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Selective extraction of vanadium from pre-oxidized vanadium slag by carbochlorination in fluidized bed reactor
Journal of Cleaner Production 237 (2019) 117765
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Selective extraction of vanadium from pre-oxidized vanadium slag by carbochlorination in fluidized bed reactor Guangchao Du a, b, c, Chuanlin Fan a, b, *, Haitao Yang a, b, Qingshan Zhu a, b, ** a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China c Pangang Group Research Institute Co., Ltd., State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, Panzhihua, 617000, Sichuan, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2019 Received in revised form 20 June 2019 Accepted 23 July 2019 Available online 24 July 2019
Most of the vanadium (V) is industrially extracted from vanadium slag, a byproduct in steelmaking, via the sodium roasting-water leaching or calcification roasting-acid leaching process, which usually suffers from environment pollution or decrease in product purity. In this paper, a novel process of vanadium extraction featuring carbochlorination of pre-oxidized vanadium slag with chlorine (Cl2) and nitrogen (N2) mixture in a fluidized bed reactor was proposed. And the selectivity between vanadium and iron (Fe) in the slag was primarily concerned, due to the high content of the latter in vanadium slag. The thermodynamic analysis reveals that there exists a favorable zone for selective chlorination of V from Fe compounds. Subsequently, carbochlorination experiments were conducted to investigate the influences of chlorination temperature and time, addition amount of petroleum coke and pressure fraction of chlorine on extraction of V and Fe. And 87.47% of V together with 18.79% of Fe can be extracted from the pre-oxidized vanadium slag by chlorination at 650 C for 120 min with chlorine pressure fraction [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and petroleum coke mass fraction in raw materials for chlorination Rc ¼ 10%. The results demonstrate that the selective chlorination process has good potential to recover vanadium from vanadium slags in efficient and clean ways. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Vladimir Strezov Keywords: Vanadium extraction Vanadium slag Selective chlorination Pre-oxidization Thermodynamics
1. Introduction Vanadium (V) is a crucial nonferrous metal extensively applied in steelmaking and alloys preparation (Li et al., 2016; Liu et al., 2016a). It also plays an important role in chemical industries such as catalysts (Enache et al., 2004), pigments (Jansen and Letschert, 2000) and batteries (Choi et al., 2017). Numerous resources like vanadium titanomagnetite (Chen et al., 2015), stone coal (Ye et al., 2012) and spent catalysts (Kim et al., 2018) can be used as the raw materials for vanadium extraction. About 88% of vanadium is extracted from vanadium titanomagnetite around the world (Li et al., 2015; Xiang et al., 2017a), with an enriched slag called
* Corresponding author. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China. ** Corresponding author. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China. E-mail addresses: [email protected] (C. Fan), [email protected] (Q. Zhu). https://doi.org/10.1016/j.jclepro.2019.117765 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
vanadium slag or converter slag from steel making as the raw material in most cases (Ji et al., 2017b; Li et al., 2015, 2017b; Xiang et al., 2017a). The current industrial practice of vanadium extraction from vanadium slag mainly involves sodium roasting-water leaching process, in which V3þ compounds in vanadium slag are first oxidized into water soluble V5þ compounds by adding Na2CO3, and then the as-obtained V-containing solution is purified and precipitated with a subsequent calcination process to produce vanadium pentoxide (V2O5) (Barolin et al., 1982; Liao and Bai, 1985; Moskalyk and Alfantazi, 2003). However, the sodium roastingwater leaching process generates plenty of waste water with high concentration of sodium and ammonium ions (about 3000050000 kg of waste water per 1000 kg of V2O5 (Li et al., 2017a)), inevitably arousing environmental and cost concerns (Yang et al., 2017; Zhang et al., 2011b). As an alternative, the calcification roasting-acid leaching process solves this problem by water recycling for the whole procedure (Mirazimi et al., 2013; Xiang et al., 2017b), but the impurities such as P and Mn are easily concentrated in the recycled water, ultimately leading to higher impurities in the obtained vanadium oxide products (Li et al., 2016, 2017a).
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Furthermore, the residual after vanadium extraction by the calcification roasting-acid leaching process, containing 30 wt% or more of Fe and many other valuable metals, cannot be further utilized as iron-making raw materials due to the high content of CaSO4 accumulated in the leaching process (Li and Xie, 2012; Xiang et al., 2017a, 2018a). And stacking of the residual will threaten the environment since it also contains a small amount of carcinogenic calcium chromate (Li et al., 2017b). Thus, it appears to be imperative for seeking new technologies to tackle the mentioned problems. Kinds of processes such as multistage recovery (Li et al., 2015; Xiang et al., 2017a), non-salt roasting coupled with ammonium leaching (Li et al., 2017b) and sub-molten salts method (Liu et al., 2013) have been recently reported, which can alleviate the environment pollution and enhance the comprehensive utilization of vanadium slag. Nevertheless, the high cost or complicated operation procedure prevents their industrial applications. Chlorination is an efficient and clean method for extraction of numerous metals like titanium (Zhang et al., 2011a), zirconium (Movahedian et al., 2011), copper and nickel (Cui et al., 2018). It is the major process for industrial production of TiCl4-an important intermediate for preparation of titanium dioxide or titanium sponge (Deng and Wang, 2010). Similar to titanium, vanadium can be extracted via the chlorination process as well, principally including chlorination of the raw materials to generate vanadium oxytrichloride (VOCl3), separation and purification of the crude VOCl3 by rectification, and final oxidation of VOCl3 to V2O5 (Fan et al., 2016a, 2016b) (shown in Fig. 1). The whole process scarcely produces high salinity and ammonium-containing waste water. And the excellent distillation efficiency of crude VOCl3 insures high purity of V2O5 (99.99 wt%) (Fan et al., 2017). Meanwhile, the residual after vanadium extraction can be dechlorinated and then recycled as the raw materials for iron-making (Metallurgy Laboratory of Central South Institute of Mining and Metallurgy, 1978; Xiang et al., 2017a). All the advantages mentioned above make chlorination process a promising solution for vanadium extraction. During chlorination of a vanadium slag, the selectivity among V and other impurities is a key issue closely related to the process efficiency and cost. The vanadium slag usually contains only 5 wt%-20 wt% of V2O3 while 30 wt%-40 wt% of FeO, and other components such as TiO2, CaO and MgO (Diao et al., 2016; Ji et al.,
2017b; Li et al., 2016). The chlorination of Fe in a vanadium slag will distinctly increase the chlorine consumption and challenges the subsequent purification of crude VOCl3, thus it should be inhibited as far as possible. In a vanadium slag, V is commonly enriched in the spinel phases and coexists with Fe (Diao et al., 2016; Ji et al., 2017b), readily causing simultaneous chlorination of Fe and V (Liu et al., 2016b). Fortunately, according to the mechanisms of vanadium slag roasting, vanadium-bearing spinel minerals can be decomposed during oxidation roasting, to form vanadates (Mn2V2O7 and Ca2V2O7) and iron-rich oxides respectively (Barolin et al., 1982; Jiang et al., 2018; Li et al., 2017b; Zhang et al., 2015), which attains separation of Fe and V in vanadium slag and may be suitable for selective chlorination of vanadium compounds. In order to confirm the feasibility of selective extraction of vanadium from vanadium slag, carbochlorination of a pre-oxidized vanadium slag was investigated in the present work. The thermodynamics for selective chlorination of V from Fe compounds in the pre-oxidized vanadium slag was analyzed. And then the carbochlorination experiments were conducted, to discuss the key factors influencing extraction efficiency and selectivity between V and Fe, in a fluidized bed reactor with chlorine and petroleum coke as the chlorination agent and reduction agent, respectively. 2. Materials and methods 2.1. Materials The petroleum coke was more than 99% of mass fraction for carbon content. Purity of chlorine (Cl2) and nitrogen (N2) in the experiments was 99.9% and 99.99%, respectively. The purified water was prepared by a Millipore Elix 3 water purifier (Millipore, the US). The original vanadium slag with less than 74 mm of particle size was supplied by Panzhihua Iron and Steel Corporation, China. Chemical compositions of the original vanadium slag determined by X-ray fluorescence spectrometry (XRF, AXIOS-MAX, PANalytical B. V., the Netherlands) are shown in Table 1. To acquire the raw material for fluidized carbochlorination, a pre-oxidation process was performed herein for the original slag as follows. Firstly, pellets with about 5e10 mm of diameter were prepared by manual agitation and oscillation for mixture of the original vanadium slag and pure water. After desiccated at 100 C for 24 h, the pellets were roasted under air atmosphere using a tube furnace. The roasting temperature and time were respectively determined at 900 C and 120 min according to the references (Jiang et al., 2018; Li et al., 2017a, 2017b; Zhang et al., 2015). Finally, the calcined pellets were crashed by a pulverizer and sieved afterwards, and only the particles of 100e300 mm which belong to Btype powders suitable for fluidization were used as the raw material for the subsequent carbochlorination process according to the Handbook of Fluidization (Guo and Li, 2010). The BET surface area of particles, measured on a specific surface area analyzer (NOVA, 2000e; QUANTACHROME INSTRUMENTS, the US), was increased from 1.15 m2 g1 for the original vanadium slag to 2.21 m2 g1 for the pre-oxidized vanadium slag. All other chemical reagents used were of analytical grade and without any further purification. 2.2. Methods
Fig. 1. Flow diagram of vanadium extraction from vanadium slag via chlorination process.
2.2.1. Thermodynamic calculations Employing HSC 6.0 software the chlorination selectivity between V and Fe was discussed in terms of the computed chemical potential diagrams (predominance diagrams). Meanwhile, the standard Gibbs free energy changes at different temperatures with respect to the possible carbochlorination reactions were also calculated by the software.
