The method was developed for a national level analysis considering temporal factors related to the metal criticality and in view of the medium-term perspective (i.e., until 2030). This methodology is applicable for all mineral producing countries. For examples, copper mine and aluminum smelter produced countries in 2015 are depicted in the Supplementary Information, Tables S6 and S7, respectively.

The overall framework of the study is visualized in Fig. 1. As the first part of the research, the identification and classification of indicators were performed through an extensive literature review. With the support of published articles, 18 indicators were suggested under a good balance of both quantitative and qualitative aspects covering geological, economic, environmental, technical, social, political, policy, and regulatory constraints (Graedel et al., 2012; Harper et al., 2015a; Harper et al., 2015b; Nassar et al., 2015b; Nuss et al., 2014; Panousi et al., 2016). Since the criticality method should reflect as a three-demensional framework: supply risk, environmental risk, and supply restriction risk, indicators were proposed after understanding requirements of each dimension thoroughly. A list of suggested indicators under each dimension is shown in Fig. 2. All required datasets obtained to quantify indicators are publicly available and single or multiple sources can be used for calculations. The criticality assessment is based on 2015 data sources; however, 2016–2018 statistics were also adopted to determine a few indicators due to some issues arisen in the 2015 data usage. The unavailability of required data may influence on the final result of the assessment, but in some cases, certain variables can be removed from the methodology, which are described in the Supplementary Information. A risk index (see Fig. 2) with a common 0–100 score scale was used to precisely present the risk intensity of each indicator under each dimension. This index was also proposed based on the scientific literature (British Geological Survey, 2015; European Commission, 2010; European Commission, 2017; Graedel et al., 2012).

Fig. 1: An overall illustration of the research framework.

The detailed flow shows steps beyond the criticality method development.

Fig. 2: A summary of the risk index and criticality indicators in the study.

The indicators to measure SR, ER, and SRR are proposed not only conducting a comprehensive investigation of previously published papers but novel indicators are also introduced to develop the methodology. The risk score determination routes of each indicator are prudently described in the Supplementary information.

After completing the prior necessitates, the national criticality method for metals was constructed. It is essential to quantify indicators to figure out a potential risk score. Therefore, equations were developed to quantify all suggested indicators as the first step. A risk value range between very low and very high risk was introduced for each indicator (see Table S1 in the Supplementary Information for an example) as a supporting method to calculating the risk score. The assessment method for each indicator is distinctive and each result was finalized to a risk vlaue as the second step. Then, the obtained risk value of each indicator was converted to the risk score. All these steps are fulfilled with the help of scientific publications. The valuation methods until the risk score determination of indicators including the used scientific literature are summarized in Table 1. A further informative discussion is presented in the Supplementary Information intensely explaining the calculations of corresponding indicators of each dimension. Combining all indicators considered in each dimension of the criticality, a common equation was used to uncover the average value of supply risk, environmental risk, and supply restriction risk that is presented below.

$${mathrm{SR/ER/SRR}} = {sum} {frac{{{mathrm{Risk}},{mathrm{score}},{mathrm{of}},{mathrm{concerned}},{mathrm{indicators}}}}{{{mathrm{Number}},{mathrm{of}},{mathrm{indicators}}}}}$$


Table 1 A summary of assessing the indicators’ criticalities. Further detail is elaborated in the Supplementary Information.

The resultant average value of supply risk (SR) or environmental risk (ER) or supply restriction risk (SRR) is equal to the final risk score of each dimension. By comparing the final score with the risk index, the risk severity/intensity of indicators/dimensions can be determined.

In this study, we applied the developed method for assessing China’s copper and aluminum criticality status to demonstrate how this method can be performed at the national scale. Although all three-dimensional assessment was performed to one country, a mult-national ER assessment was also conducted for a broad range of countries. A sensitivity analysis to determine the possible uncertainties, which can be occurred at +5% and −5% of the calculated value of each indicator and dimension is comprised in the criticality evaluations. Thus, the calculated value is denoted as “C” and results of the sensitivity analysis is presented as “W”. Nevertheless, the maximium uncertainty is represented as 100 while presenting 0 as the minimum due to the proposed risk score range is in 0–100. Then, we introduced two approaches: single-metal approach and multiple-metal approach to mapping the criticality for a well-understandable interpretation while using China as the example. In the single-metal approach, we equally weighted SR, ER, and SRR by inspecting the degree of risk to interpret net resultant as a whole. For that purpose, we selected indicators at the range between substantial risk and very high risk as prioritized indicators in each dimension to implement the approach. Furthermore, we used overall risk scores resulted in SR, ER, and SRR to model the multiple-metal approach. Here, SR and SRR are interpreted as subsets of economic risk. As in the single-metal approach, we selected the substantial risk score and over this intensity range to interpret SR, ER, and SRR for each metal in the multiple-metal approach. Finally, comprehensive sensitivity analyses for all indicators in ER and selected indicators in SR and SRR were conducted regarding copper and/or aluminum at country levels to present the degree of accuracy of the study. The performed method of the sensitivity analysis is described in the Supplementary Information.

