Attachment, Coalescence, and Spreading of Carbon Dioxide

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Attachment, Coalescence, and Spreading of Carbon Dioxide Nanobubbles at Pyrite Surfaces Behzad Vaziri Hassas, Jiaqi Jin, Liem Xuan Dang, Xuming Wang, and Jan D Miller Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02929 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Attachment, Coalescence, and Spreading of Carbon Dioxide Nanobubbles at Pyrite Surfaces Behzad Vaziri Hassas1, Jiaqi Jin1, Liem X. Dang2, Xuming Wang1, Jan D. Miller1* Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, 135 South

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1460 East, Rm 412, Salt Lake City, Utah 84112, USA Chemical and Material Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard,

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Richland, WA 99353

* Corresponding author: [email protected]

ABSTRACT Recently it was reported that using CO2 as a flotation gas increases the flotation of auriferous pyrite from high carbonate gold ores of the Carlin Trend. In this regard, the influence of CO2 on bubble attachment at fresh pyrite surfaces was measured in the absence of collector using an induction timer, and it was found that nitrogen bubble attachment time was significantly reduced from 30 ms to less than 10 ms in CO2 saturated solutions. Details of CO2 bubble attachment at a fresh pyrite surface have been examined by AFM measurements and MD simulations, and the results used to describe the subsequent attachment of a N2 bubble. As found from MD simulations, unlike the attached N2 bubble, which is stable and has a contact angle of about 90 degrees, the CO2 bubble attaches, and spreads, wetting the fresh pyrite surface and forming a multi-layer of CO2 molecules, corresponding to a contact angle of almost 180 degrees. These MDS results are complemented by in-situ AFM images, which show that after attachment, CO2 nano/micro bubbles spread to form pancake bubbles at the fresh pyrite surface. In summary, it seems that CO2 bubbles have a propensity to spread, and whether CO2 exists as layers of CO2 molecules (gas pancakes) or as nano/micro bubbles, their presence at the fresh pyrite surface subsequently facilitates film rupture and attachment of millimeter N2 bubbles, and in this way, improves the flotation of pyrite.

Keywords: Pyrite, Carbon dioxide, Atomic Force Microscopy (AFM), Bubble attachment, MDS, nanobubbles

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INTRODUCTION Gas phase composition can influence flotation separation efficiency in some mineral systems, including the flotation of auriferous pyrite. In 1997, Newmont USA Ltd. developed a sulfide mineral flotation process using nitrogen as the flotation gas. The technology, which is known as N2TEC, resulted in considerable improvement in the recovery of auriferous pyrite and other sulfide minerals.1,2 This process is known to reduce the oxygen potential of the system, which prevents surface oxidation of pyrite. It is well established that sulfide minerals, under anaerobic conditions, are naturally hydrophobic and relatively easy to float.3 Natural hydrophobicity and floatability of sulfide minerals has been reported to decrease by oxidation of the sulfide mineral surface, as shown by a decrease in contact angle.4 Formation of ferric hydroxide islands as a result of this oxidation reaction have been shown experimentally by AFM images of the oxidized pyrite surface.1,5 Thus, using N2 during pyrite flotation (N2TEC) reduces the amount of dissolved oxygen in the system, and together with lead as activator, improves the flotation separation efficiency, and the recovery of auriferous pyrite. More recently, it was reported that using CO2 as a flotation gas would increase the flotation recovery of auriferous pyrite from carbonate rich gold resources of the Carlin Trend, especially in alkaline pH.6 The presence of carbonate minerals in the carbonaceous auriferous pyrite ore of the Carlin trend increases the pH of the slurry, which results in greater surface oxidation of pyrite. The use of CO2, however, was found to control the surface oxidation of pyrite and increase its flotation recovery, even for the carbonaceous gold ores. Yet a thorough investigation of this phenomenon is needed to establish the details by which CO2 increases the flotation recovery. In this study, fundamental features associated with CO2 flotation of pyrite were investigated. Bubble attachment experiments, AFM micro and macro imaging of surface bubbles, bubble nucleation/growth, and MD simulation techniques have been used to provide information regarding the influence of CO2 on the flotation of pyrite. MATERIALS AND METHODS Pyrite Sample and CO2 Saturated Water A high quality cubic crystal of pyrite was obtained from the Geology Department at the University of Utah. The powder XRD pattern of the sample is given in Fig. 1. XRD patterns reported in this study show that the sample is of high purity, as the pattern fits the natural pyrite XRD pattern reported previously.7 Ground pyrite particles of 150×106 µm were


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prepared and stored in a refrigerator (below 5 °C) in sealed containers filled with nitrogen to prevent surface oxidation.

