WAI-985-000-REP-LC-0056 Revision 0 Page 13 of 21 Waihi North Project – Tailings and Rock Storage – Functional Need Assessment thickened and split tails are pumped to an underground paste plant for water removal and paste creation as an alternative to creating paste at the surface). This option is cost prohibitive and introduces several environmental considerations including: • Risk of leakages • Increased requirement for mine water treatment through the existing (upgraded) water treatment plant. While GOP TSF (slurry tailings) forms part of the preferred (and selected) option for WNP, consideration was also given to storing tailings in the Martha open pit or as a paste in GOP. The latter would have combined with paste-fill into WUG to increase storage capacity to the requisite volume but would have required complicated and ultimately impractical scheduling to achieve deposition of paste into GOP at the same time as mining it. The same considerations ruled out paste-fill into Martha Open Pit. Co-disposal of tailings and overburden rock placed on top of TSF1A and TSF2 and in NRS Co-disposal of tailings and overburden rock as a mixed waste is an option which would require additional thickening and a filter press plant to dewater the tailings to achieve “dry tailings” (i.e. tailings with enough water removed to make it dry enough to be placed in a truck as a wet soil). This enables the tailings to be moved mechanically with dump trucks and mixed with the overburden rock. A slurry tailings storage facility would still be required for periods when the filter press is unavailable (break downs and maintenance) to allow for continued ore processing. Mixing with overburden rock has the benefit of the rock strength and trafficability with dump trucks. Depending on the situation, the rock to tailings ratios would need to be between 8 to 1 (11%) and 4 to 1 (20%) by weight. This co-disposal option has been investigated and assessed to be impractical for WNP for the following reasons: • No precedent exists for an operation of this scale in a high rainfall environment. • High rainfall (average annual of 2100mm) will result in saturation of tailings and make disposal impractical and un-trafficable for notable periods of time. • Insufficient overburden rock to achieve satisfactory strength. • Fine grind size of tailings at Waihi makes dewatering challenging. • Conventional slurry tailings storage will still be required in periods of wet weather and when the filter plant is unavailable (filter plants do not have 100% availability). • Complex stockpiling cell arrangement would be required at Waihi to manage the variability in rock sources (PAF, NAF, wet and very weak). • Increased likelihood of acid generation from partially saturated tailings and mixing with rock. Exposure to oxygen is increased with co-disposal. • Increased environmental, geotechnical, and economic risks associated with unproven technology in the Waihi setting.
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