Rapid Vanadium Extraction from Roasted Vanadium Steel Slag via a H2SO4-H2O2 System: Process and Mechanism.

A H2SO4-H2O2 system was developed to enhance the efficacy of vanadium extraction from roasted vanadium steel slag. The optimum parameters and the behavior of vanadium extraction were investigated systematically. When 1 mL of H2O2 per gram of vanadium slag was added to a leaching mixture at 50 °C, along with 30% H2SO4, 80.5% of vanadium extraction was achieved within 15 min. However, without H2O2, only 58.5% of vanadium extraction was achieved at the same leaching time. The H2SO4-H2O2 system facilitated the dissolution of metallic ions in a short time and then triggered the production of strong oxidizing substances, such as HO• and O2 -•, via the Fenton reaction and Fenton-like reaction. Subsequently, the low-valence vanadium, existing in the leaching solution or located on the surface of the particle, was converted to pentavalent vanadium by strongly oxidizing substances, such as H2O2 and its derivatives HO• and O2. The complex oxides on the surface of the particle were destroyed, after which the vanadium inside the particle was gradually exposed to the acid leaching solution. The vanadium was oxidized to pentavalent vanadium, which then entered the leaching solution. Finally, a pathway of vanadium extraction via the H2SO4-H2O2 system was proposed to gain insight into rapid vanadium leaching.

Introduction

Vanadium is regarded as a “vitamin of modern industry,” and it is widely used in various fields, such as aerospace, chemistry, and steel manufacturing, because of its excellent physical and chemical properties.[1,2] Vanadium is widely distributed on Earth but in small proportions. In particular, black shale and vanadium titanomagnetite are two essential sources available for vanadium extraction.[3] As vanadium titanomagnetite is used in the steel-making process, a large amount of vanadium steel slag is produced, and the V2O5 content can be enriched in the range of 12–21% according to different smelting processes and raw materials.[4]

At present, the traditional method for vanadium extraction from steel slag involves alkaline salt roasting, followed by acid leaching and ammonium precipitation.[2] However, it has some disadvantages, such as high energy consumption and large high-salinity wastewater discharge.[5,6] Although technologies such as calcium roasting, direct leaching, and bioleaching[7−9] have made significant progress, most of them have seldom been applied in practical production owing to their inherent limitations. Considering that high-valence vanadium compounds favor vanadium extraction from the solution,[3] sodium salt roasting is adopted before vanadium leaching. However, this process produces chlorine, hydrogen chloride, and other harmful gases, resulting in equipment corrosion and environmental pollution if not controlled.[10] Recent research revealed that CaF2 could facilitate the process of alkaline salt roasting at a low temperature of 700–750 °C,[11] and vanadium extraction under atmospheric acid leaching increased by 7.4% when NaCl was added as an auxiliary agent. In addition, Li et al.[10] developed a novel method of nonsalt roasting that showed high vanadium recovery, and the oxidation of vanadium spinel was conducted in the following steps: (1) destruction of vanadium spinel and formation of Fe2O3·V2O3; (2) subsequent oxidation of trivalent vanadium compounds; and (3) formation of high-valence vanadates such as Mn2V2O7 and Mg2V2O7.

To enhance the efficacy of vanadium extraction, a leaching process has currently gained increasing research interest. In the method developed by Li et al.[10] ammonium bicarbonate leaching was achieved after nonsalt roasting, whereby 85.1% of vanadium leaching was achieved at 50 °C, with an NH4HCO3 mass concentration of 35%. However, ammonium salts evaporated and decomposed easily; thus, the treatment for ammonium gas must be addressed. Liu et al.[12] introduced pressure acid leaching to extract vanadium from vanadium steel slag; the maximum leaching rate was 87.8% at an oxygen pressure of 1.0 MPa and leaching temperature of 160 °C. In contrast, Wu et al.[5] proposed a pressure leaching process using NaOH solution to coextract vanadium and chromium, in which the vanadium recovery was as high as 98.3%; however, the required conditions were very rigid, such as a leaching temperature of 200 °C and a reaction time of 180 min. In addition, other methods were implemented to improve the vanadium leaching. Oxidizing agents such as MnO2 and KClO3 were added to the leaching solution to oxidize low-valence vanadium to water-soluble vanadates.[13,14] Peng et al.[1] introduced the system of NaOH–H2O2 to recover vanadium and chromium; the leaching efficiency attained through their system exceeded 85%; however, the required NaOH and H2O2 were up to 1.0 g and 1.2 mL per gram residue, respectively. In addition, some researchers also suggested that 8-hydroxyquinoline entrapping vanadium to metal chelate was an efficient means for vanadium extraction.[15,16]

