Comprehensive environmental assessment of typical Chinese industrial metal production processes

Summary

To promote energy conservation and emission reduction, a quantitative assessment of pollutants from metal production is imperative. However, conventional life cycle assessment cannot provide detail environmental impact assessment for pollutants (e.g., waste water, waste gas, and solid waste) in metal productions. In this research, by analyzing 39 kinds of pollutants, a comprehensive environmental assessment (CEA) was conducted for 31 kinds of typical industrial metal-production processes in China. For CEA, Li, In, Cr, K, and Hg have higher environmental impact. Under global warming potential (GWP), 5 metals with higher carbon emission impacts are as follows: Nb > Ta > Mo > Cr > Au. A comprehensive analysis of CEA and GWP revealed that Cr has the most severe environmental impact (CEA: 74.99; GWP: 48500 kg CO₂ eq./kg), while Ca exhibits the minimal influence (CEA: 0.05; GWP: 1.02 kg CO₂ eq./kg). Overall, this research offers valuable insights to optimize relevant production processes.

Graphical abstract

Highlights

• •Comprehensive environmental assessment was evaluated for metal production processes
• •39 kinds of pollutants in waste water∖gas∖solid were analyzed for metal production processes
• •Carbon footprint analysis reveal that Nb, Ta, and Mo production have higher carbon emission
• •Comprehensive analysis pointed out that Li, K, In, Hg, and Cr had more environment impacts

Environmental engineering; Materials science; Materials processing

Introduction

Metals have garnered widespread attention from global media and political arenas, particularly as they are pivotal in high-technology fields and various industries.^1,2 The indispensable value of high-purity metal products in electronics and energy technology has attracted global interest.³ Moreover, metal mining is a crucial source for energy production,⁴ communication systems⁵ and transportation networks.^6,7 The steadily rising global demand for metals has contributed to a corresponding increase in environmental pollution associated with industrial metal-production processes (IMPPs).^8,9 Mining and processing of metal ores not only contribute to severe environmental pollution¹⁰ but also result in increased carbon emissions while depleting energy and water resources.¹¹ For instance, the production of heavy metals has led to air pollution due to their inherent properties. Meanwhile, these pollutants decompose or disappear naturally at a slow rate and tend to accumulate in the environment.^12,13,14 Base-metal production generates large amounts of solid waste, contributing to air and water pollutions.¹⁵ It is essential for the entire industry to prioritize environmentally friendly and efficient of metal production processes while minimizing reduction of their environmental impact. Furthermore, various types of metals used in production processes have complex environmental impacts.¹⁶ Therefore, evaluating the environmental impact of pollutants during existing IMPPs can help mitigate the overall environmental effects.

The summarized results of the environmental assessment methods and perspective for IMPPs (Table 1) demonstrated that the current environmental impact assessment methods was majorly relying on life cycle assessment (LCA).^26,27 For instance, Zhu et al.²⁸ utilized LCA to evaluate the energy consumption and environment impact of Al production process. Lu et al.²⁹ applied LCA to assess the environmental impact of the Cu production process. Farjana et al.²⁰ evaluated the environmental impacts of Au and Ag production processes from the perspective of human health, ecosystems, and resources using the IMPACT 2002 + and the international life cycle reference data methods. However, the environmental impact assessment of waste water, waste gas, and solid waste discharges was often neglected in IMPP assessments.^30,31 Many investigations focused solely on the input materials, energy consumption, and discharge for certain substances in IMPPs (e.g., carbon emissions,³² greenhouse gases (GHGs)^33,34,35 and the affection of acid rain). Current research achievements were inconsistent with the actual concerns of plant operators regarding the waste discharge and environmental impacts. Therefore, it is crucial to consider the comprehensive environmental impact of pollutants in the IMPP. This research not only provides the most effective approach for enterprises to reduce production costs but also serves as a necessary method promote enterprises to meet environmental protection discharge standards.

Table 1.

The summary of the IMPPs assessment method and assessment perspective in the environment

