Estimating the rheological properties of a paste using convenient methods such as the slump technique are common practice within the industry. However, the universality of comparing slump heights to estimate the flow properties for different paste products has been questioned (Clayton et al., 2003;Paterson, 2002). The dispute relates to the fact that two paste products may in fact exhibit similar slump heights, but may not share the same shear yield strength since their densities may be different. This discrepancy is catered for by the method of Pashias et al. (1996) in which the shear yield stress can be conveniently calculated from the slurry density and the slump height.However, certain paste rheological behaviours are observed which appear puzzling and which cannot be explained on the basis of particle size and slurry density alone. The chemical conditions of pastes (particularly those containing clay minerals) have profound effects on the colloidal interactions of the suspended solids and hence on the rheological behaviour of pastes (Vietti, 2004;Dunn, 2005). This paper demonstrates that different slump heights may be observed for a kimberlite paste containing smectite at one material density, but varying paste chemical conditions. CLAY MINERAL COLLOIDAL PROPERTIESDue to the variety of resources exploited tailings generally contain a kaleidoscope of mineral types.However, in general the fine clay minerals tend to be concentrated within the tailings. Since, the colloidal properties of clay suspensions are highly sensitive to chemical conditions; their behaviour directly affects the operational performance of each aspect of the paste system from thickening to transport to deposition and rehabilitation. As such the clay minerals will be discussed in some detail in the following section. Clay size and structureClays (or phyllosilicates) are typically classified on a size basis as the -2 m fraction within a tailings slurry.However, a distinction must be made between the mineralogy of clay sized particles and true clay minerals (Fourie, 2002). True clays are classified into two major types based on the clay crystal lattice structure, namely 1:1 type clays (composed of a single octahedral layer which is bound to a single silicon tetrahedral layer), and 2:1 type clays (composed of two tetrahedral sheets sandwiching a central octahedral sheet) (Figure 1).
The potential processing risks associated with the presence of dispersive, swelling clays in mineral orebodies are well known. Uncontrolled dispersion of such clays on contact with water results in problems including poor tailings dewatering and consolidation, increased reagent demand, dirty process water and reduced mineral recoveries and increased plant maintenance with associated economic, environmental and safety implications.Dispersive clays are typically managed via approaches including high dosages of both polymer coagulant and flocculants in thickening, high-pressure secondary dewatering steps such as pressure filtration, belt press filtration or centrifugation, or inline flocculation and deposition of thickened tailings. Commonly, the secondary dewatering step requires re-dosing of additional coagulant and/or flocculant to regenerate a flocculated structure to develop acceptable dewatering rates.An alternative, more proactive approach to managing tailings containing dispersive clays is to promote controlled dispersion of the clays by conditioning the process water circuit to induce a coagulative state in the clays on first contact, reducing clay breakup and ultra-fines generation during initial wetting of the ore on entry to the plant. Clay dispersion control via process water conditioning involves reagent dosing into the process water at only a single location, however, this delivers benefits at every stage of dewatering across the tailings management flow sheet.The potential site-wide benefits of this approach are demonstrated for the ClariVie44®process water conditioner, using a combination of flocculation and settling test results, compression-permeability testwork and pressure filtration model simulations from a range of different tailings samples.The benefits demonstrated include step changes in thickener fines capture and overflow clarity, material improvements in process plant operability and reduced down time due lower fines recirculation, elimination of coagulant dosing in the thickener and downstream secondary dewatering operations, increases in pressure filtration throughputs of up to 300%, improvements in any process technology employing secondary flocculation due to more homogenous structure development, and improved tailings storage facility (TSF) operability and lower risk due to less segregation, faster consolidation and operational dry densities and improved decant water management. Test data from the Jagersfontein kimberlite tailings are also discussed in the context of the recent TSF failure and the potential role of dispersive clays as a risk factor at this site.
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