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Surface chemistry aspects of coal flotation in bore water
Int. J. Miner. Process. 92 (2009) 177–183
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Int. J. Miner. Process. j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Surface chemistry aspects of coal flotation in bore water O. Ozdemir ⁎, E. Taran, M.A. Hampton, S.I. Karakashev, A.V. Nguyen ⁎ Division of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 21 October 2008 Received in revised form 28 March 2009 Accepted 1 April 2009 Available online 8 April 2009 Keywords: Coal flotation Bore water Bubble-particle attachment Contact angle Zeta potential Atomic force microscopy
a b s t r a c t Several theories have been proposed to explain enhancement of coal flotation in salt solutions. In this paper, surface chemistry aspects of coal flotation in bore (hypersaline) water were examined using bubble-particle attachment time experiments, zeta potential measurements, cyclic measurements of contact angle and Atomic Force Microscopy (AFM). The attachment time experiments showed that the bubble-particle attachment in deionized water was instantaneous and independent of the particle size. The attachment in bore water required longer time, which increased with increasing particle size. The cyclic measurements of contact angle on a flat coal surface showed that the coal hydrophobicity as measured by the advancing (maximum) and receding (minimum) contact angle did not change in the presence of salt ions. The zeta potential measurements show that both the coal particles and air bubbles were negatively charged in bore water. The AFM studies showed that bore water reduced repulsive surface forces between the coal particles and air bubbles but had little effect on the force of adhesion. The overall results suggest that enhancement of coal flotation in hypersaline water is not entirely attributed to the surface chemistry aspects as previously proposed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the 1930s, researchers in the former USSR found that naturally hydrophobic minerals such as coal could be floated in electrolyte solutions without use of collectors and frothers (Klassen and Mokrousov, 1963). Since then, particularly for coal, there have been many studies investigating this mechanism in seawater and saline waters (Yoon, 1982; Yoon and Sabey, 1989; Li and Somasundaran, 1993; Laskowski, 2001). These studies have brought several theories to explain the coal flotation in inorganic electrolyte solutions. For example, one theory proposes that the inorganic electrolytes prevent bubble coalescence resulting in a reduction of the bubble size and increased population, which in turn increases the encounter efficiency for the bubble-particle attachment and the froth stability, hence flotation efficiency (Yoon, 1982; Paulson and Pugh, 1996). However, this theory cannot explain the enhancement of coal flotation in a Hallimond tube, where a froth phase is essentially not present and coal flotation is primarily determined by the attachment interaction between air bubbles and coal particles (Harvey et al., 2002). Another theory proposes that the presence of electrolytes compresses the electrical double-layer (EDL) between bubbles and particles which corresponds to the reduction of zeta potential of both bubble and particle. Although there are some studies showing that the flotation recovery shows a maximum at minimum zeta potentials (Reay and Ratcliff, 1975; Fuerstenau et al., 1983; Yoon and Sabey, 1989), other ⁎ Corresponding authors. Fax: +61 7 336 54199. E-mail addresses: [email protected] (O. Ozdemir), [email protected] (A.V. Nguyen). 0301-7516/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2009.04.001
studies showed that the flotation recovery has a maximum at pH values both above and below the isoelectric point (Celik and Somasundaran, 1980; Li and Somasundaran, 1993). A final theory proposes that the inorganic electrolytes destabilize the hydrated layers surrounding the coal and reduce the surface hydration of the coal (Klassen and Mokrousov, 1963). The destabilization makes the coal more hydrophobic and enhances the bubble-particle attachment. However, this hypothesis does not support experimental evidence that inorganic salts do not cause flotation of minerals which are not naturally hydrophobic. The hypotheses discussed above have only partly explained the enhancement of coal flotation performance in salt solution. The aim of this paper is conduct a comprehensive study of the role of salt ions in enhancing coal flotation. In particular, the paper focuses on surface chemistry aspects of coal flotation in bore (ground) water which is frequently used as process water in several flotation plants in Australia (George, 1996). 2. Experimental 2.1. Materials The coal samples used in this study were obtained from Peak Down and Saraji, BHP Billiton Mitsubishi Alliance (BMA), Australia. Peak Down coal is more hydrophobic than Saraji coal. The volatile matter for Peak Down and Saraji coal is less than 22% (BMA, 2007). Based on this data, these coals can be specified as low-volatile bituminous coal. The bore water used for the flotation experiments was received from the MKO plant (BHP Billiton, Australia). The chemical analysis of
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Table 1 The chemical analysis of bore water used for the flotation experiments. pH 7.5
Ca2+
K+
Mg2+
Na+
SO2− 4
Cl−
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
77.1
554
1370
2650
25600
11300
38500
Conductivity
TDS
mS/cm 12.8
bore water is presented in Table 1. As seen from the table, the main component of bore water is NaCl which is about 0.7 M (calculated). The density of bore water was 1060 kg/m3 at 20 °C. The surface tension of bore water as measured by pendant bubble tensiometry was 72 ± 0.1 mN/m at 20 °C which is the same as that of deionized (DI) water (The surface tension of similar NaCl solutions is higher. However, the bore water contains many more ions which can jointly interact at the surface to reduce the surface tension to that of DI water). DI water freshly purified using a setup consisting of a reverse osmosis RIO's unit and an Ultrapure Academic Milli-Q system (Millipore, USA) was used for contact angle and AFM experiments. 2.2. Experimental methods and procedures 2.2.1. Flotation experiments and analysis The coal samples were crushed to less than 1 mm and classified into three different particle size fractions by a sieve shaker: 0.5 × 0.25 mm, 0.25 × 0.106 mm, and 0.106 × 0.038 mm. The flotation experiments were conducted with these three particle size ranges using a small laboratory Agitair flotation cell (1.5 L). For each flotation experiment, 150 g of a coal sample was added into the cell and mixed with bore water, and the coal suspension was first conditioned at 1000 rpm for 5 min. Then, air was introduced at a flow rate of 10 L/min into the cell and froth was collected at 1, 3, 7 and 15 min. The solid concentration in the flotation feed was 10% by weight. The impeller speed for flotation tests was kept constant at 1500 rpm. Flotation concentrates and tailings were filtered, dried at 80 °C, and weighed for analysis. For the ash analysis, about 10 g of dried sample from each product was first ground using a mortar and pestle. Then, 2 g of each of the ground samples was burned in an oven at 815 °C for 2 h. The ash left over was weighed to calculate the ash content. 2.2.2. Zeta potential measurements Zeta potential measurements of coal particles were carried out using a Zetasizer Nano-ZS (Malvern, UK). Measurements were performed by the laser Doppler electrophoresis technique. The 106 × 38 mm fraction (1 min flotation product) from the flotation experiments was used for the experiments due to low ash content of the product. First, about 5 g of
the sample was wet ground using a mortar and pestle. Then, the ground sample was screened through a −38 mm sieve. The suspension coming from the wet screening (about 500 mL) was mixed vigorously for 1 min, and 10 mL of representative sample was taken from the suspension and added into a 1 mM KCl solution. The suspension was mixed for an hour. The solid content was ∼0.02%. Prior to each zeta potential measurement, particle size analysis was performed. The results showed that the average particle size for the measurements was about 1.5 mm. For pH adjustments, the suspension was mixed for about 5 min in order to reach equilibrium after adding the desired amount of HCl or NaOH. The experiments were carried out at room temperature (22 °C). In addition to coal particles, electrophoretic mobility measurements of the bubbles were determined using a zeta meter (Rank Brothers, UK). The apparatus and procedures used for the electrophoretic mobility measurements have been described in a previous paper (Jameson and Kubota, 1993). Measurements were performed in 1 mM KCl solution. The mobility values of the bubbles were then converted to zeta potential using the Smoluchowski equation. 2.2.3. Bubble-particle attachment time measurements Bubble attachment times for coal particles in distilled water and bore water in the absence of collectors were measured with attachment timer device. Three different particle sizes, 0.5× 0.25 mm, 0.25 × 0.106 mm, and 0.106 × 0.038 mm were used for the bubble attachment tests. First, the coal particles (∼1 g) were added to the distilled water or bore water (∼100 mL), and the pulp was conditioned for 5 min. Then, the conditioned particles together with the distilled water or bore water, approximately 5 mL, were transferred to a small cell under the bubble holder. Finally, the bubble attachment time experiments were performed for different particle sizes. Fig. 1 shows a schematic of the bubble-particle attachment time device and experimental details. For the Bubble-particle attachment experiments, first, a bubble of about 3 mm in diameter was generated using a micro syringe and then the distance between the bubble and the surface was decreased until the bubble contacted the particle bed using the three-dimensional micro-translation stage and the driver head. Next, the bubble was kept in contact with the bed of particles for a given time from 10 to 7000 ms. Then, the distance was increased to separate the bubble from the particle bed, and the attachment of coal particles onto the bubble was visually observed through the lens and CCD camera on the monitor. Twenty measurements were performed at different areas of the particle bed and the observations were recorded. The bubbleparticle attachment time was calculated as the time for which 50% of the observations resulted in attachment (number of observations with attachment divided by the total number of observations).
