Separation of impurities aluminum and iron during pressure acid leaching of vanadium from stone coal

Accepted Manuscript Separation of impurities aluminum and iron during pressure acid leaching of vanadium from stone coal

Nan-nan Xue, Yi-min Zhang, Jing Huang, Tao Liu, Lu-yao Wang PII:

S0959-6526(17)31870-X

DOI:

10.1016/j.jclepro.2017.08.144

Reference:

JCLP 10403

To appear in:

Journal of Cleaner Production

Received Date:

03 April 2017

Revised Date:

16 August 2017

Accepted Date:

17 August 2017

Please cite this article as: Nan-nan Xue, Yi-min Zhang, Jing Huang, Tao Liu, Lu-yao Wang, Separation of impurities aluminum and iron during pressure acid leaching of vanadium from stone coal, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.08.144

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ACCEPTED MANUSCRIPT

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Separation of impurities aluminum and iron during pressure acid leaching of vanadium from stone coal Nan-nan Xue a,b, Yi-min Zhang a,b,c,*, Jing Huang a,b,c, Tao Liu a,b,c, Lu-yao Wang a,c a b

School of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081,China; Hubei Provincial Engineering Technology Research Center of High efficient Cleaning Utilization for Shale Vanadium

Resource, Wuhan 430081,China; c

Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan 430081,China

Abstract During the extraction of vanadium from stone coal by pressure acid leaching, a large amount of aluminum and iron are dissolved along with vanadium, which increases the difficulties of purification and enrichment of the leaching solution. In this study, the separation of aluminum and iron from vanadium has been investigated. It was found that the leaching efficiencies of vanadium, aluminum, and iron reached 90.20%, 30.30%, and 5.73%, respectively, after leaching for 5 h at 190 °C under 2.0 MPa partial pressure of O2 with 15 vol% sulfuric acid solution and 7 wt% potassium sulfate. Potassium sulfate assisted the growth of alunite crystals in the holes and cracks of muscovite particles that had formed since calcium sulfate crystals debonded after interfacial growth. This process strengthened the leaching of vanadium and aluminum from muscovite. Owing to a boost of local concentration of aluminum ions on the addition of potassium sulfate, the precipitation of aluminum ions increased. Meanwhile, ferric ions were precipitated as stable yavapaiite crystals. Thus, an effective separation of aluminum and iron from vanadium could be realized from their mineral source. Keywords: Stone coal; Pressure acid leaching; Separation; Iron; Vanadium; Aluminum Highlight: 1. V is efficiently separated from Fe and Al during acid leaching of stone coal. 2. Al3+ local concentration in cracks and holes is increased by CaSO4 debonding. 3. Al3+ and Fe3+ respectively precipitates as alunite and yavapaiite at 190°C.

1. Introduction Stone coal is an important vanadium-bearing resource that has attracted significant attention in recent years (Zhang et al., 2011; Yang et al., 2016). The primary vanadium-bearing minerals in stone coal are muscovite and illite, while the gangue minerals are mainly calcite, pyrite, and quartz. Thus, stone coal is regarded as a complex ore. Compared with vanadium slag, stone coal has a lower Fe and higher Al content. Vanadium mainly hosts in the crystal lattice of muscovite or illite, where V(III) as isomorphism replaces Al(III) (Wang et al., 2014; Zeng et al., 2015; Hu et al., 2017). Therefore, vanadium can only be obtained from the structural breakdown of muscovite or illite in stone coal. *

Corresponding author：Yi-min Zhang. E-mail addresses: [email protected].

