Slime coatings in froth flotation_ A review

Minerals Engineering 114 (2017) 26–36

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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng

Slime coatings in froth flotation: A review Yuexian Yu a b

a,b

a

b

, Liqiang Ma , Mingli Cao , Qi Liu

MARK

b,⁎

School of Chemical & Environmental Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords: Froth flotation Slime coatings Heteroaggregation Clay minerals

Slime coating is a significant phenomenon in froth flotation. The adverse effects of slime coating, by which a value mineral is covered by slimes and prevented from direct contact with collectors and/or air bubbles, have been recognized for decades. It happens ubiquitously in the flotation of various minerals, including sulfide minerals (sphalerite, galena, chalcopyrite, pentlandite, etc.), oxide minerals (hematite, wolframite, scheelite, etc.), salt minerals (fluorite, potash, etc.), coal and bitumen. In this paper, an attempt was made to present a comprehensive review of slime coatings in froth flotation including particle adhesion mechanisms, slime coating measurement techniques, influencing factors, control methods and mitigation measures. It was shown that the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, and the extended DLVO theory, are the mainstream mechanisms underpinning slime coatings. According to their sources, the slimes can originate from gangue minerals and value-gangue composite particles, or from colloidal compounds formed during ore dressing due to grinding, mineral surface oxidation and chemical precipitation. The origin and property of the slimes, the types and concentrations of electrolytes, and solution pH are the main factors influencing slime coatings. Current techniques employed to study slime coatings are mainly focused on the interactions between two particles in a static and ideal environment, which cannot account for slime coatings under commercial froth flotation conditions. Physical mitigation methods such as ultrasonic treatment and high intensity conditioning are often more effective than chemical mitigation methods that usually entail the use of dispersants. Gaps in the existing literature are discussed and potential research directions are suggested.

1. Introduction With decreasing grade and increasing complexity of mineral dissemination, ores have to be ground to very fine sizes to liberate value minerals for recovery. In the process, large quantities of fine gangue minerals are generated due to either the fine grind or the clayey nature of the ore (Kusuma et al., 2014; Liu and Peng, 2014; Wang et al., 2015a). These fine gangue mineral particles are collectively called “slimes”, and they exert many detrimental effects on froth flotation, a mineral separation technique most widely used today. The slimes increase reagent consumption and pulp viscosity, and are liable to entrain into froth product (Arnold and Aplan, 1986a; Brown and Smith, 1954; Burdon et al., 1976; Forbes et al., 2014; Mishra, 1978; Wang et al., 2015b; Yu et al., 2015). The slimes can also coat the surfaces of value minerals and significantly change the flotation behavior of the latter. Intuitively, the slimes coated on value mineral surface form a hydrophilic “armor” preventing the value mineral from direct contact with collectors and/or air bubbles, lowering flotation recovery (Arnold and Aplan, 1986a, 1986b; Bandini et al., 2001; Forbes et al., 2014; Jorjani

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et al., 2011; Jowett et al., 1956; Liu et al., 2002; Tabatabaei et al., 2014; Wang et al., 2015b; Yao et al., 2016a; Zhang and Peng, 2015). A search of open literature shows that slime coatings are ubiquitous in the flotation of various minerals, including sulfide minerals (sphalerite, galena, chalcopyrite, pentlandite, etc.), oxide minerals (hematite, wolframite, scheelite, etc.), salt minerals (fluorite, potash, etc.), coal and bitumen. In most cases, the slimes are composed of kaolinite, montmorillonite, illite, serpentine, quartz, dolomite, and smithsonite. In the early days, researchers attributed slime coatings to electrostatic attraction because the slimes and value minerals carried opposite charges (Bankoff, 1943; Fuerstenau et al., 1958; Iwasaki et al., 1962; Sun, 1943). However, the ensuing research showed that slimes could coat the value mineral surface even when they carried the same sign of charges (Oats et al., 2010; Wang et al., 2013; Yao et al., 2016b; Yu et al., 2015). The classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory has been used to understand the slime coating phenomenon from a chemical standpoint (Chen et al., 1999a; Oats et al., 2010; Yao et al., 2016a; Yu et al., 2015), although some attributed slime coatings to grinding (Bandini et al., 2001; Holuszko et al., 2008). Much

Corresponding author. E-mail address: [email protected] (Q. Liu).

http://dx.doi.org/10.1016/j.mineng.2017.09.002 Received 9 January 2017; Received in revised form 26 July 2017; Accepted 14 September 2017 0892-6875/ © 2017 Elsevier Ltd. All rights reserved.


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(2016a, 2016b) reported that the interaction behaviors between magnesite, dolomite and quartz are better explained by the EDLVO theory than by the classical DLVO theory.

of the reported research on slime coatings was focused on identifying slime coatings, but few studies confirmed the existence of slime coatings unequivocally. Furthermore, the purpose of studying slime coatings is to control it, but so far, few effective ways have been reported and implemented to minimize the adverse effects caused by slime coating either in laboratory or in commercial flotation operations. It is therefore imperative to understand and consequently to control slime coatings in froth flotation. In the following, the open literature on slime coatings in froth flotation is reviewed with an objective to provide a comprehensive overview of the slime coatings in froth flotation and to seek measures that can be taken to mitigate the slime coatings problem in mineral flotation.

2.2. Deposition of colloidal compounds formed during grinding Comminution of ores is almost always carried out before froth flotation. Researchers noticed that slime coatings may be a result of comminution (especially grinding), through the formation of a hydrophilic colloidal slime layer on the value mineral surface during comminution. For example, sphalerite can be heavily oxidized during dry grinding when the mill is open to air, forming smithsonite (ZnCO3) slime coating on the surface of sphalerite, depressing the hydrophobicity of sphalerite (Holuszko et al., 2008). Steel grinding media are routinely used in flotation circuit, and the formation of colloidal iron oxide/hydroxides originating from the grinding media can form slime coatings on galena surface through van der Waals and electrostatic interactions governed by DLVO theory, exerting a significant detrimental effect on the flotation performance of galena (Bandini et al., 2001; Learmont and Iwasaki, 1984). The compounds formed during grinding are often reported as colloidal particles, and the colloidal particles may experience a stronger attraction to the mineral surface due to the compaction or impact caused by the grinding media. Fayed and Otten (2013) reported that plate-like agglomerates can be formed due to the compaction action caused by grinding media. From a practical perspective, more attention should be focused on slime coatings caused by deposition of colloidal compounds during grinding as it is a common step preceding flotation.

