Eco-Friendly Leaching and Separation of Vanadium over Iron Impurity

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Eco-Friendly Leaching and Separation of Vanadium over Iron Impurity from Vanadium-Bearing Shale Using Oxalic Acid as a Leachant Pengcheng Hu,† Yimin Zhang,*,†,‡ Jing Huang,† Tao Liu,† Yizhong Yuan,† and Nannan Xue† †

School of Resources and Environment Engineering, Hubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Science and Technology, Wuhan 430081, Hubei Province, China ‡ School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, Hubei Province, China

ABSTRACT: Oxalic acid, an eco-friendly leachant, was used to selectively separate V over Fe impurity from V-bearing shale in the direct acid leaching (DAL) and blank roasting-acid leaching (BRAL) processes, respectively. For the DAL process, 71.6% of V with only 3.2% of Fe impurity being leached at 95 °C for 6 h with a 1.0 L/kg water−mineral ratio, a 6.0 mol/kg oxalic acid dosage and a 5 wt % CaF2, indicating that the DAL process can selectively extract V from the raw shale. This result was attributed to the passivation of pyrite coated with FeC2O4·2H2O, which can block the reaction between pyrite and oxalic acid. For the BRAL process, 86.8% of V and 35.2% of Fe impurity were recovered after leaching for 4 h at 95 °C with a 1.0 L/kg water− mineral ratio, a 6.0 mol/kg oxalic acid dosage and a 5 wt % CaF2. Moreover, 92.6% of Fe in the roasted shale leachate was removed as FeC2O4·2H2O with only 2.2% of V loss when an Fe powder/Fe(III) molar ratio was 1.25 at 25 °C for 150 min. Therefore, the separation of V over Fe impurity from V-bearing shale using oxalic acid as a leachant is eco-friendly, efficient and sustainable. KEYWORDS: Vanadium-bearing shale, Vanadium, Iron, Oxalic acid leaching, Separation

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INTRODUCTION Vanadium (V) is an important strategic metal. The major V products, including V2O5, V2O3 and VN, are widely used in the fields of vanadium redox flow battery, alloy production, advanced material, catalyst and medicine.1−3 With the increasing demand for highly purified V products, extracting V from V-bearing resources through efficient and environmentally friendly methods has become increasingly important. In addition to V−Ti magnetite, V-bearing shale (also called stone coal) is another vital V-bearing resource that is widely distributed across China. V-bearing shale contains organic and inorganic components, and the organic content is much lower than that of inorganic content, similar to coal.4 In V-bearing shale, V exists dominantly as trivalent vanadium (V(III)) in addition to a little as tetravalent vanadium (V(IV)) and very scarce as pentavalent vanadium (V(V)) because V-bearing shale originates from reducing environment.4−6 As V(III) and V(IV) have similar coordination numbers, electronegativity and ionic © XXXX American Chemical Society

radius to those of Al(III), V(III) and V(IV) can isomorphically replace trivalent aluminum (Al(III)) in the mica minerals lattice.6,7 Such lattice is very stable and hard to be broken down, making V extraction difficult. The total V reserve in Vbearing shale accounts for more than 87% of the domestic V reserve in China; therefore, extracting V from V-bearing shale has drawn substantial attention.5,6 In general, thermal roasting and acid leaching are effective ways to destroy the lattice structure to release V from V-bearing shale. Direct acid leaching (DAL) and blank roasting-acid leaching (BRAL) are the main processes for extracting V from V-bearing shale and have been widely adopted in recent years.4,8−10 For the DAL process, acid can effectively destroy the V-bearing lattice of mica minerals. For the BRAL process, Received: September 17, 2017 Revised: December 2, 2017 Published: December 6, 2017 A

DOI: 10.1021/acssuschemeng.7b03311 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

V-bearing shale was also examined. The mechanisms of V and Fe leaching behaviors in the DAL process and the BRAL process were also revealed.

