**1. Introduction**

Vanadium and chromium are strategic transition elements that have been widely used in some fields such as steel-making, energy-storage, catalysts, the petrochemical industry, and green chemistry owing to their excellence hardness, high corrosion resistance, and other excellent physicochemical properties [1–5]. High-chromium vanadium slag (HCVS) is a by-product generated during the smelting process of high-chromiumvanadium-titanium-magnetite, and it is an important vanadium source in China [6–10]. During the smelting process, the vanadium and chromium are reduced into the molten enriched in the spinel, which is hard to destroy directly and restricted the large-scale utilization of HCVS [11]. Thus, some enhancement technologies were needed to recover vanadium and chromium efficiently.

To date, the basic recovery technology for vanadium has been sodium-roasting leaching technology, which was first proposed by Birck in 1912 and is widely used in the Chinese industries since the 1980s [12–14]. The vanadium-containing ores are mixed with the sodium salts (sodium carbonate (800–1000 ◦C), sodium sulfate (1200–1250 ◦C), sodium chloride (750–850 ◦C), and sodium hydroxide (400–800 ◦C)) at determined mole ratios and then roasted in a vertical kiln under a high temperature atmosphere with O<sup>2</sup> [13,15,16]. The structure of vanadium-containing ores is destroyed in the high temperature and lowvalence vanadium is exposed and oxidized to a high valence. The high-valence vanadium

**Citation:** Peng, H.; Li, B.; Shi, W.; Liu, Z. Efficient Recovery of Vanadium from High-Chromium Vanadium Slag with Calcium-Roasting Acidic Leaching. *Minerals* **2022**, *12*, 160. https://doi.org/10.3390/ min12020160

Academic Editors: Jean-François Blais, Shuai Wang, Xingjie Wang and Jia Yang

Received: 10 January 2022 Accepted: 25 January 2022 Published: 28 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

is formed as sodium vanadate and can be easily leached out with acid, alkaline, or waterleaching [17–20]. However, some environmental hazards (sulfur dioxide, chlorine, and hydrogen chloride) and large amount of wastewater have limited continuous large-scale industrial application with the high environmental standards of today. To overcome the above problems, a roasting technology called calcium-roasting technology was developed, in which the sodium salts are replaced by calcium salts [21–25]. The roasting process is similar to the sodium-roasting process; the vanadium-containing ores are mixed with lime, limestone, and calcium salts at fixed mole ratios and then roasted at high temperatures, which are higher than with sodium roasting. During the roasting process, the vanadium spinel is decomposed and reacted with calcium salts to form different kinds of calciumvanadate, which are determined by the mole ratio of the vanadium to calcium salts [26–30]. Usually, some leaching enhancing processes or multiple roasting processes accompany this process to achieve high recovery [16,31–34].

In this paper, direct acid leaching and calcium-roasting acid leaching technology were applied to leach out chromium and vanadium. The effects of experimental parameters including reaction time, liquid-to-solid ratio, reaction temperature, and concentration of H2SO<sup>4</sup> on the leaching process were investigated. The leaching kinetics were also investigated.

#### **2. Materials and Methods**

#### *2.1. Materials*

The HCVS was collected from Pangang Group Co. Ltd., Panzhihua City, Sichuan Province, China. It was dried and ground to below 75 µm for further experiments. The elemental composition of HCVS was measured by XRF. The results displayed in Table 1 indicate that the vanadium and chromium were about 5.43 wt.% and 6.84 wt.%, respectively.

**Table 1.** Elemental accounts of the HCVS (wt.%).


#### *2.2. Experimental Procedure*

The batch experiments were conducted in a 300 mL glass beaker placed in a thermostatic mixing water bath. Firstly, the water bath was heated to a determined temperature. Then, a predetermined concentration of H2SO<sup>4</sup> solution and a predetermined amount of HCVS or roasting HCVS were added to the beaker. Then, the beaker was placed in the water bath. Finally, the filtrate was collected by vacuum filtration after the required reaction time.

