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Journal of Geographical Sciences >

2024 , Vol. 34 >Issue 4: 763 - 778

DOI: https://doi.org/10.1007/s11442-024-2226-y

Research Articles

Unveiling structural differentiation in the global nickel trade network: A product chain perspective

  • CHEN Wei , 1, 2 ,
  • JIANG Yifei 1, 2 ,
  • LIU Zhigao , 1, 2, *
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  • 1. Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China
  • 2. School of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China
* Liu Zhigao (1974-), Associate Professor, specialized in economic geography and regional development. E-mail:

Chen Wei (1989-), Associate Professor, specialized in economic geography and regional development. E-mail:

Received date: 2023-07-10

  Accepted date: 2024-02-07

  Online published: 2024-04-24

Supported by

Second Tibetan Plateau Scientific Expedition and Research Program(2019QZKK1007)

National Natural Science Foundation of China(42130508)

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Abstract

This research aimed to understand supply and demand in the global nickel market from a product chain perspective. Accordingly, we established an extensive time-series database for the nickel trade network and employed network analysis methods to explore the dynamics and shifts in the global nickel trade network. The results are as follows: first, refined nickel and nickel semis dominated global nickel trade; second, the interaction between different nickel product networks profoundly affected the global nickel supply chain; third, the global nickel trade network consisted of core-periphery structures exhibiting different degrees of spatial heterogeneity; and fourth, the most connected global backbone structure was in nickel semis, followed by refined nickel, nickel ore, and nickel scrap. Together, trade in these four products constituted an overall network topology characterized by complex forms, a clear hierarchy, and uneven development. We conclude the paper by making several recommendations to secure global nickel supply chains and promote nickel circular economy.

Key words： nickel trade; nickel supply chain; complex network; product chain; circular economy

Cite this article

CHEN Wei , JIANG Yifei , LIU Zhigao . Unveiling structural differentiation in the global nickel trade network: A product chain perspective[J]. Journal of Geographical Sciences, 2024 , 34(4) : 763 -778 . DOI: 10.1007/s11442-024-2226-y

1 Introduction

Nickel, as a widely used industrial metal, holds strategic significance for national economic development owing to its distinctive properties, such as excellent plasticity, energy storage, and resistance to wear and tear, corrosion, high temperatures, and deformation (Henckens and Worrell, 2020). Due to its unique properties, nickel is mainly used in stainless steel, nickel-based alloys, electroplating, and batteries (Majeau-Bettez et al., 2011; Stankovic et al., 2022) and is indispensable to a wide range of military and civil manufacturing industries (Fernandez, 2019). The most common industrial uses of nickel are in steel and cast iron alloys (Nakajima et al., 2018). Iron-nickel alloys are often used in electronic and structural engineering applications, and copper-nickel alloys are used in coinage and marine engineering. As a key component of rechargeable battery systems, nickel is also used in electronic devices, electric tools, transportation, and crucial emergency power supplies, including nickel-metal hydride batteries. Nickel contributes to the longevity, affordability, and energy efficiency of many products (Winjobi et al., 2022). Nickel-containing products can be reclaimed and reused in the form of alloys and are one of the most recycled materials in the world (Rostkowski et al., 2007).
With the rapid development of the global economy, nickel has become one of the important nonferrous metals used by countries worldwide (Fernandez, 2019). However, the global distribution of nickel ore resources is uneven, with the largest concentrations in Indonesia, the Philippines, Russia, New Caledonia, and Australia (Reck et al., 2008; Mayyas et al., 2019). The uneven distribution of nickel resources has shaped spatial trade flows of the metal from ore to semis into a whole industry chain, forming a complex global trade network. The recent growth in green industries has boosted the global production of rechargeable batteries, in which nickel is a key component. Consequently, the metal supply and consumption have increased yearly (Sturgeon et al., 2008; Schmidt et al., 2016). It is, therefore, crucial to explore the structures and patterns of the global nickel trade to understand the supply chain that sustains it.
As global trade shifts toward globalized production (Jiang et al., 2022; Chen et al., 2023a, 2023b), the international division of labor that characterizes nickel production and manufacturing has grown more defined. Nickel ore, refined nickel, nickel semis, nickel scrap, and other products based on the metal generate diverse global spatial flows, constituting a complex trade network. Based on this background, we constructed a long-time series of global nickel trade network matrices, integrating various methods of complex network analysis. We portray the dynamic patterns of the global trade in nickel and explore its network topology, thus providing a comprehensive and in-depth account of the structural differences and spatiotemporal patterns that define trade in this vital metal.

