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Stabilization of Lead in Acidic Mine Filtercake by Addition of Alkaline Tailings

Geomega, 2995 Baseline Road, Suite 202, Boulder, CO 80303;

Timothy E. Link

Dep. of Geology, Wilkinson Hall, Oregon State Univ., Corvallis, OR 97331;

Kim Baugh

NOAA/NGDC, E/GC2, 325 Broadway, Boulder, CO 80303;

Richard Witham

Molycorp, P.O. Box 124, Mountain Pass, CA 92366.

^* Corresponding author.

ABSTRACT

The potential for disposal of acidic Pb-bearing process waste in an alkaline tailings pond was assessed to ascertain if Pb would become stabilized as opposed to diluted during mixing. This study demonstrated that codisposal of the filtercake residue, consisting of ferrihydroxide, lead sulfate and lead oxide phases (pH = 6.2, Pb = 43 000 mg/kg), with tailings containing abundant calcium and strontium carbonate (pH = 9.8, Pb = 2400 mg/kg) would result in Pb stabilization. The leachable Pb was greater (p < 0.05%) for a sand-filtercake mixture compared with the tailings-filtercake at analogous ratios, demonstrating reactivity between filter cake and tailings. Stabilization occurs because abundant remnant calcite in the tailings neutralizes the acidic filtercake, facilitating sorption of Pb to ferrihydroxide at the pH of the filtercake/tailings mix and a >98.5% decrease in Pb solubility. Sorption (Kd_Pb = 28 600 at pH 7.7) and possible precipitation of Ce-Pb and Ln-Pb oxides limit Pb solubility. These data suggest that disposal of acidic filtercake in an alkaline tailings pond would reduce Pb solubility to levels below relevant environmental standards.