the structural breakdown of mica minerals can be achieved during both roasting and acid leaching; meanwhile, the lowvalence V (V(III) and V(IV)) was oxidized to the high-valence V (V(IV) and V(V)) by O2 from the air during roasting.4,6 Inorganic acids, especially sulfuric acid, are extensively used in the leaching process. However, there are some problems with the sulfuric acid leaching process. First, sulfuric acid can cause further environmental problems, such as the generation of waste acid solutions and toxic fumes, when the leaching treatment is not performed well.11 Second, the most important problem is that the sulfuric acid leaching process lacks selectivity for V extraction because iron (Fe) impurity is inevitably leached along with V, resulting in a sulfuric acid leachate containing a high concentration of Fe impurity, which has negative effects on the subsequent processes in V extraction.12−14 The solvent extraction process is commonly used to extract and concentrate V from acid leachates. Nevertheless, as Fe is a valence-variable element and Fe(III) is sensitive to changes in pH value, the Fe impurity in acid leachates is likely to precipitate as Fe(OH)3, resulting in emulsification in the solvent extraction process.15 Meanwhile, Fe ion can also be coextracted during the solvent extraction process, reducing the purity of V-bearing products.12 Based on the above factors, the current leaching process can be improved in three ways: (i) finding leachants that are environmentally green; (ii) enhancing the selectivity of V extraction during the leaching process; (iii) separating V from Fe in acid leachates before the solvent extraction process. To date, considerable research efforts have been devoted to extracting valuable metals from minerals using organic acids as alternative leachants, such as oxalic acid, citric acid, ascorbic acid and so forth.16−25 Generally, organic acids decompose minerals through the simultaneous action of acid attack and chelation, which favor metals extraction from minerals.19,20 Oxalic acid has attracted increasing attention due to its strong acidity, extensive biological sources and good complexing ability. Compared with sulfuric acid, oxalic acid is environmentally friendly, offers simple effluent treatment and is economically attractive because oxalic acid can not only be produced from the biofermentation of cheap carbon sources, but can also be easily removed by biodegradation without pollution.21−23 Extensive studies have been conducted to efficiently and selectively extract V from spent catalysts using oxalic acid instead of sulfuric acid.21,24 Moreover, numerous researchers have recovered Fe from minerals or removed Fe from solutions as iron(II) oxalate dihydrate (FeC2O4·2H2O, Ksp = 3.2 × 10−7) precipitate, which is highly insoluble and has potential applications as solid electrolyte and adsorbent.17,26−29 However, very few systematic studies on V extraction from Vbearing shale using oxalic acid as a leachant have been performed, and no research has been published concerning the comparison of V and Fe leaching behaviors during V-bearing shale oxalic acid leaching for the DAL process and the BRAL process. Based on the above analysis, it can be seen that oxalic acid may potentially be used to extract V from V-bearing shale in an eco-friendlily and efficiently manner, and the separation of V over Fe impurity from V-bearing shale should be studied systematically. Therefore, the primary goal of this paper is to use oxalic acid for V extraction from a V-bearing shale in the DAL process and the BRAL process, and the effects of the leaching conditions on V and Fe leaching behaviors were studied. Moreover, the separation of V over Fe impurity from

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EXPERIMENTAL SECTION

Materials and Reagents. The V-bearing shale, which was crushed to a size of 0−3 mm before the leaching experiments, was obtained from Hubei, China. For the DAL process, the crushed V-bearing shale was ground to a size of minus 0.074 mm, accounting for 75% of the particles. The ground V-bearing shale is referred to as raw shale throughout this study. For the BRAL process, the crushed V-bearing shale was heated to 800 °C for 60 min in a muffle furnace and subsequently ground to minus 0.074 mm, accounting for 75% of the particles. The ground roasted V-bearing shale is known as roasted shale throughout this study. All water was deionized water, and all reagents were of analytical grade. The main chemical compositions and mineral phase compositions of the raw shale and the roasted shale are presented in Table 1 and

