Sustainability evaluation of essential critical raw materials: cobalt, niobium, tungsten and rare earth elements

Co belongs to Group 9 of the IUPAC Periodic Table. The interest in Co is due to its industrially useful properties including ductility, malleability and monetisability. These characteristics, combined with strength and heat resistance (melting point 1495 °C and boiling point 2870 °C) make Co suitable for a wide variety of industrial and military applications (Hannis and Bide 2009).

Co has been known since ancient times, and the first evidence is attributed to blue glazed pottery found in Egyptian tombs dated at 2600 BC. For centuries, Co-containing materials have been used as pigment. Co as a pure metal was isolated by Georg Brandt in 1735 (Donaldson and Beyersmann 2005).

Co is typically mined as a by-product of copper, nickel and iron and is widely distributed in different mineral deposits including sulphides, sulphosalts, arsenides, oxides and carbonates (Cheang and Mohamed 2016, Nancharaiah et al 2016). Significant Co concentrations are present in the sea floor, within Co-rich crusts and manganese nodules; considering available technologies, sea floor-based mining is not economically sustainable at present (Hannis and Bide 2009).

Table 6 summarizes important Co-bearing mineral groups. Irrespective of the starting mineral, cobalt is not naturally present in a pure metallic form, but is commonly found in  +2 and  +3 oxidation states. There are Co mines in different geographical areas and the EC estimated that the main exporters to European countries are Russia (96%) and the USA (3%), (EC 2014).

Table 6. Common cobalt mineral-associated worldwide deposits, (Cailteux et al 2005, Gál et al 2008).

Mineral name	Mineral group	Formula	Deposits in the worlda
Erythrite	Arsenate	Co3(AsO4)2 · 8 H2O	CA, DE, MA, NO, US
Skutterudite	Arsenide	(Co,Ni)As3	CA, MA, NO, US
Cobaltite	Sulphosalt	CoAsS	AU, CA, CG, NO, US
Carolite	Sulphide	Cu(Co,Ni)2S4	SE, US, ZR, ZM
Linnaeite	Sulphide	Co2+${\rm Co}_{{\rm 2}}^{{\rm 3}+}$S4	CIS, CG, DE; MA; RU, ZR
Asbolite (Asbolane)	Oxide	(Ni,Co)2-xMn4+(O,OH)4 · n H2O	NC
Sphaerocobaltite	Carbonate	CoCO3	DE; ZR

^aAbbreviations of countries according to DIN ISO EN 3116-1 (ISO 2006).

The vast majority of Co is mined in Congo. In 2016, the country accounted for 54% of global Co mining production. Furthermore, approximately half of the world's Co reserves are estimated to lie in Congo. Due to the dominance of Congo in terms of annual mining production and estimated reserves, the role of other countries is more limited with respect to supply risks, especially considering that other individual countries contribute less than 6% to the world's share of global Co mining. Figure 2 gives an overview of the geographical distribution of Co mining and reserves.

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Co is a metal of historic importance, and world global Co mining production rapidly increased from 180 t in 1901 to 1450 t in 1909 (USGS 2017a). As shown in figure 3, there was a steady increase in global Co mining production from 1960 (14 000 t) until 1986 (50 000 t). In the mid-1990s, there was a production low of 18 000 t, which was followed by a seven-fold increase to 126 000 t in 2015.

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Co is used for metallurgical applications in various industrial sectors for its several properties including high yield strengths, high work-hardening rates, resistance under cyclic stresses and the ability to absorb stresses, obtained thanks to its unstable face-centred cubic crystal structure and related low stacking-fault energy (Davis 2000). A typical use is in the aircraft industry, as a component of superalloys for aircraft engine turbines that operate under extreme temperature and pressure conditions. The application in this field started in the first half of 1900 and has continued combining the metal with additional elements (e.g. Ni, W, Al, Re, Cr, Ru) to obtain antioxidant and corrosion-resistant materials (Pollock 2016, Shen et al 2017). Further uses concern the chemical industry and the catalysts field. In this regard, Budiman et al (2012) describes a high-performance reforming process in which methane is adsorbed on the metal active surface and dissociated to produce hydrogen and hydrocarbon species. Co is also involved in the production of cemented carbides for cutting tools (CDI 2006). In this material, Co provides a ductile metal binder for carbide particles, allowing the achievement of high density thanks to its wetting or capillary action during the liquid phase sintering (Davis 2000). Additional applications include the industries of adhesives, pigments, ceramics, enamels, agricultural and medicinal chemistry. Figure 4 shows the share of prominent Co end uses.

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One of the more common applications of Co is the production of lithium-ion batteries (LIBs), which can be used for the power supply of various electronic equipment. In LIBs, Co is present as lithium cobalt oxide (LiCoO₂); this concentration is approximately 5%–20% higher than typical Co concentration in Co-containing minerals and may be attractive for recycling (Jha et al 2013a, Nancharaiah et al 2016). The EC considers LIBs as an emerging technology, which may strongly drive increased Co demand in the future (EC 2010c, EC 2017b).

