The recovery of waste aluminum generally goes through the following four basic processes.
(1) Preparation of scrap aluminum First, scrap aluminum is preliminarily classified and stacked in stages, such as pure aluminum, wrought aluminum alloy, cast aluminum alloy, and mixture. For waste aluminum products, it should be dismantled to remove the nonferrous metal parts such as steel connected with aluminum, and then made into waste aluminum materials through cleaning, crushing, magnetic separation, drying and other processes. Thin and loose aluminium scrap, such as lock arm, speed change gear bushing and aluminium scrap on automobile, should be packed with hydraulic metal baling press. 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 waste aluminum. When there is too much iron, brittle metal crystals will be formed in aluminum, which will reduce its mechanical properties and weaken its corrosion resistance. Iron content should generally be controlled below 65438 0.2%. Waste lead with iron content above 1.5% can be used as deoxidizer in iron and steel industry, and commercial aluminum alloys are rarely smelted with waste aluminum with high iron content. At present, there is no very successful method to remove excess iron from waste aluminum, especially iron in the form of stainless steel.
Waste aluminum often contains organic and nonmetallic impurities such as paint, oil, plastic and rubber. It must be removed before melting in the furnace. For wire waste aluminum, measures such as mechanical grinding or shearing stripping, heating stripping and chemical stripping can be generally adopted to remove the foreskin. At present, domestic enterprises often use high temperature ablation to remove insulators, which will produce a lot of harmful gases and seriously pollute the air. If the method of combining low-temperature baking and mechanical stripping is adopted, firstly, the insulator is softened by heat energy and its mechanical strength is reduced, and then it is stripped by mechanical kneading, so that the purpose of purification can be achieved and the insulator material can be recovered. The coating, oil stain and other pollutants on the surface of waste aluminum utensils can be cleaned with organic solvents such as acetone. If it still cannot be removed, the paint should be removed with a paint remover. The maximum temperature of paint stripping furnace should not exceed 566℃. As long as the waste remains in the furnace for enough time, the general oil and coating can be completely removed.
For aluminum foil paper, it is difficult to effectively separate aluminum foil layer and paper fiber layer with ordinary waste paper pulping equipment. The effective separation method is to put aluminum foil paper into water solution, heat and pressurize it, and then quickly discharge it into a low-pressure environment for decompression and mechanical stirring. This separation method can recover fiber pulp and aluminum foil.
Liquefaction separation of waste aluminum is the development direction of metal aluminum recovery in the future. It combines the pretreatment of waste aluminum impurities with recasting, which not only shortens the technological process, but also avoids air pollution to the maximum extent and greatly improves the recovery rate of pure metal. The working principle of the waste aluminum liquefaction separation device is shown in figure 1- 18. There is a filter that allows gas particles to pass through the device. In the liquefaction layer, aluminum is deposited at the bottom, and organic substances such as paint attached to waste aluminum are decomposed into gas, tar and solid carbon above 450℃, and then completely burned by the oxidation device inside the separator. The waste is stirred by the drum and mixed with the dissolved liquid in the silo, and impurities such as sand and gravel are separated into the sand and gravel separation area, and the dissolved ferry brought out by the waste returns to the liquefied silo through the recovery propeller.
(2) According to the preparation and quality of waste aluminum materials and the technical requirements of recycled products, the ingredients are selected and mixed, and the dosage of various materials is calculated. The degree of oxidation and burning loss of metals should be considered when batching. The oxidation burning loss of silicon and magnesium is greater than that of other alloying elements, and the burning loss rate of various alloying elements should be determined by experiments in advance. The physical specifications and surface cleanliness of waste aluminum will directly affect the quality of recycled finished products and the metal yield. As much as 20% of the effective components of waste aluminum that is not cleaned by oil removal will enter the slag.
(3) Wrought aluminum alloys such as 3003, 3 105, 3004, 3005, 5050 can be produced from waste aluminum alloys used for recycling deformed aluminum alloys, among which 3 105 alloy is mainly produced. In order to ensure that the chemical composition of the alloy material meets the technical requirements and the technological needs of pressure working, a part of primary aluminum ingots should be added when necessary.
(4) The process flow of recycled cast aluminum alloy is shown in figure 1- 19. Only a small part of the waste aluminum material is regenerated into deformed aluminum alloy, about 1/4 is regenerated into deoxidizer for steelmaking, and most of it is used to regenerate aluminum alloy for casting. Die-casting aluminum alloys A380 and ADCl0, which are widely used in the United States, Japan and other countries, are basically recovered from waste aluminum.
The main equipment of secondary aluminum is smelting furnace and refining and purifying furnace, and generally static furnace dedicated to oil or gas is used. The largest secondary aluminum enterprise in China is Shanghai Xinge Nonferrous Metals Co., Ltd. located in the suburb of Shanghai. The company has two sets of 50-ton smelting static furnaces and one set of 40-ton fuel smelting static furnaces. 12t fuel rotary furnace. Small enterprises can use tank kiln, crucible kiln and other smelting.
In recent years, developed countries have continuously introduced a series of new technological innovations in production, such as low-cost continuous smelting and treatment processes, which can upgrade low-grade waste aluminum and use it to manufacture recycled aluminum ingots for casting, die casting, rolling and master alloy. The maximum ingot weight is 13.5t, in which the remelted secondary alloy ingot (RSI) can be used to manufacture special plates for cans, and the quality of the plates reduces the weight of each can to about 14g; Some recycled aluminum is even used to make the frame of computer floppy drive.
In the process of recycling waste aluminum, the melting and melt treatment of recycled aluminum are the key processes to ensure the metallurgical quality of recycled aluminum. Modification and refinement of aluminum melt can not only change the form of silicon in aluminum-silicon alloy and purify aluminum melt, but also greatly improve the properties of aluminum alloy. At present, most of the aluminum melts are treated with chloride salts and fluoride salts such as sodium chloride, sodium fluoride, KCI, Na3AIF6, and some of them are treated with C 12 or C2C 16. For processing.
Although the effect of refining waste aluminum melt with chlorine-containing substances is good, its by-products AICI3, HCl and Cl will cause serious harm to human body, environment and equipment. In recent years, people have tried to improve the treatment process and choose non-toxic and low-toxic refining metamorphic materials to solve the environmental pollution problem, such as N2 and Ar as refining agents, but the effect is not ideal. The so-called "pollution-free" refining agent in the market consists of carbonate, nitrate and a small amount of C2C 16. However, due to the emission of a small amount of nitrogen oxides and chlorine, environmental pollution cannot be completely eliminated. In recent years, the newly developed process of modifying, refining and refining recycled aluminum with rare earth alloys is expected to completely solve the environmental pollution problem of waste aluminum recovery and smelting industry. The process makes full use of the interaction between rare earth elements and aluminum melt to refine, purify and modify aluminum melt, which can realize the integrated treatment of aluminum melt purification, refinement and modification, which is not only concise and efficient, but also can effectively improve the metallurgical quality of recycled aluminum. The whole treatment process will not produce harmful waste gas and other by-products.
References:
Jintan shuibei nonferrous metal foundry
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