Table 1. Main Chemical Compositions of the Raw Shale and the Roasted Shalea item

raw shale, wt %

roasted shale, wt %

item

raw shale, wt %

roasted shale, wt %

Mad Vad FCad S V2O5 Fe2O3 SiO2 Al2O3

0.29 5.16 9.21 1.75 0.74 4.80 51.16 10.37

ND ND 0.62 2.19 0.89 5.77 58.32 12.46

CaO K2O MgO Na2O BaO P2O5 TiO2 ZnO

5.82 2.46 1.81 1.09 1.93 1.98 0.51 0.31

6.98 2.96 2.16 1.31 2.32 2.38 0.61 0.37

M, moisture; V, volatile matter; FC, fixed carbon; ad, air-dried basis; ND, not detected.

a

Figure 1. XRD patterns of the raw shale and the roasted shale (A-raw shale and B-roasted shale). Figure 1, respectively. In the raw shale, the grades of V2O5 and Fe2O3 are 0.74% and 4.80%, respectively. The main mineral phases are quartz, muscovite, pyrite and calcite, and Fe mostly exists in the pyrite. In the roasted shale, the grades of V2O5 and Fe2O3 are 0.89% and 5.77%, respectively. The main mineral phases are quartz, muscovite, hematite and anhydrite, and most Fe exists in the hematite, which was converted from the pyrite during roasting. As mentioned, the V in V-bearing shale can replace Al(III) as an isomorphism in the mica minerals lattice. It was worth noting that the diffraction peaks intensities of muscovite in the roasted shale became B


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Figure 2. SEM micrographs with EDS element mapping of the raw shale.

⎛ C V − C 2V2 ⎞ θ=⎜ 11 ⎟ × 100% C1V1 ⎝ ⎠

weak compared with those in the raw shale, indicating that the muscovite structure was destroyed during roasting. The V occurrence state in the raw shale was studied by scanning electron microscopeenergy dispersive spectroscopy (SEM-EDS), and the result is shown in Figure 2. Figure 2 illustrates that the relevance of V, K, O, Si and Al was quite good, and the K, O, Si and Al atom percentages at point “M” were close to those of muscovite, proving that V existed in the muscovite. Therefore, the raw shale and the roasted shale belong to typical mica-type V-bearing shale, and it is difficult to extract V from them. Leaching Experiments. Leaching experiments with different leaching times, oxalic acid dosages and water−mineral ratios were performed to investigate the effects of leaching conditions on the V and Fe leaching efficiencies. The oxalic acid dosage refers to the ratio of the oxalic acid quantity (mol) to the ore sample (kg), and the water−mineral ratio refers to the ratio of water (L) to the ore sample (kg). Each leaching experiment was conducted with a 0.05 kg ore sample under the conditions with an addition of 5 wt % CaF2 at 95 °C using a glass flask heated with a temperature-controlled water bath with a magnetic stirrer and fitted with a glass condenser. The leachate and the leaching residue were obtained by filtration. The leaching efficiency (α) was calculated as follows: α=

CV × 100% Mβ

(2)

where C1 and C2 are the Fe concentration in the roasted shale leachate and the purified roasted shale leachate (ppm), respectively, and V1 and V2 are the volume of the roasted shale leachate and the purified roasted shale leachate (L), respectively. Analytical Methods. Chemical compositions were determined by an inductively coupled plasma-atomic emission spectroscopy (ICPAES, Optima-4300DV, PerkinElmer, Boston, MA, USA) in addition to C and S. Proximate analyses (including fixed carbon, volatile matter, moisture and ash) on an air-dried basis of the raw shale and the roasted shale were performed according to the national standard of China GB/T 212-2008.30,31 The content of S in the raw shale and the roasted shale was determined according to the national standard of China GB/T 214-2007.32,33 Phase compositions were identified by an X-ray diffraction (XRD, D/MAX 2500PC, Rigaku, Tokyo, Japan) using Cu Kα radiation. Microscopic observation and elemental analysis were conducted with scanning electron microscopy (SEM, JSM-IT300, JEOL, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS, X-Act, Bruker, Oxford, London, Britain). The ferrous iron (Fe(II)) concentration in the leachates was measured via the 1,10phenanthroline spectrophotometry using a spectrophotometer (UV5500, Metash, Shanghai, China).34,35 The ferric iron (Fe(III)) concentration in the leachates was calculated by mass balance.