As shown in figure 5, there have been significant price shifts for Co since 1960. A first peak occurred in 1979 with a price of 72 USD kg⁻¹, equivalent to 250 USD kg⁻¹ in 2017 currency. The most recent peak occurred in 2008 at 93 USD kg⁻¹, which was followed by a steep drop to 25–35 USD kg⁻¹ in subsequent years (Metalary 2016a).

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As confirmed by table 7, the research is pushing towards the identification of materials suitable for Co replacement, in several applications. Nevertheless, the possible new strengths are often combined with potential reductions in product performance. In this regard, Baharuddin et al (2017) describes methods for the production of cobalt-free cathodes for solid oxide fuel cells, using perovskite structured materials, such as strontium ferrite oxide, combined with dopants. The study shows satisfying results, but this research is still new, with few applications. Consistent with the examples in the table, the literature reports several applications for the samarium cobalt magnet replacement by neodymium-iron-boron technologies with a consequent loss of performance at higher temperature and an increased corrosion tendency (Smith Stegen 2013, Binnemans and Jones 2015). Many compounds are also reported as cobalt substitute for paint applications, with different resulting disadvantages. In more detail, the manganese carboxylates are the most well known. Nevertheless, they show a lower drying ability than cobalt combined with the possible formation of brown compounds that makes it unsuitable for light colouring. On the other hand, iron carboxylates show poor activity at room temperature and a yellow-brown colour that limits their use (De Boer et al 2013). Table 7 lists some possible substitutes for Co.

As summarized in tables 8 and 9, during the last decade several publications have reported Co recovery efficiencies of 60%–100% from secondary resources, including recovery processes based on recycling mining residues, zinc plant scraps and end-of-life LIBs. In more detail, table 7 identifies solvent extraction as the most common technique selected for the first two streams, often simulated by synthetic solutions. Many organic solvents are tested for this treatment, which ends with the following stripping step, carried out with sulfuric acid. An electrochemical approach can be chosen either as the final stage, after the solvent extraction, to increase the product purity (Agatzini-Leonardou et al 2009, Katsiapi et al 2010), or as the main treatment for cobalt recovery from different solutions (Wen Min et al 2013, Cheang and Mohamed 2016). Bioleaching represents an interesting alternative, which combines the positive aspect of a lower environmental impact with lower efficiency and several days long processing time (Coto et al 2008, Ahmadi et al 2015).

As confirmed by table 8, the recent literature about Co recovery from LIBs is vast. Irrespective of the optimized method, each of the described recovery processes begins with the leaching, carried out at alkali or acid pH, under different operative conditions, necessary for the cobalt extraction. In most of the cases considered here, the extraction treatment is carried out under reductive conditions with hydrogen peroxide, in order to reduce Co (III) to Co (II), which is soluble in water. Considering the composition of secondary resources, leach liquor contains many impurities, mainly copper, lithium, nickel, aluminium and manganese. To increase the Co percentage in the final product, further treatments including solvent extraction, precipitation, cementation, crystallization or electrochemical processes may be necessary. The majority of these additional treatments are readily reproducible and can be performed over short durations at moderate temperature, with the exception of vacuum pyrolysis at 600 °C, as described by Sun and Qiu (2011), and supercritical reducing leaching (carried out at 75 bar), optimized by Bertuol et al (2016).

Both the EC and the National Environmental Research Council (NERC) have confirmed the relevance of Co as a critical metal. The EC included Co in the list of CRMs of high supply risk and high economic importance, whereas the NERC included Co as an 'E-tech element' essential for technological development (Naden 2012, EC 2014)

Continued interest in this metal is based on its industrially useful properties including ductility, malleability and magnetizability. These characteristics, combined with strength and heat resistance (melting point 1495 °C and boiling point 2870 °C), make Co suitable for a wide variety of industrial and military applications (Hannis et al 2009, Nancharaiah et al 2016, Pazik et al 2016, Lison 2007). The critical value of this material is also confirmed by the 'EC substitutability index' of 1 (in a range between 0 and 1), attributed to Co by the EC, as a measure of the difficulty of substitution of individual CRMs (EC 2017c).

Considering its many uses and the recent growth in Co demand, it is important to increase Co production based on secondary resources, as a by-product from 'waste' of other industrial processes (Cheang and Mohamed 2016). As an estimate of the production of secondary Co, the EC has considered an 'end-of-life recycling input rate' of 16% for this metal (EC 2014).