Number	Metallic Material	Method	Scope	Discharged pollutant	Conclusion	References
1	Au, U, rare earth Elements, Si, Co, Ni, Zn, Cu, Mg, Mn, Pb, B, Al2O3, Zr,	LCA	The processing levels of the mining industry	No	Gold showed the highest impact on the global-warming potential (GWP), terrestrial acidification, water resource depletion, and land use. Coal had the highest GWP. The need to conduct more LCA studies on strategic metals for the clean energy transition to reach the net zero carbon target was also highlighted.	Rachid et al.17
2	V	LCA+ CEA	Three typical V2O5 production processes	Yes	The best process combined cleaner production and end-of-pipe treatment with minimal pollutant discharge and optimal benefits. Raw material section and vanadium slag were the key processes and substances, respectively.	Zhang et al.18
3	V2O5	CEA+ Life cycle costing	Three V2O5 production processes	Yes	Under CEA, proper pollutant control improved material recirculation percentage and reduced the expense of waste treatment.	Gao et al.19
4	Cu, Au, Ag, Pb, Zn	LCA	Cradle-to-gate environmental effects of the Au-Ag-Pb-Zn-Cu beneficiation process	No	Out of the five metals considered, Au and Ag beneficiation impacts the most while Pb and Zn beneficiation impacts the least. Au beneficiation had the most impact on the category of climate change and ecosystems. Electricity grid mix had a dominant effect over the fossil-fuel mix.	Farjana et al.20
5	Rare earth elements (Y, Eu)	LCA+ Carbon footprint analysis	Acid extraction and solvent extraction recover metals	No	The solvent extraction method had significantly higher extraction efficiency, even though the acid extraction method had a lower carbon footprint.	Hu et al.21
6	He, Li, Be, B, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, etc. (63 kinds)	LCA	The cradle-to-gate environmental burdens	No	Except for a few metals, the environmental impacts of the majority of elements were dominated by the purification and refining stages in which metals were transformed from a concentrate into their metallic form.	Nuss et al.22
7	Zn, Al, Cu, In, Ga, Cd, Ge, In, Mo, Se, Te.	LCA	Mining, smelting, and refining stages.	No	The extraction of metals from primary sources, but Al, Cu, In, Ga, and Se were also recovered from recycled material.	Fthenakis et al.23
8	Cu, Ni, Al, Pb, Zn, steel, stainless steel and Ti	LCA	Various metal production processes	No	As higher-grade reserves of metallic ores were progressively depleted, mined ore grades will gradually decrease. Low energy-intensive metals such as steel may replace high energy-intensive metals such as Al is plausible.	Norgate et al.24
9	A total of 62 metals, excluding certain rare metals.	LCA+ Carbon footprint analysis	Metal production process	No	Compared with other metals, the carbon emissions caused by precious metals are more significant, and the five metals with the highest GWP are Rh, Pt, Au, Ir and Sc.	Gao et al.25
This research	Li, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Cd, In, Sn, Sb, Ba, Ta, W, Au, Hg, Pb, Bi	CEA+ LCA	The IMPPs with a wide range of applications (from resource mining to factory delivery)	Yes	For the CEAW, the first few were Cr > Li > Co > Mn > In; for the CEAG, the first few were In > Co > Au > Li > Ag; for the CEAS, the first few were Hg > Cd > Au > Ag > Bi; the first 10 metals with greater CEA in IMPPs were Li > In > Cr > K > Hg > Co > Au > Ag > Bi > Cd; for the GWP, the first 10 metals were Nb > Ta > Mo > Cr > Au > Ni > Ag > Sn > Mn > W.

The environment impact in IMPPs main comes from waste water, waste gas, and solid waste, which may contain toxic, harmful and dangerous components. A comprehensive environmental assessment (CEA) method was developed to assess the environmental impact of IMPPs.^36,37,38 For instance, in addressing harmful pollutant discharge in the V₂O₅ production process, Zhang et al.¹⁸ utilized CEA to evaluate the environmental impact of As, V⁵⁺, suspended solids (SS), SO₂ and other pollutants, and pointed out that process optimization should focus on sulfuric acid input during production. Waste water emerges as the most notable waste, with the main pollutants in the V industry being Na⁺, SO₄²⁻, NH₄⁺-N, Ca²⁺, V, Cr, etc.¹⁹ Through the scientific rigor and applicability of CEA, the most serious pollutants can be identified and managed on a macro-scale in IMPPs. Furthermore, CEA has also been applied to assess the environmental impacts of light-emitting diodes (LEDs). The majority of environmental impacts occur in the chip preparation section, with major pollutants including chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), SS, and NH₄⁺-N.^39,40 CEA conducted on LEDs determined the phase of severe contamination and contributed to the criticality evaluation. However, previous researches have predominantly examined IMPPs in isolation, with limited comparative research on the environmental impacts of pollutants generated during the multiple IMPPs. Therefore, it is essential to conduct comprehensive environmental impact assessment in IMPPs for the development of green and low-carbon.⁴¹

In this research, all typical metals in the IMPPs are analyzed used the CEA and carbon footprint analysis method based on the LCA to comprehensively assess the environmental impact from the perspective of micro and macro. The environmental impact of the whole metals production process was studied through CEA, while a carbon footprint analysis⁴² was utilized to study the environmental impact of GHGs. A multi-objective assessment was employed to determine the optimal type of metal. The research serves as a valuable reference for the optimizing of IMPPs. This evaluation method proves highly practical and can effectively guide the environmental impact assessment of the global metal-production process.

Results and discussion

Analysis of pollutants in metal production

The system boundary, functional unit and data sources of this research are illustrated in Figures 1, S1, and S2 and Tables S1 and S2. The original dataset was initially organized to refine the pollutant discharge data from all IMPPs, with individual production processes excluded if they produced unique pollutants and pollutant types streamlined for more accurate evaluation.

Figure 1.

