The recycling processing of waste aluminum generally involves the following four basic processes.
(1) The preparation of scrap aluminum begins with the primary classification of scrap aluminum, which is stacked by grade, such as pure aluminum, deformed aluminum alloys, casting aluminum alloys, and mixed materials. For aluminum scrap products, disassembly should be carried out to remove steel and other non-ferrous metal parts connected to the aluminum material, followed by processes such as cleaning, crushing, magnetic separation, and drying to produce scrap aluminum. For lightweight, loose, flaky old aluminum parts, such as the locking arms on cars, speed gear sleeves, and aluminum shavings, they should be compacted into bales using a hydraulic metal baler. For steel-core aluminum stranded wire, the steel core should be separated first, and then the aluminum wire should be wound into rolls.
Iron impurities are very harmful to the smelting of waste aluminum. When the iron content is too high, it can form brittle metal crystals in the aluminum, thus reducing its mechanical properties and weakening its corrosion resistance. The iron content should generally be controlled to below 1.2%. Waste lead with an iron content of over 1.5% can be used as a deoxidizer in the steel industry, while commercially available aluminum alloys rarely use waste aluminum with high iron content for smelting. Currently, there is no successful method in the aluminum industry that can satisfactorily remove excess iron from waste aluminum, especially iron that exists in the form of stainless steel.
Waste aluminum often contains organic non-metallic impurities such as paint, oil, plastic, and rubber. Before smelting in a furnace, these must be removed. For wire-type waste aluminum, methods such as mechanical grinding, shearing peeling, thermal peeling, and chemical peeling can generally be employed to remove the insulation. Currently, domestic companies commonly use high-temperature ablation to remove insulators, which generates a large amount of harmful gases and severely pollutes the air. If a combination of low-temperature baking and mechanical peeling is used, the insulation can be softened with heat to reduce mechanical strength, and then mechanically rubbed off, achieving purification while allowing for the recovery of insulation materials. The coatings, oil stains, and other contaminants on the surfaces of waste aluminum utensils can be cleaned with organic solvents like acetone. If they cannot be removed, a paint stripping furnace should be used. The maximum temperature of the paint stripping furnace should not exceed 566°C; as long as the waste material stays in the furnace for a sufficient time, general oils and coatings can be completely removed.
For aluminum foil paper, it is difficult to effectively separate the aluminum foil layer from the paper fiber layer using ordinary waste paper pulping equipment. An effective separation method is to first place the aluminum foil paper in a water solution, heat and pressurize it, and then quickly drain it to a low-pressure environment to reduce the pressure, followed by mechanical stirring. This separation method not only allows for the recovery of fiber pulp but also the recovery of aluminum foil.
The liquefaction and separation of scrap aluminum is the future development direction for the recycling of metallic aluminum. It combines the pretreatment of scrap aluminum materials with remelting, which not only shortens the process flow but also minimizes air pollution, while significantly improving the recovery rate of pure metal.
The device has a filter that allows gas particles to pass through. In the liquefaction layer, aluminum precipitates at the bottom, and organic substances such as paint attached to waste aluminum decompose into gas, tar, and solid carbon at temperatures above 450°C, which are then completely combusted in the oxidation device inside the separator. The waste is stirred by a rotating drum, mixed with the solvent in the chamber, and impurities like sand and gravel are separated into the sand and gravel separation area, while the dissolved solution carried out by the waste is returned to the liquefaction chamber through the recovery screw.
(2) The raw materials are selected and calculated based on the preparation and quality conditions of the scrap aluminum, according to the technical requirements of the recycled products. The formulation should take into account the degree of oxidation and burning loss of metals, with silicon and magnesium experiencing greater oxidation and burning losses than other alloying elements. The burning loss rate of various alloying elements should be determined experimentally in advance. The physical specifications and surface cleanliness of scrap aluminum will directly affect the quality of the recycled products and the metal yield. Poorly degreased scrap aluminum can result in up to 20% of effective components entering the slag.
(3) Scrap aluminum alloys that can be produced into deformed aluminum alloys include 3003, 3105, 3004, 3005, 5050, among which the main one is the 3105 alloy. To ensure that the chemical composition of the alloy materials meets technical requirements and the needs of pressure processing, it may be necessary to add a certain amount of primary aluminum ingots.
(4) Only a small portion of scrap aluminum alloy is recycled into deformed aluminum alloys; about 1/4 is recycled as deoxidizers for steelmaking, and most is used in recycled casting aluminum alloys. Die-casting aluminum alloys such as A380 and ADC10, widely used in the U.S. and Japan, are essentially recycled from scrap aluminum.
In the recycling process of waste aluminum, the smelting and treatment of recycled aluminum is a key process to ensure the metallurgical quality of recycled aluminum. The modification and refining of aluminum melt can not only change the morphology of silicon in aluminum-silicon alloys, purifying the aluminum melt, but also significantly improve the properties of aluminum alloys. Currently, the refinement and purification of aluminum melt often use chlorides and fluorides such as NaCl, NaF, KCl, and Na3AlF6, and some treatments utilize C12 or C2C16.