Figure 1. XRD pattern of the pyrite sample.

N2 and CO2 saturated solutions were prepared by blowing N2 or CO2 gas into DI water for at least 2 hours prior to each experiment. Ultra-pure N2 and CO2 gases were used for water saturation and producing bubbles for experimental measurements. The pH of the N2 saturated and CO2 saturated solutions were pH 5.6 and pH 3.6, respectively. Bubble-Particle Attachment The pyrite particles for particle bed experiments were prepared by crushing and grinding natural crystals with a mortar and pestle. The ground particles were then dry sieved and sorted into different size classes (150×106 µm). Prior to the experiments, the ground samples were washed in an excessive amount of DI water to remove slimes and any remaining ions from the surface. Bubble-particle attachment times for particle beds were measured using an Electronic Induction Timer (MCT-100). A detailed explanation of the experimental method is given elsewhere.8,9 The bubble size was kept around 1 mm and at least 100 measurements were conducted for various contact times (10–80 ms), and the average of all points were taken in order to determine the attachment time for different conditions. The bubble attachment time corresponds to the contact time which results in attachment for 50% of the number of trials.


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Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) has been used extensively in surface imaging and surface force measurements since the 1980s. Interaction between two surfaces for various conditions has been the topic of numerous studies.10,11 With some advanced techniques, the interaction between bubbles and particles has also been investigated by AFM.12–14 Recently, atomic force microscopy and other sensitive surface imaging techniques have been utilized to scan for possible gas phase and nanobubbles on solid surfaces under different conditions.15–18 The presence of nanobubbles, and in some cases even microbubbles, on hydrophobic surfaces has been reported in these studies. In a thorough review of the literature, Lohse and Zhang have clearly addressed the presence of nanobubbles on solid surfaces, as well as the methods of analyzing and imaging these bubbles.19 In order to analyze the surface properties of pyrite and bubble formation at the pyrite surface, fresh pyrite particles and substrates were used in AFM experiments in the absence of collector, with 1 mM KCl as a background electrolyte. A Nanoscope V controller, a PF scanner from Veeco and a liquid cell (Bruker Nano, Santa Barbara, CA) were used in the AFM experiments. For the surface force measurements, colloidal probes were prepared using tipless silicon cantilevers obtained from Mikromasch® USA. The spring constant of the cantilever, as determined by the Thermal Tune method, was 0.827 N/m. A micromanipulator was used to attach the desired particle of around 10 µm in size to the cantilever, using “Norland Optical Adhesive.” Before attaching the particle, the adhesive was also applied to the cantilever using tungsten wire. The adhesive was then hardened by keeping the cantilever under a UV source. Fig. 2 shows a colloidal probe with a pyrite particle attached to the cantilever. Probes were characterized by scanning electron microscopy (SEM) to make sure that there was no contamination of the particles coming from the adhesive, as well as to measure the size of the attached particle. A thin layer of a fresh pyrite surface (1–2 mm) was used as a substrate to measure the interaction forces between the two pyrite particles. The spring constant of the colloidal probe cantilevers and imaging cantilevers were measured using the thermal tune method provided with the instrument. The obtained data from force measurements were analyzed using ‘‘Scanning Probe Image Processor (SPIP)” software, and force-distance curves were generated from deflection-distance data.


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Figure 2. Colloidal probe with pyrite particle attached to cantilever, as used in AFM colloidal force measurements.

