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Mining, Metallurgy & Exploration

, Volume 36, Issue 1, pp 71–80 | Cite as

The Influence of Polysaccharides on Film Stability and Bubble Attachment at the Talc Surface

  • Venkata Atluri
  • Yuesheng Gao
  • Xuming Wang
  • Lei Pan
  • Jan D. MillerEmail author
Article
  • 112 Downloads

Abstract

The wetting characteristics and water film stability at the talc surface have been studied, particularly the effect of polysaccharides such as guar gum, starch, and dextrin. Talc is a gangue mineral in the flotation of base metal sulfide ores, precious metal sulfide ores, and platinum group metal (PGM) sulfide ores. Talc surfaces were investigated using surface analysis techniques including atomic force microscopy, high-speed video bubble attachment measurements, and wetting film stability measurements using a synchronized tri-wavelength reflection interferometry microscope (STRIM). In the presence of polysaccharides, there is a significant increase in bubble attachment time at the talc surface, but only a slight change in contact angle, which suggests that polysaccharide depression of talc is due primarily to the slow rate of bubble attachment and not due to a change in contact angle. The critical rupture thickness (hc) for a hydrophobic talc surface was found to be 56 nm, while the hydrophilic phlogopite surface of similar structure has an equilibrium film thickness (he) of 25 nm. At low polysaccharide concentrations, the wetting films formed on the talc surfaces were unstable, but at high concentrations the wetting films became stable with similar thickness values as the critical rupture thickness, and bubble attachment did not occur. However, it was found that the critical and equilibrium film thickness values do not change significantly with the polysaccharide type or concentration. The results from this research help us understand further details of film rupture and displacement during bubble attachment.

Keywords

Talc Polysaccharides Hydrophobicity Bubble attachment Film thickness 

1 Introduction

Congratulations to Professor Douglas Fuerstenau on this occasion for the celebration of his 90th birthday. With appreciation, we recognize his career of teaching/research in the processing of energy and mineral resources, particularly his contributions in the area of mineral flotation chemistry. Early research under the supervision of Professor Fuerstenau reported the nature of polysaccharide (dextrin) adsorption by the naturally hydrophobic mineral, molybdenite [1]. Since then considerable research has been reported on the chemistry of polysaccharide depressants for different flotation systems. For example, polysaccharides, such as guar gum, carboxymethyl cellulose (CMC), starch, and dextrin, have been used as talc depressants in flotation for metal recovery from base metal sulfide ores, precious metal sulfide ores, and platinum group metal (PGM) sulfide ores. High molecular weight polysaccharides are better depressants for talc, but are not selective and sometimes adsorb on the valuable sulfide minerals reducing the grade and recovery. In this regard, the selectivity and effectiveness of polysaccharide depressants for talc depression have been studied by many researchers [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. In this spirit, fundamental issues associated with film stability and bubble attachment at the talc face surface, as influenced by polysaccharide depressants, are presented.

Research on the wetting characteristics of talc by Atluri et al. has been published recently [15]. In this study, it was found that the surface properties and wetting characteristics of the talc face surface change with substitution of aluminum for silicon in the tetrahedral layer of the talc crystal structure. This substitution results in a charge imbalance at the silica tetrahedral face surface, which is compensated for by a univalent cation, usually potassium. The extent of aluminum substitution in the silica tetrahedral layer for different talc samples was examined by XPS, and the corresponding experimental sessile drop contact angles were found to decrease with an increase in the aluminum substitution, decreasing from about 80° for unsubstituted talc to 0° for more extensive substitution (phlogopite). The water film was found to be stable at the phlogopite surface due to the interaction between water molecules and the increased polarity of the surface state. This stable water film restricts the air bubble from attaching to such face surfaces. However, in the absence of aluminum substitution, no interactions between the water molecules and the face surface were observed and the air bubble readily attached to the face surface of talc. It is this hydrophobic surface state of talc that accounts for its flotation and the need for depressants, such as the polysaccharides currently being used.

