EGL Ref: 9215 23 June 2022 Page 43 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. water, and water evaporation losses are less. The last two factors are important in dry climates where water resources are scarce. Thickened tailings are typically discharged centrally from the TSF with the tailings sloping down to a perimeter embankment. This is referred to as “central thickened discharge”. They are better suited to flat terrain, areas where there is low rainfall and water is scarce, and where there is limited material available for construction of the perimeter embankment. In a heavy rainfall environment like Waihi separate water storage ponds would need to be provided to store runoff and they could result in an overall larger footprint than conventional TSFs. There can be erosion of tailings in periods of heavy rain, and this needs to be allowed for in the design of the TSF. They can also be susceptible to liquefaction or strength loss, requiring underdrainage to reduce the risks of liquefaction and the perimeter embankment to be higher to contain any earthquake induced slumping of the tailings. The infrastructure and equipment to produce thickened tailings have significantly higher capital cost than conventional slurry tailings. The terrain, high rainfall, earthquake hazard and high capital cost are factors that make them less desirable than conventional slurry tailings at the Waihi Operation. Another important factor is the Waihi Operation has had large quantities of mine pit overburden to dispose and this has allowed construction of conservatively designed downstream construction embankments that can safely contain slurry tailings. 7.1.3. Paste Paste are tailings in which the water content is reduced more than thickened tailings to produce a material of higher solids content and strength. This requires additional dewatering of the tailings and requires positive displacement (piston) pumps to transport the paste to where it is disposed. The cost of producing and pumping paste is higher than thickened tailings. Paste tailings are sometimes used to fill the voids associated with underground mining. Cement is mixed into the paste along with other admixtures to result in higher strength and pumpability. 7.1.4. Filtered tailings Filtered tailing are tailings in which the water is removed by filters and mechanical presses so that they can be mechanically handled. The tailings are still in a moist condition and susceptible to strength loss with the addition of water (e.g., rainfall). In an ideal situation no embankment is required to retain filtered tailings. They are transported by truck and spread and compacted by track rolling with a bulldozer. The resulting mound of tailings is typically referred to as a “dry stack”. A perimeter bund is required to capture and direct runoff to collection ponds. There are significant capital costs associated with producing filtered tailings. They are not suited to areas of high rainfall where earthquakes are possible. In prolonged wet periods the tailings can soften, and it can be very difficult to traffic on the surface and place additional tailings. A conventional TSF is required to allow continuous operation of the Processing Plant at these times and to allow for breakdowns and maintenance of the equipment used to produce filtered tailings. In areas of high rainfall, the tailings at low elevations could become saturated and be susceptible to liquefaction if subjected to strong earthquake ground motions. To mitigate
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