Global warming potential and total material requirement in metal production: Identification of changes in environmental impact through metal substitution

Science of the Total Environment 651 (2019) 1764–1775

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Global warming potential and total material requirement in metal production: Identification of changes in environmental impact through metal substitution Shoki Kosai ⁎, Eiji Yamasue Department of Mechanical Engineering, College of Science and Engineering, Ritsumeikan University, Shiga, Japan

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Interaction of environmental consequences of mining activities is identified. • Total material requirement is employed as an environmental indicator. • Relations between GWP and TMR for 59 metals are assessed. • 40% of metal substitute options cause an additional environmental impact.

a r t i c l e

i n f o

Article history: Received 25 August 2018 Received in revised form 6 October 2018 Accepted 7 October 2018 Available online 09 October 2018 Editor: Deyi Hou Keywords: Mining Life cycle impact assessment Metal replacement GWP

a b s t r a c t In view of the increasing demand for metal use, it is of significant importance to evaluate the environmental impact of metal production. The global warming potential (GWP) in the process of metal production has often been focused upon as a major indicator for evaluating the burden on the environment. Moreover, the environmental impact and mineral exploitation arising from metal ore mining activities, which generate unavoidable mine wastes and have an impact on the ecological biodiversity, cannot be ignored. The major factors for determining the intensity of resource exploitation being the ore grades and strip ratio, the existing indicators for land use employed in the life cycle assessment (LCA) may not fully cover the criteria of the impact of metal mining on the environmental system. Therefore, this study employs the method of total material requirement (TMR) assessment, involving not only the direct and indirect material inputs but also the hidden flows, which are particularly associated with mine wastes. Firstly, the methodology of computing the TMR in the process of metal production is developed. Next, the relation between the GWP and TMR for 58 metals is assessed and finally, the environmental impact through metal substitutes is evaluated from the perspectives of the GWP and TMR. This analysis could identify some of the aspects overlooked in the previous environmental criteria that were concentrating on greenhouse gas emissions and global warming. The developed algorithm may be useful in identifying appropriate metal substitutes, considering the environmental impact. © 2018 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (S. Kosai).

https://doi.org/10.1016/j.scitotenv.2018.10.085 0048-9697/© 2018 Elsevier B.V. All rights reserved.

In the last few decades, the increase in global population, heavy industrialization in developing countries, rapid electronic innovations,

S. Kosai, E. Yamasue / Science of the Total Environment 651 (2019) 1764–1775

and infrastructure transition have dramatically changed the global metal landscape (van der Voet et al., 2013). In view of the increasing demand for metal use, an evaluation of the environmental impact of metal production is of significant importance. Life cycle (impact) assessment (LCA) is a well-developed tool for evaluating the potential environmental impact throughout the lifetime of a product (ISO, 2006). Inventory database (e.g., ecoinvent) is used to carry out LCA under the various production processes (Swiss Centre for Life-Cycle Inventories, 2007). Environmental impact of metal production has also been evaluated in the context of LCA (Kolotzek et al., 2018; Bach et al., 2016). Pizzol et al. assessed the consequences of toxicity of metals in the ecosystem by comparing various LCA techniques including Stepwise 2006, Impact 2002+, EDIP 2003, Eco-Indicator 99, CML 2001, TRACI 2, ReCiPe, and USEtox (Pizzol et al., 2011). In metal production particularly, ReCiPe endpoints (Huijbregts et al., 2017) have often been utilized to illustrate critical environmental consequences (Graedel et al., 2012; Nassar et al., 2012; Nassar et al., 2015a; Graedel and Nuss, 2014; Harper et al., 2015). Among the various types of environmental impacts, global warming issues have specifically focused on the processes from ore mining and concentration through smelting to metal refinery. Nuss and Eckelman evaluated the global warming potential (GWP) of 63 metals including the minor metals (Nuss and Eckelman, 2014). The carbon dioxide emissions and energy input throughout the process of metal production (United States Department of Energy, 2010; Morley and Eartherley, 2008) have been used as major indicators in evaluating the environmental impact, particularly associated with global warming. Various researchers expend time and effort on developing the energy and CO2 inventory data of metal processing (e.g., rare earths (Sprecher et al., 2014; Koltun and Tharumarajah, 2014)). Notwithstanding the use of GWP as a well-known indicator of environmental impact (European Commission - Joint Research Centre - Institute for Environment and Sustainability, 2010; European Commission Joint Research Centre - Institute for Environment and Sustainability, 2012), the effects of metal ore mining activities cannot be ignored (Kosai et al., 2018). All mining activities change the landscape of the earth and global mining activities move over 57 billion tons of land every year (Douglas and Lawson, 2000). As the process of extraction primarily consumes the stock of natural capital, mining activities, directly and indirectly, have multiple impacts on the environmental systems (Franks et al., 2010). The environmental impact arising from mining activities is largely the result of generation of mine waste (Bridge, 2004; Prior et al., 2012; Mudd, 2007) and changes in the land structure (Schmidt and Ostfeld, 2001; Patz et al., 2004). It has been stressed that mine waste, like physical and chemical pollution, presents a great risk to human health and the natural environment (Bridge, 2004). Physical pollution is caused by the release of suspended particulates, such as dust into the air, water or onto the land, and has been a major concern with regard to mining activities throughout the ages (Ripley et al., 1996). Chemical pollution is basically caused by the ingress of reagents, used in processing raw material, into the environment or by the natural occurrence of chemical reactions of targeted raw materials (Mudd, 2007). As the characteristics of mine waste are largely unknown, the successful rehabilitation of mining sites is of paramount importance to avoid harmful reactions in the bio-ecosystem (Prior et al., 2012; Sánchez, 1998; Jenkins and Yakovlova, 2006). Historically, b1% of global land surface being utilized for mining activities, the environmental impact was dismissed and considered inconsequential from the geographical perspective (Hodges, 1995; Marsh, 1995). The introduction of an ecological perspective has led to a reassessment of the significance of these impacts (Pascal et al., 2008; Murguía et al., 2016). Both mine waste and land change intervene in ecological systems, resulting in environmental and ecosystem damage, such as forest fragmentation, pathogen introduction, and mitigation of biodiversity (Bridge, 2004; Sandifer et al., 2015).

