Reduction of acid generation in mine tailings through the use of moisture-retaining cover layers as oxygen barriers

TLDR

Acid production in sulphidic tailings degrades water quality, driven mainly by oxygen availability and its diffusion through the upper zone of the tailings. The study proposes that applying a fine‑grained, nonreactive cover layer to tailings surfaces can reduce acid generation. The approach relies on moisture retention by capillary forces in a fine‑grained cover, enhanced by textured layering, and uses simplified Fickian diffusion calculations to preliminarily evaluate cover effectiveness before field testing. The cover layer can theoretically reduce oxygen diffusion and acid generation rates by up to four orders of magnitude, potentially lowering tailings seepage treatment costs. Keywords: pyrite oxidation, tailings, remediation, covers, acid generation, oxygen diffusion.

Abstract

Acid production in sulphidic tailings can cause severe degradation of water quality in both subsurface and surface systems. The availability of gaseous oxygen and the rate of diffusion of oxygen through the open pore spaces in the upper zone of the tailings are the critical factors controlling the rate of acid generation. Acid generation can be reduced by applying a fine-grained, nonreactive cover layer to the tailings surface. The key process involves moisture retention by capillary forces so that near-saturated conditions can be maintained even when the cover layer occurs at several metres above the water table. Textured layering of fine over coarse materials improves moisture retention in the fine layer when infiltration exceeds evapotranspiration. The application of such a cover layer can theoretically reduce oxygen diffusion coefficients and rates of acid generation by up to four orders of magnitude. This can represent a substantial difference in the potential treatment costs of tailings seepage. Simplified calculations based on Fick's first law can be applied to preliminary laboratory measurements of diffusion characteristics of potential cover materials to evaluate their effectiveness in decreasing acidification. These concepts and methods provide an initial evaluation before field-scale testing of cover performance. Key words: pyrite oxidation, tailings, remediation, covers, acid generation, oxygen diffusion.