Pit backfilling on two continents: Comparison of recent experiences in the Wismut and Flambeau projects

Commitments to backfill acid generating waste rock are becoming a common part of mine closure plans. However, the difficulties associated with translating such commitments into practice are not commonly recognized. This paper compares recent experiences with full-scale backfilling operations at two mines, one in the U.S. and one in Germany. At Wismuts’ uranium mining operations in the former East Germany Ronneburg mining district, waste rock was placed in fourteen piles. Acid generating rock was mixed with neutral rock, resulting in various combinations of acid generation, heavy metal release, and radon exhalation. The need to control further acid generation and prevent formation of a contaminated pit lake led to the decision to relocate waste rock to fill the Lichtenberg open pit. The relocation program is designed to place the waste rock with the highest potential for acid generation below the future water table, and to cover the remaining acid generating rock with a layer of net acid consuming sulfidic rock, which will act as an oxygen consuming layer. The relocation program therefore requires in-field identification of different classes of waste rock and an iterative planning and control program. In contrast, at Flambeau Mining Co.’s copper/gold/silver operation near Ladysmith, Wisconsin, waste rock was separated into non-acid generating (Type I) and potentially acid generating (Type II) waste rock during operations. The objective of the reclamation plan was to backfill the waste rock in reverse order in which it was mined. After reflooding, all of the potentially acid generating rock would be below the water table. The key problem was to quantify the acidity stored in the waste rock, and to add appropriate amounts of alkali. The paper highlights the similarities and differences between the two programs with respect to the testing undertaken to support the backfill design, methods used for material identification, classification, control of alkali addition rates, and long and short term planning.