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Metal Casting Technologies : June 2008
TECHNICAL FEATURE Local disturbance of melt surface Nitrogen Picture 6: Schematic of lance degassing reducing the partial pressure of hydrogen against the hydrogen dissolved in the aluminium melt. Lance degassing with inert gases like nitrogen or argon represents one of the first applied physical degassing methods. Picture 6 illustrates the lance degassing principle. Due to the large bubble formation the lance degassing process does not offer high degassing efficiency and is leading to local disturbance of the melt surface with increased oxide incorporation, dross formation and gas pick-up. Another static degassing process is represented by the porous plug method. The porous plug offers a medium bubble size with improved degassing efficiency compared with lance degassing and is creating less surface turbulence. Due to the uneven bubble distribution in the melt the porous plug application is restricted to smaller melt volumes. Some 20 years ago the vacuum degassing process became quite common in aluminium foundries. The melt container is either placed in a vacuum chamber for treatment or covered with a vacuum-tight lid. When the atmosphere above the melt is evacuated the hydrogen gas is taken out of the melt in balance with the partial pressure above the melt surface. This degassing process is very effective specifically in conjunction with inert gas injection stirring the melt volume but requires large fixed installations and intense maintenance of the vacuum system. State of the art degassing practice today is provided by the rotary degassing process. This type of equipment is purging inert gas into the melt through a specific rotor connected to a hollow shaft. Picture 7 is providing a schematic view of the rotary degassing set up. 44 www.metals.rala.com.au Picture 7: Schematic of rotary degassing The rotating shaft and rotor system is distributing very fine gas bubbles evenly through the entire melt volume achieving a very rapid and effective degassing. An immersed baffle plate is preventing the formation of a detrimental vortex. An additional effect is seen in the cleaning of the melt by simultaneous flotation of non-metallic inclusions. The industry is offering meanwhile a wide spectrum of rotary degassing equipments for any conceivable application and a wide range of degassing rotors of different design. One of the most effective rotor design is experienced with the impeller type rotor which is pumping melt through the rotor chamber as shown in picture 7. In picture 8 a comparison of the degassing efficiency of different degassing processes is shown. The graph illustrates clearly that the lance degassing process Picture 8: Comparison of lance, vacuum and rotary impeller degassing