Extraction of the Rare Element Vanadium from Vanadium-Containing Materials by Chlorination Method: A Critical Review

: Vanadium as a rare element has a wide range of applications in iron and steel production, vanadium ﬂow batteries, catalysts, etc. In 2018, the world’s total vanadium output calculated in the form of metal vanadium was 91,844 t. The raw materials for the production of vanadium products mainly include vanadium-titanium magnetite, vanadium slag, stone coal, petroleum coke, ﬂy ash, and spent catalysts, etc. Chlorinated metallurgy has a wide range of applications in the treatment of ore, slag, solid wastes, etc. Chlorinating agent plays an important role in chlorination metallurgy, which is divided into solid (NaCl, KCl, CaCl 2 , AlCl 3 , FeCl 2 , FeCl 3 , MgCl 2 , NH 4 Cl, NaClO, and NaClO 3 ) and gas (Cl 2 , HCl, and CCl 4 ). The chlorination of vanadium oxides (V 2 O 3 and V 2 O 5 ) by different chlorinating agents was investigated from the thermodynamics. Meanwhile, this paper summarizes the research progress of chlorination in the treatment of vanadium-containing materials. This paper has important reference signiﬁcance for further adopting the chlorination method to treat vanadium-containing raw materials.


Introduction
Vanadium is located in the fourth period and fifth (VB) group of the periodic table and which occupies the 23rd position in the periodic table of elements. The symbol of the vanadium element is V. The physical characteristics of vanadium are a melting point of 1929 • C, boiling point of 3350 • C, relative atomic mass of 50.9415, and density of 5.96 (g/cm 3 ), and it is a silver grey metal [1,2]. The valence of vanadium in compounds can be +2, +3, +4, and +5 [3]. At present, there are known vanadium oxides such as V 2 O 3 , VO 2 , V 2 O 5 , V 3 O 5 , V 3 O 7 , V 4 O 7 , V 5 O 9 , V 6 O 11 , and V 6 O 13 , among which the pentavalent vanadium compounds are the most stable [4]. The main chlorides are VOCl 3 , VOCl, VCl 5 , VCl 4 , VCl 3 , VCl 2 , and VCl, among which VOCl 3 are the most stable [5]. However, the toxicity of vanadium compounds increases with the increase of vanadium valence, and the pentavalent vanadium compounds are the most toxic. Thus, compounds containing pentavalent vanadium, such as NaVO 3 , NH 4 VO 3 , V 2 O 5 , and VOCl 3 , are the most toxic [6]. Toxic vanadium compounds can exist in both cationic and anionic forms [7].
Vanadium as a rare element has a wide range of applications in iron and steel production, vanadium flow batteries, catalysts, etc. [8,9]. The raw materials for the production of vanadium products mainly include vanadium-titanium magnetite, vanadium slag, stone coal, petroleum coke, fly ash, and spent catalysts, etc. [1,8,9]. Salt roasting (Na 2 CO 3 , NaCl, NaOH, CaO, etc.) was applied to extract vanadium from vanadium-containing materials [1,8,9]. However, the salt roasting method extraction of vanadium from vanadium slag is associated with the formation of a large amount of sludge and significant losses of vanadium [10]. Previous review articles on vanadium extraction from vanadium-containing materials mainly focused on the salt roasting process [1,8,9]. In this work, extraction of the rare element vanadium from vanadium-containing materials by chlorination method was summarized. Table 1 shows melting temperature, boiling temperature, and sublimation temperature for vanadium compounds [11,12]. According to Table 1, the melting point and boiling point of chloride are lower than those of corresponding oxides. Thus, chloride is easier to separate and enrich than oxide [13][14][15]. Chlorinated metallurgy has a wide range of applications in the treatment of ore, slag, solid wastes, etc. [16][17][18][19][20][21][22]. In the last century, the extraction of Ti from titanium ore by chlorination method has been industrialized [23]. Chlorinating agent plays an important role in chlorination