Contents lists available at ScienceDirect Calphad journal homepage: www.elsevier.com/locate/calphad Experimental studies of liquid/spinel/matte/gas equilibria in the Si-Fe-O- Cu-S system at controlled P(SO 2 ) 0.3 and 0.6 atm Mao Chen a,* , Yongqi Sun a , Eduardo Balladares b , Claudio Pizarro c , Baojun Zhao a a School of Chemical Engineering, The University of Queensland, Brisbane, 4072, Australia b Metallurgical Engineering Department, Universidad de Concepción, Concepción, 4070386, Chile c CODELCO TECH, Santiago, 8580000, Chile ARTICLE INFO Keywords: Equilibrium Liquid/spinel/matte/gas EPMA Liquidus ABSTRACT In the present study, the phase equilibria in the “FeO”-SiO 2 system in equilibrium with matte at controlled P (SO 2 ) 0.3 and 0.6 atm and fixed matte grade 72 wt% Cu were experimentally investigated. The high-temperature equilibration using primary phase material as the substrate, quenching and Electron Probe Micro-analysis (EPMA) were applied in the experiments where the P(O 2 ) and P(SO 2 ) were accurately controlled by CO/CO 2 / SO 2 gas mixtures. The correlations between oxygen partial pressure and the matte grade were determined at P (SO 2 ) 0.3 and 0.6 atm to obtain the target matte grade in the samples. The liquidus temperatures of this “FeO”- SiO 2 system in equilibrium with matte at controlled P(SO 2 ) 0.3 and 0.6 atm and fixed matte grade 72 wt% Cu were also reported. The present experimental results were also compared with the FactSage predictions and show the differences. The present results are expected to be useful for the copper smelting operations and also provide reliable information for the thermodynamic database. 1. Introduction The pyrometallurgical copper-making process remains the domi- nant process to produce copper. In the copper smelting process, the slag phase is in contact with the matte phase and the gas containing O 2 , SO 2 and N 2 . “FeO”-SiO 2 is the fundamental slag system containing other minor elements such as Al 2 O 3 , CaO, MgO and etc., and spinel is com- monly present as the solid phase in the copper smelting slags for major copper smelting processes [1]. Theoretically, the equilibrium relations in the copper smelting furnace can be specified as the “FeO”-SiO 2 based slag in equilibrium with matte (Cu·Fe·S) under certain oxygen partial pressure (P(O 2 )) and SO 2 partial pressure (P(SO 2 )). In the present condition, minor Cu + (in either oxide form or sulfide form) and S will also dissolve in the slags. Careful literature review indicates that publications were mainly related to the liquid/tridymite/matte/gas system from the previous researchers [2–11], while only limited data was found on the liquid/ spinel/matte/gas equilibrium system [12]. In the recent publications in the liquid/tridymite/matte/gas system [9–12], the advanced equili- brium techniques equilibrating slags and matte samples on the tridy- mite or spinel substrates under controlled gas atmospheres were ap- plied and the compositions of condensed phases were measured by EPMA. In the present study, the equilibrium studies were carried out using similar equilibrium techniques, and the slags in “FeO”-SiO 2 system are in equilibrium with matte under controlled P(O 2 ) and P (SO 2 ) targeting the spinel primary phase field. In the copper industry, the copper content in the matte phase is a critical index in the pro- duction, named as “matte grade”. The matte grade was controlled to be 72 wt% here, which represents a common operational value in the bath smelting copper making process [1,13]. P(SO 2 ) was controlled to be 0.3 atm and 0.6 atm, representing two levels of SO 2 concentrations in the off-gas, which can be derived from the different oxygen enrichments used in different copper smelting processes [1,13]. The corresponding P (O 2 ) will be investigated at the beginning of the study to obtain the target matte grade. It is believed that accurate equilibrium data is vital for the guidance of the current operations of copper smelting and will also provide useful information for the thermodynamic database for further process mod- eling. 2. Experimental Briefly, high-temperature equilibration using the primary phase material as the substrate, quenching and Electron Probe Micro-analysis (EPMA) are the experimental techniques used in the present study [14,15]. The initial oxide mixture with the pre-conditioned matte https://doi.org/10.1016/j.calphad.2019.101642 Received 21 March 2019; Received in revised form 1 July 2019; Accepted 7 July 2019 * Corresponding author. E-mail address: mao.chen@uq.edu.au (M. Chen). Calphad 66 (2019) 101642 0364-5916/ © 2019 Elsevier Ltd. All rights reserved. T