Vanadium sustainability in the context of innovative recycling and sourcing development

Chalmers University of Technology, Department of Chemistry and Chemical Engineering, Kemivägen 4, 421 96 Gothenburg, Sweden University of Tartu, Institute of Technology, Ravila Street 14a, 50411 Tartu, Estonia c TU Wien, Institute of Chemical Engineering, Getreidemarkt 9/166, 1060 Vienna, Austria d Polytechnic University of Marche, Department of Life and Environmental Sciences-DiSVA, Via Brecce Bianche, 60131 Ancona, Italy Riga Technical University, Scientific Laboratory of Powder Materials & Institute of Aeronautics, 6B Kipsalas Str, Lab. 110, LV-1048 Riga, Latvia


Introduction
Metals are essential materials of strategic importance, as they support the technologies which are ubiquitous in the modern world. To meet the intensifying demands of today's technologically advanced society, modern industrial applications are increasing the quantity, diversity, and quality of metals in their products.
The European Commission (EC) has recognized that the economies of individual countries as well as the European Union (EU) as a whole are highly influenced by access to raw materials and metals resources (EC, 2017a). The strategic importance of vanadium (V) is reflected by its presence in the list of 27 critical raw materials issued by the European Commission (EC, 2017a). The list represents a selection of the metals of high importance for the EU economy, which possess a high risk of the supply. The United States and Canada have also addressed the importance of raw materials. Like the European economy, the American and Canadian economies rely on vanadium and are not globally independent; vanadium is supplied from international sources and the utilization of secondary sources (petroleum residues, spent catalysts, utility ash, and vanadium-bearing pig iron slag) .
Vanadium plays a critical role in several industrial applications, especially in steel production. Vanadium, when added in small amounts to certain ferrous alloys, can significantly improve alloy properties and performance. Moreover, specific attributes of vanadium are utilized for the production new generation batteries which support the inclusion of renewable sources of electricity on the electric grid. Thus vanadium, in addition to its conventional application, plays a very significant role in the decarbonization of the energy industry. Countries can make risk-informed decisions based on a sustainability assessment of vanadium, based on comparing the demand and supply from primary and secondary sources.
Considering the strategic international relevance of vanadium and the strong interest for both the market and the scientific community, a comprehensive overview of the current state-of-the-art is essential to identify the worldwide situation and to accurately assess risks. In this regard, some recent literature has addressed the vanadium issue, but the work has concentrated on specific aspects: the processes of extraction and recovery of vanadium from several matrices (primary or secondary resources) (Gilligan and Nikoloski, 2020;Le and Lee, 2020;Peng 2019;Zhang et al. 2014); combinations of vanadium with other elements Ferella, 2020;Seredkin et al., 2016); the effects of vanadium on the environment and human health (Domingo 1996;Imtiaz et al., 2015;Gummow, 2011;Watt et al., 2018); and the biological role of vanadium (Rehder, 2015). Some literature has described possible applications of vanadium in different technologies, mainly batteries (Choi et al., 2017;Gonçalves et al., 2020;Lourenssen et al., 2019;Shi et al. 2019;Skyllas-Kazacos et al., 2016;Parasuraman et al., 2013;Xu et al., 2020). Nearly two decades ago, authors Moskalyk and Alfantazi developed an overview of vanadium based on prevailing technological conditions at that time (Moskalyk and Alfantazi, 2003). In this context, the current paper comprehensively addresses the sustainability of vanadium, taking into account the modern state-of-the-art, complementing and expanding upon previous efforts to address important gaps, including attention to the main characteristics that make vanadium strategically essential in today's technologically-advanced society, as confirmed by market economic trends. In this the present work, the full life cycle of vanadium has been considered, from metal mining to the recovery from waste, and in the context of possible applications. The further focus on the available patents and the funded projects for vanadium recovery provides insight into technological innovation changes, which can be useful in predicting future developments of vanadium sustainability.

Occurrence
In the earth's crust vanadium is a rather abundant element. It shows a concentration of just under 100 ppm in the upper continental crust / earth's crust which is much higher than most of the other critical elements (Rudnick and Gao, 2003;Yaroshevsky, 2006) as shown in Table 1. Table 1 also demonstrates that the vanadium concentration in seawater is 2 mg/m 3 and thus again significantly higher than other critical elements (Haraguchi et al., 2003). Assuming a total volume of earth's seas of about 1.34Á10 9 km 3 , approximately 3Á10 9 t vanadium are present in seawater. This is much higher than the known reserves in deposits but nevertheless it is too diluted to make extraction economically viable. Table 1 further outlines that the supply risk as proposed by the EU commission (EC, 2017a) is not directly linked to the concentration of the elements. It is, however, not clear whether vanadium may be an essential element for the human body (Prashanth et al., 2015).
Vanadium deposits may occur in four principal types (Kelley et al., 2017): vanadiferous titanomagnetite deposits; sandstone- hosted vanadium deposits; shale-hosted deposits; vanadate deposits. Vanadiferous titanomagnetite deposits are the most important source for vanadium and deposits can be found all over the world (Kelley et al., 2017). They are mainly associated with mafic igneous rocks (e.g. gabbro or anorthosite) found in South Africa, Sweden, Finland or the USA (Fischer, 1975). The most important ore minerals are magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) but also in rutile (TiO 2 ) and perovskite (CaTiO 3 ) deposits are possible sources (Fischer, 1975). Usually vanadium in titaniferous magnetite deposits is concentrated as a solid solution in magnetite-ulvospinel, where V 3+ has replaced Fe 3+ (Fischer, 1975). Table 2 shows some important minerals for vanadium extraction. Vanadium is also available as a by-product from production processes such as iron and steel, uranium, alumina, phosphorus or lead and zinc (Bauer et al., 2017). It has also to be mentioned that vanadium occurs in fossil fuels such as petroleum, coal, tar sand and oil shale (Breit, 1992).

