Comparison of surface energy and adhesion energy of surface-treated particles
Graphical abstract
Introduction
The mineral industry requires tremendous amounts of water to separate valuable minerals from ores. A common process to separate minerals is froth flotation which is commonly conducted between 25 and 40 wt% solids [1]. The requirement of water in a conventional comminution-classification-flotation circuit to process, for instance, copper sulfide ore, is approximately 1.5 to 3.5 m3 of water per metric ton of ore processed [2]. In addition, most of the copper mines in the United States are located in the arid desert southwest (e.g. Arizona and New Mexico). Thus, sustainable processing, using reduced water consumption is of critical importance to the long-term viability of such operations.
Over the past 10–15 years significant progress has been made with dry air-based separation systems. Probably the most widely adopted systems are so called sensor-based sorter systems [3]. There have been significant advancements in this technology, largely because of improved imaging, sensing and separation algorithms. The sorter systems typically transport a dry feed (particle size 2–50 mm) on a conveyer belt over the sensing area. The sensing technology used depends upon the feed material and the desired separation. In particular, sorter technology has found utilization with separation of plastics and gemstones. The separator uses sensors that can detect and sort (via an air blast) by particle chemistry, and similarly, the particles can be sorted based upon color/spectral response. Mineral sensing separations have lagged, in use, behind other applications (e.g. plastics), and are usually practiced for relatively high-grade mineral streams that liberate at a large particle size. In other words, sorter systems currently are not amenable for low grade ores that often need to be ground to less than 200 μm (an order of magnitude smaller than used with sensor-based sorter systems) to achieve acceptable liberation.
In addition to sorting technologies, air tables, air jigs and air-dense medium are other types of dry air-based separators. These can all be considered dry-based advanced sorting technologies, and in particular the magnetic air dense medium technology (MADMT) has drawn significant interest for separating inorganic ash from coal [[4], [5], [6], [7], [8]]. The MADMT separating device is a fluidized bed that includes very finely ground magnetic powders. At intermediate bed density (between the lower density coal and higher density inorganic minerals) separation is achieved by suspending the magnetic particles. The coal then levitates to the top of the fluidized bed and the inorganic minerals to the bottom. These systems suffer from the need to add an external magnetic medium, are dynamically unstable, and work best on systems that have large differences in densities and large size of particle liberation (~10 mm). These constraints have limited application almost exclusively to coal/mineral separations.
Exploitation of differences in adhesive forces between particles and a flat substrate is one additional potential gateway to develop a dry, sustainable process for mineral separation and concentration. Measurements of adhesive forces can be accomplished through various techniques [[9], [10], [11]] and are often somewhat tedious and time consuming. Regardless of these challenges, it is important to understand how the surface energy of solids contributes to the adhesion of particles to a substrate, and this was the focus of this research.
A solid's surface energy can be thought of as the amount of energy required when molecular bonds are broken to form a new solid's surface [12,13]. With regard to interfacial adhesion, there are several sets of forces in operation across the interface. One set is the van der Waals forces, which include London dispersion forces between induced dipoles (all surfaces exhibit these forces); Debye forces, between a permanent dipole and an induced dipole (polar to non-polar); and Keesom forces, between permanent dipoles (polar to polar). The other force is the electron accepting/donating behavior of Lewis acid/base pairs. Combined, these forces attract (or repel) material within a molecular proximity to a solid surface. However, if a surface is treated such that it has low polarity, particles will not have strong adherence because only weak, non-polar (London) van der Waals forces bond the materials. If there is other interference/contamination on the surface, like dirt, oils or residue from the atmosphere, the Lewis acid, base and Lifshitz-van der Waals sites could be covered and replaced with low energy material, keeping the desired material from attaching. This research compares surface energies of glass in its natural state and treated with hydrophobic coatings, which would have few polar groups, and hydrophilic coatings, whose surfaces will have significant numbers of polar groups. Furthermore, the higher the surface energy of a mineral, the more materials of high surface energy will adhere to the mineral's surface [13].
