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 thermodynamic 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 compositions: 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 experimental 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. 4 Activity curves in liquid Au-Zn system. Dashed line is that suggested by Hultgren et al.(2)

Fig. 6 Excess quantities of mixing in liquid Au-Zn and Ag-Zn systems at 750℃.

Fig. 5 Activity curves in liquid Ag-Zn system. Dashed lines are those suggested by Hultgren et al.(2)

(10) O. Kubaschewski, E. Ll. Evans and C. B. Alcock: Metallurgi- cal Thermochemistry, 4th ed.,Pergamon Press, (1967). Fig. 7 Values of RTaZn as a function of composition in (11) B. P. Burylev: Tsvetnaja Metallurgija, No. 4 (1967), 35. liquid Au-Zn, Ag-Zn and Cu-Zn systems. 422 Thermodynamic Studies of Liquid Au-Zn and Ag-Zn Systems

Table 2 Heats of mixing of 50/50 liquid alloys between the metals of IB and II(cal/mol) (4) (5)

(6)

From these equations, at 750℃,

(7) For Ag-Zn system,

(8)

(9) Table 3 Differences of electronegativity, X, and ionic radius, R (10)

(11) and at 750℃, (12) Although the above equation may be rather a cursory approximation, it seems to be true that inflection points are located at around 0.4 mole fraction of zinc in all these three systems. This result is quite interesting as viewed in the light of the theory of Engel for copper alloys(5). He assumed that IB noble metals are transition metals in which d-electrons participate in the bonding, and suggested that metallic copper has a mixture of and in sizes of ionic radii, because the electron the following electron configurations: 25% Cu 1s2 2s2 concentration must be the same for these three systems. The necessary data are tabulated in Table 3, and the p6 3s2p6d104s1 and 75%Cu 1s2 2s2p6 3s2p6d8 4s1p2. Thus, IB noble metals have around 2.5 outer electrons great significance of electronegativity factor has been and 8.5 d-electrons whereby 2.5 outer electrons and 1.5 discussed by Hume-Rothery(16) for the stability of in- d-electrons participate in the bonding. Adding the termediate phases including IB and II-group metals. normal metal such as zinc results in the breakdown of On the other hand, Zener(17)explained the stability of the d-bonding and at the solubility limit of a solid the β structures of IB metals with II-group metals in solution, i.e. at 38% Zn in the Cu-Zn system, most of connection with the difference in core radii, and pointed the copper atoms supply only one outer bonding electron out that the most stable β phase is AuMg, followed by AuZn and AgMg. Such discussions for solid alloy phases per atom and very few copper, atoms have unfilled d- are also quite interesting for the understanding of the shells. In conclusion, in the alloys of IB noble metals thermodynamic behavior of liquid alloys. with zinc, the a solid solution range may be assumed to Figs. 6 and 7 show that the Au-Zn and Ag-Zn sys- be transition metal behavior, and alloys having more tems are not considered as regular solutions. The zinc- than 0.4 mole fraction of zinc are expected to show rich liquid alloys seem to be subregular behavior(18) as normal metal behavior. Under these assumptions he shown in Fig. 7. In the figure the results for the Cu- could explain various properties of copper alloys success- Zn liquid alloy obtained by the authors(19) are also fully. His theory seems also to supply interesting illustrated. Although the behaviors of the solution suggestions for the present results shown in Fig.7. rich in gold or silver are not clear, they are assumed to The fundamental principle of the Parkes process can be regular because the regular behavior is confirmed for be attributed to the difference of affinities of the pre- the Cu-Zn liquid rich in copper. Thus, the activity cious metals for zinc and lead. Accordingly, the ternary coefficients are expressed as follows: Au-Pb-Zn and Ag-Pb-Zn systems must be taken into For Au-Zn system, account. Combining the present experimental results for Au-Zn or Ab Zn with the data for Au-Pb or Ag- (3) Pb given by Hultgren et al.(2), the free energy of mixing for the ternary Au-Pb-Zn or Ag-Pb-Zn system has (12) P. Bolsaitis and L. Skolnik: Trans. Met. Soc. AIME, 242 (1968), 215. been calculated by the method extended by Olson and (13) T. Azakami and A. Yazawa: J. Min. Metall. Inst. Japan, 84 Toop(20). The obtained results are shown in Figs. 8 and (1968),1663. (14) L. S. Darken and R. W. Gurry: Physical Chemistry of Metals, (17) C. Zener: ibid, 25. McGraw-Hill, (1953). (18) H. K. Hardy: Acts.Met., 1(1953),203. (15) L. Pauling: The Nature of the Chemical Bond, 3rd ed., (19) A. Yazawa and A. Gubcova: Bull. Research Inst. Mineral Cornell Univ. Press, Ithaca, (1960). Dressing Metall. Tohoku University, 25 (1969),147. (16) W. Hume-Rothery: Phase Stability in Metals and Alloys, (20) N. T. Olson and G. W. Toop: Trans. Met. Soc. AIME, 236 edited by P. S. Rudman et al., McGraw-Hill, (1967), p. 3. (1966),590. Akira Yazawa and Alzbeta Gubcova 423

