EVALUATION OF ACID-GENERATING POTENTIAL OF COMPLEX MINE WASTES

Acid-base accounting (ABA) is a static test commonly used in the prediction of acid production from mine wastes. In standard ABA, acid-generating potential (AP) is calculated from total sulfur content; modified ABA uses sulfide sulfur. It is assumed in the calculation of AP that the measured sulfur is derived from pyrite, yet this calculation can be inaccurate in mine wastes containing other sulfur-bearing minerals such as jarosite, pyrrhotite, gypsum, and sulfate salts. This study used a variety of leaching techniques on a representative suite of metal mine-waste samples (total S: 0.7 – 27 Wt. %) and mineral separates to understand the behavior of these minerals in order to determine accurate AP values. Six non-sequential leaches included water for soluble salts; both dilute and concentrated HCl targeting gypsum, jarosite, and pyrrhotite; Na₂CO₃ for gypsum and jarosite; NaOH for all sulfates; and HNO₃ for pyrite. Qualitative and quantitative x-ray diffraction (XRD), sulfur measurements on solid residues, and sulfate measurements of leachates were used to assess the persistence of sulfur-bearing minerals following these leaches.

XRD showed removal of jarosite by boiled concentrated HCl, by Na₂CO₃, and by NaOH; of pyrrhotite by boiled concentrated HCl; and of gypsum by boiled concentrated HCl, by Na₂CO₃, and by HNO₃. Incomplete removal of gypsum was seen following dilute HCl and NaOH treatments. In samples with high pyrite content, pyrite persisted throughout all the leaches, though a decrease in sulfur concentration was seen in samples treated with HNO₃. It is possible, therefore, to distinguish the sulfides pyrite and pyrrhotite through a boiled concentrated HCl leach and to separate these sulfides from sulfates such as jarosite and gypsum through a Na₂CO₃ leach. Gypsum and jarosite may be differentiated through a HNO₃ leach. These results suggest that current widely accepted ABA protocols may yield inaccurate results depending upon the mineralogy of mine wastes. Interpretation of ABA results can be improved through mineralogical studies of mine wastes and through modifications to leaching protocols.