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Table 1 Chemical compositions of the original vanadium slag. Oxide species
FeO
V2O3
MnO
SiO2
TiO2
Cr2O3
Al2O3
MgO
CaO
Other
Content (wt.%)
36.84
12.33
11.35
16.05
10.29
3.13
3.46
2.82
2.35
1.38
2.2.2. Carbochlorination of the pre-oxidized vanadium slag Carbochlorination of the pre-oxidized vanadium slag was conducted on a device illustrated in Fig. 2. The quartz fluidized bed reactor, having a distributor on the bottom and about 20 mm of inner diameter for the reaction section, was equipped in an electric furnace. All of the gas tubes and connectors were made of corrosive-resistant Teflon. A batch of intimately mixed raw materials, viz., the pre-oxidized vanadium slag (100e300 mm) and petroleum coke (300e600 mm), was added into the reactor. The pressure fraction of chlorine in the reactor [P(Cl2)/P(Cl2þN2)] where P(Cl2) and P(Cl2þN2) separately denote the partial pressure of chlorine (Pa) and total pressure of chlorine and nitrogen (Pa) in the inlet gas, was determined by volume flow rates of nitrogen (V(N2), L min1) and chlorine (V(Cl2), L min1) via a customized gas controller with the total volume flow rate of inlet gas V(Cl2þN2) kept 1.5 L min1 at the standard state, since [P(Cl2)/P(Cl2þN2)] has an equal value with [V(Cl2)/V(Cl2þN2)] at the same temperature. During the chlorination, the outlet gas was cooled down and then absorbed by NaOH solution before its release to the atmosphere. After the desired time of chlorination, supply of the chlorine was ceased and the reactor was moved out of the furnace for cooling. Finally, the chlorination residues were collected and weighed for various analyses. The extraction ratio of elements in the preoxidized vanadium slag was calculated according to Eq. (1).
h i M0 ya;0 ðM ya Þ 100% Ex ¼ M0 ya;0
(1)
Where Ex denotes the extraction ratio (mass fraction, %) of a certain element from the pre-oxidized vanadium slag (e.g. V and Fe), M0 (g) and ya,0 (%) are the initial mass of the slag and the mass content of element a (such as V and Fe) in the slag, respectively. M
Fig. 2. Equipment diagram for carbochlorination.
(g) represents the mass of the chlorination residue, and ya (%) is the mass content of element a in the chlorination residue. 2.2.3. Characterizations Phase compositions of the original vanadium slag, pre-oxidized slag and chlorination residue were respectively tested by powder X-ray diffraction with Cu Ka radiation on a diffractometer (XRD, X0 Pert PRO MPD, PANalytical B. V., the Netherlands), employing the software of X0 Pert HighScore Plus for data analyses. Meanwhile, the microscopic observation and elemental analysis for the slags were conducted by scanning electron microscopy (SEM, JSM-7001F, JEOL, Japan) equipped with energy dispersive spectrum (EDS, INCA XMAX, OXFORD INSTRUMENTS, the UK). 3. Results and discussion 3.1. Vanadium slag before and after pre-oxidation The phases and elemental distributions of vanadium slag before and after roasting were investigated to learn the effects of preoxidation. Fig. 3 shows that the original vanadium slag is mainly consisted of V-containing spinel (Mn, Fe) (V, Ti, Cr)2O4 (revised from (Mn, Fe) (V, Cr)2O4 according to Ji et al. (2017a)), fayalite (Fe, Mn)2SiO4, phonotephrite (Mg, Fe, Al, Ti) (Ca, Mg, Fe, Na) (Si, Al)2O6 and diopside (Mg0.6Fe0.2Al0.2)Ca(Si1.5Al0.5)O6. Moreover, as shown in Fig. 4, the V, Cr and Ti have a similar distribution, suggesting concurrent enrichment in the spinel phase (brightest particles in Fig. 4 (a)) (Ji et al., 2017b; Li and Xie, 2012). Si demonstrates a complementary distribution with V, Cr and Ti, which implies detachment of the spinel phase from silicates phases (Ji et al., 2017a; Lindvall et al., 2017). Fe and Mn are concentrated both in the spinel and silicates phases. The commensalism of V and Fe in the spinel phase is thermodynamically and dynamically adverse to selective chlorination. In comparison, the main phases of vanadium slag after preoxidation transfer into hematite Fe2O3, pseudobrookite Fe2TiO5,
Fig. 3. XRD patterns of (I) original vanadium slag and (II) pre-oxidized vanadium slag roasted at 900 C for 120 min.
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4MnOðsÞ þ O2 ðgÞ/2Mn2 O3 ðsÞ
(7)
2MnOðsÞ þ V2 O5 ðsÞ/Mn2 V2 O7 ðsÞ
(8)
2SiO2 ðsÞ þ CaOðsÞ þ Al2 O3 ðsÞ/CaAl2 Si2 O8 ðsÞ
(9)
Herein the formation of Fe2TiO5, depending mainly on the reaction temperature, usually occurs at temperatures above 850 C according to Eq. (6) (Gennari et al., 1998). Part of Mn remains in Mn2O3 in accordance with Eq. (7) and forms solid solution with Fe2O3 (Barolin et al., 1982). V and Mn are mostly concentrated in Mn2V2O7 phase of the pre-oxidized vanadium slag along the path of Eq. (8). The overlap between V and Fe, Ti indicated from Fig. 5 suggests that a small part of V exists in Fe2O3 and Fe2TiO5 phases after pre-oxidization, probably deriving from the decomposition of V-containing spinel in the original vanadium slag where oxidation of the intermediate R2O3 (R ¼ Fe, V, Cr and Mn) to generate Fe2O3$V2O5 was slightly inhibited (Barolin et al., 1982). This leads to the partial co-existence of V and Fe in the solid solutions of Fe2O3$V2O3 and Fe2O3$V2O4 in the pre-oxidized slag, and V2O4 dissolving in Fe2O3 would simultaneously cause the reserve of small quantity of vanadium in the pseudobrookite via the reaction of Eq. (6) (Barolin et al., 1982; Jiang et al., 2018; Li et al., 2017b). Even
Fig. 4. SEM images with EDS element mapping of the original vanadium slag (a. particle section).
manganese pyrovanadate Mn2V2O7, cristobalite SiO2 and anorthite CaAl2Si2O8 (see Fig. 3), as a result of decomposition of the V-containing spinel and silicates in the original vanadium slag likely according to Eq. (2)-Eq. (5) (Barolin et al., 1982; Li et al., 2015, 2017a).
ðFe; MnÞðV; Cr; TiÞ2 O4 ðsÞ þ O2 ðgÞ/Fe2 O3 ðsÞ þMnOðsÞ þ Cr2 O3 ðsÞ þ V2 O5 ðsÞ þ TiO2 ðsÞ
(2)
ðFe; MnÞ2 SiO4 ðsÞ þ O2 ðgÞ/Fe2 O3 ðsÞ þ MnOðsÞ þ SiO2 ðsÞ
(3)
ðMg; Fe; Al; TiÞðCa; Mg; Fe; NaÞðSi; AlÞ2 O6 ðsÞ þ O2 ðgÞ/Fe2 O3 ðsÞ þSiO2 ðsÞ þ TiO2 ðsÞ þ CaOðsÞ þ MgOðsÞ þ Na2 OðsÞ þ Al2 O3 ðsÞ (4) ðMg0:6 Fe0:2 Al0:2 ÞCaðSi1:5 Al0:5 ÞO6 ðsÞþO2 ðgÞ/Fe2 O3 ðsÞ þMgOðsÞþAl2 O3 ðsÞ þ CaOðsÞ þ SiO2 ðsÞ
(5)
And the new phases are possibly reconstructed in accordance with Eq. (6)-Eq. (9) (Xiang et al., 2018b; Zhang et al., 2015).
Fe2 O3 ðsÞ þ TiO2 ðsÞ/Fe2 TiO5 ðsÞ
(6)
Fig. 5. SEM images with EDS element mapping of the pre-oxidized vanadium slag obtained by roasting at 900 C for 120 min (a. particle section, b. high-magnification image for the area in red box of image (a)).
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so, Fig. 5 shows that the Mn2V2O7 phase is segregated largely from the Fe-containing phases, i.e. Fe2O3 or Fe2TiO5, revealing a satisfactory separation between V and Fe by the pre-oxidation. Furthermore, the pre-oxidized vanadium slag has a more porous microstructure compared with the original slag from the specific surface area test in section 2.1, Fig. 4 (a) and Fig. 5 (a)e(b), which would be beneficial for the chlorination process.
3.2. Thermodynamics analysis Thermodynamic feasibility for selective chlorination of V from Fe in the pre-oxidized slag was also investigated. The predominance diagrams at different temperatures for VeFeeCleO system were summarized in Fig. 6. All species presented in the predominance diagrams are the most thermodynamically stable based on minimization of Gibbs free energy (Brocchi et al., 2013). A selective region (green zone), constituted of gaseous VOCl3 and solid Fe2O3, is found at each temperature in Fig. 6. Simultaneously, the selective region shrinks with the elevated temperature, suggesting a decreased selectivity for V and Fe, e.g., selective extraction of V from Fe can be realized at the chlorine partial pressure of 101.8 Pa and oxygen partial pressure of 105.0 Pa at 500 C, while at the partial pressure of 103.3 Pa for chlorine and 100.8 Pa for oxygen with temperature increasing to 850 C. Furthermore, the standard Gibbs free energy change DG0 at different temperatures for carbochlorination of V and Fe compounds in the pre-oxidized vanadium slag was also calculated as shown in Fig. 7, which includes the reactions of Eq. (10)-Eq. (13).
2=5MnOðsÞ þ 1=5V2 O5 ðs=lÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=5MnCl2 ðs=lÞ þ2=5VOCl3 ðgÞ þ 1=2CO2 ðgÞ (10)
Fig. 7. Gibbs free energy change (standard) DG0 for carbochlorination reactions of Fe and V compounds in the pre-oxidized vanadium slag at different temperatures.
1=3V2 O5 ðs=lÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=3VOCl3 ðgÞ þ 1=2CO2 ðgÞ (11) 1=3Fe2 O3 ðsÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=3FeCl3 ðgÞ þ 1=2CO2 ðgÞ (12) 1=5Fe2 TiO5 ðsÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=5FeCl3 ðgÞ þ 1=5TiCl4 ðgÞ þ 1=2CO2 ðgÞ (13) The amount of Cl2 for each reaction was normalized at 1 mol,
Fig. 6. Predominance diagram for system VeFeeCleO at (a) 500 C, (b) 650 C and (c) 850 C (P(Cl2), Pa and P(O2), Pa refer to the partial pressure of Cl2 and O2 at 105 Pa (P0) of standard atmospheric pressure, respectively).
