Backfilling in underground mines has been practiced for close to 100 years and use is anticipated to continue at an increasing rate during the foreseeable future. Typical underground mines utilizing backfill are faced with backfilling costs ranging between $2/tonne and $23/tonne of backfill, representing between 2% to 26% of the total mining cost for these mines. Activities related to backfilling operations, such as handling, transportation and placement, require extremely large amounts of energy to conduct, resulting in an energy intensive process requiring an average energy output of 32 MJ/tonne of backfill. The typical cost of backfilling for a single mine can be as high as $53.5 million per year. Mine backfills commonly require the use of Normal Portland Cement (NPC) as a stabilizing agent. The production of cement is also an energy intensive process requiring energy amounts ranging between 4,637 and 6,600 MJ/tonne. With an average of 29 kg of cement used per tonne of backfill in typical underground mines, cement consumption can reach as high as 57 kg/tonne backfill. A single mining company can consume as much as 100 thousand tonnes of cement per year for backfilling purposes alone, corresponding to $12.3 million per year based on a purchase price of $123/tonne cement. The cost of cement per tonne of backfill placed ranges from as low as 4% to as high as 60% of the overall backfill cost.
It is evident that there exist in North America many mine backfill plants that exhibit high energy consumption and high backfilling costs. Such plants offer the potential for considerable process optimization, and reduction in energy costs and associated gas emissions.
Currently, the mining industry uses blast furnace slag and fly ash pozzolan materials as partial cement substitutes to reduce backfill costs. This paper identifies alternate, equally effective and lower cost binder agent strategies for cement in backfill that may lead to cement consumption reductions. Post-consumer glass represents one such potential alternative backfill binder. The partial replacement of cement by waste glass may address goals for reducing non-recyclable glass waste disposal; enhancing mine backfilling operations; and reducing total backfill energy costs.
Extensive engineering testing has been implemented in order to demonstrate the technical feasibility of process integration within industry. Pipe loop tests with cemented backfills and with backfills utilizing glass binder have demonstrated an equivalence in friction head loss and an equivalence in rheological properties. Strength tests have shown that glass replacement of NPC, at levels between 15 and 25 percent, can provide equivalent or improved compressive strength response relative to NPC-reinforced hydraulic backfills. Studies using paste backfills, with glass replacement of NPC at levels between 15 and 35 percent, demonstrated similar beneficial strength generation.
An economic analysis has demonstrated that glass is competitive in cost relative to cement when including sourcing, grinding, and consumer delivery operations. Assuming a 25 percent replacement of ground glass for NPC in placement of 500,000 tonnes of backfill per mine per year, individual mine savings were found to range from $0.13/tonne of backfill ($65,000/year) to C$0.46/tonne of backfill (C$230,000/year). A socioeconomic study has further indicated that the utilization of glass in mine backfill would create a new market for waste glass that requires less processing, and would both simplify glass recycling and reduce costs.
