Scrap Aluminum Recycling Pre-treatment and Melting-Recovery Technology

With the increasing global emphasis on sustainable resource utilization and environmental protection, Scrap Aluminum recycling has become increasingly important in the aluminum industry. This article will deeply explore the pre-treatment and melting-recovery technology for scrap Aluminum, elaborating on the entire process from the recycling stage to the final casting products, including crushing, sorting, de-greasing, preheating, melting, purification, filtration, casting, and disposal of aluminum slag, aiming to provide comprehensive technical references for related industries and promote the efficient development of the scrap Aluminum recycling and utilization industry.

I. Introduction

Aluminum, as an important metal widely used in construction, transportation, electronics and other fields, has a continuously increasing demand. However, the limited resources of bauxite and the high energy consumption and high pollution in the traditional electrolytic aluminum production process have made scrap Aluminum recycling and utilization an important way to alleviate resource pressure, reduce environmental pollution, and achieve sustainable development of the aluminum industry. The pre-treatment and melting-recovery technology for waste aluminum covers multiple complex and critical steps, and the technical optimization of each step is of great significance for improving the quality and output of recycled aluminum and enhancing resource utilization efficiency.

II. Scrap Aluminum Recycling

2.1 Recycling Sources

The main sources of scrap Aluminum in China are imports and domestic recycling. Due to the limited accumulation of aluminum products and components domestically, waste aluminum recycling is difficult to meet the development needs of the industry. Therefore, imported scrap aluminum has become the main raw material source for recycling aluminum enterprises. Imported scrap aluminum not only provides enterprises with abundant aluminum resources but also helps reduce the mining of bauxite in China, lower energy consumption and environmental pollution.

Domestic recycled scrap Aluminum includes domestic scrap aluminum from households and industrial scrap aluminum. Domestic scrap Aluminum comes from various sources, such as scrapped aluminum furniture, tableware, and aluminum components from electronic appliances, automobiles, and mechanical equipment; industrial scrap aluminum is generated during aluminum production processes, such as casting risers, scrap castings, and edge and corner materials, defective products, and aluminum shavings during aluminum processing. These industrial scrap aluminum has a higher quality and can be directly recycled if properly classified and stored.

2.2 Recycling Significance

The development of the scrap aluminum recycling industry has significant economic value. Producing 1 ton of recycled aluminum requires only 5% of the energy and 2.1% of the carbon emissions compared to electrolytic aluminum production, while saving 3.4 tons of standard coal, 22 cubic meters of water, and reducing 20 tons of solid waste emissions. This not only reduces production costs but also brings considerable economic benefits to enterprises.

From an environmental perspective, scrap aluminum recycling reduces the damage to the environment caused by bauxite mining and lowers the pollutant emissions during electrolytic aluminum production, playing a positive role in environmental protection. In addition, the development of the scrap aluminum recycling industry can create a large number of employment opportunities and promote the sustainable development of the economy and society.

Energy Consumption and Greenhouse Gas Emissions: Primary Aluminum vs. Recycled Aluminum

III. Pre-treatment Steps

3.1 Crushing

Large pieces of scrap aluminum need to be crushed to facilitate subsequent transportation and furnace loading, and to separate non-aluminum impurities. Different shapes and materials of scrap aluminum require appropriate crushing equipment. Long strips of scrap aluminum can be cut into blocks using shearing machines; large-section scrap aluminum requires large-tonnage hydraulic shearing machines; scrap aluminum parts such as automotive cylinder blocks and transmissions can be crushed using jaw crushers, hammer crushers, etc.; scrap wires and cables, due to their coating materials, need to be processed using dedicated stripping machines and cutting machinery. The selection of equipment and the optimization of operating parameters in the crushing process are crucial, as they directly affect the crushing effect and the difficulty of subsequent processing.

3.2 Sorting

Sorting is the key step in waste aluminum pre-treatment, aiming to improve the purity of scrap aluminum. Magnetic separation utilizes the magnetic properties of iron-containing waste materials to remove iron nails, iron blocks, and other impurities from waste aluminum. The common magnetic separation process is the cross method on a conveyor belt, where scrap aluminum pieces are attracted by the magnetic field and iron-containing waste materials are separated.

Flotation is mainly used to remove rubber, plastic, wood, and soil from waste aluminum. Using water as the medium, the waste aluminum is sent into the water by the spiral propeller. The light substances will float up and be taken away, while the soil will dissolve in the water and enter the sedimentation tank. The wastewater after sedimentation can be recycled, which not only saves water resources but also reduces environmental pollution.

