Dissolution of secondary sulfate minerals: Implication for effective acid and metalliferous drainage mitigation

Secondary hydroxysulfate minerals such as jarosite and alunite often precipitate in ferric- and aluminium-rich, acidic and oxic acid and metalliferous drainage (AMD) environments formed predominantly by pyrite oxidation. However, when the rate of pyrite oxidation passes its peak and the pH starts to increase, these secondary minerals start to re-dissolve, releasing acidity and metals. The presence of latent acidity in the forms of jarosite-alunite mineral group complicates assessment of the rates of AMD generation from potentially acid-forming waste rocks and tailings. To better enable prediction of the contributions from these secondary sulfate minerals, the relationships between pH and the rate of dissolution for natrojarosite and alunite, for comparison with pyrite, have been determined. In single mineral unstirred systems, the dissolution rates of natrojarosite and alunite in the pH range 2–7 were in the order of 10–14 to 10–13 mol m–2 s–1, slowest at pH 3.4 and 5.5, respectively. At pH above these respective minima, natrojarosite and alunite dissolution results in acid release. The surface area-normalised dissolution rates of both minerals were 2–3 orders of magnitude smaller than that of pyrite under the same conditions, and were 2–4 orders of magnitude slower than the rates of stirred dissolution of synthetic jarosite and alunite reported in the literature. In mixed mineral mini-column dissolution tests, pyrite oxidation proceeded at almost the same rate in the longer term (>50 days), regardless of the presence or absence of natrojarosite or alunite. As the presence of these secondary minerals does not impact significantly on pyrite oxidation rates, their contributions to AMD can be taken into account separately enabling accurate calculation of the neutralisation rates needed to effectively manage AMD.