Tongxiang Ma, Lang Zhao, Yu Yang, Liwen Hu, Shengfu Zhang, Meilong Hu

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

Keywords:Anode area Anode current density Carbon parasitic reactions Current efficiency

A B S T R A C T In the electro-deoxidation process,carbon parasitic reaction(CO32-+4e-=C+3O2-)usually occurs when using carbon materials as the anode,which leads to increase of the carbon content in the final metal and decrease of the current efficiency of the process. The aim of this work is to reduce the negative effect of carbon parasitic reaction on the electrolysis process by adjusting anode current density.The results indicate that lower graphite anode area can achieve higher current density, which is helpful to increase the nucleation site of CO2 bubbles. Most of CO2 would be released from the anode instead of dissolution in the molten CaCl2 and reacting with O2- to form CO32-, thus decreasing the carbon parasitic reaction of the process. Furthermore, the results of the compared experiments show that when the anode area decreases from 172.78 to 4.99 cm2, CO2 concentration in the released gases increases significantly, the carbon mass content in the final metal product decreased from 1.09%to 0.13%,and the current efficiency increased from 6.65%to 36.50%.This study determined a suitable anode current density range for reducing carbon parasitic reaction and provides a valuable reference for the selection of the anode in the electrolysis process.

1. Introduction

Molten salt electrolysis has attracted considerable attention in recent years, which can be used to extract metals and alloys from their mixed oxides, especially by the Fray-Farthing-Chen (FFC)Cambridge process [1] and Ono-Suzuki (OS) process [2]. In these electrolysis processes, carbon-based materials are commonly used as anode due to its low cost and excellent performance (e.g.,mechanical integrity at high temperatures, high temperature strength,thermal shock resistance and corrosion resistance in molten salt[3]). However, undesirable parasitic reactions will be happened when carbon anodes are used.

The carbon parasitic reaction can bring a series of the negative effects, such as reducing the current efficiency,increasing the carbon content in the final products and caused molten salt polluted.In the electro-deoxidation process, O2-removed from the cathode oxides and CO2released on the carbon anode. CO2combines with O2-dissolved in molten salt to form CO32-(Eq. (1)), CO32-migrates to the cathode and reduced to carbon (Eq. (2)) [4]. There is charge competition between the carbon parasitic reaction and the direct reduction of cathode oxide,which leads to the decrease in the current efficiency of the deoxidation process. In addition, the deposited carbon can react with the metal on the cathode, resulting in high carbon content in the cathode product.

Carbon contamination of the cathode products is a widespread risk in electrolysis process,which has been reported in many published papers [5-8]. Dringet al.[9] have studied direct electrochemical production of the Ti-10 W alloys from mixed oxide, TiC impurities were observed on the surface of the alloy. Wenget al.[10]found that VC existed in the product when preparing metallic vanadium from NaVO3, the carbon contents of vanadium obtained in the CaCl2-NaCl molten salt and CaCl2molten salt were 2.07%and 3.78%,respectively.Suzukiet al.[11]reported that the surface of the molten salt was fully covered with carbon particles during the electrolysis process, resulting in formation of TiC impurities in the product titanium. Because the high carbon content has a strong impact on the plasticity of metal materials[12],the composition standards of metals and alloys have strict requirements for carbon mass content,e.g., general purpose Ti less than 0.03%required by ASTM-B299-13 [13]. Therefore, reducing carbon parasitic reactions of the electrolysis process is great significance for improving the quality of metal products.

In recent years,researches about reducing the carbon content of cathode products have been reported. Wenget al. [8] adopted the liquid Zn as cathode to study the electrochemical reduction of soluble K2CrO4to metallic Cr. Electrodeposited metallic Cr can stably diffuse into liquid Zn,while a negligible solubility of carbon in liquid Zn, thus the product carbon contamination is effectively avoided.According to Eqs.(1)and(2),the carbon parasitic reaction can be eliminated by avoiding the formation of CO2. Some reports have converted the cathode precursor from metal oxide to metal sulfide[14,15].Gaseous sulfur was generated on the graphite anode instead of CO2and carbon deposition reaction on the cathode was avoided. Yuanet al. [16] analyzed the anode gaseous during the electrolysis of Ta2O5in MgCl2-NaCl-KCl melt, the results of CV and GC analysis showed that there is no CO2was formed on the anode, eliminating related problems caused by CO2. In addition,the development of O2evolution inert anodes is also a strategy to eliminate the cathode carbon contamination, but there is still no suitable commercial inert anode. For example, when SnO2anode is used, the surface will have an insulating layer of CaSnO3after working for a long time,which raised the resistance of the cell[6,17].CaRuO3anode has excellent stability,but the high cost limited its commercial application [18]. Therefore, carbon-based materials are still the most commonly used anode in laboratory and industry. However, there are few studies about the effect of anode current density on the carbon parasitic reaction in electrolysis process.

