Column leach study II: Heavy metal dissolution characteristics from selected lead-zinc mine tailings

Metal dissolution from tailings from six lead-zinc mine and milling sites in the western United States was examined by researchers from the Bureau of Mines using a column leaching procedure involving a formulated “western rain” leachant. This 2-year laboratory study included the effects of column waste depth, dry cycles with and without oxygen, and pore water evaporation. The results indicated that leachate metal concentrations gradually decreased with each leaching, but that the degree of enhanced metal mobilization from unsaturated tailings varied widely from waste to waste. Further studies on selected samples from this group of tailings demonstrated that the driving force behind enhanced metal dissolution is the oxidation of sulphide minerals by atmospheric oxygen during the evaporation of pore water from the columns of tailings. Leachates from water-saturated columns of these tailings gave pH values from 6.5 to 8.5; acid production and dissolved metal concentrations were enhanced by alternating wet and dry cycles. All studies to date indicate that maintaining tailings at or near saturation and/or excluding atmospheric oxygen produce leachates of nearly constant to slowly decreasing metal concentrations with each subsequent leaching.