2 Literature review

In the era of intensified globalization and heightened factor flows between nations, the production and processing of diverse products have progressively transcended national and regional borders, giving rise to global value chains (GVCs) and global production networks (GPNs) (Yeung, 2014; Werner, 2019). Recent global events have contributed to the spatial reshaping of GVCs and GPNs (Gereffi and Lee, 2012; Zheng et al., 2021), making them increasingly prominent in academic research. The normal functioning of global value chains and production networks relies on inter-country trade, and the specialized division of labor among countries and regions in product production propels the establishment and evolution of global trade networks (Neilson et al., 2014). With advancements in network science, the examination of global trade patterns from a network perspective has emerged as a crucial analytical approach (Liu et al., 2018; Chen et al., 2023c; Qin et al., 2023; Ren et al., 2023).
Currently, there is a heightened focus on the global trade network of specific products or industries. Structural characteristics and key countries in the global trade network of products like oil, food, automobiles, and minerals have been preliminarily unveiled (Blázquez and González-Díaz, 2016; Wang et al., 2022; Chen and Zhao, 2023; Niu et al., 2023). Notably, with the deepening industrialization worldwide and rapid growth in the manufacturing industry in select newly industrialized countries, the global trade of metal mineral resources, a critical raw material for various manufacturing sectors, is gaining substantial attention (Chen et al., 2024a). Similar to cobalt and zinc, the trade network for strategic metal resources also demonstrates significant spatial clustering, with a few large metal resource storage and production countries playing a central role (Sun et al., 2019; Liu et al., 2021; Yang and Chen, 2023; Yu et al., 2023). In contrast to global trade networks in industries like automobiles, heavily influenced by market effects (Blázquez and González-Díaz, 2016), the geographic distribution of metal minerals and variations in processing and smelting capacity is pivotal in shaping trade networks for metal products such as metal ores, compounds, and finished products, and their evolution. Furthermore, the patterns and structures of various metal trade networks differ somewhat due to the distinct generation conditions and geographical distribution of different metal minerals. Consequently, research on the trade networks of different metals holds great significance in elucidating the global metal trade market and ensuring the stability of the supply of key metal resources.
Nickel, one of the most essential strategic metals and raw materials for many industrial sectors, is gaining popularity in global trade networks. Scholars have studied the international nickel trade from various perspectives (Nakajima et al., 2018; Dong et al., 2021). Material flow analysis is one of the primary approaches for determining the flow of nickel resources between production stages (Daigo et al., 2010; Eckelman et al., 2012; Wang et al., 2022). Daigo et al. (2010) explored the material flow cycle of nickel in Japan, focusing on its use in stainless steel and providing a detailed elucidation of the sources of nickel in this cycle. Eckelman et al. (2012) explored the global nickel cycle by modeling inputs, outputs, and material flows. In nickel material flow studies, while some content addresses nickel trade between countries, the predominant focus remains on the production process of nickel itself, neglecting the comprehensive examination of the spatial flows of nickel and trade characteristics on a global scale. Recently, large nickel ore exporters such as Indonesia and the Philippines have introduced bans on mining or exporting metal, threatening the supply chain and security of the global nickel market. Consequently, scholars have redirected attention to research on nickel trade networks (Ma et al., 2022; Zheng et al., 2022; Yu et al., 2023). Dong et al. (2021) reconstructed nickel ore trade networks from supply and demand perspectives, and used a dynamic programming model to optimize the network structure and found that by optimising the nickel ore trade network, the trade cost was reduced, but the reduction was not uniform. Wang et al. (2022) devised a trade redistribution model to study the nickel ore trade relations among 93 countries and found that the major nickel material trade flows were concentrated in a few countries such as Indonesia, the Philippines, New Caledonia, China and Papua New Guinea. Ma et al. (2022) explored the global nickel ore trade pattern from 2005-2019 and found that the global nickel trade pattern is relatively stable, the nickel trade network has scale-free characteristics, China, the USA, and Germany are the core countries of the nickel trade network.
As a crucial strategic mineral resource, nickel is important for advancing emerging industries. The global nickel trade dynamics and supply chain have garnered considerable attention from scholars. The existing literature has preliminarily explored the global nickel cycle and trade network, drawing some conclusions. However, the existing studies have seldom conducted comparative studies on the nickel trade of different products and have neglected to investigate the global nickel trade network from the product chain perspective. To address these gaps, adopting a product chain perspective, this study explores the structural differentiation in the global nickel trade network. This study contributes to the existing literature in three main ways. First, it portrays the spatiotemporal patterns of global nickel trade networks from a product chain perspective, which builds an understanding of structural divergence in the nickel markets. Second, its depiction of global nickel trade network matrices since 2000 reveals the dynamics of global nickel trade patterns from a long-time series perspective. Third, it uses network science methods to analyze the topological structures of global nickel trade networks in depth.