(14) When the equilibrium state is established between molten lead and the η phase of the Au-Zn or Ag-Zn system, the activity of gold or silver should be equal in both phases. Thus, (15) Hence, theoretical limit of removal for gold in the Parkes process under the above assumptions is as follows: (16) Under the same condition, the limiting content of silver in lead is, (17) Under the assumed conditions, accordingly, molten lead,

Fig.8 Free enelgy of mixing for temary Au-Pb-Zn system containing 0.00003% Au or 0.011% Ag, may coexist under assumption of single liquid phase at 750℃. with zinc crust consisting of the ε and η phases in which 10 to 20 weight per cent of precious metals may be contained. The result obtained in eq. (17) is higher than the limit obtained in practice. This may be ascribed to the activity data used. Although the data obtained in the liquid state are used for solid solutions in the above calculation, if we can make avail of the data in the solid state, the lower removal limits may be expected. Nevertheless, the above calculation may be useful to understand the process of the Parkes method.

V. Summary To elucidate the fundamental principle of Parkes process, thermodynamic studies have been carried out for the liquid Au-Zn and Ag-Zn systems for which thermodynamic data could not be available over the wide range of compositions. The electromotive force between the liquid alloy and pure zinc electrodes has Fig.9 Free energy of mixing for ternary Ag-Pb-Zn system under assumption of single liquid phase at 750℃. been measured by the ordinary method. The activity curves obtained from the emf values 9. In the calculation, the existence of miscibility gap show considerably negative deviations from Raoult's starting from the Pb-Zn binary system was neglected in law, especially in the Au-Zn system, suggesting a large the same way as Hultgren et al. summarized the data affinity between zinc and precious metals. From the for the binary Pb-Zn system. Even so, the larger electromotive force curves against temperature, it was affinity of the precious metals for zinc is confirmed in found that the liquidus lines for the Au-Zn system comparison with that for lead. recommended by Hansen are not acceptable, and that In the practical Parkes process molten lead coexists higher liquidus temperature may be reasonable. These with solid zinc crust which is mainly in the form of the thermodynamic properties are discussed from the stand- ε solid solution. In the final stage of the process, how- point of the recent stability theories of alloy phases. ever, some amount of the η(Zn)solid solution should To understand the principle of the Parkes process, the remain. Accordingly, the limit of removal of the free energy of mixing for the ternary Au-Pb-Zn and precious metals may be discussed on the basis of the Ag-Pb-Zn systems has been calculated, using the equilibrium relation between liquid lead and η solid present data together with those on the Au-Pb and Ag- solution at the temperature just above the melting point Pb systems. Moreover, under the simplified assump- of lead. tions, the way of derivation for removal limit of pre- If eqs.(6) and (11) are assumed to be valid at 600゜K, cious metals from lead has been explained. the activity coefficients of gold and silver in the η Acknowledgment phases containing 3 atomic per cent precious metals, The authors wish to express their appreciation (13) to Prof. Dr. Y. K. Lee, Department of Metallurgy, On the other hand, from Hultgren's data(2), those in the Chonpuk National University, Korea, for his kind infinite dilution in the Au-Pb and Ag-Pb systems are: suggestion on the experimental apparatus.