A low-ammonium consumption method for preparing high-purity V₂O₅ from vanadium-rich liquids with high impurity content

Abstract

The conventional method for preparing V₂O₅ from vanadium-rich leachate suffers from three significant drawbacks: low purity, excessive ammonium consumption, and the generation of high-ammonia–nitrogen wastewater. To address these challenges, this study introduces an integrated process involving D2EHPA saponification extraction, hydrolysis vanadium precipitation, and ammonium purification for the production of high-purity V₂O₅ from high-impurity vanadium-rich liquid. After three-stage counter-current extraction at a 60% saponification degree, 40 vol% D2EHPA concentration, an initial pH of 1.8, a phase ratio (O/A) of 2 : 1, and an extraction time of 8 minutes, followed by three-stage counter-current stripping at 2 mol L⁻¹ H₂SO₄ concentration, a phase ratio (O/A) of 2 : 1, and stripping time of 20 minutes, the concentrations of Fe²⁺ and Al³⁺ in the stripping solution were 0.034 g L⁻¹ and 0.439 g L⁻¹, respectively. These contaminants were effectively eliminated with removal efficiencies of 98.78% and 97.93%. At an ammonium addition coefficient of 1, V₂O₅ was prepared with 99.9% purity using the hydrolysis vanadium precipitation-ammonium salt purification approach, which consumed 83% less ammonium salt compared to the ammonium precipitation method. This study significantly reduces ammonium salt usage and provides a scalable, environmentally friendly process for high-purity V₂O₅ production.

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1. Introduction

Vanadium is a strategic metal with significant importance in high-tech industries due to its unique chemical and physical properties.^1–3 V₂O₅, a key vanadium oxide, is widely used in catalysts, nanomaterials, and pharmaceuticals, and in the chemical and metallurgical industries.^4–7 The demand for high-purity V₂O₅ has particularly increased due to its application in vanadium redox flow batteries.⁸ Consequently, research on the preparation of high-purity V₂O₅ has garnered substantial interest.⁹

The vanadium-containing leachate produced through acid leaching of vanadium shale contains a significant amount of impurities. Therefore, vanadium products require extensive purification and enrichment to remove these impurities before further processing.^10,11 However, even after purification, the vanadium-rich liquid still contains substantial levels of impurity ions, which directly affect the purity of the resulting V₂O₅ products.¹² Impurities such as Fe, Al, Ca, and P can significantly decrease vanadium precipitation efficiency and the purity of V₂O₅ in ammonium salt precipitation processes.¹³ Thus, to produce high-purity V₂O₅, vanadium-rich liquids must undergo significant impurity removal before precipitation.

C. Chao et al.¹⁴ employed hydrolysis vanadium precipitation to obtain red vanadium, followed by alkali solubilization, flocculant removal, and ammonium vanadium precipitation to prepare high-purity V₂O₅. However, this method leads to high vanadium losses, excessive ammonium consumption, and the production of a large volume of ammonia–nitrogen wastewater, which poses an environmental threat. Therefore, there is a pressing need for an environmentally friendly technique that minimizes vanadium losses. Solvent extraction is an effective method for separating vanadium from impurities due to its efficiency, cost-effectiveness, and ease of implementation.^15,16 Li W. et al.¹⁷ demonstrated that solvent extraction outperforms ion exchange in removing impurities from vanadium-rich solutions while maximizing vanadium recovery. Among various extractants, Di-(2-ethylhexyl) phosphoric acid (D2EHPA) has become a widely used extractant due to its superior performance in acidic solutions.^18–20 D2EHPA is an acidic extractant, which can displace a large amount of H⁺ in the extraction process and reduce the extraction rate. After saponification, H⁺ in D2EHPA is replaced by Na⁺ or K⁺ in the saponification agent, and the content of H⁺ released during extraction is reduced, and the dimer of D2EHPA is dissociated into haploid after saponification, which greatly improves the extraction rate.²¹

The vanadium precipitation process is a critical step in vanadium recovery from vanadium shale and other vanadium-bearing deposits.^22,23 In recent years, acidic ammonium precipitation has been widely used in industrial production due to its high precipitation efficiency and excellent purity of vanadium products.^24–26 However, this method consumes large amounts of ammonium salt and produces substantial volumes of ammonia nitrogen wastewater.^27,28 Although hydrolysis vanadium precipitation results in lower-purity vanadium products (approximately 85.00%), it is highly efficient, does not generate ammonia–nitrogen wastewater, and is less environmentally damaging.^29,30 As environmental protection regulations tighten, the conventional vanadium precipitation process, which uses excessive ammonium salt and produces low-quality products, has hindered the future development of vanadium extraction technologies.