2. Mechanisms of slime coatings 2.1. DLVO theory The classical DLVO theory, which sums the universal van der Waals interaction with electrostatic interactions, has been widely used to explain the aggregation and dispersion of colloidal particles. In the minerals industry, researchers used DLVO theory to explain the interaction between mineral particles in water (Behrens et al., 2000; Celik and Bulut, 1996; Elimelech et al., 2013; Yoon and Mao, 1996). Oats et al. (2010) calculated the interaction forces between coal and clay particles using DLVO theory and reported that the calculation could account for the experimental observations in the coal and clay particles suspensions. Their results showed that the van der Waals attraction governed the clay coatings. Similarly, Chen et al. (1999a) proposed that van der Waals force was the dominant force of adhesion for gangue slimes attachment to coarse particles. There are two methods to calculate the Hamaker constant for the van der Waals interaction of mineral 1 with mineral 2 across a water medium. One is the Hamaker approach (microscopic), and the other is the Lifshitz approach (macroscopic). The Hamaker approach is based on the assumption of molecular pairwise additivity and gives a good approximation for gases and often for media interacting across a vacuum or low-pressure gas, but it often fails to predict interactions across condensed phase media like water. In the latter case, the Lifshitz theory, which is based on quantum physics, is more applicable and it often predicts values of the Hamaker constant significantly higher than the Hamaker approach (Berg, 2010). DLVO energy curves between coal and clay particles calculated by the two methods are different and even contradictory (Oats et al., 2010; Zhang et al., 2009). For example, the total DLVO energy between coal and clay particles calculated by Zhang et al. (2009) and Yu et al. (2015) using the Hamaker approach was positive in the whole range of separation distance, so the coal and clay particles repelled each other, which could not explain the clay coatings on coal surface. However, the DLVO calculation based on Lifshitz approach by Oats et al. (2010) showed that the van der Waals attraction was strong enough to overcome the electrical double-layer repulsion, resulting in a net attraction between coal and clay particles at a close separation distance. When calculating the electrostatic force between particles, zeta potentials obtained from experimental measurements are often used. However, clay particles have a plate-like structure with basal and edge surfaces that carry different charges (Gupta et al., 2010). The measured overall zeta potentials are misleading because they are not a true measure of either the basal face potential or the edge potential. In addition, it is recognized that the classical DLVO theory fails when the surfaces are very hydrophilic or very hydrophobic, necessitating consideration of additional interaction forces such as hydration and hydrophobic force (Derjaguin and Churaev, 1989). The summation of electrostatic interaction, van der Waals interaction and other interactions such as hydration and hydrophobic interactions results in the extended DLVO theory (EDLVO) (Lyklema, 2005), and it is often considered to better represent particle interactions. For instance, Yao et al.

2.3. Chemical precipitation Some researchers reported that slimes were bound to the mineral surfaces through a chemical reaction (Dorenfeld, 1953; Taggart et al., 1934). Ma et al. (2014) reported that calcium ions released by the hydrolysis of gypsum reacted with Na2CO3 to form calcium carbonate which coats the surface of a molybdenum-tungsten mineral, resulting in a lower flotation recovery (Ma et al., 2014). Through solution speciation modeling, Wang et al. (2013) proposed that precipitation of chrysotile, dolomite, hydroxyapatite and chrysotile may have occurred on the surface of coal particles. These are hydrophilic precipitates and likely have a negative effect on coal flotation (Wang et al., 2013). The type of the chemical precipitation is closely related to the solution chemistry of the pulp, and the conditions for precipitate formation and its coating mechanism need further research. 3. Slime coatings detection and quantification techniques It is suggested that slime coatings can be detected by indirect macroscopic methods such as a reduction of flotation recovery, a change in particle settling rates, and a change in rheological properties of slurry. However, these methods may not give an accurate account of true slime coatings (Arnold and Aplan, 1986a; Xu et al., 2003; Zhang et al., 2016), as there can be more than one reason for the observed macroscopic behaviors. For example, clay minerals can affect froth flotation through slime coating but also through other ways, such as affecting froth stability, changing pulp rheology and covering bubble surfaces, that also lower flotation recovery of value mineral (Cruz and Peng, 2016; Farrokhpay and Bradshaw, 2012; Forbes et al., 2014; Ndlovu et al., 2015; Ndlovu et al., 2014). Thus, it can be misleading simply attributing a lowering in flotation recovery of value minerals to slime coatings. Similarly, the settling of value mineral particles may have a “mopping” effect that can trap clay particles even when heteroaggregation does not occur. In addition, the solid concentration and the particle network structure in a slurry can significantly affect its rheological properties and transform a Newtonian fluid to a non-Newtonian fluid even when aggregation does not occur (Cruz et al., 2013; Cruz 27


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2017; Marjan et al., 2015; Xu et al., 2003). However, this method does not work for two types of particles having very similar zeta potentials. The measured zeta potentials would overlap whether aggregation occurs or not (Forbes et al., 2014).

et al., 2015). Therefore, direct visual observation of slime coating, or direct measurement of interaction forces of two particles may be more accurate to pinpoint slime coating. Belonging to these categories are some of the frequently used techniques such as scanning electron microscopy, electrokinetic measurements, induction time measurement, atomic force microscopy, and quartz crystal microbalance with dissipation.

3.3. The induction time measurement The induction time is a measure of the time required for a particle to attach to a bubble (Forbes et al., 2014; Nguyen and Schulze, 2004; Svennilsson, 1934). It depends on the surface chemistry, particle shape, particle and bubble sizes, bubble and particle trajectories, and their relative velocities (Verrelli et al., 2014; Verrelli and Koh, 2010). The induction time of value minerals becomes longer when coated by hydrophilic clay minerals. Thus, the measured induction time can be used as an indication whether slime coatings have occurred or not. There are several different methods to measure the induction time for bubble-particle attachment, such as microflotation, atomic force microscope, induction timer, integrated thin film drainage apparatus and Milli-Timer (Verrelli and Albijanic, 2015). Oats et al. (2010) showed that clay slime coatings increased the induction time of coalbubble attachment. As shown in Fig. 3, the clay coatings on the coal surface made the coal particle difficult to attach to the bubble surface. It is worth noting that in the induction time measurement experiments of Oats et al. (2010), the particle slurry was allowed to settle overnight prior to the measurements, so that the bubbles were most likely not coated by the slimes while the coal particles were. The general observation is that the longer the induction time, the more severe the clay coating. Therefore, this is a sensitive and semiquantitative way to measure slime coatings. It is only semi-quantitative since it cannot measure the mass of the coated clays.