On the basis of the existing studies, the effective oxidation of vanadium compounds in vanadium slags to high-valence vanadate and the subsequent transfer to the leaching solution are crucial for vanadium extraction. H2O2 can react with some ions, such as Fe2+, V4+, Ti3+, and Mn2+, under acidic conditions to produce free radicals with strong oxidizing properties.[17] Additionally, the Fenton reagent has been widely used to rapidly decompose these refractory components. Accordingly, this study, for the first time, introduced H2O2 under acid leaching conditions to enhance the vanadium extraction from roasted vanadium steel slag. The leaching process was straightforward, and the required leaching time was shortened to less than 20 min, which is significantly lower than that required in other leaching processes (2–3 h). In addition to the optimization of technical parameters, the process and mechanism for rapid vanadium leaching were systematically investigated. The results provide valuable support for the practical application of vanadium extraction technology.

Materials and Methods

Roasting of Vanadium Steel Slag

The original vanadium steel slag, obtained from a converter steelmaking plant, Kunming Iron and Steel Corporation in Yunnan Province, China, was crushed and ground to less than 150 μm. The obtained vanadium steel slag was mixed with sodium carbonate and calcium fluoride with a mass ratio of 80:20:3.[12] The addition of CaF2 can facilitate phase change and realize low-temperature roasting; meanwhile, sodium roasting is helpful for producing water-soluble vanadate. To ensure complete oxidation during the roasting process, the door of the muffle furnace (TC-4-10, ZHONGXING Corp., China) was opened every 10 min for 45 s. After roasting at 700 °C for 60 min, the obtained calcine was crushed to a particle size of <75 μm and then subjected to acid leaching for vanadium recovery.

The elemental compositions of the vanadium steel slags before and after roasting were measured by an X-ray fluorescence analyzer, while the vanadium contents in the slags were determined through an ICP-AES analysis after acid digestion according to the Chinese Standard Method GB/T 6730.58-2017, and the results are listed in Table S1.

Vanadium Leaching

The leaching experiments were performed at atmospheric pressure, and a 500 mL reactor equipped with a magnetic stirring device (DF-101S, YUHUA, China) was placed in hot water to maintain a constant leaching temperature. As the temperature of the water bath reached the desired value, 8 g of roasted vanadium steel slag was added to the reactor, and 32 mL of H2SO4 with a volume concentration of 25% was injected into the reactor; the stirring speed was set to 500 rpm. After 1 min, H2O2 (volume concentration, 30%) was supplied to the leaching system, as required. As the mixture was extracted by acid leaching for a certain period, the reactor was rapidly cooled by cold water. Then, the collected filtrate and residue were used for subsequent analysis.

On the basis of the volume and vanadium concentration in the leaching solution, the vanadium extraction can be described as follows:

(c: vanadium concentration; V: volume of the leaching solution; ρ: vanadium content in the roasted slag; m: mass of the roasted slag).

Analysis and Characterization

The oxidation–reduction potential (ORP) and dissolved oxygen (DO) in the leaching system were measured using an ORP meter (PHS-2F, REX, China) and a DO meter (HQ30D, HACH, USA), respectively. An inductively coupled plasma (ICP) analyzer (Iris-Advangtage1000, USA) was used to determine the concentrations of metal ions (such as vanadium, manganese, and calcium) in the filtrate. The leaching rate of vanadium extraction was calculated based on the vanadium content and volume of the filtrate.

Tetravalent vanadium was assayed using Benfield solution,[18] and the concentration of pentavalent vanadium in the leaching solution was calculated according to the difference between the total vanadium and tetravalent vanadium. Ferrous and ferric ions were acquired using phenanthroline spectrophotometry.[19]

The solid samples were dried at 100 °C for 12 h and then identified using an X-ray diffractometer (Empyrean, PANalytical Corp., USA). The chemical valence states of iron and vanadium in the solids were analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, USA).[20] All spectra were drawn and analyzed using the XPS peak software.

