Life cycle assessment and product carbon footprint for SHG zinc production

The industry’s environmental impact has moved into focus at global level. Regulators, supply chains, and the financial sector need credible, standardized, and transparent information for multiple reasons. These include regulatory targets for limiting emissions, downstream user optimizations, and developing science-based emission reduction strategies. By characterizing environmental impacts, companies can satisfy stakeholder demands and measure progress towards goals. In this paper, an introduction to life cycle assessment (LCA) for reporting on environmental impacts, including the product carbon footprint as global warming potential is given. Average results from the most recent global LCA update for special high-grade zinc production are summarized. Resulting carbon footprint information is then used to derive the zinc industry’s baseline carbon footprint as the starting point for the decarbonization roadmap for the global zinc industry.


Introduction
Modern life and increasing living standards have unintended consequences, among these emissions to the environment.Life Cycle Assessment (LCA) over the past two decades has developed into the widely used tool to quantify the environmental footprint of a process, a product, or a service.Specifiers and designers use environmental footprint information for material selection to minimize impact of future products.Regulators increasingly set targets based on LCA information.For life cycle impact assessment (LCIA) various impact categories are used.The focus today lies on the product carbon footprint expressed as global warming potential (GWP).This paper will provide an overview of the most recent global average LCA for primary, special high-grade zinc production based on 2020 and 2021 industry data.It will then be demonstrated how results feed into the derivation of the baseline carbon footprint of the zinc industry and of carbon abatement scenarios considering both Scope 1 (direct) and Scope 2 (indirect from purchased electricity) and selected Scope 3 emissions.

Life cycle assessment
LCA quantifies the environmental impact caused by a material, product, process, or service over its life cycle from cradle to gate (typical for basic raw materials and commodities) or cradle to grave (typical for products and services).LCA methodologies are by the International Organization for Standardization (ISO 14040 [1] and 14044 [2]).More specifically, for the carbon footprint the Green House Gas (GHG) Protocol "Product Life Cycle Accounting and Reporting Standard" [3] is available.It is based on International Organization of Standardization (ISO 14040 and 14044) but limited to carbon footprint calculation only.
There are four components to a typical LCA study (Figure 1): 1) Goal and scope, 2) Life cycle inventory, 3) Life cycle impact assessment, 4) Interpretation of results.

Life cycle assessment for zinc
Since 2009, the International Zinc Association (IZA) provides global average LCA information for primary zinc production [4] (cradle-to-gate, Figure 2), which established an environmental profile [5] for zinc that represents geographic differences in mining, smelting, energy use, and transportation.This Cradle-to-Gate LCA model is based on anonymized industry data from zinc mines and smelters worldwide.
Frequently, this environmental profile for special high-grade zinc (SHG; 99.995 % zinc) is updated.With each update, the number of participating companies increases.As a result, life cycle inventory data for primary (SHG) zinc production today are representativity for the sector.IZA submits global and in some cases regional average life cycle inventory data to relevant databases such as GaBi [6], Ecoinvent [7], or PEF [8].

System boundaries of zinc production
Natural zinc ores usually contain 5-15% of zinc as zinc sulfide.Accompanying elements are lead, copper, silver, cadmium, and sometimes indium, and germanium.Already at the mine site, zinc ores are crushed, ground, and concentrated by flotation.Zinc ore concentrates have a zinc concentration of about 55%.
To produce primary zinc of special high-grade quality (SHG; 99.995%), the combination of roasting, leaching and electrowinning (RLE) is the widely used methodology.Accompanying elements are concentrated in co-products such as sulfuric acid or metals concentrates for recovery in other metals smelters.More than 90% of all zinc worldwide is produced using RLE technology varying in details and single process steps.Pyrometallurgical process types such as the Imperial Smelting furnace are used in other cases.
In primary zinc production, secondary raw materials e.g.steel mill dusts with an enriched zinc concentration (Waelz or crude zinc oxide) can be processed together with zinc concentrates.
Figure 3 shows the system boundaries for the most recent LCA update for SHG zinc based on industry data from 2020 and 2021 [9].Participating companies reported relevant input and output data for the processes as included in the system boundaries (Figure 3).As a result, global average LCIA and LCIA information is available, and companies receive their company-specific environmental footprint information benchmarked against the average.

