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Preparation of V₂O₅ from converter slag containing vanadium

Chong Han Liang Li He Yang Xiang-Xin Xue

School of Materials and Metallurgy,Northeastern University

作者简介：*Chong Han e-mail: hanch@smm.neu.edu.cn;

收稿日期：4 July 2014

基金：financially supported by the Fundamental Research Fund for the Central Universities (No. N130302004);the National Major International Cooperation Program of China (No. 2012DFR60210);the National High-Tech Research and Development Program of China (No. 2012AA062304);the National Natural Science Foundation of China (Nos. 51090384 and U1360204);

Preparation of V₂O₅ from converter slag containing vanadium

Chong Han Liang Li He Yang Xiang-Xin Xue

School of Materials and Metallurgy,Northeastern University

Abstract：

Preparation of V₂O₅ from converter slag (CS)was investigated through roasting, leaching, extracting, and precipitating processes. The corresponding roles of the parameters during every procedure were analyzed in detail.(NH₄)₂SO₄ and KHSO₄ were used as fusing agents to transform compounds containing vanadium into soluble species. Mass ratio of (NH₄)₂SO₄ to CS, mass ratio of KHSO₄ to CS, and roasting temperature during the roasting process can significantly influence leaching rate of vanadium (LRV). With H₂SO₄ as leaching agent, 99.2%vanadium in CS can be leached out under the optimum leaching conditions, which mainly refer to liquid-to-solid ratio, H₂SO₄ concentration, leaching temperature, and leaching time. Extracting and back-extracting processes were introduced to purify the vanadium from the H₂SO₄lixivium. Extracting rate of vanadium (ERV) greatly depends on iron powder concentration, pH, diisooctyl phosphate (P204) content, volume ratio of extractant to H₂SO₄ lixivium, and extracting time. By adding ammonium hydroxide, 94.0%vanadium in back-extracting solution can be separated in the form of precipitates. The product from the roasted precipitate mainly consists of V₂O₅, the content of which is higher than 90.0%.

Keyword：

Converter slag; V₂O₅; Roasting; Leaching; Extracting;

Received： 4 July 2014

1 Introduction

Vanadium can enhance physical properties of materials including tensile strength,hardness,and fatigue resistance,which promotes its extensive usage in ferrous and nonferrous alloys [ 1] .Nowadays,about 85%of the vanadiumbearing products produced worldwide are consumed in the iron and steel industry [ 1] .The vanadium-titanium magnetite is the most important one of dozens of vanadiumcontaining minerals [ 2, 3] .During the steel production process with vanadium-titanium magnetite as raw materials,vanadium-containing minerals are smelted together with iron-bearing species,finally causing vanadium to be transferred to steel slag.The recovery of vanadium from steel slag attracts extensive attention since it can not only enhance economic benefits of steel slag but also avoid environmental impacts due to the presence of heavy metal vanadium [ 4, 5] .The content of V₂O₅ in steel slag greatly varies in the range of 2 wt%-25 wt%depending on the mineral types and smelting process [ 1, 2, 6, 7, 8, 9, 10] .Steel slag becomes the main raw material for the production of vanadium and its compounds [ 1, 11] .

In the last decades,many studies have been conducted to recover vanadium from steel slag [ 2, 6, 8, 9, 10, 12, 13, 14] .To recover vanadium in the form of metal vanadium or V₂O₅,steel slag containing vanadium is usually treated through several processes including roasting,leaching,extracting,and precipitating.Sodium salts such as NaCO₃,NaCl,and Na₂SO₄ are used to transform vanadium-bearing minerals into sodium vanadate during the roasting process [ 6, 8, 10, 15, 16] .Sodium vanadate can be leached from the roasted steel slag with water as leaching agent.However,the decomposition of sodium salts can result in the emission of toxic gases including Cl₂,HCl,and SO₂.In some cases,CaO is used to replace sodium salts and form waterinsoluble calcium vanadate that can be solubilized through sulfuric acid or carbonate solution during the leaching process [ 2, 6] .The recovery rate of vanadium is in the region of 85%-96%,which significantly depends on the roasting and leaching parameters [ 2, 6, 8, 10] .AarabiKarasgani et al. [ 6] demonstrated that the modified shrinking core model (SCM) well described the leaching kinetics of vanadium in steel slag by H₂SO₄.It is noticed that other ions such Fe³⁺,Fe²⁺,and Ti⁴⁺may be also leached out together with the vanadium-containing ions.Further purification of vanadium-containing ions is achieved by the extracting process with organic extractants [ 17, 18, 19, 20] .After that,the precipitate of vanadium can be formed upon adding ammonia hydroxide and was used to prepare the vanadium product [ 7, 21] .

