Aluminum dross

Aluminum dross can be classified into several different types based on its source and properties:

1. Primary Dross (also known as White Dross):

• This comes from the initial smelting process of aluminum and aluminum alloys. It contains a high proportion of aluminum materials, such as Al and Al2O3. Additionally, it is supplied by basic and optional aluminum fluxes. White dross is made up of a large amount of metallic aluminum. It can contain up to 45%, and in some cases, it can reach 70%-80%. It consists of fine powders separated from molten aluminum. It originates from the smelting of new aluminum or clean scrap aluminum, i.e., primary aluminum smelting, and is relatively pure.

1. Secondary Dross (also known as Black Dross):

• This comes from the secondary smelting process of scrap aluminum. It appears darker and contains less metallic aluminum (usually less than 30%, with 12%-18% being more common), and more oxides, salts, and other impurities. Compared to primary dross, secondary dross mainly contains a high amount of salts and gas bubbles. The bulk density of secondary dross ranges from 0.79 to 1.11 g/cm³.

1. Salt Cake:

• During the processing of aluminum dross, salts (such as sodium chloride and potassium chloride) are often added to facilitate the separation of metal and non-metal components.
• This type of dross contains a large amount of salts, along with alumina, metallic aluminum, and other impurities.

1. Oxide Dross:

• This is mainly a mixture of alumina and a small amount of other oxides.
• It typically forms in an oxidizing environment, such as when molten aluminum is exposed to air for too long.

1. Metal-Rich Dross and Metal-Poor Dross:

• Based on the aluminum metal content, dross can be classified as metal-rich (containing more metallic aluminum) or metal-poor (containing less metallic aluminum).

1. Wet Dross:

• This refers to aluminum dross that contains a significant amount of moisture, making it more difficult to process due to its water content.

Each type of aluminum dross has its specific treatment and recycling methods. White dross, due to its high metal content, has a higher economic value and recycling rate. Black dross and salt cake, due to the higher amount of impurities and salts, are more complex to treat but still have some recycling value.

Aluminum dross waste is an industrial by-product, generated exclusively in industrial environments, so various aluminum processing, aluminum production, aluminum smelting, and aluminum manufacturing companies produce large amounts of aluminum dross. To recycle aluminum dross, some companies or industrial parks set up dedicated aluminum dross collection points or facilities, while third-party service companies also specialize in providing collection, transportation, and recycling services for aluminum dross. Recycling aluminum dross not only helps with resource reuse but also contributes to reducing environmental pollution. As a result, many countries and regions encourage the recycling and processing of aluminum dross. When handling aluminum dross, it is also necessary to comply with relevant environmental protection regulations and standards to ensure that the process meets environmental protection requirements.

The value of aluminum dross primarily lies in the content of valuable metal components (mainly aluminum and aluminum alloys). In the trading of aluminum dross waste, element detection reports or spectroscopic analysis readings are essential pieces of product information.

Aluminum dross can be separated into residual aluminum metal, aluminum oxide, and impurities through mechanical recovery methods. The treatment of aluminum dross involves a unique process and system that enables the recovery of most of the metal through physical methods without chemical reduction. In this process, the dross is first crushed, and then it is sorted into particles rich in aluminum metal and those rich in aluminum oxide. This separation is typically based on density, such as flotation. Afterward, the metal-rich particles are melted in a furnace to remove the remaining oxide particles.

Aluminum dross is a common industrial waste in the aluminum refining industry. However, aluminum dross is often classified as hazardous waste, as its harmless treatment is a challenging task for the aluminum manufacturing industry. Dumping and landfilling aluminum dross waste can harm plants, animals, and the environment. Aluminum dross, particularly its impact on groundwater, is a significant environmental concern. The best solution to this issue is the recycling and harmless treatment of aluminum dross.

