Evgueni Jak

Thermodynamic modelling of the system Al2O3SiO2CaOFeOFe2O3 to predict the flux requirements for coal ash slags

Slags are formed during coal combustion processes as a result of the melting and reaction of the mineral matter. Predicting the outcome of these complex chemical reactions has long been a problem. Improvements in chemical thermodynamic models of oxide systems and computational methods now make these predictions possible. In this paper a model of the system Al2O3---SiO2---CaO---FeO---Fe2O3, developed using the F*A*C*T computer system, is presented. Examples are given of the use of the model to predict the phases formed as a function of ash composition, temperature and oxygen partial pressure of the system. These examples show how results can be presented in forms which can be readily used by practising combustion engineers, and coal production and marketing personnel.

Prediction of coal ash fusion temperatures with the F∗A∗C∗T thermodynamic computer package

This paper outlines research on the processes taking place within the coal mineral matter at high temperatures and development of the relationship between ash fusion temperatures (AFT) and phase equilibria of the coal ash slags. A new thermodynamic database for the Al-Ca-Fe-O-Si system developed by the author was used in conjunction with the thermodynamic computer package F*A*C*T for these purposes. In addition, high temperature experimental studies were undertaken that involved heat treatment and quenching of the ash cones followed by the analyses using different techniques. The study provided new information on the processes taking place during AFT test and demonstrated the validity of the AFTs predictions with F*A*C*T. Examples of practical applications of the AFT prediction method are given in the paper. The results of this study are important not only for the AFT predictions, but also in general for the application of phase equilibrium science to the characterisation of the coal mineral matter interactions at high temperature

Predicting slag viscosities in metallurgical systems

Slag viscosity is a major process variable for most pyrometallurgical smelting and refining process operations. While extensive measurements have been made of slag viscosities, it is not practicable to provide data on all the possible combinations of compositions and process conditions encountered in metallurgical practice. This article reviews the mathematical models that have been developed to predict slag viscosities and provides some advice on the factors to be considered in their selection and use.

Nickel laterite Part 2 – thermodynamic analysis of phase transformations occurring during reduction roasting

Abstract Thermodynamic and phase transformation analyses of nickel laterite ores processed in industry have been carried out. The nickel in the laterite ores is principally associated with Fe rich goethite and serpentine particles. The thermodynamic analysis suggests that the nickel recovery from the Fe rich matrix is limited by equilibrium. The nickel recovery from the single phase serpentine/olivine appears to be higher for particles with Mg/Fe molar ratio <8. The nickel partitioning to the alloy phase is sensitive to the change of temperature and oxygen partial pressure both in the case of reduction roasting of Fe rich goethite and serpentine. It has also been shown in the current study that the optimum conditions for high nickel recovery from the Fe rich goethite and serpentine are different.

Improved methodologies for the determination of high temperature phase equilibria

Improved methodologies for the investigation of the phase equilibria and thermodynamic properties have been developed based on the interactive combination of experimental investigation and thermodynamic modelling. The experimental technique involves high temperature equilibration, quenching and electron probe microanalysis. The computer thermodynamic modelling of oxide systems has been carried out using the computer system FACT. The general aspects of the quenching experimental technique with electron probe microanalysis are discussed and the advantages of using the electron probe X-ray microanalysis (EPMA) over conventional quenching techniques are highlighted. The effectiveness of the research approach is illustrated with experimental results on lead silicate glasses, zinc ferrite solid solutions and the complex oxide system PbO-ZnO-FeO-Fe2O3-CaO-SiO2.

Phase equilibria determination in complex slag systems

Abstract Despite the wealth of information available on phase equilibria of oxide systems, there remain many gaps and inconsistencies in the knowledge base. From an industry perspective, there is an ongoing need to adequately describe the phase chemistry of slag systems in order to optimise process performance and improve productivity. Since this chemical behaviour cannot be predicted from first principles it follows, there is also an ongoing need for accurate experimental data. The advancements made in recent years in the capabilities of sophisticated analytical and measurement equipment have made it possible to develop new experimental techniques for the direct determination of phase equilibria in low order and complex multicomponent slag systems. In addition, the development of powerful computer modelling tools makes it possible to provide more comprehensive descriptions of the phase chemistry, to present the information in a range of perspectives, and to critically analyse the various types of related thermodynamic and structural information available. This review provides an overview of the types of information that can be used to determine phase equilibria in slag systems, analyses the influence of uncertainty and the relative importance of these types of data, and describes the advantages and limitations of the various experimental techniques that can be employed to obtain this information. Examples of recent studies are provided that demonstrate how an integrated approach to the determination of phase equilibria in complex systems, involving the use of targeted experimental studies and systematic thermodynamic model development, can lead to a better utilisation of the current research capabilities and resources, and more accurate descriptions of these systems.