This work aims to explore a suitable anode current density range for reducing carbon parasitic reaction.Here, TiO2is selected as the precursor to study the influence of the anode current density on the carbon parasitic reaction during electrolysis,and the anode current density is adjusted by changing the anode area.The effects of anode current density on the evolution of cathode products,anode gas release, and current efficient were discussed in detail.

2. Experimental

In this paper,the electrolysis conditions of each experiment are identical. 1.5 g of TiO2powder (AR, Sinopharm Chemical Reagent,China) was selected as the oxide precursor, which was wrapped with 400 mesh (0.038 mm) stainless steel mesh and fixed on the stainless-steel electrode rod. CaCl2is used as electrolyte after dehydration in air at 350 °C for 12 h. The electrolysis process was performed in a tubular electrolytic furnace with an inert atmosphere, the detailed electrolysis procedure was described in our previous work [19]. The anode current density is adjusted by the anode area,which is changed by the anode diameter and the depth of immersion in molten salt. A 2 L foil gas sampling bag is connected to the gas outlet to collect anode gas in each stage of electrolysis. After the electrolysis, the cathode product is leached in dilute hydrochloric acid to remove the remaining molten salt,then washed in ultrasonic water bath for 30 min and dried at 100 °C.

The products phase composition and carbon content of the electrolytic products was characterized by X-ray diffraction spectroscopy (XRD, D/max 2500PC, Rigaku, Japan) and carbon-sulfur detector (CS-8820, Jinyibo, China). The anode gas was analyzed by gas analyzer (Antaris IGS, Thermo Scientific, America).

3. Results and Discussion

3.1. Effect of the anode current density on cathode product

During the electrolysis process, the current density is closely related to the frequency of anode bubble release. As the current density increases, the size of the anode bubbles gradually decreases, and the release frequency increased significantly [20](e.g., the bubble release frequency increased by 10 times when the current density increased by 4 times[21]).The variation trend of the anode current density in the electrolysis process was recorded in Fig.1.It can be found that lower anode area can obtain higher current density, especially in the early stage of electrolysis.During the electro-deoxidation process, a large amount of CO2is produced in the early stage of electrolysis[22,23].For lower anode area, the higher current density allows CO2bubbles to release rapidly from the molten salt, thus shortening the reaction time of CO2and O2-and reducing the generate of CO32-ions. Therefore,higher anode current density can be obtained by reducing the anode area, which will inhibit the carbon parasitic reaction and avoid carbon contamination of cathode products.

Fig. 1. Current density-time curve during electrolysis under different anode areas.

Fig.2 shows that the main phase of the product is Ti metal after electrolysis for 12 h.However,TiC impurities were detected in the product when the anode area is more than 8.64 cm2, which is formed by the reaction of carbon(formed by carbon parasitic reaction) and metallic titanium at the cathode (Eq. (3)). It can be seen from Eq. (3) that Ti and C can react easily at 1173 K, but no TiC impurities were detected when the anode area is 3.30 and 4.99 cm2(Fig. 2), so there may be a little carbon formed on the cathode.These results may be attributed to the inhibition of carbon parasitic reactions by higher anode current density (As discussed in Fig.1).Although low anode area can obtain higher anode current density,it will also lead to the increase of anode over potential and the uneven distribution of potential and current on the cathode,which leads to the decrease of the cathode deoxidation rate [24](e.g., cathode product is Ti6O when the anode area is 2.43 cm2).Therefore, it is necessary to select a suitable range of current density in the electrolysis process to balance the parasitic reaction and the deoxidation rate.The shaded portion of Fig.1 provides a rough range of anode current density, which ensures a suitable rate of deoxidation while inhibiting the carbon parasitic reaction.

Fig. 2. XRD patterns of products after electrolysis for 12 h at different anode area systems.

3.2. Effect of anode current density on electrolysis process

Graphite crucibles(annulus anodes)are usually used as anodes because they can provide the most uniform current density distribution and improve energy efficiencies [22,24]. However, as mentioned in Section 3.1,high anode area leads to low current density,the negative effects of the low current density on electrolysis process should also be considered. Therefore, the graphite crucible(anode area: 172.78 cm2) and 6 mm graphite rod (anode area:4.99 cm2) were selected as representative anodes for comparative experiments.The effects of anode current density on current curve,the carbon content of product and the current efficiency of process were discussed in detail.

