A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal

    A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal Fei Wang, Yimin Zhang, Tao Liu, Jing Huang, Jie Zhao, Guobin Zhang, Juan Liu PII: DOI: Reference:

S0301-7516(15)00155-6 doi: 10.1016/j.minpro.2015.06.013 MINPRO 2773

To appear in:

International Journal of Mineral Processing

Received date: Revised date: Accepted date:

23 November 2013 2 May 2015 9 June 2015

Please cite this article as: Wang, Fei, Zhang, Yimin, Liu, Tao, Huang, Jing, Zhao, Jie, Zhang, Guobin, Liu, Juan, A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal, International Journal of Mineral Processing (2015), doi: 10.1016/j.minpro.2015.06.013

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ACCEPTED MANUSCRIPT A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal Fei Wang, Yimin Zhang1, Tao Liu, Jing Huang, Jie Zhao, Guobin Zhang, Juan Liu College of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan,

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430081, PR China

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Abstract:

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A comparison between unassisted and calcium fluoride-enhanced leaching demonstrated that calcium fluoride can markedly boost the efficiency and accelerate the rate of vanadium leaching

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from stone coal. Analyses methods were adopted to identify the mechanism of calcium fluoride-enhanced vanadium leaching from stone coal, including Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), valence state of vanadium, X-ray Diffractometry

Scanning

Electron

Microscope

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(XRD), X-ray Photoelectron Spectroscopy (XPS), 19F liquid Nuclear Magnetic Resonance (NMR), (SEM)

and

thermodynamics.

The

whole

calcium

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fluoride-enhanced vanadium leaching process was that calcium fluoride reacted with sulfuric acid

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and then generated HF(aq); calcite was dissolved into the acid; chlorite and phlogopite were thoroughly disintegrated and subsequently generated quartz, K+, Mg2+, [SiF6]2-, [AlF5]2- and Al3+;

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the released V(III) was oxidized to VO2+ by O2 from the air. It is because fluorine combined with aluminum and silicon in the lattice of vanadium-bearing phlogopite to generate [SiF6]2- and [AlF5]2- that facilitated the vanadium leaching from stone coal. This generation decreased the ∆G° of phlogopite disintegration, declined the effect of chemical reaction on vanadium leaching, accelerated the leaching rate of vanadium and boosted the leaching efficiency of vanadium.

Key words: Mechanism; [SiF6]2- and [AlF5]2-; Calcium fluoride; Vanadium leaching; Stone coal

1. Introduction Vanadium is an important strategic resource that is almost exclusively used in ferrous and non-ferrous alloys (Moskalyk and Alfantazi, 2003) due to its physical properties, such as high tensile strength, hardness and fatigue resistance (Archana, 2005). China has the most abundant

1 Corresponding author E-mail address: [email protected] (F. Wang), [email protected] (Y.M. Zhang).

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ACCEPTED MANUSCRIPT vanadium-bearing mineral reserves (USGS, 2012), and the reserves of vanadium in China occupy more than 35% the world reserves of vanadium. Furthermore, stone coal (also known as black shale) is an important vanadium-bearing resource in China (Li et al., 2013; Li et al., 2012), and the

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gross reserves of vanadium in stone coal account for more than 87% of the domestic vanadium

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reserves (Bin, 2006). However, the amount of V2O5 extracted from stone coal only accounted for 30-40% of total V2O5 output of China in 2010, which is not commensurate with China’s vanadium

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reserves in stone coal. It is estimated that the need for vanadium will continue to increase (Anjum et al., 2012; Denison Mines Corp., 2010; Yang et al., 2010) and that the world’s increasing

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demand for vanadium can only be met by extracting vanadium from stone coal in the near future (Zhang et al., 2011).

