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. 2024 Feb 16;9(8):9286–9294. doi: 10.1021/acsomega.3c08452

Leaching Vanadium from the Spent Denitration Catalyst in the Sulfuric Acid/Oxalic Acid Combined Solvent

Wenting Cheng †,*, Jie Li †, Jianhang Deng ‡, Yang Li §,∥,*, Fangqin Cheng †
PMCID: PMC10905738  PMID: 38434804

Abstract

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Huge amounts of spent denitration catalysts are produced annually as waste from the flue gas denitration process, which will cause resource waste and environmental pollution. It is important to develop an efficient method for the recovery of metals from spent denitration catalysts. In this work, the leaching of vanadium (V) from the spent denitration catalyst by the sulfuric acid/oxalic acid combined solvent was investigated. Factors that influence the leaching rate of V have been studied. Results showed that the optimal leaching rate was 95.65% by 20 wt % sulfuric acid and 0.3 mol·L–1 oxalic acid with a liquid-to-solid ratio of 20 mL·g–1 at 140 °C for 7 h. For further study of the leaching process, the leaching mechanism of V was explored subsequently. Results indicated that sulfuric acid provided a strongly acidic environment, which was beneficial to transformation, complexation, and redox reactions of V in the mixed acid leaching system. Meanwhile, oxalic acid with excellent complexation and reducing–dissolving properties promoted the formation of stable water-soluble VO2+. The “complex effect” generated from the combined acids was greatly favored for leaching V from the spent denitration catalyst.

1. Introduction

The selective catalytic reduction (SCR) flue gas denitration technology is widely used in coal-fired power plants for its high efficiency, reliable operation, no secondary pollution, etc.1,2 As the core of the SCR denitration system, the catalytic activity of the V/Ti catalyst is directly related to the denitration effect.3 However, the catalytic activity of the catalyst will gradually diminish with the extension of operating time in the complex flue gas systems, and its service life is only 3–4 years generally.4 About 60% of the catalyst have no regeneration value due to their damaged physical structure or decreased mechanical properties, which typically will be landfilled.5 This has caused not only the waste of national strategic reserve metals, such as vanadium (V), tungsten (W), and titanium (Ti), but also a severe environmental risk.6 Therefore, it is of great importance to realize the extraction of these valuable metals from spent denitration catalysts.7,8

Numerous studies on the extraction and purification of V from spent denitration catalysts have been carried out.9,10 Currently, the common methods for extraction V from spent denitration catalysts include alkaline leaching,11 sodium roasting,12 and single-acid leaching.13 Among these methods, alkaline leaching is known for its high alkali consumption and wastewater generation, while sodium roasting causes high energy consumption.14 The single-acid leaching method, on the other hand, could be further classified into inorganic acid leaching and organic acid leaching.15,16 Sulfuric acid leaching, one of the inorganic acid leaching methods, is often used in industries due to its mature process and low-grade raw material requirement.17 In this method, high concentrations of sulfuric acid are required to achieve a high leaching rate of V. Thus, this process inevitably discharges large amounts of sulfuric leachate after ion exchange, causing serious environmental pollution.7 Additionally, the leaching rate of V will reduce significantly with a decrease of the concentration of sulfuric acid. For example, the leaching rate of V reduces sharply from 80 to 47% as the concentration of sulfuric acid decreases from 6.0 to 1.0 mol·L–1.18 This case might be possible because the reduction of the acidity in the leaching system would lead to the hydrolysis of the leached V.19 These problems reflect the limitation of using sulfuric acid as a single leaching agent for V leaching. Therefore, developing new methods for efficiently extracting V from spent denitration catalysts is practical and economic for the sustainable development of the catalyst industry.

