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Thermodynamic Studies of Liquid Au-Zn and Ag-Zn Systems

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Thermodynamic Studies of Liquid Au-Zn and Ag-Zn Systems Thermodynamic Studies of Liquid Au-Zn and Ag-Zn Systems By Akira Yazawa* and Alzbeta Gubcova** In order to clarify the basic principle of the Parkes process, in which precious metals are extracted from the crude lead by the addition of zinc, thermodynamic studies have been carried out for the liquid Au-Zn and Ag-Zn systems for which thermo- dynamic data have not been satisfactorily available. The electromotive force was measured using the following type of the cell: The error arising from volatilization of zinc was carefully avoided. The break points on the electromotive force curves against temperature suggest that the liquidus lines for the Au-Zn system recommended by Hansen are not acceptable, and that higher liquidus temperature may be reasonable. The activity curves obtained from the electromotive force values show considerably negative deviations from Raoult's law, especially in the Au-Zn system, suggesting the large affinity of zinc for gold and silver. In connection with the present data obtained, the properties of alloys between groups IB and II have been discussed in terms of the electronegativity, ionic radius and electron configurations. It is proved phenomenologically that the obtained data may be also interesting from the standpoint of recent stability theories of alloy phases. To understand the principle of the Parkes process, the results in the present study are combined with the data of the Au-Pb and Ag-Pb systems, and the free energy of mixing for the ternary Au-Pb-Zn and Ag-Pb-Zn systems has been calculated. Moreover, the removal limit of the precious metals from lead has been explained under the simplified assumptions. (Received June 8, 1970) I. Introduction phase stability of solid solution between IB noble metals and IIB metals. In the pyrometallurgical refining of the crude lead, Thus, the electromotive force measurements have the Parkes process, in which the precious metals (gold been carried out for the liquid Au-Zn and Ag-Zn and silver) are extracted by the addition of zinc, has been systems to clarify their thermodynamic properties. generally used. The process, however, has not yet been understood quantitatively from the standpoint of ther- II. Experimental Procedure modynamics because of the lack of reliable fundamental data. For the liquid binary systems of Pb-Zn, Au-Pb The experimental method and procedure were and Ag-Pb, rather reliable data are available, but there described already in a previous paper(6). The cell for remain doubtful points for the liquid Au-Zn and Ag- measuring the electromotive force was as follows. Zn systems although the vapor pressure measurements were carried out for these systems by Schneider et al.(1) Because the Gibbs-Duhem integration cannot be done for the Au-Zn system owing to the lack of experimental In the lower temperature range, the mixture of LiCl and data in the high gold region, the activity of gold was KCl was also ueed as a fused electrolyte. Although not given also in the compilation of Hultgren et al.(2) For the Ag-Zn system, some experimental study was pure liquid zinc metal was used as a standard electrode, it was confirmed that the error arising from volatiliza- also done partly by Kleppa et al.(3), but the data of heat tion of zinc was negligibly small. During the and entropy of mixing are not yet clear. measurement of electromotive force, effective com- These systems are also interesting from the standpoint position changes could be avoided, because the electrodes of recent progress(4)(5) of the alloy theories dealing with were covered with fused electrolytes to prevent free * Research Institute of Mineral Dressing and Metallurgy, volatilization. Due to the coexistence with pure zinc Tohoku University, Sendai, Japan. electrode, the alloy generally tend to absorb some ** Research Institute of Mineral Dressing and Metallurgy, amount of zinc, but the increase in zinc content was Tohoku University, Sendai, Japan. Present address: Depart- always less than 0.5 per cent. Slight change in com- ment of Chemical Principles of Metallurgy, Faculty of Metallurgy, Technical University, Kosice, Czechoslovakia. position was detected by weight difference before and (1) A. Schneider and H. Schmid: Elektrochem., 48 (1942), 627. after heating. (2) R. Hultgren, R. L. Orr, D. Anderson and K. K. Kelley: In the composition range having high gold or silver, Selected Values of Thermodynamic Properties of Alloys, (1963), as is suggested from Fig. 1, the measurement should J. Wiley and Sons. be carried out at a higher temperature where the vola- (3) O. J. Kleppa and C. E. Thalmayer: Phys. Chem., 63 (1959), 1953. tilization of zinc is remarkable. Thus, in these composi- (4) Phase Stability in Metals and Alloys, edited by P. S. Rudman