Rapport NEDEM 2.15.3
November 1998

EXECUTIVE SUMMARY

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.

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.

Last Modified: 2003-11-26		Important Notices

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