Anoxic dissolution of troilite in acidic media

The anoxic dissolution of troilite (FeS) in acidic medium has been investigated at 50 °C using batch dissolution experiments. Two different progress variables were followed during solid dissolution, i.e., the amounts of dissolved iron (nFenFe) and formed hydrogen sulfide (nH2SnH2S). The experimental studies performed at hydrogen ion concentrations ([H+]) ranging from 0.04 to 0.2 mol L−1 showed that anoxic dissolution of troilite is dependent on [H+]. The cumulative release of both Fe and H2S could be described by a diffusion-like rate law, with rate constants for Fe (kFep) always greater than for H2S (kH2Sp). The surplus of dissolved iron over formed hydrogen sulfide was quantified by the nFe:nH2SnFe:nH2S ratio, and ranged from 1.21 to 1.46, higher than the specific nFe:nH2SnFe:nH2S ratio of troilite bulk, i.e., 1. Rate constants are linearly related to the pH with a slope of 0.66±0.230.66±0.23 (nFenFe) or 0.63±0.130.63±0.13 (nH2SnH2S). The obtained results suggest that troilite anoxic dissolution is a process controlled by the diffusion of the reaction products across an obstructive layer, sulfur-rich layer (SRL), having a thickness that increases during reaction progress. The accumulation of H2S between the surface and the SRL, eventually leads to the mechanical destruction of this outer layer, a process that results an increased flux of reaction products.

The integral form of rate law that can describe within experimental uncertainties the troilite anoxic dissolution is: nFe=0.85(±0.50)[H+]0.66(±0.23)t0.5for dissolved iron and nH2S=0.58(±0.21)[H+]0.63(±0.13)t0.5for released H2S.