What causes inclusions in molten aluminum launder

Inclusions inside aluminum transfer launders are mainly divided into in‑situ generated in‑launder and carry‑over inclusions from upstream furnace.

1. Oxide & bifilm inclusions (dominant source)

• Exposed open‑launder surface continuously forms thin Al₂O₃ oxide skin when molten aluminum contacts air.
• Severe turbulence, sharp transitions, height drops, unstable flow at launder joints fold oxide skins into melt, forming harmful bifilm inclusions.
• Surface dross and oxide fragments accumulated inside launder get re‑entrained by flowing aluminum.

2. Refractory‑origin exogenous inclusions

• Erosion, thermal spalling, mechanical peeling of launder lining refractories under long‑term molten aluminum scouring; broken refractory particles drop into melt.
• Poor baking for new/repaired launder: moisture inside refractory causes reactions, accelerating lining degradation and particle shedding.
• Damaged joint gaskets, fiber felts and seal materials break off and enter aluminum flow.
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3. Carry‑over contaminants from upstream melting furnace

• Unremoved furnace slag, flux residues, salt‑based refining by‑products flow into launder together with melt.
• Incompletely filtered oxide particles, boride / carbide intermetallic phases coming from melting & degassing process. 
• Impurities introduced from scrap / recycled raw materials.

4. Process‑operation related factors

• Too high melt velocity, unstable flow, splashing inside launder aggravate secondary oxidation and entrainment.
• Poor launder cleaning: residual dross and sediment from previous production get stirred into new‑batch molten aluminum.
• Improper temperature control accelerates refractory corrosion and oxide generation.