As mentioned previously, talc is a major gangue mineral in the flotation of PGM ores. Batch flotation studies on the Merensky ore by Wiese et al., and O’Connor et al., indicated that the final grade and recovery of the sulfide ore is affected by the presence of naturally floatable minerals, such as talc. The PGM ores contain base metal sulfide ores, such as pentlandite, pyrrhotite, chalcopyrite, and pyrite. These ores also contain a significant quantity of talc (1.2~1.3%) which is deleterious to flotation [16, 17]. In this way, talc can impact flotation performance, concentrate grade, and recovery. Hence, the depression of talc gangue during flotation is achieved by using polysaccharide depressants, such as modified guar gum and CMC, to prevent the attachment of air bubbles at the talc particle surfaces.

Recent studies were done by Wu et al. and Shrimali et al. on bubble-particle interactions at hydrophobic surfaces in the presence of polysaccharides, such as CMC, and starch. The use of polysaccharides reduces the water contact angle at the talc surface and also increases the bubble attachment time [18, 19]. As the concentration of the dissolved polysaccharide increases adsorption increases, film stability increases, and bubble attachment time increases, reaching a point at which the wetting film is stable and attachment does not occur. In this way, polysaccharides serve as talc depressants when needed in the flotation of sulfide ores. A particularly important parameter is the effect of polysaccharide on the bubble attachment time, which is significantly greater than the bubble attachment time in the absence of polysaccharide. In addition to looking at the increase in bubble attachment time, water film stability plays a crucial role in determining bubble attachment at mineral surfaces. Still, a significant amount of research has to be done to determine what happens during bubble attachment, particularly details of how polysaccharides influence film thickness, including film thinning, film rupture, and film displacement. Now, water film thickness measurements are possible for many mineral surfaces using synchronized tri-wavelength reflection interferometry microscopy (STRIM), as recently described in the literature [20].

Examination of the effect of polysaccharides on water film thickness at the talc surface will help us understand talc depression and the efficient use of these depressants. Many researchers have identified effective polysaccharides for talc depression. In this paper, the effect of polysaccharides on the talc surface chemistry is considered and we present further research results including the measurement of film thickness using the recently developed STRIM analysis. The results from this research help us understand some of the intricacies of film rupture and displacement during bubble attachment and explain some of the previous research reported on talc depressants. According to the authors’ best knowledge, this is the first-time film thickness measurements have been done at the talc face surface, phlogopite face surface, and at the talc face surface with adsorbed polysaccharides.

2 Materials and Methods

2.1 Minerals and Reagents

High-quality talc crystals obtained from Argonaut Mine, Vermont, and phlogopite (Department of Geology, University of Utah) were used in this research for contact angle, bubble attachment time, film thickness measurements, and AFM imaging. Fresh crystal surfaces were prepared by using scotch tape to remove the top layers and expose a fresh surface. Care was taken to assure that the surface was free from imperfections and that a flat surface was prepared for contact angle and AFM measurements. The rms roughness measurements of the talc crystal surface and phlogopite crystal surface were found to be 1.6 nm and 3.2 nm, respectively.

Polysaccharides were obtained from Sigma-Aldrich and include guar gum, cornstarch, and dextrin. Stock solutions of these polysaccharides were prepared using the following methods. For guar gum, 0.05 g was added to a 100 ml solution of 1 mM KCl which was rapidly stirred in a beaker. For cornstarch, 1 g was added to 10 ml of 1 M NaOH in a beaker. After a gel was formed, 15 ml of 1 mM KCl solution was added and the solution was stirred overnight. The solution was then diluted to make a stock solution of 1000 ppm. For dextrin, 0.05 g was added to a 100 ml solution of 1 mM KCl and heated to 80 °C. All the solutions were stirred overnight for complete dissolution and were used within 3 days of preparation.

Potassium chloride (KCl), used to control ionic strength, was obtained from Sigma-Aldrich. Deionized water was obtained from a Millipore system in the laboratory with a specific conductance of 18 MΩ.cm.

2.2 Captive Bubble Contact Angle Measurements

Fresh talc and phlogopite samples were obtained by using scotch tape to remove the top layer and expose a fresh surface. The distance between the bubble needle and the sample surface was kept constant (5 mm) for each captive bubble contact angle measurement. All measurements were done in 1 mM KCl solution. Before each contact angle measurement, the sample was conditioned in the solution for 15 min. The measurement was made in the same solution. At least five contact angle measurements were obtained at different locations, and they were averaged to obtain the reported contact angle. Variation in contact angle measurements was ± 3°.