Fig. 1. Schematic of the bubble-particle attachment timer device.
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Fig. 2. Schematic of the AFM set-up (a) used for measuring attachment interaction between an air bubble and a coal particle which was glued onto a microfabricated cantilever (b).
cantilever used in the present set of data was found to have a spring constant of 0.324 N/m. A flat substrate of poly-tetrafluoroethylene (PTFE) was cleaned thoroughly by sonication in acetone, ethanol, and DI water followed by drying with a stream of high purity nitrogen. A drop of DI water or bore water was placed on the PTFE plate with a Pasteur pipette and then an air bubble of 800–900 µm in diameter was attached to the surface within the droplet by injecting air through a Hamilton 5 µL syringe (Nguyen et al., 2003). It must be noted that before usage bore water was filtered through a 0.2 µm filter.
2.2.4. Contact angle measurements A polished coal surface was used for the contact angle experiments in DI water and bore water. Advancing and receding water contact angles were measured on a given coal surface by the sessile drop method using a PAT-1 tensiometer system (Sinterface Technologies, Germany). Measurements were performed at five locations on the coal surface and the contact angles averaged. Before the measurements, the coal surface was immersed in the liquid of interest for 5 min, and dried at room temperature (22 °C). In this method, a pendant droplet of DI water or bore water was deposited on the coal surface. Then, the volume of droplet was continuously increased to a maximum volume and then continuously decreased to a minimum volume using a syringe pump while the contact area was kept constant. The air–liquid interface was imaged, digitized and fitted to the Young–Laplace equation to obtain contact angle as a function of the increasing or decreasing liquid volume, giving a cyclic loop of contact angle versus drop volume. The droplet volume changed between 6 µL and 21 µL.
3. Results and discussion 3.1. Results 3.1.1. Flotation experiments Flotation of coal was carried out in bore water as a function of particle size without a frother or collector. The results for Peak Down and Saraji coal are presented in Tables 2 and 3, respectively. As seen from the tables, the flotation of coal in bore water without any frother or collector is possible. These results also indicate that it takes only 1 min to recover 90% of the coal particles in bore water. Fig. 3 shows the combustible recovery and ash content of the flotation products with respect to particle size. These results suggest that a coal product with between 15 and 20% ash content can be beneficiated with a recovery of 90%. The results show that the recovery decreases with an increase in the particle size.
2.2.5. Atomic force microscopy measurements A MFP-3D (Asylum Research, USA) Atomic Force Microscope (AFM) was used for the force measurements. The schematic representation of the experimental setup is presented in Fig. 2(a). The measurement principle is described elsewhere (Nguyen et al., 2003). Coal particles received from Peak Down of approximately 25 µm in diameter were attached under an optical microscope to the end of a triangular cantilever (with a nominal spring constant of 0.58 N/m) by using a very small amount of epoxy resin (Selleys Araldite Super Strength, Australia) (Ducker et al., 1992; Preuss and Butt, 1998). In order to avoid oxidation, the coal sample was kept in a refrigerator at a temperature below 5 °C and was crushed immediately prior the gluing stage. A snapshot of a coal particle glued on a cantilever is presented in Fig. 2(b). The spring constants of the probes were calibrated by employing the thermal vibration method embedded in the AFM software. The
3.1.2. Zeta potentials As seen from Fig. 4, the point of zero charge (pzc) for the coal is about at pH 7, which agrees with the literature data. According to Aplan and Arnold (Aplan and Arnold, 1991), the rank of coals can be classified based on their pzc which decreases from bituminous coal to lignite coal. The pzc for low and medium volatile bituminous coals is
Table 2 Flotation results for Peak Down coal. Particle size
0.5 × 0.25 mm
Flotation time (min)
Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
0.250 × 0.106 mm Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
Yield (%)
0.106 × 0.038 mm Coal (%)
Ash (%)
Coal recovery (%)
1 3 7 15 Sink Total
76.8 7.3 5.2 4.2 6.5 100
85.5 62.9 30.3 17.4 21.1 73.9
14.5 37.1 69.7 82.6 78.9 26.1
88.8 6.2 2.1 1.0 1.9 100.0
83.7 7.4 4.0 3.1 1.8 100
81.0 44.4 15.8 15.3 19.4 72.5
19.0 55.6 84.2 84.7 80.6 27.5
93.5 4.5 0.9 0.6 0.5 100.0
81.3 4.7 3.9 5.1 5.0 100
80.6 33.4 12.6 12.1 12.6 68.9
19.4 66.6 87.4 87.9 87.4 31.1
95.2 2.3 0.7 0.9 0.9 100.0
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Table 3 Flotation results for Saraji coal. Particle size
0.5 × 0.25 mm
Flotation time (min)
Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
Yield (%)
0.250 × 0.106 mm Coal (%)
Ash (%)
Coal recovery (%)
Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
1 3 7 15 Sink Total
81.1 6.3 3.9 3.7 5.0 100
83.8 44.0 21.9 18.3 20.7 73.3
16.2 56.0 78.1 81.7 79.3 26.7
92.7 3.8 1.2 0.9 1.4 100.0
86.6 5.0 3.9 2.5 2.0 100
79.1 27.6 16.7 17.2 21.2 71.4
20.9 72.4 83.3 82.8 78.8 28.6
96.0 1.9 0.9 0.6 0.6 100.0
84.1 2.4 1.8 2.6 9.0 100
76.9 24.9 15.1 13.7 13.6 67.1
23.1 75.1 84.9 86.3 86.4 32.9
96.3 0.9 0.4 0.5 1.8 100.0
about pH 8. The pzc at pH 7 of the coal used in this study shows that the (coking) coal sample is very hydrophobic. The zeta potential measurements of the coal were also carried out in bore water. The pH (unadjusted) of the suspension was about 7.78. The result for the measurement is presented in Fig. 4. The conductivity of the bore water suspension was about 100 mS/cm which is significantly higher than the conductivity (∼ 0.3 mS/cm) of the coal suspensions prepared in 1 mM KCl. The electrophoretic mobility of particles in suspensions of high conductivity is low and can be difficult to measure accurately. In this study, the Zetasizer Nano-ZS apparatus with the PALS (Phase Analysis Light Scattering) technique was used to conduct the measurements. The mean value for the zeta potential of coal particles in bore water was about − 7.84 mV which is significantly lower than the zeta potential of coal particles (about −20 mV) in DI water at the same pH. In addition to the coal particles, the zeta potential measurements of bubbles were measured. The zeta potential values for the bubbles are shown in Fig. 4. It is seen that the bubbles have a negative zeta potential between pH 6 and 9.