Nb as well as Ta are transition elements of Group 5 of the IUPAC Periodic Table. Due to their properties such as high melting points, they are classified as refractory metals (Bauccio 1993). Nb and Ta have similar chemical properties, are co-located in natural deposits and are occasionally referred to as 'twins' (Schulz and Papp 2014). Both elements were discovered at approximately the same time. In 1801, English chemist, Charles Hatchett, derived a white oxide from a very heavy black stone, which originated from Massachusetts. Hatchett believed he had found a new element, which he named 'columbium', referring to the origin of the stone (Hatchett 1802). At the same time, Anders Ekeberg derived a white oxide from two minerals originating from Kimito, Finland and Ytterby, Sweden (Ekeberg 1803). As it was difficult to dissolve the oxide, Ekeberg assumed he had discovered a new metal and named it 'tantalum'. In 1809, English scientist, William Hyde Wollaston, concluded that 'tantalum' and 'columbium' were identical, even though he observed a distinct difference in the specific gravity of the oxides (Wollaston 1809). In 1844, Heinrich Rose found that 'tantalum' ores contain a second element, which he named 'niobium', after Niobe, the daughter of Tantalus in Greek mythology (Rose 1844). Two decades were required for the identities and naming practices of 'niobium', 'columbium', and 'tantalum' to be resolved (Deville and Troost 1865, Hermann 1865). Although the official IUPAC name is niobium (Nb), in North America the name columbium (Cb) is still sometimes used.

The abundance of Nb and Ta is linked due to their chemical similarity. In the metal extraction process, the challenge is not only to separate foreign elements, but to separate Nb and Ta. The most important minerals from an economic point of view are those of the columbite-tantalite group followed by the pyrochlore group (Crockett and Sutphin 1993).

The minerals of the columbite-tantalite group have the general formula AM₂0₆, where A  =  (Fe, Mn, and/or Mg) and M  =  (Nb in the case of columbite and Ta in the case of tantalite) (Cerny and Ercit 1989). For columbite, the Nb content is 78.2% and for tantalite the Ta content is 86.1%, according to Shaw and Goodenough (2011). However, it is common for Nb- and Ta-bearing minerals to be commingled.

Minerals of the pyrochlore group are the second most important source of Nb and Ta (Crockett and Sutphin 1993). These minerals comprise a series of oxides containing significant amounts of Nb, Ta and Ti, (either individually or in combination) and have the general formula A_2−mB₂O₆(O,OH,F)_1−n·pH₂O, where B  =  Nb, Ta and/or Ti (Hogarth 1977). Other important minerals for Nb extraction are tapiolite: (Fe, Mn)(Ta, Nb)₂O₆, lueshite: NaNbO₃ and euxenite: (Y, Ca, Ce, U, Th)(Nb, Ti, Ta)₂O₆ (Shaw and Goodenough 2011).The global share of Nb production and estimated Nb reserves are indicated in figure 6. The largest Nb deposits are situated in Brazil, which dominates global production. Canada had the second largest Nb reserves in the world in 2016, with world Nb mining production being 64 300 t, of which Brazil contributed a 90% share, or 58 000 t (Papp 2017a). A similar distribution can be noted for the world reserves of Nb, which are 4.3 Mt globally with a 95% share (4.1 Mt) concentrated in Brazil (Papp 2017a). At current usage levels, worldwide Nb reserves can be considered virtually inexhaustible (Schulz and Papp 2014), but Nb is classified as critical due to the high concentration of production and occurrence in Brazil (EC 2014).

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The use of Nb in technological applications is a relatively recent trend, and before the late 1950s there was no significant Nb mining activity (Schulz and Papp 2014). Figure 7 plots the annual global mining production of Nb since 1965. From 1965–2000, a relatively linear trend of moderate increase can be observed. However, production more than doubled over the 6 year period from 2001 (approx. 26 000 t) to 2007 (approx. 60 000 t). Thereafter, the production remained relatively constant, slightly exceeding 60 000 t.

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As demonstrated in figure 8, 88% of the mined Nb is used to produce ferroniobium, an iron niobium alloy, which is important in the production of high-strength low-alloy steel (TIC 2016a). Ferroniobium is used in the form FeNb63 (containing 58%–68% Nb, EN steel number 1.4740) or FeNb65 (containing 63%–70% Nb, EN steel number 1.4745), whereas the Ta content must not exceed 0.5% or 1.0%, respectively (DIN 2004). Ferroniobium itself is mainly used as an alloying element for Cr-, CrNi- or CrNiMo steel grades with EN steel numbers beginning with 1.45 and 1.46, which are stainless and acid resistant (DIN 2004). Table 10 shows examples of steel grades containing Nb; across all table entries, the Nb content is not higher than 1%.

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Nb is also used in some grades of superalloys. These alloys are designed for high-temperature applications such as turbine blades for aerospace engines or power generation. Table 11 shows examples of nickel-based superalloys containing Nb, some with Nb concentrations up to 5.5%.

