Recovery of Vanadium from H2SO4-HF Acidic Leaching Solution of ...

metals Article

Recovery of Vanadium from H2SO4-HF Acidic Leaching Solution of Black Shale by Solvent Extraction and Precipitation Xingbin Li, Chang Wei *, Zhigan Deng, Cunxiong Li, Gang Fan, Minting Li and Hui Huang Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (X.L.); [email protected] (Z.D.); [email protected] (C.L.); [email protected] (G.F.); [email protected] (M.L.); [email protected] (H.H.) * Correspondence: [email protected]; Tel.: +86-871-6518-8819 Academic Editor: Corby G. Anderson Received: 21 January 2016; Accepted: 3 March 2016; Published: 14 March 2016

Abstract: The recovery of vanadium from sulfuric and hydrofluoric mixed acid solutions generated by the direct leaching of black shale was investigated using solvent extraction and precipitation methods. The process consisted of reduction, solvent extraction, and stripping, followed by precipitation and calcination to yield vanadium pentoxide. The influence of various operating parameters on the extraction and recovery of vanadium was studied. Vanadium (IV) was selectively extracted using a mixture of 10% (v/v) di(2-ethylhexyl)phosphoric acid and 5% (v/v) tri-n-butylphosphate in sulfonated kerosene. Using six extraction and five stripping stages, the extraction efficiency for vanadium was 96.7% and the stripping efficiency was 99.7%. V2 O5 with a purity of 99.52% was obtained by oxidation of the loaded strip solution and precipitation of ammonium polyvanadate at pH 1.8 to 2.2, followed by calcination of the dried precipitate at 550 ˝ C for 2 h. It was concluded that the combination of solvent extraction and precipitation is an efficient method for the recovery of vanadium from a multi-element leach solution generated from black shale. Keywords: solvent extraction; vanadium precipitation; black shale; vanadium

1. Introduction Black shale (also called stone coal) is an important vanadium resource in China, where it is found extensively in the southern provinces and autonomous regions of the country. The gross reserves of vanadium in black shale in Hunan, Hubei, Guangxi, Jiangxi, Zhejiang, Anhui, Guizhou, Henan, and Shanxi provinces are 18 million tons in terms of V2 O5 , which accounts for more than 87% of the domestic reserves of vanadium [1]. The vanadium grade in black shale generally ranges from 0.13% to 1.2%, while grades higher than a cut-off of 0.5% account for about 40% of the total vanadium reserves [2]. Besides vanadium, some 60 different metallic and nonmetallic elements have been identified in these sources, including molybdenum, nickel, uranium, copper, selenium, gallium, and precious metals [3–5]. Black shale is therefore regarded as a complicated low-grade multi-element ore. Because of its relatively high vanadium grade and abundant deposits, studies of the recovery of vanadium from black shale have received considerable attention. Vanadium exists as the (II), (III), (IV), and (V) species in nature [6]. A previous study showed that vanadium in black shale is found mainly as vanadium (III), then as vanadium (IV) and vanadium (V) [7]. Vanadium (III), which usually exists in the crystal lattices of aluminosilicate clay minerals, comprises more than 90% of the total vanadium content because black shale originates from reducing environments [8–10]. The principle of recovering vanadium from black shale is first to decompose the crystal lattice of the aluminosilicate clay minerals and oxidize insoluble vanadium (III) to acid-soluble

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vanadium (IV) and/or water-soluble vanadium (V) compounds, then to dissolve these species by acid, water, or alkaline leaching. Finally, pure vanadium products are produced from the leach solution by separation and purification, using methods including solvent extraction, ion exchange, and precipitation [11,12]. The traditional technique for the extraction of vanadium from black shale was by roasting with sodium salts and water leaching [13–15]. Using this technique, the roasting efficiency is about 45%–55%, the total vanadium recovery is less than 45%, and the released exhaust gas containing Cl2 , HCl, and SO2 pollutes the environment. These serious disadvantages have limited further development of this technology and the sodium salt roasting process has been prohibited by the local government since 2006. A recently developed technique that does not involve roasting is direct acid leaching [16–22]. Investigations have found that vanadium in aluminosilicate and mica clay minerals can be leached by sulfuric acid with the addition of HF, NaF, or CaF2 to the leaching processing [23–25]. The resultant hydrofluoric acid decomposes the vanadium-bearing aluminosilicate clay minerals, allowing the exposed vanadium (III) to be oxidized to vanadium (IV) by Fe (III) and converted to soluble VOSO4 in the leach solution. The extent of vanadium leaching can exceed 85%, which is much higher than that of traditional roasting processes. Furthermore, this process completely avoids the discharge of roasting exhaust gas and lowers energy consumption. However, the various impurities elements in black shale, for instance, Ni, Al, Fe, Mg, Mn, Si, and P, leach into the solution with vanadium in the acid-leaching process. Selective extraction of vanadium is key to the recovery of pure vanadium products from the multi-element leach solutions of vanadium-bearing black shale [26,27]. Solvent extraction offers an attractive alternative for the extraction and recovery of pure metals from multi-element leach solutions [28–30]. Although the extraction of vanadium from various acidic solutions has been reported, there has been no systematic study of the recovery of vanadium from multi-element H2 SO4 -HF leach solutions from black shale by combined solvent extraction and precipitation, which is important for the practical application of this technology [31–33]. The aim of the present work was to extract and recover vanadium from such solutions using solvent extraction and precipitation. The effects of the aqueous-phase pH, extractant concentration, and organic-to-aqueous phase ratio (O/A) on the extraction of vanadium, and the effects of pH, temperature, and vanadium concentration on the precipitation of vanadium, are reported. This study provides data to develop a flow sheet for the recovery of a high purity vanadium product from the solutions generated by the direct leaching of black shale. 2. Experimental 2.1. Materials and Reagents In our previous tests, a direct acid leaching process using sulfuric acid and hydrofluoric acid solutions was proposed to recover vanadium from black shale and a high leaching ratio of 86% was obtained. In the present study, the starting solution was generated by the same conditions as our previous studies, using a liquid/solid ratio of 4 mL/g, a sulfuric acid concentration of 87.5 g/L, a hydrofluoric acid concentration of 15 g/L, a sodium hypochlorite concentration of 1 g/L, a temperature of 95 ˝ C, and a reaction time of 6 h [24]. The composition of the leach solution generated from black shale from Hunan Province, China, is listed in Table 1. D2EHPA (di(2-ethylhexyl)phosphoric acid) and TBP (tri-n-butylphosphate), supplied by Shanghai Rare-Earth Chemical Co., Ltd., Shanghai, China, were used without further purification. Commercial kerosene was contacted with 98% concentrated sulfuric acid to produce sulfonated kerosene for use as the diluent. All other reagents and chemicals were of analytical reagent grade.

