Pyrite surface

About three years after Wachtershauser s first publication appeared, an article by Christian de Duve and Stanley Miller was published in the Proceedings of the National Academy of Sciences under the title Two-Dimensional Life the title alluded to the theory of reactions at positively charged pyrite surfaces (de Duve and Miller, 1991). Their criticisms of the chemoautotrophic theory were directed particularly towards certain kinetic and thermodynamic aspects, but also to theoretical statements for which no experimental support was available. 

Bacteria that live in the dark must derive the free energy to move electrons around in chemistry and a series of reactions based on the iron pyrite surfaces have been shown to be exoergic (Figure 9.13). The cationic pyrite surface serves as the catalyst for the binding of negatively charged species, perhaps tagged with... 

Figure 9.13 Energy-yielding reactions on pyrite surfaces. (Adapted from Deaner 1997, with permission from the American Society for Microbiology)...

Figure 2.17 shows that although the metastable elemental sulfur maybe present at pyrite surface, pyrite does not exhibit self-induced collectorless floatability except in very strong acidic media and a narrow oxidized Eh range. Such behavior may be similar to that of arsenopyrite in alkaline solutions due to the formation of hydroxides, thiosulphate. 

Figure 3.11 Effects of HS" concentration on the adsorption, the amount of neutral sulphur extracted fiom pyrite surface and collectorless flotation of pyrite at pH = 11.0 (Sun et al., 1993a)...

McCarron et al. (1990) used the X-ray photoelectron spectroscopy to analyze chalcopyrite and pyrite surface after being conditioned in sodium sulphide solutions. They found that multilayer quantities of elemental sulphur were produced at the surface of both minerals in 3 x 10 and 3 x 10" mol/L sulphide solutions although for a given sulphide concentration, the surface coverage of elemental sulphur for p)uite was greater than that for chalcopyrite under open circuit conditions. Eliseev et al. (1982) concluded that elemental sulphur was responsible for the hydrophobicity of pyrite and chalcopyrite treated with sodium sulphide. Luttrell and Yoon (1984a, b) observed a shoulder due to elemental sulfixr near 164 eV in the S (2p) spectra from relatively pure chalcopyrite floated after being conditioned at different pulp potential established by different hydrosulphide concentration. 

Figure 5.4 shows the anodic oxidation of pyrite at different pH modified by CaO and NaOH. Obviously, the oxidation of pyrite is more rapid in CaO solution than in NaOH solution at the same pH. The calcium species in oxidized pyrite surface in the presence of CaO has been identified using XPS as shown in Fig. 5.5. The pyrite surface is oxidized to form Ca(OH)2, CaS04 and Fe(OH)3 in the presence of Ca lime.

The soluble complexes formed by activators will desorb cation from the lime depressed pyrite surface, which will expose a fresh pyrite surface and activate pyrite flotation. Therefore, the moderately strong acids such as oxalic acid and phosphoric acid exhibit a strong activation action on lime-depressed pyrite because of their ability to decrease pulp pH and to form soluble complexes with hydrated surface cations. 

Figure 6.27 shows the XPS spectra of the pyrite surface under different conditions. The characteristic peaks Ca(2p),Ca(2s) and Fe disappear by the addition of oxalic acid when compared to spectra in the absence of activator. In the XPS extended spectra for oxygen and carbon (presenting since the system is open to atmosphere) at the pyrite surface, no change is found after the addition of oxalic acid, indicating that no adsorption of oxalate occurs at the pyrite surface. The results in Fig. 6.27 also indicate that the soluble complexes formed by activators with cation hydroxides of Ca(OH)2 and Fe(OH)3 desorb from the pyrite surface. 

The formation of hydroxyl precipitate prevents from the transfer of electron especially between oxygen and the mineral surface. As a result of all these processes, EIS represents passivation characteristic. And the corrosive potential moves towards negatively, the surface resistance increases, and the corrosive current decreases. The formation of surface hydroxyl iron precipitates makes the pyrite surface very hydrophilic. 

The formation of hydroxyl iron and calcixxm sulphate precipitates makes pyrite surface very hydrophilic and inhibits other electrochemistry reaction on pyrite like collector giving rise to the depression of pyrite. The depression effect of lime on pyrite may be stronger than that of NaOH. 

Figure 7.14 illustrates that in the initial stage of polarization of the pyrite electrode in xanthate solution at about 120 mV, the radius of high value capacitive reactance loop increases with the increase of the polarization potential and reaches the maximum at 320 mV, indicating that the oxidation of xanthate increases gradually and collector film on pyrite surface becomes thicker. It increases the conduction resistance and the growth of collector film is the controlled step resulting in pyrite surface hydrophobic. When the polarization potential increases from 320 mV to 400 mV, the capacitive reactance loop radius decreases, indicating the decrease of transferring conduction resistance as can be seen in Fig. 7.15. It belongs to the step of film dissolution. Capacitive reactance loop radius decreases obviously when the potential is larger than 400 mV, at where the collector film falls off and the anodic dissolution of pyrite occurs. The controlled step is the anodic dissolution of pyrite and the surface becomes...

Figure 8.22 Pyrite surface in coarse particle media...

A lot of work (Opahle et al, 2000 Andrew et al, 2002a,b Muscat et al, 2002 Edelbro et al, 2003) has been performed on bulk and surface properties of FeS2 using various kinds of density functional theory. These works have shown that such methods are capable of producing calculated bulk properties such as lattice constants which agree well with experiment, and has provided a reference for the study of pyrite surfaces. 

Figure 9.21 Illustration of electron transfer between pyrite surface and collector showing the mechanism of collector flotation of pyrite...

Under adequate reducing environment dixanthogen will not be formed on the pyrite surface but lead xanthate is still formed on the galena surface, which underlies the basis of potential controlled flotation separation of galena from pjrrite by xanthate flotation. 

Gardner, J. R. and Woods, R., 1971. An electrochemical investigation of contact angle and floatation in the presence of alkyxanthates, II. galena and pyrite surfaces. Aust. J. Chem.,... 

McCarron, J. J., Walker, G. W., Buckley, A. N., 1990. An X-ray photoeletron spectroscopic investigation of chalcopyrite and pyrite surfaces after conditioning in sodium sulphide solutions. Inter. J. Miner. Process, 30 1 - 76... 

Natarajan, K. A., Riemer, S. C., Iwasaki, I., 1984. Influence of pyrrhotite on the corrosive wear of grinding balls in magnetite ore grinding. Inter. J. Miner. Process, 13(1) 73-81 Nesbitt, H. W., Bancroft, G. M., Pratt, A. R., Scaini, M. J., 1998. Sulfur and iron surface states on fractured pyrite surfaces. American Mineralogist, 83 1067 - 1076 Neeraj, K. M., 2000. Kinetic studies of sulphide mineral oxidition and xanthate adsorption. Doctor thesis of Virginia Polytechnic Institute and State University. A Bell Howell Company UMI dissertation Services... 

As shown in the following reaction, H3ASO30 may sorb onto pyrite surfaces under anaerobic conditions, which leads to the formation of FeAsS, Fe(III) hydroxides, and polysulfides, such as FeS4 (Bostick and Fendorf, 2003) ... 

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