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Materials Transactions, Vol. 58, No. 6 (2017) pp. 914 to 920 ©2017 The Institute of Metals and Materials

Carbon Dioxide (CO2) Released in the Electrochemical Reduction of Dioxide (TiO2) to Titanium Metal Galam Govinda Rajulu1,2, M. Girish Kumar1, K. Srinivas Rao1, B. Hari Babu2,* and Chaganti RVS Nagesh1,*

1Defence Metallurgical Research Laboratory, Hyderabad, TS, 500058, 2Department of , Acharya Nagarjuna University, Guntur, AP, 522510, India

Studies pertaining to kinetic aspects of the direct electrochemical reduction process of titanium preparation from TiO2 have not been substantial for determining the conversion time precisely. The solid state reduction process does not permit measuring either the rate of forma- tion of product or the rate of depletion of the feed stock. In this work an attempt has been made to study the applicability of variation of CO2 (the product ) in the vent of the electrolytic cell to approximate the bulk process kinetics of the process. From the electrolysis current trend, it might be possible to estimate CO2 pro le during the electrolysis time so as to utilize the same for inferring optimal time of the electro- chemical reduction process. [doi:10.2320/matertrans.MK201633]

(Received October 14, 2016; Accepted February 23, 2017; Published May 12, 2017)

Keywords: electrochemical reduction, , solid state conversion, CO2 in vent gases, reaction time estimation

1. Introduction intermediate titanium . Lowering of activity of CaO, (generated due to calciothermic reduction of ) as it Extraction metallurgy of titanium has been very complicat- dissolves in molten CaCl2 circumvents the thermodynamic ed and cumbersome mainly because of its high restrictions prevailing in the conventional calico-thermic re- (≈ 1663°C) and high chemical reactivity. Especially prepara- duction process in obtaining titanium suf ciently free from tion of titanium metal directly from its widely occurring ox- . Alternatively, the reduction process is also explained ide (TiO2) by employing all the conventional reduction tech- to be taking place through oxygen (generated due to ap- niques proved futile because of high stability of the oxide and plied DC voltage) transportation from the oxide through the af nity of the metal towards oxygen. The Kroll process, in to form O/CO/CO2 at the anode, which which is reduced by metal, is let out through the vent gases of the cell. Under experimen- has been the predominant method of titanium extraction. tal conditions, dioxide is however, the predominant However, since the emergence of the electrochemical reduc- product gas of the overall electrochemical reduction process. tion process popularly known as FFC Cambridge process1–3) Current exercise, aims at understanding the bulk process ki- wherein the metal oxide (TiO2) is converted into metal netics in terms of CO2 release in the electrochemical reduc- through cathodic action, expectations have been high for real- tion process. Based on experimental measurements and theo- izing an alternate cost and effective method of prepar- retical predictions on CO2 content in the vent gases of cell, it ing titanium metal of required purity. Hence the process has is possible to understand the kinetic aspects of the conversion been widely pursued for greater understanding of the process process in various stages of the experiment. Before discuss- and for scale up to produce the metal in larger batches. ing the results of these efforts, a review of current understand- In the electrochemical reduction of TiO2, the overall con- ing on various kinetic aspects of the process is brought out in version of the oxide to metal is described simply by removal the following. of oxygen in ionic form, from the oxide which passes through the conducting electrolyte (molten chloride at about 2. Mechanism and Kinetic Aspects of the Reduction 950°C) to a graphite anode to form CO/CO2 as per the fol- Process lowing: Several investigators have reported works related to the TiO + 4e = Ti + 2O2 (at cathode) (i) 2 − mechanism of the reduction process and tried to relate the 2 inuence of various experimental conditions such as type of C + 2O − 4e = CO (at anode) (ii) − 2 pre form, applied voltage, current , temperature etc. 6–16) However, the actual mechanism of conversion of TiO2 to on the mechanism and pathway of metal formation . Ac- 9) titanium metal appears to be more complex with formation of cording to Schwandt and Fray the reduction of TiO2 to Ti several sub- of titanium and calcium titanate as inter- take place through formation and of calcium mediate reaction species4,5). It has been convincingly ex- titanate and the conversion process is associated with sub- plained that the calcium metal deposited (due to the electrol- stantial changes in the microstructure. Similar ndings were 10) ysis of CaO dissolved in the CaCl2 electrolyte), on the cath- also reported by Jayashree Mohanthy et al . Lebdev et 11,12) ode (TiO2) can reduce the oxide into metal through various al discussed about possible mechanism and kinetics of processes occurring at TiO2 cathode and graphite anode inde- * Corresponding author, E-mail: [email protected], dr.b.haribabu@ pendently in CaCl2-CaO melt. According to which the CaO gmail.com concentration in the CaCl2 melt does play a role on the kinet- (CO2) Released in the Electrochemical Reduction of Titanium Dioxide (TiO2) to Titanium Metal 915

