Aluminum Dross Processing

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Introduction

Aluminum dross emerges as an inevitable byproduct during aluminum melting processes. Its composition and properties vary significantly depending on multiple factors, including scrap type, alloy treatment, and melting equipment specifications. The material primarily consists of aluminum, aluminum oxide, and oxides of alloying elements, with occasional presence of halogenide, carbide, or nitride compounds.

Dross Formation and Characteristics

A notable characteristic of dross is its substantial metallic aluminum content, which becomes mechanically fixed within an aluminum oxide cellular structure. This structure exhibits remarkable tension strength, capable of bonding aluminum at twenty times its weight. The presence of finely dispersed gas bubbles creates a foam-like structure, facilitating natural separation from the aluminum melt due to density differences.

Figure 1: Formation of Dross - Detailed illustration showing the eight-step process of dross formation from surface oxidation to cooling, with the following meaning for each step:

• Surface oxidation of melt
• Crushing of oxidic skin by bath movement
• Sinking and floating of oxidic particles
• Conglomeration of oxidic particles
• Interspace filling up by metallic aluminum
• Inside oxidation of dispersed aluminum
• Skimming of dross from melt bath surface
• Metallic aluminum drip off and follow-up oxidation of solids during dross cooling

Table 1: Composition of dross

Composition of dross
Type of Dross	Metallic Al, [%]	Residual compounds
Untreated Dross	< 80%	Al₂O₃, Al₂O₃*MgO
Flatcake & Crumbs	90 – 70%	Al₂O₃*SiO₂, Al₂O₃*FeO, etc.
Concentrate	70 – 50%	Al₂O₃, AlCl₃, NaCl, KCl
Coarse Residue	50 – 20%	CaF₂, NaF, AlF₃, etc.
Fine Residue	20 – 5%	Al₂O₃, AlN, Al₄C₃, Al₂S₃, AlP, etc.

Critical Process Parameters

Several key factors influence dross formation and properties:

• Surface-to-volume ratio of the scrap material
• Nature and quantity of impurities (organic and inorganic)
• Melting conditions and equipment specifications
• Melt composition, particularly regarding Mg, Na, Li, and Ca content
• Refining procedures
• Dross removal techniques and temperature conditions
• Cooling parameters post-removal

Figure 2: Visual representation of processed aluminum dross