3.1.3. Bubble-particle attachment time measurements The bubble-particle interaction in the attachment timer device can be described as the result of three independent steps shown in Fig. 5 (Ye et al., 1989): 1) first, the bubble-particle approach, 2) second the contact and possible attachment of the particles onto the bubble, and 3) finally, the separation of the bubble or bubble-particle aggregate from the particle bed for visualization. By controlling the contact time and visualizing the particles attached, the attachment time can be determined (Ye et al., 1989). The Bubble-particle attachment time experiments were carried out to measure the floatability of coal particles in their natural state in distilled water and bore water. The results for attachment time are
Fig. 3. Flotation recovery for Peak Down and Saraji coal and the ash contents of the flotation products with respect to particle size.
0.106 × 0.038 mm
shown in Fig. 6. As seen from Fig. 6, the bubble-particle attachment time of fine particles in pure water and bore water are almost the same (less than 10 ms). However, the increase in the particle size leads to an increase in the bubble attachment time in bore water. This dependence of attachment time on particle size is also reported in the literature (Ye and Miller, 1988; Ye et al., 1989). It is interesting to note that in bore water the flotation recovery of coarse coal particles (in the absence of reagents) is still high despite the long attachment time measured. However, in pure water the coal particles do not float in the absence of frothers and collectors and, therefore, flotation reagents are normally required. As mentioned before, the attachment time is normally determined as the contact time at 50% attachment. The attachment time for coal particles in distilled water was very short and could not be determined at 50% attachment, due to the device sensitivity. Therefore, the attachment time in distilled water was obtained as the minimum contact time at 100% attachment. Table 4 shows the attachment time determined for coal particles in bore and distilled water. It is evident that the measured attachment time of large coal particles in bore water was significantly longer compared with distilled water. Although there are some studies showing that the attachment time is reduced in NaCl solutions (Yoon and Sabey, 1989), this effect is not consistent with our results. It must also be pointed out that since bore water is not an ideal solution, it is difficult to make a final conclusion or comparison about the results. In addition, after the attachment process the bubbles were shaken in order to see whether or not the particles would detach from the bubble. The shaking tests showed that the large attached particles in bore water could be detached from the bubble surface easily, while in distilled water this was not the case. Particle aggregates were also observed during the experiments with distilled water as seen in Fig. 7. However, the coal aggregation was not observed in bore water. On the contrary, the small coal particles which had almost the same attachment time both in bore water and distilled water could not be easily detached by the shaking tests.
Fig. 4. Zeta potential of coal particles (Peak Down).
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Table 4 Measured attachment times for different coal particles in pure and bore water.
Fig. 5. Schematic of the bubble-particle attachment time measurements.
3.1.4. Cyclic contact angle results Contact angle measurements of a flat coal surface (Peak Down) in the absence of collector were performed in order to determine the relative strength of surface hydration. Fig. 8 shows the contact angles of coal with respect to the droplet volume. As seen from the figure, an increase in the droplet volume increases the contact angle of the coal. In the case of receding contact angle, the droplet volume was decreased until the three phase contact line shrinks resulting in a minimum contact angle. Experimental results for the advancing (maximum) and receding (minimum) contact angle are presented in Table 5. The measurements of the advancing contact angles differed slightly due to the heterogeneous nature of the coal surface at different locations. The measurements of the receding contact angles were very unstable due to the low interfacial profiles and small receding contact angle at small liquid volume. The receding contact angle was smaller than 10 degrees. 3.1.5. AFM results The initial step was to examine the behavior of a coal particle interacting with an air bubble in DI water and bore water. It must be mentioned that employing a particle of such unusual geometry in AFM experiments makes quantitative analysis of the force curves impossible. Consequently, the force curves are only qualitatively compared. This procedure has been successfully used before to highlight the presence of the nanobubbles and/or slime removal on ZnS surfaces after methanol treatment (Holuszko et al., 2008).
Fig. 6. The bubble-particle attachment time results for Peak Down coal in distilled and bore water.
Particle size (mm)
Attachment time in distilled water measured at 100% attachment (ms)
Attachment time in bore water measured at 50% attachment (ms)
0.5 × 0.25 0.25 × 0.106 0.106 × 0.038
b 10 b 10 b 10
∼100 ∼20 b10
Fig. 9 shows typical force curves obtained during approach of a coal particle to an air bubble in DI water and bore water, 20 min after the moment of bubble generation. In the case of interactions between two hard surfaces it is possible to plot the data as force vs. separation, but for the bubble-particle interactions the deformation of the air–water interface must be considered. Consequently the present data will be plotted as force vs. piezo movement. Prior to the bubble-particle measurements, the AFM photodetector was calibrated by pressing the cantilever against the hard surface. Here only the approaching force curves was shown because the capillary forces overcame the spring constant of the cantilever and the laser was deflected outside the photodetector window. The measurements were done at the apex of the air bubble, using a velocity of 1 µm/s and applying a maximum force of 40 nN. At large separation distances, the force is zero which corresponds to an absence of interaction. During approach, the cantilever starts to deflect (Fig. 9(a)) because of the cumulative effect of the electrostatic, van der Waals repulsive forces and hydrophobic forces (Nguyen et al., 2001). The repulsive regime ends with a jump-in corresponding to the rupture of the film. The inset in Fig. 9(a) shows the magnitude of the repulsive force and the force required for the film to rupture (0.28 nN). In different experiments the magnitude of the maximum force prior to the engulfment varied, but the force vs. piezo movement followed the same path. Unlike the pure water case, where repulsion was always recorded, in the case of bore water, the long range electrostatic contribution is totally screened as can be observed in Fig. 9(b). The measurements were repeated several times and were found to be consistent. Since the retraction force curves could not be recorded, a much stiffer cantilever (KN = 52.86 N/m) was used to study the pull off forces between a coal particle and an air bubble in DI water and bore water. The results are presented in Fig. 10. It is interesting to note that the pull off forces coincide well in both cases even though the features in approaching curves are different.
Fig. 7. Coal aggregates during the bubble-particle attachment experiments (0.5× 0.25 mm). (a) in distilled water and (b) bore water.
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Fig. 8. Contact angle measurements for the flat coal surface.
