Controlled Hydrothermal Precipitation of Alunite and Natroalunite in High-Aluminum Vanadium-Bearing Aqueous System

minerals

Article Controlled Hydrothermal Precipitation of Alunite and Natroalunite in High-Aluminum Vanadium-Bearing Aqueous System

Luyao Wang 1,2,3 , Nannan Xue 1,2,3,*, Yimin Zhang 1,2,3,4,* and Pengcheng Hu 1,2,3

1 School of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China; [email protected] (L.W.); [email protected] (P.H.) 2 State Environment Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control, Wuhan University of Science and Technology, Wuhan 430081, China 3 Collaborative Innovation Center of Strategic Vanadium Resources Utilization, Wuhan 430081, China 4 Hubei Provincial Engineering Technology Research Center of High Efﬁcient Cleaning Utilization for Cleaning Utilization for Shale Vanadium Resource, Wuhan University of Science and Technology, Wuhan 430081, China * Correspondence: [email protected] (N.X.); [email protected] (Y.Z.); Tel./Fax: +86-027-6886-2057 (Y.Z.)

Abstract: During the acid leaching process of black shale, with the destruction of the aluminosilicate mineral structure, a large amount of aluminum (Al) is leached, accompanied by the release of vanadium (V). To separate aluminum from the vanadium-containing solution, the precipitation behavior of aluminum ions (Al3+) was investigated under hydrothermal conditions with the formation of alunite and natroalunite. In the solution environment, alunite and natroalunite are able to form stably by the Al3+ hydrolysis precipitation process at a temperature of 200 ◦C, a pH value of 0.4 and a reaction time of 5 h. When Al3+ was precipitated at a K/Al molar ratio of 1, the aluminum precipitation efficiency and the vanadium precipitation efficiency were 64.77% and 1.72%, respectively. Citation: Wang, L.; Xue, N.; Zhang, However, when Al3+ was precipitated at a Na/Al molar ratio of 1, the precipitation efficiency of Y.; Hu, P. Controlled Hydrothermal Precipitation of Alunite and the aluminum decreased to 48.71% and the vanadium precipitation efficiency increased to 4.36%. Natroalunite in High-Aluminum The thermodynamics and kinetics results showed that alunite forms more easily than natroalunite, Vanadium-Bearing Aqueous System. and the reaction rate increases with increasing temperature, and the precipitation is controlled by the Minerals 2021, 11, 892. https:// chemical reaction. Vanadium loss increases as the pH value increases. It can be deduced that the ion doi.org/10.3390/min11080892 state of tetravalent vanadium (VO2+) was transformed into the ion state of pentavalent vanadium + + (VO2 ) in the hydrothermal environment. The VO2 can be adsorbed on the alunite or natroalunite Academic Editors: as a result of their negative surface charges, ultimately leading to vanadium loss. Przemyslaw B. Kowalczuk Keywords: aluminum; black shale; alunite; natroalunite; hydrothermal precipitation Received: 28 June 2021 Accepted: 12 August 2021 Published: 18 August 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Black shale is an important and abundant vanadium-bearing resource in China, and published maps and institutional affil- has attracted much attention from researchers [1–3]. In vanadium-bearing black shale, iations. vanadium (V) mainly exists as low-valence V(III) in the crystal lattice of the muscovite, re- placing aluminum (Al) due to their isomorphism [4,5]. The main distribution of vanadium grade in black shale deposits is in the range 0.1–1.0%; only 2.8% of black shale possesses a vanadium grade over 1.0%. Thus, the mica structure needs to be destroyed in order to release vanadium from the black shale. In the alkaline leaching process, the silica present in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. black shale can react with alkali to form colloidal silica, which makes it difficult to separate This article is an open access article vanadium-bearing alkali leachate from residue. Acid leaching is usually adopted for the distributed under the terms and extraction of vanadium from black shale [6,7]. The aluminum is leached along with the conditions of the Creative Commons vanadium, inevitably increasing the aluminum concentration to 15–20 g/L, which is far Attribution (CC BY) license (https:// higher than the vanadium concentration. The separation of vanadium over aluminum has creativecommons.org/licenses/by/ become the core scientific problem in the purification and concentration of high-aluminum 4.0/). vanadium-bearing leachate.