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Stone coal is a type of inferior anthracite in which the grade of vanadium is low (less than 1% usually). Most of vanadium in stone coal exists in the crystal lattice of the aluminosilicate

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minerals and isomorphically replaces Al(III) in vanadium-bearing micas (Zhu et al., 2012; Zhang

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et al., 2011; Bin, 2006). Thus, it is difficult to extract vanadium from stone coal by traditional beneficiation such as gravity concentration and magnetic concentration. Flotation is a better

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method of enrichment (Zafar and Ashraf, 2007), but the extraction efficiency is low and the grade of vanadium in the product is not sufficient to meet market requirements. Additionally, the flotation process is complex and lengthy. Therefore, it is necessary to use hydrometallurgical extraction to develop new processes for vanadium extraction from stone coal (Habbache, 2009). However, the present technologies and equipment for extracting vanadium from stone coal are relatively laggard. Most traditional processes neither are inefficient (Li et al., 2011; Li et al., 2009, 2010; Li et al., 2009) nor produce serious environmental pollutants such as HCl and Cl2 (Ye et al., 2012). Recently, an eco-friendly and hopeful process known as direct acid-leaching solvent extraction was developed (Li et al., 2009, 2010). Specifically, fluorine-bearing aid-leaching reagents (e.g., HF, NH4F, H2SiF6 and CaF2) are introduced into the leaching system to improve the efficiency of vanadium leaching (He et al., 2008). So far, a few researchers have researched the fluoride-assisted vanadium leaching. Zhou et al. (2009) used sulfuric acid with NH4F to investigate the direct acid-leaching parameters and attempted to discover the mechanism of fluoride-assisted leaching. They found that only the limonite was dissolved in the absence of

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ACCEPTED MANUSCRIPT NH4F while both micas and limonite were dissolved in the presence of NH4F. Li et al. (2010) researched the optimum parameters for the direct acid-leaching process using sulfuric acid, HF and NaClO. They found that HF can promote the decomposition of the micaceous minerals and

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increase the vanadium leaching efficiency. Zhang et al. (2011) studied the optimum parameters in

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the H2SiF6 leaching process. They preliminarily ascribed the function of H2SiF6 to the generation of HF(aq) and the destruction of the illite structure by comparing leaching by H2SO4 with leaching

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by H2SiF6. He et al. (2008) investigated the effects of HF acid, NaF and CaF2, and demonstrated that fluoride can facilitate the breakage of the crystal structure of aluminosilicate minerals.

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Besides, He (2009) researched the sulfuric acid leaching process of blank-roasted residue by calculating the standard free energies of the dissolution of pure illite, hematite and kaolinite at

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specific temperatures. This study discovered that the dissolution of vanadium-bearing illite was spontaneous. However, the mechanism about how vanadium-bearing micas break down and how

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fluoride enhances the vanadium leaching still remains unknown. It is more regrettable that there

stone coal.

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are few studies (Zhou et al., 2011) about the thermodynamics analyses of vanadium leaching from

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The main purpose of this work is to analyze the mechanism of calcium fluoride-enhanced vanadium leaching from stone coal by thermodynamic analyses and the comparison between unassisted and calcium fluoride-assisted vanadium leaching.

2. Experimental 2.1. Materials

The stone coal used in this study was obtained from Jiangxi province in China. Prior to the leaching tests, the ore was crushed to a grain size of 0-3 mm and subsequently ground to a particle size of <74 µm accounting for 75%. The obtained ore was called raw ore throughout this work. The chemical composition of the raw ore determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) is shown in Table 1. The mineral analysis by X-ray Diffractometry (XRD) method is shown in Fig. 1. The main minerals were quartz, pyrite, chlorite and phlogopite. The valence state analyses (Table 2) which depended on the method of potentiometric titration (Hu et al., 2012; Zhang, 1992) showed that vanadium of the raw ore was composed of V(III) and V(IV). There was no V(V). Phlogopite was the main vanadium-bearing 3

ACCEPTED MANUSCRIPT mineral. The calcium fluoride was supplied by Shanghai Shanpu Chemical Co., Ltd. All other reagents and chemicals used were of analytical reagent grade.

