Extraction of Vanadium from Ammonia Slag Under Near-Atmospheric Conditions

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Article Extraction of Vanadium from Ammonia Slag under Near-Atmospheric Conditions

Hirotaka Takahashi, Wataru Fujiwara, He Sun, Tsukasa Yoshida and Yuta Matsushima *

Chemistry and Chemical Engineering, Yamagata University, Yamagata 992-8510, Japan; [email protected] (H.T.); [email protected] (W.F.); [email protected] (H.S.); [email protected] (T.Y.) * Correspondence: [email protected]; Tel.: +81-238-26-3165

Received: 15 May 2018; Accepted: 31 May 2018; Published: 2 June 2018

Abstract: A process for extracting vanadium from ammonia slag is proposed in this work, taking advantage of the nature of V5+ vanadate ions condensing into a solid phase around a pH of 2. The slag is a mixture of oxides of Ca (36.0 mass %), Si (28.4 mass %), Al (9.3 mass %), Fe (7.1 mass %), V (6.9 mass %), S (3.9 mass %), Na (2.7 mass %), Ni (2.5 mass %), Mg (1.3 mass %), and other (K, P, and Ti) (1.9 mass %). The difficulty with extraction originates from the oxyanionic character of the vanadate ions, leading to the formation of vanadate salts with the concomitant cations. In contrast, a stepwise-pH-control process that is proposed in this work is effective for separating vanadium from the slag, as (1) the Si component in the slag is filtered off as the residue at the initial leaching step; (2) Fe, Al, and other cations are precipitated as the vanadate salts at pH 6, leaving Ca2+(aq) in the solution; (3) the vanadate component is transferred to the liquid phase by dissolving the precipitate in a NaOH(aq) solution of pH 13, leaving Fe, Ti, and Ni ions in the solid phase; and finally, (4) the pH of the solution is adjusted to 2. The vanadium component is solidified as sodium vanadate and V2O5. The maximum yield of vanadium from the slag is evaluated as 80.7%, obtaining for NaV6O15 and V2O5 with a purity of 97 mass %.

Keywords: vanadium extraction; pH control; ammonia slag; aqueous solution; sodium vanadate

1. Introduction Vanadium is an important element in the steel and chemical industries as it has been used to strengthen steels or as catalysts to produce sulfuric acid [1]. A high-strength alloy of aluminum, vanadium, and titanium is emerging in the aerospace field [1,2], and vanadium-redox-flow batteries have attracted much attention due to their potential applications for large-scale energy storages [3]. Since no high-vanadium-content ore is commercially available, vanadium has usually been produced from secondary sources and industrial wastes, such as titaniferrous magnetites, concentrates, slags, fly ashes, and petroleum residues [1,4,5]. The salt-roasting technique is a major process for extracting vanadium from those sources, by which a vanadium source is heated with a sodium salt to form sodium metavanadate. This material is typically converted into ammonium metavanadate in an aqueous solution by adding ammonium ions to collect it as the precipitate. This technique is quite effective for mass production of vanadium, especially in a large facility, but it is not suitable for small-to-middle-scale plants, because it requires kilns that can bear high-temperature treatment (e.g., 800–1230 ◦C) [2,6]. Energy consumption and toxic-gas emission during high-temperature treatments are also problematic [7,8]. Direct-leaching techniques have been investigated to collect vanadium under moderate conditions. The primary problem with direct leaching is the existence of concomitant cations, which makes the selective collection of vanadium difficult. The secondary sources and industrial wastes contain a range

Metals 2018, 8, 414; doi:10.3390/met8060414 www.mdpi.com/journal/metals Metals 2018, 8, 414 2 of 10 Metals 2018, 8, x FOR PEER REVIEW 2 of 10 ofcontain metals a suchrange as of Fe, metals Al, Ca, such and asMg Fe, andAl, Ca, they and are Mg usually and they extracted are usually in the extracted leachant, in together the leachant, with vanadium.together with The vanadium. effectiveness The effectiveness of using organic of using extractants organic [extractants9–15] and ion-exchanging[9–15] and ion-exchanging resins [16– 21resins] to collect[16–21] vanadiumto collect vanadium from the extractedfrom the extracted solutions solutions has been has examined. been examined. A JapaneseJapanese ammonia manufacture, for example, discharges a few thousand tons ofof ammoniaammonia slag everyevery year. The slagslag typicallytypically contains up to 5–105–10 massmass %% vanadiumvanadium as oxides,oxides, although it hashas not beenbeen used asas aa vanadiumvanadium sourcesource becausebecause thisthis contentcontent isis relativelyrelatively low.low. TheThe concentrationconcentration does not merit the costs thatthat areare involvedinvolved inin conventionalconventional vanadium-extractionvanadium-extraction processes processes,, and therefore, thethe slagslag isis wastedwasted byby solidifyingsolidifying itit inin cement.cement. This work proposesproposes aa techniquetechnique forfor collectingcollecting vanadiumvanadium from ammoniaammonia slag. The process was based on an aqueousaqueous solution, which is beneﬁcialbeneficial from the environmentalenvironmental and economiceconomic points of view.view. TheThe conceptconcept isis explainedexplained withwith FigureFigure1 1, which, which shows shows an an equilibrium equilibrium diagram diagram of of V V5+5+ vanadatevanadate ionsions in an aqueous aqueous solution solution [22]. [22]. The The key key points points are are asas follows: follows: (1) (1)several several types types of vanadate of vanadate ions ions are areformed formed in an in aqueous an aqueous solution solution depending depending on the on pH the and pH the and concentration; the concentration; (2) they (2) are they stably are soluble stably solubleover a overwide a pH wide region; pH region; and, and,(3) solid (3) solid-state-state V2O V5 2isO 5 condensedis condensed around around pH pH 2 2 at at relatively relatively higher concentrations. TheThe latter two points are contrasted with the nature of a typical cation being dissolved inin an acidic acidic solution solution and and precipitating precipitating as as hydroxide hydroxide in inthe the higher higher-pH-pH region. region. One One might might suppose suppose that thatthe concomitant the concomitant cations cations can be can separated be separated by raising by raising the pH the once pH onceto precipitate to precipitate hydroxides hydroxides and then and thenlowering lowering it to pH it to 2 pHto solidify 2 to solidify the vanadate the vanadate compounds compounds simply simply lead to lead separation to separation of vanadium of vanadium from froma solution a solution containing containing several several cations; cations; however, however, in inour our previou previouss paper paper [23], [23 ], we we reported reported that that the oxyanionic character of the vanadate ions leads to thethe formationformation ofof vanadatevanadate salts,salts, interferinginterfering withwith thethe selectiveselective collectioncollection ofof vanadiumvanadium throughthrough aa simplesimple pHpH change.change. This paper builds upon [[23]23] to establish a process for extracting vanadium from the slag by revealing the effects of the concomitant cations cruciallycrucially inﬂuencinginfluencing thethe formation formation behaviors behaviors of of vanadate vanadate salts salts and and the the role role of of the the each each step step in separatingin separating those those cations. cations.

