Phase equilibria studies of zinc-containing copper smelting slags

Abstract

The smooth operation and productivity of the copper smelters are highly dependent on the comprehension of slag chemistry under the smelting conditions. Phase equilibria of slag principally control the melting temperatures of the slags and proportions of solids at fixed operating temperatures. Due to the decrease of copper ore grade as well as the use of complex feeds, appreciable amount of ZnO is present in the copper smelting slags. The aim of the present study is to investigate the liquid temperature and phase relations of the zinc-containing copper smelting slag systems. Due to the fast vaporization of zinc under reducing conditions, experimental techniques have been developed to enable the phase equilibrium studies of zinc-containing system at copper smelting conditions to be carried out. The experimental procedures in present studies involve master slags preparation, high temperature equilibration, quenching and electron probe X-ray microanalysis (EPMA). The methodology applied in the present study allows the phase assemblages and compositions of the phases existing in the quenched samples to be spontaneously analysed by EPMA. The phase equilibria studies on the zinc-containing slag systems have been conducted under fixed oxygen partial pressure (Po2) at 10-8 atm at temperature range from 1443 K (1170 °C) to 1573 K (1300 °C). The slag systems investigated in the present studies including: (1) A reinvestigation on “FeO”-SiO2 system at temperature range from 1473 K (1200 °C) to 1573 K (1300 °C). The experimental outcomes improve the accuracy of phase diagram of this system at Po2 10-8 atm. (2) ZnO-“FeO”-SiO2 system at temperature range from 1443 K (1170 °C) to 1573 K (1300 °C). The experimental outcomes indicate that the liquidus temperature is much higher than “FeO”-SiO2 pseudo-binary system with the presence of ZnO in the slag. (3) ZnO-“FeO”-SiO2-Al2O3 system at temperatures, 1523 K (1250 °C), 1543 K (1270 °C) and 1573 K (1300 °C) with Al2O3 varying from 2 to 6 wt pct. The experimental results show that the increase of Al2O3 content in the slag phase increases the liquidus temperature in the spinel phase. The distribution of ZnO with the presence of Al2O3 tends to be in spinel than the liquid phase. (4) ZnO-“FeO”-SiO2-MgO system at temperatures, 1523 K (1250 °C), 1543 K (1270 °C) and 1573 K (1300 °C) with MgO varying from 2 to 6 wt pct. The experimental results indicate that increasing MgO content in the slag phase significantly increases the liquidus temperature in the spinel primary phase field. The presence of MgO in the ZnO-“FeO”-SiO2-MgO system has minor effect on partitioning behaviour of ZnO between in the spinel and liquid phases. (5) ZnO-“FeO”-SiO2-CaO system at temperatures, 1523 K (1250 °C), 1543 K (1270 °C) and 1573 K (1300 °C) with CaO varying from 2 to 6 wt pct and 2 wt pct sulphur. The experimental outcomes suggest that the increasing of CaO concentration in the slag phase will significantly increases the liquidus temperature in the spinel primary phase field; the introduction of CaO into the slag has minor effect on the ZnO partitioning behaviour between the spinel and liquid phases. (6) Preliminary experimental work ZnO-“FeO”-SiO2-CaO-S system at temperatures 1443 K (1170 °C) and 1473 K (1200 °C). The experiment results indicate that 2 wt pct of sulphur in the liquid phase significantly decrease the liquidus temperature, up to 70 K compared to the ZnO-“FeO”-SiO2-CaO system. The experimental work from present studies fill the gap of zinc-containing copper smelting slag under conditions relevant to that in industrial practice. With the experimental outcomes obtained in present studies, more accurate information of the slag chemistry of zinc-bearing slag systems are now available for industrial smelter operation as well as for the thermodynamic modelling of the copper smelting slags. The evaluation of minor elements (arsenic and zinc) distributions among the gas / slag / matte phases during the copper smelting process were carried out. Literature reviews of existing experimental data and models of the distributions show that the operating parameters in smelting process have major impact on the fractional distribution behaviours of As and Zn. Thermodynamic calculation software - FactSage 6.4 was applied to predict the distribution behaviours of As and Zn as function of the operating parameters during the smelting process and compared with the literatures. The study of minor element distributions will improve the understanding of thermodynamic behaviours, and benefit the management of minor elements during copper smelting process.