Vanadium in the EU
Vanadium has entered criticality in 2017, whereas it was not considered critical in the 2011 (EC, 2011(EC, 2014b assessment of the EU. According to the latest EU criticality assessment in 2017 the economic importance score (EI) is 3.7 (threshold 2.8) and the supply risk (SR) 1.6 (threshold 1.0) (EC, 2014a). Fig. 1 shows a plot of the materials which have been accessed by the EU in 2011, 2014 and 2017. Vanadium already showed a very high economic importance in 2011 and 2014 but the supply risk was not high enough that the material had to be considered critical. The 2017 plot cannot be directly compared to 2014 and 2011 as the methodology in the 2017 assessment for both the calculations of economic importance and supply risk are now different. However, it is obvious that the relative position (V close to Cr, Mn and Zn in 2011 and 2014) changed tremendously. The economic importance decreased, and the supply risk increased, as a matter of fact vanadium crossed the threshold value and moved into criticality. Table 3 demonstrates that there is no vanadium extraction in the EU at all (EC, 2017). In the period 2010-2014 the EU imported on average 9124 t vanadium per year (EC, 2017). It is also reported that about 1650 t of vanadium pentoxide is produced in the EU, whereas Belgium, United Kingdom, the Netherlands and Germany are the main producers. Furthermore, 2000 t of ferrovanadium have been produced inside the EU, mainly in the Czech Republic (EC, 2017). Table 3 summarized the available data on vanadium processing according to (EC, 2017).
The British Geological Survey (Brown, 2016) reports imports and exports of vanadium in detail. Table 4 summarizes EU's imports in 2014 structured according to the product groups of vanadiferous residues, pentoxides and metal (i.e. ferrovanadium). It is striking that for the largest volume of imports vanadiferous residues (46,000 t) are responsible and that these imports exclusively go to Austria. It is, however, likely that the volume of 46,000 t designates the total volume of the material and is not related to the vanadium content. According to the data of the BGS (Brown, 2016) the EU imported a total of about 16,000 t of vanadium pentoxides which is higher than the overall EU imports (9124 t) considered by the EU commission (EC, 2017). Again, the difference could be that the EU commission considers the vanadium content of pentoxide and not the total mass. A volume of 16,000 t (Brown, 2016) of V 2 O 5 corresponds to 8960 t vanadium which is in good agreement with 9124 t (EC, 2017). Table 4 also shows that EU's imports of vanadium metal (i.e. ferrovanadium) is as low as 645 t (Brown, 2016). It is, furthermore, again not clear if the total mass of ferrovanadium or the vanadium content is con- Table 2 Selection of important vanadium minerals (Kelley et al., 2017;Bauer et al., 2017 Table 5 summarizes the exports of vanadium from the EU to third countries (Brown, 2016). The exports of pentoxide (419 t) are much smaller than compared to the imports (16,000 t) which is based on the fact that the material is used for production of vanadium products such as ferrovanadium or vanadium bearing steel alloys. Imports (645 t) and exports (846 t) of ferrovanadium are both on a low level and almost balance each other out.
The question arises, why for Austria a large amount of vanadium imports but not exports are reported. It is known that the Austrian company ''Treibacher Industrie AG" treats end-of-life catalysts or production residues containing molybdenum, nickel and vanadium via a pyrometallurgy process by which vanadium is con-  verted into ferrovanadium which is primarily used for steel alloying (Hassan, 2003;Krutzler et al., 2012). Vanadium containing steel grades are in particular used for tool steels (DIN, 2018): alloyed cold worked steels, vanadium concentration between 0 and 1%; hot-working steels, vanadium concentration between 0.05 and 2.1%,and high-speed steels, vanadium concentration between 0.9 and 4.2%.
''Voestalpine" is an Austrian based company and has a worldwide leadership in tool steel and a leading position in high-speed steel (Voestalpine, 2016). The high-performance metals division, responsible for the production of tool steel, had a revenue of 2.7 billion € in 2016/17. The divisional revenues are mainly based in EU (49%) and third countries (47%), whereas Austria plays a minor role (4%).
It is, however, clear that the exports, inside and outside the EU, of tool steels will not account for vanadium exports in the statistics but amongst steel. Nevertheless, it is clear that most of the imported vanadiferous residues to Austria will finally end up in tool steels.

Markets
Vanadium is mainly used as alloying element for steel. As demonstrated by Table 6 different categories of steel alloys dominate the vanadium market and shows a share of 90%. The only other applications are titanium alloys, chemicals and other (mainly batteries).
Vanadium serves the purpose to improve the resistance to wear and deformation of steel. Vanadium-containing alloys are used for the hull of submarines, in structural parts, engines and landing gear, but also in gun alloy elements, armour, fuselages and wings, in the field of aeronautics and naval (Moss et al., 2011).

Mining
The world mining production is plotted in Fig. 2. From 1912 to 1960 production of vanadium was quite low and did not exceed 5000 t. A significant increase occurred between 1960 and 1980 when mine production jumped from 5000 to 38,000 t. Over the next 20 years production stagnated, and it was not until 2000 (41,000 t) that the value of 1980 was further increasing. Since 2000 a strong, near linear growth took place and mine production doubled until 2014. Since then vanadium production has reached a plateau at between 70,000 and 80,000 t (Polyak, 2017(Polyak, , 2018Survey, 2016).
World mine production is concentrated in a few countries only. As demonstrated by the charts in Fig. 3, China, Russia and South Africa are the major producers. In 2000 and 2008 no other country held a relevant share of mine production. For 2012 a small production of 272 t was reported for the USA (Polyak, 2014) which accounts less than 0.4% of worldwide production. Starting in 2014 Brazil entered the market and in 2016 the country produced 8400 t (Polyak, 2018). This volume is much smaller as compared to China (45,000 t) Russia (16,000 t) and South Africa (10,000 t) but represents a share of 10%. However, it is obvious that vanadium production is concentrated in a small number of countries only and there is no own production within the EU. Table 7 compares data from the U.S. Geological Survey (USGS) and the British Geological Survey (BGS). There are certain deviations between the references. In particular the USGS reports a significant decrease in mine production for South Africa from 21,000 t in 2015 (Polyak, 2017) to 9100 t in 2017  whereas according to the BGS the decrease is only moderate from 21,552 t in 2014 to 18,000 t 2016 and 2017 (Brown, 2018). Regarding the world production the sources report a gap of 13,000 t (73,000 t vs. 86,000 t) in 2017 (Brown, 2018;. However, Table 4 Vanadium imports into the EU in 2014 (Brown, 2016   despite this difference it is clear that the mine production is concentrated in three countries only, China, Russia and South Africa.

Reserves
The volume of extracted material is quite easy to measure. In contrast, it is difficult to estimate the amount of material that can be mined in the future. The USGS gives a classification of reserves for minerals (Menard, 1976) which is briefly summarized Table 8.
to depletion decreased from about 300 years (1994) to about 180 years (2014). It is, however, clear that our economy will not run out of vanadium in the near future. As already discussed the criticality of vanadium is not based on scarcity (Table 1) but on the concentration of mine production in a very few countries ( Fig. 3).