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and CO2 was assumed to be the only carbon oxide for simplification. Mn2V2O7 was substituted by MnO and V2O5 with 2:1 of molar ratio (MnO/V2O5) in Eq. (10) for calculation due to the lack of corresponding thermodynamical data. The state of MnCl2 in Eq. (10), V2O5 in Eq. (10) and Eq. (11) were expressed as solid or liquid (s/l) indicating a solid to liquid phase transition with temperature increased. Compared with that of the pure V2O5 (Eq. (11)), carbochlorination of Mn2V2O7 (Eq. (10)) has a more favorable thermodynamic trend as indicated in Fig. 7. Furthermore, DG0 for carbochlorination of Mn2V2O7 is apparently lower than that of Fe2O3 and Fe2TiO5 in the selected temperature range, showing an excellent chlorination priority for Mn2V2O7. And Fe2O3 has a slightly higher reactivity than Fe2TiO5 inferred from Fig. 7, where DG0 for Eq. (12) is mildly lower than that of Eq. (13). The sequence of carbochlorination is confirmed as Mn2V2O7 > Fe2O3 > Fe2TiO5. Therefore, separation of V from Fe in the pre-oxidized vanadium slag is thermodynamically feasible by carbochlorination.
3.3. Effect of chlorination temperature Temperature is an important parameter for the chlorination process, and a temperature ranging from 500 C to 850 C was chosen to explore its effect on the extraction of V and Fe, on basis of the thermodynamics predictions in section 3.2. The chlorination time, pressure fraction of chlorine ([P(Cl2)/P(Cl2þN2)]) and mass fraction of petroleum coke Rc were fixed at 30 min, 0.5 and 10%, respectively. As demonstrated in Fig. 8, the chlorination temperature has a significant influence on both of the extraction of V and Fe. With increasing temperature, the V extraction ratio increases slowly at 500 Ce600 C and then rises dramatically at 600 Ce700 C, while further increase of the temperature from 700 C to 850 C slightly augments the extraction of V. The highest V extraction ratio of about 95% can be achieved at 850 C. The extraction ratio of Fe increases mildly with temperature from 500 C to 650 C, nevertheless, it shows a sharp growth from 6% at 650 C to 82% at 850 C (see Fig. 8). The separation factor between V and Fe expressed with the rate of V extraction ratio to Fe extraction ratio in Fig. 8 presents an increase trend at 500 Ce650 C then followed by a diminution at 650 Ce850 C. The results clearly indicate that the selectivity of V and Fe decreases with increasing temperature above 650 C, consistent well with the
Fig. 8. Extraction ratio of V and Fe from the pre-oxidized vanadium slag chlorinated at different temperatures for 30 min with pressure fraction of chlorine [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10% (the rate of V extraction ratio to Fe extraction ratio is used for expression of separation factor between V and Fe).
thermodynamic predictions. A favorable selectivity of V and Fe can be obtained at 600 Ce700 C, and a maximal separation factor between V and Fe is found at 650 C. This finding differs from the chlorination of vanadium slag in molten salts where nearly equal amount of V and Fe (about 50%) was synchronously chlorinated at 700 C (Liu et al., 2016b). The increased selectivity in the present study might be attributed to the V and Fe phase separation induced by the pre-oxidization. Additionally, the chlorination kinetics related to V and Fe compounds may play an important role as well. The lower extraction ratio of V at 500 Ce600 C is possibly derived from a chemical reaction control process. And a mixed chemical reaction and pore diffusion may govern the process at 600 Ce700 C, according to the investigation by Gaballah et al. (1995) that the chlorination of V2O5 in Cl2eN2 gas mixture has apparent activation energies of 235 kJ mol1 and 77 kJ mol1 respectively at temperatures below 570 C and 570 Ce650 C. As a comparison, Fe compounds in the pre-oxidized slag seem to be very stable at temperatures lower than 650 C, possibly on account of a chemical reaction control mechanism by referring the chlorination of hematite in Cl2 ti et al., (188 kJ mol1 of activation energy at 597 Ce777 C) (Berto 1987). At temperatures higher than 650 C the chlorination ratio of Fe rapidly increases, analogous to the results found by Gennari et al. (1997) and Fuwa et al. (1978) separately studying the chlorination of Fe2O3 and Fe2TiO5. In conclusion, 650 C is a crucial dividing point for selectivity between V and Fe. Hence we determined 650 C as the optimum temperature for the subsequent experiments. 3.4. Effect of petroleum coke addition The influence of petroleum coke addition on the selectivity of V and Fe was studied by chlorination at 650 C for 30 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5. The extraction of V, indicated from Fig. 9, increases as the mass fraction of petroleum coke in raw materials for chlorination (Rc) increases from 0 to 10%. Like other reduction agents such as CO and CCl4, the petroleum coke is added mainly for decreasing the oxygen potential in chlorination systems, which can improve the kinetics of chlorination reactions and promote the formation of chlorides (Jena et al., 1999; Kakumazaki et al., 2014; Pap et al., 1989). With increase
Fig. 9. Influence of mass fraction of petroleum coke in raw materials for chlorination (Rc) on extraction ratio of V and Fe from the pre-oxidized vanadium slag chlorinated at 650 C for 30 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5.
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of petroleum coke addition in a certain range, the chlorination ratio of the targeted compositions usually increases possibly based on multiple mechanisms such as contact reinforcement between the minerals to be chlorinated and petroleum coke (Barin and Schuler, 1980; Yang and Hlavacek, 2000), and the reaction acceleration favor of the formation of CO2 (Gaballah et al., 1995). From Fig. 9, the extraction ratio of V almost keeps stable with Rc increasing from 10% to 20%. At Rc ¼ 10%, there is already near 4 times of excess for petroleum coke theoretically needed according to the stoichiometry in Eq. (10), thus further increase in addition amount can hardly increase the chlorination rate of vanadium in the slag. The extraction ratio of Fe in Fig. 9 only increases slightly with increase in petroleum coke addition, manifesting a well selectivity. Finally, the optimum mass fraction of petroleum coke in the raw materials for chlorination was fixed at 10%, with the calculated rate of V extraction ratio to Fe extraction ratio at 8.84.
3.5. Effect of chlorination time To validate the influence of reaction time on selectivity between V and Fe, chlorination of the pre-oxidized vanadium slag was carried out at 650 C, and the results were described in Fig. 10. The extraction ratio of V rapidly increases from 40% to 83% with time increasing from 15 min to 90 min, while it rises slowly from 90 min to 150 min. The extraction ratio of Fe gently increases with chlorination time and keeps under 20% over the whole range, which indicates a sufficient inhibition for chlorination of Fe. During the chlorination process, V is preferentially extracted by forming VOCl3 gas, and then diffusing out of the slag. This process is of mixedcontrol by both of the reaction and pore diffusion as mentioned above, making the mechanisms more complex. Resorting to the shrinking unreacted core model, initial rapid growth of the V extraction is probably attributed to a higher surface reactivity of Mn2V2O7 and fluent diffusion of Cl2 and VOCl3. However, with the reaction proceeding the gradually formed MnCl2, having a melting point of 650 C (Speight, 2005), will result in a molten shell on the unreacted Mn2V2O7 to hinder the diffusion of Cl2 and VOCl3 (Ahmadi et al., 2017; Gaballah et al., 1995; Sohn and Fan, 2017). Further extension of the chlorination time thus leads to a slower increase of V extraction. The most effective extraction of V accompanying an acceptable chlorination degree of Fe can be
Fig. 10. Variation of the extraction ratio of V and Fe in the pre-oxidized vanadium slag with chlorination time, at 650 C, pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10%.
7
attained at 120 min of the chlorination time, where the rate of V extraction ratio to Fe extraction ratio expressing the separation factor between V and Fe is calculated at 4.66.
3.6. Effect of pressure fraction of chlorine Fig. 11 illustrates the influence of pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] on extraction of V and Fe (chlorinated at 650 C for 120 min with Rc ¼ 10%). The extraction of V increases rapidly with [P(Cl2)/P(Cl2þN2)] from 0.1 to 0.25, demonstrating a significant dependence on the chlorine concentration. Then, further increase in the pressure fraction of chlorine hardly intensifies the vanadium extraction when [P(Cl2)/P(Cl2þN2)] exceeds 0.5 (see Fig. 11). This tendency shows an obvious resemblance with the chlorination of V2O5 using Cl2, in which a Langmuir-type rate equation was adopted to describe the dependence of chlorination on the partial pressure of Cl2 (Pap et al., 1989). An increase in Cl2 pressure fraction, e.g. from 0.1 to 0.25 in this work, can visibly promote the gas diffusion and accelerate the chlorination. However, this effect will gradually decay along with the diffusion of Cl2 arriving at the maximum. As shown in Fig. 11, the extraction ratio of Fe almost proportionally rises with increase of [P(Cl2)/P(Cl2þN2)] and up to about 27% of Fe can be extracted at [P(Cl2)/ P(Cl2þN2)] ¼ 0.65. It is probably because the chlorination of Fe is controlled by chemical reaction at 650 C and a higher Cl2 con ti et al., 1987; Fruehan centration will be kinetically beneficial (Berto and Martonik, 1973; Fuwa et al., 1978; Gennari et al., 1997). From the above, the optimum conditions for efficient and selective extraction of V from Fe in the pre-oxidized vanadium slag are determined at 650 C for 120 min with [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and Rc ¼ 10%, resulting in 87.47% of V and 18.79% of Fe synchronous extraction (4.66 for the rate of V extraction ratio to Fe extraction ratio). The extraction efficiency of vanadium by this method is higher than the results from earlier reports related to vanadium extraction by chlorination of other vanadium-containing resources, e.g. spent catalysts for valuable metals recovery (Gaballah and Djona, 1995; Gaballah et al., 1994). And the selectivity between V and Fe for this method is also intensified compared with that for chlorination of vanadium slag in molten salt system (Liu et al., 2016b). Additionally, the extraction of other main compositions in
Fig. 11. Effect of the pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] on the extraction ratio of V and Fe from the pre-oxidized vanadium slag (chlorination at 650 C for 120 min with mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10%).
