900 cyclones cut at a finer point than larger diameters and usually multiple cyclones are required to treat the full flow. It may be necessary to use two stages of cyclones with interstage pumping and dilution to achieve the desired separation. Cyclone sand is generated continuously over the life of the tailings dam creating progressive dam raises. The cyclones can be installed at equidistant points on the crest of the dam or in a central cyclone station where the underflow is pumped to spigots along the dam crest. The construction of tailings dams using hydrocyclone sands is a successful and efficient methodology. • But with increasing water scarcity, can we improve? By treating tailings from the underflow of a hydrocyclone in a dewatering screen, clays and ultrafine particles are effectively removed, improving drainage and water recovery. Cyclones of a nominal diameter around 500 mm are commonly used where depending upon the tailing’s characteristics 40% to 50% of the feed reports to the underflow producing a thick stream with up to 75% solids. This material flows down the dam’s outer wall in a stream. The underflow tails do not spread well, and the discharge point is moved between multiple points along the wall to control the deposition. Once deposited and drained the material is compacted and spread with earth moving equipment to achieve the required compaction specifications. These specifications depend on the dam design but typically the sand is compacted in thin layers, to a density higher than 95% of Standard Maximum Dry Density. The material continues to increase in density as it drains and is exposed to consolidation, dry density of 1.67 g/cm³ have been reported for tails with an sg of 2.6, changes in sg and size can vary the compacted density. Hydraulic transport The transport of tailings in a slurry is extremely cost effective when sufficient altitude is available for gravity transportation, and water is inexpensive. On smaller dams the practise of pumping tails along the crest to relocatable cyclones is energy efficient, however it can be difficult to ensure the sands have the correct geotechnical properties for dam integrity with multiple remote hydrocyclones. As dams become larger centralised cyclone stations ensure better control of the sand quality at the expense of transport, pipeline losses in high density sand transport pipelines are significant and wear can be high. Both methods rely on good drainage in the dam to ensure water required in the transport is effectively removed to prevent liquefaction. Hydraulic transport is efficient and economical when water is readily available, but it poses the greatest risk in tailings dams. Sand Yield and Quality With changes in mineral processing, ore types and declining grades there has been a trend to grind the ore more finely. In addition, the size of mining operations and their tailings dams is increasing resulting in higher lifts, larger dams and a higher quality of sand requirement. This trend is reducing the ratio of sand yield from the tailings and pushing the requirement for better control of sand quality. Particularly when clays are present the control of hydrocyclones is limited. Where the requirements and quality control of a cyclone sand construction material for the dam wall requires sharper cuts that contain less slimes, current technologies might struggle. Operators may be required to utilise borrow material at higher costs for the construction of tailings dam walls when suitable properties cannot be achieved with
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