Research Note pubs.acs.org/IECR
Three-Phase Contact Formation and Flotation of Highly Hydrophobic Polytetrafluoroethylene in the Presence of Increased Dose of Frothers Przemyslaw B. Kowalczuk,† Jan Zawala,*,‡ Dominik Kosior,‡ Jan Drzymala,† and Kazimierz Malysa‡ †
Faculty of Geoengineering, Mining and Geology, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ‡ Jerzy Haber Institute of Catalysis and Surface Chemistry PAS, Niezapominajek 8, 30-239 Krakow, Poland ABSTRACT: The time of a single bubble attachment to a rough hydrophobic solid surface with contact angle greater than 90° in aqueous solutions at different frothers (n-octanol, α-terpineol and N-octyl-trimethylammonium bromide) in the concentration ranges of 10−6 to 10−3 M was compared with flotation performed under corresponding conditions. A relationship between the time of three-phase contact (TPC) formation at the bubble/liquid/solid interface and flotation rate was obtained. It was found that above a certain frother dose, irrespective of frother type, one could observe significantly longer bubble−solid surface attachment time, which increases from about 2 ms to above 50−60 ms for higher frother concentrations. Accordingly, the flotation rate for the corresponding frothers concentrations decreased from ca. 0.8 s−1 to values below 0.01 s−1. The obtained results confirm that air can be entrapped at rough highly hydrophobic solid surfaces and show the importance of the bubble attachment time for the flotation kinetics. Anfruns and Kitchener10 showed that the flotation efficiency of highly hydrophobic (via surface methylation) spherical glass and irregular quartz was much higher in the case of irregular and rough particles. Recently, this observation was confirmed on the basis of microflotation tests.11 Similar results were obtained for a model system, in which a single rising bubble collided with highly hydrophobic Teflon plates of different surface roughness.12,13 It was found that larger surface roughness caused shortening of the bubble attachment time. Recently it was shown14 that time of the TPC formation (tTPC) at highly hydrophobic Teflon surfaces of different roughness was significantly prolonged at high frothers concentrations. The rather unexpected effect of the high frother concentration was attributed to the presence of air at highly hydrophobic solid surfaces. It was explained14,15 that when a single bubble collides with the Teflon surface, foam films are formed between the colliding bubble and nano- and microbubbles locally present at the highly hydrophobic solid/liquid interface. It is well-known that the stability of foam films increases with concentration of surface-active substances,16 therefore higher stability of the foam films formed locally at the Teflon surfaces causes prolongation of tTPC. There are papers which describe the relationship between the particle−bubble attachment time and flotation performance showing that the highest flotation recovery occurs at the shortest time of particle−bubble attachment.17−19 These studies were based on experiments conducted in either the presence or absence of collectors for hydrophobic materials. However, the literature does not provide data for the
1. INTRODUCTION Frothers are applied in flotation systems to enhance the degree of the gas dispersion, to ensure formation of a stable froth, to prevent bubble coalescence,1,2 to facilitate the three-phase contact (TPC) formation,3 and generally to accelerate flotation recovery.4 Flotation is an extremely complex process and it involves several subprocesses, which have to be taken into account considering both quantitative and qualitative characterization. Despite actual complexity of the process, one can focus on the key flotation steps, which are collisions, bubble-particle adhesion, and stability of floating aggregates.5 The rate of many flotation processes can be described by the first order kinetics equation: r = rmax ·(1 − e−kt )
(1)
where r is recovery of floating particles, rmax is the maximum possible recovery, and k is a first order kinetics constant. The constant k depends on efficiencies of collision, attachment, and stability of a particle-bubble aggregate (Ec, Ea, and Es), respectively.5,6 The attachment and stability efficiencies are influenced by kinetics of the three-phase contact (TPC) line formation. For the TPC formation, the liquid film separating a colliding bubble and a solid surface needs to be ruptured during the collision. The process of formation of a stable particle− bubble aggregate can be divided into three elementary steps:7 (i) thinning of the liquid film to a critical thickness; (ii) film rupture and formation of the three-phase contact, and (iii) expansion of the three-phase contact line. It is generally accepted that high hydrophobicity of a solid surface (with contact angle greater than 90°) is a factor ensuring fast TPC formation and bubble attachment. However, hydrophobic surface topography (including particle shape factor) can greatly affect liquid film stability8,9 and the TPC formation time. © 2016 American Chemical Society
Received: Revised: Accepted: Published: 839
November 13, 2015 January 6, 2016 January 8, 2016 January 8, 2016 DOI: 10.1021/acs.iecr.5b04293 Ind. Eng. Chem. Res. 2016, 55, 839−843
Research Note
Industrial & Engineering Chemistry Research
Figure 1. Photos showing (A) principle of single bubble tests, and (B) PTFE T220 disks conditioned in a Denver flotation cell (rotor speed 700 rpm, no airflow).