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The pressure acid leaching process has been a popular research topic in recent years because of its high recovery of valuable metals from diverse minerals (Yang et al., 2014; Ma et al., 2015; Fleuriault et al., 2016; Zhukov et al., 2017). It involves the use of a pressure field to increase the temperature and shorten the leaching time (Dorfling et al., 2011). While processing Egyptian stone coal by oxygen pressure acid leaching, Amer (1994) found that leaching under oxygen pressure promoted the oxidation of Fe2+, which favored the precipitation of iron ions as basic iron sulfate. Meanwhile, the hydrolyzation of Al could be initiated more easily than that of V above 150 °C. It has been previously reported that the pressure acid leaching process of stone coal generates alunite and iron sulfate compounds (Huang et al., 2016; Xue et al., 2016). Hence, this technique can be used to separate Al and Fe from V. During conventional pressure acid leaching, solid residual layers of high Si are formed after the leaching of vanadium from muscovite. These solid residual layers prevent the diffusion of hydrogen ions into unreacted cores, resulting in a slow chemical reaction rate and high acid consumption and thus a longer leaching time. In the published reports on vanadium-enhanced extraction from stone coal, a method to adopt potassium sulfate as an auxiliary agent has been proposed for enhancing muscovite dissolution during oxygen pressure acid leaching of stone coal (Xue et al., 2017). It was found that a strong strain concentration was produced in muscovite particles that increased the holes and cracks in the mineral through solid-phase transformation of CaSO4→CaSO4·2H2O→CaSO4 on the muscovite surface. The increasing concentration of holes and cracks expanded the reaction interfaces between muscovite and sulfuric acid solution, thus increasing the rate of leaching of vanadium. However, using this technique, the other main metallic elements in stone coal such as Al and Fe are also leached along with vanadium. The dissolution of Al and Fe hinders the follow-up solvent extraction and precipitation of vanadium. The recovery and separation of vanadium from leaching solutions that contain massive impurities is difficult. Owing to vanadium hydrolysis in the pH range of 4–5, precipitation of Al and Fe under these conditions resulted in a substantial loss of V (Cai et al., 2013). Several researchers have focused on the separation of V from Fe and Al by solvent extraction (Li et al., 2012; Hu et al., 2014; Liang et al., 2016). Bis(2-ethylhexyl)phosphoric acid (P204) is the most widely applied solvent in vanadium extraction because of its acidic adaptability and ease of stripping. While investigating the effect of different metal ions on the solvent extraction of V(IV) with P204, the increasing concentrations of Fe(II), Fe(III) and Al(III) ions greatly reduced the amount of vanadium that could be extracted (Li et al., 2013; Liu et al., 2017). Furthermore, the extraction ratio of V(IV) was not influenced by an increase in the concentrations of K, Na, and Mg ions. Therefore, this extractant presents the disadvantage of poor selectivity on Al and Fe ions (Li et al., 2012), which necessitates the separation of V from Al and Fe via the pressure acid leaching process. The leaching characteristics of Fe and Al under K2SO4 assistance may provide important guidance for the removal of Al and Fe from V. To date, such a system has not been investigated. Therefore, in this work, the effect of the main processing parameters on the leaching of Fe and Al, leaching kinetics of V in the direct precipitation of Al and Fe, and the separation mechanism of Al and Fe were studied based on the realization that the effective separation of Al and Fe is synchronous with the leaching of V.

2. Experimental

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2.1. Materials The stone coal used in this study was obtained from Tengda Mining Co. Ltd. in Tongshan county, Hubei province, China. The inductively coupled plasma atomic emission spectroscopy (ICP-AES; IRIS Advantage Radial, USA) results and X-ray diffraction (XRD; Bruker D8 Advance, Germany) pattern of the raw ore are given in Table 1 and Fig. 1. It was found that V, Al, and Fe accounted for 0.43%, 4.88%, and 3.36% of the total mass, respectively. According to Fig. 1, the major minerals in stone coal were quartz, muscovite/illite, feldspar, pyrite, and calcite. The muscovite surface was intact and compact in raw stone coal. The Fe/S molar ratio was calculated to be 0.55 from Table 1, which suggested that pyrite was the primary source of Fe. Table 1 Main chemical compositions of the stone coal by ICP-AES (wt.%) V

Si

Al

Fe

Ca

K

Mg

Na

Ba

Ti

S

C

0.43

33.62

4.88

3.36

4.16

2.03

1.20

0.81

0.67

0.25

3.60

10.30

Table 2 Chemical phase of vanadium in stone coal Total V

V in free oxide

V in silicate minerals

V in organic matter

100

5.61

85.14

9.25

Table 3 EPMA results of the major minerals in stone coal (wt.%) Mineral

V2O3

SiO2

Al2O3

FeO

MgO

CaO

Na2O

K2O

Muscovite

3.48

51.08

27.22

0.23

4.53

0.02

0.07

9.52

Illite

4.14

39.95

23.25

0.14

2.02

0.15

0.08

8.55

Albite

0.09

49.43

6.92

0.08

0.40

0.21

1.97

0.99

The chemical phases of vanadium in stone coal were analyzed by potentiometric titration (Zhang 1992) and are summarized in Table 2. The chemical composition of the silicate minerals in stone coal was analyzed by an electron probe (X-ACT, Oxford, UK), and these results are given in Table 3. According to Table 3 and Table 4, 80.64% of vanadium existed in muscovite or illite, which suggested that muscovite was the primary source of V.

