Development of a Thermodynamic Database for the Multicomponent PbO-“Cu2O”-FeO-Fe2O3-ZnO-CaO-SiO2 System for Pyrometallurgical Smelting and Recycling

Abstract

Integrated experimental and modelling research program on the phase equilibria and development of thermodynamic databases of the lead and copper metallurgical gas/ slag -matte-metal-solids systems (PbO-“Cu₂O”-FeO-Fe₂O₃-ZnO-CaO -SiO₂) is being undertaken to support improvements in the pyrometallurgical smelting and recycling processes. This is the first systematic investigation of phase equilibria of slag systems in equilibrium with Pb metal, providing information for systems in which copper coexists in slag with lead or zinc as major components. The experimental studies involve high-temperature equilibration of synthetic samples, rapid quenching, and measurement of the compositions of equilibrium phases using electron probe X-ray microanalysis (EPMA). FactSage-based thermodynamic database development is integrated with experimental research. Initial thermodynamic assessments are used to identify priority compositions and conditions for experiments, which are planned to provide specific data to assist thermodynamic optimisation. Significant improvement in the accuracy of the phase equilibria description is achieved. Continuous database improvement and extension to include new elements are targeted. Example of industrial application is given.

Introduction

High lead slags are generated in primary metal production in the smelting of lead sulphide concentrates and in pyrometallurgical metal recycling systems. The compositions of these slags are varied and increasingly, with the treatment of materials from a variety of different sources, are becoming chemically more complex; in that they contain a large number of different metals. These slags form complex non-ideal solutions that are found to coexist with stoichiometric solids, solid solutions, mattes and molten metal depending on the bulk composition and process conditions. Predicting the outcomes of smelting and refining processes is becoming increasingly difficult. To assist in the optimisation of these metallurgical processing operations, a comprehensive research program is underway to develop accurate thermodynamic databases for these non-ferrous process systems. The research described in the present paper is focussed on the on the PbO-“CuO_x”-“FeO_x”-ZnO-CaO -SiO₂ system at variable oxygen potentials, including equilibrium with metallic lead . This research is an important part of a larger study that includes not only this PbO-“CuO_x”-“FeO_x”-ZnO-CaO -SiO₂ system but also the additional slagging elements MgO and Al₂O₃, Pb-Cu-Fe-Zn-S matte (sulphide) phase, and minor elements distributions of As, Sb, Bi, Sn, Ag, Au between all of these phases.

The development of an accurate thermodynamic database relies on the availability of accurate thermodynamic data. The experimental determination of phase equilibria in these non-ferrous slag systems are particularly difficult to undertake. These difficulties are associated with the high vapour pressures of some species, the difficulties in containment of multi-phase systems and the control of the oxygen partial pressures in the systems.

Experimental Methodology

The experimental methodology that is used in the present study is based on the use of equilibration/ quenching/ microanalysis approach to phase equilibrium determination. This approach has greatly extended the range of metallurgical systems that can be characterised. This methodology has been described in more detail elsewhere [1], only a brief summary is provided below from [1].

An artificial oxide mixture is prepared from the analytically pure powders or pre-sintered solids or pre-melted master slag to obtain after equilibration a predetermined bulk composition. The starting composition is selected to obtain two or more phases in the final sample after equilibration. Experiments are performed in a vertical impervious ceramic tube in a resistance furnace . The substrate is suspended on a wire and placed in the hot zone of the furnace . The furnace temperature is monitored by using an alumina -shielded Pt/Pt-Rh 13% thermocouple with uncertainty within 5 K. After equilibration for predetermined time the samples are quenched into salt solution with ice (T = −18 °C), so the phases present at high temperature and their compositions are retained. The samples are mounted, polished and then examined initially using optical microscopy, and then Scanning Electron Microscopy (SEM ) with Energy Dispersive Detector (EDS ). The compositions of the phases (glass and solids) are measured using a JEOL JXA 8200L electron probe X-ray microanalyzer (EPMA) with Wavelength Dispersive Detectors (WDD). The appropriate standards are selected to the oxide, sulphide or metal phases. The phase compositions (total metal cation concentrations) are measured with EPMA with accuracy within 1 wt% or better. When possible, additional standards (such as stoichiometric compounds PbSiO₃, Pb₂SiO₄, Fe₂SiO₄, Zn₂SiO₄, CaFe₂O₄, Ca₂Fe₂O₅, or homogeneous glass of independently certified composition) are separately synthesized and used together with pure oxide standards, providing additional correction to resolve the JEOL probe ZAF correction uncertainty (reaching 1–1.5% in some systems).