Polyvanadate ions preferentially form alkali metal salts in the order of K⁺ > NH₄⁺ > Na⁺ > H⁺.³¹ Based on this principle, high-purity V₂O₅ products can be generated through hydrolysis vanadium precipitation and ammonium salt purification. In this approach, sodium polyvanadate is first prepared through hydrolysis vanadium precipitation and then purified with an ammonium salt solution. Polyvanadate ions exhibit a stronger affinity for NH₄⁺ than Na⁺, enabling NH₄⁺ to replace Na⁺ in sodium polyvanadate to form ammonium polyvanadate,³² which enhances the purity of the final V₂O₅ product. This method effectively reduces ammonium consumption while achieving high-purity V₂O₅.

This study aims to develop a high-purity V₂O₅ preparation process with efficient impurity removal and low ammonium consumption. The process involves D2EHPA saponification extraction for impurity removal, hydrolysis vanadium precipitation to form sodium polyvanadate, and ammonium salt purification to yield high-purity V₂O₅. The proposed method offers excellent impurity removal, reduced ammonium consumption, and environmental sustainability, making it a promising approach for the future of the vanadium extraction industry.

2. Materials and methods

2.1. Materials

In this study, vanadium-rich liquid produced by a vanadium factory in Shaanxi province is used as a raw material. The primary impurity ions in the vanadium-rich liquid used in this study are Fe²⁺ and Al³⁺ (Table 1). All solutions were prepared using deionized water, and all reagents were of analytical grade. D2EHPA was for the extraction of vanadium, with NaOH as the saponification agent, NaClO₃ to oxidize V(IV) to V(V), Na₂CO₃ and H₂SO₄ to adjust pH, and NH₄Cl for washing. The initial pH of the vanadium-rich solution was −0.4, determined using a pH meter (S-3, Shanghai Lida Instrument Factory, China).

Table 1 Composition of vanadium-rich liquid (g L⁻¹)

Element	V	Fe	Al	Zn	Na	K	Mg	P
Content	54.23	2.78	21.21	3.88	0.707	0.545	0.304	0.36

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2.2. Experimental steps

The high-purity V₂O₅ products were prepared through the D2EHPA saponification extraction-hydrolysis vanadium precipitation-ammonium salt purification approach, as outlined in Fig. 1. The vanadium-rich liquid used in step 1 is a simulated vanadium-rich liquid configured according to the composition shown in Table 1.

Fig. 1 A flowchart of the V₂O₅ preparation process.

2.2.1. Impurities elimination process. Fe²⁺, Al³⁺, and other impurity ions in the vanadium-rich liquid must be removed before hydrolysis vanadium precipitation. To establish acceptable Fe²⁺ and Al³⁺ concentrations that meet the YB/T 5304-2017 standard, the effects of Fe²⁺ and Al³⁺ concentration on the purity of the vanadium precipitation products were investigated.

D2EHPA saponification extraction was applied to remove impurity ions and produce high-purity V₂O₅. The saponification agent was added to the organic phase (mixture of D2EHPA and sulfonated kerosene) and stirred until clear. The saponification degree was calculated using eqn (1),³³ extraction percentage(E), stripping percentage (S), partition ratio (D) and separation factor (β_M1/M2) were calculated from eqn (2)–(5).

where C_F, C_R, C_S, and C_org represent the metal concentrations (g L⁻¹) in the feed solution (vanadium-rich liquid), raffinate, stripping solution, and organic phase, respectively; V_F, V_R, and V_S represent the volumes (L) of the feed solution, raffinate, and stripping solution, respectively; and M1 and M2 represent the metal elements.

2.2.2. Hydrolysis vanadium precipitation-ammonium salt purification. Different experimental influencing factors were investigated to establish the optimum parameters for hydrolysis vanadium precipitation-ammonium salt purification. Each experiment was conducted by magnetic stirring in a thermostatic water bath. First, NaClO₃ was applied to oxidize the vanadium-rich liquid's V(IV) and Fe²⁺ to V(V) and Fe³⁺. Second, Na₂CO₃ was added to adjust the liquid's pH, the pH-adjusted solution was then heated and stirred for a period. Eventually, solid–liquid separation formed the sodium polyvanadate and vanadium precipitation liquor. The ammonium polyvanadate was produced by incorporating 2% NH₄Cl solution and stirring. The dried vanadium precipitate was then calcined at 500–550 °C for 30–60 min to produce V₂O₅. Eventually, solid–liquid separation formed the precipitation fluid of vanadium and sodium polyvanadate. The ammonium polyvanadate was produced by incorporating 2% NH₄Cl solution with sodium polyvanadate and stirring. Using the ammonium ferrous sulfate titration method, the concentration of vanadium in the vanadium precipitated liquor and washing wastewater was calculated. In addition, the percentage of vanadium precipitation and vanadium loss were computed. The vanadium precipitation percentage (η), vanadium loss (L), and ammonium addition coefficient (K) were determined using eqn (6)–(8).where C_a, C_b, and C_c are the vanadium concentration (g L⁻¹) in vanadium-rich liquid, washing wastewater, and vanadium precipitated liquor, respectively; V_a, V_b, and V_c are the volume (L) of vanadium-rich liquid, washing wastewater, and vanadium precipitated liquor, respectively; n (NH₄⁺) is the number of ammonium ions in the ammonium salt solution and n (V₂O₅) is the amount of vanadium in the vanadium-rich solution.

