Backfilling, tailings

The U.S. Bureau of Mines-Spokane Research Center is conducting research on the environmental impacts of placing mine wastes underground as backfill. This work includes a review of residual cyanide in placed landfill, water quality monitorings at two mines and laboratory tests of cyanide fate in underground environments and permeability/leachate effects through cemented tailings. 

To support and reinforce an underground excavated area after the ore has been removed, the backfill (including both fine and coarse material) is thickened. The product is called full plant tailings. Steward (1996) defined the particle sharpness as the rate of directional change in the particle perimeter. 

Backfill paste is formed by dewatering slurry of tailings (thickening and filtering). Mixing dewatered slurry with cement (3%-5%) produces a stiff backfill (1.5-3.5 MPa strength, or 218 508 psi). Coarse aggregates (<20 mm, or in) must be present. Concrete pumps can be used. 

Synthetic covers are expensive and susceptible to cracking after installation due to desiccation or subsidence. The effectiveness of such covers relies on long term integrity and because they are located on the tailings surface they are most susceptible to erosion (Blowes et al. 1994). The strategy is only applicable to rock waste, spoil or backfill and not to in situ rock (Robertson 1987). 

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