R ESEARCH ARTICLE doi: 10.2306/scienceasia1513-1874.2019.45.043 ScienceAsia 45 (2019): 43–49 Effects of different alkalis on the behaviour of vanadium loss in the pretreatment of vanadium-bearing acid leaching solution Qiaoqiao Zheng a,b , Yimin Zhang a,b,c,d,* , Shenxu Bao a,b , Jing Huang c,d , Guobin Zhang a,b a School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China b Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan 430070, China c State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan University of Science and Technology, Wuhan 430081, China d Hubei Collaborative Innovation Centre for High Efficient Utilization of Vanadium Resources, Wuhan University of Science and Technology, Wuhan 430081, China * Corresponding author, e-mail: [email protected]Received 8 Jun 2018 Accepted 3 Feb 2019 ABSTRACT: This study examined the effect of different pretreating agents on vanadium loss from vanadium-bearing shale. Acid leaching solutions with added Ca(OH) 2 , CaCO 3 , NaOH, Na 2 CO 3 , and ammonia solution were evaluated. The pH of the acid leaching solution was adjusted to 2.0 to reduce vanadium loss and allow efficient removal of impurities, providing a high vanadium extraction efficiency. Ca(OH) 2 was the most effective neutralizer and its use resulted in a vanadium loss rate was as low as 4%. SEM-EDS analysis indicates that a major cause of vanadium loss is entrapment and absorption by precipitates. The low vanadium loss rate using Ca(OH) 2 as a neutralizer appears to be due to the smooth and flat surface of the precipitate, which limits vanadium entrainment. When the pH was adjusted with CaCO 3 , the crystal structure of the precipitate was incomplete. In addition, CaSO 4 and iron phosphate particles interact, promoting entrainment and absorption of vanadium. Similarly, flocculent iron phosphate was generated when the pH was adjusted with NaOH, Na 2 CO 3 , or ammonia, resulting in a crystal with a rough surface that easily entrained vanadium. KEYWORDS: vanadium, acid leaching solution, pH adjustment, alkali INTRODUCTION Vanadium is a vital rare element that is widely used in high-tech fields, including the production of redox batteries and aerospace 1 , due to its spe- cial physical and chemical properties 2, 3 . In China, the major source of vanadium is vanadium-bearing shale 4–6 . High salt roasting-water leaching, blank roasting-acid leaching, blank roasting-alkali leach- ing, and direct acid leaching 7–9 have been used to recover vanadium from vanadium-bearing shale. Among these processes, roasting-H 2 SO 4 leaching has received considerable attention for extraction of vanadium-bearing shale due to its highest recovery rate 10 . However, impurities, such as Fe, P, and other elements, can be leached along with vanadium in the acid leaching process. This results in a com- plex, low pH mixture with the vanadium-containing extract contaminated with high concentrations of impurities. These conditions prevent efficient vana- dium separation and concentration 11–13 . At present, the main methods for vanadium separation and concentration are solvent extrac- tion 14–16 and ion exchange 17, 18 . The solvent ex- traction method, with various extracting reagents, has gradually been the primary methods due to its high efficiency with no requirement for com- plex equipment 19 . Among the reagents used for vanadium extraction, the acid extractant D2EHPA (Bis(2-ethylhexyl) phosphate) provides advantages of acid system adaptability and easy stripping 20–22 . The appropriate pH range for solvent extraction of vanadium is 1.5–2.5; however, the acidity of the acid leaching solution is often excessive, requiring pretreated to allow subsequent steps in the extrac- tion process 23, 24 . Some methods for the disposal of acid leaching solution have also been developed, these include ion exchange, solvent extraction, and www.scienceasia.org
Effects of different alkalis on the behaviour ofvanadium loss in the pretreatment ofvanadium-bearing acid leaching solutionQiaoqiao Zhenga,b, Yimin Zhanga,b,c,d,∗, Shenxu Baoa,b, Jing Huangc,d, Guobin Zhanga,b
a School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070,China
b Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan 430070, Chinac State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization
and Pollution Control, Wuhan University of Science and Technology, Wuhan 430081, Chinad Hubei Collaborative Innovation Centre for High Efficient Utilization of Vanadium Resources,
Wuhan University of Science and Technology, Wuhan 430081, China
∗Corresponding author, e-mail: [email protected] 8 Jun 2018Accepted 3 Feb 2019
ABSTRACT: This study examined the effect of different pretreating agents on vanadium loss from vanadium-bearingshale. Acid leaching solutions with added Ca(OH)2, CaCO3, NaOH, Na2CO3, and ammonia solution were evaluated.The pH of the acid leaching solution was adjusted to 2.0 to reduce vanadium loss and allow efficient removal ofimpurities, providing a high vanadium extraction efficiency. Ca(OH)2 was the most effective neutralizer and its useresulted in a vanadium loss rate was as low as 4%. SEM-EDS analysis indicates that a major cause of vanadium loss isentrapment and absorption by precipitates. The low vanadium loss rate using Ca(OH)2 as a neutralizer appears to bedue to the smooth and flat surface of the precipitate, which limits vanadium entrainment. When the pH was adjustedwith CaCO3, the crystal structure of the precipitate was incomplete. In addition, CaSO4 and iron phosphate particlesinteract, promoting entrainment and absorption of vanadium. Similarly, flocculent iron phosphate was generated whenthe pH was adjusted with NaOH, Na2CO3, or ammonia, resulting in a crystal with a rough surface that easily entrainedvanadium.