As outlined in figure 7, the annual mining production of Nb increased significantly during the 2000s. As demonstrated in figure 9, the price of Nb followed a similar trend. Prices peaked around 1980 at around 15 USD kg⁻¹, corresponding to the all-time price record of approximately 50 USD kg⁻¹ when adjusted for inflation in 2017 currency. In the early 2000s, there occurred nearly a doubling of price, jumping from 19 USD kg⁻¹ in 1998 to 42 USD kg⁻¹ in 2010 (Metalary 2016c). Since then, the price of Nb has remained relatively constant around 40 US$ kg⁻¹.

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According to the DIN standard (DIN 2002), the sum of Nb and Ta is specified, as shown in table 11, thus permitting substitution of Nb with Ta. In 2010, the EC classified Ta as a CRM (EC 2010c), but Ta is not defined as critical according to more recent EU documents (EC 2014). The annual mining production is significantly lower for Ta (2015 production 1100 t; Papp 2017b) than Nb (2015 production 64 300 t; Papp 2017a). In 2015, superalloys alone accounted for 2200 t Nb use (TIC 2016b), which twice exceeds the total production of Ta. It is thus evident that Nb cannot in practice be substituted by Ta. Furthermore, the price of Ta (128 USD kg⁻¹; Metalary 2016b) is approximately three times higher than that of Nb (42 USD kg⁻¹; Metalary 2016c).

According to Papp, Nb can be substituted by other metals or materials as shown in table 12, even if performance losses or higher costs are unavoidable and must be taken into account (Papp 2017a). The listed substitutes molybdenum (Mo), vanadium (V), tantalum (Ta), titanium (Ti), tungsten (W) and ceramics are associated with additional considerations. It has already been outlined that Ta is not a feasible substitute as the annual production of Ta is currently less than 1.7% of Nb. W is itself classified as critical and is therefore not a suitable substitute. V is not in the EC list of CRMs, but the economic importance of V is extremely high as demonstrated in figure 1. As the annual mining production of V (77 800 t in 2015 according to Polyak (2017)) is within the same range as that of Nb, a significant increase in the demand for V would most probably shift V into criticality. Thus, only Mo and Ti remain as potential substitutes for Nb in certain alloys. However, such substitution may decrease performance and there are several cases where Nb is essential. It is thus clear that Nb should be recycled to a high extent in order to decrease the demand for virgin Nb ores.

The typical recycling process for Nb does not involve the recovery of Nb as a pure element; rather, Nb-containing scrap is remelted into the same alloy (Cunningham 2004). In particular superalloys with high Nb content (e.g. EN steel number 2.4868 with up to 5.5% Nb), recycling rates of up to 70% have been reported (Phillips 1990). However, only a small fraction (approximately 4%) of Nb is used for superalloys (TIC 2016a) and lower recycling rates are reported for the total Nb market; 50% in the year 1998 (Cunningham 2004) and 56% more recently (Birat and Sibley 2011). However, according to newer figures from Papp, the amount of Nb recycled is unavailable and may be approximately 20% (Papp 2017a). The Scholz company, a large scrap dealer processing 1.4 Mt steel scrap annually, has indicated that only alloying elements higher than 1% are evaluated for recycling; this implies that Nb is not considered at all (Hodecek 2017). This does not mean that Nb ends up in a landfill, but rather that it is heavily diluted into secondary products where its unique positive characteristics are underutilised. According to Graedel et al, this practice is called 'non-functional recycling'; recycling in which metal is collected as old metal scrap and incorporated into large-magnitude material streams as 'tramp' or impurity elements (Graedel et al 2011).

A separate collection of Nb-containing alloys would be a viable option to increase the recycling rate of Nb. In addition to alloys of higher Nb concentration, the importance of recycling alloys with Nb content below 1% (e.g. EN steel numbers beginning with 1.45 and 1.56) should be stressed, as these lower-concentration alloys account for 88% of the Nb market. Unfortunately, in current practice most industrial materials are recycled only to the extent that they can be separated from other waste materials at a cost less than that of new primary materials (Ayres and Peiró 2013). Recycling companies have not currently found a viable business case for recycling low-concentration Nb alloys, and the logistics cost for the separate collection of Nb-containing scrap currently overshadows the additional revenue that could be gained from the recovered Nb.

The EC has classified Nb as a CRM (EC 2014), but other sources have come to different conclusions. According to Ayres and Peiró (2013), several elements are labelled as 'hitchhikers', meaning that they are mined unintentionally with other metals. A representative calculation helps to emphasise the magnitude of these unintentionally mined 'hitchhiker' elements. In the Bayan Obo mines in China, the Baosteel company annually intends to mine 69 Mt iron ore (with 35% Fe content), to produce 24 Mt iron per year (Qi 2011). As the iron ore of the Bayan Obo mines exhibits a Nb content of 0.13% (Drew et al 1991), the described annual ore mining corresponds to 89 000 t Nb mined inadvertently, which exceeds the 64 300 t annual global production of Nb, which is mined intentionally.

