Green Electrochemical Process Solid- Oxide Oxygen-Ion-Conducting Membrane (SOM): Direct Extraction of Ti-Fe Alloys from Natural Ilmenite

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1 Green Electrochemical Process Solid- Oxide Oxygen-Ion-Conducting Membrane (SOM): Direct Extraction of Ti-Fe Alloys from Natural Ilmenite Xionggang Lu, Xingli Zou, Chonghe Li, Qingdong Zhong, Weizhong Ding & Zhongfu Zhou Metallurgical and Materials Transactions B ISSN Volume 43 Number 3 Metall and Materi Trans B (2012) 43: DOI /s

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3 Green Electrochemical Process Solid-Oxide Oxygen-Ion-Conducting Membrane (SOM): Direct Extraction of Ti-Fe Alloys from Natural Ilmenite XIONGGANG LU, XINGLI ZOU, CHONGHE LI, QINGDONG ZHONG, WEIZHONG DING, and ZHONGFU ZHOU The direct electrochemical extraction of Ti-Fe alloys from natural ilmenite (FeTiO 3 ) in molten CaCl 2 is reported in this article. The sintered porous pellet of natural ilmenite acted as the cathode of the electrochemical system, and the carbon-saturated liquid tin contained in a solidoxide oxygen-ion-conducting membrane (SOM) tube served as the anode of the electrolytic cell. The electrochemical process was carried out at 3.8 V, under 1223 K and 1273 K (950 C and 1000 C). Oxygen was ionized continuously from the cathode and discharged at the anode; solid porous Ti-Fe alloys were obtained at the cathode. The electro-deoxidation procedure of the ilmenite was characterized by analyzing partially electro-deoxidized samples taken periodically throughout the electro-deoxidation process. The findings of this study are as follows: (1) The electro-deoxidation process followed these steps: Fe 2 TiO 5 fi FeTiO 3 fi Fe 2 TiO 4 fi Fe, Ti (and/or Ti-Fe alloys); and TiO 2 fi CaTiO 3 fi Ti; and (2) two types of particle growth pattern are observed in the experiments. The first pattern is characterized with particle fusion and second pattern is interconnection of particles to form porous structure. A microhole oxygenion-migration model is suggested based on the experimental evidence. DOI: /s Ó The Minerals, Metals & Materials Society and ASM International 2012 I. INTRODUCTION THE world now faces more serious problems involved with maintaining our ecosystem while at the same time trying to support the increase of our world s population, minimizing poverty, and maintaining a high level of security. The protection of our environment has now become one of the major issues for the development of our society in the future. [1 5] It is now commonly recognized that CO 2 emissions is one a major factor contributing toward the global warming effect. [4,5] The steel industry contributes to approximately 6 to 7 pct [4] of the total global anthropogenic CO 2 emissions; thus, scientists are desperate to search for breakthrough technologies that will enable a drastic reduction in the CO 2 emissions of the steel plant. [6 12] Three types of breakthrough technologies could lead to the reduction of the CO 2 emissions in the steel industry, which include the following: the hydrogen reduction and XIONGGANG LU, CHONGHE LI, QINGDONG ZHONG, and WEIZHONG DING, Professors, and XINGLI ZOU, Ph.D. Student, are with the Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Shanghai , P.R. China. Contact luxg@shu.edu.cn ZHONGFU ZHOU, Professor, is with the Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, is also Visiting Fellow with the Department of Materials, University of Oxford, Park Road, Oxford OX1 3PH, U.K., and is also Lecturer with the Centre for Advanced Functional Materials and Devices, Aberystwyth University, Aberystwyth SY23 3BZ, U.K. Manuscript submitted December 24, Article published online February 9, electrochemical technologies, [10 13] the CO 2 capture and storage (CCS) technology, [5] and the biomass technology. [14] Amongst them, the electrochemical process, which uses the electron as reductant to replace the carbon, offers a new more promising approach to the direct separation of oxide compounds/ores into metals/alloys and oxygen. This route can eliminate the greenhouse-gas emissions effectively, and it is not surprising that its great potential has been recognized as a way forward for the future of steel industries. [9,10] Several such