3.1. Scanning electron microscopy (SEM) SEM is one of the most versatile instruments available for the examination and analysis of the microstructural characteristics of solid objects. The primary reason for the SEM’s usefulness is its higher resolution than optical microscope (Goldstein, 1977). It permits the observation and characterization of heterogeneous organic and inorganic materials and surfaces (Goldstein et al., 1992). Energy dispersive X-ray spectroscopy (EDS) is an analytical technique that is often coupled with electron-beam based techniques including SEM, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) (Koshihara et al., 1999). When combined with these imaging tools, EDS can give the elemental composition of individual sample points or to map out the lateral distribution of elements from an entire imaged area (Solé et al., 2007). Since the elemental compositions of the value mineral and gangue mineral are different, SEM coupled with EDS can distinguish clay minerals from value minerals. Many researchers observed slime coatings of flotation products by SEM-EDS analysis (Edwards et al., 1980; Oats et al., 2010; Peng and Bradshaw, 2012; Song et al., 2006; Yao et al., 2016a, 2016b; Zhang et al., 2013). This is a direct observation of slime coatings; however, it is not an in-situ technique which often renders the validity of the existence of slime coatings questionable. The observed slime coating could be due to the natural sedimentation and attachment of clay particles onto coal surfaces during filtration and drying of the samples. Although the loosely deposited particles can be rinsed off by using particle-free water, traditional scanning electron microscopy (SEM) analysis involves drying and evacuating samples which would almost certainly alter the surface properties. Wang et al. (2013) exploited cryogenic-SEM and verified the existence of slime coatings in coal flotation as shown in Fig. 1. In the cryogenic-SEM analysis, the samples were taken from the coal slurry and quickly submerged into liquid nitrogen after washing by deionized water at pH 9.0. The flash-freezing process dropped the temperature at a rate of > 800 °C per minute vitrifying the water without crystallization to ice, thus preserved the structure of the sample. This procedure overcame the aforementioned deficiencies in sample preparation. As shown in Fig. 1(b), small clay particles were seen to coat the coal surface, and EDS detected their compositions and verified them to be gangue slimes. Indeed cryogenicSEM is an objective and direct confirmation of slime coatings, but this method is not really in-situ since this method does not detect slime coatings inside the flotation pulp. Besides, the cryogenic-SEM cannot be used to measure the degree of slime coatings quantitively.

3.4. Interaction force measurement by atomic force microscopy (AFM) Atomic force microscopy (AFM) is a powerful tool which can be used to directly measure the interaction forces between two objects such as particle-particle and particle-bubble (Gui et al., 2016; Kusuma et al., 2014; Liu and Peng, 2015; Nguyen and Schulze, 2004). AFM measurement may be carried out in air, vacuum or liquid (Berg, 2010), making it suitable for in-situ determination of interaction forces between value minerals and clay particles in aqueous suspension under different solution chemistry conditions (e.g. different pH, different electrolytes and electrolyte concentrations). The measurement principle is briefly described as follows (Gui et al., 2016; Xing et al., 2016). An AFM uses a cantilever with a sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range attractive force between the surface and the tip causes the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, increasingly repulsive force takes over and causes the cantilever to deflect away from the surface. The cantilever deflections towards or away from the surface can be detected by a reflected laser beam, which is converted to the interaction forces using the Hooke’s Law once the spring constant of the cantilever is determined. The interaction forces between value minerals and clay particles have been measured by AFM, and it is shown that there is an attractive force between clay particles and many minerals under flotation pulp conditions. For instance, coal and kaolinite attracts each other in water in the presence of 3–10 mmol/L calcium ions at pH 7.0. This was explained on the basis that the electrostatic repulsive force between coal and kaolinite was decreased due to the compression of electric double layer caused by the calcium ions (Gui et al., 2016; Xing et al., 2016); The partially hydrophobic fine solids in poor processing oil sands ores show strong attractive forces to bitumen surface in process water which contains high concentrations of calcium and magnesium ions, causing problems in the extraction of bitumen by flotation from oil sands (Liu et al., 2005). The attractive hydrophobic force coupled with reduced

3.2. Electrokinetic measurements A method based on zeta potential distribution measurement to study heteroaggregation was reported by Xu et al. (2003). The working principles of this method are depicted in Fig. 2. If two minerals possessing different surface charges do not aggregate under a given suspension condition, a bimodal zeta potential distribution can be observed showing the zeta potential of each mineral. Under conditions of a full surface coverage of clays on value minerals, the zeta potential of the value mineral will be replaced by that of the clay minerals. This is a useful technique to detect the interaction of binary particulate component suspensions. The interactions of coal-clay, aluminaquartz, montmorillonite-fluorite and bitumen-clay in aqueous media have been studied by this technique (Chen et al., 2017; Liang et al., 28


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Fig. 1. SEM images and EDS spectra of the flotation concentrate (a) and tailings (b) taken from the flotation circuit. The SEM image shows that the coal surface of the concentrate is very clean (a), but the surface of tailings solids is covered by small particles suggesting that slime coatings occurred on coal surfaces (b). The EDS spectra show the point composition indicated by the arrows. A very strong signal from C element and other clear signals from O and Au were detected from the small particles on coal surface of the concentrate (a) indicating the small particle (indicated by the arrow) is coal. In contrast, on the small particles of the tailings, the signal from C is very weak, but clear signals from Si, O, Al, and Mg were detected (b) confirming that slime coatings occurred on coal surface as clay minerals are mainly composed of Si, O, Al, and Mg. The Au signal is from the gold coating (Wang et al., 2013).

in simple static binary mineral-clay system. The complex flotation conditions (e.g., the presence of multiple minerals, electrolytes and flotation reagents, and the turbulent hydrodynamic conditions) may have a significant impact on particle interactions. Therefore, the interaction forces observed in those simple binary systems by such methods may not represent the true situation in actual flotation environment, and therefore need to be used and interpreted with caution.