In conclusion, more accurate data about Nb recycling would be necessary to fully characterize the situation. The recycling rates reported in the literature can be quite high (Phillips 1990, Birat and Sibley 2011), whereas other sources draw other conclusions (Papp 2017a, Hodecek 2017). It is conceivable to significantly reduce the supply risk, considering the large amounts of Nb available in iron ores in China. From a longer perspective, more sophisticated collection schemes for Nb-containing alloys are required to decrease the demand for virgin Nb, even if they may not appear to be economically advantageous in the short term.

Tungsten (W) is one of the materials included in the EU list of CRMs (EC 2017c, EC 2014). The name 'tungsten' originates from the Swedish language meaning 'heavy stone' (Andrew 1955).

W and W-containing materials exhibit unique properties, which make them highly attractive for specific applications, in European and global industries. Tungsten is the only viable option for a variety of applications, due to certain distinguishing features including the highest melting point of all metals (3422 °C), the lowest vapour pressure of all metals, a high density (19.3 g cm⁻³) similar to gold and very high hardness of certain W compounds (such as tungsten carbide, which is close to diamond) (Zeiler et al 2007).

Table 13 summarizes the most important W-containing minerals (Schubert and Lassner 2006, Pitfield and Brown 2011).

The principal economically interesting tungsten ores are the scheelite and wolframite-group minerals. However, scheelite remains the most important raw material for the manufacturing of tungsten chemicals and pure W metal.

The content of W in the Earth's crust is 0.007%. According to US Geological Survey data (Shedd 2017b), the global W reserves account for approximately 3.1 Mt. Most global W resources are in the form of scheelite and wolframite-group minerals (Anthony et al 2005). The right-hand chart in figure 10 outlines the share of W resources by country, based on data from the US Geological Survey (Shedd 2017b). However, this global distribution does not include data for the USA, Bolivia and Rwanda. The largest deposits of W ores are in China (1.9 Mt; i.e. 60% of worldwide reserves), and other countries lag far behind, with the second largest claim of 290 000 t (i.e. 9% of worldwide reserves) located in Canada. According to Argus Media Ltd, China accounts for the majority of global W production (Seddon 2016).

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The left-hand chart in figure 10 indicates the 2015 global distribution of W mining production. China plays a predominant role, accounting for 82% of global production (i.e. 73 000 t). Based on 2015 data, Vietnam was the second largest producer (i.e. 5600 t), accounting for only 6% of the world market.

Figure 11 plots the annual production of W over time, which has generally exhibited a trend of moderate increase between 1960 and the early 2000s, fluctuating between production values of 25 000 and 50 000 t. However, from 2004–2015, a significant increase was observed, culminating at 89 400 t according to the US Geological Survey (USGS 2017c) and at 80 900 t according to Brown et al (2017).

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Modern mining techniques for W-containing ores can be divided into underground and surface mining categories (Schmidt 2012). The selection of mining technique depends on the mineral properties and the physical peculiarities of the ore body (MSP-REFRAM 2016). Due to low W content in ores (0.3%–1.5% of WO₃), an enrichment of the ore is necessary in order to produce a concentrate containing 60%–75% WO₃. Ore enrichment processes include gravity concentration and flotation processes for scheelite, and magnetic separation for wolframite-group minerals. Additional processes are implemented after enrichment; the hydrometallurgical approach (Gaur 2006) for the production of W compounds, and pyrometallurgical methods (Gostishchev and Boiko 2008) for the production of pure W powder (or ferrotungsten). W-containing scrap is an important secondary source for pure W, as it usually contains more W than ore materials.

A combination of unique properties including a very high melting point, high wear resistance, low coefficient of expansion and others, makes W an essential material for technological applications under extreme conditions. According to recent data from the British Geological Survey (Brown et al 2017), modern applications of W can be classified into four main branches as sketched in figure 12. The main application areas of W, including annual production is in 1000s of tonnes. The respective share is in percent units (Pitfield and Brown 2011, USGS 2017c). Specialised applications of tungsten comprise the production of catalysts, electrical and electronic components (Anthony et al 2005). The manufacture of cemented carbide tools is the most widespread use of W, accounting for 61% of global average W consumption (Hayashi et al 2016). Specialised applications of W-containing materials include photocatalysis based on WO₃ and Bi₂WO₆ nanomaterials (Kumar and Rao 2015) and electrocatalytic applications for fuel cells (Christian et al 2007).

