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A high-efficiency and sustainable leaching process of vanadium from shale in sulfuric acid systems enhanced by ultrasound
Journal Pre-proofs A high-efficiency and sustainable leaching process of vanadium from shale in sulfuric acid systems enhanced by ultrasound Bo Chen, Shenxu Bao, Yimin Zhang, Sheng Li PII: DOI: Reference:
S1383-5866(19)35879-4 https://doi.org/10.1016/j.seppur.2020.116624 SEPPUR 116624
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Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
19 December 2019 17 January 2020 25 January 2020
Please cite this article as: B. Chen, S. Bao, Y. Zhang, S. Li, A high-efficiency and sustainable leaching process of vanadium from shale in sulfuric acid systems enhanced by ultrasound, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116624
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A high-efficiency and sustainable leaching process of vanadium from shale in sulfuric acid systems enhanced by ultrasound Bo Chen 1, Shenxu Bao 1, 2, 3,*, Yimin Zhang 1, 2, 3, 4, Sheng Li 3 1 School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China 2 Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan 430070 3 State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan University of Science and Technology, Wuhan 430081, PR China 4 Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Science and Technology, Wuhan 430081, PR China Corresponding Author: * Shenxu Bao, Email: [email protected] Abstract: A major source of vanadium recovery and also the raw material of this research is the vanadium-bearing (V-bearing) shale. The main purpose of the study is to capture the difference between the ultrasonic-assisted and regular leaching of vanadium from roasted V-bearing shale, having a V2O5 grade of 0.96%. In this work, the effects of various factors such as ultrasonic power, sulfuric acid concentration, amount of CaF2, leaching time and leaching temperature on the leaching recovery of vanadium were investigated by the comparison of the two methods. The results showed that the optimum leaching recovery of vanadium for regular and ultrasonic-assisted leaching method were both achieved under the conditions of 15 vol% sulfuric acid, 3 wt% CaF2 and leaching temperature 95°C. It was clearly found that the leaching recovery of vanadium can be increased from 87.86% to 92.93%, furthermore, the leaching time can be reduced by as much as 87.5% (240 min vs 30 min) in the presence of ultrasound. The enhancement of ultrasound on the leaching rate and ratio was mainly ascribed to that ultrasonic cavitation can increase the specific surface area and renew the active surface of the V-bearing muscovite particles involved in the leaching reaction. The kinetics analysis proved that the vanadium leaching process is both controlled by diffusion through the product/ash layer, and the reaction constant (kd) of vanadium leaching process with ultrasound is much higher than that without ultrasound, promoting vanadium release and accelerate the diffusion rate from the shale. Therefore, ultrasonic-assisted leaching method may be a potential and sustainable technique for increasing the leaching efficiency of vanadium from low-grade V-bearing shale. Keywords: Vanadium-bearing shale; Ultrasonic-assisted leaching; Regular leaching; Ultrasonic cavitation; Sulfuric acid
1. Introduction Vanadium, as a strategic metal element, plays a crucial role in many industries, like
alloy steels, catalysts, vanadium redox flow battery, and thermistors, on account of its unique properties [1, 2]. Vanadium-bearing (V-bearing) shale is widespread in lots of regions of China, which occupying for exceeding 87% of the total vanadium reserves of China [3, 4]. Nevertheless, V-bearing shale generally contains less than 1% V2O5, which is considered to be a complex low-grade mineral. Furthermore, most vanadium exists in trivalent vanadium (V(III)) form in V-bearing shale due to the reducing environment where it forms [5]. V(III) gradually takes the place of Al(III) in the muscovite group minerals crystalline in the form of isomorphism [6], which means that the release of vanadium is difficult from V-bearing shale. Hence, the extraction of vanadium from V-bearing shale has attracted more and more attention among many researchers. At present, the technique of blank roasting with sulfuric acid leaching is extensively applied to extract vanadium from V-bearing shale owing to the advantages of efficiently breaking the muscovite group minerals structure and making easier the release of vanadium from V-bearing shale [7, 8]. But it was discovered that the regular sulfuric acid leaching process suffer from relative low leaching recovery , long leaching time, and high energy consumed. For instance, Hu et al. [3] found that 66.2% of vanadium was leached from the V-bearing shale after 4 h leaching in sulfuric acid systems. Li et al. [9] leached vanadium from V-bearing shale by sulfuric acid, and it was found that the maximum leaching recovery of vanadium, 86%, were obtained after 6 h reaction. Wang et al. [10] investigated the leaching of vanadium from V-bearing shale in sulfuric acid systems and discovered the maximal vanadium leaching recovery, just 56.50%, achieved at leaching time of 6 h. Over the past decades, using ultrasonic-assisted method in hydrometallurgy during the leaching process of metals from minerals has been given more and more attention [11-14], attributing to the effects of chemical, mechanical, thermal, etc, brought via the ultrasonic energy. The advantages of increasing the leaching recovery, shortening the leaching time and improving the leaching efficiency brought by ultrasonic extraction technique were reported by many researchers [15-20]. Sayan et al.[16] pointed out that ultrasonic leaching led to the increasing TiO2 leaching recovery of 20% in comparison to same conditions in absence of ultrasound energy. Wang et al. [17] have found a significant augment in the zinc recovery from zinc residue with the adoption of ultrasound leaching and the activation energy can be cut down to 6.57 kJ/mol with ultrasound adoption, while near 13.07 kJ/mol using conventional leaching. Zhang et al. [19] compared the leaching behaviors of germanium from the zinc metallurgy residues under the condition of ultrasound and regular leaching and discovered that the duration of leaching was lowered as 60% and the germanium leaching recovery was improved by 3 to 5% with the adoption of ultrasound. Chang et al. [20] found that the maximum recovery percent of silver from the washed sintering dust can be increased by 5% with ultrasound-enhanced leaching and higher leaching rate was obtained by using ultrasonic leaching as compared to the conventional process. It is well known that the prerequisite for vanadium release was to break the vanadium-bearing minerals lattice structure, ascribing to that most vanadium exist in
the aluminosilicate minerals structure. Specially, fluoride can react with silicon in the aluminosilicate minerals [21], which is helpful for vanadium release from V-bearing shale. Fluorine-bearing aiding leaching agents, such as H2SiF6, HF, CaF2, are brought in the leaching process to improve the vanadium leaching effectiveness form V-bearing shale [10, 22]. The research of Wang et al. [10], showed that the addition of CaF2 can decline the chemical reaction effect on vanadium leaching, accelerate the vanadium leaching rate and boost the vanadium leaching recovery. He et al. [22] have proved that fluoride can expedite the destruction of the aluminosilicate minerals structure and CaF2 is an optimum aiding leaching reagent in industrial production. At present, much work has concentrated on the extraction of vanadium by regular leaching process, and relatively few published researches in open literature devoted to the vanadium leaching process enhanced by ultrasound. In the present study, scanning electron microscope (SEM) analyses and X-ray diffraction (XRD) are utilized to identify the difference in the microstructure, the element distribution and chemical phase transformation of the leaching residues by ultrasonic-assisted and regular leaching method. Effects of various parameters, like ultrasonic power, sulfuric acid concentration, contents of CaF2, leaching time and leaching temperature on the vanadium leaching recovery were estimated in detail. Additionally, the mechanism of the enhancement for vanadium leaching recovery and rate were detailedly analyzed. This paper aims to provide an alternative and high-efficiency extraction technique for vanadium from low-grade V-bearing shale.