Substitution
According to the EU assessment in 2017 the substitution index for supply risk is 0.94 and the substitution Index for economic importance is 0.91 (EC, 2017b). Both values indicate that substitution is difficult and that the material is more or less irreplaceable in most application. Table 9 shows possible substitutes according to the literature. A closer inspection (Table 10) of the suggested substitute elements (Mn, Mo, Nb, Ti, W) reveal that it will most likely not work in practice. Niobium and tungsten are critical itself and it will make no sense to substitute a CRM by another CRM. Manganese is not considered critical but also inadequate for substitution. Even if the prices for Mn (2000 €/t) are much cheaper than for ferrovanadium (83,810 €/t), the production volume is by far too low. The annual production of manganese is only 20% of vanadium. Even a partial substitution of vanadium by Mn would upset the markets and exceed the production capacities. Regarding the volumes and prices molybdenum and titanium could be a candidate for a vanadium substitute. It is, however, clear that performance of quality by the substitutes in the final products might lag behind. It has to be considered that significant technical adjust- Table 8 Short summary of reserve classification for minerals according to USGS (Menard, 1976).

Term Description
Resource A concentration of naturally occurring solid, liquid, or gaseous material in or on the Earth's crust in such form and amount that economic extraction of a commodity from the concentration is currently or potentially feasible Identified Resources Resources whose location, grade, quality, and quantity are known or estimated from specific geologic evidence Reserve Base Part of an identified resource that meets specified minimum physical and chemical criteria related to current mining and production practices Reserves That part of the reserve base which could be economically extracted or produced at the time of determination. The term reserves need not signify that extraction facilities are in place and operative.
Since the main share of vanadium goes into steel alloys (Table 6), the substitution of vanadium in other markets is less relevant.
Titanium is a substitute for vanadium use in paints and varnishes, a specific part of the chemical applications of vanadium. Batteries using vanadium are based on the redox flow technology which is quite new in the market. It is expected that the volume of this battery will grow in the future (Johnson, 2019). Vanadium is, however, the most promising candidate for redox flow batteries (Cunha et al., 2015) and an increasing demand for vanadium from this sector is very likely.