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Table 1. Composition of the liquor generated from the leaching of black shale in the H2 SO4 -HF mixture. Component

Concentration (g/L)

V2 O 5 * Fe(II) Fe(III) Al As Zn Cu Mg Si SO4 2´ P F´

6.75 8.10 5.30 9.20 0.10 1.01 0.43 1.20 0.50 78.50 0.32 15.0

* Vanadium concentrations are all represented as V2 O5 in this paper.

2.2. Experimental Procedure 2.2.1. Solvent Extraction The metal equilibrium distributions between the organic and the aqueous phase were obtained by contacting a given volume ratio of the two liquids in a 250 mL mechanically agitated extractor. After the extraction, the two phases were separated in a separating funnel. The batch simulation counter-current experiments were performed in a box-style mixer-settler unit which contained six mixer-settler stages, while it was just used in the former five stages in the stripping. Each stage consisted of square-box type mixing and settling compartments. The active mixer volume was 0.5 L and the settler volume was 2.0 L. The agitation speed was 800 rpm. The vanadium (VO2+ and total vanadium concentrations) in the aqueous solutions was determined by the ferrous ammonium sulfate titration using 2-(phenylamino) benzoic acid as an indicator [34]. The Fe (II) concentration was determined by potassium dichromate titration using sodium diphenylamine sulfonate as an indicator. Fluoride ion selective electrode (Crison 9655, HACH, Loveland, CO, USA) was used to measure fluoride concentration and the phosphorus concentration was measured by the ammonium molybdate spectrometric method using UV-vis spectrophotometer (UV-vis, UV-2550, Shimadzu, Kyoto, Japan). The compositions of other elements in the aqueous phases were analyzed by inductively coupled, plasma atomic emission spectroscopy (ICP-AES) using an OPTIMA 5300DV (Perkin-Elmer, Waltham, MA, USA). Metal concentrations in the organic solutions were calculated by mass balance. The analysis method used for the vanadium pentoxide product is in accordance with the relevant industry standard of YB/T5304-2011 in China [35]. The solution pH was adjusted using 3 mol/L H2 SO4 or 20% ammonia water, and the pH was measured using a digital pH meter (INESA instrucment, Shanghai, China). All extraction and stripping experiments were carried out at 25 ˘ 1 ˝ C. 2.2.2. Oxidation and Precipitation The oxidation and precipitation processes were conducted at atmospheric pressure in a 1000 mL stirred glass reactor that was placed in a temperature-controlled water bath. Hydrogen peroxide was used to oxidize vanadium (IV) to vanadium (V). Aqueous ammonia was used to control the pH and precipitate ammonium polyvanadate. The solid ammonium polyvanadate derived from the precipitation process was separated by vacuum filtration (0.7 atm) and washed twice with distilled water.

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2.2.3. Calcination The ammonium polyvanadate was dried at 60 ˝ C in a constant-temperature laboratory oven. The V2 O5 product was obtained by calcining the dried ammonium polyvanadate in a muffle furnace at a determined temperature and time. 3. Results and Discussion 3.1. Ferric Ion Reduction Because the extraction order of D2EHPA is Fe3+ > VO2+ > VO2 + > Fe2+ [36], this means that Fe3+ in solution is more strongly extracted by D2EHPA than VO2+ . The presence of Fe3+ not only reduces the maximum loading of vanadium on the extractant, but also compromises the purity of the final vanadium product. To selectively extract vanadium and avoid the co-extraction of Fe3+ , it is necessary to reduce Fe (III) to Fe (II) before the extraction of vanadium. Sodium sulfite was employed as the reductant. The reaction can be expressed as: 2Fe3` ` SO23´ ` H2 O Ñ 2Fe2` ` SO24´ ` 2H`

(1)

VO2 + in the leach solution is also reduced to VO2+ in the reduction process: 2´ ` 2` 2VO` ` SO24´ ` H2 O 2 ` SO3 ` 2H Ñ 2VO

(2)

To determine the amount of reductant necessary for Fe (III) reduction, different masses of Na2 SO3 were added into 300 mL of leach solution for 2 h at 30 ˝ C at different nR /nFe(III) coefficients, where nR /nFe(III) is the molar ratio of Na2 SO3 to Fe3+ . The reduction results are shown in Table 2. With a nR /nFe(III) coefficient of 0.8, more than 99.5% of the Fe (III) present was reduced to Fe (II). The further addition of Na2 SO3 gave no further reduction of Fe (III). In subsequent experiments, the leach solution was reduced by Na2 SO3 using a nR /nFe(III) coefficient of 0.8. The reduced leach solution was used as the feed solution for the solvent extraction test work. Table 2. Effect of Na2 SO3 /Fe (III) molar ratio on Fe (III) and V (V) reduction efficiency. nR /nFe(III)

