Aluminum Flux Guide: Cover, Cleaning, Drossing and Demagging

An aluminum flux is a salt blend that protects, cleans or treats the melt, and the wrong one, or the right one used badly, costs metal and refractory. Cover, cleaning, drossing and demagging fluxes each do a different job.

This guide explains what each type does and how and when to apply it. It also covers wall cleaning and the safety rules that keep fluxing from turning into a splash or fume hazard.

What an aluminum flux does

Molten aluminum reacts with air and moisture all the time. Oxides form, inclusions build up, and dross traps metal. Fluxes are salts that melt or react at bath temperature to deal with those problems.

A technical overview on Total Materia lists the compounds commonly used. These include aluminum chloride and fluoride, borax, calcium chloride and fluoride, carnallite, zinc chloride, cryolite, lithium chloride, magnesium chloride and potassium chloride. Chlorides tend to form the fluid base. Fluorides tend to do the chemical work on oxides. For one of the best known fluoride components, see our article on cryolite in aluminum production.

The same source notes that one aluminum flux can serve one function or several. So read the data sheet for the intended use, not only the product name.

Cover, cleaning, drossing and demagging fluxes

Cover fluxes

Total Materia describes cover fluxes as designed mainly for smaller pot and crucible furnaces. They form a physical barrier against oxidation. Some are also used to clean the charge.

Cleaning fluxes

Cleaning fluxes are usually higher in chlorides and usually contain fluorides. The fluorides help wet oxide inclusions, so they separate from the melt more easily. Use a cleaning aluminum flux when inclusions drive your rejects, and pair it with degassing and filtration rather than relying on it alone.

Drossing fluxes

Drossing fluxes rely on fluorides to break up the oxide layer and release trapped metal. Rabbling with a rake helps the reaction. The payoff is large: Total Materia states that untreated dross may contain 60% to 85% free metal. That metal burns to oxide if it is not recovered. After a good treatment the dross looks dry and powdery. Wet, shiny dross still holds metal and needs more work before it leaves the furnace.

Demagging agents

Scrap often brings more magnesium than the target alloy allows. Demagging removes it by reacting magnesium into a salt that floats out. A TMS paper by Velasco, Nino and Cardoso studied chlorine injection in a reverberatory furnace. It notes that efficiency depends on kinetic factors in the chlorine and magnesium reaction. Solid fluxes and demagging agents are another route. Either way, check magnesium with a spectrometer before and after treatment.

How and when to apply an aluminum flux

Timing and dose decide most of the result. In Foundry Management & Technology, David White advises following the maker’s recommendations while erring on the lean side. For drossing flux, he gives about 0.5 lb per 1,000 lb of metal, depending on dross depth.

A practical sequence for drossing:

1. Weigh the dose. Do not broadcast by eye.
2. Spread the aluminum flux evenly over the dross layer.
3. Rake and rabble it into the dross so it can react.
4. Give it time. White describes working the dross until it turns cherry red.
5. Pull the dross to the door and drain it on the sill.
6. Skim while tapping the skimmer, so metal drops back.

Other rules of thumb:

• Add flux to liquid metal, not to a cold solid charge, unless the data sheet says otherwise.
• Keep high-fire periods during fluxing short. White suggests 5 to 10 minutes, and no longer.
• Match the aluminum flux to the alloy. Some blends are not suitable for high-magnesium or high-silicon grades.
• Log dose, furnace and dross weight together, so you can see what the flux actually saves.

Purpose-made furnace tools make rabbling and skimming faster and more even.

Wall cleaning without wrecking the lining

Oxide grows on walls where metal splashes and cleaning is infrequent. Wall cleaning fluxes soften that buildup so it can be scraped off. However, they are aggressive. White warns that wall flux can raise the temperature of what it touches to 3,100°F. Most hot-face refractories are rated around 3,000°F.

Use wall flux only where buildup needs it. Scrape in layers, clean quickly and thoroughly, and do not leave oxide on the hearth. Regular light cleaning beats an occasional heavy dose of wall flux. Plan cleaning on a schedule tied to tonnage, and record the lining condition each time. If the lining loses thickness quickly between cleanings, the routine is too aggressive.

Safety and handling of aluminum flux

Moisture is the main hazard. Foundry Management & Technology notes that water turns to steam on contact with molten metal and expands about 1,600 times. Fluxes are salts, and many absorb moisture from the air.

• Store every aluminum flux dry, sealed and off the floor. Close opened bags.
• Do not add flux that has caked or looks damp.
• Preheat tools and plungers before they enter the bath.
• Wear face shields and aluminized protective equipment during fluxing and skimming.
• Run extraction during fluxing. Fumes can contain chlorine and fluorine compounds.

Emissions are regulated in many places. In the US, the EPA standards for secondary aluminum production list hydrogen chloride, hydrogen fluoride and chlorine among the regulated pollutants. Check the rules that apply to your plant before changing flux type or dose.

FAQ

Granular or powder flux?

Granular products are often chosen to reduce dust during handling. Choose by application method and the maker’s data sheet.

Can one product cover and dross?

Some blends do both. Confirm the intended functions on the data sheet and test in your own furnace.

How do we know a flux works?

Track dross weight and its metal content, melt cleanliness and refractory condition before and after the change.

Need an aluminum flux, demagging agent or furnace tools to your specification? AL-LINX quotes each item to your requirements, and several items fit in one request. Request a quote for fluxes and furnace tools.