No abstract

The Debswana Diamond Company's water and residue strategy is aimed at reducing mine water consumption by 50% by 2008. Paste thickening of the tailings has contributed significantly to this goal, however, recent studies have indicated that a novel flocculant product (CIBA™ Rheomax™ ETD) may release significant amounts for water from the thickened tailings.The use of Rheomax™ ETD in two applications i.e. dosing prior to deposition at the disposal site and prior to a filtration step were assessed and the water savings and cost implications were compared to that achieved by conventional; high rate and paste thickening systems.

Mine site location and the sources from which raw water is drawn are fixed by geology, geography and climate. The colloidal behaviour of the tailings is therefore randomly determined by the tailings mineralogy and the chemical characteristics of the process water circuit. In some fortunate cases, the tailings slurries are non-dispersive and solid/liquid separation, either by gravity thickening or filtration, is easily achieved at low capital and operating cost. In less fortunate cases the tailings are highly dispersive and solid/liquid separation is achieved only at high capital and operating cost. This paper presents a strategy by which difficult-to-treat slurries can be modified by conditioning the process water circuit with a process water conditioner (ClariVie44®) so that gravity thickening and filtration can be enhanced.

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).

A pilot plant paste thickening campaign was conducted at the Anglo Platinum Limited Mogalakwena South Concentrator plant in South Africa in order to determine the water saving capability of paste and thickened tailings (P&TT) technology. In the process a simple water consumption model was developed for estimating the overall water consumption of the mine. The model indicates that for the Mogalakwena tailings, significant water savings are achieved by discharging thickened tailing directly to the TSF but only at densities where free water release at the TSF is close to zero.