For the non-ferrous metal waste materials such as copper, zinc, lead, and tin contained in the scrap aluminum, the parabolic separation method or heavy medium separation method can be adopted for separation. The parabolic separation method separates different objects based on the principle that they fall at different points under the same force. However, this method has a relatively high equipment cost. The heavy medium separation method separates the waste aluminum by making it float on the heavy medium while the heavy metal waste materials settle at the bottom, thereby achieving separation.

3.3 De-painting

Most of the waste aluminum surfaces are covered with substances such as paint, coating, and oil stains. If these substances are not treated and directly returned to the furnace for melting, they will increase the oxidation and loss of aluminum and the gas and slag content in the molten metal, increasing the difficulty of molten metal purification. The rotary kiln roasting method is a commonly used de-painting method. The scrap aluminum is placed in a rotating rotary kiln, and the heat generated by the hot air and the carbonization process of the covering material is used to gradually carbonize the covering material on the surface of the scrap aluminum. The rotation of the rotary kiln prompts the scrap aluminum to collide with each other, causing the carbonized substances to fall off from the surface of the waste aluminum, thereby achieving de-painting. During the de-painting process, the temperature, rotation speed, and roasting time of the rotary kiln need to be strictly controlled to ensure the de-painting effect and avoid excessive oxidation of the scrap aluminum.

IV. Preparation before smelting

4.1 Preheating

Preheating the waste aluminum has multiple advantages. Firstly, preheating can significantly reduce the heat consumption during the smelting process, thereby improving energy utilization efficiency. By preheating, the scrap aluminum reaches a certain temperature before entering the smelting furnace, which can shorten the smelting time and reduce energy consumption. Secondly, preheating helps reduce the moisture content in the waste aluminum, reducing the gas generated during smelting, thereby reducing the formation probability of defects such as pores and slag in the molten aluminum. The preheating methods can be flame heating or electric heating, and the specific choice should be determined based on the actual production conditions and the characteristics of the scrap aluminum of the enterprise.

4.2 Ingredient Mixing

Reasonable ingredient mixing is directly related to the composition and quality of the recycled aluminum alloy. During ingredient mixing, the goal should be to produce qualified products. Combining the waste aluminum in the factory’s inventory with other raw materials, the ratio of furnace materials should be optimized. For products with narrow alloy composition requirements and low content of impurity elements, pure aluminum ingots should be used as the main ingredient for mixing; for products with a wide range of alloy composition and less strict control of impurity elements, scrap aluminum materials can be used as the main ingredient for mixing. When the composition deviates, pure aluminum ingots can be added to dilute it, or intermediate alloys can be added to supplement the alloy composition, ensuring that the final product quality meets the requirements.

V. Smelting process

5.1 Smelting equipment

Domestic recycled aluminum enterprises mainly use regenerative gas-reflecting furnaces for smelting scrap aluminum. This equipment can fully utilize the residual heat, has a high thermal efficiency, and effectively reduces energy consumption. However, due to direct contact between the flame and the waste aluminum, the oxidation and loss of the scrap aluminum are relatively serious. In contrast, the double-chamber furnace divides the furnace chamber into a heating chamber and a melting chamber, avoiding the combustion flame from contacting the scrap aluminum, thereby reducing the loss of the waste aluminum and improving the recovery rate. When choosing smelting equipment, enterprises should comprehensively consider production scale, product quality requirements, and energy costs to determine the most suitable equipment.

5.2 Smelting process control

Reasonable control of the heating and melting temperature is the key to the smelting process. Excessive temperature will cause excessive oxidation and loss of the scrap aluminum, reducing the recovery rate of aluminum; while too low a temperature will prolong the smelting time and increase energy consumption. Therefore, the heating and melting temperature should be precisely controlled according to the type and composition of the waste aluminum. After the scrap aluminum melts, due to the high melting points of metal waste materials such as iron and copper, they have not yet melted at this time and should be promptly removed from the aluminum alloy liquid, that is, the “removal of iron” operation. The more timely and thorough the tapping operation is, the more conducive it is to improving the quality of recycled aluminum alloy. Besides, the operation tools such as stirring and tapping in contact with the aluminum alloy liquid should be kept dry and coated with appropriate coatings to avoid the infiltration of water and iron, and to prevent secondary pollution to the aluminum alloy liquid.

