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. [1] 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. [2] 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. [3] Used beverage containers are the largest component of processed aluminium scrap, and most of it is manufactured back into aluminium cans. [4]
The first step in aluminium recycling is the collection and sorting of aluminium scrap from various sources. [5] Scrap aluminium comes primarily from either manufacturing scrap or end-of-life aluminium products such as vehicles, building materials, and consumer products. [5] 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, [6] 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. [6] Sorting may be done manually or using technologies like eddy current separators, air classifiers, and density separators. [7] 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. [8] These can include baling, shredding, crushing, granulating, decoating, and demagnetizing. [9] Shredding and crushing reduce the particle size of the scrap and liberate it from other materials, while granulating produces fine particles ideal for melting. [10] Thermal decoating removes coatings like paint and plastic from aluminium surfaces. [7] 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. [11]
Once pre-treated, the aluminium scrap undergoes melting and liquid metal treatment to produce refined aluminium alloy suitable for casting or reprocessing. [11] 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. [10] 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. [12]
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. [7] The direct chill method sprays water onto the hot metal as it exits the mold, quickly chilling it into a solid billet form. [13] For ingots, book molds are often used, producing slabbed ingots suitable for remelting or rolling. [14] 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. [15]
Although aluminium in its pure form has been produced as early as 1825, [16] 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. [17] Carl Josef Bayer created a multi-step process to convert raw Bauxite into alumina in 1888. [18] 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. [19] 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. [19]
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. [20]
The recycling of aluminium generally produces significant cost savings over the production of new aluminium, even when the cost of collection, separation and recycling are taken into account. [21] Over the long term, even larger national savings are made when the reduction in the capital costs associated with landfills, mines, and international shipping of raw aluminium are considered.
Recycling aluminium uses about 5% of the energy required to create aluminium from bauxite; [22] the amount of energy required to convert aluminium oxide into aluminium can be vividly seen when the process is reversed during the combustion of thermite or ammonium perchlorate composite propellant.
Aluminium die extrusion is a specific way of getting reusable material from aluminium scraps but does not require a large energy output of a melting process. In 2003, half of the products manufactured with aluminium were sourced from recycled aluminium material. [23]
The benefit with respect to emissions of carbon dioxide depends in part on the type of energy used. Electrolysis can be done using electricity from non-fossil-fuel sources, such as nuclear, geothermal, hydroelectric, or solar. Aluminium production is attracted to sources of cheap electricity. Canada, Brazil, Norway, and Venezuela have 61 to 99% hydroelectric power and are major aluminium producers. However the anodes widely used in the Hall–Héroult process are made of carbon and are consumed during aluminium production, generating large quantities of carbon dioxide, regardless of electricity source. [24] Efforts are underway to eliminate the need for carbon anodes. [25] The use of recycled aluminium also decreases the need for mining and refining bauxite.
The vast amount of aluminium used means that even small percentage losses are large expenses, so the flow of material is well monitored and accounted for financial reasons. Efficient production and recycling benefits the environment as well. [26]
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. [27] 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. [28]
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. [29] 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. [30] 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. [28]
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.
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. [31] 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. [32] 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. [33]
A recent study conducted within the United States has highlighted some ways that aluminium recycling has proven to have economic benefits, including:
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.
According to 2020 data from the International Aluminium Institute, the global recycling efficiency rate is 76%. Around 75% of the almost 1.5 billion tonnes of aluminium ever produced is still in productive use today. [34]
Brazil recycles 98.2% of its aluminium can production, equivalent to 14.7 billion beverage cans per year, [35] ranking first in the world, more than Japan's 82.5% recovery rate. Brazil has topped the aluminium can recycling charts eight years in a row. [36]
Country | 2021 [37] |
---|---|
![]() | 71% |
![]() | 94% |
![]() | 83% |
![]() | 80% |
![]() | 28% |
![]() | 51% |
![]() | 84% |
![]() | 89% |
![]() | 97% |
![]() | 48% |
![]() | 99% |
![]() | 60% |
![]() | 42% |
![]() | 91% |
![]() | 62% |
![]() | 90% |
![]() | 49% |
![]() | 87% |
![]() | 82% |
![]() | 50% |
![]() | 82% |
![]() | 92% |
![]() | 79% |
![]() | 36% |
![]() | 35% |
![]() | 58% |
![]() | 64% |
![]() | 67% |
![]() | 90% |
![]() | 92% |
![]() | 82% |
![]() | 76.1% |
Aside from recycled aluminium beverage cans, the majority of recycled aluminium comes in a 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 been recycled and require higher cost operations for the alloys to meet performance requirements. [38]
White dross, a residue from primary aluminium production and secondary recycling operations, usually classified as waste, [39] still contains useful quantities of aluminium which can be extracted industrially. [40] The process produces aluminium billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia) which spontaneously ignites on contact with air; [41] contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, however, the waste has found use as a filler in asphalt and concrete. [42]
Aluminium is a chemical element; it has symbol Al and atomic number 13. Aluminium has a density lower than that of other common metals, about one-third that of steel. It has a great affinity towards oxygen, forming a protective layer of oxide on the surface when exposed to air. Aluminium visually resembles silver, both in its color and in its great ability to reflect light. It is soft, nonmagnetic, and ductile. It has one stable isotope, 27Al, which is highly abundant, making aluminium the twelfth-most common element in the universe. The radioactivity of 26Al, a more unstable isotope, leads to it being used in radiometric dating.