2. Experimental 2.1 Materials The raw V-bearing shale employed in the present study was collected from Jiangxi province, China. Firstly, the raw shale was smashed to a particle size of 0-3 mm using a jaw breaker and a double roll crushing mill. Then, the shale was heated up at a rate of 10 °C/min to 700°C and calcined about 60 min in a muffle furnace. Finally, the roasted shale was ground to -0.074 mm in a vibrating ball mill, occupying for about 75% of the whole. The final acquired shale is considered as roasted V-bearing shale in this work. CaF2 and concentrated sulfuric acid (Offered by Sinopharm Chemical Reagent Co., Ltd., China) used were analytical grade throughout this study. The specific elemental composition of the roasted shale tested by X-ray fluorescence (XRF) are listed in Table 1. As it can be seen, the roasted V-bearing shale contains Si, Al, Fe, K, Mg, Ca, V, etc, and the grade of V2O5 is 0.96%. The XRD and SEM-EDS analysis of the roasted shale are displayed in Fig. 1 and Fig. 2. As is revealed in Fig. 1, the major mineral phase of the roasted shale are quartz, hematite, muscovite, calcite, and albite. Fig. 2 reveals that the associativity of V, Si, O, Al, K and Mg were pretty good and atom composition of O, Al, Si, K and Mg resembled that of muscovite, which manifests that the vanadium exists in the structure of muscovite. As mentioned above, V(III) replaces Al(III) in the muscovite group minerals structure in the form of isomorphism and the sample is the typical mica type V-bearing shale [3, 7].
Table 1 Main elemental composition of the roasted V-bearing shale (wt%) Element
V2O5
Al2O3
Fe2O3
SiO2
MgO
K2O
CaO
P2O5
C
S
Content
0.96
10.73
4.05
70.15
1.52
2.38
1.33
0.46
0.95
0.54
1-Quartz 2-Hematite 3-Muscovite 4-Calcite 5-Albite
Intensity, CPS
1
1 3 10
1 1 5 5 2 1 1 2 1 2 5 3 3 5 2 4 23 20
30
40
50
1 21
2 1
1 1 1 1
60
70
2θ, ° Fig. 1. XRD patterns of the roasted V-bearing shale
Fig. 2. SEM and element distribution images of the roasted V-bearing shale
2.2 Leaching experimental and analytical methods 2.2.1 Leaching experiments The leaching experiments were carried out in a 500 mL double-layer glass reactor, which connected with a thermostatic bath to keep the filled reaction solution temperature constant (55-95°C), with precision of ±1°C, and the solution was stirred by a digital magnetic stirrer in regular leaching process. The mixture of the roasted shale and CaF2 were taken in the reactor with diluted sulfuric acid at the liquid-solid ratio of 1.5:1 mL/g. For the ultrasonic-assisted leaching process, the experiments were carried out in an ultrasonic chemical reaction system (Wuxi Voshin instruments Manufacturing Co., Ltd., China) shown in Fig. 3. The 20 kHz ultrasound irradiation was conducted with an ultrasonic generator equipped with an ultrasound probe (diameter 20 mm). When ultrasonic-assisted leaching experiment begins, the ultrasonic probe is inserted into the reaction solution, and the bottom of the probe is about 1 cm below the solution level. After each leaching experiment, the leaching residues and the leachate were separated by vacuum filtration.
Fig. 3. Schematic of ultrasonic-assisted leaching instrument 1- Ultrasonic generator, 2-Magnetic stirrers, 3- Double-layer glass reactor, 4-Ultrasonic probe, 5Thermostatic bath
The vanadium leaching recovery, R (%), was calculated according to Eq. (1). R=
C V 100% m
(1)
where R is the vanadium leaching recovery (%), C is the vanadium concentration in the leachate (g/L), α is the vanadium grade of the roasted shale (%), m is the weight of feeded roasted shale (g). 2.2.2 Analytical methods The vanadium concentration in the leachate and the roasted shale were determined by ferrous ammonium sulfate titration [23]. The mineral phase compositions of the
roasted shale and the leaching residues were detected by a D/MAX-RB X-ray diffraction (XRD, Rigaku, Japan) using Cu Kα radiation with the testing angel ranging from 5°-70° at the scanning rate of 15°/min. The microstructure and the element distribution of the leaching residues were detected and scanned using scanning electron microscopy (SEM, JSM-IT300, JEOL, Japan) equipped with an energy dispersive spectrometer (EDS, Oxford, UK). The particle size distribution of the leaching residues were analyzed by laser particle size analyzer (BT-9300H, Dandong Bettersize Instrument Co., Ltd., China).
3. Results and discussion 3.1 Effects of leaching parameters on the vanadium leaching recovery 3.1.1 Effects of ultrasonic power
Leaching recovery of vanadium, %
The vanadium leaching recovery were assessed at different ultrasonic power from 0 to 1200 W with an addition of 3 wt% CaF2, 15 vol% sulfuric acid, leaching time of 30 min and leaching temperature of 95°C. 96 94 92 90 88 86 84 82 80 78 76 74 72 70 0
200
400
600
800
1000
1200
Ultrasonic power, W
Fig. 4. Effects of ultrasonic power on the vanadium leaching recovery
The outcomes shown in Fig. 4 reveal that the leaching recovery of V increase from 71.89% to 92.97%, and achieve the maximal value, 92.81%, at 900 W. When the power is below 900 W, the ultrasonic energy intensity in the reaction solution is much too low to produce satisfactory cavitation effect and mechanical action on the leaching process. The action of ultrasonic on promoting the leaching process are due to cavitation phenomenon, in which bubbles are generally nucleated and collapsed in the reaction solution with the introducing of ultrasonic irradiation. The high-speed microjets and microstream or microscopic turbulent flow at the liquid–solid interface between the reaction solution and V-bearing shale particles can be produced by symmetric or asymmetric collapse [24], which is also used to explain the leaching process of many other metals augmented by ultrasound [25, 26]. When the power reach 900 W, the high intensity ultrasound field can be acquired. Thus, the number of microscopic turbulent
flow and high-speed microjets were improved, leading to the decrease of the ions mass transfer resistance from the surface of V-bearing shale into liquid, which is controlled by diffusion resistance [27]. In addition, the microscopic turbulent flow and high-speed microjets can blow off the solid surface, which would create more highly reactive surface. Therefore, 900 W is selected as the best power due to no significant increase of vanadium leaching recovery is observed beyond it. 3.1.2 Effects of initial sulfuric acid concentration Effects of initial sulfuric acid concentration were evaluated from 5 to 20 vol%, comparing regular leaching with ultrasonic-assisted leaching, as shown in Fig. 5. The leaching parameters are held at an ultrasonic power 900 W, an addition of 3 wt% CaF2, 30 min leaching time and leaching temperature of 95°C for ultrasonic-assisted leaching experiments, while the difference for regular leaching process is that the leaching time is controlled at 240 min.
Leaching recovery of vanadium, %
100 90 80 70 60 50 40
Regular leaching Ultrasonic-assisted leaching
30 4
6
8
10
12
14
16
18
20
22
Intial H2SO4 Concentration, %
Fig. 5. Effects of initial sulfuric acid concentration on the vanadium leaching recovery
As illustrated in Fig. 5, the leaching recovery of vanadium increase with the augment of initial sulfuric acid concentration for both regular and ultrasonic-assisted leaching process. When the sulfuric acid concentration exceeds 15 vol%, both of the vanadium leaching recovery for regular and ultrasonic-assisted leaching process remain constant. During the leaching process, sulfuric acid is continually consumed as it enters into the leaching residues, which results in a sulfuric acid scarce situation at relative low acid concentration and long leaching reaction, limiting the maximum recovery of vanadium from the V-bearing shale [17]. However, it is obvious that the leaching recovery of vanadium in ultrasonic-assisted leaching process is always higher than that of the regular leaching. In the ultrasonic-assisted leaching, the vanadium ions would absorb the ultrasound energy and vibrate more vehemently at the balanced site [11]. Furthermore, the tiny bubbles generated by ultrasonic cavitation effects in the liquid around the V-bearing shale particles congregate, amalgamate, and grow quickly, leading to local high pressure and temperature nearby the tiny bubbles, which would
accelerate breaking the leaching reaction limit of vanadium. 3.1.3 Effects of CaF2 concentration CaF2 is usually adopted as an effective leaching reagent to promote the efficiency of vanadium leaching from V-bearing shale, and the amount of CaF2 has great influence on destroying the crystal structure of the V-bearing shale.