0.2

0.4

0.6

0.8

1.0

Percent of Fe (III) reduction, % Percent of V (V) reduction, %

67.2 96.5

80.8 99.2

94.1 99.8

99.8 100

99.9 100

3.2. Solvent Extraction and Stripping of Vanadium (IV) 3.2.1. Effect of pH and Extractant Concentration The effect of equilibrium pH in the range of pH 1.5 to 3.0 on the extraction of vanadium, carried out using different concentrations of D2EHPA with 5% (v/v) TBP in sulfonated kerosene, is shown in Figure 1. The results show that vanadium extraction efficiency initially increases with increasing pH because this is a cation exchange process. The maximum extraction was achieved at an equilibrium pH of 2.5 for the various D2EHPA concentrations employed. Vanadium extraction reached 85.6% at an equilibrium pH of 2.5 using 10% (v/v) D2EHPA and 5% (v/v) TBP in a single-stage extraction. Furthermore, when the aqueous pH exceeded 2.5, the solution became turbid and there was a formation of a third phase during the solvent extraction process. An X-ray diffraction (XRD) of the dried third phase (Figure 2) indicates that the major phases precipitating were (NH4 )2 Fe(SO4 )2 ¨ 6H2 O, CaPO3 (OH)2 ¨ H2 O or CaSO4 ¨ 2H2 O, and NaFe9 (PO4 )6 (OH)10 ¨ 5H2 O. The optimum aqueous pH for vanadium extraction should therefore be determined by considering both the vanadium extraction efficiency and the need

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to avoid third phase formation. An equilibrium pH of 2.5 was considered optimal and chosen for Metals 2016, 6, 63 5 of 13 subsequent extraction experiments. Metals 2016, 6, 63

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100 100 90

Vanadium extraction, Vanadium extraction, % %

90 80 80 70 70 60 60 50 50 40

5% (v/v) D2EHPA+5% (v/v) TBP 8% (v/v) D2EHPA+5% (v/v) TBP 5% (v/v) 10% (v/v)D2EHPA+5% D2EHPA+5%(v/v) (v/v)TBP TBP 8% (v/v) 15% (v/v)D2EHPA+5% D2EHPA+5%(v/v) (v/v)TBP TBP 10% (v/v) D2EHPA+5% (v/v) TBP 20% (v/v) D2EHPA+5% (v/v) TBP 15% (v/v) D2EHPA+5% (v/v) TBP 20% (v/v) D2EHPA+5% (v/v) TBP

40 30 30 20 20 10 10 0 0

1.4

1.6

1.8

2.0

1.4

1.6

1.8

pH 2.0Equilibrium 2.2 2.4

2.2

2.4

2.6

2.8

3.0

3.2

2.6

2.8

3.0

3.2

Equilibrium pH

Figure Figure 1. 1. Effect Effectof ofequilibrium equilibriumpH pHon onvanadium vanadiumextraction. extraction.(O/A (O/A phase phaseratio ratio==1/1, 1/1, agitation agitation speed speed ˝ 800 rpm, time 25 C).vanadium extraction. (O/A phase ratio = 1/1, agitation speed 800 rpm, extraction time 10 10 min, min,pH 25 on °C). Figure 1.extraction Effect of equilibrium

800 rpm, extraction time 10 min, 25 °C). 3500 3500 3000 3000 2500

1

1- (NH4)2Fe(SO4)2•6H2O

1

•H O or CaSO42•2H2O 2-CaPO 1- (NH43)(OH) Fe(SO 2 4)22•6H2O 2 ) (OH) •5H2O •2H O 3-NaFe (OH) 2-CaPO9(PO 4 6 •H O10or CaSO

Intensily Intensily

3

2000 1500 1500 1000 1000 500

32 32

500 0 0

2

2

42

2

3-NaFe9(PO4)6(OH)10•5H2O

2500 2000

10

1 1 2 1 1 2 2 1 2 1 1 3 32111 1 2 1 1 2 2 3 3 11 2 1 2 2 2 20 30 40 50

60

70

2-Theta(°) 40 50 60 70 2-Theta(°) Figure 2. XRD pattern of dried third phase produced during extraction process. Figure 2. 2. XRD XRD pattern pattern of of dried dried third third phase phase produced produced during during extraction extraction process. process. Figure 10

