04 / Background
What is Dross?
Dross is the oxidation byproduct of molten aluminum — a mixture of aluminum oxide (Al₂O₃), residual metallic aluminum, and trace alloying elements. It forms continuously on every molten surface as oxygen reacts with hot aluminum.
How it forms
The moment aluminum is exposed to air at melting temperature, surface oxidation begins. The thin Al₂O₃ layer normally protects bulk aluminum at room temperature — but at 700°C it grows continuously, capturing tiny droplets of pure metal inside an expanding oxide foam.
This foam-like mixture floats to the top of the melt and gets skimmed off. That skim is dross.
How much
Melt loss depends heavily on the input material — from around 0.3% for unalloyed remelt up to 8–10% for UBC and as much as 20% for foil scrap, also driven by alloy chemistry, scrap quality, holding time, and skimming practice.
Composition
By weight, fresh dross typically contains:
- 15–80% metallic aluminum (white dross) — recoverable, if you cool it in time
- 20–85% aluminum oxide (Al₂O₃) — the unrecoverable fraction
- Trace alloying elements and minor constituents
Why cooling matters
The longer dross sits hot, the more its trapped metal oxidizes — roughly 1% of the contained aluminum is lost every minute until the temperature falls below the oxidation range. A skim left to cool slowly on the shop floor keeps losing recoverable metal before it ever reaches the recycling line.
Rapid indirect cooling — bringing the dross below 100°C in 15 minutes — stops the oxidation reaction. The metal stays metallic, the recovery line stays profitable.
Modern recovery economics make dross cooling a profit center, not a waste-handling step.
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