Leaching recovery of vanadium, %
100 90 80 70 60 50 Regular leaching Ultrasonic-assisted leaching
40 0
1
2
3
4
5
Amount of CaF2, %
Fig. 6. Effects of CaF2 addition on the vanadium leaching recovery (Ultrasonic-assisted leaching time=30 min, regular leaching time=240 min)
Fig. 6 shows that CaF2 addition in the acid leaching process can obviously improve the vanadium leaching efficiency for both regular and ultrasonic-assisted leaching process. As seen from Fig. 6, there is an interesting phenomenon that when the CaF2 addition was below 1%, the vanadium leaching recovery of ultrasonic-assisted leaching was low than that of the regular leaching, and this is clearly distinct when there are no CaF2 addition, indicating that there may be synergistic effects during acid leaching process of V-bearing shale between CaF2 and ultrasound. The leaching mechanism of vanadium from V-bearing shale in sulfuric acid and CaF2 systems can be expressed as Eqs. (2)-(4) [10]: CaF2 + 2H + + SO 4 = 3HF aq + CaSO 4
(2)
CaCO3 + 2H + + SO 4 = CaSO 4 +CO 2 +H 2 O
(3)
2-
2-
KMg V, Al Si 3 O10 OH 2 + HF aq + H + + O 2 SiO 2 + VO 2+ + K + + Mg 2+ SiF6 + AlF5 + Al3+ + H 2 O 2-
2-
(4)
When the CaF2 addition exceeds 1%, the cavitation effect during the ultrasound ultrasonic irradiation intensify the reaction (2), (3) and (4), and the structure of muscovite was destroyed more badly than in the regular process, resulting in more highly reactive surface exposed to the reaction solution. When the amount of CaF2 was 3%, the leaching recovery of vanadium both reached a plateau at 92.93% and 87.86%
for ultrasonic-assisted and regular leaching, respectively. 3.1.4 Effects of leaching time The effects of leaching time were assessed for ultrasonic-assisted and regular leaching process. In the meantime, the other leaching parameters were held as follows: 15 vol% sulfuric acid, an addition of 3 wt% CaF2, leaching temperature of 95°C, as well as an ultrasonic power 900 W.
Leaching recovery of vanadium, %
95 90 85 80 75 Regular leaching Ultrasonic-assisted leaching
70 0
10
20
30
40
50 100 150 200 250 300 350 400
Leaching time, min
Fig. 7. Effects of leaching time on the vanadium leaching recovery
The outcomes indicate that the leaching quantity of vanadium augment significantly from 5 to 30 min with ultrasound irradiation and it resemble from 30 to 240 min by regular leaching. Under the same leaching time of 30 min, ultrasonic treatment increases the leaching recovery of vanadium by 21.04%, manifesting the superiority on high leaching recovery and less time-consuming brought in by effective ultrasound energy. The remarkable positive effects can be put down to the augmented diffusion rate resulting from vigorous ultrasonic agitation reduce the diffusion layer thickness of V-bearing shale [19]. It is evident that the optimal leaching time for ultrasonic-assisted (30 min) is shorter than that for regular leaching (240 min). In the meanwhile, the vanadium leaching recovery for regular was 87.86% at leaching time of 240 min, which was increased by 5.07% to 92.93% with ultrasonic appliance after 30 min leaching. In other words, the vanadium compounds were leached more thoroughly in the ultrasonic process. It is probable that microjets weakened or eliminated the V-bearing shale boundary layer, eroded the solid surface and broke particles, which can increase the particles specific surface area, promote more V-bearing shale particles active surface involved in the reaction and accelerate the diffusion rate [28]. Hence, the ultrasonic irradiation can not only improve the leaching recovery of vanadium but also notably shorten the leaching time. 3.1.5. Kinetics analysis The process of leaching vanadium from V-bearing shale is solid-liquid heterogeneous
reactions, which are common in hydrometallurgical process. The most common solidliquid reaction model, the shrinking core model [29], is utilized so as to determine the rate controlling step and kinetic parameters of vanadium leaching process. It is well known that in the solid-liquid reaction process, the reaction rate is generally controlled by diffusion through the product/ash layer or chemical reaction [30], which can be described by Eq. (5) and Eq. (6), respectively. (a) When product/ash layer diffusion controls: 2
1+ 2 1- x - 3 1- x 3 = kd t
(5)
(b) When chemical reaction controls: 1
1- 1- x 3 = kr t
(6)
where x is the leaching recovery of vanadium, kd and kr is the kinetic parameter for product/ash layer diffusion control and chemical reaction control, respectively, t is the leaching time (min). 0.65
0.60
(a)
Regular leaching Ultrasonic-assisted leaching
0.60
(b)
Regular leaching Ultrasonic-assisted leaching
0.55
y=0.0088x+0.3697 R2=0.9918
0.50 0.45 0.40
y=0.0014x+0.2511 R2=0.9917
0.35
0.45
y=0.0009x+0.3248 R2=0.9725
0.40
0.30 0.25
y=0.0062x+0.4082 R2=0.9603
0.50
1-(1-x)1/3
1+2(1-x)-3(1-x)2/3
0.55
0.35 0
20
40
60
80
100
120
140
160
180
200
0
t, min
20
40
60
80
100
120
140
160
180
200
t, min
Fig. 8. The variation of 1+2(1-x)-3(1-x)2/3 (a) and 1-(1-x)1/3 (b) with time for vanadium leaching process
The fitting results for vanadium leaching process using the shrinking core model are shown in Fig. 8, and the fitting parameters are listed in Table 2. It is evident that Eq. (5) can fit the experimental data better than Eq. (6), verifying that the leaching of vanadium from V-bearing shale is controlled by diffusion through product/ash layer for both regular leaching and ultrasonic-assisted leaching process. In the meanwhile, the kinetic reaction constant kd for vanadium leaching process with ultrasound (8.8×10-3) is much higher than that without ultrasound (1.4×10-3), which implies that ultrasound can greatly improve the vanadium leaching rate with the appliance of ultrasound. Table 2 Fitting parameters for vanadium leaching process using the shrinking core model Leaching methods Regular Ultrasonic-assisted
R2 Product/ash layer diffusion
Chemical reaction
0.9917 0.9918
0.9725 0.9603
3.1.6 Effects of leaching temperature
kd 1.4×10-3 8.8×10-3
The effects of leaching temperature are estimated from 55 to 95°C, comparing regular leaching with ultrasonic-assisted leaching, as shown in Fig. 9.
Leaching recovery of vanadium, %
95 90 85 80 75 70 65 60 55 50 45
Regular leaching Ultrasonic-assisted leaching
40 35 40
50
60
80 70 Temperature, °C
90
100
Fig. 9. Effects of leaching temperature on the vanadium leaching recovery
As Fig. 9 is shown, the overall vanadium leaching recovery increase with the increasing leaching temperature for both the two leaching methods. Nevertheless, the overall vanadium leaching recovery of ultrasonic-assisted process is obviously higher than that of the regular process at the same leaching temperature. Generally speaking, an increase in temperature can intensify the molecular motion or reduce the diffusion resistance, resulting in an augmented leaching rate and an increased leaching recovery. Furthermore, the increase of leaching temperature would decrease the leaching reaction activation energy, which would also improve the reaction rate of V-bearing shale with the acid solution [17]. Of course, a higher leaching recovery with the appliance of ultrasonic leaching can be ascribed to the reasons discussed in the above section. The maximum vanadium leaching recovery were 92.93% and 87.86% at 95°C for ultrasonic-assisted and regular leaching, respectively. Taking the water evaporation rate into consideration when the temperature was higher than 95°C [3], the optimal leaching temperature was selected as 95°C for both leaching systems. 3.2 Mechanism of the enhancement in vanadium leaching The following characterization analyses of the leaching residues are used for exploring the role of ultrasound during the ultrasonic-assisted leaching process. Mechanisms for augment of vanadium leaching recovery using ultrasound were detailedly analyzed by studying the leaching residues collected after the ultrasonicassisted leaching process (conditions: ultrasonic power=900 W, liquid to solid ratio=1.5:1 mL/g, sulfuric acid concentration=15 vol%, CaF2 addition=3 wt%, leaching time=30 min, leaching temperature=95°C) and the leaching residues taken from the regular leaching process (conditions: liquid to solid ratio=1.5:1 mL/g, sulfuric acid concentration=15 vol%, CaF2 addition=3 wt%, leaching time=240 min, leaching temperature=95°C).