Surface imaging experiments were conducted using Bruker SNL-10 cantilevers with sharp tips (radius of ~2 nm) using the tapping mode (TM-AFM), in which the sharp tip oscillates at the cantilever’s resonant frequency. Cantilever “A” in the SNL-10 probe was chosen and the spring constant of the cantilever, as determined by the Thermal Tune method, was 0.279 N/m. The resonant frequency of the cantilever was determined by swiping the cantilever in the frequency range suggested by the manufacturer and selecting the highest magnitude of oscillation. The drive frequency was set to 31.393 kHz for the tapping mode imaging experiments. Amplitude feedback was used to control the force exerted on the surface of bubble during the imaging. The drive amplitude was set to 1000 mV, and the set-point was kept very close to the drive amplitude at around 800 mV (±100 mV). The scan rate was adjusted to achieve a tip speed of between 5-10 µm/s, as lower speed was not suitable for bubble surface scanning. Since the surface of the bubbles on the substrate are extremely soft, the imaging parameters were set to maximum precision and sensitivity to maintain the necessary distance from the surface and prevent the tip from penetrating the surface bubbles. Molecular Dynamics Simulation Molecular dynamics simulation (MDS) is another powerful tool used to investigate surface interactions and mechanisms, and to complement reported experimental results. MDS provides a deep understanding of molecular interactions at the surface and has been applied recently to surface chemistry studies in flotation and mineral processing,3 as well as the surface nanobubbles.20 5 ACS Paragon Plus Environment

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Amber, an MDS program package, which includes consideration of Columbic/electrostatic, van der Waals, and bonded interactions,21 was used for the simulation and analysis of bubble attachment. The rigid SPC/E water model, which has the closest average configurational energy to the experimental value (-41.5 kJ mol-1),22,23 was used to describe the water phase. The calculated physical properties of the SPC/E water model are comparably good, such as self-diffusion, the dielectric constant, and the water dipole moment. Thus, the SPC/E water model was selected for exploring gas bubble attachment at the pyrite surface. The Lennard-Jones parameters of pyrite (FeS2) are from the Universal Force Field (UFF).24 Atomic partial charges for the Fe and S atoms in the pyrite crystal are Mulliken charges determined from the periodic density functional theory (DFT) quantum chemical calculations of a well-defined unit cell using the Perdew-Wang 1991 (PW91) functional theory and the generalized gradient approximation.25,26 The program DMol3 was used to assign the Mulliken charges.27 The force field parameters for pyrite (FeS2) have been used to study the wetting character and interfacial water structure of a pyrite surface in a previous study.28 The crystal structure for pyrite is from the American Mineralogist Crystal Structure Database.29 To measure the bubble attachment contact angle, three steps are required, including creating the pyrite crystal, simulating a gas bubble in the aqueous phase, and assembling them together in one simulation system. A pyrite (100) surface was prepared with Crystal Maker software packages.30 Since periodic conditions are applied in the contact angle simulation, periodic images of the nanobubbles were avoided by using a crystal with sufficient surface area. In this simulation, the horizontal extent of the surfaces was about 150 Å × 150 Å. A two point model for the N2 molecules and a three point model for the CO2 molecules were used.31 The initial coordinates of a N2 bubble containing 906 nitrogen molecules in an aqueous phase containing over 100,000 water molecules was generated by the Xleap module of the Amber software packages.21 Then the isothermal-isobaric (NPT) ensemble was used to run the simulation for equilibration of the water and the N2, bubble, with a simulation period of 500 ps. The amount (N), pressure (P), and temperature (T) were conserved. The simulation temperature was set as 298 K. After the water box containing the N2 bubble reached equilibrium, a portion of the water molecules, together with the N2 bubble, were separated from the initial water box and placed adjacent to the pyrite (100) surface in another box. The initial distance between the nitrogen bubble and the pyrite surface was set at 10 Å, i.e. the initial water film thickness at the selected mineral surface was about 10 Å. A simulation period of over 1 ns, with NVT conditions (amount (N), volume (V), and temperature (T) conserved), was used to equilibrate the simulation system. A similar procedure was used to 6 ACS Paragon Plus Environment

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prepare and run the simulation of CO2 bubble attachment (1000 CO2 molecules) at the pyrite (100) surface. RESULTS AND DISCUSSION Bubble-Particle Attachment Bubble attachment times for sized pyrite particles were measured using N2 bubbles in N2 and CO2 saturated water, separately. As shown in Fig. 3, it was found that the attachment time is reduced significantly from 30 ms in N2 saturated solution to less than 10 ms (~5 ms) in CO2 saturated solution. This decrease in the attachment time accounts for the increase in the flotation response of pyrite particles. Albijanic et al.9 concluded that the short attachment time represents strong affinity of the particles to the gas phase and higher recovery in flotation, while a longer attachment time of 20 ms and longer, indicates weak affinity to the bubbles and lower flotation recovery. Such short attachment times were also reported for other naturally hydrophobic surfaces, such as talc and highly oriented pyrolytic graphite.32,33

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Figure 3. Pyrite (150×106 µm) N2 bubble attachment time in N2 saturated and in CO2 saturated DI water (1mM KCl).

