Vanadium – Valuable and toxic element in coal combustion ash: An overview

Process Safety and Environmental Protection 172 (2023) 923–940 Available online 27 February 2023 0957-5820/© 2023 The Authors. Published by Elsevier Ltd on behalf of Institution of Chemical Engineers. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Vanadium – Valuable and toxic element in coal combustion ash: An overview Lucie Bartoˇ nov´ a * , Helena Raclavsk´ a, Jan Najser ENET Centre, CEET, VSB-Technical University of Ostrava, Tˇ r. 17. listopadu 15, Ostrava-Poruba 708 33, Czech Republic ARTICLE INFO Keywords: Vanadium Coal combustion Ash Physical separation Leaching Wastes ABSTRACT The paper evaluates all important aspects related to V present in coal and its behaviour during coal combustion with particular attention paid to its potential extraction from ash and slag wastes. Due to great environmental benefits of co-combustion of coal and wastes, these alternative fuels and corresponding ash and slag counterparts were also taken into evaluation. Specifically, the paper summarizes V levels in world coals, in other related materials (alternative fuels, additives etc.) and in ash and slag residues including V speciation and discussion of its valence states. Theoretically predicted (equilibrium-based) calculations and species are compared with experimental results; due to a significant effect on mitigating CO 2 emissions, attention is paid also to oxy-fuel combustion conditions. With the aim to extract V out of the ash and slag residues, physical, chemical, and biotechnological separation methods are discussed in detail along with dominant affecting factors. The paper includes also concluding remarks and highlights the most promising trends and limitations thereby providing suggestions for future research. 1. Introduction Annual production of V is estimated to be around 100.000 t (Bielowicz, 2020; Chinach, 2020); the largest producers are China, South Africa and Russia (Bielowicz, 2020). V is widely used in industrial applications. Due to its high tensile strength, hardness, and fatigue resistance, about 85% of V is used in the production of high-strength ferrous and non-ferrous alloys (Moskalyk and Alfantazi, 2003). According to Moskalyk and Alfantazi (Moskalyk and Alfantazi, 2003) there is still no acceptable substitute for V in the production of aerospace Ti alloys. Other important applications are production of catalysts, inks, dyes, pigments for ceramics and textile industry, batteries, or materials for nuclear reactors (Bielowicz, 2020; Zhang et al., 2019a; Font et al., 2007). According to European Commission (2020), V belongs to the 2020 list of critical raw materials, i.e., materials that are the most important economically and with a high supply risk – possible interruption in V supply would have harmful consequences on economy (European Commission, 2020). V is the 22-th the most abundant element in earth crust with average concentration of 150 g/t (Fleischer, 1953; Zhang et al., 2019a). As V is not present in high concentrations in minerals, other sources are typically used – it can be recovered as a by-product during extraction of other components from petroleum residues, metallurgical slags, ore concentrates, waste materials etc. (Moskalyk and Alfantazi, 2003; Vitolo et al., 2000; Bielowicz, 2021). Combustion residues (ashes or slags) are typical industrial wastes produced in vast quantities throughout the world, e.g. in India (112 Tg/ y), China (100 Tg/y), USA (75 Tg/y), Germany (40 Tg/y), UK (15 Tg/y), Poland (4 Tg/y) etc. (´ Swietlik et al., 2012; Zhao et al., 2018a). Combustion ashes/slags are therefore intensively studied as a possible source of valuable components, such as char (Badenhorst et al., 2020; Bartoˇ nov´ a, 2015; Nunes et al., 2022; Hower et al., 2017), alumina (Guo et al., 2019), Ge-Ga (Dai et al., 2014a; Zhou et al., 2021), REE (Wagner and Matiane, 2018; Fu et al., 2022; Hower et al., 2020, 2016; Valentim et al., 2019) or Y (Bartoˇ nov´ a et al., 2018; Vassilev and Vassileva, 2020). Unfortunately, papers evaluating combustion ashes as an available source of V are very scarce or nearly non-existent. During coal combustion, V present in input fuel is redistributed among emissions and solid combustion residues – ashes or slags. Due to medium/low V volatility, environmental research is primarily focused to other (usually more volatile) elements. Therefore, papers dealing with mitigating gaseous emissions are focused predominantly to Hg (Wiero´ nska et al., 2018; Scala et al., 2011; Hower et al., 2005), As (Wiero´ nska-Wi´ sniewska et al., 2022; Fu et al., 2021), Se (Yu et al., 2022), Pb * Corresponding author. E-mail address: [email protected] (L. Bartoˇ nov´ a). Contents lists available at ScienceDirect Process Safety and Environmental Protection journal homepage: www.journals.elsevier.com/process-safety-and-environmental-protection https://doi.org/10.1016/j.psep.2023.02.070 Received 5 December 2022; Received in revised form 23 February 2023; Accepted 24 February 2023 Process Safety and Environmental Protection 172 (2023) 923–940 924 (Bartoˇ nov´ a et al., 2019; Zhao et al., 2018b, 2017), Cd (Bartoˇ nov´ a et al., 2020; Fu et al., 2019a), Cu and Zn (Zha et al., 2018, 2020) or Cr (Zhao et al., 2018b; Bartoˇ nov´ a and Raclavsk´ a, 2022; Wagner et al., 2021; Fu et al., 2019b). Hence, due to medium/low V volatility, large proportion of V is bound in combustion ashes/slags, which may hinder their further technological use, handling, storage, or landfilling. According to EPA report from 2011 (US EPA, 2011), V is included in two pollutant groups – “Heavy metals” and “Inorganic pollutants”. Exposure to V compounds may cause coughing, irritation of upper respiratory tract, gastrointestinal diseases, or conjunctivitis. The toxicity depends on V speciation and generally increases with increasing valence state – pentavalent V species are the most toxic (Barceloux and Barceloux, 1999; Navarro et al., 2007). Therefore, V is traditionally considered as an element of environmental concern (Wagner and Hlatshwayo, 2005; Vassilev et al., 2014; George et al., 2008; Goodarzi, 2002; Swaine, 1990; Duchesne et al., 2018). Out of this perspective, recovery of V from solid combustion wastes can lead to multiple benefits – providing critical metal while enhancing the perspectives of the combustion wastes for their further technological use. Due to rapid urbanization and population growth, generation of other than industrial wastes is growing fast as well (Tun et al., 2018; Tun and Juchelkov´ a, 2019a). The co-combustion of coal and biomass, municipal solid waste etc. contributes to CO 2 neutral conversion and climate change benefits (Xu et al., 2020; Zhou et al., 2014a; Růˇ ziˇ ckov´ a et al., 2018). It addresses both the depletion of fossil fuels and environmental problems with greenhouse gas emissions; moreover, in case of certain wastes, thermal treatment is taken into consideration