Recently,a new method called sub-molten salt (SMS)has been developed to recover vanadium from steel slag [ 2, 9] .NaOH-NaNO₃ binary melts are used to treat steel slag containing vanadium.During the decomposition process of steel slag,NaOH provides basic media and facilitates the dissolution of acidic oxides,while NaNO₃ is the dominative oxidant for the oxidization of vanadium(Ⅲ) through producing active oxygen species such as O ,and O^2- [ 2] .The recovery rate of vanadium can reach 93.7%after the reaction for 6 h under the optimum conditions [ 2] .However,the SMS method had a significant disadvantage that most of the silica in steel slag would be dissolved with vanadium,enhancing the viscosity of the solution and resulting in filtration and separation difficulties [ 7] .

To our knowledge,few papers investigated the whole procedure for preparing V₂O₅ from steel slag.In this work,V₂O₅ can be obtained from converter slag (CS) by means of several processes including roasting,leaching,extracting,and precipitating.CS containing vanadium was roasted after the addition of (NH₄)2SO₄ and KHSO₄ which were rarely used as fusing agents in previous studies.Effects of both roasting and leaching conditions on the leaching rate of vanadium (LRV) were investigated in detail with deionized water or dilute sulfuric acid as the leaching agent.Changes in the extracting rate of vanadium (ERV)were also measured as functions of the extracting parameters.

2 Experimental

2.1 Materials and methods

CS was collected from Chengde Iron and Steel Plant,Hebei Province,China.After being crushed and ground,CS was sieved and its size was in the range of 0.096-0.120 mm.As shown in Table 1,chemical compositions of CS were analyzed using X-ray fluorescence (XRF,ZSX100e,Japan).Various chemical reagents,including (NH₄)2SO₄,KHSO₄,H₂SO₄,ammonium hydroxide,NaOH,diisooctyl phosphate (P₂O₄),and tributyl phosphate (TBP;AR,Sinopharm Chemical Reagent Co.,Ltd.),were used during the processes of roasting,leaching,extracting,and precipitating.Sulfonated kerosene (SK) originated from the reaction of kerosene and concentrated sulfuric acid.

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Table 1 Chemical compositions of CS (wt%)

2.2 Procedure

2.2.1 Roasting of CS

To transform compounds containing vanadium into soluble species,mixtures of CS,(NH₄)2SO₄,and KHSO₄ were roasted in the furnace.(NH₄)2SO₄ and KHSO₄ were used as the fusing agents to facilitate the transformation of compounds containing vanadium.Though the structure of KHSO₄ was similar to that of (NH₄)2SO₄,the melting point(219℃) of KHSO₄ was far less than that (513℃) of(NH₄)2SO₄.According to the theory of the eutectic mixture,KHSO₄ can reduce the melting point of mixtures and enhance the reaction between (NH₄)2SO₄ and compounds containing vanadium.NH₃ and SO₂ originating from the decomposition of (NH₄)2SO4 and KHSO₄ can form NH₃·H₂O and H₂SO₄ through the reaction with water.H₂SO₄ and NH₃-H₂O can be reused as the leaching agent and precipitator in the following processes,respectively.To investigate the effects of the roasting conditions on changes in the solubility of compounds containing vanadium,mass ratio of (NH₄)2SO₄ to CS,mass ratio of KHSO₄to CS,roasting temperature,and holding time varied in the range of 2-12,0-1.65,290-390℃,and 0-45 min,respectively.Mineral compositions of the fresh and roasted CS were analyzed through X-ray diffractometer (XRD,X'Pert Pro,the Netherlands).

2.2.2 Leaching of vanadium

The roasted mixtures were magnetically stirred and leached by the deionized water or sulfuric acid (1.5 mol·L^-1) in the beaker.It was reported that the vanadium in steel slag can be leached as V0²⁺and into the acidic solution [ 7] .After a certain time,the suspension was filtered to obtain the filtrate and residue through the filter paper.The concentration of ions containing vanadium in the filtrate was measured using inductively coupled plasma atomic emission spectrometer (ICP-AES,Optima 4300DV,America).

