OPEN-PIT BACKFILL AS AN ECONOMICAL ALTERNATIVE TO THE SURFACE STORAGE OF MINE WASTE

The permanent aboveground storage of mine tailings, the wet and fine-grained material that remains after the mineral of interest has been separated from the ore, is the most problematic aspect of mine waste management. The worst-case scenario is a catastrophic failure of the tailings dam with potential loss of life and the release of a large quantity of toxic material into the environment. For this reason, the maximum backfilling of mine tailings into either open-pit or underground mine workings is regarded as a best practice. While the backfilling of exhausted open pits is less technically challenging, concurrent open-pit backfilling and mining is common in aggregate, coal and oil-sands mining, and is becoming increasingly common in metal mining. Open pits must be backfilled in California and New Caledonia, while British Columbia sets the maximization of open-pit backfill as a target and Quebec requires a feasibility study of open-pit backfill. The objective of this study was to estimate the unit cost of open-pit backfill of either tailings or waste rock based upon publicly available data from 15 case studies. Unit costs ranged from 0.28 USD/t to 15.00 USD/t, with 10 out of the 15 studies in the range 0.72-1.50 USD/t. The very high outlier included the additional cost of remediation of acid mine drainage from waste rock that had been stored on the surface at the Lichtenberg uranium mine in former East Germany, while the very low outlier was achieved through the transport of uranium tailings as a slurry by gravity directly from the ore processing plant into an exhausted open pit at the Marymia mine in Western Australia. The expected value was calculated as the geometric mean (1.20 USD/t), which suppresses the impact of outliers. The preceding estimate was applied to the proposed expansion of the Bloom Lake iron mine in eastern Quebec, which would involve the destruction of wetlands and seven lakes in order to provide space for a new tailings storage facility. The cost of backfilling the additional mine tailings (356 million USD) and the cost of constructing and operating a new facility for the permanent aboveground storage of the additional mine tailings (328.4 million USD) are remarkably similar, so that there is no purely economic reason for choosing between backfilling and the permanent aboveground storage of the additional tailings.