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(1)

RESULTS AND DISCUSSION Results of Leaching Experiments. Effect of Leaching Time. The experiments were conducted with different leaching times ranging from 1 to 7 h with a CaF2 addition of 5 wt % as well as an oxalic acid dosage of 6.0 mol/kg, a water−mineral ratio of 1.0 L/kg and a leaching temperature of 95 °C. Figure 3 clearly shows that the V leaching efficiencies grew very slowly as the leaching time exceeded 4 and 6 h for the roasted shale and the raw shale, respectively. The Fe leaching efficiency for the raw shale remained near approximately 3% as the leaching time varied from 1 to 7 h; on the contrary, the Fe leaching efficiency for the roasted shale increased with prolongation of the leaching time. Therefore, the most suitable leaching time for the roasted shale and the raw shale was

where α is the V leaching efficiency (αV) or Fe leaching efficiency (αFe), C is the V concentration (CV) or Fe concentration (CFe) in the leachate (ppm), V is the leachate volume (L), β is the V grade (βV) or Fe grade (βFe) in the ore sample and M is the mass of the ore sample (kg). Experiments on Fe Impurity Precipitation from the Roasted Shale Leachate. The precipitation of Fe from the roasted shale leachate was systematically studied for the purification treatment of roasted shale leachate containing V. A given amount of Fe powder was added to the roasted shale leachate, which was stirred for a given precipitation time at 25 °C. At the end of each experiment, the precipitate and the purified roasted shale leachate were obtained by filtration. The precipitate was washed with deionized water three times and dried in a vacuum oven. The Fe removal efficiency (θ) was calculated as follows: C


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°C and a CaF2 addition of 5 wt %. Additionally, the leaching time was maintained at 4 and 6 h for the roasted shale and the raw shale in the experiments, respectively. As illustrated in Figure 5, the Fe leaching efficiency remained low, at approximately 3%, for the raw shale with increasing

Figure 3. Effect of leaching time on the V and Fe leaching efficiencies.

considered to be 4 and 6 h, respectively. For the raw shale, 71.6% of the V was recovered with only 3.2% of the Fe impurity being leached, and for the roasted shale, 86.8% of the V leaching efficiency together with 35.2% of the Fe impurity leaching efficiency was obtained. Effect of Oxalic Acid Dosage. The effect of different oxalic acid dosages ranging from 1.5 to 7.5 mol/kg were studied with a water−mineral ratio of 1.0 L/kg, a leaching temperature of 95 °C and a CaF2 addition of 5 wt %. The leaching time was fixed at 4 and 6 h for the roasted shale and the raw shale in the experiments, respectively. Figure 4 shows that the Fe leaching efficiency exhibited an obvious difference between the two shales: the Fe leaching

Figure 5. Effect of water−mineral ratio on the V and Fe leaching efficiencies.

water−mineral ratio. The variations in the V and Fe leaching efficiencies for the roasted shale and the V leaching efficiency for the raw shale were similar. Specifically, the leaching efficiencies at 0.6 L/kg were slightly lower than those at 1.0 L/kg, resulting from slower ion diffusion rates at low water− mineral ratios. In addition, there was a decrease in leaching efficiency as the water−mineral ratio surpassed 1.0 L/kg due to the decrease in the leachant concentration. Thus, a water− mineral ratio of 1.0 L/kg was determined as the best condition. Characterization of the Leaching Residues and the Leachates. To better understand the mineral phase changes for the DAL process and the BRAL process, the leaching residues under the optimum leaching conditions were analyzed by XRD, as shown in Figure 6. For the DAL process, Figure 6a shows that the diffraction peaks of muscovite in the raw shale leaching residue disappeared or became weak compared with those in the raw shale and that the diffraction peaks of whewellite were observed in the leaching residue, indicating that the muscovite structure was destroyed during the oxalic acid leaching. It was worth noting that the diffraction peak intensity of pyrite remained nearly unchanged after leaching, illustrating that the pyrite in the raw shale did not exhibit much dissolution in oxalic acid. The XRD analysis was consistent with the results of the leaching experiments. For the BRAL process, Figure 6b shows that almost all of the diffraction peaks of muscovite in the roasted shale leaching residue disappeared and that a new phase of whewellite was also formed after the leaching process. The diffraction peak intensity of hematite in the roasted shale leaching residue underwent an evident decline compared to that in the roasted shale, thus indicating the dissolution of hematite during the oxalic acid leaching. The V and Fe concentrations in the leachates are presented in Table 2. It can be concluded from Table 2 that the Fe/V concentration ratio was as low as 0.36 in the raw shale leachate; that is to say, very little Fe impurity was leached into the leachate along with V in the DAL process. However, the Fe impurity concentration was 5538.2 ppm (Fe(III) = 4319.8 ppm, Fe(II) = 1218.4 ppm), which was approximately 3.29