3.2.1 Chemical phase transformation analyses In order to obtain a primary mineral phase transformation in the absence and presence of ultrasound energy. The XRD patterns of the leaching residues collected from two leaching systems are displayed in Fig. 10. 1
1-Quartz 2-Hematite 3-Gypsum 4-Albite 5-Hieratite 6-Muscovite 7-Anhydrite
Intensity, CPS
B 1 75 7 7 6 7 5 17 1 1 1 1 1 6 6 72 4 6 2 2 7 2 7 2 7 21 1 1
6
A
4
3 4
6 10
3 1 3 6 31 2 3 1 6 6 4 2 4 4 5 2 3 1 11 12 1 3 2 3 21 20
30
40
50
1
2 11 11 1
1 1 1
2 1 21 60
70
2θ, °
Fig. 10. XRD patterns of the residues obtained from regular (A) and ultrasonic-assisted (B) leaching process
Intensity, CPS
1-Gypsum 2-Anhydrite B
A 1 10
1 12
1 1 20
1 30
40
50
60
70
2θ, °
Fig. 11. XRD patterns of the ultrasonic-assisted leaching residues at leaching time of 10 min (A) and leaching time of 20 min (B)
It is clearly found that the muscovite diffraction peaks in both regular and ultrasonicassisted leaching residues are weaker than that in the roasted shale (Fig. 1), attributing to the disintegration of muscovite structure during the leaching process. In the meanwhile, it was noteworthy that the most distinct difference between the chemical phases in two XRD patterns (Fig. 10 A and B) is that there is gypsum in the regular
leaching residues, while anhydrite is produced in the ultrasonic-assisted leaching residues. As shown in Fig. 11, it is obvious that only gypsum exists in the ultrasonicassisted leaching residues at leaching time of 10 min (Fig. 11A), but there are not only gypsum but also anhydrite in the leaching residues with the leaching prolonging to 20 min (Fig. 11B). In other words, the ultrasound energy may cause the conversion of gypsum to anhydrite. During the solid-phase transition from gypsum to anhydrite, the anhydrite crystals would grow by spiral dislocation, which good crystal form. The strain concentration nearby the anhydrite would act on the muscovite structure octahedral and tetrahedral sites, generating vigoroso strain concentration on the muscovite particles, which would intensify the destruction of the muscovite structure and increase the specific surface of the particles of muscovite [7]. In this way, the muscovite particles dissolved much more easily with the application of ultrasound. 3.2.2 Particle size distribution analysis The particle size distribution of the leaching residues obtained after ultrasonicassisted and regular leaching process are shown in Fig. 12.
Differential distribution, %
5
4
3
2
1
Regular leaching Ultrasonic-assisted leaching
0 0
50
100
150
200
250
300
350
Particle size, μm
Fig. 12. The particle size distribution of the leaching residues
As displayed in Fig. 12, the differential distribution of particle size is mainly divided into two special areas. The size distribution percentage of ultrasonic leaching residues is higher than that of regular residues in the particle size range of 4.71 to 29.12 μm. Furthermore, in the range of 29.12 to 105.24 μm, the size distribution ratio of ultrasonic residues is visibly lower than that of regular residues. The difference in the differential distribution of the leaching residues by two leaching systems confirmed that ultrasonic energy can smash the V-bearing shale particles, which can reinforce the solid-liquid leaching reaction [20]. The results can also demonstrated the viewpoint that ultrasonic cavitation effects can improve the specific surface area of the particles and renew the product layer, which is in accordance with the leaching process of germanium from zinc metallurgy residues via ultrasonic-assisted method [19].
3.2.3 Microscopic surface and element distribution analyses In order to acquire a visual insight into the microsurface morphology and the element distribution of the residues collected from the regular and ultrasonic-assisted leaching, the SEM-EDS analyses were carried out under different magnification and the results are presented in Fig. 13.
Si
O
Al
K
Fig. 13. SEM and element distribution images of the residues obtained from regular (A), (B) and ultrasonic-assisted (C), (D) leaching process
The SEM images reveal the difference of the microstructure of the leaching residues using regular and ultrasound methods. From the contrast of Fig. 13B and Fig. 13D, it is distinct that the particle size of residues by ultrasonic-assisted method is finer than that by regular process, which is corresponding to the results of the particle size distribution analysis. In the meanwhile, it was clearly found in Fig. 13A and Fig. 13C that the particles in the regular leaching residues grown heterogeneous and dense, on the contrary, these particles are homogeneous and loose with ultrasound treatment. As is distinctly presented in the element distribution images of the two residues (Fig. 13), the muscovite particle in regular leaching residues is almost completely wrapped by CaSO4‧2H2O, which may have negative influence on the dissolution of V-bearing muscovite particle. While, most CaSO4 crystals fall off from the surface of muscovite particle with the application of ultrasound, which may promote the eroding of the muscovite particle with sulfuric acid. These difference demonstrate that the ultrasound energy can hinder the growth of the solid particles and break the V-bearing shale particles into finer pieces, which is beneficial for the renewal of the active surface and the reinforcement of the solid-liquid reaction [19], and it is meaningful for the release of vanadium from the lattice of mica group minerals.
4. Conclusions (1) Under the conditions of a sulfuric acid concentration of 15 vol%, 3 wt% CaF2 addition as well as a leaching temperature of 95°C and a liquid to solid ratio of 1.5:1 mL/g, 92.93% recovery of vanadium was obtained for a leaching duration of 30 min using 900 W ultrasonic power; However, 87.86% of vanadium was leached out with regular method at a leaching time of 240 min. (2) Ultrasonic cavitation has generated some remarkable advantages on the release of vanadium from vanadium-bearing (V-bearing) shale. First, ultrasonic energy can smash solid particles and improve the specific surface area of the V-bearing muscovite particles, as well as reduce the thickness of the diffusion layer, which can promote more active surface of the V-bearing shale particles exposed to the reaction solution and accelerate the diffusion rate. Second, the ultrasound energy makes transform from CaSO4‧2H2O to CaSO4, and the strain concentration nearby the CaSO4 would act on the octahedral and tetrahedral sites in the muscovite structure, which would further intensify the destruction of the muscovite structure and the release of vanadium from vanadium-bearing shale. (3) The kinetics fitting results show that the leaching process of vanadium from Vbearing shale is controlled by diffusion through the product/ash layer, and the reaction constant (kd) of vanadium leaching process with ultrasound is much higher than that without ultrasound, which results in higher leaching recovery and shorter leaching time with the appliance of ultrasound compared to that without ultrasound. (4) All of the traits of ultrasound are beneficial for fortifying the release of vanadium from V-bearing muscovite particles and markedly shortening the leaching time. Ultrasonic-assisted leaching may be a potential and promising technique for extraction vanadium from complex low-grade vanadium-bearing minerals.
Declaration of Competing Interest The authors declare no competing financial interest.
Appendix A. Supplementary material Supplementary data to this article http://dx.doi.org/10.17632/scx98yf4r6.1.
can
be
found
online
at:
Acknowledgements This research was supported by the National Natural Science Foundation of China (51874222) and the Major Technical Innovation Project of Hubei Province (2018ACA157).
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Bo Chen: Conceptualization; Investigation; Methodology; Software; Writing-Original draft preparation; Writing-Reviewing and Editing Shenxu Bao: Funding acquisition; Supervision; Validation Yimin Zhang: Supervision Sheng Li: Investigation; Software
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights Ultrasound is used to intensify low-grade vanadium-bearing shale leaching in H2SO4. Ultrasonic-assisted and regular leaching of vanadium were contrastively studied. Ultrasound shortens leaching time by 87.5% and increases leaching ratio about 5%. Enhancement mechanism of ultrasonic-assisted leaching has been detailedly analyzed.
