Solvent Extraction and Ion Exchange A Critical

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A Critical Review of Technology for Selective Recovery of Vanadium from Leaching Solution in V2O5 Production a

a

a

Ju-hua Zhang , Wei Zhang , Li Zhang & Song-qing Gu

b

a

School of Materials and Metallurgy, Northeastern University, Shenyang, P.R. China b

Zhenzhou Research Institute of China Aluminum Corporation, Zhenzhou, P.R. China Published online: 03 Apr 2014.

To cite this article: Ju-hua Zhang, Wei Zhang, Li Zhang & Song-qing Gu (2014) A Critical Review of Technology for Selective Recovery of Vanadium from Leaching Solution in V2O5 Production, Solvent Extraction and Ion Exchange, 32:3, 221-248, DOI: 10.1080/07366299.2013.877753 To link to this article: http://dx.doi.org/10.1080/07366299.2013.877753

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Solvent Extraction and Ion Exchange, 32: 221–248, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0736-6299 print / 1532-2262 online DOI: 10.1080/07366299.2013.877753

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A CRITICAL REVIEW OF TECHNOLOGY FOR SELECTIVE RECOVERY OF VANADIUM FROM LEACHING SOLUTION IN V2 O5 PRODUCTION Ju-hua Zhang1 , Wei Zhang1 , Li Zhang1 , and Song-qing Gu2 1

School of Materials and Metallurgy, Northeastern University, Shenyang, P.R. China 2 Zhenzhou Research Institute of China Aluminum Corporation, Zhenzhou, P.R. China In the V2 O5 production process, purification of vanadium-bearing solution plays an important role in determining the quality of the final product. The choice of purification method depends on the vanadium species, aqueous media, and associated impurities. In this review, the recent research results for the separation and recovery of vanadium from leaching solution with solvent extraction, ion exchange, and chemical precipitation separation are presented. The effects of the main operational parameters on the efficiency of purification with these three methods are compared and discussed respectively, and several problems existing in these processes are proposed. Keywords: vanadium, purification, solvent extraction, ion exchange, chemical precipitation separation

INTRODUCTION Vanadium, as a very important material, is widely used in various fields.[1] At present, the primary vanadium resources are vanadium slag, stone coal, and spent catalyst. Vanadium slag, which is a by-product of smelting of vanadium titanomagnetite, accounted for more than 38% of the world’s overall vanadium production in 2009.[2] Sodium chloride roasting followed by water leaching[3] is the conventional vanadium extraction process, though it is falling into disuse due to emission of a large amount of HCl and Cl2 and loss of vanadium resulting from the formation of volatilizable VOCl3 during chlorination roasting above 600◦ C.[4] Hence, more attention has been paid to exploring new vanadium extraction technologies, and accordingly several processes have been developed, for example, sodium carbonate roasting,[5,6] calcium roasting,[7] no-salt roasting,[8] direct acid leaching,[9,10] and molten-alkali extracting method.[11] It is acknowledged that there is no one all-purpose technique applicable to all types of vanadium resources. Processes are chosen on the basis of the compositions and the properties of vanadium feedstock. However, their operational steps are similar. First, the vanadium component transfers from solid ore or slag into Address correspondence to Ju-hua Zhang, School of Materials and Metallurgy, Northeastern University, Shenyang, 110004, P.R. China. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsei

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J-H. ZHANG ET AL.

aqueous solution; then the vanadium component contained in the solution is separated from the impurities; and lastly vanadium is concentrated into the solid phase, and vanadium pentoxide or other vanadium compounds are produced. It is realized that no matter which process of vanadium extraction is chosen, the leaching solution must be purified before the subsequent vanadium precipitating step, since impurities contained in the leaching solution seriously affect the quality of the final product. The methods for purifying the vanadium-containing solution usually include solvent extraction, ion exchange, and chemical precipitation. These three methods for recovering molybdenum and vanadium from spent hydrodesulphurization catalysts were reviewed by Li et al.[12] An investigation[13] compared ion exchange resins including ZGA414, D202, D301, D301PC, and ZGA351 (Zhejiang Zhenguang Resin Co., Ltd, China) with D2EHPA (Aladdin Chemistry Co., Ltd, also called P204 in China) modified with tributyl phosphate (TBP) in recovering vanadium from sulfuric acid leaching solutions of stone coal. However, up to now, there are few reports focusing on the development of these different methods for purifying vanadium-bearing solution, especially the leaching solution from the vanadium resources mentioned above. This paper discusses the continuing results of the previous studies on purification of vanadium-bearing solution with these three methods in detail, and some problems are proposed. MAIN COMPOSITIONS OF VANADIUM-BEARING SOLUTION The compositions and the mineralogical characteristics of feedstock determine the choice of vanadium extraction process. A typical flowchart for vanadium extraction with hydrometallurgical methods shown in Fig. 1 commonly includes roasting, leaching, purification of vanadium solution, and precipitation of vanadium.

Filtration Vanadium feedstock Residue Grinding

Solution

Purification

Additives Pelletization

Roasting

Vanadium precipitation

Ammonium polyvanadate

Mother liquor

Grinding Calcination Leaching V2O5 Figure 1 Flowchart of vanadium extraction technique.

SELECTIVE RECOVERY OF VANADIUM

223

Currently, China, South Africa, and Russia are the world-leading vanadium production countries, and their primary resources are vanadium slag and stone coal. Spent catalyst is used in small scale in Japan and the U.S. and so on, and vanadium is commonly obtained as a by-product in a molybdenum recovery process. The compositions of vanadium–bearing solutions vary with feedstock and extraction processes, and the typical compositions are shown in Table 1.


VANADIUM SPECIATION IN AQUEOUS SOLUTION The speciation of vanadium in aqueous solution directly affects its behavior in purification processes. The polymeric degrees of vanadium ranging from 2 to 10 in solution depend on the vanadium concentration and aqueous solution pH. Vanadium ions with lower concentration (< 10−4 M) are present as mononuclear species within the whole pH range. In the pH between 1 and 3, the prevailing species are the pervanadyl ions VO+ 2 , and the ions convert to the metavanadic acids, HVO3, when the pH continues rising. Vanadium at higher concentration exists in the form of isopolyanions with high degrees of polymerization, and 2− the polymeric number relates to the pH value. VO3− 4 and HVO4 are stable in alkaline 4− 3− solution, V2 O7 and HV2 O7 are stable in weakly alkaline solution, and metavanadate of tetramers (V4 O4− 12 ) is the dominant species in neutral solution. Various polynuclear anionic species exist in acidic or weakly acidic solution. When the solution pH is less than 1, the [14,15] The main species of structure of polymeric species is destroyed and converted toVO+ 2. vanadium of various concentrations in pH ranging from 0 to 14 are shown in Table 2. The Pourbaix diagram for vanadium-water system depicted as Fig. 2 (drawn by HSC Chemistry 6.0) shows that the vanadate cation V(V) is stable and predominant in a high acidity range of pH < 2 under high electrode potential, and when the pH further increases, V(V) exists as 2− 3− a series of anions, such as H2 VO− 4 , HVO4 , and VO4 , which are deprotonated according to the pH value. As the potential decreases, vanadium is present as VO2+ , V3+ , V2 (OH)4+ 2 , and V2+ cations in the solutions with pH less than 4.[16]

