Study on Vanadium Leaching from Vanadium and Ferro-Titanium Concentrate Using Calcified Roasting Pellets and Sulfuric Acid at Constant pH

Abstract

This study proposed a selective leaching method to address the challenge of excessive iron (Fe) leaching during a sulfuric acid treatment of magnetite pellets, which complicates the subsequent extraction and precipitation of vanadium (V). The approach involved constant-pH sulfuric acid leaching of calcined and roasted vanadium–titanium (V–Ti) magnetite pellets to enhance V recovery while minimizing Fe dissolution. A comparison between constant-pH leaching and conventional heap leaching was conducted. The results showed that, under optimal leaching conditions, the V leaching rate remained largely unchanged, while the Fe leaching rate was significantly reduced compared with conventional heap leaching. Specifically, under optimal conditions—acid concentration of 2 mol/L, liquid–solid ratio of 1:3, temperature of 90 °C, and leaching time of 360 h—the V leaching rate reached 72.21%, while the Fe leaching rate remained as low as 0.91%. Additionally, the valence states of V and Fe in the pellets before and after leaching, as well as the main phase compositions during the leaching process, were analyzed. The results indicated that the primary phases in the calcined and roasted pellets remain unchanged before and after leaching, and most of the V and nearly all divalent Fe were effectively leached.

1. Introduction

Vanadium (V) is a strategically important rare metal, widely used across various industrial sectors. Approximately 85% of global V production is used by the iron (Fe) and steel industry, 7% in non-ferrous alloys—such as V–titanium (V–Ti) alloys—and 3% in batteries, medicine, and other fields, with the remainder used in minor applications [1,2]. In the steel industry, V is primarily used in carbon (C) and alloy steels, while in the Ti alloy industry, it is a key component of V-containing Ti alloys. In the chemical industry, V serves as a raw material in the synthesis of various chemical products [3,4]. China possesses a diverse range of V resources, the majority of which are derived from V–Ti magnetite. Notably, V extracted from this source accounts for over half of the total V reserves in the country.

Recently, V extraction from V–Ti magnetite was divided into two main approaches: direct and indirect extraction. Direct extraction methods include sodium (Na) roasting–leaching, salt-free roasting–leaching, and calcification roasting–leaching processes [5,6]. Sodium roasting involves the addition of Na salts such as Na₂CO₃ or NaCl to convert low-valence V oxides into soluble forms, facilitating leaching and extraction. This technique is quite advanced and can produce high-quality V₂O₅ through steps, such as precipitation, sintering, pickling, and secondary precipitation [7,8]. However, it also generates corrosive gases like sulfur dioxide (SO₂), hydrogen chloride (HCl), and chlorine gas (Cl₂) during roasting, leading to equipment corrosion and environmental pollution [9,10]. Salt-free roasting, also known as blank roasting, avoids the use of additives and extracts V through acid leaching of the roasted material. The process then proceeds through solid–liquid separation, oxidation, purification, precipitation, and final sintering to yield V pentoxide [11,12]. Despite this method eliminating the release of harmful gases and simplifying roasting conditions, it suffers from drawbacks such as low leaching efficiency and poor selectivity [13]. However, the calcification roasting–leaching process has garnered attention from researchers both in China and abroad. This method is simple, cost-effective, and does not generate corrosive gases. It also offers a relatively high V recovery rate, making it a promising alternative to traditional extraction techniques [14].