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Table 2 Extraction ratio of the related compositions in the pre-oxidized vanadium slag chlorinated at 650 C for 120 min with pressure fraction of chlorine [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10% (calculated by Eq. (1)). Element
Cr
Mna
Ti
Si
Extraction ratio (mass fraction, %)
11.58
11.83
6.15
2.67
a
63.03% of Mn (mass fraction) can be leached out by the subsequent water leaching of the chlorination residue, at 85 C for 60 min and liquid-solid ratio of about 80 (L:S, kg kg1).
the pre-oxidized vanadium slag was also considered at the proposed optimum conditions (see Table 2). It reveals an excellent selectivity for Cr, Ti and Si with very low extraction ratios as well. Noteworthily, the extraction ratio of Mn is only 11.83%, which was calculated by Eq. (1) virtually based on the volatilization ratio of the elements during chlorination. And most of the formed MnCl2 might remain in the slag because of its high boiling point (1210 C) (Speight, 2005). A follow-up water leaching process for the chlorination residue obtains 63.03% of leaching ratio (mass fraction) for Mn as annotated in Table 2.
3.7. Distribution of V in chlorination residue
Fig. 12. XRD patterns of the pre-oxidized vanadium slag after chlorination at 650 C for 120 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10%.
The slag after chlorination at 650 C for 120 min with [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and Rc ¼ 10% was characterized using XRD and SEM-EDS. As indicated in Fig. 12, the chlorination residue is mainly composed of hematite Fe2O3, pseudobrookite Fe2TiO5, cristobalite SiO2, manganese chloride dihydrate MnCl2$2H2O, as well as the residual carbon (C) originated from the petroleum coke. By contrasting the patterns of Fig. 3 and Fig. 12, it can be found that Mn2V2O7 in the pre-oxidized vanadium slag transforms into MnCl2$2H2O in the chlorinated slag with other phases like hematite and pseudobrookite unchanged. MnCl2$2H2O was possibly formed by hydration of the MnCl2 generated from the carbochlorination of Mn2V2O7, via subsequent contact with air. From Fig. 13 (A), the chlorination residue has a porous structure. A magnified view taken for the selected area of Fig. 13 (A) (marked with a red box) indicates that the residue is under the architecture of granular and tabular particles as well as larger integrated blocks (see Fig. 13(B)), respectively corresponding to the hematite, pseudobrookite and complex of cristobalite & manganese chloride dihydrate from Fig. 12 and Table 3. V is principally remnant in pseudobrookite of the residue (see Table 3), potentially in the form of V4þ as lattice substitution for Ti4þ as mentioned in section 3.1 (Barolin et al.,
Fig. 13. SEM images of the pre-oxidized vanadium slag after chlorination at 650 C for 120 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10% (A. particle, B. magnified area in the red box of (A)) Note: red symbols in picture (B) are selected points for EDS analysis.
Table 3 Chemical compositions of the selected points in Fig. 13 (B) by EDS. Position
Element compositions (wt.%) Fe
Mn
Cr
V
Ti
Si
Cl
Al
Ca
Mg
Na
K
S
P
C
O
Sp1 Sp2 Sp3
43.36 25.17 0.92
3.21 2.72 10.09
2.39 0.87 /
0.46 4.29 0.36
2.67 14.67 0.20
1.46 3.28 29.93
1.49 1.28 13.01
0.98 1.43 2.08
0.25 0.36 2.95
1.48 4.72 0.31
0.02 0.10 0.26
/ 0.06 0.15
/ 0.16 0.26
0.13 0.17 0.12
3.80 2.73 4.49
38.30 38.00 34.87
Note:/in the table refers to the concentration of the corresponding element below the detection limit.
G. Du et al. / Journal of Cleaner Production 237 (2019) 117765
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Fig. 14. Schematic diagram for selective extraction of vanadium from the pre-oxidized vanadium slag by carbochlorination.
1982), and the compact microstructure as well as the inferior chlorination activity at lower temperature of pseudobrookite could prevent the extraction of vanadium in this phase (Gennari et al., 1998). From the above, the mechanism for selective extraction of V by the present process can be simply described in Fig. 14. V is firstly reenriched as Mn2V2O7 from spinel phase ((Mn, Fe) (V, Cr, Ti)2O4) of the original slag during the pre-oxidization process. Fe, Ti and Cr are separated from Mn2V2O7 by forming hematite and pseudobrookite phases in the pre-oxidized slag simultaneously. Then, Mn2V2O7 in the pre-oxidized slag is preferentially chlorinated into VOCl3 gas and solid MnCl2. Ultimately, most of V can be selectively extracted by carbochlorination, with Fe and other compositions primarily staying in the residue. 4. Conclusions In this work, a novel process for vanadium extraction from a pre-oxidized vanadium slag via selective chlorination was developed. By roasting the original vanadium slag, a porous pre-oxidized vanadium slag was firstly obtained. A well separation between V and Fe compounds in the pre-oxidized vanadium slag was identified employing XRD and SEM-EDS characterizations. According to the thermodynamic analysis, V can be selectively extracted from other Fe-containing compositions in the pre-oxidized slag by carbochlorination, and the selectivity between V and Fe weakens with the elevated temperature. On the basis of thermodynamics analysis, chlorination of the pre-oxidized slag at different temperatures, reaction time, mass fraction of petroleum coke Rc and chlorine pressure fraction [P(Cl2)/P(Cl2þN2)] were performed in a fluidized bed reactor. The experiment results agree well with the thermodynamic predictions. A resultant 87.47% of V and 18.79% of Fe can be simultaneously extracted at the optimum conditions of 650 C for 120 min with [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and Rc ¼ 10%. The surplus V is principally observed in the pseudobrookite phase of the chlorination residue. Furthermore, the extraction ratios of other main compositions in the pre-oxidized vanadium slag, viz. Cr, Ti and Si are also very low at the optimum conditions. Therefore, the proposed process can realize efficient vanadium extraction from Vcontaining resources and provides new insights for cleaner
production of vanadium industry. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgments Authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Project No. 51774262, 51504231, 51504232 and 21736010) and Open Project of State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization (Project No. CNMRCUKF1704). References Ahmadi, E., Rezan, S.A., Baharun, N., Ramakrishnan, S., Fauzi, A., Zhang, G., 2017. Chlorination kinetics of titanium nitride for production of titanium tetrachloride from nitride ilmenite. Metall. Mater. Trans. B 48B, 2354e2366. Barin, I., Schuler, W., 1980. On the kinetics of the chlorination of titanium dioxide in the presence of solid carbon. Metall. Trans. B 11B, 199e207. Barolin, H.A., Wang, C.L. (Translation), 1982. Oxidation of Vanadium Slag. Press of Metallurgy Industry, Beijing (in Chinese). ti, I., Pap, I.S., Sze kely, T., Babievskaya, I.Z., 1987. Comparative thermograviBerto metric study of chlorinations of hematite and wustite. J. Therm. Anal. 32, 281e292. Brocchi, E.A., Navarro, R.C.S., Moura, F.J., 2013. A chemical thermodynamics review applied to V2O5 chlorination. Thermochim. Acta 559, 1e16. Chen, D., Zhao, H., Hu, G., Qi, T., Yu, H., Zhang, G., Wang, L., Wang, W., 2015. An extraction process to recover vanadium from low-grade vanadium-bearing titanomagnetite. J. Hazard Mater. 294, 35e40. Choi, C., Kim, S., Kim, R., Choi, Y., Kim, S., Jung, H., Yang, J.H., Kim, H.T., 2017. A review of vanadium electrolytes for vanadium redox flow batteries. Renew. Sustain. Energy Rev. 69, 263e274. Cui, F., Mu, W., Wang, S., Xin, H., Shen, H., Xu, Q., Zhai, Y., Luo, S., 2018. Synchronous extractions of nickel, copper, and cobalt by selective chlorinating roasting and water leaching to low-grade nickel-copper matte. Separ. Purif. Technol. 195, 149e162. Deng, G.Z., Wang, W.Y., 2010. Titanium Metallurgy. Press of Metallurgy Industry, Beijing (in Chinese). Diao, J., Zhou, W., Gu, P., Ke, Z., Qiao, Y., Xie, B., 2016. Competitive growth of crystals in vanadium-chromium slag. CrystEngComm 18, 6272e6281. Enache, D.I., Bordes-Richard, E., Ensuque, A., Bozon-Verduraz, F., 2004. Vanadium oxide catalysts supported on zirconia and titania I. Preparation and characterization. Appl. Catal., A 278, 93e102. Fan, C., Yang, H., Zhu, Q., 2017. Selective hydrolysis of trace TiCl4 from VOCl3 for preparation of high purity V2O5. Separ. Purif. Technol. 185, 196e201.
