# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 2 Speciation of Surface Products
### Flotation Reagents  
Oxidation of sulfide minerals notably affects their hydrophobicity and floatability but flotation reagents play the main role in their recovery and separation [2; 5; 8; 9; 10; 18; 22; 91; 92; 93; 94]. Sulfhydryl collectors, particularly alkyl xanthates are the most common reagents [2; 5; 8; 10], so we restrict this overview to xanthates as a representative example. The main efforts of researchers have been concentrated on elucidating the chemical state of adsorbates [10; 18; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94]. Sulfhydryl collectors, particularly alkyl xanthates are the most common reagents [2; 5; 8; 10], so we restrict this overview to xanthates as a representative example. The main efforts of researchers have been concentrated on elucidating the chemical state of adsorbates [10; 18; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 72; 74; 76; 78; 79; 81; 83; 85; 86; 87; 88; 89; 90; 91; 92; 93; 95; 96; 97; 98; 99; 100; 101; 102; 103]. It has been established [92; 93; 94; 95; 96; 97; 98; 103] that alkyl xanthates of alkali metals can be attached to sulfide mineral surfaces as chemisorbed xanthate radicals, metal xanthates, or dialkyl dixanthogens, depending on the nature of the mineral, collector concentration, electrochemical potential and mineral pre-oxidation, and other conditions. Nevertheless, a large amount of uncertainty still remains because the analytical techniques have insufficient surface sensitivity and the lack of in situ measurements, particularly in technological media. In the case of XPS, it is necessary to discern weak signals of S atoms in xanthate-derived surface species having the S 2p3/2 binding energies of 162-162.5 eV for chemisorbed xanthate and metal xanthates and 163.5-164.2 eV for dixanthogens [92; 95; 98; 99; 100; 103] from the spectra of di- and polysulfides in underlying metal sulfide phase; the spectra of O and C are usually less informative due to surface oxygen and carbon contaminations. To overcome this obstacle, higher xanthate concentrations are often utilized, which may lead to unpractical results. The samples evacuated at room temperature, typically rinsed to remove solution components, could loss interfacial volatile and soluble species of interest. When sulfide minerals reacted with xanthate solutions were dried and cooled to LN temperature [98; 99; 100], or prepared as fast-frozen wet particulate paste [102; 103], XPS recognizes dialkyl dixanthogens, which otherwise evaporate in UHV. The cryo-XPS data (see Figure 2 for an example) show that dixanthogens are the major adsorbate at pyrite, dixanthogen and cuprous xanthate formed in the next stage on chalcopyrite and Cu-activated sphalerite and pyrite, while predominant chemisorbed xanthate is present on galena. In general, one can conclude that the quantities of dixanthogens were underestimated whereas those of metal xanthates overestimated previously. This may be explained in part by the fact that metal xanthates tend to form nanoparticles with lower visible surface [104; 105], whereas dixanthogen continuously (but not completely) covers the mineral surface [106]. XPS has been extensively used for characterization of mineral surfaces modified with flotation activators, depressors and other reagents. Noteworthy, organic reagents, which are physically adsorbed on sulfides and volatile in the UHV, are not well suitable for XPS examination.