Abstract
The refining process is one of the essential stages of aluminum production. Its main aim is to remove hydrogen, that causes porosity and weakens the mechanical and physical properties of casting aluminum. The process is mainly conducted by purging inert gas through the liquid metal, using rotary impellers. The geometry of the impellers and the processing parameters, such as flow rate of gas and rotary impeller speed, influence the gas dispersion level, and therefore the efficiency of the process. Improving the process, and optimization of parameters, can be done by physical modelling. In this paper, the research was carried out with the use of a water model of batch reactor, testing three different rotary impellers. Varied methods were used: visualization, which can help to evaluate the level of dispersion of gas bubbles in liquid metal; determination of residence time distribution (RTD) curves, which was obtained by measuring the conductivity of NaCl tracer in the fluid; and indirect studies, completed by measuring the content of dissolved oxygen in water to simulate hydrogen desorption. The research was carried out for different processing parameters, such as flow rate of refining gas (5–25 L·min−1) and rotary impeller speed (3.33–8.33 s−1). The obtained results were presented graphically and discussed in detail.
Highlights
Hydrogen is one of the most harmful impurities in liquid aluminum, and its alloys, due to porosity, which in turn deteriorates the physical and mechanical properties of aluminum castings [1,2,3]
There are various methods used for the refining process, the process of blowing the melt with refining gas, mainly argon, seems to be the most popular [4,5,6]
These pictures show the level of dispersion for various processing parameters
Summary
Hydrogen is one of the most harmful impurities in liquid aluminum, and its alloys, due to porosity, which in turn deteriorates the physical and mechanical properties of aluminum castings [1,2,3]. Its main purpose is to remove hydrogen from liquid metal, but flotation enables the elimination of up to 90% of nonmetallic impurities [5,6]. This method of gas injection can be carried out with the use of porous plugs, lances, diffusers, rotating nozzles and rotary impellers, the latter of which is widely used in aluminum foundries worldwide because of promising results [7,8,9]. Oldshue et al [11] proposed four different patterns of gas liquid mixture:
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