Teflon disks, 2 mm in diameter, were produced by stamping them from a 1 mm thick Teflon plate (50 mm × 50 mm). Prior to the disc stamping, both sides of the plate were roughened using the grid number 220 sand paper, to ensure similar roughness of both disk surfaces. The prepared disks (referred as T220 particles further in the text) were washed in diluted Mucasol (commercially available cleaning liquid, purchased from Sigma-Aldrich), and then rinsed with a large amount of warm distilled water. Prior to the tests, the rotor and flotation cell were washed in a large amount of distilled water. The T220 particles were added to the clean flotation cell together with 1.35 dm3 of frother solution and then conditioned for 1 min. The picture of T220 particles conditioned in the flotation cell, before admitting air, is presented in Figure 1B. The experiments were carried out in distilled water as well as in α-terpineol and CTABr solutions of various concentrations. In each test the airflow rate was adjusted to oscillate between 20 and 40 dm3/h, while the rotor speed was set as 700 rpm. The floating T220 particles, after reaching the liquid surface, were skimmed manually to different containers after certain flotation time. The numbers of the floating particles were counted in each container in order to determine the flotation recovery as a function of time. Each flotation test was repeated five times.
relationship between the time of three-phase contact formation and flotation performance of highly hydrophobic substances (with contact angle greater than 90°) as a function of a frother dosage. This paper presents the results on the influence of frother concentration on flotation kinetics (batch flotation) and time of the three-phase contact formation by the colliding (single) bubble test. A correlation was found between the three-phase contact formation time (tTPC) at a Teflon surface and flotation rate constant (k) of the Teflon particles.
2. EXPERIMENTAL METHODS AND MATERIALS 2.1. Single Bubble Tests. The experimental setup used for monitoring the dynamic phenomena occurring during bubble collisions with a hydrophobic solid surface was described in details elsewhere.20 Briefly, a single bubble was formed at a capillary orifice (diameter 0.075 mm) at the bottom of a square glass column (40 mm × 40 mm). Polytetrafluoroethylene (PTFE, Teflon) was used as a model hydrophobic solid of contact angle above 90°.14 The Teflon plate (30 × 30 mm) was positioned horizontally beneath the liquid surface at the distance L = 3 mm from the capillary orifice (Figure 1A). Bubble motion and collision with the surface of the solid plate were recorded using a high-speed video camera (SpeedCam MacroVis, 1040 fps). Sequences of recorded images of the colliding bubble were analyzed frame-by-frame using an image analysis software (ImageJ, and/or WinAnalyze moving object tracking software) in order to determine the time of the threephase contact (TPC) formation (tTPC); that is, the period from the first bubble collision to its attachment. The Teflon plate used (referenced further in the text as T600) was polished using the grid number 600 sand paper. The roughness of the plate equal to 40−60 μm was determined using microscopic measurements. Prior to each experiment the T600 plate was carefully cleaned using a chromic acid mixture, and then rinsed thoroughly with water. Mili-Q water was used in all tests, including cleaning and surfactant solution preparation. N-Octyltrimethylammonium bromide (CTABr), α-terpineol and noctanol used in the experiments were purchased from SigmaAldrich and were commercial reagents of the highest available purity (≥98%). 2.2. Flotation Tests. Flotation tests were performed using a Denver D12 flotation machine equipped with a 1.5 dm3 cell.