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In high-aluminum vanadium-bearing leachate, the aluminum ions spontaneously transform into colloidal alum (KAl(SO4)2·12H2O), which can adsorb vanadium ions, caus- ing vanadium loss [8]. Meanwhile, aluminum ions have adverse effects on the subsequent processes of solvent extraction or the precipitation process of vanadium by acidic ammonium salt. It has been reported that aluminum ions can be co-extracted with vanadium ions, which reduces the extraction efficiency of vanadium in D2EHPA(P204) solvent extraction systems [9,10]. Moreover, aluminum ions can also be adsorbed onto vanadium precipitates (such as poly-ammonium vanadate) or be precipitated in the form of alum in the vanadium precipitation process by acidic ammonium salt, leading to low crys- tallinity of poly-ammonium vanadate, and ultimately reducing the purity of the vanadium pentoxide [8,11]. It has been noticed by some scholars [12] that in the pH range of 3–5, aluminum hydroxide forms. Wang [12] adopted 8-hydroxyquinoline as the selective complexing precipitant at a pH value of 4.5, leading to the precipitation of aluminum ions in the form of 8-hydroxyquinoline aluminum. However, the vanadium is easy to hydrolyze and precipitate in the pH range of 2–4 [13,14]. Therefore, the above-mentioned methods with aluminum hydroxide or 8-hydroxyquinoline aluminum precipitation are suitable for achieving no vanadium co-existence. Guo [15] found that the cooling crystallization of AlCl3·6H2O from the high-aluminum acid leaching solution of coal gangue was an effective method for the removal of aluminum. However, the high consumption of hydrochloric acid dilutes the concentrations of valuable ions, resulting in an increase in the number of extraction stages. Shi [11] also adopted the method of cooling crystallization to remove aluminum in the form of KAl(SO4)2·12H2O from the high-aluminum vanadium-bearing leachate of black shale. After a long period of crystallization, the aluminum precipitation efficiency reached no more than 70%. Apparently, it is necessary to find an efficient and feasible method for the removal of aluminum from high-aluminum vanadium-bearing leachate. Alunite (KAl3(SO4)2(OH)6) and natroalunite (NaAl3(SO4)2(OH)6) belong to the alunite 2− 3+ supergroup (AB3(TO4)2(OH)6) and form stably in oxidizing, acidic, and SO4 /Al - enriched hydrothermal environments [16,17], especially at hydrothermal temperatures of above 100 ◦C. The increasing temperature obviously offsets the initial reaction acidity for Al3+ hydrolysis and promotes the formation of alunite and natroalunite [18]. The K/Al molar ratio also changes the stoichiometric number of alunite [19]. Some scholars have successfully synthesized natroalunite at acidic pH values (1–4) and temperatures no less ◦ − 2+ 3− than 180 C in order to immobilize As, or adsorb F , Cd , PO4 from the aqueous solution [20–22]. It has been reported that alunite can be formed in the pressure acid leaching process of black shale, and the hydrolyzation of Al could be initiated more easily than that of vanadium by reducing the residual acid of the leachate [23,24]. Therefore, both alunite and natroalunite can be considered new aluminum-containing precipitates for the separation of vanadium over aluminum on the basis of their resistance to acidic and high-temperature conditions. However, the generation difference between alunite and natroalunite has not been clarified in the existing published literature, which is important for the theoratical guidence of the purification process of high-aluminum vanadium- bearing acid leaching solutions of black shale. In this paper, to effectively separate aluminum and vanadium, the effect of the main parameters on the precipitate efficiency of aluminum was studied, namely pH values, K(or Na)/Al molar ratios and temperatures. Meanwhile, the precipitation behaviors of alunite and natroalunite were compared on the basis of analysis of their thermodynamics and kinetics. Finally, the phase composition, elemental distribution, and surface charge of the precipitate were analyzed to clarify the mechanism of vanadium and alunite co-precipitation. Minerals 2021, 11, 892 3 of 17

2. Experimental Section 2.1. The Removal Behavior of Al in Acid Solution The main metal impurity ions in the acid leachate of black shale are Al3+, Fe3+, Fe2+, 2+ + + 2+ Mg ,K , Na and Ca . In the alunite supergroup (AB3(TO4)2(OH)6), the A site is usually occupied by a monovalent cation (such as K+, Na+) or a divalent cation (such as Ca2+, Ba2+, Pb2+), the B site is principally occupied by Al3+ and Fe3+ [20]. During the formation of alunite or natroalunite, the Fe3+ and Ca2+ can be embedded in the crystal lattice of alunite. To avoid the influence of Fe3+, Ca2+ and the formation of K-Na alunite on the aluminum precipitation process, the vanadium-bearing solution was obtained by dissolving VOSO4 and Al2(SO4)3·18H2O in deionized water. The K2SO4 or Na2SO4 was fed into the vanadium-bearing solution to a certain volume with a certain K(or Na)/Al molar ratio, and mixed adequately. The concentration of vanadium and aluminum in the solution were 1.79 g/L and 0.6 mol/L, respectively. Then, the mixed solution was put into the pressurized reactor (MCT250, Beijing Century SenLong experimental apparatus Co., Ltd., Beijing, China), and kept at a certain temperature for a specified time. After the reaction, the solution and precipitate were separated by filtration. All reagents used in the tests were analytically pure. Deionized water was used throughout. The precipitation efficiency (η) of aluminum and vanadium were calculated using the following equation (see Equation (1)):

C × V η = 0 0 × 100% (1) C × V

where η is the precipitation efﬁciency of aluminum or vanadium (%), C0 is initial concentration of aluminum or vanadium in solution before reaction (g/L), V0 is the solution volume before reaction (L), C is the concentration of aluminum or vanadium ions in solution after reaction (g/L), V is the solution volume after reaction (L). The kinetic experiment was carried out in a pressurized reactor. An amount of 200 mL solution was added into the reactor, and the initial pH value of the solution was 0.6. During the 4 h reaction, 2 mL of the solution was taken out from the sampling tube of the reactor at intervals, and the concentration of K+ or Na+ was measured by the Ion chromatograph.

2.2. Detection Methods The concentration of vanadium in the solution was analyzed by titration with ferrous ammonium sulfate. The phase compositions of the samples were tested by means of X-ray diffraction (XRD, D/MAX 2500PC, Rigaku, Tokyo, Japan) using Cu Kα radiation. Microscopic observation and elemental analysis were conducted with a scanning electron microscope (SEM, JSM-IT300, JEOL, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS, X-Act, Oxford, London, UK). The pH value of the solution was measured using a pH meter (PHS-3C, INESA Scientiﬁc Instrument Co., Ltd. Shanghai, China). The aluminum concentration of the solution was obtained using ICP-AES (Optima-4300DV, PerkinElmer, Boston, MA, USA). The Na+ or K+ concentration in solution was obtained by Ion chromatography (Metrohm 883, Heriau, Switzerland). X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Multilab 2000 instrument (ThermoFisher Electron, Waltham, MA, USA). The zeta potential was measured using a Zetasizernano potentiometric titrator (Malvenpanako, Almelo, The Netherlands).

3. Results and Discussion 3.1. Effect of Initial pH Value on the Precipitation Efﬁciency of Al To determine whether the addition of sodium sulfate or potassium sulfate can effectively remove aluminum from the vanadium-bearing solution under different pH values, experiments were conducted at different pH values under the following conditions: a temperature of 200 ◦C, K(or Na)/Al molar ratio of 1, and reaction time of 5 h. The pH values after the reaction are shown in Table1, and the precipitation efﬁciency of aluminum Minerals 2021, 11, x FOR PEER REVIEW 4 of 17 Minerals Minerals2021 2021, 11,, 11 892, x FOR PEER REVIEW 5 of 418 of 17

