Methodology

Scope 1 (Direct) Greenhouse Gas Emissions are derived from sources that are owned or controlled by the reporting organisation - often to the 'minegate' boundary or smelter/refinery gate for integrated operations. They typically include emissions from consumption of diesel in haulage trucks or onsite power generators along with emissions from other sources such as natural gas, aviation fuel, fugitive emissions, use of explosives, etc. Intensity numbers are calculated as total tonnes of CO2e divided by zinc metal equivalent production.

Scope 2 (Indirect) Greenhouse Gas Emissions are generated at sources owned or controlled by another organisation. This is generally electricity purchased by the mine (or smelter for integrated assets) from the grid. The Grid Carbon Factors sheet in this model provides a high level summary of the grid emission data.

Downstream emissions for mining include a substantial portion of Scope 3 (Other Indirect) Greenhouse Gas Emissions. These typically include downstream emissions such as freight (land and ocean), port loading and smelting to produce finished metal - typically LME zinc. Intensities for downstream are divided by the primary metal (zinc) in question with by-product metal in concentrate given a 'free ride'. By including these downstream emissions this allows mining operations to be placed on the same basis with mines that are integrated with a smelter complex.

Zinc smelting technology: The dominating smelting technology is Electrolytic, which includes RLE (Roaster Leaching Electrowinning) and Direct Leach. The electricity consumption of this technology can be as high as 4.9MWh/t Zn which explains the high CO2 emission levels. Therefore for electrolytic smelters to have low CO2 intensities they require access to low carbon electricity like hydro power. ISF (Imperial Smelting Furnace) is, as expected, more intensive in combustible consumption, which results in higher scope 1 emissions. In general, zinc smelting is very energy intensive, in comparison to other processes (like copper, producing around 58% of emissions on a total tonnes basis (or 70% based on CO2e intensity).

Carbonate in ore: We have made some assumptions regarding carbonate content in ore and have assigned emission factors based on deposit type as to the amount of of CO2e liberated during smelting. Although this is relatively high level analysis we understand that this can be material for some operations. This is incorporated into the smelter emissions and range between 0-0.3 tCO2e/t Zn. For some integrated mine-smelter operations, this may already be included in their reported smelter emissions.