The CEA methodology and system boundary

Nine types of Class I pollutants in water (i.e., Hg, Cd, Cr, Cr⁶⁺, As, Pb, Ni, Be and Ag) were selected for analysis. The following data were also summarized (Figure 2 and Tables S3–S5): (1) four kinds of Class II pollutants in water (i.e., BOD₅, COD, oil, and chloride) that exhibited high concentrations in wastewater; (2) eight additional pollutants (i.e., sulfate, Na, K, dissolved organic carbon (DOC), total organic carbon (TOC), Ca, Si, and Mg); (3) ten types of pollutants in waste gas (i.e., CO₂, SO₂, NO_x, CH₄, PM_2.5, CO, H₂S, non-methane volatile organic compounds,¹⁰ and total particulate matter) and (4) eight major constituents of solid waste (i.e., industrial waste, fly ash, waste water, sand, cinder, gypsum, hazardous waste, tailings, and waste).¹⁰ To quantify the pollution emissions from IMPPs, Figure 3 summarizes the total discharge of water–gas–solid waste for each metal.

Figure 2.

The waste scale of metals in the periodic table of elements

Figure 3.

The mass of pollutants for IMPP

(A) Waste water.

(B) Gas.

(C) Solid.

(D) CO₂.

Regarding waste water volume (Figure 3A), the top 10 IMPPs ranked as follows: In > Bi > Li > Mo > Hg > Sn > W > Co > Cr > Sb. In the production of In and Bi, the primary pollutants were BOD₅, COD, oil, and chloride. For Li production, BOD₅ and chloride were the dominant pollutants.

For waste gas emissions (Figure 3B), the top 10 IMPPs were ranked as: In > Bi > Cr > Ta > Nb > Ni > Mo > Mg > Cd > Au. The primary emissions from In production included SO₂, NO_x, PM_2.5, methane,⁴³ etc. Bi production predominantly generated CO, SO₂, NO_x, methane, etc., while Cr production emitted mainly SO₂, methane, NO_x, PM_2.5, etc.

For solid waste production (Figure 3C), the top 10 IMPPs were ranked as: Mo > Bi > W > Co > In > Sb > Ni > K > Cr > Sn. The highest volumes of industrial pollutants and tailings were generated by the top three IMPPs (Mo, Bi, and W).

Although the Integrated Emission Standard of Air Pollutants⁴⁴ did not explicitly require CO₂ in order to achieve a carbon peak in 2030 and carbon neutrality in 2060,^45,46 the GREET model and the LCA-related software database provided detailed statistics on CO₂ emissions. As indicated in Figure 3D, the top 10 IMPPs are ranked as follows: In > Co > Li > Ta > Nb > Hg > Mo > Cr > Au > Bi.

Environmental impact assessment

With the exception of waste water, waste gas, industrial solid waste, and CO₂ discharges data, numerous pollutants (including Class I and Class II pollutants in water, waste gas, and hazardous waste) fall under the pollutants regulated by the Relevant National Standards of China (Table S2). Utilizing Equations 3, 4, and 5, the CEA of IMPPs can be calculated (Tables S6–S8). The CEA serves as a tool to assess the degree of environmental impact degree.

Environmental impact assessment of waste water

Waste water pollutants are primarily classified into two categories: Class I and Class II. In the CEA indicators of Class I pollutants (CEA-I) (Figure 4A), the top five IMPPs were Cr > Li > Co > Mn > In. The elevated CEA-I of the Cr production process primarily stemmed from the high hexavalent Cr content in the waste water, contributing to its substantial CEA-I. Similarly, the Li production process exhibited the higher CEA-I due to increased total Ni and hexavalent Cr in waste water (Raychaudhuri et al., 2024). Elevated CEA-I in the coproduction process, which contained more Ni. Higher total As, Pb, and Ni contents led to higher CEA-I of the Mn production process. In terms of the CEA indicators of Class Ⅱ pollutants (CEA-II) (Figure 4B), the top five IMPPs were Li > K > In > Cr > Co. Higher chloride content resulted in higher CEA-II of Li, K, and Cr production processes, higher CEA-II of In and Co were mainly determined by higher oil and chloride content. By synthesizing the CEA of Class I and Class II pollutants in waste water, the final result showed that the metals with greater CEA_W were Li > Cr > K > In > Co (Figure 4C); therefore, the research department should focus on improving the production process of five metals, reducing their environmental pollution in waste water. Optimized wastewater treatment strategies were implemented to address the predominant contaminants in each production process, achieving substantial pollutant reduction and enhanced environmental performance.

Figure 4.

The CEA of pollutants for IMPP

(A) CEA-I (first kind of pollutants).

(B) CEA-II (second kind of pollutants).

(C) CEA_W (waste water).

(D) CEA_G (waste gas).

(E) CEA_S (waste solid).

(F) CEA (all pollutants).