experiments were conducted at different pH values under the following conditions: a tem- andperaturevalues, vanadium experimentsof 200 are °C, shownK(or were Na)/Al in Figureconducted molar1. Theratio at phase of different 1, and composition reaction pH values time of the of under precipitates5 h. The the pH following values obtained atafterconditions: a pH the value reaction a temperature of 0.6 are were shown analyzed of in200 Table °C, by K(or 1, XRD,and Na)/Al the and precipitation molar the results ratio ofefficiency are 1, shownand reaction of in aluminum Figure time2 of. and 5 h. vanadiumThe pH values are shown after the in Figurereaction 1. areThe shown phase incomposition Table 1, and of the precipitationprecipitates obtained efficiency at ofa TablepHaluminum value 1. The of pH and0.6 value were vanadium of analyzed the solution are by shownXRD, before and and in the afterFigure results reaction. 1. are The shown phase in Figure composition 2. of the precipitates obtained at a pH value of 0.6 were analyzed by XRD, and the results are Tableshown 1.pH inThe ValueFigure pH value 2. of the solution before Na2SO and4 after reaction. K2SO4 Before reaction 0 0.3 0.4 0.6 0.8 0 0.3 0.4 0.6 0.8 pH Value Na2SO4 K2SO4 TableAfter 1. The reaction pH value of the 0 solution 0.29 before 0.21 and after 0.41 reaction. 0.39 −0.10 0.27 0.25 0.51 0.54 Before reaction 0 0.3 0.4 0.6 0.8 0 0.3 0.4 0.6 0.8 AfterpH reactionValue 0 0.29 Na 0.212SO 4 0.41 0.39 −0.10 0.27 K0.252SO 4 0.51 0.54 Before reaction 0 0.3 0.4 0.6 0.8 0 0.3 0.4 0.6 0.8 After reaction 0 0.29 0.21 0.41 0.39 −0.10 0.27 0.25 0.51 0.54

Figure 1. Effect of pH value on the precipitation efficiency of aluminum and vanadium.

Figure 1. Effect of pH value on the precipitation efﬁciency of aluminum and vanadium. Figure 1. Effect of pH value on the precipitation efficiency of aluminum and vanadium.

Figure 2. XRD patterns of precipitate obtained at pH value of 0.4: (a) Na2SO4; (b) K2SO4.

Figure 2. XRD patterns of precipitate obtained at pH value of 0.4: (a) Na2SO4;(b)K2SO4.

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AsAs shown shown in in Table Table1, the 1, pHthe valuepH value decreased decrea aftersed theafter reaction, the reaction, indicating indicating that H +thatwas H+ producedwas produced in the reactionin the reaction process. process. ItIt can can be be seen seen from from Figure Figure1 that 1 that the pHthe valuepH value significantly significantly influenced influenced the precipitation the precipita- behaviortion behavior of aluminum. of aluminum. With increasing With increasing pH value, pH the value, precipitation the precipitation efficiency efficiency of aluminum of alu- alsominum increased. also increased. However, However, at the same at the pH same value, pH the value, precipitation the precipitation efficiency efficiency of aluminum of alu- withminum the additionwith the ofaddition sodium of sulfate sodium was sulfate lower was than lower that than with that the additionwith the ofaddition potassium of po- sulfate.tassium When sulfate. the When pH value the pH was value 0.4, was the 0.4, precipitation the precipitation efficiency efficiency of aluminum of aluminum with thewith additionthe addition of potassium of potassium sulfate sulfate was 64.77%, was 64.77%, while while the precipitation the precipitation efficiency efficiency of aluminum of alumi- withnum the with addition the addition of sodium of sulfatesodium was sulfate only 48.71%.was only However, 48.71%. theHowever, precipitation the precipitation efficiency ofefficiency vanadium of wasvanadium 1.72% andwas 4.36%,1.72% and respectively. 4.36%, respectively. When the pHWhen value the exceededpH value 0.6,exceeded the precipitation0.6, the precipitation efficiency efficiency of vanadium of vanadium increased increased rapidly, indicating rapidly, indicating an increase an in increase the loss in ofthe vanadium. loss of vanadium. Thus, under Thus, the under same conditions,the same conditions, potassium potassium sulfate is sulfate more suitable is more for suitable the removalfor the ofremoval aluminum of aluminum than sodium than sulfate. sodium To sulfate. avoid vanadiumTo avoid vanadium loss, the initial loss, pHthe valueinitialof pH solutionvalue of should solution not should be more not than be more 0.4. than 0.4. WhenWhen the the initial initial pH pH value value is is less less than than 0.8, 0.8, the the precipitate precipitate is is white. white. Figure Figure2 clearly 2 clearly showsshows that that only only the the natroalunite natroalunite (NaAl (NaAl3(SO3(SO44))22(OH)6)) or or alunite alunite (KAl (KAl33(SO(SO4)42)(OH)2(OH)6) 6is) isde- detected,tected, without without the the appearance appearance of of any any other other diffraction diffraction peaks. peaks. Hence, Hence, it itis is feasible feasible to to re- removemove aluminum fromfrom thethe vanadium-bearingvanadium-bearing acid acid solution solution with with the the formation formation of of alunite alunite andand natroalunite. natroalunite. 3.2. Effect of K(Na)/Al Molar Ratio on the Precipitation Efficiency of Al 3.2. Effect of K(Na)/Al Molar Ratio on the Precipitation Efficiency of Al Experiments were performed using different K(or Na)/Al molar ratios under the Experiments were performed using different K(or Na)/Al molar ratios under the fol- following conditions: pH value of 0.4, temperature of 200 ◦C, and reaction time of 5 h. lowing conditions: pH value of 0.4, temperature of 200 °C, and reaction time of 5 h. The The results obtained after the reaction were as shown in Figure3. results obtained after the reaction were as shown in Figure 3.

FigureFigure 3. 3.Effect Effect of of the the K(Na)/Al K(Na)/Al molar molar ratio ratio on on the the precipitation precipitation efﬁciency efficiency of of aluminum. aluminum.

AsAs shown shown in in Figure Figure3, the 3, precipitationthe precipitation efﬁciency efficiency of aluminum of aluminum increased increased with increas- with in- ingcreasing amounts amounts of sodium of sodium sulfate orsu potassiumlfate or potassium sulfate. However,sulfate. However, when the when K(Na)/Al the K(Na)/Al molar ratiomolar was ratio more was than more 1:1, than increasing 1:1, increasing the K(Na)/Al the K(Na)/Al molar ratio molar had ratio no obvious had no effectobvious on effect the precipitationon the precipitation efﬁciency efficiency of aluminum, of aluminum, which may which be duemay tobe the due decreasing to the decreasing pH value pH with value thewith progress the progress of the reaction of the reaction inhibiting inhibiting the reaction. the reaction.