2.3 Bubble Attachment Measurements

For bubble attachment time measurements, a high-speed camera, KODAK EKTAPRO, was used to record videos at 1000 frames/s. Fresh talc and phlogopite surfaces were conditioned for 15 min before the start of the experiment. Similar to contact angle measurements, the conditioned samples were placed in a glass cell, and a captive bubble released 5 mm from the surface. By checking the frames in the video, the time required for bubble attachment was calculated, which includes film thinning time and time required for expansion of the three-phase line of contact.

2.4 X-Ray Photoelectron Spectroscopy

The surface chemical compositions of the samples were obtained by XPS using the Kratos Axis Ultra Model. The monochromatic aluminum K-alpha source (1486 keV) and detector were used at 15 kV and 10 mA (150 watts power). A spot size of 700 × 300 μm was used to collect the XPS spectra. The pressure during analysis was ~ 4 × 10−9 Torr at room temperature. Sample heating during analysis was not expected to be significant. The binding energies were determined from the spectra of each sample to determine the elemental content of the surface. Under these conditions, a penetration depth of 10 nm was expected and was indicative of the surface composition. Low-resolution survey scans were run from 1400 to 0 eV while high-resolution region scans covered the range of the peaks for specific elements.

2.5 Atomic Force Microscopy

AFM imaging was done using a Nanoscope III A (Digital Instruments), and SNL-10 silicon probes (Bruker), which are specifically designed for tapping mode images in liquids. The tip had a resonant frequency varying between 10 and 100 Hz and had a spring constant varying from 20 to 35 N/m. The cleaning procedure for the tip as well as the cantilever involved cleaning them with acetone and ethanol, rinsing with milli-Q water, followed by drying with ultra-pure nitrogen. The cleaved surfaces of talc were conditioned with 30 ppm guar gum (pH = 5.7) prepared in 1 mM KCl solution for 30 min. The contact mode images were taken in a liquid cell and subjected to second-order flattening and second-order low pass filtering. In the case of guar gum, both height and deflection error images were taken to describe molecular organization at the talc face surface.

2.6 Water Film Thickness Measurements by Synchronized Tri-Wavelength Reflection Interferometry Microscopy

The thickness of water films (or wetting films) between air bubbles and mineral surfaces was studied using a newly developed synchronized tri-wavelength reflection interferometry microscopy (STRIM) technique [20]. In each experiment, an air bubble of 400 μm in radius was generated by a gas-tight syringe and fixated on a hydrophobic quartz plate at the bottom of a glass cell. The air bubble was brought towards a flat mineral surface in 10−3 M NaCl aqueous solution at a constant nominal velocity of 1.1 μm/s. Simultaneously, the time evolution of the separation distance, i.e., the spatiotemporal thickness profile of the wetting film, was determined by monitoring the interference fringes using the STRIM technique. The STRIM deploys three synchronized high-speed cameras that record the interference fringes at three different wavelengths (λ = 460, 527, and 620 nm) simultaneously. This improved interferometry technique enables an accurate film thickness measurement with 1 nm resolution over the range of 0–1 μm. The detailed experimental method can be found elsewhere [20]. The obtained interference fringes were analyzed to determine the spatiotemporal thickness profile of the wetting film.

3 Intrinsic Hydrophobic Surface State of Talc

3.1 Contact Angle and Bubble Attachment Time

In the case of the talc sample, the contact angle is nearly 80°, and the bubble attachment time is about 25 milliseconds. But in the case of phlogopite, due to aluminum substitution in the silica tetrahedral layer, the surface state changes from a hydrophobic state to a hydrophilic state [15]. The contact angle is zero, and there is no bubble attachment. (See Table 1).
Table 1

Contact angle and bubble attachment time for talc and phlogopite surfaces [15]

Mineral sample

XPS aluminum, atomic %

Contact angle, degrees

Bubble attachment time, (s)