Vanadium is a vital rare element that is widelyused in high-tech fields, including the productionof redox batteries and aerospace1, due to its spe-cial physical and chemical properties2, 3. In China,the major source of vanadium is vanadium-bearingshale4–6. High salt roasting-water leaching, blankroasting-acid leaching, blank roasting-alkali leach-ing, and direct acid leaching7–9 have been usedto recover vanadium from vanadium-bearing shale.Among these processes, roasting-H2SO4 leachinghas received considerable attention for extraction ofvanadium-bearing shale due to its highest recoveryrate10. However, impurities, such as Fe, P, and otherelements, can be leached along with vanadium inthe acid leaching process. This results in a com-plex, low pH mixture with the vanadium-containingextract contaminated with high concentrations of
impurities. These conditions prevent efficient vana-dium separation and concentration11–13.
At present, the main methods for vanadiumseparation and concentration are solvent extrac-tion14–16 and ion exchange17, 18. The solvent ex-traction method, with various extracting reagents,has gradually been the primary methods due toits high efficiency with no requirement for com-plex equipment19. Among the reagents used forvanadium extraction, the acid extractant D2EHPA(Bis(2-ethylhexyl) phosphate) provides advantagesof acid system adaptability and easy stripping20–22.The appropriate pH range for solvent extraction ofvanadium is 1.5–2.5; however, the acidity of theacid leaching solution is often excessive, requiringpretreated to allow subsequent steps in the extrac-tion process23, 24. Some methods for the disposalof acid leaching solution have also been developed,these include ion exchange, solvent extraction, and
Table 1 Chemical composition of acid leaching solutions(g/l).
Element V Fe Na Ca Al P S Si
Conc. 3.10 1.66 1.91 1.11 3.81 1.25 29.1 1.48
alkali neutralization25–27. Although ion exchangeand solvent extraction can purify the acid leachingsolution, the pH of treated solution does not meetthe requirements for the next step in the extractionprocess28. Hence neutralization with alkali is apopular and necessary process to adjust the pHof acid leaching solution before subsequent extrac-tion29, 30. Currently, NaOH is used to adjust pH;however, this results in considerable vanadium lossand introduces many impurities.
In this study, Ca(OH)2, CaCO3, NaOH, Na2CO3,and ammonia were used to adjust the pH ofvanadium-bearing shale acid leaching solution. Themechanisms of vanadium loss and the introductionof impurities during precipitation under different al-kalis condition were investigated by XRD and SEM-EDS. Furthermore, the vanadium loss rate, removalrate of impurity ions, and vanadium extraction ef-ficiency were evaluated to select the appropriatealkali and pH for the pretreatment process.
MATERIALS AND METHODS
Vanadium-bearing shale was supplied by Ping-fanMining Co. Ltd., Zaoyang, China. The acid leachingsolution from vanadium-bearing shale was preparedby heating vanadium-bearing shale at 850 °C for1.5 h followed by leaching with 5% (wt) CaF2and 15% (v/v) H2SO4 solution (L/S= 1.5:1) at98 °C for 2 h. The chemical composition of theacid leaching solution is shown in Table 1 and theinitial pH was 0.33. Na2SO3 was used to reducethe vanadium(V) and iron(III) to vanadium(IV)and iron(II). Analytical grade Ca(OH)2, CaCO3,NaOH, Na2CO3, and NH3 ·H2O were obtained fromShanghai Rare-Earth Chemical Co., Ltd., China.The extracting solution was composed of 5% (v/v)TBP (tributyl phosphate), 20% (v/v) P204 (Bis(2-ethylhexyl) phosphate D2EHPA), and 75% (v/v)sulfonated kerosene. P204 and TBP were purchasedfrom Sinopharm Chemical Reagent Co., Ltd., China.All other reagents used were of analytical grade.