Environmental impact assessment of waste gas and solid waste

Based on the analysis of pollutants in waste gas (Figure 4D), the five metal production processes with the highest CEA_G were In, Co, Au, Li, and Ag. The elevated CEA_G in the production processes of In and Co primarily stems from emissions of SO₂, NO_x, and methane. Meanwhile, the elevated CEA_G in Au and Ag production were primarily driven by NO_x and non-methane volatile organic compounds. NO_x, SO₂, and non-methane volatile organic compounds were key contributors to the higher CEA_G in Li production.

Regarding CEA_G (Figure 4E), the Hg production process had the highest CEA_G due to greater hazardous wastes generation, followed by Cd, Au, Ag, Bi, and Co. To mitigate air pollution from metal production, strategic optimization of waste gas treatment processes during IMPPs are recommended, especially for metals contributing substantially to CEA_G. For these metal production processes, clean production methodologies were implemented to both minimize waste gas generation at source and optimize key emission-intensive process stages, while simultaneously enhancing waste gas treatment systems. For instance, in the Hg production process with the highest CEA_S, proper disposal of solid waste generated during production is essential, rather than direct disposal in landfills.

Comprehensive environmental assessment

Based on the unilateral environmental impact assessment of waste water, waste gas, and solid waste, CEA was computed (Figure 5). To facilitate comparison and observation, the data were categorized into five ranges based on CEA (less than 0.01, 0.01–0.1, 0.1–1, 1–10 and 10–100). As seen from Figure 4F, the top 10 metals with great CEA during IMPPs were Li > In > Cr > K > Hg > Co > Au > Ag > Bi > Cd. The elevated CEA in the production processes of Li, K, Cr, and Co primarily stemmed from their high CEA_W. Higher CEA_W and CEA_G of the In production process resulted in higher CEA; the high CEA of the Hg production process was mainly caused by the high Hg containing solid hazardous waste in its solid waste.

Figure 5.

The CEA of all the IMPP

According to the CEA of various metals, IMPPs were classified into four classes. The metals with the CEA of 0.01–0.1 belonged to Class Ⅰ; the metals with the CEA of 0.1–1 belonged to Class Ⅱ; the metals with the CEA of 1–10 belonged to Class Ⅲ; the metals with the CEA of 10–100 belonged to Class Ⅳ. The classification results were as follows, with corresponding element positions in the periodic table as presented in Figure 6:

Figure 6.

The environmental impact level of each metal

Class Ⅰ (7 elements): Ca, Ti, V, Fe, Cu, Ga, and Ba;

Class Ⅱ (6 elements): Na, Mg, Al, Zn, Sb, and Pb;

Class Ⅲ (10 elements): Mn, Ni, Zr, Nb, Mo, Cd, Sn, Ta, W, and Bi; and

Class Ⅳ (8 elements): Li, K, Cr, Co, Ag, In, Au, and Hg.

The four-category metal classification based on the CEA reflects the pollution levels caused by IMPPs, enabling industry workers to develop more pollution solutions.

Carbon footprint analysis

In recent years, growing concerns about global warming⁴⁷ have drawn increasing public attention to phenomena such as glacier melting⁴⁸ and the greenhouse effect.⁴⁹ This has prompted widespread international adoption of carbon reduction measures, positioning carbon footprint analysis as a key research focus.⁵⁰ Derived from the ecological footprint concept,⁵¹ the carbon footprint represents GHG emissions attributable to organizations, products or individuals,⁵² generated either directly or indirectly throughout the entire life cycle of products or product systems.^53,54

Given the escalating production of metals, reducing carbon emissions from IMPPs has become imperative. This section provides an overview of carbon emission levels in typical IMPPs using GWP as the assessment metric. After initial data processing, the results are summarized in Tables S6–S8. A comparative chart (Figure 7A) illustrates the total emission profiles of different metal production processes. The top 10 highest-emitting IMPPs are, in descending order: Nb > Ta > Mo > Cr > Au > Ni > Ag > Sn > Mn > W.

Figure 7.

Analysis of GWP for each metal

(A) The GWP for each metal.

(B) The GWP level of each metal.

To facilitate comparison and observation, metals were categorized into four categories based on their GWP within IMPPs. Metals with GWP ranging from 0 to 100 kg CO₂ eq./kg were designated as Class Ⅰ; those with a GWP of 100–2000 kg CO₂ eq./kg belonged to Class Ⅱ; the metals with the GWP of 2000–20000 kg CO₂ eq./kg belonged to Class Ⅲ; the metals with the GWP of 20000–200000 kg CO₂ eq./kg belonged to Class Ⅳ. The final metal classification results are presented below, with corresponding element positions in the periodic table shown in Figure 7B:

Class Ⅰ (7 elements): K, Ca, Pb, Zr, Li, Cd, Mg, Bi, and Hg;

Class Ⅱ (4 elements): Co, Cu, In, and Ti;

Class Ⅲ (11 elements): Al, Ga, V, Fe, Sb, Na, Ba, Zn, W, Mn, and Sn; and

Class Ⅳ (7 elements): Ag, Ni, Au, Cr, Mo, Ta, and Nb.