A variety of methods for calculating the surface free energy of solids using the contact angle a liquid makes with a solid surface are usable, which generally yield similar values for the solid surface energy [14,15]. In this research this is referred to as a ‘molecular approach’ to measuring surface free energy. The van Oss-Good-Chaudhury acid-base method (VOGC) which includes the Lifshitz-van der Waals, Lewis acid and Lewis base interactions between solid and liquid [[14], [15], [16], [17]] has been used in this work. The VOGC method uses a triad of three liquids with known surface tension components. Triads are chosen to minimize their condition number to yield accurate surface energy values [14]. To find the Lifshitz-van der Waals component, a non-polar liquid is used (e.g. diiodomethane (CH2I2)). Also needed are a liquid that is heavily dominant Lewis acid (e.g. water) and one that is highly Lewis basic (e.g. ethylene glycol (C2H6O2) or glycerol (C3H8O3)). There has been some debate in the literature with the scales used [18] but these issues are related to inter-comparison of the components, not with the measured total solid surface energy. The surface energy can be used to determine the adhesion between two solids [10]. The most accurate calculation of surface energy comes from using the advancing angle of the contact angle hysteresis (the difference between the advancing angle and the receding angle) [19]. When the surface with the adhering drop is tilted, advancing (down-hill) and receding (up-hill) angles form. The advancing angle is the angle measured just before the liquid begins to slide and is the instance of strongest adhesion for that solid [19]. Thus, using the advancing angles in the VOGC method yields solid surface energy values that represent the lowest energy regions of the surface [15].
In 1971 Johnson, Kendall and Roberts (JKR) developed a model that includes the effect of adhesion force on the deformation of an elastic sphere in contact with an elastic half space [20]. As previously stated by Zafar et al. [9] the JKR theory is an adhesion energy theory that infers that “the pressure distribution at contact is such that all short-range contact forces exist within the contact area” adding an adhesion force to the classical Hertz [21] contact theory. However, when using a solid with high elastic modulus (glass in this case), the deformation produced by the attractive forces is very small [20], thus the deformations can be neglected. Zafar et al. [9] utilized JKR theory to develop a drop test method for the determination of particle adhesion (interfacial energy). In this research the Zafar method was adapted to measure interfacial energy in what is referred to here as a ‘mechanical approach’.
Researchers have previously examined the correlation between surface energy and adhesion, particularly in polymer and polymer matrix composite samples [10,11,[22], [23], [24]]. In addition, mineral surface energy components have been correlated with mineral separation response [25,26]. In the polymer and composite studies, a strong correlation between surface energy and mechanical adhesion was often observed, although some deviation was noted, possibly related to roughness of the surface altering the contact area. With respect to mineral separation, a direct correlation between the work of adhesion and separation has been observed [25,26].
The optimal condition in this is work is to have comparable results of measured interfacial energy using a quick test (mechanical approach) with the VOGC method to calculate the surface energy (molecular approach).The specific property compared here is the surface and interfacial energy of glass under different chemical treatments. The surface and interfacial energy are compared using two different methods: a molecular and a mechanical approach. The obtained data was compared to literature results. The benefits of treating a surface with a chemical is to change its surface energy and be able to apply this in a system to separate particles of different surface energies.
Section snippets
Materials and methods
For the measurement of contact angle between the liquids elected for this investigation, and the glass slides, a Ramé-Hart Model 500 Goniometer/Tensiometer was used. The selected probe liquids for the series of tests were: distilled water (H2O, noted as W), ethylene glycol (C2H6O2, noted as E), glycerol (C3H8O3, noted as G), diiodomethane (CH2I2, noted as D), and dimethyl sulfoxide (C2H6OS, noted as S). We chose diiodomethane, because it is the nonpolar liquid with the greatest surface tension.
Results and discussion
Disks treated with TMPA and TCOD were placed in capped vials for two hours, 24 h and 36 h. Fig. 5, Fig. 6 show the results of the contact angles measured with different liquids. As the TMPA is hydrophilic and having a high surface energy, the treatment was less stable over time compared to the TCOD treatment, and hence all contact angle measurements were conducted in a timely manner (within the day of treatment). The contact angle with water showed a variation from 48.6 degrees (in 2 h) to 34.2
Conclusions
An impact test apparatus (mechanical approach) and the VOGC method (molecular approach) were used to characterize the interfacial energies of a model system with a variety of surface treatments. The values measured in the experiments are in good agreement with literature values of critical surface tension. Both approaches proved to be quite comparable despite their wide variation in experimental technique. The mechanical approach is a quick and easy way to measure the total interfacial energy
Declaration of Competing Interest
The authors declare that there is no conflict of interest.
Acknowledgments
Thanks to the National Science Foundation Grant (NSF) #1805550 Sustainable System for Mineral Beneficiation. Also, thanks to Dr. Umair Zafar for his helpful discussion and Ms. Kathryn Bozer and Mr. Bobby Santore (NSF Grants #1460912 and #1757799 (REU Site: Back to the Future)) for their generation of preliminary data.
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