## *2.3. Analytical Methods*

The concentrations of chromium and vanadium in the filtrate were measured by inductively coupled plasma-optical mission spectrometry (ICP-OES, PerkinElmer Optima 6300DV, Kyoto, Japan.) and the leaching efficiency was calculated following Equations (1) and (2):

$$
\eta\_{\rm V} = \frac{\rm V \cdot C\_{\rm V}}{\rm m \cdot \omega\_{\rm V}} \times 100\% \tag{1}
$$

$$
\eta\_{\rm Cr} = \frac{\rm V \cdot C\_{\rm Cr}}{\rm m \cdot \omega\_{\rm Cr}} \times 100\% \tag{2}
$$

where C<sup>V</sup> and CCr, are the concentration of chromium and vanadium in the filtrate in g/L; V, is the volume of the filtrate in liters; ωV, and ωCr, are the percentages of chromium and vanadium in the HCVS; and m, is the mass of the HCVS used in the batch experiments in grams.

#### *2.4. Characterization*

The element percentages of HCVS were measured by XRF (Shimadzu Lab Center XRF-1800, Kyoto, Japan) and the main phases were measured by XRD (Shimadzu Lab Center XRD-6000, Kyoto, Japan). The valences of the vanadium and chromium in the HCVS were detected by UV-Vis DRS (Shimadzu Lab Center, Kyoto, Japan) and XPS (ESCALAB-250Xi, Thermo Fisher Scientific, New York, NY, USA). The thermo-gravimetric analysis was conducted by TG-DSC (Shimadzu Lab Center DSC-60H, Kyoto, Japan) with a heating rate of 10 ◦C/min from 0 ◦C to 900 ◦C.

#### **3. Results**

#### *3.1. Characterization of HCVS*

The XRD pattern showed in Figure 1a shows that the main crystal structures in the HCVS were Fe2O3, FeCr2O4, MgFe2O4, Fe2VO<sup>4</sup> and Fe2SiO<sup>4</sup> [27,28,30]. The vanadium and chromium mainly existed as spinel structures (FeCr2O<sup>4</sup> and Fe2VO4), which are hard to destroy. Therefore, the leaching efficiency of vanadium and chromium may not be high and some enhancing technologies are needed in the further experiments. *Minerals* **2022**, *12*, x 4 of 12

**Figure 1.** Characterization of HCVS. (**a**) XRD pattern of HCVS; (**b**) UV-Vis DRS of HCVS; (**c**) XPS of vanadium; (**d**) XPS of chromium. **Figure 1.** Characterization of HCVS. (**a**) XRD pattern of HCVS; (**b**) UV-Vis DRS of HCVS; (**c**) XPS of vanadium; (**d**) XPS of chromium.

*3.2. Direct-Acid-Leaching Process*  The direct acid leaching process was conducted to leach out vanadium and chromium from the HCVS. Figure 2 summarizes the effects of the liquid-to-solid ratio, reaction time, concentration of H2SO4, and reaction temperature on the leaching process. The leaching efficiencies of vanadium and chromium were relatively low (<35% for vanadium and 8% for chromium), which is consistent with the results of XRD. Figure 2a shows that the leaching efficiency of vanadium increased with the increase of the concentration of H2SO4. During the leaching process, the H+ attacks the spinel and The UV-Vis DRS of HCVS was conducted to help understanding the composition of HCVS; the result is displayed in Figure 1b. The original spectrum signal was analyzed and the peaks were fitted to four main peaks: 280 nm, 380 nm, 482 nm, and 542 nm. The peak at 280 nm was assigned to Fe (III) and confirmed the existence of Fe2O<sup>3</sup> and MgFe2O<sup>4</sup> [35,36]. The peak at 380 nm was assigned to Cr (III) [37], which corresponds to the FeCr2O<sup>4</sup> phase. The peak at 482 nm was assigned to V=O stretching, which confirmed the existence of V (IV) and V (V) [38,39].

destroys the spinel structure to release vanadium and chromium. With an increasing of concentration of H2SO4, the corrosion process of the spinel by the highly concentrated H+