SEPARATION AND RECOVERY OF VANADIUM FROM VANADIUM-BEARING SOLUTION Solvent Extraction Solvent extraction as applied to the vanadium industry commenced in 1956. Subsequently, it was increasingly concerned with producing high purity of product due to its powerful selectivity and fast kinetics.[17] Organophosphorus acids, amines, and hydroxyoximes that are normally used to extract vanadium are listed in Table 3. Cation exchange extractants are suitable to be used in acidic solution where cationic species are more stable; basic long-chain alkylamines are favorable in extracting anionic complexes where cationic species are not predominant; and quaternary long-chain alkylammonium salts are used to extract anionic species in neutral and basic regions. Because vanadium ions could be present in +4 and +5 valences in aqueous solution and the polymeric degree of the complex is determined by aqueous solution pH and vanadium concentration, extractants should be chosen according to the solution media and vanadium valences. Effect of Aqueous Solution pH on Vanadium Extraction. Solution pH plays an important role in metal-chelates formation and the subsequent extraction because it

224

Sodium roasting-water leaching Sodium roasting-dilute acid leaching Direct acid leaching with oxidizer/pressure No salt roasting-acid leaching

Sodium roasting-water leaching Roasting-acid leaching

Stone coal/Blank shale

Spent catalyst

Roasting-sodium hydroxide leaching

Sodium roasting-water leaching Calcium roasting-dilute acid leaching Sub-molten alkali extracting

Process

Vanadium slag

Feedstock

V(V)

High alkalinity

V(V)

V(V)

∼1.0

V(V)

∼1.0 V(V)

V(IV), V(V)

[H+ ] > 1 M

8.0–9.0

V(V)

1.5–2.5

V(V)

V(V)

2–3

9–10

V(V)

8.5–10

Solution pH

Vanadium valence

Mo(VI)

Mo(VI), Si(IV), K, Al(III)

Mo(VI), Al(III), Si(IV)

Si(IV), Ca(II), P(V), Zn(II), Al(III) Fe(III), Fe(II), Al(III), Mg(II), Si(IV) Na, Ca(II), Zn(II), Si(IV)

Na, P(V), Si(IV)

Na and/or K, Si(IV), P(V), Cr(III)

Ca(II), Mn(II), Mg(II), Si(IV)

Na, Si(IV), P(V), Cr(VI)

High

Ni(II), Co(II), Cu(II), Zn(II) Si(IV), K, Na

P(V)

Al(III), K Mg(II)

Na, K, P(V), Ca(II)

Al(III)

Al(III)

Al(III), P(V)

Medium

Relative concentrations of impurities

Table 1 Compositions of vanadium-bearing solutions from various vanadium feedstocks with different vanadium extraction processes.


Fe(III)

As(V), Ni(II)

Fe(III), Ni(II)

As(V) Cr(III)

Fe(III) Cu(II), As(V)

Ca(II), Fe(III), As(V)

Fe(III), Cr(VI), Ti(IV)

Al(III)

Low


225


Table 2 Main species of V(V) of different concentrations in pH range of 0 to14. pH [V] < 10−4 M

< 3.6 3.6–4.4 VO+ HVO3 2

4.4–8.4 VO− 3

8.4–13.0 HVO2− 4

13.0–14.0 VO3− 4

pH [V] = 10−4 −10−2 M

< 1.2 1.2–3.3 4− VO+ VO+ 2 2 , H2 V10 O28 ,

3.3–8.0 8.0–13.0 6− 2− HV10 O5− , V O , HVO 10 28 28 4 − VO3

13.0–14.0 VO3− 4

pH [V] = 10−2 −10−1 M

< 0.5 0.5–6.5 6.5–8.7 5− 4− 3− VO+ H2 V10 O4− 2 28 , HV10 O28 , V4 O12 ,V3 O9 V10 O6− 28

8.7–13.0 13.0–14.0 4− HV2 O3− VO3− 7 , V2 O7 4

Eh (Volts) 2.0 VO2(+a)

1.5

H3VO4(a) H2VO4(−a)

HVO4(−2a)

1.0

VO4(−3a)

VO(+2a)

0.5

0.0 V(+3a)

V2O2(OH)2(+2a) V2(OH)2(+4a) VO(+a)

–0.5

–1.0 V(+2a) –1.5

VOH(+a) H2O Limits

–2.0

0

2

4

6

8

10

12

14

pH

Figure 2 E-pH diagram for V-H2 O system at 25◦ C ([V] =1 mol/kg, 100 kPa).

defines the charges of the vanadium complex, then determines extraction efficiency. The optimum pH ranges for different extractants to extract vanadium are listed in Table 4. P204, P507, and Cyanex 272 are acidic phosphorus extractants used in acidic solution media. They extract V(IV) in priority to V(V); thus, before extraction, the V(V) should be reduced to V(IV). Na2 SO3 is normally employed as a reductant. Many investigations have been carried out on the effect of the aqueous pH on V(IV) extraction from acidic solution with these organophosphorus extractants. Li et al. found that as the pH increased from 1.5 to 3.2, the V(IV) extraction with P204 and TBP in an organic phase/aqueous phase (O/A) ratio of 1.0 increased from 17% to 83%.[32] Li observed that V(IV) recovery increased obviously in the pH range of 0.5–1.5, and gradually leveled off when pH over 1.5.[13] Moussa et al. reported that using 20% P204 + 15% TBP to obtain a significant extraction of V(IV) pH value of the aqueous solution should be 1.5–1.8.[18] The slight difference in optimum pH ranges obtained in these three studies can be attributed

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2-Ethylhexyl phosphonic acid mono-2-ethylhexyl phosphoric ester (EHEHPA, P507)

Di(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272)

Tributylphosphate (TBP) Trialkyl phosphine oxide (Cyanex 923,TRPO)

Secondary carbon primary amine (Primene JMT, N1923) Trialkylamine (Alamine 336, N235)

Trioctylamine (TOA, Alamine 308)

Methyltrioctylammonium chloride (Aliquat 336, N263) 5,8-Diethyl-7-hydroxydodecane-6-oxime (LIX 63, N509)

5-Dodecylsalicylaldoxime (LIX 860-I)

Alkyl phosphonic acid

Alkyl phosphinic acid

Phosphoester Phosphine oxide

Primary amine

Tertiary amine

Quaternary ammonium salt Hydroxyoxime

Hydroxyoxime

Tertiary amine

Di(2-ethylhexyl)phosphoric acid (D2EHPA, P204)

Reagent

Alkyl phosphoric acid

Type

Table 3 Commonly used extractants for solvent extraction of vanadium.

O

R

P OH

O

R

O P OH

O

P OH

O

N

R

R = dodecyl

R

HO

N

OA

OH

R4 NCl R = octyl

R = octyl

R

R

N

OH

A mixture of tri-C8 C10 -alkyl amines, with the general structure R1R2R3N

(RO)3 P=O, R = butyl A mixture of R3 P=O, R2 R´P=O and RR´2 P=O, R = octyl, R´ = hexyl R2 CNH2 R, C19 ∼C23

R = CH3 CH(CH3 )2 CH2 CH(CH3 )CH2

R

R

R = CH3 (CH2 )CH(C2 H5 )CH2

R

R = CH3 (CH2 )3 CH(C2 H5 )CH2

O

R

Formula


V(V) in H3 PO4 solution[26]

V(IV) in H2 SO4 solution[25]

V(V) in HCl/NaOH/ Na3 VO4 solution[46,47]

V(V) in H2 SO4 solution[42]

V(V) in H2 SO4 / Na3 VO4 solution[24,28]

V(V) in Na3 VO4 solution[41]

V(V) in acidic chloride solution[21] V(V) in acidic solution[22,23]

V(IV), V(V)in H2 SO4 / HCl solution[35,20,52]

V(IV), V(V) in H2 SO4 / HCl solution[34,19]

V(IV), V(V) in H2 SO4 solution[32,33,13,18,52]

Vanadium valence and solution media


227

Table 4 Optimum pH values for different extractants to extract vanadiuma . Extractants

