Aluminum Degassing Methods - Picking the Best Method
There are six commonly recognized methods of degassing aluminum. These are briefly discussed below along with the pros and cons of each method.
1. Purge Gas Bubbling
How it works: Inert gas (nitrogen, argon, or blends with chlorine) is introduced through lances or porous plugs immersed in the molten aluminum. The bubbles rise to the surface, carrying hydrogen and impurities with them.
Pros: Simple, relatively inexpensive to set up. Manual lancing is a hazardous hot job.
Cons: Less efficient than rotary degassing, particularly in humid environments where moisture reintroduces hydrogen.
2. Tablet Degassing
How it works: Solid tablets (often containing hexachloroethane) are added to the melt. These tablets release inert gases as they decompose, forming bubbles throughout the melt and facilitating degassing.
Pros: Convenient and easy to apply.
Cons: Potential for introducing unwanted elements from tablet residues, issues with consistency and control, possible environmental concerns. Best results require automated or manual circulation of the melt for uniformity.
3. Vacuum Degassing
How it works: The molten aluminum is placed in a chamber where the pressure is significantly reduced. This lowered pressure promotes the rapid expansion and removal of dissolved gases, including hydrogen.
Pros: Can achieve very low hydrogen levels, effective for large melt batches.
Cons: Expensive equipment and higher operational costs, but can be a much slower process.
4. Ultrasonic Degassing
How it works: High-frequency ultrasonic vibrations are introduced into the molten aluminum, creating cavitation bubbles. These bubbles attract dissolved hydrogen and facilitate its removal.
Pros: Potentially very efficient, can improve both degassing and melt refinement (removal of inclusions).
Cons: Relatively new technology, still under development for widespread industrial adoption, potential challenges with equipment and process control.
5. Flux Degassing
How it works: Fluxes (mixtures of salts) are added to the melt. These fluxes can react with hydrogen and help trap it within the flux, or they can promote the formation of bubbles.
Pros: Relatively simple and inexpensive method.
Cons: Can introduce residues or contaminants into the melt, potentially less effective at achieving very low hydrogen levels.
6. Rotary Degassing
How It Works: Inert gas (nitrogen, argon, or blends with chlorine) is introduced through a graphite rotor attached to a hollow graphite shaft. The rotor is immersed in the molten aluminum and rotated to disperse the gas and circulate the aluminum.
Pros: Fast and efficient by combining mechanical agitation and gas bubbles. Granular flux can be injected into the gas stream for addition processes like magnesium removal. Rotary degassing can be automated to ensure the most repeatable quality results.
Cons: Regular maintenance of graphite shaft and rotor is required. Excessive rotor ware reduces the mechanical agitation. If granular flux is being introduced through the shaft and rotor, the gas path may become blocked by flux.
Choosing the Right Method
The best degassing method depends on several factors, including:
Desired hydrogen levels: Some methods are better suited for achieving very low hydrogen content.
Batch size and production volume: Some methods are more efficient for larger-scale operations.
Cost considerations: Equipment and operational costs vary between methods.
Alloy composition: Certain methods may be more suitable for specific aluminum alloys.
High Temperature Systems offers both portable and fixed rotary degassing solutions which can be found here.
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