13) ic aspects of the process. Studies by Alexander et al , indi- TiO2 to Ti can be expressed in terms of a set of thermodynam- cated that the conversion of TiO2 to titanium metal takes ically possible chemical reactions involving sub-oxides of ti- place through formation of several sub-oxides of titanium and tanium as given below: calcium titanate though formed, can decompose into titanium 8TiO2 + C = 2Ti4O7 + CO2 sub oxide and . It is also said that the formation (iii) ∆G◦ = 863178 J/mol of calcium titanate can be minimized by the use of highly 1200K − 14) sintered/less porous TiO2 pre-forms. Dring et al dwelled 16Ti4O7 + C + 2Ti = 22Ti3O5 + CO2 into thermodynamic aspects of Ca-Ti-O-C-Cl system to pro- (iv) ∆G◦ = 1124570 J/mol pose predominance area diagrams for electrochemical reduc- 1200K − tion of titanium dioxide in molten calcium chloride. Interest- 3Ti2O3 + C + Ti = 7TiO + CO2 ing results/suggestions reported by this work include favour- (v) ∆G◦ = 843871 J/mol able conditions for producing low oxygen titanium metal. 1200K − According to these studies higher pO2- values in the melt are 2TiO + C + Ti = 3Ti + CO2 favourable for rapid reduction in the initial stages and towards (vi) ∆G◦ = 525832 J/mol end of the reduction, relatively a lower temperature and re- 1200K − duced oxide activity will support for high purity titanium for- Chemical reactions associated with calcium titanate might mation. In an investigation into the kinetics of the electro- not be considered in representing the overall conversion pro- chemical reduction process reported by Songli Liu15), it is cess as it can exist only as reaction intermediate and is also stated that while the reduction process is associated with for- expected to dissociate into titanium sub-oxide and CaO. mation of sub-oxides of titanium and calcium titanate, hydro- Thus the overall electrochemical reduction process can be lysis and subsidiary reactions occurring at anode may result represented as in reduced current ef ciency of the process. According to TiO2(in contact with CaCl2) + C(graphite anode in contact with CaCl2) these works, in the rst stage, TiO2 is reduced to Ti3O5 and = Ti(left behind in the starting material) + CO2(anode gas) Ti2O3 and form CaTiO3. In the second stage CaTiO3 and Ti2O3 are converted to TiO and nally TiO is reduced to Ti. A Since it is practically dif cult to monitor the chemical few other works16,17) report on inuence of various additives changes taking place in the oxide during the process so as to to TiO2 pre-cursers on the rate of the electrochemical reduc- correlate the experimental conditions with bulk process kinet- 18) tion process. Bhagat et al conducted in situ synchrotron ics, study of CO2 generation during the course of reduction diffraction to propose electrochemical pathway of TiO2 re- process shall be useful in understanding the kinetic aspects of duction and discussed the kinetic aspects of oxygen ion gen- the overall conversion process. In this work an attempt has eration at cathode and formation of CaO and other com- been made to understand kinetic aspects of the process with 19) pounds. Hualin Chen et al studied inuence of graphite experimentally measured values of CO2 content in the vent anode area on the oxidation kinetics of O2- at the anode and gases of the electrolytic cell during the course of reduction suggested that a low anode current is bene cial for process. Measurements of CO2 in the out gasses could be reducing energy consumption. An elaborate review by Mo- made at selected time intervals of the process in two different handas20) brings out summary of various developments that scales of operation viz 500–1000 g cell and 5–8 kg cell. have been taken place over the years on understanding of the electrochemical conversion of oxides to metal in general. 3. Experimental From the above description, it is pertinent to state that si- multaneous occurrence of various cathodic and anodic cell Experimental work on electrochemical reduction of titani- reactions and a set of chemical reactions associated with the um dioxide to titanium metal has been pursued on different species TiO2, Ti, Ca, C CaO and CO/CO2 would determine scales of operation so as to develop improved understanding the bulk kinetics of the electrochemical reduction process of of the process. After establishing the feasibility of the process 21) TiO2 to titanium metal. As already presented earlier , the on 500–1000 g scale, scaled up experimentation on 5–8 kg bulk kinetics of the reduction process can be determined from per batch is continuing. Detailed description of experimental the rate constants of all the associated kinetic steps such as (i) procedures pertaining to establishment of feasibility of the oxygen ion generation at cathode and its transportation to process from gram scale to 1000 gram scale is available else- cathode/electrolyte interface, (ii) transportation of oxygen where23–25). In all the experimental works Merck make high ion through electrolyte to anode/electrolyte interface and (iii) purity titanium dioxide is used. Experimental set up em- evolution of O/CO/CO2 at the anode. These kinetic steps ployed for both the scales of operation are similar and com- however, additionally might depend on aspects such as disso- prise a cathode basket made of AISI 304 perforated sheet in lution kinetics of CaO in CaCl2, of calcium in which sintered TiO2 granules are loaded. Electrolytic grade CaCl2, related surface ows, parameters related to graphite plates (rectangular/curved shaped) are used as an- gas evolution at the anode, solid state phase/crystalline ode. The electrode rods are used both for holding the changes in the cathode feed stock etc. In the earlier work21) an cathode basket and graphite anode plates in speci ed posi- attempt has also been made to determine time of conversion tions in the cell as well as for connecting to DC power source. of the oxide granule under an assumption that oxygen ion Typical experimental procedure of 5–8 kg/batch experimen- mobility from the bulk oxide to oxide/electrolyte interface as tation includes preparation of granules in the the predominant and rate controlling kinetic step. size range of 8–12 mm which are sintered at 1000°C for 24 As reported earlier22), the overall reaction of conversion of hours before taken into the cathodic basket which takes the 916 G. G. Rajulu, M. G. Kumar, K. S. Rao, B. H. Babu and C. RVS Nagesh

