The recycling and processing of scrap and miscellaneous aluminum generally undergo the following four basic procedures.
(1) Preparation of scrap aluminum: First, conduct initial sorting of the scrap aluminum and categorize it for storage, such as pure aluminum, deformed aluminum alloys, cast aluminum alloys, and mixed materials. For scrap aluminum products, they should be disassembled to remove steel and other non-ferrous metal parts connected to the aluminum, and then processed through cleaning, crushing, magnetic separation, drying, and other steps to make scrap aluminum material. For thin, loose sheet-like scrap aluminum parts, such as locking arms on cars, speed gear bushings, and aluminum shavings, they should be compressed 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 is coiled.
Iron impurities are very harmful to the smelting of scrap aluminum. When the iron content is excessive, brittle metal crystals form in the aluminum, reducing its mechanical properties and weakening its corrosion resistance. The iron content is generally controlled below 1.2%. Scrap aluminum with an iron content above 1.5% can be used as a deoxidizer in the steel industry, but commercial aluminum alloys rarely use scrap aluminum with a high iron content for smelting. Currently, there is no highly effective method in the aluminum industry for satisfactorily removing excess iron from scrap aluminum, especially when the iron is present in the form of stainless steel.
Waste aluminum often contains organic non-metallic impurities such as paint, oil, plastic, and rubber. Before remelting, these must be removed. For scrap aluminum in the form of wires, mechanical grinding or shearing removal, heat stripping, or chemical stripping can generally be used to remove the insulation. Currently, domestic companies commonly use high-temperature incineration to remove insulating materials. This process produces a large amount of harmful gases, severely polluting the air. If a combination of low-temperature baking and mechanical stripping is used, the insulating material is first softened by heat, reducing its mechanical strength, and then mechanically removed by rubbing. This method not only achieves purification but also allows for the recovery of insulation materials. Coatings, oil stains, and other contaminants on the surface of aluminum utensils can be cleaned using organic solvents such as acetone. If they still cannot be removed, a paint removal furnace should be used. The maximum temperature of the paint removal furnace should not exceed 566℃. As long as the scrap materials remain in the furnace long enough, most oils and coatings can be thoroughly 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 method is to first place the aluminum foil paper in an aqueous solution for heating and pressurization, then quickly release it into a low-pressure environment for decompression, and perform mechanical stirring. This separation method allows for the recovery of both paper fibers and aluminum foil.
The liquefaction and separation of scrap aluminum is the future direction for recycling metallic aluminum. It combines the pretreatment of scrap aluminum impurities with remelting, which not only shortens the process but also minimizes air pollution and greatly increases 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 the scrap aluminum decompose into gas, tar, and solid carbon at temperatures above 450°C, which are then completely burned through the oxidation device inside the separator. The scrap material is stirred by a rotating drum and mixed with the solution in the tank, with impurities such as sand and gravel separated into the sand and gravel separation zone, and the solution carried out by the scrap material is returned to the liquefaction tank via a recovery propeller.
(2) Based on the preparation and quality of the scrap aluminum, the ingredients are selected and the amount of each type of material is calculated according to the technical requirements of the recycled products. When preparing the mix, the degree of metal oxidation and burn loss should be considered; the burn loss of silicon and magnesium is greater than that of other alloying elements, and the loss rates 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 product and the actual metal yield. For scrap aluminum that is not cleaned of oil, up to 20% of the effective components may end up in the slag.
(3) Recycled deformed aluminum alloys. Scrap aluminum alloys used for producing deformed aluminum alloys include 3003, 3105, 3004, 3005, 5050, etc., with 3105 alloy being the main product. To ensure that the chemical composition of the alloy meets technical requirements and process needs for pressure forming, it may be necessary to add a portion of primary aluminum ingots.
(4) Recycled casting aluminum alloys. Only a small portion of scrap aluminum is recycled into deformed aluminum alloys; about 1/4 is recycled into deoxidizers for steelmaking, while most is used for recycled casting aluminum alloys. In countries like the United States and Japan, widely used die-casting aluminum alloys such as A380 and ADC10 are basically recycled from scrap aluminum.