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ACCEPTED MANUSCRIPT Fig. 1 The XRD pattern of the stone coal

2.2 Experimental All experiments were carried out in a pressurized zirconium reactor of 2 L volume and conducted by using a programmable temperature controller with a deviation of ±3 °C. The test samples were obtained after being crushed and ground to a state in which the size of minus 0.074 mm accounted for more than 85% of the sample. The sample, K2SO4, and the prepared H2SO4 solution were then loaded into the reactor and mixed adequately through mechanical stirring at 350 rpm. The oxygen used in the experiments was introduced into the reactor when the temperature reached a predetermined value. The purity of the oxygen used was 98%. After reaction, the leaching residues and the leachate were isolated via solid-liquid separation. The morphology of the leaching residues was observed by using a scanning electron microscope (JSMIT300, Jeol, Japan). The phase changes in the leaching residues were examined by an X-ray diffractometer (Bruker D8 Advance, Germany) and X-ray photoelectron spectroscopy (Multilab 2000, VG, UK). The bonding structures of muscovite in the leaching residues were studied by a Fourier infrared (FTIR) spectrometer (Nexus, Thermo Nicolet, USA). The concentrations of Fe and Al were determined by spectrophotometry and ICP-AES, respectively. The concentration of vanadium was analyzed by titration with ferrous ammonium sulfate. The leaching efficiency of a metal element was calculated by using Eq. 1.

i 

Ci  L  100%  M

(1)

where ηi, α, Ci, L, and M refer to the leaching efficiency of a metal element (%), grade of a metal element in the raw ore (%), concentration of the metal ion in the leachate (g L−1), leachate volume (L), and feeding mass of the raw ore (g), respectively. The separation coefficient of Al or Fe from V is calculated by Eq. 2 (Seader, 2002).

Si / V =

i v

(2)

where Si/V, ηi, and ηv refer to the separation coefficient of Al or Fe, leaching efficiency of Al or Fe (%), and leaching efficiency of V (%), respectively.

3. Results and discussion 3.1. Leaching of Al, Fe, and V The effects of leaching temperature on the leaching efficiencies of Al, Fe, and V are presented in Fig. 2. The Fe leaching efficiency shows an uptrend below 150 °C. As the leaching temperature increases over 150 °C, the leaching of Fe becomes less efficient. The leaching efficiency of Al shows a similar trend with variation in temperature as that of Fe. From these results, it was speculated that the precipitation of Al and Fe was initiated over 150 °C and strengthened as the temperature increased further. However, the V leaching efficiency first increased quickly and then leveled off below 190 °C. When the temperature exceeded 190 °C, the V leaching efficiency started to decrease. Therefore, the leaching temperature for isolating V was recommended as 190 °C, at which an obvious separation of Al and Fe from V would already have taken place.

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Fig.2 Effects of leaching temperature on leaching efficiencies of V, Al, and Fe under process conditions consisting of 5 h, 7% K2SO4, 20%H2SO4 and 1.5 MPa O2

The influence of O2 partial pressure on the leaching efficiencies of Al, Fe, and V is shown in Fig. 3. It was found that the leaching efficiency of V first increased from 89.04% to 95.34% under an O2 partial pressure of 1.5 MPa and then leveled off. Similarly, the Al leaching efficiency also increased under an O2 partial pressure of 1.5 MPa and then started to decline. In contrast, the Fe leaching efficiency showed a downtrend with increasing O2 partial pressure. The dissolved oxygen concentration in solution can be improved by increasing the O2 partial pressure, which contributes to the oxidation of Fe2+ to Fe3+ and V3+ to V4+. Thus, an increasing O2 partial pressure favors the precipitation of Fe3+ and the leaching of V. The O2 partial pressure of 2.0 MPa was chosen as the optimum condition.

Fig.3 Effect of O2 partial pressure on leaching efficiencies of V, Al, and Fe under process conditions consisting of 5 h, 7% K2SO4, 20%H2SO4 and 190°C

The effect of varying acid concentration on the leaching efficiencies of Al, Fe, and V is illustrated in Fig. 4. It was found that the increase in the V leaching efficiency was initially quick and then followed a slow downward trend. The Al leaching efficiency increased slowly at acid concentrations less than 15 vol%

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but once it exceeded this value, the leaching efficiency grew rapidly. Therefore, it was concluded that V and Al cannot be separated under highly acidic conditions. The Fe leaching efficiency first declined and then increased with increasing acid concentration. Hence, it was not feasible to separate V and Fe from their highly acidic solutions. As the Fe leaching efficiency showed a minimum at the acid concentration of 15 vol%, this was chosen as the optimal concentration for separation.