Mineral compositions of the residue were also analyzed by X-ray diffractometer (XRD,X'Pert Pro,the Netherlands).Effects of liquid-to-solid ratio,H₂SO₄ concentration,leaching temperature,and leaching time on LRV were investigated in detail.

2.2.3 Extracting of vanadium

Several ions including Fe³⁺,Fe²⁺,and Ti⁴⁺may also be leached out during the leaching process.Thus,the vanadium should be further extracted from the H₂SO₄ lixivium through the appropriate method.The P₂O₄ existed in the form of the biopolymer ([H(C₈H₁₇)2PO₄]2) in the kerosene.The H⁺in[H(C₈H₁₇)2PO₄]₂ can react with VO²⁺to form VO[(C₈H_i7)2PO₄]2 through the ion exchange [ 22] .It should be pointed out that the H₂SO₄ lixivium may contain VO²⁺, ,and Fe³⁺.In acid media,P₂O₄ exhibited the strong extracting power for VO²⁺and Fe³⁺,while it showed the weak extracting power for and Fe²⁺ [ 22] .Therefore,the H₂SO₄ lixivium should be pretreated before the extracting process.Here,iron powders were added to reduce and Fe³⁺to VO²⁺and Fe²⁺,respectively.To extract vanadium from H₂SO₄ lixivium,P₂O₄,TBP,and SK were used as extraction agent,assistant extraction agent,and diluent,respectively.A mixture of50 ml pretreated H₂SO₄ lixivium and 50 ml organic extraction phase were stirred for a certain time.Then,the mixture was transferred and separated in 250 ml separating funnel.Finally,the H₂SO₄ solution was used to back-extract the vanadium from the organic extract.The backextracting solution was used to prepare V₂O₅ in the following process.

2.2.4 Preparation of V2O5

The pH of back-extracting solution was adjusted to 8.25through adding ammonium hydroxide.The precipitate would be continually formed when back-extracting solution was stirred for 45 min at 85℃.After being filtered using the filter paper,the precipitate was washed by the deionized water and dried in the oven.Then,the precipitate was roasted to prepare V₂O₅ for 2 h at 550℃in the furnace.It should be pointed out that the experimental data in four processes had slight errors within 3%.

3 Results and discussion

3.1 Effects of roasting conditions on leaching of vanadium

Figure 1 shows XRD patterns of the fresh and roasted CS.As shown in Fig.la,the main minerals containing vanadium in the fresh CS are MnV₂O₄,MgV₂O₄,Fe₂VO₄,FeV₂O₄,and V₃O₄.The fresh CS also contains iron-bearing compounds such as Fe₃O₄,Fe₂SiO₄,and Fe₂TiO₄.After the roasting process,vanadium mainly exists in the minerals of MgV₂O₅,CaV₂O₅,Ca₃(VO₄)2,and Mg(VO₃)2(Fig.1b).Iron-bearing compounds are transformed into FeSiO₃ and Fe₂(SO₄)3.Additionally,species containing calcium such as CaSiO₃ and CaSO₄ also appear.These results demonstrate that roasting process results in significant changes in mineral compositions in CS.

Fig.1 XRD patterns of a fresh CS and b roasted CS.Roasting conditions:roasting temperature of 350℃,holding time of 360 min,and mass ratio of CS,(NH₄)₂SO₄,and KHSO₄ of 1.00:8.00:0.67