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Besides these, the various inputs including water and energy required in the mining process must be taken into account. In particular, the relation between energy utilization and mining activities was highlighted in the context of global warming issues (Bridge, 2004; Mudd, 2007). The higher the energy input required, the more vulnerable is the process to energy-related issues, specifically to greenhouse gas emissions (Mudd, 2010a). Considering the significance of the relation between environmental burden and mining activities, focus on GWP alone may lead to a shortsighted analysis of the environmental impact of metal production. The following comparison between copper and aluminum is an apt example: Aluminum is obtained by the electric refining of bauxite at a high temperature with a significant input of energy, whereas the process of reducing copper oxide to obtain copper does not require such a large amount of energy. Therefore, the GWP of aluminum is greater than that of copper (Nuss and Eckelman, 2014). On the other hand, the intensity of mineral exploitation for copper mining would be greater than that for aluminum as the ore grades of copper and aluminum are 0.5–1% and 10–15%, respectively (JOGMEC, 2014). The importance of ore grade for determining the intensity of mining activity is presented in detail in Section 2. Hence, a comparative analysis of the indicators for evaluating the environmental risk in metal production is of paramount importance. Further, the comparative analysis between GWP and TMR would assist in identifying the potential to increase or decrease the environmental impact through metal substitution. Various uncertainties in metal price, processing cost, geopolitical stability and metal depletion have raised the security of continuous metal supply to an alarming extent. Critical metal strategies have been proposed in the national resource policy narratives in the US (United States Department of Energy, 2010), EU (European Commission, 2010) and Japan (Nakamura and Sato, 2011). In order to address these critical metal issues, metal substitutes are recommended, in addition to the reduction of metal utilization, reuse and recovery of metals used in products (Graedel et al., 2015a). Many researchers have put in huge efforts in identifying alternative materials to be substituted for the original critical metals (Graedel et al., 2015b). It is of interest to evaluate the change in environmental impact due to the use of metal substitutes. Therefore, the objectives of this paper are to propose an indicator related to the intensity of mineral exploitation for evaluating the environmental impact arising from mining activities, to conduct a comparative analysis with the global warming issues, and to identify the potential to increase/decrease the environmental impact through metal substitutes. The paper is structured as follows: In Section 2, a literature review on the indicators for evaluating land use and the determination factors of mining intensity is outlined to propose a quantitative evaluation of the environmental impact of mining activities. Section 3 presents the methodology of comparative analysis between global warming issues and environmental impact of mining activities in the process of metal production and its effect on metal substitution. Section 4 illustrates the results of the comparative analysis and environmental impact due to the metal substitutes. The obtained results and the limitation of the assessment are discussed in Section 5. Finally, Section 6 presents the conclusions. 2. Prospective indicators for mining activity Mining is a process of abiotic resource extraction and it affects the anthropogenic land use (Taelman et al., 2016). The proposed approaches for evaluating the overall land use and environmental systems are first surveyed, to develop an indicator appropriate for mining activities. Many research works highlighted the potential impact of land use on the environmental system in the LCA narratives (Bare, 2010) from the perspectives of biodiversity (Koellner et al., 2013; Crenna et al., 2018; Fantke et al., 2016), chemical contamination (Koellner et al., 2013;

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Souza et al., 2015), and quality of land stock (Sonderegger et al., 2017). Taelman et al. developed a cause-effect chain to identify the critical environmental impact in the process of land use (Taelman et al., 2016). In particular, Curran et al. reviewed the methodology (Mischelsen and Lindner, 2015; Fehrenbach et al., 2015) as well as the indicators of evaluating the biodiversity loss resulting from land use (Curran et al., 2010), including the number of plant species (Koellner and Scholz, 2007; Koellner and Scholz, 2008; De Baan et al., 2013) and habitat suitability matrix (Souza et al., 2015; De Baan et al., 2015). These literatures focus on the quantification of the consequences of land use on the environment and ecosystem. In addition, land is considered to be one of the natural resources, together with minerals, metals, air, fossil fuels, renewables, water, soil and biotic natural resources (Sonderegger et al., 2017). This approach attempts to analyze the scale of land use. Dewulf et al. developed a cumulative exergy extraction from the natural environment (CEENE) method (Dewulf et al., 2007) to evaluate all the natural resources in the LCA (Liao et al., 2012). Land as a natural resource has been evaluated by using indicators such as solar irradiation exergy (Dewulf et al., 2007) and net primary production loss (Alvarenga et al., 2013; Taelman et al., 2014). In both the approaches of analyzing the consequences on the ecosystem, considering the natural resources, the common subject of land includes the surface area involved in land use (Koellner et al., 2013; Milà i Canals et al., 2007) such as land occupation (m2/year, m2/capita/year) and its transformation (m2) (Mattila et al., 2012; Huijbregts et al., 2008; Aleksandrowicz et al., 2016). However, there is ample scope for conceptualizing land use in the LCA and there is no accepted consensus on the quantification method of land use impact (Finnveden et al., 2009). It must be mentioned here that the focus on land surface area alone may not fully encompass the criteria of metal mining impacts on the environmental system. As mentioned in Section 1, the environmental impact of mining activities is attributed to the change in the landscape and the mine waste left on the ground such as tailings, slag, leached ore, debris fans, sand bars, and turbid rivers (Bridge, 2004; Bridge, 2000; Lèbre, 2015), and mineral exploitation is related to the disturbances involved in the lithosphere. The degree of mineral exploitation through mining activity largely depends on two mining components: ore grade and strip ratio (Northey et al., 2014a; Norgate and Haque, 2010; Mudd, 2010b). The ore grade indicates the content rate of metal in ores, while the strip ratio depicts the ratio of the summation of surface soil and muck, or mine waste, to the mined ore, which measures the scale of mining activities, including the mine area and depth. As a higher energy input is required to extract larger amounts of ore when dealing with lower grade ores and deeper mines (Norgate and Jahanshahi, 2010), more mine waste is generated and the changes in the land structure are more profound (Mason et al., 2011; Dold, 2014; Onuaguluchi and Eren, 2012). Some research works attempted to identify the nature of mining determination factors in the environmental context. Xian et al. showed that the rapid generation of mine waste with increasing strip ratio was closely associated with mine depth (Xian et al., 2016). Sahu and Dash reported that the extent of land degradation depended on the volume of overburden removal due to the strip ratio (Sahu and Dash, 2011). Cortez et al. evaluated the bioremediation of metalliferous polluted soil caused by the large amounts of waste mine from low ore grade (Cortez et al., 2017). Different environmental components such as water footprint (Northey et al., 2014b) and energy intensity (Norgate and Jahanshahi, 2011) in mining activities were also quantitatively assessed from the perspective of ore grade. As such, a thorough reading of the literature highlights the qualitative correlation between the magnitude of the environmental impact and determination factors, including the ore grade and strip ratio.

