hexylamine concentration from 0° to 36°, and remained constant for higher concentrations. It means that the presence of hexylamine effects not only flotation of quartz but also “uncovers” its natural (flotometric) hydrophobicity. Yoon and Yordan [57] reported that the induction time for the quartz-DAH system was the lowest at pH 10.5 and decreased with amine concentration. Ren et al. [43] investigated the adsorption of Clariant EDA-C via AFM and reported that the morphology of the 7
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an important role on the rate of film thinning and rupture, and hence the overall flotation rate. It is also known from literature that the particles with sharper edges and rougher surfaces attached easily to the bubbles than smooth ones because of the increasing in the rate of film thinning and the rupture [22,19]. In summary, the ultrasonic effect on particle surface roughness might facilitate the contact angle and bubble-particle attachment. Slightly increased adsorption density of DAH at quartz particle surface with ultrasound might be due to the cavitation bubbles. As reported by Wang and Miller [59] that the adsorption of amine increased significantly in the presence of air bubbles by transferring amine molecules to the quartz/air bubble interface from air bubble/amine solution interface during bubble attachment.
their relation to flotation process, Int. J. Miner. Process. 94 (3–4) (2010) 180–191. [2] B. Albijanic, O. Ozdemir, A.V. Nguyen, D. Bradshaw, A review of induction and attachment times of wetting thin films between air bubbles and particles and its relevance in the separation of particles by flotation, Adv. Colloid Interface Sci. 159 (1) (2010) 1–21. [3] N.E. Altun, J.-Y. Hwang, C. Hicyilmaz, Enhancement of flotation performance of oil shale cleaning by ultrasonic treatment, Int. J. Miner. Process. 91 (1–2) (2009) 1–13. [4] http://wps.prenhall.com/wps/media/objects/3082/3156196/blb1107.html, Structures of Solids, Retrieved: 21 March 2018. [5] S.D. Barma, Ultrasonic-assisted coal beneficiation: a review, Ultrason. Sonochem. (2018) (in press). [6] G. Binning, H. Rohrer, C. Gerber, E. Weibel, Surface studies by scanning tunneling microscope, Phys. Rev. Lett. 49 (1) (1982) 57–61. [7] M. Breitbach, D. Bathen, S. Henner, Effect of ultrasound on adsorption and desorption processes, Ind. Eng. 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Miller, Understanding the role of ion interactions in soluble salt flotation with
3.4. AFM microscope analysis The two-dimensional AFM images of the flat surfaced quartz glass samples without and with ultrasound and DAH were taken, and the results are given in Fig. 10. The color bar in Fig. 10 shows the height of surfaces which darkest and lightest colors indicate lower and higher parts of the surface, respectively. Fig. 10(a) and (b) show the effect of ultrasound on the glass surfaces. As seen from Fig. 10(a), (b) that the distribution of lighter and darker areas in the AFM images were similar to each other without (a) and with (b) of ultrasound in the absence of DAH. This clearly proved that the ultrasound showed no significant change on the glass surfaces. Meanwhile, in order to see the effect of ultrasound on the adsorption of DAH molecules on the surfaces, the AFM analyses were carried out in the presence of DAH without/with the ultrasound. It is seen from Fig. 10(c) that there is a light-color zone while the other parts of the image is darker. This means that DAH molecules accumulated at some parts of the surface during the adsorption. When the ultrasound was applied during the adsorption process, the DAH molecules adsorbed on the glass surface more orderly as seen from the equal distribution of lighter and darker parts in Fig. 10(d). The results imply that the ultrasound may affect on the organization of adsorbed amine molecules at the quartz surface, which also will affect the bubble-particle attachment. 4. Conclusions The effect of ultrasound on the quartz flotation was studied. The results indicated that during the conditioning with ultrasound the quartz flotation recovery increased significantly. A decreased quartz flotation recovery was obtained at a higher ultrasonic power. The higher ultrasonic power may interrupt the adsorption between amine molecules and surface, which is due to the physical bonds. An increased contact angle and shorter bubble-particle attachment time were observed with ultrasonic treatment. The results show some effect of ultrasonic treatment on the particle size and surface roughness in some extent. These effects may facilitate the bubble-particle attachment. The cavitation bubbles may benefit the amine adsorption and amine molecular organization as evidence by adsorption density and AFM image. Acknowledgements This study was supported by the Research Fund of Istanbul University, Project No.: 31626, BYP-2018-29597, FBA-2017-25533, FAB-2017-25658, and The Scientific and Technological Research Council of Turkey (TUBITAK), “2214-A International Research Scholarship during Ph.D. studies”. References [1] M.M. Ahmed, Effect of comminution on particle shape and surface roughness and
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