Deionized water was used as leaching agent to investigate the effects of roasting conditions on leaching behaviors of vanadium in CS.Figure 2a shows the influences of mass ratio of (NH₄)2SO₄ to CS on LRV at mass ratio of KHSO₄ to CS of 0.67,roasting temperature of 350℃,and holding time of 36 min.LRV increases from 34.5%to67.9%with an increase in mass ratio of (NH₄)2SO₄ to CS from 2 to 8,whereas it decreases from 67.9%to 57.1%with an increase in mass ratio of (NH₄)2SO₄ to CS from 8to 12.Figure 2b shows the effects of mass ratio of KHSO₄to CS on LRV at mass ratio of (NH₄)2SO₄ to CS of 8,roasting temperature of 350℃,and holding time of36 min.LRV at mass ratio of KHSO₄ to CS of 0.67 is larger by 11.5%than that without KHSO₄.However,LRV decreases by 5.0%when mass ratio of KHSO₄ to CS increases from 0.67 to 1.67.These confirm that there are the optimal dosages of (NH₄)2SO₄ and KHSO₄ for the leaching of vanadium.The role of roasting temperature in LRV is summarized in Fig.2c when the mass ratio of CS,(NH₄)2SO₄,and KHSO₄ is 1.00:8.00:0.67 and holding time is 36 min.LRV is 62.5%at 290℃,while it is 69.0%and is larger by 6.5%at 350℃.LRV decreases by 9.7%at390℃compared to that at 350℃.Li and Xie [ 8] also found similar results about the effects of roasting temperature on LRV through direct roasting and soda leaching process.As shown in Fig.2d,LRV only exhibits slight changes in the range of 54%-57%with holding time increasing from 0 to 45 min at mass ratio of CS,(NH₄)2SO₄,and KHSO₄ of 1.00:8.00:0.67 and roasting temperature of 350℃.These results suggest that mass ratio of (NH₄)2SO₄ to CS,mass ratio of KHSO₄ to CS,and roasting temperature play important roles in LRV,while holding time has slight effects on LRV.According to the above results,the optimum roasting conditions can be confirmed to be mass ratio of (NH₄)2SO₄ to CS of 8,mass ratio of KHSO₄ to CS of 0.67,roasting temperature of350℃,and holding time of 36 min.It can be noticed that LRV is unsatisfactory with the deionized water as the leaching agent,which can be ascribed to poor water solubility of minerals containing vanadium such as MgV₂O₅,CaV₂O₅,Ca₃(VO₄)2,and Mg(VO₃)2 in the roasted CS.

Fig.2 Effects of roasting conditions on LRV:a mass ratio of (NH₄)2SO₄ to CS,b mass ratio of KHSO₄ to CS,c roasting temperature,and d holding time

3.2 Leaching of vanadium from roasted CS using H2SO4

To enhance LRV of vanadium in the roasted CS,the deionized water was replaced by H₂SO₄ as the leaching agent.Figure 3a shows the effects of liquid-to-solid ratio on LRV at H₂SO₄ concentration of 6 vol%,leaching temperature of 90℃,and leaching time of 6 h.LRV increases from 87.3%to 91.6%with liquid-to-solid ratio increasing from 2 to 8 ml·g-.Nevertheless,LRV almost remains unchanged when liquid-to-solid ratio increases from 8 to 10 ml·g^-1.Figure 3b summarizes the changes in LRV as a function of H₂SO₄ concentration at liquid-tosolid ratio of 8 ml·g^-1,leaching temperature of 90℃,and leaching time of 6 h.LRV is 71.2%at H₂SO₄ concentration of 2 vol%,while it is 94.9%at H₂SO₄ concentration of 16 vol%.It can be noticed that LRV only increases by 1.1%with H₂SO₄ concentration increasing from12 vol%to 16 vol%.Under conditions with liquid-to-solid ratio of 8 ml·g-1,H₂SO₄ concentration of 12 vol%,and leaching time of 6 h,influences of leaching temperature on LRV are shown in Fig.3c.LRV at leaching temperature of95℃is larger by 34.5%than that at leaching temperature of 50℃.LRV at leaching temperature of 98℃exhibits a slight decrease when compared to that at leaching temperature of 95℃,which may be ascribed to an experimental error.Figure 3d shows the role of leaching time in LRV at liquid-to-solid ratio of 8 ml·g-1,H₂SO₄ concentration of 12 vol%,and leaching temperature of 95℃.LRV exhibits a significant increase with leaching time prolonging.However,LRV just increases by 0.5%when leaching time increases from 5 to 7 h.The above results definitely demonstrate that liquid-to-solid ratio,H₂SO₄concentration,leaching temperature,and leaching time have significant effects on LRV during the leaching process.During the leaching process of vanadium from the roasted mixtures of CS and Na₂CO₃,Aarabi-Karasgani et al. [ 6] observed similar effects of H₂SO₄ concentration and leaching temperature on LRV.However,they found that LRV initially increased and then decreased with liquid-to-solid ratio increasing [ 6] .Here,the optimum leaching conditions can be confirmed to be liquid-to-solid ratio of 8 ml·g-1,H₂SO₄ concentration of 12 vol%,leaching temperature of 95℃,and leaching time of 5 h.