3.2. Discussion The flotation results suggest that under the present test conditions the coal product can be recovered in bore water without using any frother or collector. These results agree with the earlier findings (Yoon and Sabey, 1989; Li and Somasundaran, 1993). Li and Somasundaran (1993) which showed that above 0.1 M NaCl any increase in salt concentration increased the coal floatability. Their studies also showed that the coal flotation decreased with an increase in the salt concentration up to 0.1 M (Li and Somasundaran, 1993). As shown from the zeta potential measurements, the magnitude of zeta potential of coal particles in bore water was lower than in DI water. Previous studies with coal and bubbles showed that at high salt concentrations, dissolved ions, particularly cations such as Na+ and Mg2+, show a significant influence on the electrokinetic behavior of the bubbles and particles, and reduce the magnitude of zeta potential of bubble and particle. (Li and Somasundaran, 1991; Paulson and Pugh, 1996; Harvey et al., 2002). As known from the literature, the electrostatic interaction would be sufficiently weak at high electrolyte concentrations due to the compression of the electrical double layers (EDL). For this reason, the hydrophobic force would dominate the bubble-particle interaction. It is believed that this EDL compression would enable bubble-particle attachment to occur and better flotation response. As shown in bubble-particle attachment tests, coal is very hydrophobic in pure water with the shortest induction time for all size fractions, while the induction time of coal in bore water depends on the particle size. According to the bubble-particle attachment time results, coal particles would be more floatable in pure water than in bore water. However, the flotation tests showed the opposite results. Therefore, the increase in bubble-particle attachment time for coal particles in bore water could not have any significant effect on the coal flotation in bore water. These results clearly indicate that the attachment time experiments could not precisely correlate with the flotation behavior of the coal particles in bore water. The Bubble-particle attachment time is related to the long-range hydrophobic interaction between the bubble and the coal particles. The long-range hydrophobic interaction depends on: (i) the hydroTable 5 The contact angles for coal surface.
DI water Bore water
Advancing (°)
Receding (°)
75 ± 4 75 ± 4
b10 b10
Fig. 9. Approaching force curves for the interaction between a coal particle and an air bubble. (a) in DI water and (b) in bore water.
phobicity of the coal particles and (ii) the amount the dissolved gas in the medium (Paulson and Pugh, 1996). It is known that salty water contains less dissolved air and thus less cavitation occurs (Paulson and Pugh, 1996). Thus, the coarse coal particles (N0.25 mm) cannot effectively attach to the bubble in bore water (large bubble attachment time) as compared to coarse particles in distilled water (short bubble-particle attachment time). It is noted that liberation of coal particles from gangue increases with decreasing particle size and, therefore, small coal particles are likely more hydrophobic than large particles. Thus, the strong and long-range hydrophobic attraction occurs between the small coal particles and the bubble in both distilled and bore water. The contact angle on a coal surface in DI water is almost the same as that in bore water, which means that the friction of the three-phase contact line coal/gas/water in bore water and pure water is the same. The bubbles equally hold the trapped particles in both distilled water and bore water. Possibly, in both cases the particles detach from the bubble during the shaking tests; in the case of distilled water cavitations reappear again; while in the case of bore water, such cavitations are less probable due the decreased concentration of dissolved gas in bore water. Therefore, the large particles can be easily detached in bore water. Efficiency of flotation depends on the degree of bubble coalescence (Li and Somasundaran, 1991). In the flotation system, bubbles are highly hydrophobic, and they coalescence because of low repulsive force between them. Craig et al. (1993) showed that the bubble coalescence was inhibited in the presence of some electrolytes such as
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Fig. 10. Retraction force curves for the interaction of a coal particle and an air bubble in DI water and bore water.
NaCl, KCl, MgCl2 etc. (Craig et al., 1993). Under these conditions, smaller bubbles will be generated with a higher population. And, it is believed that the high number of the bubbles in the system increased the flotation efficiency of coal particles in bore water. 4. Conclusions Results show that coal particles can float in bore water, which contains mostly Na+ and Mg2+, without using any frother or collector and produce a coal concentrate of 85%. The flotation results showed that 90% of the coal particles float within 1 min which suggests rapid flotation kinetics in the presence of salt ions. These results also indicated that a poor flotation response for particles coarser than 0.106 mm. Coal flotation in bore water was also investigated to understand the surface chemistry of coal flotation. The bubble-particle attachment time experiments carried out to measure the floatability of coal particles in pure water and bore water showed that the bubbleparticle attachment in bore water is size dependent. Meanwhile, the contact angle values for a flat coal surface in bore water did not change compared to that in pure water. AFM experiments showed that the repulsive force between bubble and particle reduced in bore water. These results provide significant insight into coal flotation in hypersaline water and an important consideration in the development of improved flotation technology for the coal industry. However, additional research is required for a better understanding of the coal flotation in salt solutions which will reduce demands on scarce fresh water supplies. Acknowledgements The Australian Research Council is gratefully acknowledged for financial support through a Discovery grant (AVN). BHP Billiton Mitsubishi Alliance (BMA) is gratefully acknowledged for funding the BMA Chair of Minerals Processing at the University of Queensland (AVN) and the coal samples (Mr Ian Brake and Mr Ben Cronin). BHP Billiton Nickel West (Perth, Australia) is acknowledged for providing the bore water samples. We are indebted to Mr Maung Aung Min from the JKMRC, the University of Queensland for helping with the bubbleparticle attachment time measurements.