electrochemical processes have been developed in recent years, [6 9,15 32] and some of them have proven to be promising in the laboratory or even at a pilot scale, e.g., the Fray-Farthing-Chen (FFC) Cambridge process, [16 20] the molten oxide electrolysis process, [21 23] the Ono-Suzuki process, [24 26] the Quebec Iron and Titanium (QIT Fer et Titane) process [27,28] and the solid-oxide oxygen-ion-conducting membrane (SOM) process, [6 8,29 32] etc. The FFC and SOM processes have shown potential in the direct electrochemical extraction of metals or alloys from porous oxides. In particular, the SOM process has advantages in overcoming the challenges of the low current efficiency and environmental pollution, [6 8,30] which are the key obstacles to the electrochemical extractive metallurgy. Natural ilmenite (FeTiO 3 ) is a common mineral resource that is used extensively to produce TiO 2 pigment and titanium metal; it is also recognized as one of the materials for producing oxygen gas on the moon. [33] It has attracted the interests of devoted researchers. [33 38] The most common titanium metal production process from ilmenite is known as the Kroll METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 43B, JUNE

4 process [39] : The ilmenite is reduced first into rutile (TiO 2 ), which is converted into titanium tetrachloride (TiCl 4 ), and then followed by a reduction process to produce metallic titanium with an active metal such as magnesium. This process is costly and has a low yield; hence, a large amount of research efforts have been made to address the problems. Solutions are devised, in particular to discover alternative single-step processes that can produce titanium metal or titanium alloys directly from TiO 2 and/or ilmenite. [12,16,24 26] The direct reduction of ilmenite by hydrogen and carbon as well as molten silicon have been studied intensively; however, the products from these reduction processes always contain TiO x, TiC, and Si. [13,37,38] In addition, the previous studies on the production of oxygen gas from natural ilmenite are mainly focused on the hydrogen reduction process, which is carried out in a hot reactor to create water vapor, and then the water vapor is condensed and electrolyzed to produce oxygen gas. [33,34] This process consists of two steps, i.e., the reduction of natural ilmenite (FeTiO 3 +H 2 = Fe + TiO 2 +H 2 O) and the electrolysis of water (H 2 O=H 2 + 1/2O 2 ). This article will demonstrate the feasibility of the onestep direct separation of TiFe and oxygen from natural ilmenite by using the SOM process in molten CaCl 2 : FeTiO 3 = TiFe + 3/2O 2. We also try to propose a microhole oxygen-ion-migration model and a Ti-Fe alloys formation route, and we try to understand the mechanism of the electrochemical deoxidation process and the corresponding solid transformation. II. EXPERIMENTAL A. Preparation of the Ilmenite Cathode The chemical composition of the natural ilmenite used in this experiment is listed in Table I. It was first ball milled with anhydrous alcohol for approximately 10 hours, and the milled powder was sized<10 lm with an average particle size of 3.8 lm. Then, approximately 1.5 g milled powder was pressed into a cylindrical pellet sized 15 mm in diameter and ~2 mm in thickness under a pressure load of 5 MPa to 8 MPa. To obtain adequate strength and the desired porosity for the electro-deoxidation, the pressed pellet was then sintered in air at 1423 K (1150 C) for approximately 2 hours. After the sintering, the pellet was measured with a porosity of approximately 30 pct, and the sintered powder was measured with an average particle size of 7.1 lm. The sintered pellet was finally wrapped with Mo wire mesh to form a cathode. B. Preparation of the Anode The SOM anode used in our experiments was a solidoxide oxygen-ion-conducting (yttria-stabilized zirconia [YSZ]) membrane tube filled with carbon-saturated liquid tin. The liquid tin was used as the medium to transport oxygen ions during the electro-deoxidation procedure. It should be noted that the carbon contained in the YSZ tube acted as the reductant to react with O 2 in this experiment. To realize zero CO 2 emissions, hydrogen could be introduced to the anode to react with O 2 instead of carbon, [6,31] or the YSZ tube could be coated with noble metal on the inner walls to oxidize O 2 directly to produce O 2 gas. [32] C. Electrodeoxidation Experiments The SOM process electrolytic cell designed for the separation of TiFe and oxygen from natural ilmenite is shown in Figure 1. The assembled cathode and anode were placed in a graphite crucible that contained anhydrous CaCl 2 as an electrolyte. When the temperature reached the desired cell temperature, the system was equilibrated at the fixed temperature for approximately 1 hour to ensure complete melting of CaCl 2. Ultra pure argon gas was purged continuously into the crucible to maintain an inert atmosphere in the system during the experiment. The preelectrolysis process was performed between a Kanthal wire cathode and the SOM anode at low potential of 2.5 V to remove moisture and redox impurities, and it lasted until the current reached a low and stable value (the background current). The electrodeoxidation experiments were then performed systematically at 3.8 V and 1223 K (950 C) as well as 1273 K (1000 C) for 1 hour to 8 hours, respectively. A Bio- Logic HCP-803 electrochemical workstation (Bio-Logic Fig. 1 Schematic illustration of the SOM process electrolytic cell used to electro-deoxidize natural ilmenite. Table I. Chemical Composition of the Natural Ilmenite (wt pct) Composition TiO 2 Total Fe SiO 2 CaO MgO Al 2 O 3 MnO Natural ilmenite VOLUME 43B, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS B

5 SAS, Claix, France) was used to record the current curve during the electro-deoxidation. After the electrodeoxidation process finished, the cathode pellet was removed from the molten salt pool and then cooled in an argon atmosphere, it was washed with water to dissolve the solidified salt after it was cooled down and then was dried in vacuum. D. Cyclic Voltammetry The metallic cavity electrode (MCE) used in the experiments was a molybdenum pellet (15 mm in diameter and 2 mm in thickness) with one circular hole (1.0 mm in diameter) drilled through the pellet by a mechanical drill. The sintered natural ilmenite powder mixed with distilled water was pressed manually into the MCE cavity to form the working electrode. The counterelectrode was the SOM anode. The Kanthal wire was used as a pseudoreference electrode. A Bio- Logic HCP-803 electrochemical workstation was employed to record cyclic voltammograms (CVs). E. Characterization of the Cathode Products The morphology of the reduced pellet was examined using a JEOL JSM-6700F scanning electron microscope (SEM) (Jeol Ltd., Tokyo, Japan). The elemental composition was analyzed by energy-dispersive X-ray (EDX) spectroscopy (Oxford INCA EDS system; Oxford Instruments, Oxfordshire, U.K.) attached to the SEM. The phase constitution was determined by a Rigaku D/Max-2550 X-ray diffractometer (XRD) (Rigaku Corporation, The Woodlands, TX). The particle size distribution of the powder was measured by a Malvern Mastersizer 2000 laser particle size analyzer (Malvern Instruments, Ltd., Worcestershire, U.K.). III. RESULTS AND DISCUSSION A. XRD Analysis of the Electrodeoxidation Products To study the influence of the processes, such as sintering in air and immersing in molten CaCl 2 on the phase composition of the pellets, XRD patterns of the sintered-only pellet and the pellet that was sintered and then was immersed into molten CaCl 2 for 6 hours are shown in Figure 2(a). It is apparent that FeTiO 3 is oxidized into Fe 2 TiO 5 during sintering in air. After immersed into molten CaCl 2 for 6 hours, the sintered pellet (Fe 2 TiO 5 ) is partly converted into CaTiO 3 and Fe 2 O 3. This may result from the reaction of Fe 2 TiO 5 with the Ca 2+ and O 2 ions penetrated into the pellet from molten CaCl 2 following Reaction [1] (DG 0 1 = kj/mol at 1273 K [1000 C], calculated from reported thermodynamic data [40] ). The formation of such perovskites as intermediate products has been reported. [12,41 45] CaTiO 3 can also be produced from TiO 2 by Reaction [2] [29] (DG 0 2 = kj/mol at 1273 K [1000 C]). Fe 2 TiO 5 þ Ca 2þ þ O 2 ¼ CaTiO 3 þ Fe 2 O 3 ½1Š TiO 2 þ Ca 2þ þ O 2 ¼ CaTiO 3 The XRD patterns of the products obtained under different electro-deoxidation temperatures are shown in Figures 2(b) and (c). When the electro-deoxidation processes