if there is a danger of releasing harmful organic pollutants during landfilling or if mechanical recycling is for any reasons problematic (Kucbel et al., 2019; Raclavsk´ a et al., 2018). Therefore, it is not surprising that thermochemical conversion of wastes and biomass is one of the most actively studied areas in the world science (Vershinina et al., 2022; Corsaro et al., 2016; Demirbas, 2004). However, different composition and other characteristics of such ash and slag wastes require attention due to their broad variability in dependence of the waste material used (Růˇ ziˇ ckov´ a et al., 2018; Vassilev et al., 2015; Demirbas, 2004). For the aforementioned reasons, this review paper evaluates all important aspects related to V present in coal and its behaviour during coal combustion with particular attention paid to its potential extraction from ash and slag residues. Due to great environmental benefits of cocombustion of coal and wastes, these alternative fuels and corresponding ash and slag counterparts were also taken into evaluation. The scheme of the evaluated research area is depicted in Fig. 1. Specifically, Section 2 summarizes V levels in world coals as well as in other related materials (biomass, additives etc.) and Section 3 deals with thermodynamic equilibrium predictions. Section 4 evaluates concentration of V in ash and slag residues including the effect of operating conditions (e.g., oxy-fuel combustion) and V speciation. Section 5 discussed physical separation methods and their efficiency if applied on ash and slag wastes, while Section 6 provides an overview of chemical and biotechnological ways of V extraction from these materials. Section 7 includes concluding remarks and highlights the most promising trends (as well as the limitations) providing suggestions for the future research. 2. V in coals and other related materials 2.1. V concentrations Concentrations of V in coal combustion ashes/slags are given not only by operating conditions but namely by V levels in coal; in case of co-combustion with other materials (wastes) V levels therein also affect the resulting concentration in ash/slag. Examples of V concentrations in coals from world coal regions are summarized in Table 1; concentrations in coal gangue/rejects and limestone (typically used as desulfurization additive) are listed in Table 2. World averages for low-rank and high-rank coals are 22 and 28 ppm (g/t) V (Ketris and Yudovich, 2009); nevertheless, the data from Table 1 document broad fluctuation (variability) even within 1 seam or coalfield. There are coals containing a few ppm V as well as coals with a few hundred ppm V. Most world coals contain V content below 100 ppm Fig. 1. Classification diagram describing evaluated research area. L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 925 (Swaine, 2000); V concentrations in coal gangue, wastes, rejects or inclusions are generally moderately higher whereas V levels in limestone are lower compared to coals (Table 2). Absolutely the highest V concentration relates to petcoke; therefore, if such material is added to coal during combustion (gasification), the ashes/slags produced contain substantially higher V concentrations (which is undesirable from environmental point of view but advantageous in terms of searching for available V source). 2.2. Modes of occurrence of V Not only total concentration but also modes of occurrence of V in coal affect its levels in the combustion residues. According to Finkelman et al. (2018), in high-rank coals 65% of V is associated with (alumino) silicates (probably clays) and 35% is organically associated, whereas in low-rank coal the proportion of organic affinity is higher (50%) while silicate association is lower (50%). In 21 Chinese coals of different rank (from lignite up to anthracite), in addition to aluminosilicate association (R(V-Al) = +0.42), siderophile affinity was also observed (R(V-Fe) = +0.55). As relationship with S was quite low (R(V-S) = +0.21)), it was concluded that V was bound in other Fe-bearing minerals than sulphides/disulphides (Liu et al., 2016). Siderophile character of V was mentioned also in other papers (Vassilev et al., 2005; Wagner and Hlatshwayo, 2005). The decrease in the proportion of organic affinity with increasing rank observed by Liu et al. (2016) agrees with the conclusion of Finkelman et al. (2018). The results of Finkelman et al. (2018) are based on evaluation of 20 USGS coal samples from Australia, Great Britain, Canada, Brazil, and the USA (Pennsylvania, West Virginia, Kentucky, Alabama, Wyoming etc.) and were obtained by sequential leaching (discussed with the view of other available data, such as optical petrography, SEM with ED XRF, electron microprobe analysis, ion probe analysis, XRD or density fractionation). In case of Kentucky #9 and Illinois #6 coals, X-ray absorption spectroscopy data are available (Maylotte et al., 1981; Huggins and Table 1 V concentrations (in ppm or µg/g) in coals from world coal regions. Region Coal Concentrations Ref. World World average lignite (3889 coals) Arithmetic mean 37.28 Central range 1.2–191.6 Maximum 843.0 (Bouˇ ska and Peˇ sek, 1999) World low rank coal World hard coal World total (all coals) 22 ±2 28 ±1 25 (Ketris and Yudovich, 2009) Ketris and Yudovich (2009) Ketris and Yudovich (2009) World coals 2–100 (for most coals) (Swaine, 1990, 2000) Northern Europe 4 Northern European peats 0.51–11.3 (mean 4.2) (Klavins et al., 2009; Růˇ ziˇ ckov´ a et al., 2019a) Australia 35 Gunnedah Basin Coals 6.90–1436.0 (mean 153.15) (Ward et al., 1999) Australian coals 2–279 (Swaine and Goodarzi, 1995) Brazil 7 feed coals 97.54–124.07 (Silva et al., 2010) 57 Brazilian coals 23–86 (Pires and Querol, 2004) 4 coals 71–87 (Cutruneo et al., 2014) Santa Catarina coal 104 (mean) (Oliveira et al., 2012) Bulgaria Underground mine coals 17 ±7.9 (Eskenazy and Stefanova, 2007) Open-pit mine coals 19 ±5.9 (Eskenazy and Stefanova, 2007) Dobrudza deposit 42 ±20 (Eskenazy, 2009) Bobov dol 76 (Vassilev et al., 1994) 5 raw coals 51–116 (Yossifova, 2014) Canada White Wood Mine coal Alberta 4.79–56.83 (Gentzis and Goodarzi, 1999) Genesee Mine coal 2–83 (Pollock et al., 2000) 6 feed coals 8.70–18.0 (Goodarzi, 2002) Canadian coals 23–300 (Swaine and Goodarzi, 1995) Colombia 7 commercial feed coals (exported to Portugal) <8–32 (mean 18) (Santos et al., 2022) China 1324 Chinese coals 35.1 (Dai et al., 2012) 1123 Chinese coals 51.18 a (Bai et al., 2007) 1266 Chinese coals 35.05 a (Ren et al., 2006) 1257 Chinese coals 25 a (Tang and Huang, 2004) 28 coals from 28 Chinese provinces 46.81 (arithmetic mean) 35.81 (weighted average) (Liu et al., 2017) Lvshuidong mine drillhole 16.9–195, weighted average 68.4 (Dai et al., 2014b) 10 Leping coals 22–105 (Querol et al., 2001) Jungar basin coals 10–200, average 27 (Dai et al., 2008) Yudai coal mine 32–201 (Li et al., 2020) Jinqi coal mine 31–398 (Li et al., 2020) Haerjiao coal <dl - 33 (Li et al., 