Furthermore, with the continuous advancement of science and technology, the global demand for V resources has shown sustained growth, prompting the development of numerous leaching methods for V extraction. Among these, sulfuric acid leaching is widely applied in V extraction from V–Ti magnetite due to its advantages, including a relatively simple process flow, high V recovery rate, and low energy consumption [15,16]. However, in sulfuric acid leaching, various impurity ions are leached simultaneously with V ions, particularly Fe ions, whose concentrations typically far exceed those of V ions, complicating the subsequent separation and enrichment of V. Consequently, reducing the Fe ion concentration in the leachate has become a crucial step in enhancing the efficiency of V recovery [17,18]. Li Lanjie [19] investigated the leaching behavior of V–Ti magnetite concentrate using sulfuric acid as the leaching agent under roasting conditions of 10% calcium carbonate addition, 1473 K roasting temperature, and 1 h roasting time. At a liquid–solid ratio of 5 mL/g, sulfuric acid volume fraction of 5%, leaching time of 3 h, and leaching temperature of 363 K, a V leaching rate of 72.10% was achieved. Wang ZX et al. [20] explored the optimal parameters for conventional heap leaching and identified the ideal conditions at an acid concentration of 3.5 mol/L, a liquid–solid ratio of 3:1, a leaching temperature of 90 °C, and a leaching duration of 100 h. Under these conditions, the leaching rates of V, Fe, and Ti were 72.89%, 1.75%, and 2.60%, respectively. Further optimization resulted in slightly adjusted parameters, 3.0 mol/L acid concentration, a 1:1 liquid–solid ratio, same leaching temperature (90 °C) and leaching time (100 h), yielding enhanced V, Fe, and Ti leaching rates of 80.28%, 1.39%, and 1.72%, respectively. Despite enhanced V recovery, the relatively high Fe content in the leachate persisted. These studies indicated that current leaching processes lack sufficient selectivity. While extracting V, significant amounts of Fe are also leached, complicating the downstream separation process and reducing the overall V conversion efficiency in the calcification roasting–acid leaching method. Additionally, this results in the loss of valuable Fe content present in the V–Ti magnetite.

To address these challenges, this study aimed to enhance the selectivity of the leaching process and reduce complications in subsequent V extraction by maintaining a constant sulfuric acid concentration through controlled acid supplementation. This approach is expected to stabilize the pH of the leaching solution and effectively suppress Fe ion dissolution, thereby offering a more favorable leachate composition for efficient V extraction.

2. Experimental Section

2.1. Chemical Composition of the Experimental Raw Materials

The raw materials used for the calcified roasting of V-ilmenite concentrate pellets were sourced from a mining region in Panzhihua, Sichuan Province, China. After undergoing screening, drying, and pelletizing, the prepared materials were used in subsequent leaching experiments. The semi-quantitative and chemical composition analyses of the roasted pellets are presented in

Table 1. X-ray fluorescence (XRF) semi-quantitative analysis of roasted pellets.

Element	Fe	Ti	Si	Ca	Mg	Al	Cr	V	Ni	Mn
wt%	45.0	8.0	2.0	2.0	1.0	1.0	0.5	0.3	0.2	0.2

and

Table 2. The chemical composition of roasted pellets (ICP).

Element	Fe	Ti	Si	Ca	Mg	Al	V	Cr
wt%	45.46	8.40	1.71	1.36	1.67	1.17	0.30	0.04

, respectively. The X-ray diffraction (XRD) pattern of the roasted pellets is shown in Figure 1.

Given the analysis results in Table 1 and Table 2, the primary elements in the roasted pellets were Fe and Ti. Other detected elements included copper (Cu), zinc (Zn), nickel (Ni), and chromium (Cr), while V was present in a relatively low concentration of only 0.3%.

It is evident from Figure 1 that the primary phases in the roasted pellets were Fe₂O₃ and Fe₉TiO₁₅, indicating that Fe and Ti were the main constituents. This finding correlated with the results obtained from XRF and ICP analyses. The presence of secondary mixed peaks suggested that, in addition to the dominant mineral phases, small amounts of other metal compounds were also present. However, due to the low V content of only 0.3%, no V-containing phase was detected.

2.2. Experimental Methods

Constant-pH sulfuric acid leaching: An electronic balance was used to weigh a specified mass of dried, roasted pellets, which were then placed into a wide-mouth bottle. A sulfuric acid solution of known volume was added to the bottle. The bottle was then placed in a constant-temperature water bath set to a specific temperature. The pH of the sulfuric acid solution was measured before leaching, and after a specific period, the pH was re-measured. Optionally, the original acid solution was replenished to maintain the same pH level.

Acid addition method: The initial pH of the acid solution was measured using an ion meter. Acid was added manually at fixed intervals—specifically, once every 2 h—to maintain the desired pH.

Conventional heap leaching: This method followed the same procedure as the constant-pH sulfuric acid leaching, with the exception that no acid was added during the leaching process.