3 Methods and data

3.1 Data processing

Within globalized production, the international division of labor involved in producing nickel and nickel-based goods has continued to develop, and worldwide spatial flows define global trade in the metal. According to the generalized metal material cycle and nickel material flow analysis (Reck et al., 2008; Ma et al., 2022; Wang et al., 2022; Zheng et al., 2022), terms used in the UN Comtrade, and Harmonized System (HS) codes, we roughly classify nickel products into nickel ore, refined nickel, nickel semis, and nickel scrap (Table 1).
Table 1 Classification of products containing nickel and HS codes
Product category Product content HS code
Nickel ore Nickel ore and concentrates 2604
Refined nickel Unwrought nickel 7502
Nickel semis Nickel mattes, nickel oxide sinters and other intermediate products of nickel metallurgy; nickel powders and flakes; nickel bars, rods, profiles and wire; nickel plates, sheets, strip and foil; nickel tubes, pipes and tube or pipe fittings; other articles of nickel wire; nickel plates, sheets, strip and foil; nickel tubes, pipes and tube or pipe fittings; other nickel articles. 7501, 7504, 7505,
7506, 7507, 7508
Nickel scrap Nickel waste and scrap 7503
We used the HS codes for different nickel products and the UN Comtrade Database to extract the data on nickel trade flows, including the attributes of importing countries, exporting countries, trade volume, and flow direction. We used import trade data and then cleaned and converted the data to compile information on nickel trade flows between 2000 and 2020 among 215 countries and regions of the world. Given the non-comparability of different nickel products regarding weight, we used trade volumes to measure trade relations between countries and regions. We finally constructed global nickel trade networks for four different nickel products based on the above process. The nodes in these complex networks denote countries or regions, the edges represent trade relations, and trade volumes indicate the weight of international trade. Directed and undirected weighted networks were constructed to represent the global nickel trade network.

3.2 Methods

3.2.1 Core-periphery profile

The concept of a network characterized by a dense core and a sparse periphery, known as a core-periphery structure, has been extended to various disciplines (Boyd et al., 2006). Several algorithms have been proposed to identify core-periphery structures in networks; however, many of them are not suitable for weighted networks, and their robustness remains to be verified. Addressing this issue, Rossa et al. (2013) introduced the core-periphery profile algorithm, which reveals the overall network structures and the distinctive roles of specific nodes.
In most real-world networks, however, the structure is not ideal although the core-periphery structure is evident: a weak connectivity exists among the peripheral nodes. This calls for the generalized definition of α-periphery, which denotes the largest subnetwork S with the persistence probability αS ≤ α.
We define the core-periphery profile αk (k = 1, 2, …, n) of the network by the following algorithm. This is the equation:
$\begin{align} & {{\alpha }_{k}}=~\underset{h\in N/{{P}_{k-1}}}{\mathop{\min }}\,\frac{\mathop{\sum }_{i,j\in {{P}_{k-1}}\mathop{\cup }^{}\left\{ h \right\}}{{\pi }_{i}}{{m}_{ij}}}{\mathop{\sum }_{i\in {{P}_{k-1\mathop{\cup }^{}\left\{ h \right\}}}}{{\pi }_{i}}} \\ & \ \ \ \ =~\underset{h\in N/{{P}_{k-1}}}{\mathop{\min }}\,\frac{\mathop{\sum }_{i,j\in {{P}_{k-1}}}{{\pi }_{i}}{{m}_{ij}}+\mathop{\sum }_{i\in {{P}_{k-1}}}\left( {{\pi }_{i}}{{m}_{ih}}+{{\pi }_{h}}{{m}_{hi}} \right)}{\mathop{\sum }_{i\in {{P}_{k-1}}}{{\pi }_{i}}+{{\pi }_{h}}} \\ \end{align}$
We start by the node i with the weakest connectivity and generate a sequence of sets {1} = P1$\subset$P2$\subset$…$\subset$Pn=N by adding, at each step, the node attaining the minimal increase in the persistence probability. Correspondingly, we obtain the core-periphery profile, that is the sequence 0 =α1 ≤α2, …, ≤ αn = 1 of the persistence probabilities of the sets Pk.
The above algorithm provides, as byproducts, two other important tools of analysis, centralization and coreness. We define the centralization C for a core-periphery profile αk as the complement to 1 of the normalized area, namely:
$C=1-\frac{2}{n-2}\underset{k=1}{\overset{n-1}{\mathop \sum }}\,{{\alpha }_{k}}$
We can therefore quantify such similarity by measuring the area between the αk-curve of a given network and that of the star network and normalizing to assign C = 1 to the star network (maximal centralization) and C = 0 to the complete network (no centralization).