Analysis of the four metal categories found that either reducing the production of Class Ⅳ metals or implementing other effective technologies can significantly reduce GHG emissions. Prompt treatment of emitted GHG content can effectively reduce the GWP associated with IMPPs.

Discussion

The CEA provides micro-level insights into the environmental impact of IMPPs, while carbon footprint analysis offers macro-level perspectives. Combining these two approaches enable comprehensive optimization of metal production technologies. Through these analyses (Figure S3), the top 10 metals with highest CEA values during IMPPs were identified and ranked as follows: Li > In > Cr > K > Hg > Co > Au > Ag > Bi > Cd. For GWP, the top-ranking metals were Nb > Ta > Mo > Cr > Au > Ni > Ag > Sn > Mn > W. Integrated comparative analysis of CEA and GWP metrics (Figure 8A) revealed that Nb, Ta, and Cr exhibit substantial environmental impacts across both assessment dimensions. Quantitative evaluation identified numerous metals with minimal environmental footprints, suggesting their broader application potential. Multivariate classification analysis (Figure 8B) consistently positions Ca within Class Ⅰ for both GWP and CEA, indicating minimal environmental impact, corroborated by its favorable numerical performance across all metrics. As Class Ⅳ metals in both frameworks, Cr, Au, and Ag demand prioritized industrial regulation, including production process optimization and deployment of state-of-the-art waste management solutions. Convergent evidence from numerical assessment and categorical evaluation identifies Cr as the most environmentally detrimental element, whereas Ca demonstrates minimal ecological disruption. Cr primarily originates from the production of Cr via the aluminothermic process. Future research efforts should prioritize the development of advanced pollution control technologies in Cr production processes, while simultaneously promoting the sustainable manufacturing and application of environmentally friendly Ca-based alternatives.⁵⁵ Comparative analysis with precedent LCA research confirms that Au and Ag exert significant cross-category environmental impacts, aligning precisely with the classification outcomes presented in Figure 8B.^17,20 The conspicuous absence of Cr evaluation in these studies validates the methodological approach adopted herein and underscores the critical knowledge gap addressed by this research. A comparative analysis of CEA and GWP facilitates fine-tuning the impact of different metals on water quality, air pollution, and solid waste management in industrial optimization strategies, while supporting energy conservation efforts.

Figure 8.

Analysis of GWP and CEA for each metal

(A) The GWP and CEA for each metal.

(B) The GWP and CEA level of each metal.

By comparing the research results of CEA, and carbon footprint analysis of LCA (Table 2), LCA mainly analyzes the impact of products on the environment through impact categories, while CEA can accurately focus on the environmental impact caused by waste water, waste gas, and solid produced during IMPP. Currently, no comprehensive researches have systematically evaluated the environmental impacts of pollutants across all major metal production processes. This research can improve this aspect.

Table 2.

Differences between CEA, LCA, and carbon footprint analysis

Environmental impact assessment	Dataset	Methodologies	Calculation step	Scope	Environmental impact category	Meaning of the impact category
LCA	eBalance, GaBi and other software databases, and enterprise production data	EDIP, CML 2001, EPS 2000, TRACI, ReCiPe, IMPACT 2002+, Eco-indicator95/99	First, determine the purpose and scope of the research, conduct a life cycle inventory analysis, and finally analyze the results through life cycle impact assessment.	Full life cycle or multiple stages (e.g., cradle to grave)	GWP	Measures greenhouse gas impacts
					Abiotic depletion elements	Measures depletion of non-living resources
					Acidification potential	Quantifies potential to form acid rain
					Eutrophication potential	Assesses nutrient over-enrichment in ecosystems
					Freshwater aquatic ecotoxicity potential	Evaluates toxic impacts on freshwater organisms
					Human toxicity potential	Estimates harmful effects on human health
					Ozone layer depletion potential	Quantifies stratospheric ozone damage
					Marine aquatic ecotoxicity potential	Assesses toxic impacts on marine organisms
					Photochemical ozone creation potential	A chemical’s smog-forming potential
					Terrestrial ecotoxicity potential	Evaluates toxic impacts on soil organisms
Carbon footprint analysis	IPCC emission factors, LCA database, and enterprise energy consumption records	EDIP, CML 2001, EPS 2000, TRACI, ReCiPe, IMPACT 2002+, Eco-indicator95/99	First, determine the purpose and scope of the research, conduct a life cycle inventory analysis, and finally analyze the results through life cycle impact assessment.	Full life cycle or multiple stages (e.g., cradle to grave)	GWP	Measures greenhouse gas impacts
CEA	LCA database, the discharge standards issued by the Ministry of Ecology and Environment of China, and enterprise production data	$W_x=1/E_x$ $CE{A}_W/CE{A}_G/CE{A}_S=\sum _x{W}_x{m}_x$ $\begin{array}{l}CEA=\\ CE{A}_W+CE{A}_G+CE{A}_S\end{array}$	Determine the goals and scopes in sequence, collect pollutant discharges and equivalent value data, and analyze the impact of each pollutant in accordance with the discharge standards.	Full life cycle or multiple stages (e.g., cradle to grave)	CEAW	The CEA indicators of waste water
					CEAG	The CEA indicators of waste gas
					CEAS	The CEA indicators of solid waste