To investigate the effects of free radicals on vanadium leaching, 1000 μL of DMPO (5,5-dimethyl-1-pyrroline-N-oxide) trapping agent, with a concentration of 0.3 mol L–1,[21] was mixed with 100 μL of leaching mixture. Thereafter, electron paramagnetic resonance (EPR A300-6/1, Bruker Corp., Germany) spectrometry coupled with the spin trapping technique was employed to identify the free radicals.

Results and Discussion

Vanadium Extraction in the Leaching System

Effect of H2O2 on Vanadium Leaching

H2O2 is widely used as a typical strong oxidant in wastewater treatment. In this study, 8 mL of H2O2 (volume concentration, 30%) was added to the leaching system, which contained 8 g of roasted vanadium steel slag and 32 mL of H2SO4. Other technical parameters were as follows: leaching temperature of 50 °C, H2SO4 concentration of 25% (volume concentration), and stirring speed of 500 rpm. As shown in Figure , the vanadium extraction in the leaching system with H2O2 was 70.5% at 10 min, which increased to 80.5% at 15 min, followed by no significant increase. In contrast, vanadium leaching in the control system without H2O2 was less than 60% before 15 min, which gradually increased from 66.7% at 30 min to 74.2% at 120 min. Therefore, the addition of H2O2 effectively enhanced the rapid vanadium extraction from the leaching system. The optimal leaching time in the conventional acid leaching process is 2–3 h. However, in this study, the time required for vanadium extraction, with the addition of H2O2, was only 15 min. In addition, the leaching rate was significantly improved.

Figure 1

Vanadium extraction in two comparative leaching systems.

Vanadium Extraction at Different Reaction Temperatures

The effects of temperature on vanadium extraction were investigated in the test and control leaching systems, and the leaching time was 15 min. Other parameters were the same as above. When the reaction temperature increased from 30 to 50 °C, vanadium leaching in the H2O2 system exhibited a gradual increase; however, it presented a moderate decrease from 80.5% at 50 °C to 73.9% at 60 °C (Figure ). For the leaching system without H2O2, the vanadium extraction increased gradually from 44.1 to 62.9% in the range of 30–60 °C. Nevertheless, the leaching rate at any temperature in the leaching system was lower than that in the test system.

Figure 2

Variations in vanadium leaching at different reaction temperatures.

In general, increasing the temperature can accelerate the rate of chemical reaction;[2] therefore, vanadium leaching in the control system showed an increasing trend at the temperature of 30–60 °C. However, for the test system, the addition of H2O2 to the acidic mixture intensified the leaching reaction, and the higher temperature was not helpful for vanadium extraction. Moreover, H2O2 rapidly decomposed with increasing temperature, while the water in the leaching system likely evaporated at high temperatures, which led to less vanadates entering the leaching solution at the lower liquid–solid ratio. Thus, 50 °C was selected as the suitable ambient temperature for this study.

Mutual Effects of H2SO4 Concentration and H2O2 Dosage

This study investigated the effects of H2SO4 and H2O2 on vanadium extraction. On the basis of the previous tentative experiments, the typical parameters were as follows: the leaching time was 15 min, reaction temperature was 50 °C, volume concentration of H2SO4 varied from 10 to 30%, and H2O2 dosage was 0–1.5 mL/g. Vanadium leaching increased with acid concentration and showed no significant increase as the volume concentration of H2SO4 exceeded 25% (Figure ). Although acid leaching is an important step for vanadium extraction, a high acid concentration may reduce the oxygen solubility in the solution and decrease the oxidation of vanadium,[12] which is unfavorable for vanadium extraction.

Figure 3

Effects of acid concentration and H2O2 dosage on vanadium extraction.

As shown in Figure , the leaching system exhibited higher vanadium extraction after the addition of H2O2 to the mixture of H2SO4 and vanadium steel slag. As the acid concentration was 25%, the test systems, with their respective H2O2 dosage of 1.0 and 1.5 mL/g, maintained a relatively higher vanadium extraction than the leaching system with a H2O2 dosage of 0.5 mL/g. However, the vanadium leaching only presented a slight increase when the H2O2 dosage increased from 1.0 to 1.5 mL/g. On the basis of the above experiment results, the suitable acid concentration and H2O2 dosage were 25% and 1.0 mL/g, respectively.