The vanadium concentration in solution wasmeasured indirectly using iron ammonium sulphatetitration, with the iron concentration determinedcolorimetrically using 1,10-phenanthroline. Theconcentrations of other ions in the solution were de-
termined using inductively coupled plasma-opticalemission spectroscopy (ICP-OES, Optima 4300DV,Perkin-Elmer, USA). The mineralogical composi-tion of the neutralizing filtered residue was iden-tified by XRD spectra pattern, recorded with a D/-MAX 2500PC X-ray powder diffractometer (Rigaku,Japan) at room temperature. Microscopic observa-tion and elemental analysis (SEM with EDS) wereconducted using a JEOL JSM-6610 scanning elec-tronic microscope (JEOL, Japan) equipped with aBRUKER QUANTAX 200-30 energy dispersive spec-trometer (BRUKER, Germany). The pH of the solu-tion was measured with a pHS-3C digital pH meter(Shanghai Rex Instruments Factory, China).
The pH adjustment experiments were carriedout using a magnetic stirrer. In each pH adjust-ment experiment, 100 ml acid leaching solutionwas reduced with Na2SO3 for 30 min. The pHof the solutions was then adjusted with Ca(OH)2,CaCO3, NaOH, Na2CO3, or NH3 ·H2O. After pHadjustment, the solutions were filtered and washedfor a selected duration, preparing the feed solutionfor solvent extraction. The solvent extraction wasperformed by magnetically stirred with the organicand aqueous phases at a 1:2 ratio, for 8 min at 25 °Cin a water bath. Phase separation was achievedby gravity using separatory funnels. After phaseseparation, the ion concentrations in the raffinatewere determined with the concentration of ions inorganic phase deduced from mass-balance calcu-lations. The distribution ratio (D) and extractionefficiency (E) were calculated by D = Corg/Caq andE = D/[D+(Vaq/Vorg)]×100%, respectively, whereCorg is the concentration of vanadium presented inthe organic phase, Caq is the content of vanadiumin the raffinate, and Vaq and Vorg are the volumes ofaqueous and organic phases used in the extraction,respectively.
RESULTS AND DISCUSSION
Effects of pH on ion precipitation
To determine the optimal pH in the neutralizationprocess, the effect of pH on vanadium loss and re-moval of impurity ions was investigated. Ca(OH)2,CaCO3, NaOH, Na2CO3, and NH3 ·H2O were usedas alkali neutralizers to adjust the pH of acid leach-ing solution from 1.4–2.4. The results shown inFig. 1 indicate that the solution pH is a factor thatsubstantially influences the vanadium loss rate andimpurity ion removal efficiency. The vanadiumloss rate and impurity ion (Fe, Al, P, S) removalefficiency increases as pH increases; however, the
Fig. 1 Effect of pH on ion precipitation when adjusting pHwith: (a) Ca(OH)2, (b) CaCO3, (c) NaOH, (d) Na2CO3,and (e) NH3 ·H2O.
0
20
40
60
80
100
1 . 4 1 . 6 1 . 8 2 . 0 2 . 2 2 . 4
pH
calcium hydroxide
calcium carbonate
sodium carbonate
sodium hydroxide
ammonia
Van
adiu
m e
xtra
ctio
n ef
fici
ency
, %Fig. 2 Effect of pH on vanadium extraction using differentalkali neutralizers.
removal efficiency of Si decreases as pH increases.The removal efficiency of Fe and P increases as pHincreases. As shown in Fig. 1a,b, when pH wasadjusted with Ca(OH)2 and CaCO3, the removal rateof S was enhanced 60%, this may be due to thegeneration of CaSO4 from dissolved Ca(OH)2 andCaCO3. Possible chemical reactions are described inTable 4(1,2).