Fig. 1 (a) Schematic of experimental set up (5–8 kg scale), (b) Schematic of nozzles and vent line arrangement on the cell lid (5–8 kg scale).

dimensions of 250 × 450 × 100 mm. As shown in Fig. 1 (a), grade anhydrous calcium chloride is taken into the reactor graphite plates of 250 × 400 × 15 mm thick placed on either and melted under gas and then cooled to side of the cathode basket form anode and the cathode- anode room temperature. At room temperature, after placing the assembly is placed inside a stainless steel reactor (450 mm electrodes assembly, the reactor is closed with a stainless ID × 1300 mm height × 12 mm thick). with the help of stain- steel lid that has necessary openings and nozzles for (i) taking less steel lead rods. These are connected to a DC power out the electrode lead rods, (ii) for evacuation of the reactor source for applying DC voltage during the electrolysis. A system, (iii) argon gas supply and (iv) for the exit of vent 40 kW three zone electrical resistance furnace is employed to gases. tightness of the assembly is achieved with the provide heating requirements to heat the electrolyte and elec- use of appropriate high temperature gaskets between the reac- trodes assembly along with stainless steel reactor to a tem- tor ange and the lid ange (Fig. 1 (b)). The annular gap be- perature of 970°C and maintain the temperature all through tween the electrode lead rods and nozzles are lled with high the experimental period. Initially about 500 kg of laboratory temperature seals so as to ensure pressure tightness even at Carbon Dioxide (CO2) Released in the Electrochemical Reduction of Titanium Dioxide (TiO2) to Titanium Metal 917

Table 1 Variation of applied DC voltage in the experiments.