2019) India Meghalaya coalfield Upper Assam coalfield Jammu coalfield 25–500 (in coal ash) 200–600 (in coal ash) 30–150 (in coal ash) (Mukherjee et al., 1992) Mukherjee et al. (1992) Mukherjee et al. (1992) Iran 4 Alborz coals 11–16 (Goodarzi et al., 2006) South Africa Highveld coalfield 23–37 (Wagner and Hlatshwayo, 2005; Wagner and Tlotleng, 2012) Witbank coalfield (no. 4 seam) 39.2 Table 1 (continued) Region Coal Concentrations Ref. (Bergh et al., 2011; Wagner and Tlotleng, 2012) Witbank coalfield (no. 2 seam) 27 (Cairncross et al., 1990; Wagner and Tlotleng, 2012) Waterberg coalfield 85–150 (Faure et al., 1996; Wagner and Tlotleng, 2012) Spain Spanish subbituminous coal 58.2 (Querol et al., 1995) 4 Puertollano coals 66–265 (Font et al., 2010) Puertollano coals (2 mines) 13–89 (Alastuey et al., 2001) Turkey 143 Turkish coals 5.5–270 (mean 65, median 52) (Palmer et al., 2004) 13 Turkish coals 65–219 (Karayigit et al., 2000) Soma coals 97 and 163 (Karayigit et al., 2007) UK 24 UK coals 6.6–93 (average 31.6) (Spears and Zheng, 1999) Parkgate coal 25.1 (median) (Spears, 2015, 2017) USA 142 Gulf Coast coals 205 Fort Union coals 4.9–110 (median 28) 1.1–110 (median 7.4) (Palmer et al., 2004) Average in U.S. coals 22 (Orem and Finkelman, 2003) U.S. coals 0.14–370 (Swaine and Goodarzi, 1995) 3 Appalachian coals 4.61, 3.63 and 20.54 (Hower et al., 2015) a Papers in Chinese reviewed by Dai et al (Dai et al., 2012). L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 926 Huffman, 1996, 2004; Huggins et al., 2009, 1997) and agree with the aforementioned sequential extraction ones (Finkelman et al., 2018). The XAFS results are consistent – revealing two different V associations in coal – mineral and maceral association. The former is attributed to aluminosilicate affinity, probably with V-rich muscovite K(V 3+ , Al) 2 AlSi 3 O 10 (OH) 2 (roscoelite) or illite containing V(3 +). In both these cases, V(3 +) prevails (Huggins and Huffman, 1996; Huggins et al., 1997). In organic (maceral) association, V(4 +) is more common than V (3 +) that dominates in aluminosilicates (Maylotte et al., 1981; Huggins and Huffman, 2004). It was hypothesized that maceral association of V relates to poorly crystalline oxide V 2 O 4 or oxyhydroxide VO(OH) 2 species (Huggins and Huffman, 2004). 3. Thermodynamic equilibrium calculations Under standard oxidizing conditions and lower temperatures (below 630 K), according to Frandsen et al. (1994), vanadyl sulfates are formed. At ca. >630 K, VOSO 4 (s) decomposes to V 2 O 5 (s,l) according to Eq. (1) (Frandsen et al., 1994): 2 VOSO 4 (s) ↔ V 2 O 5 (s,l) +2 SO 2 (g) +½ O 2 (g) (1) Between 1350 and 1645 K, V 2 O 5 (s,l) is converted to VO 2 (g) (which is stable at 1640 K) according to Eq. (2) (Frandsen et al., 1994): V 2 O 5 (s,l) ↔ 2 VO 2 (g) +½ O 2 (g) (2) Under reducing conditions, V also forms O-species but V 2 O 5 (s,l) is converted to VO 2 at 1500–1750 K, i.e. at higher temperatures (Frandsen et al., 1994). As the only gaseous form of V in these equations is VO 2 , it could be deduced that at oxidizing conditions V is more volatile than at the reducing ones, which is in agreement with the conclusion of Yan et al. (2001a) that unlike many other trace elements (Mn, Sb, Zn etc.), only V, As and Sn exhibited increased volatility under oxidizing conditions (vs. reducing ones), whatever the coal type (i.e., in both high-ash and low-ash coals). In this study, the difference in temperature for the same V volatility in oxidizing and reducing conditions was ca. 100 ◦C as well. It also agrees with the conclusions of Duchesne et al. (2018) that in combustion systems V is more volatile than in gasification ones. As higher oxidation states of V (that are more toxic) are expected during combustion (vs. gasification), it might result also in environmental consequences. According to Yan et al. (2001b), V belongs to elements with high affinity to O, which has the order: As >Se (Co, Cr, Mn, Pb, Sb, Te, V)>Ni >Sn >Zn > > (Cu, Hg) >other trace elements (Cd, Tl). V forms O-combined species even at low mole fraction of O (O≈0.16); with increasing amount of O, more V-O species appeared. The Table 2 V concentrations (in ppm or µg/g) in coal gangue, rejects, waste materials cocombusted with coal and limestones. Material Details Concentrations Ref. Coal gangue Coal gangue from Guqiao Coal Mine, China 54.3 (Zhou et al., 2014a) World coal gangue Clarke value-coal gangue Huainan Coalfield China 100–400 135 10.1–83.1 (average 49.5) (Zhou et al., 2014b) Zhou et al. (2014b) Zhou et al. (2014b) Coal gangue Fired gangue (Coal) 77 88 (34) (Querol et al., 2008) Querol et al. (2008) Querol et al. (2008) Coal processing wastes Coal wastes from dumps 203–295 (average 243) (Ciesielczuk et al., 2014) Coal pulveriser rejects From <1% S coal From 1% to 2% S coal From >3% S coal 16 (ppm of ash) 46 (ppm of ash) 55 (ppm of ash) (Hower et al., 2005) Hower et al. (2005) Hower et al. (2005) Coal inclusions Coal inclusions Bed coals 640 a 150 b (Yudovich, 2003) Yudovich (2003) Raw/washed coals G. Wood, UK Cerrejon, Colombia Harworth, UK Raw/washed 46.5/16.3 24.5/7.75 50.9/33.9 (Quick and Irons, 2002) Quick and Irons (2002) Quick and Irons (2002) Quick and Irons (2002) Petcoke Petcoke 1560 (Mukherjee et al., 2003; Sloss, 2007) Petcoke from petroleum rafinery 586–1237 (Font et al., 2010) Coal (70%)/Petcoke (30%) feed blend 7950 (ash basis, 11.3% ash) (Silva et al., 2013) Schlams/coal slurry/ petcoke (2:7:1) 442 (George et al., 2008) Agricultural biomass World reference plant 0.5 c (Vassilev et al., 2014) Clarke for angiospermous plants worldwide Clarke for gymnospermous plants worldwide 1.6 0.69 (Vassilev et al., 2014) Vassilev et al. (2014) Peanut shell Wheat straw 2.81 7.17 (Zhou et al., 2014a) Zhou et al. (2014a) Soybean stalk 1.15 (Zhou et al., 2015) Corn stalk Sawdust 2.92 2.64 (Zhou et al., 2016) Zhou et al. (2016) Wood pellets ca. 2 (from graph) (Guo and Zhong, 2018) Straw 2.24 (George et al., 2010) Waste wood Biomass mix (green wood, garden waste, straw, roadside grass and manure) 10 6 (Mukherjee et al., 2003; Sloss, 2007) Mukherjee et al. (2003);Sloss (2007) Coal-Biomass feed blends Coal – wet saw dust (3:1) Coal slurry – dry saw dust (3:7) Coal-undried bark (1:3) 44.3 90.4 139 (George et al., 2008) George et al. (2008) George et al. (2008) Sludge Table 2 (continued) Material Details Concentrations Ref. Paper sludge Municipal sewage sludge 5 24 (Mukherjee et al., 2003; Sloss, 2007) Mukherjee et al. (2003);Sloss (2007) Sewage sludge 15.61 (George et al., 2010) Limestone 3 High-Cl limestones 1.3, 5.8 and 15.1 (Font et al., 2010) Limestone Limestone slurry 11 8 (C´ ordoba et al., 2012) C´ ordoba et al. (2012) 2 Czech limestones <2.1 (Klika et al., 2001) a Ash from the coal inclusions b Ash from the bed coals c Worldwide average of all parts of plants L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 927 most abundant are V-Ca (or Fe) oxides, such as FeV 2 O 6 (s), CaV 2 O 6 (s), CaV 2 O 