Materials Transactions, Vol. 49, No. 6 (2008) pp. 1338 to 1341 #2008 The Japan Institute of Metals
Activity Coefficient of Strontium in Liquid Copper and the Standard Free Energy of Formation for SrOÁ6Al2O3 and SrOÁ2Al2O3
Shigeru Ueda1;2, Keita Utagawa1;2 and Katsunori Yamaguchi1
1Faculty of Engineering, Iwate University, Morioka 020-8551, Japan 2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
The activity coefficient of strontium in a copper melt has been investigated by a chemical equilibrium technique in the temperature range from 1473 to 1773 K. A graphite crucible was used to control the oxygen partial pursuer in CO gas. The activity coefficient was derived from the standard free energy of formation of SrO, the oxygen partial pressure, and the concentration of strontium in copper. The standard free energies of formation for SrO 6Al2O3 and SrO 2Al2O3 at 1723 K are also derived by equilibrating Al2O3, SrO 6Al2O3, and SrO 2Al2O3 with Cu. [doi:10.2320/matertrans.MRA2008044]
(Received February 5, 2008; Accepted March 10, 2008; Published April 23, 2008) Keywords: activity coefficient, standard free energy, strontium, copper, strontium aluminate
1. Introduction Graphite with a purity of 99.9% was used as a crucible to hold 2 g of the Cu melt with 1 g of a pellet of a saturating Alkaline earth metals have a strong affinity for oxygen and oxide. When measuring the activity coefficient of Sr in Cu, a are as stable as their oxides. The oxides of alkaline earth SrO pellet was use as the oxide. SrO was prepared by the metals behave as basic oxides in oxide melts. Basic oxides decomposition of SrCO3 reagent at 1573 K. The specimen in such as CaO and MgO are used as the contents of the the crucible was equilibrated in a CO gas atmosphere for 6 h refractory, slag, and flux for the refining of metals. Therefore, at a controlled temperature. A continuous flow of CO gas at thermodynamic data for CaO, MgO, and oxide systems 100 mL/min was used to control the atmosphere. After the containing these oxides have been reported. Strontium is an equilibrating process, the crucible was withdrawn from the alkaline earth metal. Strontium oxide might be a candidate furnace and quenched by flushing Ar gas. The oxide phase component of the flux or refractory in pyrometallurgy. was carefully removed from the copper surface. The In an actual process, strontium oxide would be used in a concentration of Sr in Cu was determined by analyzing multi-component system. However, few thermodynamic data with ICP-AES. for oxide compounds containing strontium have been reported. 2.2 Measurement of the standard free energy of for- In the present study, a basic system, namely, the SrO- mation for SrOÁ6Al2O3 and SrOÁ2Al2O3 Al2O3 binary oxide system, is focused on. The phase diagram SrO 6Al2O3-Al2O3 and SrO 6Al2O3-SrO 3Al2O3 mix- 1–3) for the SrO-Al2O3 system is reported. SrO 6Al2O3, SrO tures were used as the saturating oxides for the measurement 2Al2O3, SrO Al2O3, and 3SrO Al2O3 exist as double oxide of the formation energy of SrO 6Al2O3 and SrO 2Al2O3; compounds in air. Their phase diagram is similar to that of these were prepared by sintering a mixture of SrO and the CaO-Al2O3 system. The enthalpy and entropy of forma- reagent grade Al2O3 at 1573 K for 72 h. The formation of tion for a double oxide including strontium and aluminum the compounds was confirmed by X-ray diffraction. The 4) oxide at high temperature are reported only for SrO Al2O3. experimental procedure used for measuring the standard free A chemical equilibrium technique using a Cu melt was energies of formation of SrO 6Al2O3 and SrO 2Al2O3 was employed for the measurement. First, the activity coefficient the same as that used for measuring the activity coefficient of of Sr in liquid Cu is determined, and then strontium- Sr. A graphite crucible with CO gas was used to control the aluminum oxide was equilibrated with Cu in an oxygen oxygen partial pressure. The concentration of Sr in Cu was pressure controlled atmosphere. The