were carried out at 1223 K (950 C) and 3.8 V, the intermediate products appear sequentially with the prolonged electrolysis time as stated: CaTiO 3, FeTiO 3,andFe 2 TiO 4. This suggests that the multistep electro-deoxidation process might follow the sequences of reactions: Fe 2 TiO 5 fi FeTiO 3 fi Fe 2 TiO 4 fi Ti, Fe, and/or TiFe x (x = 1, 2); and TiO 2 fi CaTiO 3 fi Ti. It has been noticed that element Fe is identified as the first element electro-deoxidized from compound (Figure 2(c)), this may promote subsequent electrochemical reactions effectively owing that the metallic Fe could enhance electronic conduction. [45] After being electro-deoxidized for 6 hours, the oxygen in the compounds has been removed completely and Ti-Fe alloys are obtained (as shown in Figure 2(b)). When the cell temperature increased to 1273 K (1000 C), the results given in Figure 2(c) suggest that the electro-deoxidation also proceeds through the similar steps, some of which involve the formation and decomposition of intermediate compounds (such as FeTiO x and CaTiO 3 ). The general trend that the amount of metallic Fe gradually decreases as the increasing of electrolysis time is shown in Figure 2(c); this phenomenon has not been noticed and studied in detail, and it will be one focus of our future study in the project. B. Current Features and Weight Loss of the Pellet During Electrodeoxidation The typical current vs time curves of the electrodeoxidation process carried out at different temperatures are drawn in Figure 3. Figure 3(a) shows the curves corresponding to electro-deoxidation at 1223 K (950 C) and 3.8 V for 2 hours, 4 hours, and 6 hours, respectively, and Figure 3(b) shows the curves for electro-deoxidation at 1273 K (1000 C) and 3.8 V for 1 hours, 3 hours, and 5 hours, respectively. From Figure 3(a), it can be observed that the current declines with the increasing of time at the beginning of the electro-deoxidation process, and then it reaches approximately 800 ma to 1000 ma in the first few minutes. This suggests that the electrolytic cell reaches its equilibrium quickly, and it is consistent with our previous results. [29] Then, the current maintains at a broad peak of 1100 ma to 1200 ma for approximately 1 hour, which is in agreement with what observed in the electro-deoxidation taking place at the metal/compounds/molten salt three-phase interlines (3PIs). [46,47] As discussed, [47] the oxides contacting with the metal wire of the cathode are electro-deoxidized to metal, and new 3PIs are formed, which leads to the current increases. The current reaches its maximum when the pellet surface is totally metallized. With 3PIs moving into the interior of the pellet, the diffusion of oxygen ions becomes slower because the oxygen ions have to ½2Š METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 43B, JUNE

6 Fig. 2 XRD patterns of the products obtained from different conditions. (a) The natural ilmenite pellets before sintering, after being sintered in air at 1423 K (1150 C) for 2 h, and the pellet sintered and immersed into molten CaCl 2 for 6 h. (b) The natural ilmenite pellets electro-deoxidized at 3.8 V and 1223 K (950 C) for different times. (c) The natural ilmenite pellets electro-deoxidized at 3.8 V and 1273 K (1000 C) for different times. pass the metal layer formed, which results in the decrease of the current. The characteristics of the curves obtained at 1223 K and 1273 K (950 C and 1000 C) show great similarity; the only differences exhibited are current maximum and peak width. The weight loss of the pellet during the electrodeoxidation process is depicted in Figure 3(c); the inset in Figure 3(c) shows the photos of the cross section of the pellets at different electrolysis times and the schematic illustration of the changes of the pellet during the electro-deoxidation process. The migrations of the 3PIs are observed clearly at the cross section of the pellet, which is consistent with the previous study. [47] The weight of pellet decreases quickly within the first 2 hours because of the loss of the oxygen. When the pellet is electro-deoxidized completely, the weight of product obtained at the cathode is approximately wt pct of the initial pellet. It is slightly less than the theoretical value wt pct (when the