References Aplan, F.F., Arnold, B.J., 1991. Part 2: wet fine particle concentration. Section 3: Flotation. Coal Preparation. J. W. L. a. B. C. Hardinge. SME, Littleton, Colorado. BMA (2007)."http://www.bmacoal.com/about_BMA/documents/FY07_Coal_Resources_ Summary.pdf." Celik, M.S., Somasundaran, P., 1980. Effect of pretreatments on flotation and electrokinetic properties of coal. Colloids Surf. 1 (1), 121–124. Craig, V.S.J., Ninham, B.W., et al., 1993. Effect of electrolytes on bubble coalescence. Nature 364 (6435), 317–319. Ducker, W.A., Senden, T.J., et al., 1992. Measurement of forces in liquids using a force microscope. Langmuir 8 (7), 1831–1836. Fuerstenau, D.W., Rosenbaum, J.M., et al., 1983. Effect of surface functional groups on the flotation of coal. Colloids Surf. 8 (2), 153–173. George, C., 1996. The Mt Keith operation. Australasian Institute of Mining and Metallurgy, pp. 9–23. Harvey, P.A., Nguyen, A.V., et al., 2002. Influence of electrical double-layer interaction on coal flotation. J. Colloid Interface Sci. 250 (2), 337–343. Holuszko, M.E., Franzidis, J.P., et al., 2008. The effect of surface treatment and slime coatings on ZnS hydrophobicity. Miner. Eng. 21 (12–14), 958–966. Jameson, G.J., Kubota, K., 1993. A study of the electrophoretic mobility of a very small inert gas bubble suspended in aqueous inorganic electrolyte and cationic surfactant solutions. J. Chem. Eng. Jpn. 26 (1), 7–12. Klassen, V.I., Mokrousov, V.A., 1963. An Introduction to the Theory of Flotation. Butterworths, London. Laskowski, J.S., 2001. Coal flotation and fine coal utilization. Elsevier, Amsterdam. Li, C., Somasundaran, P., 1991. Reversal of bubble charge in multivalent inorganic salt solutions — effect of magnesium. J. Colloid Interface Sci. 146 (1), 215–218. Li, C., Somasundaran, P., 1993. Role of electrical double layer forces and hydrophobicity in coal flotation in sodium chloride solutions. Energy Fuels 7 (2), 244–248. Nguyen, A.V., Evans, G.M., et al., 2001. Prediction of van der Waals interaction in bubbleparticle attachment in flotation. Int. J. Miner. Process. 61 (3), 155–169. Nguyen, A.V., Nalaskowski, J., et al., 2003. A study of bubble-particle interaction using atomic force microscopy. Miner. Eng. 16 (11), 1173–1181. Paulson, O., Pugh, R.J., 1996. Flotation of inherently hydrophobic particles in aqueous solutions of inorganic electrolytes. Langmuir 12 (20), 4808–4813. Preuss, M., Butt, H.J., 1998. Direct measurement of particle-bubble interactions in aqueous electrolyte: dependence on surfactant. Langmuir 14 (12), 3164–3174. Reay, D., Ratcliff, G.A., 1975. Experimental testing of the hydrodynamic collision model of fine particle flotation. Can. J. Chem. Eng. 53 (5), 481–486. Ye, Y., Khandrika, S.M., et al., 1989. Induction-time measurements at a particle bed. Int. J. Miner. Process. 25 (3–4), 221–240. Ye, Y., Miller, J.D., 1988. Bubble/particle contact time in the analysis of coal flotation. Coal Prep. (Gordon & Breach) 5 (3–4), 147–166. Yoon, R.H., 1982. Flotation of coal using micro-bubbles and inorganic salts. Min. Congr. J. 68, 76–80. Yoon, R.H., Sabey, J.B., 1989. Coal flotation in inorganic salt solution. In: Botsaris, G.D., Glazman, Y.M. (Eds.), Interfacial phenomena in coal technology. Marcel Dekker, New York, pp. 87–114.
Contents lists available at ScienceDirect
Int. J. Miner. Process. j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Surface chemistry aspects of coal flotation in bore water O. Ozdemir ⁎, E. Taran, M.A. Hampton, S.I. Karakashev, A.V. Nguyen ⁎ Division of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 21 October 2008 Received in revised form 28 March 2009 Accepted 1 April 2009 Available online 8 April 2009 Keywords: Coal flotation Bore water Bubble-particle attachment Contact angle Zeta potential Atomic force microscopy
a b s t r a c t Several theories have been proposed to explain enhancement of coal flotation in salt solutions. In this paper, surface chemistry aspects of coal flotation in bore (hypersaline) water were examined using bubble-particle attachment time experiments, zeta potential measurements, cyclic measurements of contact angle and Atomic Force Microscopy (AFM). The attachment time experiments showed that the bubble-particle attachment in deionized water was instantaneous and independent of the particle size. The attachment in bore water required longer time, which increased with increasing particle size. The cyclic measurements of contact angle on a flat coal surface showed that the coal hydrophobicity as measured by the advancing (maximum) and receding (minimum) contact angle did not change in the presence of salt ions. The zeta potential measurements show that both the coal particles and air bubbles were negatively charged in bore water. The AFM studies showed that bore water reduced repulsive surface forces between the coal particles and air bubbles but had little effect on the force of adhesion. The overall results suggest that enhancement of coal flotation in hypersaline water is not entirely attributed to the surface chemistry aspects as previously proposed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the 1930s, researchers in the former USSR found that naturally hydrophobic minerals such as coal could be floated in electrolyte solutions without use of collectors and frothers (Klassen and Mokrousov, 1963). Since then, particularly for coal, there have been many studies investigating this mechanism in seawater and saline waters (Yoon, 1982; Yoon and Sabey, 1989; Li and Somasundaran, 1993; Laskowski, 2001). These studies have brought several theories to explain the coal flotation in inorganic electrolyte solutions. For example, one theory proposes that the inorganic electrolytes prevent bubble coalescence resulting in a reduction of the bubble size and increased population, which in turn increases the encounter efficiency for the bubble-particle attachment and the froth stability, hence flotation efficiency (Yoon, 1982; Paulson and Pugh, 1996). However, this theory cannot explain the enhancement of coal flotation in a Hallimond tube, where a froth phase is essentially not present and coal flotation is primarily determined by the attachment interaction between air bubbles and coal particles (Harvey et al., 2002). Another theory proposes that the presence of electrolytes compresses the electrical double-layer (EDL) between bubbles and particles which corresponds to the reduction of zeta potential of both bubble and particle. Although there are some studies showing that the flotation recovery shows a maximum at minimum zeta potentials (Reay and Ratcliff, 1975; Fuerstenau et al., 1983; Yoon and Sabey, 1989), other ⁎ Corresponding authors. Fax: +61 7 336 54199. E-mail addresses: [email protected] (O. Ozdemir), [email protected] (A.V. Nguyen). 0301-7516/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2009.04.001
studies showed that the flotation recovery has a maximum at pH values both above and below the isoelectric point (Celik and Somasundaran, 1980; Li and Somasundaran, 1993). A final theory proposes that the inorganic electrolytes destabilize the hydrated layers surrounding the coal and reduce the surface hydration of the coal (Klassen and Mokrousov, 1963). The destabilization makes the coal more hydrophobic and enhances the bubble-particle attachment. However, this hypothesis does not support experimental evidence that inorganic salts do not cause flotation of minerals which are not naturally hydrophobic. The hypotheses discussed above have only partly explained the enhancement of coal flotation performance in salt solution. The aim of this paper is conduct a comprehensive study of the role of salt ions in enhancing coal flotation. In particular, the paper focuses on surface chemistry aspects of coal flotation in bore (ground) water which is frequently used as process water in several flotation plants in Australia (George, 1996). 2. Experimental 2.1. Materials The coal samples used in this study were obtained from Peak Down and Saraji, BHP Billiton Mitsubishi Alliance (BMA), Australia. Peak Down coal is more hydrophobic than Saraji coal. The volatile matter for Peak Down and Saraji coal is less than 22% (BMA, 2007). Based on this data, these coals can be specified as low-volatile bituminous coal. The bore water used for the flotation experiments was received from the MKO plant (BHP Billiton, Australia). The chemical analysis of
