Castelmella Plant

Primary smelting aluminium, or primary aluminium, is the metal produced by alumina electrolysis; secondary smelting aluminium, or secondary aluminium, is the metal produced by casting and refining waste, scrap, swarf, spare parts of aluminium and its alloys.

 

These 'raw materials' contain alumina and, depending on their origin, other components such as paints, oils, plastic parts and organic substances that must be separated before processing.

Aluminum oxide is created as a result of the surface being in contact with the atmosphere; the thin oxidized film is very dense and prevents oxidation within the metal, thus guaranteeing resistance to corrosion. The other components are usually part of the original product, such as paints or plastic, or come from the processing of raw materials, such as lubricants.

During the casting of scrap, alumina must be separated from the metal bath in order to preserve the light alloy ingots, which is the finished product. This technology, currently used on a global level, almost exclusively involves the use of rotary kilns working with salt waste. Using a mixture of potassium chloride and sodium chloride the liquid aluminium is protected from oxygen in the atmosphere thus preventing oxidation of the metal, which is highly reactive due to the high temperatures involved; moreover, the alumina enters into solution with the aforementioned mixture that acts like a solvent. Besides its function as a solvent and as a protector against oxidation, during the casting process the waste also traps inert materials and all the other contaminants that 'degrade' the metal bath.

A number of products remain at the end the skimming process: metallic aluminium, alumina, other oxides composing light alloys and inert materials held inside a chloride mix. The weight of the waste produced is equal to 0.5-0.7 of the metal weight. This large volume of waste cannot be considered 'normal' waste as it is made up of metals that are considered to be raw materials, such as alumina, or that can be recycled within the productive process itself, such as sodium chloride and potassium chloride.