Thermodynamics of Indium-Tin-Oxygen Ternary System

Transcription

1 Materials Transactions, Vol. 52, No. 6 (211) pp to 1199 #211 The Mining and Materials Processing Institute of Japan Thermodynamics of Indium-Tin-Oxygen Ternary System Satoshi Itoh 1, Hiroki Osamura 2; * 1 and Kimihiko Komada 3; * 2 1 Innovation Plaza, Graduate School of Engineering, Tohoku University, Sendai , Japan 2 Graduate School of Environmental Studies, Tohoku University, Sendai , Japan 3 Faculty of Engineering, Tohoku University, Sendai , Japan A new pyrometallurgical process for the recovery of metallic indium and tin from ITO scrap has been proposed based on the present thermodynamic study on the In--O ternary system. In the present study the phase relations and the equilibrium partial pressures of oxygen in the ternary system were determined at the temperature of 1173 K. Then by applying the Gibbs-Duhem equation to the phase relation of alloyoxide equilibrium, the activities of the components were obtained at 1173 K using the equilibrium partial pressures of oxygen. Moreover, the vapor pressures of indium and tin were calculated at temperatures of 1173 and 14 K to discuss the proposed process. The process consists of two stages of reaction: first is the reduction of ITO by CO to In- alloy at a low temperature. Second is the preferential vaporization of indium at a high temperature due to the three orders of magnitude difference between the vapor pressure of indium and that of tin. [doi:132/matertrans.m-m21186] (Received June 24, 21; Accepted February 18, 211; Published April 2, 211) Keywords: indium-tin-oxygen ternary system, indium tin oxide (ITO), phase equilibrium, activity, rare metal, recycling, pyrometallurgy, reduction, vaporization 1. Introduction Indium is one of the rare metals and demand for transparent electrode ITO (Indium Tin Oxide) target material has been increased rapidly in Japan. In order to supply raw materials to frontier industries, indium has been mainly provided by import, since the resource of indium is poor in Japan. However, there are many problems such as an overseas uneven distribution of the resource, economic strategy, uneasy political situation, and so on. 1) Securing of raw materials, in which amount and price are both stable, is strongly desired from the viewpoint of state stockpiling. Consequently, the stable supply of rare metal like indium and/or establishment of recycling technologies are highly required. Indium has been recovered and recycled previously from in-process scrap in ITO manufacturing, but has been done very few from end products. In the past few years, a hydrometallurgical method has been developed, where liquid crystal panels are chemically treated and recovered as indium hydroxide. 2) More recently, a reduction of indium oxide to metallic indium with lithium nitride was achieved through mechanochemical route from a basic research. 3) However, they have problems such as extensive use of acid-alkaline agents, a large amount of leaching residue, low rate of recovery, use of dangerous and expensive chemicals, and so on. Therefore, most appropriate material recycling process from artificial rare metal resource like ITO scrap, which is considered to be an urban mine, with economic and environmentally friendly is desired ardently. The objective of the present work is to clarify the relation between the phase relations and the equilibrium partial pressures of oxygen in the In--O ternary system at the * 1 Graduate Student, Tohoku University. Present address: DOWA ECOSYSTEM-RECYCLING Co., Ltd., Saitama 367-2, Japan * 2 Undergraduate Student, Tohoku University. Present address: RICOH Co., Ltd., Tokyo , Japan temperature of 1173 K in order to discuss the recovery of metallic indium and tin from ITO scrap by means of pyrometallurgy. In the present work two experiments were mainly conducted: one is the equilibrium-quenched experiment and the other is the CO-CO 2 gas equilibriumgravimetric experiment as mentioned below. Additionally, the phase stability of 3 O 4 and the solubility of oxygen in liquid tin at 1173 K, which have not been determined yet, have also been investigated. 2. Experimental 2.1 Phase relations in In--O ternary system (equilibrium-quenched experiment) In the equilibrium-quenched experiment the phase relations in the In--O ternary system were determined by equilibrating synthetic samples under argon gas followed by chemical composition analyses and X-ray diffraction studies of the quenched samples. The starting materials were metallic indium (99.99 mass% purity), tin ( mass% purity), analytical reagent grade In 2 O 3 (99.9 mass% purity) and O 2 (99.8 mass% purity). The sample was prepared as follows: the above-mentioned materials were weighed for 5 g at the desired composition, which were represented by the symbols of diamonds in the later Fig. 3. Then the sample was put in an alumina crucible of 1 mm inner diameter (i.d.), 13 mm outer diameter (o.d.), 5 mm length. The sample was heated in a quartz reaction tube with 4 mm i.d. at 1173 K by a kanthal wound furnace under argon gas. After holding at 1173 K for a desired holding time, the sample was quenched and separated into alloy and oxide phases. The experimental setup is given elsewhere. 4) The indium and tin contents in liquid alloy phase were determined by inductively coupled plasma (ICP) spectrometry, while those in solid oxide phase were determined by electron probe X-ray microanalysis (EPMA) using atomic-number (Z), absorption and fluorescence corrections, that is, the ZAF method. The phases were identified by X-ray diffraction (XRD).