standard free energy determined by analyzing with ICP-AES. was derived from the concentration of Sr in the Cu melt. 3. Results and Discussion 2. Experimental Procedures 3.1 Activity coefficient of strontium in copper melt 2.1 Measurement of activity coefficient of Sr The activity coefficient of Sr in Cu was measured by An electric resistance furnace connected to a proportional- equilibrating liquid Cu saturated with SrO in the graphite integral-differential controller with a Pt/Pt-13%Rh thermo- crucible in a CO atmosphere. The mole fraction of Sr in Cu couple was used for the experiments. The temperature was equilibrated with SrO in a C/CO atmosphere is shown in controlled within 2 K over the length of the crucible in a Fig. 1. The mole fraction increases with temperature. mullite tube (OD: 50 mm, ID: 42 mm, length: 1000 mm) in According to eq. (1), the oxygen partial pressure in the the furnace. system is controlled by the C/CO equilibrium as follows: Activity Coefficient of Strontium in Liquid Copper and the Standard Free Energy of Formation for SrO 6Al2O3 and SrO 2Al2O3 1339
-8 0
-10 -1 Sr in Cu X Sr in Cu
ln -12 γ -2 ln
-14 -3 1400 1500 1600 1700 1800 Temp. [K] 5.5 6 6.5 7 Fig. 1 Mole fraction of Sr in Cu equilibrated with SrO in C/CO 10,000/ T [1/K] atmosphere. Fig. 2 The activity coefficient of Sr inCu equilibrated with SrO in C/CO atmosphere. Table 1 Mole fraction of Sr in liquid Cu saturated with SrO and activity coefficient of Sr in Cu.
T [K] Po2 aSr XSr Sr 0 1473 7:55 10 18 6:76 10 8 1:37 10 6 0.0492 1523 1:40 10 17 2:45 10 7 2:64 10 6 0.0928 1573 2:50 10 17 8:15 10 7 5:58 10 6 0.146 1623 4:32 10 17 2:52 10 6 2:50 10 5 0.101 1673 7:23 10 17 7:27 10 6 3:95 10 5 0.184 -5 1723 1:18 10 16 1:97 10 5 7:57 10 5 0.260 in Cu γ 1773 1:86 10 16 5:04 10 5 1:49 10 4 0.338 ln
Present study 1 Activity coefficinet of Ca in Cu C(s) þ O (g) ¼ CO(g) ð1Þ -10 Activity coefficient of Mg in Cu 2 2 and 5Þ 6 7 8 9 G1 ¼ 11;600 85:1T J/mol ð2Þ 10,000/T [1/K] where G1 is the standard free energy of eq. (1). The standard free energy of formation of SrO and the Fig. 3 The activity coefficient of alkaline earth metals in Cu. equilibrium constant are given as follows: 1 Sr(l) þ O2(g) ¼ SrO(s); ð3Þ the interaction parameters on the activity of Sr in Cu are 2 not found. The influence of Sr on the activity of Sr may be 5Þ G3 ¼ 595;000 þ 103T J/mol ; ð4Þ supposed to be negligible due to the small amount of Sr in Cu and melts. The interaction parameters of O and C on Sr in liquid a Cu are also unknown. There have been many reports on the K ¼ SrO(s) ð5Þ 3 1=2 activity coefficient of O in liquid Cu,6) and the values range aSr(l) PO 2 from 0.1 to 0.5. The maximum mole fraction of O is less than Where, the activity of Sr in Cu is referred to as pure liquid 3 10 5 at the temperature range from 1473 to 1773 K; Sr, while those of SrO and compounds including SrO are therefore, the influence of O on the activity of Sr may be referred to as pure solid. The activity of Sr would be supposed to be negligible in the present study. calculated from that of SrO and the oxygen partial pressure. Since PCO ¼ 1 atm, the activity of Sr can be derived The activity coefficient of Sr in Cu, namely, Sr,is assuming that there is no influence of C on the activity calculated from that of Sr and the mole fraction of Sr in Cu. of Sr. The Arrhenius plot between the temperature and the a ¼ X ð6Þ Sr Sr Sr activity coefficient is shown in Fig. 2. The mole fraction of Sr in liquid Cu saturated with SrO and The data can be arranged as a line from 1473 to 1773 K as the activity coefficient of Sr in Cu are shown in Table 1. The ln ¼ 7:84 15;700=T: ð7Þ mole fraction of Sr is less than 1:5 10 4 at 1773 K and it Sr decreases with temperature. The activity coefficient is compared with that of other The activity coefficient is influenced by solutes. However, alkaline earth metals in copper. (Fig. 3) However, they differ 1340 S. Ueda, K. Utagawa and K. Yamaguchi
Table 2 Equilibrated oxide phase and the mole fraction of Sr in Cu.