Conclusions

Metals underpin industrial systems and fuel economic expansion through material supply chains. Therefore, it is crucial to conduct CEA analysis on typical IMPPs. This research collected the data of pollutants discharged during IMPPs, explored the environmental impact of waste water, waste gas, and solid waste, used the CEA method, and analyzed the carbon footprint based on the greenhouse effect. The top 10 metals in terms of CEA during IMPPs were Li > In > Cr > K > Hg > Co > Au > Ag > Bi > Cd; according to the CEA results, the metals were divided into four categories in descending order of environmental impact severity. In the CEA_W, for the CEA-I, the first few metals were Cr > Li > Co > Mn > In, for CEA-Ⅱ, the first few metals were Li > K > In > Cr > Co. The comprehensive environmental effects analysis of the Class I and Class Ⅱ pollutants in wastewater demonstrated that the metals with greater CEA_W were Li > Cr > K > In > Co. For the CEA_G, the first few metals were In > Co > Au > Li > Ag. For the CEA_S, the first few metals were Hg > Cd > Au > Ag > Bi; the Hg production process ranked first because it produced more hazardous wastes. Meanwhile, the GWP of IMPPs are also analyzed, and the top 10 metals were Nb > Ta > Mo > Cr > Au > Ni > Ag > Sn > Mn > W. Comprehensive analysis of CEA and GWP indicated that Cr had most serious environment impact because of the aluminothermic process, and Ca had the least severe impact on the environment. Future research efforts should prioritize the development of advanced pollution control technologies in Cr production processes, while simultaneously promoting the sustainable manufacturing and application of environmentally friendly Ca-based alternatives.

By calculating the CEA of all typical IMPPs, the effects on waste water, waste gas and solid in the production process are summarized, and the macro and micro analysis are carried out. The metals were classified accordingly according to the CEA and carbon footprint analysis for IMPPs. This classification informed the selection of metals for production processes involving metal-containing products and contributed to the formulation of national policy macro-controls. Meanwhile, metal producers can leverage these findings to attain desired environmental impact levels across all stages of metal production processes. The research provides macro support for industrial optimization and technological upgrades within the industry.

Owing to data availability constraints, this research was limited to publicly accessible databases. The next research could incorporate database updates or newly available industry data to enable recalibration of the results. Future research should systematically examine the environmental implications across different metal production processes to provide more comprehensive insights.

Limitations of the study

In this research, a CEA was conducted for 31 kinds of typical IMPP in China. However, the data in this research are sourced from the CLCD-China 0.8, CLCD-China Public 0.8, Ecoinvent 3.1, and Ecoinvent-Public 2.2 database, rather than actual operational data from the factory. As the factory’s operational conditions and emerging technologies will be improved in the future, the data and results may change depending the conditions. Therefore, this research may not provide accurate predictions or in-depth analysis for future scenarios.

Resource availability

Lead contact

Requests for further information and resources should be directed to and will be fulfilled by the lead contact, Wenfang Gao (wfgao@hebut.edu.cn).

Materials availability

This study did not generate new unique reagents.

Data and code availability

A bullet point for Data and Code: Data will be made available on request: eBalance 4.7 (http://localhost:8082).

One for all other items: Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Acknowledgments

This paper is supported by National Natural Science Foundation of China (No. 52300232), Science and Technology Project of Tianjin (23JCQNJC00970), Education Commission Project of Tianjin (2022KJ098).

Author contributions

Z.W. contributed to manuscript review and editing. H.C. led the writing of the original draft, data visualization, and designed the methodology. W.Y. participated in manuscript review and editing. G.Z. performed data visualization and investigation. P.W. contributed to data visualization. X.L. and Z.R. supervised the research. Z.S. and W.G. assisted in manuscript review and editing.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Deposited data
The selected metal production database and the corresponding metal production process (CLCD-China 0.8, CLCD-China Public 0.8, Ecoinvent 3.1, Ecoinvent-Public 2.2)	eBalance 4.7 (http://localhost:8082)	Table S1
The pollutants in the metal production processes (CLCD-China 0.8, CLCD-China Public 0.8, Ecoinvent 3.1, Ecoinvent-Public 2.2)	eBalance 4.7 (http://localhost:8082)	Tables S2–S5
The data for GWP of Li	Rolinck et al.56	N/A
The data for GWP of Cr	Nuss et al.22	N/A
Software and algorithms
eBalance 4.7 (http://localhost:8082)	The software of LCA	N/A
GHGs, Regulated Emissions, and Energy use in Technologies (GREET) model	Argonne National Laboratory	N/A

Method details

CEA methodology

This research focuses on the environmental assessment as a typical research direction. Micro-environmental impact and macro-interpretation of IMPPs were analyzed using CEA and GWP.⁵⁷ The CEA and GWP were employed to comprehensively analyze the pollution levels throughout the entire IMPPs (Figure 1).