2 Thermodynamics of Indium-Tin-Oxygen Ternary System 1193 The mutual solid solubility between In 2 O 3 and O 2 in air was also determined. In the experiment the above-mentioned reagent In 2 O 3 and O 2 were mixed at the desired composition and the mixture was pressed to the 1 mm diameter, 2 g briquette under about 4 MPa. The briquette sample was heated at 1573 K in air for four days followed by heating at the experimental temperature 1173 K for eight days. Then it was quenched and the indium and tin contents were analyzed by EPMA. Additionally, the solubility of oxygen in liquid tin at 1173 K was investigated by inert gas fusion-infrared absorption method. The sample was prepared by heating the mixture of metallic tin and O 2 under argon gas at 1173 K for fourteen days followed by quench. argon gas electronic balance cooling water CO-CO 2 gas inlet alumina crucible cooling water gas outlet on adjusting the electronic balance to zero kanthal wire hook stainless steel wire quartz reaction tube In--O sample kanthal wound furnace thermocouple 2.2 Equilibrium partial pressures of oxygen in In--O ternary system (CO-CO 2 gas equilibrium-gravimetric experiment) In the CO-CO 2 gas equilibrium-gravimetric experiment the relation between the phase relations and the equilibrium partial pressures of oxygen in the In--O ternary system were investigated at 1173 K using a gravimetric method in a CO-CO 2 gas mixture at constant temperature. The sample was prepared in much the same way as the above-mentioned mutual solid solubility experiment. That is, In 2 O 3 and O 2 were mixed at the desired composition, and then the mixture was pressed to the 2 g briquette. The briquette sample was heated at 1573 K in air for four days followed by heating at the experimental temperature 1173 K for eight days. Then it was quenched and crushed to identify the phase present by XRD and confirmed to be the desired phase, that is, the In 2 O 3 +O 2 two-condensed-phase combination. Figure 1 shows the experimental setup used in this study. The powder sample was put in an alumina crucible of 18 mm i.d., 22 mm o.d., 2 mm length. The crucible was suspended from the electronic balance into a quartz reaction tube with 33 mm i.d. by a piece of :5 mm stainless steel wire to measure the sample mass. While the sample was being heated to the experimental temperature 1173 K, the reaction tube was purged with argon gas. After the sample was weighed at 1173 K by the electronic balance in argon gas with a flow rate of m 3 (STP)s 1, the inert argon gas was switched to CO-CO 2 gas mixture with the same flow rate as that of argon, and then the equilibrium measurement was commenced. When the sample mass change was not observed for more than one hour, each equilibrium of the sample with some definite gas ratio of CO/CO 2 was judged to be accomplished, and then another equilibrium measurement was repeated by varying the gas ratio of CO/CO 2 with the CO-CO 2 total flow rate of m 3 (STP)s 1. The sample was also weighed in argon which gas density was almost the same as CO-CO 2 gas mixture to confirm the buoyancy effect to be negligible small within the experimental errors. In each run the adjustment of the electronic balance to zero was made prior to the sample weighing by means of the kanthal wire hook shown in Fig. 1. During the experiments argon gas with a flow rate of :5 1 6 m 3 (STP)s 1 was passed through the electronic balance case to protect from heat and CO-CO 2 gas mixture. Fig. 1 Experimental setup for the CO-CO 2 gas equilibrium-gravimetric experiment. In the gravimetric technique using CO-CO 2 gas mixture, the sample mass is measured to obtain the composition of the equilibrated sample with a known ratio of CO/CO 2 gas mixture at constant temperature 1173 K as with the previous work. 5) Equilibrium partial pressure of oxygen can be calculated from eq. (1) which is the equilibrium constant of the reaction (2). ðp CO2 =1 5 PaÞ K ¼ ðp CO =1 5 PaÞðP O2 =1 5 PaÞ 1=2 ð1þ where P i is the pressure of gas i and the standard Gibbs energy change of the reaction (2) is given as eq. (3). 