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Phases of industrial activity in China along with reduced demand for tungsten carbide have influenced the volatility of W prices. EU directives (EC 2009, 2012) on phasing-out W filament and incandescent light bulbs have also affected the demand for W used in lighting devices. Figure 13 shows a dramatic fluctuation of W prices after 2005, including peaks in 2006 (37 USD kg⁻¹) and 2012 (57 USD kg⁻¹). However, over recent years the price of W has steadily decreased to reach the current value of 35 USD kg⁻¹ (Metalary 2016d).

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Novel approaches to cemented carbides and cermets can enable substitution possibilities for W. The main substitution routes are directed to the replacement of W by molybdenum and titanium (Ishida 2011). Possible substitutes for tungsten in technical applications are listed in table 14 (MSP-REFRAM 2016).

Cemented carbides are among the most demanding W-containing materials for recycling. There are several processes to recycle cemented carbide to form ammonium paratungstate (5(NH₄)₂O·12WO₃·5H₂O) (Itoh 2014). The conventional process used by European industries consists of multiple technological stages as sketched in figure 14.

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A new process developed and launched in 2011 by the Japanese corporation, Sumitomo Electric Industries Group, comprises fewer stages compared to the standard process and is sketched in figure 15 (Hayashi et al 2016).

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According to the International Tungsten Industry Association data of 2013, the percentage of recycled W-containing materials in Europe and the USA was approximately 50%, compared to only 30% in Japan (Hayashi et al 2016). The effectiveness of recycling secondary W strongly depends on the origin of the feed materials. For example, the recycling of W-containing high-speed steel is more favourable for recycling than W used in lamps and electrodes, due to the higher W content in the former. At the same time, processes for W recycling require less energy compared to raw mineral processing (Leal-Ayala et al 2015). Recently, a low-cost hydrometallurgical process for W recycling was suggested by Ishida et al (2012). Moving towards a circular economy, Shishkin et al (2012) investigated a recycling of industrial residues that consisted of structural composite reinforced by tungsten filaments into aluminium and copper matrix composites suitable for polishing applications (figure 16).

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Distinctive mechanical and chemical resistance position tungsten as a very important material for use under extreme conditions. There are several major application areas for W and its compounds: the production of cemented carbides, production of catalysts, W-containing alloys, electrical and electronic components, and many other specialised applications. However, an efficient application and recycling of tungsten-containing materials remain a key point of supply risk minimisation for technological innovation in modern circular economies (Leal-Ayala et al 2015, Ku et al 2017).

The REEs are scandium (Sc), yttrium (Y) and the 15 lanthanide elements of the IUPAC Periodic Table (Connelly et al 2005). This group of 17 elements share chemical and physical similarities: cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb) and yttrium (Y). As already outlined in table 4, REEs can be subdivided into LREEs and HREEs.

The name 'rare earth element' is somewhat misleading, as these elements are actually not rare, but can be quite abundant. As outlined by Klinger (2015), the name is mainly indicative of certain assumptions made at the time of their discovery. As there is no natural occurrence of promethium (Pm), and all isotopes of Pm are radioactive, the majority of this section will deal with the other 16 REEs, for which there is relevant information on markets, applications, recycling and substitution.

The average concentration of REEs in the Earth's crust is higher than the concentration of some metals mined industrially on a large scale (see table 5). The abundance of REEs in the Earth's crust is for example higher than the abundance of copper (55 ppm) or zinc (70 ppm). The main sources of REEs are alkaline igneous rocks and carbonatites. Pegmatites, iron-oxide copper-gold deposits, placer deposits, residual deposits and marine phosphates (Long et al 2010) are considered to be potential sources as well, since the concentration of REEs is sufficiently high from an economic point of view. However, bastnäsite, monazite, xenotime and ion-adsorbed clays are the typical sources from which REEs are recovered on the commercial level. Monazite and xenotime occur in heavy mineral sand deposits. For those minerals, the beneficiation processes are already developed and they need minimal or no comminution, which make monazite and xenotime economically advantageous. Bastnäsite has been processed on industrial scale in Mountain Pass, USA and Bayan Obo, China (Jordens et al 2013). Ion-adsorbed clays require limited or even no beneficiation.

Figure 17 shows the world mining production of REEs, annually from 1960–2015. In recent times, China has been the dominant global supplier of REEs, although excess supply has caused a decrease in price. China's Rare Earth Industry Association has forecast that the use of REE oxides in China will increase to 149 000 t by 2020, while it was only 98 000 t in 2015. Magnets, abrasives and catalysts were the main applications of REEs in China in 2015. Approximately 35% of REEs were used for magnet production, 18% for abrasives and 15% for catalysts. During 2015, the export of REEs from China increased by up to 20% in comparison to 2014 (USGS 2016).

