No abstract

Interest in the potential use of high density, thickened tailings has recently increased significantly. Reasons for considering this new technology vary across projects, but commonly include the need to conserve water, perceived lower risk of catastrophic failure, potential easier closure or even reduced overall costs. As with any new technology, there has been some tendency to overstate its potential benefits. This paper reflects on whether or not the potential benefits that have been attributed to paste and thickened tailings have been realised. Using a grading system, thirteen benefits that were ascribed to the new technology some years ago are evaluated. Data are taken from case studies, and it is suggested that the key proven benefits appear to be reduced operating costs in some cases, reduced wall-building costs and reductions in water consumption. One key potential benefit that has not been universally achieved is a reduction in the footprint of the tailings facility. Reports in the literature highlight the sometimes confusing nature of trade-off studies. Unless full life-of-mine costs are considered, these studies can be misleading and incorrect. The need to establish a consistent basis for comparative studies is discussed.

During 1994, a 3-D seismic reflection survey was undertaken at Vaal Reefs No. 10 shaft with the objective of mapping the detailed structure of the Ventersdorp contact reef gold orebody. This would provide vital input into future mine planning and development. The survey benefitted from 10 years of 2-D seismic experience and one previous 3-D mine survey, conducted in the Witwatersrand Basin. The seismic survey at No. 10 shaft accurately and spectacularly delineated the 3-D structure of the Ventersdorp contact reef at depths ranging from 1000 to 3500 m, imaging faults with throws in the 20- to 1200-m range. The resultant structure plans were satisfactorily validated by subsequent surface drilling and underground mapping mining operations during the period 1994 to 1996. These plans have been merged with drillhole, underground, and sampling data into an integrated mine modeling, gold reserve estimation, and mine scheduling package. The geology department now manages the planning function at No. 10 shaft, and 3-D seismics has played a significant role in placing this important responsibility firmly within the geologists’ domain. Building on the success of the No. 10 shaft survey, two other 3-D seismic surveys were concluded over mines during 1996 and 1997.

Since the beginning of the Paste and Thickened Tailings seminar series, there has been ongoing discussion as to whether an increase in the deposited (initial) slurry density will result in an increased density at depth within an accreting deposit, all else being equal. Despite frequent speculation and theorising, there has been relatively little laboratory data or other evidence presented at the conference series to address this question. To address the above deficiency, a literature review was undertaken to identify experimental programs where the same material was prepared for soil testing from different initial slurry densities. Twelve such studies were identified, of which nine indicate that an increase in initial slurry density resulted in increased density at subsequent significant vertical effective stresses. Case studies presented on three tailings deposits are investigated, to assess the implications of the presented results on the potential for initial slurry density to influence in situ density. For many of the studies reviewed, there appears to be an indication that an increased initial slurry density resulted in an increased in situ density at significant vertical effective stresses. Finally, a series of consolidation tests were undertaken by the authors, wherein four tailings types were prepared from different non-segregating initial slurry densities. For three of these materials, a dependence on situ density was observed, based on changes to initial slurry density. On the basis of the review and experimental data presented in this study, the authors conclude that for at least some materials, initial slurry density can result in an increased density across a wide range of vertical effective stresses.

One method for the placement of thickened tailings is the central discharge technique. The beach slopes achieved with this method have generally been in the order of 2 to 3%. With such small beach slopes, it is important to be able to predict the slope angles accurately. This is often done using laboratory flume tests. However, these predictions have tended to overestimate slope angles. This paper presents a model that takes account of the wall friction in flume tests and illustrates the folly of using flume tests indiscriminately. The model does not account for issues such as deposition rate or initial velocity, but serves to quantify the potential errors of using flume data for direct extrapolation to field applications.

Sampling error and bias, especially the Increment Preparation Error (IPE), are introduced when the tool allocated to do the job fails to extract a representative sample. This is the case with chip sampling; the tool can only extract haphazard shapes of loose or fractured material and not always from within the demarcated sample area. Increment Extraction Error (IEE) and Increment Delimitation Error (IDE) could be severe, but are found to be relatively unbiased. The uncontrolled action of the sampler in discarding excess sample material after sample collection has a significant impact on the error and bias introduced with each and every extraction. A new type of bias, referred to as the 'waste discard bias', arises when samplers select what portion of the excess sample material to discard at the sample site. Material visually identified as waste is discarded in preference to mineralized broken ore if there is an excess of sample material. This results in a considerable error in the analytical results. The bias is proportional to the reef-waste ratio in the demarcated sample.sampling, error, bias, mine call factor.

This paper presents full-scale experiments on cemented paste backfilling in two operating mine stopes located at Raleigh mine, Kalgoorlie, Western Australia. The experiments compared the behaviour of the cemented paste backfill (CPB) in a drained stope and an undrained stope. The drained stope was equipped with a drainage system at the barricade, whereas the undrained stope was not. Total stress and pore water pressure sensors were installed to measure the stress within the fill mass during and after backfilling. The stress behaviour was expressed by the normalised total horizontal pressure and the normalised pore water pressure. It was found that the normalised total horizontal pressure and the normalised pore water pressure behaviour at the two stopes were significantly different. The drainage system increased the consolidation of the fill and reduced the pressure acting on the barricade. The pore water pressure measurement in the undrained stope showed a non-linear accumulation at the barricade. In contrast with the undrained stope, the stope with the drainage system showed significant pore water pressure reduction and the accumulation of pore water pressure at the barricade no longer occurred. The findings described in the paper contribute to critical design and management aspects such as barricade stress, filling rate, and curing strategy, with an ultimate goal of reducing costs while ensuring safety underground.

In this paper, a simple time-dependent failure criterion has been proposed for cemented paste backfills. In addition to curing time, it allows to take into account the binder content that will be responsible for the material strength development. The properties described by this time-dependent criterion are those undrained since the backfilled mine stopes have been categorised as undrained in long term.