6 8) CO (g, 1 5 Pa) þ 1/2O 2 (g, 1 5 Pa) ¼ CO 2 (g, 1 5 Pa) ð2þ G r/jmol 1 ¼ 2814 þ 85:28 (T/K) ð3þ The vapor pressures of pure indium and tin are Pa and Pa 9 12) at the experimental temperature 1173 K, respectively. The sample composition was thus obtained directly from the sample mass change, since the mass change was considered the oxygen mass gain or loss in the sample as the result of oxidation or reduction by CO-CO 2 gas mixture. 3. Results 3.1 Phase relations in In--O ternary system (equilibrium-quenched experiment) The relation between the sample composition in In- alloy phase and holding time at 1173 K is shown in Fig. 2. The sample was the number 5 and was the three-condensedphase combination of In 2 O 3 (s)+o 2 (s)+in-(l). The result indicated that the sample reached equilibrium in five days. Thus, all the sample was heated for seven days at 1173 K in order to establish the equilibrium. The composition of all the phases in the samples was determined by ICP for liquid alloy phase and EPMA for solid oxide phase, respectively. The results are summarized in Table 1, where N i and N i denote the mole fraction in the In- -O ternary system in alloy and oxide phase, respectively. The phases identified by XRD are listed in Table 2, showing that the oxide phase in the sample number 1 4 is In 2 O 3, while

3 1194 S. Itoh, H. Osamura and K. Komada 1.8 O 2 In 2 O 3 In 2 O 3 (s)+o 2 (s) + In-(l) mole fraction, N i N In N In In 2 O 3 (s) + In-(l) 5 6 NO O 2 (s) + In-(l) Time, t / d Fig. 2 Relation between the sample composition in In- alloy phase and holding time at 1173 K for sample No. 5. Table 1 Phase compositions of the quenched samples after equilibrium at 1173 K by ICP for liquid alloy phase and EPMA for solid oxide phase, respectively. Sample In- alloy(l) In 2 O 3 (s) O 2 (s) No. N In N In N N O N In N N O : balance. Table 2 Phase present in the quenched samples after equilibrium at 1173 K, identified by X-ray diffraction (XRD). Sample No. Phase present 1, 2, 3, 4 In 2 O 3 (s)+in-(l) 5 In 2 O 3 (s)+o 2 (s)+in-(l) 6 O 2 (s)+in-(l) O 2 (s)+(l) that in the sample number 6 and is O 2. The 3 O 4 was not observed in XRD patterns for all the samples. Then, it was found that the stable oxides coexisting with In- alloy were In 2 O 3 (s) and O 2 (s) at 1173 K from the present work. From these results, the phase diagram of the In--O ternary system at 1173 K is presented in Fig. 3. In the figure the symbols such as the circles, and triangles correspond to the compositions in the two-condensed-phase and threecondensed phase field, respectively. The solid lines connecting the solid and open circles are the tie lines. The dotted area surrounded by the triangle indicates the three-condensedphase field of In 2 O 3 (s)+o 2 (s)+in-(l). The phase relation in the In--O 2 -In 2 O 3 system at 1173 K consists of the three-condensed-phase field and two kinds of the twocondensed-phase fields, that is, In 2 O 3 (s)+in-(l) and O 2 (s)+in-(l). The tie lines are almost through the starting sample compositions shown as the diamonds in the figure. This indicates that the present experiment and chemical analyses by ICP and EPMA have been conducted appropriately. In.8 Fig. 3 Phase diagram of the In--O ternary system at 1173 K, where circles, and triangles correspond to the compositions in the twocondensed-phase and three-condensed phase field, respectively. The cross denotes the composition of the experiment for the solubility of oxygen in liquid tin. Table 3 Result of the mutual solid solubility between In 2 O 3 and O 2 in air at 1173 K. No. In 2 O 3 (s) O 2 (s) N In N N O N In N N O #2, # The result of the mutual solid solubility between In 2 O 3 and O 2 in air by EPMA is given in Table 3. The value is similarly small as with the mutual solid solubility between In 2 O 3 and O 