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Aluminium recycling is the process in which secondary 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 aluminum from raw bauxite via electrolysis of aluminum oxide (Al2O3) during the Hall–Héroult and Bayer 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 aluminum for every metric ton of primary aluminum produced. Over the same time period, secondary aluminum accounted for 34% of the total new supply of aluminum including imports. Used beverage containers are the largest component of processed aluminum scrap, and most of it is manufactured back into aluminium cans.

Collection & Sorting
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.

Pre-Treatment
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.

Melting
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.

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.

History
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.

Environmental
Because many countries continue to rely on coal-generated electricity for aluminium production, the aluminium industry contributes to 2% of global greenhouse gas emissions, around 1.1 billion tons of carbon dioxide. Many countries now seek to decarbonize aluminium not only as it is the second most used metal in the world, but also because it would heavily address the total greenhouse gas emissions in an effort to slow climate change.

As one of the most recyclable –and recycled– materials in use today, aluminium can be virtually infinitely recycled. Since recycled aluminium takes 5% of the energy used to make new aluminium, around 75% of aluminium manufactured continues to be in use today. According to the Aluminium Association, in industrial markets such as automotive and building, aluminium is recycled at rates of up to 90%.

Since 1991, greenhouse gas emissions from aluminium cans have dropped about 40%, similar to energy demand levels. This can be attributed to a reduction in the carbon intensity of primary aluminium production, improving the efficacy of manufacturing operations, and lighter cans. While primary aluminium only accounts for 26.6% of the can, it makes up the major source of the can’s carbon footprint. For example, as of 2020, 86% of China’s aluminium production relies mostly on coal-generated electricity. On the other hand, Canada sources roughly 90% of its primary aluminium production using hydroelectric power, considering it to be the most sustainable in the world.

Aluminium and its applications are wide and numerous–from defense construction and electrical transmission to playing a key role in emission-reducing goods (electric vehicles and solar panels). As such, countries have begun to decarbonize aluminium to combat global climate change.

Economic
Aluminium recycling has several economic benefits when done properly. In fact, the Environmental Protection Agency considers recycling a “critical” part of the United States economy, contributing to tax revenue, wages, and job creation. By facilitating scrap handling and improving its efficiency –from “end of life” scrap to repurposing scrap throughout the production stage (“in-house” scrap) –aluminium recycling helps in achieving the goals of a circular economy. This type of economy focuses on minimizing the extraction of natural resources, leading to a reduction of consumer and industrial waste. A few examples of countries that have adopted the shift to a circular economy include the European Union, Finland, France, Slovenia, Italy, Germany, and the Netherlands.

A recent study conducted within the United States has highlighted some ways that aluminium recycling has proven to have economic benefits, including:


 * Job creation: Contributing to more than 100,000 jobs in the reprocessing to the United States economy.
 * Economic activity: Bring about $1.6 billion in material sales
 * Wage Increases: Increasing the wages of waste management and the recycling industry from $2.1 billion to $5 billion.
 * Energy Conservation: Save enough energy to power 1.5 million homes per year.
 * Waste Management: Maintain more than 1 million tons of waste out of landfills every year.

As countries take note of the various economic and environmental benefits of aluminium recycling, increased efforts are expected to improve the efficacy of this process.

Challenges
Aside from recycled aluminium beverage cans, the majority of recycled aluminium comes mixture of different alloys. Those alloys generally have high percentages of silicon (Si) and require additional refinement during the shredding, sorting, and refining process to reduce impurities. Due to the levels of impurities found after refinement, the applications of recycled aluminium alloys are limited to castings and extrusions. The aerospace industry often restrict impurity levels of Si and Fe in alloys to a 0.40% maximum. Controlling the appearance of these elements is increasingly difficult the more often the metal has be recycled and require higher cost operations for the alloys to meet performance requirements.