Elsevier

Calphad

Volume 71, December 2020, 102011
Calphad

Experimental study and thermodynamic optimization of the ZnO–FeO–Fe2O3–CaO–SiO2 system

https://doi.org/10.1016/j.calphad.2020.102011Get rights and content

Abstract

Liquidus phase equilibrium experimental data from the present study for the ZnO-“Fe2O3”-CaO-SiO2 system in air, combined with phase equilibria and thermodynamic data from the literature on the ZnO-“Fe2O3”-CaO system in air and ZnO-“FeO”-CaO-SiO2 system in equilibrium with metallic Fe, have been used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in the ZnO–FeO–Fe2O3–CaO–SiO2 system. The modified quasichemical model is used for the liquid slag phase; spinel (Fe,Zn,Ca)tetr (Fe,Zn,Ca,Va)oct2O4, melilite Ca2(Fe2+,Fe3+,Zn)(Fe3+,Si)2O7 and olivine (Fe,Zn,Ca)I(Fe,Zn,Ca)IISiO4 are described with compound energy formalism; lime and wustite (monoxide) (Ca,Fe,Zn)O, zincite (Zn,Fe,Ca)O1+x, calcium-zinc ferrites Ca2Fe2O5-“CaZnO2” and CaFe4O7-“ZnFe4O7”, α- and α′-dicalcium silicate (Ca,Fe,Zn)2SiO4 and tricalcium silicate (Ca,Fe,Zn)3SiO5 and silicoferrite of calcium (SFC) Ca9Fe46SiO80–Ca12Fe40Si4O80 are described within Bragg-Williams formalism; for other phases, previous assessments have been adopted. The phase diagrams are back calculated with the optimized model parameters. Present study is a part of research program on the characterization of the multicomponent PbO–ZnO–FeO–Fe2O3-“Cu2O”-CaO-SiO2 system.

Introduction

A thermodynamic “optimization” of a system using the CALPHAD approach involves critical evaluation of all available phase equilibria and thermodynamic data to obtain one set of model equations for the Gibbs energies of all phases as functions of temperature and composition [[1], [2], [3]]. The thermodynamic properties and the phase diagrams can then be back-calculated from the model equations to ensure that all data have been reproduced within the experimental error limits. Thermodynamic property data, such as activity data, can aid in assessing the phase diagram, and phase diagram measurements can be used to derive the thermodynamic properties. If a discrepancy in the available data is found during the development of the model, new experimental measurements are undertaken to provide the data essential for further refinement of the model equations. The present thermodynamic optimization was performed using FactSage thermodynamic software package [4] as part of the development of the self-consistent thermodynamic database for the seven-component gas-slag-solid-metal PbO–ZnO–FeO–Fe2O3–Cu2O–CaO–SiO2 system for zinc/lead/copper smelting and recycling industries by integrated experimental and thermodynamic modelling studies. For the molten slag phase, a modified quasichemical model has been used [[5], [6], [7], [8]]. Spinel, melilite and olivine are described with compound energy formalism [[9], [10], [11], [12]]; lime and wustite (monoxide), zincite, calcium-zinc ferrites Ca2Fe2O5-“CaZnO2” and CaFe4O7-“ZnFe4O7”, α- and α′-dicalcium silicate, tricalcium silicate, and silicoferrite of calcium (SFC) are described within Bragg-Williams formalism.

Section snippets

Literature review

The following data are available for the selection of model parameters for the thermodynamic database. The CaO–ZnO and CaO–ZnO–SiO2 systems have been recently reassessed [13] based on new experimental data [14,15]. The FeO–Fe2O3–CaO and FeO–Fe2O3–CaO–SiO2 systems are taken from Hidayat et al. [16,17] with minor modifications described below. The ZnO–FeO–Fe2O3–SiO2 system has been reoptimised by the authors [18].

Liquidus of the ZnO-“Fe2O3”-CaO system in air has been studied by the authors in air

Experimental technique and results

The technique used for experiments are the same as in the previous study by the authors [19,27]. Initial mixtures were made by mixing high-purity powders of Fe2O3 (99.945 wt% purity), ZnO (99.8 wt% purity), CaCO3 (99.9–99.99 wt% purity), SiO2 (99.9 wt%) supplied by Alfa Aesar, MA, USA. Pt foil (>99.9% purity, 0.05 mm thickness, provided by Johnson Matthew, Australia) envelope substrates were used for all experiments. Synthetic samples were equilibrated at controlled conditions, quenched in H2

Thermodynamic optimization

In the present study, the Modified Quasichemical model (MQM) is used to describe the liquid phase in the binary, ternary and multicomponent systems. The liquid oxide phase is assumed to form an ionic liquid where Zn2+, Fe2+, Fe3+, Ca2+ and Si4+ cations mix on one sublattice, while oxygen anions occupy the other sublattice. The model has been described in detail by Pelton and co-workers [[5], [6], [7], [8]].

The CaO–ZnO–Fe2O3, CaO–ZnO–FeO, and CaO–FeO–SiO2 parameters of the liquid slag are

Modelling results and discussion

Optimized thermodynamic properties of the liquid slag solution and the solid phases (stoichiometric compounds and solutions) are listed in Table 2.

The optimized thermodynamic model calculations are compared to the experimental results for the pseudo-ternary liquidus ZnO-“Fe2O3”-CaO in air and ZnO-“FeO”-CaO in equilibrium with metallic Fe in Fig. 2, Fig. 3. The calculated ZnO-“Fe2O3”-CaO diagram in air is an improved version of the diagram published in Ref. [19].

The CaO-“Fe2O3”-SiO2 phase

Conclusions

A critical evaluation of phase equilibria and thermodynamic data for the ZnO–FeO–Fe2O3–CaO–SiO2 system has been carried out using previous as well as most recent experimental data, the latter produced by the authors as part of the integrated research program. The modified quasichemical model is used to describe the Gibbs energy of the slag phase. The new model parameters reproduce the available data within experimental error limits. The optimized database can be used as a basis for the

Data availability

The raw/processed data (thermodynamic parameters) required to reproduce these findings are provided in supplementary file.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Nyrstar (Australia), Outotec Pty Ltd (Australia), Aurubis AG (Germany), Umicore NV (Belgium), and Kazzinc Ltd, Glencore (Kazakhstan), and Australian Research Council Linkage project LP150100783 for their financial support for this research. The authors are grateful to Prof. Peter C. Hayes (UQ) for valuable comments and suggestions.

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