Aluminum and its 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.
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.
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
abeats = 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.
Intensity of oxidation of molten aluminum on the 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. These losses are often called "aluminum intoxication».
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.
Source: Ch. Schmitz, Handbook of Aluminium Recycling, 2006.