# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 5 Conclusions and Outlook  
XPS became at present a routine technique for characterization of solid surfaces in mineral processing and related areas. XPS and surface-sensitive SR-XPS successfully specified products formed at mineral surfaces upon their interaction with flotation reagents, including flotation collectors. The method discovered the phenomenon of surface non-stoichiometry caused by the preferential release of metal cations from the sulfide lattices, while depth-resolved high-energy XPS (HAXPES) found that the metal-depleted near-surface is composed of several layers with polysulfides occurring only in the outmost one. It should be stressed that many surface species (elemental sulfur, disanthogens, polysulfides, etc.) are volatile in the spectrometer vacuum at room temperature so the samples should be cooled before or during their evacuation. The cryogenic XPS of fast-frozen particulate minerals is one of the most promising techniques for (quasi) in situ exploration not only of the unstable surface species, but also adjacent aqueous phase, double electrical layers and adsorbates. This technique is worthy of further development and wider application. The near-ambient pressure XPS, whose application rapidly  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Technique** & \multicolumn{2}{c}{**Possibilities and**} & \multirow{2}{*}{**Drawbacks**} & \multirow{2}{*}{**Mineral Samples**} \\ \cline{3-3} \cline{5-5}  & & **Findings** & & \\ \hline \multirow{4}{*}{Conventional XPS} & Analysis of surface & & & \\  & reaction products and & & decay of solid/water & numerous minerals, ores \\  & adsorbates, & interface & & \\  & non-stoichiometry & & & \\  & Detection of volatile & & & \\  & species & & & \\  & (S, dixanthogens, etc.) & & & \\  & Characterization of volatile & & & \\ Fast-freezing & species and solid/water & & & \\  & interfaces & & & \\  & & & & \\  & Tunable excitation energy & & facilities, potential sample & \\  & and surface sensitivity; & damage and loss of volatile & & \\  & high spectral sensitivity & species, decay of & & \\  & and resolution & & solid/water interface & NaES [131; 132], \\  & & & & \\  & & & & \\  & & & & \\ HAXPES & Non-destructive depth & Mainly buried layers and & & \\ (hv = 2000–1000 eV) & profiling & interfaces, need of SR & & \\  & & facility access (mostly) & & \\  & & & & \\  & Lateral resolution down to & & & \\  & & 100 nm & & \\  & & & & \\ PEEM & Lateral resolution down to & & & \\  & 10 nm & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular}
\end{table}
Table 1: Principal possibilities and shortcomings of X-ray photoelectron spectroscopy techniques, and examples of the application to sulfide minerals.  
progresses in catalysis and other fields, practically is not in use in mineral processing. Photoelectron microscopy (SPEM and PEEM) methods having high lateral resolution could provide interesting results but are not well suited for common measurements yet. HAXPES, a powerful method for the characterization of thick layers and buried interfaces, is underestimated in the mineral processing studies, especially as commercial laboratory instruments equipped with innovative hard energy X-ray sources are becoming available. The problem of surface studies on real ores is far from being solved; development of spatially resolved methods both in the depth and lateral dimensions together with appropriate sample preparation procedures is still pressing.  
The author declares no conflict of interest.