DC voltage (V) Time (hrs) 2.0 2–4 2.6 4–6 2.8 4–6 3.2 4–6 3.8 6–10 4.5 Remaining period of experiment

Fig. 2 A photograph of cathode basket loaded with sintered TiO2 granules. Table 2 Typical experimental operating conditions and important process parameters in a 5–8 kg experiment.

S. No. Parameter Value

1 Weight of TiO2 10–14 kg ble 2 presents typical operating conditions and important pro- 2 Level of molten electrolyte ≈ 650 mm cess parameters recorded during the course of electrolysis 3. DC voltage 2.5–5.2 V time. Figure 2 shows a photograph of the cathode basket 4 Electrolysis temperature 960°C loaded with the oxide granules employed in the experimenta- tion. 5. pressure in the cell 1.1 atm After the electrolysis experiment, the entire assembly is 6. Free (gas) volume in the cell 0.1 m3 cooled to room temperature under argon gas cover. Then the cathode basket is carefully taken out, examined and after vi- sual inspection of the same, it is taken up for further handling. the higher temperatures experienced during the experimenta- The granules are taken out from the cathode basket and are tion. The system is evacuated with the help of an oil rotary subjected to a series of washings with , dilute acetic vacuum pump to 10–15 mm of Hg and lled back with argon and dilute HCl for completely removing the ad- gas to about 2 psig. The evacuation and lling back with ar- hered electrolyte and other species. The anode after visual gon gas is repeated to ensure withdrawal of residual air (up to inspection is separately taken away and water washed. In ev- a lowest limit) that initially present in the system. ery experiment new graphite plates are used as anode. Before each experiment, samples of calcium chloride are analyzed for CaO content by titration with dilute acid and the 4. Prediction of CO2 Pro les data is recorded. As a general practice voltage drops associat- ed with the given cell con guration are experimentally deter- As per the understanding of the mechanism of conversion mined under ‘no load’ conditions (basket without TiO2 gran- of TiO2 to titanium metal, the overall reduction process is de- ules) prior to the actual reduction experiment. Accordingly scribed (as mentioned before) by eqs. (i) and (ii) and practi- the experimental procedure pertaining to DC voltage applica- cally CO2 can be considered as the predominant products gas. tion is evolved. This helps in understanding the applicable The essential requirement for CO2 evolution at anode is voltage window (avoiding CaCl2 decomposition and oxygen ion transportation from cathode through molten calci- generation) for the given experiment. um chloride which is governed by applied voltage and trans- The experiment begins with the heating of the set up to a port properties of the associated species. Electrolysis current temperature of 970°C as measured at the reactor outer wall, in a way represents the rate of transportation of oxygen under argon gas cover. After ensuring an electrolyte tempera- to anode and hence the DC current pro le, recorded during ture of 930°C, application of DC voltage is initiated and in- the electrolysis time can be applied to infer the rate of CO2 creased stepwise from 2.0 to 4.5 V (Table 1). During the en- formation at the anode. However, CO2 formation cannot be in tire electrolysis period argon gas purge into the cell system is proportion to the magnitude of electrolysis current as there continued at a rate of 100–200 cc/min. The exit gases let out can be many electron transfer subsidiary reactions that are not during the experiment are led through a /rubber tube associated with the conversion of the oxide to metal/CO2 for- to a CO2 detection and system. A carbon dioxide mation. Still, the electrolysis current pro le during the reac- transmitter cum infrared sensor, ADT-D3 1164, Intertek, Ger- tion time is expected to describe the trend of CO2 formation. man make of (i) 0–5000 and (ii) 0–50000 ppm range were In a simple and preliminary approach, the electrolysis current employed for measuring CO2 content in the exit gases at se- variation is correlated to rate of formation of CO2 and at- lected points of time during the experimentation (The higher tempts have been made to predict CO2 variation in the vent range detector was used in the 5–8 kg experimentation where- gases and compare with the experimental measurements. as the lower range one is used in the experiments on 5000– 1000 g scale set up). Otherwise the vent gases pass through a 5. Results & Discussion set of baryta ( ) bubblers which helps in detecting CO2 in the gases qualitatively and enables It is generally observed that during the electrochemical re- monitoring of the same as formation of barium carbon- duction process, as the voltage is applied and increased say ate precipitate indicates the presence of CO2 in the gases. Ta- from 2.0 to 4.5 volts, DC current raises from a lower value to 918 G. G. Rajulu, M. G. Kumar, K. S. Rao, B. H. Babu and C. RVS Nagesh a higher value which continues predominantly over a period tected in the vent gases. In Fig 4 (a) and (b), measured values of time and then the current gradually decreases. As can be of CO2 content in the exit gases at various selected points of seen in Fig. 3 the DC current – time pro le broadly consists electrolysis time are presented. In the rst part of the experi- of three phases viz initial phase during which current increas- ment after reaching the applied higher cell voltage, CO2 con- es with voltage and in the second phase higher current per- tent above 5000 ppm in the case of 500–1000 g experiment sists at a constant applied higher voltage and this is followed and above 50000 ppm in the case of 5–8 kg experiment were by nal phase during which current decreases to a lower val- observed. In the second half of the experiment, the CO2 con- ue even at the highest applied voltage. This kind of current tent is seen gradually reducing from 5000 ppm to as low as variation can be understood to represent the pattern of oxygen 300 ppm in the case of small scale experiment and from ion transportation from the cathode to graphite anode through 50000 ppm to about 500 ppm in the case of 5–8 kg experi- the electrolyte resulting in the formation of CO2 which is de- ment. In both the scales of operation the last/terminal phase of electrolysis is denoted by a lower and constant CO2 value. It is interesting to note that the variation of CO2 content in the exit gases, with time during the experiment follows the trend similar to the DC current pro le as can be seen in Fig. 5. At a given time in the midst of the experiment, it is seen that the CO2 content is increasing with the applied voltage as can be seen from Table 3. This indicates that the overall conversion rate could be increasing with increased voltage within the ap- plicable voltage range. The observations made on the conditions of electrodes af- ter the experiment are highly informative. In Fig. 6 (a) and (b) photographs of graphite anode plates before and after the ex- periment are shown. The area of the anode immersed in the electrolyte is seen to be signi cantly corroded. The regions of the anode that are at the gas-electrolyte interface are general- ly seen to be highly corroded. The gas- electrolyte interface at Fig. 3 Variation of electrolysis current with time (5–8 kg scale). the anode is apparently experiencing erosion due to intense gas evolution during the electrolysis time as was also ob- served and reported by Srimaha Vishnu et al26).