The key advantage of this approach is independence of the accuracy of results from the changes of bulk composition during the experiment (provided the achievement of equilibrium between phases is confirmed), since the compositions of phases are measured after rather than before the experiment. This eliminates uncertainties due to interaction with the containment material (crucible, substrate), vaporisation of some elements during equilibration, uncertainties in the initial mixture composition and the changes in phase compositions due to the gas/ metal/ matte/ slag / solid(s) interactions. The small size (down to a several micron film) and direct exposure to gas and quenching media of the liquid slag /matte/ metal phases equilibrated on solid substrates enhances achievement of (a) equilibrium of condensed phases with the gas phase as well as (b) high quenching rates, thus extending the applications to the systems where these factors are the limitations for research. The compositions of all liquid and solid solution phases at equilibrium are measured directly providing data for further thermodynamic modelling . The achievement of equilibrium and other factors affecting uncertainties can be directly investigated from microstructural analysis and compositional profiles at micro (~ <20 µm) and macro (~ >20 μm) scale measured with EPMA or other microanalysis techniques. The key tests to ensure the achievement of equilibrium include [2]:

1.
Changing the equilibration time to confirm that no further changes take place with time.
2.
Confirming the chemical homogeneity of phases and samples.
3.
Approaching equilibrium from different directions.
4.
Analysing possible reactions taking place during equilibration.

Available analytical techniques including SEM imaging and EPMA analysis of the compositional gradients across the phases used in the present study are particularly effective for the analysis of possible signs of incomplete reaction pathways during equilibration.

In lead and zinc-containing systems Pb and Zn species can be transferred to the vapour through reactions, such as,

${\text{PbO}}({\text{slag}}) = {\text{PbO(gas)}}$

(1)

${\text{PbO}}({\text{slag}}) + 2{\text{FeO}}({\text{slag}}) = {\text{Pb}}({\text{gas}}) + {\text{Fe}}_{ 2} {\text{O}}_{3} ({\text{slag}})$

(2)

${\text{Pb}}({\text{metal}}) = {\text{Pb}}({\text{gas}})$

(3)

${\text{ZnO}}({\text{slag}}) + 2{\text{FeO}}({\text{slag}}) = {\text{Zn}}({\text{gas}}) + {\text{Fe}}_{ 2} {\text{O}}_{ 3} ({\text{slag}})$

(4)

Importantly, the reactions (2, 4) increase the effective oxygen potential in slag . Therefore, it is not reliable to study such systems at low p(O₂) fixed by CO /CO ₂ mixtures, since the effective oxygen potential in slag will be much higher than intended. These experiments need to be replaced with closed system approach developed at PYROSEARCH.

In addition, these reactions usually cause fast change in sample bulk composition. If this change corresponds to precipitation /dissolution of any solid, it will be questionable if the measured liquidus corresponds to the equilibrium, or oversaturation/undersaturation, respectively.

If a solid phase represents a solution—its composition may be not in equilibrium with slag . For example, Ca₂SiO₄-(Zn₂SiO₄-Fe₂SiO₄) solid solutions [3] show peculiar tendency—adding small amounts of Fe to the system greatly increases Zn solubility in solid (Ca₂SiO₄) solution. Formed during the first seconds of equilibration, C₂S solid phase keeps high initial Zn concentration, while the slag is losing ZnO due to reaction ZnO + Fe = Zn(g) + FeO. This reaction has high rate since the estimated equilibrium p(Zn) >> 1 atm (Fig. 1).