3. RESULTS AND DISCUSSION Figure 2 shows the surface tension isotherms of the investigated frothers. The solid lines represent the Frumkin adsorption isotherms based on the equilibrium surface tensions determined using the pendant drop technique (Kruss DSA100 apparatus).21 The sequences of photos recorded during the single bubble tests, illustrating the bubble collision with the T600 surface in water and n-octanol solutions are presented in Figure 3. The moment of the first collision is denoted as t = 0. Thus, the negative time values refer to the time for a bubble to approach the surface. As seen in Figure 3, for distilled water and at low concentration of n-octanol solution (3 × 10−5 M) the tTPC was similar and very short (3.8 ms). It means that the separating liquid film formed by the colliding bubble was unstable and rapidly reached its critical thickness of rupture. However, at high concentration (1 × 10−3 M) of n-octanol, tTPC was much longer and equal to 72 ms. It indicates that the liquid film formed between the T600 surface and colliding bubble was more stable since its drainage was prolonged, and 840
DOI: 10.1021/acs.iecr.5b04293 Ind. Eng. Chem. Res. 2016, 55, 839−843
Research Note
Industrial & Engineering Chemistry Research
the frother type (ionic or nonionic surface-active substance). It has to be emphasized that the flotation kinetics for distilled water could not be obtained because the flotation was extremely fast. After the air-valve opening, all T220 particles floated immediately (in time less than 5 s) and stayed at the water/air interface. The fast flotation in distilled water indicates that Teflon is a naturally highly hydrophobic material with contact angle greater than 90°,14,21−24 and can be easily recovered in surface flotation in the frothers absence. This observation corresponds to the reports by Drelich and Bowen,25 who demonstrated theoretically that hydrophobic asperities of rough particles can significantly reduce the energy barrier in interactions between gas bubbles and particles during flotation. A similar trend was observed for the single bubble tests. The TPC formation in distilled water and at low concentrations was very fast, while at higher frothers concentrations the tTPC values were significantly higher. Figure 5A presents dependencies of the time of the tTPC on concentrations of cationic (CTABr) and nonionic (α-terpineol and n-octanol) reagents. Figure 5A shows that the values of tTPC obtained at low frother concentrations are similar to these obtained for distilled water. However, above some threshold concentration, the tTPC values start to increase significantly, especially in the case of nonionic frothers. The data presented in Figure 5 panels A and B show a strong correlation between the time of the tTPC and inverse of the first order flotation rate constant k−1. It can be seen there that both flotation kinetics k (s−1) and tTPC (ms) depend on the frother concentration. At very low frother concentrations (see insert in Figure 5A) a slight decrease in the tTPC with respect to distilled water was observed. It was caused by adsorption layer formation at the colliding bubble, for which the frother coverage was large enough to dampen the bubble bouncing,15 but insufficient for stabilization of the intervening liquid film formed between the bubble and hydrophobic Teflon surface. However, at higher concentrations of α-terpineol, n-octanol and CTABr, the tTPC values start to be prolonged due to increasing stability of the liquid films formed by the colliding bubble. It should be noted that the k−1 values vary in a similar manner, increasing equally within similar concentration ranges of CTABr and α-terpineol solutions (Figure 5B). Both, tTPC and k−1 values increase with the frother concentration. Prolongation
Figure 2. Surface tension isotherms of frothers studied.
the film critical thickness of rupture was reached during 72 ms. A quite similar effect of prolongation of the tTPC values at high solution concentrations on was observed also for α-terpineol and CTABr (Figure 5). The flotation recovery (r) of T220 particles as a function of time is presented in Figure 4. The recovery of floating particles (expressed in %) was calculated as n r = t ·100 n0 (2) where n0 is the total number of T220 particles added to the flotation cell (precisely determined number of 100−105 pieces) and nt is the number of particles recovered after time t. Figure 4 shows the flotation kinetics of the Teflon discs in frothers solutions at different concentrations based on the first order kinetics k (eq 1). The kinetics of flotation in the form of k (s−1) were calculated by fitting eq 1 to the experimental points (Figure 5) The results presented in Figure 4 clearly show that with the increasing frother concentration the flotation rate decreased. It means that longer time was needed to recover a similar number of particles. This effect was quite significant and independent of
Figure 3. Sequence of photos of the bubble colliding with T600 surface in water and n-octanol solutions at concentrations of 3 × 10−5 and 1 × 10−3 M. 841
DOI: 10.1021/acs.iecr.5b04293 Ind. Eng. Chem. Res. 2016, 55, 839−843
Research Note
Industrial & Engineering Chemistry Research
Figure 4. Flotation recovery of T220 as a function of time for (A) α-terpineol and (B) CTABr solutions of various concentrations.
Figure 5. Dependencies of the tTPC values, (A) determined in the single bubble tests, and (B) inversed values of the kinetic constant, determined from the flotation tests, on concentration of different frothers.
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CONCLUSIONS It was found that the rate of collectorless flotation of PTFE (Teflon), performed in aqueous frother solutions (n-octanol, αterpineol, CTABr), decreased with increasing frother concentration, irrespective of its type (ionic, nonionic). In the case of n-octanol and α-terpineol a sharp increase in the tTPC from ca. 2 ms observed for distilled water to 50−60 ms, occurring for frother concentrations between 10−6 and 10−3 M, was observed. For CTABr in the studied concentration range of 10−6 to 10−5 M, the jump in the tTPC values was smaller (the tTPC increased ca. 3-fold as compared to pure water), nevertheless evident. The decreased flotation rate, measured as the first order rate constant (k), correlated well with the increasing values of the tTPC for the bubble colliding with the PTFE surface at high frothers concentrations. The determined k values decreased for corresponding concentrations from ca. 0.8 s−1 to the values
