| Dispersion Modelling Methodology | WNP Processing Plant Air Discharge Assessment | 4397169-66885702-113 | 16/06/2022 | 14 Sensitivity: General Table 5-1. Summary of the modelled annual average emission scenario Source Scenario 1 (existing) Scenario 2 (proposed) Electrowinning Uncontrolled Uncontrolled Oven Uncontrolled Controlled* Furnace Uncontrolled Controlled* Carbon Regeneration Uncontrolled Uncontrolled *Assumes a retort oven has been installed 5.4 Modelled Mercury Emission Rates 5.4.1 Annual average mercury emission rates The modelled annual average emission rates conservatively assumed an annual ore processing rate of 2.25MTPA, of which 0.611 MTPA is GOP ore. The modelled emission scenario is therefore a theoretical worst-case emission scenario as the annual ore processing rate is not expected to exceed 1.76MPTA. The modelled annual average mercury emission rates for the emission scenarios are shown in Table 5-2. The modelled annual average emission rate assumes the total amount of mercury emitted from each of the modelled emission sources per year is evenly spread over the simulated year. Table 5-2. Summary of the modelled annual average mercury emission rates (g/s). Source Scenario 1 emission rates (g/s) Scenario 2 emission rates (g/s) Electrowinning 0.00017 0.00017 Oven/Furnace 0.0082 0.00012 Carbon Regeneration 0.0039 0.0039 Total 0.0123 0.0042 5.5 Non-Mercury Metal Emission Scenario and Emission Rates A screening approach has been used to assess the discharge of non-mercury metals from the Processing Plant. Screening modelling is used to assess whether emissions to air could theoretically have an adverse effect and whether a more detailed assessment would be required. For this assessment, the modelled metal emission rates assume that discharges from the electrowinning and combined oven/furnace stacks are occurring 24 hours per day for every day of the simulated year. This approach is highly conservative and will substantially overestimate the number of hours per year discharges from the plant will occur and therefore, the contribution of site emissions to ambient air contaminant concentrations. The modelled metal contaminant emission rates have been based on the results of the 2021 emission testing programme (refer Table 2-7). The modelled emission rates are presented in Table 5-3 and assume that discharges are occurring continuously. The assessment has assumed all the emitted chromium is in the more toxic form (i.e chromium VI). The potential effect that the proposed retort may have on limiting the emission rate of the more volatile metals has also not been considered in the assessment.
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