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Mining, Metallurgy & Exploration

, Volume 36, Issue 1, pp 35–53 | Cite as

Recent Advances in Studying Colloidal Interactions in Mineral Processing

  • Z. XuEmail author
  • Z. Li
  • Q. Liu
Article
  • 203 Downloads

Abstract

Colloidal interactions play a critical role in mineral processing, including grinding, physical separation (particularly flotation), dewatering, and tailings management. Despite great energy input in comminution to liberate valuables from gangues, hetero-coagulation between valuables and gangues would prevent the separation of valuables from gangues. On the other hand, selective coagulation/flocculation to increase the size of desired fine particles could enhance physical separation and dewatering, while dispersion is needed for fine grinding. To control the state of colloidal dispersions by creating favorable conditions, it is of paramount importance to study colloidal interactions in a relevant system and understand underlying mechanisms. This review summarizes recent advances in developing the-state-of-the-art techniques and novel methods of measuring colloidal forces, including atomic force microscope, surface force apparatus, zeta potential distribution measurement, quartz crystal microbalance with dissipation, and our recently developed integrated dynamic force apparatus. The basic principle of each technique was introduced first, followed by a summary of critical information derived for the relevant mineral processing systems. Finally, the pros and cons of each technique were discussed to emphasize the use of complementary techniques that assist in solving fundamental problems in mineral processing.

Graphical Abstract

Keywords

Mineral processing Hetero-coagulation Selective coagulation Colloidal forces 

1 Introduction

Measurement of colloidal forces is becoming an increasingly important subject in mineral processing since the inevitable depletion of high-grade ore deposits requires the processing of finely disseminated low-grade ores (Fig. 1). The extensive grinding to obtain adequate liberation of valuables results in the production of a large proportion of fine particles [1]. One of the challenges in fine particle processing is the slime coatings of fine gangue particles on valuables (hetero-coagulation), which leads to a low recovery and poor selectivity [2]. In contrast, homo-coagulation between valuable minerals increases the apparent particle size, promoting their attachment to air bubbles compared with single smaller particles [3, 4]. In dewatering, both homo- and hetero-coagulation to form loosely packed floc structures are beneficial to improve settling velocity and filtration rate. On the other hand, dispersion is highly desirable if not necessary for fine grinding [5] to increase the degree of liberation and to minimize energy consumption.
Fig. 1

Schematics illustrating the need for fine grinding and the corresponding consequences in mineral processing

Dispersion is stabilized by the net repelling force between the particles [6], whereas the attraction between the particles gives rise to coagulation. The colloidal stability of a dispersion can be described by the classical or extended DLVO theory, with the former considering only van der Waals and electric double-layer interactions [7, 8], and the latter involving additional forces such as attractive hydrophobic forces for hydrophobic surfaces [9, 10], repulsive hydration forces for hydrophilic surfaces [7, 11, 12], and steric repulsion [13] or attractive bridging forces [14] for polymer bearing surfaces. To control the state of colloidal dispersions by creating favorable conditions, it is of paramount importance to study the colloidal interactions in a relevant system, including such considerations as different electrolyte concentrations [15, 16], the presence of divalent cations [17], the co-existence of various cation species and natural surfactants in industrial process water [18], different pH values [19, 20], the presence of reagents such as collectors, dispersants, and activators [1, 21, 22, 23], and the existence of other surfactants [24].

Compared with the traditional diagnostic methods such as settling test [25], particle size analysis [26] and turbidity measurements [27], surface force measurement with atomic force microscope (AFM) and surface force apparatus (SFA) is a more direct approach of high-resolution for studying colloidal interactions, which could provide insights into molecular mechanisms of operating colloidal forces. With the ability to directly measure the absolute separation of two surfaces, SFA has been utilized to clarify the interactions of varying ranges, magnitudes, and signs between atomically smooth mica surfaces under different conditions [8]. Modified SFA, for which one coated mica surface was replaced by an air bubble or droplet, can monitor the thickness changes in the thin liquid film between a fluid drop and a flat mica surface [28, 29, 30], making it possible for the study of bubble-particle attachment, although a straightforward force measurement was not available due to the absence of force sensor [31]. However, it is the interactions in the relevant or practical systems that are of interest, and AFM is advantageous in such cases as compared with SFA that is restricted to transparent materials [32]. Since its invention AFM has become one of the most important tools in colloid and interface science [33] to directly measure the interactions between one particle and a substrate in a liquid, and even the single molecular force involved in the rupture of a single chemical bond and the stretching of polymer chains [34]. Not limited to hard or non-deformable surface, AFM is also used to measure the forces between one solid particle and an air bubble [35, 36, 37] or an oil drop [38]. Although progress has been made in studying bubble-particle interactions using AFM in recent years, the deformation of the bubble triggered by both the hydrodynamic and surface forces remains a bottleneck in determining the absolute separation and the surface forces [31]. In addition, AFM is only suitable to determine forces with Reynolds numbers < 10−2 which is much lower than the typical hydrodynamic conditions encountered in flotation. The first measurement of the interaction forces between air bubbles and a solid surface over the hydrodynamic conditions of intermediate Reynolds numbers (10−2 < Re < 102) was conducted using the integrated thin film drainage apparatus (ITFDA) by Wang, Shahalami, and their co-workers [39, 40]. A new integrated dynamic force apparatus (IDFA) was developed recently to measure the force and film profiles simultaneously between a deformable surface and a solid surface over a wider range of Reynolds numbers by Zhang et al. [41, 42].

However, the abovementioned methods of force measurements are usually restricted to the materials of regular geometry or optical transparency. Complementary methods are highly desirable when studying the colloidal interactions of non-ideal systems. Zeta potential distribution measurement, originally developed to investigate fine clay coatings on bitumen in aqueous solutions, is one of such techniques not restricted by the geometry of particle and can be used to study hetero-coagulations for the binary or tertiary colloidal systems of industrial importance, as long as the components involved have distinct zeta potential distributions under the testing conditions [43]. Yet for specific types of minerals with anisotropic surface properties on basal planes and edge surfaces, such as clay, it is necessary to probe the properties of basal planes and edge surfaces separately to obtain comprehensive understanding of surface properties and the corresponding interactions in colloidal systems. With the preparation of smooth edge surfaces of anisotropic platy minerals using the ultramicrotome method that are suitable for AFM force measurement [44], the surface potential of anisotropic minerals have been determined by AFM force measurements through fitting the force profiles in aqueous solutions with the DLVO theory [18]. For particles of little difference in surface charge properties, quartz crystal microbalance with dissipation monitoring (QCM-D) shows an exclusive advantage to determine the slime coatings in oil sands extraction, hetero-coagulation in mineral flotation, and adsorption of flocculants on clay surfaces for tailings treatment by coating QCM-D crystal sensor surfaces with a thin layer of materials to be studied [45].

