Olympias Mine is operated by Hellas Gold S.A., a subsidiary of Eldorado Gold Corporation. The orebody shape and size are suitable for a drift and fill mining method. The mining sequence is overhand and the demand for backfill strengths are generally low except for the initial sill cuts. The design fill strengths are determined from the planned stope exposures to allow for safe extraction of the ore in adjacent drifts and immediately below the initial sill drifts with minimum dilution. Due to the permit constraints imposed on mining at Olympias Mine, after an environmental impact assessment, there is a requirement that the final backfill strength must reach a uniaxial compressive strength (UCS) of 4.0 MPa at 28-day cure age. By developing a suit of mix recipes incorporating superplasticiser admixtures, it was possible to achieve the strength demands and the workability of the backfill. This paper presents the results from comprehensive test work conducted on whole mill tailings and cyclone mill tailings to produce high strength backfill.
Traditional paste filling materials adopt industrial solid wastes such as unclassified tailings and waste rocks as filling aggregates, but the binder used is dominantly commercial cement, which raises the filling cost. Hemihydrate phosphogypsum (HPG) is one of the main by-products of the phosphorus chemical industry, and the high-efficiency utilisation technology of HPG has been a problem worldwide. The potential gelling property of HPG is found in the laboratory at first. It is then modified by appropriate chemical and physical methods to produce new filling binder with the purpose of replacing cement completely. A lot of experiments show that the modified HPG (MHPG) has outstanding properties of quick setting (setting time less than 90 minutes), high early strength (UCS for three days more than 8 MPa) and good flowability (yield stress less than 50 Pa). In addition, MHPG can bind different aggregates according to the filling needs to prepare a homogeneous, non-settling, and non-bleeding filling paste. In the past three years, two mines in Guizhou, China have adopted the underground paste backfill technology and surface open pit cemented filling technology based on MHPG paste materials. In practice, this new technology reduces the material cost by 30%, increases the flow capacity by 50%, and has a solid wastes rate of more than 98% compared to traditional methods.