Characteristics of the Leaching Process

Variations of Ferric and Vanadic Components in the Solution

To reveal the characteristics of the leaching system, the concentrations of typical metal ions, such as V5+/V4+ and Fe3+/ Fe2+, were determined under the conditions mentioned in section . In the acid leaching system without H2O2, the concentration of total vanadium gradually increased during the entire leaching process. The pentavalent vanadium fluctuated from 3897 to 4588 mg/L, and the tetravalent vanadium moderately increased from 973 mg/L at 10 min to 4588 mg/L at 120 min (Figure a). Conversely, the vanadium extraction in the H2O2 system showed a moderate increase before 15 min, following which it exhibited minor fluctuations. Moreover, the concentrations of tetravalent and pentavalent vanadium presented the same variations (Figure b). During the leaching process, the H2O2 system maintained a stable ratio of tetravalent vanadium to total vanadium, which slightly fluctuated between 13.5 and 20.3%. In addition, it obtained relatively higher concentrations of total vanadium and pentavalent vanadium compared to the control system. This result is consistent with the vanadium extraction derived from the two systems.

Figure 4

Variations of iron and vanadic concentrations: (a) vanadic concentration in the control system; (b) vanadic concentration in the H2SO4–H2O2 leaching system; (c) iron concentration in control system; (d) iron concentration in the H2SO4–H2O2 leaching system.

For the acid leaching system without H2O2, the Fe3+ ion and total iron concentrations exhibited an increasing tendency as the leaching time increased. Fe3+ ions were predominant in the solution, and the proportion of Fe2+ ions was less than 3.5% (Figure c). In contrast, the concentration of Fe2+ ions gradually increased from 2227 mg/L at 10 min to 3234 mg/L at 45 min, and the percentage of Fe2+ to total iron was approximately 58–60%. As the leaching time was extended to 120 min, Fe2+ ions rapidly declined to 668 mg/L, whereas the concentration of Fe3+ ions increased to 5700 mg/L (Figure d), which was 89.5% of the total iron. The variations of ferric ions in the two leaching systems indicated that adding H2O2 to the acid leaching system may facilitate the dissolution of ferrous ions, leading to an increase in the total iron of the leaching solution.

In the vanadium extraction process, vanadium in the mineral exists primarily as iron vanadium oxide and surrounds the matrix of olivine phases (Fe2SiO4),[22] and the high-valence vanadate is more easily dissolved in the leaching solution.[3] Although alkaline salt roasting may be helpful for vanadium extraction, the roasted slag or the leached residual still contains a certain amount of low-valence vanadium compounds.[12] Thus, the effective conversion of low-valence vanadium to water-soluble vanadates is crucial for vanadium extraction. After H2O2 was added to the leaching system, it reacted with Fe2+ under acidic conditions and produced HO• via the Fenton reaction. The strong oxidizing potential, derived from H2O2 and its derivatives HO• and O2, oxidized the low-valence vanadium in the leaching solution or on the surface of the vanadium-bearing particle, after which the tetravalent vanadium in the liquid phase decreased. However, the total vanadium and pentavalent vanadium maintained relatively higher concentrations (Figure a,b). The addition of H2O2 facilitated the dissolution of ferrous compounds (such as Fe2SiO4 and Na4FeO3) and caused a significant increase in the Fe2+ ion and total iron concentrations (Figure c,d), thereby accelerating vanadium leaching (Figure b) as more enveloped vanadium was exposed. At a later stage of the leaching reaction, the promotion of vanadium leaching was no longer evident because of the decomposition of H2O2, and the total iron increased moderately under acid leaching for a long time. However, the Fe2+ ion could be oxidized by high-potential substances, such as O2 and vanadate. Thus, the ferrous concentration in the H2O2 leaching system decreased from 3234 mg/L at 45 min to 668 mg/L at 120 min (Figure d).

