Thermodynamic analysis on the direct preparation of metallic vanadium from NaVO3 by molten salt electrolysis☆

Wei Weng ,Mingyong Wang *,Xuzhong Gong Zhi Wang Zhancheng Guo

1 State Key Laboratory of Advanced Metallurgy,University of Science and Technology Beijing,Beijing 100083,China

2 National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

1.Introduction

Metallic vanadium is widely used in the fields of steel industry,nuclear industry,super conductive alloy materials,and refractory alloys due to a high melting point,high corrosion resistance and small absorption cross-section of fast neutrons[1-4].At present,the metallic vanadium with a purity of about 95%is usually produced by calciothermic or aluminothermic reduction of V2O5powders[5-8].Then,the vacuum thermal refining or electron beam refining is used to obtain high-purity vanadium[9].V2O5is the most commonly used raw material for the production of metallic vanadium.However,V2O5is not a natural substance and generally obtained by complex metallurgical processes from vanadium minerals or vanadium slag which is a by-product of iron making[2].A representative process for the production of V2O5is the sodium salt roasting process,as shown in Fig.1[10-12].NaVO3solution obtained from the vanadium slag by a roasting-leaching procedure is transformed to NH4VO3crystals by ammonia precipitation.Then,NH4VO3is calcinated to produce V2O5.Obviously,the process for the production of V2O5is very complex.In addition,the discharges of high concentration ammonia-nitrogen waste water and NH3during the ammonia precipitation and calcination processes result in serious environmental pollutions.Therefore,it is significant to develop a simple and environmentally friendly route for the production of metallic vanadium.

As shown in Fig.1,high concentration ammonia-nitrogen wastewater and NH3are only discharged during the production of V2O5from NaVO3.It was reported that NaVO3crystals can be directly obtained from NaVO3solution by cooling/evaporation crystallization[13].If NaVO3can be directly converted to metallic vanadium,the ammonia precipitation and calcination processes are avoided.Therefore,the pollution problems due to the discharges of ammonia-nitrogen wastewater and NH3are solved.It is well known that molten salt electrolysis is a common method for metal production[14,15].The low-cost alkali and alkali earth metal chloride salts with a good conductivity and low melting point are usually used as the molten salts[16].NaVO3is an ionic compound and can be ionized to Na+and VO3-due to a low melting point of 630°C.Thus,it may be feasible to directly produce metallic vanadium from NaVO3by molten salt electrolysis.In this paper,thermodynamic analysis and experimental verifications for the preparation of metallic vanadium from NaVO3by electrolysis in chloride molten salts are conducted.

Fig.1.The comparison of the conventional and our suggested processes for the production of metallic vanadium.

2.Calculation Method and Experiment

The standard Gibbs free energy change,ΔG?(kJ·mol-1),was calculated by the HSC thermodynamic software and the theoretical decomposition voltages(U?)were obtained by Eq.(1)[17]:

where n is the electron transfer number and F is a Faraday's constant(96,485 C·mol-1).

All reagents used in this paper were of analytical grade.Before electrolysis,NaVO3,CaCl2and NaCl were dehydrated at673 K in air for 24 h,cooled slowly to room temperature and then transferred quickly to a vacuum drying oven at 373 K.To verify the thermodynamic analysis,NaVO3electrolysis was carried out in the eutectic CaCl2-NaCl molten salt.The content of NaVO3was 5 wt%.Before electrolysis,NaVO3and the chloride salts were homogeneously mixed and then quickly charged into an alumina crucible(outer diameter 80 mm,height 108 mm).The crucible containing the salts was placed inside a stainless steel chamber(as shown in Fig.2)which was continuously flushed with high-purityargon(99.999%).To ensure a good electric conductivity and low viscosity,NaVO3electrolysis was conducted at 800°C in the eutectic CaCl2-NaCl molten salt[18,19].The melting point of the eutectic CaCl2-NaCl salts is 504°C[20].The cell voltage was 3.2 V.The electrolysis temperature was measured by a type-K thermocouple.Both the anode and cathode were graphite rods(d=6 mm).

Fig.2.The illustration of electrolysis equipment.

