MEND Report 2.12.1e
March 2001

EXECUTIVE SUMMARY

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:

1. Post-test solids characterization to confirm geochemical processes taken place
2. Correlation of results of the column studies with field monitoring data
3. Lysimeter testing of a 1.0 m water cover with less alkalinity content to clarify its long-term performance under well oxygenated conditions
4. 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.

Last Modified: 2003-11-26		Important Notices

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