GENERATION OF ACIDS FROM MINE WASTE - OXIDATIVE LEACHING OF PYRRHOTITE IN DILUTE H2SO4 SOLUTIONS AT PH 3.0

Pyrrhotite (Fe7S8) grains 3 x 3 x 6 mm were reacted in solutions of H2 SO4 (pH 3.0) for eight hours and analyzed using secondary electron mic roscopy (SEM), Auger electron spectroscopy (AES), and X-ray photoelect ron spectroscopy (XPS). SEM images of reacted surfaces display an arra y of reaction textures, which are interpreted to represent a five-stag e (T1-T5) paragenetic alteration sequence. Leached pyrrhotite surfaces are initially featureless (T1 texture). Surfaces leached more extensi vely develop a mottled felty texture (T2). Subsequent drying of reacte d surfaces causes dehydration, producing cracked, tiled surfaces (T3 t extures). Prolonged drying intensifies the effects of desiccation, pro ducing rubbly (T4) textures. The rubble is readily spalled, exposing s mooth underlayers (T5 textures). AES and XPS data collected from T1 th rough 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 s palling) detect only Fe and S, with S significantly enriched over Fe. XPS and modelled AES data show T5 textured regions are mainly ferric i ron bonded to disulphide and/or polysulphide species. The accumulation of S in the underlayer is accomplished by preferential migration of F e to the overlying oxyhydroxide layer to the pyrrhotite surface, thus, promoting spallation. Spalling of Fe(III)-oxyhydroxides is promoted i n waste rock dumps and tailings situated above the water table by peri odic 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 t here are dramatic seasonal overturns of lake or pond water. Exposure b y spalling of S-rich sublayers to aqueous solutions is an effective me ans for producing sulphuric acid-rich mine waste runoff, and of produc ing periodic flushes of sulphuric acid-rich drainage waters.