2 coexisting with In- alloy shown in Table 1. In other words, the mutual solid solubility between In 2 O 3 and O 2 at 1173 K is small regardless of the coexistent phase. The experiment for the solubility of oxygen in liquid tin at 1173 K was carried out three times using the samples prepared from the mixture of metallic tin and O 2, whose composition was shown as the cross in Fig. 3. The solubility of oxygen in liquid tin was observed as 124 2:5 ppm. This value corresponds to.9 in mole fraction of oxygen. It is very small and not much different from the solubility of oxygen in liquid indium coexisting with In 2 O ) at 1173 K. 3.2 Equilibrium partial pressures of oxygen in In--O ternary system (CO-CO 2 gas equilibrium-gravimetric experiment) Figure 4 presents the relation between the oxygen pressure and the oxygen mole fraction of the sample in the In--O ternary system at 1173 K. In the figure the vertical line corresponds to the three-condensed-phase field of In 2 O 3 (s)+o 2 (s)+in-(l). The points where the curves intersect at the horizontal axis N O ¼ and many breaks in the figure denote the phase boundaries as given in the next Figs. 5 and 6. The phase diagram of the In--O ternary system at 1173 K is presented in Fig. 5, together with the isobars of oxygen. The dashed-dotted and solid lines correspond to the isobars by the CO-CO 2 gas equilibrium-gravimetric and equilibrium-quenched experiment, respectively. The area

4 Thermodynamics of Indium-Tin-Oxygen Ternary System 1195 N O K 4 3 CO% 2 #18, 2 #4 #6 # log(p O2 /1 5 Pa) 1 #75, 83 #6 #4 #18, 2 #18 #2 #4 #6 #75 #83 Fig. 4 Relation between the oxygen pressure and the oxygen mole fraction of the sample in the In--O ternary system at 1173 K, where the cross denotes the composition of quenched sample for identifying the phase present. surrounded by the solid triangle corresponds to the dotted area in Fig. 3 and is the three-condensed-phase field of In 2 O 3 (s)+o 2 (s)+in-(l). Numerals are the equilibrium partial pressure of oxygen in log unit. The equilibrium partial pressure of oxygen in the three-condensed-phase field is logðp O2 =1 5 PaÞ ¼ 15:3. The cross point in Figs. 4 and 5 indicates the composition of quenched sample for identifying the phase present. The phase was confirmed by XRD to be the two-condensed-phase combination of In 2 O 3 (s)+in- (l). The relation between the partial pressure of oxygen and the composition of condensed phase at 1173 K is presented in Fig. 6, where X ¼ =ðn In þ Þ and X ¼ N = ðn In þ N Þ denote the molar ratio in alloy and oxide phase, respectively. In the figure the triangles and circles, correspond to the symbols shown in Fig. 3. The squares with one diagonal stroke in Fig. 6 are depicted as the points where the isobars shown as the dashed-dotted line intersect at the base N O ¼ in Fig. 5, indicating the phase boundaries. The squares with two diagonal strokes are the phase boundaries as seen in Fig. 4. In Fig. 6 the left and right hand side corresponds to In/InO 1:5 (In 2 O 3 ) and /O 2, respectively. The symbols such as the triangles and circles, denote the compositions in the three-condensed phase combination of In 2 O 3 (s)+o 2 (s)+in-(l) and the twocondensed-phase combinations, where the open circle and solid circle correspond to the oxide and alloy phase, respectively. The two oxide phases of In 2 O 3 (s)+o 2 (s) are stable above the horizontal line connecting the triangles, while In- alloy phase is stable below the two lines O In 2 O 3 (s) + In-(l) In #18,2 O 2 In 2 O 3 N In #4 #6 #75 log(p O2 /1 5 Pa) # In 2 O 3 (s)+o 2 (s).8 + In-(l) O 2 (s) + In-(l) N O Fig. 5 Phase diagram of the In--O ternary system at 1173 K, together with the isobars of oxygen, where, the dashed-dotted and solid lines correspond to the isobars by the CO-CO 2 gas equilibrium-gravimetric and equilibrium-quenched experiment, respectively. The cross denotes the composition of quenched sample for identifying the phase present.