Fig. 5 Variation of Electrolysis current and CO2 in vent gases with time (5–8 kg batch).

Table 3 Measured values of CO2 content in vent gases.

Applied DC voltage (V) Measured CO2 content in the exit gases (ppm) 3.0 2630 3.5 4130 3.8 5120 4.0 5310 Fig. 4 (a): Measured CO2 content in the vent gases of 1 kg experiment, (b): 4.2 6260 Measured CO2 content in the vent gases of 5–8 kg experiment. Carbon Dioxide (CO2) Released in the Electrochemical Reduction of Titanium Dioxide (TiO2) to Titanium Metal 919

through estimation of time at which CO2 in the exit gases is minimum represent completion of the reduction process. This is because the lowest values of CO2 content in the vent gases is expected to describe the completion of the reduction pro- cess.

6. Conclusions

Some kinetic aspects of electrochemical reduction of TiO2 to titanium metal have been discussed. Based on the experi- mental work carried out on two different scales of operation, it could be seen that variation of electrolysis current during the course of reduction process has a correlation with CO2 content in the vent gases of the electrolytic system. It has Fig. 6 Photographs of graphite anode (a) before electrolysis (b) after the been noticed that variation of CO2 content in the exit gases use in electrolysis. follows a trend similar to DC current – time pro le. Under- standing of CO2-electroysis time pro le shall be useful for optimizing the process cycle time.

Acknowledgements

Authors wish to express their deep sense of gratitude to Dr. S.V. Kamat, Outstanding Scientist & Director, DMRL for the constant support and encouragement extended to this devel- opmental activity and also for according permission to pub- lish this technical . Authors also wish to acknowledge DRDO for providing funds to this R& D work on develop- ment of the electrochemical reduction process for titanium extraction.

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