With the great importance of colloids and surfaces in mineral processing, we dedicate this review on investigations of the colloidal interactions in mineral processing using both well-defined techniques such AFM, SFA, and IDFA, and techniques suitable for non-ideal systems such as zeta potential distribution measurement and QCM-D along with AFM force measurements of basal and edge surfaces prepared by ultramicrotome to Professor Douglas Fuerstenau’s 90th birthday and his lifelong research contributions to advancing the field of colloids and surface science in mineral processing. The objectives are to provide an overview on the measurement of colloidal forces in mineral processing, analyzing advantages and disadvantages of each technique to understand the phenomena involved in mineral processing, and to emphasize the importance of using suitable and complementary methods as necessary.

2 Principles of Main Techniques

2.1 Atomic Force Microscope

Designed to provide high-resolution topographical analysis, atomic force microscope (AFM) uses a light lever to detect the deflection of a sensitive cantilever spring as it interacts with the substrate surface attached to a piezoelectric translation stage (Fig. 2a) [46]. A laser light focused onto the back of the cantilever spring is reflected and directed onto a split photodiode detector which produces an electrical signal proportional to the cantilever deflection due to the undulations on the surface. With the feedback loop holding the spring deflection constant, the corresponding movement of the piezoelectric translation stage generates an image. To determine colloidal and adhesion forces, a probe particle of interest is attached to the end of the cantilever and the interaction of the particle with a sample of choice can be studied [47]. The approach speed and relative particle-surface position are accurately controlled by applying a voltage across the piezoelectric ceramics. In the force measurements, motion in the x and y directions is disabled, and the surface is solely moved in the z direction by the piezoelectric tube while the cantilever deflection is continuously measured, which can be converted to a force using Hooke’s law with the known spring constant of the cantilever. A typical force measurement between a sphere and a substrate plotted as the deflection of the cantilever vs. the height position of the sample is shown in Fig. 2b. No deflection is observed at large separation distances (Fig. 2b1) while a small repulsive force acts at smaller separation distances between the tip and sample as shown by the upwards bending of the cantilever (Fig. 2b2). The tip jumps onto the sample as the distance between them is so close that the gradient of attractive forces exceeds the spring constant of the cantilever (Fig. 2b3). The tip usually sticks to the surface to large retraction due to the adhesion between the tip and the surface (Fig. 2b4).
Fig. 2

(a) Schematic diagram of an AFM with the sample placed on the piezoelectric scanner (modified from [46], with permission. Copyright © 1995 Elsevier B.V.). (b) A typical measurement cycle of colloidal forces between a sphere and a mineral (substrate) surface plotted as the deflection of the cantilever (Δz) vs. the displacement position of the sample (adapted from [48], with permission. Copyright © 2005 IUPAC)

AFM has been used extensively to measure the interaction forces relevant to mineral processing between solid sphere and solid substrate and between solid sphere and air bubble/oil droplet in water of different chemistries, with the surfaces being hydrophilic or hydrophobic (Table 1).
Table 1

AFM force measurements used in mineral processing

Surface A

Surface B

Water conditions

Main findings

Hydrophilic negatively charged silica sphere

Hydrophilic negatively charged silica and mica flat surfaces

pH 2.2–8.8; NaNO3 10−4–10−3 M

Good correlation between fitted force measurements and zeta potentials at low ionic strength; deviation observed at higher ionic strength due to the increased dependence on surface roughness; magnitude of the repulsion decreased with decreasing pH [49].

Hydrophilic negatively charged and hydrophobic negatively charged glass sphere

Hydrophobic air bubble

None

Repulsive forces measured between hydrophilic particle and air bubble; attractive forces measured between hydrophobic particle and air bubble [35].

Hydrophobic silica sphere with cationic surfactant adsorbed

Hydrophobic silica with cationic surfactant adsorbed

NaCl 0.1 M

Long-range attractions between hydrophobic surfaces remains in the presence of high salt; the forces increases following neutron irradiation, decreases upon removal of dissolved gas and decreases upon increasing the approach velocity of the surfaces [50].

Hydrophobic MoS2 (basal plane)

Hydrophobic stimuli-responsive copolymer molecule

NaCl 0–2 M

Single-molecule adhesion force of stimuli-responsive copolymer increases with increasing NaCl concentration due to the more dominant hydrophobic attraction at higher salt concentrations [51].

2.2 Surface Force Apparatus

SFA (Fig. 3) [52] consists of two crossed silica cylinders with an almost atomically smooth mica sheet of a few micron thick and a thin layer (~ 100 nm) of silver film being glued on each cylinder surfaces. The interaction forces between two mica surfaces in a liquid or air are measured using force measuring springs on which the lower surface is mounted. Mica surfaces are moved toward and away from each other using a combination of upper micrometer for initial positioning within about 1 μm, a lower high precision micrometer for more precise position to about 1 nm and a piezoelectric crystal tube for positioning to 0.1 nm. Multiple beam interferometry is used to visualize the surfaces and measure the thickness of adsorbed layers and the absolute distance between the two surfaces. Typical force measurements using SFA for systems related to mineral processing are given in Table 2.
Fig. 3

(a) Schematic diagram of SFA, (b) fringes of equal chromatic order (FECO) generated after multiple reflections between the silver layers for distance measurement, and (c) configuration of surfaces with the white light entering through the window in the bottom of the chamber (a adapted from [52], with permission. Copyright © 2011 Elsevier Inc.; b and c adapted from [53], with permission. Copyright © 1999 Elsevier Science B.V.)

Table 2

SFA force measurements in systems relevant to mineral processing

Surface A and B

Water conditions

Main findings

Hydrophilic negatively charged mica surfaces

Mg2+, Ca2+, Sr2+, and Ba2+ 5 × 10−6 − 5 M

Increase in concentration of electrolytes caused a reduction in Debye length; short-range hydration forces measured at high concentration of cationic ions due to the residual hydration of the adsorbed cation [54].

