Brian J Monaghan

Effect of changing slag composition on spinel inclusion dissolution

Abstract The rate of MgAl2O4 spinel inclusion dissolution in CaO–SiO2–Al2O3 slags at 1504°C has been measured using a laser scanning confocal microscope. It was found that the mechanism of spinel inclusion dissolution was at least in part controlled by mass transfer in the slag phase for the slag compositions used. Evidence in support of this finding was that the calculated diffusion coefficient was inversely proportional to the slag viscosity and that the diffusion coefficients were in reasonable agreement with those obtained in a separate study on alumina dissolution. The diffusion coefficients obtained were in the range of 0·76–2·2 × 10−10 m2 s−1.

Comparative study of oxide inclusion dissolution in CaO–SiO2–Al2O3 slag

Abstract The dissolution of alumina, spinel and zirconia inclusions in a liquid CaO–SiO2–Al2O3 slag has been investigated using laser scanning confocal microscopy. Over the experimental temperature range 1477–1577°C it is found that alumina inclusions have a slower dissolution rate than that of spinel, and that zirconia inclusions form gas bubbles at the inclusion surface during dissolution. The results have been analysed assuming mass transfer control in the slag phase, and the slower rate of dissolution of the alumina inclusions is explained by the observation that they have a greater mass of aluminium oxide, the rate controlling species, than that of spinel.

High temperature oxidation behaviour of ferritic stainless steel SUS 430 in humid air

The high temperature oxidation behaviour of ferritic stainless steel SUS 430 was investigated over the temperature range from 1000 to 1150 °C in humid air containing 18% water vapour. Isothermal thermogravimetric analyses were performed to study the oxidation kinetics. The microstructure, composition and thickness of the oxide scale formed were investigated via optical microscopy (OM), X-ray diffraction and a scanning electron microscope equipped with an energy dispersive spectrometer. The results indicate that breakaway oxidation occurs at all temperatures and that its onset is accelerated by increasing temperature. The growth rate of the multilayer oxide scale follows a parabolic law with apparent activation energy of 240.69 kJ/mol, and the formation of FeO is decreased when the temperature is higher than 1120°C. The inner oxide scale, Fe-Cr spinel, grows mainly inward and internal oxidation is observed even in a short oxidation test at 1150°C for 105 s. The mechanism of high temperature oxidation of SUS 430 in humid air containing 18% water vapour is discussed.

Alternative Al production methods: Part 1 - a review of indirect carbothermal routes

Abstract The production of Al from its ores at present relies on the Bayer (alumina production) and the Hall–Heroult (Al production) process. The cost associated with alumina production and apparent disadvantages of the Hall–Heroult process have led to intensive research to find alternative routes for Al production. The direct carbothermal reduction process has been thoroughly investigated as an alternative technique. Another alternative includes the indirect carbothermal reduction route where alumina (or aluminous ores) is first reduced to intermediate Al compounds before reduced further to Al. The present study reviews and provides systematic thermodynamic analyses of alternative Al production routes. In this paper (Part 1), a comprehensive review of alternative Al production techniques focusing on the indirect carbothermal reduction routes is presented. These include carbochlorination, carbonitridation and carbosulphidation routes for the formation of intermediate Al compounds, followed by various Al extraction processes.

Thermophysical properties of slags for process control

Abstract Thermophysical properties have been shown to be important in a number of industrial processes. Unfortunately, for a number of properties there are scant data available and/or the data that are available are of dubious quality. In the Thermo-Physical Property Group at the National Physical Laboratory (NPL) the aim has been to develop measurement techniques capable of working in the difficult environments that are typical of industrial processes. This paper presents current developments at NPL in techniques to measure the thermal diffusivity of liquid slags using a laser flash and insights into the copper smelting process using interfacial tension techniques.

