Slag treatment method for reducing aluminum loss

[China Aluminum Network] During the smelting of aluminum alloys, non-recoverable metal loss, known as burnout, occurs due to oxidation of aluminum and its interaction with furnace walls and refining agents. The metals contained in both the burnout and slag are collectively referred to as melt loss. When using a flame reverberatory furnace for melting aluminum alloys, the amount of slag generated typically ranges from 2% to 5% of the charge weight, with about 40–60% of that slag containing recoverable aluminum. Therefore, it is crucial to properly treat aluminum slag and recover the aluminum within it to minimize melting losses. This article discusses practical methods used in 5-ton flame reverberatory furnaces to reduce melt loss by effectively managing aluminum slag.

To reduce aluminum slag, several key measures can be taken during the melting process. As slag increases, so does the melting loss, and factors such as melting temperature, charge condition, and production procedures all influence slag formation. Here are some effective strategies:

(1) Optimize furnace geometry and the order of material addition.
(2) Maintain strict control over the melting temperature to avoid overheating. The flame should be directed at an appropriate angle to speed up melting and shorten the overall process.
(3) Use the right flux and refining time, ensuring stable stirring without breaking the oxide layer on the surface of the melt, and cover the molten aluminum promptly.
(4) Sort and clean scrap aluminum, comparing the surface area of finely divided charges with those pressed into compact forms.
(5) Treat the slag before removal to improve recovery efficiency.

Slag treatment can be divided into two stages. First, before slag removal, the slag floating on the surface after refining has good wettability with the melt, with a wetting angle less than 90 degrees. At this stage, a significant amount of melt is mixed into the slag, and when the temperature is low, more melt adheres to the slag. Adding a slagging agent at a rate of 1‰ to 2‰ evenly across the melt surface helps reduce the aluminum content in the slag. The chemical reactions involved include: Na₂SiF₆ → 2NaF + SiF₄; 2NaF + Al₂O₃ → NaAlO₂ + NaAlOF₂; 4NaF + 2Al₂O₃ → 3NaAlO₂ + NaAlF₄; 6NaF + Al₂O₃ → 2AlF₃ + 3Na₂O. The resulting AlF₃ reacts exothermically with aluminum and oxygen, transforming viscous slag into dry, powdery material. This reduces the wettability between the melt and the oxides in the slag, allowing aluminum droplets to separate and return to the melt.

After tapping, the slag still contains small aluminum droplets. To recover them, first dip the slag into a perforated iron container, allowing some of the adhered melt to drain out. Then, pour the slag into a prepared pit, add some slagging agent, and stir thoroughly. After 5–10 minutes, remove the slag from the pit. The slagging agent rapidly heats the slag to around 950°C, causing the oxide layers around aluminum droplets to crack, allowing the droplets to fall to the bottom under gravity. After secondary treatment, the remaining slag contains only minimal aluminum particles, which can be manually recovered once cooled. This method is simple, efficient, and widely applicable. Today, melting losses have been reduced to as low as 1.6%, sometimes even 1.4%. For example, if 5,000 tons of aluminum alloy are produced annually, the economic loss due to melting can be cut by approximately 400,000 yuan, making it a highly beneficial practice.

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