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In addition to China, India also has significant sources of REEs. In India, the main reserves are based on heavy mineral sand deposits and are located along the coastline. Only three government-owned companies extract those minerals: Tamil Nadu (Manavalakurichi), Kerala (Chavara) and Odisha (Zepf 2013, Roskill 2015). In Russia, there are two important deposits at Khibiny and Lovozero (Walters et al 2011). Within the EU, the Norra Kärr mine in Sweden is a potential source of REEs, which could last for 20 years at a mining rate 1.15 Mt yr⁻¹, with a stripping ratio of 0.73:1 (Schreiber et al 2016).

Table 15 summarizes essential data of the individual 17 REEs. Ce occupies the most important position, as it covers 42% (i.e. 24 000 t) of the tonnage produced followed by La with 22% (i.e. 12 500 t). Among the other REEs, only Y (8900 t corresponding to 16 %) and Nd (7300 t corresponding to 13%) are notable amounts. All the other REEs show a share of less than 5% and may be as low as 400 kg per year, as is the case for Sc.

Table 15 also demonstrates the relation between the estimated reserves and the annual production. The 'time to depletion' was calculated under the assumption of constant consumption and not considering the potential discovery of new deposits. Based on this calculation, it could be estimated that humankind will not run out of REEs in the short term. The shortest time until depletion was determined for Eu, which would last for 600 years, given these assumptions. Under these assumptions, Ce and La as the most relevant REEs in terms of volume, would not expire before 1300 and 1800 years, respectively.

Catalysts, glass production, fluorescent lamp production and metallurgy consume 59% of the total production of REEs, whereas the remaining 41% is used for ceramics, alloys in batteries, and permanent magnets. Approximately 80% of Ce and La is used for catalysts, glass, fluorescent lamp production and metallurgy. This sector consumes 45% of cerium oxides, 39% of lanthanum oxides and 8% of yttrium oxides. About 85% of Nd, Pr and Dy is applied in batteries, ceramics and magnets (Charalampides et al 2015). Neodymium, gadolinium, praseodymium and dysprosium oxides and other REE oxides represent 7% of the total consumption of REE oxides (Goonan 2011, Charalampides et al 2015). Ce is applied as the oxygen storage promoter, while La and Nd are used as promoters or stabilizers. The most significant property of Ce is the oxygen storage capacity (Cuif et al 2005). Cerium oxide possesses cubic structure similar to a fluorite type. This structure of cerium oxide enables fast diffusion of oxygen as a result of the oxygen vacancies, because the atoms of the oxygen in the structure are arranged in plane. The increased number of vacancies promotes movement of the oxygen, which leads to the reduction and oxidation (Dahle and Arai 2015). The use of lanthanum originates from the fact that the trivalent lanthanum generates Brønsted acid sites by hydrolysis of the water (which improves the activity of the catalyst), but at the same time modifies the electronic environment and causes the change of the silanol (ceramic support) acidity strength (Dalla Costa et al 2014). In the glass industry, Ce is used for glass discoloration, since tetravalent cerium oxide oxidizes the iron as the impurity, which results in the colour change from blue to light yellow (Cuif et al 2005). Due to the hardness and chemical reactivity, cerium oxide is largely applied for glass polishing. Cerium oxide reacts with the glass silica at the interface forming the silicate stratum, which makes glass more fragile and not so resistant to physical modification (Cuif et al 2005). Fluorescent lamps are another important application of REEs. Those lamps use the different mixes of green, red and blue REE phosphors in order to produce visible light (Tunsu et al 2014). Tb³⁺ is a promising activator for phosphors applied in fluorescent tubes, plasma display panel devices, etc. Tb³⁺ shows a strong VUV/UV excitation characteristic due to its spin-allowed 4f–5d transition, and an intense visible emission can be achieved in an appropriate host lattice upon the VUV/UV excitation (Liu et al 2016). The red emission is obtained by Y₂O₃:Eu. Emission occurs within the f-levels of the Eu³⁺ ions (Ronda et al 1998). Eu³⁺ activated Y₂O₃ exhibits sharp red emissions at ~610 nm arising from the ⁵D₀ f ⁷F₂ intra-4f electronic transitions of the Eu³⁺ ions, upon UV excitation into the charge transfer band at ~254 nm (Li et al 2008).