The leaching rates of V and Fe were calculated using the following formula:

$$\beta =\left[\left(c\times v\right)/\left(\omega \times m\right)\right]\times 100\%$$

(1)

In Formula (1), β and c are the leaching rate and the concentration of V and Fe in the leachate (mol/L), respectively. v is the volume of leached liquid (L); ω is the grade of V and Fe in the original; m is the original sample mass (g).

2.3. Thermodynamic Analysis

The Eh-pH diagram is a valuable tool for analyzing aqueous solution systems. It provides insights into the electrochemical potential, ionic activity, and equilibrium conditions of various reactions [21], allowing for a clearer understanding of reaction directions and limitations [22]. Additionally, it helped in controlling the pH of aqueous systems, which, in turn, enables better regulation of reaction processes. This study investigated the thermodynamic behavior of vanado-ilmenite concentrate under atmospheric pressure. Hence, Eh-pH diagrams for the V–Fe–H₂O system across all four different temperatures (25 °C, 50 °C, 75 °C, and 100 °C) were constructed to further understand the acid leaching behavior of the material [23].

Thermodynamic analysis showed that when the ion concentration of the V–Fe–H₂O aqueous system reached 0.1 mol/Kg, the stability regions of V³⁺, VO²⁺, and VO⁺ significantly overlapped with those of Fe³⁺ and Fe²⁺ under acidic conditions. This overlap suggested that simply adjusting the acid concentration and redox potential in conventional acid leaching of V–titanite concentrate was insufficient to enhance the leaching rate of soluble V while suppressing the Fe leaching. Hence, it was difficult to achieve effective separation of V and Fe via conventional means.

As shown in Figure 2, the steady-state redox potentials of vanadium (V) and iron (Fe) increase progressively with rising temperature, while the pH value decreases correspondingly. This pH variation significantly influences the leaching efficiency of both valuable vanadium and impurity iron. To better evaluate the effects of such pH changes, systematic acid leaching experiments should be performed to verify the validity of the thermodynamic calculations. Therefore, during the acid leaching process, acid supplementation was introduced as a strategy to inhibit the leaching of Fe.

3. Experimental Results and Discussion

3.1. Sulfuric Acid Leaching Under Constant-pH Conditions

3.1.1. Influence of Acid Concentration on Vanadium and Iron Leaching Rates

The experiments were conducted at 25 °C with a liquid-to-solid ratio of 1:3 and a leaching duration of 72 h to investigate the effects of five different acid concentrations (1 mol/L, 1.5 mol/L, 2 mol/L, 2.5 mol/L, and 3 mol/L) on vanadium and iron leaching efficiency. The leaching rates of V and Fe increased with increasing acid concentration, indicating a direct relationship. Furthermore, as the acid concentration reached 2 mol/L, the leaching rate of V approached a plateau, indicating minimal enhancement with increasing concentrations. However, the leaching rate of Fe continues to increase beyond this point. Given this observation, 2 mol/L was selected as the optimal acid concentration for effective V extraction while minimizing excessive Fe dissolution.

3.1.2. Influence of Temperature on the Leaching Rate of Vanadium and Iron

The influence of temperature on the leaching efficiency of V and Fe was investigated across five different temperatures of 25 °C, 35 °C, 50 °C, 75 °C, and 90 °C. The experiments were conducted using an acidic solvent with a concentration of 2 mol/L, a liquid–solid ratio of 1:3, and a leaching time of 72 h. The results are shown in Figure 3b. As the leaching temperature increased, the leaching rates of both V and Fe exhibited an increasing trend. At 90 °C, the leaching rate of V increased by 3.91% compared with that at room temperature (25 °C), while the Fe leaching rate increased by only 0.56%. This indicated that elevated temperatures significantly enhanced V extraction with a relatively minor effect on Fe. Therefore, 90 °C was set as the optimal leaching temperature under the given conditions.

3.1.3. Influence of Liquid–Solid Ratio on Vanadium and Iron Leaching Rates

The effect of varying liquid–solid ratios on the leaching efficiencies of V and Fe was investigated under the following fixed conditions: acid concentration of 2 mol/L, temperature of 90 °C, and leaching duration of 72 h. Five different liquid–solid ratios (1:3, 1:2, 1:1, 2:1, and 3:1) were tested. As shown in Figure 3c, both V and Fe leaching rates decreased progressively with increasing liquid volume. Upon comparing the liquid–solid ratio of 1:3 with 1:2, the leaching rate of Fe decreased slightly by 0.1%, while the V leaching rate significantly decreased by 2.45%. Therefore, the optimal liquid–solid ratio for maximizing leaching efficiency was 1:3.