Optimum aqueous pH

P204

2.3, 2.5, 2.0–2.5

Cyanex 272


P507 Cyanex 923 N 1923 Alamine 336

1.5 V(V) octanone, while for Cyanex 272 the order followed kerosene > octanone > decanol = isooctanol = octanol. Only a ketone had a positive effect on extraction with Cyanex 272. In concentrated mineral acid solution, a significant increase in the extractive ability of organophosphoric acids was observed in aliphatic alcohols compared with inert diluents. In this case, the maximum effect was observed with isooctanol compared with normal alcohol structure like decanol and octanol.[52] Strong interactions of the diluent with the neutral extractant Cyanex 923 can result in lower extraction of V(V), so the extraction of V(V) is higher in cyclohexane than in aromatic hydrocarbons. The diluents with low dielectric constants, such as kerosene, benzene, and toluene, show the higher extraction of V(V) by Cyanex 923, and a similar tendency is obtained for TBP. The polymerization degrees of amine extractants vary with the polarity of diluents. The smaller is the polarity, the greater is the extent of polymerization for extractants (usually dimer), and this leads to a larger distribution ratio. Kerosene and n-dodecane are usually used as diluents for V(V) extraction with amine extractants in many studies. For V(V) extraction from neutral solution by primary amine N1923 via a solvation mechanism, either “acceptor type” diluents like chloroform or aliphatic or “donor type” diluents like esters or ethers decreased the vanadium recovery compared with inert diluents such as carbon tetrachloride, cyclohexane, or kerosene.[48] Small differences were observed with aliphatic and aromatic hydrocarbons as diluents in the process of V(V) extraction with Primene 81R.[49] Extraction Equilibrium and Kinetics. The extraction processes for P204, P207, and Cyanex 272 were exothermic, and the values of G for these three extraction equations were negative. The pH50 values for 0.1 M P204, P507, and Cyanex 272 were 1.17, 1.42, and 1.72 respectively, so the extractability for these three extractants was in an increasing order of P204 > P507 > Cyanex 272.[35] The equilibrium constant for the reaction VO2+ aq + H2 R2org ⇔VOR2org +2H+ aq by which V(IV) is extracted from acidic sulfate solution with P204 was found to be (1.86 ± 0.02) × 10−1 at equilibrium pH of 0.95 when the complexes of V(IV) and sulfate anions were taken into account to calculate the equilibrium concentration of vanadium.[50] The equilibrium constants for reactions of V(V) and V(IV) extraction from chloride solutions with P507 in kerosene were 3.14 and 0.32, respectively.[19] The apparent equilibrium constant for reaction of V(IV) extraction from sulfate solution with Cyanex 272 in Exxsol D 80 was 8.7 × 10−1 when the complex equilibrium of vanadyl ion with sulfate group was considered.[51] The organophosphoric acids in inert diluents could be arranged in an order of increasing capacity for V(V) extraction as follows: P204 (technical grade) < P204 (purified) < Cyanex 272. The branching of carbon chains can affect both the extractive ability and selectivity. Cyanex 272 possesses two more substituents (methyl radicals) at the fourth carbon, which suggests a higher selectivity of Cyanex 272 with respect to V(V) compared with P204. An opposite phenomenon was observed in V(V) extraction from weakly acid solution where cation exchange mechanism is operative. These differences lie in the reasons that the two ester groups (–OR) in P204 withdraw the electron density from the hydroxyl group –OH and give the proton higher mobility.[52] It is known that organophosphoric acids are readily dimerized in pure form and in inert solvents due to the simultaneous presence of proton-donor (–OH) and protonacceptor (P = O) groups. This gives rise to hydrogen bonds and leads to a decrease in thermodynamic activities of the extractants, and then the access for V(V) to the functional groups of the extractants is hindered. The active solvents, neutral oxygen-containing reagents such as TBP, sulfoxides, ketones, and alcohols, can change the degree of


231

dimerization of the organophosphoric acids and improve their extractive abilities. For example, petroleum sulfoxide (PSO) has an obvious synergistic extraction effect on [53] P204 extracting VO+ 2 (V), and the extraction reaction is expressed as Eq. 2:


+ VO+ 2 + 2(HA)2 + PSO → VO2 A(HA)3 PSO + H

(2)

The equilibrium constant of the synergistic extraction is 3.34, larger than that of P204 in HCl medium with a low acidity. Navarro discovered that the smallest amount of extractant was required to achieve complete V(V) extraction at pH 3 with Aliquat 336 compared with Primene JM-T, Alamine 336, Alamine 304, and Amberlite LA-2. The average values of the extraction equilibrium constants for Aliquat 336 from alkaline (pH > 10) and acidic (pH < 2) media were (2.50 ± 0.21) × 103 and (5.50 ± 0.56) × 102 , respectively.[44] In the temperature range of 15 to 45◦ C, the values of H for V(V) extraction from both media were positive, which indicate the endothermic nature of the extraction process. The G value for extraction in 3 M HCl solution was negative under the given conditions of 0.5 M Aliquat 336 and 0.1 g/L vanadium, while it was positive in 0.1 M NaOH solution in the same case.[46] The contact time for vanadium extraction to reach equilibrium with different extractants is summarized in Table 6. As shown in Table 6, apart from LIX 860-I, the contact times for vanadium extraction to reach equilibrium with acidic and neutral organophosphorus and amine extractants are all no more than 15 min. Additionally, a magnetic field[54] has great effect on increasing Table 6 Contact time for vanadium extraction to reach equilibrium. Conditions Extractant P204

Contact Time, min 3–4, 10

In separating funnels, mechanical stirring

25-40

5

In glass stoppered bottles, mechanical shaking In separating funnels, mechanical shaking Mechanical shaking

5 10

8 P507 Cyanex 272 Cyanex 923

5–8

N1923

15

Aliquat 336

15 1–2

LIX 860-I

T, ◦ C

Contact type

3h

Medium

Diluent

References [18,32,33]

Kerosene

25

H2 SO4 solution at pH 1.5, 2.3-2.5 1.5

25 ± 1

HCl solution

Kerosene

25 ± 2

HCl solution

Toluene

[20]

Mechanical stirring

30

HCl solution

Kerosene

[19]

In separating funnels, mechanical stirring In separating funnels, shaken by a oscillator Thermostatic and shaking water bath Handing shaking

Room H2 SO4 solution temperature 20 Alkaline solution 25 HCl or NaOH solution >20 Na3 VO4 solution at pH 8.34-8.64 1.47 M H3 PO4 solution

Kerosene

[23]

Kerosene

[41]

Kerosene

[46]

Kerosene

[47]

Toluene

[26]

In special stoppered tubes, mechanical shaking

[13] [19,34]


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J-H. ZHANG ET AL.

the reaction rate for TOA extracting vanadium; however, it does not change the mechanism of extraction. Ultrasonic irradiation can also improve the V(V) recovery of solvent extraction compared with mechanical shaking in the same time, because ultrasonic irradiation enhances the emulsion formation, which increases the interfacial area between the aqueous and organic phases.[55] Though the reactions of vanadium extraction with various extractants proceed fast, a single extraction stage usually cannot obtain a satisfactory recovery. Taking the P204 for example, a McCabe-Thiele plot for 10% P204 and 5% TBP in kerosene showed that a complete extraction of vanadium is theoretically possible in six stages using an O/A phase ratio of 1:1.[32] With an O/A ratio of 1:2 and contact time of 10 min, 95.9% of the vanadium was extracted from a real acidic leaching solution of stone coal by P204, and after a continuous six-stage counter-flow extraction, the vanadium in the loaded phase was 7.56 g/L, while it was 0.16 g/L in the raffinate phase.[33] From the McCabe-Thiele plot for V(IV) extraction, it was found that vanadium was nearly completely extracted in three stages by P507 with an O/A ratio of 1.0.[34] Complete extraction of vanadium (< 0.001% g/L V in aqueous raffinate) was theoretically possible in two counter-current stages using both Primene 81 R and Alamine 336 when the initial vanadium concentration in solution was 3.88 g/L.[24] EI-Nadi compared the V(V) extraction by Aliquat 336 from acidic and alkaline media and found that the vanadium loaded onto the organic phase increased with the number of stages, and after twelve stages the loaded amount of vanadium extracted from alkaline medium was twice the amount from acidic media. [46] Selectivity. The high selectivity of vanadium over other associated impurities for different extractants is summarized in Table 7. The extractability of P204 for metal ions in an increasing order is Fe3+ > VO2+ > + VO2 > Ca2+ > Mn2+ > Mg2+ > Fe2+ > K+ ≈ Na+ .[56] The primary amine can selectively