Fig. 4 Effect of sulfuric acid concentration on leaching efficiencies of V, Al, and Fe under process conditions consisting of 5 h, 7% K2SO4, 190°C, and 2.0 MPa O2

The influence of increasing K2SO4 dosage on the leaching efficiencies of Al, Fe, and V are shown in Fig. 5. As the dosage of K2SO4 increased, the V leaching efficiency first increased and then decreased, indicating that muscovite dissolution was first increased and then decreased. The Al leaching efficiency showed a similar trend as that of V with increasing amounts of added K2SO4. Meanwhile, Al3+ precipitated at 190 °C, which further reduced the Al leaching efficiency. In contrast, the Fe leaching efficiency declined below K2SO4 dosage of 7% and then rapidly increased. As an excessive amount of K2SO4 was adverse for Fe3+ precipitation, the optimal K2SO4 dosage was chosen as 7%.

Fig. 5 Effect of K2SO4 dosage on leaching efficiencies of V, Al, and Fe under process conditions consisting of 5 h,

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ACCEPTED MANUSCRIPT 20%H2SO4, 190°C, and 2.0 MPa O2

The effect of leaching time on the leaching efficiencies of Al, Fe, and V with and without K2SO4 assistance is presented in Fig. 6. The V leaching efficiency under K2SO4 assistance was higher than that without any assistance. The Al leaching efficiency first increased for two hours and then decreased to the stable value (30.60%) in 1 h in the presence of K2SO4. However, the Al leaching efficiency increased with the passage of time without assistance. The Fe leaching efficiency first increased for three hours and then decreased to the stable value (5.76%) in another two hours assisted by K2SO4. However, it is worth noting that the Fe leaching efficiency without assistance from K2SO4 increased gradually over time. Therefore, an optimal leaching time of 5 h was selected. Based on these experimental results, the optimal process conditions were 7 wt% dosage of K2SO4, leaching temperature of 190 °C, acid concentration of 15 vol%, and oxygen partial pressure as 2.0 MPa for a total leaching time of 5 h. The leaching efficiencies of Al, Fe, and V were 30.30%, 5.76%, and 90.20%, respectively. According to Eq. 2, the separation factors SAl/V and SFe/V were 0.336 and 0.064 with K2SO4 assistance and 0.820 and 0.218 with no assistance, respectively. The smaller the value of the separation factor, the better is the separating effect. Therefore, the addition of K2SO4 enabled a good separation of Al and Fe from V.

Fig. 6 Effect of leaching time on leaching efficiencies of V, Al, and Fe with and without K2SO4 assisting under conditions of 190°C, 20%H2SO4 and 2.0 MPa O2

3.2 Mineral phases of Al and Fe in residues The leaching residues obtained by following the conditions outlined in Table 4 were taken as study objects to analyze the variation of mineral phases of Al and Fe.

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ACCEPTED MANUSCRIPT Table 4 Experimental conditions for obtaining the leaching residues. Leaching conditions

Leaching residue

7% K2SO4, 190 °C，20% H2SO4, 2.0 MPa O2, 5 h

KP190

7% K2SO4, 150 °C，20% H2SO4, 2.0 MPa O2, 5 h

KP150

190 °C，20% H2SO4, 2.0 MPa O2, 5 h

BP190

150 °C，20% H2SO4, 2.0 MPa O2, 5 h

BP150

Fig. 7 XRD patterns of leaching residues: (a) KP190, (b) KP150, (c) BP190, (d) BP150

