| Discharges to Air and Associated Mitigation Methods | Waihi North Project Air Discharge Assessment - Waihi Surface Facilities | 4396626-1331619777-99 | 16/06/2022 | 24 During blasting operations, the area of the mine in proximity to the blast is evacuated and machinery emissions cease, therefore the emissions generated from diesel engines, vehicle movements and excavation will not occur at the same time as blasting emissions. 3.4.2 Discharges from above-ground blasting OGNZL has not carried out any specific monitoring of the discharges to air from above-ground blasting as it is impractical. The nature and quantity of the discharges will be the same as the discharges to air from underground blasting, but the discharges will be dispersed into the atmosphere at the source. Concentrations of contaminants are expected to decrease rapidly with distance from the source and to be well below guideline levels where members of the public may be exposed. OGNZL manages and plans each blast taking into consideration the likely effects on nearby residents, the size of the blasts and weather conditions. 3.5 Vegetation Removal Vegetation will need to be removed in some locations prior to earthworks commencing. There will be no burning of vegetation on the WNP sites. 3.6 Processing Plant Emissions from the Processing Plant will continue to be managed in accordance with site’s Air Quality Management Plan. There are no material discharges to air of contaminants from the thickeners and leach tanks and hence, the addition of a new larger thickener and three additional leach tanks at the Processing Plant are not expected to result in any additional air discharges. Small amounts of hydrogen cyanide (HCN) are volatilised to the atmosphere from leach tanks. Emission rates are controlled by maintaining leach tank pH above 10, with HCN gas levels diminishing as pH rises. At a pH of 11 and above, no HCN emissions would be expected20. Provided the pH of the tanks are operated in accordance with the management plan, emissions from the additional leach tanks are highly unlikely to result in an exceedance of HCN ambient air quality criteria limits (refer Table 4-1). Therefore, discharges from the additional leach tanks would not be expected to have any adverse health effects. 3.6.1 Metal Emission Rates Metals are emitted from the electrowinning and furnace stack when smelting is occurring. Annual metal emission rates can be expected to increase with the increase in ore processing rates once the proposed WNP is commissioned. Emissions of metals from the electrowinning and oven/furnace stack were measured by WaterCare Laboratory Services in February 202121. Table 3-2 summarises the reported emission rates and discharge concentrations for the metals for which ambient air guidelines are published (refer Table 4-1). 20 Australian Government Department of Environment and Heritage. NPI Emission Estimation Technique Manual for Gold Processing Version 2.0, 2006 21 WaterCare Laboratory Services. Oceana Gold Waihi: Selected Metals – Electrowinning and Gold room Furnace Stacks, February 2021.
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