electrostatic repulsive force due to the presence of Ca2+ and Mg2+ between the fine solids and bitumen are responsible for this enhanced interaction. Since AFM can directly measure the interaction forces between two particles, it is an indispensable tool to investigate slime coatings. 3.5. The quartz crystal microbalance with dissipation technique (QCM-D)

4. Principal factors affecting slime coatings

Quartz crystal microbalance with dissipation monitoring (QCM-D) is an ultrasensitive tool that has been used to study microcosmic solidsolid and solid-liquid interactions (Alagha et al., 2011; Fatisson et al., 2009; Ivanchenko et al., 1995; Notley et al., 2005). The QCM-D works by monitoring the resonance frequency and dissipation of a piezoelectric quartz crystal sensor. When slime particles coat value mineral surface immobilized on the quartz sensor, the increased mass of the sensor changes its resonance frequency and dissipation. Therefore, by monitoring the resonance frequency, it is possible to detect the deposition of slime particles on the mineral-immobilized sensor. Recently, Marjan et al. (2015) used QCM-D to study the interaction of montmorillonite, kaolinite and illite with bitumen surface, showing that QCM-D is an effective technique to study the slime coatings. Moreover, the QCM-D can quantify the mass of clay particles deposited on the bitumen surface. For instance, the mass of montmorillonite deposited on bitumen surface was found to be 8.80 μg/cm2 in 1 mM KCl solution with 40 ppm of Ca2+ (1 mM CaCl2) at pH 8.5 (Marjan et al., 2015). However, the immobilization of a mineral particle onto the quartz sensor is a key challenge. All the measurement techniques discussed above are reported only

4.1. Solution chemistry 4.1.1. The pH of the slurry Pulp pH controls the activities of hydrogen ions (H+) and hydroxide ions (OH−), the two most important counter-ions of the mineral surface. The mineral surface charges can be changed by adjusting the pH of the pulp. When the pH of the solution is lower than the isoelectric point (IEP), the minerals charge positively; on the contrary, when the pH is above IEP, the minerals charge negatively. As pH changes away from IEP in both directions, the absolute value of zeta potential also increases gradually (Franks, 2002; Parks, 1967). Therefore, changing the pH of the pulp can result in a shift of zeta potential of the mineral surface, with consequent changes in the electrostatic force between the value mineral and clay particles, affecting the dispersion and aggregation of the particles (DoymuŞ, 2007). If the signs of the zeta potential of value mineral and clay mineral surfaces are opposite at the same pulp pH, the electrostatic interaction force will be attractive. Coupled with the attractive van der Waals forces, the clay mineral and value mineral 29


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Fig. 2. Schematic zeta potential distributions for a binary particulate component system. In this figure, the black and white circles represent mineral matter (M) and coal (C) particles, respectively. (a) Superimposed zeta potential distribution of the two components measured separately; (b) a binary mixture without heteroaggregation; (c) weak attraction (coal partially covered by mineral matter with some remnant free mineral matter particles); (d) strong attraction (coal fully covered with mineral matter particles); (e) strong attraction (coal partially covered with some but not sufficient mineral matter particles available for full surface coverage) (Xu et al., 2003).

Fig. 3. The attachment of the coal particles onto air bubbles in the absence (left figure) and presence of (0.1% by weight) fine clay particles (middle figure) under the same conditions. The left bubble surface is firmly attached by the fresh coal particles. By contrast, in the presence of fine clay particles, the coal surface is clay-like and difficult to attach to the bubble firmly. As to the bubble-particle contact time (right figure), the presence of fine clay particle requires a longer time to achieve a 100% attachment (Oats et al., 2010).

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ions increased the positive charge of coal particles below the isoelectric point (IEP) and decreased the negative charge above the IEP and eventually shifted the IEP of coal particles (Arnold and Aplan, 1986b). Base on the forementioned description, the adsorption of calcium ions on coal surface should belong to specific adsorption. And this is in agreement with the other authors’ findings (Celik and Somasundaran, 1986). Overall, if the cations of the added electrolytes does not show a specific adsorption on negatively charged mineral surfaces, then they only compress the electric double layer and shift zeta potential to less negative values, and hence decrease the electrostatic repulsion between the mineral particles and promote aggregation, aggravating slime coatings (Chorom and Rengasamy, 1995; Gui et al., 2016; Vane and Zang, 1997; Xing et al., 2016; Zhang and Liu, 2015; Zhang et al., 2013). If the cations of the added electrolytes exhibit a specific adsorption on mineral surfaces, even a low concentration of the electrolyte can cause an apparent compression of the electric double layer, and a high concentration can lead to charge reversal. Anions may also have an effect on zeta potential. For example, coal particles exhibit a strong negative zeta potential in high concentration of SO42− and CO32−, but not in the solution of the NO3− ions due to greater adsorption of SO42− and CO32− ions (DoymuŞ, 2007). Other authors also found that the presence of sulfate ions increased the negative charges of sphalerite and pyrite (Bulut and Yenial, 2016). In general, the addition of electrolyte affects the magnitude and even the sign of the zeta potential, which in turn changes the electrostatic interactions between value mineral and clay particles.

Fig. 4. Adsorption isotherms for iron oxide slimes on galena particles (25–38 μm) (Bandini et al., 2001).

attracts each other and slime coating occurs. In contrast, the two particles may repel each other if the two carry the same signs of charges, offsetting the van der Waals force and experiencing dispersion. The effect of pH on the attachment of oxide slimes on galena surfaces are shown in Fig. 4. As can be seen, with the increase of pH, the quantity of iron oxide slimes coated on galena particles significantly decreases (Bandini et al., 2001). The mean IEP of iron oxide is about 8.4 according to the reported literature (Carlson and Kawatra, 2013). Therefore, as pH increases from 4 to 8, the magnitude of the positive zeta potential of the iron oxide slimes decreases. Consequently, the electrostatic attraction force between iron oxide and galena, which is negatively charged from pH 4 to 10 (Vergouw et al., 1998), decreases between pH 4 and 8. When pH continues to rise to 10, the zeta potential of iron oxide slimes turns negative and an electrostatic repulsion occurs between the iron oxide and galena. As a result, the quantity of iron oxide slimes attached to galena decreases further.

