Uranium mill tailings neutralization: contaminant complexation and tailings leaching studies

Abstract

Laboratory experiments were performed to compare the effectiveness of limestone (CaC0 3) and hydrated lime [Ca(OH)2] for improving waste water quality through the neutralization of acidic uranium mill tailings liquor. The experiments were designed to also assess the effects of three proposed mechanisms-carbonate complexation, elevated pH, and colloidal particle adsorption-on the solubility of toxic contaminants found in a typical uranium mill waste solution. Of special interest were the effects each of these possible mechanisms had on the solution concentrations of trace metals such as Cd, Co, Mo, Zn, and U after neutralization. Results indicated that the neutralization of acidic tailings to a pH of 7.3 using hydrated lime provided the highest overall waste water quality. Both the presence of a carbonate source or elevating solution pH beyond pH= 7.3 resulted in a lowering of previously achieved water quality, while adsorption of contaminants onto colloidal particles was not found to affect the solution concentration of any constituent investigated. At solution pHs greater than pH = 6.3, carbonate complexation affects the solid phase control for uranium and perhaps zinc concentrations, while molybdenum concentrations were found to be a function of elevated solution pH. Arsenic, Cd, Cr, Pb, and V were effectively removed from solution at all solution pHs exceeding pH = 6.5, regardless of solution carbonate content. Cobalt removal is best at pH values above 7 .3, and copper removal may be enhanced in carbonate-bearing solutions. Selenium removal is only 50% for either reagent at all pH values studied. Uranium solution concentrations in the presence of excess carbonate reagents were significantly higher than in solutions created by hydrated lime-only neutralization, but all solutions were below U.S. Nuclear Regulatory Commission radiation protection limits. Molybdenum was effectively removed from solution at pH = 6.3 but was present at pH = 9.0 in concentrations corresponding to those found in the unneutralized tailings liquor. Aqueous speciation calculations with the computer code MINTEQ were used to clarify these observed results. Acidic untreated solid tailings from two mill sites and tailings neutralized with lime were leached with a simulated ground water for several pore displacement volumes. Analyses performed on the column effluents indicate that prior neutralization results in a significant reduction in the concentration of all pH-dependent constituents. In comparing the effluent data from the neutralized and untreated tailings, ions such as Al, As, Cd, Co, Cr, Cu, Fe, Si, Sr, V, and Zn displayed reduction of several orders of magnitude and, in most cases, were below detectable limits in the effluents generated from the neutralized tailings. It appears that more molybdenum leaches from the neutralized tailings column for the Lucky Me sample than for the untreated tailings column. Ions such as Ca, Mg, Na, Cl, and so 4 comprised the majority of the total dissolved solids content in the lime-treated tailings e...

Extraction of uranium and molybdenum from aqueous solutions: a survey of industrial materials for use in chemical barriers for uranium mill tailings remediation

“…The effectiveness of hydrated lime for extracting uranium from solution is reported in studies with hydratedlime-treated uranium mill effluents (10,26) and hydrated-lime treatment of uranium mill tailings (9). Fly ash is a waste product from coal combustion and has a significant lime (CaO) component.…”

“…However, the addition of peat decreased dissolved uranium, molybdenum, and other contaminants. Other studies focused on the extraction of contaminants from uranium mill effluent using lime, limestone (10), or biosorbents (11). Opitz et al (10) determined that uranium concentrations were reduced from 40.0 to 0.10 mg/L by adding lime to reach a final pH of 7.3. In the Barnes et al (11) study, calcium alginate beads extracted 78% of the uranium and 74% of the molybdenum from uranium-contaminated mill tailings pore fluid.…”