3.2.2 Disparity filter

A network’s backbone is a sparse and (un)weighted subgraph that contains only the most ‘important’ or ‘significant’ edges. Backbone structure can be useful when the original network is too dense or when edge weights are difficult to interpret (Serrano et al., 2009; Domagalski et al., 2021). Among these methods, Serrano et al. (2009) proposed the disparity filter algorithm, which exploits local heterogeneity and local correlations and can filter out the backbone of dominant connections in weighted networks with strong disorder, preserving structural properties and hierarchies at all scales. As a result, the disparity filter significantly reduces the number of edges in the original network and simultaneously retains almost all of the weight and a large proportion of nodes.
To assess the effect of inhomogeneities in the weights at the local level, for each node with i with k neighbors, one can calculate the function:
${{\omega }_{i}}\left( k \right)=k{{Y}_{i}}\left( k \right)=k\underset{j}{\mathop \sum }\,p_{ij}^{2}$
where Yi(k) characterizes the level of local heterogeneity. Under perfect homogeneity, when all the links share the same amount of the strength of the node, ωi(k) equals 1 independently of k, while in the case of perfect heterogeneity, when just one of the links carries the whole strength of the node, this function is ωi(k) = k.
The null model we use to define anomalous fluctuations provides the expectation for the disparity measure of a given node in a pure random case. It is based on the following null hypothesis: random assignment from a uniform distribution produces the normalized weights corresponding to the connections of a certain node of degree k. The probability density function for one of these variables taking a particular value x is:
$\text{ }\!\!\rho\!\!\text{ }\left( x \right)dx=\left( k-1 \right){{\left( 1-x \right)}^{k-2}}dx$
which depends on the degree k of the node under consideration.
The disparity filter proceeds by identifying which links should be preserved for each node in the network. The null model allows this discrimination by the calculation for each edge of a given node of the probability the calculation for each edge of a given node of the probability αij that its normalized weight pij is compatible with the null hypothesis. The statistically relevant edges will be those whose weights satisfy the relation:
${{\alpha }_{ij}}=1-\left( k-1 \right)\underset{0}{\overset{{{p}_{ij}}}{\mathop \int }}\,{{\left( 1-x \right)}^{k-2}}dx<\alpha $
Note that this expression depends on the number of connections k of nodes to which the link under consideration is attached.

4 Results and analysis

4.1 Global nickel import and export trade dynamics

To comprehend the post-2000 evolution of the global nickel trade, we obtained statistical data on imports of nickel-containing products among 215 countries and regions worldwide since 2000 to depict the structural changes in the trade volume of different nickel products (Figure 1). Whereas imports and exports of refined nickel, semis, and ore were broadly consistent with overall global nickel trade fluctuations, the scrap trade remained stable across the period. Between 2000 and 2020, influenced by shifts in the global supply and demand of nickel as well as geopolitical and financial factors, the import and export volume of nickel products, except for scrap, fluctuated greatly. In 2007, the global trade volume of refined nickel and nickel semis peaked at US\$26.04 billion and US\$21.05 billion, respectively, followed by two troughs in 2009 and 2016. On the other hand, nickel ore reached its maximum trade value of US\$6.97 billion in 2011, followed by a fluctuating downward trend. Indeed, between 2011 and 2016, nickel ore, semis, and refined nickel displayed a significant downward trend in trade volumes. While nickel semis volumes rose considerably after 2016, global trade in refined nickel showed only a slight increase. Compared with the other three nickel products, global trade volumes of nickel scrap remained low and did not significantly change during the period.
Figure 1 Structural change in global nickel product trade
Regarding product structure, different aspects of the nickel trade demonstrated obvious structural divergence, with refined nickel and semis constituting the bulk of global nickel trade, followed by ore, while trade in scrap nickel accounted for only 4% of the total volume. Except for 2012 and 2013, the combined trade in refined nickel and nickel semis accounted for more than 80% of the total. From 2000 to 2015, refined nickel held the largest trade share, followed by nickel semis, nickel ore, and nickel scrap. Refined nickel had a slightly larger share of trade volume than semis until 2008, after which the two maintained similar shares until 2016, when the nickel trade changed, and the trade in semis began to exceed that of refined nickel. While trade volume in semis grew, refined nickel fluctuated downward. Imports at the back end of the nickel product chain increased, but the global trade in scrap nickel remained below US\$1 billion for most of the period.

