| Processing Plant Emissions | WNP Processing Plant Air Discharge Assessment | 4397169-66885702-113 | 16/06/2022 | 5 Sensitivity: General For this assessment, a maximum annual average mercury emission rate has been derived assuming a total ore 2.25 MTPA annual and GOP ore processing rate of 0.611 MTPA. Remaining ore which is processed (1.640 MTPA) is assumed to 1.12 MPTA of WUG ore and 0.52 MPTA is MUG ore. The ratio of WUG and MUG ore is based on the ratio processed in Year 5 of the project. The Processing Plant’s estimated annual mercury emissions for the proposed processing rate is shown in Table 2-6. Approximately 96% of the peak annual mercury emissions are associated with the processing of GOP ore. The derived OGNZL emission rates are based on the plant’s existing configuration. Currently, no emission control equipment has been installed on site. However, OGNZL proposes to install a retort oven prior to processing GOP ore which would substantially reduce the mercury discharges from the main processing oven and furnace as predicted in the OGNZL mass balance analysis. The retort oven would be used to remove the mercury from the gold bearing sludge. The sludge is dried in the oven under a partial vacuum at a temperature of approximately 750°C. The mercury which is present in the sludge is then vaporised. The vaporised mercury is drawn through a water-cooled retort condenser which condenses mercury back into a liquid state which is then collected. A small proportion of the mercury is discharged to the atmosphere in the main oven’s exhaust gas. The mercury content of the sludge entering the oven retort has been estimated from the OGNZL mass balance analysis to be 0.303 mg/tonne3 for MUG ore, 23.1mg/tonne4 for WUG ore and 383 mg/tonne5 for GOP ore, based on the mercury emitted from the oven and furnace (refer to Table 2-2). Emissions from the retort oven have assumed that 99% of the mercury in the sludge is vaporised in the oven. Of this mercury, 99.5% is assumed to be recovered in the retort condenser (i.e. 0.5% is emitted to the atmosphere). The assumed emission control efficiencies are based on those published by the USEPA6 and WHO/Minamata Convention7. The 1% of the mercury in the sludge which is not vaporised in the oven is subsequently assumed to be emitted to atmosphere when the dry solids from the retort oven are heated in the furnace. No additional emission control system is currently proposed for the carbon regeneration. OGNZL is seeking to increase the Processing Plant processing rate from 1.25 MTPA to 2.25 MTPA. The maximum GOP ore processing rate is expected to be 0.611 MTPA. Table 2-6 shows the plant’s predicted annual mercury emission rate for the ore processing rate of 2.25 MTPA when a retort oven is installed. 3 0.303mg/tonne = 0.116mg/tonne + 0.188mg/tonne 4 23.1mg/tonne = 14.3mg/tonne + 10.9mg/tonne 5 383 mg/tonne = 145.8 mg/tonne + 237.1 mg/tonne 6 United States Environmental Protection Agency (USEPA). 2001. Draft Mercury Mass Balance Emission Factor Estimates for Gold Processing Facilities. 7 United Nations Environmental Programme/Secretariat of the Minamata Convention on Mercury. 2019 Guidance on the Best Available Techniques and Best Environmental Practices.
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