Talc

< 0.1

79

0.025

Phlogopite

7.0

0

NA

NA—no attachment

3.2 Molecular Dynamics Simulation

The hydrophobic state of talc has been described based on molecular dynamics (MD) simulation involving the talc face surface and a nitrogen bubble suspended in water. Detailed discussion of the bubble attachment can be found in the literature [15]. Analysis of the nitrogen bubble as it moves closer to the talc surface is shown in Fig. 1. The water film starts to rupture and displacement occurs at 0.1 ns. The three-phase line of contact expands on the talc surface until an elapsed time of 1 ns. The diameter of the three-phase line of contact gradually increases during this time. From the interfacial water analysis, there is an insignificant interaction between the water molecules and surface atoms of talc, which accounts for water film rupture and displacement causing the nitrogen bubble to attach at the surface of the talc face surface.
Fig. 1

Sequence of MD simulation snapshots for nitrogen bubble attachment at the face surface of a talc crystal for interaction times of 0, 0.1, 0.5, and 1 ns. The color codes for atoms are yellow, K; cyan, Al; green, Si; purple, Mg; red, O; white, H; Blue, N (reprinted with permission) [15]

4 Adsorption State of Polysaccharides

As discussed by Professor Fuerstenau in his early paper on dextrin adsorption by molybdenite [1], polysaccharides frequently adsorb at naturally hydrophobic surfaces, which can be described by Langmuir adsorption isotherms [21, 22]. In the case of dextrin, adsorption is about the same regardless of the hydrophobic mineral surface. See Fig. 2. The adsorption of polysaccharide depressants on highly hydrophobic surfaces is due to hydrophobic interactions between hydrophobic moieties of the polysaccharide molecule and the non-polar mineral surface as described in the literature [1, 3, 14, 19]. Some of the research on polysaccharide adsorption by talc is presented for guar gum, starch, and dextrin in Fig. 3 [10, 13].
Fig. 2

Adsorption isotherms for dextrin adsorption at hydrophobic mineral surfaces [21, 22]

Fig. 3

Adsorption isotherms for guar gum, starch, and dextrin adsorption by talc [10, 13]

AFM images of the talc surface with 30 ppm guar gum are particularly interesting since an ordered structure was found, as shown in Fig. 4. The 100 nm by 100 nm images were subjected to second-order flattening and second-order low pass filtering. As seen from Fig. 4, the clean freshly cleaved surface of talc is smooth with an rms roughness of 4 nm. The same surface with adsorbed guar gum reveals some ordering of the adsorbed guar gum molecules with a typical cylindrical structure size of approximately 7 nm in length and a height of about 0.7 nm, as determined from a number of line profiles.
Fig. 4

Contact mode AFM images taken in a liquid cell for fresh talc (left) and talc conditioned with 30 ppm guar gum (right)

5 Effect of Polysaccharides on Bubble Attachment

Captive bubble contact angle measurements in the presence of low concentrations of polysaccharide solution show that there is a slight decrease in the contact angle, but a significant increase in bubble attachment time values, as shown in Tables 2, 3, and 4. The contact angle decreases by only a few degrees. For example, at low concentrations of polysaccharides, the contact angle changes from 80 to 60°, but the bubble attachment time increases from 30 milliseconds to nearly 100 s, in the case of guar gum and starch.
Table 2

Bubble attachment with guar gum adsorbed at the talc face surface

Conc. (guar gum, ppm)

Contact angle (degrees)

Bubble attachment time (s)

0

81

0.03

5

65

120

15

NA

NA

25

NA

NA

100

NA

NA

Table 3

Bubble attachment with starch adsorbed at the talc face surface

Conc. (starch, ppm)

Contact angle (degrees)

Bubble attachment time (s)

0

79

0.025

5

72

110

25

56

200

50

NA

NA

100

NA

NA

Table 4

Bubble attachment with dextrin adsorbed at the talc face surface

Conc. (dextrin, ppm)

Contact angle (degrees)

Bubble attachment time (s)

0

81

0.023

5

76

0.065

25

70

0.100

50

60

0.128

100

NA

NA

It should be noted that this corresponds to a 1000-fold increase in the value of bubble attachment time, from milliseconds in the absence of guar gum (Table 2) and cornstarch (Table 3) to seconds at low concentrations of these polysaccharides. The increase in bubble attachment time was similar to that observed by Beaussart et al., where the adsorption of dextrin-based polymers (glucose monomers) led to a significant increase in bubble attachment time, but only a slight change in contact angle [23]. Our results, as presented for guar gum, starch, and dextrin (Tables 2, 3, and 4, respectively), and those of Beaussart et al. suggest that polysaccharide depression of talc is due to the slow rate of bubble attachment, and not due to a change in contact angle.