20

30

From the results of Figure 1, it also can be observed that the extraction of vanadium increased of Figure 1, it alsoin can be observed that the extraction vanadium increased fromFrom 65.1%the to results 85.6% with the increase D2EHPA concentration from 5% of (v/v) to 10% (v/v), and From the results of Figure 1, it also can be observed that the extraction of vanadium increased from 65.1% to 85.6% with the increase in D2EHPA concentration to 10% extraction (v/v), and upon the further increase in the D2EHPA concentration to 20% from (v/v),5% the(v/v) vanadium from 65.1% to 85.6% with the increase in D2EHPA concentration from 5% (v/v) to 10% (v/v), and upon theslowly. furtherBased increase in the D2EHPA concentration to 20% (v/v), the increase on the experimental results, 10% (v/v) D2EHPA wasvanadium sufficient extraction to extract upon the further increase in the D2EHPA concentration to 20% (v/v), the vanadium extraction increase increase slowly. Based the experimental results, 10% (v/v) D2EHPA was sufficient to extract vanadium from the leachonsolution. slowly. Based on the experimental results, 10% (v/v) D2EHPA was sufficient to extract vanadium from vanadium from the leach solution. the leach solution. 3.2.2. Effect of Fluoride Ion Concentration 3.2.2. Effect Effect of of Fluoride Fluoride Ion Ion Concentration 3.2.2. The effect of fluoride Concentration ion concentration on the extraction of vanadium (IV) was studied. The The effect of fluoride ion concentration concentration ondifferent the extraction extraction of vanadium vanadium (IV) was was studied. The fluoride ion concentration varied in step with concentrations of sodium fluoride fromThe 15 The effect of fluoride ion on the of (IV) studied. fluoride ion concentration varied in step with different concentrations of sodium fluoride from 15 g/L to 35 g/L, the results of which are shown in Figure 3. It was observed that the change in fluoride fluoride ion concentration varied in step with different concentrations of sodium fluoride from 15 g/L g/L35to 35 g/L, results of which are shown in Figure It was observed the change in fluoride ion concentration in of thewhich investigated range had almost no effect on that the that vanadium (IV) extraction, to g/L, the the results are shown in Figure 3. It3. was observed the change in fluoride ion ion concentration in the investigated range had almost no effect on the vanadium (IV) extraction, and the extraction of fluoride ion is very low, which indicates that the fluoride ion does not concentration in the investigated range had almost no effect on the vanadium (IV) extraction, and the and the extraction of fluoride ion is very low, which indicates that the fluoride ion does not participateofinfluoride the extracted complexes. extraction ion is very low, which indicates that the fluoride ion does not participate in the participate in the extracted complexes. extracted complexes.

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90 90

80

Extraction, %

Extraction, %

80

70

70

Vanadium Vanadium Fluoride ion Fluoride ion

60 60 50 50 4 3 2 1 0

4 3 2 1 0 15

20

15

25

30

35

20 25 30 Fluoride ion concentration, g/L

35

Fluoride ion concentration, g/L

Figure 3. Effect of fluoride ion concentration on the extraction of vanadium (IV) with D2EHPA and

Figure 3. Effect of of fluoride ion concentration on the the extraction of of vanadium vanadium (IV) (IV) with D2EHPA D2EHPA and Figure 3. (D2EHPA Effect fluoride ion concentration on TBP = 10%(v/v), TBP = 5% (v/v), O/A phaseextraction ratio = 1/1, agitation speed 800 with rpm, extraction and TBP (D2EHPA = 10% (v/v), TBP = 5% (v/v), O/A phase ratio = 1/1, agitation speed 800 rpm, extraction TBP (D2EHPA 10%(v/v), TBP = 5% (v/v), O/A phase ratio = 1/1, agitation speed 800 rpm, extraction time 10 min,=˝25 °C) time 10 min, 25 C). time 10 min, 25 °C) 3.2.3. McCabe-Thiele Plot for Vanadium (IV) Extraction

3.2.3. McCabe-Thiele McCabe-Thiele Plot Plot for for Vanadium Vanadium (IV) 3.2.3. (IV) Extraction Extraction To determine the theoretical number of stages required for the maximum extraction of To determine theoretical number of stages required for the maximum extraction ofdrawn vanadium vanadium at a the chosen phase ratio and D2EHPA concentration, an extraction isotherm was To determine the theoretical number of stages required for the maximum extraction of from the results obtained by contacting the leach solution with organic phase (10% (v/v) D2EHPA at a chosen phase ratio and D2EHPA concentration, an extraction isotherm was drawn from the vanadium at a chosen phase ratio and D2EHPA concentration, an extraction isotherm was drawn and 5% (v/v)by TBP in sulfonated kerosene at different O/A phase ratios (O/A = 1/8, 1/5, 1/2, and 1/1, 2/1, results obtained contacting the leach solution with organic phase (10% (v/v) 5% (v/v) from the results obtained by contacting the leach solution with organic phaseD2EHPA (10% (v/v) D2EHPA 5/1). The McCabe-Thiele plot (Figure 4) shows that quantitative extraction of vanadium can be TBP 5% in sulfonated kerosene at different O/A phase ratios 1/8, 1/5, 1/1, 2/1, and (v/v) TBP in sulfonated kerosene at different O/A(O/A phase=ratios (O/A1/2, = 1/8, 1/5, 1/2,5/1). 1/1, The 2/1, theoretically achieved in five stages of counter-current extraction at an O/A phase ratio of 1.0. McCabe-Thiele plot (Figure 4) shows that quantitative extraction of vanadium can be theoretically 5/1). Because The McCabe-Thiele plot (Figure 4) number shows that quantitative extraction can be the pH will decrease as the of extraction stages increases of in vanadium a continuous achieved in five stages in of counter-current extraction at anextraction O/A phase ratio of 1.0. Because the pH theoretically achieved five stages of counter-current at an O/A phase ratio of 1.0. counter-current extraction process, which influences the extraction efficiency, one more stage was will decrease thewill number of extraction stages increases in a extraction continuous counter-current extraction Because theAas pH decrease number of extraction stages was increases continuous added. batch simulation of as six the stages of counter-current carried in outa and the process, which influences the extraction efficiency, one more stage was added. A batch simulation of raffinates were analyzedprocess, for vanadium other metals concentration (Table one 3). The raffinate counter-current extraction which and influences the extraction efficiency, more stage was six stages counter-current was carried out and the raffinates were contained 222simulation mg/L V2O5, extraction which 96.7% extraction efficiency. The initial andfor final pH the added. A ofbatch of sixcorresponds stages of to counter-current extraction wasanalyzed carried outvanadium and and other metals concentration (Table 3). The raffinate contained 222 mg/L V O , which corresponds of the solutions are 2.5 and 1.52, respectively. The loaded organic, containing 6.45 g/L V 2 O 5 , was then 2 5 raffinates were analyzed for vanadium and other metals concentration (Table 3). The raffinate usedextraction to carry outefficiency. stripping studies. to 96.7% The initial and final pH of the solutions are 2.5 and 1.52, respectively. contained 222 mg/L V 2O5, which corresponds to 96.7% extraction efficiency. The initial and final pH The loaded organic, containing g/L V2 O5The , was then used to carry out stripping of the solutions are 2.5 and 1.52,6.45 respectively. loaded organic, containing 6.45 g/Lstudies. V2O5, was then Table 3. Chemical analysis of the raffinate. used to carry out stripping studies. raffinate. Component Table 3.V2Chemical O5 Feanalysis Alof the As Zn Concentration (g/L) 0.222 13.0 9.0 0.93 0.93 Table 3. Chemical analysis of the raffinate. Component V2 O5 Fe Al As Zn