Table 7 Selectivity of vanadium over other associated impurities for different extractants. Extractant P204

P507 TBP Cyanex 923 N1923 TOA Aliquat 336 8-Quinolinol LIX 63

Selectivity V(IV) over Fe(II) in a real acidic solution from stone coal with initial pH of 2.3 V(IV) and Fe(III) over Fe (II), Si(IV), Al(III), Mg(II), Na, and K in H2 SO4 leaching solution of stone coal V(V) and Mo(VI) over Al(III), Co(II), and Fe(II) V(V) and Fe(III) over Mg(II), Al(III), Ti(IV), Cr(III), and Mn(II) in a simulated chloride liquor V(V), Ti(IV), and Fe(III) over Mg(II), Al(III), Cr(III), and Mn(II) in an industrial chloride liquor V(V) over Cr(VI) in a real solution from Huihong Nonferrous Metal Co., Ltd V(V) over Ca(II), Mg(II), Al(III), P(V), and Si(IV) in a dilute acid leaching solution of stone coal V(V) over Mo(VI) at pH range of 8.0–8.5 in a synthetic solution V over Ca(II), Mg(II), Al(III), Fe(III), Cu(II), Cr(VI), and NO− 2 in a synthetic solution V(IV) and Mo(VI) over Ni(II), Co(II), Fe(III), and Al(III) in a synthetic solution V(V) and Mo(VI) over Ni(II), Co(II), Fe(III), and Al(III) in a synthetic solution of spent catalyst

Reference [33] [32,13] [34] [21] [22] [41] [30] [59] [55] [25] [59]



233

extract V in priority from nearly neutral aqueous solution by a solvation mechanism but not Cr with a separation factor over 170. [41] The extraction and separation of Mo and V over Al were achieved with P507 at a low pH on the basis of the differences in extraction equilibriums and kinetics of these metals, and then the loaded Mo and V can be separated with different strippants, NH4 OH + NH4 Cl and pure acid solution, respectively.[57] V(IV) was selectively separated from Mo(VI), Ti(IV), U(VI), and Fe(III) at 5 M HCl with Cyanex 272, leaving V(IV) behind in the aqueous phase, while at 1.0 × 10−3 M HCl, V(IV) and Fe(III) were co-extracted.[20] In a low pH range (1.2–2.0), V(IV) and Mo(VI) were preferentially extracted and completely separated from the co-existing metals, such as Ni(II), Co(II), Fe(III), and Al(III), by LIX 63 dissolved in Exxsol D80 with a phase ratio of O/A = 1, and then the loaded V(IV) and Mo(VI) can be separated with different strippants.[25] Similarly, at equilibrium pH more than 2.0, both V(IV) and Mo(VI) were extracted by LIX 84I, and up to pH more than 3.0, there was no extraction of Al, Ni, and Fe; however, at pH lower than 2.0, the extraction efficiency of Mo(VI) was much higher than V(IV).[58] Li reported that almost all the V(V) and Mo(VI) were extracted from sulfuric acid solution by 0.5 M LIX 63 in Shellsol 70 at an A/O ratio of 1:1 within the pH range of 1–2, while the extractions of Ni(II), Co(II), Fe(III), and Al(III) were negligible.[59] When 8-quinolinol was used to extract V(V) in acid medium, the recovery of vanadium was 97% or more while only 0.2%–0.3% or less of Ca(II), Mg(II), Al(III), Fe(III), Cu(II), or Cr(VI) was extracted.[55] LIX 860-I was effective for V(V) extraction from acidic media but not for V(IV); however, the increase in the extractant concentration gives rise to the conversion of V(V) to V(IV) in aqueous phase.[26] Stripping. Different strippants and stripping conditions for the loaded organic extractants above mentioned are listed in Table 8. The V(IV) extracted by acidic organophosphorus extractants can be stripped by pure acid solution in several stages. As the number of stripping stages increases, the stripping degree normally increases.[32,34] The study on the effect of concentration of sulfuric acid on stripping efficiency showed that vanadium stripping increased from 41% to 98% as the concentration of sulfuric acid increased from 0.5 to 2.5 M; however, the free H2 SO4 in stripping solution increased as well, and this was not desirable. The optimum concentration of sulfuric acid was chosen as 1.5 M when both vanadium stripping ratio and the free sulfuric acid in stripping solution were considered.[32] When 15% H2 SO4 was used to strip the loaded metal-P204 organic phase with a phase ratio of O/A = 5:1 at 45◦ C for 15 min, after five stages counter-flow back-extraction, the stripping yields of vanadium and iron could reach 99.14% and 19.35%, respectively, with a mass ratio (V/Fe) of 62 in aqueous phase of stripping, which indicates separation performance was satisfactory.[33] The stripped organic phase can be recycled after a wash with NH4 HCO3 solution to remove Fe(II), followed by water washing and re-acidification.[32] In stripping, the co-extracted metal ions on loaded Cyanex 923, such as V(V), Ti(IV), and Fe(III) can be selectively recovered by different acidities of stripping agent.[22] The stripping of vanadium from high-molecular-mass quaternary ammonium salt is difficult and slow. A solution of 1 M NaOH can be used to strip V(V) extracted from both acidic and alkaline media by Aliquat 336.[46] Although the NaOH solution is thermodynamically efficient to achieve the stripping of V(V), the kinetics is slow. The limiting step of the stripping process lies in the slow transformation of the extracted polyoxometallate species H2 V10 O4− 28 3− [31] into HVO2− Navarro discovered that quantitative recovery of vanadium can 4 or VO4 . be achieved using concentrated mixed solutions of ammonia and ammonium salt (1.5 M

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J-H. ZHANG ET AL.

Table 8 Stripping vanadium from loaded organic phases.

Extractant P204

Stripping reagent 1.5 M H2 SO4


15% H2 SO4 P507

1.0 M H2 SO4

TBP Cyanex 923

4 M HCl 1.5 M HCl

Primine 81 R TOA

1 M NH3· H2 O

Alamine 336 Aliquat 336

LIX63

0.5–0.7 M Na2 CO3 1.5 M NaOH 10% NH3· H2 O 1 M NaOH 4 M NaCl + 1 M NaOH 400 g/L H2 SO4 10% NH3· H2 O 1 M NaOH

Conditions O/A = 5:1, three stages of counter-current stripping O/A = 5:1 at 45◦ C for 15 min, 5-stage counter flow O/A = 2:1, three stages of countercurrent stripping O/A = 2:3, four stages O/A =1:6, three counter-current stages O/A = 1:1, single stage, 180 min

Stripping efficiency

Reference

Completely stripped 99.14%

[32]

>99.5%

[34]

Completely Completely

[21]

65%

[49]

O/A =1:1, pH > 12, two stages

99.9%

[30]

One stage, O/A = 2:1, ambient temperature Single stage, O/A = 2:1, 30◦ C One stage Single stage, O/A = 1:2, 1 min, 28 ◦ C

96.02%

[28]

99.85% 99% 99%

[29]

11.3% >99.9%

[59]

>99.9

[59]

Single stage, A/O = 1:1, 40◦ C Single stage, A/O = 1:1, 40◦ C, pH = 1.01–1.39 Single stage, A/O = 1:2, 40◦ C