Furthermore, it was found that the CO2 bubbles demonstrate more elasticity when compared with the N2 bubbles. Fig. 4 shows the last step in the bubble attachment time measurement, in 7 ACS Paragon Plus Environment

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which the bubble is retracted after being in contact with the particle bed for a certain time. Bubble elongation after attachment to the particles in the bed during bubble attachment time measurements was observed for the CO2 bubbles, unlike the N2 bubbles for which there was no elongation. In the case of CO2, the bubble did not release after attachment, but rather elongated as shown by the top photos in Fig. 4. Results are indicative of a strong interaction of CO2 with the pyrite surface. The elasticity of the CO2 bubbles improves the possibility of bubble-particle attachment. This phenomenon is important in flotation as the change in the shape of the bubble at the moment of collision with the particle, increases the energy dissipation and results in a far-from-perfect elastic collision (in which kinetic energy is totally conserved), which in turn increases the attachment probability. Zawala et al.34 reported that the higher kinetic energy at the moment of collision reduces the probability of attachment. They also discuss that the kinetic energy of the bubble transfers into the surface energy at the collision. Consequently, when the bubble surface is more elastic, a portion of the kinetic energy is consumed in surface deformation and decreases the probability of bouncing on the surface and prolongs the contact time, which in turn increases the attachment probability.35

CO2 Bubble

N2 Bubble

Figure 4. Elasticity and shape of CO2 (top) and N2 (bottom) bubbles after retraction from the pyrite particle bed during attachment time measurements.


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The lack of bubble release from the particle bed was observed, as well as significant aggregation of pyrite particles, for the case of CO2 nano/micro bubbles at the particle surfaces. Although quantitative results of bubble attachment time measurements show that the pyrite particles have a shorter attachment time in CO2 saturated water, qualitative observations also should be taken into account. Photos in Fig. 5 show the difference in bubble-pyrite particle aggregates during attachment for both N2 and CO2 saturated water.

N2 Saturated Solution

CO2 Saturated Solution

Figure 5. N2 bubble-pyrite aggregates in N2 saturated water (Left) and CO2 saturated water (Right), dp = 150×106 µm, db = ~1 mm.

Both photos show successful attachment, but particle attachment in CO2 saturated water (Fig. 5-Right) is expected to exhibit better flotation recovery since greater bubble-particle aggregation (aerofloc) is observed. Results from the literature suggest that the surface nanobubbles achieve a dynamic stable equilibrium with Laplace pressure, when the gas outflux is compensated for by the gas influx.36 This equilibrium may also be achieved in the case of CO2 nanobubbles on the pyrite surface when its outflux rate becomes equal to the influx rate. The increase in coagulation of hydrophobic particles (methylated synthetic silica spheres) in CO2 saturated solution has been investigated,37 the results of which coincide well with those reported in this research. It has been known for some time that mass transfer influences film stability and coalescence. For example, in solvent extraction systems, the drop coalescence time can be reduced by at least a factor of 10 when the system is not at chemical equilibrium and mass transfer is 9 ACS Paragon Plus Environment