Figure 4. Effect of oxalic acid dosage on the V and Fe leaching efficiencies.

efficiency remained nearly constant for the raw shale but increased for the roasted shale during the leaching experiments with increasing oxalic acid dosage. However, the oxalic acid dosage significantly affected the V leaching efficiency. The increase in V leaching efficiency started to leveled off for both the roasted shale and the raw shale when the oxalic acid dosage was greater than 6.0 mol/kg. Thus, an oxalic acid dosage of 6.0 mol/kg was selected as the optimal condition in the following experiments for both the roasted shale and the raw shale. Effect of Water−Mineral Ratio. It was found that the leaching slurry was difficult to stir and filter when the water− mineral ratio was less than 0.6 L/kg; therefore, the V and Fe leaching efficiencies were systematically investigated at different water−mineral ratios ranging from 0.6 to 2.5 L/kg under an oxalic acid dosage of 6.0 mol/kg, a leaching temperature of 95 D


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Figure 6. XRD patterns of the ore samples and the leaching residues.

The effect of Fe powder/Fe(III) molar ratio was investigated at 25 °C with a reaction time of 150 min, as shown in Figure 7a. The growth trend in Fe removal efficiency slowed after the Fe powder/Fe(III) molar ratio exceeding 1.25. Meanwhile, the effect of reaction time was also studied at 25 °C with an Fe powder/Fe(III) molar ratio of 1.25, as shown in Figure 7b. It can be concluded that the reaction time had little impact on Fe removal efficiency when the reaction time exceeded 150 min. Figure 7 also illustrates that the V loss remained at a low level, approximately 2%, throughout the Fe impurity precipitation experiments. From the experiments results, the most suitable conditions for the maximum Fe impurity removal efficiency of 92.6% with a V loss of only 2.2% were determined as follows: an Fe powder/Fe(III) molar ratio of 1.25 and a reaction time of 150 min at 25 °C. Figure 8 shows the analysis of the obtained FeC2O4·2H2O precipitate, which matches well with the characteristic FeC2O4· 2H2O diffraction peaks (Figure 8c). This precipitate was an aggregate of cuboid particles (Figure 8d). The sharp peaks and regular particle shapes indicate that the obtained FeC2O4·2H2O precipitate was in a high-quality crystalline state. Figure 8b shows the chemical composition of the obtained FeC2O4·2H2O precipitate, in which the Fe composition was 30.64 wt %. Because Fe theoretically accounts for 31.11 wt % in FeC2O4· 2H2O, the obtained FeC2O4·2H2O precipitate exhibited a highpurity of 98.5%. Results of V and Fe Behaviors. According to the experiments, the Fe impurity leached during the BRAL process