7 (s), Ca 3 V 2 O 8 (s), FeV 2 O 4 (s), etc. (Yan et al., 2001b). It is consistent with V species predicted by George et al. (2008, 2010) – VO 2 , V 2 O 5 , FeO.V 2 O 5 , CaO.V 2 O 5, and 2CaO.V 2 O 5 . Predicted formation of V 4 O 10 was not observed experimentally, which was explained by kinetic constraints. In addition to the effect of oxidizing or reducing atmosphere, there is also an effect of the rest of the atmosphere, which means that not only % O 2 but also traditional (conventional) or oxy-fuel atmosphere could have as effect on V volatility (Jano-Ito et al., 2014). The comparison of temperatures where more than 50% V was volatilized revealed that this temperature was lower at air-fired combustion (>660 ◦C) than at oxy-fuel combustion with 55.9% and 72.1% gas recycling (>690 ◦C and >670 ◦C). However, this difference was not attributed to replacement of N 2 by CO 2 ; rather, it was explained by larger concentration of S (due to gas recycling) that bind Ca leaving its lower amount available for the formation of Ca-V oxides which results in more abundant V 2 O 5 formation (Jano-Ito et al., 2014). V interacts also with Cl, the temperature of VOCl 3 occurrence is lower at oxy-fuel combustion (100–200 ◦C) than at air-combustion (ca. 500 ◦C) (Jano-Ito et al., 2014). 4. V in ash and slag wastes 4.1. Concentrations of V in bottom ash (BA)/slags, fly ash (FA) and desulphurization residues World averages for V concentrations in brown-coal and hard-coal ashes are 140 and 170 ppm (Ketris and Yudovich, 2009). V concentration ranges in FAs from Australia, Europe, and Canada-USA are 49–274 (median 143) ppm, 154–514 (median 202) ppm and 49–3120 (median 413) ppm, respectively (Riley, 2007; Sloss, 2007). Elevated V content was observed in industrial FA originated from cocombustion of coal, tire-derived fuel and petcoke (1400 ppm) (Hower et al., 2001); extremely high concentration of V in FBC FA was obtained if petroleum coke was used as input fuel – 5473 ppm V (Gonz´ alez et al., 2009). Co-combustion of coal and solid recovered fuel (SRF) containing plastics, wood, paper, fabric, rubber etc. produced FAs with V levels similar to those of conventional coal combustion – FAs from hard coal and SRF contained 103 and 175 ppm V (Wasielewski et al., 2020) or 353 ppm (Wojtaszek et al., 2021), FA originated from lignite and SRF contained 233 ppm V (Bartoˇ nov´ a et al., 2010). World reference for biomass ash (calculated as worldwide average for all parts of all plants and 4.9% ash yield) is 10 ppm (Vassilev et al., 2010, 2014); Clarke value for plant ash is 61 ppm V (Vassilev et al., 2014). V concentrations in desulphurization products have been published in literature as well. European (mostly from Germany), Japanese, and USA FGD gypsums contained 8–16, 21 and 10–12 ppm V, respectively (Berland et al., 2003; Sloss, 2007). V concentrations in FGD sulfate and sulfite were 8 and 15 ppm (Hower et al., 2005); in gypsum and gypsum slurry it was 7 and 5 ppm V (C´ ordoba et al., 2012). All these values are substantially lower than V contents in ashes or slags (mentioned above). There are abundant papers reporting V concentrations in BA, slag, and FA samples from coal (co-) combustion; typically, V levels in FAs are moderately higher than those in BAs/slags (and than those in coals). Fig. 2 depicts enrichment factors (EF) of V in FA compared to BA/slag (within the same combustion unit) and it was created using the literature data (top to bottom) (Font et al., 2010; Silva et al., 2010; Dahl et al., 2009; Silva et al., 2014; Parzentny and R´ og, 2020; C´ ordoba et al., 2012; Querol et al., 1995; Tang et al., 2013; Hower et al., 1996; Wu et al., 2019; Chen et al., 2019; Karayigit et al., 2007; Spears and Martinez-Tarrazona, 2004; Silva et al., 2010; Chen et al., 2019; Narodoslawsky and Obernberger, 1996). In most combustion tests, EFs are >1 indicating V enrichment in FA vs. BA/slag. The only exceptions are two white columns (Narodoslawsky and Obernberger, 1996; Chen et al., 2019) related to grate-fired units; the lowest value in the whole diagram (Narodoslawsky and Obernberger, 1996) relates to the test operating without coal – therein, only bark and wood were combusted in grate-fired facility. In contrast, it is interesting that the 4 highest EFs relate to units operating with coal and other materials, such as petcoke (EF=1.82) (Font et al., 2010), oil (EF =1.52) (Silva et al., 2010), forest residue (EF =1.47) (Dahl et al., 2009), or switchgrass (EF =1.38) (Silva et al., 2014). Enrichment factors for normal PCC or FBC of (sole) coal are within the range of 1.09–1.37; despite combusting different types/ranks of coal in various units, the EF values are quite similar. Due to CO 2 neutral conversion and climate change benefits, the replacement of certain part of coal (gangue) by biomass during the combustion is intensively studied (Zhou et al., 2014a, 2015, 2016; Guo and Zhong, 2018). General conclusion of all these studies is decreased V percentage in FA (vs. BA) during combustion due to adding biomass. Even if V percentages of gaseous emissions remained low (ca. 1–2%) during combustion of coal gangue and soybean stalk, peanut shell, or sawdust (or their blends with the gangue), V proportion in BA during combustion of sole biomass was always higher than that in sole gangue – in case of soybean stalk and sawdust by ca. 5–10%, in case of peanut shell even by ca. 25%. Although the effect of the overall distribution of the ash particles during the combustion experiment cannot be neglected, higher levels of alkali and alkali earth elements were supposed to play a dominant role (Zhou et al., 2014a, 2015). Co-combustion of anthracite with wood pellets in fluidised bed have led to similar conclusion – adding wood pellets inhibit volatilization of V (Guo and Zhong, 2018). 4.2. Distribution of V within ESP/baghouse rows V concentrations in 3 (or more) rows of the same ESP or baghouse are for various combustion tests shown in Fig. 3. The diagram was created using the literature data (left to fight) (Mardon et al., 2008; Tang et al., 2013; Santos et al., 2022; Santos et al., 2022; Santos et al., 2022; Santos et al., 2022; Kostova et al., 2016; Kostova et al., 2016). V levels in these FAs are mostly a few hundred ppm, only in some individual cases they exceed 400 ppm. In most combustion tests, V levels gradually increase with increasing number of the row (i.e., in the last rows its content is typically higher). FA from the last rows contain more V; hence, such last-row ashes should be more suitable when searching for the cheap V source; nevertheless, amount of FA collected at the 3rd or later rows is rather low in comparison with FA from the 1st row which is more abundant. This general trend (V increase in later rows FAs) is valid also for cocombustion of 70% pulverized coal +30% pet coke (in Kentucky power station) (Silva et al., 2013). FA from the 1st row was not evaluated (due to difficulties with the FA collection), in the 2nd and the 3rd sections the V levels were 30,800 ppm and 35,600 ppm V -these concentrations are high due to high V content in 70% coal/30% petcoke feed fuel – 7, Fig. 2. Enrichment factors of V in FA vs. BA/slag. *Numbers in parentheses indicate the number of FA-BA/slag pairs evaluated (and averaged). L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 928 950 ppm (ash basis; 11.3% ash). Increasing concentration of V (and other pollutants) (Czech et al., 2020) with decreasing particle size of FA is undesirable from environmental point of view because the finest PM particles can be distributed to large surrounding areas releasing toxic pollutants there (Kantor et al., 2021; ˇ Sv´ edov´ a et al., 2019). 