Chemical Phase Analysis of the Solids

Four types of solid samples were subjected to XRD analysis. The first was the roasted vanadium steel slag, and the second was residual 1 collected at a leaching time of 15 min in the control system. The remaining two samples were residuals 2 and 3 collected at 15 and 120 min, respectively, in the H2SO4–H2O2 system. Section describes the technical parameters. As shown in Figure a, the diffraction peaks observed in the XRD patterns of the roasted vanadium steel slag are complex, but the solid sample presents distinct diffraction peaks of iron vanadium oxides, such as FeVO4, Fe2V4O13, Fe0.33V2O5, and FeV3O8. Furthermore, Fe2O3, NaFeTiO4, (Ti0.5V0.5)2O3, and Fe2SiO4 (fayalite) appeared in the solid. In addition to pentavalent vanadium, low-valence vanadium, such as in FeV3O8 and (Ti0.5V0.5)2O3, was detected simultaneously, indicating that roasting cannot convert all vanadium compounds in various valence states to pentavalent vanadium. After acid leaching (Figure b–d), three typical substances, namely, Fe0.33V2O5, Fe2SiO4, and (MnO)0.441(CaO)0.559, could hardly be detected in the XRD patterns. However, CaSO4 presented strong diffraction peaks, which is because Ca2+ combined with the anion SO42– to produce the sediment. Compared to the control system, the H2O2 leaching system had similar diffraction peaks; however, some of the peaks, such as Fe2V4O13 and FeVO4, were relatively weak, indicating that the H2SO4–H2O2 system can enhance the dissolution of vanadium compounds. For residuals 2 and 3, the diffraction peaks showed no significant difference, which conforms with the fact that the two leaching systems at 15 and 120 min maintained a similar vanadium extraction.

Figure 5

XRD patterns of the solids: (a) roasted vanadium steel slag; (b) residual 1 at 15 min in the control system; (c) residual 2 at 15 min in the H2SO4–H2O2 system; (d) residual 3 at 120 min in the H2SO4–H2O2 system.

Three solid samples, residuals 1–3, were used for the XPS analysis. In this work, Gauss–Lorenz was used to fit the V 2p and Fe 2p peaks, and the results are shown in Figure , where the dotted lines indicate the Shirley background level. The peaks of V 2p (514–520 eV) and Fe 2p (708–718 eV) exhibited significant variations, revealing that the addition of H2O2 had evident effects on the leaching process. Figure a reveals that the valence states of V4+ and V5+ coexist in residual 1, and the main binding energy V 2p3/2 peaks at 516.8 and 517.6 eV correspond to V4+ and V5+ species, respectively.[12] The estimated percentage of V4+ species in the leached residual was 58.3%. As H2O2 was added to the acid leaching system, the low-valence vanadium was oxidized to the V5+ species by high-potential reagents, such as HO• (Eθ (HO•/H2O) = 2.85 V), H2O2 (Eθ (H2O2/H2O) = 1.763 V), and O2 (Eθ (O2/H2O) = 1.229 V), making the peak of V4+ species relatively weak (Figure c,e). As for the existence of iron compound, the binding energy Fe 2p3/2 peaks at 709.8–710.4 eV correspond to Fe2+–O bonds, whereas those at 711.4–712.5 eV correspond to Fe3+–O bonds.[23] The Fe2+ and Fe3+ species presented characteristic peaks, as demonstrated in Figure b–d. The percentage of Fe2+ species was 28.9% in residual 1, whereas it changed to 22.9% in residual 2 and then decreased to 14.3% in residual 3, which was collected at 120 min in the H2O2 leaching system. The results indicate that adding H2O2 to the leaching system enhanced the dissolution of ferrous compounds. This finding was also supported by the high concentration of Fe2+ ions in the H2O2 system (Figure d).

Figure 6

XPS spectra for the solids: (a) V4+/V5+ and (b) Fe2+/Fe3+ in residual 1; (c) V4+/V5+ and (d) Fe2+/Fe3+ in residual 2; (e) V4+/V5+ and (f) Fe2+/Fe3+ in residual 3.

Free Radicals in the H2SO4–H2O2 Leaching System

EPR spectrometry coupled with the spin trapping technique was employed to investigate the generation of radicals in the control system and the H2SO4–H2O2 leaching system. According to the characteristic peaks of the EPR spectra,[24] hydroxyl radicals (HO•) and superoxide anion radicals (O2–•) can be detected simultaneously in the H2SO4–H2O2 leaching system, but neither of them exists in the control system without H2O2. According to the principle of chemical reaction, the Fenton reaction can trigger the production of free radicals such as HO• and HO2•, and O2–• is also a potential reactive oxygen species produced by the following reaction:[25]

It was not surprising that HO• and O2–• appeared in the H2SO4–H2O2 system. The reactive oxygen species have strong oxidizing properties; for example, the standard electrode potential of HO•/H2O is up to 2.85 V, which provides a favorable external environment for the oxidation of low-valence vanadium compounds. Thus, the H2SO4–H2O2 system maintained lower tetravalent vanadium and higher total vanadium than the control system. The results revealed that these reactive oxygen species are beneficial for rapid vanadium leaching from roasted vanadium steel slag.