Table 2. V and Fe Concentrations in the Leachates item

V, ppm

Fe, ppm

Fe/V concentration ratio

raw shale leachate roasted shale leachate

1662.5 1685.2

602.4 5538.2

0.36 3.29

times greater than the V concentration in the roasted shale leachate. Many researchers have found that hematite was easy to be dissolved as Fe(C2O4)33− and Fe(C2O4)22− because of the complexation and reduction reactions between the hematite and oxalic acid, which exhibits good acid strength, complexing power and reducing ability.36,37 Therefore, to avoid the negative effects of Fe impurity on the subsequent processes in V extraction, it is essential to separate the Fe impurity from V in the roasted shale leachate. Results of Experiments on Fe Impurity Precipitation from the Roasted Shale Leachate. Currently, numerous studies have focused on the recovery or removal of Fe from solutions as FeC2O4·2H2O precipitate (Ksp = 3.2 × 10−7), which is highly insoluble and has potential applications as solid electrolyte and adsorbent.17,26−29 It has been reported by Bruyère et al.38 and Shyue et al.39 that V in the oxalic acid solution can exist as stable complex ion of VO(C2O4)22−. Therefore, the Fe impurity may potentially be selectively removed as FeC2O4·2H2O through the reduction of Fe(III) to Fe(II) in the roasted shale leachate. In this study, Fe powder is regarded as the most suitable reductant because no external impurities will be introduced into the roasted shale leachate.

Figure 7. Effects of Fe powder/Fe(III) molar ratio and reaction time on the Fe removal efficiency and V loss. E


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Figure 8. Analysis of the obtained FeC2O4·2H2O precipitate.

Figure 9. (a) SEM micrograph of the raw shale leaching residue; (b) EDS element mapping of the raw shale leaching residue; (c) Line-scan analysis from points A to B; (d) schematic diagram of the DAL process.

addition, 92.6% of the Fe impurity removal efficiency with a V loss of only 2.2% was achieved through the Fe impurity precipitation process. Mechanisms of V Separation over Fe Impurity for the DAL Process and the BRAL-P Process. The mechanisms of V separation over Fe impurity for the DAL process and the BRAL-P process are proposed to offer deep insights into the behaviors of Fe impurity in the two processes, as shown in Figure 9 and Figure 10. For the DAL process, the V(III) was oxidized to VO(C2O4)22− by O2 from the air and the V(IV) was dissolved into

can be efficiently removed from the roasted shale leachate as FeC2O4·2H2O via Fe impurity precipitation process. The BRAL process combined with the Fe impurity precipitation process is hereafter referred to as the BRAL-P process. For the DAL process, 71.6% of the V was recovered with only 3.2% of the Fe impurity being leached, indicating that the DAL process had a highly selective extraction of V over Fe from the raw shale using oxalic acid as the leachant. For the BRAL-P process, 86.8% of the V leaching efficiency and 35.2% of the Fe impurity leaching efficiency were obtained during the BRAL process using oxalic acid as the leachant. In F


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hematite is easily dissolved in oxalic acid solution as Fe(C2O4)33−, resulting in the roasted shale leachate containing a high concentration of Fe impurity. As mentioned, the Fe impurity in the roasted shale leachate can be removed as highly insoluble FeC2O4·2H2O (Ksp = 3.2 × 10−7) via the reduction of Fe(III) to Fe(II) using Fe powder as the reductant and the V can exist as stable complex ion of VO(C2O4)22−. Therefore, the V would not be precipitated with the precipitation of FeC2O4· 2H2O. The main chemical reactions are as follows. 2V2O3 + 8H+ + 8C2O4 2 − + O2 = 4VO(C2O4 )2 2 − + 4H 2O

V2O4 + 4H+ + 4C2O4 2 − = 2VO(C2O4 )2 2 − + 2H 2O Figure 10. Schematic diagram of the BRAL-P process.

= 2VO(C2O4 )2 2 − + 2CO2 ↑ +3H 2O

(8)

Fe2O3 + 6H 2C2O4 = 3Fe(C2O4 )33 − + 6H+ + 3H 2O (9)

Fe + 2Fe(C2O4 )33 − + 6H 2O

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= 3(C2O4 )2 − + 3FeC2O4 ·2H 2O↓

(10)