Kinetic analysis of silicothermic process under flowing argon atmosphere

Abstract The Pidgeon process, a silicothermic reduction of calcined dolomite under vacuum, is the dominant process to make magnesium metal. Experimental data from Morsi et al., were utilised for kinetics analysis of silicothermic reduction of calcined dolomite under argon atmosphere. A number of kinetic models were assessed to evaluate the rate-controlling step in the process. The results suggest that the reaction is controlled by the solid-state diffusion of reactants with the Jander and Ginstling–Brounshtein model providing the best representation of the process kinetics. Mass transfer effects of magnesium vapour from the surface to the bulk gas phase was also analysed. These results suggest that gas–film mass transfer is not the limiting step of the kinetics. Pore diffusion through briquettes is postulated to have some effect on the kinetics but solid-state diffusion is the major rate-limiting step. Le procédé Pidgeon, une réduction silicothermique sous vide de dolomie calcinée, est le procédé dominant de production de magnésium métallique. On a utilisé les données expérimentales de Morsi et al pour l’analyse cinétique de la réduction silicothermique de dolomie calcinée sous une atmosphère d’argon. On a estimé un certain nombre de modèles cinétiques afin d’évaluer l’étape du procédé cinétiquement limitante. Les résultats suggèrent que la diffusion à l’état solide des réactants contrôle la réaction, le modèle de Jander et Ginstling-Brounshtein fournissant la meilleure représentation de la cinétique du procédé. On a également analysé les effets du transfert massique de la vapeur de magnésium de la surface vers le cœur de la phase gazeuse. Ces résultats suggèrent que le transfert de masse gaz-film n’est pas l’étape limitante de la cinétique. On postule que la diffusion par pore à travers les briquettes a un certain effet sur la cinétique mais la diffusion à l’état solide est l’étape majeure cinétiquement limitante.

Alternative Al production methods Part 2 - thermodynamic analyses of indirect carbothermal routes

Abstract It has been widely claimed that direct and indirect carbothermic reduction of alumina for the production of aluminium have the potential to consume less energy, generate fewer perfluorocarbons and have lower total greenhouse gas emissions compared to the Hall–Héroult process. A review of alternative aluminium production techniques focusing on the indirect carbothermal reduction routes has been presented in the Part 1 paper. In this Part 2 paper, systematic thermodynamic analyses of indirect carbothermal reduction routes are presented. These include the analyses of the Stage 1 of the process where alumina is converted to intermediate aluminium compounds before they are further reduced to aluminium metal in the subsequent stages. The results suggested that in the Stage 1, high alumina conversion at the process conditions studied can be achieved through three process routes: halide (chloride), nitride and sulphide routes. Thermodynamic analyses were also carried out for the Stage 2 where the intermediate aluminium nitride, chlorides or sulphides are reduced to aluminium through thermal dissociation, thermal disproportionation and/or electrolysis. Analysis of the results suggests aluminium may be extracted from AlN using thermal dissociation, while disproportionation and electrolysis are more suitable for AlCl3 and Al2S3.

Strength and bonding in reduced ironsand–coal compacts

Abstract In this investigation, the strength and bonding within reduced ironsand–coal compacts were studied, with the aim of better understanding the binding mechanisms in the reduced compacts and, based on this understanding, to improve their strength. Ironsand ore and sub-bituminous coal were mixed and pressed into compacts, which were reduced by heating in a thermogravimetric furnace to temperatures between 1273 and 1573 K under argon. The progress of the reaction was monitored by measuring the weight loss with time. The reduced compacts were found to have low strength in compression testing. The main form of bonding between the reduced ironsand particles in the compact was by the formation of a slag-like material. Increasing the final reduction temperature was found to have a profound effect on the strength of the compacts by promoting the formation of this slag-like material.

Determination of thermal histories of coke in blast furnace through X-ray analysis

Abstract A study has been undertaken to identify the source of coke fines sampled from the deadman area of the blast furnace. Using measurements of the coke crystallite dimension L C, it was established that some of the fines found in the deadman area of a blast furnace were not simply the degradation products of the lump coke in this area. The L C was measured using standard X-ray analysis techniques. The coke fines had a higher L C than the coke lump and therefore have experienced a higher maximum temperature than the associated coke lump. This finding has been interpreted as at least some of the coke fines found in the deadman area (lower temperature region) have been blown there from the raceway by the high velocity hot blast.

Morphology and composition changes of spinel (MgAl2O4) inclusions in steel

Abstract In this study, spinel inclusions of close to stoichiometric MgO.Al2O3 composition and known size distribution were added to a liquid steel bath before assessing their reactivity. The inclusions were then tracked for changes in size, morphology and composition with time in an aluminium killed steel at 1600°C, by sampling of the melt followed by automatic SEM based inclusion analysis techniques. The majority of the inclusions in the melt were alumina and complex sulphide inclusions, with the added spinel inclusions being a small proportion of the total inclusions. The proportion of alumina inclusions increased with time. The spinel inclusions were found to be evenly distributed through the melt and there was little change in their average size during the reaction. However, the composition of the inclusions did change, with the Mg/Al mass-% ratio changing from ∼0·5 to 0·08.