metallurgy, which is divided into solid (NaCl, KCl, CaCl 2 , AlCl 3 , FeCl 2 , FeCl 3 , MgCl 2 , NH 4 Cl, NaClO, NaClO 3 ) and gas (Cl 2 , HCl, CCl 4 ) [24][25][26][27][28][29]. Compared with gaseous chlorinating agents, the solid chlorinating agents are easier to handle and more environmentally friendly. The traditional chlorination method of extracting vanadium with NaCl as an additive will produce NaVO 3 and then ammonia nitrogen wastewater will be produced in the process of preparing V 2 O 5 . The carbochlorination method of extracting vanadium to prepare VOCl 3 will not produce ammonia nitrogen wastewater. Molten salt chlorination of extracting vanadium will obtain VCl 3 , and metal V will be obtained by molten salt electrolysis. In this work, these two new processes will be introduced. Table 2 shows the world's vanadium ore reserves in 2018. More than 99% of the world's vanadium ore reserves are concentrated in China, Russia, South Africa and Australia [30]. Meanwhile, China has the largest vanadium reserves. According to statistics, in 2018, about 16% of the world's vanadium products directly came from vanadium-titanium magnetite, about 68% of the vanadium products came from the vanadium-rich steel slag (and a small amount of phosphorus-rich vanadium slag) obtained by vanadium-titanium magnetite after iron and steel metallurgical processing, and approximately 16% of vanadium products were produced from recovered vanadium-containing by-products (vanadium-containing fuel ash, waste chemical catalysts) and vanadium-containing stone coal [30]. Table 3 shows the overview of major vanadium producers in the world in 2018 [30]. In 2018, the world's total vanadium output calculated in the form of metal vanadium was 91,844 t [30]. The global market share of vanadium products in 2018 was approximately 90.8% ferroalloy products (FeV, VN, ferrovanadium nitride, etc), approximately 4.2% non-ferrous metals such as Ti, and about 5% of vanadium compounds (vanadium oxide, ammonium vanadate, VOSO 4 , etc.) for the chemical industry, energy storage and other fields [30]. Like the consumption pattern of the global vanadium market, more than 90% of China's vanadium is used in the steel industry in the form of vanadium alloys [30].

Chlorination Thermodynamics of Vanadium Oxides
Vanadium in vanadium-titanium magnetite, vanadium slag, and stone coal mainly exists in trivalent form. Meanwhile, the V 5+ compounds are the very stable. Thus, V 2 O 3 and V 2 O 5 were selected as the reactants for thermodynamic calculation by HSC Chemistry 6.4. The possibilities of V 2 O 3 reacting with different chlorinating agents are calculated from the thermodynamic viewpoint as shown in Equations (1)- (10). Figure 1 shows the standard Gibbs free energies of reactions between V 2 O 3 and chlorination agents at 0-1300 • C. V 2 O 3 can be chlorinated to VCl 3 by AlCl 3 , CCl 4 and COCl 2 . Gibbs free energies of reaction between V 2 O 3 and AlCl 3 increases with increasing temperature. Thermodynamically, increasing temperature is not conducive to AlCl 3 chlorination. However, V 2 O 3 cannot be chlorinated to VCl 3 by the NaCl, CaCl 2 , FeCl 2 , FeCl 3 , MgCl 2 , HCl or Cl 2 at 0-1300 • C.
2V2O3 + 6Cl2 (g) + C = 4VOCl3 + CO2 (g) (21) V2O5 + 3Cl2 (g) + 1.5C = 2VOCl3 + 1.5CO2 (g) Metals 2021, 11, x FOR PEER REVIEW 6 of 18 Under an oxygen atmosphere, the equations for the NaCl roasting reaction of V2O3 and V2O5 are as shown in Equations (23) and (24). Figure 4 shows the variation of standard Gibbs free energy of reactions 23-24 with temperature. Under the same conditions, V2O3 is more easily chlorinated. Thermodynamically, increasing temperature is not conducive to NaCl chlorination of V2O3. The reaction of V2O3 and different chlorinating agents (FeCl2 and FeCl3) are as shown in Equations (25) and (26). It can be seen from Figure 4 that V2O3 can be chlorinated by FeCl2 and FeCl3 under an oxygen atmosphere.