Different pollutants exhibit varying levels of the environmental impact. Therefore, CEA results for pollutants were primarily determined by the weight coefficient of pollution. Due to the limitation of data acquisition, the pollutant data in the research is based on the standard of China. Therefore, the establishment of the CEA model based on pollutants weight fully considered China’s national environment protection standards. W_x was determined according to the discharge and concentration of waste water and waste gas¹⁸:

$$\{\begin{array}{l}{R}_x=0,S_{\max ,x,y}\le S_x^{\ast }\\ R_x=\frac{1}{y}\sum _y\left(S_{\max ,x,y}/S_x^{\ast }\right),S_{\max ,x,y}>S_x^{\ast }\end{array}$$ (Equation 1)

$$W_x=R_x/\sum _x{R}_x\text{,}$$ (Equation 2)

where x is the type of pollutants in waste water, waste gas and solid waste (such as COD, fluoride and NH₄⁺-N), y is the type for waste (such as waste water after a work station), $S_x^{\ast }$ is the discharge standard of pollutant x,⁵⁸ S_max,x,y is the highest concentration for pollutant x in waste y, R_x was the correlation ratio of pollutant x⁵⁹ and W_x is the weight coefficient for pollution factor x in all pollutant.⁵⁹

The data source for all pollutants conforms to quality data rather than concentration data. The W_x for IMPPs was left uncalculated according to Equation 2. Hence, considering the data characteristics, W_x can be determined based on the appropriate pollution equivalent value (E_x) stipulated in the pollutant discharge charge and declaration.⁶⁰ The larger pollution equivalent value, the smaller pollution level and the corresponding W_x is smaller. W_x had been determined following Equation 3:

$$W_x=1/E_x\text{.}$$ (Equation 3)

According to the obtained quality which based on the weight coefficient for pollution and combined with the quality data from the data source, the CEA of waste water, waste gas, and solid waste shall be carried out according to Equation 4.

$$CE{A}_W/CE{A}_G/CE{A}_S=\sum _x{W}_x{m}_x\text{,}$$ (Equation 4)

where the CEA indicators of waste water (CEA_W), the CEA indicators of waste gas (CEA_G) and the CEA indicators of solid waste (CEA_S) are the CEA indicators for waste water, waste gas and solid waste, respectively.^36,37,61 m_x is the mass of pollutant x.

Based on environmental assessment which impacts the results for waste water, waste gas and solid waste, the CEA of the entire production process can be calculated:

$$CEA=CE{A}_W+CE{A}_G+CE{A}_S\text{.}$$ (Equation 5)

This Equation 5 is entirely derived from the calculation method for pollution equivalent value (dimensionless) that is outlined in the pollutant discharge charge and declaration.⁶⁰ For pollutant found in waste water, waste gas or solid waste, the cost of treating the waste water (or air) for an equivalent unit reflects the isovalent harm to the environment. Through CEA, the pollution levels induced by different metals in the production process can be comprehensively evaluated from various perspectives, elucidating the distribution of various pollutants and emphasising the types of pollution that warrant attention in current environmental pollution control measures.

Functional unit

The functional unit of a product serves as a quantified reference for all associated inputs and outputs. In this research, the functional unit is defined as one kilogram of the produced metal (Figure 1 and Table S1). All quantity of the product is based on this unit.

System boundary

The system boundary⁶² encompasses the process from resource mining to factory delivery, aiming to explore the model’s application beyond merely focusing on the production process for key metal resources. Considering the complexity of each metal’s production process(i.e., distinct methods for production, purification and productization), the research to streamline the analysis of the IMPPs. Therefore, the research selected the production method with broad applications and omitted the detailed production processes. The analysis primarily focuses on the generated of waste water, waste gas and solid waste at each production stage. Waste water quality was classified according to Class I and Class II pollutants,^63,64 as outlined in the Integrated Wastewater Discharge Standard⁵⁸ (Table S2). Furthermore, the transportation, labor, recovery and other post-production processes of metal products were excluded from the analysis. The scope of this research was exclusively limited to the production process of metals.

To enhance the credible and broader applicability of the results in evaluating metal production process, the metal element production processes was extensive coverage (Table S1). The low criticality metals (e.g., Fe, Al and other metals)⁶⁵ in the actual industrial production process were the primary focus of CEA. Therefore, the approach expanded beyond the traditional critical evaluation of metal resources to encompass the whole production process from resource extraction to factory delivery for all metals (Figure 1 and Figure S1). The research selected 31 metal elements and excluded the metals with limited application scope and lacked sufficient raw data in the database (Figure S2). Detailed data and process descriptions for the selected of metals are provided in Table S1.