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Fan, C.L., Zhu, Q.S., Yang, H.T., 2016a. System and Method for Highly Effective Chlorination and Vanadium Extraction from Vanadium Slag. PCT/CN2016/ 072522. Fan, C.L., Zhu, Q.S., Yang, H.T., 2016b. System and Method for Preparation of High Purity Vanadium Pentoxide by Chlorination. CN107555478A. Fruehan, R.J., Martonik, L.J., 1973. The rate of chlorination of metals and oxides: Part III. The rate of chlorination of Fe2O3 and NiO in Cl2 and HCl. Metall. Trans. 4, 2793e2797. Fuwa, A., Kimura, E., Fukushima, S., 1978. Kinetics of iron chlorination of roasted ilmenite ore, Fe2TiO5 in a fluidized-bed reactor. Metall. Trans. B 9B, 643e652. Gaballah, I., Djona, M., Allain, E., 1995. Kinetics of chlorination and carbochlorination of vanadium pentoxide. Metall. Mater. Trans. B 26B, 711e718. Gaballah, I., Djona, M., Mugica, J.C., Solozobal, R., 1994. Valuable metals recovery from spent catalysts by selective chlorination. Resour. Conserv. Recycl. 10, 87e96. Gaballah, I., Djona, M., 1995. Recovery of Co, Ni, Mo, and V from unroasted spent hydrorefining catalysts by selective chlorination. Metall. Mater. Trans. B 26B, 41e50. Gennari, F.C., Andrade Gamboa, J.J., Pasquevich, D.M., 1998. Formation of pseudobrookite through gaseous chlorides and by solid-state reaction. J. Mater. Sci. 33, 1563e1569. Gennari, F.C., Bohe, A.E., Pasquevich, D.M., 1997. Effect of reaction temperature on the chlorination of a Fe2O3-TiO2-C mixture. Thermochim. Acta 302, 53e61. Guo, M.S., Li, H.Z., 2010. Handbook of Fluidization. Press of Chemical Industry, Beijing (in Chinese). Jansen, M., Letschert, H.P., 2000. Inorganic yellow-red pigments without toxic metals. Nature 404, 980e982. Jena, P.K., Brocchi, E.A., Dos Reis, M.L., 1999. Kinetics of chlorination of zirconia in mixture with petroleum coke by chlorine gas. Metall. Mater. Trans. B 30B, 375e381. Jiang, T., Wen, J., Zhou, M., Xue, X., 2018. Phase evolutions, microstructure and reaction mechanism during calcification roasting of high chromium vanadium slag. J. Alloy. Comp. 742, 402e412. Ji, Y., Shen, S., Liu, J., Xue, Y., 2017a. Cleaner and effective process for extracting vanadium from vanadium slag by using an innovative three-phase roasting reaction. J. Clean. Prod. 149, 1068e1078. Ji, Y., Shen, S., Liu, J., Yan, S., Zhang, Z., 2017b. Green and efficient process for extracting chromium from vanadium slag by an innovative three-phase roasting reaction. ACS Sustain. Chem. Eng. 5, 6008e6015. Kakumazaki, J., Kato, T., Sugawara, K., 2014. Recovery of gold from incinerated sewage sludge ash by chlorination. ACS Sustain. Chem. Eng. 2, 2297e2300. Kim, H.I., Moon, G., Choi, I., Lee, J.Y., Jyothi, R.K., 2018. Hydrometallurgical process development for the extraction, separation and recovery of vanadium from spent desulfurization catalyst bio-leach liquors. J. Clean. Prod. 187, 449e458. Liao, S.M., Bai, T.L., 1985. Abroad Vanadium Metallurgy. Press of Metallurgy Industry, Beijing (in Chinese). Li, H.Y., Fang, H.X., Wang, K., Zhou, W., Yang, Z., Yan, X.M., Ge, W.S., Li, Q.W., Xie, B., 2015. Asynchronous extraction of vanadium and chromium from vanadium slag by stepwise sodium roastingewater leaching. Hydrometallurgy 156, 124e135. Li, H.Y., Wang, K., Hua, W.H., Yang, Z., Zhou, W., Xie, B., 2016. Selective leaching of vanadium in calcification-roasted vanadium slag by ammonium carbonate. Hydrometallurgy 160, 18e25. Li, M., Du, H., Zheng, S., Wang, S., Zhang, Y., Liu, B., Dreisinger, D.B., Zhang, Y., 2017a. Extraction of vanadium from vanadium slag via non-salt roasting and ammonium oxalate leaching. JOM (J. Occup. Med.) 69, 1970e1975. Li, M., Liu, B., Zheng, S., Wang, S., Du, H., Dreisinger, D.B., Zhang, Y., 2017b. A cleaner vanadium extraction method featuring non-salt roasting and ammonium
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Selective extraction of vanadium from pre-oxidized vanadium slag by carbochlorination in fluidized bed reactor Guangchao Du a, b, c, Chuanlin Fan a, b, *, Haitao Yang a, b, Qingshan Zhu a, b, ** a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China c Pangang Group Research Institute Co., Ltd., State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, Panzhihua, 617000, Sichuan, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2019 Received in revised form 20 June 2019 Accepted 23 July 2019 Available online 24 July 2019
Most of the vanadium (V) is industrially extracted from vanadium slag, a byproduct in steelmaking, via the sodium roasting-water leaching or calcification roasting-acid leaching process, which usually suffers from environment pollution or decrease in product purity. In this paper, a novel process of vanadium extraction featuring carbochlorination of pre-oxidized vanadium slag with chlorine (Cl2) and nitrogen (N2) mixture in a fluidized bed reactor was proposed. And the selectivity between vanadium and iron (Fe) in the slag was primarily concerned, due to the high content of the latter in vanadium slag. The thermodynamic analysis reveals that there exists a favorable zone for selective chlorination of V from Fe compounds. Subsequently, carbochlorination experiments were conducted to investigate the influences of chlorination temperature and time, addition amount of petroleum coke and pressure fraction of chlorine on extraction of V and Fe. And 87.47% of V together with 18.79% of Fe can be extracted from the pre-oxidized vanadium slag by chlorination at 650 C for 120 min with chlorine pressure fraction [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and petroleum coke mass fraction in raw materials for chlorination Rc ¼ 10%. The results demonstrate that the selective chlorination process has good potential to recover vanadium from vanadium slags in efficient and clean ways. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Vladimir Strezov Keywords: Vanadium extraction Vanadium slag Selective chlorination Pre-oxidization Thermodynamics
1. Introduction Vanadium (V) is a crucial nonferrous metal extensively applied in steelmaking and alloys preparation (Li et al., 2016; Liu et al., 2016a). It also plays an important role in chemical industries such as catalysts (Enache et al., 2004), pigments (Jansen and Letschert, 2000) and batteries (Choi et al., 2017). Numerous resources like vanadium titanomagnetite (Chen et al., 2015), stone coal (Ye et al., 2012) and spent catalysts (Kim et al., 2018) can be used as the raw materials for vanadium extraction. About 88% of vanadium is extracted from vanadium titanomagnetite around the world (Li et al., 2015; Xiang et al., 2017a), with an enriched slag called
* Corresponding author. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China. ** Corresponding author. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China. E-mail addresses: [email protected] (C. Fan), [email protected] (Q. Zhu). https://doi.org/10.1016/j.jclepro.2019.117765 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
vanadium slag or converter slag from steel making as the raw material in most cases (Ji et al., 2017b; Li et al., 2015, 2017b; Xiang et al., 2017a). The current industrial practice of vanadium extraction from vanadium slag mainly involves sodium roasting-water leaching process, in which V3þ compounds in vanadium slag are first oxidized into water soluble V5þ compounds by adding Na2CO3, and then the as-obtained V-containing solution is purified and precipitated with a subsequent calcination process to produce vanadium pentoxide (V2O5) (Barolin et al., 1982; Liao and Bai, 1985; Moskalyk and Alfantazi, 2003). However, the sodium roastingwater leaching process generates plenty of waste water with high concentration of sodium and ammonium ions (about 3000050000 kg of waste water per 1000 kg of V2O5 (Li et al., 2017a)), inevitably arousing environmental and cost concerns (Yang et al., 2017; Zhang et al., 2011b). As an alternative, the calcification roasting-acid leaching process solves this problem by water recycling for the whole procedure (Mirazimi et al., 2013; Xiang et al., 2017b), but the impurities such as P and Mn are easily concentrated in the recycled water, ultimately leading to higher impurities in the obtained vanadium oxide products (Li et al., 2016, 2017a).