CONCLUSIONS In this work, the V-bearing shale was leached with oxalic acid, which is eco-friendly and cost-effective. The V and Fe behaviors were investigated in both the DAL process and the BRAL-P process. (1) The DAL process had an efficient and selective extraction of V over Fe impurity from the raw shale, and 71.6% of the V was recovered with only 3.2% of the Fe impurity being leached under the conditions of a leaching time of 6 h, an oxalic acid dosage of 6.0 mol/kg, a water−mineral ratio of 1.0 L/kg, a leaching temperature of 95 °C and a CaF2 addition of 5 wt %. (2) The BRAL-P process can achieve an efficient separation of V over Fe impurity from the roasted shale, leaching efficiencies of 86.8% for V and 35.2% for Fe impurity were obtained in the BRAL process with a leaching time of 4 h, an oxalic acid dosage of 6.0 mol/kg, a water− mineral ratio of 1.0 L/kg, a leaching temperature of 95 °C and a CaF2 addition of 5 wt %. In addition, 92.6% of the Fe in the roasted shale leachate was removed as FeC2O4·2H2O with only 2.2% of the V loss through Fe impurity precipitation process when an Fe powder/ Fe(III) molar ratio was 1.25 at 25 °C with a reaction time of 150 min. (3) The mechanism of the selective extraction of V over Fe impurity in the DAL process was attributed to the passivation of pyrite coated with the FeC2O4·2H2O coating, which can inhibit the reaction of pyrite with oxalic acid. (4) The mechanism of the efficient separation of V over Fe impurity in the BRAL-P process mainly lay in two aspects. First, the hematite in the roasted shale was dissolved in oxalic acid. Second, the Fe in the roasted Vbearing shale leachate was efficiently precipitated as FeC2O4·2H2O.

2V2O3 + 8H+ + 8C2O4 2 − + O2

V2O4 + 4H+ + 4C2O4 2 − = 2VO(C2O4 )2 2 − + 2H 2O

(7)

V2O5 + 6H+ + 5C2O4 2 −

leachate as VO(C2O4)22− based on V extraction from shale in sulfuric acid leaching system.4,6,9 The reactions can be expressed as eqs 3 and 4. Figure 9b shows that the relevance of Fe and S was quite good, indicating that the mineral particle in Figure 9a was pyrite in the raw shale leaching residue. Numerous studies have been conducted to remediate acid mine drainage (AMD) production through the passivation treatment of pyrite, on which a protective coating is formed to suppress pyrite reactions with air and water. Oxalic acid has been used as a passivation agent by many researchers.40−43 Lalvani et al.41 reported that the oxalic acid treatment can decrease the corrosion current of pyrite by 17−40%; meanwhile, Ogunsola et al.42 showed that the pyrite oxidation rate was decreased in the presence of oxalic acid. These researchers attributed their experimental results to the formation of FeC2O4·2H2O protective coating on pyrite and this reaction can be expressed as eq 5. For the results of our leaching experiments, a further study of the pyrite particle in the raw shale leaching residue was conducted by a line-scan analysis, as shown in Figure 9c. It can be clearly seen that the C and O mass compositions of the outer region were significantly higher than those of the central region. Furthermore, the O signal distribution of the outer region was clearly greater than that of the central region in the element mapping. Thus, the line-scan and elemental mapping analyses confirmed the enrichment of C and O in the outer region, which was attributed to the formation of FeC2O4·2H2O coating on the pyrite in the raw shale during oxalic acid leaching. The FeC2O4·2H2O coating can prevent further chemical reactions between pyrite and oxalic acid, thereby allowing a highly selective extraction of V from the raw shale to be achieved using oxalic acid as the leachant. = 4VO(C2O4 )2 2 − + 4H 2O

(6)

(3) (4)

FeS2 + H 2C2O4 + 2H 2O = H 2S + S + FeC2O4 ·2H 2O↓ (5)

For the BRAL-P process, the V(III) was oxidized to VO(C2O4)22− by O2 from the air and the V(IV) was dissolved into leachate as VO(C2O4)22−. The V(V) was reduced to VO(C2O4)22− because oxalic acid is a reductive agent.39,44,45 A schematic diagram of Fe impurity behavior in the BRAL-P process is shown in Figure 10. It has been reported that G


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AUTHOR INFORMATION

Corresponding Author

*Yimin Zhang. E-mail: [email protected]. Tel: +86-02768862057. Fax: +86-027-68862057. ORCID

Pengcheng Hu: 0000-0002-0356-953X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Project of National Natural Science Foundation of China (No. 51474162, No.51774215 and No.51774216). The authors cordially thank anonymous the reviewers and the editor for their constructive comments on this paper.

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Eco-Friendly Leaching and Separation of Vanadium over Iron Impurity

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