Changes in Other Indicators

The indicators DO and ORP were detected in the two comparative leaching systems (Figure ). The DO values in the H2SO4–H2O2 system always exceeded the upper limit of detection (22 mg/L), and the reason is related to the fact that abundant oxygen was released into the leaching solution after the decomposition of H2O2. For the control system with no addition of H2O2, the mechanical agitation and the addition of diluted H2SO4 solution resulted in the presence of dissolved oxygen in the leaching solution, and the DO decreased from 7.5 mg/L at startup to 1.0 at 3 min, then gradually increased and maintained a stable value of 7.6 mg/L after 12 min (Figure a). In the early stage of acid leaching, ferrous ions rapidly entered the leaching solution and consumed the dissolved oxygen to be oxidized to ferric compounds (shown in Figure c,d). Consequently, the DO decreased significantly and maintained a relatively lower value during a leaching time of 1–6 min.

Figure 8

Changes in (a) DO and (b) ORP during the leaching process.

For the control system, the ORP fluctuated moderately between 1020 and 1060 mV. However, the H2SO4–H2O2 system presented a relatively lower ORP during a leaching time of 15 min. Although the DO was always more than 22 mg/L, the leached ferrous ion in the H2SO4–H2O2 system was also up to 2227 mg/L at 10 min (Figure d), resulting in an ORP value of less than 1000 mV, and varied from 930 mV to 970 mV (Figure b).

In addition to the vanadic and ferric compounds, the leaching of other metallic elements was investigated in this study, and the ICP analysis is shown in Table S2. For the control system, the leaching of Mn, Cr, and Ti significantly increased with an increase in leaching time; however, the concentration of Ca2+ in the leaching solution moderately decreased due to the formation of CaSO4 (Figure ). After H2O2 was added to the leaching mixture, the compounds Mn, Cr, and Ti could be leached out in a short time of 15 min; however, there was no significant increase in chromium leaching.

Chemical Reaction and Pathway for the Rapid Vanadium Extraction

To discuss the feasibility of a chemical reaction, the electrode potentials[2,26,27] were compared in this study. As shown in Table , the standard electrode potentials of HO•/H2O, H2O2/H2O, HO2•/H2O, O2/H2O, VO2+/VO2+, Fe3+/Fe2+, and VO2+/V3+ are 2.85, 1.776, 1.70, 1.229, 1.00, 0.771, and 0.331 V, respectively. Therefore, most of the free radicals have a stronger oxidizing ability, and low-valence metal compounds such as ferrous ions and trivalent or tetravalent vanadium in the leaching system can be oxidized by HO•, H2O2, and O2. In addition, ferrous ions can be oxidized by pentavalent vanadium because of the difference in the electrode potentials. The speed of the oxidation process depends on the chemical reaction rate, which is determined by the strength of the bonds and external factors, such as temperature, concentration, and mass transfer.

Table 1

Redox Potential of the Specific Reaction for the Vanadium Leaching[2,26,27]

electrode reaction	EΘ/V	electrode reaction	EΘ/V
VO2+ + 4H+ + 3e– = V3+ + 2H2O	0.337	O2 + 4H+ + e– = 2H2O	1.229
H2O2 + H+ + e– = HO· + H2O	0.71	HO2· + 3H+ + 3e– = 2H2O	1.70
Fe3+ + e– = Fe2+	0.771	H2O2 + 2H+ + 2e– = 2H2O	1.776
VO2+ + e– = VO2+	1.00	HO· + H+ + e– = H2O	2.85

On the basis of the principles of physical chemistry and the obtained results, a pathway for vanadium leaching in the H2SO4–H2O2 system was proposed (Figure ). According to the simulation of the HSC Chemistry 6.0 software, as well as some related references, the potential chemical reactions during vanadium extraction are described in Table . In the acid leaching system, the soluble compounds on the outer surface of the particles dissolved in the H2SO4 and entered the leaching solution. The fluorine in the roasted vanadium steel slags accelerated the etching of the outer surface,[33] causing the iron vanadium compounds and others in the inner layer of the particles to gradually be exposed to the acid solution. This process provides the prerequisites for the enhancement of vanadium extraction. However, two factors adversely affected the leaching process; on one hand, silicate compounds such as Fe2SiO4 and MgSiO3 dissolved in highly acidic solution, and they were converted into colloidal H2SiO3,[34] which can absorb positive cations of vanadium such as VO2+ or VO2+.[32] On the other hand, Ca2+ reacted with SO42– to produce a poorly soluble compound CaSO4 (shown in Figure ). Both H2SiO3 and CaSO4 wrap around the particle or the vanadic compounds,[34,35] resulting in adverse effects on the dissolution and diffusion of vanadium ions.