4.2. Mechanical energy input Mechanical energy input (such as mixing, agitation and high intensity conditioning) exerts kinetic energy on particles and thus may improve collision and adhesion of fine particles (Schubert, 1999; Yu et al., 2017a). This is especially the case when a flotation collector is present, which is usually the case in froth flotation. In fact, Warren observed significant aggregation of ultrafine scheelite particles (90% by weight of the particles were between 0.75 and 1.4 μm diameter) in the aqueous solution of sodium oleate only when a certain minimum mechanical agitation energy was exerted, and he coined the phrase “shear flocculation” to describe this phenomenon (Warren, 1975). This was explained by an energy barrier in the DLVO interaction curve, and the energy barrier must be overcome before particle aggregation or slime coating occurs (Oats et al., 2010; Warren, 1975). Therefore, it can be inferred that the mechanical energy input may provide external kinetic energy to the suspension and render particles to overcome the energy barrier more easily than that in the absence of mechanical energy input, producing more serious slime coatings. However, the effect of mechanical energy input on slime coatings still need further study and will be further discussed in Section 5.2.1.

4.1.2. The addition of electrolyte The added electrolytes can interact with mineral particles in one of two distinct ways: (a) non-specific ion adsorption which has no effect on the position of IEP (i.e., the sign of the charge) but can cause changes of the magnitude of the zeta potential of the mineral particles. (b) Specific ion adsorption which can change the position of IEP and sometimes give rise to charge reversal (Fokkink et al., 1987; Peter Horsman and Yeager, 1985; Trefalt et al., 2015). In the case of nonspecific ion adsorption, the cations of the added electrolytes can compress the double layer of negatively charged mineral surfaces and shift the zeta potential to less negative values. The higher the concentration of the electrolyte, the lower the magnitude of the negative charges. For example, the IEP of alumina is independent of the concentration of NaNO3. With increasing concentration of NaNO3, the zeta potential above IEP becomes less negative and the zeta potential below IEP becomes less positive (Reyes Bahena et al., 2002). In the case of specific ion adsorption, the cations of the added electrolytes can enter the Stern layer through specific adsorption, greatly compressing the double layer at a very low concentration. With the increasing concentration of the electrolyte, the specifically adsorbed cations can even change the sign of the zeta potential. It is reported that multivalent cations are more strongly adsorbed to clay particles than monovalent cations (Lagaly and Dékány, 2013). For instance, AlCl3 changed the sign of zeta potential of montmorillonite to positive at a concentration of 5 × 10−4 mol/L at pH 5.3. With further increase of the AlCl3 concentration, the positive zeta potential rises rapidly. Specific adsorption is thought to have occurred in the interaction between Al3+ and the montmorillonite surface (Saka and Güler, 2006). It has been reported in the literature that the addition of calcium

4.3. Surface oxidation Sulfide minerals are reactive and prone to oxidation in the presence of air. Different sulfide minerals have different propensity to oxidation. For example, the oxidation of chalcocite (a secondary copper sulfide mineral) is faster than that of chalcopyrite (Peng and Zhao, 2011). Mild oxidation results in a surface that is rich in polysulfides together with some metal hydroxides (Buckley and Woods, 1984). Extensive oxidation results in high quantities of metal hydroxides on the mineral surface (Senior and Trahar, 1991). The surface oxidation may take place in ambient air or in the grinding process, and sometimes this oxidation can be accelerated by hydrogen peroxide generated during grinding (Borda et al., 2003; Nooshabadi et al., 2013). The oxidation products themselves have an influence on the floatability of the sulfide minerals. For instance, the colloidal lead hydroxide particles formed by oxidation reduce the kinetics of flotation recovery of galena (Bandini, 2000). Similarly, metal hydroxides on the mineral surface decrease the flotation 31


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of copper sulfides even in the presence of collector (Senior and Trahar, 1991). On the contrary, polysulfides on the surface as a result of mild surface oxidation facilitate the collectorless flotation of copper sulfide minerals due to the improved hydrophobicity by the formed polysulfides (Ekmekçi and Demirel, 1997; Lekki and Drzymala, 1990). However, the generated oxidation products may have different electrochemical properties from the initial sulfide minerals. Unoxidized sulfide minerals such as pyrite, sphalerite, chalcopyrite and galena are negatively charged in a wide pH range and their IEP has been found between 1 and 2 (Fornasiero et al., 1992; Fornasiero et al., 1994). Oxidation of these sulfide minerals makes the sulfide minerals behave like the corresponding metal oxides and shows distinct IEP of the metal oxides (Fairthorne et al., 1997; Fornasiero et al., 1992; Fornasiero et al., 1994; Fullston et al., 1999; Witika and Dobias, 1993). Therefore, this sulfide to oxide conversion alters the electrostatic interactions between sulfide minerals and clays. For example, Peng and Zhao (2011, 2012) reported that bentonite slime coatings on oxidized chalcocite was more severe than on unoxidized chalcocite, and depressed the flotation of chalcocite due to the alteration of electrostatic forces before and after surface oxidation (Peng and Zhao, 2011; Zhao and Peng, 2012).

Fig. 5. The EDLVO energy (van der Waals energy, electrostatic energy and interfacial polar energy) of different sizes of kaolinite (radius = 1, 3 and 5 μm) with coal particle (radius = 45 μm) at pH = 7 (Yu et al., 2015).

depressed the flotation to a much less extent. The authors proposed that the great depression caused by the medium density fraction was due to the partial hydrophobicity of the clay-coal composite particles. They argued that the hydrophobic side of the composite particles could adhere to the coal surface by hydrophobic force, exposing the hydrophilic side to water (Hou et al., 2016). Liu et al. (2005) reported that fine solids (which were slightly hydrophobic) in poor processing oil sands ores showed strong attractive forces to bitumen in process water than those from good processing oil sands ores, causing problems in the extraction of bitumen by flotation from oil sands. Thus, the coatings of partially-hydrophobic composite particle play an important role in the flotation of naturally hydrophobic minerals. Therefore, the different properties of slimes such as crystallinity, cation exchange capacity, swelling, and partial hydrophobicity (composite particles), etc. have a significant contribution to slime coatings. In addition, it is also reported that finer particles have a lower energy barrier in the total energy curve as shown in Fig. 5, indicating that smaller clay particles are more likely to attach to value mineral surface (Yu et al., 2015). However, this has not been demonstrated experimentally yet.

