899 Permeability and drainage To determine the permeability Laboratory Permeability testing can be undertaken using (Australian Standards AS1289 or similar) using a flexible wall permeameter with the Falling Head method for mixtures of sands/silts /clays, and the Triaxial method for high plasticity/ low permeability Clays. For soils the typical permeability for sands silty sands to sand clay mixtures range up to 1E-5 m/s down to 1E-6 to 1E-8 m/s, and clays less than 1E-9 m/s. Clays being the major cause of poor permeability. The presence of swelling clays (montmorillonite and illite) even in small quantities can adversely affect sand permeability. (Barrera, 2015) reports acceptable permeability of the sand in the dam must be greater than 2E-6 m/s for the sand, noting the permeability coefficient decreases as the confining stress increases, product of the decrease in density and the generation of fines by breaking of particles, although this last effect is relatively a minor one at least for tailings sands with 15 to 21% fines content. The sand properties are largely driven by the methodology of producing the sand. Clay-rich ores retain high volumes of water due to their porosity, which reduces dewatering efficiency and increases operating costs. Therefore, variability in mineralogy creates disturbances in concentrator operations that must be anticipated to avoid bottlenecks in downstream tailings handling. Methods of production and transport Hydrocyclones are the technology currently utilised to produce sands it is a simple device to construct that consists of a cylindrical upper body and a conical lower body (Figure 1). The principle of operation is to use centrifugal forces to separate faster settling large particles from fine slow settling solids, a slurry entering to the conical body of the cyclone is forced to a spiralling movement creating a centrifugal force that sends the larger particles to the outside where they are transported to the Apex spigot at the bottom(underflow). The finer particles and most of the water concentrate to the centre where they are transported to the vortex finder and exit from the pipe at the top and centre (overflow). . Figure 1 – Hydrocyclones The separation is not perfect, the time in the cyclone is short and fines can be entrained with the underflow, a serious drawback is that it is a poor classifier where the cyclones are fed with high pulp densities. To control the cyclone, it should be fed with a constant flow and the resistance to flow from the Apex and vortex require a pressure loss of 40 to 150kPa. The separation is affected by the diameter of the cyclone, the ratio of coarse to fine material, the density of the feed and the density of the solids in the slurry. Smaller
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