Aluminum dross is one of the most important—and often misunderstood—byproducts in the aluminum industry. As the demand for recycled aluminum grows, so does the need to better understand and manage the residues generated during melting and processing.
This article dives deep into the formation, classification, and composition of aluminum dross, laying the groundwork for understanding its value and challenges.
How is Aluminum Dross Formed?
When aluminum is melted for casting, alloying, or recycling, it reacts with oxygen in the air. This reaction forms aluminum oxide (Al₂O₃) on the surface of the molten metal. Mechanical stirring, skimming, and fluxing operations further mix this oxide layer with the underlying metal, forming a semi-solid byproduct known as dross.
Dross formation is an inevitable part of aluminum production. On average, 1 to 10% of the aluminum charge may end up as dross, depending on the process efficiency and furnace type.
Types of Aluminum Dross
There are three main categories of dross, each with different physical and chemical characteristics:
1. White Dross
Origin: Produced in primary and secondary aluminum melting without salt.
Composition: High metallic aluminum content (up to 80%), mixed with aluminum oxide and minor impurities.
Appearance: Silvery, reflective, often still hot and semi-molten when skimmed.
2. Black Dross
Origin: Typically from secondary aluminum recycling or dirty scrap.
Composition: Lower aluminum content (~10–30%), high in oxides and non-metallic inclusions.
Appearance: Dark gray or black, powdery or granular.
3. Salt Slag (Salt Cake)
Origin: Generated in rotary salt furnaces, which use a mixture of NaCl and KCl to prevent oxidation.
Composition: Aluminum oxide, aluminum nitride (AlN), salts (NaCl, KCl), small amounts of metallic aluminum.
Environmental concern: Classified as hazardous in many jurisdictions due to leachable salts and ammonia emissions from AlN.
Typical Composition of Dross
Values are approximate and vary depending on scrap quality and process parameters.
Why It Matters
Understanding the type and composition of dross is essential for:
– Selecting the right recovery technology
– Estimating metal yield
– Ensuring environmental compliance
– Reducing operational costs in foundries and recycling plants
In many cases, white dross is reprocessed in-house, while black dross and salt slag require specialized treatment due to their lower value and environmental risks.
From Waste to Resource
Traditionally viewed as waste, aluminum dross is increasingly being recognized as a recoverable resource. With appropriate treatment, it’s possible to:
– Recover metallic aluminum
– Neutralize or reuse salt residues
– Repurpose oxide content in building materials or ceramics
This shift is critical for advancing circular economy goals in the metal industry
Does this topic interest you? Discover our thematic dossier:
→ Aluminium dross recycling: overview and key technical challenges