33.90%, as the concentration of H2SO4 increased from 10 vt.% to 50 vt.%. As the chromium spinel was more stable than the vanadium spinel, the chromium was harder to leach out, with a leaching efficiency below 8%. Otherwise, the leaching efficiency showed no obvious increase when the concentration of H2SO4 increased from 40 vt.% to 50 vt.%, and high concentrations bring high impurities [40]; thus, the concentration of 40 vt.% was selected

The results showed in Figure 2b indicate that only 10.40% vanadium and 2.18% chromium can be leached out at 30 ℃. Higher temperatures enhances the activity of vanadium and chromium ions and further favors the reaction intensity [41]. The leaching efficiency increased to 33.89% for vanadium and 7.56% for chromium at 90 °C. In other words, high

reaction temperature was beneficial to the leaching process.

as optimal for further experiments.

The XPS results showed that most vanadium existed as V(III) and V(IV) (515.7 eV and 516.4 eV), and the Cr (III) accounted for about 82.84% of the HCVS (576.1 eV, 577.3 eV and 586.3 eV), while there were no V (III) and V (V) phases in the XRD pattern. According to our previous study, V(III), V(IV), and V(V) co-exist in HCVS, which means that some V(III) and V(V) compounds in the HCVS exist amorphously and could not detected by XRD [28]. contribution to the leaching process; thus, the reaction time of 3 h was selected in the following experiments. During the leaching process, the HCVS particles was ground fine enough for good contact with the concentrated H2SO4 solution. The leaching process was most controlled by parameters such as the reaction temperature and concentration of H2SO4, but less by

Usually, in order to produce more products, long reaction times are utilized. Figure 2c shows that the leaching efficiency of vanadium and chromium increased with the reaction time, but the increase amplitude was slow. Longer reaction times may not make any

#### *3.2. Direct-Acid-Leaching Process* the liquid-to-solid ratio, as the liquid-to-solid ratio had no obvious effect on the leaching efficiency (seen in Figure 2d).

The direct acid leaching process was conducted to leach out vanadium and chromium from the HCVS. Figure 2 summarizes the effects of the liquid-to-solid ratio, reaction time, concentration of H2SO4, and reaction temperature on the leaching process. The leaching efficiencies of vanadium and chromium were relatively low (<35% for vanadium and 8% for chromium), which is consistent with the results of XRD. As the spinel structure was hard to destroy directly, the leaching efficiencies of vanadium and chromium were 33.89% and 7.56%, respectively, at the selected optimum conditions: reaction time of 3 h, liquid-to-solid ratio of 4 :1 mL/g, concentration of H2SO4 of 40 vt.%, reaction temperature of 90 °C, and stirring rate of 500 rpm.

*Minerals* **2022**, *12*, x 5 of 12

**Figure 2.** Effect of parameters on leaching efficiency of vanadium and chromium: (**a**) concentration of H2SO4; (**b**) reaction temperature; (**c**) reaction time; (**d**) liquid-to-solid ratio. **Figure 2.** Effect of parameters on leaching efficiency of vanadium and chromium: (**a**) concentration of H2SO<sup>4</sup> ; (**b**) reaction temperature; (**c**) reaction time; (**d**) liquid-to-solid ratio.

*3.3. Characterization of Roasting HCVS*  In order to achieve efficient leaching performance of HCVS, calcium-roasting technology was applied to oxidize the low valence compounds. The obtained TG-DSC curves shown in Figure 3a indicate that there was a dehydration step, with weight loss of 1.23% from 0 °C to 400 °C, and an obvious exothermic peak of the DSC curve at 400 °C was observed, which corresponds to the decomposition of the spinel structure. After the temperature increased to 620 °C, a dramatic mass gain of 6.46% was obtained due to the oxidative decomposition of the vanadium spinel phase. This means that the oxidative roasting of vanadium spinel should be conducted above 620 °C. Thus, the calcium-roasting Figure 2a shows that the leaching efficiency of vanadium increased with the increase of the concentration of H2SO4. During the leaching process, the H<sup>+</sup> attacks the spinel and destroys the spinel structure to release vanadium and chromium. With an increasing of concentration of H2SO4, the corrosion process of the spinel by the highly concentrated H<sup>+</sup> was intensified and the leaching efficiency of vanadium was increased from 17.87% to 33.90%, as the concentration of H2SO<sup>4</sup> increased from 10 vt.% to 50 vt.%. As the chromium spinel was more stable than the vanadium spinel, the chromium was harder to leach out, with a leaching efficiency below 8%. Otherwise, the leaching efficiency showed no obvious increase when the concentration of H2SO<sup>4</sup> increased from 40 vt.% to 50 vt.%, and high concentrations bring high impurities [40]; thus, the concentration of 40 vt.% was selected as optimal for further experiments.

