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Although oxygen is widely employed as the oxidant of choice in gold leaching by cyanide, its low aqueous solubility presents some drawbacks in practical application; hydrogen peroxide has therefore been considered as a possible alternative. The aim of this investigation was to study the catalytic decomposition of hydrogen peroxide, which generates an oxidizing intermediate species, and to understand its effect on cyanide destruction. Operating conditions that facilitated the effective decomposition of hydrogen peroxide were established by varying the pH and catalyst type and concentration. The oxidizing intermediate, detected using an indirect technique, was found to be the hydroxyl radical (OH). OH' is commonly generated in acidic solutions, but this work demonstrated that it is also produced at the alkaline pH values necessary for cyanide gold leaching. The effects of free and complexed iron and copper catalysts on the oxidation and consumption of hydrogen peroxide and cyanide were also investigated. It was shown that the cyano complexes of Fe(II) and Cu(I) are also effective as decomposition catalysts. Hydrogen peroxide concentrations above 0.01 M decreased the free cyanide concentration, which was attributed to the probable formation of the cyanate anion (CNO-). Although cyanide consumption increased due to its oxidation in the presence of OH', excessive cyanide consumption in the presence of copper was attributed primarily to its complexation by the unstable copper(I) cyanide species. Rate constants for the decompositions of H2O2 and cyanide by ferrocyanide and copper cyanide were calculated; the latter was identified as being a better catalyst.
As shown above, leaching also requires the presence of an oxidant, so a source of oxygen (typically air) is introduced into the leach pulp. Leach kinetics are, however, limited by the slow rate at which oxygen transfers from the gaseous to the liquid phase and the resulting low levels of dissolved oxygen. Even with the use of enhanced aeration techniques, such as compressed air or pure oxygen (Adams, 2016; Loroesch, 1990), the maximum aqueous solubility of dissolved oxygen is about 20 mg/L (Loroesch, 1990). To overcome this limitation of oxygen mass transfer, the application of a liquid oxidant, such as hydrogen peroxide (H2O2), has been proposed (Ball et al., 1989; Knorre et al., 1993, 1994).
This decomposition is well known to be catalysed by metallic species, specifically iron and copper, in what are termed Fenton and Fenton-like reactions, respectively (Fenton, 1894; Haber and Weiss, 1934; Watts and Teel, 2005). H.J.H. Fenton discovered in 1894 that several metals exhibit a strong catalytic effect that generates highly reactive hydroxyl radicals; these impart oxygen transfer properties that improve the use of hydrogen peroxide. Iron and copper commonly occur as impurities in gold ores (usually as sulphides), so leaching can benefit from the enhanced dissolved oxygen content resulting from the catalytic dissociation of H2O2into free oxygen and water. However, if the pH is too high, iron will precipitate as Fe(OH)3 and H2O2 will decompose to oxygen.
NMR spectra of the hydroxylation products compared well with that of the standard 2,3-DHBA isomer (National Institute of Advanced Industrial Science and Technology, 2013): peaks characteristic of the hydrogen atoms of this isomer were observed at chemical shifts of 6.5 to 8.5 ppm. IR spectra indicated the presence of carboxylic acid, alcohol, and carbonyl groups, which are the functional groups present in this isomer. Identification of the hydroxylation product of salicylic acid as 2,3-DHBA by NMR and IR spectroscopy, irrespective of whether the reaction was carried out in acidic or alkaline media, indirectly confirmed the presence of OH' and showed that H2O2 decomposed to this radical under both pH conditions. It was important to confirm the presence of this intermediate because it is highly reactive (standard reduction potential of 1.4 V) compared with the ferryl ion (0.9 V) (Petri et al., 2011) and could possibly react with contaminants in a gold leach pulp, leading to further losses in such application. This high reduction potential of OH' may also improve Au leaching kinetics.
The aim of this work was to study how the addition of H2O2 affects the cyanide concentration, the Eh, and the DO concentration in cyanide gold leaching. The effects of transition-metal cation catalysts, such as iron and copper (free and complexed), and pH on the stability of H2O2 were established. It was found that the cyano complexes of Fe(II) and Cu(I) effectively act as hydrogen peroxide decomposition catalysts in alkaline media. Increasing pH and catalyst concentration increased the rate of decomposition of H2O2. Copper (free and complexed) cyanide was found to be more effective in decomposing H2O2 than the corresponding iron(II) species.
It was established that a H2O2 concentration greater than 0.01 M caused loss of free cyanide. In the presence of copper, loss of free cyanide by complexation was attributed to the formation of stable higher copper(I) cyanide complexes. No loss of cyanide by complexation was observed in the presence of ferrocyanide, in accordance with the known relatively high stability of this complex. Additional losses of cyanide in the presence of hydrogen peroxide were attributed to the presence of the hydroxyl radical in Fenton-like reactions for both iron and copper.