As shown in Fig. 1c,d, when the pH was ad-justed with NaOH and Na2CO3, the removal effi-ciency of Si sharply decreases as pH increases from2.0–2.2. When pH was adjusted with NH3 ·H2O,vanadium loss rate and impurity ion (Al, S, Na,Ca) removal efficiency increases as pH increasesslightly, however, the removal efficiency of Fe andP increases as pH increases significantly (Fig. 1e).
Based on the experiments described above, weconclude that the removal efficiency of P and Feincreases significantly when pH increases, however,the vanadium loss rate also increases. Hence theoptimal pH range was determined to be 1.8–2.2 topromote precipitation of less vanadium and moreimpurities.
Effects of pH on vanadium extraction
The effect of pH on vanadium extraction efficiencywas also investigated. Solution extraction experi-ments were carried out under the following con-ditions: contact time, 8 min; temperature, 25 °C;and organic to aqueous phase ratio (O/A), 1:2.Under these conditions, the extraction efficiency ofvanadium increases as pH increases (Fig. 2). WhenpH was greater than 2.0, the extraction of vanadiumwas almost constant even with further increases in
Fig. 3 XRD pattern of precipitate when pH was adjustedwith alkalis.
pH. Hence the optimal pH for acid leaching solutionwas 2.0.
Comparison of different alkali neutralizers
Alkali neutralizers, to adjust the pH of the acidleaching solution, were investigated with regard tovanadium loss rate and impurity ion removal effi-ciency. The lowest vanadium loss rate, 4%, and ef-ficient removal of other impurity ions was achievedwith Ca(OH)2 used to adjust pH (Table 2). Whenthe pH was adjusted with Ca(OH)2 and CaCO3, thenumber of calcium ions generated was relativelysmall. However, using NaOH and Na2CO3 as al-kali neutralizers, a large amount of Na ions wasproduced. The use of NH3 ·H2O resulted in thegeneration of ammonia nitrogen wastewater, whichis difficult to treat. Considering the above factors,the best alkali neutralizer for the neutralizationprocess was determined to be Ca(OH)2.
XRD and SEM-EDS analysis
pH adjustment with Ca(OH)2 and CaCO3
XRD and SEM-EDS were used to analyse precipitatesgenerated after adjusting pH to 2.0 to determine
(a) (b) (c)
(d) (e) (f)
(Ca) (S)
(O) (Fe) (P)
1 2
Fig. 4 (a) SEM image of precipitate when pH was adjustedwith Ca(OH)2, EDS elemental distribution: (b) Ca; (c) S;(d) O; (e) Fe; (f) P.
(a)
(Ca) (S)
(O) (Fe) (P)
(b) (c)
(d) (e) (f)
3
Fig. 5 (a) SEM image of precipitate when pH was adjustedwith CaCO3, EDS elemental distribution: (b) Ca; (c) S;(d) O; (e) Fe; (f) P.
the mechanism of vanadium loss. When Ca(OH)2and CaCO3 were used to adjust pH, the major com-ponents of the precipitate were CaSO4 ·2 H2O andCaSO4·0.5H2O (Fig. 3). The SEM-EDS electronicimages from precipitates formed when adjusting thepH with Ca(OH)2 indicates the presence of pris-matic gypsum crystals with surfaces that are smoothand flat, limiting the potential for vanadium to beentrained (Fig. 4). The type of crystal structureformed with Ca(OH)2 likely contributes to the low-est vanadium loss rate with this alkali neutralizer.In contrast, the precipitate formed when adjustingpH with CaCO3 (Fig. 5) exhibits many small gypsumparticles stuck together. This crystal structure couldmore easily intercalate vanadium consistent withthe increased loss rate when the pH was adjustedwith Ca(OH)2.
The EDS elemental distribution reveals that thecontribution of O, P, and Fe was limited, suggestingthat the Fe, P, and O might exist in the form of ironphosphate. However, the content of iron phosphateis too low to account for the EDS result, as there isno peak for iron phosphate in the XRD pattern. Theresult of the EDS spot analysis (Table 3) indicates
Fig. 6 (a) SEM image of precipitate when pH was adjustedwith NaOH, EDS elemental distribution: (b) Na; (c) Si;(d) F; (e) Fe; (f) O; (g) P.