Component Concentration (g/L)

V2O513.0 8 0.222

Concentration (g/L)

0.222

O/A=5 O/A=8 Fe 9.0 Al 0.93As O/A=2 13.0 9.0 0.93

6

[V2O5]org, g/L

[V2O5]org, g/L

0.93

Cu 0.41

0.41

Mg 1.15 1.15

O/A=2 4

A/O=2

6

2

n tio era Op

O/A=1 A/O=5

0A/O=2 0

1

2

A/O=5

1:1 A= O: e lin

1:1 A= O: e lin4 5

4

2

Zn 0.93

O/A=5 O/A=8

O/A=1

8

Cu Mg 0.41 1.15 Cu Mg

3 ion rat[V2O5]aq, g/L e Op

6

7

8

Figure 4. Extraction isotherm for vanadium (D2EHPA = 10% (v/v), TBP = 5% (v/v), equilibrium pH = 2.5, 25 °C). 0

3.2.4. Stripping of Vanadium0 (IV) 1

2

3

4

5

6

7

8

[V2O5]aq, g/L

Figure 4. Extraction isotherm for vanadium (D2EHPA = 10% (v/v), TBP(v/v), = 5% (v/v), = 2.5, 25 °C). Figure 4. Extraction isotherm for vanadium (D2EHPA = 10% TBP equilibrium = 5% (v/v),pH equilibrium ˝ pH = 2.5, 25 C).

3.2.4. Stripping of Vanadium (IV)

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3.2.4. Stripping of Vanadium (IV) The loaded organic was scrubbed using 0.15 mol/L sulfuric acid solution at an O/A phase ratio The loaded organic was scrubbed using 0.15 mol/L sulfuric acid solution at an O/A phase ratio of 2.0 to remove the entrainment impurities. The scrubbed loaded organic phase was stripped with of 2.0 to remove the entrainment impurities. The scrubbed loaded organic phase was stripped with sulfuric acid. Different concentrations of sulfuric acid were contacted with the loaded organic phase sulfuric acid. Different concentrations of sulfuric acid were contacted with the loaded organic phase at at various O/A phase ratios. The results (Figure 5) show that the vanadium stripping increases from various O/A phase ratios. The results (Figure 5) show that the vanadium stripping increases from 40.6% to 97.9% as the sulfuric acid concentration increased from 0.5 to 2.5 mol/L at an O/A phase 40.6% to 97.9% as the sulfuric acid concentration increased from 0.5 to 2.5 mol/L at an O/A phase ratio of 2/1. Additionally, the residual sulfuric acid in the spent strip solution also increased with the ratio of 2/1. Additionally, the residual sulfuric acid in the spent strip solution also increased with the increasing acid concentration. As the initial concentration of H2SO4 increased from 0.5 to 2.5 mol/L, increasing acid concentration. As the initial concentration of H2 SO4 increased from 0.5 to 2.5 mol/L, the residual H2SO4 in the strip solution increased from 0.26 to 2.1 mol/L. Greater residual sulfuric the residual H2 SO4 in the strip solution increased from 0.26 to 2.1 mol/L. Greater residual sulfuric acid in the strip solution means that more aqueous ammonia will be consumed during the acid in the strip solution means that more aqueous ammonia will be consumed during the subsequent subsequent vanadium precipitation process. The optimum amount of H2SO4 was determined by vanadium precipitation process. The optimum amount of H2 SO4 was determined by considering both considering both the vanadium stripping efficiency and the residual sulfuric acid concentration in the vanadium stripping efficiency and the residual sulfuric acid concentration in the strip solution. the strip solution. For the present study, 1.5 mol/L H2SO4 was chosen as optimum. For the present study, 1.5 mol/L H2 SO4 was chosen as optimum. 100 90

O/A ratio=1/1 O/A ratio=2/1 O/A ratio=5/1 O/A ratio=8/1 O/A ratio=10/1

Stripping percentage, %

80 70 60 50 40 30 20 10 0 0.5

1.0

1.5

2.0

2.5

3.0

Concentration of H2SO4, mol/L

Figure Figure 5.5. Effect Effect of of H H22SO SO44 concentration concentration and and organic-to-aqueous organic-to-aqueous (O/A) (O/A) phase ratio on on vanadium vanadium ˝ C). stripping stripping (agitation (agitation speed speed 1000 1000 rpm, rpm, stripping strippingtime time10 10min, min,25 25 °C).