tion ranges, the alloys having the composition of AuZn et al., McGraw-Hill, (1967). (5) N. Engel: Acta Met., 15 (1966), 557. (6) A. Yazawa and Y. K. Lee: Trans. JIM, 11 (1970), 411. Trans. JIM 1970 Vol.11 420 Thermodynamic Studies of Liquid Au-Zn and Ag-Zn Systems confirm the liquidus temperature, a thermal analysis was carried out for the alloys corresponding to the com- positions: AuZn, and two eutectics of E1 and E2 in Fig. 1. Table 1 shows that the thermal results obtained here, together with the data of other investi- gators, are in good agreement with those estimated from the present electromotive force measurement. The liquidus line thus obtained is illustrated in Fig. 1 with the dashed line, showing rather good agreement with those obtained by Vogel(8) and Kubaschewski(9), and hence Hansen's diagram seems to need some corrections. Table 1 Melting temperatures in the Au-Zn system(℃) Fig. 1 Phase diagram of Au-Zn system. Dashed liquidus line is assumed from this study. or AgZn were used as the reference electrodes instead of pure zinc. In Fig. 3 the electromotive force measured for the Ag-Zn system is plotted against temperature, showing III. Experimental Results a linear relation having a slightly positive temperature dependence. The liquidus temperatures estimated It was confirmed in the preliminary experiments from the data in Fig. 3 are in good agreement with that the electromotive force was zero when both of the those recommended by Hansen(7). positive and negative electrodes were pure zinc. The electromotive forces of the alloys having less Fig. 2 shows the relationship between electromotive than 0.5 atomic fraction of zinc were measured in force and temperature for the Au-Zn system, indicating reference to the AuZn or AgZn alloy. In such cases, the electromotive force values of the AuZu or AgZn alloy in reference to the pure zinc electrode were added to the electromotive force values obtained from experi- mental measurements, and the resulting values are illustrated in Figs. 2 and 3. The activities of zinc, azn, are given by (1) where E is the electromotive froce obtained, F is the Faraday constant, and ΔGZn is the partial molar free energy change of zinc. The activities of gold or silver were calculated by the Gibbs-Duhem integration. The Fig. 2 Plots of emf values against temperatures for the liquid Au-Zn system. a linear function of temperature. As shown in the figure, when the temperature is decreasing gradually from homogeneous liquid range, the value of the electromotive force represents a break point at the liquidus temperature, and again shows another break point corresponding to solidus temperature. In the solid solution range the linear behavior was also observed. Such a line of electromotive force measurements suggests that the liquidus line is located at a considerably higher Fig. 3 Plots of emf values against temperatures for the temperature than that recommended by Hansen(7). To liquid Ag-Zn system. (7) M. Hansen and K. Anderko: Consuauion of Binary Alloys, (8) R. Vogel: Z. anorg. Chem., 48 (1906), 319. McGraw-Hill, (1958). (9) O. Kubaschewski: Z. phys. Chem.,192 (1943),292. Akira Yazawa and Alzbeta Gubcova 421 activity curves thus obtained are illustrated in Figs. 4 the law of regular solutions, but this equation gives and 5 at 750℃. Both of the systems show considerably activity curves in good agreement with the present negative deviations from Raoult's law, especially the experimental results on both terminal regions. Au-Zn system, suggesting a large affinity between zinc and gold. The activity data presented by Hultgren et IV. Discussions al.(2) are also shown in the figures with dashed lines, indicating the trend similar to the authors' results. The partial and integral molar quantities were derived The activity of zinc derived from Kleppa's experiment from the experimental data, and the excess integral for the Ag-Zn system agrees very well with the present quantities are illustrated in Fig. 6. In Table 2, the results, although their data were obtained just only in a heats of mixing of 50/50 liquid between the metals of high zinc region. group IB and II are tabulated. Although the reliability From the knowledge of the heat of fusion(10) of of the data is not so high, when the metal of group II, AuZn and the liquidus line estimated in the present Me, is fixed, the data of Me-Cu and Me-Ag are com- study, the activity coefficients were derived from parable but large negative values are always observed in Burylev's equation(11) under the assumption of the the Me-Au system. A similar tendency is also observed regular solution. in Fig. 7 in which the a function(12) multiplied by RT is taken as the ordinate. (2) Such a great affinity found in the Au-Zn system in As will be discussed, the Au-Zn liquid does not obey comparison with the Ag-Zn or the Cu-Zn system may be ascribed to the differences in the electronegativities Fig.
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