Properties and applications; future trends
Vanadium is one of the group five elements, and has two naturally occurring isotopes: 51 V (stable, 99.75% abundance) and 50 V (t 1/2 = 1.7*10 17 years, 0.25% abundance) (Greenwood and Earnshaw, 1997). It is shiny, silvery metal with typical metallic bcc structure; it is soft and ductile in its very pure form but Table 9 Possible substitutes for vanadium according to USGS  and EU (EC, 2017a (Metalary, 2019a, b, c, d, e). *** average price for ferrovanadium (80%) in 2018 according to (Vanadiumprice.com, 2019).  becomes harder and more brittle when impure. Similar to other members of its group, niobium and tantalum, vanadium has high resistance to corrosion. This is due to the formation of protective oxide films on its surface. Vanadium exhibits oxidation states from +2 to +5, the most stable one being +5 (e.g., V 2 O 5 ). These include the vanadyl ion VO 2+ , one of the most stable diatomic ions known. High temperatures are required for chemical interactions with most non-metals. Vanadium is attacked by hot concentrated mineral acids and is resistant to fused alkali. When it comes to the +5 state, vanadium forms a pentahalide with fluorine. With other non-metals, such compounds are based on oxyhalides and pentoxide. Vanadium pentoxide (V 2 O 5 ) is the final product upon heating with excess oxygen but lower oxides can also form, e.g., VO 2 , V 2 O 3 or VO. Upon thermal treatment, V 2 O 5 loses oxygen reversibly, making it a versatile catalyst. Industrial applications include the oxidation of SO 2 to SO 3 for production of sulfuric acid, and the air oxidation or hydrogen reduction of organic compounds, e.g., oxidizer for the production of maleic anhydride. Notable advantage for using V 2 O 5 instead of platinum catalyst include reduced costs, and reduced impurity poisoning, e.g., arsenic (Greenwood and Earnshaw, 1997). For sulfuric acid production, sulfur is oxidized from +4 to +6 and vanadium is reduced from +5 to +4 (Eq. (4.1)); the pentoxide catalyst is regenerated by oxidation with air (Eq. (4.2)).
In aqueous solutions, vanadium can form a wide range of oxyanions, depending on pH and concentration. Orthovanadate VO 4 3À , which forms at high pH, is important for crystallographic investigations of phosphoryl transfer enzymes (Davies and Hol, 2004), and is a powerful competitive inhibitor of purified alkaline phosphatase from human liver, intestine and kidney (Seargeant and Stinson, 1979). Phosphate and vanadate compete for the same binding site on the enzyme. The inhibition is reversible, and full enzymic activity can be restored in the presence of adrenaline. The decavanadate ion predominates at lower pH (pH 2-4) and can form from orthovanadate via condensation (Eq. (4.3)).
Potential applications in catalysis have led to studying reduced polyvanadates containing a wide range of vanadium (V) and (IV) ratios, e.g., decavanadates (Soghomonian et al., 1993) or octodecavanadates (Müller et al., 1990). The latter is able to encapsulate negatively charged ions and ions with significantly different radii. Higher nuclearities have been attained, e.g., K 10 [V 34 O 82 ]*20H 2 O (Müller et al., 1991). The studies of such mixed valence species are important not only in catalysis but also geochemistry and, most importantly, biochemistry. The VO n+ unit in aforementioned polyvanadates resembles Fe n+ in biochemical systems. This can provide models for biological systems, e.g., incorporation of VO 2+ into the biological iron transport, and storage proteins transferrin and ferritin, which have important implications in medicine (e.g., rheumatic arthritis). While vanadium is a versatile catalyst, its predominant application is in the iron and steel industry, as hardening and strengthening agent (Table 11).
Vanadium interacts with carbon present in steel, and forms stable carbides, which strengthen the alloy. Addition of vanadium to steel provides good castability, rollability, reduced roll wear, relative insensitivity to finish rolling temperatures in structural steels, good weldability of structural steels, and protection against oxidation via formation of a protective surface oxide layer (EC, 2011). The amounts used vary from subpercentage up to several percent vanadium (e.g., 5%). The latter is used in high speed steel with high hardness and abrasion resistance, e.g., for power-saw blades and drill bits. In the beginning of the 2000 s, it was estimated that vanadium consumption in the iron and steel sector amounted to 85% of the vanadium-containing products produced worldwide (Moskalyk and Alfantazi, 2003). More recent reports put this at 90% and conclude that future demand is expected to increase if the steel demand experiences steady growth (EC, 2011). Vanadium is also used together with aluminum, chromium, iron, nickel, titanium and others in various alloys. The different formulations serve diverse purposes, from common ones such as train rails to specific alloys for aerospace use. Alloys with titanium and aluminum have applications in jet engines and highspeed airframes (EC, 2011), and, currently, there is no substitute for vanadium in aerospace titanium alloys (Moskalyk and Alfantazi, 2003). In combination with dysprosium and other elements, vanadium is used in laser materials. It was forecasted that vanadium will play an important role in carbon capture and storage technologies. The main use is high specification steel alloys, e.g., for pipelines. Modelling the future metal demands for carbon capture and storage applications showed that vanadium may have the largest metal requirements as a percentage of current world supply in 2030 (1.3%). Other applications of vanadium include colour modifiers in mercury vapor lamps, target material for X-rays, photographic developer, drying agent in various paints and varnishes, production of pesticides and black dyes (mordants), inks and pigments used in ceramics, printing and textile industries (Moskalyk and Alfantazi, 2003). Its non-metallurgical use is predicted to rise due to applications of vanadium in batteries (EC, 2011). Lithiumvanadium phosphates can function well as cathodic material in lithium ion batteries, and gives good cyclability, high voltage and capacity (Nitta et al., 2015). Li 3 V 2 (PO 4 ) 3 has one of the highest voltage and highest energy cathode identified for lithium ion batteries (EC, 2011). Other energy-related applications include vanadium redox batteries, which exploit the different oxidation states of vanadium (Alotto et al., 2014). Various systems were investigated, e.g., magnesium-vanadium, vanadium-cerium and vanadiumpolyhalide but the most promising commercial technology is VRB (vanadium/vanadium redox battery), e.g., vanadium/vanadium dissolved in aqueous sulfuric acid. This system uses the same metal ions in the electrolyte, electrodes and membrane. This prevents cross-contamination, which positively affects the cell capacity in time, and allows for longer lifetime of the battery. During charging, VO 2+ ions are oxidized to VO 2 + ions (Eq. (4.4)); at the negative electrode V 3+ ions are reduced to V 2+ ions (Eq. (4.5)).  Vanadium-bromine (V-Br) cells and hybrid vanadium-oxygen redox fuel cells (VOFC) are other applications for energy storing for which vanadium is needed. The VRB technology has been successfully tested, with applications mainly in Asia (Japan) but also South Africa and Europe, and commercial exploitation is ongoing (Alotto et al., 2014). Due to their bulkiness, vanadium batteries are best suited for grid storage. (ways, industrial processes, companies, etc.) Primary sources of vanadium include ore feedstock, concentrates, metallurgical slags and petroleum residues (Moskalyk and Alfantazi, 2003). A significant amount of vanadium is also recycled. According to a recent report, the end-of-life recycling input rate of vanadium in the European Union amounts to 44%, the highest contribution of recycling to meet the Union's demand of critical raw materials (EC, 2019). Two main types of secondary streams are targeted for recycling: steel scrap, which is recycled along with the vanadium content, and spent catalysts. Vanadium can be recovered and refined via several processes, e.g., calcium reduction, roasting and leaching, solvent extraction and ion exchange (Moskalyk and Alfantazi, 2003). Final commercial products are vanadium metal, ferrovanadium (iron-vanadium alloy), vanadium pentoxide but also various vanadium compounds, depending on the desired application. The main ore feedstock to recover metallic vanadium is titaniferous magnetites. Vanadium and vanadium compounds can also be recovered as by-products in several industrial operations (Moskalyk and Alfantazi, 2003). Such operations include recovery of aluminum and magnesium metal from smelters and refineries, and production of uranium. For the last one, the vanadium left in the raffinate after the solvent extraction of uranium from ore leachate is extracted to an organic phase in a subsequent solvent extraction step, back-extracted with sodium carbonate solution, and precipitated with ammonium sulphate as ammonium metavanadate. This is finally calcined to produce vanadium pentoxide. Other commercial processes, e.g., lead and zinc production in Namibia, processing of iron ore deposits in Finland and Norway (operations stopped during the 1980s), and processing of bauxite residues in France also generated vanadium by-products. Typical processing of primary vanadium-bearing materials involved crushing and roasting with sodium chloride or sodium carbonate at 850°C, which forms water-soluble sodium metavanadate. After leaching with water, a polyvanadate (red cake) is precipitated with sulfuric acid at pH 2-3. This is heated at 700°C to produce black, technical grade vanadium pentoxide. Reduction is carried out to obtain vanadium metal (Fig. 8). Pure metal can be obtained via reduction of vanadium pentachloride with hydrogen or magnesium, reduction of vanadium pentoxide with aluminum or calcium, or electrolysis of partly refined vanadium in fused alkali metal halides (Greenwood and Earnshaw, 1997;Moskalyk and Alfantazi, 2003). Vanadium-aluminum alloys can be refined using molten salt electrolysis. Carbon is not preferred for reduction to avoid formation of vanadium carbides. Because the vast majority of vanadium is used in the steel industry as additive (as presented in section 4), ferrovanadium is usually produced via reduction in an electric furnace in the presence of iron or iron ore. The obtained ferrovanadium can be used without further refinement. The pig iron containing vanadium can be oxygen lanced to produce a slag containing 12-24% vanadium. This can be further smelted at high temperature or chilled and processed using solvent extraction to produce vanadium pentoxide (Moskalyk and Alfantazi, 2003).

Production
The main global supplier of vanadium is China, which had a market share of 53% in 2017 (EC, 2017b). South Africa and Russia follow, with a market share of 25% and 20%, respectively. With worldwide reserves totalling over 13.5 million tonnes (about 10 million in China and Russia, and 3.5 million in South Africa), it was estimated that demand can be met for at least another century at the present rate of consumption (EC, 2011). There is no extraction of vanadium in Europe. This is either to limited knowledge of the availability of vanadium in the European Union, or to economic and societal factors that negatively affect exploration (EC, 2017b). Despite China being the global supplier, the main share of European sourcing (60%) comes from Russia.
In China, mining is done in the Sichuan and Anhui provinces. In addition, vanadium-containing slags are imported from New Zealand, Russia and South Africa (Moskalyk and Alfantazi, 2003). Chengde Xinghua Vanadium Chemical Co. Ltd., Emei Ferro-Alloy Co. Ltd., Hengyang Manganese Product Works, Jinzhou Ferro-Alloy Co. Ltd., Nanjing Ferro-Alloy Works, Panzhihua Iron and Steel Group, and Shanghai Non-Ferrous Metals Research Institute produce a wide range of vanadium products, including metal, pentoxide and ferrovanadium. In South Africa, in the beginning of the century, four companies were operational: Highveld Steel and Vanadium Corporation, Vametco Minerals Corp., Vanadium Technologies (Vantech), and Rhombus Vanadium Holdings Ltd. (Rhovan). The first in the list was the largest and operated two plants, the Vantra (production of vanadium pentoxide from ore and slag) and Wapadskloof facilities (production of vanadium trioxide and others). In Russia, deposits of vanadium are located in the Ural Mountains (Katschkanor and Sverlov regions), Siberia, and some areas in the far east, and northwest. The vast majority of the pentoxide is produced from slag by Nizhny Tagil Iron & Steel Works. Pentoxide and ferrovanadium are also produced by Kachkanar Vanadium Mine and Concentrator, Chusovskoy Iron and Steel Works, and JSC Vanadiy-Tula.