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2.2. Methods 2.2.1. Leaching tests

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Leaching tests were carried out to determine the effects of sulfuric acid and time on the

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unassisted and calcium fluoride-assisted vanadium leaching from stone coal. The tests about the effect of sulfuric acid concentration on the efficiency of calcium fluoride-assisted vanadium

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leaching were carried out in magnetic temperature-controlled stirrers (SZCL-2A) at liquid/solid ratio (L/S) of 1 mL/g, 368±1 K and 5% (w/w) calcium fluoride for 4 h. The H2SO4 concentration

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was in the range of 5%-25% (v/v). Similarly, the effect of leaching time on the efficiency of calcium fluoride-assisted vanadium leaching was determined in magnetic temperature-controlled stirrers (SZCL-2A) at 15% (v/v) H2SO4, 1 mL/g, 368±1 K and 5% (w/w) calcium fluoride. The

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leaching time was in the range of 0-6 h. Leaching tests in the absence of calcium fluoride were

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carried out under the same conditions except for 5% (w/w) calcium fluoride. Each leaching test consumed 100 g raw ore. The slurry was then divided into the leachate and residue via vacuum

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filtration.

2.2.2. Analyses

The leachate and residue obtained from the leaching tests at 15% (v/v) H2SO4, 1 mL/g and 368±1 K for 4 h were used for analyses. The chemical composition of the leachate was analyzed by ICP-AES. The ICP-AES was performed on the IRIS Advantage ER/S instrument (Thermo Elemental, USA). The forms of fluorine in the leachate were determined by

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F liquid Nuclear

Magnetic Resonance (NMR) and X-ray Photoelectron Spectroscopy (XPS). The NMR analysis was performed on the Avance-III-500 instrument (Bruker, Germany). The XPS analysis, which consisted of loading anion resin 201×7 (commercial Chinese name) with the leachate with fluorine and subsequent vacuum drying of the resin, was performed on the VG Multilab 2000 instrument (Thermo Electron, USA). The mineral composition and morphology of the obtained residue was analyzed by XRD and Scanning Electron Microscope (SEM) respectively. The XRD analysis was performed on the D/max-III instrument (Rigaku, Japan), and the SEM analysis employed the JSM-5610LV instrument (JEOL, Japan). In addition, the valence state of vanadium was

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ACCEPTED MANUSCRIPT determined by potentiometric titration performed on the ZDJ-4 automatic potentiometric titrator from INESA Scientific Instrument Co., Ltd. The vanadium concentration in the aqueous phase was determined by ferrous ammonium

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sulfate titration using 2-(phenylamino)-benzoic acid as an indicator (GB/T 8704.5, 2007). The

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fluorine concentration was determined by fluorine ion selective electrode (GB/T 7484, 1987) and the iron concentration was determined by 1,10-phenanthroline spectrophotometry (HJ/T 345,

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2007). The content of other elements in the aqueous phase was analyzed using ICP-AES. By comparing the analyses of the raw ore with those of the leachate and residue, it is possible

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to infer the whole leaching process. Furthermore, comparing calcium fluoride-assisted and unassisted leaching behaviors can identify the mechanism of calcium fluoride-enhanced vanadium

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leaching from stone coal. Thermodynamic analyses were conducted to further confirm the reactions that described calcium fluoride-assisted vanadium leaching from stone coal and the

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3. Results and Discussion

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associated mechanism.