Fig.3 Effects of leaching conditions on LRV:a liquid-to-solid ratio,b H₂SO₄ concentration,c leaching temperature,and d leaching time

Figure 4 shows XRD patterns of the residues under different leaching conditions.The peak at 35.117°is ascribed to species containing vanadium such as MnV₂O₄,Fe₂VO₄,and MgV₂O₄.The peak intensity significantly decreases with the increase in liquid-to-solid ratio,H₂SO₄concentration,leaching temperature,and leaching time.This indicates the decrease in the content of species containing vanadium in the residues,leading to the increase in LRV as shown in Fig.3a-d.However,the peaks of species containing vanadium incompletely disappear,which suggests that some of species containing vanadium remain in the residue due to the blocking by gangue minerals.Additionally,it is noticed that the peak intensity of SiO₂ at21.915°shows an increase with the corresponding parameters increasing,suggesting the enrichment of SiO₂ in the residues.Additionally,the residues also consist of minerals containing Fe,Si,and Ti.According to the previous studies [ 23] ,the residues may be used as raw materials of fertilizers to enhance the growth of crops.

Table 2 reports LRV from the slag under the corresponding optimum conditions using different methods in previous studies and this work.LRV is in the range of 93.7%-95.0%using SMS technology with NaOH-NaNO₃ or KOH as leaching agents [ 2, 9] .According to the results of Li and Xie [ 8] ,LRV (90.0%) is unsatisfactory through direct roasting and Na₂CO₃ leaching.Though LRV can reach 98.8%without pretreatments of the slag and using HCl-H₂O₂ as leaching agents,gaseous HCl would be produced and lead to environmental risks during the heated leaching process [ 14] .Compared to that of the roasted slag with Na₂CO₃ [ 6, 10] ,LRV of the roasted slag with (NH₄)2SO₄ and KHSO₄ exhibits an increase of 3.2%-4.2%with H₂SO₄ as the leaching agent.This suggests that (NH₄)2SO₄ and KHSO₄ may be more favorable than Na₂CO₃ for the transformation of compounds containing vanadium into H₂SO₄-soluble species.Additionally,LRV in this work is also larger by 0.4%-9.0%than that using other methods.These results prove that the roasting with(NH₄)2SO₄ and KHSO₄ and the leaching with H₂SO₄ are very attractive and promising processes for recovering vanadium from CS.

Fig.4 XRD patterns of leaching residues under different conditions:a liquid-to-solid ratio,b H₂SO₄ concentration,c leaching temperature,and d leaching time

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Table 2 LRV from slag under optimum conditions using different methods