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Table 1 The chemical analysis of bore water used for the flotation experiments. pH 7.5
Ca2+
K+
Mg2+
Na+
SO2− 4
Cl−
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
77.1
554
1370
2650
25600
11300
38500
Conductivity
TDS
mS/cm 12.8
bore water is presented in Table 1. As seen from the table, the main component of bore water is NaCl which is about 0.7 M (calculated). The density of bore water was 1060 kg/m3 at 20 °C. The surface tension of bore water as measured by pendant bubble tensiometry was 72 ± 0.1 mN/m at 20 °C which is the same as that of deionized (DI) water (The surface tension of similar NaCl solutions is higher. However, the bore water contains many more ions which can jointly interact at the surface to reduce the surface tension to that of DI water). DI water freshly purified using a setup consisting of a reverse osmosis RIO's unit and an Ultrapure Academic Milli-Q system (Millipore, USA) was used for contact angle and AFM experiments. 2.2. Experimental methods and procedures 2.2.1. Flotation experiments and analysis The coal samples were crushed to less than 1 mm and classified into three different particle size fractions by a sieve shaker: 0.5 × 0.25 mm, 0.25 × 0.106 mm, and 0.106 × 0.038 mm. The flotation experiments were conducted with these three particle size ranges using a small laboratory Agitair flotation cell (1.5 L). For each flotation experiment, 150 g of a coal sample was added into the cell and mixed with bore water, and the coal suspension was first conditioned at 1000 rpm for 5 min. Then, air was introduced at a flow rate of 10 L/min into the cell and froth was collected at 1, 3, 7 and 15 min. The solid concentration in the flotation feed was 10% by weight. The impeller speed for flotation tests was kept constant at 1500 rpm. Flotation concentrates and tailings were filtered, dried at 80 °C, and weighed for analysis. For the ash analysis, about 10 g of dried sample from each product was first ground using a mortar and pestle. Then, 2 g of each of the ground samples was burned in an oven at 815 °C for 2 h. The ash left over was weighed to calculate the ash content. 2.2.2. Zeta potential measurements Zeta potential measurements of coal particles were carried out using a Zetasizer Nano-ZS (Malvern, UK). Measurements were performed by the laser Doppler electrophoresis technique. The 106 × 38 mm fraction (1 min flotation product) from the flotation experiments was used for the experiments due to low ash content of the product. First, about 5 g of
the sample was wet ground using a mortar and pestle. Then, the ground sample was screened through a −38 mm sieve. The suspension coming from the wet screening (about 500 mL) was mixed vigorously for 1 min, and 10 mL of representative sample was taken from the suspension and added into a 1 mM KCl solution. The suspension was mixed for an hour. The solid content was ∼0.02%. Prior to each zeta potential measurement, particle size analysis was performed. The results showed that the average particle size for the measurements was about 1.5 mm. For pH adjustments, the suspension was mixed for about 5 min in order to reach equilibrium after adding the desired amount of HCl or NaOH. The experiments were carried out at room temperature (22 °C). In addition to coal particles, electrophoretic mobility measurements of the bubbles were determined using a zeta meter (Rank Brothers, UK). The apparatus and procedures used for the electrophoretic mobility measurements have been described in a previous paper (Jameson and Kubota, 1993). Measurements were performed in 1 mM KCl solution. The mobility values of the bubbles were then converted to zeta potential using the Smoluchowski equation. 2.2.3. Bubble-particle attachment time measurements Bubble attachment times for coal particles in distilled water and bore water in the absence of collectors were measured with attachment timer device. Three different particle sizes, 0.5× 0.25 mm, 0.25 × 0.106 mm, and 0.106 × 0.038 mm were used for the bubble attachment tests. First, the coal particles (∼1 g) were added to the distilled water or bore water (∼100 mL), and the pulp was conditioned for 5 min. Then, the conditioned particles together with the distilled water or bore water, approximately 5 mL, were transferred to a small cell under the bubble holder. Finally, the bubble attachment time experiments were performed for different particle sizes. Fig. 1 shows a schematic of the bubble-particle attachment time device and experimental details. For the Bubble-particle attachment experiments, first, a bubble of about 3 mm in diameter was generated using a micro syringe and then the distance between the bubble and the surface was decreased until the bubble contacted the particle bed using the three-dimensional micro-translation stage and the driver head. Next, the bubble was kept in contact with the bed of particles for a given time from 10 to 7000 ms. Then, the distance was increased to separate the bubble from the particle bed, and the attachment of coal particles onto the bubble was visually observed through the lens and CCD camera on the monitor. Twenty measurements were performed at different areas of the particle bed and the observations were recorded. The bubbleparticle attachment time was calculated as the time for which 50% of the observations resulted in attachment (number of observations with attachment divided by the total number of observations).
Fig. 1. Schematic of the bubble-particle attachment timer device.
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179
Fig. 2. Schematic of the AFM set-up (a) used for measuring attachment interaction between an air bubble and a coal particle which was glued onto a microfabricated cantilever (b).
cantilever used in the present set of data was found to have a spring constant of 0.324 N/m. A flat substrate of poly-tetrafluoroethylene (PTFE) was cleaned thoroughly by sonication in acetone, ethanol, and DI water followed by drying with a stream of high purity nitrogen. A drop of DI water or bore water was placed on the PTFE plate with a Pasteur pipette and then an air bubble of 800–900 µm in diameter was attached to the surface within the droplet by injecting air through a Hamilton 5 µL syringe (Nguyen et al., 2003). It must be noted that before usage bore water was filtered through a 0.2 µm filter.
2.2.4. Contact angle measurements A polished coal surface was used for the contact angle experiments in DI water and bore water. Advancing and receding water contact angles were measured on a given coal surface by the sessile drop method using a PAT-1 tensiometer system (Sinterface Technologies, Germany). Measurements were performed at five locations on the coal surface and the contact angles averaged. Before the measurements, the coal surface was immersed in the liquid of interest for 5 min, and dried at room temperature (22 °C). In this method, a pendant droplet of DI water or bore water was deposited on the coal surface. Then, the volume of droplet was continuously increased to a maximum volume and then continuously decreased to a minimum volume using a syringe pump while the contact area was kept constant. The air–liquid interface was imaged, digitized and fitted to the Young–Laplace equation to obtain contact angle as a function of the increasing or decreasing liquid volume, giving a cyclic loop of contact angle versus drop volume. The droplet volume changed between 6 µL and 21 µL.
3. Results and discussion 3.1. Results 3.1.1. Flotation experiments Flotation of coal was carried out in bore water as a function of particle size without a frother or collector. The results for Peak Down and Saraji coal are presented in Tables 2 and 3, respectively. As seen from the tables, the flotation of coal in bore water without any frother or collector is possible. These results also indicate that it takes only 1 min to recover 90% of the coal particles in bore water. Fig. 3 shows the combustible recovery and ash content of the flotation products with respect to particle size. These results suggest that a coal product with between 15 and 20% ash content can be beneficiated with a recovery of 90%. The results show that the recovery decreases with an increase in the particle size.