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3.3. Effect of Temperature on the Precipitation Efﬁciency of Al 3.3.Experiments Effect of Temperature were carried on th oute Precipitation at different Efficiency temperatures of Al under the following conditions: K(or Na)/AlExperiments molar were ratio carried of 1, pH out value at different of 0.4, and temperatures reaction time under of 5 the h. Thefollowing results condi- are showntions: inK(or Figure Na)/Al4. molar ratio of 1, pH value of 0.4, and reaction time of 5 h. The results are shown in Figure 4.

Figure 4. Effect of temperature on the precipitation efficiency of aluminum. Figure 4. Effect of temperature on the precipitation efﬁciency of aluminum.

AsAs shown shown in in Figure Figure4, the4, the temperature temperature significantly significantly influenced influenced the the precipitation precipitation bebe- haviorhavior of of aluminum. aluminum. With With increasing increasing temperature, temperature, the precipitationthe precipitation efficiency efficiency of aluminum of alumi- increased.num increased. However, However, at the sameat the temperature,same temperat theure, precipitation the precipitation efficiency efficiency of aluminum of alumi- withnum the with addition the addition of sodium of sodium sulfate was sulfate lower was than lower that than with that the additionwith the ofaddition potassium of potas- sul- fate.sium When sulfate. the temperatureWhen the temperature was 220 ◦C, was the 220 precipitation °C, the precipitation efficiency ofefficiency aluminum of aluminum with the additionwith the of addition potassium of sulfatepotassium was sulfate 81.75%, was while 81.75%, the precipitation while the precipitation efficiency of aluminumefficiency of withaluminum the addition with ofthe sodium addition sulfate of sodium was 60.83%. sulfate The was concentration 60.83%. The of concentration aluminum decreased of alumi- fromnum 0.6 decreased mol/L (16.2 from g/L) 0.6 mol/L to 2.95 (16.2 g/L org/L) 6.34 to g/2.95 L, g/L respectively. or 6.34 g/ L, respectively.

3.4.3.4. The The Precipitation Precipitation Thermodynamics Thermodynamics and an Kineticsd Kinetics of of Alunite Alunite and and Natroalunite Natroalunite ◦ WithinWithin the the temperature temperature range range of of 100–350 100~350C, °C, water water can can act act as as a catalyticallya catalytically active active speciesspecies in in chemical chemical reactions reactions and and possesses possesses a strong a strong tendency tendency to ionize to ionize [25,26 ].[25,26].Alunite Alunite can + + 3+ 2− − becan formed be formed by the by reaction the reaction of K of(Na K+ ),(Na Al+), ,Al SO3+4, SOand42− and OH OH[−20 [20,22,27].,22,27]. After After the the reaction, reaction, thethe pH pH value value decreased, decreased, andand the the formation formation of of na natroalunitetroalunite (or (or alunite) alunite) caused caused by bythe thereac- reactiontion of ofNa Na2SO2SO4 (or4 (or K2SO K24SO) and4) and Al2 Al(SO2(SO4)3 can4)3 canbe expressed be expressed by Equations by Equations (2) (2)and and (3). (3). The TheGibbs Gibbs free free energy energy of of the the reactions reactions was was calculated usingusing HSCHSC software.software. The The results results are are shownshown in in Figure Figure5. 5.

+ 3+ 2− + Na +Na3Al+3Al+ 2SO+2SO4 + 6H+6H2O =O=NaAlNaAl3(SO(SO4)2()OH(OH))6 + +6H6H (2)(2)

+ 3+ 2− + K +K3Al+3Al+ 2SO+2SO4 + 6H+6H2O =O=KAlKAl3(SO(SO4)2()OH(OH))6 + +6H6H (3)(3)

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θ FigureFigure 5. The5. The∆G ΔθGvalues values of of Equations Equations (2) (2) and and (3) (3) at at different different temperatures. temperatures. Figure 5. The ΔGθ values of Equations (2) and (3) at different temperatures. AsAs illustrated illustrated in in Figure Figure5, the 5, the∆G ΔθGvaluesθ values were were negative negative at at temperatures temperatures higher higher than than θ 100100◦C,As °C, and illustrated and decreased decreased in Figure with with increasing 5, increasing the ΔG temperature.values temperature. were Equations negative Equations (2)at temperatures and (2) (3)and were (3) were spontaneous higher spontane- than when100ous °C, thewhen and temperature decreasedthe temperature waswith higher increasing was higher than temperature. 100 than◦C. 100 In °C. contrast, Equations In contrast, the ∆ (2)G theθ andvalues ΔG (3)θ values were of Equation spontane- of Equation (3) wereous(3) when lowerwere the thanlower temperature those than obtained those was obtained usinghigher Equation thanusing 100 Equation (2), °C.which In contrast, (2), indicates which the indicates thatΔGθ alunitevalues that wasof aluniteEquation formed was more(3)formed were easily lower more than thaneasily natroalunite. those than obtained natroalunite. using Equation (2), which indicates that alunite was formedTheThe more equilibrium equilibrium easily than compositions compositions natroalunite. of of alunite alunite and and natroalunitenatroalunite werewere alsoalso analyzed using us- ingHSC HSCThe software equilibrium software under under compositions the the same same thermody thermodynamicof alunitenamic and natroaluniteconditions. conditions. The were The amount amountalso analyzed of of Al Al3+,3+ Kusing,K+, Na+, +, + 2− 2− - − 3+ + + NaHSCSO, 4software SO and4 OHand under were OH the setwere sameat 1 setmol, thermody at 1 1 mol, mol, namic1 1mol, mol, conditions.1 mol, 1 mol, and 1 mol,3The mol, andamount respectively. 3 mol, of Al respectively. The, K ,equilib- Na , TheSOrium42 equilibrium− and composition OH- were composition set diagrams at 1 mol, diagrams are 1 mol,shown 1 are mol, in shown Figure 1 mol, in 6. and Figure 3 mol,6. respectively. The equilibrium composition diagrams are shown in Figure 6.

