Aitik, Gällivare, Sweden
There have been recent problems with instability of the Tailings Storage Facility (TSF) main embankment which has necessitated costly repairs and an on-going programme of waste rock placement at the downstream toe. Aitik wish to develop a long term plan for future management of the dam that minimises both risk and cost. Down Valley Discharge (DVD) of thickened tailings is a method that could potentially achieve these aims.
A failure of the dam occurred at the southern end of embankment E-F in September 2000. It is believed that the cause was internal erosion of materials along the buried concrete culvert from the spillway tower. The collapse led to tailings and water washing into the clear-water dam. Fortunately there was no escape of solids from the clear-water dam and no significant environmental impact beyond the mine site. The southern end of the embankment and the spillway system were rebuilt (Section E-F2) but there was subsequently a separate incident when a small slump apparently occurred in the central section of embankment E-F on the downstream face at an elevation above the starter dam crest level.
Two principal alternatives were investigated:
- Discharge of the existing unthickened tailings from the east, north and west sides of the impoundment, predominantly from the west initially, in order to push the decant pond away from the west embankment and against the southern hillside instead. Construct a new spillway arrangement from this new pond location. Continue to raise all of the embankments via upstream methodology, including the spillway, for the duration of mine life.
- Thicken the tailings so that a steeper tailings beach can be developed. Continue to discharge the tailings from the eastern end. The steeper beach slope will have the effect of “stacking” the tailings at the eastern end of the dam. Manage the process so that no further tailings arrive at the E-H embankment and therefore negates the need to raise the facility. Re-design the spillway to minimise water ponding.
The laboratory testing and option study confirmed the feasibility of adopting down valley discharge of thickened tailings for future disposal at the Aitik mine. The tailings are very fast settling and relatively low in fines. A corresponding high segregation threshold of 66% solids has been measured for the tailings in the laboratory testing but this is still well below the predicted thickener underflow densities of 71% for a High Density thickener and 70-74% solids for Deep-Cone thickeners.
Layout schemes for both types of thickeners were prepared. The HD thickener option, has lower start-up, operating, and NPV cost and is recommended as the preferred option.
The following benefits are foreseen as a result of implementing the DV discharge of thickened tailings:
- Substantially reduced long term costs compared to the existing disposal method.
- Reduced risk of embankment instability (the ongoing waste rock stabilisation work will cease).
- Reduced water make-up requirement from river pumping.
- Warmer return water to the plant and hence better copper recovery.
- A final tailings surface shape would be self-shedding and avoid ponding on the surface at closure. This is important in order to minimise infiltration and the consequent transport of contamination from oxidation and acid generation to surface water and groundwater.
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