4.4. Properties of the slimes It is reported that at the same pH (9.0), the flotation of pentlandite is more severely depressed by chrysotile than lizardite, possibly due to the stronger electrostatic attraction between pentlandite and chrysotile caused by the higher positive charge on the surface of chrysotile compared with lizardite at this pH (Edwards et al., 1980). This indicates that the electrical property of clay surface plays an important role in slime coatings when it is governed by electrostatic interactions. Arnold and Aplan (1986a) investigated the effect of clay type on coal flotation and indicated that different clays influenced coal flotation differently. They suggested that for this reason, slimes in a froth flotation system should be well characterized. Kaolinite and illite clays were found to cause little or no coal depression. The presence of as little as 2% bentonite clay in the flotation feed, however, caused significant depression of all but the most hydrophobic coals (Arnold and Aplan, 1986a). Xu et al. (2003) also claimed that it was montmorillonite, the major constitute clay mineral in bentonite, that significantly decreased coal recovery instead of kaolinite (Xu et al., 2003). Xu et al. (2003) reported that the zeta potential of kaolinite was less negative than that of montmorillonite at pH 5, and thus should be attracted to the coal more than montmorillonite. It is puzzling that Xu et al. (2003) reported the opposite coal flotation behavior. The montmorillonite coatings on coal surface at pH 5 was confirmed by zeta potential distribution measurement by Xu et al. (2003), but the same measurement technique could not establish kaolinite coatings on coal surface at pH 5 due to the overlapping of the zeta potentials of kaolinite and coal. It is interesting to note that Gui et al. (2016) found that the overall forces between coal and kaolinite at pH 4 measured by AFM was attractive. In Gui et al.’s case, the zeta potentials of both of kaolinite and coal at pH 4 were more negative than that reported by Xu et al. (2003) at pH 5. Therefore, in the case of Xu et al. (2003), the kaolinite should have coated the coal surface. It is possible that the kaolinite coating on coal surface did not influence coal flotation but Xu et al. (2003) did not comment on this possibility. The reason accounting for this discrepancy is still unclear although it is noted that montmorillonite does have specific properties that are different from kaolinite, such as higher cation exchange capacity and the tendency to swell in water. Hou et al. (2016) extracted three fractions of slimes (< 74 μm) with different densities (light, medium and heavy) from the original coal slimes and artificially mixed them with coarse pure coal (−710 + 74 µm) and then attempted to separate them by flotation. They found that the medium density fraction which mainly contains clay-coal composite particles greatly decreased the flotation recovery of the pure coal, but the heavy fraction which mainly comprises clays only

5. Slime coating mitigation methods 5.1. Slime coating removal by chemical means 5.1.1. Dispersant Dispersants are widely used to modify the colloidal interactions between particles by modulating electrostatic and steric interactions which offset the contribution of the van der Waals attraction to the total net force (Oats et al., 2010). These dispersants are mostly anionic polymers which adsorb on mineral surfaces, making them more negatively charged. In froth flotation, sodium silicate (Na2SiO3) has been widely reported to depress silicate and carbonate minerals by adsorbing onto these gangue mineral surfaces and making them more hydrophilic. Also, it is an effective dispersant due to the SiO32− and HSiO3− ions which adsorb on the gangue mineral surface and enhancing the electrostatic repulsion between negatively charged value minerals and gangue minerals. Sodium silicate can also have different “module number”, i.e., the SiO2/Na2O ratio, that alters its function (Taner and Onen, 2016). However, sodium silicate does not significantly improve the flotation performance of coal slimes (Oats et al., 2010). Carboxymethyl cellulose (CMC), comprising a linear chain structure with β-Dglucose as the basic unit, is another common anionic polymer which is used to disperse gangue minerals. It is a typical dispersant which utilizes the strong steric repulsion between particles with adsorption layers of a long chain polymer, leading to a high stability of the colloidal dispersion. Edwards et al. (1980) reported that CMC can modify the surface charge of serpentine slimes and reduce the detrimental 32


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impact of the slime coatings on the flotation of pentlandite. Effective dispersion results by CMC were achieved in fluorite–quartz/calcite flotation separation. In this case, CMC chemically adsorbed on the surfaces of fluorite and calcite at pH 9.0 through the formation of calcium carboxylate (Crozier, 1992), and the adsorption made fluorite and calcite negatively charged, and hence produced an electrostatic repulsion force between fluorite and calcite. Meanwhile, since CMC is a macromolecule, a strong steric repulsion between fluorite and calcite was also present. The two repulsive forces overcame the van der Waals attraction forces and mitigated the calcite coatings on fluorite surface. In the case of fluorite/quartz flotation, although CMC could not adsorb on the surface of quartz due to the strong negative charges on quartz surfaces at pH 9.0, the strong electrostatic repulsion between quartz and CMC-coated fluorite prevented the quartz coatings on fluorite particles (Song et al., 2006). However, the effect of CMC may deteriorate when the water quality changes. For instance, in nickel plants using fresh or low salinity saline water, CMC has a good performance in controlling serpentine slime coatings on pentlandite surfaces. However, CMC does not work in nickel plants using saline water. This difference is attributed to the different ionic compositions of the water used during flotation (Liu and Peng, 2015; Peng and Seaman, 2011). Recently, lignosulfonate was reported to improve copper and gold recovery by dispersing clay minerals in flotation using fresh water; this is attributed to the enhanced electrostatic repulsion at the low dosage or steric repulsion at the high dosage (Liu and Peng, 2015; Seaman et al., 2012; Wei et al., 2013). However, high dosages of dispersants may depress not only the gangue minerals but also value minerals due to the non-selective adsorption of dispersants on both minerals (Huynh et al., 2000). Thus, the selective adsorption of dispersants is the key to ensure the success of dispersion.