Recycling
Data on recycling rates of vanadium vary widely. In 2014 the EU Commission estimated that vanadium is not recycled at all (EC, 2014b). Only recently, for the EU an end of life recycling input rate of 44% was reported (EC, 2017b;Eurostat, 2018). For the USA, according to the U.S. Geological Survey about 40% of vanadium is recycled from chemical process catalysts , petroleum residues, utility ash, and vanadium-bearing pig iron slag.
The amount of spent desulfurization catalysts grows rapidly due to the increasing demand of related markets, and a limitation of the regeneration. The reasons for significant increase in the waste production can be summarized as follows: (1) a substantial expansion in the capacity of distillate hydrotreating to sustain a sulfur with extra-low level, (2) decreased operation times as the consequence of harsh operation environment and source having high content of sulfur, and (3) prompt deactivation together with deficiency of reactivation process (Kim et al., 2018). Currently, the market for fresh hydro-processing catalysts approaches 120,000 tons per year (Kim et al., 2018). Composition of the catalyst waste differ. Erust et al. determined composition with approximately 6% V 2 O 5 , 2% Al 2 O 3 , 1% Fe 2 O 3 and 60% SiO 2 ; and the rest constituting several other oxides were in traces/minute quantities (Erust et al., 2016). Another work reported that catalysts contained 27% of V 2 O 5 but much larger content of Al 2 O 3 (40%) (Villarreal et al., 1999).
Similarly, to catalysts, the increasing demand of steel products leads to the higher volumes of steel making slags generated and it is constantly expanding. Approximately 90% of vanadium is used for steel alloys.
The slag is usually composed of oxides such as: CaO, Fe 2 O 3 , SiO 2 and MgO, with smaller amount of Al 2 O 3 , MnO, P 2 O 5 and TiO 2 . The vanadium ends up in a BOF slag containing approximately 5% of vanadium pentoxide (V 2 O 5 ) (Waligora et al., 2010). The U.S. Geological Survey also reports that only some tool steel scrap was recycled primarily for its vanadium content. It is further outlined that this only accounted for a small percentage of total vanadium used (Polyak, 2018).
Another secondary source of vanadium is the oil fly ash produced by the combustion of crude oil (Navarro et al., 2007). Depending on the chemical composition and process, fly ash can contain from 2 to 50% of vanadium.
Both, vanadium and niobium are primarily used as an alloying element for different steel grades. About 90% of vanadium (EC, 2014b) and niobium (TIC, 2016) go into steel alloys. It is thus evident that for both elements end-of-life steel grades containing vanadium and/or niobium are potential sources for recycling. It has been reported that in practice niobium is not recycled as pure metal but a niobium bearing alloy is re-melted into a similar alloy (Cunningham, 2004). It has further been outlined that due to the low content of niobium in steel (<1%) niobium bearing steels are not collected separately and during recycling of steel niobium is diluted. This is a so called non-functional recycling in which niobium is lost (Tkaczyk et al., 2018). As a matter of fact the recycling rate of niobium is de facto zero (EC, 2017b). The situation of vanadium is quite similar. Even if concentration of vanadium in steel alloys is higher than niobium, it is well below 4.2% (DIN, 2018).
It can be concluded, that end-of-life steel alloys are the most important source for vanadium recycling as this product group covers 90% of the market. As it makes no sense to recover vanadium from scrap, scrap is re-melted into similar alloys. Recycling of vanadium from steel scrap is thus rather a logistical than a technological issue. However, markets for vanadium other than steel will gain an increasing importance in the future. In particular batteries using vanadium are estimated an emerging technology. In the future vanadium containing batteries must be considered as a source for vanadium recycling.
Stone coal is considered to be an alternative source of vanadium. However, due to the low vanadium grade, complex chemical composition and various occurrences of vanadium of stone coal, vanadium recovery from stone coal is usually confronted with the problems of enormous ore handling quantity, significant acid consumption and notable production cost  Currently vanadium recycling results into the production of V 2 O 5 from V-bearing raw materials including secondary materials are reviewed in a general flowsheet in Fig. 9 displaying the hydrometallurgical route.

Roasting and leaching of vanadium from the secondary sources
Vanadium is recovered as sodium vanadate after roasting the slag in a multi-hearth furnace or rotary kiln with addition of sodium carbonate or sodium sulphate/chloride. In some cases, more additives such as lime are added into the process . However, the salt roasting is a traditional route for production of soluble vanadium substances, it is time-demanding process, which requires high temperature (800-900°C) conditions and it is related to the formation of hydrogen chloride or elemental chlorine . The mechanism of the roasting is according to the reactions below. They represent roasting conditions using the oxygen (reaction (6.1.1)) and with the presence of water (reaction (6.1.2)). Either oxygen or water are required to drive the reaction towards the formation of sodium metavanadate V 2 O 5 + 2NaCl + 1/2O 2 ¡ 2NaVO 3 + Cl 2 (without vapor) ð6:1:1Þ V 2 O 5 + 2NaCl + H 2 O(g) ¡ 2NaVO 3 + 2HCl (with vapor)

