Author: wal0017

Physical Modelling of Aluminium Melt Rotary Refining by Blowing of Inert Gas

Karel Michalek, Josef Walek, Markéta Tkadlečková

Methodology
The use of physical modelling to study the degassing process of aluminium melts in the refining ladle by blowing inert gas through a rotating impeller. The efficiency of this rotary refining depends on the creation of fine bubbles with a high interphase surface, wide-spread distribution, the residence time of its effect in the melt, and mostly on the wide-spread dispersion of bubbles into the whole volume of the refining ladle. The decrease of gas (hydrogen) content in molten aluminium during refining with inert gas is simulated at physical model by a decrease of dissolved oxygen in model liquid (water). The main advantage of using water is primarily its low cost, good availability, and in their physical properties. The physical modelling is conducted in accordance with the theory of similarity between the model and the prototype, based on the identity of Froude’s criterion. It is necessary to observe, in particular, geometrical similarity of the prototype and its model, and the dynamic similarity of fluid in prototype and in its model.

Model Description
The physical model of the cylindrical refining ladle is made of transparent plexiglass. The advantages of this polymer are, in especially, excellent optical properties, in primarily high transparency, which is very important when observing the course of the refining process. The graphite impeller, shaft and baffles are also an integral part. Their construction and geometry, both on the model and on the operating equipment, are designed in such a way that the impeller shaft can be connected to the argon supply. The change of the impeller immersion is solved by a hydraulically controlled platform, on which the physical model of the ladle is placed. Variable and regulated rotary impeller speeds are solved by using an asynchronous motor powered by an inverter. Rotary impeller speed control is therefore performed by changing the frequency of the alternating current. Mass flow meters with needle regulate valves are used to measure and regulate the flow of gases. The gas is fed from the pressure bottles through a line to the inside of the impeller itself, from its lower part it then exits into the bath during rotation. The optical probes are used for the continuous measurement of oxygen content, allowing measurement of dissolved oxygen content up to 26 ppm.

Physical model control
Ar flow setting
Blowing O2 (0.2 MPa); 350 rpm; to the value of 4.5 V (23 ppm)
Rotary impeller speed setting
Switch O2 to Ar
Launch of experiment
End of experiment: 1 ̴1.05 V (0 ppm)

Intellectual property rights
Functional sample of physical model for study of the refining aluminium melts; Internal result identification code 048/18-11-2020_F; Registration date 18.11.2020

Acknowledgement
The result of the functional sample type was created within the support of the Czech Ministry of Industry and Trade within the frame of the programme TRIO within the solution of the project reg. No. FV10080 “Research and Development of Advanced Refining Technologies of Aluminium Melts for Increase in Product Quality”.