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3.1. Results of leaching tests

To study the function of calcium fluoride in the leaching process, the effects of sulfuric acid concentration and leaching time on the vanadium leaching efficiency were tested in the presence and absence of calcium fluoride. Fig. 2 indicates that with sulfuric acid concentration increasing, the vanadium leaching efficiency increased apparently to 95.83% in the presence of calcium fluoride. This effect was maintained in the absence of calcium fluoride, where the vanadium leaching efficiency rose from 13.95% at 5% (v/v) H2SO4 to 80.37% at 25% (v/v) H2SO4. However, this effect reached a plateau at 92.39% of vanadium leaching efficiency at 15% (v/v) H2SO4 in the presence of calcium fluoride while the vanadium leaching efficiency continued to rise in the absence of calcium fluoride despite only 47.46% at 15% (v/v) H2SO4. Therefore, calcium fluoride can significantly enhance the vanadium leaching efficiency and reduce the sulfuric acid consumption, which was consistent with the results of Zhou et al. (2009). Fig. 3 clearly shows that prolongation of leaching time increased sharply the vanadium leaching efficiency in the presence of calcium fluoride to 78.76% at only 1 h. This effect was preserved in the absence of calcium fluoride, where the vanadium leaching efficiency increased to 5

ACCEPTED MANUSCRIPT 56.50% at 6 h. Nevertheless, the vanadium leaching efficiency remained stable at 92.39% beyond 4 h in the presence of calcium fluoride, but continued to rise with time in the absence of calcium fluoride. Thus, calcium fluoride can accelerate the leaching rate as well. It can be concluded that

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calcium fluoride is an effective aid-leaching reagent that can enhance the vanadium leaching.

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3.2. The mechanism of calcium fluoride-enhanced vanadium leaching from stone coal

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3.2.1 Analyses of leachate

To investigate the reason for the significant differences in the efficiency of unassisted and

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calcium fluoride-assisted vanadium leaching from stone coal, the leachate was analyzed by ICP-AES, as shown in Fig. 4. The vanadium leaching efficiency increased by 44.93%, and the

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content of other elements in the leachate rose dramatically when calcium fluoride was introduced into the leaching system. The leaching efficiency of aluminum, potassium and magnesium rose by

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25.86%, 26.31% and 18.52%, respectively; the leachate retained 60.52% of fluorine from calcium

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fluoride. Meanwhile, the silicon content in the leachate obtained from the calcium fluoride-assisted leaching process was slightly increased; the content of sodium and calcium that

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were attributed to the introduced elements remained steady, and the ferric iron and total iron content decreased slightly. Table 3 indicates that there was no V(V) in the raw ore and that V(III) was the dominant valence state. Furthermore, the vanadium (V(III) and V(IV)) in the raw ore occupied the crystal lattice of the aluminosilicate minerals, isomorphismally replacing Al(III) in phlogopite (Zhang et al., 2011; Li et al., 2010; Wang et al., 2008; Perron, 2001). However, the valence state of vanadium in the fluorine-bearing leachate was only V(IV), which was in the form of VO2+ (Zhang et al., 2011). That was to say, the V(III) in vanadium-bearing aluminosilicate minerals was oxidized and went into leahcate. Due to the strong electronegativity of fluorine, it was necessary to investigate the forms of fluorine present in the leachate obtained from calcium fluoride-assisted leaching process. The 19F liquid NMR analysis using trifluoroacetic acid as the internal standard showed that fluorine in the leachate was composed of two forms (Fig. 5). To further investigate these two forms, the XPS of the loaded resin with fluorine confirmed the presence of these two forms in the leachate as shown in Fig. 6. XPS database of chemical bonds (Database of XPS, 2012; Liu et al., 1988) and fluorine NMR database (Fluorine NMR Data, 2012) indicated the presence of [SiF6]2- (Machavaram et al., 6

ACCEPTED MANUSCRIPT 2007) and [AlF5]2- in the leachate. It has been known that the electronegativities of aluminum and silicon are 1.5 and 1.8 respectively (Berry et al., 1983). What is more, the effective radii of Al(3V) and Si(4VI), where the Arabic number represents the oxidation state and the Roman number

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represents the corresponding coordination number, are 0.048 nm and 0.040 nm respectively. Al(3V)

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and Si(4VI) are similar in character. Meanwhile, the bond energies of Al-F and Si-F (664 and 552 kJ/mol, respectively) are stronger than those of Al-O and Si-O (585 and 460 kJ/mol, respectively),

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which demonstrates that Al-F and Si-F are more stable than Al-O and Si-O. The similarity and

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stability suggest that fluorine in the leachate consisted of [SiF6]2- and [AlF5]2-.