Although the land surface area as a major indicator, widely utilized for land use, may cover a part of the environmental impact of mining activities, its shortcomings need to be highlighted. First, the total mine waste is not considered since the aspect of mine depth is not fully included. Second, all types of mining approaches such as open-pit and underground mines are not considered. The intensity of an open-pit mine could be correlated to the land surface area involved in mining activity, whereas that of an underground mine is hardly assessed without the consideration of ore grade and strip ratio. In the flow study of raw materials, the volume, weight and degree of earth movement due to the target mineral extracted as well as the hidden flows associated with mine waste need to be considered (World Resources Institute, 1997). Because of these two shortcomings and the qualitative correlation between the magnitude of the environmental impact and determination factors, the development of a new indicator dedicated to the environmental burden caused by mining activities, on the basis of ore grade and strip ratio, is of significant importance. It must be noted that the mining determination factors depict the magnitude of its scale, or the potential to affect the environment as a consequence of mining activity, i.e., the generation of mine waste and change in the land structure. In comparison with the potential of global warming, or GWP, it can be assumed that the mining determination factors are the new indicators to evaluate the potential of environmental impact of mining activity. For an assessment of the environmental impact of mining activity, the ore grade as well as the mine depth of the target metal need to be taken into consideration. Each step, from cradle to refinery, needs to be considered to assess the magnitude of mining activity i.e., besides the targeted ore mining, the potential environmental impact arising from the exploitation of the associated input material and energy utilized throughout, up to the refinery stage must be taken into account. The new indicator proposed in this study is defined as the total potential magnitude of mining activities at each step of metal processing. The proposed new indicator appears to match the concept of Total Material Requirement (TMR). TMR was originally developed as an indicator for economy-wide material flow analysis (Fischer-Kowalski et al., 2011) and involves the evaluation of direct and indirect inputs and their accompanied hidden flows. Bringezu et al. (Bringezu et al., 2004) stated that hidden flows included the unused extraction related to mine waste, such as overburden and erosion in non-economic activity, that burden the environment through the change of landscape. The hidden flows, included in a TMR analysis, account for the mine waste and land change, and TMR assessment takes into consideration ore grades as well as strip ratio, hence, TMR encompasses the definition of the newly proposed indicator in this study.

3. Methodology 3.1. Assessed metals This study assessed 58 metals based on data availability and earlier work (Nuss and Eckelman, 2014): viz., iron (Fe), manganese (Mn), chromium (Cr), titanium (Ti), nickel (Ni), copper (Cu), cobalt (Co), vanadium (V), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), tantalum (Ta), tungsten (W), hafnium (Hf), rhenium (Re), silver (Ag), platinum (Pt), gold (Au), palladium (Pd), rhodium (Rh), osmium (Os), iridium (Ir), scandium (Sc), yttrium (Y) lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), dysprosium (Dy), ytterbium (Yb), erbium (Er), terbium (Tb), gadolinium (Gd), lithium (Li), barium (Ba), calcium (Ca), strontium (Sr), boron (B), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), aluminum (Al), zinc (Zn), gallium (Ga), lead (Pb), cadmium (Cd), magnesium (Mg), bismuth (Bi), selenium (Se), tin (Sn), beryllium (Be), indium (In), mercury (Hg), and thallium (Tl).