[33]

[22]

[46] [47]

[59]

NH3 + 1.5 M NH4 NO3 /NH4 Cl), while other stripping reagents like 1.0 M NH3 +1.0 M NH4 NO3 /NH4 Cl and 1 M NaOH only released vanadium partially.[44] When a stripping system of 5 M NH4 Cl + l M NH4 OH with a phase ratio of 1:2 was used to strip V(V) from the loaded organic phase of Aliquat 336, NH4 VO3 precipitated. NaCl solution has efficient performance in stripping V(V) from Aliquat 336, and the extractant is regenerated effectively after stripping with chloride solution. By contrast, after stripping with NH4 NO3 solution, the regenerated extractant in the nitrate form performs inefficiently compared with fresh Aliquat 336 in the chloride form.[47] While the V(V) loaded on LIX 63 was substantially scrubbed by NH3· H2 O and NaOH solutions, V(IV) was stripped by pure acid solution, and the stripping efficiency reached 90.15% by 2.5 M H2 SO4 with O/A ratio of 1/1 at 30◦ C.[25] Ultrasonic irradiation with the additives including KBr, NaBrO3 , and HNO3 solutions can greatly shorten the time for NaOH stripping V(V) from the organic phase of CHCl3 and alcohols to only 5 min with a satisfied recovery of 97%, while shaking with the additives needed 60 min to obtain a recovery of 91%.[55] Ion Exchange In comparison with solvent extraction, ion exchange methods are limited to smallscale industrial applications. Generally, the ion exchange resins that are used to separate and extract vanadium belong to strongly basic and weakly basic exchange resins and other chelating resins as shown in Table 9. Pentavalent vanadate ions react with anions

235

Strongly acidic cationic resin

Weakly basic anionic resin Chelating resin

Strongly basic anionic resin

Type

−N(CH3 )2 −NHCH2 PO3 Na3 Py–CH2 NH2 −N(CH2 COOH)2 −CH2 N–(CH2 COOH)2 R–CH2 N(CH2 COO–)2 −CH2 N(CH2 COO)2 2–

DDAS CW-2 CUW Chelating resin A Chelex 100 SK1B

−N(CH3 )3

717(201×7)

D314

−N(CH3 )3

Functional group

D296

Exchanger Hangzhou Zhengguang Chemical Company, China Jiangsu Suqing Water Treatment Engineering Group Co., Ltd Hangzhou Zhengguang Chemical Company of China Suzhou Bojie Resin Technology Co., Ltd Orient Chemical Industries, Ltd Orient Chemical Industries, Ltd Jiangsu Sekesaisi resin Co., Ltd Bio-Rad, Richmond, CA Mitsubishi Chemical Co.

Manufacturer

Table 9 commonly used ion exchange resins for purification of vanadium-bearing solutions.

V(V) in a synthetic solution containing Mo and V V(V) in a synthetic solution containing Mo and V V(V) in a synthetic solution containing Mo and V V(IV) in a synthetic solution V(IV) and V(V) in a synthetic solution V(V) in a synthetic solution containing V and Fe

V(V) in a synthetic solution/an alkaline solution of stone coal V(V) in an industrial leaching solution of stone coal

V(V) in a molybdate solution from spent catalyst

Vanadium valence


[67]

[66]

[65]

[69]

[69]

[69]

[64,72]

[63,71]

[62]

Reference


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J-H. ZHANG ET AL.

on exchange groups, and then are absorbed on the resin so as to separate from the other impurities in solution. Besides these resins, ion exchange fiber (chitosan[60] ) and hybrid ion exchanger,[61] which is formed through combining ion exchange resin with ion exchange fiber, have been investigated to absorb V(V). V(IV) cannot react with anionic resin because it does not form an anion; therefore, it is impossible to extract V(IV) with anionic resins. Several chelating resins are used to extract V(IV) from acidic media. Effect of Solution pH, Temperature, and Time on Absorption Rate. The effects of pH, contact time, and temperature on the capacities of different exchange resins and vanadium recovery are summarized in Table 10. Since the loading capacity of vanadium on a resin depends on the vanadium polynuclear species and anionic charge, the pH value is one of the important factors for V(V) absorption with exchanger resins from solution. It was found that the V(V) absorption capacity of resin D296 increases with pH rising from 6.7 to 7.12, and then decreases with pH further increase. When pH is below 6.5, the ammonium crystal forms. The max3− imum value occurrs at pH 7.12. This is due to the conversion of V10 O6− 28 to V3 O9 at − 3− pH 6.5–7.12 and to VO3 over pH 7.12, of which V3 O9 has a smaller radius and higher charge.[62] The sorption degree of V(V) increases as the temperature increases, and the sorption of V(V) by resin 717 decreases with increase in pH from 9.06 to 10.39.[63] The flow rate was seen to have an obvious effect on absorption ability of resin when it is beyond 0.09 mL/(min·g).[70,71] Dai et al. discovered that at a flow rate of 0.09 mL/(min·g), pH of 7.2–8.2, and temperature of 40–50◦ C, the absorption performance of resin 717 was superior to that of resin 714 (from Jiangsu Suqing Water Treatment Engineering Group Co., Ltd), and the purity of the final product obtained from the alkaline vanadium solution of stone coal that was purified by resin 717 satisfied the requirement of grade 99.[71] The weakly basic resin D314 is normally applied in acidic or neutral solution. At very low pH, V(V) predominantly exists in the form of VO+ 2 , which cannot be absorbed by the anionic resin, while at higher pH, this resin has no ion-exchange capacity. At pH 4, the loading capacity of V(V) was found to be 264 mg/mL for wet resin D314, and it increased with contact time.[72] However, this capacity value is much less than the theoretical one. The theoretical exchange capacity of V(V) on resin D314 at different pH values in a decreasing order is pH 2–6 > pH 6–8 > pH 8–12 > more than 12 > less than 2, and the maximum capacity can be inferred to be 556 mg/L. An investigation[68] of the loading capacities of different vanadium species was carried out on resin D314, where the contact time was increased to 24 h. The maximum loading capacity of the resin for V(V) was obtained in the pH range of 2.5–4.5, 492 mg/mL at pH 3.2 and 501 mg/L at pH 4.46 respectively, which are consistent with the theoretical values. However, at pH 2.5–3.5, Mo(VI), V(V), P(V), Si(IV), and As(V) were simultaneously absorbed on resin D314.[64] Temperature is also one of the important factors influencing the distribution ratio of vanadium. It was observed that the absorption of V(V) on resin D314 was highest at 35◦ C, with a distribution ratio of 1475.[68] It was reported that V(V) can be completely separated from Mo(VI) from an ammonium molybdate solution by the chelating resins CUW, CW-2, and DDAS in the pH range 7.4–8.0, whereas it is difficult for anionic resins to extract it quantitatively.[69] There are few investigations of V(IV) extraction from acidic media with ion exchange resins. Huang et al. studied the absorption performance of an amino phosphonic acid chelating resin for V(IV) and found that the aqueous solution pH had great effect on absorption rate. The absorption increased with pH from 1 to 4 and tended to decline at

237

Chelating resin A

DDAS CW-2 CUW Chelex 100

Batch-wise Batch-wise Continuous Batch-wise Batch-wise Batch-wise

3–4.5

Batch-wise

SO4 2– -D314

7.2–8.2 9.14 7-8 4

Continuous Batch-wise Continuous Continuous

717

7.44 7.44 7.4–8.0 3–6 V(V) >3 V(IV) 4

7.12

Batch-wise

Cl-D296

Optimum pH

Operation type

Exchanger

Ambient Ambient Ambient 25 25 60

15

50 26 40-50 Ambient

25

Temperature, ◦ C

80 60 60 60 60 4h

24 h

0.09 mL/(min·g) 15 min 0.09 mL/(min·g) 60 min

16 h

Time

Absorption capacity

184 mg/g resin

55 mg/g resin 260 mg/mL wet resin 501 mg/mL wet resin 17.1 g/L 27.1 g/L 31.4 g/L

16.83 g/L wet resin 149 mg/L

Table 10 Optimum pH values, temperature and contact time for the maximum loading capacities of different exchange resins.