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3.2.2 Analyses of residue

The residue was analyzed by XRD and SEM. Fig. 7 shows that the characteristic peaks of phlogopite and chlorite disappeared and that new characteristic peaks of gypsum appeared when

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calcium fluoride was introduced into the leaching system. The peaks of phlogopite declined

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slightly but persisted when the raw ore was leached without calcium fluoride. In addition, it was

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worth noting that the intensity of quartz peaks of the residue from calcium fluoride-assisted vanadium leaching was weaker than that without calcium fluoride. Fig. 8 shows that the morphology of the leaching residue became more fragmented and the gypsum was generated when calcium fluoride was introduced. Therefore, the structures of phlogopite and chlorite were thoroughly disintegrated when calcium fluoride was introduced into the leaching system. In contrast, when the leaching process was carried without the addition of calcium fluoride, only a small fraction of phlogopite and chlorite was dissolved and chlorite was transformed into glauconite. Besides, the increased intensity in the quartz peaks relative to the raw ore can be ascribed to the fact that the phlogopite and chlorite structures disintegrated. However, this increase was less pronounced when the raw ore was leached with calcium fluoride. Thus, a portion of silicon may have entered the leachate, which can be seen in Fig. 4. 3.2.3 Thermodynamics Analyses of the leachate and residue suggest that the leaching process with calcium fluoride can be described as follows: calcium fluoride reacted with sulfuric acid and then generated HF(aq) 7

ACCEPTED MANUSCRIPT and CaSO4; calcite was dissolved into the acid; chlorite and phlogopite were thoroughly disintegrated and subsequently generated SiO2, K+, Mg2+, [SiF6]2-, [AlF5]2- and Al3+; V(III) was oxidized to VO2+ by O2 from the air. The chemical reactions for this process are outlined in

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equations (1) to (10), as shown in Table 4. Herein, equations (6) and (7) are the two ultimate

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reactions generating [AlF5]2- or Al3+ for phlogopite. So are equations (8) and (10) for chlorite. The content of [SiF6]2- remained stable in our analyses because the content of silicon in the leachate

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was very low. When the raw ore was leached without calcium fluoride, the leaching process can be summarized as follows (Equations (11) and (12)): chlorite and phlogopite disintegrated and

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released SiO2, K+, Mg2+ and Al3+.

However, the feasibility of the reactions described in these equations should be theoretically discussed using thermodynamic analyses. Thermodynamics were rarely researched in the fields of

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chemical engineering and hydrometallurgy. Fortunately, Yang et al. (1983) and Lin et al. (1985)

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calculated serial thermodynamic data of ions and minerals (The thermodynamic data of phlogopite and chlorite were derived from Lin et al. (1985), and the other data were obtained from Yang et al.

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(1983)). In this work, the activities of all species involved in the reactions have been considered to be equal to 1. Based on the known G° of phlogopite and chlorite at specific temperatures, functions relating G°(T) and T were fitted by EXCEL. Each of these fitted curves had an R2 greater than 0.99, which indicated that they were usable and reliable. The ∆G° (standard free energy of reaction) can be calculated using the G°(T) (standard free energy) of the substances involved in the chemical reaction (Yang et al., 1983). The functions describing the ∆G°(368K) for the reaction equations are shown in Table 4. Table 4 shows that ∆G°(368K) was negative for all reaction equations except for equation (4). Therefore, all reactions above are spontaneous at 368 K, except for the dissolution of pyrite. It means that the breakage of phlogopite and chlorite can react spontaneously as well in absence of calcium fluoride. However, why did the vanadium leaching efficiency depend so strongly on the presence of calcium fluoride? According to the function ∆G°(T) in Table 5, the ∆G° of equation (11) is always higher than that of equations (6) and (7) for phlogopite dissolution. The ∆G° of equation (12) is always higher than that of equations (8), (9) and (10) for chlorite dissolution. In