After electrolysis,the powders mixed with the solidified salt in different regions were leached in an aqueous HCl solution(pH=2-3),rinsed with distilled water and then dried at 373 K under vacuum for 24 h.The obtained powders were characterized by XRD(X'Pert PRO MPD,Philips,Holland),SEM(JSM-7001F,JEOL,Japan)and EDS(INCA X-MPAX,Oxford Instruments,UK).

3.Results and Discussions

3.1.Thermodynamic analysis on the electrochemical reduction of NaVO3 to metallic vanadium in chloride molten salts

The low-cost alkali and alkali earth metal chloride salts with a low melting point,wide electrochemical window and good conductivity are suitable molten salts for the direct preparation of metallic vanadium from NaVO3by molten salt electrolysis[16,21,22].By taking graphite as the anode material,the theoretical decomposition voltages(U?)of NaVO3(reaction(2)and(3))are compared with those of the commonly used alkali and alkali earth metal chloride salts(reactions(4)-(7)).The results are shown in Fig.3.

The theoretical decomposition voltages(U?)of all the chloride salts and NaVO3gradually decrease as a function of temperature.Fig.4 presents the melting points of NaVO3and the chloride salts.The melting points of all the chloride salts are lower than 801°C.However,the melting point of NaVO3is only 630°C.When electrolysis is carried out at a temperature higher than 630°C,NaVO3exists in a liquid state.As shown in the insert of Fig.3,the theoretical decomposition voltages(U?)of the alkali and alkali earth metal chloride salts are higher than 3.24 V at 800°C.However,the theoretical voltage for decomposing NaVO3to metallic vanadium is lower than 0.47 V,which is much lower than those of the chloride salts.Theoretically,metallic vanadium can be obtained from NaVO3by molten salt electrolysis in the cell voltage range of 0.47 V-3.24 V.Under this condition,the chloride salts cannot be decomposed to produce Cl2on the anode.

Fig.3.The theoretical decomposition voltages(U?)of NaVO3 and alkali or alkali earth metal chloride salts vs.temperature(T).(Insert:theoretical decomposition voltages at 800°C).

Thermodynamically,by controlling the cell voltage,the direct preparation of metallic vanadium from NaVO3by molten salt electrolysis is feasible in the alkali and alkali earth metal chloride molten salts.

3.2.Thermodynamic analysis on the electrochemical reduction of NaVO3 to low-valence vanadium oxides

It is confirmed by the thermodynamic analysis in Fig.3 that metallic vanadium can be directly prepared from NaVO3according to reaction(2)or(3)by electrolysis in the chloride molten salts.However,vanadium is a multivalent element.During the electro-reduction of NaVO3,the stable low-valence vanadium oxides such as VO2,V2O3and VO and nonstoichiometric vanadium oxides such as V6O13,V8O15,V7O13,V6O11,V5O9,V4O7and V3O5may be produced.Thus,the thermodynamic analysis on the electrochemical reduction of NaVO3to low-valence vanadium oxides(reactions(8)-(17))was also conducted and the results are shown in Fig.5.When graphite is used as the anode material,the main anode product is CO2during the molten salt electrolysis[21-23].Thus,to simplify the calculation,CO2is considered as the only component in the anode gas.

Fig.4.The melting points of the commonly used chloride salts and NaVO3.

Fig.5(a)shows the relationship between the theoretical decomposition voltages of reactions(8)-(17)and temperature.It is found that reaction(16)has the lowest theoretical decomposition voltage in the temperature range of 300-1000°C.At 1000°C,the value is only 0.26 V.The lowest energy is needed for the production of V2O3from NaVO3.It is easier for NaVO3to be electro-reduced to V2O3thermodynamically.What's more,the theoretical decomposition voltages of reactions(15)and(17)are slightly higher than that of reaction(16).It means that V3O5and VO can be also easily obtained from NaVO3.

The theoretical decomposition voltages of reactions(8)-(17)at 800°C are shown in Fig.5(b).The theoretical voltage for decomposing NaVO3to V2O3is the lowest and only 0.32 V.Theoretical voltages for decomposing NaVO3to V3O5and VO are only 0.36 V and 0.34 V,respectively.The values are lower than that for decomposing NaVO3to metallic vanadium(0.47 V).Thus,V3O5and VO have a great possibility to be co-generated with V2O3.According to the results in Fig.5,compared to metallic vanadium,V2O3,V3O5and VO are easier to be produced from NaVO3by reaction(15)-(17)thermodynamically.During the practical electrolysis of NaVO3,the reactions to produce low-valence vanadium oxides will compete with the reaction for the production of metallic vanadium,which may result in the decrease of current efficiency.Low-valence vanadium oxides,especially V2O3,V3O5and VO,may be suspended or dissolved in the molten salts and the molten salt composition will become more complex.