2.2.5. Atomic force microscopy measurements A MFP-3D (Asylum Research, USA) Atomic Force Microscope (AFM) was used for the force measurements. The schematic representation of the experimental setup is presented in Fig. 2(a). The measurement principle is described elsewhere (Nguyen et al., 2003). Coal particles received from Peak Down of approximately 25 µm in diameter were attached under an optical microscope to the end of a triangular cantilever (with a nominal spring constant of 0.58 N/m) by using a very small amount of epoxy resin (Selleys Araldite Super Strength, Australia) (Ducker et al., 1992; Preuss and Butt, 1998). In order to avoid oxidation, the coal sample was kept in a refrigerator at a temperature below 5 °C and was crushed immediately prior the gluing stage. A snapshot of a coal particle glued on a cantilever is presented in Fig. 2(b). The spring constants of the probes were calibrated by employing the thermal vibration method embedded in the AFM software. The
3.1.2. Zeta potentials As seen from Fig. 4, the point of zero charge (pzc) for the coal is about at pH 7, which agrees with the literature data. According to Aplan and Arnold (Aplan and Arnold, 1991), the rank of coals can be classified based on their pzc which decreases from bituminous coal to lignite coal. The pzc for low and medium volatile bituminous coals is
Table 2 Flotation results for Peak Down coal. Particle size
0.5 × 0.25 mm
Flotation time (min)
Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
0.250 × 0.106 mm Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
Yield (%)
0.106 × 0.038 mm Coal (%)
Ash (%)
Coal recovery (%)
1 3 7 15 Sink Total
76.8 7.3 5.2 4.2 6.5 100
85.5 62.9 30.3 17.4 21.1 73.9
14.5 37.1 69.7 82.6 78.9 26.1
88.8 6.2 2.1 1.0 1.9 100.0
83.7 7.4 4.0 3.1 1.8 100
81.0 44.4 15.8 15.3 19.4 72.5
19.0 55.6 84.2 84.7 80.6 27.5
93.5 4.5 0.9 0.6 0.5 100.0
81.3 4.7 3.9 5.1 5.0 100
80.6 33.4 12.6 12.1 12.6 68.9
19.4 66.6 87.4 87.9 87.4 31.1
95.2 2.3 0.7 0.9 0.9 100.0
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Table 3 Flotation results for Saraji coal. Particle size
0.5 × 0.25 mm
Flotation time (min)
Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
Yield (%)
0.250 × 0.106 mm Coal (%)
Ash (%)
Coal recovery (%)
Yield (%)
Coal (%)
Ash (%)
Coal recovery (%)
1 3 7 15 Sink Total
81.1 6.3 3.9 3.7 5.0 100
83.8 44.0 21.9 18.3 20.7 73.3
16.2 56.0 78.1 81.7 79.3 26.7
92.7 3.8 1.2 0.9 1.4 100.0
86.6 5.0 3.9 2.5 2.0 100
79.1 27.6 16.7 17.2 21.2 71.4
20.9 72.4 83.3 82.8 78.8 28.6
96.0 1.9 0.9 0.6 0.6 100.0
84.1 2.4 1.8 2.6 9.0 100
76.9 24.9 15.1 13.7 13.6 67.1
23.1 75.1 84.9 86.3 86.4 32.9
96.3 0.9 0.4 0.5 1.8 100.0
about pH 8. The pzc at pH 7 of the coal used in this study shows that the (coking) coal sample is very hydrophobic. The zeta potential measurements of the coal were also carried out in bore water. The pH (unadjusted) of the suspension was about 7.78. The result for the measurement is presented in Fig. 4. The conductivity of the bore water suspension was about 100 mS/cm which is significantly higher than the conductivity (∼ 0.3 mS/cm) of the coal suspensions prepared in 1 mM KCl. The electrophoretic mobility of particles in suspensions of high conductivity is low and can be difficult to measure accurately. In this study, the Zetasizer Nano-ZS apparatus with the PALS (Phase Analysis Light Scattering) technique was used to conduct the measurements. The mean value for the zeta potential of coal particles in bore water was about − 7.84 mV which is significantly lower than the zeta potential of coal particles (about −20 mV) in DI water at the same pH. In addition to the coal particles, the zeta potential measurements of bubbles were measured. The zeta potential values for the bubbles are shown in Fig. 4. It is seen that the bubbles have a negative zeta potential between pH 6 and 9.
3.1.3. Bubble-particle attachment time measurements The bubble-particle interaction in the attachment timer device can be described as the result of three independent steps shown in Fig. 5 (Ye et al., 1989): 1) first, the bubble-particle approach, 2) second the contact and possible attachment of the particles onto the bubble, and 3) finally, the separation of the bubble or bubble-particle aggregate from the particle bed for visualization. By controlling the contact time and visualizing the particles attached, the attachment time can be determined (Ye et al., 1989). The Bubble-particle attachment time experiments were carried out to measure the floatability of coal particles in their natural state in distilled water and bore water. The results for attachment time are
Fig. 3. Flotation recovery for Peak Down and Saraji coal and the ash contents of the flotation products with respect to particle size.
0.106 × 0.038 mm
shown in Fig. 6. As seen from Fig. 6, the bubble-particle attachment time of fine particles in pure water and bore water are almost the same (less than 10 ms). However, the increase in the particle size leads to an increase in the bubble attachment time in bore water. This dependence of attachment time on particle size is also reported in the literature (Ye and Miller, 1988; Ye et al., 1989). It is interesting to note that in bore water the flotation recovery of coarse coal particles (in the absence of reagents) is still high despite the long attachment time measured. However, in pure water the coal particles do not float in the absence of frothers and collectors and, therefore, flotation reagents are normally required. As mentioned before, the attachment time is normally determined as the contact time at 50% attachment. The attachment time for coal particles in distilled water was very short and could not be determined at 50% attachment, due to the device sensitivity. Therefore, the attachment time in distilled water was obtained as the minimum contact time at 100% attachment. Table 4 shows the attachment time determined for coal particles in bore and distilled water. It is evident that the measured attachment time of large coal particles in bore water was significantly longer compared with distilled water. Although there are some studies showing that the attachment time is reduced in NaCl solutions (Yoon and Sabey, 1989), this effect is not consistent with our results. It must also be pointed out that since bore water is not an ideal solution, it is difficult to make a final conclusion or comparison about the results. In addition, after the attachment process the bubbles were shaken in order to see whether or not the particles would detach from the bubble. The shaking tests showed that the large attached particles in bore water could be detached from the bubble surface easily, while in distilled water this was not the case. Particle aggregates were also observed during the experiments with distilled water as seen in Fig. 7. However, the coal aggregation was not observed in bore water. On the contrary, the small coal particles which had almost the same attachment time both in bore water and distilled water could not be easily detached by the shaking tests.
Fig. 4. Zeta potential of coal particles (Peak Down).
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Table 4 Measured attachment times for different coal particles in pure and bore water.
Fig. 5. Schematic of the bubble-particle attachment time measurements.
3.1.4. Cyclic contact angle results Contact angle measurements of a flat coal surface (Peak Down) in the absence of collector were performed in order to determine the relative strength of surface hydration. Fig. 8 shows the contact angles of coal with respect to the droplet volume. As seen from the figure, an increase in the droplet volume increases the contact angle of the coal. In the case of receding contact angle, the droplet volume was decreased until the three phase contact line shrinks resulting in a minimum contact angle. Experimental results for the advancing (maximum) and receding (minimum) contact angle are presented in Table 5. The measurements of the advancing contact angles differed slightly due to the heterogeneous nature of the coal surface at different locations. The measurements of the receding contact angles were very unstable due to the low interfacial profiles and small receding contact angle at small liquid volume. The receding contact angle was smaller than 10 degrees. 3.1.5. AFM results The initial step was to examine the behavior of a coal particle interacting with an air bubble in DI water and bore water. It must be mentioned that employing a particle of such unusual geometry in AFM experiments makes quantitative analysis of the force curves impossible. Consequently, the force curves are only qualitatively compared. This procedure has been successfully used before to highlight the presence of the nanobubbles and/or slime removal on ZnS surfaces after methanol treatment (Holuszko et al., 2008).
Fig. 6. The bubble-particle attachment time results for Peak Down coal in distilled and bore water.