It is interesting to note that the air bubble is prevented from attachment at different concentrations depending on the polysaccharides used. For example, the minimum concentrations to prevent attachment are 15 ppm for guar gum, 50 ppm for starch, and 100 ppm for dextrin. This order can be explained by the extent of polysaccharide adsorption, as described by the adsorption isotherms. When the surface is saturated with polysaccharides, bubble attachment is not possible, as is evident from the adsorption isotherms shown in Fig. 3. The decreasing order of polysaccharide effectiveness in preventing bubble attachment is as follows: guar gum > starch > dextrin, which is the same order observed for effectiveness as a depressant in flotation experiments [10, 11].

Also, in the presence of polysaccharides, water displacement is not complete during attachment and small droplets of water (~ 80 μm) have been observed at the talc surface inside the attached air bubble, as shown in Fig. 5 for dextrin. In the absence of dextrin, water is completely displaced and water micro-drops were not observed. It is expected that adsorbed dextrin molecules are present as islands at the surface of talc [24]. The water binds to adsorbed dextrin molecules and the water droplets are not displaced, but are stabilized at the talc surface inside the attached air bubble. This is an important observation, because it helps to explain the increase in bubble attachment time as well as formation of the three-phase line of contact.
Fig. 5

Presence of water droplets inside the attached air bubble with dextrin adsorbed at the talc surface (magnification × 10)

6 Effect of Polysaccharides on Film Stability

Synchronized tri-wavelength reflection interferometry microscopy (STRIM) experiments on bubble attachment for determination of film thickness and stability were conducted by bringing an air bubble towards a flat mineral surface at a constant nominal velocity of 1.1 μm/s, while monitoring the time evolution of separation distance. At Re < < 1, the bubble-mineral attachment process is considered as a quasi-static approach [25]. Figure 6 shows spatiotemporal thickness profiles of the wetting films formed on a talc and a phlogopite surface in 1 mM NaCl solutions. The radii of the air bubbles are in the range of 0.35–0.45 mm.
Fig. 6

A comparison of the spatiotemporal thickness profile of wetting films formed on a a fresh talc surface and b a fresh phlogopite surface in 1 mM NaCl solution

The results show that on both talc and phlogopite surfaces, the air bubble remained spherical at a separation distance of greater than 300 nm, where both hydrodynamic and surface forces were negligible. As the film continued to thin to a thickness of less than 300 nm, the bubble started to flatten due to an arising repulsive hydrodynamic force. The wetting film ruptured when the closest separation distance reached a critical rupture thickness (hc). We have shown in the present work that hc = 56–60 nm for fresh talc surfaces. On the contrary, the wetting film formed on the phlogopite surface is stable and the equilibrium film thickness (he) was determined to be 25 nm; bubble-mineral attachment did not occur. The equilibrium was reached when the arising disjoining pressure became equal to the capillary pressure (pc). Table 5 shows a summary of the film thickness measurements on talc and phlogopite surfaces. The present results confirm that wetting films are stable on hydrophilic surfaces, while unstable on hydrophobic surfaces.
Table 5

Contrast between water film thickness values at talc and phlogopite surfaces in the absence of polysaccharides

Sample

Talc

Phlogopite

Critical rupture thickness (nm)

56

 

Equilibrium film thickness (nm)

 

25

Air bubble

Attachment

No Attachment

Surface state

Hydrophobic

Hydrophilic

The effect of the adsorbed polysaccharides on the stability of wetting films formed on hydrophobic talc surfaces was studied. Table 6 shows the results obtained with talc surfaces in the presence of 0–100 ppm guar gum. At guar gum concentrations less than 5 ppm, the wetting films formed on talc surfaces were unstable with a hc in the range of 56–58 nm. When the concentration of guar gum was increased to 15 ppm or above, the wetting films became stable with no bubble-mineral attachment. The present results confirm that the adsorption of guar gum on talc surfaces lowers the surface hydrophobicity of talc surfaces. The equilibrium film thickness (he) of 55–58 nm at 15–100 ppm guar gum concentrations is about the same as the critical rupture thickness at lower concentrations, and indicates that the adsorption of guar gum on talc surfaces does not change the surface potentials.
Table 6