-6 Sr T [K] Po X a Equilibrated phase Ca 2 Sr SrO 16 6 Mg 1723 1:18 10 3:14 10 0.0415 Al2O3-SrO 6Al2O3 -8 16 5 1723 1:18 10 1:82 10 0.0578 SrO 6Al2O3-SrO 2Al2O3 M
X -10 ln SrO þ 6Al O ¼ SrO 6Al O ; ð12Þ -12 2 3 2 3 aSrO 6Al2O3 1 K12 ¼ 6 ¼ ð13Þ aSrO a aSrO -14 Al2O3 1400 1500 1600 1700 1800 Here, K12 is the equilibrium constant of eq. (12). Accord- ing to the phase diagram for the SrO-Al O system,1–3) SrO Temp. [K] 2 3 is not soluble in the Al2O3 phase. Al2O3 and SrO 6Al2O3 coexist in the oxide phase; therefore, the activities of these Fig. 4 The mole fractions of alkaline earth metals in Cu equilibrated with that oxide in C/CO atmosphere. oxides are unity. According to eqs. (3), (5), and (6), the relationship between the mole fraction of Sr and the activity of SrO is with the measurement temperature; the value of the activity aSrO(s) 7) ¼ ð Þ coefficient of Sr appears similar to that of Mg, and it is much K4 1=2 14 Sr XSr P greater than that of Ca.8) O2 The standard free energies of formation of CaO and MgO6) The oxygen partial pressure is controlled by the C/CO are reported as follows: atmosphere. The standard free energy of formation of SrO 6Al O is obtained from eqs. (13) and (14) as 1 2 3 Ca(l) þ O2(g) ¼ CaO(s); ð8Þ 2 G12 ¼ RT ln K12 ¼ 45;900 ðJ/molÞ: ð15Þ G8 ¼ 643;000 þ 112T J/mol; ð9Þ 1 3.3 Standard free energy of the formation of SrOÁ Mg(l) þ O2(g) ¼ MgO(s) ð10Þ 2 2Al2O3 at 1723 K and The standard free energy of formation of SrO 2Al2O3 was derived as above. G10 ¼ 609;000 þ 115T J/mol: ð11Þ A pellet of the SrO 6Al2O3-SrO 2Al2O3 mixture is used as The mole fractions of alkaline earth metals in Cu the saturating oxide; therefore, the activities of SrO 6Al2O3 equilibrated with that oxide in a C/CO atmosphere are and SrO 2Al2O3 are unity. The relationship between these calculated in the temperature range from 1473 to 1773 K, and oxides and Sr is given as follows: these are compared in Fig. 4. The activity coefficients of Ca and Mg in liquid Cu at 1723 K were derived from the 2SrO þ SrO 6Al2O3 ¼ 3(SrO 2Al2O3) ð16Þ reported values7,8) based on the regular solution approxima- a3 1 K ¼ SrO 2Al2O3 ¼ ð17Þ tion. The solubilities of these alkali earth metals in Cu have 16 2 2 aSrO aSrO 6Al2O3 aSrO similar values. The solubility of Sr is less than that of Ca and Mg. The slope of Sr is similar to that estimated from the The activity of SrO is derived from the activity coefficient regular solution approximation. and mole fraction of Sr in Cu using eq. (14). Then, the standard free energy of eq. (16) is 3.2 Measurement of the standard free energy of the G16 ¼ RT ln K16 ¼ 40;800 ðJ/molÞ: ð18Þ formation of SrOÁ6Al2O3 at 1723 K Using the activity coefficient of Sr obtained above, the The formation of SrO 2Al2O3 from SrO and Al2O3 is standard free energy of formation of SrO 6Al2O3 can be written as derived by measuring the mole fraction of Sr in liquid Cu. SrO þ 2Al O ¼ SrO 2Al O : ð19Þ The interaction parameter between Al and Sr on the activity 2 3 2 3 of Sr in liquid Cu is not available. The activity coefficient of The standard free energy for the reaction given in eq. (19) 0 3 Al in the Cu melt is reported as Al in Cu ¼ 5:2 10 at would be derived from eqs. (15) and (18) as 1773 K.9) The activity coefficient of Al in liquid Cu at 1723 K G ¼ RT ln K ¼ 28;900 ðJ/molÞ: ð20Þ would be derived using the regular solution approximation as 19 19 4:5 10 3. Therefore, the mole fraction of Al in liquid Cu in the condition of the present experiment is less than 0.002. 3.4 Comparison of the standard free energy of the The influence of Al on the activity of Sr is supposed to be formation of the double oxide in the SrO-Al2O3 and negligible due to the small amount of Al in the Cu melt. CaO-Al2O3 system Table 2 shows the equilibrated phases and the mole The standard free energies of formation of the compound fraction of Sr in Cu. The formation of SrO 6Al2O3 and the for the SrO-Al2O3 and CaO-Al2O3 systems at 1723 K are equilibrium constant are given as shown in Fig. 5. Those of SrO 6Al2O3 and SrO 2Al2O3 are Activity Coefficient of Strontium in Liquid Copper and the Standard Free Energy of Formation for SrO 6Al2O3 and SrO 2Al2O3 1341 ) temperature obtained using a method for measuring the Y 12) + 0 enthalpies of dissolution of SrO Al2O3 in 4.3 N HCl. The X
/( standard free energy and entropy of formation for SrO Al2O3 3
O have not been measured. The standard free energy is derived 2 -10
Al from many data sources and includes some thermodynamic Y suppositions. Since the temperature range of the present
MO. -20 study is much higher than room temperature, the thermody- X namic suppositions might cause errors at high temperature. [J/kmol]
for for In the present study, the derivation of the standard free -30 XSrO.YAl2O3 XCaO.YAl2O3 energy is more simple. The activity of Sr was obtained from an indirect measurement with equilibrating liquid Cu and formation 0 oxide. However, that is same to be decided from the ratio of
G -40 0 0.1 0.2 0.3 0.4 0.5 ∆ the concentration of Sr in Cu equilibrated with SrO-Al2O3 to X/(X+Y ) [-] that equilibrated with SrO. Errors may be caused only in the chemical analysis of the concentration of Sr. Fig. 5 The standard free energies of formation of the compound for SrO- Al O and CaO-Al O systems at 1723 K. 2 3 2 3 4. Conclusions
Table 3 The standard free energy for double oxides at 1723 K. The activity coefficient of strontium in a copper melt and the standard free energy of the formation of compound, for MCaSr the SrO-Al2O3 system have been investigated by a chemical 5Þ MAl12O19 77700 J/mol 45900 J/mol equilibrium technique and the following conclusions were 5Þ MAl4O7 70400 J/mol 28900 J/mol obtained. 