Huang et al., 2016; Yu et al., 2017b). The hydrodynamic forces must be responsible for the removal of slime coatings, which overcome the adhesion force and remove particles from surfaces by rolling and sliding. Chen et al. (1999a) indicated that the finer the slime particle, the higher the critical shear velocity required for the slime to detach from the surface. Overall, the high-intensity conditioning is proposed to remove the coated slimes from the mineral surface, but certain energy input is likely to increase slime coatings as discussed in Section 4.2, and thus the intensity of conditioning is a key parameter that needs to be determined to remove slime coatings. The successful industrial application of HIC has been reported in Kailuan coal mining Ltd, China (Ma et al., 2013). However, whether the detached slime particles after HIC will re-attach onto mineral surface remains unknown. 5.2.2. Desliming before flotation Since the slimes exhibit great detrimental effects on flotation efficiency, it is desirable to remove the slimes before flotation (Taner and Onen, 2016). Removing fine particles (which are mostly fine clay particles) from coal by sieving or classification (by a hyrocyclone) significantly improved the coal flotation performance (Oats et al., 2010; Quast et al., 2008). Desliming using a hydrocyclone also gave improved flotation performance of pentlandite (Chen et al., 1999b). However, desliming can only work when the slime fraction does not contain high concentration of value minerals. If the slime fraction does contain high concentration of value minerals, separate flotation circuits for slimes and coarse fractions should be considered. Kienko and Voronova (2014) treated fluorite ores by separate coarse and fine flotation at a cut size of 15 μm in laboratory tests, and the results showed that the overall recovery of fluorite increased by 18 percentage points. These separate flotation circuits have been used in the Mt Keith nickel plant in Western Australia (Peng and Seaman, 2012). In practice, the success of desliming on subsequent flotation is still a matter of debate, i.e., it is still unknown whether the improved flotation performance is only attributed to the reduction of reagent adsorption on slime particles, or whether it is truly due to the removal of slime coatings from the value mineral surfaces.

5.1.2. Clay binder Tao et al. (2007) developed a novel dispersant called “clay binder” that reportedly significantly improved coal and phosphate flotation by agglomerating clay minerals and removing them from the surface of value minerals. It is a low molecular weight polymer that is the condensation product of urea and formaldehyde reacted under acidic conditions. Depending on the application, the properties of the condensation polymers can be tailored to suit the specific application requirements. The main factors affecting their performance are the molar ratio of formaldehyde to urea, the addition of functional groups, the degree of functionalization, molecular weight and crosslink density. The two main adsorption mechanisms of the clay binders are dipoledipole interactions and strong hydrogen bond. Because of the amphiprotic characteristics and multiple binding and chelating sites, binding can be very selective (Taner and Onen, 2016; Tao et al., 2007). However, the authors did not clearly validate whether the “clay binder” could really mitigate the slime coatings.

5.2.3. The ultrasonic treatment Celik et al. (1998) reported that the ultrasonic treatment increased the flotation recovery of the value minerals by removing the clay particles from boron minerals (Celik et al., 1998). Also, it was reported that the rate of iron oxide removal from quartz surface in the presence of Calgon was increased ∼5 times following ultrasonic treatment (Bandini et al., 2001). Here the dispersant Calgon adsorbed on the surface of iron oxide and reversed the surface charge to negative, and hence increased the electrostatic repulsion between iron oxide and quartz. Meanwhile, the sonication provided an extra mechanical force to expedite the removal of iron oxide slimes. In China, the flotation of some problematic coal slimes is suffering from slime coatings, it is found that ultrasonic treatment increased the coal flotation recovery through removing the fine slimes from coal surface (Kang and Lv, 2006). Gurpinar et al. (2004) compared the SEM images of barite surfaces before and after ultrasonic treatment and found that fine particles previously coated on the surfaces were completely removed (Gurpinar et al., 2004). It was proposed that, during ultrasonic treatment, a clean surface was created by removing impurities from value mineral surfaces, which was then followed by the formation of microbubbles on the hydrophobic solid surface, leading to enhanced bubble-particle attachment (Celik, 1989). These studies show that ultrasonic pre-treatment is an effective technique to remove slimes from mineral surfaces. However, the ultrasonic treatment has not found use in commercial production.

5.2. Slime coating removal by physical means 5.2.1. High intensity conditioning Increasing energy input during conditioning has been shown to substantially improve recovery and selectivity (Bulatovic, 1994; Bulatovic and Salter, 1989; Chen et al., 1999b; Huang et al., 2016; Sun et al., 2016). This energy input is termed “High Intensity Conditioning (HIC)”. Previous studies demonstrated that the improvement derived from selective collector adsorption, the fast diffusion of soluble collectors, the emulsification of oily collectors and the creation of fine mineral particle aggregates (Bulatovic and Salter, 1989; Huang et al., 2016; Ma et al., 2013; Sun et al., 2008). However, the study from Chen et al. (1999) showed that the “surface cleaning” of value minerals induced by high intensity conditioning played an important role in this improvement, removing the impurities from the mineral surface and decreasing the slime coatings (Chen et al., 1999a, 1999b). Surface cleaning was also reported by other researchers (Bandini et al., 2001;

5.2.4. Adding another mineral Feng et al. (2012) described a method to mitigate the detrimental effect of serpentine coatings on pentlandite. They reported that the addition of quartz (−150 + 75 μm) to the suspension of pentlandite/ 33