Oxalic acid is one of the most promising organic acids acting as both leaching and reducing reagents.10 It has been intensively applied for metal extraction such as Fe, V, W, etc. from minerals or secondary resources.20 For example, Hu et al.21 reported that 71.6% of the V was extracted by 6.0 mol·kg–1 oxalic acid with a leaching time of 4 h at 95 °C. Wu et al.22 also found that 84% of V could be leaching by 1.0 mol·L–1 oxalic acid at 90 °C. Compared with sulfuric acid, oxalic acid has better environmental adaptability but a higher price.23 Moreover, the acidity of oxalic acid is weaker than that of sulfuric acid at the same concentration.24 To efficiently leach metals and achieve the economic and environmental goals of the industry, an inorganic acid and organic acid mixture was used as a leaching solvent.25 For instance, Wang and Yang.26 leached V from the spent SCR catalyst by sulfuric acid–ascorbic acid with a leaching rate of 96.25%. Li et al.27 extracted metals from spent lithium-ion batteries using a sulfuric acid–malonic acid mixture. The results showed that 99.46% Ni, 97.24% Co, and 96.88% Mn were extracted under optimal conditions. However, leaching V from the spent denitration catalyst using the sulfuric acid/oxalic acid mixture as leaching solvents was rarely reported. Moreover, more attention should be given to the leaching mechanism of V in the sulfuric acid/oxalic acid combined solvent as it plays an important role for improving the leaching rate of V from the spent denitration catalyst.

In this work, the leaching of V from the spent denitration catalyst was conducted with sulfuric acid/oxalic acid combined solvent. Liquid-to-solid (L/S) ratio, concentrations of sulfuric acid and oxalic acid, reaction temperature, and leaching time were investigated as key factors to determine the optimum leaching condition. The leaching rate, mineral phase transformation, and valence change of V during the leaching process were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP–OES), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, and UV–vis analysis. Eventually, the leaching mechanism of V in the sulfuric acid/oxalic acid combined system was investigated based on the kinetics analysis.

2. Experimental Section

2.1. Materials

The spent denitration catalyst was collected from Jiangsu Longking–Coalogix Environmental Protection Technology Co., Ltd., China. The collected sample was dried at 80 °C for 24 h and ground to obtain powdered samples with a size smaller than 75 μm. It was then stored in an airtight container. The particle size of the spent denitration catalyst could strongly affect the leaching rate of the metals from it.28 Therefore, all the leaching experiments were conducted using spent denitration catalyst particles with a size smaller than 75 μm to avoid the decrease of the leaching rate.29

Sulfuric acid (H2SO4, 98.0%), oxalic acid (H2C2O4·2H2O, 99.8%), vanadyl sulfate (VOSO4, 99.9%), and vanadyl oxalate (VOC2O4·5H2O, 98.5%) were purchased from Sinopharm Chemical Reagents Co., Ltd., China. The water used in this work for solution preparation, dilution, sample washing, etc. was double distilled water (conductivity <0.1 μS·cm–1).

2.2. Characteristics

The elemental composition analysis of the spent denitration catalyst was performed by using X-ray fluorescence (XRF, S8 TIGER, Brook Ax, Germany). Morphological and elemental analyses of the spent denitration catalyst were performed using a scanning electron microscope equipped with an energy-dispersive X-ray spectroscope (SEM–EDS, JSM–IT500HR, Sharp Corporation, Japan). The chemical form of V, W, and Ti in the spent denitration catalyst was determined through XPS (ESCALAB 250 Xi, Thermo Fisher Scientific, America). The concentrations of elements in the leaching solution were analyzed by using ICP–OES (Optima 5800VD, Agilent, America). The composition of the solid phases was identified using XRD (SmartLab, Rigaku Corporation, Japan). The FTIR spectra of the spent denitration catalyst was recorded using a Bruker instrument (FTIR, INVENIO–S, Bruker Corporation, Germany). The final valence state of V in the leaching system was determined by using UV–vis spectrophotometry (UV 2600, Shimadzu, China).