Since indicators represent relevant constraints to reflect SR, ER, and SRR of a certain metal, a further discussion on these constraints and reasons for why these indicators are selected to assess the metal criticality with appropriate literature evidences is articulated below.

Supply risk (SR)

Supply risk (SR) of a metal was calculated in view of the primary resource availability and production of a country. The national SR consists of nine indicators as shown in Fig. 2. The first five indicators are enclosed the geological, economic, and technical nature of a country while the last four indicators are based on social, political, and policy status of the same location.

The five indicators representing geological, economic, and technical scales are featured to demonstrate the parameters, such as primary metal availability, abundance, and production capacity that would be more influential to justify SR in a given country. It is important to address these parameters of mineral interest in measuring SR since the feasibility of acquiring primary sources is the main challenge at a national system. The geological reserve accumulation is much subjected to mineral supply due to its economic potential in mine extraction at the time of determination (Graedel and Nassar, 2013; United States Geological Survey, 2019) while supporting to fulfill the demand of the host country in a substantial proportion only through a marginal or zero dependence on others. In this case, SR becomes a lower stage since the existing primary resources play a pivotal role in demand reduction. Likewise reserve concentration, high-production concentration is advantageous in decreasing the supply risk. Not only a country obtains a monopoly or oligopoly in global mineral availability and/or production but these indicators are also concentrated to several countries that can considerably affect SR fluctuations. The reserve depletion period is in local importance to detect how long a country will likely balance the supply and demand for a certain metal. In the medium-term perspective, reserves would be a good relative measurement in determining the depletion time (Graedel et al., 2012; Nassar et al., 2012). Comprising entitlements to improve the national level resource production such as overseas mine entitlements to increase the primary production capacity is an efficient measure in stabilizing the supply (Hatayama and Tahara, 2015). Mineral adequacy (MA) is an indicator introduced to interpret such necessitates. Although a few commonly used metals such as aluminum, iron, and copper are typically abundant in the earth’s crust (Mookherjee and Panigrahi, 1994), a range of metals like gallium, germanium, and indium faces challenges in supply since they are combined with “carrier/major metals” in relatively minor quantities as “by-products” (European Commission, 2010; Nassar et al., 2015a; Schwarz-Schampera and Herzig, 2002). In such cases, the by-production is driven as a crucial and fragile concern to altering circumstances towards supply stability.

The social, political, and policy configuration of a particular country can either obstruct or accelerate the development of economic minerals. The governance and social approaches are interlinked with mining operations since metal supply can constrain according to these jurisdictions. Generally, negative socio-economic and ecological consequences due to mining activities are led to cultivate rigid government policies/regulations and societal objections. Policy Perception Index (PPI), a composite index that measures the mining policy desirability in jurisdictions at the country level (Stedman and Green, 2017; Wilson, 2018) is used to quantify the policy scale in SR assessment. One indicator, Human Development Index (HDI), is a synopsized calculation to presume the country-specific average performance in the human development process (United Nations Development Program, 2010; United Nations Development Program, 2018), is also employed to quantify social attitudes in this valuation. Out of six composite governance dimensions of the Worldwide Governance Indicators (WGI) (World Bank, 2018), two [i.e., Political Stability and Non-violence (PSNv) and Government Effectiveness (GE)] are represented in the study to quantify the political scale in SR determination. Whilst the political instability of a nation may lead to a higher risk of metal supply (Graedel et al., 2012), the attractive government efficacy would help to develop prolific mining/industrial policies and inspire the local and foreign business investment.