The substances that pose environmental risks in the metal production process are the focus of attention. This primarily encompasses data on environmental discharges (i.e., waste water, waste gas, solid waste, radioactive materials and waste heat) from the production of metal resources recorded in the database. In the interest of practical quantified CEA, only discharge data were incorporated, while data related to material radioactivity and waste heat were disregarded. Given the diverse array of pollutant data, discharge standards for waste water, waste gas and solid waste were based on relevant standards in China (Table S2). Concerning waste water discharge, Class I pollutants, which substantially impact the environment, were comprehensively accounted for regardless of volume. For Class II pollutants in waste water, only environmental discharges exceeding 0.001 kg per kg of products were considered. Similarly, for waste gas emissions, only those exceeding 0.001 kg per kg of products were taken into consideration. Additionally, in alignment with international initiatives⁶⁶ and efforts to reduce carbon emissions,⁶⁷ calculations of the GWP for related metals sourced from the eBalance database were integrated into the analysis. Regarding solid pollutants, aside from hazardous wastes, other forms such as tailings, waste rock and fly ash,⁶⁸ despite their large production volumes, were found to have minimal environmental impacts and could be directly stored or reused as ceramics and bricks. Consequently, relevant data were initially collected, but only the CEA of hazardous wastes was factored into the calculation process, while the CEA of other solid wastes was temporarily disregarded.

The influence of identical pollutants on different substances or environments were omitted from the comprehensive dataset. For streamline comparative analysis, Chinese domestic data and average data from various technologies available in the market were predominantly utilised.

Data collection sources

To ensure the robustness and comparability of the results and maximise the quality of the outputs of the research, the availability and quality of the data in this research should be guaranteed. The appropriate pollution equivalents were determined based on the relevant pollutant discharge standards in China to calculate the CEA and expand the application range for results. In the selection of data sources, the widely recognized data mainly come from China with Chinese national conditions.⁶⁹ The pollutant data of various metal production processes were collected from the GHGs, Regulated Emissions, and Energy use in Technologies (GREET) model of the Argonne National Laboratory (ANL),⁷⁰ the Manual of the First National Pollution Source Survey Industrial Pollution Source Emission Coefficient of China,⁷¹ and the software of LCA (eBalance) with four databases (CLCD-China 0.8, CLCD-China Public 0.8, Ecoinvent 3.1, Ecoinvent-Public 2.2). The data for GWP of Li and Zr came from the works of Rolinck et al.⁵⁶ and Nuss et al.²² due to the limited availability of the database.

The pollution discharge standards for CEA comprehensively incorporate the discharge standards issued by the Ministry of Ecology and Environment of China. Effluent pollutants adhere to the Integrated Wastewater Discharge Standard (GB8978-1996), while waste gas emission align with the Integrated Emission Standard of Air Pollutions (GB16297-1996). Additionally, the industrial water pollutant discharge standards for industries such as Sn, Sb, Hg, V, steel, Al, Mg, Ti, Pb, Zn, ferroalloy, Cu, Ni and Co industries were thoroughly taken into account. Specific standard information was provided in Table S2.

The E_x of pollutants served as the benchmark for evaluation, and its determination dictated whether the evaluation could proceed accurately. Consequently, the E_x of each pollutant was individually determined through the pollutant discharge charge and declaration.⁶⁰ The E_x of pollutants involved in the evaluation process outlined in Table 3.

Table 3.

The identification of pollutant equivalent value E_x

Waste type		Pollutant	Ex
Waste watera	I	Total mercury/kg	0.0005
		Total cadmium/kg	0.005
		Total chromium/kg	0.040
		Hexavalent chromium/kg	0.020
		Total arsenic/kg	0.020
		Total lead/kg	0.025
		Total nickel/kg	0.025
		Total beryllium/kg	0.010
		Total silver/kg	0.020
	II	BOD5/kg	0.500
		COD/kg	1.000
		Oil/kg	0.100
		Chloride/kg	0.040
Waste gasb	SO2/kg		0.950
	NOx/kg		0.950
	CH4/kg		0.380
	PM2.5/kg		4.000
	CO/kg		16.700
	H2S/kg		0.290
	Non-methane volatile organic compounds/kg		0.230
	Total particulate matter/kg		2.180
Waste solid	Hazardous waste/kg		1.540

^aAlthough benzoapyrene is Class I pollutants in water stipulated by Chinese national standards, it is excluded in this because there is no discharge data of this pollutant in the database. Although BOD₅ and COD in actual industrial waste water partly coincide with each other in terms of organic composition, the environmental impact of the two is still considered separately in this, and the sum of their comprehensive environmental impact is finally calculated, because environmental standards limit the two separately.

^bThe E_x of waste gas: PM_2.5 is the value of general dust. Total particulate matter is the value of smoke. Methane is the value of average of formaldehyde and methanol. Non methane volatile organic compounds is the value of the average value of organic hydrocarbon or hydrocarbon.

Quantification and statistical analysis

There are no quantification or statistical analyses to include in this study.

Published: June 26, 2025