2.3. Experimental Procedure

The experimental flowchart is shown in Figure 1. The leaching experiment was performed in a 100 mL homogeneous reactor. A standard volume of the sulfuric acid/oxalic acid combined solvent with a certain concentration was poured into the reactor. The reactor was then electrically stirred and heated. When the solution attained the required temperature (90–150 °C), a certain amount of the spent denitration catalyst was added and kept stirring for 1–9 h. The stirring rate of 500 rpm was selected to eliminate the effect of external diffusion on the leaching process.28 After each experiment, the slurry was filtered, and the residue was washed three times with distilled water. The leach liquor along with washing water was analyzed for elements using ICP. The leaching rate was calculated according to eq 1

2.3. 1

where xi was the leaching rate of metals, %; n was the dilution multiple; V was the volume of leaching filtrate, L; Ci was the concentration of metals, mg·L–1; m was the mass of the added spent denitration catalyst, and g; wi was the content of metals in the spent denitration catalyst, wt %.

Figure 1.

Figure 1

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Flowchart of V leaching from spent denitration catalysts.

3. Results and Discussion

3.1. Characterization of Spent Denitration Catalysts

The chemical composition of the spent denitration catalyst was analyzed and is listed in Table 1. It can be seen that the main contents of the spent denitration catalyst were TiO2, WO3, V2O5, SiO2, CaO, and Al2O3. The corresponding mass fraction was 88.76, 3.49, 1.11, 2.88, 1.03, and 0.75%, respectively. The spent denitration catalyst also contained trace amounts of Mg, Na, and K, with mass fractions below 0.1%.

Table 1. Main Chemical Analysis of Spent Denitration Catalysts.

component TiO2 WO3 V2O5 SiO2 CaO Al2O3 MgO Na2O K2O others
amount (%) 88.76 3.49 1.11 2.88 1.03 0.75 0.095 0.068 0.027 1.79
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The SEM–EDS analysis of the spent denitration catalyst is shown in Figure 2. The SEM images present irregular-shaped and rod-shaped morphologies of the raw spent denitration catalyst. The EDS spectra analyses of regions (a) and (b) indicated that the major irregular particles contained W and Ti, whereas rod-shaped particles as binders mainly contained other elements, such as Al, Si, and V. Moreover, the V was encapsulated in the mineral phase with the V-bearing lattice structure (Al–Si–O–V compound) attributed to the prolonged high-temperature sintering during the denitration catalytic process.30−32 Obviously, only if the V-bearing lattice structure was destroyed by the leaching solvent could ensure a high leaching rate of V.33

Figure 2.

Figure 2

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SEM image and EDS spectra of spent denitration catalysts: (a) rod-shaped and (b) irregular-shaped.

For further investigating the valence states of element V in the spent denitration catalyst, the element composition analysis was performed by XPS (Figure 3). The C 1s peak at 248.8 eV was used as the internal reference for correcting the binding energy. It could be observed that the primary elements of Ti, W, V, and O along with other minor elements were detected in the spent denitration catalyst (Figure 3a). As shown in Figure 3b, the 2p3/2 spectrum of V had three peaks at 517.1, 516.0, and 515.2 eV, and their corresponding area ratios were 61.24, 30.22, and 8.54%, respectively.34 The peak at 517.1 eV could be assigned to V5+. The peaks at 516.0 and 515.2 eV could be assigned to V4+ and V3+, respectively.35 Therefore, the V existed in the forms of V2O5, V2O4, and V2O3 in the spent denitration catalyst.1,22

Figure 3.

Figure 3

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(a) XPS survey spectra and (b) V 2p spectrum of the spent denitration catalyst.

3.2. Leaching Analysis of V

The effect of different factors, including L/S ratio, sulfuric acid concentration, oxalic acid concentration, reaction temperature, and leaching time, on the leaching rate of V from the spent denitration catalyst was investigated.

3.2.1. Effect of the L/S Ratio

The effect of L/S ratios (8–22 mL·g–1) on the leaching rate of V was studied with 30 wt % sulfuric acid and 0.5 mol·L–1 oxalic acid at 90 °C for 6 h (Figure 4). It could be seen that the leaching rate increased from 75.32 to 82.16% as the L/S ratio from 8 to 20 mL·g–1 since a higher L/S ratio was beneficial to the mass diffusion between the combined acids and solid particles of the spent denitration catalyst.22 Moreover, the leaching rate of V was relatively stable when the L/S ratio was further increased from 20 to 22 mL·g–1. It could be seen that there was no use for further improving the leaching rate of V by increasing the L/S ratio when it reached 20 mL·g–1. That was to say the leaching rate of V was saturated under this condition.27 Therefore, 20 mL·g–1 was selected as the best L/S ratio for its corresponding optimal leaching rate of 82.16%.