5+ − Figure 1. Predominance diagramdiagram forfor VV5+–OH–OH− spspeciesecies at at 25 25 °C◦C[ [22].22]. The The solid solid lines lines represent represent conditions under whichwhich thethe predominantpredominant speciesspecies inin adjacentadjacent regionsregions containcontain equalequal amountsamounts ofof VV5+5+.. TheThe dasheddashed lines represent the solubility of V2O5 in terms of the V5+ 5+concentration. mV(V): moles of V5+ per5+ kg of lines represent the solubility of V2O5 in terms of the V concentration. mV(V): moles of V per kgwater. of water. (Reprinted (Reprinted with permission with permission from “The from Hydrolysis “The Hydrolysis of Cations”, of Copyright Cations”, (1976) Copyright John Wiley (1976) & Sons). John Wiley & Sons). 2. Experimental Section

Metals 2018, 8, 414 3 of 10

2. Experimental Section

2.1. Chemical Composition of the Ammonia Slag The ammonia slag was offered from a Japanese ammonia manufacturer in Yamaguchi Prefecture. The slag is a mixture of oxides (and probably sulﬁdes) of several metals. The chemical composition of the ash content excluding oxygen is shown in Table1. The concentrations were evaluated by X-ray ﬂuorescence spectroscopy (XRF) (EDXL 300, Rigaku, Tokyo, Japan), and they are shown as mass-percentages in elemental form. The examined slag contained Ca (36.0 mass %), Si (28.4 mass %), Al (9.3 mass %), Fe (7.1 mass %), V (6.9 mass %), S (3.9 mass %), Na (2.7 mass %), Ni (2.5 mass %), Mg (1.3 mass %), and other (K, P, and Ti) (1.9 mass %). Ca originated in CaCO3, which is an additive that acted as a melting-point depressant for coke ash. The other components were from the coke used to produce hydrogen for ammonia production via the water gas reaction. The composition of the slag was somewhat different from that used in our previous paper, and this difference was due to deviation of the usual nonuniformity of the composition of the industrial-waste mixture.

Table 1. Chemical composition of the ash content of ammonia slag.

Element Concentration (mass %) Ca 36.0 Si 28.4 Al 9.3 Fe 7.1 V 6.9 S 3.9 Na 2.7 Ni 2.5 Mg 1.3 Other (K, P, and Ti) 1.9

