Oxidation of liquid aluminum

Molten aluminum alloys have two inherent characteristics:

• propensity to absorb hydrogen gas and
• ability to easily oxidize.

Reactions on the surface of aluminum melt

Hydrogen appears on the surface of molten aluminum alloys as a result of the reaction of aluminum melt with water vapor., which is present to some extent in the atmosphere of a smelting furnace or other smelting equipment. The reaction between water vapor and molten aluminum produces more than just hydrogen gas, but also a film of amorphous aluminum oxide (Al₂O₃). This film acts as a protective barrier against oxidation for the molten metal., which is underneath. Such amorphous films are called “young films” [1].

Two oxidation reactions, that occur when molten aluminum comes into contact with air, proceed as follows:

2Al + 3H₂O → Al₂O₃ + 3H₂

4Al + 3O₂ → 2Al₂O₃

Hydrogen, in turn, dissociates into its atomic form on the surface of the melt, and then diffuses through the film of amorphous alumina and then quickly dissolves in the aluminum melt. Aluminum oxide films are an integral part of the melting process; they protect the metal under the film from further oxidation.

oxide films and slag

However, in real casting operations, the surface of the melt pool always experiences movements and disturbances during various metallurgical operations., such as:

• Charging
• Skimming
• Cleaning
• Degassing
• Transferring
• Ladling

Any of these operations causes rupture and re-oxidation of thin films of alumina., which leads to a rapid thickening of these films. The constant movement of the metal and the destruction of aluminum oxide films lead to their grinding, thickening and capture of non-oxidized molten aluminum. As a result, the so-called “wet” slag.

A typical representation of this phenomenon is well observed, for example, when filling pouring ladles, as shown in fig.. 1. As soon as the liquid metal is drained from the melting furnace into the discharge ladle, then not only the melt jet is disturbed, but there is also a significant amount of splashing and seething of the melt. All this contributes to the oxidation of aluminum, as a result of which this very “wet” slag. Typically, the aluminum content in wet slags is in the order of 60–85%. The remaining 40-15% is aluminum. The amount of liquid metal captured by the slag depends on the methods of handling the molten metal.

Fig. 1 – Metal being tapped from a holding furnace into a transfer ladle, causing aluminum oxide films
to crumble, thicken, and encapsulate unoxidized molten aluminum, generating wet dross [1]

In addition to the growth of oxide films and slag on the surface of the melt, caused by exposure to general atmospheric conditions, in industrial smelters there are a number of factors, which can make a significant contribution to the intensity of aluminum oxidation. These include turbulence, caused by the type of burners, formation of atmospheres, which may be chemically more reactive, and even adding oxidants or contaminants to the melt composition.

Complex Interactions, that occur between the atmosphere above the melt, the capture of useful aluminum alloy by crushed oxide films and the resulting buoyancy of oxide inclusions in the melt due to hydrogen adsorption on complex inclusion surfaces are shown in Fig.. 2.

Additional surface oxides enter the furnace with the charge. Organic compounds react with aluminum melt, adding inclusions in the form of carbides and nitrides. These suspended inclusions and oxides themselves have a higher density, than molten aluminum (Fig. 3).

Fig. 2 – Complex interactions that occur between the atmosphere above the melt [2]

Fig. 3 – Addition of supplementary surface oxides that are on the metal stocks entering the furnace molten metal [2].

Aluminum and aluminum oxide

Aluminum has a negative redox potential (-1.66 V), and magnesium, it is an important alloying element, has an even lower potential (-2.38 V). therefore, like most other metals, Aluminum only occurs in nature as a very stable oxide. Chemically, it is the most stable state to the lowest energy level. In the electrolysis, metal forced to separate from the oxygen by raising its energy potential. In contact with oxygen, aluminum tends to return to a lower energy level as alumina. Due to its high affinity for oxygen, this reaction is instantaneous.

The oxidation reaction of aluminum

The oxidation reaction of aluminum The following reactions

4Al + 3O₂ -> 2Al₂O₃

The positive change in enthalpy ΔH of the reaction indicates, what oxidation of aluminum It is exothermic, that is, there is a release of energy. it is logical, so as aluminum passes into a state with a lower energy level.

