1 Introduction
1.1 The Reason for the Research
The degradation of molten aluminium is very common during melting processes, also when optimum conditions are used. Usually the molten aluminium must be cleaned from contaminants such as hydrogen gas, melt oxidation and several other contaminations. Hydrogen is the only gas that is appreciably soluble in aluminium, and its solubility varies directly with temperature. The solubility of hydrogen in aluminium, just above and below the melting temperature, is 0.65 and 0.034 ml/100 g respectively1-3, therefore,
during the solidification of molten aluminium, the excess of dissolved hydrogen gas generates what is known as hydrogen porosity. Aluminium oxidizes quickly by direct contact with air or water vapour; this oxidation causes a decrease of elemental aluminium production, an unwanted alteration in its chemistry and the formation of complex oxide particles that could be entrained into the bulk. In addition to oxides, a number of other inclusions, e.g. alkali4 causing edge
cracking in aluminium sheet production, can be present in molten aluminium. Hydrogen porosity and all the other possible particles in aluminium castings make the mechanical properties of the final product get worse.
Consequently, different solutions have been adopted which aim to minimize the hydrogen concentration and the incidence of unwanted particles in molten aluminium. One of the most widely used techniques is the introduction of chlorine/inert gas mixture, using a static lance, in the molten aluminium which creates large gas bubbles, the later fact causes ineffective mass transfer and reaction, and a poor utilization of chlorine gas. To compensate for the inefficiency gas utilization, fluxing, excess chlorine gas is used, which leads to high production cost; furthermore the chlorine is environmental unfriendly.
Over the last five years or so, reducing or eliminating the use of chlorine gas in the holding furnaces has become the top priority for most aluminium producing companies. A solution has been proposed by a flux supplier based on the techniques of rotary degassing and the use of granulated halide fluxes. The industrial use of this kind of process has proven to be a metallurgical and environmental benefit to the traditional chlorine fluxing in terms of emissions requirements and removal efficiency of dissolved hydrogen, alkali metals and inclusions found in molten aluminium metals. The rotation of the impeller aims both to produce a very fine and widely dispersed bubbles pattern and to obtain a good global mixing of the liquid; in this way, a great transfer coefficient is guaranteed. Furthermore, the purge gas bubbles collect the unwanted particles from the melt and, on rising to the surface by flotation, can be easily removed. The turbulent field inside the melt increases the probability of capture of the particles by the gas bubbles. Therefore, the efficiencies of hydrogen and particles removal by the rotary degasser depend on
vessel size, the impeller position, the mixing time and the thermodynamic factors are strictly interrelated to previously mentioned parameters.
1.2 Scope of this Thesis
The IRC (Interdisciplinary Research Centre) and the Department of Chemical School of Engineering of the Engineering in the University of Birmingham are carrying on a three years research project, with the participation of aluminium producers and suppliers of cleaning equipment and quality measurement devices, on non-chlorine cleaning of aluminium in furnaces. The project falls in 4 topics:
q Understanding the fluxes in molten aluminium, using static
testing and observation in real time x-ray equipment;
q Cold modelling of impeller shape and location, furnace design
and processing parameters, using water based models and a range of analysis techniques including mixing by decolourisation and Particle Image Velocimetry (PIV);
q Computational Fluid Dynamic (CFD) modelling techniques to
perform computation experimentation prior to use in real furnaces;
q Industrial trials in a range of furnace configurations, using
furnaces in the aluminium suppliers’ plants.
This thesis develops the second point of the project; all the experiments have been conducted in a scale-down, parallelepiped shaped, vessel simulating a typical geometrical industrial furnace 1,
the gas flow rate has also been scaled down to give two values typical of the aluminium industry.
1.3 Thesis Layout
The general introduction to mixing and statements of the problems to be studied were given in the previous sections. Chapter 2 provides a brief introduction to the theory used in the rest of the thesis, especially concerning the power consumption in mixing processes. Chapter 3 provides a description of the equipment and techniques used in the experimental work. In Chapter 4 are collected and shown all the results concerning the subject of this thesis. This chapter is divided in 4 parts, relatively to the subject analysed: power drawn, mixing time, fluid flow pattern and bubble size. Chapter 5 provides a modelling process aiming at the comparison between theory and experimental work. Finally, Chapter 6 concludes the thesis highlights of the main outcomes of the research and provides some further work programmed. After this last chapter are reported most of raw data in the Appendix.