4.2 Global nickel trade network patterns

In this section, we constructed undirected and weighted trade relationship matrices covering 215 countries and regions for the four nickel product types, allowing the global networks to be visualized. Figure 2 shows that the lines’ thickness and colors represent trade flows between the two countries. Complex international trade networks have formed due to the uneven distribution of nickel ore. The increase in nodes and trade links, as more countries trade nickel products in an increasingly complicated network of relations, demonstrates the growing complexity of the nickel supply chain.
Figure 2 Global nickel trade network patterns by product type
Varying endowments and demands for nickel ore resources have shaped the development of a complex trade network. Generally, countries with abundant nickel resources but no technology or energy to refine the ore tend to be the largest exporters. Figure 2a shows that global nickel ore trade links are significantly more concentrated in East and Southeast Asia and the South Pacific, with China’s central position in the network highlighted. The top three global nickel ore trade volumes flow through China, the Philippines, and New Caledonia, with China mainly importing nickel ore and exporting it to the Philippines and New Caledonia. In 2020, the trade volume of nickel ore between China and the Philippines reached US\$1751.65 million, accounting for 44.3% of the world’s total trade volume, 58.8% of China’s nickel ore trade, and 97.8% of the Philippines’ foreign trade in nickel ore. These figures demonstrate the close trade links between the two countries while emphasizing how the Philippines has increased its share of nickel ore exports during the period. Since 2010, China has consistently ranked first in the global trade volumes of nickel ore.
Countries with sizeable global trade volumes of refined nickel include Russia, Canada, Australia, China, and the USA, the first three of which are primarily exporters of refined nickel and the latter three importers (Figure 2b). Russia has always been the leading refined nickel exporter, while China’s market share has climbed since it acceded to the WTO. In 2020, the three countries with the largest global trade volumes were China, Russia, and Australia, totaling US\$49.80 billion and accounting for 28.60% of the global trade in refined nickel. Regarding trade linkages, the largest global trade flow in refined nickel in 2020 was worth US\$765.4 million between the USA and Canada. Canada’s advantageous resource endowment and high demand from the USA, alongside the close political and economic cooperation between the two countries, ensure that supply and demand are closely matched. Meanwhile, South Africa possesses nickel ore resources, an industrial base, and the technological capabilities necessary to sustain a refined nickel trade network in the African region.
Compared with nickel ore and refined nickel, bilateral agreements to trade nickel semis increased significantly during the period, with higher network density and greater node coverage forming a complex nickel trade network. The industrial production of nickel semis requires high technology and know-how, so trade was mainly concentrated in the USA, China, Japan, Canada, Norway, Germany, and other developed economies or emerging market countries. Some countries maintain their core position in the nickel semis trade network by importing nickel ore and refined nickel, which they manufacture into nickel semis for export. For example, the USA, Germany, and other countries in the nickel industry chain all have robust production and processing capacities and advanced scientific expertise that has supported their positions in the nickel semis trade for many years. During the past 20 years, demand for nickel semis rose in Japan and Norway while China’s share of the semis trade increased and occupied an important position in the network by the end of the period.
The global trade in scrap nickel is relatively small, with stable nodes, simple trade relationships, and trade links concentrated in a few developed economies or emerging market countries. During the period under study, the USA, Germany, the UK, Japan, and Canada enjoyed relatively large shares in the scrap nickel trade. The volume of trade in scrap nickel in the USA increased from US\$118.9 million in 2000 to US\$292.7 million in 2020, with its share of trade rising from 14.42% to 24.02%. As an important raw material for many industrial processes, nickel recycling helps to build a circular economy. The scrap nickel trade significantly lowers the resource consumption rate and eases the environmental burden. Although trade in scrap nickel remains relatively undeveloped, its potential value is apparent and likely to attract much greater interest in the future.

