Controls on 226Ra during raffinate neutralization at the Key Lake uranium mill, Saskatchewan, Canada

This study was conducted to define the geochemical controls on Ra-226 during raffinate (pH = 1.2) neutralization to pH 10 at the Key Lake U mill in northern Saskatchewan, Canada. High activities (120-150 Bq/L) of aqueous phase Ra-226 are present in raffinate produced during milling of U ore. The solubility control of Ra-226 in the SO4-rich, hydrometallurgical raffinate solutions often involves the addition of BaCl2 to form a radium-barite co-precipitate (Ba(Ra)SO4). As such, neutralization experiments were conducted with samples of mill raffinate using Ca(OH)(2) or NaOH with and without the addition of BaCl2. Radium-226 activity decreased from 150 to <4 Bq/L for all combinations of neutralizing agents with Ca(OH)(2) + BaCl2 being the most effective combination (final activity similar to 1.0 Bq/L; similar to 99.3% removal). In the absence of BaCl2, Ca(OH)(2) more efficiently removed Ra-226 than NaOH between pH 4 and 8, due to the co-precipitation of Ra-226 with gypsum. Overall, neutralization with the addition of BaCl2 reduced Ra-226 activities at lower pH values (by pH 4.5), due to co-precipitation of Ra-226 with BaSO4. At varying concentrations of BaCl2, aqueous phase activities of Ra-226 converged, but did not attain steady-state values during neutralization and would continue to decrease with time. Sequential extractions indicated that Ra-226 in precipitates formed during neutralization of the mill raffinate is dominated by amorphous and crystalline Fe hydroxide phases, consistent with raffinate neutralization experiments that showed that adsorption onto ferrihydrite can remove most Ra-226 in the raffinate. Data generated in this study are being used to define the long-term geochemical controls on Ra-226 in U mill processes and tailings. (C) 2011 Elsevier Ltd. All rights reserved.