2.3. Analysis methods

Ammonium ferrous sulfate titration was used to determine the concentration of vanadium in solution (GB/T 8704.5-2020). The V₂O₅ products' elemental content and the ionic content in solution were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES; Thermo Elemental, USA). The physical composition of vanadium-containing products was determined using X-ray diffraction (XRD; Rigaku, Japan). The vanadium-containing products' microstructures and corresponding elemental distributions were examined using a field emission scanning electron microscope (ESEM; Quattro S, USA) equipped with an energy-dispersive spectrometer (EDS; BRUKER, Germany). The content of crystal water in vanadium precipitation products was determined using thermogravimetric (TG; Netzsch, Selb, Germany). To document the changes in chemical bonding throughout the reaction, Fourier transform infrared (FTIR) spectra (Thermo Fisher Scientific Co., MA, USA) were employed. The purity of V₂O₅ products was determined according to the standard YB/T 5304-2017.

3. Results and discussion

3.1. Impurities elimination process

3.1.1. Effect of Fe²⁺ and Al³⁺ on V₂O₅ purity. To avoid interference from other impurities, a vanadium-rich liquid free of impurity ions was prepared, and different concentrations of Fe²⁺ and Al³⁺ were added. The effects of Fe²⁺ and Al³⁺ concentration on the purity of the V₂O₅ product were analyzed (Fig. 2).

Fig. 2 Effect of (a) Fe²⁺ and (b) Al³⁺ concentration on V₂O₅ purity (vanadium precipitation conditions: initial pH 1.8, temperature 95 °C, time 1.5 h).

Fig. 2(b) demonstrates that the Al³⁺ concentration has a minimal effect on the purity of V₂O₅. Specifically, as the Al³⁺ concentration increased from 5 g L⁻¹ to 25 g L⁻¹, the purity of V₂O₅ decreased slightly from 99.98% to 99.96%, while the aluminum content increased from 0.0007 wt% to 0.001 wt%. Fig. 2(a) indicates that the iron content in V₂O₅ products increased with the rise in Fe²⁺ concentration in the vanadium-rich liquid. When the Fe²⁺ concentration was 0.2 g L⁻¹, the iron content in the V₂O₅ products was 0.082 wt%, whereas at a Fe²⁺ concentration of 1 g L⁻¹, the iron content increased to 0.276 wt%. The purity of the V₂O₅ products meets the V₂O₅ 98-P standard outlined in the YB/T 5304-2017 specification when the Fe²⁺ concentration in the vanadium-rich solution is less than 0.2 g L⁻¹. Therefore, considering various iron removal methods, D2EHPA extraction was chosen as the optimal process for eliminating iron and producing high-purity V₂O₅ products that meet the required standard.

3.1.2. Extraction process. The vanadium-rich liquid used in this study exhibited a high vanadium concentration. Direct extraction led to a significant decrease in the pH of the aqueous phase due to the displacement of vanadium and hydrogen ions by D2EHPA, resulting in a low extraction rate.³⁴ By employing NaOH as a saponifier, sodium ions replace the hydrogen ions in D2EHPA during the saponification process, leading to a lower pH in the aqueous phase, which facilitates the smooth progression of the extraction reaction.³⁵ The impact of the saponification degree of D2EHPA on vanadium extraction was examined under the following conditions: an initial pH of 1.8, a D2EHPA concentration of 40 vol%, a phase ratio (O/A) of 2 : 1, and an extraction time of 8 minutes.

As shown in Fig. 3, the vanadium extraction percentage increased from 55.93% to 89.11%, while the Fe²⁺ extraction percentage increased from 13.61% to 24.66% as the saponification degree of D2EHPA was raised from 0% to 60%. However, no significant increase in vanadium extraction was observed as the saponification degree increased from 60% to 80%, whereas the Fe²⁺ extraction percentage rose from 24.66% to 35.91%. When the saponification degree was increased from 80% to 100%, the vanadium extracted proportion dropped to 70.08%. Therefore, a saponification degree of 60% was determined to be optimal for achieving high vanadium recovery and effective vanadium–iron separation.

Fig. 3 Effect of saponification degree on extraction.