2.2. Extraction Procedure The optimized extraction process for collecting vanadium is shown in Scheme1. The ammonia slag was dried at 95 ◦C on a hot plate and ground in an alumina mortar. A given amount of the slag was weighed for the extraction experiment. Typically, 2.0 g of the ammonia slag was treated in ◦ 50 mL HNO3(aq) (1.3 mol/L) in an autoclave at 95 C for 48 h. After the insoluble residue was filtered 4+ 5+ off, 0.10 g of KClO3 was added to the filtrate to oxidize V to V [23], which led the solution to change the color from green to yellow. The solution was condensed on a hot plate at 95 ◦C to halve its volume. Then, the solution’s pH was adjusted to 6 using NaOH(aq) (15 mol/L) to form the precipitate (Precipitate A in Scheme1). After being separated from the solution by filtration, Precipitate A was treated with 30 mL NaOH(aq) (1.5 mol/L). After stirring for 1 h, the insoluble residue (Precipitate B in Scheme1) was filtered off from the solution. As discussed later in detail, the vanadium component was precipitated as Precipitate A at Step 2, and was transferred back to the liquid phase by treatment with NaOH(aq) at Step 3. Then, the pH of the filtrate was adjusted to 2 by adding HNO3(aq) (13 mol/L) to condense the vanadate ions into the solid phase (Precipitate C) in the solution during aging at 95 ◦C for 120 h. The chemical reagents used in this work are shown in Table2. The control experiments were carried out using the chemical reagents V2O5, Fe(NO3)3·9H2O, Al(NO3)3·9H2O, and CaCO3 (Table2) to examine the individual effects of iron, aluminum, and calcium 3+ 3+ 2+ ions coexisting. Fe , Al , or Ca were added to a 50 mL HNO3(aq) (mol/L) solution, dissolving 0.3 g of V2O5 at the molar ratio that was determined by XRF. The precipitating behaviors were investigated at several pHs, and the precipitates were heat-treated at 700 ◦C to improve the crystallinity to allow for the identification of the crystallographic phases by X-ray diffraction (XRD) (Rigaku, Tokyo, Japan). Metals 2018, 8, x FOR PEER REVIEW 3 of 10 composition of the ash content excluding oxygen is shown in Table 1. The concentrations were evaluated by X-ray fluorescence spectroscopy (XRF) (EDXL 300, Rigaku, Tokyo, Japan), and they are shown as mass-percentages in elemental form. The examined slag contained Ca (36.0 mass %), Si (28.4 mass %), Al (9.3 mass %), Fe (7.1 mass %), V (6.9 mass %), S (3.9 mass %), Na (2.7 mass %), Ni (2.5 mass %), Mg (1.3 mass %), and other (K, P, and Ti) (1.9 mass %). Ca originated in CaCO3, which is an additive that acted as a melting-point depressant for coke ash. The other components were from the coke used to produce hydrogen for ammonia production via the water gas reaction. The composition of the slag was somewhat different from that used in our previous paper, and this difference was due to deviation of the usual nonuniformity of the composition of the industrial-waste mixture.

Table 1. Chemical composition of the ash content of ammonia slag.

Element Concentration (mass %) Ca 36.0 Si 28.4 Al 9.3 Fe 7.1 V 6.9 S 3.9 Na 2.7 Ni 2.5 Mg 1.3 Other (K, P, and Ti) 1.9

2.2. Extraction Procedure The optimized extraction process for collecting vanadium is shown in Scheme 1. The ammonia slag was dried at 95 °C on a hot plate and ground in an alumina mortar. A given amount of the slag was weighed for the extraction experiment. Typically, 2.0 g of the ammonia slag was treated in 50 mL HNO3(aq) (1.3 mol/L) in an autoclave at 95 °C for 48 h. After the insoluble residue was filtered off, 0.10 g of KClO3 was added to the filtrate to oxidize V4+ to V5+ [23], which led the solution to change the color from green to yellow. The solution was condensed on a hot plate at 95 °C to halve its volume. Then, the solution’s pH was adjusted to 6 using NaOH(aq) (15 mol/L) to form the precipitate (Precipitate A in Scheme 1). After being separated from the solution by filtration, Precipitate A was treated with 30 mL NaOH(aq) (1.5 mol/L). After stirring for 1 h, the insoluble residue (Precipitate B in Scheme 1) was filtered off from the solution. As discussed later in detail, the vanadium component was precipitated as Precipitate A at Step 2, and was transferred back to the liquid phase by treatment with NaOH(aq) at Step 3. Then, the pH of the filtrate was adjusted to 2 by adding HNO3(aq) (13 mol/L) to condense the vanadate ions into the solid phase (Precipitate C) in the solution during aging at 95 °C for 120 h. Metals 2018, 8, 414 4 of 10 The chemical reagents used in this work are shown in Table 2.

Step 1 Step 2

HNO3 aq. KClO3 NaOH aq.

95 ℃ 48 h Filtration Condensation Filtration Washing Precipitate A Ammonia slag pH< 1 V(IV, V) V(V) pH 6

Step 3 Step 4

NaOH aq. HNO3 aq. Filtration 95 ℃ 120 h Filtration Precipitate A Washing Precipitate C pH 13 pH 13 pH 2 Precipitate B SchemeScheme 1 1.. ProcedureProcedure for for extracting extracting vanadium vanadium from from ammonia ammonia slag slag by by pH pH control. control.

Table 2. Chemical reagents used in this work.

Reagent Reagent Information 1 KClO3 99.5% (Kanto Chem. * ) NaOH 95% (Kanto Chem.) HNO3(aq) 60–61% (Kanto Chem.) H2SO4 96% (Kanto Chem.) (COOH)2·2H2O 99.5–100.2% (Kanto Chem.) H2O Deionized water (for control experiments)

V2O5 99.0% (Kanto Chem.) 2 Fe(NO3)3 9H2O 99.9% (Wako * ) Al(NO3)3·9H2O 98.0% (Kanto Chem.) CaCO3 99.99% (Kanto Chem.) *1 Kanto Chemical, Tokyo, Japan; *2 Wako Pure Chemical Industries, Osaka, Japan.

