- Email: [email protected]
Determination of the kinetic pathway in the electrochemical reduction of titanium dioxide in molten calcium chloride
Electrochimica Acta 51 (2005) 66–76
Determination of the kinetic pathway in the electrochemical reduction of titanium dioxide in molten calcium chloride C. Schwandt, D.J. Fray ∗ Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 27 September 2004; received in revised form 24 January 2005; accepted 27 March 2005 Available online 23 May 2005
Abstract An investigation into the kinetics of the electrochemical reduction of titanium dioxide (TiO2 ) to titanium metal (Ti) in molten calcium chloride has been performed. Partially reduced samples were prepared by terminating the reduction process after different reaction times and characterised by means of X-ray diffraction analysis. Based on the time-dependent changes of phase composition as well as thermodynamic and kinetic considerations, the reaction path has been derived. The key result is that the reduction proceeds through a number of individual stages some of which involve the formation and decomposition of calcium titanates. Several of the partially reduced samples were examined further through scanning electron microscopy and energy-dispersive X-ray analysis. The results demonstrate that the electrochemical reduction of titanium dioxide to titanium metal is accompanied by substantial changes in the microstructure. © 2005 Elsevier Ltd. All rights reserved. Keywords: Electrochemical reduction; Calcium chloride; Titanium dioxide; Titanium; Calcium titanate
1. Introduction Over the recent past, a novel molten salt electrochemical technique has been developed, which makes it possible to gain metals and alloys through direct electrochemical reduction of the respective oxides in fused salts [1–3]. In this process, commonly molten calcium chloride, either pure or mixed with other chlorides, is employed as the electrolyte, the metal oxide forms the cathode, and graphite serves as the anode. At elevated temperatures, a voltage is applied that is lower than the thermodynamic decomposition potential of the electrolyte but higher than the decomposition potential of the metal oxide, thereby avoiding continuous electrolysis of the electrolyte and thus deposition of calcium metal at the cathode. Under these conditions, the overall reaction is the removal of oxygen from the cathode and the formation of carbon monoxide or carbon dioxide at the anode, while the electrolyte enables the transfer of oxide ions between the ∗
Corresponding author. Tel.: +44 1223 334479; fax: +44 1223 335637. E-mail address: [email protected] (D.J. Fray).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.048
electrodes. It has been demonstrated that a number of metals and alloys may be prepared in this way [1–3]. In particular, the method is of interest for the production of titanium metal, as it may have the potential to render titanium cost affordable and hence more widely utilised. The objective of the present study has been to gain an insight into the reaction pathway that occurs in the course of the electrochemical reduction of titanium dioxide to titanium metal in molten calcium chloride. The experimental approach consists in the preparation and investigation of partially reduced specimens. As the most important characterisation technique, X-ray diffraction analysis is applied in order to identify phase composition of the different samples. Further quantities to be measured are mass change and oxygen content of the partially reduced samples as well as amount of charge passed throughout an experiment. The results of a scanning electron microscopic and energy-dispersive X-ray examination are also presented. Based on this information it will be elucidated how the chemical composition and the microstructure of the material being processed change during electrochemical reduction.
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
2. Experimental A commercially available titanium dioxide powder (43047, Alfa Aesar) was selected as the starting material for the preparation of the oxide precursor. The material is specified as rutile of 99.5% purity with an average particle size between 1 and 2 m. It was found that pressing the as-received powder into pellets results in significant delamination followed by crack formation or distortion during sintering. The compact titanium dioxide preforms were therefore prepared via the following ceramic powder processing route, which includes impregnating the powder with a binder/plasticiser system as well as pressing and sintering under optimised conditions. Firstly, the titanium dioxide powder was dried in air at temperatures around 100 ◦ C for several days. 1% by mass of a mixture of PVB/PVA (polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate, approximately 80% butyral) and 0.5% by mass of PEG (polyethylene glycol, average molecular mass 200) were added, and the components were mixed by wet milling in iso-propanol for 24 h. The powder was dried in air at around 100 ◦ C and passed through a 53 m stainless steel sieve. Secondly, the treated titanium dioxide powder was made into solid bodies. To that end, a quantity of powder was first pressed uniaxially at 50–75 MPa into a pellet and then further densified isostatically at 175 MPa. Thirdly, the titanium dioxide bodies were sintered. Following the results of a pre-investigation, sintering was carried out at 1100 ◦ C in air for 150 min. Properties of the pellets prepared were: mass between 4.0 and 16.0 g, diameter 23 mm, thickness 4.0–16.0 mm depending on mass, relative density 65–70%, and open porosity 25–30%, while mechanical strength and integrity were sufficient to permit handling and processing. The electrochemical experiments were performed in a vertical tubular Inconel® reactor, which was located inside a programmable electrical furnace. The upper end of the reactor was closed by a stainless steel cover, which was equipped with feedthroughs for the electrode leads and a thermocouple as well as gas inlet and outlet. The interior of the reactor was continuously purged with argon, the latter dried over self-indicating calcium sulphate. The electrochemical experiments comprised the pretreatment of the calcium chloride as well as the actual electrochemical reduction of titanium dioxide. The pre-treatment, done with the aim to eliminate redox-active species from the electrolyte and ensure a low electronic background current in the subsequent reduction experiment, involved thermal predrying of the solid calcium chloride and pre-electrolysis of the molten calcium chloride, and was performed by adhering to previously established procedures [3]. A quantity of nominally anhydrous calcium chloride (21079, Fluka), usually between 110 and 220 g, was placed in an alumina crucible that served as the reaction vessel. Following the assumption that residual moisture can be removed by applying slow heating rates and long dwell times far below the melting point, the temperature was first ramped to 150 ◦ C at 2 ◦ C/min and
67
held for at least 8 h, then raised to 300 ◦ C at 2 ◦ C/min and held for an additional 5 h, before ramping to the target temperature of 900 ◦ C. To further remove water and possible other redox-active impurities, pre-electrolysis was carried out. In this, a direct voltage of 1.5 V was applied for several hours between a graphite rod anode (EC4, Graphite Technologies) and a graphite rod or titanium sheet cathode until a small and time-independent current, typically in the range of 2 mA/cm2 electrode surface, was established. The applied voltage was high enough to eliminate hydrogen present in water or its derivatives like calcium hydroxide, whilst formation of calcium metal was impossible. Possibly existing impurities with higher decomposition potentials cannot be removed by this procedure, but the early application of high voltages had to be avoided since these are likely to affect the electronic properties of the electrolyte, as will be discussed later. Acid–base titration was applied to monitor the content of calcium oxide in the calcium chloride. In agreement with previous expectations it was found that the calcium oxide content in the as-received material was below 0.1 mol%, increased by a fraction of a percent during drying, melting and pre-electrolysis, and always remained below 1 mol%. The actual electrochemical reduction experiments were carried out as follows. A titanium dioxide pellet prepared as outlined before was made the cathode, a graphite rod was used as the anode, and a voltage versus time regime comprising up to three stages was applied. During the first phase, the applied voltage was 2.5 V and this value was maintained for typically 8 h; in the second phase, the voltage was increased to 2.7 V and kept for typically 24 h; in the third phase, the voltage was set to 2.9 V and then left unchanged. The conditions were thus similar to those selected in previous studies in that no continuous electrolysis of calcium chloride could occur. Electrical contact to the electrodes used in pre-electrolysis and reduction was made through nickel wire of 2 mm in diameter, this rendering the IR-drop in the current leads insignificant. The first part of each experiment was performed with a computercontrolled potentiostat (Powerstat, Sycopel Scientific) and the current was recorded as a function of time. Thereafter either the same apparatus or a manually controlled voltage source (LS 30-10, Wayne Kerr) was used. In order to produce partially reduced titanium dioxide specimens, the electrochemical reduction was interrupted after different reaction times, these ranging from 0.5 to 120 h. A run was terminated by removing the cathode from the salt melt and locating it in the upper and cooler part of the reactor. As the temperature of the quenched sample decreased rapidly and caused adherent salt to solidify, and as the reactor was continuously flushed with argon, it is assumed that this procedure did not cause a significant increase of the oxygen content. The recovered samples were vigorously rinsed with tap water and placed in 1.0 N hydrochloric acid whereafter a mild vacuum of around 20 mbar was applied. Preinvestigations had shown that materials like titanium dioxide, titanium sesquioxide, Ti2 O3 , titanium monoxide, TiO, and calcium titanate, CaTiO3 , are inert under these conditions,
68
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
whereas calcium oxide and calcium metal may easily and completely be removed. Occasionally, a purple colouration of the aqueous phase was observed, which indicates the reaction of freshly gained and chemically reactive titanium metal with the hydrochloric acid. In such a case, the acid was slightly diluted with distilled water until the colour intensified no further. After around 10–12 h, the hydrochloric acid solution was replaced with distilled water, again assisted by vacuum impregnation. After another 10–12 h, the samples were washed with acetone and dried in air at around 100 ◦ C. The surface of each sample was ground in order to remove surface impurities and provide an even surface for X-ray diffraction analysis, with some of the samples being investigated at different depths. Fractured surfaces were prepared for electron microscopic examinations. Phase composition of the various partially reduced specimens was determined through X-ray diffraction analysis (PW 1710, Philips). Each scan ranged from 10◦ to 80◦ . Microstructure and chemical composition of the samples were investigated by means of scanning electron microscopy and energy-dispersive X-ray analysis (JSM-5800LV, JEOL). The acceleration voltage was 15 keV, secondary electrons were detected, and images were taken at magnifications between 1000 and 5000 times. Except for the oxide precursor, all samples were examined in the as-prepared state, that is, without a sputter-coated conductive layer. Oxygen content of the samples was quantitatively determined by means of an analyser that makes use of the hot extraction technique and infrared detection of the carbon dioxide evolved (ONH-2000, Eltra). For each sample typically three individual measurements were done and averaged.