Hydrophobic mica surfaces with monolayer adsorption of cationic surfactant

pH 5.4–10.4; NaCl and KBr 0–0.1 M

Measured attractive force, decaying exponentially with distance, is about an order of magnitude larger than the maximum possible van der Waals force; the force is not sensitive to the type and concentration of the electrolyte present, nor to the pH [10].

Hydrophobic mica surfaces coated with octadecyltriethoxysilane

pH 5.6; KNO3 10−2–10−1 M

Attractive force with no electrolytic dependence was measured; the range of the force (~ 17 nm) is longer than van der Waals attraction but shorter than the hydrophobic force reported elsewhere [55].

Hydrophilic mica surfaces

Glycopolymers: 10 ppm

Strong adhesion between polymer and particle, which increases with increasing molecular weight of the glycopolymer [56].

2.3 Integrated Dynamic Force Apparatus

A custom designed instrument, IDFA (Fig. 4a) [42] is used to measure simultaneously the interaction force and the profile of the thin liquid film between a deformable surface whose approach and retract are controlled by a speaker diaphragm or a motorized actuator, and a transparent solid surface in liquids of interest. To get the bubble profile, an image of interference rings with monochromatic interference band-pass filters (Fig. 4b1) with the peak and valley appearing alternatively from the center to the edge of the film (Fig. 4b2) was converted to a bubble profile and corresponding film thickness (Fig. 4b3). In the meanwhile, the solid disc is attached at the free end of a bimorph which determines directly time-dependent interaction forces between a deformable surface (bubbles or droplets) and a solid plate. The transparency of the solid makes it possible to observe the interference fringes for calculation of film profiles and thickness along with the detection of forces simultaneously. With the displacement velocity of the deformable surface ranging from 2 μm/s to 50 mm/s, IDFA can be used to study interactions over a wide range of hydrodynamic conditions involved in mineral processing.
Fig. 4

(a) Schematic diagram of IDFA. (b) A representative analysis of the film profile between an air bubble and a silica surface (contact angle ~ 0°) in Milli-Q water (adapted from [42], with permission of The Royal Society of Chemistry)

2.4 Zeta Potential Distribution Measurement

Zeta potential distribution measured by ZetaPhoremeter (Fig. 5a) could be used to interpret interactions of dispersed phases including solid particles, oil droplets, and air bubbles [57]. By referencing the interactions between bitumen (B) and fines (F) from good/poor processing ore as an example, the working principle of zeta potential distribution measurement to determine particle interactions is as follows: with the zeta potential distribution peaks of bitumen and fines measured separately as a background, the zeta potential distribution of the mixture of the bitumen and fines is then measured. The presence of two distribution peaks observed (Fig. 5c2) for the mixture of bitumen and fines from good processing ores demonstrates the negligible attachment/or no attraction of fines to bitumen, while the presence of only one peak (Fig. 5c1) for the poor processing ore indicates strong coagulation/strong attraction between the two components. Hetero-coagulation of two types of minerals of distinct zeta potential distributions can be investigated based on this principle.
Fig. 5

Principle of zeta potential distribution measurement with Zetaphoremeter (a) to study interactions among colloidal particles taking poor (1) and good (2) processing ores (bd) as an example. (a, adapted from [58]; b–d, adapted from [57], with permission. Copyright © 2005 Elsevier Inc.)

2.5 Quartz Crystal Microbalance with Dissipation Monitor

QCM-D measures the changes in resonance frequency (Δf) of a conducting metal-coated quartz crystal in response to the mass added to the crystal (Δm), allowing detection of interactions between the depositing particles and functionalized surfaces (coated on crystal) in liquids [45]. A decrease in the resonance frequency is observed upon attachment of new substance to the surface of a QCM-D sensor. Disconnecting the power source causes the amplitude of crystal oscillation to decrease slowly if the substance forms a rigid layer. In the case that the adsorbed substance forms a soft layer, the amplitude of oscillation decreases rapidly, due to the high energy loss in the viscoelastic film. The change in mass (Δm) on the quartz surface is related to the changes in frequency (Δf) of the crystal through the Sauerbrey relationship for homogenous, rigid and thin deposited layers, or Voigt model for soft adsorbed layers [59].

3 Critical Information on Colloidal Interactions

Colloidal and surface forces influence mineral processing in many ways (Fig. 6). Particle coagulation controls rheology and hence affects grinding efficiency. Selective homo-coagulation/flocculation is beneficial for fine particle flotation of valuables or rejection of gangues by reduced mechanical entrainment, carrier flotation, or dewatering. Flotation of fine particles is also dependent on bubble-bubble interaction through two stages of aeration, where micro-size bubbles are generated in situ on fine particles by hydrodynamic cavitation or gas nucleation under reduced pressure, followed by the attachment of mineral particles frosted with micro-size bubbles to conventional flotation size bubbles [60], while bubble-particle interaction is critical for flotation of both fine and coarse particles. Hetero-coagulation/flocculation that is detrimental to selective flotation by slime coating is, however, desirable for thickening/dewatering.
Fig. 6

Colloidal and surface forces involved in mineral processing

3.1 Colloidal Stability

The stability of colloidal system is mainly affected by interactions among the particles. A quantitative theory developed by Hogg et al. early in 1966 described the kinetics of coagulation of multicomponent colloidal systems with consideration of the dissimilar electrical double layers for each dispersed species [6]. Reduction in diffuse layer potential by means of surfactant adsorption for example [61] may result in coagulation. Hydrophobicity is another important factor affecting the coagulation of mineral particles and therefore influencing the rheology of suspensions. This is especially true when the particles are sufficiently hydrophobic, with the strong hydrophobic attractive forces being able to counteract electrostatic repulsion [9, 62].

