PSI - Issue 13

Igor Bunin et al. / Procedia Structural Integrity 13 (2018) 1971–1976 Author name / Structural Integrity Procedia 00 (2018) 000–000

1974

4

The electrokinetic potential ( ζ –potential) of mineral particles less than 10 μm in size before and after HPEMP treatment was studied by dynamic (electrophoretic) light scattering using the universal Zetasizer Nano ZS system (Malvern Instruments). Variations in the electrostatic potential of the surfaces ( V , V) of Ca-bearing mineral samples relative to the probe potential as a result of HPEMP exposure were investigated via Kelvin probe force microscopy (KPFM, INTEGRA Prima, NT-MDT). The surface potential was measured in air under standard conditions using silicon-based NSG10/TiN AFM cantilevers with conductive TiN coatings from the tip side. The contact angle of the wetting of mineral polished surfaces before and after HEMP treatment was determined using the sessile drop approach with a modified KP-CK5 contact unit designed by V.A. Glembotskii, equipped with a Moticam 2300 digital camera with software to input and process images (Motic Image Plus 2.0 ML). A drop of distilled water with a diameter of no more than 2–3 mm was placed on each sample’s surface and held under standard conditions for 40 s until the drop profile was determined. The angles of wetting were estimated using the ImageJ analysis program with special plug-ins. The reagent regime of the flotation of Ca-bearing minerals (weighed portions of 1 g; pH 10) was selected such that the initial scheelite (without HEMP treatment) flotation resulted in maximum mineral extraction. The time of mineral contact with water was 1 min; the time of contact with liquid glass (100 g tn −1 ) was 3 min; and the time of contact with sodium oleate (300 g tn −1 ) was 3 min. The flotation time was 1.5 min. 3. Results and discussion 3.1. Structural-chemical properties and softening of natural dielectric minerals under HPEMP-impact Adsorption of Hammett color indicators was used to establish the basic mechanisms of the evolution (mutual transformation, or transmutation) of the acid–base centers of Ca-bearing mineral surfaces under the non-thermal action of nanosecond HPEMP: an increase in the electron donor capacity of the fluorite surface and enhancement of acceptor properties of scheelite upon electric pulse treatment. The adsorption of acid–base indicators showed that the Bronsted acid site with   pK 1.3 predominated in the initial state on the calcite surface. We also discovered the electron donating sites of the Lewis base type with   pK –4.4 and Bronsted base with   pK 12.8. As a result of HPEMP treatment for ~ treat t 10 s, the mineral’s surface dehydroxilated (the content of the Bronsted base sites with   pK 12.8 fell by a factor of more than six). There was also an increase in the concentration of Lewis–type aprotonic electron donating sites with   pK –4.4 by a factor of ~1.3, along with proton-donating sites of the Bronsted type with   pK 1.3 and   pK 4.1. The opposite effect was observed when the duration of pulse treatment was increased to ~ treat t 100–150 s: the calcite surface hydroxilated (the content of Bronsted base type with   pK 12.8 grew by a factor of 4.6–9.3), and the concentration of both Lewis base and Bronsted acid sites fell. Overall, the mutual transformation of base and Bronsted acid centers of the calcite surface at  treat t 30 s, reducing the donor capacity and enhancing the acceptor properties of the mineral’s surface; and recovery (enhancement) of the surface’s donor properties at  treat t 30 s. For a detailed analysis of the chemical (valent) state of silicon and oxygen atoms contained in the surface layer of serpentine , we studied the X-ray photoelectron spectra of the Si 2 p and O 1 s electron levels. Two components with binding energies of 102.4 and 103.4 eV that correspond to three-coordinate silicon Si 3+ and silicon Si 4+ were identified in the electron spectrum of the Si 2 p –level. The spectrum of the O 1 s –level was decomposed with respect to three states: bridge oxygen Si–O–Si ( E bond = 532.51 eV), oxygen with bonds of Si–O–Mg ( E bond = 531.61 eV), and oxygen bound with magnesium Mg–O ( E bond = 530.8 eV). Analysis of the XPS data showed that pulse actions during  treat t 10 s reduced fraction (at %) of trivalent silicon Si 3+ and raised the atomic concentration of quadrivalent silicon Si 4+ . This could have been due to the emission of electrons at the valent level of the atom under the action of HPEMP and/or interaction between the mineral surface and the active products of the radiolytic decomposition of water. In contrast, increasing the duration of pulse action to ~ treat t 100 s simultaneously reduced the atomic concentration of silicon Si 4+ and the surface atomic concentration of oxygen bound in the state Si–O–Mg. These results indicate breaking of the bonds between the layers of magnesium–oxygen octahedrons and silicon tetrahedrons, resulting in disorder in the surface structure due to the octahedral layer escaping from its initial bound state. At the same time, triple valent silicon Si 3+ was likely formed as a result of one electron from an oxygen ion O 2– being captured at the summit of the silicon tetrahedron.