ð6:1:2Þ
In general sodium salts roasting process generates harmful gases, such as Cl 2 , SO 2 or HCl. Those by-products can cause the corrosion of the equipment and same way can negatively affect the environment if not treated properly. The efficiency of the roasting is around 80%, thus, several roasting steps are required to achieve sufficient extraction.
The calcification roasting is considered to be a cleaner alternative to the roasting using sodium carbonate. In the calcination roasting the vanadium slag is processed with limestone or lime. Vanadium is transformed from the vanadium-bearing spinels into calcium vanadates. Vanadium in the vanadium slag is present as FeV 2 O 4 spinels which are enraptured by the olive phase Fe 2 SiO 3 (Zhang et al., 2012). After the roasting, the olive phase Fe 2 SiO 4 is degraded and transformed to CaSiO 3 and Fe 2 O 3 . The spinel phase FeV 2 O 4 is then subsequently oxidized and transformed to Ca 2 V 2 O 7 and Ca(VO 3 ) 2 . The calcification roasting is environment-friendly due to the elimination of pollutant gas and it is also cost-effective due to the inexpensiveness of lime and limestone. However, during calcification roasting, the phosphorus in vanadium slag is transformed to calcium triphosphate which can react with sulfuric acid and dissolve into the leach liquor of vanadium (Gao et al., 2017a;Li et al., 2016a;Li et al., 2017a Innovative NaOH-added pellet was applied to replace traditional Na 2 CO 3 -Na 2 SO 4 -NaCl-added pellet (Ji et al., 2017). Vanadium extraction was increased from traditional 80% at 800°C to current 99% at 700°C.
Vanadium recovery from the sources starts directly with the leaching or alkaline roasting followed by the leaching. Valent state of vanadium in the solution can be +2, +3, +4 and +5. If the leaching is performed in the presence of the oxygen or in in the presence of the oxidative agent, vanadium oxidizes to a stable form of pentavalent vanadium. Controlling parameters for vanadium valent state are vanadium molarity in the solution, pH of the solution, and potential (Eh) of the solution. In the acid region the predominant specie of vanadium is VO 2 + . In the alkaline region at pH above 13, VO 4 3À is the predominant specie (Gupta and Krishnamurthy, 1992). Ability to control vanadium valent state via pH is utilized in the leaching process. Secondary source of vanadium -slag is processed via acidic leaching using mostly mineral acids. Acidic leaching is then followed by the neutralization with alkaline media. Using acid, the vanadium in the slag can be recovered as VO 2+ or VO 2 + ions. In the neutralization step, the VO 2+ and VO 2 + ions can be converted to sodium vanadate. In general, leaching of vanadium is performed using sulfuric acid (and oxidation/reduction agent) can be expressed by following chemical reactions of vanadium oxides: Novel approach to improve leaching with the sulfuric acid was developed using microwave heating (Tian et al., 2019) as the alternative to the electric heating. Leaching efficiency of vanadium was improved by 50%. The electro-oxidation is another alternative method for the improvement of the leaching efficiency due to the oxidation of vanadium .
Alkaline leaching with and without the oxidation has been performed to recover vanadium from the secondary sources. Sev-eral oxidative agents were used for the leaching to recover vanadium, and chromium. Yang et al. (2010) applied NaOH and H 2 O 2 . Air was applied as the oxidative agent for the leaching with NaOH (Guo et al., 2015), and oxidizing roasting followed by leaching with NaOH (Yang et al., 2014a) was also tested. The vanadium leaching efficiency in each process was in the range of 75-90%. Selective oxidation of V 4+ to V 5+ was performed using manganese dioxide . Alkaline leaching using NaOH was also applied for vanadium recovery from spent catalysts (Kim et al., 2015).
The research focused the recovery of vanadium from the primary and secondary sources is significantly extensive. There are several works applying roasting or direct leaching summarized in the Table 12. A novel acid leaching method for direct recovery of vanadium from Linz-Donawitz converter slag avoiding pyrometallurgical roasting process is suggested by Mirazimi et al. (2015). At the same time, a direct leaching of vanadium-containing magnetite by mixture of nitric and sulfuric acids is suggested by Nejad et al. (2018) as an optimized approach without pyrometallurgical step.
Vanadium can be recovered from the solution by precipitation using from using ammonium salts (Biswas et al., 1985;Liu and Sui, 2002;Wen et al., 2019;Ye et al., 2012;Zeng and Yong Cheng, 2009). In general, the precipitation efficiency depends mostly on the concentration of ammonium salt and very often 100% recovery rate is achieved. Use of ammonium chloride is preferred due to economic reasons. However, it is necessary to achieve requested efficiency, ammonium chloride has to be applied in excess, since it desalinizes the ammonium vanadate (V) due to the occurrence of the common ion effect (Mazurek, 2013). Satisfactory efficiency can be achieved by precipitating vanadium using barium hydroxide, since the Ba 3 (VO 4 ) 2 is weakly soluble in aqueous solutions (Zeng and Yong Cheng, 2009). Short kinetics and use of stoichiometric amounts of Ba(OH) 2 can lead to the efficiency of 90% for vanadium recovery. Despite the advantage vanadium recovery from the secondary source, such approach leads to the generation of the secondary products. For example, approximately 30-50 tons of ammonia waste water based on Na 2 SO 4 , (NH 4 ) 2 SO 4 and/or NH 4 Cl per ton of V 2 O 5 product is generated  after the precipitation and recovery of the vanadium.
Common methods to recover and purify vanadium from the solution is solvent extraction or ion exchange.