5 1196 S. Itoh, H. Osamura and K. Komada K O 2 (s) + In- metal 1 log(p O2 /1 5 Pa) In 2 O 3 (s) + O 2 (s) In 2 O 3 (s) + In- metal In- metal CO% In 2 O 3 (s) + In- metal O 2 (s) + In- metal enlarged illustration composition in the three-condensed -phase field from equilibrium-quenched experiment composition in the two-condensed -phase field from equilibrium-quenched experiment phase boundary from Fig. 4 phase boundary from base N O = in Fig. 5 In/InO 1.5 ( In 2 O 3 ) X, X /O 2 Fig. 6 Relation between the partial pressure of oxygen and the composition of condensed phase at 1173 K, where X ¼ =ðn In þ Þ and X ¼ N =ðn In þ N Þ denote the molar ratio in alloy and oxide phase, respectively. connecting the solid circles and the squares,. Between the horizontal line and these two lines, alloy and oxide phases coexist. The symbols diamond in Fig. 6 denote the equilibrium partial pressures of oxygen in the systems In-InO 1:5 (In 2 O 3 ) 9,14,15) 11,15,16) and -O 2 at 1173 K, respectively. Incidentally, the two-condensed-phase field of O 2 (s)+in-(l) is quite small as shown in the enlarged illustration. 4. Discussions log γ In log γ In =.364(1 N In )2 ± Activities of components in In--O ternary system The activities of components in the In--O ternary system can be obtained by applying the Gibbs Duhem equation, since the oxygen partial pressure has been known by the present work as mentioned above. The derivation of activities is given below. When an alloy is equilibrated with oxide, by applying the Gibbs-Duhem equation to the phase relation between alloy and oxide in the In--O ternary system, the following two equations are given: N In d log a In þ d log a þ N O d logðp O2 =1 5 PaÞ 1=2 ¼ ð4þ N Ind log a In þ N d log a þ N Od logðp O2 =1 5 PaÞ 1=2 ¼ ð5þ where a In and a are the activities of indium and tin in the condensed phase, respectively. P O2 is the partial pressure of oxygen. As mentioned above, N i and N i are the mole fraction in alloy and oxide, respectively. Thus, N O corresponds to the solubility of oxygen in alloy, and then N O can be approximated by zero, since the solubility of oxygen in In- alloy is.9 to.1 at 1173 K as mentioned above, which is negligibly small..8 1 In (1 - N In ) 2 Fig. 7 Relation between log In and ð1 N In Þ 2 at 1173 K. Equation (6) is derived from eqs. (4), (5) as with the previous work, 17) and then the activity of indium can be calculated. Z logðpo2 =1 5 PaÞ N O=2 log a In ¼ ðn = ÞN In N In logðp O2 =1 5 PaÞ d logðp O2 =1 5 PaÞ ð6þ where P O2 is the equilibrium partial pressure of oxygen in the system In-In 2 O 9,14,15) 3 at 1173 K. The activity of indium was obtained from eq. (6). Then the activity coefficient of indium was calculated and plotted against ð1 N In Þ 2 as shown in Fig. 7. The linear relation between log In and ð1 N In Þ 2 is observed except for -rich side. Thus, by assuming that In- binary alloy behaves as regular solution, 18) the activity coefficient of indium is expressed by the method of least squares as follows with the standard deviation: log In ¼ :364ð1 N In Þ 2 :26 at 1173 K ð7þ

6 Thermodynamics of Indium-Tin-Oxygen Ternary System a InO1.5.8 estimated at 1173 K after Hultgren et al..8 a In2O3 a O2 Activities, a, a In a In a Activities of oxide 1173 K Fig. 8 Present work 14 K 1173 K.8 1 In The activity coefficient of tin is then obtained by applying the Gibbs-Duhem relation to the In- binary system as follows: log ¼ :364ð1 Þ 2 at 1173 K ð8þ The activities of indium and tin in the In- binary system can be thus calculated at 1173 K by using eqs. (7), (8) and shown in Fig. 8 as solid lines. As seen in the figure, the activities in the In- binary system are symmetrical and both the activities exhibit negative deviations from Raoult s law. The activity of indium obtained by the present work at 1173 K is not much different from the estimated value from the literature. 