The mineral phases of the leaching residues were first analyzed by XRD and these results are shown in Fig. 7. Under no assistance from K2SO4, the diffraction peaks of muscovite disappeared and while those of alunite (KAl3(SO4)2(OH)6) appeared as the temperature increased from 150 °C to 190 °C. Although the diffraction peaks of pyrite disappeared, no new Fe-bearing mineral phase was found. The iron leaching efficiency was 54.57% at 150 °C and 19.77% at 190 °C without K2SO4, which indicated that Fe3+ could be precipitated during unassisted pressure acid leaching. Under K2SO4 assistance, the diffraction peaks of muscovite disappeared at 150 °C and those corresponding to alunite appeared over 190 °C. Meanwhile, the diffraction peaks of yavapaiite (KFe(SO4)2) also appeared at 190 °C. The equilibrium composition of muscovite-K2SO4-H2O system is calculated by the factsage software and the results at different temperatures are shown in Fig.8. The stable components are Al3+ at 150 °C, KAl3(SO4)2(OH)6 and Al3+ at 190 °C, which also proved the formation of KAl3(SO4)2(OH)6 at 190 °C. KAl3(SO4)2(OH)6 is the hydrolyzate of Al3+ formed at high temperatures, which can be expressed as Eq. 3 (Acero et al., 2015; Luo et al., 2016). K   3Al3  2SO 4 2   6H 2 O  KAl3  SO 4 2  OH 6  6H 

(3)

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Fig. 8 Equilibrium compositions of muscovite-K2SO4-H2O system

Subsequently, the chemical states of Fe2p in KP190 and BP190 were studied by XPS and the results are shown in Fig. 9. The Fe(III) valence state of Fe2O3 is characterized by an asymmetric 2p3/2 peak and by the satellite features at higher binding energy. The binding energy of the spin-orbital split peaks Fe 2p3/2 and 2p1/2 is 711.0 and 724.4 eV, respectively (Gabriela et al., 2017). The Fe(II) valence state of FeS2 is characterized by a Fe2p3/2 peak with a binding energy of about 708 eV (Eggleston et al., 1996). Without K2SO4 assistance, the Fe2p fine spectrum was split into four peaks. The peak with a binding energy of 707.24 eV belonged to FeS2 while those with binding energies of 711.35 eV and 723.34 eV indicated that Fe in BP190 existed as Fe2O3, with the exception of FeS2. If Fe3+ hydrolyzes, Fe2O3 is usually formed as the precipitate at high temperatures. However, with increasing acidity, a large amount of a substance such as Fex(OH)y(SO4)z is formed in the hydrolysis equilibrium phase of the sulfuric acid solution, which leads to poor results in the iron-removal processes (Yue et al., 2012; Yang et al., 2014). The aqueous pH value is usually less than zero during acid leaching. Fe3+ hydrolysis also decreases the aqueous pH value, which gives rise to a further increase in the amount of Fex(OH)y(SO4)z. Therefore, it is difficult to separate Fe from V under highly acidic conditions via the formation of Fe2O3. Under K2SO4 assistance, the Fe2p fine spectrum was split into five peaks. The Fe2p peak with the binding energy of 707.96 eV belonged to FeS2 while the other Fe2p peaks with binding energies of 711.01 eV and 725.25 eV belonged to Fe2O3. A new Fe2p peak with a binding energy of 713.80 eV was also observed, which indicated the chemical state of the Fe(III)-SO4 bond (Khoshkhoo et al., 2014). Based on XRD analysis, it was determined that this chemical state corresponded to KFe(SO4)2. The final Fe-bearing precipitates in the leaching residue were identified as KFe(SO4)2 and Fe2O3. Therefore, the intervention of K2SO4 made it possible to precipitate Fe3+ ions as stable KFe(SO4)2 crystals, circumventing the formation of Fex(OH)y(SO4)z in sulfuric acid leaching solutions.

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Fig. 9 XPS fine spectrums of Fe2p in (a) BP190 and (b) KP190

3.3 Bonding structure of muscovite in residues The bonding structures of muscovite in KP190 and BP190 were compared by FTIR analysis, and the results are shown in Fig. 10.

Fig. 10 FTIR spectrum patterns of (a) raw stone coal, (b) BP190 and (c) KP190.

While the Al2OH vibration in the muscovite structure was located at 3615 cm−1, the band at 1020 cm−1 with shoulder bands at 1164 and 911 cm−1 on each side and those at 694 and 713 cm−1 belonged to the Si– O–AlIV stretching vibrations in the muscovite structure (Wen et al., 1988). The band at 3615 cm−1 disappeared when K2SO4 assisted the breakdown of muscovite structure. This band shifted to 3604 cm−1 and weakened in the absence of K2SO4.. So the hydroxyl groups in the muscovite structure were eliminated under K2SO4 assistance. The disappearance of the bands at 1020 cm−1, 713 cm−1, and 694 cm−1 under K2SO4 assistance indicated complete dissolution of AlIV in the tetrahedron while the band at 694 cm−1 was still preserved under no assistance. At acidic pH, the dissolution rate of muscovite appeared to be controlled by the breaking of tetrahedral Si–O bonds after the adjoining tetrahedral Al had been removed by a proton exchange reaction (Crundwell et al., 2014). However, as a result of the heterogeneous dissolution of muscovite, Si could not be dissolved out. Thus, the dissolution rate of muscovite was determined by the