4.3. Influence of combustion procedure and operating parameters Relative enrichment (RE) of V in FA (vs. coal) compared for 3 types of industrial combustion units with grate fired, pulverized coal and fluidised bed boilers revealed moderate increase in RE in FA in the order: gratefired boiler (700 ◦C) <pulverized coal boiler (1400 ◦C) <fluidised bed boiler (950 ◦C) – all these values ranged from ca. 0.7 to ca. 0.8 documenting in all cases V depletion in FA vs. coal (Chen et al., 2019). Apart from the effect of the different V associations in the studied coals, interestingly, this sequence did not follow the combustion temperature increase that was in the different order: grate-fired boiler (700 ◦C) <fluidised bed boiler (950 ◦C) <pulverized coal boiler (1400 ◦C). Since V is not a typical volatile element and generally tends to remain in condensed fractions, it might rather reflect the overall redistribution of mineral matter. Another possible effect relates to unburned carbon levels in these three industrial ashes because the RE value was calculated according to the traditional formula: RE =(ci)ash (ci)coal •(%ash)coal 100 where c i are the concentrations of the i-th elements (herein of V) in ash and coal. The (%ash)coal relates to the ash content in the coal and 100 in the denominator is used for the approximate ash content in the evaluated ash or slag. Even if unburned carbon levels in these samples were not published (Chen et al., 2019) it can be presumed that unburned carbon levels (generally) decrease in the order: grate fired boiler > pulverized coal boiler >fluidised bed boiler. Hence, as the RE differences among the boiler types are quite low (0.7–0.8) it could be hypothesized that high unburned carbon levels in the ash could “dilute” V contents there leading to moderately lower RE values (in comparison with RE in the ash containing negligible amount of unburned carbon). The comparison of industrial combustion and household stove combustion (Pędziwiatr et al., 2021) led to the conclusion that ash and soot samples from the household stoves contained lower V concentrations (< 92 ppm) in comparison with industrial BA and FA samples (140–248 ppm), probably due to wood that was co-combusted with bituminous coal in household stove (co-combustion of wood or other biomass types with coal is quite common practice) (Růˇ ziˇ ckov´ a et al., 2019b). 4.3.1. Oxy-fuel combustion and effect of water Oxy-fuel combustion. As coal combustion is an important contribution to anthropogenic greenhouse gas (CO 2 ), modern technologies mitigating CO 2 emissions are intensively studied (Mikulˇ ci´ c et al., 2022). One of the most feasible options in the mid-term horizon is oxy-fuel combustion (Senneca et al., 2017). In comparison with traditional air-combustion, coal is burned usually in a mixture of oxygen and recycled flue gas, which enables to attain high CO 2 concentration in exhaust gas (Davidson and Santos, 2010). Experimental results of co-combustion of bituminous coal and sawdust under air (21%O 2 /79%N 2 ) and oxy-fuel combustion conditions (21%O 2 /79%CO 2 ) revealed practically no effect of changed atmosphere. Zhou et al. (2020) observed nearly the same ratios of V in FAs at both atmospheres for 800 ◦C, 900 ◦C, and 1000 ◦C combustion experiments (ca. 40–48% according to the temperature); the volatilization ratios remained also unchanged - in both atmospheres it was 2–3%. In the study of Oboirien et al. (2014), the V concentrations in 900 ◦C BAs generated at 21%O 2 /79%N 2 and 21%O 2 /79%CO 2 atmospheres were exactly the same. The results of Jano-Ito et al. (2014) (described in detail in Section 3) and Zhou et al. (2020) or Oboirien et al. (2014) are quite similar; nevertheless, moderately lower V volatility under oxy-fuel combustion conditions (vs. air combustion) concluded by Jano-Ito et al. (2014) was not observed by Zhou et al. (2020) and Oboirien et al. (2014). This minor difference might be brought about by mixing pure gases by Zhou et al. (2020) and Oboirien et al. (2014), while Jano-Ito et al. (2014) recycled the whole flue gas including S, Cl, H 2 O etc.; the elevated concentrations of S, Cl, and H 2 O might result in minor decrease in V volatility. In addition to oxy-fuel combustion, there are also other promising technologies with environmental benefits, such as IGCC (combined optimally with CO 2 capture) (Ordorica-Garcia et al., 2006), co-firing with low C or C-neutral fuels (biomass (McIlveen-Wright et al., 2007), H 2 or ammonia (Xu et al., 2022) or waste-derived fuels (Mikulˇ ci´ c et al., 2016)) or carbon capture technologies (e.g., chemical looping combustion using particulate solid oxygen carrier (Coppola and Scala, 2021) or using mineral carbonation technology (Coppola et al., 2022)). As V behaviour and the ash characteristics are expected to be different and highly variable, more research attention is needed in this field. Effect of water. Effect of water (moisture) on behaviour of trace elements is generally more pronounced in case of oxy-fuel combustion due to flue gas recycling (thereby increasing levels of H 2 O) (Roy et al., 2013) or if coal-water slurry is combusted (Vershinina et al., 2016, 2020; Glushkov et al., 2016). When waste materials containing higher moisture levels are co-combusted with coal, such as sewage sludge, agricultural or forest residues – then different drying methods might also have certain effect (Raclavska et al., 2011; Tun and Juchelkov´ a, 2019b). Papers on effect of water (e.g., during coal water slurry combustion) Fig. 3. Concentrations of V in individual rows of ESP or baghouse filters. *Values plotted herein for the 3rd row are in fact mixtures of the 3rd and the 4th row FAs. L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 929 on behaviour of elements are extremely scarce or nearly non-existent. In case of Cr, Mn, and Ni, the bottom emission stream was higher in coalwater slurry fuel than in case of pulverized coal combustion, which was attributed to lower temperatures during coal-water slurry fuel combustion and possibly also due to larger agglomerates formed from coalwater droplets (Nodelman et al., 2000). Nevertheless, V was not included in this study. Increase of H 2 O content in