process was conducted at 650–850 °C and the HCVS was mixed with CaO at a mole ratio

of n(CaO)/n(V2O5) = 1.1

The results showed in Figure 2b indicate that only 10.40% vanadium and 2.18% chromium can be leached out at 30 ◦C. Higher temperatures enhances the activity of vanadium and chromium ions and further favors the reaction intensity [41]. The leaching efficiency increased to 33.89% for vanadium and 7.56% for chromium at 90 ◦C. In other words, high reaction temperature was beneficial to the leaching process.

Usually, in order to produce more products, long reaction times are utilized. Figure 2c shows that the leaching efficiency of vanadium and chromium increased with the reaction time, but the increase amplitude was slow. Longer reaction times may not make any contribution to the leaching process; thus, the reaction time of 3 h was selected in the following experiments.

During the leaching process, the HCVS particles was ground fine enough for good contact with the concentrated H2SO<sup>4</sup> solution. The leaching process was most controlled by parameters such as the reaction temperature and concentration of H2SO4, but less by the liquid-to-solid ratio, as the liquid-to-solid ratio had no obvious effect on the leaching efficiency (seen in Figure 2d).

As the spinel structure was hard to destroy directly, the leaching efficiencies of vanadium and chromium were 33.89% and 7.56%, respectively, at the selected optimum conditions: reaction time of 3 h, liquid-to-solid ratio of 4: 1 mL/g, concentration of H2SO<sup>4</sup> of 40 vt.%, reaction temperature of 90 ◦C, and stirring rate of 500 rpm.

## *3.3. Characterization of Roasting HCVS*

In order to achieve efficient leaching performance of HCVS, calcium-roasting technology was applied to oxidize the low valence compounds. The obtained TG-DSC curves shown in Figure 3a indicate that there was a dehydration step, with weight loss of 1.23% from 0 ◦C to 400 ◦C, and an obvious exothermic peak of the DSC curve at 400 ◦C was observed, which corresponds to the decomposition of the spinel structure. After the temperature increased to 620 ◦C, a dramatic mass gain of 6.46% was obtained due to the oxidative decomposition of the vanadium spinel phase. This means that the oxidative roasting of vanadium spinel should be conducted above 620 ◦C. Thus, the calcium-roasting process was conducted at 650–850 ◦C and the HCVS was mixed with CaO at a mole ratio of n(CaO)/n(V2O5) = 1.1

The XRD pattern was used to analyze the phase changes during the calcium-roasting process. The results showed in Figure 3b indicate that some new peaks appeared, corresponding to the new phases of Ca2V2O5, CaFe (Si2O6), and Ca2V2O7. These three new phases appeared at 650 ◦C, and the crystal structures became more stable as the roasting temperature increased from 650 ◦C to 850 ◦C. During the calcium-roasting process, the Fe2VO<sup>4</sup> decomposed (seen in Equation (3)) to form V2O<sup>4</sup> at nearly 400 ◦C according to DG-TSC results, and then reacted with CaO to form Ca2V2O5. With the increasing roasting temperature, partial Ca2V2O<sup>5</sup> was further oxidized to Ca2V2O7, which means that in the calcium roasting of HCVS, the V(IV) and V(V) co-existed.