The effects of initial pH, D2EHPA concentration, phase ratio (O/A), and extraction time on the vanadium extraction and vanadium–iron separation were further investigated under the optimal saponification degree of 60%, as presented in Fig. 4.

Fig. 4 Effect of initial pH (a), D2EHPA concentration (b), phase ratio (O/A) (c), and extraction time (d) on the extraction rate of vanadium and Fe²⁺ and β_V/Fe.

The impact of initial pH on vanadium extraction was assessed at an extraction time of 8 minutes, D2EHPA concentration of 40 vol%, and phase ratio (O/A) of 2 : 1. Fig. 4(a) illustrates that the vanadium extraction rate increased significantly from 55.64% to 89.15% as the initial pH of the feed solution was raised from 0.4 to 1.8. The vanadium extraction rate remained constant beyond this pH range. Similarly, the Fe²⁺ extraction rate increased steadily as the pH increased. The optimal vanadium extraction rate and vanadium–iron separation coefficient were observed at an initial pH of 1.8, which was thus selected as the optimal pH.

D2EHPA concentration and phase ratio are critical factors influencing vanadium recovery and vanadium–iron separation. Given the high vanadium concentration in the vanadium-rich liquid, the extractant dosage plays a significant role in vanadium recovery. The effect of D2EHPA concentration was examined at an initial pH of 1.8, a phase ratio (O/A) of 2 : 1, and an extraction time of 8 minutes. The results in Fig. 4(b) indicate that the vanadium extraction rate increased from 15.35% to 92.59%, while the Fe²⁺ extraction rate increased from 12.55% to 30.19% as the D2EHPA concentration was raised from 10 vol% to 50 vol%. Considering both vanadium recovery and vanadium–iron separation, a D2EHPA concentration of 40 vol% was selected as optimal.

The effect of phase ratio (O/A) on vanadium extraction was studied at an initial pH of 1.8, a D2EHPA concentration of 40 vol%, and an extraction time of 8 minutes. As shown in Fig. 4(c), the extraction rates of vanadium and Fe²⁺ increased from 9.79% to 98.36% and from 6.69% to 46.16%, respectively, as the phase ratio (O/A) was adjusted from 1 : 2 to 3 : 1. While a higher phase ratio improved vanadium recovery, the vanadium–iron separation effect diminished. The optimal phase ratio (O/A) for maximizing vanadium–iron separation, with a vanadium extraction rate of 89.65%, was determined to be 2 : 1.

The impact of extraction time on vanadium extraction was investigated at an initial pH of 1.8, D2EHPA concentration of 40 vol%, and a phase ratio (O/A) of 2 : 1. Fig. 4(d) shows that the vanadium extraction rate increased from 74.09% to 87.31% as the extraction time was extended from 2 to 6 minutes, with the extraction rate stabilizing after 8 minutes. The vanadium–iron separation coefficient remained unchanged with longer extraction times. Thus, an extraction time of 6 minutes was identified as optimal.

The McCabe–Thiele diagram for vanadium concentrations in the aqueous and organic phases under optimal conditions (pH of 1.8, D2EHPA concentration of 40%, and extraction time of 8 minutes) was plotted, as shown in Fig. 5. The phase ratio (O/A) was set to 2 : 1. Theoretically, the vanadium concentration in the raffinate can be reduced to 50 mg L⁻¹ after two-stage counter-current extraction. However, three-stage counter-current extraction was performed in practice to achieve the optimal experimental parameters. The vanadium extraction rate reached 99%, with vanadium and Fe²⁺ concentrations in the loaded organic phase of 26.84 g L⁻¹ and 0.88 g L⁻¹, respectively.

Fig. 5 McCabe–Thiele diagram for determining the number of vanadium extraction stages (conditions: initial pH of 1.8, D2EHPA concentration of 40 vol%, time of 8 minutes).

3.1.3. Stripping process. The concentration of Fe²⁺ in the stripping solution is directly linked to the purity of V₂O₅ products. As the extraction process removes most impurity ions and the vanadium concentration in the feed solution meets the requirement for hydrolysis vanadium precipitation, the stripping process must ensure effective vanadium recovery while minimizing the entry of Fe²⁺ into the stripping solution. The impacts of phase ratio (O/A), H₂SO₄ concentration, and stripping time on vanadium and Fe²⁺ stripping rates, as well as the vanadium–iron separation coefficient, were evaluated, as shown in Fig. 6.

Fig. 6 Effect of phase ratio (a), H₂SO₄ concentration (b), and time (c) on vanadium and Fe²⁺ stripping rates and β_V/Fe.

