Surface Science and Flotation

X-ray Photoelectron Spectroscopy in Mineral Processing Studies

“…It has been stated that lead sulfate is the major product of oxidation of PbS in air and aqueous environments, while elemental sulfur formed in the early reaction stages; another product is PbO followed by slow adsorption of hydroxide ions. Brion [41] has compared oxidation of main sulfide minerals in air and water and found sulfate and, in the case of Fe-bearing minerals, Fe(III) (hydr)oxides as the main surface products, with the rate of sulfation decreasing in the order FeS 2 > CuFeS 2 ≥ PbS > ZnS, which looks debatable at present [10,[35][36][37][38][39]. Evans and Raftery [42] have studied the initial surface oxidation of natural PbS crystals using angle-resolved photoelectron spectroscopy and pointed out that elemental sulfur tends to be lost in vacuum but some S remained trapped under an oxidized layer.…”

“…Surface composition and structure are critically important for minerals behavior in a variety of natural processes and for mineral processing, particularly the recovery and separation of base metal sulfides by flotation, as well as their leaching in hydrometallurgy [1][2][3][4]. The products of surface oxidation and reactions of sulfides with flotation reagents and precipitates from the mineral slurries, which render hydrophobic/hydrophilic properties and phenomena that determine the ability of mineral particles to break the wetting water film and attach to a millimeter air bubble in the flotation process, are not well understood so far [5][6][7][8][9][10]. The characters of the reacting and modified surfaces as well as the near-surface regions affect the rates of dissolution and leaching, which are often very slow and considered as "passivation" of still debatable nature (for example, [11][12][13][14]).…”

“…Application of surface-sensitive techniques to this field, which is necessary in order to gain insights into the surface composition and chemistry, however, faces with a number of difficulties. The X-ray photoelectron spectroscopy (XPS, also known as electron spectroscopy for chemical analysis ESCA) is a powerful method for surface studies in materials science, catalysis, and other areas [15,16], and is widely utilized for characterization of mineral surfaces and interfaces [10,[17][18][19][20][21][22][23][24].…”

“…The intensities and chemical shifts of the photoelectrons provide information about the surface composition and the chemical state of the elements. The probing depth can be estimated by the electron inelastic mean free path (IMFP) in solid that depends on the kinetic energy of the electron (and on the solid constituents) and is usually in the range of 1-5 nm for laboratory instruments with Mg Kα (1253.6 eV) or Al Kα (1486.6 eV) irradiation of the X-ray tube anodes, but can be substantially varied with the excitation photon energy, particularly, using synchrotron radiation facilities (SR-XPS) [10,15,16,24]. XPS measurements traditionally have required ultra-high vacuum (UHV) because: (i) the detector needs to operate under this pressure regime, (ii) the surface contamination should be minimized, and (iii) potential inelastic collisions of the ejected electrons in their way to the detector should be also minimized since if the photoelectrons lose energy, then information is also lost.…”

“…The metal sulfides are the main source of base metals in nature, and are also of vivid interest for materials science. In comparison with the previous reviews of mineral surface studies [5][6][7][8][9][10][17][18][19][20][21][22][23][24], this contribution is focused on methodological aspects, modern trends and developments of XPS along with the limitations concerning elusive (volatile, unstable or buried) entities on sulfide minerals, and the techniques aimed at (quasi) in situ approach. Only some important papers from the XPS application history are overviewed here, and only selected results are discussed in detail as examples.…”