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ACCEPTED MANUSCRIPT this regard, calcium fluoride can decrease the ∆G° of the breakage of phlogopite and chlorite because of the generation of [SiF6]2- and [AlF5]2-, which facilitated the vanadium leaching. In order to further confirm the leaching process, the leaching kinetics was applied into the

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analyses. The SEM images of the raw ore (Fig. 9) show most of small particles appeared

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approximately spherical, and a few big particles with irregular morphology belonged to quartz. The main vanadium-bearing mineral was phlogopite, so the effect of the big irregular quartz on

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the application of the SCM (shrinking core model) could be ignored (Zhu et al., 2012). In this work, the effect of external diffusion can be eliminated due to the stirring rate above 250 rpm.

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According to the SCM, leaching kinetics for chemical reaction and internal diffusion control respectively are following:

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1-2α/3-(1-α)2/3=kbt

(14)

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1-(1-α)1/3=kat

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Where: t is reaction time (min); α is leaching efficiency of vanadium (%); ka is a rate constant

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of chemical reaction; ka is a rate constant of internal diffusion.

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The effect of introducing calcium fluoride into the leaching system on the controlling step was investigated. The results were shown in Fig.10. According to Fig.10, the control of chemical reaction fitted the leaching data well for leaching without calcium fluoride. By contrast, internal diffusion was suitable for the leaching process of 0-1 h with calcium fluoride. Therefore, introduction of calcium fluoride transformed vanadium leaching from the control of chemical reaction into the control of internal diffusion in a short time. Calcium fluoride decreased the effect of chemical reaction on vanadium leaching and accelerated the leaching rate of vanadium. Based on the above discussions, the calcium fluoride-assisted leaching process with can be summarized as follows: chlorite and phlogopite were thoroughly disintegrated, and released V, Al, K, Mg, Si etc. The V(III) was oxidized into VO2+ by O2 from the air and was released into the leachate. The Al in the leachate consisted in part of Al3+, while the remaining Al combined with F to generate [AlF5]2-. Similarly, the Si consisted in part of SiO2 (quartz) of the residue, while the remaining Si combined with F to generate [SiF6]2-. The calcium fluoride-assisted leaching process can be described in its entirety by equations (1), (3), (15) and (16). 9

ACCEPTED MANUSCRIPT CaF2 + 2H+ + [SO4]2- = 2HF(aq) + CaSO4↓ CaCO3 + 2H+ + [SO4]2- = CaSO4↓+ CO2↑+ H2O

(1) (3)

KMg3 (V, Al)Si3O10(OH)2+HF(aq)+H++O2→ (15)

Mg5Al2Si3O10(OH)8+HF(aq)+H+ → SiO2↓+Mg2++[SiF6]2-+[AlF5]2-+Al3++H2O

(16)

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SiO2↓+VO2++K++Mg2++[SiF6]2-+[AlF5]2-+Al3++H2O

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The mechanism of calcium fluoride enhancing vanadium leaching from stone coal is that fluorine combined with aluminum and silicon in the lattice of vanadium-bearing phlogopite to

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generate [SiF6]2- and [AlF5]2- respectively; this generation decreased the ∆G° of phlogopite disintegration, declined the effect of chemical reaction on vanadium leaching, accelerated the

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leaching rate of vanadium and boosted the leaching efficiency of vanadium.

4. Conclusion

The whole calcium fluoride-assisted vanadium leaching process from stone coal was

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(1)

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From the results of this study, the following conclusions can be drawn:

that calcium fluoride reacted with sulfuric acid and then generated HF(aq); calcite

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was dissolved into the acid; chlorite and phlogopite were thoroughly disintegrated and subsequently generated quartz, K+, Mg2+, [SiF6]2-, [AlF5]2- and Al3+; the released V(III) was oxidized to VO2+ by O2 from the air.

