Recycling and Processing Technology Flow and Detailed Explanation of Scrap Aluminum Regeneration

After the recycling of scrap aluminum, restoring it to pure aluminum is nearly unfeasible due to the varying aluminum content in the scrap. Even if achievable, the cost would likely be substantial. In contrast, our aluminum ingots generally refer to remelting-grade aluminum ingots with an aluminum content of 99.7%, which can be considered highly pure. Therefore, scrap aluminum is typically subjected to tempering processes and then utilized for direct processing into aluminum alloys or casting into aluminum rods.

Since the raw material for recycled aluminum mainly consists of various types of scrap aluminum, including scrap aluminum castings (primarily Al-Si alloys), scrap aluminum forgings (Al-Mg-Mn, Al-Cu-Mn alloys), profiles (Al-Mn, Al-Mg alloys), and waste cables (mainly pure aluminum), among others, sometimes even non-aluminum alloy scrap parts (such as Zn, Pb alloys) are mixed in. This poses significant challenges to the preparation of qualified recycled aluminum ingots.

The core issue in aluminum recycling production is how to blend these complex raw materials with various compositions into recycled aluminum ingots with the desired composition. Thus, the first step in the recycling aluminum production process is the sorting and classification of scrap aluminum. The finer the sorting and the more accurate the classification, the easier it is to achieve control over the chemical composition of recycled aluminum.

Scrap aluminum parts often contain embedded components, many of which are non-aluminum parts primarily made of steel or copper alloys. If these embedded components are not removed promptly during the melting process, they can introduce unwanted elements (such as Fe, Cu) into the recycled aluminum composition. Therefore, during the early stages of aluminum recycling, when the scrap aluminum is just melting, there must be a process to remove these embedded components (commonly referred to as the "iron removal process"). The more timely and thorough the removal of embedded components, the easier it is to control the chemical composition of recycled aluminum.

During the iron removal process, the temperature of the melt should not be too high. Higher temperatures can cause elements like Fe and Cu from the embedded components to dissolve into the aluminum melt. Scrap aluminum collected from different places often has dirt on its surface due to various reasons, and some might even be heavily rusted. These impurities and rusted surfaces can enter the melt during melting, forming slag phases and oxide inclusions in the melt, which can severely degrade the metallurgical quality of recycled aluminum. Removing these slag phases and oxide inclusions is also an important step in the recycling aluminum melting process.

Adopting multi-stage purification, such as conducting a preliminary purification followed by secondary rare earth refining, and then using inert gas to further enhance the refining effect, can effectively remove impurities from the aluminum melt. The oil and moisture adsorbed on the surface of scrap aluminum materials introduce a significant amount of gas into the aluminum melt. If these gases are not effectively removed, the metallurgical quality will be greatly reduced. Strengthening the degassing process in recycled aluminum production to reduce the gas content is an important measure to achieve high-quality recycled aluminum.

The recycling and processing of miscellaneous scrap aluminum typically involve the following four fundamental steps:

(1) Preparation of Scrap Aluminum Materials:

Firstly, scrap aluminum is initially categorized and sorted into different grades, such as pure aluminum, deformed aluminum alloys, cast aluminum alloys, mixed materials, etc. For scrap aluminum products, disassembly is necessary to remove steel and other non-ferrous metal parts connected to the aluminum. After disassembly, the materials undergo processes such as cleaning, crushing, magnetic separation, and drying to form scrap aluminum materials. For light and loose sheet-like scrap aluminum components, such as locking arms and speed gear shafts from vehicles, and aluminum shavings, hydraulic metal balers are used to compress and bundle them. In the case of aluminum-clad steel wire, the steel core should be separated first, and then the aluminum wire is coiled.

Iron impurities are harmful in aluminum smelting. Excessive iron content can lead to the formation of brittle metal crystals in aluminum, reducing mechanical properties and corrosion resistance. The iron content should generally be controlled below 1.2%. Scrap aluminum with iron content above 1.5% can be used as a deoxidizer in the steel industry, but it is rarely used for smelting commercial aluminum alloys with high iron content. Currently, there is no satisfactory method in the aluminum industry to effectively remove excess iron from scrap aluminum, especially in the form of stainless steel.

Scrap aluminum often contains organic non-metallic impurities such as paint, oil, plastic, rubber, etc. These impurities must be removed before recycling. For wire-type scrap aluminum, mechanical grinding or cutting and heating methods can be used to remove insulation. Currently, high-temperature incineration is commonly used in China to remove insulation from wire-type scrap aluminum, but it generates a large amount of harmful gases, causing severe air pollution.

By combining low-temperature baking with mechanical peeling, the insulation is softened by heat, reducing its mechanical strength, and then mechanically rubbed off, achieving the purpose of purification and recycling of insulation materials. Coatings, oil residues, and other contaminants on the surface of scrap aluminum utensils can be cleaned using organic solvents such as acetone. If cleaning is still insufficient, 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, most oils and coatings can be effectively removed.

