Generation of acids from mine waste: Oxidative leaching of pyrrhotite in dilute H 2SO 4 solutions at pH 3.0

Abstract

Pyrrhotite (Fe 7S 8) grains 3 × 3 × 6 mm were reacted in solutions of H 2SO 4 (pH 3.0) for eight hours and analyzed using secondary electron microscopy (SEM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). SEM images of reacted surfaces display an array of reaction textures, which are interpreted to represent a five-stage (T1–T5) paragenetic alteration sequence. Leached pyrrhotite surfaces are initially featureless (T1 texture). Surfaces leached more extensively develop a mottled felty texture (T2). Subsequent drying of reacted surfaces causes dehydration, producing cracked, tiled surfaces (T3 textures). Prolonged drying intensifies the effects of desiccation, producing rubbly (T4) textures. The rubble is readily spalled, exposing smooth underlayers (T5 textures). AES and XPS data collected from T1 through T4 textured surfaces indicate primarily Fe-oxyhydroxide reaction products. AES depth profiles show that S varies antipathetically with oxygen. AES analysis of T5 textured surfaces (underlayer exposed by spalling) detect only Fe and S, with S significantly enriched over Fe. XPS and modelled AES data show T5 textured regions are mainly ferric iron bonded to disulphide and/or polysulphide species. The accumulation of S in the underlayer is accomplished by preferential migration of Fe to the overlying oxyhydroxide layer to the pyrrhotite surface, thus, promoting spallation. Spalling of Fe(III)-oxyhydroxides is promoted in waste rock dumps and tailings situated above the water table by periodic wetting, drying and desiccation of the oxyhydroxide layer. These circumstances may, in turn, lead to high concentrations of suspended Fe-oxyhydroxide in tailings ponds during flooding and in ponds where there are dramatic seasonal overturns of lake or pond water. Exposure by spalling of S-rich sublayers to aqueous solutions is an effective means for producing sulphuric acid-rich mine waste runoff and of producing periodic flushes of sulphuric acid-rich drainage waters.