3. Results Fig. 1a presents four current versus time curves, which were recorded during the first 8 h of electrochemical reduction experiments performed with compact titanium dioxide preforms at an applied voltage of 2.5 V. The plots commence with a current peak of significant height that extends over about 1 h, and this is followed by a current shoulder that spans over about 3–5 h. Thereafter the current assumes relatively small values. Fig. 1b and c shows current traces obtained during the subsequent stages of an experiment in which voltages of 2.7 and 2.9 V were applied. The current remains relatively small and changes only little as a function of time. When the voltage is raised, the current increases to some extent but as before does not vary markedly over the course of the experiment. Fig. 2 presents the X-ray diffraction spectrum of the compact titanium dioxide preform after sintering, and Figs. 3–8 show the X-ray diffraction spectra obtained from specimens the polarisation treatment of which was terminated after different durations. Samples quenched after around 0.5 h of polarisation were found to consist predominantly of the titanium suboxide Ti3 O5 and the calcium titanate CaTiO3 ,
Fig. 1. Current vs. time curves of electrochemical reduction experiments. Electrolyte: calcium chloride; cathode: titanium dioxide pellets (mass 8 g); anode: graphite (geometric surface area 12 cm2 ); anode-to-cathode distance: 4 cm; temperature: 900 ◦ C; atmosphere: dried argon. (a) Four nominally identical experiments performed at a voltage of 2.5 V; (b) continuation of one of the experiments at a voltage of 2.7 V; (c) continuation of the same experiment at a voltage of 2.9 V.
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
69
Fig. 2. X-ray diffraction spectrum of the compact titanium dioxide preform after sintering in air at 1100 ◦ C for 2.5 h, showing rutile-type TiO2 ((䊉), JCPDS 21-1276).
Fig. 4. X-ray diffraction spectrum of a sample quenched after about 1 h of polarisation, showing Ti2 O3 ((䊉), JCPDS 43-1033) and CaTiO3 ((), JCPDS 42-0423); oxygen content ∼35% by mass.
the latter possessing titanium in the oxidation state +IV. The X-ray diffraction pattern taken from a section that crosses the centre of the pellet showed the additional presence of a small amount of the titanium suboxide Ti4 O7 . Samples recovered after about 1 h were primarily composed of titanium sesquioxide, Ti2 O3 , and CaTiO3 , and samples removed after about 4 h comprised mainly titanium monoxide, TiO, and CaTiO3 . The majority of the TiO was cubic, while small amounts of the monoclinic modification were always present too. Samples retrieved at reaction times longer than that contained progressively less TiO and CaTiO3 , while there was an increase in the amount of the lower-valent calcium titanate CaTi2 O4 , in which titanium is present in the oxidation state +III. From around 8–16 h of polarisation, CaTi2 O4 was found to be the prevailing phase, with some
samples being almost single phase while others contained detectable quantities of TiO and CaTiO3 . After longer reaction times, the amount of CaTi2 O4 declined while more TiO was identified. The latter formed preferentially on the outside of the pellet and was exclusively monoclinic. After about three days, the X-ray diffraction spectra obtained were those of titanium metal. Two issues are noteworthy with respect to the results presented above. Firstly, owing to the relatively weak sensitivity of X-ray diffraction analysis, only the major phases of each sample could be identified. In particular, small amounts of TiO are difficult to discern in the presence of titanates. Secondly, the X-ray diffraction patterns of the ordered phases Ti3 O and Ti6 O are very similar to that of Ti. This renders the technique less convenient to analyse phase composition in the final stage of the reduction process.
Fig. 3. X-ray diffraction spectrum of a sample quenched after about 0.5 h of polarisation, showing Ti3 O5 ((䊉), JCPDS 40-0806) and CaTiO3 ((), JCPDS 42-0423); oxygen content ∼36% by mass.
Fig. 5. X-ray diffraction spectrum of a sample quenched after about 4 h of polarisation, showing cubic TiO ((䊉), JCPDS 08-0117) and CaTiO3 ((), JCPDS 42-0423); oxygen content ∼31% by mass.
70
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
Fig. 6. X-ray diffraction spectrum of a sample quenched after about 12 h of polarisation, showing CaTi2 O4 ((), JCPDS 11-0029); oxygen content ∼29% by mass. (Small amounts of CaTiO3 and TiO were also present but could only be seen at high resolution.)
Fig. 8. X-ray diffraction spectrum of a sample retrieved after about 120 h of polarisation, showing ␣-Ti ((䊉), JCPDS 44-1294); oxygen content ∼3000 ppm by mass. (The small peaks at 43.4◦ and 44.6◦ are not present in the reference file but were also found in titanium samples of commercial purity.)
Overall, reproducibility and consistency of the results were good for the first stage of reduction but deteriorated once CaTi2 O4 was the predominant component. The specimens quenched during the early stages of the reduction process were subjected to gravimetric analysis. The samples consisting predominantly of Ti3 O5 and CaTiO3 , Ti2 O3 and CaTiO3 , as well as TiO and CaTiO3 were found to have gained in mass on average by approximately 12, 16 and 23%, respectively. Samples containing significant amounts of CaTi2 O4 were not considered in the gravimetric analysis because their friable character did not allow for quenching and post-treatment to be carried out without noticeable mass loss, whereas the oxygen content of metallic samples was
too small to permit meaningful measurements. Quantitative determination of the oxygen content provided values in the order of 29–36% by mass for all samples quenched in the early stages of the reduction process, while numbers became smaller as the reduction proceeded further, with approximately 3000 ppm by mass being a typical endpoint. Also for the samples retrieved after relatively short processing times, the amount of charge passed during the polarisation experiment was measured. This amount of charge was compared with that needed theoretically to attain complete reduction, and the degree of reduction was expressed in terms of a percentage value. For samples consisting of primarily Ti3 O5 and CaTiO3 , Ti2 O3 and CaTiO3 , as well as TiO and CaTiO3 these numbers were approximately 13, 18 and 31%, respectively. Samples recovered at the later stages were disregarded in this analysis, since the rate of reduction decelerated to such an extent that the charge contributed by the background current became significant. Fig. 9 shows a micrograph of the compact titanium dioxide preform as obtained after sintering at 1100 ◦ C in air. Figs. 10–12 present micrographs of some of the partially reduced specimens together with typical results obtained from energy-dispersive X-ray analysis. Fig. 13 is an image of the final product.
4. Discussion
Fig. 7. X-ray diffraction spectrum of a sample quenched after about 52 h of polarisation, showing CaTi2 O4 ((), JCPDS 11-0029); and monoclinic TiO ((䊉), JCPDS 23-1078); oxygen content ∼26% by mass.