4.3 Core-periphery structures of global nickel trade networks

We used core-periphery profile algorithm to measure the structure of the global nickel import-export network for the four sub-types of scrap and refined nickel, ores, and semis. The concentration coefficients of the core-periphery structures for each network type were all greater than 0.9, indicating the core-periphery organization of the overall network. We classified global nickel trading partners based on their degree of coreness: nodes with a coreness of above 0.3 were classed as core countries, those whose coreness was 0.3-0.1 were designated as sub-core, those with a coreness of 0.1-0.01 were sub-peripheral, and those whose coreness was below 0.01 were classified as peripheral countries (Figure 3).
Figure 3 Core-periphery structures of global nickel product trade networks
In the nickel ore trade network, China and Canada were the core nodes, South Korea, Japan, and North Macedonia constituted the sub-core, while Ukraine and Finland were classified as sub-peripheral importers (Figure 3a). Most of China’s nickel ore imports were from Pacific countries such as the Philippines, Australia, and New Caledonia, while Canada imported most of its ore from other countries in the Americas and Europe, such as the USA, Brazil, and Switzerland. In 2020, the total volume of nickel ore imported by China and Canada reached US\$2.5 billion, accounting for 63.4% of the global total and positioning them as the core controllers of the market for nickel ore—although the coreness of China in the network was much larger than that of Canada. As nickel ore exporters, the Philippines, New Caledonia, Indonesia, and other countries occupied peripheral locations in the network due to their low volume of exports or sole trade focus on one other country.
The concentration coefficient of the refined nickel trade network was 0.94, and the core and peripheral structures were more complex, incorporating more core countries (Figure 3b). Countries focusing on exports included Russia, Australia, Canada, Norway, Japan, Finland, and the UK. Germany was primarily an importer while Japan and the Netherlands balanced larger import and export volumes of refined nickel, highlighting their role in channeling global flows of the metal. Whereas China was a core country in the ore trade, it was sub-peripheral in the refined nickel trade, with much lower coreness in the nickel industry and less influence within the nickel trade network.
The concentration coefficient of nickel semis was 0.95 in 2020, a very similar value to that of the refined nickel trade network, although there were fewer core countries (Figure 3c). The USA, Japan, and Russia were the large exporters of nickel semis. China, Norway, and the UK were the primary importers, while Germany maintained a balance of imports and exports. Notably, despite Russia and Australia’s substantial nickel ore reserves, neither they nor other ore exporters were core traders in the semis network, showing that resource endowment advantages played a minor role in this aspect of the trade.
In 2020, core nations in the nickel scrap trade network were the USA, Germany, the UK, Japan, and India, with South Korea, the Netherlands, Spain, and Malaysia comprising the sub-core. Compared with the upstream nickel industry, the scrap trade network was relatively simple and involved only a few countries (Figure 3d). The scrap nickel recycling and reuse level was highest in the USA, reflecting the country’s coreness. In Western Europe, nickel scrap trade revolves around the core countries of Germany and the UK. The UK primarily exports its scrap to the USA, whereas Germany maintains a more balanced trade in scrap nickel, involving both exports and imports. The distribution of core countries suggests that Western Europe and East Asian countries generally pay more attention to the recycling and reuse of nickel resources, which helps promote the circular economy of global nickel resources.