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Furthermore, the residual after vanadium extraction by the calcification roasting-acid leaching process, containing 30 wt% or more of Fe and many other valuable metals, cannot be further utilized as iron-making raw materials due to the high content of CaSO4 accumulated in the leaching process (Li and Xie, 2012; Xiang et al., 2017a, 2018a). And stacking of the residual will threaten the environment since it also contains a small amount of carcinogenic calcium chromate (Li et al., 2017b). Thus, it appears to be imperative for seeking new technologies to tackle the mentioned problems. Kinds of processes such as multistage recovery (Li et al., 2015; Xiang et al., 2017a), non-salt roasting coupled with ammonium leaching (Li et al., 2017b) and sub-molten salts method (Liu et al., 2013) have been recently reported, which can alleviate the environment pollution and enhance the comprehensive utilization of vanadium slag. Nevertheless, the high cost or complicated operation procedure prevents their industrial applications. Chlorination is an efficient and clean method for extraction of numerous metals like titanium (Zhang et al., 2011a), zirconium (Movahedian et al., 2011), copper and nickel (Cui et al., 2018). It is the major process for industrial production of TiCl4-an important intermediate for preparation of titanium dioxide or titanium sponge (Deng and Wang, 2010). Similar to titanium, vanadium can be extracted via the chlorination process as well, principally including chlorination of the raw materials to generate vanadium oxytrichloride (VOCl3), separation and purification of the crude VOCl3 by rectification, and final oxidation of VOCl3 to V2O5 (Fan et al., 2016a, 2016b) (shown in Fig. 1). The whole process scarcely produces high salinity and ammonium-containing waste water. And the excellent distillation efficiency of crude VOCl3 insures high purity of V2O5 (99.99 wt%) (Fan et al., 2017). Meanwhile, the residual after vanadium extraction can be dechlorinated and then recycled as the raw materials for iron-making (Metallurgy Laboratory of Central South Institute of Mining and Metallurgy, 1978; Xiang et al., 2017a). All the advantages mentioned above make chlorination process a promising solution for vanadium extraction. During chlorination of a vanadium slag, the selectivity among V and other impurities is a key issue closely related to the process efficiency and cost. The vanadium slag usually contains only 5 wt%-20 wt% of V2O3 while 30 wt%-40 wt% of FeO, and other components such as TiO2, CaO and MgO (Diao et al., 2016; Ji et al.,
2017b; Li et al., 2016). The chlorination of Fe in a vanadium slag will distinctly increase the chlorine consumption and challenges the subsequent purification of crude VOCl3, thus it should be inhibited as far as possible. In a vanadium slag, V is commonly enriched in the spinel phases and coexists with Fe (Diao et al., 2016; Ji et al., 2017b), readily causing simultaneous chlorination of Fe and V (Liu et al., 2016b). Fortunately, according to the mechanisms of vanadium slag roasting, vanadium-bearing spinel minerals can be decomposed during oxidation roasting, to form vanadates (Mn2V2O7 and Ca2V2O7) and iron-rich oxides respectively (Barolin et al., 1982; Jiang et al., 2018; Li et al., 2017b; Zhang et al., 2015), which attains separation of Fe and V in vanadium slag and may be suitable for selective chlorination of vanadium compounds. In order to confirm the feasibility of selective extraction of vanadium from vanadium slag, carbochlorination of a pre-oxidized vanadium slag was investigated in the present work. The thermodynamics for selective chlorination of V from Fe compounds in the pre-oxidized vanadium slag was analyzed. And then the carbochlorination experiments were conducted, to discuss the key factors influencing extraction efficiency and selectivity between V and Fe, in a fluidized bed reactor with chlorine and petroleum coke as the chlorination agent and reduction agent, respectively. 2. Materials and methods 2.1. Materials The petroleum coke was more than 99% of mass fraction for carbon content. Purity of chlorine (Cl2) and nitrogen (N2) in the experiments was 99.9% and 99.99%, respectively. The purified water was prepared by a Millipore Elix 3 water purifier (Millipore, the US). The original vanadium slag with less than 74 mm of particle size was supplied by Panzhihua Iron and Steel Corporation, China. Chemical compositions of the original vanadium slag determined by X-ray fluorescence spectrometry (XRF, AXIOS-MAX, PANalytical B. V., the Netherlands) are shown in Table 1. To acquire the raw material for fluidized carbochlorination, a pre-oxidation process was performed herein for the original slag as follows. Firstly, pellets with about 5e10 mm of diameter were prepared by manual agitation and oscillation for mixture of the original vanadium slag and pure water. After desiccated at 100 C for 24 h, the pellets were roasted under air atmosphere using a tube furnace. The roasting temperature and time were respectively determined at 900 C and 120 min according to the references (Jiang et al., 2018; Li et al., 2017a, 2017b; Zhang et al., 2015). Finally, the calcined pellets were crashed by a pulverizer and sieved afterwards, and only the particles of 100e300 mm which belong to Btype powders suitable for fluidization were used as the raw material for the subsequent carbochlorination process according to the Handbook of Fluidization (Guo and Li, 2010). The BET surface area of particles, measured on a specific surface area analyzer (NOVA, 2000e; QUANTACHROME INSTRUMENTS, the US), was increased from 1.15 m2 g1 for the original vanadium slag to 2.21 m2 g1 for the pre-oxidized vanadium slag. All other chemical reagents used were of analytical grade and without any further purification. 2.2. Methods
Fig. 1. Flow diagram of vanadium extraction from vanadium slag via chlorination process.
2.2.1. Thermodynamic calculations Employing HSC 6.0 software the chlorination selectivity between V and Fe was discussed in terms of the computed chemical potential diagrams (predominance diagrams). Meanwhile, the standard Gibbs free energy changes at different temperatures with respect to the possible carbochlorination reactions were also calculated by the software.
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Table 1 Chemical compositions of the original vanadium slag. Oxide species
FeO
V2O3
MnO
SiO2
TiO2
Cr2O3
Al2O3
MgO
CaO
Other
Content (wt.%)
36.84
12.33
11.35
16.05
10.29
3.13
3.46
2.82
2.35
1.38
2.2.2. Carbochlorination of the pre-oxidized vanadium slag Carbochlorination of the pre-oxidized vanadium slag was conducted on a device illustrated in Fig. 2. The quartz fluidized bed reactor, having a distributor on the bottom and about 20 mm of inner diameter for the reaction section, was equipped in an electric furnace. All of the gas tubes and connectors were made of corrosive-resistant Teflon. A batch of intimately mixed raw materials, viz., the pre-oxidized vanadium slag (100e300 mm) and petroleum coke (300e600 mm), was added into the reactor. The pressure fraction of chlorine in the reactor [P(Cl2)/P(Cl2þN2)] where P(Cl2) and P(Cl2þN2) separately denote the partial pressure of chlorine (Pa) and total pressure of chlorine and nitrogen (Pa) in the inlet gas, was determined by volume flow rates of nitrogen (V(N2), L min1) and chlorine (V(Cl2), L min1) via a customized gas controller with the total volume flow rate of inlet gas V(Cl2þN2) kept 1.5 L min1 at the standard state, since [P(Cl2)/P(Cl2þN2)] has an equal value with [V(Cl2)/V(Cl2þN2)] at the same temperature. During the chlorination, the outlet gas was cooled down and then absorbed by NaOH solution before its release to the atmosphere. After the desired time of chlorination, supply of the chlorine was ceased and the reactor was moved out of the furnace for cooling. Finally, the chlorination residues were collected and weighed for various analyses. The extraction ratio of elements in the preoxidized vanadium slag was calculated according to Eq. (1).
h i M0 ya;0 ðM ya Þ 100% Ex ¼ M0 ya;0
(1)
Where Ex denotes the extraction ratio (mass fraction, %) of a certain element from the pre-oxidized vanadium slag (e.g. V and Fe), M0 (g) and ya,0 (%) are the initial mass of the slag and the mass content of element a (such as V and Fe) in the slag, respectively. M
Fig. 2. Equipment diagram for carbochlorination.
(g) represents the mass of the chlorination residue, and ya (%) is the mass content of element a in the chlorination residue. 2.2.3. Characterizations Phase compositions of the original vanadium slag, pre-oxidized slag and chlorination residue were respectively tested by powder X-ray diffraction with Cu Ka radiation on a diffractometer (XRD, X0 Pert PRO MPD, PANalytical B. V., the Netherlands), employing the software of X0 Pert HighScore Plus for data analyses. Meanwhile, the microscopic observation and elemental analysis for the slags were conducted by scanning electron microscopy (SEM, JSM-7001F, JEOL, Japan) equipped with energy dispersive spectrum (EDS, INCA XMAX, OXFORD INSTRUMENTS, the UK). 3. Results and discussion 3.1. Vanadium slag before and after pre-oxidation The phases and elemental distributions of vanadium slag before and after roasting were investigated to learn the effects of preoxidation. Fig. 3 shows that the original vanadium slag is mainly consisted of V-containing spinel (Mn, Fe) (V, Ti, Cr)2O4 (revised from (Mn, Fe) (V, Cr)2O4 according to Ji et al. (2017a)), fayalite (Fe, Mn)2SiO4, phonotephrite (Mg, Fe, Al, Ti) (Ca, Mg, Fe, Na) (Si, Al)2O6 and diopside (Mg0.6Fe0.2Al0.2)Ca(Si1.5Al0.5)O6. Moreover, as shown in Fig. 4, the V, Cr and Ti have a similar distribution, suggesting concurrent enrichment in the spinel phase (brightest particles in Fig. 4 (a)) (Ji et al., 2017b; Li and Xie, 2012). Si demonstrates a complementary distribution with V, Cr and Ti, which implies detachment of the spinel phase from silicates phases (Ji et al., 2017a; Lindvall et al., 2017). Fe and Mn are concentrated both in the spinel and silicates phases. The commensalism of V and Fe in the spinel phase is thermodynamically and dynamically adverse to selective chlorination. In comparison, the main phases of vanadium slag after preoxidation transfer into hematite Fe2O3, pseudobrookite Fe2TiO5,
Fig. 3. XRD patterns of (I) original vanadium slag and (II) pre-oxidized vanadium slag roasted at 900 C for 120 min.
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4MnOðsÞ þ O2 ðgÞ/2Mn2 O3 ðsÞ
(7)
2MnOðsÞ þ V2 O5 ðsÞ/Mn2 V2 O7 ðsÞ
(8)
2SiO2 ðsÞ þ CaOðsÞ þ Al2 O3 ðsÞ/CaAl2 Si2 O8 ðsÞ
(9)
Herein the formation of Fe2TiO5, depending mainly on the reaction temperature, usually occurs at temperatures above 850 C according to Eq. (6) (Gennari et al., 1998). Part of Mn remains in Mn2O3 in accordance with Eq. (7) and forms solid solution with Fe2O3 (Barolin et al., 1982). V and Mn are mostly concentrated in Mn2V2O7 phase of the pre-oxidized vanadium slag along the path of Eq. (8). The overlap between V and Fe, Ti indicated from Fig. 5 suggests that a small part of V exists in Fe2O3 and Fe2TiO5 phases after pre-oxidization, probably deriving from the decomposition of V-containing spinel in the original vanadium slag where oxidation of the intermediate R2O3 (R ¼ Fe, V, Cr and Mn) to generate Fe2O3$V2O5 was slightly inhibited (Barolin et al., 1982). This leads to the partial co-existence of V and Fe in the solid solutions of Fe2O3$V2O3 and Fe2O3$V2O4 in the pre-oxidized slag, and V2O4 dissolving in Fe2O3 would simultaneously cause the reserve of small quantity of vanadium in the pseudobrookite via the reaction of Eq. (6) (Barolin et al., 1982; Jiang et al., 2018; Li et al., 2017b). Even
Fig. 4. SEM images with EDS element mapping of the original vanadium slag (a. particle section).
manganese pyrovanadate Mn2V2O7, cristobalite SiO2 and anorthite CaAl2Si2O8 (see Fig. 3), as a result of decomposition of the V-containing spinel and silicates in the original vanadium slag likely according to Eq. (2)-Eq. (5) (Barolin et al., 1982; Li et al., 2015, 2017a).