The experimental results obtained within the scope of the present study have provided strong evidence that the electrochemical reduction of titanium dioxide to titanium metal in molten calcium chloride proceeds through a number of welldefined reaction steps. In the following, the individual stages of reduction will be discussed in greater detail. The required
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
71
Fig. 9. Scanning electron microscopic image of the TiO2 precursor after sintering in air at 1100 ◦ C for 2.5 h.
thermodynamic data was taken from a database [4] and apply to the temperature of 900 ◦ C. All quoted standard electrode potentials, E◦ , are referred to the Ca/Ca2+ equilibrium, and both cathodic and anodic processes are expressed in terms of the respective reduction potentials. The standard states for solids, gases, and ions are, respectively, the pure compound, the atmospheric pressure, and the unit mole fraction of the relevant ionic sublattice. The first stage of the electrochemical reduction of titanium dioxide is characterised by the formation of mixtures that consist of the various titanium suboxides Tix Oy as well as the calcium titanate CaTiO3 . The successive formation of these mixtures may be described by the following sequence of cathodic reaction steps: 5TiO2 + Ca2+ + 2e− = Ti4 O7 + CaTiO3 , E◦ ≈ +1807 mV
(1)
4Ti4 O7 + Ca2+ + 2e− = 5Ti3 O5 + CaTiO3 , E◦ = +1678 mV
(2)
3Ti3 O5 + Ca2+ + 2e− = 4Ti2 O3 + CaTiO3 , E◦ = +1575 mV
(3)
2Ti2 O3 + Ca2+ + 2e− = 3TiO + CaTiO3 , E◦ = +1200 mV
(4)
It should be noted that the electrode potential calculated for Eq. (1) is an approximate value. This is because the formation of the mixture of Ti4 O7 and CaTiO3 takes place in several steps and involves numerous Magn´eli phases that
Fig. 10. (a) Scanning electron microscopic image of a sample quenched after about 1 h of polarisation, showing a mixture of Ti2 O3 and CaTiO3 ; (b) energy-dispersive X-ray spectrum obtained from point analysis 1; (c) energy-dispersive X-ray spectrum obtained from point analysis 2.
72
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
Fig. 11. (a) Scanning electron microscopic image of a sample quenched after about 12 h of polarisation, showing the overwhelming presence of CaTi2 O4 ; (b) energy-dispersive X-ray spectrum obtained from point analysis 1.
have compositions between those of TiO2 and Ti4 O7 [5]. The approximation is nevertheless realistic since all the oxide phases concerned are of similar thermodynamic stability [6]. Eqs. (2)–(4) involve one reaction step, and accurate potentials may be given. A minor inaccuracy arises from the fact that TiO is a compound of considerable stoichiometric width, which renders its Gibbs free energy of formation somewhat composition-dependent. Through combination of Eqs. (1)–(4), the overall cathodic reactions leading to the formation of the experimentally observed mixtures may be written as follows: 4TiO2 + Ca2+ + 2e− = Ti3 O5 + CaTiO3
(5)
3TiO2 + Ca2+ + 2e− = Ti2 O3 + CaTiO3
(6)
2TiO2 + Ca2+ + 2e− = TiO + CaTiO3
(7)
Fig. 12. (a) Scanning electron microscopic image of a sample quenched after about 52 h of polarisation, showing a mixture of CaTi2 O4 and TiO; (b) energy-dispersive X-ray spectrum obtained from point analysis 1; (c) energy-dispersive X-ray spectrum obtained from point analysis 2.
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
73
The anodic reaction may be the discharge of either chloride ions or oxide ions, which results in the generation of either chlorine gas or oxygen, carbon monoxide and carbon dioxide. Cl2 + 2e− = 2Cl− ,
E◦ = +3214 mV
(8)
1 O2 + 2e− = O2− , 2
E◦ = +2407 mV
(9)
CO + 2e− = C + O2− ,
Fig. 13. Scanning electron microscopic image of a sample retrieved after about 120 h of polarisation, showing Ti.
The theoretical mass gains of the cathode due to the incorporation of calcium may be calculated from the stoichiometries of Eqs. (5)–(7) and amount to 12.5, 16.7 and 25.1%, respectively. This is close to the experimental results. The percentages of reduction that correspond to the three equations are 12.5, 16.7 and 25.0%, respectively. This is somewhat lower than the measured numbers. However, a very good agreement arises under the assumption that a portion of the current in the order of 50–80 mA is associated with phenomena other than those described by the above reactions; this will be discussed later. Overall, the experimental observations are in accordance with the proposed sequence of reactions. It may therefore be deduced that the formation of the different mixtures of titanium suboxides Tix Oy and calcium titanate CaTiO3 proceeds according to well-defined electrochemical reactions and leads to quantitatively predictable compositions of the cathode material. The mechanism underlying the cathodic reactions that occur in the first stage of the electrochemical reduction of titanium dioxide, involves the incorporation of calcium into the cathode and the redistribution of oxygen inside the cathode. Kinetically, the three phases involved, i.e., the molten salt electrolyte, the oxide cathode, and the electronically conductive current collector, interact in the following manner: Ca2+ ions are inserted into the oxide cathode from the molten salt, electrons are provided to the oxide by the current lead, and oxygen is rearranged inside the cathode via diffusion along relatively short distances. As the various titanium suboxides are electronic conductors [7], the cathode becomes conductive throughout its volume very shortly after reduction has commenced. The reduction is then no longer confined to the points of contact between the three phases but may take place over the entire interface of electrolyte and cathode.
E◦ = +1291 mV
(10)
1 1 CO2 + 2e− = C + O2− , E◦ = +1381 mV (11) 2 2 Eqs. (8)–(11) demonstrate that the formation of carbon monoxide and carbon dioxide as the anodic off-gases is thermodynamically more favourable than the formation of chlorine and oxygen gas. However, under the governing experimental conditions, the initial content of calcium oxide dissolved in the calcium chloride was small, so chlorine evolution may have occurred. This issue was clarified by way of a control experiment. In this, as much as 16 g of titanium dioxide, corresponding to 0.2 mol, were processed in as little as 1 mol of calcium chloride. After a few hours of polarisation the mass increase of the cathode due to calcium uptake was almost 4 g, whilst the calcium oxide content present at the beginning of an electrochemical reduction experiment was less than 1 mol%. As can readily be inferred from these numbers, the major uptake of calcium must have been from the calcium chloride. Accordingly, this result provides evidence that the reactive character of the oxide cathode and the stabilising effect of the calcium titanate formed enforce the decomposition of a quantity of calcium chloride and the release of chlorine at the anode in the beginning of the reduction process, although the voltage necessary for continuous calcium chloride electrolysis is not exceeded. Indeed, it was possible to prove the release of chlorine during the first stage of the reduction process by passing the exhaust gas through silver nitrate solution and identifying precipitated silver chloride. A quantitative analysis was not carried out, as the predominant part of the chlorine liberated was likely to be gettered in the heated part of the metal reactor. The assumption of chlorine being the main anodic off-gas during the first stage of the reduction process is also in agreement with two further observations. Firstly, in the present set of experiments no mixture of Ti and CaTiO3 has ever been encountered and, in fact, the formation of this composition would be possible under a voltage of 2.5 V only if carbon monoxide or carbon dioxide could be generated but not if chlorine has to be released. Secondly, it was observed that during the occurrence of the initial current peak virtually no consumption of the anode material took place. The second stage of the electrochemical reduction of titanium dioxide is characterised by the dominating presence of CaTi2 O4 . There are two obvious routes toward the formation of this compound. One possibility is the comproportionation reaction of TiO and CaTiO3 . CaTiO3 + TiO = CaTi2 O4
(12)
74
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
This reaction is chemically driven, because no change in the average oxidation state of the titanium occurs, and does not contribute to the current. The second possibility is the electrochemical removal of oxygen from CaTiO3 . 2CaTiO3 + 2e− = CaTi2 O4 + Ca2+ + 2O2− , E◦ > −146 mV
(13)
It has to be assumed that, under the present experimental conditions, Eq. (12) proceeds more rapidly than Eq. (13), because only in this way it is possible to explain the abundant presence of CaTi2 O4 and the temporary decrease of TiO during this interval of the process. Unfortunately, the Gibbs free energy of formation of CaTi2 O4 appears to be unknown and so no accurate quantitative thermodynamic considerations can be made. It may however be stated that the CaTi2 O4 forms from the previously generated mixture of TiO and CaTiO3 . Therefore, the free energy of formation of CaTi2 O4 must be more negative than the sum of the free energies of formation of TiO and CaTiO3 , and on this basis a lower potential limit for the reaction in Eq. (13) may be calculated. Moreover, it may be deduced that the reactions during the first stage of the reduction process are to some extent kinetically controlled. The third stage of the electrochemical titanium dioxide reduction involves the decomposition of the CaTi2 O4 and the formation of TiO. As no compounds are known that comprise titanium in the oxidation state +II as well as calcium and oxygen, this reaction step must be accompanied by the release of CaO. CaTi2 O4 + 2e− = 2TiO + Ca2+ + 2O2− , E◦ < +417 mV
(14)
The upper limit of the potential for the reaction given in Eq. (14) is computed on the same basis as the lower limit for Eq. (13). The TiO formed is then further reduced to solid solutions of oxygen in titanium Ti[O]δ . TiO + 2(1 − δ)e− = Ti[O]δ + (1 − δ)O2− , E◦ ≈ +177 mV
(15)
At temperatures above 920 ◦ C there is a direct transition between the compound phase TiO and the solution phase Ti[O]δ , with δ denoting the oxygen content in the solid solution that is in equilibrium with TiO. Below this temperature the Ti3 O2 phase is present in between. As the Gibbs free energy of formation of the latter compound is apparently unknown, the potential for the above reaction has been calculated using the free energy of formation of TiO and is thus to some extent approximate. At much lower temperatures additional ordered phases like Ti2 O, Ti3 O and possibly Ti6 O exist [5]. As opposed to the shorter reduction experiments discussed earlier, substantial erosion of the anode was now observed, and this indicates that the transfer of oxygen ions from the cathode to the anode and the subsequent release of carbon oxides took place at this stage of the process.