../images/468727_1_En_71_Chapter/468727_1_En_71_Fig1_HTML.gif — Fig. 1
Comparison of experimental points for slag -“C₂S” equilibria in the ZnO-“FeO”-CaO -SiO₂ system at metallic Fe saturation [3] and current model (projection onto Ca₂SiO₄-Fe₂SiO₄-Zn₂SiO₄ plane), showing synergistic solubility of Fe and Zn in “C₂S” and disagreement in the tielines direction due to fast evaporation of Zn from slag . Labels show estimated p(Zn), atm

In this case, it is not possible to get reliable points for liquid-solid C₂S equilibria , but may be possible at higher p(O₂) to reduce Zn volatility <1 atm, use sealed ampoule + additional inside substrate (e.g. spinel, wustite, iridium), and p(O₂) control by Cu/Cu₂O or Pb/PbO metal/slag equilibrium. The findings demonstrate the potential dangers of conducting experiments without careful analysis of all variables by parallel modelling studies.

Thermodynamic Modelling

Thermodynamic databases are developed through thermodynamic optimization that involves selection of proper thermodynamic models for all phases in a system, critical simultaneous evaluation of all available thermodynamic and phase equilibrium data and optimization of thermodynamic model parameters to obtain one self-consistent set best reproducing all experimental data as functions of temperature and composition.

In the thermodynamic “optimization ” of a system, all available thermodynamic and phase equilibrium data for the system are evaluated simultaneously to obtain one set of model equations for the Gibbs energies of all phases as functions of temperature and composition. From these equations, the thermodynamic properties and the phase diagrams can be back-calculated. Thermodynamic property data, such as activity data, can aid in the evaluation of the phase diagram, and phase diagram measurements can be used to deduce thermodynamic properties. Discrepancies in the available data can be identified during the development of the model . These discrepancies can then be resolved through new experimental studies that, if possible, are undertaken in areas essential for further thermodynamic optimizations. Multicomponent data, if available, are used to derive and test low-order (binary and ternary) model parameters, and if multicomponent data for a system are lacking, the low-order parameters are extrapolated. In this way, the thermodynamic databases are developed and all the data are rendered self–consistent and consistent with thermodynamic principles. FactSage computer system [4] has been used by the authors for the thermodynamic modelling . The molten slag phase is modelled by the Modified Quasichemical Model [5–7] in which short-range-ordering is taken into account. Oxide solid solutions are described with a polynomial model or with the Compound Energy Formalism [8].

Integrated Thermodynamic Database Development Using Modelling and Experimental Studies

The integrated combination of experimental and thermodynamic modelling studies carried out in parallel is an important factor in the present study to ensure high productivity and quality of research outcomes. The initial thermodynamic assessment is used (a) to evaluate existing experimental data, (b) to identify areas of importance for experimental research, (c) to focus new experimental work to resolve discrepancies of previous or acquire new data, and (d) to assist in detailed planning of the individual experiments in complex systems. There is usually a lack of experimental information to test model predictions in the multi-component slags and therefore multi-component databases are frequently developed on the basis of only binary and ternary data thus effectively extrapolating the low order binary and ternary parameters into a multi-component area without test. The present experimental program, in addition to the work on the binary and ternary systems, specifically focuses on multi-component systems in the composition and oxygen partial pressure ranges close to the important industrial slags. The thermodynamic model then is checked and corrected to agree with those multi-component measurements in the vicinity of the industrial slags. This is an important feature of the present study—the optimisation is performed in a number of cycles from binary and ternary to the multi-component systems and back so that binaries and ternaries are reoptimized to reach agreement also with the extensive data set in the multi-component area.