S1383-5866(19)35879-4 https://doi.org/10.1016/j.seppur.2020.116624 SEPPUR 116624
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Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
19 December 2019 17 January 2020 25 January 2020
Please cite this article as: B. Chen, S. Bao, Y. Zhang, S. Li, A high-efficiency and sustainable leaching process of vanadium from shale in sulfuric acid systems enhanced by ultrasound, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116624
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A high-efficiency and sustainable leaching process of vanadium from shale in sulfuric acid systems enhanced by ultrasound Bo Chen 1, Shenxu Bao 1, 2, 3,*, Yimin Zhang 1, 2, 3, 4, Sheng Li 3 1 School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China 2 Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan 430070 3 State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan University of Science and Technology, Wuhan 430081, PR China 4 Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Science and Technology, Wuhan 430081, PR China Corresponding Author: * Shenxu Bao, Email: [email protected] Abstract: A major source of vanadium recovery and also the raw material of this research is the vanadium-bearing (V-bearing) shale. The main purpose of the study is to capture the difference between the ultrasonic-assisted and regular leaching of vanadium from roasted V-bearing shale, having a V2O5 grade of 0.96%. In this work, the effects of various factors such as ultrasonic power, sulfuric acid concentration, amount of CaF2, leaching time and leaching temperature on the leaching recovery of vanadium were investigated by the comparison of the two methods. The results showed that the optimum leaching recovery of vanadium for regular and ultrasonic-assisted leaching method were both achieved under the conditions of 15 vol% sulfuric acid, 3 wt% CaF2 and leaching temperature 95°C. It was clearly found that the leaching recovery of vanadium can be increased from 87.86% to 92.93%, furthermore, the leaching time can be reduced by as much as 87.5% (240 min vs 30 min) in the presence of ultrasound. The enhancement of ultrasound on the leaching rate and ratio was mainly ascribed to that ultrasonic cavitation can increase the specific surface area and renew the active surface of the V-bearing muscovite particles involved in the leaching reaction. The kinetics analysis proved that the vanadium leaching process is both controlled by diffusion through the product/ash layer, and the reaction constant (kd) of vanadium leaching process with ultrasound is much higher than that without ultrasound, promoting vanadium release and accelerate the diffusion rate from the shale. Therefore, ultrasonic-assisted leaching method may be a potential and sustainable technique for increasing the leaching efficiency of vanadium from low-grade V-bearing shale. Keywords: Vanadium-bearing shale; Ultrasonic-assisted leaching; Regular leaching; Ultrasonic cavitation; Sulfuric acid
1. Introduction Vanadium, as a strategic metal element, plays a crucial role in many industries, like
alloy steels, catalysts, vanadium redox flow battery, and thermistors, on account of its unique properties [1, 2]. Vanadium-bearing (V-bearing) shale is widespread in lots of regions of China, which occupying for exceeding 87% of the total vanadium reserves of China [3, 4]. Nevertheless, V-bearing shale generally contains less than 1% V2O5, which is considered to be a complex low-grade mineral. Furthermore, most vanadium exists in trivalent vanadium (V(III)) form in V-bearing shale due to the reducing environment where it forms [5]. V(III) gradually takes the place of Al(III) in the muscovite group minerals crystalline in the form of isomorphism [6], which means that the release of vanadium is difficult from V-bearing shale. Hence, the extraction of vanadium from V-bearing shale has attracted more and more attention among many researchers. At present, the technique of blank roasting with sulfuric acid leaching is extensively applied to extract vanadium from V-bearing shale owing to the advantages of efficiently breaking the muscovite group minerals structure and making easier the release of vanadium from V-bearing shale [7, 8]. But it was discovered that the regular sulfuric acid leaching process suffer from relative low leaching recovery , long leaching time, and high energy consumed. For instance, Hu et al. [3] found that 66.2% of vanadium was leached from the V-bearing shale after 4 h leaching in sulfuric acid systems. Li et al. [9] leached vanadium from V-bearing shale by sulfuric acid, and it was found that the maximum leaching recovery of vanadium, 86%, were obtained after 6 h reaction. Wang et al. [10] investigated the leaching of vanadium from V-bearing shale in sulfuric acid systems and discovered the maximal vanadium leaching recovery, just 56.50%, achieved at leaching time of 6 h. Over the past decades, using ultrasonic-assisted method in hydrometallurgy during the leaching process of metals from minerals has been given more and more attention [11-14], attributing to the effects of chemical, mechanical, thermal, etc, brought via the ultrasonic energy. The advantages of increasing the leaching recovery, shortening the leaching time and improving the leaching efficiency brought by ultrasonic extraction technique were reported by many researchers [15-20]. Sayan et al.[16] pointed out that ultrasonic leaching led to the increasing TiO2 leaching recovery of 20% in comparison to same conditions in absence of ultrasound energy. Wang et al. [17] have found a significant augment in the zinc recovery from zinc residue with the adoption of ultrasound leaching and the activation energy can be cut down to 6.57 kJ/mol with ultrasound adoption, while near 13.07 kJ/mol using conventional leaching. Zhang et al. [19] compared the leaching behaviors of germanium from the zinc metallurgy residues under the condition of ultrasound and regular leaching and discovered that the duration of leaching was lowered as 60% and the germanium leaching recovery was improved by 3 to 5% with the adoption of ultrasound. Chang et al. [20] found that the maximum recovery percent of silver from the washed sintering dust can be increased by 5% with ultrasound-enhanced leaching and higher leaching rate was obtained by using ultrasonic leaching as compared to the conventional process. It is well known that the prerequisite for vanadium release was to break the vanadium-bearing minerals lattice structure, ascribing to that most vanadium exist in
the aluminosilicate minerals structure. Specially, fluoride can react with silicon in the aluminosilicate minerals [21], which is helpful for vanadium release from V-bearing shale. Fluorine-bearing aiding leaching agents, such as H2SiF6, HF, CaF2, are brought in the leaching process to improve the vanadium leaching effectiveness form V-bearing shale [10, 22]. The research of Wang et al. [10], showed that the addition of CaF2 can decline the chemical reaction effect on vanadium leaching, accelerate the vanadium leaching rate and boost the vanadium leaching recovery. He et al. [22] have proved that fluoride can expedite the destruction of the aluminosilicate minerals structure and CaF2 is an optimum aiding leaching reagent in industrial production. At present, much work has concentrated on the extraction of vanadium by regular leaching process, and relatively few published researches in open literature devoted to the vanadium leaching process enhanced by ultrasound. In the present study, scanning electron microscope (SEM) analyses and X-ray diffraction (XRD) are utilized to identify the difference in the microstructure, the element distribution and chemical phase transformation of the leaching residues by ultrasonic-assisted and regular leaching method. Effects of various parameters, like ultrasonic power, sulfuric acid concentration, contents of CaF2, leaching time and leaching temperature on the vanadium leaching recovery were estimated in detail. Additionally, the mechanism of the enhancement for vanadium leaching recovery and rate were detailedly analyzed. This paper aims to provide an alternative and high-efficiency extraction technique for vanadium from low-grade V-bearing shale.