Under the conditions of the present set of experiments, the applied voltage exceeds the electrolysis voltage of calcium oxide. As the potential distribution across a galvanic cell of the given type and in particular the magnitude of the overpotentials are unknown, a quantitative assessment is at present impossible, but it is probable that the calcium oxide liberated during decomposition of the intermediate compounds will react further. At the cathode a small quantity of divalent calcium ions will be discharged, and at the anode the release of carbon monoxide or carbon dioxide will take place. The latter proceeds in accordance with the reactions given in Eqs. (10) and (11), although the relevant potentials will be somewhat different from the quoted ones, as neither the calcium oxide nor the calcium metal are in their standard states. The reduced calcium species formed at the cathode dissolve in the molten calcium chloride and make a significant contribution to the electronic conduction of the melt. The relatively constant current during the later stages of the process is therefore likely to be due to the compensating effects of the slowly decaying oxide ion current, resulting from the removal of oxygen from the cathode, and the slowly rising electronic background current, originating from the generation of dissolved calcium species. It should be emphasised that the occurrence of reduced calcium species is an inevitable sideeffect, which occurs under high applied potentials as they are necessary in the processing of titanium dioxide. Due to the high applied potentials and the solubility of calcium metal in calcium chloride, the calcium activity in the melt rises to values that are significantly larger than zero but smaller than unity. The enhanced calcium activity may assist the reduction process chemically although this is not a prerequisite to accomplish reduction. One experimental proof, for instance, is the finding that titanium dioxide pellets that are attached to the cathodic current collector are substantially more reduced than samples that are exposed to the melt but detached from the cathode. An in-depth discussion has been provided in a publication that is exclusively dedicated to this topic [8]. Nevertheless, it is acknowledged that an unambiguous differentiation between the electrochemical and the chemical contributions to the reduction process is difficult to achieve once the final stage is reached. Based on the interpretation put forward above, the shape of the current versus time curves may be explained. The current peak is caused by fast reactions of the cathode with calcium ions from the electrolyte, which lead to the formation of mixtures containing Ti4 O7 , Ti3 O5 and Ti2 O3 in conjunction with CaTiO3 . The current shoulder is due to the formation of a mixture of TiO and CaTiO3 . While compound formation is ongoing, the removal of oxygen from the cathode is a minor process that contributes only little to the overall current. The aforementioned reaction stages are denoted in Fig. 14, which features a particularly distinctly resolved current versus time curve. During the later stages of the reduction process, when compound formation has ceased, the removal of oxygen from the cathode becomes the predominant process. However, the current associated with the transfer of oxygen is relatively
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
75
the CaTi2 O4 particles reduce in size, whereby a much finer and more homogeneous microstructure is again produced. As before, energy-dispersive X-ray analysis confirmed the anticipated cation ratios. Fig. 13 shows the microstructure of fully reduced titanium metal. Particle size is in the range of a few microns and the overall appearance is similar to that of titanium sponge prepared by conventional methods. 5. Conclusion
Fig. 14. A particularly well-resolved current vs. time curve taken from Fig. 1a, with cathode composition given for various points in time during the first stage of reduction.
small, and thus the electronic current now contributes significantly to the overall current. This renders the current versus time curve rather unstructured, and it is not possible any longer to assess directly the degree of reduction at a given point in time. As an additional consequence, the overall current efficiency for the preparation of titanium of low oxygen contents falls to relatively small values in the range of 10–20%, which is unsatisfactory from the viewpoint of application. As is visible in Fig. 9, the particle size of the compact titanium dioxide preform, prepared as described earlier, is around 1 m and rather uniform, and there is a large extent of porosity. Fig. 10 illustrates that the microstructure of the sample composed of Ti2 O3 and CaTiO3 is relatively fine and homogeneous, with an average particle size somewhat above 1 m. Energy-dispersive X-ray analysis reveals the presence of an intimate mixture of particles of two different compositions. The titanium to calcium ratios as measured for both kinds of crystallites typically ranged from 91:9 to 97:3 and from 40:60 to 53:47, respectively, and are thus close to the expected values for Ti2 O3 and CaTiO3 , while the deviations are probably due to the fact that particle size and sampling area are of the same order of magnitude. Similar results were obtained for the mixtures of Ti3 O5 and CaTiO3 as well as TiO and CaTiO3 . In Fig. 11 it may be seen that the microstructure of CaTi2 O4 is comparatively coarse and heterogeneous. Particles are elongated and some reach a length of up to 100 m. This finding explains readily the poor mechanical stability that partially reduced samples exhibit during the intermediate interval of reduction. The titanium to calcium ratio as determined by energy-dispersive X-ray analysis was always very close to the expected one of 67:33. Fig. 12 reveals the unique microstructure of samples containing both TiO and CaTi2 O4 . The TiO emerges in the form of individual and separate grains, which grow together as they expand and as
Titanium dioxide specimens were reduced electrochemically and to different extents in molten calcium chloride. Analysis of the partially reduced samples proved that the reduction process advances through a series of defined reaction steps. In the first stage, calcium species from the electrolyte are inserted into the cathode and oxygen is redistributed in the cathode, such that mixtures consisting of the various titanium suboxides, Tix Oy , and calcium titanate, CaTiO3 , are formed. The second stage is characterised through the overwhelming presence of a lower-valent calcium titanate, CaTi2 O4 , which is thought to originate mainly from the chemical reaction between the previously generated compounds. In the third stage, the CaTi2 O4 is decomposed and TiO formed, which is then reduced further to solid solutions of oxygen in titanium metal, Ti[O]δ . The results have demonstrated that the process is fast as long as the reduction may be accomplished through the uptake of calcium but becomes sluggish as soon as the reduction may only be achieved through the removal of oxygen. One objective of future work is to elucidate the reason for the slow rate of oxygen removal and to identify improved experimental conditions. A scanning electron microscopic examination has revealed that the microstructure of the specimens being reduced evolves through a very coarse and fragile state when CaTi2 O4 is the predominant component, while the microstructure is considerably finer and more uniform during the earlier and the later stages of reduction. A particularly noteworthy conclusion from the present study is that the formation and decomposition of calcium containing intermediate compounds are inherent steps in the kinetic pathway of the molten salt electrochemical reduction of titanium dioxide. A thermodynamic analysis shows that the temporary occurrence of similar compounds is to be expected in the reduction of most other metal oxides too, and compounds of this type have indeed been observed in incompletely reduced samples of niobium oxide [9,10] and chromium oxide [11,12]. Accordingly, the precise reaction path ought to be investigated for each feed material that is considered appropriate for the present reduction technique. Acknowledgements Financial support of this work by the European Office of Aerospace Research and Development (EOARD) of the
76
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
US Air Force is gratefully acknowledged. Dr. G. Doughty, of British Titanium plc., is thanked for discussions and critically reading the manuscript.
References [1] [2] [3] [4]
D.J. Fray, T.W. Farthing, Z. Chen, Patent WO9964638. G.Z. Chen, D.J. Fray, T.W. Farthing, Nature 407 (2000) 361. G.Z. Chen, D.J. Fray, J. Electrochem. Soc. 149 (2002) E455. A. Roine, HSC Chemistry, version 4.1, Outokumpu Research Oy, Pori, Finland.
[5] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, vol. 3, second ed., ASM International, Materials Park, OH, 1990, p. 2924, O–Ti (oxygen–titanium). [6] D.C. Lynch, D.E. Bullard, Metall. Mater. Trans. B 28 (1997) 447. [7] R.L. Clarke, S.K. Harnsberger, Am. Lab. 20 (1988) 8. [8] D.J. Fray, G.Z. Chen, in: F.H. Froes, M.A. Imam, D.J. Fray (Eds.), Cost-Affordable Titanium, TMS (The Minerals, Metals & Materials Society), 2004, p. 9. [9] X.Y. Yan, D.J. Fray, Metall. Mater. Trans. B 33 (2002) 685. [10] X.Y. Yan, D.J. Fray, J. Mater. Res. 18 (2003) 346. [11] E. Gordo, G.Z. Chen, D.J. Fray, Electrochim. Acta 49 (2004) 2195. [12] G.Z. Chen, E. Gordo, D.J. Fray, Metall. Mater. Trans. B 35 (2004) 223.