3.1.1 Methods to Study Colloidal Stability

Past several decades have seen the golden age of accurate and direct measurements of forces acting between particles as a function of the surface separation in liquids. Hydration forces, which can be dominant in short range (< 5 nm) and therefore is of great importance in a wide range of surface and colloid phenomena, have been measured between mica surfaces using SFA [54]. It was the hydration of adsorbed cations on the mica surface that induced this measured short-range repulsive force between cleaved mica sheets when they were immersed in electrolyte solutions. On the other hand, a long-range attractive force operating between hydrophobic surfaces was measured in water and aqueous solutions using SFA [10, 63, 64]. With the advances in techniques of force measurement and distance control associated with the development of AFM, Ducker et al. [47] conducted the first direct measurement of the forces on an individual colloidal silica particle as it approached a flat silica surface in NaCl aqueous solutions. The chosen spherical geometry of the particle interacting with a flat silica substrate made it easy to compare the measured forces with the predictions from the theory. While the results were consistent with the double-layer theory of colloidal forces, deviation was observed at very short distances, due mostly to hydration forces or surface roughness. This type of measurement has been expanded to a wide range of particulates and fibrous materials. Although hydration forces have been observed with both of mica [54] and silica [47] surfaces, the presence of cations in aqueous phase, which allows the hydrated cations to adsorb on the solid surface is responsible for the resulting hydration force between mica surfaces, while hydration of silica is more relevant in flotation.

On the other hand, zeta potential distribution measurement has been utilized to predict hetero-coagulation such as slime coating. Strong hetero-coagulation between apatite and hematite was inferred by a single peak on the zeta potential distribution of the mixture of apatite and ultrafine hematite, in contrast to two distinct peaks detected when measured separately [65]. For a binary particulate component systems containing mineral fines and coal particles, no attraction, weak attraction, or strong attraction between the coal and fines can be distinguished from the zeta potential distributions of their mixture [66]. Strong attractive interaction between pentlandite and serpentine detected by zeta potential distribution measurements predicted slime coating of serpentine on pentlandite surface [67]. Furthermore, strong attachment and little attachment were revealed between micron-sized alumina and gas bubbles, and between micron-sized alumina and silica, respectively, by the same technique for a complex tertiary particle system [68].

3.1.2 Effect of Water Chemistry

Water chemistry has a great impact on interaction forces between solid surfaces in aqueous solution. Increase in electrolyte concentration caused a reduction in the Debye length and sometime in surface potential [54]. Similarly, anionic surfactant (sodium dodecyl sulfonate) adsorbs at low concentration as individual ions on positively charged alumina surfaces, reducing the surface potential and as a result inducing coagulation of alumina dispersions [61]. In fact, for systems where the concentration is in the order of 0.1 M or higher, the dispersion forces become dominant [69]. After double-layer repulsion between silica and oxidized silicon was suppressed to be negligible at certain concentration of electrolytes, a pure attractive force contributed mainly by van der Waals force was measured. This attractive interaction then decreased with further increasing concentration of electrolyte (1 to 4 M) due to the increase in non-DLVO repulsive hydration force at higher electrolyte concentrations up to saturation [70]. The findings from such studies expound on flotation mechanisms of coal and halide minerals in saline groundwater and seawater.

The effect of divalent cations is especially significant on the interactions between mineral particles. Slime coating of bitumen by montmorillonite clay was promoted by the addition of calcium ions, as illustrated by the single peak on the zeta potential distribution of the mixture of bitumen emulsion and montmorillonite clay suspension with the addition of 1 mM calcium ions [43]. On the other hand, the interaction between bitumen and kaolinite clay is much weaker, as demonstrated by the bimodal zeta potential distribution obtained for the mixture of kaolinite and bitumen in the presence of the same concentration of calcium ions [43]. The detrimental effect of divalent cations on bitumen liberation from sand grains, which decrease the long-range repulsive force and increase the adhesion force between silica and bitumen, was revealed by AFM force measurements and zeta potential distribution measurements [18]. The observed phenomenon is probably linked to divalent cations as a binder to bridge the negatively charged silica and bitumen surfaces. As a result, anionic surfactants if present in process water, which bind with divalent cations to decrease the number of free divalent cations in the liquid, are able to alleviate the undesirable impact of divalent cations and thus facilitate bitumen liberation. As disclosed by the deposition behavior of silica nanoparticles on sphalerite surface and SiO2-coated surface with QCM-D, high-calcium concentration in gypsum supersaturated solution promoted both the hetero-aggregation between silica and sphalerite, and the homo-aggregation among silica particles [59].

To reveal the mechanism of the observed contamination of sphalerite concentrate by silica if floated from a neutral pH slurry rather than an alkaline slurry, detailed force measurements between a fractured mineral sphalerite and a silica sphere in the context of selective sphalerite flotation were conducted [20]. The entrainment of silica was found to be brought about by the weaker electrostatic repulsion between sphalerite and silica at neutral pH than at pH 10, due to the alternation of surface charge by pH. Monomodal and bimodal electrophoretic spectra were used to study the hetero-coagulation and dispersion of chalcopyrite and pyrite minerals, suggesting hetero-coagulation at pH value between 2.2 and 5.5 where chalcopyrite minerals were positively charged and pyrite minerals were negatively charged [71]. Both long-range forces and adhesion forces between asphaltene films were significantly affected by the solution pH, salinity, and calcium concentration [72]. Since asphaltenes, a solubility class of macromolecules present in crude oil and bitumen, stabilize water droplets in crude oil or bitumen and oil droplets in aqueous phase [73], the study of Liu et al. shed light on the stabilization of water or oil droplets coated with asphaltene films involved in bitumen extraction and crude oil recovery. To provide a fundamental understanding of bitumen emulsion stability and a mechanism of bitumen “aeration” in bitumen recovery processes from oil sands, colloidal forces between bitumen surfaces in aqueous solutions were measured with an atomic force microscope [74]. It was found that lower solution pH, higher salinity, and higher calcium concentration all decreased long-range repulsive forces, and lower solution pH, salinity and calcium concentration made the adhesion forces stronger, while both decreasing long-range repulsive forces and increasing adhesion forces are beneficial for bitumen coagulation and therefore the extraction of bitumen from oil sands.