The thickness of the oxide film on the solid aluminum

The thickness of the natural oxide film is quite thin - from 1 to 3 nm depending on the alloy and the temperature of oxide formation (up to 300 ° C). The figure 1 It shows a gradual increase in the thickness of the oxide film on pure aluminum during its formation at a temperature of from room temperature to 400-500 ° C. Then there is a break in the oxidation rate and a dramatic increase in oxide film thickness to 20 nm. The reason for this is considered to be a transition from the amorphous alumina structure to its crystalline structure. That is why the particulate aluminum scrap drying and firing it with organic coatings is not heated above its 400 °, to avoid excessive oxidation.

Figure 1 – Natural oxide film on aluminum [3]

Figure 2 – The thickness of the oxide film on solid aluminum [4]

In the solid state of aluminum oxide, aluminum plays a positive role, since the oxide film is shaped γ-Al₂O₃ and a thickness of several nanometers. It reliably insulates the aluminum surface and stops further oxidation. At constant temperature, the thickness of the first oxide film grows rapidly, but then the growth rate is slowing, and reduces to almost zero.

Secondary aluminum oxidation

Oxidation of aluminum chips

With the feature of oxide film growth, shown in Figure 1, An interesting phenomenon associated. It occurs when stored aluminum scrap, chips. This type of aluminum scrap arise in machining aluminum and enters the remelting essentially as turning and drilling chips. This has chips after machining fresh, clean surface, which immediately begins to oxidize. Since the chips before remelting stored in compressed packages, the, it would seem that, shall oxidize them, only the outer layer, and the inner layers of the packet stored without oxidation. However, to change the weight of the package was installed, Oxidation it generally lasts for a long time. The reason for this is, that the package has cracks and cavities, through which air is slowly, but surely it penetrates into the inner layers. Most of the individual chips are very thin, and an oxide layer, although even thinner, It provides a significant share of the total weight of the package. Therefore, long-term storage of metal shavings losses occur just out of nowhere. The conclusion from this may be only one - the chips necessary to melt immediately upon its receipt.

The specific surface area of ​​aluminum scrap

The loss of alumina due to its oxidation during remelting furnace in some scrap load is proportional to the specific area of ​​the scrap. The specific area is expressed by

a_beats = m/A,

where m - the total weight of the scrap Party, A - the total surface area of ​​all the scrap pieces, constituting the load.

The specific surface area of ​​aluminum wastes is a critical parameter. Its magnitude increases with decreasing particle size scrap. So, a cube with a side 10 cm surface area is 600 quarter. look, while the equivalent weight 1000 cubes with side 1 cm - 10 times more. Therefore, the rate of oxidation of these blocks will be 10 times more, than large cube.

The oxide film on the molten aluminum

Except for drying of organic coatings operations and firing all aluminum scrap oxidation takes place in a liquid state. During melting the protective oxide film is destroyed, and aluminum oxidation begins again, but at higher temperature. In the unperturbed surface of molten aluminum is installed stable oxide film, wherein the thickness slowly increases in time.

Dependence of liquid aluminum oxidation on temperature

With increasing temperature of the melt of aluminum oxidation rate increases. It grows rather slowly up to the temperature range of 760 to 780 ° C, and then followed by a sharp increase in the rate of oxidation, as shown in Figure 2. Heating of the aluminum melt above these temperatures leads to higher loss of aluminum from its oxidation.

Figure 3 – Dependence of aluminum oxidation rate on temperature [4]

The optimum temperature for the aluminum melt

Given the dramatic growth of oxidation of aluminum at a temperature above the melt 760-780 ° C, if there is no particular reason for the high melt temperature (for example, long length of transmitting metal lines), molten aluminum is heated to a temperature just, which is optimal for its casting. In most cases, this temperature ranges from 730 to 750 ° C.

Sources:

1. Aluminum Fluxes and Fluxing Practice / R. Gallo, D. Neff //ASM Handbook, Volume 15: Casting (2008)
2. Dross, Melt Loss, and Fluxing of Light Alloy Melts / D. Grotesque, D. Neff // ASM Handbook, Volume 15: Casting (2008)
3. Corrosion and Corrosion Protection – TALAT 1252
4. Ch. Schmitz, Handbook of Aluminium Recycling, 2006.