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breaking of AlIV–O bond. Under K2SO4 assistance, the disappearance of Si–O–AlIV stretching vibration in the muscovite structure indicated that the original structure no longer existed after leaching started with the assistance of K2SO4. 3.4 Morphology of precipitates of Al and Fe The morphology of KP190 and BP190 was studied by scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) and the results are shown in Fig. 11 and Fig. 12. Alunite crystals grew as aggregates and showed good crystallinity and cubical shape. These aggregates of alunite were embedded in the muscovite particles, as shown in Fig. 11b. A few calcium sulfate crystals also grew in the muscovite particles (Fig. 11c). A lot of fragments that originated from muscovite also came into contact with the alunite and calcium sulfate crystals. Fig. 12 shows that the alunite crystals also grew into the muscovite particle surface without assistance from K2SO4, although the surface generally kept intact. Assisted by K2SO4, the muscovite particle surface crumbled because of the growth of alunite and calcium sulfate crystals. Under these conditions, the morphology of KP150 was observed by SEM-EDS, and the results are shown in Fig. 13. Compared with the surface appearance of muscovite in KP190, a lot of calcium sulfate crystals grew into the muscovite particle instead of alunite. In this manner, the muscovite particle surface was in a broken state in the presence of K2SO4 because of the interfacial growth of calcium sulfate crystals. These observations suggested that the higher temperature resulted in the interfacial debonding of calcium sulfate crystals.

b

a

B

A

c

A

B C C Fig. 11 SEM-EDS images of KP190

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A

B

A

B

Fig. 12 SEM-EDS images of BP190

b

a

B A

c A C B

C Fig. 13 SEM-EDS images of KP150

Based on a previous study (Xue et al., 2017), the surface solid-phase transformation of calcium sulfate dihydrate (CSD) to calcium sulfate anhydrous (CSA), CaSO4·2H2O → CaSO4 induces CSA growth on the muscovite surface under assistance from K2SO4, which was validated by the results shown in Fig. 13. The sedimentary process of alunite deposition in the holes and cracks of a muscovite particle is shown in Fig. 14. After the interfacial debonding of CSA, the holes and cracks remained in the muscovite particles. The H+ ions would enter these holes and cracks and react with fresh surfaces. Thus, the local Al3+ concentration exploded and then precipitated as alunite within the holes and cracks. Alunite crystals grew and aggregated

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over time, deforming and expanding the holes and cracks in the muscovite particle.

Fig. 14 Sketch of sedimentary process of alunite deposition in muscovite particle

3.5 Kinetics of vanadium leaching with and without the assistance of K2SO4 The shrinking core model (SCM) considers that the leaching rate is controlled by either the diffusion of the reactant through solid residual layers, or the interfacial chemical reaction (Zhang et al., 2015a; Zhang et al., 2015b; Zhang et al., 2016). The pressure acid leaching process seems to conform to the SCM because Si is not leached and is instead left behind in the muscovite structure. However, the boundary condition of SCM demands that the linear-fitting curve should pass through the origin. In practice, the test sample must experience a temperature-rising stage where the leaching of V has already been taking place for a certain time. When considering the starting time of the leaching process, it is evident that the leaching efficiency is not zero, and thus, the linear fitting curve cannot pass through the origin. Therefore, the leaching efficiency (α1) calculated for the holding time of 0 h (t1) acts as the boundary condition. The kinetic equations of the SCM should be revised on boundary condition, which results in the following equations after mathematical integration. Assuming that the pressure acid leaching of stone coal conforms to the SCM, when the leaching rate is controlled by interior diffusion, Eq. 4 can be used to describe the leaching kinetics of the process (ArabiKarasgani et al., 2010; Ke et al., 2016).  1  k D  t  t1   1  3    1  1 

23



 2 1  1  1 

2 3

   1  

(4)

When the leaching rate is controlled by the interfacial chemical reaction, Eq. 5 is employed to describe the leaching kinetics (Arabi-Karasgani et al., 2010; Ke et al., 2016). 13