oxygen is electro-deoxidized completely and the impurities have been removed), which might attribute to the loss of the cathode powder, which falls into the molten CaCl 2. From the current time curves recorded from electrodeoxidation at 3.8 V (as shown in Figures 3(a) and (b)), the current efficiencies are calculated to be pct (for 1273 K [1000 C], 5 hours) and pct (for 1223 K [950 C], 6 hours), and the energy consumptions are kwh/kg (for 1273 K [1000 C], 5 hours) and kwh/kg (for 1223 K [950 C], 6 hours), respectively. In contrast, the energy consumption using the current industrial technologies for producing the titanium metal is 45 kwh/kg to 55 kwh/kg and that of primary steel is 4 kwh/kg to 6 kwh/kg. [12,42] It should be noted that the product of the SOM process is Ti-Fe alloy, not metallic Ti, which means that a significant energy savings can be achieved using the SOM process to produce Ti-Fe alloys directly from natural ilmenite. Furthermore, the SOM process has been proven to be 506 VOLUME 43B, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS B

7 Fig. 3 Current-time curves of the electro-deoxidation process performed at 3.8 V and 1223 K (950 C) (a) and 1273 K (1000 C) (b) for different times. (c) The weight loss of the cathode pellet during electro-deoxidation, the inset in (c) shows the photos of the cross-section of the pellets at different electrolysis times and the schematic illustration of the changes of the pellet during the electro-deoxidation process. more efficient than the FFC process. [30] It should be noted also that the current efficiency depends on several factors, such as the porosity and thickness of the pressed pellet and the particle size of the feed material. The electronic conductivities of the molten salt and the zirconia membrane also inevitably create a residual current and decrease the current efficiency. Although the current efficiency is approximately 40 pct in the SOM electro-deoxidation process, it is evident that with the future optimization of the process parameters, the current efficiency of the SOM process can be improved. [30,31] C. Cyclic Voltammetry Figure 4 shows the CV curves of the MCE without (Figure 4(a)) and with (Figure 4(b)) natural ilmenite in molten CaCl 2. In the experiment absent of natural ilmenite, the CV curve has no plateau, which coincides with the theoretical speculation: After 2 hours of preelectrolysis, the residual dissolved oxygen and redoxactive impurities in the molten salt have been removed completely. Thus, there is almost no migration of oxygen ion. In the experiment with the natural ilmenite (Figure 4(b)), the electro-deoxidation starts at ~0.9 V and follows two peaks at ~1.5 V and ~2.0 V, which might imply that the electro-deoxidation of natural ilmenite involves three steps corresponding to Reactions [3] through [5], respectively. The CaTiO 3 of Reaction [5] is generated by Reactions [1], [2], and [7]. The XRD results given in Figure 2 indicate that the electrodeoxidation process involves two intermediate products (FeTiO 3 and Fe 2 TiO 4 ); this finding might imply that Fe 2 TiO 4 is also presented in the electro-deoxidation process although no corresponding peak of Reaction [6] in CV curves appears. Therefore, this compound Fe 2 TiO 4 is possibly generated by chemical Reaction [7]. Reaction [8] shows that Fe 2 TiO 4 electro-deoxidizes directly to Fe 2 Ti; its theoretical decomposition potential is 1.34 V, which corresponds with the peak risen in cycle 4. The intermetallic TiFe x might form by Reaction [9] between the metallic Fe and Ti. The Gibbs free energy change (DG 0 ) and the theoretical decomposition potential (DE d ) of these reactions at the electrolysis temperature METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 43B, JUNE