ðFe; MnÞðV; Cr; TiÞ2 O4 ðsÞ þ O2 ðgÞ/Fe2 O3 ðsÞ þMnOðsÞ þ Cr2 O3 ðsÞ þ V2 O5 ðsÞ þ TiO2 ðsÞ
(2)
ðFe; MnÞ2 SiO4 ðsÞ þ O2 ðgÞ/Fe2 O3 ðsÞ þ MnOðsÞ þ SiO2 ðsÞ
(3)
ðMg; Fe; Al; TiÞðCa; Mg; Fe; NaÞðSi; AlÞ2 O6 ðsÞ þ O2 ðgÞ/Fe2 O3 ðsÞ þSiO2 ðsÞ þ TiO2 ðsÞ þ CaOðsÞ þ MgOðsÞ þ Na2 OðsÞ þ Al2 O3 ðsÞ (4) ðMg0:6 Fe0:2 Al0:2 ÞCaðSi1:5 Al0:5 ÞO6 ðsÞþO2 ðgÞ/Fe2 O3 ðsÞ þMgOðsÞþAl2 O3 ðsÞ þ CaOðsÞ þ SiO2 ðsÞ
(5)
And the new phases are possibly reconstructed in accordance with Eq. (6)-Eq. (9) (Xiang et al., 2018b; Zhang et al., 2015).
Fe2 O3 ðsÞ þ TiO2 ðsÞ/Fe2 TiO5 ðsÞ
(6)
Fig. 5. SEM images with EDS element mapping of the pre-oxidized vanadium slag obtained by roasting at 900 C for 120 min (a. particle section, b. high-magnification image for the area in red box of image (a)).
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so, Fig. 5 shows that the Mn2V2O7 phase is segregated largely from the Fe-containing phases, i.e. Fe2O3 or Fe2TiO5, revealing a satisfactory separation between V and Fe by the pre-oxidation. Furthermore, the pre-oxidized vanadium slag has a more porous microstructure compared with the original slag from the specific surface area test in section 2.1, Fig. 4 (a) and Fig. 5 (a)e(b), which would be beneficial for the chlorination process.
3.2. Thermodynamics analysis Thermodynamic feasibility for selective chlorination of V from Fe in the pre-oxidized slag was also investigated. The predominance diagrams at different temperatures for VeFeeCleO system were summarized in Fig. 6. All species presented in the predominance diagrams are the most thermodynamically stable based on minimization of Gibbs free energy (Brocchi et al., 2013). A selective region (green zone), constituted of gaseous VOCl3 and solid Fe2O3, is found at each temperature in Fig. 6. Simultaneously, the selective region shrinks with the elevated temperature, suggesting a decreased selectivity for V and Fe, e.g., selective extraction of V from Fe can be realized at the chlorine partial pressure of 101.8 Pa and oxygen partial pressure of 105.0 Pa at 500 C, while at the partial pressure of 103.3 Pa for chlorine and 100.8 Pa for oxygen with temperature increasing to 850 C. Furthermore, the standard Gibbs free energy change DG0 at different temperatures for carbochlorination of V and Fe compounds in the pre-oxidized vanadium slag was also calculated as shown in Fig. 7, which includes the reactions of Eq. (10)-Eq. (13).
2=5MnOðsÞ þ 1=5V2 O5 ðs=lÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=5MnCl2 ðs=lÞ þ2=5VOCl3 ðgÞ þ 1=2CO2 ðgÞ (10)
Fig. 7. Gibbs free energy change (standard) DG0 for carbochlorination reactions of Fe and V compounds in the pre-oxidized vanadium slag at different temperatures.
1=3V2 O5 ðs=lÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=3VOCl3 ðgÞ þ 1=2CO2 ðgÞ (11) 1=3Fe2 O3 ðsÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=3FeCl3 ðgÞ þ 1=2CO2 ðgÞ (12) 1=5Fe2 TiO5 ðsÞ þ 1=2CðsÞ þ Cl2 ðgÞ/2=5FeCl3 ðgÞ þ 1=5TiCl4 ðgÞ þ 1=2CO2 ðgÞ (13) The amount of Cl2 for each reaction was normalized at 1 mol,
Fig. 6. Predominance diagram for system VeFeeCleO at (a) 500 C, (b) 650 C and (c) 850 C (P(Cl2), Pa and P(O2), Pa refer to the partial pressure of Cl2 and O2 at 105 Pa (P0) of standard atmospheric pressure, respectively).
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and CO2 was assumed to be the only carbon oxide for simplification. Mn2V2O7 was substituted by MnO and V2O5 with 2:1 of molar ratio (MnO/V2O5) in Eq. (10) for calculation due to the lack of corresponding thermodynamical data. The state of MnCl2 in Eq. (10), V2O5 in Eq. (10) and Eq. (11) were expressed as solid or liquid (s/l) indicating a solid to liquid phase transition with temperature increased. Compared with that of the pure V2O5 (Eq. (11)), carbochlorination of Mn2V2O7 (Eq. (10)) has a more favorable thermodynamic trend as indicated in Fig. 7. Furthermore, DG0 for carbochlorination of Mn2V2O7 is apparently lower than that of Fe2O3 and Fe2TiO5 in the selected temperature range, showing an excellent chlorination priority for Mn2V2O7. And Fe2O3 has a slightly higher reactivity than Fe2TiO5 inferred from Fig. 7, where DG0 for Eq. (12) is mildly lower than that of Eq. (13). The sequence of carbochlorination is confirmed as Mn2V2O7 > Fe2O3 > Fe2TiO5. Therefore, separation of V from Fe in the pre-oxidized vanadium slag is thermodynamically feasible by carbochlorination.
3.3. Effect of chlorination temperature Temperature is an important parameter for the chlorination process, and a temperature ranging from 500 C to 850 C was chosen to explore its effect on the extraction of V and Fe, on basis of the thermodynamics predictions in section 3.2. The chlorination time, pressure fraction of chlorine ([P(Cl2)/P(Cl2þN2)]) and mass fraction of petroleum coke Rc were fixed at 30 min, 0.5 and 10%, respectively. As demonstrated in Fig. 8, the chlorination temperature has a significant influence on both of the extraction of V and Fe. With increasing temperature, the V extraction ratio increases slowly at 500 Ce600 C and then rises dramatically at 600 Ce700 C, while further increase of the temperature from 700 C to 850 C slightly augments the extraction of V. The highest V extraction ratio of about 95% can be achieved at 850 C. The extraction ratio of Fe increases mildly with temperature from 500 C to 650 C, nevertheless, it shows a sharp growth from 6% at 650 C to 82% at 850 C (see Fig. 8). The separation factor between V and Fe expressed with the rate of V extraction ratio to Fe extraction ratio in Fig. 8 presents an increase trend at 500 Ce650 C then followed by a diminution at 650 Ce850 C. The results clearly indicate that the selectivity of V and Fe decreases with increasing temperature above 650 C, consistent well with the
Fig. 8. Extraction ratio of V and Fe from the pre-oxidized vanadium slag chlorinated at different temperatures for 30 min with pressure fraction of chlorine [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10% (the rate of V extraction ratio to Fe extraction ratio is used for expression of separation factor between V and Fe).
thermodynamic predictions. A favorable selectivity of V and Fe can be obtained at 600 Ce700 C, and a maximal separation factor between V and Fe is found at 650 C. This finding differs from the chlorination of vanadium slag in molten salts where nearly equal amount of V and Fe (about 50%) was synchronously chlorinated at 700 C (Liu et al., 2016b). The increased selectivity in the present study might be attributed to the V and Fe phase separation induced by the pre-oxidization. Additionally, the chlorination kinetics related to V and Fe compounds may play an important role as well. The lower extraction ratio of V at 500 Ce600 C is possibly derived from a chemical reaction control process. And a mixed chemical reaction and pore diffusion may govern the process at 600 Ce700 C, according to the investigation by Gaballah et al. (1995) that the chlorination of V2O5 in Cl2eN2 gas mixture has apparent activation energies of 235 kJ mol1 and 77 kJ mol1 respectively at temperatures below 570 C and 570 Ce650 C. As a comparison, Fe compounds in the pre-oxidized slag seem to be very stable at temperatures lower than 650 C, possibly on account of a chemical reaction control mechanism by referring the chlorination of hematite in Cl2 ti et al., (188 kJ mol1 of activation energy at 597 Ce777 C) (Berto 1987). At temperatures higher than 650 C the chlorination ratio of Fe rapidly increases, analogous to the results found by Gennari et al. (1997) and Fuwa et al. (1978) separately studying the chlorination of Fe2O3 and Fe2TiO5. In conclusion, 650 C is a crucial dividing point for selectivity between V and Fe. Hence we determined 650 C as the optimum temperature for the subsequent experiments. 3.4. Effect of petroleum coke addition The influence of petroleum coke addition on the selectivity of V and Fe was studied by chlorination at 650 C for 30 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5. The extraction of V, indicated from Fig. 9, increases as the mass fraction of petroleum coke in raw materials for chlorination (Rc) increases from 0 to 10%. Like other reduction agents such as CO and CCl4, the petroleum coke is added mainly for decreasing the oxygen potential in chlorination systems, which can improve the kinetics of chlorination reactions and promote the formation of chlorides (Jena et al., 1999; Kakumazaki et al., 2014; Pap et al., 1989). With increase
Fig. 9. Influence of mass fraction of petroleum coke in raw materials for chlorination (Rc) on extraction ratio of V and Fe from the pre-oxidized vanadium slag chlorinated at 650 C for 30 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5.
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of petroleum coke addition in a certain range, the chlorination ratio of the targeted compositions usually increases possibly based on multiple mechanisms such as contact reinforcement between the minerals to be chlorinated and petroleum coke (Barin and Schuler, 1980; Yang and Hlavacek, 2000), and the reaction acceleration favor of the formation of CO2 (Gaballah et al., 1995). From Fig. 9, the extraction ratio of V almost keeps stable with Rc increasing from 10% to 20%. At Rc ¼ 10%, there is already near 4 times of excess for petroleum coke theoretically needed according to the stoichiometry in Eq. (10), thus further increase in addition amount can hardly increase the chlorination rate of vanadium in the slag. The extraction ratio of Fe in Fig. 9 only increases slightly with increase in petroleum coke addition, manifesting a well selectivity. Finally, the optimum mass fraction of petroleum coke in the raw materials for chlorination was fixed at 10%, with the calculated rate of V extraction ratio to Fe extraction ratio at 8.84.