3.3.Thermodynamic analysis on the electrochemical reduction of low-valence vanadium oxides to metallic vanadium

Based on the analysis in Figs.3 and 5,the theoretical voltage for decomposing NaVO3to metallic vanadium is slightly higher than those for decomposing NaVO3to low-valence vanadium oxides such as V2O3,V3O5and VO.The low-valence vanadium oxides are easily formed during the electrolysis of NaVO3,which leads to the current loss.It was reported that tungsten oxide suspended in the molten chlorides can be electro-reduced to metallic tungsten[22].If the lowvalence vanadiumoxides can be also further electro-reduced to metallic vanadium,current for the production of low-valence oxides is effectively used.Thus,the feasibility that the low-valence vanadium oxides are electro-reduced to metallic vanadium is analyzed.

Fig.5.(a)The relationships between the theoretical decomposition voltages(U?)of NaVO3 and temperature,(b)the theoretical decomposition voltages at 800°C.

The theoretical voltages for decomposing low-valence vanadium oxides to metallic vanadium(reactions(18)-(20))are presented in Fig.6.Obviously,the values are lower than 0.68 V at 800°C.Especially,the theoretical voltage for decomposing V2O3to metallic vanadium is only 0.59 V and also much lower than those of the chloride salts.So,the low-valence vanadium oxides suspended in the molten salts can be further electro-reduced to metallic vanadium thermodynamically.

4.Experimental Verification on the Electrochemical Reduction of NaVO3 to Metallic Vanadium

Fig.6.The theoretical decomposition voltages(U?)of low-valence vanadium oxides.

According to the thermodynamic analysis,NaVO3can be directly electro-reduced to metallic vanadium.However,it is difficult for NaVO3to be one-step reduced to metallic vanadium by a five electron transfer process.The reduction reaction may undergo a multi-step process[19].To verify the results of thermodynamics analysis,NaVO3electrolysis was performed in the nontoxic and cheap eutectic CaCl2-NaCl molten salt[14,19,22,23].A higher cell voltage is favorable to promote the electro-reduction of NaVO3.However,if the cell voltage is larger than 3.24 V,NaCl and CaCl2may be decomposed[21].So,3.2 V was chosen as the cell voltage for the electrolysis of NaVO3.

After washed by the deionized water,metallic vanadium was detected in the residues obtained from the solidified salt in the nearcathode region,as shown in Fig.7(a).However,the main component was Ca(OH)2which was formed due to the hydrolysis of CaO.Na2O was released during the electrolysis of NaVO3,as shown in reactions(2),(3)and(8)-(17).CaO was formed due to the reaction between CaCl2and Na2O by reaction(21).

The main component of the residues in the near-anode region was CaCO3,as shown in Fig.7(c).CaO produced nearcathode has a high solubility in the eutectic CaCl2-NaCl molten salt and can be transferred to the anode region[20,21,23].CaCO3was easily formed due to the reaction between CaO and CO2produced on the anode.In addition,CO2produced on the anode may combine with O2-to form CO32-[21],which also led to the formation of CaCO3.

The solid residues were further washed in a dilute HCl solution.Ca(OH)2and CaCO3disappeared,as shown in Fig.7.For residues obtained from the bulk molten salt(Fig.7(b))and near-anode region(Fig.7(c)),the low-valence vanadium oxides such as V3O5,V2O3and VO were the main components.Particularly,V2O3was dominant and also detected before washed in a dilute HCl solution(Fig.7(b)and Fig.7(c)).The results were consistent to the thermodynamic analysis in Fig.5.The stable low-valence vanadium oxides were produced more easily.For the residues obtained from the near-cathode region(Fig.7(a)),the diffraction peaks of metallic vanadium were preferred and no low-valence vanadium oxides were found.It meant that metallic vanadium was successfully produced from NaVO3.In other words,the low-valence vanadium oxides in the near-cathode region can be further reduced to metallic vanadium.It confirmed that the electrochemical reduction of NaVO3may be a multi-step reaction process.The further research about the reaction mechanism is underway.The metallic vanadium obtained in the near-cathode region was spongiform and the vanadium content was up to 96.8 wt%,as shown in Fig.8.So,it is feasible to directly produce metallic vanadium from NaVO3by molten salt electrolysis.However,the current efficiency for the production of metallic vanadium at 3.2 V was only 29%.The yield of vanadium was 69.5%.The low-valence vanadium oxides produced in the nearcathode region may be transferred to the anode region and be reoxidized,which decreased the current efficiency.In our opinion,the design of electrolytic cell to avoid the transfer of low-valence vanadium oxides to the anode region is a key method to increase the current efficiency[21].