3.1.4. Influence of Leaching Time on the Leaching Rate of Vanadium and Iron

The experiment was conducted under the following conditions: an acid concentration of 2 mol/L, a temperature of 90 °C, and a liquid–solid ratio of 1:3. This study investigated the effect of different leaching times (120 h, 144 h, 168 h, 192 h, and 216 h) on the leaching efficiency of V and Fe. As shown in Figure 3d, the leaching rates of both V and Fe were directly proportional to the leaching time. From 120 h to 360 h, the leaching rates of both V and Fe increased with time. After 360 h, the V leaching rate stabilized, while the Fe leaching rate continued to increase steadily. This was because when the leaching time reached 360 h, most of the pentavalent V in the roasted pellets had been leached. The remaining V was primarily in the trivalent form, which was more resistant to leaching by sulfuric acid, thereby leading to the stabilization of the V leaching rate. At this stage, the leaching rate of Fe was only 0.8%, suggesting the presence of bivalent Fe and a small amount of trivalent Fe in the pellets that had not yet been leached. These Fe forms continue to participate in the reaction, leading to a gradual increase in the Fe leaching rate. Conclusively, 360 h was selected as the optimal leaching time for this process.

3.2. Experiment on Conventional Reactor Leaching Conditions

The results of the conventional heap leaching experiment are shown in Figure 4. Similar to cyclic leaching, Figure 4a shows that the optimal acid concentration was 2 mol/L. Figure 4b shows that the optimal temperature was 90 °C. According to Figure 4c, the optimal liquid–solid ratio was 1:3. As shown in Figure 4d, the optimal leaching time was 360 h. Under these ideal conditions, the leaching rates of V and Fe were 72.89% and 1.75%, respectively. Under the optimal conditions of the constant-pH leaching process, the leaching rates of V and Fe were 72.21% and 0.91%, respectively. While constant-pH leaching did not significantly enhance the V leaching rate, it substantially reduced the Fe leaching rate, simplifying the subsequent separation of V and Fe.

3.3. Acid Supplementation Under Different Experimental Conditions

As shown in Figure 5a, as the acid concentration increased, the amount of acid supplementation remained relatively constant. However, higher acidity levels required a greater initial amount of acid, indicating that an acid concentration of 2 mol/L was optimal. As shown in Figure 5b, temperature exhibited a minimal impact on the amount of acid added, suggesting that an increase in temperature did not significantly affect acid consumption. Therefore, a temperature condition of 90 °C was feasible. As shown in Figure 5c, at a 1:3 liquid–solid ratio, despite increased acid consumption, this condition resulted in a higher V leaching rate and a lower Fe leaching rate compared with other conditions, hence substantiating the increased acid usage. Finally, as shown in Figure 5d, acid consumption continued to increase with time but plateaued after 360 h, which correlated with the stabilization of the Fe–V leaching rate after this period.

3.4. Composition Analysis of Leach Solution and Leached Pellets

The concentration of each element (ICP) in the leach solution is presented in

Table 3. Concentration of each element in the leach solution (ICP).

Element	Fe	Al	Mg	V	Ti	Ca	Cr	Si
Concentration/g/L	1.94	1.49	0.75	0.60	0.38	0.26	0.25	0.02

. It was observed that Fe exhibited the highest concentration, followed by aluminum (Al) and magnesium (Mg). The concentrations of these three elements exceeded that of V in the solution, which was 0.60 g/L. The relatively low V concentration with impurities suggested that the solution was not suitable for a direct V precipitation process.

The chemical composition analysis of the leached pellets is presented in

Table 4. Chemical composition analysis of leach pellets (ICP).