3.1.3 Effect of Reagents

Flotation reagents affect the hetero-coagulation that could lead to the loss of liberated valuables to the tailings due to slime coating, while selective homo-coagulation of fine valuable particles improves fine particle recovery kinetics. The role of flotation reagents in controlling the colloidal interactions between sphalerite and synthesized ZnS (analogous to sphalerite) and between silica and synthesized ZnS in aqueous solutions was investigated using direct AFM force measurements and zeta potential distribution measurement [1]. The results showed attractive hydrophobic interactions and strong adhesion forces between sphalerite and synthesized ZnS probe triggered by treatment with copper sulfate and xanthate, which are activators and collectors respectively for the flotation of sphalerite. Such long-range attraction in the presence of xanthate led to strong coagulation between sphalerite particles, whereas the long-range forces dominated by electrostatic double-layer force without treatment by xanthate led to dispersion of fine sphalerite particle. On the other hand, the collector-triggered hydrophobic interaction and strong adhesion between ZnS and silica can be effectively suppressed by adding depressants (sodium silicate), changing the coagulation nature between ZnS and silica to be dispersive. Deactivating the ferric-activated quartz with disodium EDTA or potassium fluoride was found to facilitate the selective flocculation of hematite with polyacrylic acid adsorption in a mixture of quartz and hematite [75].

Inorganic salts and synthetic high-molecular-weight polymers are widely used to coagulate the solid particles in treating mine wastes and oil sands tailings [76, 77, 78]. The potential of using high-biocompatible glycopolymers in solid-liquid separation has been investigated following its biomedical applications through settling test, SFA surface force measurements, and AFM imaging [56]. Repulsion was measured on the approaching force curve, due to the steric interaction of opposing polymer loops and tails (brushers) adsorbed on mica surfaces, whereas the strong adhesion between the glycopolymers and the particles increases with increasing molecular weight of the glycopolymer because of the stronger hydrogen bonding with more hydroxyl groups for glycopolymers of higher molecular weight, accounting for the observed impact of molecular weight on their settling performance. Likewise, AFM study revealed the effect of the charge density of polymers on the adsorption of copolymer, allowing fine-tuning of the surface interactions between similarly charged silica surfaces [79]. Because the dispersion of nanosize molybdenum disulfide (MoS2) is essential for its various applications, molecular interactions of a biocompatible stimuli-responsive copolymer with the basal plane surface of MoS2 was studied using the single molecular force spectroscopy feature of AFM [51]. The single-molecule adhesion force of the copolymer on the MoS2 basal plane surface (Fig. 7) was found to increase with increasing salt concentration, which can be used to modulate the single-molecule interactions for desired applications. The interactions measured between titanium dioxide surfaces or between titanium dioxide and silica surfaces by AFM in the presence of linear polyphosphate solutions were found to be a combination of DLVO forces and an additional steric repulsion from the steric barrier formed by the adsorbed polymers [80]. By measuring the interactions between a titanium dioxide colloid probe and a titanium dioxide substrate, the thickness of the steric layer was determined to be within the limits of the molecular dimensions calculated from molecular modeling.
Fig. 7

(a) Single molecular adhesion (detachment) force profile measured using AFM with the oligo (ethylene glycol) methacrylate copolymers attached to an AFM cantilever and retracted from a MoS2 basal plane at 500 nm/s in 0.14 M NaCl background solution. (b) Schematics showing multiple polymer chains probed during the experiment (adapted from [51], with permission. Copyright © 2017 American Chemical Society)

3.2 Bubble-Particle Interaction

The interactions between air bubbles and colloidal particles are of paramount importance for flotation which is used extensively in mineral concentration and water treatment.

3.2.1 Methods to Study Bubble-Particle Interaction

By attaching a bubble to a stationary solid surface and a particle to an AFM cantilever, it is possible to measure the bubble-particle attachment forces [36]. For a hydrophobic particle and an air bubble, strong long-range attractive forces were measured before any double-layer and van der Waals forces could be detected, with the gas bubble behaving like a hydrophobic surface. In such case, the entire particle and cantilever often burst into the air phase of the bubble and the sudden deflection of the cantilever spring was well beyond the measuring limit of the device. Butt [35] described a technique to overcome the measuring limit through recording the snapping-in process, from which kinetic force versus-distance curves were constructed based on Newton’s equation of motion. To account for repulsive nature of electrostatic repulsion and van der Waals forces between particles and air bubbles encountered in flotation, the attractive hydrophobic attraction between the solid-water and the water-gas interfaces is believed to be the main driving force for film rupture and the attachment of air bubbles to hydrophobic mineral particles [81, 82, 83, 84, 85, 86]. In contrast, the repulsive interaction on approach was observed between hydrophilic particles and air bubbles in aqueous electrolyte as anticipated [37, 87, 88, 89]. In addition to repulsive van der Waals forces, this increasing repulsive force with increasing SDS concentration was believed to originate from electrostatic double-layer force since both silica particles [90, 91] and air bubbles [92, 93, 94] are negatively charged, and the surface charge of bubbles is known to increase with increasing SDS concentration up to the critical micelle concentration (cmc) [93].

Deformation of bubble surface during force measurement makes it difficult to determine the point where the probe starts to touch the surface, which is however critical in evaluation of AFM force curves [95]. To get precise information of the particle-bubble distance, AFM was combined with film drainage model to capture the essential physical characteristics in dynamic force measurements [96, 97, 98], or coupled with bubble deformation and film drainage to understand the underlying mechanism responsible for bubble-particle attachment measured experimentally [31]. The effect of surface hydrophobicity on interactions between bubbles and gold surfaces in water was measured with a force apparatus for deformable surfaces (FADS) developed by Pan and Yoon in 2016, which measures both the forces between an air bubble and a surface, and the spatiotemporal profiles of the wetting films simultaneously [99]. However, few techniques mentioned above could measure interaction forces over a wide range of hydrodynamic conditions which have a great impact on particle-bubble interactions. For example, hydrodynamic interactions between an air bubble and a solid particle were studied with AFM by different approaching velocity from 0.6 to 97.8 μm/s [88]. The maximum approaching velocity allowed by the AFM has been restricted to the experiments at low Reynolds numbers (< 10−2). The first measurement of the interaction between air bubbles and particles at intermediate Reynolds numbers (10−2 < Re < 102) was conducted using integrated thin film drainage apparatus (ITFDA) [39]. Recently, a new integrated dynamic force apparatus (IDFA) based on the original ITFDA was developed to measure force and spatiotemporal film thickness over a wide range of Reynold numbers, with the displacement velocity of deformable surfaces ranging from 2 μm/s to 50 mm/s [41, 42], which is an important supplement to the techniques for study of particle-bubble interactions including flotation kinetics. The formation of dimple resulting from the hydrodynamic pressure larger than the internal pressure of the bubble is highly dependent on the approach velocity of the air bubble. While higher bubble approach velocity corresponds to more pronounced dimple, the surface hydrophobicity needs to be taken into consideration to determine the film thickness of the first dimple and the shape of the film for the systems with hydrophobic solid surface. As shown in Fig. 8, the film on hydrophobic silica surface with 63° contact angle ruptured at 200-nm film thickness at the barrier rim under an attractive force of 95 μN (Fig. 8) [42]. The dimple was formed at film thickness of around 1000 nm for this hydrophobic surface, much smaller than the film thickness of 2000 nm for a hydrophilic surface at the same approach velocity of 1 mm/s. This distinct feature was attributed to the change in the hydrodynamic boundary condition of the liquid/solid interface by changing the surface wettability [100]. Evidently, the attractive hydrophobic force also plays an important role in bubble-particle attachment in the high Reynold number region. Combined with AFM, IDFA was used to investigate the interactions between surfaces of various hydrophobicity and microscopic air bubbles (AFM) or macroscopic air bubbles (IDFA) [101], making it possible to simultaneously measure the interaction forces with sub-nN resolution and the drainage dynamics of thin films down to nanometer thickness. More importantly, the simultaneous observation of dynamic forces and spatiotemporal film thickness allows accurate determination of separation distance between deformable and solid surfaces, which is difficult or impossible for other techniques like AFM, and therefore the measurement of surface forces in wetting films.
Fig. 8