8 Fig. 4 CVs of the MCE without (a) and with (b) natural ilmenite in molten CaCl 2 at 1273 K (1000 C) using the SOM process. have been calculated from thermodynamic data, [40] as follows: Fe 2 TiO 5 þ 4e ¼ FeTiO 3 þ Fe þ 2O 2 DG 0 3 ¼ 368:01 kj=mol; DE d3 ¼ 0:95 V FeTiO 3 þ 6e ¼ FeTi þ 3O 2 DG 0 4 ¼ 878:11 kj=mol; DE d4 ¼ 1:52 V CaTiO 3 þ 4e ¼ CaO þ Ti þ 2O 2 DG 0 5 ¼ 799:13 kj=mol; DE d5 ¼ 2:07 V Fe 2 TiO 5 þ 2e ¼ Fe 2 TiO 4 þ O 2 DG 0 6 ¼ 93:43 kj=mol; DE d6 ¼ 0:48 V 2FeTiO 3 þ Ca 2þ þ O 2 ¼ Fe 2 TiO 4 þ CaTiO 3 DG 0 7 ¼ 88:15 kj=mol ½7Š Fe 2 TiO 4 þ 8e ¼ Fe 2 Ti þ 4O 2 DG 0 8 ¼ 1037:65 kj=mol; DE d8 ¼ 1:34 V xfe þ Ti ¼ TiFe x ðx ¼ 1; 2Þ ½9Š DG 0 9 < 33:71 kj=mol During the backward scan (positive direction), no peak arises, which indicates that the reduction of natural ilmenite by the SOM process is irreversible. Because the oxygen ions need to transport through the YSZ membrane before being oxidized at the anode, [6,32] no reoxidation current appears until ~0.3 V, which is also caused by the low inherent oxide concentration and the dissipation of electrochemically generated oxide ions in the bulk molten CaCl 2. [16,48] As shown in Figure 4(b), ½3Š ½4Š ½5Š ½6Š ½8Š the recorded CVs show that the current increases with increasing the number of potential cycles. The current peaks of cycle 4 shift to more negative potentials, and the reduction starts at ~1.5 V, ~2.0 V, and ~2.5 V, respectively, which indicates that Reactions [4], [5], and [8] but without [3] are coexisting in this cycle. The peak at ~2.5 V is caused by reactions Ca 2+ +e =Ca + and Ca + +e = Ca. [32] D. Morphology Observations Figure 5 shows the SEM images of natural ilmenite before (Figures 5(a) and (b)) and after being electrolyzed at 1223 K (950 C) for different electrolysis times (Figures 5(c) and (d)). The initial pressed pellet seems compact and consists of rough particles; the pellet becomes more porous and consists of smooth particles after sintering in air at 1423 K (1150 C) for 2 hours. The sintered pellet possesses higher mechanical strength both in air and in molten salt than the initial pressed pellet; this would be beneficial to the electro-deoxidation. Figure 5(c) shows the typical morphology of the intermediate products obtained from the electro-deoxidation at 1123 K (950 C) and 3.8 V for 2 hours. It is obvious that the particles have grown up to approximately 10 lm, which is attributed mainly to the formation of intermediate compounds (such as CaTiO 3 ). This finding is supported by the results of EDX (the inset in Figure 5(c)) and XRD (Figure 2(b)). An SEM image of the completely electro-deoxidized products is shown in Figure 5(d), the particles are interconnected homogeneously spheres with size approximately 5 lm. Comparing Figures 5(a) through (d), it can be observed clearly that, during the electro-deoxidation process, the particles of natural ilmenite first grow up by reacting with the Ca 2+ and O 2 ions contained in the molten CaCl 2 to form CaTiO 3 compound, and then they shrink during electro-deoxidizing. Figure 6 shows the typical evolutions of the morphology of particles during electro-deoxidation. After being electro-deoxidized at 1273 K (1000 C) and 3.8 V 508 VOLUME 43B, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS B

9 Fig. 5 SEM images of the pressed natural ilmenite pellets before sintering (a) and after being sintered in air at 1423 K (1150 C) for 2 h (b). (c) and (d) SEM images of the sintered pellets electro-deoxidized at 1223 K (950 C) and 3.8 V for 2 h (c) and 6 h (d), respectively. The insets in (b) through (d) are the EDX spectra measured over the image areas, respectively. Fig. 6 SEM images of the pressed natural ilmenite pellets electro-deoxidized at 1273 K (1000 C) and 3.8 V for 1 h (a), 3 h (b), 5 h (c 1 ), and 8h(c 2 ). The inset in (c 2 ) is the EDX spectra measured over the image area. for 1 hour, the particles possess different morphologies, as shown in Figure 6(a); the cubic cone particles are identified as CaTiO 3 by EDX, and the irregular particles are confirmed as FeTiO x compounds. In contrast, after being electro-deoxidized at 1273 K (1000 C) and 3.8 V for 3 hours, the particles have been converted to small scattered particles and their sizes are from 1 lmto5lm, which are much smaller than that of the intermediate compounds particles (as shown in Figure 6(a)). Experiments have also been performed for a longer electrolysis time, to investigate the subsequent growth of the particles. Figures 6(c 1 ) and (c 2 ) show the SEM images of the cathode pellet after being electro-deoxidized for different times. It is apparent that the particle growth pattern has two types; the first pattern is characterized with particle fusion (as shown in Figure 6(c 1 )) and the second pattern is the