Results
Overall results for SHG zinc, zinc ore concentrate, and Waelz oxide are presented in Table 1.Based on zinc content, 86% of SHG production is sourced from concentrate and 14% from Waelz oxide.[9] Table 1.Life cycle impact analysis (LCIA) results per metric ton [9]. Figure 4 shows the results for SHG zinc production, broken down by process steps.It can be seen that smelting often accounts for most of the burden (47% and 61%; excluding ADPe).Concentrate burdens range from 24% to 52%, while Waelz oxide burdens range from 1% to 15%.Finally, intermediate transport of concentrate and Waelz oxide can range from 0% to 7%, depending on the impact category.Based on zinc content, 86% of the zinc is coming from concentrate and 14% from Waelz oxide.[9] Figure 4. Relative results of the life cycle impact assessment for SHG zinc production [9].
Detailed results for the smelting process are shown in Figure 5.All results, except from ADPe, and Blue Water Consumption (BWC) are driven by electricity consumption.Direct emissions and credit from sulfuric acid production are the second highest impact contributors.Smelting contributes 56% to total ADPe, of which the main impact is coming from the production of raw materials (mainly Zinc oxide).The consumption of electricity impacts the Blue Water Consumption impact category higher than the water consumption itself, mainly because of the hydro power sources.Direct emissions of SO2 from the smelting process are also a noticeable contributor to overall AP and POCP impacts.[9] Figure 5. Relative life cycle impact results for smelting and refining in SHG zinc production [9].

Representativity
The most recent LCA update for SHG zinc production provided by IZA [9] includes primary data from 25 mines and 24 smelters from 10 companies, which cover 2.5 million metric tons of mined zinc (30% of world mined zinc production) and 3.97 million metric tons of SHG zinc (31% of the world zinc production).The best covered region was Europe (including Norway) with up to 94% coverage of the regional production while Asia was underrepresented (13% of Asian zinc production).
Exemplarily, the variation of environmental footprints between zinc smelters is depicted in Figure 6.The anonymized carbon footprints for SHG zinc produced in participating smelters are shown as global warming potential (GWP) in kg carbon dioxide equivalents (CO2e) per t of SHG zinc produced.Huge variations become visible spanning over a range from below 1000 up to 9000 kg CO2e/kg, depending largely on the type of electricity supply (renewables versus coal-fired power plants (Figure 6).

Chinese zinc production in the LCA subsection
Based on the recent IZA LCA update for SHG zinc (2020/2021), three Chinese mines and three Chinese smelters participated.Due to conditions for confidentiality not being satisfied (number participating companies), information on an average environmental footprint for the Chinese zinc industry is not yet viable.However, the global average can be compared with the result of study published by Qi et al. (2017; Table 2).The global carbon intensity for SHG zinc production is currently estimated to be 3.8 kg CO2e per kg SHG zinc, while the Chinese average published by Qi et al. (2017) was 6.12 kg CO2e per kg SHG zinc.

Company-specific product carbon footprint
Companies are asked for the specific product carbon footprint of zinc produced at their individual sites.For reference, companies and customers may compare their site-specific product-specific carbon footprint to the global average for marketing purposes.For this purpose, IZA launched the Carbon Footprint Guidance [13] to harmonize calculation and reporting.
This guidance is an excerpt of the LCA methodology and model used for calculating the full environmental footprint in IZA's LCA update projects with the product carbon footprint for special high-grade zinc.Version 1.0 of the ISO-compliant reporting guidance for GWP reporting has been recognized by the London Metals Exchange (LME) as the single methodology to be used when submitting carbon footprints for registered zinc brands to the LMEpassport (Figure 7).Anticipated update (version 2.0, 2023) will account for carbon footprint allocation to secondary raw materials as a substitute for zinc ore concentrates [14] and seek alignment with end-use product sectors requirements such as e.g., the Pathfinder Framework [21] by the Partnership for Carbon Transparency (PACT).PACT represents a large group of companies from the automotive and various companies.