19) Incidentally, the activities at 14 K presented by the dashed-dotted line in the figure will be discussed later. The activities of In 2 O 3 and O 2 were then calculated using the equilibrium constants 9,11,14 16) of the In 2 O 3 and O 2 formation reaction. The equilibrium constants and the formation reactions are described by eqs. (9) to (12). a In2 O 3 2=3 K In ¼ ð9þ a 4=3 In ðp O2 =1 5 PaÞ a O2 K ¼ ð1þ a ðp O2 =1 5 PaÞ 4/3In(l) þ O 2 (g, 1 5 Pa) ¼ 2/3In 2 O 3 (s) ð11þ (l) þ O 2 (g, 1 5 Pa) ¼ O 2 (s) ð12þ Thus, the activities of In 2 O 3 and O 2 were obtained using the activities of indium, tin in the In- binary system and the partial pressures of oxygen. The activities of In 2 O 3 and O 2 in the InO 1:5 -O 2 pseudo-binary system coexisting with In- alloy at 1173 K are presented against X ¼ N =ðn In þ N Þ in Fig. 9. In the figure the activity of InO 1:5 is also presented as with SbO 1:5, BiO 1:5. 4) In the In 2 O 3 (s)+o 2 (s)+in-(l) threecondensed-phase field, the activities of InO 1:5 and In 2 O 3 are.91 and.83 at 1173 K, respectively. These values might be consistent with the mutual solid solubility, while the activity of O 2 in the three-condensed-phase field was calculated as.79. However, the activity of O 2 is estimated at nearly unity because the mutual solid solubility is quite small as Activities of indium and tin in the In- binary system. Raoult's law.8 1 InO 1.5 (In 2 O 3 ) X' O 2 Fig. 9 Activities of In 2 O 3 and O 2 in the InO 1:5 -O 2 pseudo-binary system coexisting with In- alloy at 1173 K, where X ¼ N = ðn In þ N Þ. shown in Table 1. Considering eq. (1), this would be understood. Equation (13) is given from eq. (1). a O2 ¼ K a ðp O2 =1 5 PaÞ ð13þ The In- alloy composition coexisting with the two oxide phases of In 2 O 3 (s)+o 2 (s) is ¼ :81 as shown in Table 1 and Fig. 3. The activity of tin is.79 at the alloy composition. In eq. (13) the K is constant at constant temperature. The oxygen partial pressure exhibits little increase in the composition range from /O 2 to the alloy composition coexisting with In 2 O 3 (s) and O 2 (s) as shown in Fig. 6. Then, the activity of O 2 was calculated to be.79 by substituting.79 for the activity of tin in eq. (13). 4.2 Vapor pressures of components in In- binary system The vapor pressures of indium and tin in In- alloy were calculated at 1173 and 14 K using the activities obtained by the present work, where the activities at 14 K were calculated by using eqs. (7), (8) and assumption of regular solution, namely, RT log i ¼ constant. 18) The vapor pressures of indium and tin are presented as a function of the composition of In- alloy in Fig. 1. In the figure the solid and dashed-dotted lines correspond to the vapor pressures of indium and those of tin, respectively. The point denotes the In- alloy composition ¼ :93, where ITO composition 2) is taken in the In- binary system to discuss the case that ITO is reduced to In- alloy by CO. The most important observation is that the vapor pressure of indium is three orders of magnitude larger than that of tin in In- alloy at the ITO composition at both the temperature. This indicates that the separation of indium and tin from In- alloy by vaporization is basically possible. The In- alloy can be obtained by reduction of ITO with CO. However, at the lower temperature 1173 K the vapor pressures of both the components in In- alloy are less than 1 Pa at the ITO composition, suggesting that both the components cannot vaporize at a low temperature like 1173 K. At the higher temperature