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Behrens, S.H., Christl, D.I., Emmerzael, R., Schurtenberger, P., Borkovec, M., 2000. Charging and aggregation properties of carboxyl latex particles: experiments versus DLVO theory. Langmuir 16, 2566–2575. Berg, J.C., 2010. An Introduction to Interfaces & Colloids: The Bridge to Nanoscience. World Scientific, Hackensack, U.S. Borda, M.J., Elsetinow, A.R., Strongin, D.R., Schoonen, M.A., 2003. A mechanism for the production of hydroxyl radical at surface defect sites on pyrite. Geochim. Cosmochim. Acta 67, 935–939. Brown, D., Smith, H., 1954. Continuous testing of frothers. Colliery Eng. 31, 245–250. Buckley, A.N., Woods, R., 1984. An X-ray photoelectron spectroscopic study of the oxidation of chalcopyrite. Aust. J. Chem. 37, 2403–2413. Bulatovic, S., 1994. High-intensity conditioning: development, testing methodology, and applications. Int. Miner. Met. Technol. 103–109. Bulatovic, S., Salter, R., 1989. High intensity conditioning: a new approach to improving flotation of mineral slimes. In: Conference of Metallurgists. Springer, Halifax, pp. 182–197. Bulut, G., Yenial, Ü., 2016. Effects of major ions in recycled water on sulfide minerals flotation. Miner. Metall. Process. 33, 137–144. Burdon, R., Booth, R., Mishra, S., 1976. Factors influencing the selection of processes for the beneficiation of fine coal. The Seventh International Coal Preparation Congress, Sydney, Australia. Carlson, J., Kawatra, S., 2013. Factors affecting zeta potential of iron oxides. Miner. Process. Extr. Metall. Rev. 34, 269–303. Celik, M., 1989. Effect of ultrasonic treatment on the floatability of coal and galena. Sep. Sci. Technol. 24, 1159–1166. Celik, M., Bulut, R., 1996. Mechanism of selective flotation of sodium-calcium borates with anionic and cationic collectors. Sep. Sci. Technol. 31, 1817–1829. Celik, M., Elma, I., Hancer, M., Miller, J., 1998. Effect of in-situ ultrasonic treatment on the floatability of slime coated colemanite. In: Balkema, A.A. (Ed.). International Symposium; 7th, Innovations in Mineral and Coal Processing, Istanbul, Turkey, pp. 153–166. Celik, M., Somasundaran, P., 1986. The effect of multivalent ions on the flotation of coal. Sep. Sci. Technol. 21, 393–402. Chen, G., Grano, S., Sobieraj, S., Ralston, J., 1999a. The effect of high intensity conditioning on the flotation of a nickel ore, part 2: Mechanisms. Miner. Eng. 12, 1359–1373. Chen, G., Grano, S., Sobieraj, S., Ralston, J., 1999b. The effect of high intensity conditioning on the flotation of a nickel ore. Part 1: Size-by-size analysis. Miner. Eng. 12, 1185–1200. Chen, T., Zhao, Y., Song, S., 2017. Electrophoretic mobility study for heterocoagulation of montmorillonite with fluorite in aqueous solutions. Powder Technol. 309, 61–67. Chorom, M., Rengasamy, P., 1995. Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type. Eur. J. Soil Sci. 46, 657–665. Crozier, R.D., 1992. Flotation. Theory, Reagents and Ore Testing. Pergamon Press, UK 356. Cruz, N., Peng, Y., 2016. Rheology measurements for flotation slurries with high clay contents–a critical review. Miner. Eng. 98, 137–150. Cruz, N., Peng, Y., Farrokhpay, S., Bradshaw, D., 2013. Interactions of clay minerals in copper–gold flotation: Part 1–Rheological properties of clay mineral suspensions in the presence of flotation reagents. Miner. Eng. 50, 30–37. Cruz, N., Peng, Y., Wightman, E., 2015. Interactions of clay minerals in copper–gold flotation: Part 2—Influence of some calcium bearing gangue minerals on the rheological behaviour. Int. J. Miner. Process. 141, 51–60. Derjaguin, B., Churaev, N., 1989. The current state of the theory of long-range surface forces. Colloids Surf. 41, 223–237. Dorenfeld, A.C., 1953. 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serpentine mitigated the serpentine coatings and improved the pentlandite recovery significantly. They attributed this to the stronger attraction between serpentine slimes and quartz particles than between serpentine and pentlandite since the quartz was more negatively charged than pentlandite in the pH range of 4–11 in their tests. This method provides a new concept to control undesired slime coatings. However, this may meanwhile increase the entrainment of the new mineral additives into the froth if their sizes are too small and lower the grade of the concentrate (Liu and Peng, 2014; Wang et al., 2015a). 6. Summary Slime coatings have long been recognized to have detrimental effects on froth flotation. The occurrence of slime coatings is attributed to the interactions between particles which are interpreted by classical DLVO or extended DLVO theories as well as chemical precipitation or deposition of colloidal particles during grinding. According to the sources, the coated slimes can be classified into two types: (1) original particles such as associated gangue minerals and composite particles; (2) secondary compounds caused by grinding, mineral surface oxidation and chemical precipitation. Slime coatings are influenced by several factors such as pH, electrolyte, mechanical energy input and slimes properties. So far, much research has been focused on the factor of solution chemistry. The literature reports many measurement methods for slime coatings. However, these methods often have limitations and lack general applicability. A number of methods to control slime coatings were proposed which can be divided into two categories, chemical means and physical means. Considering the practical perspective, it is worth noting that high intensity conditioning (HIC) should be highlighted. This is because HIC not only mitigates slime coatings but also increases the diffusion of soluble collectors and the collision of particles with oily collectors, therefore, improves flotation recoveries. Further studies are required to thoroughly understand slime coatings and mitigate its detrimental effects: (1) The study of slime coatings under actual flotation conditions (complex slurries of polymetallic ores in the presence of flotation reagents). (2) In-situ technique to directly detect and quantify slime coatings in laboratory scale and commercial scale flotation. (3) Effective and applicable control methods to mitigate slime coatings especially in commercial flotation circuits. Acknowledgements Financial support to the project was provided by National Natural Science Foundation of China (No. 51604280) and Natural Sciences and Engineering Research Council of Canada (NSERC). Yuexian Yu appreciates a scholarship (Grant No. 201606430041) from the China Scholarship Council (CSC) to carry out a visiting study at the University of Alberta. References Alagha, L., Wang, S., Xu, Z., Masliyah, J., 2011. Adsorption kinetics of a novel organic–inorganic hybrid polymer on silica and alumina studied by quartz crystal microbalance. J. Phys. Chem. C 115, 15390–15402. Arnold, B., Aplan, F., 1986a. The effect of clay slimes on coal flotation, part I: The nature of the clay. Int. J. Miner. Process. 17, 225–242. Arnold, B., Aplan, F., 1986b. The effect of clay slimes on coal flotation, part II: the role of water quality. Int. J. Miner. Process. 17, 243–260. Bandini, P., 2000. Surface Chemical Studies and Heterocoagulation in Metal Sulphide and Oxide Systems. School of Chemical Technology, University of South Australia, Australia. Bandini, P., Prestidge, C., Ralston, J., 2001. Colloidal iron oxide slime coatings and galena particle flotation. Miner. Eng. 14, 487–497. Bankoff, S., 1943. Experiments with slime-coatings in flotation. Trans. AIME 153, 473–478.

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