After roasting, some V(III) and V(IV) were oxidized to V(V). The XPS results indicate that only 9.55% V(III) was retained in the roasted HCVS, while Cr(III) still accounted for about 80.32%. As the Cr spinel was more stable than the V spinel [13], the Cr was not oxidized and still existed in FeCr2O4, according to the XRD results. It was concluded that the chromium was still hard to leach out.

$$\text{2Fe}\_2\text{VO}\_4 \to \text{4FeO} + \text{V}\_2\text{O}\_4\tag{3}$$

$$\text{V}\_2\text{O}\_4 + \text{CaO} \rightarrow \text{CaV}\_2\text{O}\_5\tag{4}$$

$$2\text{CaV}\_2\text{O}\_5 + \text{O}\_2 + 2\text{CaO} \rightarrow 2\text{Ca}\_2\text{V}\_2\text{O}\_7\tag{5}$$

cium roasting of HCVS, the V(IV) and V(V) co-existed.

the chromium was still hard to leach out.

**Figure 3.** Characterization of roasting HCVS. (**a**) TG and DSC; (**b**) XRD pattern of roasted HCVS; (**c**) XPS of vanadium; (**d**) XPS of chromium. **Figure 3.** Characterization of roasting HCVS. (**a**) TG and DSC; (**b**) XRD pattern of roasted HCVS; (**c**) XPS of vanadium; (**d**) XPS of chromium.

The XRD pattern was used to analyze the phase changes during the calcium-roasting process. The results showed in Figure 3b indicate that some new peaks appeared, corresponding to the new phases of Ca2V2O5, CaFe (Si2O6), and Ca2V2O7. These three new phases appeared at 650 °C, and the crystal structures became more stable as the roasting temperature increased from 650 °C to 850 ℃. During the calcium-roasting process, the Fe2VO4 decomposed (seen in Equation (3)) to form V2O4 at nearly 400 °C according to DG-TSC results, and then reacted with CaO to form Ca2V2O5. With the increasing roasting temperature, partial Ca2V2O5 was further oxidized to Ca2V2O7, which means that in the cal-

After roasting, some V(III) and V(IV) were oxidized to V(V). The XPS results indicate that only 9.55% V(III) was retained in the roasted HCVS, while Cr(III) still accounted for about 80.32%. As the Cr spinel was more stable than the V spinel [13], the Cr was not oxidized and still existed in FeCr2O4, according to the XRD results. It was concluded that

**2Fe VO 4FeO + V O 2 4 24** → (1)

**V O + CaO CaV O 24 25** → (2)

**2CaV O + O +2CaO 2Ca V O 2 5 2 22 7** → (3)

## *3.4. Acid Leaching for Roasting HCVS*

The acid-leaching experiments with calcium roasting of HCVS were conducted to investigate the effect of calcium roasting on the leaching process under the same reaction conditions as the direct acid leaching process described above. As the Cr spinel was still stable under the roasting temperature, the leaching efficiency of chromium showed no obvious increase compared with the direct acid leaching process; therefore, the leaching behavior of chromium is not discussed in this part.

Figure 4a shows that the calcium roasting made a great contribution to the leaching process. The leaching efficiency of vanadium was increased by nearly 40 percentage (up to 57.54%) after roasting, compared with the direct acidic leaching process (at the concentration of 10 vt.% H2SO4). During the roasting process, most V(III) and V(IV) were oxidized to V(V), making a contribution to the great leaching performance of roasting HCVS. The leaching efficiency increased quickly at the beginning and then smoothly with the increase of H2SO<sup>4</sup> concentration. The highest leaching efficiency was up to 90.12% at a concentration of 50 vt.%, which showed nearly a 58% improvement compared to the direct acid leaching process. Compared to our previous study, the leaching efficiency might be increased more with some enhancing technologies, such as oxidative leaching and electro-oxidative leaching [28]. Otherwise, the formation of the by-product CaSO4, which is a villous particle, might adsorb on the surface of leaching residue and have negative effects on the leaching process [40]; thus, a too high concentration of H2SO<sup>4</sup> is not suitable for leaching while calcium roasting HCVS. Meanwhile, the leaching efficiency showed little increase as the concentration increased from 40 vt.% to 50 vt.%; thus, a concentration of H2SO<sup>4</sup> of 40 vt.% was selected for further experiments.

