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At present, the scrap aluminum used by aluminum recycling plants in our country comes mainly from two sources: scrap aluminum imported from abroad, and scrap aluminum generated domestically. 1. Imported scrap aluminum In recent years, China has imported large quantities of scrap aluminum from abroad. Regarding the composition of imported scrap aluminum, most of it is mixed, with only a few having clear classifications. It can generally be divided into the following main categories: ① Waste aluminum of a single type. This type of waste aluminum usually consists of certain types of waste components, such as pistons from internal combustion engines, car gearbox casings, car wheel hubs, and front and rear bumpers of cars. Aluminum doors and windows, etc. These scrap aluminums are clearly classified upon import, consist of a single type, and are imported in bulk; therefore, they are high-quality raw materials for recycled aluminum. ②Scrap aluminum sheets Scrap aluminum sheets, also simply referred to as sheets, are a higher-quality type of scrap aluminum. It is called slices because many developed countries use mechanical methods to break down end-of-life cars, waste equipment, and various types of used household appliances into fragments, and then carry out mechanical sorting; the aluminum waste that results from this sorting process is what is known as aluminum waste slices. Additionally, the recycling sector also uses crushing methods to break down larger aluminum waste components into fragments; such fragments are also referred to as aluminum waste chips. Recycled aluminum sheets are easy to transport and sort, have a relatively pure composition, and thus represent high-quality recycled aluminum material. In the current international trade of scrap aluminum, sliced materials account for the largest share, and various types of slices are moving toward standardization. In terms of the composition of the scrap, it is generally classified into several grades. The higher-grade scrap consists of relatively pure mixtures of various types of aluminum waste and their alloys; most of this can be fed directly into the melting furnace without any processing. Lower-grade scrap contains varying amounts of other impurities, with aluminum waste accounting for 80% to 90% or more of its composition. The main impurities are non-ferrous metals such as scrap steel and copper, as well as small amounts of scrap rubber. After manual sorting, pure aluminum waste material is obtained. Smelting waste aluminum in slabs is also relatively easy; it is convenient to feed them into the furnace during melting, impurities can be removed easily, less flux is required, the metal recovery rate is high, energy consumption is low, and processing costs are low as well. This makes it very popular among users, and generally, large-scale aluminum recycling plants use slabs as their main raw material. High-quality scrap aluminum sheets are more expensive than other types of scrap aluminum; they are suitable for large-scale modern enterprises, and it is difficult to obtain them in the international market. As a result, aside from wholly-owned or joint venture companies that import them on their own, few aluminum recycling plants in China use this type of scrap material. ③Mixed scrap aluminum: This type of scrap aluminum has a complex composition and varies in physical shape. In addition to aluminum scrap, it also contains certain amounts of scrap steel, scrap lead, scrap zinc and other metals, as well as scrap rubber, scrap wood, scrap plastic, stones, etc. Sometimes, some aluminum scrap and steel scrap are mechanically combined together. This kind of waste material has a complex composition; some pieces of aluminum scrap are larger in size and have a smooth surface, which makes sorting easier. Such waste must undergo pre-sorting before smelting, that is, scrap steel and other impurities are manually removed. ④Aluminum-containing scrap material after incineration: This type of aluminum material is a lower-grade form of aluminum-containing waste; it consists of crushed pieces from various discarded household appliances, and it is obtained by separating out some of the steel waste before incinerating the remaining material. The purpose of incineration is to remove combustible materials such as waste rubber and waste plastic. Such aluminum-containing waste generally has an aluminum content of around 40% to 60%; the rest consists mainly of waste materials such as bricks and stones, scrap steel, and very small amounts of non-ferrous metals like copper (copper wires). The size of aluminum particles is generally under 10 centimeters. During the burning process, some aluminum along with substances with low melting points such as zinc, lead, and tin melt, forming a surface-glass-like substance that is difficult to distinguish with the naked eye, making it impossible to separate them. ⑤Mixed scrap aluminum – This type of waste aluminum is of the lowest quality; its composition is very complex. It contains 40% to 50% various types of aluminum scrap, with the remainder consisting of scrap steel, small amounts of lead and copper, as well as a large amount of trash, stones and soil, waste plastic, and waste paper. Soil accounts for about 25%, scrap steel for 10% to 20%, and stones for 3% to 5%. 2. Recycled scrap aluminum domestically Most of the waste aluminum recycled in China is pure, containing little to no impurities (except for those introduced intentionally). It can be broadly divided into three categories: what recycling agencies refer to as recycled used aluminum, recycled virgin aluminum, and recycled alloy aluminum. Recycled virgin aluminum mainly consists of recycled cast aluminum and recycled alloy aluminum. It is mostly made from waste machine parts (such as used car parts, waste molds, waste cast aluminum pots and pans, internal combustion engine pistons, etc.). Waste mature aluminum generally refers to waste aluminum with an aluminum content of over 99% (such as waste cables, waste household tableware, kettles, etc.). Waste alloy aluminum (such as waste aircraft aluminum, aluminum frames, etc.). In terms of the production sector, it can be divided into domestic waste aluminum and industrial waste aluminum. ①Waste aluminum originating from everyday life includes waste household tableware, waste cast aluminum pots and pans, waste aluminum components from old household appliances, waste wires, and waste packaging materials. Waste machine parts made of aluminum and its alloys from scrapped mechanical and electrical equipment (such as waste auto parts, waste aircraft aluminum, waste molds, waste internal combustion engine pistons, waste cables, waste aluminum pipes, etc.). ②Waste aluminum generated by manufacturing enterprises. This type of waste aluminum is generally referred to as new waste; it mainly includes aluminum and its alloys that are produced as waste during the manufacturing process, as well as scraps and defective materials that arise during the processing of aluminum products ； Scrap, aluminum dust, and waste products of aluminum and its alloys generated by machining systems ； Waste aluminum cables from cable factories, as well as sprues and waste castings generated in the casting industry, are all high-quality aluminum waste materials, with the exception of debris contaminated with oil. If such waste is properly sorted and stored at the point of generation within a company, its value for reuse is very high. ③The slag generated during the melting of aluminum and aluminum alloys, as well as during their processing, is what is commonly referred to as aluminum dross. Aluminum dross is produced wherever molten aluminum is present; for example, it is generated in large quantities during the production, melting, processing of aluminum, and the recycling of scrap aluminum. Particularly large amounts of aluminum dross are produced during the recycling and melting of mixed scrap aluminum. The more complex the composition of the scrap aluminum and the higher the level of impurities, the greater the surface contamination, and thus the more aluminum dross is produced. The aluminum content in aluminum ash is related to the coating agent used and the smelting technique; it is generally below 10%, with higher values reaching over 20%. Basic principles for the pretreatment of domestic scrap aluminum Due to the fairly complex composition of scrap aluminum, it is necessary to carry out appropriate pretreatment of this raw material before using it as the main ingredient in the secondary processing of alloys. Theoretically, all impurities should be removed completely; however, in actual industrial processes, cost considerations mean that only the most significant problems can be addressed. The general principle for handling is to classify raw materials into broad categories based on their material composition, with the criterion being to bring them as close as possible to the composition of a certain alloy grade. The classified aluminum alloy waste is subjected to necessary disassembly to remove large pieces of non-aluminum metals or organic impurities. Perform the necessary cleaning of the raw materials, including washing with water or organic solvents, sandblasting, etc. The waste and scrap aluminum that has undergone the above treatments can then be used as a basic raw material for alloy melting.