(2)

The mechanism of calcium fluoride enhancing vanadium leaching was that fluorine combined with aluminum and silicon in the lattice of vanadium-bearing phlogopite to generate [SiF6]2- and [AlF5]2- respectively. It is the generation of [SiF6]2- and [AlF5]2- that decreased the ∆G° of phlogopite disintegration, declined the effect of chemical reaction on vanadium leaching, accelerated the leaching rate of vanadium and boosted the leaching efficiency of vanadium.

Acknowledgments This study was supported by the key science and technology support program (2011BAB05B01) from the Ministry of Science and Technology of China and the special program (51344001) from the Committee of National Natural Science Foundation of China. The authors express sincere gratitude to Professor D.W. Feng of the University of Melbourne. 10

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acid. Chem. Eng. J. 131, 41-48.

Industry Press, Beijing, pp. 326-329.

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Zhang, H.B., 1992. Chemical phase analyses of ores and industry products, first ed. Metallurgical

Zhang, X.Y., Yang, K., Tian, X.D., et al., 2011. Vanadium leaching from carbonaceous shale using

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fluosilicic acid. Int. J. Miner. Process. 100, 184-187.

Zhang Y.M., Bao S.X., Liu T., et al., 2011. The technology of extracting vanadium from stone coal

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in China: history, current status and future prospects. Hydrometallurgy 109, 116-124.

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Zhou, X.W., Wei, C., Li, M.T., et al., 2011. Thermodynamics of vanadium-sulfur-water systems at 298K. Hydrometallurgy 106, 104-112.

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Zhou, X.Y., Li, C.L., Li, J., et al., 2009. Leaching of vanadium from carbonaceous shale. Hydrometallurgy 99, 97-99. Zhu, X.B., Zhang, Y.M., Huang, J., et al., 2012. A kinetics study of multi-stage counter-current circulation acid leaching of vanadium from stone coal. Int. J. Miner. Process. 114-117, 1-6.

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Table 1 The composition of the raw ore.

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Table 2 The valence state of vanadium in the raw ore.

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Table 3 ∆G°(368 K) of reaction equations (1) to (12).

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ACCEPTED MANUSCRIPT Table 1 The composition of the raw ore. V

TFe

K

Ca

Mg

Ba

3.39

2.18

1.41

0.31

0.90

0.043

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Content/ wt.% 0.57

Al

15

Si

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Element

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C

S

16.16

2.58

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Content/ %

64.84

35.16

V(V)

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V(III)

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Valence state

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ACCEPTED MANUSCRIPT Table 3 ∆G°(368 K) of reaction equations (1) to (12). ∆°G(368 K)/kJ·mol-1

Reaction equations CaF2 + 2H++[SO4]2- = 2HF(aq) + CaSO4↓

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Al3+ + 5F- = [AlF5]2-

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CaCO3 + 2H+ + [SO4]2- = CaSO4↓ + CO2↑+ H2O

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FeS2 + 2H+ = H2S↑ + S↓ + Fe2+

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2V2O3 + 8H+ + O2 = 4VO2+ + 4H2O

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2SiO2↓+K++3Mg2++[SiF6]2-+[AlF5]2-+8H2O

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KMg3AlSi3O10(OH)2 +6HF(aq)+8H+ = 7

-16.7462

-148.8819

KMg3AlSi3O10(OH)2+11HF(aq)+3H+ = 6

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1

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No.