For aluminum foil, it is difficult to effectively separate the aluminum foil layer and the paper fiber layer using ordinary waste paper pulping equipment. An effective separation method is to first heat and press the aluminum foil paper in a water solution, and then rapidly release it to a low-pressure environment for depressurization and mechanical stirring. This separation method can recycle both fiber pulp and aluminum foil.

The liquefaction separation of scrap aluminum is the future direction of aluminum metal recovery. It combines the pretreatment of scrap aluminum with re-melting, shortening the process flow, maximizing avoidance of air pollution, and greatly increasing the recovery rate of pure metal.

The device has a filter that allows gas particles to pass through. In the liquefaction layer, aluminum precipitates to the bottom. Organic substances such as paint attached to the scrap aluminum decompose into gas, tar, and solid carbon above 450°C, and then fully combust through the oxidation device inside the separator. The waste is mixed with the dissolving solution by rotating the drum, and impurities such as sand and stone are separated in the sand separation area. The dissolved solution carried by the waste through the recycling propeller returns to the liquefaction chamber.

(2) Batching:

Based on the preparation and quality of the scrap aluminum materials, and following the technical requirements of recycled products, the quantities of various materials are selected and calculated for mixing. Batching should consider the degree of oxidation and combustion loss of metals. Silicon and magnesium have higher combustion loss than other alloying elements, and the combustion loss rate of various alloying elements should be determined in advance through experiments. The physical specifications and surface cleanliness of scrap aluminum materials will directly affect the quality of the regenerated products and the actual metal yield. Except for scrap aluminum with unclean oil, up to 20% of effective components may enter the slag.

(3) Recycling of Deformed Aluminum Alloys:

Scrap aluminum alloys can be used to produce various deformed aluminum alloys, such as 3003, 3105, 3004, 3005, 5050, among others. Among these, the production of the 3105 alloy is the primary focus. To ensure that the chemical composition of the alloy meets technical requirements and is suitable for processes involving pressure forming, it may be necessary to add a certain amount of primary aluminum ingots during the alloying process.

(4) Recycling of Cast Aluminum Alloys:

Only a small portion of scrap aluminum materials are recycled into deformed aluminum alloys, with about 1/4 being used as deoxidizers in the steel industry. Most of the scrap aluminum is used to produce recycled aluminum alloys for casting. Widely used die-casting aluminum alloys like A380 and ADC10 are mostly produced using recycled scrap aluminum.

The main equipment for recycling aluminum includes melting furnaces and refining purification furnaces, typically using dedicated stationary furnaces fueled by oil or gas. The largest recycling aluminum enterprise in China is Shanghai Xinge Nonferrous Metals Co., Ltd., located in the suburbs of Shanghai. The company has two sets of 50-ton stationary melting furnaces, one set of 40-ton oil-fired stationary melting furnaces, and one 12-ton oil-fired rotary furnace. Small-scale enterprises may use pit kilns or crucible kilns for smelting.

In recent years, developed countries have continuously introduced a series of new technological innovations, such as low-cost continuous smelting and processing processes, which can upgrade low-grade scrap aluminum for manufacturing recycled aluminum ingots used for casting, die-casting, rolling, and producing master alloys. The largest ingots weigh up to 13.5 tons. Secondary alloy ingots (RSI) from remelting can be used to manufacture special thin plates for beverage cans, reducing the weight of each can to around 14 grams. Some recycled aluminum is even used to make frames for computer floppy disk drives.

In the process of recycling scrap aluminum, the smelting and treatment of recycled aluminum and its melt are key processes to ensure the metallurgical quality of recycled aluminum. Refining and purifying the aluminum melt can not only change the form of silicon in aluminum-silicon alloys and purify the aluminum melt, but also significantly improve the performance of aluminum alloys. The refining and purifying of the aluminum melt are currently often performed using chloride salts such as NaCl, NaF, KCl, and Na3AlF6, as well as chlorine or chlorine compounds such as C12 or C2C16.

Although refining the scrap aluminum melt with chlorine-containing substances yields good results, the byproducts such as AlCl3, HCl, and Cl can cause severe damage to humans, the environment, and equipment. In recent years, efforts have been made to improve the treatment process by using non-toxic or low-toxic refining agents to address environmental pollution, such as using N2 or Ar as refining agents, but the results have not been entirely satisfactory. "Environmentally friendly" refining agents available on the market mostly consist of carbonates, nitrates, and small amounts of C2C16, and they still release small amounts of nitrogen oxides and chlorine gas, thus not completely eliminating environmental pollution.

In recent years, a new process has emerged using rare earth alloys to refine, refine, and purify recycled aluminum. This process aims to completely solve the environmental pollution problem in the scrap aluminum recycling and smelting industry. The process makes full use of the interaction between rare earth elements and the aluminum melt, utilizing the refining, purifying, and modifying functions of rare earth elements on the aluminum melt. This integrated treatment of the aluminum melt achieves purification, refining, and modification, which is not only efficient but also effectively improves the metallurgical quality of recycled aluminum. Throughout the entire process, no harmful exhaust gases or other byproducts are produced.