 1  kC  t  t1   1     1  1 

(5)

When the leaching rate is under mixed control, Eq. 6 may be used instead (Arabi-Karasgani et al., 2010; Ke et al., 2016).  1   1  13 k  t  t1   1 3 1  1  ln   1   1    1   1  1 

1 3

(6)

In these equations, t, α, kD, kc, and k refer to the reaction time (s), vanadium recovery (%), and rate

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constants of internal diffusion, chemical reaction, and mixed control, respectively. The experimental leaching data at different temperatures was analyzed by using these three kinetic equations of the shrinking core model. The results of this analysis are shown in Fig. 15 and Table 5. In Fig. 15, the linear fitting curves of Eq. 6 all pass though the origin at different temperatures. The correlation coefficients from Eq. 6 were higher than those determined from Eq. 4 and Eq. 5, as summarized in Table 5. This suggests that the leaching of V may be controlled by both interior diffusion and interfacial chemical reaction. The apparent rate constants from Table 5 can be substituted into the Arrhenius equation, ln k = ln A − Ea/RT to construct the Arrhenius plot. The activation energy (Ea) of leaching of V is determined from the slope of this plot. In general, the activation energy is less than 12 kJ mol−1 when leaching takes place under control of interior diffusion, more than 40 kJ mol−1 under the control of the interfacial chemical reaction, and 12–40 kJ mol−1 under mixed control (Zhang et al., 2016). Accordingly, the calculated values of Ea under control of interior diffusion and of interfacial chemical reaction were inconsistent with the abovementioned reported values. However, the calculated values of Ea under mixed control were in the range of 12–40 kJ mol−1 (see Fig. 16), which suggested that the leaching rate of V was controlled by both interior diffusion and interfacial chemical reaction. The value of Ea decreased from 19.38 to 13.12 kJ mol−1 under K2SO4 assistance, which indicated that the leaching rate of V was higher with K2SO4 assistance than without. The resistance of interior diffusion and interfacial chemical reaction decreased in the presence of K2SO4. Therefore, the sedimentary process of alunite deposition made more reaction interfaces exposed and accelerated the interfacial reaction rate of H+ with muscovite. Meanwhile, this sedimentary process broke the blocking caused by the solid residual layers, which further decreased the mass transfer resistance of H+. So, the leaching rate of V accelerated with assistance from K2SO4. The leaching rate of Al also increased because V replaced Al in muscovite structure as a result of their isomorphism. The strong release of Al caused a big boost in the Al3+ local concentration, moving the equilibriums of Eq. 3 towards the right side. Meanwhile, the addition of K2SO4 implies that more K+ is available for the transformation outlined in Eq. 3, which results in an accelerated formation of alunite. Therefore, the precipitation of Al3+ increased assisted by K2SO4. Table 5 Apparent rate constants (k) and correlation coefficients (R2) from linear fitting results of Eq. 4-Eq. 6 Temperatur e

Control by interior diffusion No assistance

K2SO4 assistance

Control by chemical reaction No assistance

K2SO4 assistance

Mixed control No assistance

K2SO4 assistance

(°C)

kD

R2

kD

R2

kC

R2

kC

R2

k

R2

k

R2

130

0.084

0.973

0.043

0.969

0.035

0.941

0.043

0.965

0.010

0.995

0.021

0.996

150

0.108

0.958

0.064

0.985

0.032

0.981

0.039

0.959

0.013

0.992

0.027

0.997

170

0.114

0.981

0.073

0.980

0.032

0.961

0.050

0.977

0.017

0.996

0.030

0.998

190

0.122

0.976

0.079

0.948

0.054

0.918

0.056

0.972

0.021

0.997

0.035

0.997

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Fig. 15 Linear-fitting curves of (a) Eq.4, (b) Eq.5 and (c) Eq.6 at different temperatures

Fig. 16 Linear fitting results of Arrhenius equation under mixed control

4. Conclusions In this work, an effective separation of Al and Fe from V was realized from the mineral source. Under optimized process conditions consisting of K2SO4 dosage of 7 wt.%, leaching temperature as 190 °C, acid concentration of 15 vol.%, oxygen partial pressure as 2.0 MPa, and leaching time of 5 h, the leaching efficiencies of V, Al, and Fe were 90.20%, 30.30%, and 5.73%, respectively. The addition of K2SO4 made CSA crystals grow up on muscovite surface. The increasing temperature resulted in the interfacial debonding of calcium sulfate crystals, which left holes and cracks in the muscovite particles. Al3+ ions quickly precipitated as alunite within the holes and cracks because of an increase in the local concentration of Al3+ ions. The leaching kinetic results showed that the sedimentary process broke the blocking of the solid residual layers, which also decreased the mass transfer resistance of H+. Hence, the holes and cracks were expanded since alunite crystals grew into aggregates over time, which exposed more fresh surfaces to degradation. In this manner, the leaching rates of V and Al from muscovite accelerated.