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occurring.38 So, it is expected that film rupture and N2 bubble attachment at a pyrite surface decorated with CO2 nanobubbles is facilitated by transport of surface CO2 molecules through the separating film into the N2 bubble, since the system is not at equilibrium. In this way a significant decrease in bubble attachment time is explained, due to the accommodation of CO2 molecules at the pyrite surface, as expected from DFT calculations. Oliveira et al.39 also reported that the presence of hydrophobic sites on the solid surface ease the mass transfer for the air bubble, which triggers bubble attachment. Atomic Force Microscopy (AFM) Surface force measurements Pyrite-pyrite interaction was investigated in DI water with 1 mM KCl as the background electrolyte, and in CO2 saturated solution, in order to investigate the effect of the presence of dissolved CO2 on particle interactions. Force measurements were conducted at three different pH values: 3, 6, and 10. For pH adjustments, 0.1 M NaOH and 0.1 M HCl solutions were used. Fresh pyrite-pyrite interactions were measured at the three different pH values and the results show that by increasing the pH, attraction between the pyrite particle and the pyrite surface decreases (Fig. 6). Despite the fact that at pH 3 the surface of pyrite is positively charged, and an electrostatic repulsion is expected in this region, an attraction and jump to contact from a distance of ~8 nm was observed. This attraction was also observed at pH 6 after a region of slight repulsion. It has been reported that this repulsion before jump to contact is due to the presence of nanobubbles at hydrophobic surfaces.40 These attractions, which overcome electrostatic repulsion and result in a jump to contact, appear to be caused by a hydrophobic (non-DLVO) interaction which is thought to be due to an attractive van der Waals force41 and the presence of nanoscale bubbles at the interacting surfaces.42–44 By increasing the pH, the rate of pyrite surface oxidation increases and consequently the hydrophobicity of the pyrite surface decreases.45 Under these conditions electrostatic repulsion becomes dominant. This repulsion is also obvious in oxidized pyrite experiments. Oxidation of a pyrite substrate in 1% H2O2 for 240 s shows that attraction at both pH 3 and 6 no longer exists (Fig. 7).


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Figure 6. AFM surface force profiles for pyrite-pyrite (fresh surface) interactions at different pH values.

It was also observed that the repulsion force and corresponding interaction distance increase by increasing pH which means electrostatic interaction is more dominant at higher pH values. Comparing Figs. 6 and 7 clearly shows that the hydrophobic attraction at the pyrite surface decreases with oxidation. 30

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Figure 7. AFM surface force profiles for pyrite-pyrite (oxidized in 1% H2O2 for 240 s) interactions at different pH values.

The force measurement experiments were conducted in CO2 saturated solutions as well. As the results suggest (Fig. 8), the attractive force at pH 3 increases in CO2 saturated solutions. It was also found that there is an earlier jump to contact at pH 3 around 40–50 nm from the surface, which can be an indication of the presence of nanobubbles at the pyrite surface. This kind of jump to contact and its relation to the presence of nanobubbles on hydrophobic surfaces has been reported previously.17 This significant attractive force that caused the 11 ACS Paragon Plus Environment

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earlier jump to contact, however, cannot be seen in higher pH values, perhaps related to the stability of CO2 bubbles in the solution. Tabor et al.14 reported that the CO2 nanobubbles are more stable at an acidic pH (pH < 6), while at higher pH values other species and ions form, i.e., carbonic acid (H2CO3) and bicarbonate (HCO3-). It was found that at pH 6, the repulsion force starts at a separation distance of about 30 nm, which decreases at closer distances (Fig. 8). The distance of the jump to contact coincides well with the range that is reported by Zhang et al. for nanobubbles at hydrophobic surfaces.46 15

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Surface nanobubbles Bubble-particle attachment in flotation occurs when the thin liquid film at the hydrophobic mineral surface ruptures and a three phase (gas-liquid-solid) line of contact starts to form and grow.47–50 Since air bubble zeta potentials have been reported to be negative,51 and mineral surfaces are also negatively charged above the iep, the electrostatic interaction should be repulsive between the bubble and particle. Considering the Hamaker constant, which is also negative between bubble and particle,52 the van der Waals (vdW) interaction also should be repulsive. These two force components should result in a significant repulsive force between the bubble and particle in water, yet it is known that the bubble shows affinity towards hydrophobic mineral surfaces in aqueous systems.53 In order to have an attractive interaction, the sign of the Hamaker constant should be positive to render the vdW interaction attractive. As the refractive index is a main parameter in the Hamaker constant calculation, the only way to have a positive Hamaker constant in a medium next to a liquid is to have a medium with a refractive index lower than that of the liquid, which can be a gas.54 Considering the fact that the hydrophobic solid surface must have a higher refractive index, the positive Hamaker 12 ACS Paragon Plus Environment