**Figure 4.** Effect of parameters on leaching efficiency of vanadium and chromium: (**a**) concentration of H2SO4; (**b**) reaction temperature; (**c**) reaction time; (**d**) liquid-to-solid ratio. **Figure 4.** Effect of parameters on leaching efficiency of vanadium and chromium: (**a**) concentration of H2SO<sup>4</sup> ; (**b**) reaction temperature; (**c**) reaction time; (**d**) liquid-to-solid ratio.

*3.5. Leaching Kinetics*  In order to understand the reaction mechanism, the leaching kinetics of vanadium were analyzed (leaching out chromium was very difficult; thus, it is not analyzed here). Usually, the leaching kinetics followed the shrinking core model described in Equation The same phenomenon can also be observed in Figure 4b. The leaching process was greatly enhanced by calcium roasting; the leaching efficiency was increased from 10.4% to 89.12% as the reaction temperature increased from 30 ◦C to 90 ◦C. Usually, metal ions have high solubility at high temperatures, accompanied by high activity; thus, 90 ◦C was chose in further experiments.

(6), which was used to describe the liquid-solid reaction [40,42–45]: 1/3 [(1 ) 1] 1 / 3 Ln (1 ) k t <sup>−</sup> −η − + ⋅ −η = ⋅ (6) where *η* is the leaching efficiency of vanadium, in percentage. The experimental data was fitted to Equation (6) and the results are shown in Figure The results displayed in Figure 4c,d indicate that the liquid-to-solid ratio and reaction time showed similar effects on the leaching process, and a suitable liquid-to-solid ratio and a long reaction time could achieve high leaching efficiency. As can be seen, the calciumroasting process can oxidize low valence vanadium to high valence vanadium and enhance the leaching process to achieve high leaching efficiency of vanadium, but has no influence on the change trend of leaching efficiency affected by the experimental parameters.

5. Based on the fitting results, the Ea for vanadium leached out was calculated following the Arrhenius equations (Equation (7)). Figure 6 shows that the Ea for vanadium leached out was 58.95 kJ/mol for the direct acid leaching process and 62.98 kJ/mol for the calciumroasting acid leaching process, which indicates that the controlling step for vanadium leaching is the surface chemical reaction [40,43–45]. Compared with the references [40,43,45,46], the Ea was much larger, indicating that the vanadium in the HCVS was hard To summarize, low valence vanadium in V spinel was decomposed and oxidized to V(V) during the calcium-roasting process, but Cr spinel was too stable to decompose. For vanadium, 89.12% was leached out under the optimal reaction conditions: reaction time of 3 h, reaction temperature of 90 ◦C, liquid-to-solid ratio at 4:1 mL/g, concentration of H2SO<sup>4</sup> at 40 vt.%, and stirring rate at 500 rpm. Most chromium existing as FeCr2O<sup>4</sup> was hard to leach out and was retained in the leaching residue.

#### to leach out by both direct acid leaching technology and calcium-roasting acid leaching *3.5. Leaching Kinetics*

mole gas constant.

technology. In order to improve the leaching efficiency and enhance the leaching process, some more efficient pre-treatment technologies are needed. Lnk LnA Ea / RT = − ( ) (7) where Ea is the apparent activation energy, A is the pre-exponential factor, and R is the In order to understand the reaction mechanism, the leaching kinetics of vanadium were analyzed (leaching out chromium was very difficult; thus, it is not analyzed here). Usually, the leaching kinetics followed the shrinking core model described in Equation (6), which was used to describe the liquid-solid reaction [40,42–45]:

$$\frac{1}{2}\left[\left(1-\eta\right)^{-1/3}-1\right]+1/3\cdot\text{Ln}(1-\eta)=\mathbf{k}\cdot\mathbf{t}\tag{6}$$

where η is the leaching efficiency of vanadium, in percentage.