(a) Evolution of film profiles with the experimental time of profiles for a, b, c, d, e, f, g, h, and i being 0.3565 s, 0.3650 s, 0.3800 s, 0.3925 s, 0.4150 s, 0.4450 s, 0.4850 s, 0.5350 s, and 0.6000 s, respectively. (b) Force profile during the measurement of interaction forces between an air bubble and a surface (contact angle 62.51°). The inset in b is the enlarged force curve between point A and point B. (adapted from [42], with permission of The Royal Society of Chemistry)

3.2.2 Alteration of Hydrophobicity

Bubble-particle attachment is highly affected by the surface properties of particles. First of all, composition of particles and the liberation of gangue from valuable minerals plays an important role [102], with minerals of higher grade or higher degree of liberation collected faster by air bubbles [102]. This is due to the difference in surface hydrophobicities, which is the basis of froth flotation and controls not only the thermodynamics of flotation but also the kinetics of particle-bubble attachment. It is well established that the efficiency of bubble-particle attachment increases with increasing the contact angle [103].

In flotation, collectors are used extensively to increase the hydrophobicity of minerals, so as to improve the attachment of target mineral particles to air bubbles [104]. For better selection of collectors, useful information was provided through measuring the interaction force between collectors and mineral surfaces. As revealed by AFM force measurements, better flotation response for fluorite than calcite with the oleate collector was attributed to the stronger attractive interaction of calcium dioleate with fluorite than with calcite, with the former having higher density of calcium ions than the latter to work as the reaction sites for collector adsorption [105, 106]. The interaction forces between paraffin/stearic acid and fresh/oxidized coal particles were measured directly using the AFM colloidal probe technique. It was proven that the attractive force between coal surface and collector is the key to improve the flotation recovery of coal [107]. To determine the nature of the forces controlling the adsorption of the octadecyltrimethylammonium chloride (C18TAC) to a negatively charged surface, force measurements were conducted between a C18TAC-treated AFM tip (C18TAC-si) and a silica surface in the presence of varying concentrations of NaCl. The screening of the measured attractive force with increasing NaCl concentration indicated the electrostatic nature of collector adsorption [108].

Activators facilitate the function of collectors. High solubility of short-chain xanthate–zinc compounds in water limits the application of short-chain thiol collectors in flotation of sphalerite, which can be resolved by the addition of transition metal ions such as cupric, silver, lead, and cadmium ions, all of which are effective activators used in sphalerite flotation with short-chain thiol collectors. Kinetics of sphalerite activation was studied in situ by the QCM-D technique [109]. Efficient activation of ZnS by silver was confirmed by the mass increase on the sensor of a thin layer zinc sulfide coating, attributed to the substitution of zinc atoms by silver followed by xanthate adsorption in the form of a densely packed monolayer, as demonstrated by the relatively large frequency drop and low dissipation. The silver uptake as a function of time features two distinct domains: a logarithmic kinetics region followed by a parabolic kinetics region, consistent with findings from previous studies using other techniques [110].

Altering hydrophobicity with/without collector can be optimized by adjusting the reaction conditions such as electrochemical potentials and solution pH. Electrochemical reactions on sulfide surfaces resulted in metal deficient sulfide, elemental sulfur or polysulfides, making sulfide surfaces non-polar and hydrophobic [111]. Chalcocite can be made hydrophobic by adjusting the potential of the system in the presence of collectors such as ethyl xanthate, with the more oxidizing potential resulting in more hydrophobic surfaces [112]. Oleate as the collector for flotation of manganese dioxide adsorbs physically in the form of ion pairs to the positively charged solid at low pH values, but adsorbs chemically in the form of molecules to the negatively charged surface at alkaline pH values [113]. Combined effects of pH and electrochemical potentials have been studied by collector-less flotation using electrochemical cells [114]. The direct force measurements between a cleaved covellite (CuS) plate and hydrophobized glass sphere (θ = 109°) (simulating the air bubble) were conducted in potassium ethyl xanthate (KEX) solution under different electrochemical potentials using AFM equipped with an electrochemical attachment to investigate the effect of electrochemical potential on the reaction between KEX and sulfide minerals [22]. The non-DLVO hydrophobic force measured in KEX solution increased with increasing electrochemical potential of the mineral electrode until 445 mV, above which the hydrophobic force decreased with further increasing electrochemical potentials. As revealed by SFA measurement of interparticle forces in the presence of surfactants, the adsorption of a hydrolysable dodecylammonium chloride on mica surfaces was found in the form of monolayer, bilayer, or multilayer, depending on the pH of adsorption [115]. The optimum flotation of the dodecylamine/mica system was found to occur at pH around 8, where dodecylamine adsorbed on mica in the form of a compact hydrophobic monolayer [116]. Synergistic co-adsorption of a mixture of the amine and oleate collectors at the mineral water interface was observed to give rise to high flotation recovery over a narrow pH range [117]. The long-range attractive force was found to decrease upon degassing of the system. This attractive force was found to be much stronger than the van der Waals forces between moderately hydrophobic surfaces prepared by the adsorption of octadecyltrimethylammonium chloride (C18TACl) [118]. SFA study revealed a dense monolayer of hydrophobic sodium dodecylsulfate (SDS) on the surface of alumina which is highly positively charged at pH below 9 [119]. The results from SFA measurement provide a plausible explanation as to why optimum flotation starts only at pH 6 or below for corundum whose isoelectrical point is 9.0.