3.2.5. 3.2.5. Stripping Stripping Isotherm Isotherm for for Vanadium Vanadium(IV) (IV) The The stripping stripping isotherm isotherm for for vanadium vanadium was was constructed constructed to to determine determine the the number number of of stages stages required stripping at at aachosen chosenphase phaseratio. ratio.The The loaded organic phase 1.5 mol/L SO4 required for for stripping loaded organic phase andand 1.5 mol/L H2SOH4 2were were contacted at different 10:1 1:2, while keeping the total volume constant. contacted at different O/A O/A phasephase ratiosratios from from 10:1 to 1:2,towhile keeping the total volume constant. The McCabe-Thiele plotplot (Figure 6) illustrates that greater than be The McCabe-Thiele (Figure 6) illustrates that greater than99% 99%ofofthe theloaded loaded vanadium vanadium can can be theoreticallystripped strippedusing usingfour fourcounter-current counter-current stages at an O/A flow ratio 8. Considering theoretically stages at an O/A flow ratio of 8.ofConsidering thatthat the the sulfuric acid concentration will decrease as the number of stripping stages which increases, which sulfuric acid concentration will decrease as the number of stripping stages increases, influences influences the strippingone efficiency, onewas more stageAwas A batch simulation five stages of the stripping efficiency, more stage added. batchadded. simulation of five stages ofof counter-current counter-current stripping was out of at 8anusing O/A 1.5 ratio of 8 using mol/L H 2 SO 4 . The stripped stripping was carried out at ancarried O/A ratio mol/L H2 SO1.5 . The stripped organic phase 4 organic phase contained 20 mg/L V2O5, that which indicates that greater than 99.7% vanadium contained only 20 mg/L V2only O5 , which indicates greater than 99.7% vanadium stripping efficiency stripping efficiency was achieved. was achieved.

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O/A=10 50

O/A=8

[V2O5]aq, g/L

40

30

O/A=5

n tio era Op

20

8:1 :A= O e lin

O/A=2

10

O/A=1 O/A=1/2

0 0

1

2

3

4

5

6

7

[V2O5]org, g/L

Figure Figure6.6.McCabe-Thiele McCabe-Thieleplot plotfor forvanadium vanadiumstripping stripping(V(V 2O inloaded loadedorganic organicphase phase==6.45 6.45g/L, g/L,strip strip 2O 5 5in ˝ solution H22SO SO44, ,25 25°C). C). solution== 1.5 1.5 mol/L mol/L H

3.3.Oxidation Oxidationand andPrecipitation Precipitation 3.3. 3.3.1. 3.3.1.Oxidation Oxidation 2+) The The vanadium vanadium (VO (VO2+ ) in in the the strip strip solution solution should should be be oxidized oxidized to toVO VO22++ before before precipitation. precipitation. KMnO , NaClO , H O , and H SO are common oxidants that can be used. Among these, KMnO44, NaClO33, H22O22, and H22SO55 are common oxidants that can be used. Among these,HH 2O was 2O 2 2was chosen chosenfor forits itsstrong strongoxidizing oxidizingability, ability,low lowcost, cost,and andeasy easyoperation: operation:

2+ `+ +H O 2VO +H H22OO2 2“=2VO 2VO 2VO2` ` 2 H2 O2 2 `

(3). (3)

2O2 at six times the stoichiometric Inthe thepresent presentstudy, study,oxidation oxidationwas wascarried carriedout outby byadding addingHHO In 2 2 at six times the stoichiometric amountat at50 50˝°C for 11h. h. The The color color of of the the strip strip solution solution changed changed from fromblue blueto toyellow yellowat atthe theend endof ofthe the amount C for reaction. Sample testing showed that the vanadium (IV) concentration in the oxidized solution was reaction. Sample testing showed that the vanadium (IV) concentration in the oxidized solution was below0.1 0.1g/L, g/L, which which meant meant that that almost almost all all of of the the vanadium vanadium (IV) (IV) was was oxidized oxidized to to vanadium vanadium(V). (V).The The below vanadium(V) (V)solution solutionwas wasused usedas asthe thefeed feedfor forthe theprecipitation precipitationexperiments. experiments. vanadium

3.3.2.Precipitation Precipitation 3.3.2. After the the oxidation oxidation of of the the strip strip solution, solution, vanadium vanadium can can be be precipitated precipitated from from the the vanadium vanadium After (V)-containing solution by adding an ammonium salt, such as (NH 4 ) 2 SO 4 , (NH 4 ) 2 CO 3 , or NH44Cl. Cl. (V)-containing solution by adding an ammonium salt, such as (NH4 )2 SO4 , (NH4 )2 CO3 , or NH Becauseaqueous aqueousammonia ammonia was added to feed the feed solution to neutralize the residual acidpresent in the Because was added to the solution to neutralize the residual acid in the present study, a large amount of (NH 4 ) 2 SO 4 was produced during the neutralization reaction and study, a large amount of (NH4 )2 SO4 was produced during the neutralization reaction and additional additional ammonium salt was not required. ammonium salt was not required. The effect effect of of temperature temperature on on vanadium vanadium precipitation precipitation was was studied studied in in the the range range of of 25 25 to to 95 95 ˝°C. The C. The results shown in Figure 7 indicate that precipitation efficiency increases with increasing reaction The results shown in Figure 7 indicate that precipitation efficiency increases with increasing reaction ˝ Ccaused temperature. Increasing Increasing the the solution extent of temperature. solution temperature temperaturefrom from60 60to to90 90°C causedananincrease increaseininthe the extent vanadium precipitation; little effect. effect. According According to to of vanadium precipitation;however, however,a afurther furtherincrease increasein in temperature temperature had had little ˝ these data, vanadium precipitation should be carried out between 90 and 95 °C. these data, vanadium precipitation should be carried out between 90 and 95 C.