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The TMR weight is obtained by summing the material weight itself in the following equation:

3.2. Computation of GWP and TMR The data of GWP (kg-CO2 -eq per kg) in the production of each element from cradle to refinery gate was taken from the research works conducted by Nuss and Eckelman (Nuss and Eckelman, 2014). The concept of TMR is presented by obtaining the TMR coefficient (kg-TMR/kg) in the production of each element from cradle to refinery gate. The steps of the calculation are as follows: In order to compute the TMR coefficient of the processed metal, the TMR coefficient of ore mining is obtained first, referring to the work (Halada, 2012), by using the following equation: TMR coefficient ðOreÞ ¼

ð1 þ strip ratioÞ 100  ðore grade½%Þ

ð1Þ

Then, the TMR involved in the steps other than ore mining for metal production is also accounted for. The TMR assessment in the other processing steps is based on three constituents: the material weight (kg), the TMR weight (kg-TMR), and the TMR coefficient (kg-TMR/kg). The TMR weight means the total weight on the basis of TMR, including hidden flows. The TMR coefficient is the ratio of the TMR weight to the material weight. As the input material is processed to generate output material, the material weight for the input is represented by Ax , where x is the input material component, and the material weight for the output is represented by By, where y is the output material component. For example, in matte production, one of the copper (Cu) processing methods, the input materials, or x, such as copper ore, silicon dioxide, coal, heavy oil, and electricity are processed to obtain the output materials, or y, such as copper matte and sulfur dioxide (Kosai et al., 2018). The total weight of the input and output materials are presented in the following equations: A¼ B¼

X X

Ax

ð2Þ

By

ð3Þ

The TMR coefficient of each input material is represented by Cx. The TMR weight of all the input materials, represented by T, is computed by the following equation: T¼

X

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T y ¼ T 0y þ By

ð7Þ

Finally, the TMR coefficient is computed by using the following equation: Cy ¼

Cy ¼

Ty By X

ð8Þ

Ax C x −

X

 D y By P þ1 By Dy

ð9Þ

The TMR coefficient of the output material (Cy) through a given process could be obtained by the aforementioned steps on the basis of one of the input materials (Cx). Beginning with the calculation of the TMR coefficient of the ore, this method is successively applied to each of the processing steps until the TMR coefficient of the target metal is obtained as the final output. A more detailed calculation to obtain the TMR coefficient of the target material is presented in an earlier work (Kosai et al., 2018). 3.3. Relation between GWP and TMR The computed indicators are plotted on the TMR versus GWP graph and the common trends and differences between them are presented in Fig. 1. In particular, metal combinations of similar GWP are selected and the gap in their TMR values is evaluated, for further analysis of differences between GWP and TMR. The metal combination of similar GWP is selected by the following equation: GWP difference ¼ jGWP m −GWP n j≤100

ð10Þ

where m, n refer to any two different metals. The difference in the TMR coefficients is given by the following equation: TMR difference ¼ jC m −C n j

ð11Þ

where C is the TMR coefficient. The computed values of the selected metal combinations are plotted on the graph of GWP difference versus TMR difference. 3.4. Potential changes in the environmental impact due to metal substitutes

Ax C x

ð4Þ

The TMR weight must be properly allocated to determine the TMR coefficient. Taking into account the fact that monetary return is the main incentive of mining activities, this study selects the economic allocation technique. The unit price of each output material is represented by Dy, and the allocation rate (ry) is computed by the following equation: By Dy ry ¼ P By Dy

ð5Þ

In accordance with the law of conservation of mass in a given process, the allocated total weight is the difference between the total weight of the input materials, based on the TMR, and the total weight of the output materials. The weight, on the basis of TMR, allocated to each of the output materials expressed as Ty′, is obtained by the following equation: T 0y ¼ ðT−BÞr y

ð6Þ

On the basis of the GWP and TMR of the assessed metals presented in Fig. 1, the environmental impact caused by metal substitutes was evaluated. Graedel et al. summarized the potential substitute options and their applications (Graedel et al., 2015b). This study selected 75 options of metal substitutes to be assessed. The metal alloy and compound substitutes are excluded as this study focuses on the potential of individual elements. For each of the metal substitutes, the differences in GWP and TMR were computed using logarithms to clarify the essence of the change in environmental impact, with the help of the following equations. GWP change due to metal substitute ¼ logSp − logSq

ð12Þ

TMR change due to metal substitute ¼ logC p − logC q

ð13Þ

where p and q signify the substitute and original metals, respectively. Each of the metal substitutes is plotted on a graph of change in TMR due to metal substitute versus change in GWP due to metal substitute. In addition, in order to evaluate the global environmental impact, this study focused on the potential annual metal production of the original metal corresponding to its application. It was computed by using the

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Fig. 1. Relations between GWP and TMR.

data on world metal production (Nuss and Eckelman, 2014; U.S. Geological Survey, 2010) and the ratio of applications potentially designated for replacement to the world production of original metals (Graedel et al., 2015b). 4. Results 4.1. Relations between GWP and TMR As a result of the methodology developed for computing the TMR of assessed metals, the relations between GWP and TMR are presented in Fig. 1. This study categorizes the assessed metals into 7 groups viz., transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Ta, W, Hf, Re), precious metals (Au, Ag, Pt, Pd, Rh, Os, Ir), rare earth elements (Sc, La, Ce, Pr, Nd, Sm, Eu, Dy, Yb, Y, Er, Tb, Gd), alkali metals (Li), alkali earth metals (Ba, Ca, Sr), metalloids (B, Ge, As, Sb, Te), and base metals (Al, Zn, Ga, Cd, In, Sn, Hg, Pb, Bi, Be, Mg, Tl, Se). Although precious metals and rare earth elements are grouped under transition metals, these are specifically separated due to their independent characteristics, such as a similar refining process for the elements in each group. To generalize, the metals with higher GWP are also the ones with higher TMR. For metals with a large TMR, a significant amount of energy is required at the mining stage, which would be dominant among the processing steps in terms of energy input. This results in a large GWP. Different trends are observed among the metal groups. The GWP and TMR for the rare earth elements are correlated in a linear manner. The number of samples of alkali metals and alkali earth metals was not sufficient to determine the trend between their GWP and TMR. Moreover, although the basic trend might be seen, other metal groups such as precious metals (excluding perhaps Ag), transition metals, metalloids, and base metals do not indicate a strong relation between GWP and TMR. There are many cases where the value of the GWP is almost the same while the TMR is significantly different or vice versa, even in the same metal group. For example, in spite of a similar value of GWP, the TMR differs by more than two orders of magnitude in the same metal group (e.g., Fe-Zr, Mn-Zr, and Ti-Mo in the transition metals, Sb-Te in metalloids, Al-Hg, Al-Sn, and Bi-In in the base metals). Conversely, despite a similar value of TMR, the GWP differs by more than two orders of magnitude in the same metal group (e.g., Ru-Hf in the transition metals, Sb-As in metalloids, Tl-Sr in the base metals). In addition, significant relations for the same companion metals, obtained as by-products (Nassar et al., 2015b; Reuter and V.E.V., 2004), are also not observed.

