The recycling and processing of scrap miscellaneous aluminum generally goes through the following four basic processes.
1) Preparation of scrap aluminumFirst of all, the scrap aluminum is primarily classified and stacked in grading, such as pure aluminum, deformed aluminum alloy, cast aluminum alloy, mixture, etc. For scrap aluminum products, they should be dismantled, the steel and other non-ferrous metal parts connected to the aluminum should be removed, and then scrap aluminum should be made into scrap aluminum through cleaning, crushing, magnetic separation, drying and other processes. For light and loose sheet scrap aluminum parts, such as locking arms, speed gear bushings and aluminum chips on automobiles, they should be pressed into bales with hydraulic metal balers. For steel core aluminum stranded wire, the steel core should be separated first, and then the aluminum wire should be wound into a coil.
Iron impurities are very harmful to the smelting of aluminum scrap, and when there is too much iron, brittle metal crystals will form in aluminum, thereby reducing its mechanical properties and weakening its corrosion resistance. The iron content should generally be controlled below 1.2%. For scrap lead with an iron content of more than 1.5%, it can be used as a deoxidizer in the steel industry, and commercial aluminum alloys rarely use scrap aluminum smelting with high iron content. At present, there is no very successful method in the aluminium industry to satisfactorily remove excess iron from aluminium scrap, especially in the form of stainless steel.
Aluminum scrap often contains organic and non-metallic impurities such as paint, oil, plastics, and rubber. Before returning to the furnace for smelting, it must be removed. For wire aluminum scrap, the foreskin can generally be removed by mechanical grinding or shear peeling, heating peeling, chemical peeling and other measures. At present, domestic enterprises often use high-temperature ablation to remove insulators, and a large number of harmful gases will be produced during the ablation process, which will seriously pollute the air. If the combination of low-temperature baking and mechanical stripping is adopted, the insulator is first softened by heat energy, the mechanical strength is reduced, and then peeled off by mechanical kneading, which can not only achieve the purpose of purification, but also recover the insulator material. The coating, oil stains and other contaminants on the surface of scrap aluminum utensils can be cleaned with organic solvents such as acetone, and if they still cannot be removed, they should be removed by a paint stripping furnace. 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 period of time, the general oil and coating can be removed.
For aluminum foil, it is difficult to effectively separate the aluminum foil layer and the paper fiber layer with ordinary waste paper pulping equipment, and the effective separation method is to put the aluminum foil paper in an aqueous solution to heat and pressurize, and then quickly discharge to a low-pressure environment to decompress and carry out mechanical stirring. This separation method allows for the recovery of both fiber pulp and aluminum foil.
The liquefaction and separation of aluminum scrap is the development direction of recycling aluminum metal in the future, which combines the pretreatment of aluminum scrap miscellaneous materials with remelting and casting, which not only shortens the process flow, but also avoids air pollution to the greatest extent, and greatly improves the recovery rate of net metal.
In the liquefaction layer, the aluminium precipitates at the bottom, and the organic matter such as paint attached to the scrap aluminium decomposes into gas, tar and solid charcoal above 450°C, and then completely burns through the oxidation device inside the separator. The waste is stirred by the rotating drum and mixed with the dissolved liquid in the silo, and the impurities such as sand and gravel are separated into the sand and gravel separation area, and the dissolving ferry brought out by the waste is returned to the liquefaction bin through the recovery propeller.
2) According to the preparation and quality status of scrap aluminum materials, and in accordance with the technical requirements of recycled products, the ingredients are selected and matched and the amount of various materials is calculated. The oxidative burn loss of silicon and magnesium is greater than that of other alloying elements, and the burn loss of various alloying elements should be determined by experiments in advance. The physical specifications and surface cleanliness of the scrap aluminum will directly affect the quality of the recycled finished product and the actual yield of the metal, and the scrap aluminum that is not clean in oil removal will have up to 20% of the active ingredients into the slag.
(3) The deformed aluminum alloys that can be produced by scrap aluminum alloys for recycled deformed aluminum alloys include 3003, 3105, 3004, 3005, 5050, etc., of which 3105 alloys are mainly produced. In order to ensure that the chemical composition of the alloy material meets the technical requirements and the technological needs of pressure processing, a part of primary aluminum ingots should be added if necessary.
(4) Only a small part of the recycled casting aluminum alloy scrap is recycled into deformed aluminum alloy, and about 1/4 is regenerated into deoxidizer for steelmaking, and most of it is used for recycled casting aluminum alloy. The die-casting aluminum alloy A380 and ADCl0 widely used in the United States, Japan and other countries are basically recycled from scrap aluminum.
The main equipment for recycled aluminum is a smelting furnace and a refining purification furnace, which generally uses a special static furnace for oil or gas. China's largest recycled aluminum enterprise is Shanghai Xinge Nonferrous Metals Co., Ltd., located in the suburbs of Shanghai, which has two sets of 50t smelting static furnaces and one set of 40t oil-fired smelting static furnaces; A 12-ton oil-fired rotary furnace. Small enterprises can use pool kilns, crucible kilns and other smelting.
In the process of recycling aluminum scrap, the smelting of recycled aluminum and the treatment of melt are the key processes to ensure the metallurgical quality of recycled aluminum. The deterioration and refining purification of aluminum melt can not only change the form of silicon in aluminum-silicon alloy, purify aluminum melt, but also greatly improve the performance of aluminum alloy. At present, chloride and fluorine salts such as Nacl, NaF, KCI and Na3AIF6 are used for refining and purification of aluminum melt, and C12 or C2C16 is also used for treatment.
Although the effect of refining scrap aluminum melt with chlorine-containing substances is good, its by-products such as AICI3, HCl and Cl will cause serious damage to the human body, the environment and equipment. In recent years, people are trying to improve the treatment process, the selection of non-toxic, low-toxicity refining and metamorphic materials to solve the problem of environmental pollution, such as the use of N2, Ar and other refining agents, but the effect is not satisfactory. The so-called "pollution-free" refining agent on the market is basically composed of carbonate, nitrate and a small amount of C2C16, because there is still a small amount of nitrogen oxides and chlorine emissions, and it cannot completely eliminate environmental pollution.