The phase ratio (O/A) determines the vanadium concentration in the stripping solution. The effect of phase ratio was investigated at an H₂SO₄ concentration of 2 mol L⁻¹ and a stripping time of 30 minutes. As shown in Fig. 6(a), the vanadium stripping rate decreased significantly, and the vanadium–iron separation coefficient continuously declined as the phase ratio increased. At a phase ratio of 2 : 1, the vanadium extraction rate was 84.95%, and the vanadium–iron separation coefficient was 91.15. Since the aim of this investigation was not to further concentrate vanadium, the optimum phase ratio (O/A) was determined to be 2 : 1.

The effect of H₂SO₄ concentration, ranging from 1 to 4 mol L⁻¹, was examined at a stripping time of 30 minutes and a phase ratio of 2 : 1. The results, shown in Fig. 6(b), indicate a sharp increase in vanadium stripping rate from 46.14% to 84.95% as the H₂SO₄ concentration was raised from 1 mol L⁻¹ to 2 mol L⁻¹. Further increasing the H₂SO₄ concentration to 4 mol L⁻¹ raised the vanadium stripping rate to 93.76%, but the vanadium–iron separation coefficient decreased significantly. Considering the impact of Fe²⁺ concentration on the purity of the V₂O₅ products, the optimal H₂SO₄ concentration was selected as 2 mol L⁻¹.

The effect of stripping time was studied at an H₂SO₄ concentration of 2 mol L⁻¹ and a phase ratio (O/A) of 2 : 1. As shown in Fig. 6(c), from 10 minutes to 20 minutes, the vanadium stripping rate increased from 75.02% to 82.22%, continuing to rise with longer stripping times. However, with extended stripping time, the vanadium–iron separation coefficient decreased, and the vanadium stripping rate reached a plateau. Therefore, the optimal stripping time was determined to be 20 minutes.

The McCabe–Thiele diagram was constructed based on the optimal stripping conditions (H₂SO₄ concentration of 2 mol L⁻¹ and stripping time of 20 minutes). As depicted in Fig. 7, the vanadium concentration in the raffinate phase was reduced to 30 mg L⁻¹ after two-stage counter-current stripping. In practice, a three-stage counter-current stripping process was employed to achieve the optimal experimental parameters, with the vanadium stripping rate reaching 99.8%. The main components of the stripping solution are shown in Table 2, where the concentrations of vanadium and Fe²⁺ were 53.57 g L⁻¹ and 0.034 g L⁻¹, respectively.

Fig. 7 McCabe–Thiele diagram for determining the number of vanadium stripping stages (conditions: H₂SO₄ concentration of 2 mol L⁻¹, stripping time of 8 minutes).

Table 2 Composition of stripping solution (g L⁻¹)

Element	V	Fe	Al	Zn	Na	K	Mg	P
Content	53.57	0.034	0.439	0.185	0.017	0.004	0.014	0.012

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3.1.4. D2EHPA saponification extraction mechanism. To investigate the saponification extraction mechanism of D2EHPA, FTIR analysis was carried out on fresh, saponified, loaded, and poor organic phases. The results are shown in Fig. 8.

Fig. 8 FTIR spectra of organic phases.

In the unsaponified D2EHPA organic phase, the molecules form strong hydrogen bonds with each other, predominantly as dimers.³⁶ As shown in Fig. 8, the P–O–H bond vibration peak at 2731 cm⁻¹ in the fresh organic phase is hydrogen-bonded. In contrast, the P–O–H vibration peak in the saponified organic phase is significantly weakened, suggesting that some hydroxyl groups have been broken, with sodium ions replacing hydrogen ions. The P=O absorption peak at 1230 cm⁻¹ in the loaded organic phase splits into two distinct P=O peaks at 1209 cm⁻¹ and 1240 cm⁻¹. Additionally, the absorption peaks at 933 cm⁻¹ and 614 cm⁻¹ correspond to the V–O and V–O–V bonds, respectively, indicating that vanadium is transferred from the aqueous phase into the organic phase during extraction. The O–H bond stretching vibration peaks at 3410 cm⁻¹ in both the saponified organic and raffinate phases, suggesting the incorporation of water molecules into the organic phase during saponification. The spectra of the raffinate and fresh organic phases are similar, and the disappearance of the V–O–V and V–O peaks following sulfuric acid stripping, alongside the recovery of the P=O peaks, indicates that the organic phase can be regenerated and reused.

3.2. Hydrolysis vanadium precipitation-ammonium salt purification

3.2.1. Hydrolysis vanadium precipitation process. This study investigated the key parameters (pH, reaction time, and temperature) influencing the hydrolysis vanadium precipitation process. The experimental results are presented in Fig. 9.

Fig. 9 Effect of initial pH (a), reaction time (b), and temperature (c) on vanadium precipitation rate.

The effect of initial pH on the hydrolysis vanadium precipitation was examined at a reaction time of 2 hours and a temperature of 98 °C. As shown in Fig. 9(a), the vanadium precipitation rate increased consistently with the rise in initial pH. The precipitation rate reached 99.22% when the initial pH was 1.8, and no further increase in precipitation rate was observed for pH values above 1.8. Therefore, the optimal initial pH was determined to be 1.8.