Determination of the kinetic pathway in the electrochemical reduction of titanium dioxide in molten calcium chloride C. Schwandt, D.J. Fray ∗ Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 27 September 2004; received in revised form 24 January 2005; accepted 27 March 2005 Available online 23 May 2005
Abstract An investigation into the kinetics of the electrochemical reduction of titanium dioxide (TiO2 ) to titanium metal (Ti) in molten calcium chloride has been performed. Partially reduced samples were prepared by terminating the reduction process after different reaction times and characterised by means of X-ray diffraction analysis. Based on the time-dependent changes of phase composition as well as thermodynamic and kinetic considerations, the reaction path has been derived. The key result is that the reduction proceeds through a number of individual stages some of which involve the formation and decomposition of calcium titanates. Several of the partially reduced samples were examined further through scanning electron microscopy and energy-dispersive X-ray analysis. The results demonstrate that the electrochemical reduction of titanium dioxide to titanium metal is accompanied by substantial changes in the microstructure. © 2005 Elsevier Ltd. All rights reserved. Keywords: Electrochemical reduction; Calcium chloride; Titanium dioxide; Titanium; Calcium titanate
1. Introduction Over the recent past, a novel molten salt electrochemical technique has been developed, which makes it possible to gain metals and alloys through direct electrochemical reduction of the respective oxides in fused salts [1–3]. In this process, commonly molten calcium chloride, either pure or mixed with other chlorides, is employed as the electrolyte, the metal oxide forms the cathode, and graphite serves as the anode. At elevated temperatures, a voltage is applied that is lower than the thermodynamic decomposition potential of the electrolyte but higher than the decomposition potential of the metal oxide, thereby avoiding continuous electrolysis of the electrolyte and thus deposition of calcium metal at the cathode. Under these conditions, the overall reaction is the removal of oxygen from the cathode and the formation of carbon monoxide or carbon dioxide at the anode, while the electrolyte enables the transfer of oxide ions between the ∗
Corresponding author. Tel.: +44 1223 334479; fax: +44 1223 335637. E-mail address: [email protected] (D.J. Fray).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.048
electrodes. It has been demonstrated that a number of metals and alloys may be prepared in this way [1–3]. In particular, the method is of interest for the production of titanium metal, as it may have the potential to render titanium cost affordable and hence more widely utilised. The objective of the present study has been to gain an insight into the reaction pathway that occurs in the course of the electrochemical reduction of titanium dioxide to titanium metal in molten calcium chloride. The experimental approach consists in the preparation and investigation of partially reduced specimens. As the most important characterisation technique, X-ray diffraction analysis is applied in order to identify phase composition of the different samples. Further quantities to be measured are mass change and oxygen content of the partially reduced samples as well as amount of charge passed throughout an experiment. The results of a scanning electron microscopic and energy-dispersive X-ray examination are also presented. Based on this information it will be elucidated how the chemical composition and the microstructure of the material being processed change during electrochemical reduction.
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
2. Experimental A commercially available titanium dioxide powder (43047, Alfa Aesar) was selected as the starting material for the preparation of the oxide precursor. The material is specified as rutile of 99.5% purity with an average particle size between 1 and 2 m. It was found that pressing the as-received powder into pellets results in significant delamination followed by crack formation or distortion during sintering. The compact titanium dioxide preforms were therefore prepared via the following ceramic powder processing route, which includes impregnating the powder with a binder/plasticiser system as well as pressing and sintering under optimised conditions. Firstly, the titanium dioxide powder was dried in air at temperatures around 100 ◦ C for several days. 1% by mass of a mixture of PVB/PVA (polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate, approximately 80% butyral) and 0.5% by mass of PEG (polyethylene glycol, average molecular mass 200) were added, and the components were mixed by wet milling in iso-propanol for 24 h. The powder was dried in air at around 100 ◦ C and passed through a 53 m stainless steel sieve. Secondly, the treated titanium dioxide powder was made into solid bodies. To that end, a quantity of powder was first pressed uniaxially at 50–75 MPa into a pellet and then further densified isostatically at 175 MPa. Thirdly, the titanium dioxide bodies were sintered. Following the results of a pre-investigation, sintering was carried out at 1100 ◦ C in air for 150 min. Properties of the pellets prepared were: mass between 4.0 and 16.0 g, diameter 23 mm, thickness 4.0–16.0 mm depending on mass, relative density 65–70%, and open porosity 25–30%, while mechanical strength and integrity were sufficient to permit handling and processing. The electrochemical experiments were performed in a vertical tubular Inconel® reactor, which was located inside a programmable electrical furnace. The upper end of the reactor was closed by a stainless steel cover, which was equipped with feedthroughs for the electrode leads and a thermocouple as well as gas inlet and outlet. The interior of the reactor was continuously purged with argon, the latter dried over self-indicating calcium sulphate. The electrochemical experiments comprised the pretreatment of the calcium chloride as well as the actual electrochemical reduction of titanium dioxide. The pre-treatment, done with the aim to eliminate redox-active species from the electrolyte and ensure a low electronic background current in the subsequent reduction experiment, involved thermal predrying of the solid calcium chloride and pre-electrolysis of the molten calcium chloride, and was performed by adhering to previously established procedures [3]. A quantity of nominally anhydrous calcium chloride (21079, Fluka), usually between 110 and 220 g, was placed in an alumina crucible that served as the reaction vessel. Following the assumption that residual moisture can be removed by applying slow heating rates and long dwell times far below the melting point, the temperature was first ramped to 150 ◦ C at 2 ◦ C/min and
67
held for at least 8 h, then raised to 300 ◦ C at 2 ◦ C/min and held for an additional 5 h, before ramping to the target temperature of 900 ◦ C. To further remove water and possible other redox-active impurities, pre-electrolysis was carried out. In this, a direct voltage of 1.5 V was applied for several hours between a graphite rod anode (EC4, Graphite Technologies) and a graphite rod or titanium sheet cathode until a small and time-independent current, typically in the range of 2 mA/cm2 electrode surface, was established. The applied voltage was high enough to eliminate hydrogen present in water or its derivatives like calcium hydroxide, whilst formation of calcium metal was impossible. Possibly existing impurities with higher decomposition potentials cannot be removed by this procedure, but the early application of high voltages had to be avoided since these are likely to affect the electronic properties of the electrolyte, as will be discussed later. Acid–base titration was applied to monitor the content of calcium oxide in the calcium chloride. In agreement with previous expectations it was found that the calcium oxide content in the as-received material was below 0.1 mol%, increased by a fraction of a percent during drying, melting and pre-electrolysis, and always remained below 1 mol%. The actual electrochemical reduction experiments were carried out as follows. A titanium dioxide pellet prepared as outlined before was made the cathode, a graphite rod was used as the anode, and a voltage versus time regime comprising up to three stages was applied. During the first phase, the applied voltage was 2.5 V and this value was maintained for typically 8 h; in the second phase, the voltage was increased to 2.7 V and kept for typically 24 h; in the third phase, the voltage was set to 2.9 V and then left unchanged. The conditions were thus similar to those selected in previous studies in that no continuous electrolysis of calcium chloride could occur. Electrical contact to the electrodes used in pre-electrolysis and reduction was made through nickel wire of 2 mm in diameter, this rendering the IR-drop in the current leads insignificant. The first part of each experiment was performed with a computercontrolled potentiostat (Powerstat, Sycopel Scientific) and the current was recorded as a function of time. Thereafter either the same apparatus or a manually controlled voltage source (LS 30-10, Wayne Kerr) was used. In order to produce partially reduced titanium dioxide specimens, the electrochemical reduction was interrupted after different reaction times, these ranging from 0.5 to 120 h. A run was terminated by removing the cathode from the salt melt and locating it in the upper and cooler part of the reactor. As the temperature of the quenched sample decreased rapidly and caused adherent salt to solidify, and as the reactor was continuously flushed with argon, it is assumed that this procedure did not cause a significant increase of the oxygen content. The recovered samples were vigorously rinsed with tap water and placed in 1.0 N hydrochloric acid whereafter a mild vacuum of around 20 mbar was applied. Preinvestigations had shown that materials like titanium dioxide, titanium sesquioxide, Ti2 O3 , titanium monoxide, TiO, and calcium titanate, CaTiO3 , are inert under these conditions,
68
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
whereas calcium oxide and calcium metal may easily and completely be removed. Occasionally, a purple colouration of the aqueous phase was observed, which indicates the reaction of freshly gained and chemically reactive titanium metal with the hydrochloric acid. In such a case, the acid was slightly diluted with distilled water until the colour intensified no further. After around 10–12 h, the hydrochloric acid solution was replaced with distilled water, again assisted by vacuum impregnation. After another 10–12 h, the samples were washed with acetone and dried in air at around 100 ◦ C. The surface of each sample was ground in order to remove surface impurities and provide an even surface for X-ray diffraction analysis, with some of the samples being investigated at different depths. Fractured surfaces were prepared for electron microscopic examinations. Phase composition of the various partially reduced specimens was determined through X-ray diffraction analysis (PW 1710, Philips). Each scan ranged from 10◦ to 80◦ . Microstructure and chemical composition of the samples were investigated by means of scanning electron microscopy and energy-dispersive X-ray analysis (JSM-5800LV, JEOL). The acceleration voltage was 15 keV, secondary electrons were detected, and images were taken at magnifications between 1000 and 5000 times. Except for the oxide precursor, all samples were examined in the as-prepared state, that is, without a sputter-coated conductive layer. Oxygen content of the samples was quantitatively determined by means of an analyser that makes use of the hot extraction technique and infrared detection of the carbon dioxide evolved (ONH-2000, Eltra). For each sample typically three individual measurements were done and averaged.
