In 1995, a collaborative project using the Louvicourt site to demonstrate the
effectiveness of subaqueous tailings disposal in artificial containment structures
under shallow water covers was initiated under the Mine Environment Neutral
Drainage Program. The project was managed by Golder Associés as the lead
consultants with Aur Resources, Canada Centre of Mining and Energy Technoloy
(CANMET), INRS-EAU (University of Quebec), the University of British
Columbia, SENES Consultants and the Noranda Technology Centre as
collaborators. The role of CANMET in the project was to investigate, through
column studies, four scenarios of subaqueous tailings disposal that might be
applicable to the Louvicourt site upon mine closure.
Tailings samples collected over a period of a month by site personnel from the
backfill circuit of the Louvicourt mill were shipped to CANMET with a layer of
process water maintained over the tailings at all times. These were transferred in
slurry and allowed to settle and form the bottom layer in four series of triplicate
columns (0.3 m inner diameter), each simulating a different scenario of subaqueous
tailings disposal. The column setup was as follows:
- Series 1: 0.3 m water directly overlying tailings
- Series 2: 0.3 m water overlying 0.3 m peat as an intermediate layer over the
tailings - Series 3: 0.3 m water overlying 0.3 m sand as an intermediate layer over the
tailings - Series 4: 1.0 m water directly over tailings
In other words, the column experiment was designed to evaluate the effectiveness
of two water depths (0.3 m and 1.0 m) and two intermediate barriers (peat and
sand) to prevent weathering of submerged tailings. Both the peat and sand used in
the column studies were locally available material sampled by mine personnel
from within the Louvicourt property. To facilitate replenishment of the water cover
as needed during the study, the process water in each column was replaced by
untreated Ottawa River water at startup.
The column studies consisted of two major phases. Phase I, which lasted for 200
days, focused on oxygen diffusion and ionic fluxes under conditions of a circulated
water cover. In the first 100 days, the water cover in each column was circulated
but not aerated. In the second 100 days, aeration of the water cover was also
included. Phase II, which lasted for 13 months, incorporated precipitation, runoff
and drawdown events at rates comparable to those observed in the field. The
impact on the chemistry of the water cover and porewater in each series of
columns was investigated. Both Phases I and II commenced with a new batch of
natural water as water cover such that only the porewater in each column retained
remnant effects of the previous stage of testing.
The test results showed that, especially during Phase I, sulfide oxidation, efflux of
porewater sulfate and perhaps also dissolution of minor secondary sulfates
contributed to increasing sulfate concentrations in the water covers directly
overlying the tailings. A slight pH depression was observed in the overlying water
in the peat and sand columns during Phase I. This could be caused by acidity
released in the hydrolysis of Fe and Mn near the water/solids interface and/or
oxidation of entrapped sulfide contamination. After the initial flushing of stored
weathering products, however, both peat and sand provided an effective diffusion
barrier to suppress chemical weathering of the underlying tailings. Largely
controlled by the alkalinity balance in the water covers, the 1.0 m simple water
cover without an intermediate barrier layer appeared to outperform the 0.3 m water
cover in suppressing sulfide oxidation and metal leaching in the submerged tailings
under the laboratory test conditions. In any case, precipitation of iron
oxyhydroxides at the water/tailings interface and drawdown limited the efflux of
undesirable metals to the overlying water column. Only minor dissolved zinc was
found diffusing from the tailings porewater to the overlying water column.
Since the column studies were conducted under laboratory conditions that differed
significantly from those occurring in the field, caution must be exercised in
extending conclusions drawn from the column studies directly to the Louvicourt
site. Further work recommended to supplement the current investigation include
the following:
- Post-test solids characterization to confirm geochemical processes taken
place - Correlation of results of the column studies with field monitoring data
- Lysimeter testing of a 1.0 m water cover with less alkalinity content to
clarify its long-term performance under well oxygenated conditions - Investigation of the rate of carbonate depletion in the test solids and its
long-term implications
A thorough understanding of the important and relevant physical, chemical and
biological processes associated with subaqueous tailings disposal will facilitate the
design of practical, walk-away solutions for the decommissioning of mine sites,
like the Louvicourt Mine, which utilize the water cover technology to suppress
sulfide oxidation in reactive tailings.