The effect of reaction time on hydrolysis vanadium precipitation was evaluated at an initial pH of 1.8 and a temperature of 98 °C. As depicted in Fig. 9(b), the vanadium precipitation rate increased progressively with longer reaction times. The rate reached 99.20% after 1.5 hours, and no significant change in the precipitation rate was observed beyond this time. To optimize energy usage and manufacturing efficiency, the optimal reaction time was determined to be 1.5 hours.

The effect of reaction temperature on hydrolysis vanadium precipitation was studied at an initial pH of 1.8 and a reaction time of 1.5 hours. As shown in Fig. 9(c), the vanadium precipitation rate increased with temperature, reaching a maximum of 99.13% at 95 °C. Further temperature increases did not lead to any additional improvement in precipitation. Consequently, the optimal reaction temperature was selected to be 95 °C.

The main parameters of the hydrolysis vanadium precipitation process were optimized to ensure the complete precipitation of vanadium from the vanadium-rich solution while minimizing vanadium loss during the process. As illustrated in Fig. 9, the vanadium precipitation rate reached 99.13% under the optimal conditions: an initial pH of 1.8, a reaction time of 1.5 hours, and a temperature of 95 °C.

3.2.2. Ammonium salt purification process. NH₄⁺ exhibit a stronger binding affinity for vanadate ions to vanadate ions compared to Na⁺, enabling the purification of the hydrolysis vanadium precipitation product using an ammonium salt solution to produce high-purity V₂O₅. In this study, NH₄Cl solution was used as the purification agent. The effects of the ammonium addition coefficient, purification temperature, purification time, and purification frequency on the purity of V₂O₅ were investigated, with the experimental results shown in Fig. 10.

Fig. 10 Effect of ammonium addition coefficient (a), temperature (b), time (c), and purification frequency (d) on purity.

The effect of the ammonium addition coefficient on purity was examined at a purification temperature of 25 °C, a purification time of 20 min, and a purification frequency of 1. As shown in Fig. 10(a), the purity of V₂O₅ increased significantly as the ammonium addition coefficient was increased from 0 to 1. However, as the ammonium addition coefficient continued to increase, the purity of V₂O₅ stabilized, while the vanadium loss rate increased. Therefore, the optimum ammonium addition coefficient was determined to be 1, balancing V₂O₅ purity improvement with reduced ammonium usage and vanadium loss.

The effect of purification temperature on purity was studied at an ammonium addition coefficient of 1, a purification time of 20 min, and a purification frequency of 1. The results, shown in Fig. 10(b), indicate that V₂O₅ purity gradually increased with the rise in temperature from 25 °C to 55 °C. However, further temperature increases led to a decline in purity and an increase in vanadium loss. Hence, the optimal purification temperature was determined to be 55 °C.

The effect of purification time on purity was investigated at an ammonium addition coefficient of 1, a purification temperature of 55 °C, and a purification frequency of 1. As shown in Fig. 10(c), the purity of V₂O₅ improved as the purification time was extended from 0 to 15 minutes. Beyond 15 minutes, the purity remained constant, while vanadium loss increased. Therefore, the optimal purification time was chosen to be 15 minutes.

The effect of purification frequency on V₂O₅ purity was studied at an ammonium addition coefficient of 1, a purification temperature of 55 °C, and a purification time of 15 minutes. As illustrated in Fig. 10(d), with the total ammonium consumption kept constant, the purity of V₂O₅ increased as the purification frequency was raised from 0 to 2. However, the purity remained unchanged when the purification frequency exceeded 2, indicating that the system had reached equilibrium. To ensure optimal V₂O₅ purity while enhancing manufacturing efficiency and reducing energy consumption, the optimal purification frequency was selected to be 2.

To assess the reduction in ammonium consumption, the hydrolysis vanadium precipitation-ammonium salt purification process was compared with the ammonium salt vanadium precipitation process. Ammonium salt (NH₄Cl) vanadium precipitation experiments were conducted on the solution obtained after D2EHPA saponification extraction, using an initial pH of 1.8, a temperature of 95 °C, and a reaction time of 1.5 hours. The results of these experiments are presented in Fig. 11.

Fig. 11 Effect of ammonium addition coefficient on vanadium precipitation rate and purity.

As shown in Fig. 11, both the vanadium precipitation rate and purity increased with the ammonium addition coefficient. When the ammonium addition coefficient increased from 1 to 6, the vanadium precipitation rate increased from 98.33% to 99.43%, while the purity rose from 93.51% to 99.23%. When the ammonium addition coefficient exceeded 6, both the precipitation rate and purity remained constant. Therefore, the optimal ammonium addition coefficient for vanadium precipitation was selected to be 6.