The recycling and regeneration of aluminum dross mainly involves the following steps:

1. Collection and Crushing: First, the aluminum dross must be collected. The top dross is generally in a semi-liquid form floating on the surface of the molten aluminum and can be collected by skimming and then cooling. The bottom dross, which is solid, settles at the bottom of the furnace and is collected during the furnace cleaning process. The collected dross is often stored in appropriately sized containers and, after cooling, forms aluminum dross ingots that can be directly used. Some large dross ingots may not be suitable for furnace charging, so they are crushed into smaller aluminum dross pieces for subsequent processing.
2. Screening and Separation: Some processing plants further separate the crushed aluminum dross using screening, magnetic separation, gravity separation, and other methods to extract aluminum metal and non-metallic components. The aluminum metal portion can be recycled, while the non-metallic portion requires further treatment.
3. Melting and Purification: This is the pyrometallurgical recovery process. The separated aluminum metal is remelted, and various purification techniques, such as slag removal, degassing, and filtration, are used to remove impurities and gases to improve the purity of aluminum. Due to its low raw material requirements and minimal waste liquid generation, the pyrometallurgical recovery method is suitable for large-scale industrial waste recycling. Specialized aluminum dross recovery equipment and processes, such as rotary salt furnaces and salt-free technology, have been industrialized. Many hydrometallurgical techniques, which are patented, consume aluminum dross to produce aluminum sulfate and other industrial products.
4. Chemical Treatment: For aluminum dross containing a significant amount of non-metallic impurities, chemical methods can be employed. The remaining dross residue contains aluminum oxide, alloying elements, and salts such as sodium chloride (NaCl) and potassium chloride (KCl). These residual drosses can pollute adjacent areas when piled up, as the salts may leach into water streams and release harmful gases. Currently, domestic aluminum dross is being used to develop appropriate processes to obtain η-alumina. It is a highly valuable product, and acid leaching of the dross can increase the aluminum oxide recovery rate from 70% to about 84%. Washing the dross is crucial for maximizing the recovery of aluminum oxide. It is also used for recycling salts for reuse.
5. The liquid obtained from acid-leaching aluminum dross can be treated with water-soluble ammonia water alkali to hydrolyze aluminum sulfate, producing amorphous aluminum hydroxide. This aluminum hydroxide, when heated to 900°C, forms η-alumina.
6. Utilization of Residual Dross: Through both pyrometallurgical and hydrometallurgical methods, the metallic components in aluminum dross can be recovered. A significant amount of non-metallic components in the dross can be used to produce potassium alum. The dross powder is filtered and slowly added to sulfuric acid while continuously stirring the solution, heating, and cooling. After several hours, potassium alum crystals are obtained in the solution. Analysis of the product potassium alum, mother liquor, and residual liquid shows that nearly 89.2% of the aluminum has been successfully transferred to the product potassium alum crystals. Potassium alum is widely used in wastewater treatment, the pharmaceutical industry, cosmetics, leather tanning, and other fields. Therefore, extracting potassium alum from aluminum dross is a very positive solution to landfill waste problems.
7. Environmental Treatment: For the final residual dross that cannot be recycled, environmental treatment is necessary to reduce environmental pollution. Aluminum dross can be used as filler for concrete, providing higher flexural and compressive strength compared to pure cement; it can also be used as a source of calcined alumina in refractory materials. It can be used to produce sintered calcium aluminate as a protective covering layer for molten metals. It can also be used as an alternative to green sand in aluminum alloy casting molds. In steel mills, it can be used as a desulfurizing and deoxidizing agent for refining furnaces.

In summary, the concept of aluminum dross recycling is entirely different from that of waste aluminum recycling. Waste aluminum recycling refers to collecting the pure aluminum metal portion and remelting it into aluminum ingots for use in new aluminum products. In contrast, aluminum dross recycling not only recovers its metal components but also recycles its non-metallic components for harmless treatment. In addition to recovering metals from aluminum dross, there are several other uses, such as producing zeolite, ion exchangers, refractory materials, composite materials, cement and concrete products, as well as generating hydrogen, ammonia, and methane gases.

Factories with aluminum slag processing capabilities

When buying and selling aluminum slag, it is essential to have a chemical analysis report to facilitate pricing and quality control.