interconnection of particles to form a porous structure (as shown in Figure 6(c 2 )). The sizes of the particles are measured to be approximately 5 lm to 10 lm. It is worth noting that the first growth pattern is not conducive to the subsequent electro-deoxidation because the diffusion of the oxygen ions is blocked by the compact metallic film. The inset in Figure 6(c 2 ) is the EDX spectra measured over the imaging area, which indicates that element Fe disappeared gradually with the continuous electrolysis. E. Microhole Oxygen-Ion-Migration Model and Ti-Fe Alloys Formation Mechanism The knowledge about the model of the solid oxide electro-deoxidation process, especially as the oxygen ions migrate from the inner of the particles to its surface, is still unclear. [49,50] As shown in Figure 7(a), it is clearly METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 43B, JUNE

10 Fig. 7 Representative SEM images and the corresponding schematic model showing the electro-deoxidation and particle growth processes. (a) SEM image of the microhole particles. (b) and (c) SEM images of the growth lines of particles. observed first that the electro-deoxidation process involves the emergence of many microholes. When the molten CaCl 2 emerges into the interspaces of the particles, it diffuses around the particles, which means that the 3PIs are first formed on the surface of the particles. After the compounds on the surface are reduced to metals and/or alloys, the 3PIs extend into the inner part of the particles along the depth direction, which leads to the change of the particle volume. [50] Accordingly, microholes on the surface of particles are created because local pieces shrunk and/or broke. In addition, the difference in the reduction speeds around the surface of particles also contributes to the formation of the microholes. The microholes could act as the direct channels for the transmissions of molten salt and oxygen ions, and also could increase the surface area to provide more reaction interface. Figure 7(b) shows the growth lines of particles when the electro-deoxidation finishes and the microholes shrink and close; it is obvious that these small metals and/or alloys particles are interconnected to form larger ones. Figure 7(c) shows the growth lines of particles after these small particles have been combined together (as shown in Figure 7(b)); this growth is caused mainly by the sintering during the continuous electrolysis process. Based on the experimental results and previous reports, [12,29,41 45] a multistep mechanism of the electrochemical extraction of Ti-Fe alloys from ilmenite is suggested as the follows, and a schematic illustration is given in Figure 8: (a) Natural ilmenite is oxidized to Fe 2 TiO 5 and TiO 2 during sintering in air, and then it combines partially with Ca 2+ and O 2 to form CaTiO 3 in the electrolytic cell. This process begins when the pellets immerse into molten CaCl 2, and it continues during the electro-deoxidation process. (b) Element Fe is first electro-deoxidized from compounds, whereas the compounds FeTiO 3 and Fe 2 TiO 4 are formed as intermediate products. Then, the intermediate compounds electro-deoxidize gradually to TiFe x. This process is supported by the XRD and CVs results given in Figures 2 and 4, respectively. (c) CaTiO 3 is decomposed finally to Ti and CaO, and then CaO dissolves in molten CaCl 2 because of its solubility in the molten salt. [29] Thermodynamically, CaTiO 3 is more stable than other compounds and stays in the cathode longer. This phenomenon is also observed elsewhere. [12] (d) TiFe 2 and TiFe alloys are formed finally and the particles grow up. Two points are worth mentioning. First, there is a general trend that TiFe 2 is first formed and then reacts with Ti to form TiFe if the sufficient titanium is available. Second, the Ti-Fe alloys can be electro-deoxidized directly from FeTiO x and can be synthesized from the metals Ti and Fe, which are first electro-deoxidized from compounds. [45] It should be noted that the trace metallic impurities (such as Ca, Mg, Al, Mn, and Si) contained in natural ilmenite are not detected by XRD and EDX analyses in the final products. This finding attributed mainly to their large solubilities in molten CaCl 2 and their low melting point. The mechanism of the removal of these alkaline earth metallic impurities has been proposed and discussed in our previous work. [51] As observed previously, [52] the interactions between different metal atoms may be responsible for the loss or removal of metallic 510 VOLUME 43B, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS B