An alloy is a mixture of chemical elements of which in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements. Most alloys are metallic and show good electrical conductivity, ductility, opacity, and luster, but may have properties that differ from those of the pure metals, such as increased strength or hardness. In some cases, an alloy may reduce the overall cost of the material while preserving important properties. In other cases, the mixture imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength.
In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold that contains a negative impression of the intended shape. The metal is poured into the mold through a hollow channel called a sprue. The metal and mold are then cooled, and the metal part is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.
The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) in molten cryolite and electrolyzing the molten salt bath, typically in a purpose-built cell. The Hall–Héroult process applied at industrial scale happens at 940–980 °C and produces 99.5–99.8% pure aluminium. Recycling aluminum requires no electrolysis, thus it is not treated in this way.
Steelmaking is the process of producing steel from iron ore and/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon are removed from the sourced iron, and alloying elements such as manganese, nickel, chromium, carbon and vanadium are added to produce different grades of steel.
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), the rest being a mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide must be further purified before it can be refined into aluminium.
Industrial processes are procedures involving chemical, physical, electrical, or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.
Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.
Scrap consists of recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap has monetary value, especially recovered metals, and non-metallic materials are also recovered for recycling. Once collected, the materials are sorted into types — typically metal scrap will be crushed, shredded, and sorted using mechanical processes.
Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low-melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. For higher melting point metals and alloys such as steel and silver, oxidized impurities melt and float making them easy to pour off.
A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.
Aluminium carbide, chemical formula Al4C3, is a carbide of aluminium. It has the appearance of pale yellow to brown crystals. It is stable up to 1400 °C. It decomposes in water with the production of methane.
Aluminium smelting is the process of extracting aluminium from its oxide, alumina, generally by the Hall-Héroult process. Alumina is extracted from the ore bauxite by means of the Bayer process at an alumina refinery.
Semi-solid metal casting (SSM) is a near net shape variant of die casting. The process is used today with non-ferrous metals, such as aluminium, copper, and magnesium, but also can work with higher temperature alloys for which no currently suitable die materials are available. The process combines the advantages of casting and forging. The process is named after the fluid property thixotropy, which is the phenomenon that allows this process to work. Simply, thixotropic fluids flow when sheared, but thicken when standing. The potential for this type of process was first recognized in the early 1970s. There are three different processes: thixocasting, rheocasting, thixomolding. SIMA refers to a specialized process to prepare aluminum alloys for thixocasting using hot and cold working.
A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: gas porosity, shrinkage defects, mould material defects, pouring metal defects, and metallurgical defects.
Recycling can be carried out on various raw materials. Recycling is an important part of creating more sustainable economies, reducing the cost and environmental impact of raw materials. Not all materials are easily recycled, and processing recyclable into the correct waste stream requires considerable energy. Some particular manufactured goods are not easily separated, unless specially process therefore have unique product-based recycling processes.
The aluminum industry in the United States in 2023 produced 860 thousand metric tons of aluminum from refined metal ore, at six smelters. In addition, US industry recycled 3.4 million tons of aluminum . Total annual imports of metal and alloy for use in secondary production stood at 2.6 million metric tons in the year to August 2023, with the previous decade seeing a fundamental shift toward recycled production.
Aluminium metal is very rare in native form, and the process to refine it from ores is complex, so for most of human history it was unknown. However, the compound alum has been known since the 5th century BCE and was used extensively by the ancients for dyeing. During the Middle Ages, its use for dyeing made it a commodity of international commerce. Renaissance scientists believed that alum was a salt of a new earth; during the Age of Enlightenment, it was established that this earth, alumina, was an oxide of a new metal. Discovery of this metal was announced in 1825 by Danish physicist Hans Christian Ørsted, whose work was extended by German chemist Friedrich Wöhler.
Aircraft recycling is the process of scrapping and disassembling retired aircraft, and re-purposing their parts as spare parts or scrap. Airplanes are made of around 800 to 1000 parts that can be recycled, with the majority of them made from metal alloys and composite materials. The two most common metal alloys are aluminum and titanium and the main composite material is carbon fiber.