99.5% 99.7% 99.9% Completely Completely

[71]

>99% 90.6% 93.8% 99.41%

[65]

[66]

[66]

[69]

[69]

[69]

[68]

[72]

[70]

[63]

[62]

Reference 99.5%

Recovery of V


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J-H. ZHANG ET AL.

higher pH. However, the V(IV) in the solution began to hydrolyze at pH above 4. The V(IV) absorption capacity of this resin reached 184.2 mg/g at pH of 4, stirring time of 4 h, and reaction temperature of 60◦ C.[65] For a lower initial concentration of V(V) or V(VI), the fractions of V(V) and V(VI) absorbed on chelating resin Chelex 100 were both present as a function of pH of the solution. A 0.1 g amount of Chelex 100 in the H+ form absorbed the maximum fraction of V(V) in the pH range 3–6 from 0.1 M NaNO3 solution, while the absorption of V(IV) reached the maximum at pH higher than 3 under the same conditions.[66] As shown in Table 8, ion exchange resins used for absorbing vanadium typically require a long contact time. Ion exchange fibers with high absorption rate of metal ions overcome this weakness of ion exchange resins; however, they have low packing densities, which lead to a decrease in volumetric ion exchange capacity. A novel hybrid exchanger[61] that combines PONF-g-GMA fibrous exchanger with IRA-96 anion exchanger with the hot-melt spraying method can be an alternative to compensate the disadvantages of these exchangers. Selectivity. Resin D296 can separate Mo(VI) from V(V) in a pH range 7.0–8.0, 2− because the charge of V3 O3− 9 is higher than that of MoO4 , which makes the sorption affinity with resin Cl-D296 of Mo(VI) less than that of V(V).[62,64] The presence of Cl− ion has a sharp effect on the absorption capacity of resin Cl-D296 for V(V) in a continuous absorption experiment. Since the absorption reaction is reversible, the less the Cl− in the solution is, the higher is the absorption capacity of resin.[62] Resin 717 is a kind of polymer of styrene and diethylene benzene containing − 2− ions quarternary ammonium groups, and it can absorb V4 O4− 12 , Al(OH)4 , and SiO3 simultaneously; however, the adsorption of V(V) is prior to that of Si(IV) and Al(III).[63] Within the pH range 2−4 vanadium ions in solution exist in the forms of V10 O6− 28 , 4− , and H V O . When D314, a slightly basic resin, was used to purify a weakly HV10 O5− 2 10 28 28 3− 2− acidic solution, the V(V) in the form of anions and PO3− 4 ,AsO4 ,and SiO3 were absorbed 2+ 2+ 2+ 2+, 2+ by the resin while cations, such as Fe , Ni , Cu , Ca and Mg were left in the initial − [72] solution. The absorption capacities for SO2− 4 and Cl types of resins differed slightly. Chelating resins have excellent performances in absorption of some transition metal cations, such as Cu2+ , Ni2+ , Co2+ , Zn2+ , and Au2+ , from aqueous solution. They can also be transformed into amphoteric resins by reacting with H+ or Na+ or other cations to form complexes that are used to extract anions. Li et al. investigated the absorption capacity and the selectivity of several chelating resins and found that DDAS, CW-2, and CUW perform best in V(V) extraction with more than 99% recovery in 3 h at pH 7.42 and ambient temperature, while the extraction of Mo with these three resins was less than 0.83%.[69] The mechanism of V(V) absorption with H-type chelating resins is the protonation of the N atom in the functional group, which makes the resin readily able to absorb anions with smaller radius from solution, like VO3- . The separation factor of V/Fe on the chelating resin C467 (supplied by Duolite, France) was about 3 in the pH range 0.42–1.43, and then a multistage operation resulted in a mutual separation of V and Fe.[73] In order to separate vanadium from other associated metal ions that form polyanions, like W, V(V) is reduced to V(IV) and then absorbed by a cation exchanger.[74,75] The associated impurity cations in solution, such as Al3+ , Fe2+ , K+ , Zn2+ , and Mg2+ , compete with VO2+ and consequently have detrimental effects on the vanadium absorption, while Ca2+ does not show any obvious effect on VO2+ absorption with chelating resin A.[65] Eluant. The elution conditions for different resins are shown in Table 11.


239

Table 11 Conditions for vanadium eluted from the exchange resins.


Exchanger

Elution reagent

Cl-D296

6 M HCl

717

2 M NaOH

SO4 2– D314

3 M NaOH

DDAS CUW CW-2 Chelating resin A

5% NH3 ·H2 O 5% NH3 ·H2 O 5% NH3 ·H2 O 5% HCl

Chelex 100

NH3 ·H2 O at pH 10 for V(V) HNO3 at pH 0.8 for V(IV)

Conditions Volume ratio of effluent/resin = 5, 60 min, room temperature Volume ratio of effluent/resin = 5, 5 min, 26 ◦ C Volume ratio of eluate/resin = 3, 60 min, ambient temperature 60 min 60 min 60 min Volume ratio of effluent/resin = 3, 60 min

Elution yield

Reference

98.68%

[62]

81.7%

[63]

quantitatively

[72]

99.61% 99.83% 99.78% 65.52%

[69]

quantitatively quantitatively

[66]

[69] [69] [65]

[66]

The V(V) loaded on the resin is easily desorbed by strongly acidic solution and converts the vanadium to the cationic speciesVO+ 2 . At the same time, the resin is regenerated by the eluant. It was reported that the peak value of vanadium concentration eluted from resin D296 appears when the ratio of effluent/resin is near 2. Then when the ratio is above 5, the concentration of vanadium in the effluent is less than 0.1 g/L, which can be considered as the terminal point of the stripping.[62] Somewhat differently, Li found that the stripping of V(V) from loaded D314 with NaOH solution is better than with HCl solution, and the maximum vanadium concentration in the eluate reaches as high as 160 g /L when 3 M NaOH solution is used.[72] When NaOH solution is used as an eluant for loaded resin 717, the desorption of V(V) increases with increase in concentration of the eluant.[63] For V(IV), only acid solutions can be used as eluants. The co-absorbed polyanions 6− HV10 O5− 28 with Mo7 O24 on a tertiary amine resin Amberlite IRA 94S can be selectively separated with 30 g/L H2 SO4 eluant, and a decreasing current intensity occurs after V(V) reduction to VO2+ .[76] V(V) and V(IV) can be separated by two-stage stripping from the chelating resin Chelex 100 at pH values of 10 and 0.8, respectively. Only V(V), but not V(IV), can be eluted from resin Chelex 100 by basic solution. This is due to the hydrolysis of V(V) in the form of VO+ 2 , which is dominant in acidic solution and is strongly absorbed , which is not absorbed at all by this resin.[66] by Chelex 100, to VO− 3 Chemical Precipitation for Purification in Alkaline Media As a separation and purification method, precipitation is widely used in extractive metallurgy field. Compared with alkali extraction of vanadium, the impurity content in acidic vanadium solutions is relatively higher owing to the weak selectivity of acid. Chen stated that in an acidic leaching solution within the pH range 2.5–3.2, it was necessary to increase the pH to 8–12 to remove impurities, including Mn, Ca, Mg, Al, Si, and so on. Then in the


240

J-H. ZHANG ET AL.