Particle size (mm)
Attachment time in distilled water measured at 100% attachment (ms)
Attachment time in bore water measured at 50% attachment (ms)
0.5 × 0.25 0.25 × 0.106 0.106 × 0.038
b 10 b 10 b 10
∼100 ∼20 b10
Fig. 9 shows typical force curves obtained during approach of a coal particle to an air bubble in DI water and bore water, 20 min after the moment of bubble generation. In the case of interactions between two hard surfaces it is possible to plot the data as force vs. separation, but for the bubble-particle interactions the deformation of the air–water interface must be considered. Consequently the present data will be plotted as force vs. piezo movement. Prior to the bubble-particle measurements, the AFM photodetector was calibrated by pressing the cantilever against the hard surface. Here only the approaching force curves was shown because the capillary forces overcame the spring constant of the cantilever and the laser was deflected outside the photodetector window. The measurements were done at the apex of the air bubble, using a velocity of 1 µm/s and applying a maximum force of 40 nN. At large separation distances, the force is zero which corresponds to an absence of interaction. During approach, the cantilever starts to deflect (Fig. 9(a)) because of the cumulative effect of the electrostatic, van der Waals repulsive forces and hydrophobic forces (Nguyen et al., 2001). The repulsive regime ends with a jump-in corresponding to the rupture of the film. The inset in Fig. 9(a) shows the magnitude of the repulsive force and the force required for the film to rupture (0.28 nN). In different experiments the magnitude of the maximum force prior to the engulfment varied, but the force vs. piezo movement followed the same path. Unlike the pure water case, where repulsion was always recorded, in the case of bore water, the long range electrostatic contribution is totally screened as can be observed in Fig. 9(b). The measurements were repeated several times and were found to be consistent. Since the retraction force curves could not be recorded, a much stiffer cantilever (KN = 52.86 N/m) was used to study the pull off forces between a coal particle and an air bubble in DI water and bore water. The results are presented in Fig. 10. It is interesting to note that the pull off forces coincide well in both cases even though the features in approaching curves are different.
Fig. 7. Coal aggregates during the bubble-particle attachment experiments (0.5× 0.25 mm). (a) in distilled water and (b) bore water.
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Fig. 8. Contact angle measurements for the flat coal surface.
3.2. Discussion The flotation results suggest that under the present test conditions the coal product can be recovered in bore water without using any frother or collector. These results agree with the earlier findings (Yoon and Sabey, 1989; Li and Somasundaran, 1993). Li and Somasundaran (1993) which showed that above 0.1 M NaCl any increase in salt concentration increased the coal floatability. Their studies also showed that the coal flotation decreased with an increase in the salt concentration up to 0.1 M (Li and Somasundaran, 1993). As shown from the zeta potential measurements, the magnitude of zeta potential of coal particles in bore water was lower than in DI water. Previous studies with coal and bubbles showed that at high salt concentrations, dissolved ions, particularly cations such as Na+ and Mg2+, show a significant influence on the electrokinetic behavior of the bubbles and particles, and reduce the magnitude of zeta potential of bubble and particle. (Li and Somasundaran, 1991; Paulson and Pugh, 1996; Harvey et al., 2002). As known from the literature, the electrostatic interaction would be sufficiently weak at high electrolyte concentrations due to the compression of the electrical double layers (EDL). For this reason, the hydrophobic force would dominate the bubble-particle interaction. It is believed that this EDL compression would enable bubble-particle attachment to occur and better flotation response. As shown in bubble-particle attachment tests, coal is very hydrophobic in pure water with the shortest induction time for all size fractions, while the induction time of coal in bore water depends on the particle size. According to the bubble-particle attachment time results, coal particles would be more floatable in pure water than in bore water. However, the flotation tests showed the opposite results. Therefore, the increase in bubble-particle attachment time for coal particles in bore water could not have any significant effect on the coal flotation in bore water. These results clearly indicate that the attachment time experiments could not precisely correlate with the flotation behavior of the coal particles in bore water. The Bubble-particle attachment time is related to the long-range hydrophobic interaction between the bubble and the coal particles. The long-range hydrophobic interaction depends on: (i) the hydroTable 5 The contact angles for coal surface.
DI water Bore water
Advancing (°)
Receding (°)
75 ± 4 75 ± 4
b10 b10
Fig. 9. Approaching force curves for the interaction between a coal particle and an air bubble. (a) in DI water and (b) in bore water.
phobicity of the coal particles and (ii) the amount the dissolved gas in the medium (Paulson and Pugh, 1996). It is known that salty water contains less dissolved air and thus less cavitation occurs (Paulson and Pugh, 1996). Thus, the coarse coal particles (N0.25 mm) cannot effectively attach to the bubble in bore water (large bubble attachment time) as compared to coarse particles in distilled water (short bubble-particle attachment time). It is noted that liberation of coal particles from gangue increases with decreasing particle size and, therefore, small coal particles are likely more hydrophobic than large particles. Thus, the strong and long-range hydrophobic attraction occurs between the small coal particles and the bubble in both distilled and bore water. The contact angle on a coal surface in DI water is almost the same as that in bore water, which means that the friction of the three-phase contact line coal/gas/water in bore water and pure water is the same. The bubbles equally hold the trapped particles in both distilled water and bore water. Possibly, in both cases the particles detach from the bubble during the shaking tests; in the case of distilled water cavitations reappear again; while in the case of bore water, such cavitations are less probable due the decreased concentration of dissolved gas in bore water. Therefore, the large particles can be easily detached in bore water. Efficiency of flotation depends on the degree of bubble coalescence (Li and Somasundaran, 1991). In the flotation system, bubbles are highly hydrophobic, and they coalescence because of low repulsive force between them. Craig et al. (1993) showed that the bubble coalescence was inhibited in the presence of some electrolytes such as
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Fig. 10. Retraction force curves for the interaction of a coal particle and an air bubble in DI water and bore water.
NaCl, KCl, MgCl2 etc. (Craig et al., 1993). Under these conditions, smaller bubbles will be generated with a higher population. And, it is believed that the high number of the bubbles in the system increased the flotation efficiency of coal particles in bore water. 4. Conclusions Results show that coal particles can float in bore water, which contains mostly Na+ and Mg2+, without using any frother or collector and produce a coal concentrate of 85%. The flotation results showed that 90% of the coal particles float within 1 min which suggests rapid flotation kinetics in the presence of salt ions. These results also indicated that a poor flotation response for particles coarser than 0.106 mm. Coal flotation in bore water was also investigated to understand the surface chemistry of coal flotation. The bubble-particle attachment time experiments carried out to measure the floatability of coal particles in pure water and bore water showed that the bubbleparticle attachment in bore water is size dependent. Meanwhile, the contact angle values for a flat coal surface in bore water did not change compared to that in pure water. AFM experiments showed that the repulsive force between bubble and particle reduced in bore water. These results provide significant insight into coal flotation in hypersaline water and an important consideration in the development of improved flotation technology for the coal industry. However, additional research is required for a better understanding of the coal flotation in salt solutions which will reduce demands on scarce fresh water supplies. Acknowledgements The Australian Research Council is gratefully acknowledged for financial support through a Discovery grant (AVN). BHP Billiton Mitsubishi Alliance (BMA) is gratefully acknowledged for funding the BMA Chair of Minerals Processing at the University of Queensland (AVN) and the coal samples (Mr Ian Brake and Mr Ben Cronin). BHP Billiton Nickel West (Perth, Australia) is acknowledged for providing the bore water samples. We are indebted to Mr Maung Aung Min from the JKMRC, the University of Queensland for helping with the bubbleparticle attachment time measurements.
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