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constant is not possible. In this regard, it has been argued that there is a low density region called the water exclusion zone, perhaps even a thin gas film at hydrophobic surfaces which accounts for the attractive van der Waals force between the bubble and the hydrophobic surface, the attraction increasing with increased thickness of the water exclusion zone.55,56 The interaction between two hydrophobic surfaces, which was first reported by Israelachvili and Pashley,57,58 has since been referred to as a “long range hydrophobic force”.59 Later, the interaction was discussed widely in the literature as a “bridging effect” of the nanobubbles on the hydrophobic surfaces that may interact with the exclusion zone at hydrophobic surfaces,55,60–62 as discussed in the literature.53 Through the use of various imaging and analytical methods, nanobubbles and their clusters have been found to form microbubbles and other geometries (i.e., doughnut shapes). Among these methods, tapping mode AFM is of significance, as it provides information on the dimension and the shape of the nano/microbubbles at hydrophobic surfaces.63–67 Such bubbles are not found at hydrophilic surfaces. Adsorption of CO2 at hydrophobic surfaces leads to the nucleation of CO2 nanobubbles on hydrophobic sites, as reported in the literature.68–70 However, the study of modestly hydrophobic fresh pyrite surfaces has not been reported. Snoswell et al.37 studied the aggregation of hydrophilic and hydrophobic silica particles in the presence of dissolved CO2. They reported that the aggregation between hydrophobic silica particles increases with an increase in CO2 concentration in the solution, while it has no effect on the aggregation of hydrophilic particles. They concluded that the presence of nanobubbles on the surface of hydrophobic particles increases the attractive force between the particles, which in turn increases the aggregation kinetics. Experimental data from this study of pyrite in CO2 saturated solution, including the shorter bubble attachment time and the more attractive interaction between particles, support the formation of CO2 nanobubbles at the relatively hydrophobic fresh pyrite surface. In order to investigate these nanobubbles, pyrite surfaces were scanned by tapping mode AFM using a cantilever with a sharp tip (Bruker SNL-10) with a tip radius of around 2 nm. As shown in Fig. 9, fresh pieces of pyrite were chipped and separated from a pure crystal and fixed on a mica disc using a thin layer of UV adhesive, as discussed previously. All imaging experiments were conducted in the AFM liquid cell with 1 mM KCl as the background electrolyte. First, an area of 45 µm×45 µm on the fresh pyrite surface was scanned in DI water, before the CO2 saturated solution was introduced. This surface was scanned in contact mode to establish the exact natural features and properties, and to be sure that preexisting bubbles were not 13 ACS Paragon Plus Environment

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present at the surface. It is noteworthy that a slightly lower set point was selected to make sure the cantilever would not scan any preexisting bubble on the surface. Fig. 10 shows the 2D, 3D, and profile of the scanned area. It was found that the maximum height (maximum peak and minimum valley) within this area does not exceed 120 nm (surface profile). Later on, after introducing the CO2 saturated solution to the cell, numerous smaller regions (5 µm×5 µm and 10 µm×10 µm) from exactly the same area were scanned by tapping mode AFM in CO2 saturated solution (Fig. 11). It was found that the nanobubbles nucleate on the fresh pyrite surface immediately after the CO2 saturated solution comes in contact with the pyrite surface. However, with time these nanobubbles grow, merge, and spread on the surface, resulting in an increase in size, and the formation of various shapes of microbubbles, observations which are complemented by the MDS results presented in section 3.3.

Figure 9. Micrograph of pyrite substrates used in imaging experiments.

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Figure 10. Surface topography of pyrite substrates used in imaging experiments (Contact mode AFM).


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Within the scanned area of 45 µm×45 µm, around 136 bubbles were detected, the size of which varied from 200 nm to 2 µm, equivalent circular diameter. It was found that the bubble formation does not follow any specific trend, and that the bubbles are not uniformly distributed on the surface. The fraction of the surface occupied by CO2 bubbles was found to be around 4.9% of the total area. The average bubble area was 0.417 µm2, with a mean diameter of 0.561 µm and standard deviation of 0.447 µm, over the entire surface bubble population for the scanned area.