2SiO2↓+K++3Mg2++[SiF6]2-+Al3++8H2O

-101.9596 +44.2782 -396.6075 -869.2576

-837.4097

2SiO2↓+5Mg2++[SiF6]2-+2[AlF5]2-+14H2O

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Mg5Al2Si3O10(OH)8+16HF(aq)+4H+ =

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Mg5Al2Si3O10(OH)8+11HF(aq)+9H+ = 2SiO2↓+5Mg2++[SiF6]2-+[AlF5]2-+Al3++14H2O

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-1374.2857

Mg5Al2Si3O10(OH)8+6HF(aq)+14H+ = 10

-1342.4378

2SiO2↓+5Mg2++[SiF6]2-+2Al3++14H2O

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KMg3AlSi3O10(OH)2+10H+ = 3SiO2↓+K++3Mg2++Al3++6H2O

-823.9602

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Mg5Al2Si3O10(OH)8+16H+ = 3SiO2↓+5Mg2++2Al3++12H2O

-1328.9756

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ACCEPTED MANUSCRIPT Figure Captions

Fig. 1. XRD patterns of the raw ore.

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Fig. 2. Effects of sulfuric acid concentration on the vanadium leaching efficiency and a

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comparison of calcium fluoride-assisted and unassisted leaching at liquid/solid ratio of 1 mL/g, and 368±1 K for 4 h.

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Fig. 3. Effects of leaching time on the vanadium leaching efficiency and a comparison of calcium fluoride-assisted and unassisted leaching at 15% (v/v) H2SO4, liquid/solid ratio of 1 mL/g, and

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368±1 K.

Fig. 4. Leachate composition changes in the presence and absence of calcium fluoride. 19

F liquid NMR spectra of the leachate obtained from the calcium fluoride-assisted

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Fig. 5.

vanadium leaching process (trifluoroacetic acid as internal standard).

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Fig. 6. XPS spectra of loaded resin (on 201×7 resin) and 201×7 resin.

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Fig. 7. XRD patterns of the residue obtained from the vanadium leaching process a) in the presence of calcium fluoride and b) in the absence of calcium fluoride.

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Fig. 8. SEM images of the residue obtained from the vanadium leaching process left) in the presence of calcium fluoride and right) in the absence of calcium fluoride. Fig. 9. The SEM images of the raw ore. Fig. 10. (a) Plot of 1-(1-α)1/3 versus time. (b) Plot of 1-2α/3-(1-α)2/3 versus time.

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Fig. 1. XRD patterns of the raw ore.

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Fig. 2. Effects of sulfuric acid concentration on the vanadium leaching efficiency and a comparison of calcium fluoride-assisted and unassisted leaching at liquid/solid ratio of 1 mL/g,

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and 368±1 K for 4 h.

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Fig. 3. Effects of leaching time on the vanadium leaching efficiency and a comparison of calcium fluoride-assisted and unassisted leaching at 15% (v/v) H2SO4, liquid/solid ratio of 1 mL/g, and

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368±1 K.

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Fig. 4. Leachate composition changes in the presence and absence of calcium fluoride.

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F liquid NMR spectra of the leachate obtained from the calcium fluoride-assisted

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Fig. 5.

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vanadium leaching process (trifluoroacetic acid as internal standard).

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Fig. 6. XPS spectra of loaded resin (on 201×7 resin) and 201×7 resin.

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Fig. 7. XRD patterns of the residue obtained from the vanadium leaching process a) in the

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Fig. 8. SEM images of the residue obtained from the vanadium leaching process left) in the

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presence of calcium fluoride and right) in the absence of calcium fluoride.

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Fig. 9. The SEM images of the raw ore.

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Fig. 10. (a) Plot of 1-(1-α)1/3 versus time. (b) Plot of 1-2α/3-(1-α)2/3 versus time.

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Graphical abstract

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Presentation of the whole process of vanadium leaching from stone coal.




Essence of the mechanism of CaF2 in enhancing vanadium leaching.




Fluorine existed in acid leachate in the forms of [AlF5]2- and [SiF6]2-.




Introducing CaF2 decreased the ∆G° of vanadium-bearing phlogopite disintegration.




CaF2 declined the effect of the control of chemical reaction on vanadium leaching.

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