Aluminum Salt Slag

What is aluminum salt slag?

Aluminum salt slag is a byproduct of both the production and recycling of aluminum.

During the production of aluminum using the smelting process, aluminum oxide and other impurities are melted in an electrolysis process to produce pure aluminum. The resulting slag, known as aluminum salt slag, consists primarily of aluminum oxide and is contaminated with salts such as sodium chloride, potassium chloride, and fluorides.

Aluminum salt slag is also produced during the reprocessing of used or spent aluminum in the recycling process.
The collected aluminum is melted down in smelters or furnaces to purify it for use in the manufacture of new aluminum products. The impurities and foreign materials contained in the recycled aluminum alloy are deposited as slag, which also consists mainly of aluminum oxide and salts.

Formation of aluminum salt slag

Aluminum salt slags are produced as a residual material during the melting of aluminum scrap and dross, which mainly takes place in rotary drum furnaces. During this particular process, smelting salt, a mixture of e.g. 70% NaCl, 28 % KCl and 2 % CaF2, is added. This salt floats on top of the molten metal and prevents oxidation of the liquid aluminum metal and additionally absorbs the impurities of the scrap. Aluminum metal and salt blanket are then successively discharged from the rotary drum furnace.

The last „tapped“ salt mixture contains residues of aluminum metal and various metal oxides, primarily aluminum oxide. This mixture solidifies in tubs, forming what is known as aluminum salt slag. The typically produced aluminum salt slag is composed of approx. 60% NaCl/KCl mixed salts, 30% aluminum oxides and 5 – 10% metallic aluminum.

Typical process description

First, the metallic aluminum is separated from other components through various crushing stages, including eccentric vibrating mills, and by dry mechanical separation processes, such as screening, and extracted from the aluminum salt slag.

A combination of processes is used in the subsequent wet processing stage. The molten salt still contained in the slag is recovered in multi-stage dissolution and recrystallization processes and dewatered using SHS pusher centrifuges or CONTURBEX screen scroll centrifuges.

The alumina residue in the wet section is thoroughly washed and dewatered. Special equipment such as vacuum belt filters and DZ decanters are used to reduce the moisture content and convert the toner residues into a transportable form. These residues are reused in the construction materials industry.

An important aspect of processing is the environmental compatibility of the process. The wet end generally operates with a closed water circuit to minimize water consumption and environmental impact. All dust and gases generated during the process are captured and separated in an exhaust gas cleaning system. Harmful gases such as ammonia are converted to ammonium sulfate in special column scrubbers and dewatered using our pusher centrifuges and screen scroll centrifuges.

The treatment of aluminum salt slag is a highly sophisticated process in which our equipment solutions play several key roles. By efficiently recovering aluminum and other valuable materials from the slag, this process makes a valuable contribution to promoting recycling and reducing the environmental impact of the aluminum industry.