For further analysis, the TMR difference for the metal combination with negligible GWP difference is presented. The result given in Fig. 2 shows that for many metal combinations, a significant gap in the TMR is obtained, although the GWP is almost the same. In particular, the TMR is scattered in the range of 10–550 kg-TMR/kg, with the same GWP, for 9 metal combinations. Considering the difference of two orders of magnitude between the GWP and TMR based on Fig. 1, in this research work, 102 times of GWP is considered to be of the same impact magnitude of TMR. On the basis of the ratio of the TMR difference to GWP difference, the magnitude of the TMR gap is divided into three groups (presented in Table 1). Those in group A, where the ratio of TMR difference to GWP difference is in the range of 0–102, are considered to have a small gap of TMR. Those in group B, with the ratio of TMR difference to GWP difference in the range of 102–103, are considered to have some gap of TMR, and those in group C, with the ratio of TMR difference to GWP difference above 103, are considered to have a huge gap of TMR. Among 77 of the total metal combinations with similar GWP, there are 18 in group A (23%), 24 in group B (31%), and 35 in group C (46%) i.e., nearly half of the metal combinations with similar GWP indicate a significant difference in TMR. In the case of the GWP in the range of 0–100, all the metal combinations are categorized in either group B or C, except Ti-Al. This may be because different processing techniques have been employed for each of the metals and the energy input for the ore mining activity is not dominant among the steps of metal production. Moreover, in the case of the GWP in the range of 140–240, there are many metal combinations

Fig. 2. TMR difference for the metal combination with negligible GWP difference.

S. Kosai, E. Yamasue / Science of the Total Environment 651 (2019) 1764–1775 Table 1 Groups of metal combinations on a basis of ratio of TMR difference to GWP difference. Group Range of ratio of TMR Metal combination difference to GWP difference A

0–102

B

102–103

C

103–

Ti-Al, La-Gd, La-Y, La-Be, Ce-Gd, Ce-Y, Ce-Be, Sm-Gd, Sm-Y, Sm-Be, Er-Gd, Er-Y, Er-Be, Pr-Yb, Nd-Yb, Nd-Ge, Ge-Ag, Ga-Ta Cr-Cu, Cd-Zn, Sr-Se, Mg-Ni, Mo-Ni, Mo-Li, Ni-Ti, Li-Co, W-Sb, Sn\ \Sb, Te-V, In-Be, Gd-Y, Y-Be, Gd-Hf, Y-Hf, Be-Hf, Hf-Pr, Hf-Yb, Pr-Ge, Yb-Ge, Ge-Ga, Ag-Ga, Tl-Re Ba-As, Mn-Ca, Zr-Mn, Zr-Ca, Zr-Fe, B-Fe, Cd-Cu, Sr-Zn, Mg-Mo, Ni-Li, Li-Ti, Li-Al, Ti-Co, Al-Co, Hg-Nb, Hg-W, Hg-Sb, Nb-W, Nb-Sb, W-Sb, Sn-Sb, Sb-Te, Sn-Te, Te-V, La-Ce, La-Sm, La-Er, Ce-Sm, Ce-Er, Sm-Er, La-In, Ce-In, Sm-In, Er-In, In-Gd, In-Y, Hf-Nd, Pr-Nd

categorized in group A and this may be because the increase in energy input for the ore mining activity correlates the TMR with GWP. In summary, significant discrepancies in the TMR for similar values of GWP in certain metal combinations could potentially indicate differences of magnitude between greenhouse gas emissions and resource exploitation. This result emphasizes the importance of assessing not only the GWP but also the TMR for a broad evaluation of the environmental impact of metal production.

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detail. A summary of the metal substitutes in each of the regions and associated information is presented in Table 2. Metal substitutes in each of the regions are shown in descending order of magnitude of environmental indicator differences. 4.2.1. Region I Both the GWP and TMR increase in Region I, which has 15 metal substitute options, accounting for 20% of the overall metal substitutes. The potential annual increase in the environmental impact in Region I is presented in Fig. 4. In terms of the GWP and TMR, the metal substitutes of (Fe → Al), (Ag → Au) and (Cr → Al) have the largest magnitude of environmental impact. For the metal substitutes of (Fe → Al) and (Cr → Al), the GWP and TMR per kg do not indicate a great deal of gaps, while the annual production of Fe and Cr in the associated applications is significant, particularly in transport vehicles. Fe used in the vehicle body is replaced by Al to achieve weight reduction and increase fuel economy (Liedl et al., 2011; Bai et al., 2017), and the metal substitute of (Cr → Al) is evident in automotive exhaust catalytic systems, bicycles, and ships (Johnson and Schewel, 2006). In addition, despite the high cost of Au, the more critical depletion time of Ag (Graedel et al., 2015a) may lead to the metal substitute of (Ag → Au) in future, for applications in jewelry and investments (S. Institute, 2018). Metal substitutes of (As → Cu) used in wood preservation and agricultural chemicals, (Re → Pt) used in petroleum refining catalysts and (Se → Te) used in plastics, inks, paints, and catalysts in chemical reactions are also likely to transpire from the perspective of supply risk measured in the metal criticality assessment (Graedel et al., 2012).