According to the above thermodynamic analysis, the valence state of vanadium, re-  Under an oxygen atmosphere, the equations for the NaCl roasting reaction of V 2 O 3 and V 2 O 5 are as shown in Equations (23) and (24). Figure 4 shows the variation of standard Gibbs free energy of reactions 23-24 with temperature. Under the same conditions, V 2 O 3 is more easily chlorinated. Thermodynamically, increasing temperature is not conducive to NaCl chlorination of V 2 O 3 . The reaction of V 2 O 3 and different chlorinating agents (FeCl 2 and FeCl 3 ) are as shown in Equations (25) and (26). It can be seen from Figure 4 that V 2 O 3 can be chlorinated by FeCl 2 and FeCl 3 under an oxygen atmosphere.

Chlorination Extraction of Vanadium from Vanadium Titanomagnetite
Vanadium-titanium magnetite is mainly composed of iron (Fe), vanadium (V) and titanium (Ti) elements, which is multi-element symbiotic iron ore containing a small amount of cobalt (Co), nickel (Ni), chromium (Cr), scandium (Sc) and gallium (Ga) [31,32]. The reserves of vanadium-titanium magnetite in the Panzhihua-Xichang regions in China amount to about 9.66 billion tons [33]. The content of vanadium pentoxide in vanadiumtitanium magnetite is 0.1 wt%-2 wt% [34]. Jena et al. [23] proposed that under the action of oxygen and water, NaCl as an additive reacts with the vanadium in the vanadium bearing titaniferrous magnetite. The roasted samples were leached with hot water. More than 90% of V was extracted. The reaction Equations are as follows in Equations (27)- (31).

Chlorination Extraction of Vanadium from Vanadium Titanomagnetite
Vanadium-titanium magnetite is mainly composed of iron (Fe), vanadium (V) and titanium (Ti) elements, which is multi-element symbiotic iron ore containing a small amount of cobalt (Co), nickel (Ni), chromium (Cr), scandium (Sc) and gallium (Ga) [31,32]. The reserves of vanadium-titanium magnetite in the Panzhihua-Xichang regions in China amount to about 9.66 billion tons [33]. The content of vanadium pentoxide in vanadiumtitanium magnetite is 0.1 wt%-2 wt% [34]. Jena et al. [23] proposed that under the action of oxygen and water, NaCl as an additive reacts with the vanadium in the vanadium bearing titaniferrous magnetite. The roasted samples were leached with hot water. More than 90% of V was extracted. The reaction Equations are as follows in Equations (27)-(31).
To some extent, the presence of SiO 2 and the formation of HCl and Cl 2 can promote the extraction of vanadium [23,35,36]. Zheng et al. [37] first calculated the feasibility of extraction vanadium from vanadium-rich resources with FeCl 2 and FeCl 3 . Thermodynamic calculations show that the higher the valence of vanadium in vanadium titanomagnetite, the easier it is to extract vanadium. Therefore, the chlorinated atmosphere was selected as the oxygen atmosphere. The reaction Equations are as follows in Equations (32)- (36). Under the optimal experimental conditions (827 • C, reactant (vanadium titanomagnetite)chlorination agent (FeCl 3 ) molar ratio of 1:2, 2 h, oxygen atmosphere), the extraction ratio of vanadium is 32%.
Chloride extraction of vanadium from vanadium-titanium magnetite has long been used. However, the content of vanadium in vanadium-titanium magnetite is low, and the cost of directly extracting vanadium in vanadium slag using chlorination method is high. Thus, it is not recommended to extract vanadium directly from vanadium-titanium magnetite by chlorination method.