Characteristics of wetting films on talc surfaces at different guar gum concentrations

Dosage (ppm)

hc (nm)

he (nm)

Stability

0

56

Unstable

5

58

Unstable

15

58

Stable

25

55

Stable

100

56

Stable

Note that the critical rupture thicknesses (hc) obtained at 0 and 5 ppm of guar gum were the same, despite the significant difference in bubble attachment times as shown in Table 2. Such discrepancy is probably associated with different hydrodynamic conditions in these two experiments. For wetting film stability measurements (i.e., quasi-static measurements), in which the bubble approaching velocity was 1.1 μm/s, the film rupture might be governed dominantly by the surface hydrophobicity of substrates [26]. In this regard, the talc surface switched from a hydrophobic state to a hydrophilic state when the guar gum dosage was increased from 5 ppm to 15 ppm. On the contrary, for dynamic bubble attachment time measurements, in which the free rising air bubble was impacting a flat solid plate, the terminal rising velocity before impact was estimated to be 0.3 m/s [27]. Such high impacting velocity results in a large interfacial deformation and potentially a turbulence that benefits the film rupture. However, the microscopic mechanisms associated with the effect of hydrodynamic conditions on bubble-mineral attachment are still ambiguous, requiring future research.

Tables 7 and 8 show the results of wetting film stability measurements obtained with starch and dextrin as the depressant. Starch molecules have larger molecular weight (MW) than dextrin. As shown, the wetting films formed on talc surfaces were unstable and ruptured at a thickness of 56–64 nm for starch concentrations less than 5 ppm. As the starch dosage was further increased to 25 ppm, the film became metastable. In this case, the wetting films ruptured in half of the total events while remained stable for the other half of the events. When the starch concentration was further increased to 50 ppm and 100 ppm, the wetting film became stable with an equilibrium film thickness (he) of 47–50 nm. It is evident that the inherent hydrophobicity of the fresh talc surface was altered by the adsorption of starch molecules at concentrations exceeding 25 ppm.
Table 7

Characteristics of wetting films on talc surfaces at different starch concentrations

Dosage (ppm)

hc (nm)

he (nm)

Film stability

0

56

Unstable

5

64

Unstable

25

56

Metastable

50

50

Stable

100

47

Stable

Table 8

Characteristics of wetting films on talc surfaces at different dextrin concentrations

Dosage (ppm)

hc (nm)

he (nm)

Film Stability

0

56

Unstable

5

61

Unstable

25

66

Unstable

50

55

Unstable

100

51

Stable

The results obtained with dextrin are slightly different from those obtained with starch. Dextrin exhibits a similar molecular structure to starch, but it has a lower MW. The results show that the wetting films became stable when the dextrin dosage was increased from 50 to 100 ppm. It should be noted that the critical dosage, at which the surface lost its hydrophobic character and the wetting film became stable, is slightly different between starch and dextrin. The present results suggest that starch is a more effective depressant than dextrin for naturally hydrophobic talc surfaces, but less effective than guar gum.

7 Discussion

For guar gum, it can be seen from Fig. 3 that the Langmuir adsorption isotherm plateaus at a concentration of about 20 ppm, and a saturation adsorption density of nearly 1.2 mg/m2 is observed. Under these conditions the water film is stable and the bubble does not attach at the talc face surface. (See Table 6).

Starch, being of higher molecular weight, exhibits an adsorption density of nearly 3 mg/m2 at about 30 ppm. (See Fig. 3). At such high adsorption density, the surface changes to a hydrophilic state, and the film becomes stable at 50 ppm; the bubble does not attach, as shown in Table 7.