The experimental data was fitted to Equation (6) and the results are shown in Figure 5. Based on the fitting results, the Ea for vanadium leached out was calculated following the Arrhenius equations (Equation (7)). Figure 6 shows that the Ea for vanadium leached out was 58.95 kJ/mol for the direct acid leaching process and 62.98 kJ/mol for the calciumroasting acid leaching process, which indicates that the controlling step for vanadium leaching is the surface chemical reaction [40,43–45]. Compared with the references [40,43,45,46], the Ea was much larger, indicating that the vanadium in the HCVS was hard to leach out by both direct acid leaching technology and calcium-roasting acid leaching technology. In order to improve the leaching efficiency and enhance the leaching process, some more efficient pre-treatment technologies are needed.

$$\text{Lnk} = \text{LnA} - \text{Ea/(RT)}\tag{7}$$

where Ea is the apparent activation energy, A is the pre-exponential factor, and R is the mole gas constant. *Minerals* **2022**, *12*, x 9 of 12 *Minerals* **2022**, *12*, x 9 of 12

**Figure 5.** Plot of leaching kinetics of vanadium at various reaction temperatures. (**a**) direct acid leaching process; (**b**) calcium-roasting acid leaching process. **Figure 5.** Plot of leaching kinetics of vanadium at various reaction temperatures. (**a**) direct acid leaching process; (**b**) calcium-roasting acid leaching process. **Figure 5.** Plot of leaching kinetics of vanadium at various reaction temperatures. (**a**) direct acid leaching process; (**b**) calcium-roasting acid leaching process.

**Figure 6.** Natural logarithm of reaction rate constant versus reciprocal temperature of vanadium. **Figure 6.** Natural logarithm of reaction rate constant versus reciprocal temperature of vanadium. reaction, with an *Ea* of 58.95 kJ/mol for the calcium-roasting acid leaching process. **Figure 6.** Natural logarithm of reaction rate constant versus reciprocal temperature of vanadium.

were conducted. The following conclusions were obtained:

(1) The chromium and vanadium existed as spinel structure in the HCVS, which are too stable to destroy directly; only 33.89% of vanadium and 7.56% of chromium could be leached out at the selected conditions during the direct acid leaching process: reaction time of 3 h, liquid-to-solid ratio at 4:1 mL/g, concentration of H2SO<sup>4</sup> at 40 vt.%, reaction temperature of 90 °C, and stirring rate at 500 rpm. The *Ea* of the vanadium leached out was 62.98 kJ/mol, which indicates that the vanadium was hard to leach out directly; (2) Most low valence vanadium could be oxidized to high valence during the calciumroasting process, and the leaching efficiency could achieve 89.12% under the optimal conditions: reaction time of 3 h, liquid-to-solid ratio at 4:1 mL/g, reaction temperature of 90 °C, concentration of H2SO<sup>4</sup> at 40 vt.%, and stirring rate at 500 rpm. The leaching behavior followed the shrinking core model well, and the controlling step was the surface chemical

(1) The chromium and vanadium existed as spinel structure in the HCVS, which are too stable to destroy directly; only 33.89% of vanadium and 7.56% of chromium could be leached out at the selected conditions during the direct acid leaching process: reaction time of 3 h, liquid-to-solid ratio at 4:1 mL/g, concentration of H2SO4 at 40 vt.%, reaction temperature of 90 °C, and stirring rate at 500 rpm. The *Ea* of the vanadium leached out was 62.98 kJ/mol, which indicates that the vanadium was hard to leach out directly; (2) Most low valence vanadium could be oxidized to high valence during the calciumroasting process, and the leaching efficiency could achieve 89.12% under the optimal conditions: reaction time of 3 h, liquid-to-solid ratio at 4:1 mL/g, reaction temperature of 90 °C, concentration of H2SO4 at 40 vt.%, and stirring rate at 500 rpm. The leaching behavior followed the shrinking core model well, and the controlling step was the surface chemical reaction, with an *Ea* of 58.95 kJ/mol for the calcium-roasting acid leaching process.

**4. Conclusions** 

**4. Conclusions**