2. Experimental 2.1 Materials The raw V-bearing shale employed in the present study was collected from Jiangxi province, China. Firstly, the raw shale was smashed to a particle size of 0-3 mm using a jaw breaker and a double roll crushing mill. Then, the shale was heated up at a rate of 10 °C/min to 700°C and calcined about 60 min in a muffle furnace. Finally, the roasted shale was ground to -0.074 mm in a vibrating ball mill, occupying for about 75% of the whole. The final acquired shale is considered as roasted V-bearing shale in this work. CaF2 and concentrated sulfuric acid (Offered by Sinopharm Chemical Reagent Co., Ltd., China) used were analytical grade throughout this study. The specific elemental composition of the roasted shale tested by X-ray fluorescence (XRF) are listed in Table 1. As it can be seen, the roasted V-bearing shale contains Si, Al, Fe, K, Mg, Ca, V, etc, and the grade of V2O5 is 0.96%. The XRD and SEM-EDS analysis of the roasted shale are displayed in Fig. 1 and Fig. 2. As is revealed in Fig. 1, the major mineral phase of the roasted shale are quartz, hematite, muscovite, calcite, and albite. Fig. 2 reveals that the associativity of V, Si, O, Al, K and Mg were pretty good and atom composition of O, Al, Si, K and Mg resembled that of muscovite, which manifests that the vanadium exists in the structure of muscovite. As mentioned above, V(III) replaces Al(III) in the muscovite group minerals structure in the form of isomorphism and the sample is the typical mica type V-bearing shale [3, 7].
Table 1 Main elemental composition of the roasted V-bearing shale (wt%) Element
V2O5
Al2O3
Fe2O3
SiO2
MgO
K2O
CaO
P2O5
C
S
Content
0.96
10.73
4.05
70.15
1.52
2.38
1.33
0.46
0.95
0.54
1-Quartz 2-Hematite 3-Muscovite 4-Calcite 5-Albite
Intensity, CPS
1
1 3 10
1 1 5 5 2 1 1 2 1 2 5 3 3 5 2 4 23 20
30
40
50
1 21
2 1
1 1 1 1
60
70
2θ, ° Fig. 1. XRD patterns of the roasted V-bearing shale
Fig. 2. SEM and element distribution images of the roasted V-bearing shale
2.2 Leaching experimental and analytical methods 2.2.1 Leaching experiments The leaching experiments were carried out in a 500 mL double-layer glass reactor, which connected with a thermostatic bath to keep the filled reaction solution temperature constant (55-95°C), with precision of ±1°C, and the solution was stirred by a digital magnetic stirrer in regular leaching process. The mixture of the roasted shale and CaF2 were taken in the reactor with diluted sulfuric acid at the liquid-solid ratio of 1.5:1 mL/g. For the ultrasonic-assisted leaching process, the experiments were carried out in an ultrasonic chemical reaction system (Wuxi Voshin instruments Manufacturing Co., Ltd., China) shown in Fig. 3. The 20 kHz ultrasound irradiation was conducted with an ultrasonic generator equipped with an ultrasound probe (diameter 20 mm). When ultrasonic-assisted leaching experiment begins, the ultrasonic probe is inserted into the reaction solution, and the bottom of the probe is about 1 cm below the solution level. After each leaching experiment, the leaching residues and the leachate were separated by vacuum filtration.
Fig. 3. Schematic of ultrasonic-assisted leaching instrument 1- Ultrasonic generator, 2-Magnetic stirrers, 3- Double-layer glass reactor, 4-Ultrasonic probe, 5Thermostatic bath
The vanadium leaching recovery, R (%), was calculated according to Eq. (1). R=
C V 100% m
(1)
where R is the vanadium leaching recovery (%), C is the vanadium concentration in the leachate (g/L), α is the vanadium grade of the roasted shale (%), m is the weight of feeded roasted shale (g). 2.2.2 Analytical methods The vanadium concentration in the leachate and the roasted shale were determined by ferrous ammonium sulfate titration [23]. The mineral phase compositions of the
roasted shale and the leaching residues were detected by a D/MAX-RB X-ray diffraction (XRD, Rigaku, Japan) using Cu Kα radiation with the testing angel ranging from 5°-70° at the scanning rate of 15°/min. The microstructure and the element distribution of the leaching residues were detected and scanned using scanning electron microscopy (SEM, JSM-IT300, JEOL, Japan) equipped with an energy dispersive spectrometer (EDS, Oxford, UK). The particle size distribution of the leaching residues were analyzed by laser particle size analyzer (BT-9300H, Dandong Bettersize Instrument Co., Ltd., China).
3. Results and discussion 3.1 Effects of leaching parameters on the vanadium leaching recovery 3.1.1 Effects of ultrasonic power
Leaching recovery of vanadium, %
The vanadium leaching recovery were assessed at different ultrasonic power from 0 to 1200 W with an addition of 3 wt% CaF2, 15 vol% sulfuric acid, leaching time of 30 min and leaching temperature of 95°C. 96 94 92 90 88 86 84 82 80 78 76 74 72 70 0
200
400
600
800
1000
1200
Ultrasonic power, W
Fig. 4. Effects of ultrasonic power on the vanadium leaching recovery
The outcomes shown in Fig. 4 reveal that the leaching recovery of V increase from 71.89% to 92.97%, and achieve the maximal value, 92.81%, at 900 W. When the power is below 900 W, the ultrasonic energy intensity in the reaction solution is much too low to produce satisfactory cavitation effect and mechanical action on the leaching process. The action of ultrasonic on promoting the leaching process are due to cavitation phenomenon, in which bubbles are generally nucleated and collapsed in the reaction solution with the introducing of ultrasonic irradiation. The high-speed microjets and microstream or microscopic turbulent flow at the liquid–solid interface between the reaction solution and V-bearing shale particles can be produced by symmetric or asymmetric collapse [24], which is also used to explain the leaching process of many other metals augmented by ultrasound [25, 26]. When the power reach 900 W, the high intensity ultrasound field can be acquired. Thus, the number of microscopic turbulent
flow and high-speed microjets were improved, leading to the decrease of the ions mass transfer resistance from the surface of V-bearing shale into liquid, which is controlled by diffusion resistance [27]. In addition, the microscopic turbulent flow and high-speed microjets can blow off the solid surface, which would create more highly reactive surface. Therefore, 900 W is selected as the best power due to no significant increase of vanadium leaching recovery is observed beyond it. 3.1.2 Effects of initial sulfuric acid concentration Effects of initial sulfuric acid concentration were evaluated from 5 to 20 vol%, comparing regular leaching with ultrasonic-assisted leaching, as shown in Fig. 5. The leaching parameters are held at an ultrasonic power 900 W, an addition of 3 wt% CaF2, 30 min leaching time and leaching temperature of 95°C for ultrasonic-assisted leaching experiments, while the difference for regular leaching process is that the leaching time is controlled at 240 min.
Leaching recovery of vanadium, %
100 90 80 70 60 50 40
Regular leaching Ultrasonic-assisted leaching
30 4
6
8
10
12
14
16
18
20
22
Intial H2SO4 Concentration, %
Fig. 5. Effects of initial sulfuric acid concentration on the vanadium leaching recovery
As illustrated in Fig. 5, the leaching recovery of vanadium increase with the augment of initial sulfuric acid concentration for both regular and ultrasonic-assisted leaching process. When the sulfuric acid concentration exceeds 15 vol%, both of the vanadium leaching recovery for regular and ultrasonic-assisted leaching process remain constant. During the leaching process, sulfuric acid is continually consumed as it enters into the leaching residues, which results in a sulfuric acid scarce situation at relative low acid concentration and long leaching reaction, limiting the maximum recovery of vanadium from the V-bearing shale [17]. However, it is obvious that the leaching recovery of vanadium in ultrasonic-assisted leaching process is always higher than that of the regular leaching. In the ultrasonic-assisted leaching, the vanadium ions would absorb the ultrasound energy and vibrate more vehemently at the balanced site [11]. Furthermore, the tiny bubbles generated by ultrasonic cavitation effects in the liquid around the V-bearing shale particles congregate, amalgamate, and grow quickly, leading to local high pressure and temperature nearby the tiny bubbles, which would
accelerate breaking the leaching reaction limit of vanadium. 3.1.3 Effects of CaF2 concentration CaF2 is usually adopted as an effective leaching reagent to promote the efficiency of vanadium leaching from V-bearing shale, and the amount of CaF2 has great influence on destroying the crystal structure of the V-bearing shale.