2.3. Instruments The concentrations of vanadate ions in the solution were determined using ultraviolet-visible-ray (UV-Vis) spectroscopy (PD-303, APEL, Saitama, Japan) with an absorbance curve calibrated with reference solutions prepared at several concentrations. (COOH)2·2H2O was used as a reduction agent from V5+ to V4+ to use the absorption of VO2+(aq) at 760 nm to determine the concentration [23]. XRD (MiniFlex600, Rigaku, Tokyo, Japan) with Cu Kα radiation was used to identify the compounds with the samples being heat-treated at elevated temperatures to improve the crystallinity beforehand because they did not necessarily exhibit appreciable diffraction peaks in their as-prepared conditions. The compositions were analyzed with energy-dispersive X-ray spectroscopy (EDS) (Link ISIS, Oxford Instruments, Oxon, UK) attached to scanning electronic microscopy (SEM) (JSM-T330, JEOL, Tokyo, Japan) at an accelerating voltage of 20 kV with a working distance of 20 mm, for which the powder samples were dried at 300 ◦C in air and were pressed into compacts with a small amount of polyvinyl alcohol (PVA) (Kuraray Poval PVA-217) as a binder. The carbon coating was vacuum-evaporated onto the pellet samples before SEM-EDS and a Co foil was used as a calibration standard.

3. Results and Discussion

3.1. Extraction Process from the Slag (Step 1 in Scheme1) Figure2 compares the EDS spectra of the starting slag and the precipitates that are formed at the subsequent extraction steps. EDS detected 12 elements in the slag (Figure2a) other than vanadium, which was consistent with the elemental analysis using XRF (Table1). The slag was mainly composed MetalsMetals2018 2018, 8,, 414 8, x FOR PEER REVIEW 5 of5 of10 10

the solution at this step was estimated to be above 97%, based on the vanadium content in the slag of Si, Ca, Al, V, and Fe. The carbon peak was attributed to the residual coke in the slag and to carbon and the VO2+(aq) concentration in the solution. coating for the SEM-EDS experiment. Metals 2018, 8, x FOR PEER REVIEW 5 of 10

V the solution at this step was estimated to be above 97%, based on the vanadium● Fe content in the slag ■ Ni 2+ and the VO (aq) concentration in the solution. ▼ Na ★ Mg C O V Na Al S (e) Precipitate C V ● Fe (d) Precipitate B■ Ni Fe ▼ Na C O Al ★ ● ■ Si Ti V Fe Ni Mg C ★ S V Ni O Al V Na Al S (e)(c) PrecipitatePrecipitateCA

Intensity C O S V Fe ● ■ ★ Si Ti V (d) PrecipitateFe NiB Ni CO Fe C Al Si Ni ● ■ ★ Si S Ti V V Fe Ni O Al S (b) Slag residue (c) Precipitate A

Intensity CCO S V ● ■ ★ Si Ti V Fe Fe Ni Ni (a) Slag C Si S AlSi Ca O ● ■ ▼ ★ P Ca Ti V V Fe Ni 0 O 1 2 S 3 4 5 (b)6 Slag residue7 8 C Energy (keV)

(a) Slag Si S Figure 2. Energy-dispersiveAl X-ray spectroscopyCa (EDS) spectra of the slag (a) and the precipitates Figure 2. Energy-dispersiveO ● ■ ▼ ★ X-rayP spectroscopyCa (EDSTi ) VspectraV of Fe the slag Ni (a) and the precipitates obtained in the subsequent extraction steps in Scheme1;( b) Residue after extraction at Step 1; obtained in the 0subsequent1 extraction2 steps3 in Scheme4 51. (b) Residue6 after7 extraction8 at Step 1, (c) (c)Precipitate A, A; ( (dd) )Precipitate Precipitate B, B; and and (e ()e Precipitate) Precipitate C (see C (see Scheme Scheme 1). 1). Energy (keV)

3 FigureFigure2b 2. shows Energy the-dispersive EDS spectrum X-ray spectroscopy of the residue (EDS0.04) spectra after the of the extraction slag (a) and by nitric the precipitates acid atStep 1. －5 The residueobtained was in composedthe subsequent of C,extraction O, Si, andsteps S.in TheScheme peaks 1. (yb of=) 0.046xResidue Ca,－ V,6.5 after and×10 extraction Fe were clearlyat Step 1, weakened, (c) indicatingPrecipitate that they A, (d were) Precipitate effectively B, and transferred (e) Precipitate into C (see0.03 the Sc ﬁltrateRheme = 1.00 1 from). the slag.

2 3 0.02 0.04 － 0.01 y = 0.046x－6.5×10 5

0.03 R = 1.00 Absorbance

Vanadium concentration(mol/L) Vanadium 0 1 2 0 0.2 0.4 0.6 0.8 0.02 Absorbance

0.01 Absorbance

0 concentration(mol/L) Vanadium 0 1 400 500 600 700 0 0.2800 0.4 9000.6 0.8 1000 Wavelength (nm) Absorbance