The overall approach to integration of experiments with modelling consists of:

	Thermodynamic modelling : assess existing data and plan experiments
	$\downarrow$
	Experiments on low-order systems: fundamental knowledge
	$\downarrow$
	Experiments on multicomponent systems: usually in support of industrial processes
	$\downarrow$
	Thermodynamic modelling : final database development

Focus and Scope of the Investigation

All subsystems of the multicomponent system PbO-FeO-Fe₂O₃-CaO -ZnO-Cu₂O-SiO₂ are arranged systematically as shown in Fig. 2. There are 21 binary and 35 ternary subsystems in this multicomponent system. Each of the subsystems is described by a separate set of parameters, usually between 1 and 10. No quaternary parameters are currently used in the quasichemical model . Therefore, a reliable multicomponent model needs to include several hundreds optimised parameters, selected from many thousands of theoretically possible parameters. Mathematically, this task is unsolvable unless approached carefully following a special procedure. Importantly, while the industrially important systems are multicomponent in nature, it would not be efficient to use exclusively multicomponent data representing the typical industrially occurring compositions, as their thermodynamic properties depend on many hundreds of parameters simultaneously. Instead, it is essential to obtain all binary and ternary parameters from direct experimental study of the corresponding low-order systems.

../images/468727_1_En_71_Chapter/468727_1_En_71_Fig2_HTML.gif — Fig. 2
Chemical sub-systems within the PbO-FeO-Fe₂O₃-CaO -ZnO-Cu₂O-SiO₂ system

Among the 21 binary subsystems, only eight can be considered as well studied. Four systems (PbO-SiO₂, PbO-“Fe₂O₃”, PbO-ZnO, PbO-“Cu₂O”) have been selected as most important to reinvestigate due to inaccuracies or very limited ranges of investigation in the previous studies. Then, eight other systems have been studied only partially but not included in the scope of the present work, mostly due to significant experimental difficulties. Finally, one system (PbO-“FeO”) is chemically incompatible.

Among the 35 ternary subsystems, only nine are relatively well studied. The systems PbO-“Fe₂O₃”-SiO₂, PbO-“FeO”-SiO₂, PbO-“Cu₂O”-SiO₂, PbO-CaO -“Fe₂O₃”and ZnO-“Cu₂O”-SiO₂ had only very limited studied ranges with large inaccuracies, so these systems were investigated extensively here. They are also the key systems most close to some industrial operation compositions. Several systems such as PbO-ZnO-SiO₂, PbO-CaO -SiO₂, CaO -ZnO-SiO₂ were studied within selected ranges of composition, to resolve discrepancies and fill some gaps. There are also nine ternary systems with no data, which are planned for at least partial study during the continuation of the current research project; and some “incompatible” systems.

Results

As examples of present experimental studies, phase diagrams of a binary system PbO-SiO₂ (Fig. 3) and a ternary projection PbO-“FeO”-SiO₂ in equilibrium with Pb metal (Fig. 4) are given together with recent optimised liquidus lines. A significant improvement in accuracy has been achieved in the PbO-SiO₂ binary, particularly in the high-SiO₂ region where the two-liquid immiscibility predicted by previous models has been shown to be metastable (subsolidus). The PbO-“FeO”-SiO₂-Pb system has been experimentally studied and optimised for the first time.

../images/468727_1_En_71_Chapter/468727_1_En_71_Fig3_HTML.gif — Fig. 3
Experimental points and modelled liquidus curves (literature and present study [11]) for PbO-SiO₂ system

../images/468727_1_En_71_Chapter/468727_1_En_71_Fig4_HTML.gif — Fig. 4
Experimental points [12] and modelled liquidus isotherms in the PbO-“FeO”-SiO₂ system in equilibrium with Pb metal

Other systems being studied over the course of the present work, are: PbO-FeO-Fe₂O₃ [9], PbO-“Fe₂O₃”-SiO₂ in air, PbO-“Cu₂O”-SiO₂ [10], PbO-CaO -“Fe₂O₃” in air, etc.

As indicated in the introduction to the paper, this research on the PbO-FeO-Fe₂O₃-CaO -ZnO-Cu₂O-SiO₂ is part of a larger study that includes not only PbO-“CuO_x”-“FeO_x”-ZnO-CaO -SiO₂ system but also the additional slagging elements MgO and Al₂O₃, Pb-Cu-Zn-Fe-S matte (sulphide) phase, and minor elements distributions of As, Sb, Bi, Sn, Ag, Au between all of these phases [13]. It would be impractical to attempt to optimise the whole of the 16 component system at once. The approach taken here is to prepare an initial database optimisation on the 7-component system, review and analyse the agreement with experimental data available in multicomponent systems and identify where there are inconsistencies in the database descriptions. On this basis, targeted experiments on low-order systems are then undertaken, and a revised database for the 7-component system prepared. Thus, the optimisation follows an iterative process. By incorporating data from a range of different systems and using different types of data, e.g. activity data, phase equilibria , minor element distributions, the accuracy of the database is improved.