7 1198 S. Itoh, H. Osamura and K. Komada log (P i / 1 5 Pa) Fig P In 14 K P In 1173 K P 14 K P 1173 K In Vapor pressures of indium and tin in In- alloy. at a low temperature at a high temperature ITO scrap CO Reduction In- alloy Vaporization metallic metallic In Fig. 11 Flow sheet of pyrometallurgical process for the recovery of metallic indium and tin from ITO scrap. 14 K, however, the vapor pressure of indium in In- alloy is more than 1 Pa at the ITO composition. Thus, indium would vaporize preferentially from In- alloy at the high temperature 14 K until the difference between the vapor pressure of indium and that of tin reaches about two orders of magnitude. The two orders of magnitude difference corresponds to the alloy composition of almost ¼ :55. Based on the above-mentioned discussions, a new pyrometallurgical process for the recovery of metallic indium and tin from ITO scrap is proposed below. 4.3 Pyrometallurgical process for recovery of metallic indium and tin from ITO scrap Figure 11 shows the concept of the principle of pyrometallurgical process for the recovery of metallic indium and tin from ITO scrap. The process consists of two stages of reaction for ITO scrap as a starting material. At the first stage, ITO, that is, indium tin oxide is reduced to In- alloy by CO at a low temperature. At the second stage, the temperature is higher and indium in the In- alloy preferentially vaporizes due to the difference between the vapor pressure of indium and that of tin. Indium vapor formed is cooled and recovered as metallic indium. The basic reactions at the first stage are expressed by eqs. (14), (15). 1/3In 2 O 3 (s) þ CO(g, 1 5 Pa) ¼ 2/3In(l) þ CO 2 (g, 1 5 Pa) ð14þ 1/2O 2 (s) þ CO(g, 1 5 Pa) ¼ 1/2(l) þ CO 2 (g, 1 5 Pa) ð15þ Since the ITO composition 2) taken in the In- binary system is ¼ :93 as mentioned above, it is found that ITO is easily reduced to In- alloy under a reducing atmosphere of more than 6 vol% CO at 1173 K from the upper scale of the horizontal axis in Fig. 4 and the right-hand scale of the vertical axis in Fig. 6. At the second stage, the basic reaction is expressed by eq. (16). In(l) ¼ In(g, 1 5 Pa) ð16þ As presented in Fig. 1, the vapor pressure of indium in In- alloy at the ITO composition exhibits more than 1 Pa and is three orders of magnitude larger than that of tin at 14 K. Thus, indium will vaporize preferentially from In- alloy under vacuum or reduced pressure. The indium will easily vaporize until the alloy composition reaches almost ¼ :55 as mentioned above. Consequently, the recovery of metallic indium and tin from ITO scrap by means of pyrometallurgy is thermodynamically understood. Some demonstration experiments of the proposed process are now in progress by using practical ITO samples. 5. Conclusions The phase relations and the equilibrium partial pressures of oxygen in the In--O ternary system, which are fundamental data on the pyrometallurgical process for the recovery of metallic indium and tin from ITO (indium tin oxide) scrap proposed in the present work, have been determined at the temperature of 1173 K. The results are summarized as follows: Phase relation in the In--O 2 -In 2 O 3 system consists of the three-condensed-phase field of In 2 O 3 (s)+o 2 (s)+in- (l) and two kinds of the two-condensed-phase fields of In 2 O 3 (s)+in-(l) and O 2 (s)+in-(l) at 1173 K. By applying the Gibbs-Duhem equation to the phase relation of alloy-oxide equilibrium, the activities of the components were obtained using the equilibrium partial pressures of oxygen in the In--O ternary system at 1173 K. Then the vapor pressures of indium and tin in In- alloy were calculated at temperatures of 1173 and 14 K. Based on the experimental results and discussions, a new pyrometallurgical process for the recovery of metallic indium and tin from ITO scrap has been proposed. The process consists of two stages of reaction. At the first stage, ITO is reduced to In- alloy by CO at a low temperature. At the second stage, indium in the In- alloy preferentially vaporizes at a high temperature due to the difference between the vapor pressure of indium and that of tin. Acknowledgments The authors would like to express appreciations to Mr. K. Suda (Tohoku University) in analyses by EPMA. Financial support from Dowa Metals & Mining Co., Ltd. in 27 is gratefully acknowledged.

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