Figure 3. Ultraviolet-visible-ray (UV-Vis) spectrum of the extracted solution. The vanadate species in 2+ 2+ the solution were 0reduced to VO by (COOH)2∙2H2O so that the concentration of VO (aq) could be determined by the400 absorption500 at 760 nm.600 The inset700 shows800 the calibration900 curve1000 drawn with the Wavelength (nm) prepared solutions at several concentrations. FigureFigure 3. 3.Ultraviolet-visible-ray Ultraviolet-visible-ray (UV-Vis) (UV-Vis) spectrumspectrum of the extracted solution. solution. The The vanadate vanadate species species in in 3.2. Precipitation of Aluminum Vanadate and Iron Vanadate (Step 2) 2+2+ 2+ 2+ thethe solution solution were were reduced reduced to to VO VO by (COOH) (COOH)2∙2H2·2H2O2 Oso sothat that the the concentration concentration of VO of VO(aq) could(aq) could be bedeterminedThe determined EDS spectrum by by the the absorption absorptionof Precipitate at at 760 760 A nm. is nm. shown The The inset insetin Figure shows shows 2 thec, the where calibration calibration Precipitate curve curve drawn A drawn was with collected with the the at Stepprepared prepared2 from solutions thesolutions solution at at several several at pH concentrations. concentrations. 6. Figure 2c indicates that Precipitate A was mainly composed of aluminum, vanadium, and iron. The crystallographic phases of Precipitate A were identified as Fe3O4 3.2. Precipitation of Aluminum Vanadate and Iron Vanadate (Step 2) (ICDDFigure #263 -shows1136) and the FeVO absorption4 (ICDD spectrum #38-1372) of (Figure the extracted 4). Altho solutionugh vanadate at Step ions 1 after are thestably addition soluble of 5+ 2+ (COOH) The2·2H EDS2O spectrum to reduce of V Precipitatevanadate A ionsis shown to VO in Figure(aq), which 2c, where exhibits Precipitate a broad A absorption was collected peak at at Step 2 from the solution at pH 6. Figure 2c indicates that Precipitate A was mainly composed of aluminum, vanadium, and iron. The crystallographic phases of Precipitate A were identified as Fe3O4 (ICDD #26-1136) and FeVO4 (ICDD #38-1372) (Figure 4). Although vanadate ions are stably soluble

Metals 2018, 8, 414 6 of 10 around 760 nm. The inset shows the calibration curve, indicating an absorbance proportional to the VO2+(aq) concentration with a correlation coefﬁcient of R = 1.00. The extraction yield from the slag to the solution at this step was estimated to be above 97%, based on the vanadium content in the slag and the VO2+(aq) concentration in the solution.

3.2. Precipitation of Aluminum Vanadate and Iron Vanadate (Step 2) The EDS spectrum of Precipitate A is shown in Figure2c, where Precipitate A was collected at Step 2 from the solution at pH 6. Figure2c indicates that Precipitate A was mainly composed of Metals 2018, 8, x FOR PEER REVIEW 6 of 10 aluminum, vanadium, and iron. The crystallographic phases of Precipitate A were identiﬁed as Fe3O4 (ICDD #26-1136) and FeVO4 (ICDD #38-1372) (Figure4). Although vanadate ions are stably soluble in this pH region (Figure 1), the vanadium component precipitated as vanadate salts with Fe and Al in this pH region (Figure1), the vanadium component precipitated as vanadate salts with Fe and (FeVO4 and AlVO4) at this step. Since an Al-related crystallographic phase was not observed in Figure Al (FeVO4 and AlVO4) at this step. Since an Al-related crystallographic phase was not observed in 4, the Al component was considered to exist as amorphous or to be incorporated into FeVO4. Fe3O4 Figure4, the Al component was considered to exist as amorphous or to be incorporated into FeVO 4. originated from the conversion of iron hydroxide into Fe3O4 during heat treatment for XRD. The Fe3O4 originated from the conversion of iron hydroxide into Fe3O4 during heat treatment for XRD. absence of a Ca peak at 3.69 keV in EDS is characteristic of Precipitate A, indicating that Ca2+(aq) was The absence of a Ca peak at 3.69 keV in EDS is characteristic of Precipitate A, indicating that Ca2+(aq) left in the solution. The solution after the filtration was colorless.

was left in the solution. The solution after the ﬁltration was colorless. Intensity Intensity

Fe3O4

FeVO4

5 15 25 35 45 55 65 75 85 2θ (°)

FigureFigure 4. 4.X-ray X-raydiffraction diffraction(XRD) (XRD)pattern pattern ofof Precipitate Precipitate A A after after the the heat heat treatment treatment at at 700 700◦ °C.C. TheThe lineline diagram below the pattern indicates the peak positions of Fe3O4 (ICDD #26-1136) and FeVO4 (ICDD diagram below the pattern indicates the peak positions of Fe3O4 (ICDD #26-1136) and FeVO4 (ICDD#38-1372). #38-1372).

3.3.3.3. SeparationSeparation ofof VanadiumVanadium fromfrom Fe Fe and and Al Al (Step (Step 3) 3) FigureFigure2 d2d presents presents the the EDS EDS spectrum spectrum of of Precipitate Precipitate B, B, which which was was the the residue residue at at pH pH 13 13 after after PrecipitatePrecipitate AA waswas dissolveddissolved inin NaOH(aq).NaOH(aq). TheThe peakpeak intensitiesintensities ofof Fe,Fe, Ni,Ni, andand TiTi increasedincreased afterafter thisthis stepstep relativerelative toto PrecipitatePrecipitate A A (Figure (Figure2 d).2d). The The vanadate vanadate compound compound was was preferentially preferentially dissolved dissolved into into thethe solution solution from from Precipitate Precipitate A, A, leaving leaving the the Fe, Ni,Fe, andNi, and Ti components Ti components in Precipitate in Precipitate B. The B. XRD The peaks XRD peaks of Precipitate B after heat treatment◦ at 700 °C (Figure 5) were attributed to Fe2O3 (ICDD #33- of Precipitate B after heat treatment at 700 C (Figure5) were attributed to Fe 2O3 (ICDD #33-664) and 664) and Fe3O4 (ICDD #26-1136), and the main component of the residue (Precipitate B) was assumed Fe3O4 (ICDD #26-1136), and the main component of the residue (Precipitate B) was assumed to be iron hydroxideto be iron withhydroxide impurities with impurities of Ni and Ti.of TheNi and Al componentTi. The Al component was not effectively was not effectively separated fromseparated V at thisfrom step V at as this the peakstep as intensity the peak of intensity Al decreased of Al from decreased Figure from2c to Figure 22cd. to The 2d. peaks The peaks of V were of V stillwere still recognized in EDS (Figure 2d), and vanadium partly remained in Precipitate B.