Decarbonization Roadmap for the global zinc industry
The zinc sector decarbonization roadmap has characterized the baseline carbon footprint and identified goals for net-zero.It builds on zinc stock and flow analysis, future demand and availability scenarios, and carbon footprint information from IZA LCA updates and other sources [5][15][16].As a result, the baseline carbon footprint of the global zinc industry (2019), was estimated to be 59 million tons (Mt) CO2e (0,12% global GHG emissions; [17]).The total GHG emissions were calculated relative to applying global mining, recycling, and smelting emission factors for production amounts -1030 kg CO2e / t Zn concentrate production, 2500 kg CO2e / t Zn smelting, 4030 kg CO2e / t Zn Waelzing, [22] and 78 kg CO2e / t Zn from remelting [18], respectively (Figure 8).Plausibility was checked with the zinc database by Skarn Associates [19].
For zinc mining, 73% of all GHG emissions are associated with electricity consumption.Further 23% of emissions come from diesel fuel consumed by mine trucks.About 90% of all GHG emissions from zinc smelting come from the use of electricity due to a majority of zinc smelters using the roastleach electrowinning process.More than 50% of all zinc today is produced in China.Due to the importance of purchased energy sourcing, governmental decarbonization strategies for the power impact the success of industry initiatives.
Looking at zinc production technologies used today, Figure 9 shows the broad range of sitespecific carbon footprints including information on tonnage and production technology.For sites producing zinc pyrometallurgically, via the Imperial Smelting Furnace or vertical retorts, the carbon footprint is related to the use of carbon as a reductant.Carbon abatement strategies for these technologies will face their own challenges.It is estimated from 1.2% global domestic product growth that the baseline carbon footprint of the zinc industry of approximately 59 Mt CO2e annual emissions (~%0.12 global artificial emissions) continuing with a "business-as-usual" scenario would increase to approximately 95 Mt by 2050 (17.1 Mt associated with Scope 1 emissions).Based on current electrical utility decarbonization trends reported by the International Energy Agency [20], the impact of purchased energy will decrease by 13.4 Mt CO2e by 2030.Assuming that country ambitions are met between 2030 and 2050, the impact of purchased energy is expected to decrease by an additional 58.1 Mt CO2e.Consequently, the zinc industry is responsible for decreasing its Scope 1 footprint by 15.4 Mt, thus achieving 90% total Scope 1 and 2 reductions by 2050.

Summary and conclusions
Lifecycle assessment is the established tool for regulators, specifiers, and designers used for calculating environmental impacts of products, processes, and services.Providing credible and transparent life cycle information has become key for market access.
The International Zinc Association regularly updates the global average environmental profile for special high-grade zinc and submits the associated data sets to relevant LCA data bases.
Increasingly, end use product sectors demand company specific information on environmental footprints including the carbon footprint as global warming potential.Companies can use the IZA carbon footprint calculation and reporting guidance when reporting information to the London Metals Exchange LMEpassport.With this guidance, IZA contributes to harmonizing calculation and reporting thus enhancing transparency, comparability, and credibility for the whole zinc sector.
Carbon footprint and mass flow analysis information was used for developing the Decarbonization Roadmap for the global zinc industry.Scenario analysis shows that impact opportunities for decarbonization lie in decarbonizing electricity production and coal-based reduction processes used in zinc production and recycling such as vertical retorts, Imperial Smelting, and Waelzing.

Figure 6 .
Figure 6.Global average and anonymized Global Warming Potential (GWP) for special high-grade zinc production sites participating in the LCA update based on 2020 and 2021 industry data [9] Includes zinc mining, concentration, crude zinc oxide production, and smelting/refing.

Figure 8 .
Figure 8. Baseline carbon footprint and carbon intensity for the global zinc industry, reference year 2019 [14].

Table 2 .
[9]parison of the carbon footprint of SHG zinc as reported in literature and as a result of the IZA update of the global average carbon footprint of SHG zinc production based on 2021 industry data[9].