Figure 4.

Figure 4

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Effect of the L/S ratio on the V leaching rate (conditions: 30 wt % H2SO4, 0.5 mol·L–1 H2C2O4, 90 °C, 6 h).

3.2.2. Effect of the Sulfuric Acid Concentration

The effect of the sulfuric acid concentration (0–30 wt %) on the leaching of V was investigated under the fixed condition: L/S ratio was 20 mL·g–1 and 0.5 mol·L–1 oxalic acid at 90 °C for 6 h (Figure 5). The leaching rate of V increased from 75.16 to 83.89% as the sulfuric acid concentration increased from 0 to 20 wt % and then decreased to 82.75% as the sulfuric acid concentration increased to 30 wt %. The contact area between sulfuric acid and catalyst particle increased with increasing initial concentration of sulfuric acid (0–20 wt %), thus accelerated the leaching reaction.27 However, at a higher initial concentration of sulfuric acid (20–30 wt %), the spent denitration catalyst powder had a lower diffusion rate due to the higher viscosity of initial solution, which greatly hindered their release from the solid phase into the liquid phase.36 Therefore, 20 wt % sulfuric acid concentration was selected for its corresponding optimal leaching rate of 83.89%.

Figure 5.

Figure 5

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Effect of sulfuric acid on the V leaching rate (conditions: 0.5 mol·L–1 H2C2O4, 90 °C, 6 h, 20 mL·g–1).

3.2.3. Effect of the Oxalic Acid Concentration

The effect of the oxalic acid concentration (0.3–1.3 mol·L–1) on the leaching of V was studied at an L/S ratio of 20 mL·g–1 and 20 wt % sulfuric acid at 90 °C for 6 h (Figure 6). It could be seen that the leaching rate of V initially decreased from 82.75 to 79.84% and subsequently increased to 82.94% with increasing oxalic acid concentration. However, when the concentration of oxalic acid increased to 1.3 mol·L–1, the leaching rate of V was only slightly higher than that of 0.3 mol·L–1. By considering the economy and energy consumption, 0.3 mol·L–1 oxalic acid was selected for its corresponding optimal leaching rate of 82.75%.

Figure 6.

Figure 6

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Effect of oxalic acid on the V leaching rate (conditions: 90 °C, 6 h, 20 wt % H2SO4, 20 mL·g–1).

3.2.4. Effect of Reaction Temperature

The effect of reaction temperature (90–150 °C) on the leaching rate of V was illustrated under a L/S ratio of 20 mL·g–1 with 20 wt % sulfuric acid and 0.3 mol·L–1 oxalic acid for 6 h (Figure 7). It was indicated that the leaching rate of V significantly increased as the temperature increased from 90 to 140 °C. This case might be attributed to the continued enhancement of the molecular collision.37 However, the leaching rate of V decreased from 90.02 to 87.54% with further increasing the temperature from 140 to 150 °C. This case might be possible because higher temperature led to the generation of H2WO4, which could cover the surface of the spent denitration catalyst and prevent the leaching of V.27 Therefore, 140 °C was determined as the optimized reaction temperature for its corresponding optimal leaching rate of 90.02%.

Figure 7.

Figure 7

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Effect of reaction temperature on the V leaching rate (conditions: 6 h, 0.3 mol·L–1 H2C2O4, 20 wt % H2SO4, 20 mL·g–1).

3.2.5. Effect of Leaching Time

The effect of leaching time (3–9 h) on the V leaching rate was depicted under the following conditions: L/S ratio of 20 mL·g–1, 20 wt % sulfuric acid, and 0.3·mol L–1 oxalic acid at 140 °C (Figure 8). The leaching rate of V increased continuously from 83.04 to 95.65% as the leaching time increased from 3 to 7 h and stabilized after 7 h. It indicated that enough leaching time promoted the reactions between the acid solvent and spent denitration catalyst, resulting in a higher leaching rate of V. This case might be possible because sufficient time should be spent on completely released V from the spent denitration catalyst since the V-bearing lattice structure was very stable and hard to break down.24,38 Therefore, a leaching time of 7 h was selected by considering the maximum leaching rate of 95.65%.

