Evaluation of the rate of dissolution of secondary sulfate minerals for effective acid and metalliferous drainage mitigation

The presence of latent acidity in the form of the jarosite-alunite mineral group complicates assessment of the rates of acid and metalliferous drainage (AMD) generation from potentially acid-forming waste rocks and tailings. To enable better 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 range of 10 ⁻¹⁴ to 10 ⁻¹³ mol m ⁻² s ⁻¹ and were found to be slowest at pH 3.4 and 5.5, respectively. At pH above these respective minima, alunite and natrojarosite dissolution results in acid release. The surface area-normalised dissolution rates of both minerals were 2–3 orders of magnitude slower 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 sulfate minerals does not impact significantly on pyrite oxidation, their contributions to AMD can be taken into account separately enabling accurate calculation of the neutralisation rates needed to effectively manage AMD.