The use of shallow water cover (up to 2 m) to flood reactive sulphide mine tailings is a
popular method of acid drainage prevention used by the mining industry. In flooded
tailings, wind-induced turbulence can increase the oxygen flux from air into water by
creating turbulence at the air-water interface, thus promoting mechanical mixing of
oxygen from air into the water and thus keeping the dissolved oxygen concentrations
at saturation levels. Turbulence can also resuspend tailings particles in the
oxygen-saturated water cover and expose tailings to greater contact with oxygen,
possibly leading to increased oxidation and metal release.

In late 1996, the Geotechnical Research Centre at The University of Western Ontario
initiated a study to investigate the contribution of resuspension to tailings oxidation and
acid generation under laboratory conditions. The study was undertaken on behalf of
MEND and sponsored by Battle Mountain Canada Limited, Falconbridge Limited, INCO
Limited, Noranda Mineral and Exploration Limited, Teck Corporation, Ontario Ministry
of Northern Development and Mines and Canada Centre for Mineral and Energy
Technology (CANMET) through the CANADA/Northern Ontario Development
Agreement (NODA). This report presents the results of the study.

The study involved a series of laboratory experiments performed in Plexiglas columns
packed with unoxidized pyrrhotite tailings, which were then flooded with 45, 60 and 80
cm deep water covers. The tailings were obtained directly from the mill discharge
pipelines at the Falconbridge Strathcona Mines, near Sudbury, Ontario. The water
cover was stirred at 140, 170 and 200 revolutions per minute to suspend the
underlying tailings. Control experiments involving water-covered tailings without stirring
were also conducted for comparison. To facilitate the analysis of the results, the stirrer
speed and the depth of water cover were used to define a mixing index, a
dimensionless parameter that measures the degree of mixing in the water cover.
Oxygen mass transfer from air to the water cover was measured for each mixing index.
The water cover was also monitored for dissolved oxygen (DO), pH, conductivity,
sulphate and metals. At the end of the experiments (126 days), suspended tailings,
surficial tailings and undisturbed solid tailings and pore water were sampled for
chemical analysis. The bulk and surface mineralogy of the tailings was also examined.

Key findings and conclusions arising from the study are as follow:

1. Resuspension increases sulphide tailings oxidation, acid generation and metal
   release. This is based on sulphide (pyrrhotite) depletion, oxygen consumption,
   pH, sulphate and metal loadings.
2. Oxidation products are lighter than the original tailings (specific gravity of 2.7
   versus 3.9-4.4 for the original tailings).
3. Oxidation products include iron oxyhydroxides (possibly goethite) and gypsum.
4. Suspended tailings are finer than tailings at rest: 80% of the suspended tailings
   are finer than 0.018 mm, compared to only 30% of the original tailings.
5. Unoxidized suspended tailings contain sulphide mineral (pyrrhotite) as well as
   gangue minerals.
6. The rate at which oxygen is transferred into the water cover is independent of
   mixing index.
7. The release of nickel, zinc, aluminium, and manganese increases with mixing
   index.
8. Significant bed erosion occurs at a threshold (critical) mixing index. This
   threshold value would occur in shallow water covers (possibly 60 cm or
   shallower).

The results of the study suggest that when tailings are resuspended, whether by
mechanical stirring or by intense wind and wave activity, sulphide oxidation is
accelerated with consequent precipitation of secondary iron oxyhydroxide minerals.
These minerals tend to have a large surface area when freshly precipitated and can
adsorb or scavenge trace metals released during the primary oxidation reactions.
Further research is recommended to assess the long-term stability of the
oxyhydroxides, especially as conditions in the water become reducing, under which
iron hydroxides tend to dissolve and are likely to release scavenged metals. Metals
released in the water cover, as a result of increased oxidation due to resuspension, do
not infiltrate deep into the underlying tailings. The impact of this resuspension-induced
oxidation and metal release on the overall water quality in the field should be
assessed. Various factors including dilution and contributions from groundwater and
precipitation will have an influence on the overall water quality in the field. These
factors are not evaluated in the present laboratory study.