Leaching recovery of vanadium, %
100 90 80 70 60 50 Regular leaching Ultrasonic-assisted leaching
40 0
1
2
3
4
5
Amount of CaF2, %
Fig. 6. Effects of CaF2 addition on the vanadium leaching recovery (Ultrasonic-assisted leaching time=30 min, regular leaching time=240 min)
Fig. 6 shows that CaF2 addition in the acid leaching process can obviously improve the vanadium leaching efficiency for both regular and ultrasonic-assisted leaching process. As seen from Fig. 6, there is an interesting phenomenon that when the CaF2 addition was below 1%, the vanadium leaching recovery of ultrasonic-assisted leaching was low than that of the regular leaching, and this is clearly distinct when there are no CaF2 addition, indicating that there may be synergistic effects during acid leaching process of V-bearing shale between CaF2 and ultrasound. The leaching mechanism of vanadium from V-bearing shale in sulfuric acid and CaF2 systems can be expressed as Eqs. (2)-(4) [10]: CaF2 + 2H + + SO 4 = 3HF aq + CaSO 4
(2)
CaCO3 + 2H + + SO 4 = CaSO 4 +CO 2 +H 2 O
(3)
2-
2-
KMg V, Al Si 3 O10 OH 2 + HF aq + H + + O 2 SiO 2 + VO 2+ + K + + Mg 2+ SiF6 + AlF5 + Al3+ + H 2 O 2-
2-
(4)
When the CaF2 addition exceeds 1%, the cavitation effect during the ultrasound ultrasonic irradiation intensify the reaction (2), (3) and (4), and the structure of muscovite was destroyed more badly than in the regular process, resulting in more highly reactive surface exposed to the reaction solution. When the amount of CaF2 was 3%, the leaching recovery of vanadium both reached a plateau at 92.93% and 87.86%
for ultrasonic-assisted and regular leaching, respectively. 3.1.4 Effects of leaching time The effects of leaching time were assessed for ultrasonic-assisted and regular leaching process. In the meantime, the other leaching parameters were held as follows: 15 vol% sulfuric acid, an addition of 3 wt% CaF2, leaching temperature of 95°C, as well as an ultrasonic power 900 W.
Leaching recovery of vanadium, %
95 90 85 80 75 Regular leaching Ultrasonic-assisted leaching
70 0
10
20
30
40
50 100 150 200 250 300 350 400
Leaching time, min
Fig. 7. Effects of leaching time on the vanadium leaching recovery
The outcomes indicate that the leaching quantity of vanadium augment significantly from 5 to 30 min with ultrasound irradiation and it resemble from 30 to 240 min by regular leaching. Under the same leaching time of 30 min, ultrasonic treatment increases the leaching recovery of vanadium by 21.04%, manifesting the superiority on high leaching recovery and less time-consuming brought in by effective ultrasound energy. The remarkable positive effects can be put down to the augmented diffusion rate resulting from vigorous ultrasonic agitation reduce the diffusion layer thickness of V-bearing shale [19]. It is evident that the optimal leaching time for ultrasonic-assisted (30 min) is shorter than that for regular leaching (240 min). In the meanwhile, the vanadium leaching recovery for regular was 87.86% at leaching time of 240 min, which was increased by 5.07% to 92.93% with ultrasonic appliance after 30 min leaching. In other words, the vanadium compounds were leached more thoroughly in the ultrasonic process. It is probable that microjets weakened or eliminated the V-bearing shale boundary layer, eroded the solid surface and broke particles, which can increase the particles specific surface area, promote more V-bearing shale particles active surface involved in the reaction and accelerate the diffusion rate [28]. Hence, the ultrasonic irradiation can not only improve the leaching recovery of vanadium but also notably shorten the leaching time. 3.1.5. Kinetics analysis The process of leaching vanadium from V-bearing shale is solid-liquid heterogeneous
reactions, which are common in hydrometallurgical process. The most common solidliquid reaction model, the shrinking core model [29], is utilized so as to determine the rate controlling step and kinetic parameters of vanadium leaching process. It is well known that in the solid-liquid reaction process, the reaction rate is generally controlled by diffusion through the product/ash layer or chemical reaction [30], which can be described by Eq. (5) and Eq. (6), respectively. (a) When product/ash layer diffusion controls: 2
1+ 2 1- x - 3 1- x 3 = kd t
(5)
(b) When chemical reaction controls: 1
1- 1- x 3 = kr t
(6)
where x is the leaching recovery of vanadium, kd and kr is the kinetic parameter for product/ash layer diffusion control and chemical reaction control, respectively, t is the leaching time (min). 0.65
0.60
(a)
Regular leaching Ultrasonic-assisted leaching
0.60
(b)
Regular leaching Ultrasonic-assisted leaching
0.55
y=0.0088x+0.3697 R2=0.9918
0.50 0.45 0.40
y=0.0014x+0.2511 R2=0.9917
0.35
0.45
y=0.0009x+0.3248 R2=0.9725
0.40
0.30 0.25
y=0.0062x+0.4082 R2=0.9603
0.50
1-(1-x)1/3
1+2(1-x)-3(1-x)2/3
0.55
0.35 0
20
40
60
80
100
120
140
160
180
200
0
t, min
20
40
60
80
100
120
140
160
180
200
t, min
Fig. 8. The variation of 1+2(1-x)-3(1-x)2/3 (a) and 1-(1-x)1/3 (b) with time for vanadium leaching process
The fitting results for vanadium leaching process using the shrinking core model are shown in Fig. 8, and the fitting parameters are listed in Table 2. It is evident that Eq. (5) can fit the experimental data better than Eq. (6), verifying that the leaching of vanadium from V-bearing shale is controlled by diffusion through product/ash layer for both regular leaching and ultrasonic-assisted leaching process. In the meanwhile, the kinetic reaction constant kd for vanadium leaching process with ultrasound (8.8×10-3) is much higher than that without ultrasound (1.4×10-3), which implies that ultrasound can greatly improve the vanadium leaching rate with the appliance of ultrasound. Table 2 Fitting parameters for vanadium leaching process using the shrinking core model Leaching methods Regular Ultrasonic-assisted
R2 Product/ash layer diffusion
Chemical reaction
0.9917 0.9918
0.9725 0.9603
3.1.6 Effects of leaching temperature
kd 1.4×10-3 8.8×10-3
The effects of leaching temperature are estimated from 55 to 95°C, comparing regular leaching with ultrasonic-assisted leaching, as shown in Fig. 9.
Leaching recovery of vanadium, %
95 90 85 80 75 70 65 60 55 50 45
Regular leaching Ultrasonic-assisted leaching
40 35 40
50
60
80 70 Temperature, °C
90
100
Fig. 9. Effects of leaching temperature on the vanadium leaching recovery
As Fig. 9 is shown, the overall vanadium leaching recovery increase with the increasing leaching temperature for both the two leaching methods. Nevertheless, the overall vanadium leaching recovery of ultrasonic-assisted process is obviously higher than that of the regular process at the same leaching temperature. Generally speaking, an increase in temperature can intensify the molecular motion or reduce the diffusion resistance, resulting in an augmented leaching rate and an increased leaching recovery. Furthermore, the increase of leaching temperature would decrease the leaching reaction activation energy, which would also improve the reaction rate of V-bearing shale with the acid solution [17]. Of course, a higher leaching recovery with the appliance of ultrasonic leaching can be ascribed to the reasons discussed in the above section. The maximum vanadium leaching recovery were 92.93% and 87.86% at 95°C for ultrasonic-assisted and regular leaching, respectively. Taking the water evaporation rate into consideration when the temperature was higher than 95°C [3], the optimal leaching temperature was selected as 95°C for both leaching systems. 3.2 Mechanism of the enhancement in vanadium leaching The following characterization analyses of the leaching residues are used for exploring the role of ultrasound during the ultrasonic-assisted leaching process. Mechanisms for augment of vanadium leaching recovery using ultrasound were detailedly analyzed by studying the leaching residues collected after the ultrasonicassisted leaching process (conditions: ultrasonic power=900 W, liquid to solid ratio=1.5:1 mL/g, sulfuric acid concentration=15 vol%, CaF2 addition=3 wt%, leaching time=30 min, leaching temperature=95°C) and the leaching residues taken from the regular leaching process (conditions: liquid to solid ratio=1.5:1 mL/g, sulfuric acid concentration=15 vol%, CaF2 addition=3 wt%, leaching time=240 min, leaching temperature=95°C).
