OPTIMIZING THE TERMINAL PH OF URANIUM MILL TAILINGS TO CONTROL AS, SE, NI, AND MO: A PILOT SCALE STUDY

The Athabasca Basin in northern Saskatchewan, Canada has the largest deposits of high-grade uranium (U) ore in the world. The acidic (pH <1 with H₂SO₄) and highly oxidizing conditions used in the leaching of U results in the leaching of elevated concentrations of elements of concern (EOCs) such as As, Mo, Ni and Se together with Al, Fe and Mg from the ore. At the Cameco Key Lake mill, the acidic solution left after U extraction (raffinate), is step-wise neutralized with Ca(OH)₂ to a terminal pH of 10.5-11.0 to decrease the dissolved concentrations of EOCs by sequestering them in secondary mineral phases before being deposited in the Deilmann tailings management facility. However, despite its importance, the effect of lowering the terminal pH for raffinate neutralization to 7.5-8.5 on the long-term sequestration of EOCs at the Key Lake mill is not known. This study investigated the impact of changing the terminal pH in a pilot-scale raffinate neutralization model of the Key Lake mill on solids and pore water chemistry, including EOCs. In the model, the pH of the synthetic raffinate was increased from <1.0 to 8.5 in three steps (step 1: <1.0 to 4.0, step 2: 4.0 to 6.5, step 3: 6.5 to 8.5) by mixing with slaked lime. During neutralization 99.9% As, 99.9% Fe, 99.6% Mo and 99.3% Se was removed from solution at pH 4.0. Although, 87.9% of Al was also removed at this pH, removal of 99.9% was completed at pH 6.5. Nickel was gradually removed from the raffinate over the pH ranges tested and was completed (99.8%) at pH 8.5. Only 48.0% of Mg was removed at pH ≤8.5, while in a companion study, 70% of Mg was removed by pH 10.1. Preliminary mineralogical analyses suggest the precipitation of amorphous ferrihydrite and Al(OH)₃ at pH 4.0 and calcite and hydrotalcite like compounds at the terminal pH 8.5. The results of geochemical modelling using PHREEQC support the experimental results. The results of this study indicate that, except for Mg, lowering the terminal pH from 10.5 to 8.5 should not affect the targeted percentage removal of EOCs as well as elements such as Fe and Al. However the lower Mg removal at the terminal pH of 8.5 may decrease the formation of Mg-Al hydrotalcite, which is known to sequester EOCs.