The above experimental results demonstrate that the hydrolysis vanadium precipitation-ammonium salt purification process can significantly reduce the ammonium addition coefficient from 6 to 1, resulting in an 83% reduction in ammonium consumption compared to the conventional ammonium salt vanadium precipitation process. This makes the process more efficient and environmentally friendly for the production of high-purity V₂O₅.

3.2.3. Mechanisms analysis of ammonium salt purification. To explore the mechanisms underlying the ammonium salt purification process, the hydrolysis vanadium precipitation and ammonium salt purification products, prepared under optimal conditions, were analyzed using XRD and ESEM-EDS. These analyses aimed to identify changes in the physical phases of sodium polyvanadate and ammonium polyvanadate.

Fig. 12 shows the XRD patterns of both the hydrolysis vanadium precipitation and ammonium salt purification products. It can be observed that the NaV₃O₈·xH₂O and Na₂SO₄ phases present in the hydrolysis vanadium precipitation product gradually disappear during the ammonium salt purification process. Concurrently, the formation of the (NH₄)₂V₆O₁₆·1.5H₂O phase is evident. This suggests that NH₄⁺ in the ammonium salt solution substitutes Na⁺ in the hydrolysis vanadium precipitation product. In the XRD spectrum of the ammonium salt purification product, only the (NH₄)₂V₆O₁₆·1.5H₂O phase is observed, confirming the low impurity content and the high purity of the resulting (NH₄)₂V₆O₁₆·1.5H₂O.

Fig. 12 XRD patterns of the hydrolysis vanadium precipitation product (K = 0) and ammonium salt purification product (K = 1).

Fig. 13 and 14 reveal that the morphologies of both the hydrolysis vanadium precipitation and ammonium salt purification products are similar, exhibiting an irregular, layered structure. These observations suggest that the purification procedure does not significantly alter the microscopic morphology of the product. In Fig. 13, the hydrolysis vanadium precipitation product shows a strong correlation between V, O, and Na, based on face-scanning EDS analysis, which, in conjunction with the XRD results, indicates that its primary component is NaV₃O₈·xH₂O. In contrast, Fig. 14 shows that the ammonium salt purification product exhibits a strong correlation between V, O, and N, with a lower correlation between V and Na compared to the hydrolysis vanadium precipitation product. This supports the XRD results, suggesting that the ammonium salt purification process has effectively substituted Na⁺ with NH₄⁺, resulting in the formation of the new (NH₄)₂V₆O₁₆·1.5H₂O phase. Additionally, it is hypothesized that nitrogen from (NH₄)₂V₆O₁₆ · 1.5H₂O may be released as gas during the purification process, further enhancing the purity of the final V₂O₅ product.

Fig. 13 ESEM micrographs with EDS element mapping of hydrolysis vanadium precipitation product.

Fig. 14 ESEM micrographs with EDS element mapping of ammonium salt purification product.

Thermogravimetric (TG) and derivative thermogravimetry (DTG) analyses were conducted to determine the crystal water content in NaV₃O₈·xH₂O, the primary component of the hydrolysis vanadium precipitation product. Fig. 15 illustrates the weight loss profile of the hydrolysis vanadium precipitation product, which can be divided into two stages. The first stage, between 0 °C and 125 °C, exhibits a 3.4% weight loss, indicating the evaporation of adsorbed water. The second stage, between 125 °C and 600 °C, shows an 8.1% weight loss, corresponding to the evaporation of crystal water. Based on the weight loss observed in the second stage, the crystal water content in NaV₃O₈·xH₂O can be calculated, confirming the chemical formula of sodium polyvanadate as NaV₃O₈·1.5H₂O.

Fig. 15 TG and DTG analysis of hydrolysis vanadium precipitation products under a nitrogen atmosphere.

To further elucidate the mechanism of the ammonium salt purification process, FTIR spectroscopy was performed on both the hydrolysis vanadium precipitation and ammonium salt purification products. Fig. 16 shows that the peaks at 540 cm⁻¹ and 729 cm⁻¹ in the FTIR spectrum of the hydrolysis vanadium precipitation product correspond to V–O–V stretching vibrations. In contrast, the peaks at 543 cm⁻¹ and 738 cm⁻¹ in the FTIR spectrum of the ammonium salt purification product correspond to V–O–V bending vibrations.³⁷ This shift from lower to higher wavenumbers after ammonium salt purification suggests a change in the crystal structure of the hydrolysis vanadium precipitation product. Additionally, the V=O peaks at 966 cm⁻¹ and 1001 cm⁻¹ in the hydrolysis vanadium precipitation product disappear after purification, while a new V–O–V peak appears at 970 cm⁻¹, indicating the formation of a V₆O₁₆²⁻ layers by combining two V₃O₈⁻ layers. The peaks at 1116 cm⁻¹ and 1186 cm⁻¹, which are characteristic of SO₄²⁻, vanish in the ammonium salt purification product, while a peak at 1400 cm⁻¹ corresponding to NH₄⁺ bending vibrations appears. This suggests that SO₄²⁻ is leached from the solid product into the liquid phase, and NH₄⁺ ions enter the solid phase during the ammonium salt purification process.