Fig.7.XRD patterns of solid residues obtained from the solidified salt in different regions after washed in the deionized water and dilute HCl solution:(a)in the near-cathode region;(b)in the bulk molten salt;(c)in the near-anode region.

Fig.8.SEM and EDS of metallic vanadium powders obtained in the near-cathode region.

On the other hand,C in the form of VC was the main impurity(Fig.7(a)and the insert in Fig.8).C may originate from the graphite cathode due to the reaction between the metallic vanadium and carbon.To verify the analysis,△G?of reactions(22)-(27)were calculated and presented in Fig.9.All△G?values of reactions(22)-(27)are negative.It means that the reactions between the metallic vanadium and carbon can proceed spontaneously.However,as shown in Fig.7(a),only VC was detected and the peak intensity of VC in the XRD pattern of cathode products was much weaker compared to that of metallic vanadium.It is ascribed to a low kinetic reaction rate between the metallic vanadium and carbon at a temperature lower than 850°C[7,24,25].Carbon content may be effectively reduced by using a metal cathode.

As shown in Fig.1,two high-temperature processes are needed to produce metallic vanadium from NaVO3in the conventional process.The calcination of NH4VO3for the production of V2O5is performed at about 550°C[26].The thermal reduction of V2O5for the production of metallic vanadium is usually ignited by a reaction booster such as barium peroxide at about 750°C[5,8].For the suggested new process,NaVO3is one-step electrochemical reduced to metallic vanadium at 800°C by molten salt electrolysis.Only one high-temperature process is needed.

Fig.9.The standard Gibbs free energy(ΔG?)changes of reactions between C and V.

On the other hand,the electrolysis of NaVO3for the verification experiments in this paper was performed at 800°C in the eutectic CaCl2-NaCl molten salt.However,the melting point of the eutectic CaCl2-NaCl molten salt is only 504°C[4].So,there is a huge potential to decrease the operational temperature for the suggested new process[20].For example,a temperature of 550°C was chosen for the study of reduction behavior of titanium ions in the equimolar CaCl2-NaClmolten salt[27].

In addition,the treatments of the high concentration ammonianitrogen waste water and NH3released in the conventional process are also high energy-consuming and cost.However,in the suggested new process,the release of high concentration ammonia-nitrogen waste water and NH3is avoided.So,the suggested new process might be superior to the conventional process in the energy-saving aspect.The optimization of electrolytic reduction of NaVO3to metallic vanadium is still underway.The energy consumption of the suggested and conventional processes can be better compared when the optimization for the electrolysis of NaVO3is completed.

5.Conclusions

The feasibility for the production of metallic vanadium from NaVO3by molten salt electrolysis is analyzed thermodynamically.It is found that the theoretical voltage for decomposing NaVO3to metallic vanadium is only 0.47 V at800°C,and much lower than those of the alkali and alkali earth metal chloride salts.However,the theoretical voltages for decomposing NaVO3to low-valence vanadium oxides(such as V2O3,V3O5and VO)are slightly lower than those for decomposing NaVO3to metallic vanadium.Thermodynamically,the low-valence vanadium oxides are firstly produced from NaVO3by molten salt electrolysis,and then can be further electro-reduced to metallic vanadium.To verify the thermodynamic analysis,the electrolysis of NaVO3was performed in the eutectic CaCl2-NaCl molten salt.Metallic vanadium with a purity of about 96.8 wt%is obtained on the near-cathode region.Both the thermodynamic analysis and experimental results confirm that the electrochemical reduction of NaVO3to metallic vanadium is feasible.Although carbon is introduced due to the reaction between the metallic vanadium and graphite cathode,it may be avoided by using a metal cathode.

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