5Þ 4Þ MAl2O4 51900 J/mon 77310 J/mol The activity coefficient of strontium in a Cu melt is ln Sr ¼ 7:84 15;700=T in the temperature range from 1473 to 1773 K. measured in the present study and others are reported from The standard free energies of formation for SrO 6Al2O3 4) elsewhere. The standard free energies of formation for and SrO 2Al2O3 are XSrO YAl O /(X þ Y) and XCaO YAl O /(X þ Y) are 2 3 2 3 SrO þ 6Al O ¼ SrO 6Al O ; shown on the vertical axis in Fig. 5. The energies for the 2 3 2 3 G ¼ 45;900 (J/mol) at 1723 K; CaO-Al2O3 system show a concave curve. However, the relationship between SrO 6Al2O3 and SrO Al2O3 is a convex and curve. Namely, the weighted mean standard free energy of SrO þ 2Al2O3 ¼ SrO 2Al2O3; SrO 6Al O and SrO Al O is less than the standard energy 2 3 2 3 G ¼ 28;900 (J/mol) at 1723 K: of SrO 2Al2O3. This is thermodynamically incorrect. Table 3 shows the standard free energies of formation of the compound for 1 mole of the SrO-Al2O3 and CaO-Al2O3 REFERENCES systems. The standard free energies of formation of SrO 6Al2O3 and SrO 2Al2O3 are greater than those of CaO 1) F. Massazza: Chim. Ind. (Milan) 41 (1959) 108–115. 6Al2O3 and CaO 2Al2O3, respectively. The phase diagram 2) F. Hanic, T. Y. Chemekova and Y. P. Udalov: Zh. Neorg. Khim. 24 for the SrO-Al2O3 system is similar to that of the CaO-Al2O3 (1979) 471–475. system. The standard free energies increase with the mole 3) F. Hanic, T. Y. Chemekova and Y. P. Udalov: Russ. J. Inorg. Chem. (Engl. Transl.) 24 (1979) 260–263. fraction of alkali earth oxides for the CaO-Al2O3 system. 4) Thermochemical Properties of Inorganic Substances, ed. by I. Barin, However, that of SrO Al2O3 is much lesser than those of O. Knacke, and O. Kubaschewski (Springer-Verlag, Berlin, 1977). SrO 6Al2O3 and SrO 2Al2O3. This does not agree with the 5) JANAF Thermochemical Tables, ed. by D. R. Stull and H. Prophet tendency of the formation energy in the CaO-Al2O3 system. (U.S. Department of Commerce, Washington, 1985). Cp Fitted by CRCT, Montreal. The standard free energy of SrO Al2O3 at the temperature 4) 6) T. Oishi and K. Ono: Bull. Jpn. Inst. Met. 25 (1986) 291–299. used in the present study is listed in Table 3. The data is 7) S. P. Gory, Y. J. Bhatt and C. V. Sundaram: Metall. Trans. 4 (1973) obtained from the heat capacity and enthalpy of formation of 283. 10,11) SrO Al2O3. The original measurement of enthalpies of 8) R. A. Sharma: J. Phys. Chem. 74 (1970) 3896. SrO Al2O3 in the temperature range from 289 to 1595 K was 9) S. Ueda, K. Morita and N. Sano: ISIJ inter. 38 (1998) 1292. determined by the mixing method in the calorimeter with the 10) B. N. Bokeria, D. Sh. Tsagareishvili and G. G. Gvelesiani: Bull. Acad. 10) Sci. Geo. SSR 58 (1970) 601–603. isothermal shell. The estimation method of the heat of 11) K. Schwitzgebel, P. S. Lowell and T. B. Parsons: J. Chem. Eng. Data 16 11) formation of Sr Al2O3 is reported in literature. The (1971) 418. estimated value agrees with the measured value at room 12) C. Brisi and F. Abbattista: Ann. Chim. (Rome) 50 (1960) 165.