3.2.1. Current curve

Fig. 3(a) is the current-time curve during the electrolysis process.It can be seen that the current of the crucible as anode is significantly higher than that of the rod anode. It can be explained that the anode over potential being lower when higher anode surface area is used, which leading to larger currents based on a higher cathodic potential at constant voltage [6,23].

Interestingly, when the graphite crucible is used as anode, the current will rise after the current reaches a low point, rather than become stable as the rod anode (Fig. 3(a)). These features can be attributed to the high background currents and the undesirable short circuit currents.When the graphite crucible is used as anode,the low anode current density leads to the decrease of bubble release frequency[20,25],most of the CO2bubbles will be captured by O2-to form CO32-during the slow coalescence and growth of bubbles. CO32-is reduced to C and O2-at the cathode, the rereleased O2-then migrates to the anode to produce CO2, which is converted to CO32-again.During this cycle,more parasitic reactions result in higher background currents. It can be also observed from Fig.3(b)that the CO2concentration of the anode gas in the graphite crucible anode system is much lower than that of the graphite rod anode,which confirms that most of the CO2is dissolved in the molten salt instead of being released. When a graphite rod is used as the anode, the higher bubble release frequency shortens the reaction time of CO2with O2-in the molten salt, and inhibits the formation of CO32-ions, so the contribution of parasitic reactions to the background current is reduced. On the other hand, when graphite crucible as anode,there is a thick carbon layer on the surface of molten salt (Fig. 3(a)). As discussed above, high anode area will promote carbon parasitic reactions, which leading to more carbon generation on the cathode.In addition,more and more of O2-to be released back into the melt (Eq. (2)) to allow further carbon consumption at graphite anode [26], which aggravates the corrosion of the graphite anode and causing a large amount of carbon powder to fall off. The conductance of the solidified dense carbon precipitate was 1.1 × 104S·m-1(～7.7 × 104S·m-1of pure graphite)[5].Therefore,with the continuous accumulation of carbon powder on the molten salt surface, the short-circuit current between the cathode and anode gradually increases [27], which causes the abnormal current curve. However, when graphite rod is used as anode, there is less carbon powder on the surface of molten salt(Fig. 3(a)), which is not enough to connect anode and cathode to form short circuit.

Fig. 3. (a) Current-time curves during electrolysis and photograph of molten salt;(b) concentration of anodic CO2 gases during electrolysis.

3.2.2. Cathode product

Fig. 4 shows the XRD patterns of the cathode products after electrolysis for different time. As shown in Fig. 4(a) and (b), the cathode product of crucible anode cell is Ti after electrolysis for 3 h, while that of rod anode cell is Ti3O, so the reduction rate of the cathode oxide is faster when the graphite crucible is used as the anode. Compared with the graphite rod, the graphite crucible anode can reduce the anode over potential and provide more uniform potential and current distribution on the cathode[24],which is conducive to the rapid deoxidization of cathode oxides in the early stage of electrolysis. However, the final product is Ti after electrolysis for 12 h in both anode systems. Thermodynamic results show that as the oxygen content in titanium decreases,the oxygen potential of the oxide gradually decreases and becomes more stable,so the deoxidation of the Ti-O solid solution becomes more and more difficult [28]. The anode area has almost no effect on the reduction of titanium-oxygen solid solution. Therefore, the effect of anode area on the oxygen content of product gradually decreases with the prolongation of electrolysis time.

Generally, the oxygen content of cathode product can be reduced to the acceptable standard by prolonging the electrolysis time, it will cause another problem that the carbon content of the product increases.As can be seen from Fig.4(a),when the graphite crucible was used as the anode, TiC impurities were present once the cathode was reduced to Ti. However, when the graphite rod was used as anode, TiC was not observed in cathode products(Fig. 4(b)), which can be seen more clearly in the XRD local enlarged image of Fig. 4(c) and (d). The carbon content of the product was detected by carbon-sulfur detector. As shown in Fig. 5,the carbon content in the cathode products increased with the prolongate of electrolysis time, and the carbon content increased rapidly when the cathode was reduced to titanium.When graphite rod is used as anode, the carbon mass content of the final product is only 0.13%,which is significantly lower than that of graphite crucible as anode(1.09%).Therefore,the results indicate that the parasitic reaction can form carbon at the cathode during the electrolysis process,and it will react with cathode to form carbide impurities when the oxide precursor is reduced to the metal phase.The lower anode area can obtain higher anode current density,which significantly limit the parasitic reaction of carbon and reduce the carbon content of the product.