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Vanadium precipitationratio,% ratio,% Vanadium precipitation

9 of 13 9 of 13 9 of 13 100 100 95 95 90 90 85 85 80 80 75 75 70 70 65 65 60 60 55 55 50 50

20 20

30 30

40 40

50 50

60 60

70 70

80 80

Temperature, oC Temperature, oC

90 90

100 100

Figure 7. Effect of temperature on vanadium precipitation (V2O5 concentration 52 g/L, equilibrium Figure7.7.Effect Effectofoftemperature temperatureon onvanadium vanadiumprecipitation precipitation(V(V 2O5concentration 52g/L, g/L, equilibrium equilibrium Figure 2O 5 concentration52 pH of 1.8–2.2, reaction time of 2 h). pH of 1.8–2.2, reaction time of 2 h). pH of 1.8–2.2, reaction time of 2 h).

Vanadiumprecipitation precipitationratio,% ratio,% Vanadium

The effect of pH on vanadium precipitation was studied under conditions of 90 °C for a reaction The effect of pH on vanadium precipitation was studied under conditions of 90˝°C for a reaction pH onare vanadium precipitation wasresult studied under conditions of 90 C forreaction a reaction time The of 2 effect h. Theofresults shown in Figure 8. This suggests that the precipitation is time of 2 h. The results are shown in Figure 8. This result suggests that the precipitation reaction is time of 2 h. The results are shown in Figure 8. This result suggests that the precipitation reaction is promoted in the pH range of 1.8–2.2. promoted in the pH range of 1.8–2.2. promoted in the pH range of 1.8–2.2. 100 100 95 95 90 90 85 85 80 80 75 75 70 70 65 65 60 60 55 55 50 500 0

1 1

2 2

3 3

4 4

Equilibrium pH Equilibrium pH

5 5

6 6

7 7

Figure 8. Effect of equilibrium pH on vanadium precipitation (V2O5 concentration 52 g/L, reaction Figure8.8.Effect Effectofofequilibrium equilibriumpH pHon onvanadium vanadiumprecipitation precipitation(V(V 2O concentration52 52g/L, g/L, reaction reaction Figure 2O 5 5concentration time of 2 h, 90 °C). timeof of22h, h,90 90˝°C). time C).

3.4. Calcination 3.4. Calcination Calcination 3.4. The dried ammonium polyvanadate was calcined at 550 °C˝ for 2 h in a muffle furnace under an The dried dried ammonium ammonium polyvanadate inin a muffle furnace under an The polyvanadate was wascalcined calcinedatat550 550°CCfor for2 2h h a muffle furnace under oxidizing atmosphere; the XRD analysis for this (Figure 9) fitted the vanadium pentoxide pattern oxidizing atmosphere; thethe XRD vanadium pentoxide pentoxide pattern pattern an oxidizing atmosphere; XRDanalysis analysisfor forthis this(Figure (Figure9) 9) fitted fitted the the vanadium well. The composition of the vanadium pentoxide product is shown in Table 4 and compared with well. The composition of the vanadium pentoxide product is shown in Table 4 and compared well. The composition of the vanadium pentoxide product is shown in Table 4 and compared withwith the the requirements of the relevant industry standard in China (YB/T5304-2011) [34]. As shown, the purity the requirements of the relevant industry standard in China (YB/T5304-2011)[34]. [34].As Asshown, shown,the thepurity purity requirements of the relevant industry standard in China (YB/T5304-2011) of the product reached 99.52% V2O5, with trace amounts of sodium, iron, and phosphorus impurities. of the the product product reached amounts ofof sodium, iron, and phosphorus impurities. of reached 99.52% 99.52%V V22OO5,5with , withtrace trace amounts sodium, iron, and phosphorus impurities.

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10 of 13 10 of 13 7000

∆

6000

∆-V2O5

5000

Intensity

4000 3000

∆

∆

∆

2000

∆

1000

∆

∆

∆

∆ ∆ ∆ ∆ ∆ ∆∆

∆∆

0 10

20

30

40

50

60

70

80

90

2-Theta(°)

Figure9.9.XRD XRDpattern patternof ofthe theVV22O O55 product. product. Figure Table4.4. Chemical Chemicalanalysis analysisof ofvanadium vanadiumpentoxide. pentoxide. Table

Component (%) Product Product V2O5 99 YB/T 5304-2011 V2 O5 99 V2O5 98 YB/T 5304-2011

V2O5 V2 O5 99.52 99.52 ≥99.0 ě99.0 ≥98.0

Si Si 0.005 0.005 ≤0.20 ď0.20 ≤0.25

Fe P S As K2O + Na2O Fe P S As K2 O + Na2 O 0.08 0.01 0.007 0.006 0.15 0.08 0.01 0.007 0.006 0.15 ≤0.20 ≤0.03 ≤0.01 ≤0.01 ≤1.0 ď0.20 ≤0.05 ď0.03 ≤0.03 ď0.01 ≤0.02 ď0.01 ď1.0 ≤0.30 ≤1.5

V2 O5 98

ě98.0

ď0.25

ď0.30

Component (%)