Thermodynamic optimisation of the FeO-Fe2O3-SiO2 (Fe-O-Si) system with FactSage

Abstract Phase equilibrium and thermodynamic experimental data available in the literature on the FeO – Fe2O3 – SiO2 (Fe – O – Si) system were critically reviewed and used to obtain a self-consistent set of parameters for thermodynamic models for all oxide phases using the FactSage computer package. The present optimisation covers the range of oxygen partial pressures from equilibrium with pure oxygen to metal saturation and temperatures from 25 °C to above the liquidus. The present thermodynamic optimisation was performed as part of the development of a thermodynamic database for the multi-component system Al – Ca – Fe – Mg – O – Pb – Si – Zn; the thermodynamic parameters for the Fe – O – Si system therefore were chosen to be consistent not only with the experimental data in this ternary system, but also with the data in higher-order systems. The modified quasichemical model was used for the liquid slag phase. Sublattice (based upon the compound-energy formalism) and polynomial models were used for the spinel (magnetite) and monoxide (wustite) solid solutions, respectively. The use of physically reasonable models means that the models can be used to predict thermodynamic properties and phase equilibria in composition and temperature regions where experimental data are not available. From these model parameters, the optimised ternary phase diagram of the FeO – Fe2O3 – SiO2 (Fe – O – Si) system was back calculated. The database of the model parameters can be used in conjunction with computer software for Gibbs-free-energy minimisation in order to calculate all thermodynamic properties and any type of phase-diagram section in the FeO – Fe2O3 – SiO2 (Fe – O – Si) system.

Nickel laterite Part 1 – microstructure and phase characterisations during reduction roasting and leaching

Abstract Detailed microstructure and phase characterisations of processed nickel laterite ore feed, reduced ore and leached ore have been carried out using scanning electron microscope, synchrotron X-ray powder diffraction and electron probe X-ray microanalysis as a part of study aimed to increase Ni recovery from the laterite ores. The majority of the nickel in the ore feed were found to be associated with fine grained goethite (Fe rich matrix of limonite composite) and serpentine particles. These phases were transformed to magnetite and olivine respectively upon reduction roasting. Nanosize metallic particles were also observed on the free surface of limonite composite particles following reduction roasting. Upon leaching, >90% of the nickel in the Fe rich matrix was leached out, while <50% was removed from the olivine and its composites. The characterisation results from this study were used to assist the analysis of thermodynamic and phase changes during reduction roasting and leaching described in Part 2 of the paper.

Investigation of freeze linings in copper-containing slag systems: Part II. Mechanism of the deposit stabilization

A major industrial problem in high-temperature liquid reaction systems is the attack of furnace components by chemically aggressive molten reactants. Freeze-lining technologies involving the deliberate formation of controlled frozen deposits are increasingly being applied to extend the range of liquid bath compositions and process temperatures that can be used; this has resulted in significant increases in process performance and productivity. It has been widely assumed that the interface between the stationary frozen layer and the agitated molten bath at steady state consists of the primary phase, which stays in contact with the bulk liquid at the liquidus temperature, Tliquidus. It has been shown in the current laboratory-based studies through the use of a cold finger technique that, at steady state and in selected ranges of process conditions and bath compositions, the phase assemblage present at the deposit/liquid interface is not that of the primary phase alone. The microstructural observations clearly demonstrate that the temperature of the deposit/liquid bath interface, Tf, can be lower than the liquidus temperature of the bulk liquid, Tliquidus. These observations point to a significant change in the mechanism and behavior of the systems. To explain this phenomenon, it is proposed that the steady-state thickness of freeze linings is not the result of equilibrium freezing but rather represents a state of dynamic equilibrium that is critically dependent on the relative rates of crystallization, mass, and heat transfer processes, occurring close to and at the deposit interface. The mechanisms taking place in the boundary liquid layer involve both partial crystallization/remelting and continuous removal of solids. This finding has important implications for the design of the high-temperature industrial reactors and selection of ranges of melt chemistries and conditions that can be used. This finding means that temperatures below the liquidus can be selected for some processes, resulting potentially in significant savings of energy and increases in throughput of pyrometallurgical reactors. The findings are generic and are not limited to the specific chemical systems reported in the article.

Thermodynamic Optimization of the Ca-Fe-O System

The present study deals with the thermodynamic optimization of the Ca-Fe-O system. All available phase equilibrium and thermodynamic experimental data are critically assessed to obtain a self-consistent set of model parameters for the Gibbs energies of all stoichiometric and solution phases. Model predictions of the present study are compared with previous assessments. Wüstite and lime are described as one monoxide solution with a miscibility gap, using the random mixing Bragg-Williams model. The solubility of CaO in the “Fe3O4” magnetite (spinel) phase is described using the sublattice model based on the Compound Energy Formalism. The effect of CaO on the stability of the spinel phase is evaluated. The liquid CaO-FeO-Fe2O3 slag is modeled using the Modified Quasichemical Formalism. Liquid metal phase is described as a separate solution by an associate model.