3. Results Fig. 1a presents four current versus time curves, which were recorded during the first 8 h of electrochemical reduction experiments performed with compact titanium dioxide preforms at an applied voltage of 2.5 V. The plots commence with a current peak of significant height that extends over about 1 h, and this is followed by a current shoulder that spans over about 3–5 h. Thereafter the current assumes relatively small values. Fig. 1b and c shows current traces obtained during the subsequent stages of an experiment in which voltages of 2.7 and 2.9 V were applied. The current remains relatively small and changes only little as a function of time. When the voltage is raised, the current increases to some extent but as before does not vary markedly over the course of the experiment. Fig. 2 presents the X-ray diffraction spectrum of the compact titanium dioxide preform after sintering, and Figs. 3–8 show the X-ray diffraction spectra obtained from specimens the polarisation treatment of which was terminated after different durations. Samples quenched after around 0.5 h of polarisation were found to consist predominantly of the titanium suboxide Ti3 O5 and the calcium titanate CaTiO3 ,
Fig. 1. Current vs. time curves of electrochemical reduction experiments. Electrolyte: calcium chloride; cathode: titanium dioxide pellets (mass 8 g); anode: graphite (geometric surface area 12 cm2 ); anode-to-cathode distance: 4 cm; temperature: 900 ◦ C; atmosphere: dried argon. (a) Four nominally identical experiments performed at a voltage of 2.5 V; (b) continuation of one of the experiments at a voltage of 2.7 V; (c) continuation of the same experiment at a voltage of 2.9 V.
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
69
Fig. 2. X-ray diffraction spectrum of the compact titanium dioxide preform after sintering in air at 1100 ◦ C for 2.5 h, showing rutile-type TiO2 ((䊉), JCPDS 21-1276).
Fig. 4. X-ray diffraction spectrum of a sample quenched after about 1 h of polarisation, showing Ti2 O3 ((䊉), JCPDS 43-1033) and CaTiO3 ((), JCPDS 42-0423); oxygen content ∼35% by mass.
the latter possessing titanium in the oxidation state +IV. The X-ray diffraction pattern taken from a section that crosses the centre of the pellet showed the additional presence of a small amount of the titanium suboxide Ti4 O7 . Samples recovered after about 1 h were primarily composed of titanium sesquioxide, Ti2 O3 , and CaTiO3 , and samples removed after about 4 h comprised mainly titanium monoxide, TiO, and CaTiO3 . The majority of the TiO was cubic, while small amounts of the monoclinic modification were always present too. Samples retrieved at reaction times longer than that contained progressively less TiO and CaTiO3 , while there was an increase in the amount of the lower-valent calcium titanate CaTi2 O4 , in which titanium is present in the oxidation state +III. From around 8–16 h of polarisation, CaTi2 O4 was found to be the prevailing phase, with some
samples being almost single phase while others contained detectable quantities of TiO and CaTiO3 . After longer reaction times, the amount of CaTi2 O4 declined while more TiO was identified. The latter formed preferentially on the outside of the pellet and was exclusively monoclinic. After about three days, the X-ray diffraction spectra obtained were those of titanium metal. Two issues are noteworthy with respect to the results presented above. Firstly, owing to the relatively weak sensitivity of X-ray diffraction analysis, only the major phases of each sample could be identified. In particular, small amounts of TiO are difficult to discern in the presence of titanates. Secondly, the X-ray diffraction patterns of the ordered phases Ti3 O and Ti6 O are very similar to that of Ti. This renders the technique less convenient to analyse phase composition in the final stage of the reduction process.
Fig. 3. X-ray diffraction spectrum of a sample quenched after about 0.5 h of polarisation, showing Ti3 O5 ((䊉), JCPDS 40-0806) and CaTiO3 ((), JCPDS 42-0423); oxygen content ∼36% by mass.
Fig. 5. X-ray diffraction spectrum of a sample quenched after about 4 h of polarisation, showing cubic TiO ((䊉), JCPDS 08-0117) and CaTiO3 ((), JCPDS 42-0423); oxygen content ∼31% by mass.
70
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
Fig. 6. X-ray diffraction spectrum of a sample quenched after about 12 h of polarisation, showing CaTi2 O4 ((), JCPDS 11-0029); oxygen content ∼29% by mass. (Small amounts of CaTiO3 and TiO were also present but could only be seen at high resolution.)
Fig. 8. X-ray diffraction spectrum of a sample retrieved after about 120 h of polarisation, showing ␣-Ti ((䊉), JCPDS 44-1294); oxygen content ∼3000 ppm by mass. (The small peaks at 43.4◦ and 44.6◦ are not present in the reference file but were also found in titanium samples of commercial purity.)
Overall, reproducibility and consistency of the results were good for the first stage of reduction but deteriorated once CaTi2 O4 was the predominant component. The specimens quenched during the early stages of the reduction process were subjected to gravimetric analysis. The samples consisting predominantly of Ti3 O5 and CaTiO3 , Ti2 O3 and CaTiO3 , as well as TiO and CaTiO3 were found to have gained in mass on average by approximately 12, 16 and 23%, respectively. Samples containing significant amounts of CaTi2 O4 were not considered in the gravimetric analysis because their friable character did not allow for quenching and post-treatment to be carried out without noticeable mass loss, whereas the oxygen content of metallic samples was
too small to permit meaningful measurements. Quantitative determination of the oxygen content provided values in the order of 29–36% by mass for all samples quenched in the early stages of the reduction process, while numbers became smaller as the reduction proceeded further, with approximately 3000 ppm by mass being a typical endpoint. Also for the samples retrieved after relatively short processing times, the amount of charge passed during the polarisation experiment was measured. This amount of charge was compared with that needed theoretically to attain complete reduction, and the degree of reduction was expressed in terms of a percentage value. For samples consisting of primarily Ti3 O5 and CaTiO3 , Ti2 O3 and CaTiO3 , as well as TiO and CaTiO3 these numbers were approximately 13, 18 and 31%, respectively. Samples recovered at the later stages were disregarded in this analysis, since the rate of reduction decelerated to such an extent that the charge contributed by the background current became significant. Fig. 9 shows a micrograph of the compact titanium dioxide preform as obtained after sintering at 1100 ◦ C in air. Figs. 10–12 present micrographs of some of the partially reduced specimens together with typical results obtained from energy-dispersive X-ray analysis. Fig. 13 is an image of the final product.
4. Discussion
Fig. 7. X-ray diffraction spectrum of a sample quenched after about 52 h of polarisation, showing CaTi2 O4 ((), JCPDS 11-0029); and monoclinic TiO ((䊉), JCPDS 23-1078); oxygen content ∼26% by mass.
The experimental results obtained within the scope of the present study have provided strong evidence that the electrochemical reduction of titanium dioxide to titanium metal in molten calcium chloride proceeds through a number of welldefined reaction steps. In the following, the individual stages of reduction will be discussed in greater detail. The required
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
71
Fig. 9. Scanning electron microscopic image of the TiO2 precursor after sintering in air at 1100 ◦ C for 2.5 h.
thermodynamic data was taken from a database [4] and apply to the temperature of 900 ◦ C. All quoted standard electrode potentials, E◦ , are referred to the Ca/Ca2+ equilibrium, and both cathodic and anodic processes are expressed in terms of the respective reduction potentials. The standard states for solids, gases, and ions are, respectively, the pure compound, the atmospheric pressure, and the unit mole fraction of the relevant ionic sublattice. The first stage of the electrochemical reduction of titanium dioxide is characterised by the formation of mixtures that consist of the various titanium suboxides Tix Oy as well as the calcium titanate CaTiO3 . The successive formation of these mixtures may be described by the following sequence of cathodic reaction steps: 5TiO2 + Ca2+ + 2e− = Ti4 O7 + CaTiO3 , E◦ ≈ +1807 mV
(1)
4Ti4 O7 + Ca2+ + 2e− = 5Ti3 O5 + CaTiO3 , E◦ = +1678 mV
(2)
3Ti3 O5 + Ca2+ + 2e− = 4Ti2 O3 + CaTiO3 , E◦ = +1575 mV
(3)
2Ti2 O3 + Ca2+ + 2e− = 3TiO + CaTiO3 , E◦ = +1200 mV
(4)
It should be noted that the electrode potential calculated for Eq. (1) is an approximate value. This is because the formation of the mixture of Ti4 O7 and CaTiO3 takes place in several steps and involves numerous Magn´eli phases that
Fig. 10. (a) Scanning electron microscopic image of a sample quenched after about 1 h of polarisation, showing a mixture of Ti2 O3 and CaTiO3 ; (b) energy-dispersive X-ray spectrum obtained from point analysis 1; (c) energy-dispersive X-ray spectrum obtained from point analysis 2.