recognized in EDS (Figure2d), and vanadium partly remained in Precipitate B. Intensity

Fe3O4

Fe2O3

5 15 25 35 45 55 65 75 85 2θ (°)

Metals 2018, 8, x FOR PEER REVIEW 6 of 10

in this pH region (Figure 1), the vanadium component precipitated as vanadate salts with Fe and Al (FeVO4 and AlVO4) at this step. Since an Al-related crystallographic phase was not observed in Figure 4, the Al component was considered to exist as amorphous or to be incorporated into FeVO4. Fe3O4 originated from the conversion of iron hydroxide into Fe3O4 during heat treatment for XRD. The absence of a Ca peak at 3.69 keV in EDS is characteristic of Precipitate A, indicating that Ca2+(aq) was

left in the solution. The solution after the filtration was colorless. Intensity Intensity

Fe3O4

FeVO4

5 15 25 35 45 55 65 75 85 2θ (°) Figure 4. X-ray diffraction (XRD) pattern of Precipitate A after the heat treatment at 700 °C. The line diagram below the pattern indicates the peak positions of Fe3O4 (ICDD #26-1136) and FeVO4 (ICDD #38-1372).

3.3. Separation of Vanadium from Fe and Al (Step 3) Figure 2d presents the EDS spectrum of Precipitate B, which was the residue at pH 13 after Precipitate A was dissolved in NaOH(aq). The peak intensities of Fe, Ni, and Ti increased after this step relative to Precipitate A (Figure 2d). The vanadate compound was preferentially dissolved into the solution from Precipitate A, leaving the Fe, Ni, and Ti components in Precipitate B. The XRD peaks of Precipitate B after heat treatment at 700 °C (Figure 5) were attributed to Fe2O3 (ICDD #33- 664) and Fe3O4 (ICDD #26-1136), and the main component of the residue (Precipitate B) was assumed to be iron hydroxide with impurities of Ni and Ti. The Al component was not effectively separated from V at this step as the peak intensity of Al decreased from Figure 2c to 2d. The peaks of V were Metals 2018, 8, 414 7 of 10

still recognized in EDS (Figure 2d), and vanadium partly remained in Precipitate B. Intensity

Fe3O4

Fe2O3

5 15 25 35 45 55 65 75 85 Metals 2018, 8, x FOR PEER REVIEW 7 of 10 2θ (°)

FigureFigure 5. 5.XRD XRD pattern pattern of of Precipitate Precipitate B B after after heat heat treatment treatment at at 700 700◦ C°C for for 1 1 h. h.

3.4.3.4. Precipitation Precipitation of of Vanadate Vanadate Compounds Compounds (Step (Step 4) 4)

TheThe pH pH of of the the ﬁltrated filtrated solution solution at at Step Step 3 3 was was adjusted adjusted to to 2 2 with with HNO HNO3(aq).3(aq). A A reddish-brown reddish-brown precipitateprecipitate (Precipitate (Precipitate C) C) was was obtained obtained during during aging aging at at 95 95◦ °C.C. The The EDS EDS (Figure (Figure2 e)2e revealed) revealed that that the the mainmain component component of of Precipitate Precipitate C C was was vanadium vanadium with with minor minor impurities impurities of of Na, Na, Al, Al, and and S. S. QuantitativeQuantitative analysisanalysis by by EDS EDS gave gave a molar a molar ratio ratio of V:Na of V:Na= 7.7:1, = which 7.7:1, iswhich consistent is consistent with the observationwith the observation of NaV6O 15 of

(ICDDNaV6O #24-1155)15 (ICDD and#24-11 V255O5) (ICDDand V2O #41-1426)5 (ICDD in#41 XRD-1426 for) in Precipitate XRD for Precipitate C (Figure6 C). (Figure 6). Intensity

V2O5

NaV6O15

5 15 25 35 45 55 65 75 85 2θ (°)

FigureFigure 6. 6.XRD XRD pattern pattern of of Precipitate Precipitate C C after after heat heat treatment treatment at at 500 500◦ C°C for for 1 1 h. h.

Sodium vanadates are used together with vanadium oxides as raw materials in vanadium Sodium vanadates are used together with vanadium oxides as raw materials in vanadium metallurgy [2], and therefore, Precipitate C can be regarded as a goal of our extraction process. The metallurgy [2], and therefore, Precipitate C can be regarded as a goal of our extraction process. yield of vanadium from the slag was evaluated at 80.7% based on the vanadium content in the slag The yield of vanadium from the slag was evaluated at 80.7% based on the vanadium content in and the mass of obtained Precipitate C as a mixture of NaV6O15 and V2O5 at molar ratio V:Na = 7.7:1. the slag and the mass of obtained Precipitate C as a mixture of NaV6O15 and V2O5 at molar ratio The estimated content of the vanadate compounds (NaV6O15 and V2O5) was 97 mass % in Precipitate V:Na = 7.7:1. The estimated content of the vanadate compounds (NaV O and V O ) was 97 mass % C. 6 15 2 5 in Precipitate C. The Al component that was dissolved in the NaOH(aq) solution together with V at Step 3 was The Al component that was dissolved in the NaOH(aq) solution together with V at Step 3 was separated at Step 4 by leaving it as Al3+(aq) in the solution at pH 2. Contamination by Al was separated at Step 4 by leaving it as Al3+(aq) in the solution at pH 2. Contamination by Al was recognized in Precipitate C in Figure 2e and lowering the pH below 2 could suppress it to some degree, but reduce the V yield instead.

