Showing papers by "Peter C. Hayes published in 2014"

Investigation of Freeze-Linings in Aluminum Production Cells

Abstract: The molten cryolite bath creates chemically a very aggressive environment in the Hall–Heroult cell, and thus, the formation of a protective solid layer (freeze-lining) on the cell wall is essential for the operation of the present cell designs. To provide further information on the formation of the freeze-lining deposit in this system, laboratory-based studies were undertaken using an air-cooled probe technique The effects of process conditions, i.e., time, bath agitation, and superheat on the microstructures, morphologies of the phases, and the phase assemblages adjacent to the deposit/bath interface were investigated. A detailed microstructural analysis of the steady-state deposits shows that a dense sealing primary-phase layer of cryolite solid solution was formed at the interface of the bath deposit for the process conditions examined. The formation of sealing primary-phase layer at the bath/deposit interface explicitly indicates that the deposit/liquid bath interface temperature is equal to that of the liquidus of the bulk bath. The experimentally investigated liquidus temperature and subliquidus equilibria differ significantly from those previously reported.

Investigation of the Freeze-Lining Formed in an Industrial Copper Converting Calcium Ferrite Slag

Abstract: Pyrometallurgical coppermaking processes are operated under intensive reaction conditions; high process temperatures and vigorous bath agitation is used to increase the kinetics of reactions and to achieve high smelter throughput. Slag freeze-lining reactor wall protection is a widely used technology in coppermaking processes, such as flash smelting and converting reactors. Freeze-linings mitigate and resist the effects of thermal and chemical attack by aggressive slags. In this laboratory-based study, a water-cooled probe “cold finger” technique has been used to investigate freeze-lining formation with calcium ferrite slags in equilibrium with metallic copper; the slag composition reflects that used in the industrial copper flash converting furnace of Rio Tinto—Kennecott Utah Copper. The effects of probe immersion times on the thickness and microstructures in the freeze-lining deposits have been investigated. A range of complex oxide solutions and copper-containing phases have been found in the deposits. The phase assemblages formed from the industrial calcium ferrite slag in the steady-state deposit are very complex and information on the phase equilibria of the multi-component systems with addition of minor elements may not be available. Subsolidus and subliquidus phase equilibria in the Cu-Ca-Fe-O system at metallic copper saturation along with interpolated temperature across the deposit, microstructural changes and compositional trends in the phases in the deposit have been used to understand the formation and characteristics of the phases in the steady-state freeze-lining. Also, it has been shown that under steady-state conditions a dense sealing layer consisting primarily of the spinel primary phase is formed at the deposit/liquid interface; however, the interface temperature is below the liquidus temperature. The findings of the study have potentially important implications for the operation of the converting furnace and the design of freeze linings in metallurgical systems.

Investigation of Freeze-Linings in a Nonferrous Industrial Slag

Abstract: Slag freeze-lining reactor wall protection is a widely used technology in high temperature reaction systems. An air-cooled probe technique was used to investigate the formation of the freeze-linings in an industrial blast furnace slag. The compositions of the phases and the microstructures within the deposits have been characterized. It has been demonstrated that an industrial air-cooled probe can be used to take bath samples from actual smelter operations. In addition, a laboratory-scale experiment was undertaken to investigate the formation, stability, and bath/deposit interface temperature at steady-state conditions. Importantly, the current study has shown that stable steady-state freeze-linings can be obtained in metallurgical reactors operating below the slag liquidus temperature. In spite of the fact that solids are present in the bulk slag, the deposit thickness remains unaltered due to the dynamic conditions present at the deposit/bath interface. The results are consistent with findings obtained on a number of other different slag systems and the proposed dynamic mechanism of deposit stabilization. The findings demonstrate the basis for, and potential benefits that may follow from, operating the high temperature reactors at temperatures below the liquidus temperature, i.e., with solids present, without a catastrophic build-up of solids. This change in design concept could result in significant decreases in operating temperature, energy, and operating cost savings.

Further Experimental Investigation of Freeze-Lining/Bath Interface at Steady-State Conditions

Abstract: In design of the freeze-lining deposits in high-temperature reaction systems, it has been widely assumed that the interface temperature between the deposit and bath at steady-state conditions, that is, when the deposit interface velocity is zero, is the liquidus of the bulk bath material. Current work provides conclusive evidence that the interface temperature can be lower than that of the bulk liquidus. The observations are consistent with a mechanism involving the nucleation and growth of solids on detached crystals in a subliquidus layer as this fluid material moves toward the stagnant deposit interface and the dissolution of these detached crystals as they are transported away from the interface by turbulent eddies. The temperature and position of the stable deposit/liquid interface are determined by the balance between the extent of crystallization on the detached crystals and mass transfer across the subliquidus layer from the bulk bath. A conceptual framework is developed to analyze the factors influencing the steady-state deposit/interface temperature and deposit thickness in chemical systems operating in a positive temperature gradient. The framework can be used to explain the experimental observations in a diverse range of chemical systems and conditions, including high-temperature melts and aqueous solutions, and to explain why the interface temperature under these conditions can be between Tliquidus and Tsolidus.