(a) (b) (c)
(d) (e) (f) (g)
(Na) (Si)
(F) (Fe) (O) (P)
5
Fig. 7 (a) SEM image of precipitate when pH was adjustedwith Na2CO3, EDS elemental distribution: (b) Na; (c) Si;(d) F; (e) Fe; (f) O; (g) P.
that the surface of CaSO4 crystals does not containvanadium. However, adherence of small iron phos-phate particles to the CaSO4 surface can promotethe absorption of vanadium, resulting in vanadiumloss (vanadium content of 0.57% and 0.37%).
pH adjustment with NaOH and Na2CO3
The major component of precipitate when pH wasadjusted with NaOH was Na2SiF6 (Fig. 3). Whendissolved NaOH and Na2CO3 released Na+, whichreacted with SiF2–
6 in the acid leaching solutiongenerating Na2SiF6
31. However, the large num-ber of dispersion peaks in the XRD pattern indi-cates poor crystallization of sodium fluorosilicate.Possible chemical reaction equations are shown in
(a) (b)
(c) (d)
(Fe)
(O) (P)
6
Fig. 8 (a) SEM image of precipitate when pH was adjustedwith NH3 ·H2O, EDS elemental distribution: (b) Fe; (c) O;(d) P.
Table 4(3,4)The results of SEM-EDS (Fig. 6 and Fig. 7) indi-
cate that sodium fluorosilicate crystals are incom-plete and covered with floccule. The precipitatesurface structure allows for vanadium to be en-trained at higher levels compared with Ca(OH)2and CaCO3 as alkali neutralizers. Meanwhile, thefloccule consists of O, P, and Fe suggesting the pres-ence of iron phosphate. When the pH was adjustedwith Na2CO3, the floccule surface was rougher, en-training more vanadium than when using NaOH.The result of EDS spot analysis shows the vanadiumcontent was 0.47% and 1%, indicating that theiron phosphate entrained and absorbed vanadium,resulting in vanadium loss (Table 3).
pH adjustment with NH3 ·H2O
The XRD diffraction pattern of precipitate whenpH was adjusted with NH3 ·H2O was a dispersionpeak, indicating that the residue was an amorphousmaterial (Fig. 3). The results of SEM-EDS electronicimage and elemental distribution (Fig. 8) reveal
that floccule iron phosphate was formed. Fe canexist as the cation Fe3+ and P is often present asneutral H3PO4 and H2PO–
4. Possible chemical reac-tions with Fe3+, H3PO4 and H2PO4–
4 were describedin Table 4(5,6). The result of EDS spot analysis(Table 3) shows that the content of vanadium was0.61 wt%, indicating that the iron phosphate en-trained and absorbed vanadium resulting in vana-dium loss.
Thermodynamics
The feasibility of the reactions described in theseequations should be theoretically discussed usingthermic analyses. The ∆°G (standard free energychange of reaction) can be calculated using the°G (T) (standard free energy) of the substancesinvolved in the chemical reaction32. The functionsdescribing the ∆°G (298K) for the reaction equa-tions are shown in Table 4.
Table 4 shows that∆°G (298K) was negative forall reaction equations. Hence all reactions above arespontaneous at 298K.
CONCLUSIONS
To obtain a low vanadium loss rate, high impurityions removal rate and high vanadium extractionefficiency, the pH of acid leaching solution shouldbe adjusted to 2.0 with Ca(OH)2.
The mechanism of vanadium loss is that vana-dium was entrained and absorbed by precipitate.When adjusting pH with Ca(OH)2, the lowest vana-dium loss rate is achieved because crystal structurewas the most integrated compared with using otheralkalis. When adjusting pH with CaCO3, crystalstructure was relatively integrated with tiny calciumsulphate particles stuck together, making it easier toentrain vanadium compared with Ca(OH)2. Mean-while, the tiny iron phosphate particles adhered toCaSO4 surface would entrain and absorb vanadiumand result in vanadium loss.
Compared with Ca(OH)2, when NaOH andNa2CO3 were used to adjust pH, the structure ofsodium fluorosilicate was incomplete, and the floc-
cule iron phosphate was produced, which makesthe surface rough and causes higher vanadium loss.When NH3 ·H2O was used to adjust pH, amorphousflocculent materials were formed, making it easierto entrain vanadium than when adjusting pH withCa(OH)2.
Acknowledgements: This study was financially sup-ported by the National Natural Science Foundationof China (51774215), the National Natural ScienceFoundation of China (51874222), and the NationalKey Science Technology Support Programs of China(2015BAB03B05).
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