Fig. 16 FTIR spectra of hydrolysis vanadium precipitation product (K = 0) and ammonium salt purification product (K = 1).

3.3. Characterization of V₂O₅ products

Vanadium precipitation at an initial pH of 1.8, a reaction time of 1.5 hours, and a temperature of 95 °C achieved a vanadium precipitation rate of 99.13%. With an ammonium addition coefficient of 1, a purification temperature of 55 °C, a purification time of 15 min, and a purification frequency of 2, the purity of V₂O₅ products exceeded 99.9%. Table 3 presents the ICP detection results for the V₂O₅ products, which exhibit a low impurity level, meeting the requirements of the V₂O₅ 99.8-P grade as specified in the YB/T 5304-2017 standard. Fig. 17 shows the XRD spectrum of the V₂O₅ product, which confirms that its main composition is V₂O₅, with virtually no impurity peaks, supporting the high purity observed in the ICP results. Thus, high-purity V₂O₅ can be efficiently produced using the hydrolysis vanadium precipitation-ammonium salt purification process with reduced ammonium usage.

Table 3 Composition of V₂O₅ products (wt%)

	V2O5	Fe	P	S	Al	K2O + Na2O
V2O5 products	99.9	0.0084	0.016	0.010	0.001	0.022
Standard sample	≥99.8	≤0.03	≤0.02	≤0.03	—	≤0.1

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Fig. 17 XRD pattern of V₂O₅ products.

4. Conclusion

This study investigates the preparation of high-purity V₂O₅ using a D2EHPA saponification extraction-hydrolysis vanadium precipitation-ammonium salt purification process, utilizing vanadium-rich liquids with high impurity content as the raw material. The following conclusions can be drawn:

(1) The D2EPHA saponification extraction process effectively eliminates impurities from the vanadium-rich liquid. The vanadium extraction rate reached 99% after three-stage counter-current extraction under the following conditions: 60% saponification degree of D2EHPA, an initial pH of 1.8, a D2EHPA concentration of 40 vol%, a phase ratio (O/A) of 2 : 1, and an extraction time of 8 minutes. After three-stage counter-current stripping, the vanadium stripping rate achieved 99.8% under a phase ratio (O/A) of 2 : 1, an H₂SO₄ concentration of 2 mol L⁻¹, and a stripping time of 20 minutes. The concentrations of Vanadium, Fe²⁺, and Al³⁺ in the stripping solution were 53.57 g L⁻¹, 0.034 g L⁻¹, and 0.439 g L⁻¹, respectively, with Fe²⁺ and Al³⁺ elimination rates of 98.78% and 97.93%. These results demonstrate the effectiveness of the purification and decontamination processes.

(2) The hydrolysis vanadium precipitation-ammonium salt purification process resulted in high-purity V₂O₅. The vanadium precipitation rate reached 99.13% under the optimal conditions of pH 1.8, a reaction time of 1.5 hours, and a temperature of 95 °C. The purity of the V₂O₅ products exceeded 99.9% under the following conditions: ammonium coefficient of 1, temperature of 55 °C, time of 15 minutes, and purification frequency of 2. Compared to the ammonium salt vanadium precipitation process, this method reduced the ammonium addition coefficient from 6 to 1, resulting in an 83% reduction in ammonium consumption. This makes the process more environmentally friendly while achieving high-purity V₂O₅.

(3) The D2EHPA saponification extraction process replaced H⁺ in the O–H group with Na⁺, facilitating Na and V substitution during the extraction process. This resulted in a pH shift and an increased vanadium extraction rate. During the ammonium salt purification process, vanadate preferentially interacts with NH₄⁺ over Na⁺, leading to the substitution of Na⁺ with NH₄⁺ in the V₃O₈⁻ layer. This substitution disrupts the interaction between Na⁺ and V₃O₈⁻, and NH₄⁺ induces a structural transformation in the V₃O₈⁻ layer, converting the V=O bond into V–O–V. This promotes polymerization of the V₃O₈⁻ layer into V₆O₁₆²⁻layer, eventually forming (NH₄)₂V₆O₁₆ · 1.5H₂O.

Data availability

All data included in this study are available from the corresponding author upon request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the Science and Technology Innovation Talent Program of Hubei Province (2022EJD002) and Hubei Provincial Natural Science Foundation of China (2024AFB817).

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