Applications

As an example of an industrial application of the developed multicomponent thermodynamic model , a lead sinter system PbO-ZnO-“Fe₂O₃”-CaO -SiO₂ in air may be considered. To display such a complex system on a triangle diagram, a projection should be used to eliminate two degrees of freedom. In the original study [14–17], the results were represented on pseudo-ternary sections “Fe₂O₃”-ZnO-“PbO+CaO +SiO₂”, at several fixed PbO:CaO :SiO₂ ratios. This is possible due to high melting points of spinel (Zn,Fe)Fe₂O₄ and zincite ZnO, which do not dissolve significant amounts of either PbO, CaO , or SiO₂, so the mixture of the latter may be considered as a flux of constant composition. In Fig. 5, another kind of projection is selected. This is a pseudo-liquidus projection of liquidus of a third phase, precipitating from the slag already in equilibrium with two most stable solids—spinel and zincite, at p(O₂) = 0.21 atm. The possible third phases are willemite (Zn₂SiO₄), larsenite (PbZnSiO₄), melilite (Ca,Pb)₂ZnSi₂O₇, dicalcium silicate (Ca,Pb,Zn)₂SiO₄, calcium ferrite Ca₂Fe₂O₅, ganomalite (Ca,Pb)₃Pb₂Si₃O₁₁, and massicot PbO. The apexes of this projected triangle correspond to the ternary systems ZnO-“Fe₂O₃”-CaO , ZnO-“Fe₂O₃”-PbO and ZnO-“Fe₂O₃”-SiO₂; none of them has been studied systematically along the (spinel + zincite + slag ) lines, which imposes significant difficulties in creating a reliable model of the complex system. Currently, each point of a multicomponent system depends on ten sets of ternary parameters, which are impossible to unbind from each other. To create a robust model , it is recommended to study the three corners first, followed by selective studies of the edges (ZnO-“Fe₂O₃”-CaO -PbO, ZnO-“Fe₂O₃”-CaO -SiO₂, ZnO-“Fe₂O₃”-PbO-SiO₂).

../images/468727_1_En_71_Chapter/468727_1_En_71_Fig5_HTML.gif — Fig. 5
Projection of liquidus of a third phase precipitating from slag already in equilibrium with spinel and zincite, from ZnO and “Fe₂O₃” corners onto CaO -PbO-SiO₂ plane. Experimental liquidus temperatures and observed phases other than spinel and zincite [14–17] are compared with current optimised database prediction (boundaries and isotherms). A highlighted point in the triangle insert in the left bottom corner represents the location of a current projection relative to projections used in [14–17]

Conclusions

A systematic and in-depth investigation of phase equilibria in the system PbO-FeO-Fe₂O₃-CaO -ZnO-Cu₂O-SiO₂ is being undertaken. The study involves both experimental measurements and thermodynamic database development, which are undertaken simultaneously. The databases are used to advise the experimental work to be undertaken and the new data are used to improve the thermodynamic description of the system using the database.

Initial studies have included the revision of the PbO-SiO₂ binary system and the PbO-“FeO”-SiO₂ in equilibrium with Pb metal. Experimental techniques have been developed to study the phase equilibria in the Pb-containing slag -metal systems at selected oxygen potentials.

Acknowledgements

The authors would like to thank Australian Research Council Linkage program, Altonorte Glencore, Atlantic Copper , Aurubis, BHP Billiton Olympic Dam Operation, Kazzinc Glencore, Nyrstar, PASAR Glencore, Outotec (Espoo and Melbourne), Anglo-American Platinum , and Umicore for the financial and technical support.

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Development of a Thermodynamic Database for the Multicomponent PbO-“Cu₂O”-FeO-Fe₂O₃-ZnO-CaO-SiO₂ System for Pyrometallurgical Smelting and Recycling