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.
Description:
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in Environmental Systems Engineering, University of Regina. xii, 145 p.