Manganese and iron are two major alloying elements in various aluminum products. Since these elements have high melting points and low diffusivities in molten aluminum, their dissolution rates are very slow, when they are added to aluminum melts. In order to improve the kinetics of dissolution, several alloying methods have been introduced. All methods of alloying use mechanical stirring of some form or other to enhance dissolution rates by promoting forced convective mass transfer. In the present study, a comparison between the kinetics of dissolution of iron and manganese when added to the melt as discrete alloying particles or as compact briquettes (consisting of alloying elements and aluminum particles) was carried out. This study revealed that exothermic reactions and the local accumulation of heat within the briquettes can accelerate the kinetics of the alloy-making process. / Using the results obtained, a model for the dissolution of briquettes in molten baths of aluminum is proposed. Following disintegration of the briquettes into the melt, fine particles disperse freely into the bath, while their dissolution being controlled by mass transfer. To predict the dissolution rates, the relative velocity between fine particles entrained in the turbulent field and the fluid must be known. However, such velocities are extremely difficult, if not impossible, to clarify, since the particles will be moving erratically with a turbulent liquid, in which, random velocity fluctuations will be superimposed on mean bulk flow velocities. As such, the hydrodynamic interactions which control particle-fluid mass transfer in an agitated vessel are highly stochastic and impossible to predict, ab initio. In order to quantify such phenomena two theoretical approaches have been introduced by aqueous model researchers. In this investigation, for the first time, these theories have been evaluated for a high temperature liquid metal system involving the dissolution of alloying particles. / In the evaluation of the terminal velocity theory in high temperature systems, water modelling experiments in conjunction with dimensional analysis between cold model and hot temperature system were carried out to study the entrainment behaviour of (heavy) alloying particles. Then, through continuous sampling of the melt, mass transfer coefficients and rates of dissolution of suspended particles in the melt were measured. A comparison between predicted values and measured results indicated that the terminal velocity theory, in conjunction with correlations proposed for aqueous systems, was not applicable to the high temperature alloying systems studied. / In Kolmogoroff's theory of local isotropy, mass transfer in a turbulent system can be treated by a single parameter, i.e. energy dissipation rate. High temperature experiments revealed that this approach could be applied to stirred alloying particles-molten aluminum systems. However, at higher rates of mixing, when a fully suspended condition is reached, further increases in input mixing energy have negligible effects on dissolution rates. Thus, as a practical measure, very high mixing rates are not recommended. In this regard, a correlation for mass transfer coefficient as a function of the rate of input energy was presented.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.40249 |
| Date | January 1996 |
| Creators | Shafyei Najafabadi, Ali |
| Contributors | Guthrie, R. I. L. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Mining and Metallurgical Engineering.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001498845, proquestno: NN12481, Theses scanned by UMI/ProQuest. |
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