Recovery of vanadium using solvent extraction
For vanadium recovery by solvent extraction several extractants were used. Among these D2EHPA (Di(2ethylhexyl)phosphoric acid) was used in plat operations. D2EHPA extracts V 4+ more strongly than V 5+ and extraction coefficients of V 4+ are high enough to be useful in a practical process. The pH-extraction isotherms exhibit that the extraction order would be Fe 3+ > VO 2+ > VO 2 + > Ca 2+ > -Mg 2+ > Fe 2+ > K + , Na + (Li et al., 2011). It follows from there that at given conditions, D2EHPA will extract Fe 3+ together with vanadium but not Fe 2+ . In order to selectively recover vanadium and to avoid the extraction of iron, it is needful to reduce trivalent iron to divalent. Mechanism of D2EHPA reaction with metal ions is as following ( In the industrial processing the concentration of DEHPA is sustained from 0.2 to 0.4 M and the operational pH is 2. Also, Na 2 S or NaHS is used as the reduction agent. This treatment reduces pentavalent vanadium. For the recovery of the vanadium to the aqueous phase, dilute sulfuric acid is used (Gupta and Krishnamurthy, 1992). The SOTEX process which recovered vanadium and nickel from ash and soot residues has been developed by MEAB. The dominating metal constituents of the residues are vanadium, nickel and iron, together with high concentration of magnesium. The leaching with sulfuric acid yielded to about 55% of V 4+ and for Ni 2+ it was 95%. A final (post) leach with sodium hydroxide dissolved remaining V 5+ . During leaching, the RedOx potential was controlled by SO 2 addition to keep vanadium in its V 4+ form. The extraction of vanadium was achieved with a mixture of DEHPA and TBP diluted in kerosene. Vanadium was stripped from the organic solution with 1.5 M sulfuric acid. The concentration of vanadium in the stripping products was approximately 50 g/L. Ammonium polyvanadate (APV) was precipitated by the oxidation and addition of ammonia. The plant was however closed in 1985.
In the more recent research, the mixture of D2EHPA and TPB was still applied in many processes of vanadium recovery (Li Table 12 Some of the studies dealing with recovery of vanadium from different souces applying roasting and/or leaching. V, W, Ti Huo et al., 2015Huo et al., et al., 2013aLi et al., 2011;Zeng and Yong Cheng, 2009). Addition of TBP to the organic phase reduces the unfavourable effects of M2EHPA (mono (2ethylhexyl)phosphoric acid) which is present in commercial D2EHPA and moreover, it is an effective phase modifier since it improves separation efficiency and phase separation (Cheraghi et al., 2015). It was reported that D2EHPA extracts Fe 3+ but not Fe 2+ and V 4+ is extracted more strongly than V 5+ . Sodium sulphide, sodium hydrosulphide or sodium sulphate can be used as reduction agents to reduce Fe 3+ to Fe 2+ and V 5+ to V 4+ . Extraction is usually carried on at pH around 1-2. Vanadium stripping can be done using sulfuric acid. D2EHPA can be used for recovery of Mn and Mg by increasing pH at 3-4. One of the developments in the field of solvent extraction is the introduction of some new organophosphines and their sulfur analogues as extractants as Cyanex 272 and Cyanex 301. Cyanex 272 can extract V 4+ and the comparison study  showed that D2EHPA is a stronger extractant than Cyanex 272. Cyanex 272 also extracts Fe 3+ and there is a lack of the information about extraction characteristic related to V 5+ . On the other hand, Cyanex 301 can extract V 4+ and vanadium can be separated from Fe 3+ at pH 1. Cyanex 923 was successively applied for V 5+ recovery at pH 1. Extractant LIX 63 was used for V 4+ (Zhang et al., 1996) and V 5+ (Zeng and Cheng, 2010) at pH around 1 was used and excellent separation from Fe 3+ has been reported. Even good separation from Al 3+ was mentioned. Cyanex 272 (0.6 M) was also applied for the separation of Ni 2+ and V 4+ recovered from the fly ash (Noori et al., 2014). Addition of TBP as a modifier raised separation factor of V over Ni (SF V = 440). McCabe-Thiele diagrams showed that two extraction stages are needed for the separation. Solvating extractant Cyanex 923 was applied to separate V 5+ from Fe 3+ (Tavakoli and Dreisinger, 2014). Cyanex 923 showed the best selectivity for V 5+ over Fe 3+ by extracting of VO 2 SO 4in the system.
Amines have been also used in vanadium recovery (Wen et al., 2017). The advantage of amines is that it has two active H atoms and one active N atom, and can extract vanadium selectively (Jing et al., 2017;Ning et al., 2014). Only pentavalent vanadium forms anionic complexes. It the tetravalent state, vanadium is not extracted by amines. Among the various amines, the tertiary and quaternary amines have found maximum use and have been applied in praxis. Alamine 336 and Alamine 308 was tested for vanadium recovery after leaching of the spent catalysts (Kim et al., 2018). It was determined that optimal pH was 2.5. It was concluded that Alamine 308 performed better than Alamine 336, since over 2000 mg/L of vanadium was recovered. Research dealing with the extraction of vanadium and impurities via solvent extraction are summarized in Table 13.
Comparison of ion exchange and solvent extraction for vanadium recovery from sulfuric acid leach solutions of stone coal was published in a work . Leaching of roasted stone coal with sulfuric acid resulted in a mixed vanadium solution containing Fe 3+ , Fe 2+ , Al 3+ and Mg 2+ . An anion exchange resin ZGA414 was tested as its optimum adsorption capacity compared with D202, D453, D301FC and ZGA351 resins, and D2EHPA and TBP diluted with kerosene were employed in solvent extraction. Ion exchange tests indicated that only V 5+ was loaded from the synthetic solution at pH > 1.5, while it was difficult to separate V 5+ from Fe 3+ . Solvent extraction experiments revealed that V 4+ had a better extraction ratio than that of V 5+ , while Fe 3+ had a serious effect on the extraction of V 4+ . The co-extraction ratio of Al 3+ and Mg 2+ can be decreased by controlling their concentrations lower than 10 g/L. Counter-current experiments with D2EHPA presented that 99% of V 4+ was extracted from the real leach solution after reduction process, leaving most of Fe 2+ , Al 3+ and Mg 2+ in the raffinate. Both solvent extraction and ion exchange can be applied in vanadium recovery. D2EHPA has been already applied for vanadium recovery in the industrial applications and seems to be promising extractant since it allows to extract V 4+ and V 5+ . However, D2EHPA extracts Fe 3+ and it can cause the difficulties in vanadium recovery and use of reduction agent would be required. LIX 63 seems to be also suitable extractants since it extracts V 4+ and V 5+ over the iron.

Recovery of vanadium using ion exchange
Ion exchange has been also applied for vanadium recovery. A strong anion exchange resin such as Amberlite IRA-400 has been used in uranium ore process. It may be added that the resin extracts along with the vanadium the major impurity -iron. This is, however selectively removed from the loaded resin with NH 4 Cl and HCl prior to the elution of vanadium. Ion exchange is mostly used in the processes of spent hydrodesulfurication catalysts to recover molybdenum and vanadium (Chen et al., 2003;Li et al., 2009). To recover V 5+ from the sulfuric media strong anion exchange resin DOWEX 21 K (Zipperian and Raghavan, 1985), weak anion exchange resin D314  and strong cation exchange resin DOWEX 50-W, have been used for vanadium recovery. The possibility of vanadium (V 4+ , V 5+ ) separation from other metals was performed using DOWEX50-X8 (Fritz and Abbink, 1962). Vanadium was separated from Mn 2+ which was not absorbed. Subsequently V 5+ was separated from Ti 4+ and Fe 3+ with HClO 4 . Seeing that leaching solution contains also other metals, it should be mentioned that heavy metals can be selectively removed from acid leach solutions using DOWEX TM M4195 resin. Metal removal from very strong acids may be effective using strong base anion exchange resin such as DOWEX 21 K resin. Ion exchange has been applied for recovery of vanadium from several primary and secondary sources. Ion exchange has been applied for vanadium recovery from the stone coal (Li et al., 2013c;, from the waste generated in the Bayer process (Zhao et al., 2010), from the wastewater (Keränen et al., 2015), spent catalysts (Nguyen and Lee, 2013), and steel making slags (Gomes et al., 2017). Recovery from the alkaline media using D403 has been reported in . Several other works applying ion exchange for vanadium separation and recovery are summarized in the Table 14.
A designated system model was prepared (Lundkvist et al. 2013) by simulating the implementation of a two-step BOF blowing procedure and a slag reduction process in an integrated steel production system. The aim was to analyse the system effects from extracting vanadium as an FeV alloy and improve the slag recycling and total material efficiency in the system. Smelting reduction was applied for the recovery of vanadium and chromium from an LD slag. An in-plant by-product melting process (IBPM) was tested using steel slags, fine-grained wastes, such as EAF dust, millscale, oily millscale, BOF dust, BF dust, hydroxide sludge and scrap resi-due (Ye et al. 2003). Iron, vanadium, chromium and nickel were recovered in a metal (alloy) phase after applying the IBPM.
A German company, GfE Metalle and Materialien GMBH, developed a combination of pyrometallurgical and hydrometallurgical processes, where a pyrometallurgical approach is used for obtain-ing cast vanadium concentrate for the further extraction of pure vanadium (Marafi et al., 2010).
Microwave-assisted pyrometallurgical processes for the calcination (Salakjani, Nikoloski, and Singh 2017) of ores are emerging applications for vanadium-containing ore processing. Specifically,  when applied to a vanadium slag, it facilitates materials conversion at lower temperature (Gilligan and Nikoloski 2020).