In order to extract vanadium, the traditional method is to oxidize insoluble low-valent vanadium to soluble high-valent vanadium in aqueous solution [42,43]. Figure 5 shows a flow chart of extracting vanadium from vanadium slag by NaCl roasting. The roasting temperature is about 800 • C. After roasting, vanadium in the solid exists in the form of NaVO 3 , and then dissolves to obtain NaVO 3 solution. Vanadium is precipitated in the form of ammonium vanadate by adding ammonium salt (NH 4 Cl, NH 4 HCO 3 , (NH 4 ) 2 SO 4 , (NH 4 ) 2 CO 3 ). Ammonium vanadate is calcined to obtain V 2 O 5 at about 550 • C. Under the action of oxygen, NaCl as an additive reacts with the vanadium spinel in the vanadium slag. The reaction Equation is as follows in Equation (37). The conversion rate of vanadium can reach 85% [44,45].
12FeCl2 + 4V2O5 + 3O2 = 6Fe2O3 + 8VOCl3 Chloride extraction of vanadium from vanadium-titanium magnetite has long been used. However, the content of vanadium in vanadium-titanium magnetite is low, and the cost of directly extracting vanadium in vanadium slag using chlorination method is high. Thus, it is not recommended to extract vanadium directly from vanadium-titanium magnetite by chlorination method.
In order to extract vanadium, the traditional method is to oxidize insoluble low-valent vanadium to soluble high-valent vanadium in aqueous solution [42,43]. Figure 5 shows a flow chart of extracting vanadium from vanadium slag by NaCl roasting. The roasting temperature is about 800 °C. After roasting, vanadium in the solid exists in the form of NaVO3, and then dissolves to obtain NaVO3 solution. Vanadium is precipitated in the form of ammonium vanadate by adding ammonium salt (NH4Cl, NH4HCO3, (NH4)2SO4, (NH4)2CO3). Ammonium vanadate is calcined to obtain V2O5 at about 550 °C. Under the action of oxygen, NaCl as an additive reacts with the vanadium spinel in the vanadium slag. The reaction Equation is as follows in Equation (37). The conversion rate of vanadium can reach 85% [44,45].
1/3V2O5 (s/l) + 1/2C (s) + Cl2 (g) → 2/3VOCl3 (g) + 1/2CO2 (g) 1/3Fe2O3 (s) + 1/2C (s) + Cl2 (g) → 2/3FeCl3 (g) + 1/2CO2 (g) (41) In order to extract vanadium from vanadium slag, Liu et al. [49][50][51][52][53][54][55] proposed to use selective chlorination method to extract vanadium. Because to the existence form and value of valuable metal elements (Fe, Mn, V, Cr and Ti) in vanadium slag, NH4Cl was selected to chlorinate Fe and Mn in vanadium slag. Thermodynamic calculations show that the iron and manganese in vanadium slag could be chlorinated by hydrogen chloride, but the V, Cr and Ti could not be chlorinated in the temperature range from 0 to 1000 °C. Under optimal chlorination conditions, the chlorination ratio of iron and manganese were 72% and 95%, respectively. Meanwhile, the enrichment ratio of V, Cr and Ti was obtained as 48%. In addition, AlCl3 was selected to chlorinate V, Cr and Ti in vanadium slag. Figure  7 shows a flow chart of extracting vanadium from vanadium slag by AlCl3 chlorination. The chlorination temperature is about 900 °C. Vanadium after chlorination exists in the form of VCl3. Metal V was obtained by molten salt electrolysis at 900 °C. The effects of In order to extract vanadium from vanadium slag, Liu et al. [49][50][51][52][53][54][55] proposed to use selective chlorination method to extract vanadium. Because to the existence form and value of valuable metal elements (Fe, Mn, V, Cr and Ti) in vanadium slag, NH 4 Cl was selected to chlorinate Fe and Mn in vanadium slag. Thermodynamic calculations show that the iron and manganese in vanadium slag could be chlorinated by hydrogen chloride, but the V, Cr and Ti could not be chlorinated in the temperature range from 0 to 1000 • C. Under optimal chlorination conditions, the chlorination ratio of iron and manganese were 72% and 95%, respectively. Meanwhile, the enrichment ratio of V, Cr and Ti was obtained as 48%. In addition, AlCl 3 was selected to chlorinate V, Cr and Ti in vanadium slag. Figure 7 shows a flow chart of extracting vanadium from vanadium slag by AlCl 3 chlorination. The chlorination temperature is about 900 • C. Vanadium after chlorination exists in the form of VCl 3 . Metal V was obtained by molten salt electrolysis at 900 • C. The effects of reaction temperature, reaction time, mass ratio of AlCl 3 /slag and mass ratio of salt/AlCl 3 on the chlorination ratio of valuable elements were investigated. Under optimal chlorination conditions (AlCl 3 -slag mass ratio of 1.5:1, (NaCl-KCl)-AlCl 3 mass ratio of 1.66:1, at 900 • C, 8 h.), the chlorination ratio of iron, vanadium, chromium and manganese were 90.3%, 76.5%, 81.9% and 97.3%. The volatilization ratio of titanium was 79.9%. The results of kinetic study indicate that the rate-control step of vanadium chlorination process was the surface chemical reaction. The vanadium and chromium in vanadium slag after AlCl 3 chlorination were present in the form of VCl 3 and CrCl 3 in molten salt. The main reaction was as follow (42). 8AlCl 3 +3FeV 2 O 4 = 3FeCl 2 + 4Al 2 O 3 + 6VCl 3 (42) reaction temperature, reaction time, mass ratio of AlCl3/slag and mass ratio of salt/AlCl3 on the chlorination ratio of valuable elements were investigated. Under optimal chlorination conditions (AlCl3-slag mass ratio of 1.5:1, (NaCl-KCl)-AlCl3 mass ratio of 1

Chlorination Extraction of Vanadium from Stone Coal
Black shale is one of China's most important vanadium resources, accounting for more than 87% of domestic vanadium reserves [58,59]. It is estimated that the reserves of vanadium in the form of V2O5 in stone coal are 118 million tons [60]. However, the ordinary grade of vanadium in black shale is usually below 2 wt% [58,59]. In China, vanadium in most of the stone coal replaces trivalent aluminum in mica minerals in a quasi-homogeneous form. The chemical formula of vanadium-containing illite is K(Al,V)2(OH)2[Si3Al]O10. The mica mineral structure is very stable. It is difficult to destroy the lattice structure by general concentration of acid and alkali. Thus, in order to extract vanadium from vanadium-containing mica, the lattice structure of vanadium containing mica first needs to be destroyed [61].
Under the action of oxygen and water, NaCl as an additive reacts with the pre-decarburized stone coal [62,63]. The reaction Equation is expressed as follow (43):

Chlorination Extraction of Vanadium from Stone Coal
Black shale is one of China's most important vanadium resources, accounting for more than 87% of domestic vanadium reserves [58,59]. It is estimated that the reserves of vanadium in the form of V 2 O 5 in stone coal are 118 million tons [60]. However, the ordinary grade of vanadium in black shale is usually below 2 wt% [58,59]. In China, vanadium in most of the stone coal replaces trivalent aluminum in mica minerals in a quasihomogeneous form. The chemical formula of vanadium-containing illite is K(Al,V) 2 (OH) 2 [Si 3 Al]O 10 . The mica mineral structure is very stable. It is difficult to destroy the lattice structure by general concentration of acid and alkali. Thus, in order to extract vanadium from vanadium-containing mica, the lattice structure of vanadium containing mica first needs to be destroyed [61].
Under the action of oxygen and water, NaCl as an additive reacts with the predecarburized stone coal [62,63]. The reaction Equation is expressed as follow (43) (43) where m is the number of vanadium ions replacing aluminum ions in hydromica octahedron.
