Abstract
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:
Zn (l) |NaCl+KCl, ZnCl2|Zn–M (l)
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.