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Height Profiles

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Figure 11. Tapping mode AFM images of bubbles at the fresh pyrite surface in CO2 saturated solution (left: 2D, middle: 3D, and right: height profile for indicated line on 2D image) scanned in tapping mode AFM.


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These results confirm previous reports of the presence of CO2 bubbles at hydrophobic surfaces.69 It should be noted that bubbles were not detected at the pyrite surface in DI water. The profiles and images of the bubbles on the surface indicate that the CO2 bubbles are in micrometer scale in all three dimensions. The thickness (height) of the bubbles clearly indicates that no such height was observed at the pyrite surface before the CO2 saturated solution was introduced. The maximum change in elevation (roughness) of the pyrite surface was less than 120 nm while the height profile of the bubbles revealed a thickness of about 1 µm. Yang et al.69 reported that the kinetics of CO2 adsorption at a hydrophobic surface (roughness of 1.8 nm) follows two steps, namely, nucleation and diffusion, of which the latter has lower activation energy. They argued that once the CO2 nanobubbles nucleate on the surface, further adsorption occurs simply by CO2 molecules crossing the water/gas interface and diffusing into the nanobubbles, which results in growth and/or spreading of the CO2 nanobubbles at the pyrite surface as found from MD simulation. Once the nucleation of CO2 nanobubbles was revealed from microscopic examinations, further experiments were conducted to determine the rate and mechanism of steps after nucleation. It was found that the bubbles, which can be detected by the naked eye (~100 µm), form in 30 s after the CO2 saturated solution comes in contact with the walls of a plastic hydrophobic container. Previous studies also reported that the required time for a CO2 bubble to grow to around 100-200 µm would take up to around 30 s.71 It is noteworthy that the nucleation and merging of bubbles were not detected in glass containers as the surface is not hydrophobic enough to trigger the nucleation.71 Also, it should be taken into account that surface roughness plays a significant role on the apparent hydrophobicity of the surface10,72 and would have a considerable effect on the nucleation of CO2 bubbles. These facts coincide well with our previous discussion in which we did not find many bubbles nucleating and growing on oxidized pyrite surfaces. Molecular Dynamics Simulations The AFM results are complemented by MD simulations of bubble attachment. Only recently have MD simulations of nanobubble attachment at hydrophobic surfaces been reported.73,74 In this way, bubble attachment at the pyrite (100) surface was studied by MDS. Attachment comparisons of N2 vs. CO2 bubbles Thin sections (2 nm thick) from 3D images for initial and equilibrium states of N2 and CO2 bubbles at the pyrite (100) surface are shown in Fig. 12. The MD simulated N2 bubble at the 17 ACS Paragon Plus Environment

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pyrite (100) surface took less than 1 ns simulation time to reach the equilibrium state, whereas simulation of the CO2 bubble at the pyrite (100) surface took about 3 ns. Most interesting, it was found that N2 bubble attachment at the pyrite (100) surface was similar to that observed for molybdenite.73,74 Film thinning, film rupture, and film displacement processes occur for N2 bubble attachment at the pyrite (100) surface. Then, the attached N2 bubble at the pyrite (100) surface equilibrates forming a hemispherical shape, indicating a contact angle of 90 degrees.

N2 Bubble Attachment at Pyrite (100) Surface

CO2 Bubble Attachment at Pyrite (100) Surface

Figure 12. Thin sections of initial and equilibrium states (1ns for N2 and 3 ns for CO2) for MDS N2 nanobubble (top) and CO2 nanobubble (bottom) at pyrite (100) surface. Atom color code: Fe, green; S, yellow; O, red; H, white; N, lime; C, orange.