Six. Purification and Filtration of Melt

6.1 Purification Methods

Gas pores and inclusions are common defects in aluminum alloys, which seriously affect their properties. Furnace injection refining is a commonly used purification method. Inert gases such as nitrogen or argon are used as carriers to spray powder refining agents into the aluminum alloy liquid. After the refining agent fully contacts with the aluminum alloy liquid, it can effectively capture and remove hydrogen and inclusions. For high-quality recycled aluminum alloys, in addition to furnace injection refining, online degassing and filtration treatment outside the furnace are also required. The aluminum alloy liquid flows through the degassing box and filtration box set on the flow channel successively, further removing gases and impurities to ensure that the gas and slag content of recycled aluminum alloys meets customer requirements.

6.2 Filtration Technology

Filtration is an important means to remove inclusions from the aluminum alloy liquid. Commonly used filtration materials include foam ceramic filter plates and alumina balls, etc. Foam ceramic filter plates have uniform pores and high filtration accuracy, which can effectively block inclusions in the aluminum alloy liquid. During filtration, it is necessary to ensure that the aluminum alloy liquid passes through the filtration material evenly to avoid short-circuiting, so as to fully exert the role of the filtration material and improve the purity of the aluminum alloy liquid.

Seven. Casting Process

7.1 Selection of Casting Process

According to the shape, size and performance requirements of the product, different casting processes can be selected, such as gravity casting, high-pressure die casting, low-pressure casting and extrusion casting. Gravity casting is suitable for products with simple shapes and low requirements for mechanical properties; high-pressure die casting has high production efficiency and can produce products with high dimensional accuracy and good surface quality; low-pressure casting and extrusion casting can effectively improve the density and mechanical properties of products. Enterprises should reasonably select the appropriate casting process based on their own product characteristics and market demands.

7.2 Control of Casting Process

Strict control of parameters such as casting temperature, speed and pressure is crucial in the casting process. Excessive casting temperature will lead to an increase in the absorption of air by the aluminum alloy liquid, resulting in defects such as gas pores; while low casting temperature will affect the fluidity of the aluminum alloy liquid, thereby affecting the forming quality of the product. The control of casting speed and pressure is also important, as they directly affect the dimensional accuracy and internal quality of the product. In addition, attention should be paid to the design and maintenance of the casting mold to ensure the accuracy and surface quality of the mold, so as to ensure the quality of the final product.

Eight. Disposal of Aluminum Ash

Aluminum ash is the floating slag produced during the melting of aluminum and aluminum alloys. Although its aluminum content is relatively low, due to its large production volume, it still has certain recycling value. Aluminum ash contains metallic aluminum, aluminum oxide and a small amount of other impurities. If not properly treated, it will not only cause waste of resources but also cause pollution to the environment.

At present, the disposal methods of aluminum ash mainly include pyrometallurgy and hydrometallurgy. Pyrometallurgical treatment is to melt aluminum ash at high temperatures to re-melt the metallic aluminum into aluminum liquid for recycling. This method has high treatment efficiency, but it consumes a lot of energy and generates certain exhaust gas pollution. Hydrometallurgical treatment uses chemical reagents to dissolve aluminum ash, and then undergoes subsequent separation and purification processes for aluminum recovery. The advantage of hydrometallurgical treatment is that it has low energy consumption and low environmental pollution, but its process is relatively complex and the cost is relatively high.

During the disposal of aluminum ash, it is necessary to attach great importance to environmental protection requirements and take effective measures for waste gas, wastewater and waste residue treatment to ensure that it does not cause pollution to the environment. At the same time, efforts should be strengthened in the research and development of aluminum ash disposal technology to improve the recycling rate of aluminum ash, reduce processing costs and achieve the resource utilization of aluminum ash.

Conclusion

The pre-treatment and melting-refining recycling technology for scrap aluminum is a complex and systematic project, covering multiple links from recycling, pre-treatment to melting, casting, and disposal of aluminum slag. Each link has significant influence on the quality and output of recycled aluminum. Only by continuously optimizing the technologies in each link, strengthening equipment research and development and process innovation, strictly controlling various parameters in the production process, and paying attention to environmental protection requirements, can the quality of recycled aluminum be improved, the efficiency of resource utilization be enhanced, the high-quality development of the scrap aluminum recycling industry be promoted, and the sustainable development goals of the aluminum industry be achieved.