Figure 6. The equilibrium composition diagrams of the K-Na-SO42−-H2O system. Figure 6. The equilibrium composition diagrams of the K-Na-SO242−−-H2O system. Figure 6. The equilibrium composition diagrams of the K-Na-SO4 -H2O system. As shown in Figure 6, the amount of alunite is much greater than the amount of natroalunite.As shown inHowever, Figure6 6,,with thethe amountincreasingamount ofof alunitetempaluniteerature, isis muchmuch the greater greateramount thanthan of thealunitethe amountamount increased, ofof natroalunite.while the amount However, However, of natroalunite with with increasing increasing first increa temp temperature,sederature, and thenthe the amountdecreased. of In alunite combination increased, with while the amount of natroalunite first increased and then decreased. In combination with

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Minerals 2021, 11, x FOR PEER REVIEW 8 of 17 while the amount of natroalunite ﬁrst increased and then decreased. In combination with Figure5, it can be concluded that increasing temperature promotes the formation of alunite and natroalunite. It is speculated that it is possible to replace Na with K, with the reaction beingFigure expressed 5, it can be as concluded in Equation that (4), increasi and theng relationship temperature between promotes Gibbs the formation free energy of andalu- temperaturenite and natroalunite. being as shown It is speculated in Figure7 .that it is possible to replace Na with K, with the reaction being expressed as in Equation (4), and the relationship between Gibbs free en- + + ergy and temperatureNaAl 3being(SO4) as2( OHshown)6 + inK Figure= KAl 7.3 (SO4)2(OH)6 + Na (4) NaAl(SO)(OH) ++K =KAl(SO)(OH) +Na+ (4) KAl3(SO4)2(OH)6 + Na = NaAl3(SO4)2(OH)6 + K (5) KAl(SO)(OH) +Na =NaAl(SO)(OH) + K (5)

Figure 7. The ΔGθ values of Equations (4) and (5) at different temperatures. Figure 7. The ∆Gθ values of Equations (4) and (5) at different temperatures.

AsAs shownshown inin FigureFigure7 ,7, the the∆ ΔGGθθ values are negative for thethe reactionreaction inin EquationEquation (4),(4), butbut the Δ∆GGθθ valuesvalues are are positive positive for forthe thereaction reaction in Equation in Equation (5). The (5). ΔG Theθ values∆Gθ ofvalues Equation of Equation(4) decrease (4) decreasewith increasing with increasing temperature, temperature, which which means means that thatthe theNa+Na at+ theatthe A-site A-site of ofnatroalunite natroalunite can can theoretica theoreticallylly be be replaced replaced with with K K++ andand transformed transformed into alunite, butbut thethe alunitealunite cannotcannot bebe transformedtransformed intointo natroalunite.natroalunite. AccordingAccording to the thermodynamic thermodynamic calculation calculation of of HSC, HSC, under under the the same same conditions, conditions, al- aluniteunite forms forms more more easily easily than than natroalunite. natroalunite. Ther Therefore,efore, the the effect effect of oftemperature temperature on onthe the ki- kineticsnetics of of natroalunite natroalunite and and alunite alunite were were preliminarily preliminarily investigated, investigated, and andthe apparent the apparent acti- activationvation energy energy was was calculated. calculated. TheThe curvecurve of K ++ oror Na Na+ +concentrationconcentration with with reaction reaction time time was was drawn, drawn, as shown as shown in Fig- in Figureure 8. 8The. The tangent tangent of ofthe the curve curve was was obtained obtained for for a specified a speciﬁed time, time, and and the the rate rate equation equation for forthe thereaction reaction can can be bewritten written as shown as shown in Equation in Equation (6). (6).

d[C𝑑[𝐶]] ri =𝑟 =−− (6)(6) dt𝑑𝑡i

wherewherer rii isis thethe reactionreaction raterate constantconstant atatt ti,i, ttii isis thethe timetime ofof thethe reactionreaction (min),(min), andand [[CC]] isis thethe + + concentrationconcentration ofof KK+ or Na+ atat tti i(g/L).(g/L).

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FigureFigure 8. 8.Variation Variation inin thethe concentrationconcentration of Na Na++ oror K K+ +withwith hydrothermal hydrothermal reaction reaction at different at different tem- temperatures:peratures: (a ()a Na) Na2SO2SO4; (4b;() bK)K2SO2SO4. 4.

TheThe reaction reaction rate rate and and the the concentration concentration of of K +Kor+ or Na Na+ conform+ conform to to the the following following rela- rela- tionship,tionship, see see Equation Equation (7), (7), as as shown shown in in Figure Figure9. 9.

ln ri𝑙𝑛= 𝑟ln=𝑙𝑛𝑘+𝑚𝑙𝑛[𝐶]k + m ln [C] (7)(7)

wherewherek isk is the the reaction reaction rate rate constant, constant, and andm mis is the the reaction reaction order. order.

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Figure 9. Fitting results of Equation (7) at different temperature: (a) Na SO ;(b)K SO . Figure 9. Fitting results of Equation (7) at different temperature: (a) Na2 2SO4 4; (b) 2K2SO4 4. Finally, according to the relationship between the reaction rate constant (k) and tem- Finally, according to the relationship between the reaction rate constant (k) and temperature (T), the apparent activation energy Ea of natroalunite and alunite were calculated perature (T), the apparent activation energy Ea of natroalunite and alunite were calculated using the Arrhenius formula (see Equation (8)) [28–30], as shown in Figure 10. using the Arrhenius formula (see Equation (8)) [28–30], as shown in Figure 10.

Ea ln k = − +EA (8) ln k =RT − +A (8) RT where k is the reaction rate constant, Ea is the apparent activation energy (kJ/mol), R is the molarwhere gas k constantis the reaction (J·mol rate−1·K constant,−1), T is theEa is reaction the apparent temperature activation (K), energy and A is(kJ/mol), the constant. R is the molar gas constant (J·mol−1·K−1), T is the reaction temperature (K), and A is the constant.