Arslan, F., Ozdamar, D.Y., and Muduroglu, M. 2003. Cyanidation of Turkish gold-silver ore and the use of hydrogen peroxide. European Journal of Mineral Processing and Environmental Protection, vol. 3, no. 3. pp. 309-315. [ Links ]
Griffiths, Α., Knorre, H., Gos, S., and Higgins, R. 1987. The detoxification of gold-mill tailings with hydrogen peroxide. Journal of the South African Institute of Mining and Metallurgy, vol. 87, no. 9. pp. 279-283. [ Links ]
Haber, F. and Weiss, J. 1934. The catalytic decomposition of hydrogen peroxide by iron salts. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, vol. 147, no. 861. pp. 332-351. [ Links ]
Knorre, H., Griffiths, Α., Loroesch, J., and Fischer, J. 1994. Process for the leaching of gold and silver cyanide leaching solution and controlled addition of hydrogen peroxide. US patent 5275791. [ Links ]
Lin, S.S. and Gurol, M.D. 1998. Catalytic decomposition of hydrogen peroxide on iron oxide: kinetics, mechanism, and implications. Environmental Science and Technology, vol. 32, no. 10. pp. 1417-1423. [ Links ]
Petri, B.G., Watts, R.J., Teel, A.L., Huling, S.G., and Brown, R.A. 2011. Fundamentals of ISCO using hydrogen peroxide. In Situ Chemical Oxidation/or Groundwater Remediation. 1st edn. Siegrist, R.L., Crimi, M., and Simpkin, T.J. (eds.). Springer, New York. pp. 33-88. [ Links ]
Salem, I.A., El Maazawi, M., and Zaki, A.b. 2000. Kinetics and mechanisms of decomposition reaction of hydrogen peroxide in presence of metal complexes. International Journal of Chemical Kinetics, vol. 32, no. 11. pp. 643-666. [ Links ]
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Abstract The influence of an electrolyte flux injection over the detection side on pure iron membranes on electrochemical hydrogen permeation test was studied. A modified Devanathan's cell was used in the experiments. This cell allows the injection of an electrolyte flux over the hydrogen exit side during the permeation tests. In the hydrogen generation compartment a NaOH 0.1M + 2mgL-1 As2O3 solution was used applying a constant current of 2.85 mA. The detection side was maintained under potentiostatic control at potential values in the passivity range for iron. The solution used in the exit compartment was a borate buffer (pH=8.4). It was not observed a significant variation of the permeation current when an electrolyte flow was injected on iron samples, however, a slightly raise both of the steady state of permeation current density and the calculated permeation parameters was observed in the tests in which an electrolyte flux was applied.
The interaction of hydrogen with metals has been widely studied by electrochemical permeation tests due to their simplicity and flexibility [1-8]. These measurements, generally, involve the use of the electrochemical cell developed by Devanathan and Stachurski [9]. The technique is based on a hydrogen concentration gradient created in a metallic sample to achieve hydrogen diffusion through the metal thickness. The cell is a double compartment type, the sample is a flat sheet and it is located between the two compartments, on one side of the sample hydrogen is generated, adsorbed, absorbed and diffused through the metal membrane by cathodic polarization (generation compartment). The opposite side is maintained over potentiostatic control in order to oxide the emergent hydrogen (detection compartment).
The measured permeation current is expected to be proportional to the hydrogen flux. However, the relatively poor reproducibility of the technique may conducted to misinterpretation of the results obtained [3]. Additionally, electrochemical hydrogen permeation on iron and its alloys depends on the surface cleaning, sample preparation, defects and the surface oxidation state [10-13]. The exit side surface state has a higher influence on the hydrogen permeation [14]. Furthermore, the models used to calculate permeation parameters like apparent diffusion coefficient and quantity of emergent hydrogen are proposed assuming well-defined boundary conditions. It is assumed that the hydrogen concentration at the entrance side is imposed instantaneously by both potentiostatic and galvanostatic charging. Furthermore, in all models it is assumed that the hydrogen concentration at the exit side is zero [3, 15-19]. However, some works has been demonstrated that the boundary conditions assumed are not respected during the entire permeation time [14, 20-23]. It is not a linear relationship between hydrogen steady state flow and the inverse of the membrane thickness when the hydrogen charge was made by cathodic way [24-26]. The calculation of the permeation parameters is made considering the stability of the iron surface, any interaction between hydrogen and both surfaces (in and out) is not considered [14,22,23]. 59ce067264
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