In contrast, attachment of the CO2 nanobubble proceeds to complete spreading and the formation of a multi-layer of CO2 molecules at the pyrite (100) surface, corresponding to a contact angle of ~180 degrees. To compare the attachment kinetics of N2 and CO2 bubbles at the pyrite (100) surface, film rupture time for the N2 bubble at the pyrite (100) surface was about 0.1 ns, which is similar to the previous study for N2 bubble attachment at the 18 ACS Paragon Plus Environment

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molybdenite face surface.73,74 However, water film rupture time for the CO2 bubble at the pyrite (100) surface was less than 0.05 ns, much faster than that for the N2 bubble attachment. Attachment and spreading of CO2 bubble As discussed in previous studies,73,74 the number of molecules (906) in the N2 bubble for MD simulation is close to the calculated number of molecules in a nitrogen bubble with a diameter of 7 nm (1086). For the CO2 bubble with a diameter of 6 nm (R = 3 nm) at 298 K, the pressure inside of the bubble is 48.5×107 dyne/cm2. A previous study has confirmed this relationship.75 At high pressure, the ideal gas law is not valid, so the corrected real gas law at high pressure is applied for the CO2 bubble, p(V-nb) = nRT, where V, p, and n are the volume, pressure, and moles of CO2. For CO2 gas, the constant b is 0.043 L·mol-1, T is 298 K, and R is the molar gas constant (8.314 J·K-1·mol-1). The calculation of the number of molecules in the CO2 bubble with a diameter of 6 nm is 756, which is close to the number of molecules (955) in the CO2 bubble at the beginning of the MD simulation. According to the simulation, CO2 molecules are initially dispersed in the bubble at a density of 522 g/L, significantly higher than the CO2 gas density at standard temperature and pressure (1.98 g/L), but still less than the density of liquid CO2 at saturation (1101 g/L).


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Figure 13. Snapshots of CO2 bubble attachment at pyrite (100) surface for 0, 1, 2, and 3 ns. Atom color code: Fe, green; S, yellow; O, red; C, orange.

The thin section snapshots in Fig. 13 reveal the behavior of the CO2 nanobubble at the pyrite (100) surface. Note that after attachment, the CO2 bubble is not stable, but spreads to form a multilayer of CO2 molecules, completely wetting the pyrite surface, and corresponding to a contact angle of about 180 degrees. Snapshots of the CO2 bubble attachment simulation at the pyrite (100) surface are shown for 0, 1, 2, and 3 ns. N2 bubble attachment at the CO2 decorated pyrite surface In view of these MDS results, it might be anticipated that N2 bubble attachment at the CO2 decorated pyrite surface is facilitated, and this expectation has been confirmed. Specifically, it has been found that initial film rupture time for a N2 bubble at the CO2 decorated pyrite surface is about the same as initial film rupture time for a CO2 bubble at a fresh pyrite surface, 0.05 ns, in contrast to the film rupture time for a N2 bubble at a fresh pyrite surface,


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0.10 ns. Complementary flotation results are reported in the patent literature.6 The recovery of pyrite was found to improve significantly using N2 for flotation after initial treatment with CO2. SUMMARY AND CONCLUSIONS Generally, pyrite flotation is facilitated at low pH in the absence of oxidation and hydroxylation of surface sites. Under these circumstances clean pyrite surfaces have modest hydrophobicity. However, the flotation recovery of low grade auriferous pyrite from Carlin trend carbonate ores has been less than satisfactory due, in part, to pH control difficulty. Recently, significant progress has been made to improve pyrite recovery from these ores using CO2/N2 gas phase mixtures for more efficient flotation. In this way, better pH control is achieved, and it is expected that the hydrophobic character of the pyrite surface is improved by minimizing oxidation and hydroxylation, the acidic pH being maintained with CO2. In addition to elimination of these well recognized effects (oxidation and hydroxylation), current collectorless flotation research has shown that bubble attachment and pyrite flotation is improved with CO2, due to the formation of nanobubbles and the spreading of these CO2 molecules at the pyrite surface, as demonstrated from AFM measurements and MD simulations. The decrease in N2 bubble attachment time at CO2 treated pyrite surfaces accounts, in part, for the improved flotation response. The CO2 nanobubbles and/or the CO2 molecular multilayers at pyrite surfaces facilitate film rupture and displacement during subsequent millimeter N2 bubble attachment at the CO2 decorated pyrite surface for more efficient flotation recovery. ACKNOWLEDGMENTS The authors are grateful to Newmont USA Ltd. for the financial support of this project and thank Ms. Dorrie Spurlock for her assistance in the preparation of the manuscript. The authors declare no competing financial interest.


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Table of Contents Graphic


Attachment, Coalescence, and Spreading of Carbon Dioxide

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