Depositional and Dewatering Behaviour of Uranium Mill Tailings

Abstract

The Key Lake operation in Saskatchewan, Canada, is the largest uranium mill in the world. This mill process generates tailings that are deposited into an onsite storage area called the Deilmann Tailings Management Facility (DTMF). An effective tailings management scheme requires a clear understanding of slurry behaviour throughout the life-cycle, starting from production thorough the deposition to dewatering in the storage facility. The main objective of this research was to investigate the depositional and dewatering behaviour of uranium mill tailings (4%, 5%, and 6% mill tailings) from the Key Lake operation under laboratory and field conditions. All of the samples exhibited the same trend for yield strength development during the tests for rheological properties. A negligible strength (0.4 kPa) was found to have at 60% solids content (s) followed by a rapid increase thereafter. The settling and segregation tests were performed under different initial solids contents (si). The 4% mill tailings exhibited a lower rate and total amount of settlement than 5% and 6% mill tailings in the settling tests. The initial hydraulic conductivity (ki) decreased by two orders of magnitude (10-2 m/s to 10-4 m/s) with a decrease in initial void ratio (ei) from 16 to 4 (15% < si < 40%) and a decrease in final void ratio (ef) from 8 to 4 (30% < si < 45%) such that 4% mill tailings showed one order of magnitude lower values than the 5% and 6% mill tailings. The corresponding settling potential (SP) decreased ten times (50% to 5%) for 4% mill tailings and four times (60% to 15%) for 5% and 6% mill tailings. The effective stress (σ') increased from 80 Pa to 260 Pa in the settling tests. The average solids content after settling was 35% (20% < s < 42%) for 4% mill tailings, 40% (15% < s < 60%) for 5% mill tailings, and 39% (18% < s < 54%) for 6% mill tailings with a corresponding normalized solids content deviation of ±3%, ±8%, ±6%, respectively. The 4% tailings were less prone to segregation when compared with 5% and 6% tailings. Nevertheless, all materials were essentially non-segregating at 40% initial solids content. The large strain consolidation tests were conducted by using a customized and fabricated consolidation test system. During the tests, the total strains were 31% to 42% for all investigated mill tailings in an effective stress range of 0.3 kPa to 8 kPa. The change in void ratio was higher for 4% mill tailings (Δe = 2.5) than 5% and 6% mill tailings (Δe = 1.3 to 1.7). The lowest measurable effective stress was 0.3 kPa for all investigated mill tailings. The void ratios were found to be 3.8, 3.1, and 3.4 at σ' of 1 kPa and further reduced to 3.3, 2.8, and 3.1 at σ' of 8 kPa for 4%, 5%, and 6% mill tailings. The k values showed an initial scatter before attaining a steady value and were found to range from 10-7 m/s to 10-8 m/s. The test results provided the volume compressibility and hydraulic conductivity relationships for current (4%) and future (5% and 6%) mill tailings. The large strain consolidation behaviour in the DTMF was investigated by analyzing survey data from 1996 to 2008, laboratory testing of the current (4%) mill tailings, and history matching of the deposited tailings using numerical modeling. The numerical modeling results closely approximated the consolidated tailings elevations and effective profiles in the DTMF over the period of 1996 to 2008. The field effective stress values correlated quite well with the modeling results thereby validating the predictions. Overall, the results indicate that the effective stress increased from 0 kPa at the surface to the following values at the DTMF bottom: 200 kPa in 1999, 530 kPa in 2005, and 680 kPa in 2008.