4.2. Potential changes in the environmental impact due to metal substitutes The importance of considering the GWP and TMR separately was pointed out in Section 4.1. This section analyzes the environmental impact due to metal substitutes from the perspective of both the environmental indicators. The change in the environmental impact due to metal substitutes, shown in Fig. 3, can be divided into 4 regions: Region I Both the GWP and TMR increase due to the metal substitutes, which is the most environmentally critical group, Region II - The GWP increases while the TMR decreases, which is environmentally critical from the GWP perspective, Region III - Both the GWP and TMR decrease, which is favorable for the environment, and Region IV - The GWP decreases while the TMR increases, which is environmentally critical from the TMR perspective. Although many proposed and implemented metal substitutes could reduce the GWP and TMR, nearly 40% of them potentially have an additional impact on the environmental system from the perspective of either the GWP or TMR. Each of the regions is further evaluated in

4.2.2. Region II The GWP increases and TMR decreases in Region II, in which there are 9 metal substitute options, accounting for 12% of the total metal substitutes. The potential annual GWP increase and TMR reduction in Region II is presented in Fig. 5. The order of magnitude of GWP increase and TMR reduction is almost the same between the two indicators. In terms of the GWP and TMR, the metal substitutes of (Cu → Al), (Zr → Al) and (Pd → Pt) have the largest magnitude of environmental impact. For the metal substitutes of (Cu → Al) and (Zr → Al), the GWP per kg does not indicate a large gap, while the demand for Cu and Zr in the associated applications is significant. The metal substitute of (Cu → Al) can be seen in various applications due to the light weight and low price of Al. However, its performance as a substitute is poor in some applications such as electrical lines and wiring, transport vehicle system, and electronic devices (Graedel et al., 2015b). The metal substitute of (Zr → Al) is used in the form of alumina in opacifiers for tile glazes with an adequate performance (U.S. Geological Survey, 2010).

Fig. 3. Environmental impacts through the metal substitute.

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Table 2 Summary of environmental impacts caused by the metal substitute (based on the literature (Reuter and V.E.V., 2004)). Metal substitute

Application

GWP gap (kg-CO2-eq/kg)

TMR gap (kg-TMR/kg)

Potential annual metal production (kg/year)

Region I Ir → Rh Ag → Au Re → Pt Ru → Ir Pd → Au Er → Tb Se → Te Nb → Ta Nb → V As → Cu Co → Nd Gd → Nd Fe → Al As → Sb Cr → Al

Chemical Jewelry, investment Catalyst Electrical, electrochemical Investment Phosphor Chemical and pigments Stainless steel Construction, transportation, oil and gas industry Wood preservation and pesticide Magnet Neodymium magnet Transportation, metal goods Copper alloy Transportation, household appliances and electronics

2.62E + 04 1.23E + 04 1.21E + 04 6.75E + 03 8.62E + 03 1.03E + 03 1.83E + 01 2.48E + 02 2.06E + 01 2.50E + 00 1.32E + 02 3.30E + 01 6.70E + 00 1.26E + 01 5.80E + 00

1.90E + 06 1.10E + 06 5.12E + 05 3.92E + 05 2.90E + 05 1.42E + 04 9.55E + 03 6.16E + 03 8.60E + 02 3.30E + 02 2.29E + 02 1.64E + 02 4.00E + 01 3.00E + 01 1.20E + 01

2.38E + 03 6.83E + 06 8.24E + 03 2.34E + 04 8.46E + 03 1.06E + 06 2.37E + 05 4.00E + 06 3.08E + 07 3.63E + 07 4.01E + 06 1.68E + 06 2.09E + 11 3.74E + 06 3.72E + 09

Region II Pd → Pt In → Ga Se → Bi Te → Bi Zr → Al Mo → Nb Cu → Al Ni → Al Gd → Y

Autocatalyst, jewelry Solder and alloy Metallurgy Ferrous product Ceramics Superalloy Electrical, transportation, cooling, electronics, architecture Transportation Phosphor

8.62E + 03 1.03E + 02 5.53E + 01 3.70E + 01 7.10E + 00 6.80E + 00 5.40E + 00 1.70E + 00 1.00E + 00

−2.80E + 05 −4.70E + 03 −2.30E + 02 −9.78E + 03 −5.02E + 02 −1.10E + 02 −3.12E + 02 −2.12E + 02 −2.17E + 02

1.12E + 05 4.86E + 04 5.94E + 05 6.74E + 04 8.38E + 08 2.18E + 07 8.12E + 09 1.10E + 08 5.12E + 05

Region III Rh → Ni Rh → Co Rh → Pt Au → Ag Pd → Ni Os → Ru Pt → Co Pt → Mo Ir → Mo Ir → Ru Pt → Ir Sc → Ti Sc → Al Tb → Er Te → Pb Te → Se Hf → Zr Ta → Al Ta → Zr Tb → Dy Ta → Nb Ag → Fe Ag → Cu Sn → Pb Sn → Al Li → Ca Li → Ni Pr → Mg Pr → Ce V → Nb Nd → Mg Nd → Ce Co → Mn Y → Ca Cu → Fe Co → Ni Bi → Pb La → Mg Ce → Mg Al → Fe Cr → Mn Sb → Ti La → Ce Sr → Ba

