Biooxidation and Cyanidation for Gold and Silver Recovery from Acid Mine Drainage Generating Tailings (Ticapampa, Peru)

The acid mine drainage (AMD) generating sulfidic tailings have a total mass of 1,639,130 t containing 1.65 g/t Au, 34.5 g/t Ag, 7.74 % Fe, 5.91 % S, 3.2 % As, 0.75 % Zn and 0.05 % Cu. The precious metals Au and Ag are enriched in the fine fractions. Approximately 35 % of the material is below 25 /m in size and 53 % below 63 /m. Electron microprobe analysis of a sulfide concentrate of the tailings, produced by gravity separation, proved the occurrence of pyrite and arsenopyrite with appreciable sphalerite and galena. Refractory gold (up to 316 g/t) is hosted in Asrich zones of some arsenopyrites. Approximately 200 g of the sulfide concentrate of the tailings was biooxidized in laboratory shake flasks using an adapted mixed culture of Acidithiobacillus ferrooxidans (Ram 6F), Acidithiobacillus thiooxidans (Ram 8T) and Leptospirillum ferrooxidans (R3). During biooxidation, arsenopyrite was preferentially dissolved and the secondary mineral tooeleite (Fe8(AsO4)6(OH)5·H2O) precipitated. The following cyanidation of the biooxidized sulfide concentrate showed a recovery of 97 % and 50 % for Au and Ag, respectively. The values were 56 % and 18 % for the untreated concentrate. The recovery of Au and Ag from the tailings significantly reduces the costs for the tailings remediation to mitigate AMD release.

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

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.