Decreasing hydrophobicity either intentionally or inadvertently is observed in mineral processing. For example, the addition of montmorillonite clays, particularly when co-added with calcium ions, increased the long-range repulsive forces and decreased adhesion forces between two bitumen surfaces [74, 120]. Originally, hydrophobic ZnS particles were found to have high wettability due to the slime coating of ZnCO3 resulted from extensive oxidation of ZnS surfaces [121]. In addition, the adsorption or precipitation of these oxidation products on sulfide mineral prevented collectors from reaching their targeted minerals, making the reagents less selective [122]. Adsorption of colloidal iron oxide/hydroxides, originating from the steel grinding media, iron sulfide minerals and non-sulfide gangue, on sulfide mineral particles increased the difficulty of sulfide flotation [123]. As a result, dissolution of slime coating increased the hydrophobicity of zinc sulfide by exposing the originally hydrophobic surface, as disclosed by the measured attractive forces between these mineral surfaces [15]. Adsorbing selectively onto the titanium surface to reverse the electrostatic attraction between silica and titanium, phosphate has the potential to aid in the removal of pre-formed slime coatings from valuable mineral surfaces [23]. On the other hand, the addition of depressants, typically low molecular polymers of high density of hydroxyl and/or carboxyl groups, was found to decrease the hydrophobicity or increase the wettability of unwanted minerals in flotation [124, 125, 126, 127]. Similarly, dextrin-based polymers adsorbed on talc not only reduced the final contact angle (hydrophobicity) of the talc but also prevented the bubble-particle attachment of talc, which is highly desirable to decrease the flotation recovery of the unwanted minerals [126]. The investigation of the adsorption of three dextrin-based polymers on alkanethiol-coated hydrophobic gold surface using QCM-D revealed that the substitution of an additional aromatic group on dextrins increased the interaction between polymer and the thiol substrate, and promoted the affinity of the dextrins to the hydrophobic substrate [128].

3.2.3 Effect of Water Chemistry

Bubble-particle attachment is highly affected by the water chemistry of the aqueous environment where the attachment occurs. For the already hydrophobic particles, electrolytes in water compress the electrical double layer and hence lower the energy barrier formed between hydrophobic particles and air bubbles during collision, which is beneficial for the bubble-particle attachment [129]. This explains the fact that flotation of naturally hydrophobic graphite particles was assisted by the inorganic electrolytes in aqueous solutions [130] or coal flotation in seawater. On the other hand, the adsorption of ionic SDS surfactant on the hydrophobic mineral surface and at the air-water interface increased the electrostatic repulsion between the bubbles and solid mineral particles, hindering the bubble-hydrophobic particle attachment [131]. Depression is usually more effectively achieved by polymers such as dextrin [132] and other polysaccharide polymers [133]. These polymeric depressants adsorb on the surface of hydrophobic unwanted minerals, rendering them less hydrophobic during the flotation of valuable minerals. The adsorption of dextrin could be promoted by electrostatic interactions. As a result, the depression of pyrite with dextrin in flotation of base metal sulfides is optimized by adjusting the pH to a range between the isoelectric point of dextrin and pyrite [134]. In addition, pH conditions of the aqueous phase can affect the surface hydrophobicity of minerals such as zinc sulfide which acquires hydrophobicity upon superficial oxidation [21]. This is because the zinc hydroxide present on the partially oxidized zinc sulfide surface dissolves under acidic pH conditions, exposing the strongly hydrophobic sulfur-rich zinc sulfide.

3.3 Anisotropic Surface Characteristics of Platy Minerals

Anisotropic minerals, with different surface properties on different sides of the crystal, are important constituents of many ores. This group covers both valuable minerals, i.e., molybdenite in Cu-Mo ores as well as gangue minerals, i.e., clay minerals in all types of ores [135].

3.3.1 Surface Charge

The surface charges of the edges and faces of the plate-like kaolinite crystallites may be different [6]. The adsorption of the hydrolyzed metal cations affected the face and edge surfaces of talc differently [136].

Yield stress measurements as a function of solution pH in conjunction with electrophoretic and point of zero charge measurements can be used to detect the surface charge characteristics of particles [137]. While the coagulation peak, the isoelectric point, and the point of zero charge all converge on the same value for isotropic minerals, they spread out over a wide range for anisotropic minerals, and this difference between isotropic and anisotropic minerals is even accentuated with the addition of anionic polymer as dispersant to the mineral suspensions [137]. As a result, other techniques are required to measure the properties of aqueous suspensions of anisotropic minerals. Inspired by preferential deposition of silica nanoparticles on an alumina substrate, and alumina nanoparticles on a silica substrate by managing attractive electrostatic forces between substrates and particles through adjusting the ionic concentrations and pH [138], Gupta and Miller [139] described an experimental procedure to arrange kaolinite particles with desired face exposed (Fig. 9) for colloidal force measurements with AFM, where a negatively charged glass substrate is used to order kaolinite particles with the silica face exposed, and a positively charged alumina substrate is used to expose the alumina face of the kaolinite particles. The force measurement results revealed the negative charge on the silica tetrahedral face at pH > 4, and the pH dependence of the surface charge on the alumina octahedral face, which is positive at pH < 6 and negative at pH > 8. In other words, the isoelectric point of the silica tetrahedral face and the alumina octahedral face is at pH < 4, and between pH 6 and 8, respectively. This is consistent with the maximum coagulation of kaolinite suspensions at approximately pH 5.5, where the largest difference between the electrical charges of the two different sites of kaolinite platelets exists [135]. The dependency of surface charge on pH for the silica and aluminum faces of kaolinite may be related to the possible broken bonds on the silica and aluminum faces. On silica tetrahedral surfaces, the silicon is bonded with oxygen to form relatively stable siloxane bonds, while on aluminum octahedron surfaces, the presence of hydroxyl groups make the surface to exhibit pH-dependent protonation-deprotonation and hence more pH-dependent surface ionization characteristics. For the case of mica, the basal plane is made mainly of silica tetrahedra, the same as the silica face of kaolinite. The less-pH dependent negative surface charge of mica basal planes comes mainly from isomorphic substitution of silicon atoms by lower valency aluminum atoms in its crystal structure. This charging characteristic explains why mica could be floated by cationic collectors at pH as low as 2.
Fig. 9

Schematic representation for the organization and ordering of kaolinite particles on fused silica and fused alumina (adapted from [139], with permission. Copyright © 2010 Elsevier Inc.)