The study of technological innovation change
The rich literature about the vanadium recovery proves the strong interest for this topic, showing innovative experimental approaches for the exploitation of several kinds of waste. Nevertheless, many treatments need specific reaction conditions that make the scale-up difficult. In order to have an overview of the options designed for a real implementation, a deepened study of published patents and the European projects was carried out, following the approach used by Amato and Beolchini (2018). The study of technological innovation change allows the identification of innovative markets, promising from an economic point of view Garcia and Calantone, 2002;Rocchetti et al., 2018).
As concern the patents, the free access Espacenet platform was chosen as research tool (http://worldwide.espacenet.com), since it ensures the overview of the worldwide inventions. The ''vanadium recycling" was used as keyword for the patent researches and an interval around 50 years (between 1954 and 2018) was taken into Table 15 Pyrometallurgical processing to improve vanadium recovery.

Approach
Main products Reference Aluminothermic reduction of vanadium sludge Ferrovanadium Suri et al., 1983 Electro-aluminothermic processing Vanadium droplets and vanadium oxides Bellemans et al. 2018 CO 2 addition to oxygen and bubbling into melt Vanadium Ray et al. 2013 Furnaces with open slag bath configuration (Evraz Highveld Steel & Vanadium process) Vanadium oxide and ferrovanadium Steinberg, Geyser, and Nell 2011 Pyrometallurgical processing of vanadiferous slag using silicothermic reduction and byaluminothermic reduction and plasma induction heating Ferrovanadium Richards et al. 1992 Fig. 10. Technological evolution of the processes for the vanadium recovery. The ball size is proportional to the available number of patents. Recovery of valuable metals from the TiO 2 production process [1983][1984][1985] United Kingdom TiO 2 production process waste Vanadium and associated materials beneficiation out of residues and complex ores [1983][1984][1985] Italy various residues Valuable metal recovery from residues: Mo, Co and Ni from spent catalysts 1988-1990 Spain Spent catalysts Table 17 Patents dealing with vanadium recovery.

Patent number Patent title
Year Reference  (Table 17), mainly from: tailings, slags, spent catalysts, different kinds of industrial waste, industrial wastewater (or vanadium rich solutions which simulates real flows). The high number of patents proves the actual interest in the implementation of an urban mining strategy, with an increasing trend since 2011, for all the considered scraps (Fig. 10). Around 92% of the gathered inventions was developed in China, whereas the remaining percentage is shared among Taiwan, USA, Great Britain, Hungary and South Korea. The reason could be the Chinese willpower to increase the already high vanadium primary production, around 53% of the whole market (EC, 2017b), with the supply from secondary raw materials.
The collected patents include matrices in which the metal target is combined with different metals: titanium, iron, chromium, manganese, tungsten, molybdenum, aluminum, silicon, cobalt, nickel, silver, selenium. Overall, the described processes include a pre-treatment, specific for the waste type. Usually, this preliminary step is followed by hydrometallurgical treatments for the vanadium extraction, mainly acid or alkaline leaching, at different conditions, irrespective of the starting matrix. A consecutive stage allows the vanadium recovery and the solvent extraction (combined with a precipitation) is one of the most common options to ensure the highest metal purity Li, H. et al., 2018;Shikun et al., 2010;Wang, Y. et al., 2017b;Xi et al., 2018;Xia et al., 2016;Zhang, T. et al., 2015).
On the other hand, the adsorption by resins for the selective vanadium recovery was described for the treatment of both wastewater or metal rich synthetic solutions (He et al., 2016;Li, H. et al., 2015b;Mingyu et al., 2012;Yin, D. et al., 2013;Zibi et al., 2012aZibi et al., , 2012b and spent catalysts (Sun et al., 2013b;Xia et al., 2017).
An additional study focused on the European Commission funded projects to better understand the research evolution over time. With this aim, the European Commission Cordis portal was used and the two keywords: ''vanadium recycling" and ''vanadium recovery" were selected (https://cordis.europa.eu/). Table 16 shows the research results, proving an interest which started in 1983 with an Italian task for the exploitation of different residues. The relevance of this topic continues until today, as confirmed by the ERA-MIN French project. Overall, the main waste targets for the vanadium recovery include the spent catalysts and the steel slags, in agreement with the study of patents.

Discussion
The discussion in the present review highlighted how the increasing criticality of vanadium has pushed towards the utilization of secondary sources to replace the traditional mining activities. The study of different kinds of sources (the scientific literature combined with available patents and European funded projects) has proved a particular attention to the development of more sustainable technologies with lower environmental impact and generation of the harmful by-products. Current research activities, summarized in this review, show that it is possible to design the alternatives to traditional methods with better efficiencies and lower footprint, in agreement to the circular economy principles. In this regard, the combination of hydrometallurgical and pyrometallurgical approaches can lead to increase the material recovery rates since hydrometallurgy allows for more selective metal separation and provides the ability to re-utilize chemical reagents or byproducts within the production. Therefore, future perspectives include the sustainability increase of the recovery processes, also combining established techniques with innovative approaches. impact, www.crm-extreme.eu. Furthermore, the authors would like to acknowledge networking support from COST CA15102.