subsequent vanadium precipitation step, the pH value should be adjusted to 1.5–2.5 again, so a large amount of acid and alkali gets consumed.[77] Moreover, a large vanadium loss occurs when increasing the solution pH to remove the above impurities due to the strong adsorption of colloidal precipitates of Al(OH)3 , Fe(OH)3 , and so on. Up to now, there are few reports referring to the purification of acidic vanadium-containing solution by precipitation methods. In the course of alkali extraction of vanadium, metal ions such as Al, Fe, Ca, Mn, and Mg transform into deposit, while Si and P move into the leaching solution in the form of anions, which brings difficulty to the subsequent step of vanadium precipitation.[78] Thus, the aim of purification of alkali leaching solutions is therefore mainly to remove Si and P. Silicon Removal. If the feedstock containing a large amount of silicate minerals is adopted for alkaline extraction of vanadium, the obtained solution will contain a high content of silicon, and the silicon is bound to influence the quality of final product. Generally, magnesium salt, aluminum salt, and neutral method are employed in industry to remove the silicon from the alkaline solution. The solution produced from alkaline extraction of vanadium typically remains in the pH range 8–10. It was discovered that under the conditions of temperature of 90◦ C, standing time of 30 min, adding MgSO4 according to a mole ratio of Mg/Si of 1–1.5, the removal of silicon and loss of vanadium reached 98.8% and 15.1%, respectively.[79] Similarly, the principle of aluminum salt removing silicon is by adding a soluble aluminum salt which reacts with sodium and silicon in the solution to produce sodium aluminosilicate precipitate, while the aluminum salt simultaneously plays a role of flocculation as well. It was found that high pH and high temperature were better for silicon removal. Under the conditions of reacting temperature of 98 ◦ C, initial pH of 10.5, final pH of 9.0, and reacting time of 1 h, about 90% of silicon was removed, and loss of vanadium was less than 1.5%.[80] Colloidal silicon dioxide was formed through adjusting the pH of alkaline vanadiumbearing solution to 5.5–7.5. During this process, the loss of vanadium pentoxide could be controlled to less than 1%, and a by-product of white carbon black with 96.96% SiO2 was obtained.[81] However, this neutralization precipitation method usually takes a long time. A combination of flocculation and film separation was used to improve it, and the experimental results showed that the better silicon removal was obtained, and the operational time was shortened to 30 min.[82] Xiao et al. improved this method through combining neutral depositing with solvent extraction. After adjusting the pH of the solution to 9.5 to remove silicon partially, an extraction system of 18.5% N236 + 8.5% Octanol + 73% sulfonate kerosene was adopted to extract vanadium and the remaining silicon in solution, and then the loading organic phase was washed by Na2 CO3 solution at pH 10 to further remove silicon. Finally, the total silicon removal reached 99.6% in two stages. [83] Phosphorus Removal. Provided that converter vanadium slag was used as the feedstock for vanadium extraction with alkali, about 30% of the phosphorus in the slag would be converted to soluble phosphate. Phosphorus in the leaching solution can react with ammonium and vanadium to form complicated heteropolyacids, so it influences the extent of vanadium precipitation and the quality of final product. Hence it is necessary to remove phosphorus before vanadium precipitation. Adding magnesium, calcium, or aluminum salt to remove phosphorus is currently used in the vanadium pentoxide industry, which is characterized by high phosphorus removal and easy settling. The optimum pH range for magnesium salt removing phosphorus is 9.5–11, pH 8.0–9.0 for the calcium salt[84] , and pH 5–7 for the aluminum salt. When the pH is lower,



241

dihydric phosphate and hydrophosphate with larger solubility in comparison with phosphate are generated. Conversely, the concentrations of soluble Ca2+ , Mg2+ , and Al3+ decrease due to the hydrolysis, which then leads to a drop in phosphorus removal. The performance of magnesium salt was investigated to remove phosphorus from a sodium vanadate solution at pH 10 that had been treated by ion exchange. Under the conditions of reaction temperature of 45◦ C and time of 50 min, the P removal was over 96%, and vanadium loss was less than 6%. Then after ammonium precipitation and calcinations steps, the content of P in the final product decreased from 0.114% to 0.017%, and the grade of product increased from 99.12% to 99.92%.[85] Song et al. observed that high temperature and low concentration of vanadium were beneficial to calcium salt removing phosphorus; thus, taking into account of practical production conditions and the quality of final product, the temperature should be controlled between 80 and 95◦ C, and the concentration of vanadium should be 19–22 g/L.[84] Aluminum salts play two functions in the phosphorus removal process. On one hand, Al3+ precipitates phosphate; on the other hand, multinuclear complexes of aluminum attract ions with negative charge because they have highly positive charges and have large specific surface areas. Chengde Steel Company in China is employing aluminum salt to remove phosphorus in its new plant. The content of phosphorus in the purified solution decreases to lower than 8 mg/L from 20–30 mg/L in the initial solution.[86] DISCUSSIONS There are presently three main methods to purify vanadium-bearing solutions: solvent extraction, ion exchange, and chemical precipitation. It is essential to choose a suitable purification process according to the vanadium polymeric state, valence, concentration, associated impurities, and solution media. Ion exchange and solvent extraction are more suitable to treat solutions with lower concentration of vanadium from stone coal and spent catalyst to concentrate (the concentration of V in solution for precipitation step commonly higher than 20 g/L) and separate vanadium from the other impurities, while chemical precipitation is more favorable to treat the alkaline solution from vanadium slag. Presently, more than 75% of vanadium products originate from vanadium titanomagnetite (including vanadium slag and vanadium titano-magnetite with high content of vanadium). Most vanadium production manufactures in the world[87] (Chengde iron and steel group co., LTD and Panzhihua iron and steel group co., LTD in China, Vantra and Vametco in South Africa, and GfE in Germany, and so on) still employ the conventional process, sodium roasting followed by water leaching to treat this kind of vanadium resource. Since the impurities in the obtained alkaline medium are relatively simple, only including P, Si, and Cr, and a small amount of Al, and Cr has little effect on the vanadium precipitation step, chemical precipitation is commonly used to remove P and Si to satisfy the requirement for the next step. Final product with V2 O5 content higher than 98% can be produced in this process. The technology of chemical precipitation for purification of alkaline solution from vanadium slag is mature in industry, and it has advantages in lower cost, larger operational capacity, and easier operation in comparison with solvent extraction and ion exchange. An explorative test[78] has been conducted to purify an acidic solution from stone coal containing more Si, Al, Fe, P, and less Ca, Cu, Zn with the chemical precipitation method, and the results showed when the removal of Ca and Si were both 50%, removal of Fe, Cu, Zn, and P was 100%, removal of Al was 90%, and the