ď0.05

ď0.03

ď0.02

ď1.5

3.5. Development of a Process Flow Sheet to Recover Vanadium 3.5. Development of ainvestigation Process Flow Sheet to Recover Vanadium Based on the and discussion above, a process flow sheet to extract and recover vanadium theinvestigation H2SO4-HF solution generatedabove, by thealeaching black shale solvent extraction Based from on the and discussion processof flow sheet toby extract and recover and precipitation proposed in Figure 10. In this flow sheet, vanadium (IV) in the leach solution can vanadium from theisH SO -HF solution generated by the leaching of black shale by solvent extraction 2 4 be effectively extracted using 10% (v/v) D2EHPA with 5% (v/v) TBP at an aqueous pH of 2.5 and an and precipitation is proposed in Figure 10. In this flow sheet, vanadium (IV) in the leach solution can be O/A phaseextracted ratio of using 1/1. With six counter-current extraction stages, 96.7% vanadium is extracted. effectively 10% (v/v) D2EHPA with 5% (v/v) TBP at an aqueous pH of 2.5 and an O/A Vanadium in1/1. the loaded phase can be completely by 1.5 mol/L H2SO4 Vanadium at an O/A phase ratio of With sixorganic counter-current extraction stages, stripped 96.7% vanadium is extracted. phase ratio oforganic 8/1. Then, about times of thestripped stoichiometric requirement 2OO/A 2 is added the in the loaded phase can six be completely by 1.5 mol/L H2 SO4ofatHan phaseto ratio stripping solution (IV) to vanadium (V)of at °Cadded for 1 to h.the Ammonium of 8/1. Then, aboutto sixoxidize times ofvanadium the stoichiometric requirement H250 O2 is stripping polyvanadate can vanadium be successfully precipitated oxidized solution polyvanadate by adding aqueous solution to oxidize (IV) to vanadium (V)from at 50 ˝the C for 1 h. Ammonium can be ammonia to precipitated adjust the pH to 1.8–2.2, at 90 °C for 2 h.byThe driedaqueous ammonium polyvanadate calcined successfully from the oxidized solution adding ammonia to adjustisthe pH to at 550 °C to obtain a high of V2O5polyvanadate product. 1.8–2.2, at for 90 ˝2Ch for 2 h. The driedpurity ammonium is calcined at 550 ˝ C for 2 h to obtain a high purity of V2 O5 product.

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Figure flow sheet sheetfor for extraction recovery of vanadium from 2SO 4-HFsolution leach Figure10. 10. Proposed Proposed flow extraction andand recovery of vanadium from H -HF leach 2 SO4H solution fromshale blackbyshale by solvent extraction and precipitation. from black solvent extraction and precipitation.

4.4.Conclusions Conclusions Based in this thispaper, paper,a aprocess process extraction and recovery of vanadium Basedon onthe thedata data presented presented in forfor extraction and recovery of vanadium from from H 2 SO 4 -HF solutions generated by the leaching of black shale is suggested. The conclusions H2 SO4 -HF solutions generated by the leaching of black shale is suggested. The conclusions from the from test work are presented test the work are presented below: below: (1) A vanadium extraction efficiency of 96.7% was achieved using 10% (v/v) D2EHPA with 5% (1) TBP A vanadium extraction efficiency 96.7% achieved using 10% (v/v) with 5% (v/v) in sulfonated kerosene in six of stages ofwas counter-current extraction at D2EHPA an equilibrium pH(v/v) of TBP in sulfonated kerosene in six stages of counter-current extraction at an equilibrium pH of 2.5 2.5 and an O/A phase ratio of 1/1. Fluoride ions in the leaching solution had almost no effect on and an phase ratio of 1/1. Fluoride ions in the leaching solution had almost no effect on vanadium (IV)O/A extraction. vanadium (IV) extraction. (2) 99.7% of the loaded vanadium was stripped from the organic phase by 1.5 mol/L H2SO4 in (2) 99.7% of the loaded vanadium stripped five counter-current stages at an O/A was phase ratio offrom 8/1. the organic phase by 1.5 mol/L H2 SO4 in five counter-current stages at an O/A phase ratio of 8/1. (3) 98% of vanadium was precipitated as ammonium polyvanadate by oxidation and (3) 98% of vanadium was precipitated as ammonium polyvanadate by oxidation precipitation. Following calcination of the dried ammonium polyvanadate at 550 °Cand forprecipitation. 2 h, a high ˝ C for 2 h, a high purity Following calcination of the dried ammonium polyvanadate at 550 purity 99.52% V2O5 product was obtained. 99.52% V2sheet O5 product obtained. (4) A flow for thewas extraction and recovery of vanadium from a H2SO4-HF mixed leach A flow sheet for the extraction and recovery vanadium from a H2 SO4 -HF mixed leach solution (4) solution by solvent extraction and precipitation is of proposed. by solvent extraction and precipitation is proposed. Acknowledgments: This work has been supported by the National Basic Research Program of China (Grant No. 2014CB643404), NSFC Foundation of China, Grant No. 51174104, 51304093 and 51474115). Acknowledgments: This (Natural work hasScience been supported by the National Basic Research Program of China (Grant No. 2014CB643404), NSFC Xingbin (Natural Li Science Foundation of China, Grant No. 51174104, 51304093 Author Contributions: conducted the experiments and wrote the initial draft ofand the51474115). manuscript. Author Contributions: Li conducted experiments the initial draft of the Chang Chang Wei designed the Xingbin experiments. Zhigan the Deng, Cunxiongand Li wrote and Gang Fan analyzed themanuscript. data and wrote Wei designed the experiments. Zhigan Cunxiong andcontributed Gang Fan analyzed therevised data and wrote the All final the final manuscript. Minting Li and HuiDeng, Huang reviewedLiand to the final manuscript. manuscript. Minting Li and Hui Huang reviewed andthe contributed to the final revised manuscript. All authors authors contributed to the analysis of the data and read final paper. contributed to the analysis of the data and read the final paper. Conflicts of Interest: The author declares no conflict of interest.

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Conflicts of Interest: The author declares no conflict of interest.

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