Usually, BaMgAl₁₀O₁₇ with the addition of Eu²⁺ is applied as blue phosphor. In this substance, the photons are absorbed by the Eu²⁺ ions and the absorption is the result of 4f  →  5d transition (Ronda et al 1998). The metallurgy consumes 45% of cerium oxides, 39% of lanthanum oxides and 8% of yttrium oxides (Charalampides et al 2015) for mischmetal production. Mischmetal is an alloy of REEs with inexactly defined composition. In general, mischmetal contains 50% Ce and 25% La, together with a small concentration of Pr, Y and Nd. Mischmetal binds the impurities in the metal and forms solid compounds and thus reduces the effect of the impurities on metal properties (Dahle and Arai 2015). About 85% of Nd, Pr and Dy is applied in batteries, ceramics and magnets (Charalampides et al 2015). The electrode material of NiMH batteries uses REEs in a form of metal hydride (MH). In MH, the REEs are present as A2B7 (LaCePrNdNiCoMnAl plus Mg) or AB5 (LaCePrNdNiCoMnAl) or as AB2 (VTiZr-NiCrCoMnAlSn), where the 'ABx' represents the ratio of the A metals (LaCePrNd or TiZr) to B metals (VNiCrCoMnAlSn) (Ruetschi et al 1995). Neodymium and dysprosium are applied in the production of powerful magnets. Those magnets are based on the intermetallic material, which in general consists of Nd, Fe and B with the addition of Dy. The boron is used to increase the interatomic distance from Fe, which fixes the orientation of the magnetic moment. The origin of the magnetic moment of REEs is the spin and the orbital momenta of unpaired 4f electrons (Jensen and Mackintosh 1991). The orbital magnetic moment is well preserved, since the electrons in the inner 4f shell are shielded by the 5s²5p⁶ shell (Shirsath et al 2014). Dy is used to enhance the performance of the magnets. The magnet's coercive force is improved by replacing the Nd on the periphery of crystalline particles with Dy (TechJournal 2011, Smith et al 2012), which have potential for utilization in refrigerator applications (Shah et al 2014).

Higher demand for REEs can be expected due to the development of new materials and alloys for various applications. For example, newly developed alloys such as Gd₅(Si₂Ge₂), Gd₅(Si_xGe_1−x)₄, La(Fe_xSi_1−x)₁₃H_x and MnFeP_1−xAs_x have large magnetocaloric effects (Smith et al 2012), which have potential for utilization in refrigerator applications (Shah et al 2014).

Figure 18 shows the price of REE oxides from 1960–2015 in USD kg⁻¹ (red dots) and inflation-adjusted USD kg⁻¹ (blue dots). No significant price jumps can be observed until 2010, after which there was a major increase and temporary peaking in 2011–2012, and after 2013 prices dropped to the pre-peak level. The peak in the price between 2011 and 2012 may be attributed to export quotas, duties and licenses introduced by the Chinese government; these export restrictions were introduced in 2006 and gradually tightened. The quotas have become increasingly detailed (targeting LREEs and HREEs separately), with specific local regulations on reserves and the number of licensed exporters (ERECON 2015).

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The prices of individual REEs vary, but show a similar trend. Figure 19 plots the trend for selected REEs (Ce, Ce–La mischmetal, La, Nd), which are in the range of the low price level. For all the mentioned REEs, there has been a price drop since 2010. By analogy to the trend shown in figure 17, Nd showed a jump in price during 2011, peaking at almost 200 USD kg⁻¹. The current price level is 40 USD kg⁻¹, i.e. only one fifth compared to 2011. For La, Ce and mischmetal there has been no peak within the investigated time period. However, the price decreased massively, and current prices are only at 4% (La) and 5% (Ce) compared to 2010.

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As shown in figure 20, the same trend occurred for selected REEs, which are in the range of the high price level. Dy (1400 USD kg⁻¹), Tb (2800 USD kg⁻¹) and Eu (3800 USD kg⁻¹) peaked in 2011. Since then, prices have dropped tremendously. The Eu price in 2016 was 2% (66 USD kg⁻¹) compared to 2011.

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Due to the widespread application of REEs, there are a substantial number of end-of-life products that are considered to be a valuable source of those metals. Among them, NiMH batteries, fluorescent lamps and permanent magnets are the most common REE-based commodities that are being recycled, or there are a lot of efforts to develop recycling technologies for REEs recovery from these sources.

3.5.5.1. Spent NiMH batteries.

3.5.5.2. Fluorescent lamps.

3.5.5.3. Permanent magnets.

REEs have strategic importance in the production of advanced materials utilized in modern and 'green' applications. Since REEs possess specific properties, there exist only limited or no possibilities for their substitution (ERECON 2015). Moreover, due to preferably controlled exploration, which occurs only in a few countries, the accessibility of REEs contributes to their criticality for the EU, USA, Japan and other industrialised economies. The export quotas, duties and licenses introduced by the Chinese government played a role in the record peak in the prices of REEs in 2011 and 2012 (ERECON 2015, USGS 2017d). Subsequently, significant effort has been made in the EU and worldwide to determine and implement alternative sources to REEs. Spent portable and car NiMH batteries, obsolete fluorescent lamps and magnets are considered as promising and feasible secondary sources of REEs. Several technologies and recycling processes have been developed and designed to recover valuable REEs using pyrometallurgical or hydrometallurgical approaches. However, due the recent decline in the price of REEs, the majority of these technologies are currently not commercially applied. Nevertheless, their existence is a strategic solution in the case of future limited accessibility of REEs.