Figure 8.

Figure 8

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Effect of leaching time on the V leaching rate (conditions: 140 °C, 0.3 mol·L–1 H2C2O4, 20 wt % H2SO4, 20 mL·g–1).

In summary, the optimized leaching parameters were determined to be as follows: L/S ratio of 20 mL·g–1, 20 wt % sulfuric acid, and 0.3 mol·L–1 oxalic acid at 140 °C for 7 h. Under these conditions, the optimized V leaching rate was 95.65%. It was observed that the leaching rate of V was higher under mixed acid conditions compared to single acid conditions.22,38

3.3. Characterization of the Leaching Residue

The XRD diffraction patterns of the solid before and after leaching are shown in Figure 9. It can be seen that all of the diffraction peaks in both XRD patterns were matched with the standard JCPDS no. 01–84–1285 of anatase type TiO2. It was verified that the main solid phase TiO2 of the spent denitration catalyst had no obvious change in the leaching process.39 After the leaching process, the support TiO2 can be reused as a carrier to produce a new SCR catalyst.

Figure 9.

Figure 9

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XRD spectra for the solid phase (a) before leaching and (b) after leaching.

The phase transformation of V before and after leaching was further determined by FTIR analysis.40 In Figure 10a, the peak at 1049 cm–1 was attributed to characteristic of V=O while the peak at 1635 cm–1 was attributed to asymmetrical deformation vibrations of C=O.41 It indicated that element V obviously existed in the solid phase before leaching. In Figure 10b, the peak of V=O at 1049 cm–1 disappeared while two peaks of sulfates at 1113 and 1094 cm–1 were generated in the solid phase after leaching.42,43 Comparing Figure 10a,b, it can be seen that there was no detection of V=O in the solid phase after leaching, which indicated that the V had been released from the solid phase into the leaching solvent. In addition, some sulfate precipitations might be generated during the leaching process.

Figure 10.

Figure 10

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FTIR spectra for the solid phase (a) before leaching and (b) after leaching.

3.4. Analysis of the Leaching Mechanism

As concluded from the above analysis, sulfuric acid/oxalic acid combined leaching was a complex process, and many factors influenced the leaching rate of V. Therefore, the leaching mechanism of V in this process was essentially required for the description, practical design, and operation of large-scale leaching.

3.4.1. Leaching Kinetics

As an important connection between the leaching process and mechanism, the leaching kinetics of V was studied using the shrinking core model (SCM). According to the SCM, the rate-determining step of the dissolution process could be attributed to the diffusion of materials through the solution boundary, the solid material layer, or the surface chemical reactions.44

The leaching kinetics of V under different temperatures were analyzed using SCM based on the solution boundary diffusion (eq 2) and surface chemical reaction (eq 3).38,45

3.4.1. 2
3.4.1. 3

where x was the leaching rate; k1 and k2 were the apparent constants of different control steps, min–1; and t was the leaching time, min.

Comparing Figure 11a,b, it can be seen that the latter fitting results were obviously better than the former results. The data in Figure 11b show a good linear relationship for all of the temperatures with correlation coefficients greater than 0.9804. Therefore, the leaching process was primarily controlled by the surface chemical reaction (eq 3), and the calculated rate constants (k2) were equal to the values of the slopes. This was in accordance with the observation reported by Zhang et al. that the dissolution of V could be promoted by enhancing the surface chemical reaction.46 Furthermore, the inhibition of the surface chemical reaction was weakened, which resulted in a faster leaching of V.

Figure 11.

Figure 11

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(a) X vs time for V and (b) 1 – (1 – X)1/3 vs time for V.