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loss of vanadium reached15.52%. All in all, presently, the chemical precipitation method is commonly employed by some larger-scale vanadium factories to treat the solutions with relatively simple impurities, especially with less Fe and Al. Using the chemical precipitation method to purify acidic solution is still under development. Stone coal is abundant in China, and it occupies more than 87% of Chinese vanadium reserves. It is widely distributed in southern provinces and Shanxi province and now is mainly utilized by some small and medium-sized enterprises in China. The vanadium mainly exists in illite and muscovite in the forms of V(III) and V(IV) as replacements of Al(III). After conventional sodium roasting and water leaching, impurities in the obtained solution are similar to those of vanadium slag, and the solution can also be purified using chemical precipitation. When acid leaching is used to treat the stone coal, many metallic impurities such as Fe, Al, Mg, K, Na, etc., are leached along with vanadium. Solvent extraction has been commercially used to purify vanadium solution from stone coal owing to its fast extraction, large operational capacity, high purity of product, and continuous and automatic operations. Acidic organophosphorus extractants like P204 (D2EHPA), P507, and Cyanex 272 in sulfonated kerosene are effective for V(IV) extraction from weakly acidic media over impurities such as Al(III), Fe(II), Si(IV), Mg(II), and so on, and neutral phosphorus extractants, such as TBP and Cyanex 923, are effective for vanadium recovery from solutions with higher acidity, especially the solution from direct acid leaching, where vanadium mainly exists as V(IV); however, they are not suitable for purifying solutions with high concentrations of Fe(III) due to the poor selectivity of organophosphorus extractants for V(IV) vs Fe(III). Furthermore, since the P204, P507, Cyanex 272, Cyanex 301, and Cyanex 923[23] are weakly efficient systems for extraction of V(V), V(V) in the solution from roasting-dilute acid leaching should be reduced to V(IV) before extraction, and then be oxidized to V(V) again before vanadium precipitation, which makes this purification step more complicated and expensive. Compared with organophosphorus extractants, amine extractants, like primary amine, tertiary amine, and quaternary ammonium salt, are more suitable to extract V(V) from solutions after roasting– acid leaching (water leaching) with initial pH above 2. Among these extractants, quaternary ammonium salts perform efficiently in V(V) extraction over a wide pH range, for example, pH 1.5–12 for Aliquat 336. The loaded organophosphorus phases are normally stripped with pure acid solutions, while loaded amines are stripped with alkaline solutions. The stripping process for the loaded high molecular mass quaternary ammonium salt phase with 1 M NaOH is difficult and slow. Kerosene is a promising diluent for both V(IV) and V(V) extraction from dilute mineral acid solutions with P204, P507, and Cyanex 272, and V(V) extraction with amines, while in the V(V) extraction from concentrated mineral acid solutions process with organophosphoric acids, aliphatic alcohols are more efficient than inert diluents. In the ion exchange process, V(V) from stone coal can be extracted and separated from associated impurities ions with strongly basic and weakly basic exchange resins and several chelating resins. D296 or some chelating resins are usually applied to separate vanadium from neutral solution, and 201 × 7 (commercial name in China is 717) resin is used to adsorb vanadium from alkaline solution. In acidic solution, the ion exchange D314 is used to recover vanadium. Strongly basic resins, like D296 and 717, have the maximum loading capacities at pH of 7–8. Since the V(V) exists in the form of VO+ 2 in solutions with pH < 2, it cannot be extracted through anion exchange mechanism by the above resins. Some chelating resins can be transformed into amphoteric resins to extract V(V) and V(IV) at lower pH. Exchange reactions usually take a long time to reach equilibrium. The



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V(V) loaded on strongly basic and weakly basic resins is easily desorbed by concentrated 3− acid solution and concentrated alkali solution and then converted into VO+ 2 and VO4 , respectively. Though anionic exchange resins have powerful selectivity of V(V) over Al3+ , 3− 2− Fe2+ , K+ , Mg2+ , Ca2+ cations, and so on, anions such as PO3− 4 , AsO4 , and SiO3 have detrimental effects on V(V) absorption. Hence, P and Si are usually further removed by chemical precipitation from alkaline vanadium-containing eluates.[88] Additionally, as the aqueous pH is higher than 1.5, V(V) is difficult to be separated by anion exchange resins from Fe(III), Si(IV), P(V), and Al(III), which make resin toxic. Because the content of Mo is much higher than V in spent catalysts (2–3 wt% Mo, 0.5–1.0 wt% V), the concentration of Mo is also higher than V (Mo:10–13 g/L, V:2–2.5 g/L) in the obtained solution, and then recovery of Mo is first considered while V is recovered as a by-product from the leaching solution of spent catalyst. To date these recycling efforts on vanadium in spent catalyst have not proved economically viable at any scale. Recently, due to stringent environmental restrictions on the release of harmful and toxic elements/gases during processing, hydrometallurgical processes are considered for extracting vanadium from spent catalysts. The main impurities of vanadium-bearing solution from spent solution is Mo(VI). In acidic solution, both the cationic species of V(IV) and Mo(VI) can be extracted by acidic organophosphorus extractants, such as P204, P507, and Cyanex 272, and the formation of an emulsion or a third phase in the stripping of Mo(VI) and V(IV or V) from the loaded solvent with aqueous ammonia solution usually occurs, so solvent extraction is not commonly used to separate the V from Mo. Although the scale of application of ion exchange in industry is limited it can be used to separate molybdenum and vanadium almost completely and to produce highly pure products. Mo(VI) and V(IV) are both absorbed on the anionic exchange resin, and they can be separated with different strippants, while some chelating resins can separate Mo(VI) from V(V) in the loading stage. In summary, although some techniques for purification of vanadium-bearing solution have been proposed and developed, the current processes of solvent extraction and ion exchange are complicated and expensive compared with the chemical precipitation method commonly used in conventional vanadium extraction process of sodium roastingwater leaching. It is acknowledged that the conventional sodium roasting process pollutes the environment seriously, and new directions are needed for clean vanadium extraction to address the problems of recycling waste water with high salinity and comprehensive utilization of the residue after leaching. Subsequently several new processes for vanadium extraction have been proposed. However, up to now, there are some problems existing in the purification of solutions from these new vanadium extraction technologies.

1. Conventionally, solvent extraction requires a large amount of highly pure organic extractants and diluents, and this makes the solvent extraction more expensive. It also cause environmental and safety problems due to the volatility, toxicity, and flammability of these organic solvents. Besides, chemical degradation of extractants and phase modifier occur to a certain extent in contact with V(V).[89] Solvent extraction is timeconsuming due to its complicated operational procedures and regenerations. In addition, a small amount of organic substance contained in the raffinate brings difficulty to complete recycling of wastewater. Ion exchange method has good separation efficiency, but it has problems in long exchange cycles, a large amount of waste water from stripping, and there is no suitable exchange system for commercial scale production.


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2. For a solution from sub-molten extraction of vanadium, to date, there is no suitable solvent extraction or ion exchange system to separate the V(V) from the other associated impurities directly. The alkalinity of solution has to be decreased to around pH 10 through neutralization or electrodialysis.[90] Consequently, purification procedure becomes complicated and acid-consuming. Similarly, the direct acid leaching for vanadium extraction from stone coal has received considerable attention, because it can obtain higher vanadium recovery; however, diffusion dialysis should be used to recover the H2 SO4 from the acidic vanadium solution before solvent extraction.[91] 3. If an acidic solution contains high contents of Fe, P, Si, and Al, like the solution from calcium roasting-dilute acid leaching of vanadium slag or acid/dilute acid leaching solution from stone coal, precipitation is likely to occur in the course of adjusting the solution pH to an optimum value for solvent extraction or ion exchange. This leads to emulsion phenomena, third phases,[41,92] and high loss of vanadium, and also makes the ion exchange resin toxic, so it is still necessary to do more research on removing P, Si, Fe, and Al from acidic media using chemical precipitation methods. 4. Stripping V(IV) from loaded organophosphorus extractants with concentrated acid solutions results in a high concentration of free acid (about 2 M) in the V(IV) stripped solution, and then lots of ammonium water would be consumed during the subsequent precipitation of ammonium vanadate. In the same way using concentrated NaOH or NaCl solutions to strip V(V) from loaded amines results in wastewater with high salinity. Though the wastewater with high salinity can be treated by membrane-thermal method,[93] it makes the production cost higher. Additionally, some useful components, such as Fe and Ti in the vanadium feedstock, follow the concept of zero emission in the new clean vanadium extraction process, so it is desirable to recover them from the residue after vanadium extraction, especially the residue from vanadium slag containing about 35% Fe2 O3 and 8.0% TiO2 . However, the raffinate and mother liquor with high salinity may bring lots of alkali metals (Na and K, for example) into residue in the whole water-recycle course, and then make Fe and Ti difficult to be recovered with pyrometallurgical methods due to the serious corrosion of refractory materials caused by Na or K.

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