3.2.1 Chemical phase transformation analyses In order to obtain a primary mineral phase transformation in the absence and presence of ultrasound energy. The XRD patterns of the leaching residues collected from two leaching systems are displayed in Fig. 10. 1
1-Quartz 2-Hematite 3-Gypsum 4-Albite 5-Hieratite 6-Muscovite 7-Anhydrite
Intensity, CPS
B 1 75 7 7 6 7 5 17 1 1 1 1 1 6 6 72 4 6 2 2 7 2 7 2 7 21 1 1
6
A
4
3 4
6 10
3 1 3 6 31 2 3 1 6 6 4 2 4 4 5 2 3 1 11 12 1 3 2 3 21 20
30
40
50
1
2 11 11 1
1 1 1
2 1 21 60
70
2θ, °
Fig. 10. XRD patterns of the residues obtained from regular (A) and ultrasonic-assisted (B) leaching process
Intensity, CPS
1-Gypsum 2-Anhydrite B
A 1 10
1 12
1 1 20
1 30
40
50
60
70
2θ, °
Fig. 11. XRD patterns of the ultrasonic-assisted leaching residues at leaching time of 10 min (A) and leaching time of 20 min (B)
It is clearly found that the muscovite diffraction peaks in both regular and ultrasonicassisted leaching residues are weaker than that in the roasted shale (Fig. 1), attributing to the disintegration of muscovite structure during the leaching process. In the meanwhile, it was noteworthy that the most distinct difference between the chemical phases in two XRD patterns (Fig. 10 A and B) is that there is gypsum in the regular
leaching residues, while anhydrite is produced in the ultrasonic-assisted leaching residues. As shown in Fig. 11, it is obvious that only gypsum exists in the ultrasonicassisted leaching residues at leaching time of 10 min (Fig. 11A), but there are not only gypsum but also anhydrite in the leaching residues with the leaching prolonging to 20 min (Fig. 11B). In other words, the ultrasound energy may cause the conversion of gypsum to anhydrite. During the solid-phase transition from gypsum to anhydrite, the anhydrite crystals would grow by spiral dislocation, which good crystal form. The strain concentration nearby the anhydrite would act on the muscovite structure octahedral and tetrahedral sites, generating vigoroso strain concentration on the muscovite particles, which would intensify the destruction of the muscovite structure and increase the specific surface of the particles of muscovite [7]. In this way, the muscovite particles dissolved much more easily with the application of ultrasound. 3.2.2 Particle size distribution analysis The particle size distribution of the leaching residues obtained after ultrasonicassisted and regular leaching process are shown in Fig. 12.
Differential distribution, %
5
4
3
2
1
Regular leaching Ultrasonic-assisted leaching
0 0
50
100
150
200
250
300
350
Particle size, μm
Fig. 12. The particle size distribution of the leaching residues
As displayed in Fig. 12, the differential distribution of particle size is mainly divided into two special areas. The size distribution percentage of ultrasonic leaching residues is higher than that of regular residues in the particle size range of 4.71 to 29.12 μm. Furthermore, in the range of 29.12 to 105.24 μm, the size distribution ratio of ultrasonic residues is visibly lower than that of regular residues. The difference in the differential distribution of the leaching residues by two leaching systems confirmed that ultrasonic energy can smash the V-bearing shale particles, which can reinforce the solid-liquid leaching reaction [20]. The results can also demonstrated the viewpoint that ultrasonic cavitation effects can improve the specific surface area of the particles and renew the product layer, which is in accordance with the leaching process of germanium from zinc metallurgy residues via ultrasonic-assisted method [19].
3.2.3 Microscopic surface and element distribution analyses In order to acquire a visual insight into the microsurface morphology and the element distribution of the residues collected from the regular and ultrasonic-assisted leaching, the SEM-EDS analyses were carried out under different magnification and the results are presented in Fig. 13.
Si
O
Al
K
Fig. 13. SEM and element distribution images of the residues obtained from regular (A), (B) and ultrasonic-assisted (C), (D) leaching process
The SEM images reveal the difference of the microstructure of the leaching residues using regular and ultrasound methods. From the contrast of Fig. 13B and Fig. 13D, it is distinct that the particle size of residues by ultrasonic-assisted method is finer than that by regular process, which is corresponding to the results of the particle size distribution analysis. In the meanwhile, it was clearly found in Fig. 13A and Fig. 13C that the particles in the regular leaching residues grown heterogeneous and dense, on the contrary, these particles are homogeneous and loose with ultrasound treatment. As is distinctly presented in the element distribution images of the two residues (Fig. 13), the muscovite particle in regular leaching residues is almost completely wrapped by CaSO4‧2H2O, which may have negative influence on the dissolution of V-bearing muscovite particle. While, most CaSO4 crystals fall off from the surface of muscovite particle with the application of ultrasound, which may promote the eroding of the muscovite particle with sulfuric acid. These difference demonstrate that the ultrasound energy can hinder the growth of the solid particles and break the V-bearing shale particles into finer pieces, which is beneficial for the renewal of the active surface and the reinforcement of the solid-liquid reaction [19], and it is meaningful for the release of vanadium from the lattice of mica group minerals.
4. Conclusions (1) Under the conditions of a sulfuric acid concentration of 15 vol%, 3 wt% CaF2 addition as well as a leaching temperature of 95°C and a liquid to solid ratio of 1.5:1 mL/g, 92.93% recovery of vanadium was obtained for a leaching duration of 30 min using 900 W ultrasonic power; However, 87.86% of vanadium was leached out with regular method at a leaching time of 240 min. (2) Ultrasonic cavitation has generated some remarkable advantages on the release of vanadium from vanadium-bearing (V-bearing) shale. First, ultrasonic energy can smash solid particles and improve the specific surface area of the V-bearing muscovite particles, as well as reduce the thickness of the diffusion layer, which can promote more active surface of the V-bearing shale particles exposed to the reaction solution and accelerate the diffusion rate. Second, the ultrasound energy makes transform from CaSO4‧2H2O to CaSO4, and the strain concentration nearby the CaSO4 would act on the octahedral and tetrahedral sites in the muscovite structure, which would further intensify the destruction of the muscovite structure and the release of vanadium from vanadium-bearing shale. (3) The kinetics fitting results show that the leaching process of vanadium from Vbearing shale is controlled by diffusion through the product/ash layer, and the reaction constant (kd) of vanadium leaching process with ultrasound is much higher than that without ultrasound, which results in higher leaching recovery and shorter leaching time with the appliance of ultrasound compared to that without ultrasound. (4) All of the traits of ultrasound are beneficial for fortifying the release of vanadium from V-bearing muscovite particles and markedly shortening the leaching time. Ultrasonic-assisted leaching may be a potential and promising technique for extraction vanadium from complex low-grade vanadium-bearing minerals.
Declaration of Competing Interest The authors declare no competing financial interest.
Appendix A. Supplementary material Supplementary data to this article http://dx.doi.org/10.17632/scx98yf4r6.1.
can
be
found
online
at:
Acknowledgements This research was supported by the National Natural Science Foundation of China (51874222) and the Major Technical Innovation Project of Hubei Province (2018ACA157).
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Bo Chen: Conceptualization; Investigation; Methodology; Software; Writing-Original draft preparation; Writing-Reviewing and Editing Shenxu Bao: Funding acquisition; Supervision; Validation Yimin Zhang: Supervision Sheng Li: Investigation; Software
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights Ultrasound is used to intensify low-grade vanadium-bearing shale leaching in H2SO4. Ultrasonic-assisted and regular leaching of vanadium were contrastively studied. Ultrasound shortens leaching time by 87.5% and increases leaching ratio about 5%. Enhancement mechanism of ultrasonic-assisted leaching has been detailedly analyzed.