EGL Ref: 9215 23 June 2022 Page 51 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 9.1.1.1. Martha Open Pit The use of Martha Open Pit as a TSF was not considered due to its close interaction with the Martha Underground Mine and ore resources surrounding it, and potential health and safety risks. 9.1.1.2. Gladstone Open Pit The use of Gladstone Open Pit as a TSF is a realistic option. 9.1.2. Underground paste backfill Underground backfill using paste is a potential option for tailings disposal. To create the paste tailings are thickened and mixed with a binder, like cement. Pastes are expensive due to high capital cost and high operating costs including binders, dewatering and pumping. Pastes are placed in the ground without a liner; however, cement binders will reduce acid generation until groundwater rises following mining. If pastes are not properly mixed and placed, that can present a hazard if mining occurs close to previous backfilled stopes, which could result in sterilising resources. The benefit of paste backfill is that it utilises the underground voids created in the mining process for tailings storage. Underground paste backfill is an option. However, it would only be able to provide a small proportion of tailings storage requirements for the Waihi Operation. This is because not all the ore is from underground mining and the density of paste backfill is lower than the insitu mined rock. The feasibility of underground paste depends on costs, risks around ground water contamination, and sterilisation of resource. At the Waihi Operation the use of paste backfill has not been necessary because there has been surplus mine pit overburden material available to backfill voids associated with underground mining. Pumping of pastes present a technical challenge and paste plants are typically located close to the deposition location for this reason. 9.1.3. Central thickened discharge Thickened tailing can be discharged from the centre of a TSF with the surface of the tailings sloping down to the perimeter. The tailings are stronger than conventional slurry tailings and a smaller embankment is required. Central thickened tailings are best suited to flat terrain where circular shaped facilities can be constructed, dry climates, and where there is low earthquake hazard. These conditions do not exist at Waihi and combined with higher capital cost are not considered practical or to offer any additional benefit compared to downstream construction embankments with conventional slurry tailings.
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