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The hydrolysis precipitation of Al3+ under K2SO4 assistance was increased on account of the stronger release of Al3+ ions at 190 °C under K2SO4 assistance. The final Fe-bearing precipitates were Fe2O3 and KFe(SO4)2 in the presence of K2SO4. Fe3+ ions precipitated as stable KFe(SO4)2 crystals, which avoided the generation of Fex(OH)y(SO4)z in the sulfuric acid solution. Thus, the separation of Fe and Al from V was enhanced. ACKNOWLEDGEMENTS This research was funded by the National Natural Science Foundation of China (No.51404174 and No.51474162) and the Project in the National Science & Technology Pillar Program of China (No. 2015BAB18B01). References Zhang, Y.M., Bao, S.X., Liu, T., Chen, T.J., Huang, J., 2011. The technology of extracting vanadium from black shale in China: history, current status and future prospects. Hydrometallurgy, 109, 116-124. Yang, X., Zhang, Y.M., Bao, S.X., Shen, C., 2016. Separation and recovery of vanadium from a sulfuric-acid leaching solution of stone coal by solvent extraction using trialkylamine. Sep. Purif. Technol. 164, 49-55. Wang, F., Zhang, Y.M., Liu, T., Huang, J., Zhao, J., Zhang, G.B., Liu, J., 2014. Comparison of direct acid leaching process and blank roasting acid leaching process in extracting vanadium from black shale. Int. J. Miner. Process. 128, 40-47. Zeng, X., Wang, F., Zhang, H.F., Cui, L.J., Yu, J., Xu, G.W., 2015. Extraction of vanadium from stone coal by roasting in a fluidized bed reactor. Fuel, 142, 180-188. Hu, P.C., Zhang, Y.M., Liu, T., Huang, J., Yuan, Y.Z., Yang, Y.D., 2017. Separation and recovery of iron impurity from a vanadium-bearing stone coal via an oxalic acid leaching-reduction precipitation process. Sep. Purif. Technol. 180, 99-106. Yang, X.L., Zhang, J.W., Fang, X.H., Qiu, T.S., 2014. Kinetics of pressure leaching of niobium ore by sulfuric acid. Int. J. Refract. Met. Hard Mater. 45,218-222. Ma, B.Z., Yang, W.J., Yang, B., Wang, C.Y., Chen, Y.L., Zhang, Y.L., 2015. Pilot-scale plant study on the innovative nitric acid pressure leaching technology for laterite ores. Hydrometallurgy, 155, 88-94. Fleuriault, C.M., Anderson, C.G., Shuey, S., 2016. Iron phase control during pressure oxidation at elevated temperature. Minerals Engineering, 98, 161-168. Zhukov, V.V., Laari, A., Lampinen, M., Koiranen, T., 2017. A mechanistic kinetic model for direct pressure leaching of iron containing sphalerite concentrate. Chem. Eng. Res. Des. 118, 131-141. Dorfling, C., Akdogan, G., Bradshaw, S.M., Eksteen, J.J., 2011. Determination of the relative leaching kinetics of Cu, Rh, Ru and Ir during the sulphuric acid pressure leaching of leach residue derived from Ni-Cu converter matte enriched in platinum group metals. Minerals Engineering, 24(6), 583-589. Amer, A.M., 1994. Hydrometallurgical processing of Egyptian black shale of the Quseir-Safaga region. Hydrometallurgy, 36(1), 95-107. Huang, J., Zhang, Y.M., Huang, J., Liu, T., Cai, Z.L., Xue, N.N., 2016. Selective leaching of vanadium from roasted stone coal by dilute sulfuric acid dephosphorization-two-stage pressure acid leaching. Minerals, 6(3), 75-87. Xue, N.N., Zhang, Y.M., Liu, T., Huang, J., 2016. Study of the Dissolution Behavior of Muscovite in Stone Coal by Oxygen Pressure Acid Leaching. Metall. Mater. Trans. B, 47(1), 694-701.

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