3.5. Control Experiments with Chemical Reagents and Mechanisms for Vanadium Extraction Several cationic species are contained in the slag, as shown in Table 1, and this makes it difficult to specify reactions that occur at Steps 2, 3, and 4. Then, the control experiments were carried out using chemical reagents for each pair of V and Fe, Al, or Ca (Table 3), where a chemical reagent of V2O5 was dissolved in HNO3(aq) and Fe(NO3)3∙9H2O, Al(NO3)3∙9H2O, or CaCO3 was added to the solution. The solution’s pH was initially lower than 1. This was then adjusted to 2, 6, or 13 with NaOH(aq) to collect the precipitate at each pH. The precipitates were heat-treated at 700 °C for 1 h in order to improve the crystallinity prior to XRD. As shown in Figure 1, vanadate ions themselves are stably soluble over a wide-pH region, except around pH 2. On the other hand, the coexistence of Fe3+(aq) led to the insolubility of iron vanadate, with FeVO4 being collected at pH 2. The precipitate at pH 6 was composed of FeVO4 and Fe2O3, indicating that Fe3+ precipitated as vanadate and hydroxide until pH 6. On the other hand, the main phase at pH 13 became Fe2O3, and therefore, the vanadium component was dissolved back into the liquid phase from FeVO4 in the higher-pH region.

Metals 2018, 8, 414 8 of 10 recognized in Precipitate C in Figure2e and lowering the pH below 2 could suppress it to some degree, but reduce the V yield instead.

Table 3. Crystallographic phases identiﬁed in the precipitates at pH 2, 6, and 13 with combinations of vanadate ions and (a) Fe3+, (b) Al3+, or (c) Ca2+.

Product Phase Coexisting Cation pH 2 pH 6 pH 13 3+ (a) Fe FeVO4 FeVO4 & Fe2O3 Fe2O3 3+ (b) Al V2O5 & NaV6O15 AlVO4 No precipitation 2+ (c) Ca V2O5 & NaV6O15 No precipitation Ca10V6O25 & CaO

As shown in Figure1, vanadate ions themselves are stably soluble over a wide-pH region, except around pH 2. On the other hand, the coexistence of Fe3+(aq) led to the insolubility of iron vanadate, with FeVO4 being collected at pH 2. The precipitate at pH 6 was composed of FeVO4 and Fe2O3, indicating that Fe3+ precipitated as vanadate and hydroxide until pH 6. On the other hand, the main phase at pH 13 became Fe2O3, and therefore, the vanadium component was dissolved back into the liquid phase from FeVO4 in the higher-pH region. 3+ Al (aq) did not form a vanadate salt at lower pH as the precipitate at pH 2 was V2O5 and NaV6O15, with sodium coming from NaOH to adjust the pH. The AlVO4 that was recognized in the product at pH 6 was dissolved again at higher pH with no precipitate at pH 13. Ca2+(aq) did not interact with the vanadate ions at a pH of 6 or lower. Ca10V6O25 and CaO were recognized in the precipitate at pH 13, where CaO resulted from Ca(OH)2 during the heat treatment for XRD. The above results reveal rather complicated correlations between the vanadate ions and Fe3+, Na+, 3+ 2+ Al , and Ca , depending on the solution’s pH. FeVO4 is more favorably formed in the lower-pH + region around 2, although Na also tends to form the vanadate salt NaV6O15. AlVO4 precipitates near the neutral-pH region when the solution’s pH is raised. These vanadates (FeVO4, NaV6O15, and AlVO4) are dissolved in the NaOH(aq) solution, whereas calcium vanadate stably exists in the higher-pH region even up to pH 13. Scheme2 summarizes the vanadium-condensation procedure from the slag through Scheme1. Si is left in the residue during the extraction at Step 1. Precipitate A at Step 2 is composed of iron and aluminum vanadates with minor metals (Ni, Ti, etc.) and Ca2+ are separated at this step, being left in the solution. The vanadate compounds are dissolved in the solution by raising its pH, leaving Fe and other metals, such as Ni and Ti in the solid phase, as Precipitate B. The vanadate component is condensed from the solution into the solid phase (NaV6O15,V2O5, etc.) during aging at pH 2, leaving Al3+(aq) in the solution. Metals 2018, 8, x FOR PEER REVIEW 8 of 10

