Aluminium recycling

Aluminium recycling is the process in which secondary commercial aluminium is created from scrap or other forms of end-of-life or otherwise unusable aluminium. It involves re-melting the metal, which is cheaper and more energy-efficient than the production of virgin aluminium by electrolysis of alumina (Al₂O₃) refined from raw bauxite by use of the Bayer and Hall–Héroult processes.

Recycling scrap aluminium requires only 5% of the energy used to make new aluminium from the raw ore. In 2022, the United States produced 3.86 metric tons of secondary aluminium for every metric ton of primary aluminium produced. Over the same time period, secondary aluminium accounted for 34% of the total new supply of aluminium including imports. Used beverage containers are the largest component of processed aluminium scrap, and most of it is manufactured back into aluminium cans.

The first step in aluminium recycling is the collection and sorting of aluminium scrap from various sources. Scrap aluminium comes primarily from either manufacturing scrap or end-of-life aluminium products such as vehicles, building materials, and consumer products. Manufacturing scrap includes shreds, shavings, cuttings, and other leftover aluminium from manufacturing processes. Post-consumer scrap consists of obsolete or discarded aluminium products. Aluminium cans, in particular, are a major source of recyclable aluminium scrap. Once collected, aluminium scrap is sorted based on alloy type, grade, impurity levels, and other factors. Sorting may be done manually or using technologies like eddy current separators, air classifiers, and density separators. The scrap is sorted into categories like wrought alloy scrap, casting alloy scrap, used beverage cans, automobile scrap, and mixed scrap. Proper sorting is essential for producing high-quality recycled aluminium.

After sorting, the scrap may undergo pre-treatment processes to prepare it for melting. These can include baling, shredding, crushing, granulating, decoating, and demagnetizing. Shredding and crushing reduce the particle size of the scrap and liberate it from other materials, while granulating produces fine particles ideal for melting. Thermal decoating removes coatings like paint and plastic from aluminium surfaces. Demagnetizing removes iron particles clinging to the aluminium scrap. Pre-treatment improves the density of the scrap charge and removes contaminants, resulting in faster melting, cleaner metal, reduced dross formation, and lower energy consumption.

Once pre-treated, the aluminium scrap undergoes melting and liquid metal treatment to produce refined aluminium alloy suitable for casting or reprocessing. Different furnace types are used based on the type of scrap, desired metal quality, and economics. Smaller scrap is typically processed in rotary or reverberatory gas-fired furnaces, while large individual pieces of scrap can be charged directly into reverb furnaces through side wells. Electric induction furnaces are also used. As the scrap melts, fluxes are added to bind and absorb impurities which are scraped off the top as dross. Chlorine gas may also be injected to remove impurities through flotation. The melt can then undergo refining processes like flux injection to further reduce hydrogen and impurities. Degassing removes dissolved hydrogen while chemical filtration removes solid impurities and inclusions. The final result is molten aluminium alloy ready for casting.

The molten recycled aluminium is cast into solid forms such as ingots, sows, or directly into sheets or extrusion billets. Direct-chill casting is commonly used to solidify the liquid aluminium into large cylindrical billets for extrusion or rolling. The direct chill method sprays water onto the hot metal as it exits the mold, quickly chilling it into a solid billet form. For ingots, book molds are often used, producing slabbed ingots suitable for remelting or rolling. Continuous casting directly shapes the aluminium into rolling slabs without an intermediate ingot casting step. Twin-belt or twin-roll strip casting produces alloy strips 6-7mm thick directly from the melt for subsequent rolling. The casting method depends on the subsequent processing and use of the recycled aluminium alloy.

Although aluminium in its pure form has been produced as early as 1825, secondary aluminium production, or recycling, rose in volume with the introduction of industrially viable primary aluminium processes, namely the combination of the Bayer and Hall-Héroult processes. The Hall-Héroult process for aluminium production from alumina was invented in 1886 by Charles Hall and Paul Héroult. Carl Josef Bayer created a multi-step process to convert raw bauxite into alumina in 1888. As aluminium production rose with the use of these two processes, aluminium recycling grew too. In 1904, the first two aluminium can recycling plants were built in the United States; one recycling plant was built in Chicago, Illinois and the other was built in Cleveland, Ohio. Aluminium recycling increased most significantly in volume when metal resources were strained during WWI, as the U.S. government campaigned for civilians to donate old products such as aluminium pots, pans, boats, vehicles, and toys to recycle for the construction of aluminium airframes.

Aluminium is an infinitely recyclable material, and it takes up to 95 percent less energy to recycle it than to produce primary aluminium, which also limits emissions, including greenhouse gases. Today, about 75 percent of all aluminium produced in history, nearly a billion tons, is still in use.