NaCl has a melting point of 801 • C, which tends to keep the structure stable and does not decompose at high temperature. However, due to the presence of V, Al, Fe, and other oxides in the stone coal, NaCl can be decomposed at lower temperature to generate Cl 2 with high chemical reactivity. The reaction is described as follows in Equations (44)- (47). Cl 2 can react with low-valent vanadium to form VOCl 3 , and VOCl 3 is an intermediate product that can be further oxidized to V 2 O 5 . The presence of Cl 2 promotes the high temperature roasting to destroy the crystal structure of illite. The oxidation of the exposed trivalent vanadium changes to a higher valence state. Cl 2 is more active than oxygen at high temperature and is more easily adsorbed on the surface of minerals. The promotion of Cl 2 on the oxidation of low-cost vanadium cannot be ignored. Thus, NaCl as an additives agent was selected for extracting vanadium from stone coal [63][64][65][66].
The possible chemical reaction between vanadium oxide (V 2 O 3 , VO 2 , and V 2 O 5 ) and the solid chlorinating agent (NaCl, CaCl 2 and FeCl 3 ) was calculated by FactSage 7.1 (Montreal, Canada) using the database of FactPS, FToxid and FT salt. The results show that vanadium oxide cannot be directly chlorinated thermodynamically by NaCl and CaCl 2 as solid chlorinating agents. However, V 2 O 4 and V 2 O 5 can be chlorinated by FeCl 3 . Meanwhile, V can be separated from black shale by controlled roasting temperature of chlorination volatilization [67]. In the air, the structure of illite and muscovite in stone coal is hard to be destroyed by roasting without additives. Zhang et al. [68] studied that the vanadium-bearing stone coal was roasted in chlorine, and 90% of V in the form of VOCl 3 was extracted at 1000 • C for 1 h. Li et al. [69] investigated extraction of vanadium by leaching. Under the optimal leaching conditions (liquid-to-solid ratio of 2, oxygen partial pressure of 1200 kPa, 90 • C, 6 h, 1.5 g/L NaClO, 15 g/L HF, 100 g/L H 2 SO 4 ), 91% of V in vanadium slag was extracted by NaClO-H 2 SO 4 -HF system under atmospheric pressure. V 3+ in stone was oxidized by NaClO as a chlorinating agent and oxidant.

Chlorination Extraction of Vanadium from Spent Catalysts
Catalysts are extensively used in sulfuric acid production and petroleum refining [70,71]. More than 100,000 tons of spent hydrodesulphurization catalysts are produced every year, which usually contain the valuable elements V, Mo, Ni, and Co [72]. Vanadium in spent catalyst is present in the form of sulfide (V 2 S 3 or V 3 S 4 ) [73]. Oxidation roasting of spent catalyst and subsequent NaCl/H 2 O roasting of oxide were proposed by Biswas et al. [74], and 81.9% of V was extracted. The reactions were as follows in Equations (48) and (49): There are two processes (direct chlorination and roasting chlorination) for recovering vanadium from spent hydrodesulphurization catalysts by Cl 2 chlorination.
In addition to metal elements such as vanadium and molybdenum, spent catalysts also contain elemental carbon and sulfur. Direct chlorination of spent catalysts was investigated by Gaballah et al. [75]. In order to recover Mo, V, Ni and Co, Cl 2 /N 2 , Cl 2 /air, and Cl 2 /CO/N 2 , gas mixtures were used to chloride spent catalysts. Vanadium sulphide was chlorinated to vanadium chloride as expressed in the reaction Equations (50)- (52). A total of 75% of V in the form of VCl 4 and/or VOCl 3 was recovered by Cl 2 /air gas mixture at less than 600 • C.
The affinity of metal to oxide is stronger than that of metal to sulfur. Under the same conditions, sulfides are easier to chlorinate than oxides. Thus, direct chlorination of vanadium sulfide is better than chlorination after oxidation of vanadium sulfide [77].