4.4 Backbone structures of global nickel trade networks

To extract this network’s backbone and supporting structure, we extended our use of the disparity filter algorithm to the four nickel products, visualizing our results cartographically (Figure 4). Overall, the backbone structures of the global nickel trade networks diverge across different geographical regions, with the nickel semis trade network demonstrating the highest network density, number of nodes, linkage strength, and trade volume, followed by the trade networks in refined nickel, ore, and scrap. Together, these constitute global nickel trade network patterns with complex forms, a clear hierarchy, and uneven development.
Figure 4 Backbone structures of global nickel product trade networks
The global trade in nickel ore was manifested as the main structure, with the importing countries at its core supporting the spatial flow of raw nickel materials. Figure 4a shows that this trade was concentrated in East and Southeast Asia and the Pacific nations. By 2020, China was the main nickel ore importer, and ten countries in this backbone network shared trade links with the country, accounting for 16.13% of the backbone trade network. These trade volumes were huge, and China’s core position in the network was prominent; conversely, Europe’s involvement in the backbone structure was associated with relatively small volumes of trade in nickel ore. As the core of the European backbone, Germany has established nickel ore trade links with seven countries, a much smaller number than China, Japan, South Korea, and other countries enjoy. Canada, which imports a large amount of nickel ore from the USA, Brazil, Finland, Switzerland, and other nations, is also a key node and the other core country in the network alongside China.
In the global refined nickel trade, exporters like Russia, Australia, Canada, and Norway and importers like China, the USA, Germany, and India comprise the network’s backbone, with obvious spatial hierarchical characteristics (Figure 4b). The refined nickel trade backbone exhibits denser connections than ore, marked by increased interactions between importing and exporting countries. South Africa and Australia are key exporters of refined nickel to Europe and East Asia, playing a crucial role in the refined nickel backbone network. Russia, the leading exporter of refined nickel, engages in substantial and diversified trade, primarily with China, Germany, the Netherlands, and the USA. The USA was also a core node in this backbone network, predominantly importing refined nickel from Canada and forming strong trade links with European countries.
The nickel semis trade network demonstrates even more profound connections than the preceding two networks (nickel ore and refined nickel), exhibiting a complex topology with spatial interactions and a distinct hierarchy (Figure 4c). Global nickel semis tended to converge on the USA, which enjoyed trade ties with 55 countries and was the core of the backbone network. China, Japan, and other Asian countries also established backbone networks with the USA, forming a trans-Pacific nickel semis trade backbone. By 2020, China had established trade backbone network links with 35 countries, initially showing a pattern of trade links that radiated out to the world, while Germany enjoyed trade backbone links with 44 countries and maintained firm control over the global nickel semis network. Although these core countries were closely linked, Indonesia and Australia had limited connectivity to the backbone network, and the volume of South African trade in nickel semis trade declined significantly over the period, indicating a weakening of its backbone network. In addition, some countries in South America began to appear in the backbone semis trade network by 2020.
Finally, the nickel scrap backbone network formed a system with several developed countries at its core, and a low network density (Figure 4d). Global flows in nickel scrap were clustered around Germany and the USA: 13 and 14 countries had established scrap trade links with these countries, accounting for 13.36% and 12.5%, respectively, of the total. A more complex backbone structure had formed among European countries by 2020, although this lacked the participation of China. Overall, the trade volume of nickel scrap was and remains significantly lower than that of other nickel products, and spatial clustering was not evident.

5 Discussion and policy implications

Nickel is a metal found in many industrial processes and objects indispensable to everyday life. It is widely used in infrastructure, novel green applications, high-tech products, the military industry, and many other fields. The overall global volume of trade in nickel overgrew at first before slowing after 2008. The rapid development of green vehicles and other technologies has stimulated demand in recent years, and the global nickel supply chain has been studied more closely. The uneven distribution of nickel resources has led to a complex global nickel trade network. This is structurally differentiated and increasingly complex, with the number of countries and trade links increasing steadily since 2000. Moreover, the different nickel-based trade networks increasingly interact, profoundly altering global markets and the supply chain system.
Due to distinct links in the industrial chain, significant structural variations exist between the global nickel trade network and trade networks of manufactured products like automobiles. Trade in manufactured products tends to exhibit market orientation, reflecting globalization and decentralized layout trends (Blázquez and González-Díaz, 2016). In contrast, all four types of nickel products demonstrate evident hierarchical characteristics and spatial agglomeration trends. Specifically, the trade network of upstream products, such as nickel ores, reveals a pronounced main structure centered on nickel resource-producing countries. Regions like the South Pacific emerge as concentrated hubs for the nickel ore trade, with certain countries forming a crucial part of the backbone of the nickel ore trade. Unlike most global trade networks for manufactured goods, this region usually resides on the periphery with limited participation in global trade (Setayesh et al., 2022; Li et al., 2023).
The global nickel trade network shares similarities with other metal trade networks in macro characteristics, exhibiting apparent polarization effects and hierarchical features. However, owing to variations in the geographical distribution of different metal resources, the core nodes and backbone edges in the nickel trade network differ from those in other metal trade networks, such as cobalt (Sun et al., 2019; Yang and Chen, 2023), antimony (Zhao et al., 2023), and lead (Chen et al., 2024b). The nickel ore trade network displays distinct spatial patterns in the global nickel trade, with nickel resource countries serving as the primary exporters. In the middle of the industrial chain, influenced by the level of manufacturing industry and downstream demand market, developed countries and emerging economies hold dominant positions.
Compared with the existing nickel trade research, this paper investigates the global trade network encompassing nickel ore, refined nickel, nickel semis, and nickel scrap. By delineating commonalities within all nickel product trade networks, this study unveils nuanced distinctions among them. This study underscores the concentration of global nickel trade in select countries and regions, emphasizing the central roles of large industrial countries such as China, Germany, and the USA. Simultaneously, certain nickel resource-producing countries influence the overall connectivity of the trade network, aligning with existing conclusions (Dong et al., 2021; Ma et al., 2022; Zheng et al., 2022; Yu et al., 2023). Furthermore, this paper illustrates the differentiated characteristics of global trade networks within the four product types upstream in the nickel industrial chain and identifies key nodes in their respective trade networks. It preliminarily demonstrates that trade networks for upstream nickel products are significantly influenced by mineral distribution, whereas midstream product trade networks exhibit characteristics indicative of both productivity and market tendencies. Consequently, the findings will contribute to expanding and extending existing research on global nickel trade.
This paper employed a product chain perspective to comprehensively analyze the global nickel trade network’s dynamic changes, topological structures, and spatiotemporal patterns. Several policy suggestions are proposed to contribute to international cooperation in nickel trade and enhance the security of strategic metal reserves. Firstly, optimization of the nickel supply chain and spatial allocation of nickel resources is essential. Given the structural differences in trade for the four nickel products, each with its distinct industrial applications, there is a need to reassess their complementarity and substitutability in industrial production. Enhancing interactions across industrial chains’ upstream, midstream, and downstream segments and promoting integrity and diversity in global nickel product supply chains are imperative. Secondly, there is a necessity to optimize the structure of the nickel trade network and bolster its resilience. Given the variable distribution of nickel resources, the complex trade network requires ongoing efforts to improve its structure and robustness, fostering cooperation between matching importers and exporters. Countries should also diversify the nickel products they import and export to mitigate the impact of unforeseen events on the trade network. Thirdly, the establishment of emergency reserve mechanisms for nickel resources is crucial to reduce the impact of unexpected events on trade cooperation. Considering the profound influence of major global events on the global economy, countries should establish emergency reserve mechanisms for nickel resources to navigate uncertainty arising from growing domestic production demands and supply chain instability. Finally, a focus on nickel recycling and resource efficiency is paramount. More countries should participate in the scrap nickel trade network, leveraging technologies from developed nations to recycle and reuse mature nickel resources efficiently. This will enhance the overall utilization of nickel resources and related products, maximizing their role and significance in the circular economy.