72
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
Fig. 11. (a) Scanning electron microscopic image of a sample quenched after about 12 h of polarisation, showing the overwhelming presence of CaTi2 O4 ; (b) energy-dispersive X-ray spectrum obtained from point analysis 1.
have compositions between those of TiO2 and Ti4 O7 [5]. The approximation is nevertheless realistic since all the oxide phases concerned are of similar thermodynamic stability [6]. Eqs. (2)–(4) involve one reaction step, and accurate potentials may be given. A minor inaccuracy arises from the fact that TiO is a compound of considerable stoichiometric width, which renders its Gibbs free energy of formation somewhat composition-dependent. Through combination of Eqs. (1)–(4), the overall cathodic reactions leading to the formation of the experimentally observed mixtures may be written as follows: 4TiO2 + Ca2+ + 2e− = Ti3 O5 + CaTiO3
(5)
3TiO2 + Ca2+ + 2e− = Ti2 O3 + CaTiO3
(6)
2TiO2 + Ca2+ + 2e− = TiO + CaTiO3
(7)
Fig. 12. (a) Scanning electron microscopic image of a sample quenched after about 52 h of polarisation, showing a mixture of CaTi2 O4 and TiO; (b) energy-dispersive X-ray spectrum obtained from point analysis 1; (c) energy-dispersive X-ray spectrum obtained from point analysis 2.
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
73
The anodic reaction may be the discharge of either chloride ions or oxide ions, which results in the generation of either chlorine gas or oxygen, carbon monoxide and carbon dioxide. Cl2 + 2e− = 2Cl− ,
E◦ = +3214 mV
(8)
1 O2 + 2e− = O2− , 2
E◦ = +2407 mV
(9)
CO + 2e− = C + O2− ,
Fig. 13. Scanning electron microscopic image of a sample retrieved after about 120 h of polarisation, showing Ti.
The theoretical mass gains of the cathode due to the incorporation of calcium may be calculated from the stoichiometries of Eqs. (5)–(7) and amount to 12.5, 16.7 and 25.1%, respectively. This is close to the experimental results. The percentages of reduction that correspond to the three equations are 12.5, 16.7 and 25.0%, respectively. This is somewhat lower than the measured numbers. However, a very good agreement arises under the assumption that a portion of the current in the order of 50–80 mA is associated with phenomena other than those described by the above reactions; this will be discussed later. Overall, the experimental observations are in accordance with the proposed sequence of reactions. It may therefore be deduced that the formation of the different mixtures of titanium suboxides Tix Oy and calcium titanate CaTiO3 proceeds according to well-defined electrochemical reactions and leads to quantitatively predictable compositions of the cathode material. The mechanism underlying the cathodic reactions that occur in the first stage of the electrochemical reduction of titanium dioxide, involves the incorporation of calcium into the cathode and the redistribution of oxygen inside the cathode. Kinetically, the three phases involved, i.e., the molten salt electrolyte, the oxide cathode, and the electronically conductive current collector, interact in the following manner: Ca2+ ions are inserted into the oxide cathode from the molten salt, electrons are provided to the oxide by the current lead, and oxygen is rearranged inside the cathode via diffusion along relatively short distances. As the various titanium suboxides are electronic conductors [7], the cathode becomes conductive throughout its volume very shortly after reduction has commenced. The reduction is then no longer confined to the points of contact between the three phases but may take place over the entire interface of electrolyte and cathode.
E◦ = +1291 mV
(10)
1 1 CO2 + 2e− = C + O2− , E◦ = +1381 mV (11) 2 2 Eqs. (8)–(11) demonstrate that the formation of carbon monoxide and carbon dioxide as the anodic off-gases is thermodynamically more favourable than the formation of chlorine and oxygen gas. However, under the governing experimental conditions, the initial content of calcium oxide dissolved in the calcium chloride was small, so chlorine evolution may have occurred. This issue was clarified by way of a control experiment. In this, as much as 16 g of titanium dioxide, corresponding to 0.2 mol, were processed in as little as 1 mol of calcium chloride. After a few hours of polarisation the mass increase of the cathode due to calcium uptake was almost 4 g, whilst the calcium oxide content present at the beginning of an electrochemical reduction experiment was less than 1 mol%. As can readily be inferred from these numbers, the major uptake of calcium must have been from the calcium chloride. Accordingly, this result provides evidence that the reactive character of the oxide cathode and the stabilising effect of the calcium titanate formed enforce the decomposition of a quantity of calcium chloride and the release of chlorine at the anode in the beginning of the reduction process, although the voltage necessary for continuous calcium chloride electrolysis is not exceeded. Indeed, it was possible to prove the release of chlorine during the first stage of the reduction process by passing the exhaust gas through silver nitrate solution and identifying precipitated silver chloride. A quantitative analysis was not carried out, as the predominant part of the chlorine liberated was likely to be gettered in the heated part of the metal reactor. The assumption of chlorine being the main anodic off-gas during the first stage of the reduction process is also in agreement with two further observations. Firstly, in the present set of experiments no mixture of Ti and CaTiO3 has ever been encountered and, in fact, the formation of this composition would be possible under a voltage of 2.5 V only if carbon monoxide or carbon dioxide could be generated but not if chlorine has to be released. Secondly, it was observed that during the occurrence of the initial current peak virtually no consumption of the anode material took place. The second stage of the electrochemical reduction of titanium dioxide is characterised by the dominating presence of CaTi2 O4 . There are two obvious routes toward the formation of this compound. One possibility is the comproportionation reaction of TiO and CaTiO3 . CaTiO3 + TiO = CaTi2 O4
(12)
74
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
This reaction is chemically driven, because no change in the average oxidation state of the titanium occurs, and does not contribute to the current. The second possibility is the electrochemical removal of oxygen from CaTiO3 . 2CaTiO3 + 2e− = CaTi2 O4 + Ca2+ + 2O2− , E◦ > −146 mV
(13)
It has to be assumed that, under the present experimental conditions, Eq. (12) proceeds more rapidly than Eq. (13), because only in this way it is possible to explain the abundant presence of CaTi2 O4 and the temporary decrease of TiO during this interval of the process. Unfortunately, the Gibbs free energy of formation of CaTi2 O4 appears to be unknown and so no accurate quantitative thermodynamic considerations can be made. It may however be stated that the CaTi2 O4 forms from the previously generated mixture of TiO and CaTiO3 . Therefore, the free energy of formation of CaTi2 O4 must be more negative than the sum of the free energies of formation of TiO and CaTiO3 , and on this basis a lower potential limit for the reaction in Eq. (13) may be calculated. Moreover, it may be deduced that the reactions during the first stage of the reduction process are to some extent kinetically controlled. The third stage of the electrochemical titanium dioxide reduction involves the decomposition of the CaTi2 O4 and the formation of TiO. As no compounds are known that comprise titanium in the oxidation state +II as well as calcium and oxygen, this reaction step must be accompanied by the release of CaO. CaTi2 O4 + 2e− = 2TiO + Ca2+ + 2O2− , E◦ < +417 mV
(14)
The upper limit of the potential for the reaction given in Eq. (14) is computed on the same basis as the lower limit for Eq. (13). The TiO formed is then further reduced to solid solutions of oxygen in titanium Ti[O]δ . TiO + 2(1 − δ)e− = Ti[O]δ + (1 − δ)O2− , E◦ ≈ +177 mV
(15)
At temperatures above 920 ◦ C there is a direct transition between the compound phase TiO and the solution phase Ti[O]δ , with δ denoting the oxygen content in the solid solution that is in equilibrium with TiO. Below this temperature the Ti3 O2 phase is present in between. As the Gibbs free energy of formation of the latter compound is apparently unknown, the potential for the above reaction has been calculated using the free energy of formation of TiO and is thus to some extent approximate. At much lower temperatures additional ordered phases like Ti2 O, Ti3 O and possibly Ti6 O exist [5]. As opposed to the shorter reduction experiments discussed earlier, substantial erosion of the anode was now observed, and this indicates that the transfer of oxygen ions from the cathode to the anode and the subsequent release of carbon oxides took place at this stage of the process.