Element	Fe	Ti	Si	S	Ca	Al	Cr	Mg	Ni	Mn
Content/wt%	54.2	8.2	2.8	3.0	1.7	0.6	0.3	0.4	0.1	0.1

. In comparison with the calcined pellets, the Fe grade in the leached pellets increased by 8.74%, while the Ti grade remained largely unchanged. The silicon (Si) grade increased by 1.09%, and the calcium (Ca) grade increased by 0.34%. Additionally, the grades of impurities such as Al, chromium (Cr), and Mg exhibited a significant decrease. Notably, the sulfur (S) grade increased from below the detection limit before leaching to 3% after leaching. The reasons for the presence of S in the pellets will be explored in a later section. Furthermore, ICP analysis revealed that the S content in the pellets after secondary roasting was only 0.014%, indicating that secondary roasting effectively achieved desulfurization. The S content of the desulfurized pellets meets the requirements for blast furnace ironmaking, with the allowable limit set at ≤ 0.1% [24]. The V grade decreased from 0.30% before leaching to below the detection limit following the leaching process.

3.5. Repetitive Experiment

The stability of process parameters for constant-pH leaching was verified by single-factor experiments. Three repeatability experiments were conducted to systematically evaluate the reproducibility and reliability of the optimal parameter combination of the process in practical applications. The experimental results are shown in

Table 5. Repeatable experimental data.

Number of Experiments	1	2	3
Leaching rate (Fe)/%	72.21	73.12	71.14
Leaching rate (V)/%	0.91	0.88	0.98

.

After calculation, the standard deviation of the iron leaching rate in the three experiments was 0.8097, and the standard deviation of the vanadium leaching rate was 0.0455. The standard deviations of the leaching rates of both elements were less than 1, indicating that the experimental data were relatively reliable and the experiments could be reproduced.

4. Mechanism Analysis of Selective Vanadium Leaching

This section presents an analysis of the calcified calcined pellets and leached pellets from V and ilmenite concentrate via XRD, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM-EDS) techniques. The phase transformations of the pellets before and after leaching, as well as the mechanism underlying selective leaching, were analyzed and discussed.

4.1. XRD Analysis

Figure 6 shows the XRD analysis of both calcined and leached pellets. The results revealed that the morphology of the primary phase in the calcined pellets remained unchanged after leaching. The main phases identified were Fe₂O₃ and Fe₉TiO₁₅. The intensity of the corresponding diffraction peaks did not show significant variation, suggesting that the acid leaching process did not affect the primary phases of the roasted pellets. The presence of a few mixed peaks indicated the existence of minor metal compounds in addition to the primary mineral phases. The detection did not identify any vanadium phase, likely because the vanadium content in the pellets was too low, falling below the detection limit.

4.2. XPS Analysis

Figure 7 shows the V 2p XPS spectrum of the leached pellets at constant pH. As shown, the O 1s peak corresponding to the V₂O₅ peak with a binding energy of 529.6 eV was notably weak. The O 1s peak at 517.1 eV correlated with V₂O₅, which later disappeared. This indicated that the leaching process successfully extracted most of the V from the fired pellets.

Figure 8 shows the Fe 2p XPS spectrum of the leached pellets. As shown, the satellite peaks corresponding to Fe 2p_1/2 (FeO) at binding energies of 709.6 eV and 714.8 eV have disappeared. This indicated that the Fe²⁺ present in the calcined pellets was completely extracted during the constant-pH leaching process. However, the peaks associated with Fe₂O₃ at 710.8 eV and 724.4 eV exhibited a minimal change compared with the pre-leaching state. Additionally, given that the majority of Fe in the calcined pellets existed as Fe₂O₃, with only a small fraction present as FeO, the overall Fe leaching rate during the process remained low. The constant-pH leaching process demonstrated excellent selectivity for vanadium (V) extraction.

4.3. SEM-EDS Analysis

Figure 9 shows the SEM-EDS spectrum of leached pellets. It was observed that, under the same magnification, there was no change in the shape or size of the pellets before and after the leaching process. The elemental composition of Fe, oxygen (O), Ti, and other elements remained largely consistent. However, the S content increased significantly after leaching. Distributions of Fe and Ti exhibited a strong correlation, suggesting that the dominant phases in the leached pellets were Fe₂O₃ and Fe₉TiO₁₅. However, S exhibited a high spatial correlation with Al and Mg, indicating S existed in the form of metal sulfate compounds. It was observed that these leached pellets permeated the materials in the form of metal sulfate compounds, which were produced via the reaction between metal oxides in the pellets and sulfuric acid.