MSW from 0 to 65 wt% during the pyrolysis at 650 ◦C led to increase of V content in solid phase (by ca. 4 mg/kg) while its levels in oil and liquids remained low (< 1 mg/kg) and nearly the same regardless the water level (Raclavsk´ a et al., 2015). Results from two multi-burner coal-water slurry gasification plants (with ca. 60 wt% coal in slurry containing 9.1 and 9.6 ppm V) revealed lower V levels in coarse slags (114 and 116 ppm V) and higher V levels in fine slags (152 and 155 ppm V) (Jiang et al., 2021). Higher water levels influence also the catalytic effect of V 2 O 5 on oxidation of SO 2 to SO 3 during coal combustion – higher H 2 O levels increase the formation of SO 3 ; the most effective catalyst was V 2 O 5 while Fe 2 O 3 and CuO exhibited somewhat lower effect (Duan et al., 2015). Moreover, according to Galetz (Galetz, 2015), combined effect of low-quality fuels containing high V concentrations (oil sands, refinery residues etc.) and reducing atmosphere with high levels of CO, water and H 2 S can accelerate the corrosion of boilers or heat exchangers if made from low Cr steel. 4.3.2. Combustion additives Silica-based additive used for the inhibition of ash slagging and fouling (Low et al., 2015) in industrial pulverized coal-fired boiler (1300 ◦C) provided noticeable reduction in enrichment across all boiler positions not only for V but also for As, Cr, and Mn. Addition of the silica-based additive slightly increased the tendency of V to remain in the gas phase. Thermodynamic equilibrium predictions on V states during coal combustion at 1300 ◦C revealed significant increase of gaseous (VO 2 ) fraction if silica-based additive was added. In both cases (with or without silica-based additive), the most abundant solid phase was (CaO) 3 (V 2 O 5 ). Formation of Ca-V species has been mentioned also in other studies (Folgueras et al., 2007; Díaz-Somoano et al., 2006) which could be important in terms of using traditional Ca-bearing additives (limestone, dolomite, lime etc.). Calcium vanadium oxide hydrate (Ca 2 V 2 O 7 . 2 H 2 O) was identified also in ash from fluidised bed combustion of 100% petcoke where limestone was used as desulphurization additive (Jia et al., 2002). However, in FBC (850 ◦C) the correlation coefficients V-Ca in BAs from 4 combustion tests with CaCO 3 or CaO added to bituminous coal or lignite were in all cases negative (R was from −0.3 to −0.5) (Bartoˇ nov´ a and Klika, 2014), which is in agreement with the literature results described in the next section (Bielowicz, 2020; Spears and Martinez-Tarrazona, 2004; Singh et al., 2011; Huggins and Huffman, 1996; Huggins et al., 1997). Therefore, it could be hypothesized that synergetic effect of more factors leads to the formation of Ca-V compounds. The effect of Ca-bearing minerals (in this context) is probably enhanced by organic or other volatile V association in the fuel (coal, petcoke etc.). And, the effect of the combustion temperature is important as well because if V is firmly bound in aluminosilicates, low combustion temperature (e.g., in fluidised bed combustion) is not sufficient for V volatilization thereby hindering its interaction with Ca-bearing additives. 4.4. V speciation in ash and slag wastes As mentioned in Section 2.2., V in coal is associated predominantly with aluminosilicates and organic matter (macerals) (Finkelman et al., 2018; Maylotte et al., 1981; Huggins et al., 2009). Relationships of V with major elements in coal ashes were evaluated (i.a.) by Singh et al. (2011), Spears and Martinez-Tarrazona (2004) or Bielowicz (2020). Correlation coefficients calculated for V-Al 2 O 3 relationship in coal ash by Singh et al. (2011), Spears and Martinez-Tarrazona (2004) and Bielowicz (2020) were +0.89, +0.54 and +0.40, respectively. Despite quite big differences in these values, all these three R values are statistically significant. As critical value of correlation coefficient (for 2-tailed probability) decreases with the increase of the number of evaluated data, the critical value for the 9 measurements of Singh et al. (2011) is 0.798 ( α =0.01) or 0.666 ( α =0.05), for the data of Spears and Martinez-Tarrazona (2004) it is 0.39 ( α =0.01) and for the 28 samples of Bielowicz (2020) it is 0.374 ( α =0.05). These results document at least partial association of V with aluminosilicates in the studied coal ash samples. Moreover, high R(V-K) calculated by Spears and Martinez-Tarrazona (2004) is also consistent with previously published V association with illite or roscoelite in coal (Huggins and Huffman, 1996; Huggins et al., 1997). In some FAs, siderophile character of V has also been observed. In Eggborough FA, Spears and Martinez-Tarrazona (2004) based this conclusion on the correlation coefficient calculation where R(V-Fe) = +0.61 (critical value for α =0.01 is 0.39), while Vassilev and Menendez (2005) based it on V enrichment in heavy-mineral fraction (heavy concentrate with ρ >2.89 g/cm 3 ). Unlike ashes originated from coals, BAs prepared at 500 ◦C (2 h) from 8 biomass samples (beech wood chips, corn cobs, marine macroalgae, weathered plum pits, rice husks, switchgrass, sunflower shells, and walnut shells) did not reveal significant association of V with silicates or Fe (Vassilev et al., 2014). In contrast to coal ashes, V exhibited the affinity to chlorides, sulphates, Na, Cr, Sr, U, and Ni. The strongest association was observed with S (R 2 (V-S) =0.99) (Vassilev et al., 2014). In co-combustion of pulverized coal (70%) and petcoke (30%), the XPS binding energy of V was primarily attributed to V(5 +) (Vassilev et al., 2014). HR-TEM/EDS/SAED documented (in FA) multiwalled nanotubes encapsulating V. In ultrafine particles, V-spinels were incorporated into the magnetite structure (replacing Fe 2+ and Fe 3+ ions in crystal lattice) (Silva et al., 2013). It could correspond with quite high R (V-Fe) = +0.61 (statistically significant value for α =0.01 is 0.39) in Eggborough PCC FA (Spears and Martinez-Tarrazona, 2004). Results presented by Izquierdo et al. (2007) for 2 FAs from Spanish power plants document practically no effect of % petcoke added to coal during co-combustion on V speciation – in both FAs (4% and 24% petcoke), the prevalent form of V was vanadate VO 4 3with minor proportion (<30%) in lower oxidation states (mainly V(4 +)). In contrast, the effect of combustion/gasification atmosphere on V speciation is more significant (Duchesne et al., 2018). Synchrotron-based XAS revealed that petcoke gasification products contain 79–95% V(3 +) and the rest is V(4 +) while combustion products contain 40–71% V(5 +) and 12–60% V(4 +) (Duchesne et al., 2018), NaVO 3 (Bacci et al., 1983) and vanadyl sulphate (Huffman et al., 2000) were identified at oil-fired power plants. Details on V speciation are important not only in terms of toxicity or possible V extraction. There is also an effect on corrosion and fouling of combustors that is attributed mainly to V(5 +) while V(4 +) and V(3 +) are considered innocuous (Duchesne et al., 2018). 