Under the conditions of the present set of experiments, the applied voltage exceeds the electrolysis voltage of calcium oxide. As the potential distribution across a galvanic cell of the given type and in particular the magnitude of the overpotentials are unknown, a quantitative assessment is at present impossible, but it is probable that the calcium oxide liberated during decomposition of the intermediate compounds will react further. At the cathode a small quantity of divalent calcium ions will be discharged, and at the anode the release of carbon monoxide or carbon dioxide will take place. The latter proceeds in accordance with the reactions given in Eqs. (10) and (11), although the relevant potentials will be somewhat different from the quoted ones, as neither the calcium oxide nor the calcium metal are in their standard states. The reduced calcium species formed at the cathode dissolve in the molten calcium chloride and make a significant contribution to the electronic conduction of the melt. The relatively constant current during the later stages of the process is therefore likely to be due to the compensating effects of the slowly decaying oxide ion current, resulting from the removal of oxygen from the cathode, and the slowly rising electronic background current, originating from the generation of dissolved calcium species. It should be emphasised that the occurrence of reduced calcium species is an inevitable sideeffect, which occurs under high applied potentials as they are necessary in the processing of titanium dioxide. Due to the high applied potentials and the solubility of calcium metal in calcium chloride, the calcium activity in the melt rises to values that are significantly larger than zero but smaller than unity. The enhanced calcium activity may assist the reduction process chemically although this is not a prerequisite to accomplish reduction. One experimental proof, for instance, is the finding that titanium dioxide pellets that are attached to the cathodic current collector are substantially more reduced than samples that are exposed to the melt but detached from the cathode. An in-depth discussion has been provided in a publication that is exclusively dedicated to this topic [8]. Nevertheless, it is acknowledged that an unambiguous differentiation between the electrochemical and the chemical contributions to the reduction process is difficult to achieve once the final stage is reached. Based on the interpretation put forward above, the shape of the current versus time curves may be explained. The current peak is caused by fast reactions of the cathode with calcium ions from the electrolyte, which lead to the formation of mixtures containing Ti4 O7 , Ti3 O5 and Ti2 O3 in conjunction with CaTiO3 . The current shoulder is due to the formation of a mixture of TiO and CaTiO3 . While compound formation is ongoing, the removal of oxygen from the cathode is a minor process that contributes only little to the overall current. The aforementioned reaction stages are denoted in Fig. 14, which features a particularly distinctly resolved current versus time curve. During the later stages of the reduction process, when compound formation has ceased, the removal of oxygen from the cathode becomes the predominant process. However, the current associated with the transfer of oxygen is relatively
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
75
the CaTi2 O4 particles reduce in size, whereby a much finer and more homogeneous microstructure is again produced. As before, energy-dispersive X-ray analysis confirmed the anticipated cation ratios. Fig. 13 shows the microstructure of fully reduced titanium metal. Particle size is in the range of a few microns and the overall appearance is similar to that of titanium sponge prepared by conventional methods. 5. Conclusion
Fig. 14. A particularly well-resolved current vs. time curve taken from Fig. 1a, with cathode composition given for various points in time during the first stage of reduction.
small, and thus the electronic current now contributes significantly to the overall current. This renders the current versus time curve rather unstructured, and it is not possible any longer to assess directly the degree of reduction at a given point in time. As an additional consequence, the overall current efficiency for the preparation of titanium of low oxygen contents falls to relatively small values in the range of 10–20%, which is unsatisfactory from the viewpoint of application. As is visible in Fig. 9, the particle size of the compact titanium dioxide preform, prepared as described earlier, is around 1 m and rather uniform, and there is a large extent of porosity. Fig. 10 illustrates that the microstructure of the sample composed of Ti2 O3 and CaTiO3 is relatively fine and homogeneous, with an average particle size somewhat above 1 m. Energy-dispersive X-ray analysis reveals the presence of an intimate mixture of particles of two different compositions. The titanium to calcium ratios as measured for both kinds of crystallites typically ranged from 91:9 to 97:3 and from 40:60 to 53:47, respectively, and are thus close to the expected values for Ti2 O3 and CaTiO3 , while the deviations are probably due to the fact that particle size and sampling area are of the same order of magnitude. Similar results were obtained for the mixtures of Ti3 O5 and CaTiO3 as well as TiO and CaTiO3 . In Fig. 11 it may be seen that the microstructure of CaTi2 O4 is comparatively coarse and heterogeneous. Particles are elongated and some reach a length of up to 100 m. This finding explains readily the poor mechanical stability that partially reduced samples exhibit during the intermediate interval of reduction. The titanium to calcium ratio as determined by energy-dispersive X-ray analysis was always very close to the expected one of 67:33. Fig. 12 reveals the unique microstructure of samples containing both TiO and CaTi2 O4 . The TiO emerges in the form of individual and separate grains, which grow together as they expand and as
Titanium dioxide specimens were reduced electrochemically and to different extents in molten calcium chloride. Analysis of the partially reduced samples proved that the reduction process advances through a series of defined reaction steps. In the first stage, calcium species from the electrolyte are inserted into the cathode and oxygen is redistributed in the cathode, such that mixtures consisting of the various titanium suboxides, Tix Oy , and calcium titanate, CaTiO3 , are formed. The second stage is characterised through the overwhelming presence of a lower-valent calcium titanate, CaTi2 O4 , which is thought to originate mainly from the chemical reaction between the previously generated compounds. In the third stage, the CaTi2 O4 is decomposed and TiO formed, which is then reduced further to solid solutions of oxygen in titanium metal, Ti[O]δ . The results have demonstrated that the process is fast as long as the reduction may be accomplished through the uptake of calcium but becomes sluggish as soon as the reduction may only be achieved through the removal of oxygen. One objective of future work is to elucidate the reason for the slow rate of oxygen removal and to identify improved experimental conditions. A scanning electron microscopic examination has revealed that the microstructure of the specimens being reduced evolves through a very coarse and fragile state when CaTi2 O4 is the predominant component, while the microstructure is considerably finer and more uniform during the earlier and the later stages of reduction. A particularly noteworthy conclusion from the present study is that the formation and decomposition of calcium containing intermediate compounds are inherent steps in the kinetic pathway of the molten salt electrochemical reduction of titanium dioxide. A thermodynamic analysis shows that the temporary occurrence of similar compounds is to be expected in the reduction of most other metal oxides too, and compounds of this type have indeed been observed in incompletely reduced samples of niobium oxide [9,10] and chromium oxide [11,12]. Accordingly, the precise reaction path ought to be investigated for each feed material that is considered appropriate for the present reduction technique. Acknowledgements Financial support of this work by the European Office of Aerospace Research and Development (EOARD) of the
76
C. Schwandt, D.J. Fray / Electrochimica Acta 51 (2005) 66–76
US Air Force is gratefully acknowledged. Dr. G. Doughty, of British Titanium plc., is thanked for discussions and critically reading the manuscript.
References [1] [2] [3] [4]
D.J. Fray, T.W. Farthing, Z. Chen, Patent WO9964638. G.Z. Chen, D.J. Fray, T.W. Farthing, Nature 407 (2000) 361. G.Z. Chen, D.J. Fray, J. Electrochem. Soc. 149 (2002) E455. A. Roine, HSC Chemistry, version 4.1, Outokumpu Research Oy, Pori, Finland.
[5] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, vol. 3, second ed., ASM International, Materials Park, OH, 1990, p. 2924, O–Ti (oxygen–titanium). [6] D.C. Lynch, D.E. Bullard, Metall. Mater. Trans. B 28 (1997) 447. [7] R.L. Clarke, S.K. Harnsberger, Am. Lab. 20 (1988) 8. [8] D.J. Fray, G.Z. Chen, in: F.H. Froes, M.A. Imam, D.J. Fray (Eds.), Cost-Affordable Titanium, TMS (The Minerals, Metals & Materials Society), 2004, p. 9. [9] X.Y. Yan, D.J. Fray, Metall. Mater. Trans. B 33 (2002) 685. [10] X.Y. Yan, D.J. Fray, J. Mater. Res. 18 (2003) 346. [11] E. Gordo, G.Z. Chen, D.J. Fray, Electrochim. Acta 49 (2004) 2195. [12] G.Z. Chen, E. Gordo, D.J. Fray, Metall. Mater. Trans. B 35 (2004) 223.