4.4. EPMA Analysis

Table 6. Leaching pellets EPMA quantitative analysis of elemental content.

	Point 1	Point 2	Point 3
Element	Mass%
Si	0.002	0.498	0.378
Ca	0.031	0.395	0.084
Ti	0.282	5.474	4.69
V	0.168	0.069	0.078
Cr	0.016	0.282	0.383
Fe	61.805	50.418	54.62

demonstrates a substantial reduction in vanadium content alongside a remarkable increase in iron content within the leached pellets, while chromium levels remained relatively constant. Figure 10 presents the electron probe microanalysis (EPMA) results of the leached pellets. The elemental mapping reveals that iron and titanium exhibit the most extensive spatial distribution with high correlation, confirming that ilmenite (FeTiO₃) persists as the predominant phase in the leached pellets. Post-leaching, vanadium and chromium concentrations diminished to levels below the detection threshold of EPMA. Comparative analysis indicates a relative decrease in the calcium distribution compared to the roasted pellets, whereas silicon displayed more widespread dispersion. The experimental data reconfirm the excellent vanadium (V) selectivity achieved by this leaching process, while simultaneously demonstrating successful iron (Fe) leaching inhibition.

5. Conclusions

(1)

Given the Eh-pH diagram, it is difficult to achieve selective leaching of V and Fe via conventional acid leaching.

(2)

In this study, calcined pellets were subjected to both conventional acid leaching and constant-pH leaching. The optimal conditions for conventional heap leaching were an acid concentration of 2 mol/L, a liquid–solid ratio of 1:3, a leaching temperature of 90 °C, and a leaching duration of 360 h. Under these conditions, the leaching rates of V and Fe were 72.89% and 1.75%, respectively. For constant-pH leaching, the optimal conditions remained the same: acid concentration of 2mol/L, liquid–solid ratio of 1:3, leaching temperature of 90 °C, and leaching time of 360 h. Under these conditions, the leaching rates of V and Fe were 72.21% and 0.91%, respectively. These results indicated that constant-pH leaching significantly enhanced the selectivity of V extraction.

(3)

X-ray photoelectron spectroscopy analysis revealed that constant-pH leaching removed nearly all of the Fe²⁺ from the pellets, while trivalent iron (Fe³⁺), which is the dominant form in pellets, remained largely unaffected. This inhibition of Fe³⁺ leaching was attributable to the stable acid concentration during the process, which accounted for the lower overall Fe leaching rate, thus enabling the enhanced selective extraction of vanadium (V).

(4)

X-ray diffraction and XPS characterization confirmed that the constant-pH leaching process effectively extracted a significant amount of V from the roasted pellets. Importantly, the concentrations of Fe and Ti in the leached pellets remained unchanged, and the primary mineral phases were preserved. The absence of detectable V in the leached pellets further confirmed that most of the V was successfully extracted. These results provide additional evidence that iron (Fe) is largely retained in the solid phase, highlighting the remarkable selectivity of constant-pH sulfuric acid leaching toward vanadium (V) recovery.

(5)

Scanning electron microscopy–energy-dispersive X-ray spectroscopy analysis showed that the morphology and particle size of the pellets were largely unchanged before and after leaching. The elemental composition remained consistent, except for a significant increase in S content after leaching. The X-ray diffraction results indicated that the main phases present in both roasted and leached pellets were Fe₂O₃ and Fe₉TiO₁₅. The high roasting temperature was attributable to the formation of silicate compounds. The S in the leached pellets permeated the materials in the form of metal sulfate compounds, which were produced via the reaction between metal oxides in the pellets and sulfuric acid.

(6)

EPMA analysis revealed that both roasted pellets and leached pellets predominantly consist of iron and titanium, indicating that ilmenite remains one of the major components in the leached pellets. In the roasted pellets, vanadium primarily exists in the form of calcium vanadate, while silicon mainly occurs as calcium silicate. After the leaching process, both vanadium and chromium contents were reduced below the detection limit.

(7)

Based on the aforementioned leaching experimental results and verification through multiple characterization methods, the proposed leaching process demonstrates excellent selectivity for vanadium extraction, effectively addressing the long-standing challenge of vanadium–iron separation. This technological advancement provides higher-quality raw materials for subsequent vanadium product manufacturing.