Electrical Chemical Glass Jewelry, official transactions and private investment, electronics, dental and medical Electrical, dental, chemical Electron microscopy, chemical Chemical Petroleum Electrical Electrochemical Glass Sports equipment Aerospace and defense Phosphors metallurgy: nonferrous products Chemicals and catalysts Aerospace, superalloy: fuel reprocessing plants, manufacturing: plasma cutting tools Capacitors Sputtering targets Neodymium magnets alloy additives, tantalum carbide Silverware Electrical and electronics Chemicals Tinplate Lubricating greases Batteries Metallurgy, except batteries Automobile catalystic converters Full alloy steel, high-strength low-alloy steel, carbon steel Metallurgy, except batteries Ceramics, automobile catalytic converters Batteries Ceramics Industrial, building plant Superalloys, catalyst Fusible alloys, solders, and ammunition cartridges, metallurgical additives Metallurgy, except batteries Metallurgy, except batteries Transportation, building and construction, packaging, machinery building and infrastructure Chemicals Glass additive Ferrite ceramic magnets, pigments and fillers, electrolytic production of zinc

−3.51E + 04 −3.51E + 04 −2.26E + 04 −1.23E + 04 −3.87E + 03 −2.45E + 03 −1.25E + 04 −1.25E + 04 −8.85E + 03 −6.75E + 03 −3.64E + 03 −2.89E + 03 −2.89E + 03 −1.03E + 03 −2.06E + 01 −1.83E + 01 −1.30E + 02 −2.52E + 02 −2.59E + 02 −3.27E + 02 −2.48E + 02 −1.95E + 02 −1.93E + 02 −1.58E + 01 −8.90E + 00 −6.10E + 00 −6.00E-01 −1.35E + 02 −4.90E + 01 −2.06E + 01 −1.35E + 02 −4.90E + 01 −7.30E + 00 −1.07E + 02 −1.30E + 00 −1.80E + 00 −5.76E + 01 −8.56E + 01 −8.56E + 01 −6.70E + 00 −1.40E + 00 −4.80E + 00 0.00E + 00 −3.00E + 00

−2.30E + 06 −2.30E + 06 −1.77E + 06 −1.10E + 06 −8.10E + 05 −5.32E + 05 −5.29E + 05 −5.29E + 05 −3.99E + 05 −3.92E + 05 −1.30E + 05 −6.40E + 04 −6.39E + 04 −1.42E + 04 −9.97E + 03 −9.55E + 03 −9.45E + 03 −6.75E + 03 −6.25E + 03 −6.24E + 03 −6.16E + 03 −4.79E + 03 −4.44E + 03 −2.47E + 03 −2.45E + 03 −1.41E + 03 −1.24E + 03 −9.62E + 02 −8.89E + 02 −8.60E + 02 −7.69E + 02 −6.96E + 02 −6.00E + 02 −3.68E + 02 −3.52E + 02 −3.50E + 02 −1.90E + 02 −8.39E + 01 −7.32E + 01 −4.00E + 01 −2.60E + 01 −2.00E + 01 −1.07E + 01 0

2.39E + 02 1.92E + 03 7.18E + 02 2.16E + 06 4.91E + 04 2.46E + 03 9.63E + 03 5.78E + 03 1.70E + 03 2.83E + 03 7.70E + 03 2.00E + 03 5.00E + 03 2.85E + 05 1.69E + 04 3.51E + 04 1.60E + 07 6.12E + 05 1.40E + 05 3.53E + 04 3.44E + 05 1.28E + 06 4.91E + 06 4.33E + 07 5.67E + 07 1.69E + 07 3.53E + 07 3.49E + 05 9.96E + 04 5.13E + 07 1.43E + 06 8.94E + 05 1.26E + 07 4.46E + 06 3.53E + 09 1.89E + 07 1.16E + 07 2.06E + 06 4.79E + 06 2.97E + 10 4.65E + 09 1.28E + 07 1.81E + 06 3.28E + 08

Region IV

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Table 2 (continued) Metal substitute

Application

GWP gap (kg-CO2-eq/kg)

TMR gap (kg-TMR/kg)

Potential annual metal production (kg/year)

Pt → Au Pt → Pd W → Mo Al → Cu Ce → Se Nb → Mo Ce → La

Investment Jewelry, autocatalyst, medical and biomass, electrical Mill product, steel Electrical, consumer durable Class additive Niobium metal and alloys Automobile catalytic converter

0 −8.62E + 03 −6.90E + 00 −5.40E + 00 −8.74E + 01 −6.80E + 00 0

5.70E + 05 2.80E + 05 5.50E + 02 3.12E + 02 3.07E + 02 1.10E + 02 1.07E + 01

1.35E + 04 8.67E + 04 1.46E + 07 7.52E + 09 6.49E + 06 2.00E + 06 5.47E + 06

In addition, despite the relatively low demand, the metal substitute of (Pd → Pt) applied in jewelry and vehicle exhaust catalytic systems may potentially aggravate global warming issues. Owing to the critical supply risk of indium in the metal criticality assessment (Graedel et al., 2012), particularly for its depletion time (Lokanc et al., 2015), the metal substitute of (In → Ga) in solders and alloys, in transport vehicles, and in transparent electrodes (Arvidsson and Sandén, 2017; Bae et al., 2012) would be highly imminent among the metal substitute options in this region. 4.2.3. Region III Both the GWP and TMR decrease in Region III, which has 44 metal substitute options, accounting for 59% of the total metal substitutes. The potential annual reduction of the environmental impact in Region III is presented in Fig. 6. Although the order of magnitude of reduction in GWP and TMR is correlated to some extent, some metal substitutes contribute to either one of these. The metal substitutes such as (Sr → Ba), (Bi → Pb), (Y → Ca), (Ce → Mg), (Nd → Mg), and (La → Mg) contribute to the reduction of GWP rather than that of TMR, whereas the metal substitutes such as (Pd → Ni), (Li → Ca), (Rh → Co), (Co → Ni), (Li → Ni), and (Os → Ru) contribute to the reduction of TMR rather than that of GWP. In terms of the GWP and TMR, the metal substitutes of (Au → Ag), (Cu → Fe), (Al → Fe), and (Cr → Mn) have the highest magnitude of environmental impact. For the metal substitutes of (Cu → Fe), (Al → Fe), and (Cr → Mn), the GWP and TMR per kg do not indicate large changes, while the demand of Cu, Al and Cr in the associated applications is significant. The metal substitute of (Cu → Fe) is used in industrial equipment machine tools (International Copper Study Group, 2017), (Al

