Pyrrhotite oxidation in weakly acidic solutions

Complete text of publication follows: The kinetics and mechanism of pyrrhotite (for simplicity noted FeS) oxidation by dissolved oxygen was investigated in weakly acidic solutions as a function of pH (2.75 to 3.45) and temperature (25 to 45 C). The changes in Eh, pH and total dissolved iron were monitored to quantify the reaction progress. Additionally, Fourier transform infrared spectroscopy (FTIR) was used to characterize the oxidation products of FeS. The experimental results demonstrate the importance of temperature and initial pH for the FeS oxidative dissolution. The amounts of dissolved iron (nFe) and removed H+ (nH) increase with temperature and initial [H+]. The activation energy of FeS oxidative dissolution is 41.6 ± 10.7 kJ mol-1 at initial pH=3.00 suggesting that the kinetic regime is controlled by a mix of diffusion and surface reaction (De Guidici et al., 2005). It was found that the reaction order of oxidative dissolution of FeS is 1.0 ± 0.02 (25 C) with respect to initial [H+]. FTIR spectroscopy indicated the presence of several sulfur species (S0, Sn2-, S2O32-, SO32- and SO42-) and ferric hydroxides or oxyhydroxide (Fe(OH)3 and goethite) on residual FeS surface. It is important to note that the experimental ratios of nH over nFe (nH:nFe) observed at 25 C decrease over a first period of time (0-4 h) of FeS oxidative dissolution from 7.97 down to 2.01. Afterwards, the nH:nFe ratio becomes lower than 2 and remains roughly constant (4-72 h). At higher temperatures (35 and 45 C) and pH 3.00, nH:nFe≤2 and is quasi-invariant over the reaction time. The experimental observations suggest a mechanism based on the protonation of FeS surface (Chirita and Descostes, 2006) followed by oxidation of FeS by dissolved oxygen to produce Fe2+, S0 and Sn2-. Fe2+ is unstable in oxidative conditions (Descostes et al., 2002) and transforms into Fe(OH)3(s) and goethite after approximately 30 h of reaction. References:1) De Guidici G., Rossi A., Fanfani L., Lattanzi P. (2005) Geochim. Cosmochim. Acta 69 2321-2331; 2) Chirita, P. and Descostes, M. (2006) J. Colloid Interface Sci. 294 376-384; 3) Descostes M., Beaucaire C., Mercier, F., Savoye, S., Sow, J. and Zuddas, P. (2002) Bull. Soc. Geol. France 173, 265-270