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FigureFigure 10. 10.Fitting Fitting line line of of the the Arrhenius Arrhenius equation. equation.

ItIt can can be be concluded concluded from from Figure Figure8 that8 that the th reactione reaction rate rate increased increased with with increasing increasing temperature,temperature, and and the the precipitation precipitation efficiency efficiency of of aluminum aluminum increased increased too. too. The The rate rate of of ion ion concentrationconcentration slowed slowed down down with with increasing increasing reaction reaction time. time. However, However, the the reaction reaction rate rate of of sodiumsodium sulfate sulfate and and potassium potassium sulfate sulfate were were different. different. When When the the temperature temperature was was 130 130◦C, °C, natroalunitenatroalunite was was barely barely formed, formed, whereas whereas alunite alunite was was formed. formed. The The precipitation precipitation efficiency efficiency ofof natroalunite natroalunite was was lower lower than than that that of of alunite alunite at at the the same same temperature. temperature. SinceSince the the reaction reaction is is greatly greatly affected affected by by the the temperature, temperature, natroalunite natroalunite was was barely barely ◦ ◦ formedformed at at a temperaturea temperature of of 130 130C, °C, so so only only the the data data obtained obtained at at temperatures temperatures of of 150 150C, °C, ◦ ◦ ◦ 180180C, °C, 200 200C, °C, and and 220 220 C°C were were used used for for the the calculation calculation of of the the apparent apparent activation activation energy energy EaE.a With. With increasing increasing reaction reaction time, time, the the instantaneous instantaneous reaction reaction rate rate decreased decreased and and tended tended towardstowards flat. flat. Therefore, Therefore, the the apparent apparent activation activation energy energy was was calculated calculated on on the the basis basis of of the the reactionreaction data data for for the the first first hour. hour. As As shown shown in in Figure Figure 10 10,, according according to to the the Arrhenius Arrhenius formula, formula, thethe apparent apparent activation activation energy energy of of natroalunite natroalunite was was 96.52 96.52 kJ/mol, kJ/mol, while while that that of of alunite alunite was was 58.8658.86 kJ/mol, kJ/mol, meaning meaning that that alunite alunite was was more more easily easily formed formed than than natroalunite, natroalunite, and and these these reactionsreactions were were controlled controlled by by the the chemical chemical reaction. reaction. On the basis of the calculation of the thermodynamic and kinetic properties, it can be On the basis of θthe calculation of the thermodynamic and kinetic properties, it can be seenseen that that both both the the∆ GΔGvaluesθ values and and the the activation activation energy energy of of alunite alunite were were lower lower than than those those ofof natroalunite. natroalunite. It It can can be be concluded concluded that that alunite alunite is is more more readily readily formed formed than than natroalunite natroalunite underunder the the same same conditions. conditions. 3.5. Mechanism for the Co-Precipitation of Vanadium and Alunite (Natroalunite) 3.5. Mechanism for the Co-Precipitation of Vanadium and Alunite (Natroalunite) To analyze the composition of the precipitate, XRD analyses were performed on To analyze the composition of the precipitate, XRD analyses were performed on pre- precipitates obtained at different pH values, as shown in Figure 11. cipitates obtained at different pH values, as shown in Figure 11.

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Figure 11. XRD patterns of precipitate obtained by aluminum precipitation experiment: (a) Na2SO4; Figure 11. XRD patterns of precipitate obtained by aluminum precipitation experiment: (a) Na2SO4; (b) K2SO4. (b)K2SO4.

AsAs shownshown inin FigureFigure 11 11,, thethe intensityintensity ofof thethe diffraction diffraction peaks peaks inin the the sample sample is is strong strong andand sharp,sharp, and and therethere are are no no other other impurity impurity peaks. peaks. Figure Figure 11 11aa (with (with the the addition addition of of sodium sodium sulfate)sulfate) clearlyclearly shows that that only only natroalunite natroalunite was was formed, formed, and and the the diffraction diffraction peaks peaks at dif- at differentferent pH pH values values are are almost almost the the same. same. Figure Figure 11b 11b (with (with the the addition of potassiumpotassium sulfate)sulfate) clearlyclearly shows shows that that only only alunite alunite was was formed, formed, and and the the diffraction diffraction peaks peaks at differentat different pH pH values val- areues almost are almost the same. the same. Thus, Thus, in the in suitable the suitable pH range, pH aluniterange, alunite and natroalunite and natroalunite can be formed can be stably.formed However, stably. However, the vanadium the vanadium compounds compou werends not were detected, not detected, indicating indicating that vanadium that va- maynadium be precipitated may be precipitated in the form in the of amorphous form of amorphous or adsorbed or adsorbed on the natroalunite on the natroalunite or alunite or particlesalunite particles during the during reaction the reaction process. process. WhenWhen thethe pHpH valuevalue waswas aboveabove 0.4, 0.4, part part of of vanadium vanadium was was co-precipitated co-precipitated during during the the formationformation processesprocesses ofof natroalunitenatroalunite andand alunite. alunite. ToTo directly directly visualize visualize the the behaviors behaviors of of Al Al andand vanadiumvanadium duringduring thethe experimentalexperimental process, process, SEM-EDS SEM-EDS analyses analyses were were conducted, conducted, and and thethe results results are are shown shown in in Figure Figure 12 12..

