Pit Backfilling - Managing Chemically Reactive Mine Wastes and Achieving Rehabilitation Objectives

摘要

The placement of chemically reactive mine wastes in mined out pits ('pit backfilling') is increasingly seen by industry as best practice to manage long-term environmental risks. This is primarily because leaving wastes at the surface, or allowing pit lakes to develop, can result in unacceptable environmental risks. The costs and practicality of mitigation options such as installation of covers and community expectations of landform re-construction to achieve a post-mining, beneficial land use are also important drivers for pit backfilling. Pit backfilling can significantly reduce the post-closure risks to receiving environments, by minimising seepage due to tailings, acid/solute generating waste rock and/or salts derived from mine water treatment. Environmental risks are often highest where wastes are located close to the ground surface, due to relatively high rates of groundwater movement in near-surface aquifers and/or failure of engineered structures due to rainfall/erosion processes. Placement of these wastes at significant depth in pits, subject to mine planning and environmental assessment, can address these risks by increasing the path length and resistance from contaminant source to receptor, thus enhancing encapsulation of the wastes. Pit backfilling can also be cost-effective for mining companies. This is because existing pits are developed on the basis of an economic return and thus provide an opportunity to store tailings in an existing 'engineered structure' at little or no incremental cost when compared with building and operating a new tailings or waste rock storage facility, often encompassing complex cover/ drainage management systems. Key issues for consideration are mine planning, the long-term geotechnical behaviour of the wastes (eg consolidation of tailings which leads to release of acid/solutes in tailings porewaters), seepage rates to downstream receptors/agreed end point criteria and potential resource sterilisation. Internationally, mine closure guidelines are also showing a greater recognition of pit backfilling in the context of allowing mining companies to meet agreed post-closure land use objectives. In summary, pit backfilling can enhance the encapsulation of chemically reactive mine wastes and environment protection, and with agreement on post-closure beneficial land use, can also help to deliver post-closure benefits to communities.