Al3+(aq) did not form a vanadate salt at lower pH as the precipitate at pH 2 was V2O5 and NaV6O15, with sodium coming from NaOH to adjust the pH. The AlVO4 that was recognized in the product at pH 6 was dissolved again at higher pH with no precipitate at pH 13. Ca2+(aq) did not interact with the vanadate ions at a pH of 6 or lower. Ca10V6O25 and CaO were recognized in the precipitate at pH 13, where CaO resulted from Ca(OH)2 during the heat treatment for XRD. The above results reveal rather complicated correlations between the vanadate ions and Fe3+, Na+, Al3+, and Ca2+, depending on the solution’s pH. FeVO4 is more favorably formed in the lower- pH region around 2, although Na+ also tends to form the vanadate salt NaV6O15. AlVO4 precipitates near the neutral-pH region when the solution’s pH is raised. These vanadates (FeVO4, NaV6O15, and AlVO4) are dissolved in the NaOH(aq) solution, whereas calcium vanadate stably exists in the higher- pH region even up to pH 13. Scheme 2 summarizes the vanadium-condensation procedure from the slag through Scheme 1. Si is left in the residue during the extraction at Step 1. Precipitate A at Step 2 is composed of iron and aluminum vanadates with minor metals (Ni, Ti, etc.) and Ca2+ are separated at this step, being left in the solution. The vanadate compounds are dissolved in the solution by raising its pH, leaving Fe and other metals, such as Ni and Ti in the solid phase, as Precipitate B. The vanadate component is condensed from the solution into the solid phase (NaV6O15, V2O5, etc.) during aging at pH 2, leaving Al3+(aq) in the solution.

Table 3. Crystallographic phases identified in the precipitates at pH 2, 6, and 13 with combinations of vanadate ions and (a) Fe3+, (b) Al3+, or (c) Ca2+.

Product Phase Coexisting Cation pH 2 pH 6 pH 13 (a) Fe3+ FeVO4 FeVO4 & Fe2O3 Fe2O3 (b) Al3+ V2O5 & NaV6O15 AlVO4 No precipitation Metals 2018, 8, 414 9 of 10 (c) Ca2+ V2O5 & NaV6O15 No precipitation Ca10V6O25 & CaO

Step 1 Step 2 pH < 1 pH 6 C, Si, Ca, Al, Ca, Al, Fe, V, Ni Fe, V, Ni Ca (Extracted solution) (Slag) Step 3 Step 4 pH 13 pH 2 Al, Fe, V, Ni V, Al Al C, Si (Precipitate A) (Residue) Solution (Liquid phase) Precipitate Fe, Ni V (Solid phase) (Precipitate B) (Precipitate C)

Scheme 2. Major streams of elemental transfers of cations in the slag during the extraction process of Scheme 2. Major streams of elemental transfers of cations in the slag during the extraction process of thisthis work.work. TheThe contaminationcontamination byby impuritiesimpurities isis ignored.ignored.

4. Conclusions 4. Conclusions The extraction of vanadium from the ammonia slag was demonstrated by pH control based on The extraction of vanadium from the ammonia slag was demonstrated by pH control based on the aqueous solution near ambient conditions (≤95 °C ). The effects of the concomitant cations of Fe3+, the aqueous solution near ambient conditions (≤95 ◦C). The effects of the concomitant cations of Fe3+, Al3+, and Ca2+ were investigated on the precipitation behaviors of the vanadate salts were Al3+, and Ca2+ were investigated on the precipitation behaviors of the vanadate salts were investigated. investigated. Fe3+ and Al3+ brought the vanadate salts up to a pH of 6. The vanadate components in Fe3+ and Al3+ brought the vanadate salts up to a pH of 6. The vanadate components in these salts these salts were dissolved in the NaOH(aq) solution until a pH of 13, which left Fe3+ in the insoluble were dissolved in the NaOH(aq) solution until a pH of 13, which left Fe3+ in the insoluble residue. residue. The vanadate component was separated from the Al component at pH 2 by forming solid- The vanadate component was separated from the Al component at pH 2 by forming solid-state state NaV6O15 and V2O5 during aging. The maximum yield of vanadium was estimated to be 80.7%, NaV6O15 and V2O5 during aging. The maximum yield of vanadium was estimated to be 80.7%, obtaining for NaV6O15 and V2O5 with a purity of 97 mass %. obtaining for NaV6O15 and V2O5 with a purity of 97 mass %.

AuthorAuthor Contributions:Contributions:Conceptualization, Conceptualization, H.T. H.T and. and Y.M.; Y.M. Funding; Funding acquisition, acquisition, H.T. H and.T. and Y.M.; Y Investigation,.M.; Investigation, H.T., W.F.H.T. and, W.F H.S.;. and Methodology, H.S.; Methodology, H.T. and H Y.M.;.T. and Project Y.M. administration,; Project administration, Y.M.; Supervision, Y.M.; Supervision, Y.M.; Validation, Y.M.; Validation, H.T., H.S., T.Y. and Y.M.; Visualization, H.T.; Writing—original draft, H.T.; Writing—review & editing, H.T., T.Y. and Y.M. Funding: This work is financially supported in part by Sasakawa Scientific Research Grant from The Japan Science Society (28-216). Acknowledgments: The authors thank A. Sasaki and S. Mizunuma in Yamagata University for the XRF measurements. Poval PVA-217 used as a binder was offered by Kuraray co., ltd. The authors would like to thank Enago (www.enago.jp) for the English language review. The authors thank John Wiley & Sons Ltd. for permission to reuse Figure1, which originally appears as Figure 10.7 in “The Hydrolysis of Cations (Charles Frederick Baes, Robert Eugene Messmer) (1976)”. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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