3.4.2. Chemical Reaction in the Leaching Process

As demonstrated in the above kinetics analysis, the leaching process of V with sulfuric acid/oxalic acid combined solvent was mainly controlled by the surface chemical reaction. The schematic representation of the surface chemical reaction in the leaching process that included three steps is illustrated in Figure 12.47

Figure 12.

Figure 12

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Chemical reaction in the leaching process.

3.4.2.1. Transformation

The difficulty of releasing V from the spent denitration catalyst was determined by the interaction intensity between the leaching solvent and the mineral phases of the spent denitration catalyst.24 In the sulfuric acid/oxalic acid combined leaching system, the crystal lattice structure of the V-bearing mineral phase was disrupted by hydrogen ions which were provided by combined acids.20,27 In this work, the maximum concentration of hydrogen ions reached 1.99 mol·L–1, which could strongly enhance the interaction between the combined acid and the mineral phase.

From the above XPS analysis in Figure 3b, it can be seen that V existed in the forms of V(V), V(IV), and V(III) in the spent denitration catalyst. There were many different ionic species for V in the liquid phase (Table 2).10,48 However, in a highly acidic environment such as a sulfuric acid/oxalic acid combined solvent, V(V), V(IV), and V(III) were transformed from the mineral phase into the liquid phase by the possible reactions as follows

3.4.2.1. 4
3.4.2.1. 5
3.4.2.1. 6
Table 2. States of V Species in Solution.
existence form V in solution
cationic species VO+, VO2+, VO2+
neutral species VO(OH)3
anion species V10O26(OH)24–, V10O27(OH)5–, V10O286–
  VO2(OH)2–, VO3(OH)2–, VO43–
  V2O6(OH)3–, V2O74–, V3O93–, V4O124–
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The Gibbs free energy values of eqs 4–6 were calculated by HSC [enthalpy (H), entropy (S), and heat capacity (C)] chemistry thermodynamic software for verifying the possibility of the three reactions at 0–180 °C.49,50 The calculation results are shown in Figure 13. It was found that the Gibbs free energy values of eqs 4–6 were far below zero and slightly increased as the temperature increased, indicating that all three reactions could occur spontaneously. Moreover, the spontaneous trend of the three reactions at the same temperature was arranged as (4) > (6) > (5) due to the comparison of Gibbs free energy values.

Figure 13.

Figure 13

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Gibbs free energy of eqs 4–6 at 0–180 °C.

3.4.2.2. Complexation

Oxalic acid was available to complex with metal cations due to its strong complexability.1 The oxalate ion was a bidentate anionic ligand that could donate two pairs of electrons to a metal ion.23 This ligand was commonly known as a chelate because of its ability to attach to a metal cation in two places.23 Most metals formed either simple oxalate compounds or oxalate complexes, or both.28 V was reported to form only water-soluble oxalate complexes.51

In the soluble ions generated in the transformation mentioned above, the stable VO2+ ions almost no longer took part in the subsequent complexation.22 Other soluble ions, such as VO2+ and VO+, partly reacted with oxalic acid to form oxalate complexes. The related reactions are listed as follows

3.4.2.2. 7
3.4.2.2. 8
3.4.2.2. 9
3.4.2.3. Redox

The potential difference and oxidation–reduction properties of substances were employed to determine whether a redox reaction occurred or not. On the one hand, some parts of the VO2+ and VO+ ions were complex with oxalic acid, while the other part of them participated in redox reactions with combined acids. This case was possible because HC2O4– and VO+ had reducibility, whereas VO2+ had strong oxidability for its high standard redox potential of Eθ (VO2+/VO2+) = 1.00 V, which was compared with the values Eθ (Fe3+/Fe2+) = 0.77 V, and even Eθ (O2/H2O2) = 0.68 V.47 Therefore, VO2+ rapidly reacted with VO+ to form VO2+ in the combined acids with high acidity (eq 10).10