→ Fe) used in building equipment, packaging cans and foil, and machinery tools, besides the body of transport vehicles as explained in the reverse substitute of Region I (World Aluminium, 2013), and (Cr → Mn) is used in elevators, steel furniture and steel reinforced concrete (Johnson and Schewel, 2006). These metal substitutes are highly recommended to globally reduce the environmental impact, from the GWP and TMR perspectives. 4.2.4. Region IV The TMR increases and GWP decreases in Region IV in which there are 7 metal substitute options, accounting for 9% of the total metal substitutes. The potential annual GWP reduction and TMR increase in Region IV is presented in Fig. 7. In terms of the GWP and TMR, the metal substitutes of (Al → Cu) and (Pt → Pd) have the largest magnitude of environmental impact. For the metal substitute of (Al → Cu), while the gap of GWP per kg between Al and Cu is marginal, the annual production of Al in the associated applications is significant. In addition to its use in the transmission and distribution line as a reverse substitute as presented in Region II, this metal substitute is used in cooking and home appliances. The nature of the metal substitute of (Pt → Pd) is similar to the reverse substitute as presented in Region II. The metal substitutes of (Pt → Au) and (Ce → La) do not produce a reduction in the GWP while increasing the TMR. The metal substitute of (Pt → Au), like (Pt → Pd), has a significant gap of TMR per kg with low demand and this is used for investment. The metal substitute of (Ce → La) is used in the oxygen-exchange system of automobile exhaust emission control. In addition, the metal substitute of (W → Mo) can be potentially used in high-temperature high-strength materials besides the applications presented in Table 2.

Fig. 4. Potential Annual GWP and TMR increase through the metal substitute in Region I.

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Fig. 5. Potential annual GWP increase and TMR reduction through the metal substitute in Region II.

5. Discussion The changes in the environmental impact, due to the proposed and implemented metal substitutes, are categorized into 4 regions from the perspectives of the GWP and TMR. Notwithstanding the high concentration of greenhouse gas emissions and global warming as major research topics and the non-consideration of environmental impact of mining activities for metal production in earlier LCA research, the present analysis could identify the region IV that was missing in the previous environmental criteria. This categorization, on the basis of environmental considerations, may be of use in the identification of the suitability or otherwise of metal substitutes. An increasing growth in empirical research on the availability of metal substitutes is largely expected in response to metal criticality issues. It is important to evaluate the differences in the potential environmental impact of metal production, for future metal substitutes. For example, the metal substitute of (Pb → Bi) was

recently proposed for the application of piezoelectric materials (Kim et al., 2017). However, as it is categorized in Region I, where both the GWP and TMR increase, this substitute is not optimal from the environmental point of view. In addition, the prioritization of GWP and TMR in the environmental impact is influenced by the differences in culture, economy, geopolitical location, and society. Considering the acceptable range of environmental impact, each of the parameters needs to be adjusted suitably. The proposed approach contributes to the identification of the aspects that were overlooked in the previous environmental criteria associated with global warming issues, however, its limitations must be highlighted. Owing to excessive resource exploitation, the mining component factors, such as ore grade and strip ratio, vary with time depending on the geographical location of the mining site. Technological innovations may change the amount of energy input for processing metals. Another point to be considered is that the grade of ores is on a decline and it is

Fig. 6. Potential annual GWP and TMR reduction through the metal substitute in Region III.

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Fig. 7. Potential annual GWP reduction and TMR increase through the metal substitute in Region IV.

unlikely that new high-grade ore deposits will be found (Crowson, 2012; Watling, 2015). After the higher-grade ores have been fully exploited, mining activities would inevitably move to the lower grade ores. Therefore, the link between an increase in the environmental impact and a decline in the grade of ores is significant in mining activities (Norgate and Jahanshahi, 2010). These uncertainties may affect the justification of the computation of the TMR and GWP. Furthermore, this study focuses only on the virgin metal. Preempting a circular economy in future, the assessment must include a recycling process. In addition, based on the literature review, the qualitative relation between the magnitude of environmental impact and TMR is identified, i.e., a greater TMR corresponds to a greater magnitude of environmental impact. However, its quantitative correlation, e.g., linear, exponential, or probably another numerical model, has yet to be analyzed. The identification of its quantitative relation, which is similar to that between the greenhouse gas emissions and the temperature increase in the global warming scheme, would assist in the justification of the use of the TMR as an indicator for evaluating the environmental impact associated with mining activity. The comparison of the TMR with reported land use indicators is beyond the scope of this study. Although each of the land use indicators has its merits and demerits, it is considered that the TMR assessment method is highly appropriate for the intensity of mining activity due to its inclusion of ore grades and strip ratio covering different mining approaches, such as open-pit and underground mines. The comparative indicator analysis under the different types of mining sites may be useful to distinguish the various land use indicators and emphasize the importance of TMR utilization. 6. Conclusions A literature review identified the significance of environmental consequences of mining activities and highlighted the importance of developing a suitable indicator. The TMR concept was employed to evaluate the potential to affect the environment as a consequence of mining activity, i.e., mine waste and landscape change, in the metal production system. The relations between the GWP and TMR for 58 metals were assessed and the environmental impact due to metal substitutes, from the perspectives of GWP and TMR, was analyzed. It was found that the metals with higher GWP had a higher TMR. The metals with a domination of mining impact in the process of production could be plotted on the almost linear line between the GWP and TMR,

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