3.5. Effect of chlorination time To validate the influence of reaction time on selectivity between V and Fe, chlorination of the pre-oxidized vanadium slag was carried out at 650 C, and the results were described in Fig. 10. The extraction ratio of V rapidly increases from 40% to 83% with time increasing from 15 min to 90 min, while it rises slowly from 90 min to 150 min. The extraction ratio of Fe gently increases with chlorination time and keeps under 20% over the whole range, which indicates a sufficient inhibition for chlorination of Fe. During the chlorination process, V is preferentially extracted by forming VOCl3 gas, and then diffusing out of the slag. This process is of mixedcontrol by both of the reaction and pore diffusion as mentioned above, making the mechanisms more complex. Resorting to the shrinking unreacted core model, initial rapid growth of the V extraction is probably attributed to a higher surface reactivity of Mn2V2O7 and fluent diffusion of Cl2 and VOCl3. However, with the reaction proceeding the gradually formed MnCl2, having a melting point of 650 C (Speight, 2005), will result in a molten shell on the unreacted Mn2V2O7 to hinder the diffusion of Cl2 and VOCl3 (Ahmadi et al., 2017; Gaballah et al., 1995; Sohn and Fan, 2017). Further extension of the chlorination time thus leads to a slower increase of V extraction. The most effective extraction of V accompanying an acceptable chlorination degree of Fe can be
Fig. 10. Variation of the extraction ratio of V and Fe in the pre-oxidized vanadium slag with chlorination time, at 650 C, pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10%.
7
attained at 120 min of the chlorination time, where the rate of V extraction ratio to Fe extraction ratio expressing the separation factor between V and Fe is calculated at 4.66.
3.6. Effect of pressure fraction of chlorine Fig. 11 illustrates the influence of pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] on extraction of V and Fe (chlorinated at 650 C for 120 min with Rc ¼ 10%). The extraction of V increases rapidly with [P(Cl2)/P(Cl2þN2)] from 0.1 to 0.25, demonstrating a significant dependence on the chlorine concentration. Then, further increase in the pressure fraction of chlorine hardly intensifies the vanadium extraction when [P(Cl2)/P(Cl2þN2)] exceeds 0.5 (see Fig. 11). This tendency shows an obvious resemblance with the chlorination of V2O5 using Cl2, in which a Langmuir-type rate equation was adopted to describe the dependence of chlorination on the partial pressure of Cl2 (Pap et al., 1989). An increase in Cl2 pressure fraction, e.g. from 0.1 to 0.25 in this work, can visibly promote the gas diffusion and accelerate the chlorination. However, this effect will gradually decay along with the diffusion of Cl2 arriving at the maximum. As shown in Fig. 11, the extraction ratio of Fe almost proportionally rises with increase of [P(Cl2)/P(Cl2þN2)] and up to about 27% of Fe can be extracted at [P(Cl2)/ P(Cl2þN2)] ¼ 0.65. It is probably because the chlorination of Fe is controlled by chemical reaction at 650 C and a higher Cl2 con ti et al., 1987; Fruehan centration will be kinetically beneficial (Berto and Martonik, 1973; Fuwa et al., 1978; Gennari et al., 1997). From the above, the optimum conditions for efficient and selective extraction of V from Fe in the pre-oxidized vanadium slag are determined at 650 C for 120 min with [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and Rc ¼ 10%, resulting in 87.47% of V and 18.79% of Fe synchronous extraction (4.66 for the rate of V extraction ratio to Fe extraction ratio). The extraction efficiency of vanadium by this method is higher than the results from earlier reports related to vanadium extraction by chlorination of other vanadium-containing resources, e.g. spent catalysts for valuable metals recovery (Gaballah and Djona, 1995; Gaballah et al., 1994). And the selectivity between V and Fe for this method is also intensified compared with that for chlorination of vanadium slag in molten salt system (Liu et al., 2016b). Additionally, the extraction of other main compositions in
Fig. 11. Effect of the pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] on the extraction ratio of V and Fe from the pre-oxidized vanadium slag (chlorination at 650 C for 120 min with mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10%).
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Table 2 Extraction ratio of the related compositions in the pre-oxidized vanadium slag chlorinated at 650 C for 120 min with pressure fraction of chlorine [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10% (calculated by Eq. (1)). Element
Cr
Mna
Ti
Si
Extraction ratio (mass fraction, %)
11.58
11.83
6.15
2.67
a
63.03% of Mn (mass fraction) can be leached out by the subsequent water leaching of the chlorination residue, at 85 C for 60 min and liquid-solid ratio of about 80 (L:S, kg kg1).
the pre-oxidized vanadium slag was also considered at the proposed optimum conditions (see Table 2). It reveals an excellent selectivity for Cr, Ti and Si with very low extraction ratios as well. Noteworthily, the extraction ratio of Mn is only 11.83%, which was calculated by Eq. (1) virtually based on the volatilization ratio of the elements during chlorination. And most of the formed MnCl2 might remain in the slag because of its high boiling point (1210 C) (Speight, 2005). A follow-up water leaching process for the chlorination residue obtains 63.03% of leaching ratio (mass fraction) for Mn as annotated in Table 2.
3.7. Distribution of V in chlorination residue
Fig. 12. XRD patterns of the pre-oxidized vanadium slag after chlorination at 650 C for 120 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10%.
The slag after chlorination at 650 C for 120 min with [P(Cl2)/ P(Cl2þN2)] ¼ 0.5 and Rc ¼ 10% was characterized using XRD and SEM-EDS. As indicated in Fig. 12, the chlorination residue is mainly composed of hematite Fe2O3, pseudobrookite Fe2TiO5, cristobalite SiO2, manganese chloride dihydrate MnCl2$2H2O, as well as the residual carbon (C) originated from the petroleum coke. By contrasting the patterns of Fig. 3 and Fig. 12, it can be found that Mn2V2O7 in the pre-oxidized vanadium slag transforms into MnCl2$2H2O in the chlorinated slag with other phases like hematite and pseudobrookite unchanged. MnCl2$2H2O was possibly formed by hydration of the MnCl2 generated from the carbochlorination of Mn2V2O7, via subsequent contact with air. From Fig. 13 (A), the chlorination residue has a porous structure. A magnified view taken for the selected area of Fig. 13 (A) (marked with a red box) indicates that the residue is under the architecture of granular and tabular particles as well as larger integrated blocks (see Fig. 13(B)), respectively corresponding to the hematite, pseudobrookite and complex of cristobalite & manganese chloride dihydrate from Fig. 12 and Table 3. V is principally remnant in pseudobrookite of the residue (see Table 3), potentially in the form of V4þ as lattice substitution for Ti4þ as mentioned in section 3.1 (Barolin et al.,
Fig. 13. SEM images of the pre-oxidized vanadium slag after chlorination at 650 C for 120 min with pressure fraction of chlorine [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and mass fraction of petroleum coke in raw materials for chlorination Rc ¼ 10% (A. particle, B. magnified area in the red box of (A)) Note: red symbols in picture (B) are selected points for EDS analysis.
Table 3 Chemical compositions of the selected points in Fig. 13 (B) by EDS. Position
Element compositions (wt.%) Fe
Mn
Cr
V
Ti
Si
Cl
Al
Ca
Mg
Na
K
S
P
C
O
Sp1 Sp2 Sp3
43.36 25.17 0.92
3.21 2.72 10.09
2.39 0.87 /
0.46 4.29 0.36
2.67 14.67 0.20
1.46 3.28 29.93
1.49 1.28 13.01
0.98 1.43 2.08
0.25 0.36 2.95
1.48 4.72 0.31
0.02 0.10 0.26
/ 0.06 0.15
/ 0.16 0.26
0.13 0.17 0.12
3.80 2.73 4.49
38.30 38.00 34.87
Note:/in the table refers to the concentration of the corresponding element below the detection limit.
G. Du et al. / Journal of Cleaner Production 237 (2019) 117765
9
Fig. 14. Schematic diagram for selective extraction of vanadium from the pre-oxidized vanadium slag by carbochlorination.
1982), and the compact microstructure as well as the inferior chlorination activity at lower temperature of pseudobrookite could prevent the extraction of vanadium in this phase (Gennari et al., 1998). From the above, the mechanism for selective extraction of V by the present process can be simply described in Fig. 14. V is firstly reenriched as Mn2V2O7 from spinel phase ((Mn, Fe) (V, Cr, Ti)2O4) of the original slag during the pre-oxidization process. Fe, Ti and Cr are separated from Mn2V2O7 by forming hematite and pseudobrookite phases in the pre-oxidized slag simultaneously. Then, Mn2V2O7 in the pre-oxidized slag is preferentially chlorinated into VOCl3 gas and solid MnCl2. Ultimately, most of V can be selectively extracted by carbochlorination, with Fe and other compositions primarily staying in the residue. 4. Conclusions In this work, a novel process for vanadium extraction from a pre-oxidized vanadium slag via selective chlorination was developed. By roasting the original vanadium slag, a porous pre-oxidized vanadium slag was firstly obtained. A well separation between V and Fe compounds in the pre-oxidized vanadium slag was identified employing XRD and SEM-EDS characterizations. According to the thermodynamic analysis, V can be selectively extracted from other Fe-containing compositions in the pre-oxidized slag by carbochlorination, and the selectivity between V and Fe weakens with the elevated temperature. On the basis of thermodynamics analysis, chlorination of the pre-oxidized slag at different temperatures, reaction time, mass fraction of petroleum coke Rc and chlorine pressure fraction [P(Cl2)/P(Cl2þN2)] were performed in a fluidized bed reactor. The experiment results agree well with the thermodynamic predictions. A resultant 87.47% of V and 18.79% of Fe can be simultaneously extracted at the optimum conditions of 650 C for 120 min with [P(Cl2)/P(Cl2þN2)] ¼ 0.5 and Rc ¼ 10%. The surplus V is principally observed in the pseudobrookite phase of the chlorination residue. Furthermore, the extraction ratios of other main compositions in the pre-oxidized vanadium slag, viz. Cr, Ti and Si are also very low at the optimum conditions. Therefore, the proposed process can realize efficient vanadium extraction from Vcontaining resources and provides new insights for cleaner
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