5. Physical separation Physical separation methods the most widely applied on coal combustion wastes are particle-size fractionation, magnetic separation, density separation, flotation, electrostatic separation etc. 5.1. Particle-size fractionation In case of 2 industrial coal combustion FAs (390 and 270 ppm V), the highest V concentrations within particle size fractions were observed in <25 µm fractions (480 and 320 ppm V) (Hower et al., 1999). The finest and the coarsest fractions of FA (with bulk 507 ppm V concentration) contained 1236 and 227 ppm V (Lanzerstorfer, 2018); particle-size fraction of another coal FA (with 496 ppm V) revealed the enrichment of V in the finest fraction (596 ppm V) and the depletion in the coarsest one (120 ppm) as well (Mardon and Hower, 2008). Size fractionation L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 930 applied on the non-magnetic residue of coal FA (after magnetic separation) is also consistent with the aforementioned trend – the highest V concentration (435.9 ppm) was obtained in the finest fraction, the coarsest fraction contained substantially lower V content (136 ppm) (Querol et al., 1995). The same particle-size distribution trend in FA was observed even if peat was co-combusted with forest residues in fluidised bed boiler (the highest V content 150 ppm V relates to <0.074 mm fraction). In this fraction, around 94% of V was present. In BA, the highest level of V was in 0.25–0.5 mm fraction (also ca. 150 ppm) and it decreased with increasing particle size to below ca. 20 ppm V (Dahl et al., 2009). Overall, literature data on particle-size distribution of V in FA provide a clear and consistent trend – enrichment of V in the finest FA fractions. 5.2. Magnetic separation Amount of magnetic concentrate separated from FA mostly fall within the range of 0.5%−18.1% (Vassilev et al., 2004). The most abundant magnetic minerals present in magnetic fractions are magnetite, hematite, ferrian spinel, Caand Ca-Mg spinels with some minor occurrence of maghemite, martite, mushketovite, wustite, ilmenite, chromite, native Fe, Mn ferrite, ferrosilicon, Fe hydroxides and Fe silicates (Vassilev et al., 2004) (and the references therein). Comparison of V levels in magnetic and non-magnetic fractions are summarized in Table 3. Results summarized in Table 3 are quite inconclusive: there are samples exhibiting V enrichment in magnetic fractions (Santos et al., 2022; Cornelius et al., 2021), in non-magnetic fractions (Querol et al., 1995) or showing no clear trend (Vassilev et al., 2005; Strzałkowska, 2021; Myazin et al., 2018). These differences could be brought about by different association of V in coal combustion wastes and/or by different separation techniques used in evaluated papers. According to Myazin et al. (2018), approximate minimum commercial content of a valuable component for V is 500 g/t (ppm or µg/g). Comparing this value with data in Table 3, the only magnetic concentrate meeting this requirement is magnetic fraction prepared from heavy concentrate of the only 1 FA sample (656 ppm V on air-dried basis) (Vassilev et al., 2005). It is worth mentioning in this context that another FA from the same thermal power station treated in the same way provided only 273 ppm V – which clearly documents the effect of input coal (ash) characteristics (Vassilev et al., 2005). Quite high V levels were reported also by Santos et al. (2022) for 2 magnetic concentrates of 2 ESP FAs – 420 and 372 ppm V. Enrichment factor calculated for V concentration in magnetic concentrate (vs. bulk FA) for 5 Spanish FAs was 1.0 showing no enrichment (Vassilev et al., 2004). Therefore, the combination of more methods is generally needed if greater enrichment is required. 5.3. Density separation Density fractionation from slag (155.2 ppm V) and FA (208.3 ppm V) from the same power station provided the highest V content (160 ppm) in 2.4 – 2.8 g/cm 3 fraction of slag and 2.4–2.8 g/cm 3 fraction of FA (229.9 ppm V), which could be given by prevalent aluminosilicate affinity in these samples (Querol et al., 1995). Average concentration of V in 5 heavy concentrates prepared by sink-float technique in bromoform ( ρ =2.89 g/cm 3 ) from 5 FAs generated from 4 Spanish thermal power stations was 256 ppm. As average concentrations of V (ash basis) in these original FAs were 201 ppm, V was enriched in heavy concentrates, which corresponds with predominant siderophile character (Vassilev and Menendez, 2005). 5.4. Char concentrates (from ash) and electrostatic separation There are numerous procedures used for the separation of carbon concentrates from coal ash, such as triboelectrostatic separation, density separation, oil agglomeration, fluidization or vibrational separation etc. (Lv et al., 2022). Char concentrates separated manually (when dry) from two >0.1 mm FAs provided 111 and 261 ppm V; the comparison with V levels in bulk FAs (266 and 316 ppm) indicates depletion of V in char from Turkish FAs (Vassilev et al., 2005). In 5 Spanish FAs, average V enrichment factor in carbon concentrates (vs. V content in bulk FAs) was 1.1 showing moderate enrichment. Table 3 V concentrations (in ppm or µg/g) in magnetic and non-magnetic fractions of coal combustion ashes/slags. Combustion Separation Ash/slag Magnetic fraction Non-magnetic fraction Ref. 1050 MW European power station Magnetic separator with ultrasonic bath (in acetone) FA (from bituminous coal) Slag (from bituminous coal) 227 150 237 160 (Querol et al., 1995) PCC thermal power station Dry separation FA1 FA2 273 a 656 a 423 b /111 c 198 b /261 c (Vassilev et al., 2005) Thermal power plant in Eastern Transbaikalia Laboratory magnetic separation at i)10 kA/m ii)30 kA/m i)FA1 FA2 ii)FA1 FA2 70 5.0 60 30 60 20 60 30 (Myazin et al., 2018) Power plants in Poland, bituminous coals Manual magnetic separator Slag 1 Slag 2 182.9 174.2 272.6 157.7 (Parzentny and R´ og, 2020) Arnot power station (South Africa) Wet separation in water FA 126 119 (Cornelius et al., 2021) Energetic coal combustion (power plants in Poland) Dry manual separation using magnet Class F FA Class C FA 204 274 235 208 (Strzałkowska, 2021) PCC combustion at 1500 ◦C, Colombian coal Manual wet separation (ferrite magnet) BA 1 Economizer ash 1 ESP FA 1 BA 2 Economizer ash 2 ESP FA 2 228 236 420 205 229 372 204 159 229 192 166 239 (Santos et al., 2022) 444 MW Kentucky power plant (bitum. coal +petcoke) 2-step separation (dry Eriez separator + wet in water) FA ESP 1st row FA ESP 2nd row 6159 d 5635 d 8188 d 8079 d (Henke, 2005) a Magnetic fraction from heavy ( >1 g/cm 3 ) fraction b Light fraction ( <1 g/cm 3 ) c Char concentrate (manually separated from >0.1 mm fraction) d After ashing at 750◦C L. Bartoˇ nov´ a et al. Process Safety and Environmental Protection 172 (2023) 923–940 931 Electrostatic separation applied on 2 coal combustion wastes (at U = 10 kV) provided for the first sample moderate