Investigations on Iron Precipitates accumulating from underground workings and Mud Lake at South Bay

Abstract

In pyritic mining wastes, the release and subsequent oxidation of Fe(II) from mining sites may produce non-negligible amounts of high sulfate and extremely low pH waters. The resulting acid mine drainage (AMD) may be divided into three types: • iron sulfide oxidation, • dissolution of soluble iron sulfate minerals, and • the dissolution of less soluble sulfate minerals of the alunite-jarosite series. The oxidation of iron sulfide minerals such as pyrite (FeS2) and pyrrhotite is responsible for the majority of acid production from mining wastes. In addition to metals, acid, sulfate is also released to ground and surface water. If sulfate is present in higher concentrations a variety of iron minerals may form, i.e.: jarosite (XFe3(SO4)2(OH)6, (X being a monovalent cation) • Schwertmannite (Fe8O8(OH)6SO4). These secondary minerals are not stable and the release of sulfate by dissolution of these minerals may result in the formation of Fe(III) hydroxides with subsequent acid (H+ ) release. Secondary mineral formation, with hydrogen-ion generation leading to pH values as low as pH 1, was reported in stored South Bay tailings pore water. Investigations on the secondary mineral phases, along with microbial investigations, were carried out and Schwertmannite minerals were detected as reported in Kalin, 2003, "The acid generation potential of iron precipitates and their sludge in Decommissioning with Ecological Engineering". This is a matter of some concern, given the high number of hydrogen ions, generated by the formation of Schwertmannite, The natural precipitation of stable, iron-hydroxide sludge, which is not a source of acidity, is an important component of the Ecological Engineering decommissioning approach. Thus Schwertmannite formation is undesirable, and the conditions under which it occurs must well understood. Samples of sludge were collected from relevant locations at the South Bay site for an investigation into the formation of this secondary mineral.