Figure 9

Pathway of vanadium leaching in the H2SO4–H2O2 system.

Table 2

Potential Chemical Reactions and Equations in the H2SO4–H2O2 System

reaction equation (50 °C)		ΔG	log K	related refs
	eq 1			(26)
	eq 2			(29)
	eq 3			(28)
	eq 4			(30)
	eq 5	–49.69	33.61	HSC Chemistry 6
	eq 6			(25)
	eq 7	–24.46	16.56	HSC Chemistry 6
	eq 8	–44.21	30.05	HSC Chemistry 6
	eq 9			(27)
	eq 10	–37.78	25.55	HSC Chemistry 6
	eq 11	–4.24	2.87	HSC Chemistry 6
	eq 12			(31)
	eq 13			(32)
	eq 14			(26)
	eq 15	–108.9		HSC Chemistry 6
	eq 16	–384.54	26.01	HSC Chemistry 6
	eq 17			(33)
	eq 18	–13.82	9.34	HSC Chemistry 6
	eq 19	–94.77	64.1	HSC Chemistry 6
	eq 20	–54.49	36.85	HSC Chemistry 6
	eq 21	–21.07	14.25	HSC Chemistry 6
	eq 22	–7.64	5.16	HSC Chemistry 6

After H2O2 was added to the leaching system, the Fe2+ leaching increased significantly in a short time (Figure d), and the concentrations of Mn and Ti in the solution improved moderately (Table S2). H2O2 facilitated the dissolution of low-valence metallic ions and then triggered the production of strong oxidizing substances (Figure ), such as HO• and HO2•, via Fenton and Fenton-like reactions.[17] Considering that the Fe ion presented a relatively higher concentration than Mn and Ti ions, the correlation between Fe ions and vanadium extraction was discussed systematically in this paper. In general, the increase in total vanadium in the H2SO4–H2O2 system is closely related to the oxidation of vanadium compounds with low valence. When no matter exists in the leaching solution or on the surface of the particle, trivalent and tetravalent vanadium can be converted to pentavalent vanadium by strongly oxidizing substances, such as H2O2 and its derivatives, HO• and O2 (Table ). Table lists the potential chemical reactions. Furthermore, the vanadium inside the particle gradually is exposed to the acid leaching solution after the complex oxides on the surface of the particle are destroyed and can then be oxidized to high-valence vanadium, which favors vanadium leaching (Figure ). For the H2SO4–H2O2 system, the leaching reaction was rapid (performed within 6 min), and it was very difficult to determine the activation energy and chemical reaction rate for this rapid leaching process. Thus, this requires further investigation.

Figure 7

EPR spectra of the mixture derived from the H2SO4–H2O2 leaching system (note: 1 - characteristic peaks of O2–•; 2 - characteristic peaks of HO•).

Conclusion

H2O2 is an effective reagent for enhancing vanadium leaching in a short time. The vanadium extraction in the H2SO4–H2O2 system improved by 22%, compared to that in a control system with no addition of H2O2. The leaching time in the proposed system reduced to 15 min owing to the addition of H2O2.

H2O2 and its derivatives have high redox potentials and can oxidize Fe2+ ions and low-valence vanadium. Because of the rapid dissolution of metallic ions and complex compounds, vanadium in the inner particle begins to be exposed to the acid leaching solution, following which it can be oxidized to pentavalent vanadium, resulting in a significant increase in the total vanadium.

In the H2SO4–H2O2 system, the metallic ions of Mn, Cr, and Ti were leached out simultaneously, but there was no significant increase in chromium leaching. The leaching reaction was rapid, and the potential chemical reactions were speculated based on the obtained results and the simulation of the HSC chemistry. The determination of the activation energy and chemical reaction rate for this rapid leaching process requires further investigation.