Fig.4. XRD patterns of products after electrolysis for 0.5,1,3,6 and 12 h:(a)graphite crucible as anode,(b)6 mm diameter graphite rod as anode;local enlarged images of XRD patterns after electrolysis for 3, 6 and 12 h: (c) graphite crucible as anode, (d) 6 mm diameter graphite rod as anode.

Fig. 5. C content in cathode product after electrolysis for different time.

During the electrolysis process,the carbon powder comes from the carbon parasitic reaction on the cathode(Eq.(2))and the physical shedding on the anode[26].As discussed above(Section 3.2.1),when the graphite crucible is used as anode,both carbon parasitic reactions and anode erosion are enhanced. Therefore, a reactor with corundum tube was designed to verify the source of carbon in the cathode product (Fig. 6(a)). In this reactor, the cathode precursor is suspended in a bottom-opening corundum tube (Fig. 6(c)),which allows the ions to move freely and avoids the migration of carbon powder from the graphite crucible into the cathode region.Because the carbon powder is very light,the carbon powder produced from the cathode and anode will float on the molten salt surface in their respective region(the inside of the corundum tube is the cathode region and the outside is the anode region).It can be seen from Fig. 6(b) that there is a thick carbon layer on the anode region, which indicates that a large amount of carbon powder is shed form graphite crucible during the electrolysis. However, the TiC impurities still exist in the cathode product in the case of isolating the anode carbon powder (Fig. 6(d)), and the carbon mass content is not significantly different compared to that without corundum tube (with corundum tube: 0.948%, without corundum tube: 1.089%). Therefore, the carbon contamination of the product is mainly caused by the parasitic reaction on the cathode, and the carbon powder shed from the graphite anode floats on the molten salt surface, which has little effect on the carbon content of the cathode.

Fig.6. (a)Schematic of electrochemical reactor;(b)photo of reactor after electrolysis;(c)photo of corundum tube;(d)XRD pattern and carbon mass content of product after electrolysis for 12 h.

3.2.3. Current efficiency

The current efficiency is calculated from the ratio between the theoretical charge required to remove the oxygen from the TiO2pellet and the actual charge passed over the electrolysis time[29]. Fig. 7(a) shows that the current efficiency of each of the experiments, the current efficiency of graphite crucible cell is significantly lower than that of graphite rod after the same electrolysis time. As shown in Fig. 7(b), the actual charge consumption is higher when higher surface areas are used, and the extra charge is mainly caused by parasitic reactions and short-circuit currents[6]. The lower anode area can promote the release of CO2at the anode (Fig. 3(b)), thereby inhibiting the generation of parasitic reactions, reducing additional current consumption and avoiding short-circuit current caused, resulting in higher current efficiency.In addition, once cathodic reduction becomes difficult in the later stage of electrolysis, the contribution of cathode deoxidation to the total current gradually decreases, and most of electricity will be consumed by the parasitic reactions,conductivity and short circuit [30]. Therefore, Fig. 7(a) shows that the current efficiency decreases with the prolongation of electrolysis time.

Fig.7. (a)Current efficiency of crucible anode and rod anode after electrolysis for different time(3,6,12 h);(b)actual charge consumption after electrolysis for different time.

4. Conclusions

In this paper,TiO2is selected as the precursor to study the influence of the anode current density on the carbon parasitic reaction during electrolysis and determined a suitable anode current density range for reducing carbon parasitic reaction.The results show that the anode current density is closely related to the release frequency of bubbles on anode, the lower anode surface area can obtain higher anode current density and promote the release of CO2bubbles, and most of the CO2will release from molten salt instead of being captured by O2-to form CO32-. Therefore, the carbon parasitic reaction is inhibited,which decrease the carbon content in products and improve the current efficiency of the process.When the anode area decreases from 172.78 to 4.99 cm2, the concentration of CO2in the anode gas increases significantly, the carbon mass content in the product decreased from 1.09% to 0.13%,and the current efficiency increased from 6.65%to 36.50%.In addition, an electrochemical reactor is designed to determine the source of carbon in the product, the results show the carbon contamination of the product is mainly caused by the carbon parasitic reaction,while the carbon powder off the anode has little effect on the carbon content of the product.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (51674054), and supported by the Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials, Chongqing University, China.