The reliable prediction or management of beach slope and stack geometry is integral to realisation of the potential benefits of thickened tailings technology. Stack geometry not only controls the storage capacity for a given footprint, but also strongly influences post-deposition geotechnical and geoenvironmental performance. Unfortunately, accurate prediction in design has proved difficult. However, there has been significant improvement in understanding the behaviour of tailings after they exit the pipe, and a number of methods for beach slope prediction have been proposed and developed. This paper reviews a selection of these methods, and examines their applicability to different deposition scenarios. Some recommendations are made to assist engineers and operators to achieve a given stack geometry.

No abstract

The development of the thickened tailings disposal technique presents a new paradigm in tailings disposal. It results in a total revision of the risk profile of a tailings storage facility (TSF), and may provide significant savings in both cost and water consumption. The method was developed by Robinsky (1975), who identified that thickening the tailings stream would create a non-segregating slurry and that a steeper overall beach angle could be achieved. The outcome is the elimination, or the substantial reduction in size of retaining embankments. This has obvious benefits.The paper reviews the development history, and the key design and behavioural parameters that are needed for thickened tailings schemes, with an emphasis on the understanding of the fundamental geotechnical issues. Brief historyThe concept of thickened tailings storage was developed in Canada in the early 1970s by Robinsky (1975Robinsky ( , 1999. He identified that thickening the tailings stream sufficiently would create a non-segregating slurry i.e. hydraulic sorting would not occur on a sub-aerial beach, and that a steeper overall beach angle would be achieved. These properties permit tailings to be 'stacked', either as a low conical hill on flat ground, referred to as central thickened discharged (CTD), or as down-valley discharge (DVD).There are now about thirty known thickened discharge schemes operating worldwide (with some that have now closed). These are summarised in Figure 1, based on data presented in Williams et al. (2008). This figure is limited to schemes that are (or have been) fully operational, and are referenced in the literature or details are known to the authors.

No abstract

A controlling variable for a thickened tailings scheme design is the beach slope that will be formed by the tailings as they deposit. The slope will be mainly related to the discharge flow rate and the slurry rheology. The latter will, in turn, be dependent on the solids concentration of the thickener underflow. A higher solids concentration can be achieved by combining thickening and filtration. The objective is to still maintain a slurry that is pumpable and self-distributing at the tailings storage facility (TSF), rather than having to rely on trucks or on conveyors and stacking systems to manage a 'cake', but one that will deposit at a steeper beach slope than can be achieved by thickening alone. The use of ultra paste has the potential to broaden the range of topographic conditions that will suit thickened tailings discharge.The ultra paste thickened option is when a portion of the tailings is separated and dewatered in a more advanced dewatering unit (in this case filter presses). The filtered cake will then be re-mixed with the paste thickener underflow stream.This paper presents a case study for a large copper mine in which underflow from a series of paste thickeners with a nominal tonnage of 3,660 tph at 59% solids concentration would be mixed with 1,363 tph of filter cake at 80% solids concentration. The combined tailings (ultra paste) would be discharged equally into two open channels (flumes) at the combined solids concentration of 63.5%.The ultimate goal of adopting an 'ultra paste' scheme is to produce homogenous consistency tailings, hence proper mixing of the paste and filter cake has to occur. In this study based on a series of tests, the mixing requirement has been assessed. The study also covers a conceptual investigation of natural turbulent mixing (NTM) of the filtered tailings with paste thickened tailings and issues associated with the deposition of the combined tailings, the ultra paste, into the existing TSF.In the conclusion, the study indicates that NTM can possibly occur only for the case in which the total tailings are discharged into one channel, which is not practical due to limitations with regards to tailings management. Hence, mechanical mixing has been recommended in this case.