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Figure 12. SEM-EDS patterns of samples obtained at pH value of 0.6: (a) Na:Al = 1:1; (b) Na:Al = 1:1; (c) K:Al = 1:1; (d) K:Al Figure 12. SEM-EDS patterns of samples obtained at pH value of 0.6: (a) Na:Al = 1:1; (b) Na:Al = 1:1; (c) K:Al = 1:1; =Figure 1:1. 12. SEM-EDS patterns of samples obtained at pH value of 0.6: (a) Na:Al = 1:1; (b) Na:Al = 1:1; (c) K:Al = 1:1; (d) K:Al (=d )1:1. K:Al = 1:1. ForFor the the detection detection of of SEM-EDS, SEM-EDS, Na Na and and K Khave have good good relevance relevance for for the the Al, Al, S and S and O elements,O elements,For thewhile whiledetection only only a small of a SEM-EDS, small amount amount ofNa V andwas of V detectedK was have detected good in a small relevance in anumber small for number ofthe particles. Al, ofS and parti- The O particlescles.elements,The shown particleswhile in only Figureshown a small 12a,b in amount Figure are natroalunite, 12 ofa,b V was are natroalunite,detected while the in aparticles small while number theshown particles ofin particles.Figure shown 12c,d The in areFigureparticles alunite. 12 shownc,d Inare Figure alunite.in Figure 12a,c, In 12a,b the Figure particles are 12 natroalunite,a,c, po thessess particles a while good possess thecrystal particles a form good withoutshown crystal in formthe Figure detection without 12c,d oftheare vanadium. detectionalunite. In However, ofFigure vanadium. 12a,c, in Figure the However, particles 12b,d, in thepo Figuressess crystal a 12 good morphologyb,d, crystal the crystal form is not morphology without as complete the detection is as not that as showncompleteof vanadium. in Figure as that However, 12a,c, shown and in Figure Figurea small 12b,d,12 amounta,c, the and ofcrystal a vanadium small morphology amount was ofdetected. vanadiumis not as Thus, complete was vanadium detected. as that mayThus,shown be vanadium adsorbedin Figure maydue12a,c, to be andincomplete adsorbed a small due crystalamount to incomplete growth. of vanadium crystal was growth. detected. Thus, vanadium mayTo Tobe furtheradsorbed analyzeanalyze due to thethe incomplete precipitation precipitation crystal behavior behavior growth. of of vanadium, vanadium, the the samples samples were were analyzed ana- lyzedby XPS.To by ThefurtherXPS. XPS The analyze analysis XPS analysis the was precipitation carried was carr outied asbehavior out illustrated as illustrated of vanadium, in Figure in Figure 13 the. samples13. were analyzed by XPS. The XPS analysis was carried out as illustrated in Figure 13.

Figure 13. The XPS analysis of samples obtained at pH value of 0.6: (a) Na2SO4; (b) K2SO4. Figure 13. The XPS analysis of samples obtained at pH value of 0.6: (a) Na2SO4; (b) K2SO4. Figure 13. The XPS analysis of samples obtained at pH value of 0.6: (a) Na2SO4;(b)K2SO4. As shown in Figure 13, a V 2 p peak with a binding energy of 517.47 eV and 524.01 was detectedAs shown in innatroalunite, Figure 13, awhile V 2 p a peakV 2 p with peak a with binding a binding energy energy of 517.47 of 517.25 eV and eV 524.01 was detectedwas detected in alunite. in natroalunite, The V 2 p while peaks a with V 2 pbi peaknding with energies a binding of 517.47 energy eV, of 517.28 517.25 eV eV andwas detected in alunite. The V 2 p peaks with binding energies of 517.47 eV, 517.28 eV and

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As shown in Figure 13, a V 2 p peak with a binding energy of 517.47 eV and 524.01 eV Minerals 2021, 11, x FOR PEER REVIEW 14 of 17 was detected in natroalunite, while a V 2 p peak with a binding energy of 517.25 eV was detected in alunite. The V 2 p peaks with binding energies of 517.47 eV, 517.28 eV and 524.01 eV belong to the V(V)-O bond [31–33]. In the acid leachate of black shale, vanadium mainly524.01 exists eV belong in a tetravalent to the V(V)-O form, bond and [31–33]. the valence In the state acid ofleachate the vanadium of black shale, used vanadium in the experimentmainly exists was tetravalent in a tetravalent too. This form, indicates and the a change valence in state the chemical of the vanadium environment used of in V the duringexperiment the precipitation was tetravalent process too. of alunite.This indicate A fews a V(IV) change ions in werethe chemical oxidized environment to V(V) ions, of V whichduring were the precipitated precipitation with process aluminum. of alunite. A few V(IV) ions were oxidized to V(V) ions, whichUnder were hydrothermal precipitated conditions, with aluminum. the dissolution efﬁciency of oxygen is increased [26], vanadiumUnder can be hydrothermal oxidized by oxygen, conditions, and transformedthe dissoluti intoon aefficiency pentavalent of oxygen form. According is increased to the[26], potential–pH vanadium can diagram be oxidized for vanadium–water by oxygen, and transformed systems at into 298 Ka pentavalent [34], the form form. of Ac- vanadiumcording present to the potential–pH is different at diagram different for concentrations vanadium–water and valence systems states at 298 of K vanadium, [34], the form 2+ + −5 V(IV)of existsvanadium in the present form of is VO differ, andent V(V)at different exists in concentrations the form of VO and2 or valence HV10O 28statesat of low vana- pH values.dium, V(IV) The V(IV)exists oxidationin the form reaction of VO2+ can, and be V(V) expressed exists byin the Equations form of (9)VO and2+ or (10), HV10 andO28−5 at θ the ∆lowG pHvalues values. are shownThe V(IV) in Figure oxidation 14. reaction can be expressed by Equations (9) and (10), θ and the ΔG values are shown2+ in Figure 14. + + 4VO + O2 + 2H2O = 4VO2 + 4H (9) 4VO +O +2HO=4VO +4H (9) 2+ + + = 5− + + 20VO 5O2 26H2O 2HV10O28 50H (10) 20VO +5O + 26HO=2HVO + 50H (10)

Figure 14. The ΔGθ values of Equations (9) and (10) at different temperatures. Figure 14. The ∆Gθ values of Equations (9) and (10) at different temperatures.

As shownAs shown in Figure in Figure 13, the13, ∆theGθ ΔvaluesGθ values were were negative negative for Equationsfor Equations (9) and(9) and (10), (10), with with ∆GθΔvaluesGθ values decreasing decreasing with with an increasean increase in temperature. in temperature. Therefore, Therefore, the the oxidation oxidation of V(IV) of V(IV) by oxygenby oxygen can can occur occur spontaneously spontaneously under under hydrothermal hydrothermal conditions. conditions. ZetaZeta potential potential analysis analysis of alunite of alunite with with and and without without vanadium vanadium was was carried carried out, out, and and the resultsthe results are shownare shown in Figure in Figure 15. 15.