Chlorination of V 2 O 5
Mink et al. [78] reported that CCl 4 reversibly dissociate and adsorbs on the two exposed vanadium atoms of the basic (001) plane of V 2 O 5 before the chlorination reaction. The mechanism of chlorination of V 2 O 5 by CCl 4 was analyzed by MS and XPS. Before the formation of the volatile final product VOCl 3 , the surface vanadium atoms gradually acquire two chlorine atoms [79]. The kinetics of chlorination of V 2 O 5 by CCl 4 was investigated by Jean et al. [80]. A total of 87% of V 2 O 5 could be chlorinated by CCl 4 at 480 • C in 30min. Chlorination reaction conforms to topochemical reaction model. According to analysis of kinetics results, the following mechanisms, Equations (53) The whole reaction can be expressed by the following Formula (59) V 2 O 5 + 3CCl 4 = 2VCl 4 + 2CO 2 + COCl 2 + Cl 2 Gaballah et al. [81] studied kinetics of chlorination of V 2 O 5 with Cl 2 -CO-N 2 , Cl 2 -N 2 , and Cl 2 -air gas mixtures. Thermodynamic calculation showed that chlorinated product was mainly VOCl 3 during the Cl 2 chlorination of V 2 O 5 . However, VCl 4 may be formed during the carbochlorination of vanadium pentoxide. The results of kinetics indicated that the rate-control step of V 2 O 5 chlorination process between 500 • C and 570 • C with Cl 2 -N 2 was a chemical reaction. Pore diffusion and chemical reaction were the limiting step for the V 2 O 5 chlorination in the temperature range of 570 • C to 650 • C. In Cl 2 -CO-N 2 atmosphere, the limiting step of carbochlorination of V 2 O 5 at 400-620 • C was the chemical reaction. Brocchi et al. [6] systematically studied the carbon-chlorination of V 2 O 5 from thermodynamics between 627 • C and 1327 • C. In carbon-chlorination reaction of V 2 O 5 , the most stable vanadium oxychloride and vanadium chloride are VOCl 3 and VCl 4 , respectively. E. Mccarley et al. [82] reported a process for preparing high-purity V 2 O 5 (maximum of 100 ppm impurities) by carbon chlorination using V red cake (88 wt% of V 2 O 5 ) as raw materials. Pap et al. [83] reported that the chlorination of V 2 O 5 by three chlorinating agents (Cl 2 , COCl 2 and CCl 4 ) is compared from the aspects of thermodynamics and kinetics. The results showed that V 2 O 5 can be chlorinated thermodynamically by COCl 2 and CCl 4 at 127 • C, and the chlorinated products were VOCl 3 and CO 2 . However, the reaction of V 2 O 5 with Cl 2 can occur obviously when the temperature exceeds 477 • C. The chlorinated product was VOCl 3 and O 2 . The chlorination kinetics showed that the apparent activation energy of V 2 O 5 chlorinated by Cl 2 , CCl 4 , and COCl 2 were 126 kJ/mol, 77 kJ/mol, and 48 kJ/mol, respectively. High-purity V 2 O 5 (99.95 wt%) is prepared by chlorinating industrial grade V 2 O 5 (96.7 wt%) with AlCl 3 . The reactions involved are as follows in Equations (60)- (64). Under the protection of purity Ar, the chlorination ratio of V 2 O 5 at a V 2 O 5 :AlCl 3 mole ratio of 1:6, 180 • C, and 3.5 h was 62%, and a large amount of VOCl 3 was collected. When NaCl is added to the chlorination reaction system, the chlorination ratio of vanadium can reach 83.4% at mole ratio of AlCl 3 :V 2 O 5 of 6:1 and mole fraction of NaCl of 0.6 in the NaCl-AlCl 3 system [84,85].