6 Conclusions

Nickel, possessing exceptional plasticity and resistance to wear, corrosion, high temperatures, and deformation, serves as a crucial strategic resource for national economic development. Its widespread applications in stainless steel, alloys, electroplating, batteries, and other products underscore its significance. Exploring the structure and pattern of the global nickel market and its supply chain is thus imperative. In this regard, we constructed extensive time series matrices for the global nickel trade network, utilizing network analysis methods to depict dynamic patterns and network topology. The resulting study comprehensively explains structural variations and spatiotemporal patterns within the global nickel trade network.
The results show that: (1) Global nickel trade has exhibited rapid growth followed by stable development since 2000. Refined nickel, nickel semis, and nickel ore trades align closely with the overall trend, while nickel scrap trade has remained relatively stable. Regarding product structure, refined nickel and nickel trade dominate the global nickel trade, followed by nickel ore trade, with nickel scrap trade representing a smaller proportion. (2) Global nickel trade network exhibits clear structural differentiation. The interaction among various nickel product types has significantly altered the supply and demand dynamics, reshaping the global nickel industry’s overall supply chain system. (3) The global nickel trade network exhibits distinct core-periphery structures, highlighting notable variations in the roles assumed by different countries across various product trade networks. (4) The global trade backbone structure of various nickel products exhibits a reduction in network density, linkage strength, and trade scale. The trade networks of different nickel products constitute a pattern with complex forms, clear hierarchical structure and non-equilibrium.
This study comprehensively examines the global nickel trade network’s dynamic changes, topological structures, and spatiotemporal patterns from a product chain perspective. However, certain limitations warrant acknowledgement. First, the analysis focuses solely on the product chain, neglecting the extension to final products using refined nickel—an area necessitating further exploration. Second, while directed and weighted networks are constructed, only trade volume is considered network weight, overlooking the actual weight of traded nickel and the influence of international market price fluctuations on network links. Third, the prevailing international political and economic uncertainty and increasing trade protectionism significantly impact the global nickel trade market. Future studies will delve into long-time series data, offering a deeper understanding of the spatiotemporal evolution and structural resilience of the global nickel trade network, including its comprehensive effects on nickel’s industrial and supply chains.

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