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Figure 15. The zeta potentials of the precipitates at a pH value of 0.6 ((K) alunite; (Na) natroalunite; Figure 15. The zeta potentials of the precipitates at a pH value of 0.6 ((K) alunite; (Na) natroalunite; (K-V) alunite with co-precipitated vanadium; (Na-V) natroalunite with co-precipitated vanadium). (K-V) alunite with co-precipitated vanadium; (Na-V) natroalunite with co-precipitated vanadium). As shown in Figure 15, the zeta potentials of alunite and natroalunite without co- As shown in Figure 15, the zeta potentials of alunite and natroalunite without co- precipitated vanadium are negative. In the lattice structures of natroalunite and alunite, precipitated vanadium are negative. In the lattice structures of natroalunite and alunite, the octahedral structure of AlO2(OH)4 is connected with the tetrahedral structure of SO2− 42−, the octahedral structure of AlO2(OH)4 is connected with the tetrahedral structure of SO4 , and the Na(K) ions are coordinated with the O atoms and OH groups as AO6(OH)6, lo- and the Na(K) ions are coordinated with the O atoms and OH groups as AO6(OH)6, located − incated an icosahedra in an icosahedra site. In addition,site. In addition, the hydroxyl the hydroxyl anion, anion, OH−, isOH located, is located at the at junction the junction of octahedralof octahedral and icosahedraand icosahedra [34]. The[34]. hydrogen The hydrogen bonds bonds between between the apical the apical oxygen oxygen of the S-Oof the bondS-O andbond the and hydroxyl the hydroxyl group group from thefrom octahedra the octahedra cause cause deformations deformations of the of tetrahedra the tetrahe- anddra octahedra and octahedra [16]. According[16]. According to zeta to zeta potential potential analysis, analysis, the the zeta zeta potentials potentials of of alunite alunite andand natroalunite natroalunite with with co-precipitated co-precipitated vanadium vanadium were were increased increased and and positive. positive. It canIt can be be concluded that the V(V) is present, and was adsorbed in the form of cation VO+ 2+, with concluded that the V(V) is present, and was adsorbed in the form of cation VO2 , with vanadiumvanadium loss loss being being caused caused by by electrostatic electrostatic adsorption. adsorption.

4.4. Conclusions Conclusions ItIt is is feasible feasible to to separate separate aluminum aluminum overover vanadiumvanadium through the the formation formation of of alunite alu- niteand and natroalunite. natroalunite. In hydrothermal In hydrothermal environments, environments, alunite alunite and andnatroalunite natroalunite are able are ableto sta- tobly stably form form during during the process the process of Al of3+ hydrolysis Al3+ hydrolysis precipitation precipitation at a temperature at a temperature of 220 °C, of a ◦ 220pHC value, a pH of value 0.4, and of 0.4,a reaction and a reactiontime of 5 time h. When of 5 h. Al When3+ was Alprecipitated3+ was precipitated at a K/Al atmolar a K/Alratio molar of 1, ratiothe precipitation of 1, the precipitation efficiency efﬁciencyof aluminum of aluminum was 81.75% was via 81.75% the formation via the forma- of alu- tionnite, of alunite,and the andconcentration the concentration of aluminum of aluminum decreased decreased from 16.2 from g/L 16.2to 2.95 g/L g/L. to 2.95When g/L Al. 3+ Whenwas Alprecipitated3+ was precipitated with an Na/Al with anmolar Na/Al ratio molar of 1, the ratio precipitation of 1, the precipitation efficiency of efﬁciency aluminum ofwas aluminum 60.83% was via 60.83%the formation via the formationof natroalunite, of natroalunite, and the concentration and the concentration of aluminum of alu- de- θ minumcreased decreased from 16.2 from g/L 16.2to 6.34 g/L g/L. to 6.34The Δ g/L.Gθ values The ∆ Gof thevalues formation of the formationof alunite and of alunite natroal- ◦ andunite natroalunite are negative are at negative temperatures at temperatures higher than higher 100 °C than and 100 decreaseC and with decrease increasing with in-tem- θ creasingperature. temperature. At the sameAt temperature, the same temperature, the ΔGθ value the of∆ aluniteG value is lower of alunite than is that lower of natroal- than thatunite; of natroalunite; therefore, alunite therefore, forms alunite more formseasily morethan natroalunite. easily than natroalunite. The apparent The activation apparent en- activationergy of natroalunite energy of natroalunite is 96.52 kJ/mol, is 96.52 whereas kJ/mol, th wherease apparent the activation apparent activationenergy of alunite energy is of58.86 alunite kJ/mol. is 58.86 The kJ/mol reaction. The rate reaction increases rate with increases increasing with temperature, increasing temperature,and precipitation and is precipitationcontrolled by iscontrolled the chemical by thereaction. chemical The reaction. zeta potentials The zeta of potentials the alunite of theand alunite natroalunite and natroalunitesurfaces are surfaces negative, are whereas negative, the whereas zeta potentia the zetals of potentials alunite and of alunite natroalunite and natroalunite with co-pre-

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with co-precipitated vanadium are increased and are positive. Vanadium loss increases with increasing pH value. It can be deduced that the ion state of tetravalent vanadium 2+ + (VO ) transformed into a pentavalent vanadium (VO2 ) ion state in the hydrothermal + environment. The VO2 can be adsorbed onto the alunite or natroalunite due to their negative surface charges, ultimately leading to vanadium loss. To avoid vanadium loss, the pH value should not be over 0.4.

Author Contributions: Conceptualization, L.W. and N.X.; methodology, Y.Z., P.H.; software, L.W.; validation, N.X.; formal ananlysis, L.W. and N.X.; resources, Y.Z.; data curation, L.W.; writing— original draft preparation, L.W.; writing—review and editing, L.W. and N.X.; visualization, Y.Z.; project administrtion, Y.Z.; funding acquisition, N.X. and P.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Project of National Natural Science Foundation of China (No. 51804226& 52004187) and the Outstanding Young and Middle-aged Science and Technology Innovation Team Project of Hubei Province (T201802). Data Availability Statement: Data are available on request to the authors. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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