3.3 Extraction of vanadium from H2SO4 lixivium

As mentioned in experimental section,further purification of vanadium from H₂SO₄ lixivium is necessary due to the existence of other ions.Addition of iron powders in H₂SO₄lixivium is aimed to reduce and Fe³⁺to VO²⁺and Fe²⁺,respectively.Figure 5 a shows the changes in the ERV as a function of iron powders content when the extracting organic phase is composed of 15 vol%P₂₀₄,5 vol%TBP,and 80 vol%SK.ERV obviously decreases with the concentration of iron powders increasing.ERV is99.0%at iron powder concentration of 2 g·L^-1,while it decreases to 90.0%at iron powder concentration of8 g·L^-1.To investigate the effects of pH on ERV,NaOH solution was used to adjust pH of H₂SO₄ lixivium.As shown in Fig.5b,ERV increases from 88.6%to 98.9%with pH increasing from 1.75 to 3.25.However,ERV exhibits a slight decrease with pH increasing in the range of3.25-3.75.The influences of extractant compositions on ERV are shown in Fig.5c,d when pH is fixed at 3.25.ERV continually increases with P₂₀₄ content increasing at TBP content of 5 vol%(Fig.5c).ERV at P₂₀₄ content of15 vol%is larger by 4.9%than that at P₂₀₄ content of5 vol%,whereas it just increases by 0.6%with P₂₀₄ content increasing from 15 vol%to 25 vol%.As shown in Fig.5d,ERV only exhibits a slight decrease when TBP content varies in the range of 5 vol%-25 vol%at P₂₀₄content of 15 vol%.The effects of SK on ERV were not investigated since the SK was just used as the diluent.Figure 5e shows the effects of volume ratio of extractant to H₂SO₄ lixivium on ERV at pH of 3.25 and extractant compositions of 15 vol%P₂₀₄,5 vol%TBP,and 80 vol%SK.ERV increases by 7.6%with volume ratio of extractant to H₂SO₄ lixivium increasing from 0.2 to 1.0.As shown in Fig.5f,ERV greatly increases with time increasing at pH of 3.25,extractant compositions of15 vol%P₂₀₄,5 vol%TBP,and 80 vol%SK,and volume ratio of extractant to H₂SO₄ lixivium of 1.ERV increases by 29.0%when extracting time increases from 2 to10 min.However,ERV at 30 min is just larger by 3.1%than that at 10 min.These results confirm that ERV significantly depends on iron powder concentration,pH,P₂₀₄content,volume ratio of extractant to H₂SO₄ lixivium,and extracting time.ERV can reach 96.5%under the optimum extracting conditions.El-Nadi et al. [ 19] demonstrated that the vanadium of 93.0%can be extracted from acidic media through aliquat-336.ERV is in the range of92.76%-94.32%using PRIMENE 81R to extract vanadium from the sulfate solution [ 20] .It suggests that the mixtures of P₂₀₄ and TBP may exhibit stronger extracting power than aliquat-336 and PRIMENE 81R for the vanadium in acidic media.To facilitate the preparation of V₂O₅,the vanadium in organic extract was back-extracted by1.5 mol·L^-1 H₂SO₄ solution,and the back-extracting rate of the vanadium is higher than 92.5%.

Fig.5 Effects of extracting conditions on ERV:a content of iron powders,b pH,c P₂₀₄ content,d TBP content,e volume ratio of extractant to H₂SO₄ lixivium,and f extracting time

3.4 Preparation of V2O5

It was determined that vanadium can be separated from the solution in the form of ammonium metavanadate [ 20, 21] .The precipitate appears when the pH of H₂SO₄ back-extracting solution is adjusted to 8.25 through adding ammonium hydroxide.According to the change in vanadium concentration in H₂SO₄ back-extracting solution,94.0%vanadium can be precipitated.As shown in Fig.6,the precipitate,which was washed,dried,and roasted,mainly consists of V₂O₅ according to the XRD results.Based on the analysis of XRF,V₂O₅ content in the precipitate is higher than 90.0%.Additionally,the precipitate also contains a small amount of FeVO₄.Though extracting and back-extracting procedures are used to separate vanadium ion with other ions,little iron may still participate in the transfer of vanadium during leaching,extracting,and back-extracting process,leading to the existence of FeVO₄ in the precipitate.Therefore,parameters during the extracting process should be further optimized to obtain V₂O₅ products with higher content.In a word,these results definitely demonstrate that the product of V₂O₅ with the content of higher than 90.0%can be obtained from CS containing vanadium of 13.79%through roasting,leaching,extracting,and precipitating processes.As shown in Fig.7,a promising technology road map can be proposed to enhance the efficient use of vanadium in CS.

Fig.6 XRD pattern of washed,dried,and roasted precipitates

Fig.7 Technology road map of preparation of V₂O₅ from CS

4 Conclusion

V₂O₅ can be obtained from CS through several processes including roasting,leaching,extracting,and precipitating.The LRV greatly depends on mass ratio of (NH₄)2SO₄ to CS (8),mass ratio of KHSO₄ to CS (0.67),and roasting temperature (350℃),but not on holding time (36 min)during the roasting process.The numbers in parenthesis show the optimum values of the corresponding parameters.The parameters including liquid-to-solid ratio (8 ml·g^-1),H₂SO₄ concentration (12 vol%),leaching temperature(95℃),and leaching time (5 h) can greatly affect LRV during the leaching process.LRV can reach 99.2%under the optimum leaching conditions.It is determined that iron powder concentration (2 g-L^-1),pH (3.25),P₂₀₄ content(15 vol%),volume ratio of extractant to H₂SO₄ lixivium(1),and extracting time (10 min) play important roles in the ERV during the extracting process.Through adding ammonium hydroxide,94.0%vanadium can be separated in the form of the precipitate from the solution.The product of V₂O₅ with the content of higher than 90.0%can be prepared by roasting the precipitate.