On the other hand, oxalate complex VO2(C2O4)− formed in the above complexation further reacted with the combined acids because they were unstable and also had strong oxidability (eq 11).52 Meanwhile, VO(C2O4)− could also react with VO2(C2O4)− or VO2(C2O4)23– due to the large potential difference between them [Eθ (VO2(C2O4)−/VOC2O4) = 1.13 V and Eθ (VO2(C2O4)23–/VO(C2O4)22–) = 1.22 V] (eqs 12 and 13).10

Moreover, the redox reactions were enhanced by the large amounts of hydrogen ions from sulfuric acid.21,27 The V existed as a stable form of VO2+ after the leaching process in the aqueous phase, which was in accordance with the findings reported by Al-Sheeha et al.51 By the “complex effect” from the combined acids, the redox reactions occurring are listed as follows

3.4.2.3. 10
3.4.2.3. 11
3.4.2.3. 12
3.4.2.3. 13

3.4.3. Verification the Valence State of V

The above analysis indicated that V existed in the form of stable VO2+ in the liquid phase after leaching. To verify this result, the final valence state of V in the leaching solution was further investigated by the UV–vis full spectrum scanning method (Figure 14).53 In the experiment, the interference for detecting the V valence state had been eliminated by the chosen sulfuric acid/oxalic acid combined solvent (the optimal reaction condition of 0.3 mol/L H2C2O4 and 20 wt % H2SO4) as background since the leaching solution contained C2O42– and SO42–. Moreover, a mixture of 0.005 mol·L–1 VOC2O4 and 0.005 mol·L–1 VOSO4 was chosen as the standard reagent for getting the most suitable absorption intensity after a series of tests.

Figure 14.

Figure 14

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UV–vis spectrophotometry of V in the leaching solution.

Figure 14 shows that the valence state determination of V was not influenced by the selected background of sulfuric acid/oxalic acid combined solvent because there were no characteristic peaks of them in the scanning wavelength range. The characteristic wavelengths of V in the leach solution were consistent with those of the VOC2O4 /VOSO4 standard reagent, with a peak at 765 nm. Hence, the final valence state of V in the leaching solution was VO2+, which was consistent with the chemical reaction analysis.

4. Conclusions

V was extracted from the spent denitration catalyst using a leaching process by the sulfuric acid/oxalic acid combined solvent. Effects of several parameters on the leaching rate of V were studied first. It had been shown that the optimal leaching rate was 95.65% by 20 wt % sulfuric acid and 0.3 mol·L–1 oxalic acid with a L/S ratio of 20 mL·g–1 at 140 °C for 7 h. To better describe the leaching process, the leaching kinetics of V was investigated subsequently. It was indicated that the V leaching conformed to the SCM, which meant that the surface chemical reaction played a predominant role in controlling the whole leaching process.

Moreover, the leaching mechanism of V was investigated based on the above findings. XPS analysis indicated that V primely existed in the form of V(V), V(IV), and V(III) in the spent denitration catalyst. Then, VO2+, VO2+, and VO+ ions formed with the transformation of V from the solid phase into the liquid phase. The transformation was highly driven by hydrogen ions providing from the sulfuric acid/oxalic acid combined solvent, which thoroughly disrupted the crystal lattice structure of the V-bearing mineral phase. Subsequently, some parts of VO2+ and VO+ ions complexed with oxalic acid to form VO2(C2O4)−, VO2(C2O4)23–, and VO(C2O4)−, whereas their other part participated in redox reactions with combined acids. Moreover, owing to the large potential difference, the ions formed in the above reactions tend to form stable VO2+ by redox in the strong acidic environment, which was further verified by UV–vis analysis.

In conclusion, the “complex effect” generated from the combined acids was beneficial for the transformation, complexation, and redox, thus greatly enhancing the leaching rate of V. In addition, the main solid phase TiO2 of the spent denitration catalyst had no obvious change in the leaching process, which suggested that it was potential to prepare fresh SCR catalysts using the leaching residual as raw material. These findings could provide a new method for leaching V and pave the way for extracting other valuable metals from spent denitration catalysts.

Acknowledgments

The support of National Energy-Saving and Low-Carbon Materials Production and Application Demonstration Platform Program (no. TC220H06N) is gratefully acknowledged.

The authors declare no competing financial interest.

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