Environmental risk (ER)

The environmental degradation is probably connected to the metal extraction and processing activities occurring significant negative impacts in diverse ecosystems such as eco-toxicity, greenhouse gas emissions to three natural spheres (i.e., atmosphere, hydrosphere, lithosphere) and high-energy carriage (Reller, 2011; Zepf et al., 2014). The environmental country risk is sufficient to qualify a mineral for criticality when it is aggregated with economic risk (European Commission, 2010; Frenzel et al., 2017) that is associated with supply risk (SR) and supply restriction risk (SRR) of the current study. Moreover, preserving the environmental threshold by responsible industries and authorities is important in mining and processing operations since it is vulnerable to assemble the national economic risk in a substantial proportion pushing them to disburse an extra amount on nature safeguard. For an appropriate environmental justification, a WGI indicator, Regulatory Quality (RQ) (World Bank, 2018), as well as Environmental Performance Index (EPI) (Hsu et al., 2016) published by Yale University and Columbia University in collaboration with the World Economic Forum are used to implement the ER dimension in the criticality method.

Regulations are indispensable to analyze the environmental commitment towards industrial metal congregation activities at the country level. Especially, mining, refining, and smelting activities are more susceptible to releasing a significant amount of emissions to three spheres. The metal toxicity would also act as a negative agent for human and ecosystem health when industrial metal operations are accountable to incline emissions. Thus, the government regulatory framework should have a key role in strengthening environmental safeguard and good governance that is also combined with economic security. The EPI ranks the nations’ efficacy in solving prioritized environmental issues under two broad categories: protection of human health and conservation of ecosystems. Furthermore, EPI scores are compiled to evaluate the national performance in nine issue areas consisted of >20 indicators. While measuring the country’s proximity to compromise internationally conferred targets or, in the absenteeism of covenant targets, EPI also compares each nation’s ability towards environmental conservancy (Hsu et al., 2016).

Supply restriction risk (SRR)

Supply restriction risk (SRR) of a metal was estimated considering the global supply shortage and the importance of the national economic development. It is rather necessary to understand how much risk a country to be tackled due to a global supply restriction. Any country is not self-sufficient to overcome own commodity needs itself. This obstacle range fluctuates depending on several factors; however, the influential weights of these factors are different for each country. As in Fig. 2, SRR encompasses seven indicators in this study. It is necessary to assess the end-use application of each metal in the evaluation of SRR that varies depending on the degree of national importance and the substitutability of the metal (DeYoung et al., 2006; United States National Research Council, 2008). Country-specific requirements to meet certain circumstances referring to self-production capacity, import reliance, economic resilience, and knowledge & technology development also consist at the indicator list of SRR.

Metals would contribute at great importance for national economic development in a country (Graedel et al., 2012; Hatayama and Tahara, 2015; Nassar et al., 2012). Large quantities of metal production, particularly for sourcing-nations, are noticeably responsible for the national gross domestic product (GDP) inflation. Thus, this necessity is interpreted as an indicator under SRR. The self-ability to produce mineral resources is essential to meet a supply shortage. Therefore, Self-Sufficiency of Mineral (SSM) indicator tries to measure a country’s capacity to overcome a global supply restriction. Dependency on imports is one of the major threats, a country should address if a global supply shortage of minerals occurs. The high susceptibility for importations indeed leads to many issues in the case of a supply interruption. Recycling can apply at various stages in the anthropogenic life cycle of mineral production and manufacturing while declining SRR. The end-of-life (EoL) product recycling is strongly encouraged in today’s world since its substantial benefits, i.e., cost-effectiveness, technical feasibility, energy savings in the process (Rademaker et al., 2013). A higher EoL recycling rate may perhaps lead to a decrease in primary resource demand, material substitution, and energy demand (British Geological Survey, 2015; European Commission, 2010). Substitutes are potential applications to replace one commodity to another at a supply interruption that may reduce the risk while decreasing the raw material use of the corresponding commodity (British Geological Survey, 2015; Glöser et al., 2015); however, the substitution is specifically suitable to alter potentially scarce and hard recycling critical metals (European Commission, 2010). Although some characteristics of metals are irreplaceable, a consumable substitute can be discovered with time (Graedel et al., 2012).

A more innovative country means that it inherently obtains a good capacity to find new solutions and ways to overcome a potential supply restriction; thus, these nations are less vulnerable to an SRR (Graedel et al., 2012). Economic freedom provides greater opportunities for innovation, progress, and human flourishing. It is a good measurement to evaluate the market openness for novelty that is also beneficial to overcome the impact of global supply interruption towards a country. To measure these requirements, Global Innovation Index (GII) (Cornell University, INSEAD and WIPO, 2015) and Economic Freedom Index (EFI) (Miller and Kim, 2016) are utilized as indicators in the SRR determination.

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