11 Fig. 8 Schematic illustration of the mechanism of direct extraction of Ti-Fe alloys from natural ilmenite using the SOM electro-deoxidation process. impurities during the electro-deoxidation, which may be conducive to the separation of metal/alloy from particular complex multicomponent compounds. IV. CONCLUSIONS Ti-Fe alloys have been extracted successfully directly from natural ilmenite in molten CaCl 2 at 3.8 V and 1223 K (950 C) as well as 1273 K (1000 C) by using the SOM process. The experimental results indicate that the extraction process consists of four steps: (1) compounding, (2) FeTiO x electro-deoxidation, (3) CaTiO 3 decomposition, (4) Ti-Fe alloys formation and particle growth. The morphology of the product was examined to investigate the mechanisms of the particle growth and the electro-deoxidation process. Particle growth occurs in two patterns: The first pattern is characterized with particle fusion and the second pattern is interconnection of particles to form the porous structure. The microhole oxygen-ion-migration model was proposed and discussed, and the mechanism of the direct extraction of Ti-Fe alloys from natural ilmenite was suggested. The laboratory-scale experiments show that the SOM process is a promising emission-free process to produce Ti-Fe alloys and oxygen gas simultaneously from natural ilmenite. ACKNOWLEDGMENTS The authors thank the National Nature Science Foundation of China (Grant No ), the National Basic Research Program of China (Grant No. 2007CB613606), and the Science and Technology Commission of Shanghai Municipality (Grant No , and 11DZ ) for financial support. X. Zou also would like to thank the 5th Postgraduate Innovation Foundation of Shanghai University (No. SHUCX111004). REFERENCES 1. J.L. Hennessy: Nature, 2010, vol. 463, pp A. Allanore, J. Feng, H. Lavelaine, and K. Ogle: J. Eletrochem. Soc., 2010, vol. 157, pp. E24 E K. Ju ttner, U. Galla, and H. Schmieder: Electrochim. Acta, 2000, vol. 45, pp Y. Kim and E. Worrell: Energ. Pol., 2002, vol. 30, pp S. Pacala and R. Socolow: Science, 2004, vol. 305, pp A. Krishnan, X.G. Lu, and U.B. Pal: Metall. Mater. Trans. B, 2005, vol. 36B, pp U.B. Pal, D.E. Woolley, and G.B. Kenney: JOM, 2001, vol. 53, pp U.B. Pal: JOM, 2008, vol. 60, pp A. Allanore, H. Lavelaine, G. Valentin, J.P. Birat, P. Delcroix, and F. Lapicque: Electrochim. Acta, 2010, vol. 55, pp B. Mishra and D.L. Olson: J. Phys. Chem. Solids, 2005, vol. 66, pp Y. Ito and T. Nohira: Electrochim. Acta, 2000, vol. 45, pp M. Ma, D.H. Wang, X.H. Hu, X.B. Jin, and G.Z. Chen: Chem. Eur. J., 2006, vol. 12, pp M.L. de Vries, I.E. Grey, and J.D. Fitz Gerald: Metall. Mater. Trans. B, 2007, vol. 38B, pp J.V. Dam, M. Junginger, A. Faaij, I. Ju rgens, G. Best, and U. Fritsche: Biomass Bioenerg., 2008, vol. 32, pp T.H. Okabe, T.N. Deura, T. Oishi, K. Ono, and D.R. Sadoway: J. Alloys Compd., 1996, vol. 237, pp G.Z. Chen, D.J. Fray, and T.W. Farthing: Nature, 2000, vol. 407, pp X.B. Jin, P. Gao, D.H. Wang, X.H. Hu, and G.Z. Chen: Angew. Chem. Int. Ed., 2004, vol. 43, pp A.M. Abdelkader, D.J.S. Hyslop, A. Cox, and D.J. Fray: J. Mater. Chem., 2010, vol. 20, pp J.J. Peng, K. Jiang, W. Xiao, D.H. Wang, X.B. Jing, and G.Z. Chen: Chem. Mater., 2008, vol. 20, pp T. Nohira, K. Yasuda, and Y. Ito: Nat. Mater., 2003, vol. 2, pp D.R. Sadoway: J. Mater. Res., 1995, vol. 10, pp D.R. Sadoway: U.S. Patent US 2008/ A1, A.H.C. Sirk, D.R. Sadoway, and L. Sibille: ECS. Trans., 2010, vol. 28, pp R.O. Suzuki, K. Teranuma, and K. Ono: Metall. Mater. Trans. B, 2003, vol. 34B, pp R.O. Suzuki: J. Phys. Chem. Solids, 2005, vol. 66, pp K. Ono and R.O. Suzuki: JOM, 2002, vol. 54, pp F. Cardarelli: Canada Patent CA A1, F. Cardarelli: U.S. Patent US 7,504,017 B2, X.L. Zou, X.G. Lu, C.H. Li, and Z.F. Zhou: Electrochim. Acta, 2010, vol. 55, pp METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 43B, JUNE

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