After that, successful preparation of sufficiently smooth clay edge surfaces using the ultramicrotome method together with basal plane freshly cleaved with a sticky tape in a dust-free environment made it possible to measure the colloidal force between the AFM tip and the basal and edge surfaces respectively using atomic force microscope (AFM) [44]. By fitting the measured force profiles with DLVO theory, surface charge characteristics of basal planes and edge surfaces of talc at different pH values were determined [140]. As shown in Fig. 10, the basal planes of talc were found to carry a permanent negative charge, while the charge on its edge surfaces is highly pH-dependent. Interaction energy for various associations—basal-basal, basal-edge, and edge-edge—was calculated based on the AFM-derived surface potential values of the basal planes and edge surfaces. This interaction energy actually affected the rheological behavior of talc.
Fig. 10

Typical force profiles measured between the tip and talc basal plane (a1) and edge surface (a2) in 1 mM KCl solutions as a function of aqueous solution pH, the calculated total interaction energy of association of basal-basal (b1), edge-edge (b2), and basal-edge (b3), and the yield stress of talc particles at various pH (c) (adapted from [140], with permission. Copyright © 2013 Elsevier Ltd.)

Followed the successful preparation of smooth basal plane and edge surfaces, Yan et al. investigated the effect of two divalent cations, Mg2+ and Ca2+, on the Stern potential of different surfaces of phyllosilicate minerals (talc, molybdenite) by AFM force measurements. It was observed that the Stern potential of the edge surface reversed at low concentration of divalent cations due to the prevailing specific adsorption of cations on edges, while the Stern potential of the basal planes became less negative due to double-layer compression but did not reverse to a positive value even at high concentration up to 5 mM [141]. Similar technique was used to study chrysotile rode [142]. Furthermore, rheology measurements showed that the maximum of the pH sensitive shear yield stress was located between point of zero charge (PZC) values of chrysotile basal and edge planes. Based on the measurements of surface interaction and suspension rheology, microstructures of chrysotile rode suspension formed at different pH conditions were predicted [142]. This finding shed a light on minimizing the deleterious effect of chrysotile, which increases the yield stress and viscosity of the suspension due to its rod-like shape and heterogeneous surface charges on nickel recovery in the processing of ultramafic ores [143].

3.3.2 Surface Hydrophobicity

Anisotropic minerals can be classified into clay and hydrophobic anisotropic minerals (molybdenite, graphite, talc, etc.), while the most characteristic feature of the anisotropy of hydrophobic minerals is the inherent hydrophobicity of the basal surfaces of these particles [135], with the edge surface being hydrophilic. It was postulated that the anisotropic surface properties of molybdenite may contribute to the effect of particle size on flotation recovery [144]. To better understand the wettability of molybdenite, interaction forces between hydrophobized tip and basal or edge surface of molybdenite were measured with AFM [145]. The results showed an attractive force for the basal surface but not for the edge surface, demonstrating the hydrophobic properties of the basal surface. It also suggested that small molybdenite particles with low face/edge ratio would be less hydrophobic, consistent with the sharp decrease of floatability with decreasing particle size of molybdenite [135].

3.3.3 Applications

When used in solid-liquid separation, polymers adsorb on fine clay mineral surfaces to enhance particle settling and hence solid-liquid separation. QCM-D was utilized to investigate the adsorption characteristics of polyacrylamide-based polymers (PAMs) on anisotropic basal planes of kaolinite [146]. By depositing kaolinite particles on substrates bearing different charges, such as silica and alumina at desired pH, anisotropic basal planes can be differentiated as tetrahedron (silica)/octahedron (alumina) basal planes. The results revealed that anionic PAM adsorbed preferentially on alumina basal planes through weak electrostatic attractions and hydrogen bonds, while cationic PAM adsorbed strongly on silica basal planes via both strong and long-range electrostatic attractive forces and hydrogen bonding. Due to the preferential adsorption of two types of polymers on different basal planes, it is proposed that dual polymer flocculant systems composed of both of cationic and anionic PAMs can achieve holistic improvement in settling of fine solids in solid-liquid separation.

4 Conclusions

Colloidal interactions play an important role in mineral processing. Developments of various techniques allow us to study the phenomena involving colloidal interactions, including slime coating, homo-coagulation, coalescence of bitumen droplets, and attachment of air bubble to minerals.

Clearly, none of the techniques is perfect or exclusive for all the studies: AFM with high sensitivity for force measurement is not able to measure the absolute distance between the surfaces, especially for deformable surfaces; SFA makes it possible to directly measure surface separations and the thickness of adsorbed layers, but the applicable samples are restricted to smooth and transparent solid surfaces of limited importance in mineral processing. Although IDFA accurately measures the interactions between deformable droplets/bubbles and rigid solid surfaces under a wider range of hydrodynamic conditions than AFM, it is limited to transparent substrates as well. Simplicity in concept and high sensitivity in mass detection has made QCM-D a good choice for the study of particle-particle interaction; however, it fails to distinguish selective deposition of multiple component systems and is also limited to the types of sensor surfaces available, which can be solved by zeta potential distribution measurement except for particles of little difference in surface charge characteristics. Therefore, it is important to use complementary techniques in the study of colloidal and surface forces in mineral processing.

Notes

Compliance with Ethical Standards

Conflict of Interest

The authors declare that there is no conflict of interest.

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Copyright information

© The Society for Mining, Metallurgy & Exploration 2018

Authors and Affiliations

  1. 1.Department of Chemical and Materials EngineeringUniversity of AlbertaEdmontonCanada
  2. 2.Department of Materials Science and EngineeringSouthern University of Science and TechnologyShenzhenChina

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