Chlorination Extraction of Vanadium from Other Vanadium-Containing Materials
Petroleum coke, fly ash and carbonaceous gold ore also contain a certain amount of vanadium. 0.6 Mt/year of petroleum coke was produced from Syrian petroleum refineries. The extraction ratio of vanadium can reach 60% by NaCl-roasting [86]. The vanadium content in fly ash is as low as 1-7%. The fly ash was treated by acid leaching, oxidation of NaClO 3 , and precipitation [87]. Murase et al. [88] investigated extraction and separation of vanadium from a fly ash of Orimulsion. Air-Cl 2 or N 2 -Cl 2 gas mixture were used to chlorinate valuable elements (V, Ni and Mg). The separation of V and Fe were achieved by controlled temperature of chlorination. V and Fe was selectively extracted by chlorination at 400 • C and 500 • C, respectively. Mg and Ni in residue were extracted by chlorination of N 2 -Cl 2 -Al 2 Cl 6 (g) at 600 • C. The extraction and separation of V, Ni and Mg were successfully achieved by the method of chlorination. The content of vanadium in refractory carbonaceous gold ore is 1.1 wt%. Wang et al. [89] investigated extraction and separation of vanadium from carbonaceous gold ore by NaCl roasting. After NaCl roasting, Au volatilizes in the form of AuCl 3 and V in the form of NaVO 3 remains in the roasted solid. The reactions were as follows in Equations (65) 2Au + 3Cl 2 = 2AuCl 3 The influences of experiment conditions including NaCl dosage, time and temperature were studied. Under optimal process conditions, (air gas flow rate 1 L/min, 4 h, 800 • C, NaCl 10%), the extraction ratios of V and Au are 85.3% and 92%.

Treatments of Chlor-Containing Compounds in Gas, Solid and Solution
Regarding the Cl 2 and HCl off-gas generated in the chlorination process, the first method is recycling, and the second method is the absorption of alkaline solution. The chloride in the solid can be washed to remove chloride ions. The chlor-containing wastewater can be treated by solvent extraction, the electrochemical method, separation interception method, the principle of precipitation, and ion exchange [90].

Conclusions and Outlook
The research progress on the treatment of vanadium-containing materials with various chlorinating agents (solid and gas) is summarized in terms of thermodynamics and kinetics.
The NaCl roasting method is used to treat vanadium titanomagnetite, vanadium slag, stone, spent catalysts, petroleum coke, and carbonaceous gold ore. The NaCl roasting method has the characteristics of short process, less investment and less equipment, etc. In the 1970s, in China, the price of vanadium was very high and hundreds of small-scale vanadium extraction plants adopted the NaCl roasting method to extract vanadium from vanadium-containing materials (stone coal) [63]. However, Cl 2 and HCl generated during the NaCl roasting process makes it highly demanding for the equipment's anti-corrosion performance. The environmental pollution caused by Cl 2 and HCl gas and the threat of Cl 2 and HCl gas to workers' health are also fatal defects of NaCl roasting. Due to increasingly strict environmental protection policies, the NaCl roasting method has become outdated and has gradually been replaced by other roasting methods.
The demand for high-purity vanadium pentoxide is increasing in all-vanadium flow batteries and high-purity metal vanadium. Therefore, efficient preparation of high-purity vanadium pentoxide is urgently needed [91]. Due to the low melting point and boiling point of vanadium chloride, vanadium chloride has a greater advantage than vanadium oxide in separation and enrichment. The advantages of chlorination method in the preparation of high-purity vanadium are very obvious, and it has very good development prospects.
Trivalent vanadium oxide is difficult to leach. Thus, the traditional vanadium extraction method is to oxidize vanadium to pentavalent vanadium for extraction. However, the toxicity of vanadium compounds increases with the increase of vanadium valence, and the pentavalent vanadium compounds are the most toxic. More than 90% of vanadium produced in industry is added to steel in the form of vanadium alloys. Trivalent vanadium oxide can be chlorinated to VCl 3 by AlCl 3 . Metal V can be obtained by reduction or electrolysis of VCl 3 . Thus, the direct chlorination of low-valent vanadium is also a very promising process.

Data Availability Statement:
The data presented in this study are available from the corresponding author, upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.