Dextrin exhibits Langmuir adsorption as shown in Fig. 3. Adsorption remains constant after 50 ppm, and the surface becomes more hydrophilic, with a stable film which prevents bubble attachment. (See Table 8). At 50 ppm dextrin, the talc surface with adsorbed dextrin still has some hydrophobicity, but at 100 ppm the surface is fully covered. The adsorption density is relatively low (0.6 mg/m2) at 50 ppm compared to guar gum and starch.

Thus, at higher concentrations and higher adsorption densities the talc surface state changes from a hydrophobic state to a hydrophilic state. These adsorption density results from the literature are in good agreement with results from air bubble attachment time measurements and with the results from water film thickness measurements.

It is useful to put these film thickness measurements in perspective with respect to the adsorbed polysaccharides. Based on AFM measurements and other measurements, such as electrophoresis, the adsorbed polysaccharides are believed to extend, at most, 10 nm from the talc face surface into solution representing about 20% of the stable, equilibrium film thickness [10, 13, 19]. The polysaccharide adsorption is considered to be due to hydrophobic interaction between the polymer and the naturally hydrophobic talc face surface [13, 14, 19]. Of course, portions of the extended adsorbed polysaccharides are extensively hydrated, which facilitates H-bonding with free water molecules and consequently stabilizes the film, creating a hydrophilic state at the talc face surface.

8 Summary and Conclusions

Experimental results associated with bubble attachment at the talc face surface in the presence of polysaccharides have been presented based on contact angle, bubble attachment time, and water film thickness measurements. Of course, bubble attachment consists of water film thinning, film rupture, attachment and formation of a three-phase line of contact, and these issues have been considered.

Contact angle and bubble attachment time measurements have been presented for the talc surface, phlogopite surface, and talc surface with adsorbed polysaccharides. In the absence of polysaccharides, the talc surface is hydrophobic, with a contact angle of 80°. Under these circumstances, the water film thins, ruptures, and the bubble attaches to form a three-phase line of contact. In contrast, the hydrophilic phlogopite surface, with a structure similar to that of talc, has a water contact angle of zero and a stable water film.

Addition of polysaccharides has a significant influence on film stability and bubble attachment at the talc face surface. As the concentration of the polysaccharides increases, the contact angle decreases slightly, whereas the bubble attachment time increases significantly. Sometimes the increase in bubble attachment time is 1000-fold, as seen in the case of guar gum and starch. At high concentrations of the polysaccharides, after the talc surface is saturated, the surface state changes from a hydrophobic state to a hydrophilic state. The water film becomes stable and does not rupture, thus, preventing the air bubble from attachment at the talc face surface.

Film thickness measurements provide insights into bubble-mineral attachments on various mineral surfaces. The present findings show that wetting films on fresh talc surfaces in 1 mM NaCl solution are unstable, with a critical rupture thickness of 56 nm, while the wetting films formed on a phlogopite surface have an equilibrium thickness of 25 nm for a bubble radius of 0.4 mm. In the case of talc surfaces with adsorbed polysaccharides, the wetting film gained stability and no bubble-mineral attachment occurred when the polysaccharide concentration reached a critical value. It has been found that guar gum is more effective than starch and dextrin in altering the surface hydrophobicity of talc surfaces.

Finally, results from bubble attachment time measurements and water film thickness measurements suggest that talc depression by polysaccharides is achieved by the creation of a stable wetting film. The stability of the film and its thickness has been established under quasi-static conditions, but as expected, bubble attachment time measurements suggest that film rupture and attachment time will depend on the impact velocity, as well as bubble size. The results from this research help us understand further details of film rupture and displacement during bubble attachment.

Notes

Acknowledgments

Thanks go to Ms. Dorrie Spurlock for her assistance in the preparation of the manuscript.

Funding Information

This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant No. DE-FG03-93ER14315. We appreciate funding from Newmont USA, Ltd., which helped to support the research program.

Compliance with Ethical Standards

Conflict of Interest

The authors declare that they have no conflict of interest.

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Copyright information

© The Society for Mining, Metallurgy & Exploration 2018

Authors and Affiliations

  1. 1.Department of Metallurgical Engineering, College of Mines and Earth SciencesUniversity of UtahSalt Lake CityUSA
  2. 2.Department of Chemical EngineeringMichigan Technological UniversityHoughtonUSA

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