Phase relations and thermodynamic properties of slag and metal systems related to the processing of copper anode slime

Abstract

Due to the sustainability related issues, metallurgical industries in the world have to improve and optimize their metal production processes. These efforts require many tasks including the collection of vital thermodynamic data related to the processed materials. However, in the processing of copper anode slime, for example, there are still lack of data of the melting point for the slag phase and the thermodynamic properties of some precious metals in the metal phase. Therefore, an extensive research is required.

To obtain accurate experimental data in a slag phase containing Na2O, PbO, SiO2 and TeO2 and a metal phase carrying Pt, Ag, Au and Pd, two experimental approaches were adopted in the present study. By using these developed techniques, some difficulties and errors occurred in the previous studies were resolved. To study phase equilibria in the Na2O-PbO-SiO2-TeO2 system, advanced technique involving equilibration of mixtures at high temperature, rapid quenching of equilibrated samples into ice cold water, and compositional measurement of the phases using electron probe micro analyzer (EPMA), has been employed. To obtain silver activity data, solid-state electromotive force (EMF) method incorporating equilibration of silver alloys in a galvanic cell was used. Darken method was employed to calculate integral thermodynamic properties in the ternary system. In the present investigation, liquidus data of binaries Na2O-SiO2 and Na2O-TeO2, as well as ternaries Na2O-SiO2-PbO and Na2O-SiO2 TeO2 systems were determined at several crystal saturations. Activity of silver in the Ag-Pt and Ag-Au Pd alloys were measured.

Results for the Na2O-TeO2-SiO2 system obtained in the present investigation are the first data published in the literature. Effect of Na2O and TeO2 on the liquidus is outlined. Data for the phase relations in the Na2O-SiO2, Na2O-TeO2 and Na2O-SiO2-PbO systems were compared with previous investigations. Notable differences between liquidus data measured in the present study and those reported by previous investigations were observed and evaluated. It indicated that, in general, the systems were more sensitive to Na2O than to TeO2 or PbO2. The effects of the results on the processing of the materials were outlined.

Results for the Ag-Au-Pd and Ag-Pt systems are also the first thermodynamic data published in the literature. Deviation of silver activity from ideal was discussed. The results indicated that in the Ag-Pt system silver activity has a positive deviation from ideal. In the Au-Ag-Pd system, silver activity, on the other hand, has a negative deviation from Raoult's law in almost entire range of the composition. Thermodynamic data such as Gibbs energy, enthalpy and entropy were calculated. Validation of the calculated data in the Ag Au-Pd system was also performed to evaluate the calculation approach. It was shown that the method was powerful.