Understanding Slag Freeze Linings

Abstract: Slag freeze linings, the formation of protective deposit layers on the inner walls of furnaces and reactors, are increasingly used in industrial pyrometallurgical processes to ensure that furnace integrity is maintained in these aggressive, high-temperature environments Most previous studies of freeze-linings have analyzed the formation of slag deposits based solely on heat transfer considerations These thermal models have assumed that the interface between the stationary frozen layer and the agitated molten bath at steady-state deposit thickness consists of the primary phase, which stays in contact with the bulk liquid at the liquidus temperature Recent experimental studies, however, have clearly demonstrated that the temperature of the deposit/liquid bath interface can be lower than the liquidus temperature of the bulk liquid A conceptual framework has been proposed to explain the observations and the factors influencing the microstructure and the temperature of the interface at steady-state conditions The observations are consistent with a dynamic steady state that is a balance between (I) the rate of nucleation and growth of solids on detached crystals in a subliquidus layer as this fluid material moves toward the stagnant deposit interface and (II) the dissolution of these detached crystals as they are transported away from the interface by turbulent eddies It is argued that the assumption that the interface temperature is the liquidus of the bulk material represents only a limiting condition, and that the interface temperature can be between T liquidus and T solidus depending on the process conditions and bath chemistry These findings have implications for the modeling approach and boundary conditions required to accurately describe these systems They also indicate the opportunity to integrate considerations of heat and mass flows with the selection of melt chemistries in the design of future high temperature industrial reactors

Copper processing method

Abstract: A method of processing a copper-containing source material is provided whereby an aqueous acidic leach solution of the copper-containing source material is formed and then contacted with a pH increasing agent to thereby cause the precipitation of a copper-containing intermediate. The copper- containing intermediate can then be collected and exposed to a high temperature treatment, such as would be encountered in smelter or converter operations.

Phase equilibria studies of the “MnO”–Al2O3–SiO2 system in equilibrium with metallic alloy. Part 1: Development of the technique and determination of liquidus isotherms between 1 423 K and 1 523 K

Abstract: Phase equilibria in the "MnO"-AlO-SiO pseudo-ternary system in equilibrium with metallic alloy have been experimentally investigated in the temperature range from 1423 K to 1523 K. This study is a part of a broader research program on the phase equilibria in the AlO-CaO-LiO- "MnO"-SiO system, which is of importance to the slags used in a novel pyrometallurgical process for recycling of electric car batteries. The experimental procedures involve equilibration of high purity powder mixtures at high temperatures, rapid quenching, and accurate measurement of phase compositions using electron probe X-ray microanalysis, which allow the slag liquidus temperatures to be determined. This paper is part 1 of a series of two papers and focuses on the improvement of the experimental methodology. A number of elementary reactions taking place in the samples have been identified, including the formation of a tridymite ring around the alloy particles, manganese oxidation and manganese vaporization. This enabled relevant modifications to the experimental methodology to be introduced. The liquidus at 1423 K, 1473 K and 1523 K in the high silica area and the solid solubility data in the tridymite and rhodonite phases have been reported.

A synergistic hydro- and pyro-metallurgical process for low-energy copper production

Abstract: The demand for copper continues to rise with the increased standards of living enjoyed by the world’s growing population. At the same time, we are faced with decreasing ore grades, increasing ore body complexity, cost of electrical energy, and the need to reduce greenhouse gas emissions. As a result, it is becoming more important to accommodate these challenging ores, including both low grade sulphides and oxides, in an efficient manner.In this paper, a new low-energy process route for copper production that utilises synergies between hydro- and pyro-metallurgical processes is proposed. The process involves precipitating copper from a leach solution by pH adjustment. The controlled precipitation step separates copper from impurity elements and concentrates it in a solid phase. The precipitate is then added to a copper smelter or converter to supplement the standard sulphide feed. The individual processing steps and select potential applications of the process are described. Process mass and energy balancing indicates that by using this supplementary feed, copper production through a copper converter could be increased by up to 34% using existing excess energy.

From Phase Equilibrium and Thermodynamic Modeling to Freeze Linings — The Development of Techniques for the Analysis of Complex Slag Systems

Abstract: Modern analytical tools, such as electron probe X-ray microanalysis (EPMA), are used in experimental characterization of phase equilibria and micro structure s of complex slags. Integrated thermodynamic computer packages, such as FactSage, are used to provide more accurate descriptions of complex slag systems. These advanced methodologies have been applied to characterize dynamic steady state freeze linings in slag systems using submerged cold finger probes at controlled laboratory conditions, establishing the effects of bath chemistry, temperature, heat extraction rate and bulk fluid flow.

Factors influencing reaction area

Abstract: ...Attention is now turned to the parameter A, the interfacial area. Clearly, this is also a key process parameter determining rates of these heterogeneous reactions. To emphasis its importance, this factor is explicitly addressed and discussed in the following section. ...

The challenges for professional metallurgical education

Abstract: The changes that are happening in the world of metallurgy, and with it university education, are generational and require generational responses. Some of the key issues facing metallurgical education and ongoing professional training are explored.