# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 1 Introduction  
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 [17; 18; 19; 20; 21; 22; 23; 24]. Alternative techniques such as Auger electron spectroscopy (AES) [25; 26] and secondary ion mass-spectrometry (SIMS, TOF-SIMS) [27] are less informative regarding chemical bonding and also require an ultra-high vacuum environment (UHV). X-ray absorption spectroscopy (XANES/EXAFS), infrared spectroscopy (ATR-FTIR) and Raman spectroscopy are not generally surface-sensitive and unable to determine the total surface composition [18; 22; 23; 24; 28; 29; 30; 31; 32]. The photoelectron spectroscopy is based on the analysis of electrons emitted under X-ray irradiation from the core levels and the valence band in atoms of materials located in the near-surface region as a function of the binding energy (BE) [15]. The binding energies of the photoelectrons are derived from the known energy of the incident X-ray photons (hv) and the measured electron kinetic energy (KE) as \(\mathrm{BE}=\mathrm{hv}-\mathrm{KE}-\varphi\), where \(\varphi\) is the work function of the photoelectron analyzer. 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 K\(\alpha\) (1253.6 eV) or Al K\(\alpha\) (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. This is the main problem of XPS in studies of the solid/gas and solid/liquid interface [33]. Another problem is very complex multi-phase and multi-component composition of aqueous dispersions of minerals with practical solid-to-liquid weight ratios from 1:1 to 1:5 [1; 2; 3; 4]. In this review, we consider applications of XPS with regard to the most common metal sulfide minerals (galena PbS, sphalerite ZnS, chalcopyrite CuFeS\({}_{2}\), pyrrhotite Fe\({}_{1\text{-}}\)xS, pyrite FeS\({}_{2}\), and some others), and their ones.