enrichment in the conductors´fraction and enrichment in the dielectrics fraction for the second one. When U =20 kV was used, in case of both these samples, the highest V content was observed in the intermediate (semiconductors) fraction. Nevertheless, V content in none of these fractions did not exceed 200 ppm (Myazin et al., 2018). Thus, overall, individual physical–separation techniques typically do not lead to significant V enrichment that could be sufficient for commercial utilization of coal combustion wastes as a cheap source of V. Therefore, the combination of these techniques is recommended optimally after evaluation of input fuel characteristics. Nevertheless, the “pre-concentrate” thereby obtained might become an advantageous input material for further application of chemical / biotechnological methods (that are discussed in detail in Section 6). And at the same time, such techniques could be used in order to decrease V levels in ashes/ slags. Utilization perspectives of fractions depleted in V (and other trace elements) in terms of their storage, handling, landfilling or further technological use are thereby better. 6. Chemical extraction from ash and slag wastes Methods of elemental extraction from ash and slag wastes include the following approaches (Yermagambet et al., 2018; Vitolo et al., 2000; Ye et al., 2012): i) Acid leaching (of basic or amphoteric compounds using solutions of H 2 SO 4 , HCl etc.) ii) Alkali leaching (of acidic or amphoteric compounds using solutions of NaOH, NH 4 OH etc.) iii) Extraction under severe conditions (high temperature and pressure, using aggressive chemicals, roasting etc.) iv) Biotechnology – leaching using microorganisms. 6.1. Leachability in water In general, leached amount of elements in water is given by their concentration in ash, mode of occurrence in ash, operating conditions during combustion, adsorption/desorption processes, redox conditions and namely by pH (Vassilev et al., 2020). V leachability in water from coal combustion ashes/slags is generally low. Proportions of V leached from 9 Bulgarian coal FAs (by water) was within the range of 0.1–1.0% (Vassilev et al., 2020). Around 0.1–4% V water leachable fraction from coal FAs reviewed by Izquierdo and Querol (2012) is also quite low and consistent with other authors (Lieberman et al., 2020; Pires and Querol, 2004; Querol et al., 2000; Spears, 2013). However, co-combustion of coal with wastes or even the combustion of biomass without coal might substantially elevate water leachability of V. High V solubilities in water was obtained from biomass ashes (prepared at 500 ◦C for 2 h) where V was preferably associated with water-soluble fraction – sulphates and chlorides (Vassilev et al., 2014). Therefore, it is recommended to remove the water-soluble phases prior to such ash is used (Vassilev et al., 2014). When low and high amount of petcoke was co-fired with coal (in large power station) (Izquierdo et al., 2007), increasing petcoke fraction added to coal increased the leachable content of V, not only absolutely (7 mg/kg and 190 mg/kg V in the leachates) but also relatively (1.8% from 380 mg/kg V in FA1 and 4.9% from 3890 mg/kg V in FA2). As it cannot be fully attributed to pH change (pH1 =8.1 and pH2 =8.3), rather, leachable amount reflects total V levels and probably also different modes of occurrence in lowand highpetcoke FAs (Izquierdo et al., 2007). The authors concluded that higher proportion of organically bound V (in 24% petcoke blend) somewhat hinders its assimilation to aluminosilicates thereby increasing its leachability in water. It is worth mentioning in this context that V concentration in the leachate from high-petcoke FA is quite high (190 mg/kg) and might be potentially harmful (Izquierdo et al., 2007); therefore, extraction of V from such FA would be practical thereby enhancing its utilization options/possibilities. 6.2. Effect of pH During the leaching, one of the most crucial affecting factors is pH; its effect on dissolved V concentration in the leachate is in some instances even more important than total V concentration in ash/slag (Silva et al., 2010). Literature results related to the effect of pH on V leachability from coal combustion ash and slag wastes are listed in Table 4. The data summarized in Table 4 document that pH of the solution plays an important role in elemental mobility, which was in most studies the highest at pH≈2 (Zhao et al., 2020; Zhang et al., 2019b; Stefaniak et al., 2018; Sandeep et al., 2016). In such studies, pH of the solution was adjusted by adding HNO 3 (or NaOH). In the review paper by Izquierdo and Querol (2012), maximum V mobility was at pH=7–11 (from evaluated range of 4–14), which corresponds with moderately higher (local maximum) leaching of V in pH=7–9 in the study of (Zhao et al. (2020)). Other studies evaluate V mobility in dependence of natural pH of the suspension ash/slag–water without any pH adjustment (Lieberman et al., 2020; Silva et al., 2010). In case of high-S and -Ca FAs, V mobility (in 24-h experiments) decreased with pH increase from 9.98 to 12.17 (Lieberman et al., 2020; Silva et al., 2010), which agrees with the conclusion of Nugteren et al. (2002) that due to Ca extraction, pH increases with time in longer experiments and it might result in re-precipitation of elements that have already been extracted at earlier stages. According to Izquierdo and Querol (2012), in higher-Ca systems, the formation of calcium vanadates is an important factor in terms of V solubility. This might be one of the reasons why some authors (Nugteren Table 4 The effect of pH on leachability of V from ash and slag wastes. Combustion Leaching results Ref. 5 PCC FAs (4 ESP, 1 FF), sole coal US EPA LEAF 1313 test, pH≈2–13 (HNO 3 -NaOH), L/S=10 Maximum V solubility at pH≈2, high also at pH≈7–9, the lowest at pH≈4 or 11–12 (Zhao et al., 2020) 1FA (FF) +1BA, CFB, alkaline coal US EPA LEAF 1313 test, pH≈2–13 (HNO 3 -NaOH), L/S=10 Maximum V solubility at pH≈2 (FA and BA) (Zhang et al., 2019b) PCC co-firing with off gas fuel, class F FA of sialic type US EPA LEAF 1313 test, pH≈2–13 (HNO 3 -NaOH), L/S=10 Maximum V solubility at pH≈2 (FA from sole coal and from co-firing with off gas fuel) (Stefaniak et al., 2018) 2 low-grade coal FAs, Indian power plants pH≈2–12 (HNO 3 -NaOH), L/S=20 Maximum V solubility at pH≈2 (both FAs) (Sandeep et al., 2016) 7 high S and Ca coal FAs EU standard Batch Leaching Test EN 12457–2, L/S=10 L/kg, 24 h, no pH adjustment) Solubility of V decreases as pH increases from 9.98 to 12.27 (max at 9.98) (Lieberman et al., 2020) 7 PCC units of the same power plant (7 ESP FAs, 5 normal +2 oil cofiring) EU standard Batch Leaching Test EN 12457–2, L/S=10 L/kg, 24 h, no pH adjustment i)5 FAs from sole coal (less V in FAs): pH 9.9–10.4, higher V mobility ii)2FAs from oil cofiring (more V in FAs): pH 5.3–6.2, lower V mobility (Silva et al., 2010) L. 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