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.
The authors report a robust estimate of the shear yield stress of a slurry from inline measurement of a flowing slurry process stream. At present, slurry density is often used as a proxy in estimating the shear yield stress based on laboratory-scale measurements performed as a function of solids concentration. However, this approach leads to inaccuracies due to poor translation of laboratory data to operational situations and fluctuations in the sensitive yield stress-concentration dependence due to material and process variations. A robust inline measurement that directly relates to the in situ yield stress is therefore both a valuable and significant advance. In order to provide continual, inline rheology data for process control, the OnLine Rheometer Series 1000 (OLR) was piloted for a high clay, gold process thickener underflow in a pilot pipe loop facility. Benchtop measurements of the vane yield stress, and solids concentration were performed on samples drawn from the line as the circulating liquid was progressively diluted. The measured vane yield stress was compared with the Yield Stress Index (YSI), a native function available in the software of the OLR, over a concentration (c) ranging between 49.4 wt% < c < 64.5 wt%. Over the concentration range studied, the vane yield stress exponentially increased with concentration and ranged between 4 and 65 Pa while the YSI values ranged between 16 and 131 units showing an identical dependence on concentration. Importantly, it was found that the YSI correlated linearly with the yield stress values measured using the vane. It follows that the YSI metric can be used to estimate the yield stress within a small factor in actual operations. This finding supports the use of the YSI as an inline control variable in mineral processing and tailings management operations; in thickening and paste preparation for example.
This paper investigates the work carried out by DeGrussa Mine and the Weir Technical Centre (WTC) of Weir Minerals Australia, in developing a process to produce a consistent cemented paste backfill that is fully sheared and presents the lowest possible yield stress and pressure gradients during the underground transport phase. The existing twin shaft paste mixer on DeGrussa Mine was not providing sufficient shear or time of shear to deliver a fully homogenised product. The project to improve the paste backfill rheology involved quantifying the performance of the twin shaft mixer as well as the required rheological parameters of yield stress and pressure gradient of the paste backfill product. In order to achieve the required outcome, a centrifugal pump was installed after the mixer to provide the shear energy required to produce a fully sheared paste backfill rapidly and continuously. A fully sheared consistent paste backfill is required to ensure predictable transport of the paste backfill throughout the DeGrussa Mine reticulation system. This predictability of paste backfill performance results in a safe and robust reticulation system, together with ensuring pipeline integrity. The lower pressure gradients, manifest by the fully sheared paste backfill, also allow DeGrussa Mine to fill stopes that are at a distance that would otherwise require a positive displacement pumped system.
The storage capacity and rate of rise of a tailings storage facility (TSF) constructed utilising the tailings to form the TSF is dependent on the outer side slopes of the facility. A number of TSFs in South Africa are operated utilising cyclones to construct the outer wall zone of the TSF. The outer slope is dependent on the stacking angle of the cyclone underflow and hence it is critical that the design of the TSF accurately estimates the stacking angle. There are no clear guidelines to make this estimation other than to compare to similar projects. This methodology has resulted in a few TSFs in which the stacking angle was either over or underestimated leading to an inaccurate life assessment. This paper hypothesises that the yield stress of the underflow has the most significant effect on the stacking angle. The hypothesis is tested by measuring various tailings properties on a tailings operation in relation to the stacking angle achieved to attempt to develop a method to predict the stacking angle for future projects.