Secondary aluminum alloys, which are made from aluminum scrap, usually cheaper, than the alloys of the same chemical composition of the primary metal. The reason for this lies in the extent of metal contamination of the secondary, which is generally higher, than the primary. Development of molten aluminum purification technology from impurities and contaminants gradually reduces the gap, that allows to expand the scope of secondary aluminum.
Selection aluminum purification technology
The choice of technology for purification of secondary aluminum melt depends on several factors:
- the type of aluminum scrap used;
- the type of equipment used, primarily a smelting furnace;
- type of products;
- customer requirements.
Therefore, there is no universal treatment technology molten aluminum. Moreover, usually to achieve a given chemical composition of the alloy or the level of contamination in the melting furnace mixed primary and secondary aluminum. It means, that much of the technology of purification of recycled metal is used in the purification of primary and.
The main impurities and contaminants in liquid aluminum
The main impurities and impurities in molten aluminum are:
- impurity alkali metals;
Their content in the primary and secondary aluminum is significantly different.
The hydrogen content in the aluminum
The hydrogen content in molten aluminum is:
- in primary aluminum: from 0,1 to 0,3 look3 on 100 g of metal;
- in the secondary aluminum - from 0,4 to 0,6 look3 on 100 g of metal.
The content of inclusions in aluminum
The content of inclusions in molten aluminum is:
- in primary aluminum: mainly Al carbides4C3 - near 1 mm2/kg on the PoDFA scale;
- secondary aluminum - Al2O3, MgO, MgAl2O4, Al4C3, tube2 - from 0,5 to 5,0 mm2/kg on the scale PoDFA.
Scale content inclusions PoDFA
The dimension of the units of content on a scale PoDFA inclusions - mm2/kg - looks unusual. It arises from the applicable measurement method. Until the 1980s, the inclusion was considered in the analysis under the microscope of the microstructure or macrostructure. It was later invented several analytical methods, but all of them are complex and expensive. The PoDFA (Porous Disc Filtration Apparatus) method consists of vacuum filtration of a known volume of liquid metal through a porous filter to collect impurities. They are then converted and measured. The total area of inclusions per kilogram of metal and is a measure of the content of inclusions in metal.
The content of alkali metals in aluminum
The content of impurities of alkali metals in molten aluminum is:
- in primary aluminum: sodium - from 30 to 150 ppm, Calcium - from 2 to 5 ppm, cast - to 20 ppm;
- in secondary aluminum: sodium - up to 10 ppm , Calcium - from 5 to 40 ppm, cast - to 1 ppm.
Sources of hydrogen in the molten aluminum
Dissolved hydrogen in the aluminum melt occurs from reaction between water vapor and molten aluminum – the chemical reaction of aluminum with water. Magnesium in molten aluminum is also reacted with water to form hydrogen.
3H2O + 2Al = Al2O3 + 6H
H2O + Mg = MgO + 2H
Both these reactions are thermodynamically very "beneficial" and their continued limited only by the formation of an oxide film on the melt surface.
Water vapor - the main source of hydrogen in aluminum
factors, which promote these reactions and increase the concentration of dissolved hydrogen in the metal, include the following:
- high pressure water vapor in the atmosphere – high humidity;
- metal turbulence, which destroys the oxide film and allows the reactions to continue;
- moist batch, which makes the water vapor directly into the melt.
Water vapor, are present in the fossil fuel combustion products, They are also sources of hydrogen. for instance, Reinforced use gas heating in the transmission of liquid aluminum from the furnace to the casting assemblies also will contribute to the increase in hydrogen content in the aluminum.
Magnesium in aluminum melt
The amount of magnesium in the aluminum scrap depends on the type remelt aluminum alloys and can reach 5 % and higher. It is believed that if the magnesium melt alloying element or impurity depends, alloy which is the end product. Since magnesium is more expensive, than aluminum, usually from a melt with a high content of magnesium make the corresponding alloys alyuminevomagnievye. However, in some cases produce magnesium removal from the melt.
Control of the content of alkali metals
For efficient control of the content in the melt of alkali metal impurities in the "advanced" enterprises use optical emission spectrometry.
The inclusions in aluminum melt
are the inclusions, usually, solid particles, suspended in molten aluminum. The number and size of these particles depends on many factors, in particular, the initial quality of the molten scrap aluminum and impurities, that there were.
The main inclusions in aluminum are non-metallic inclusions, typically up to 100 m. They consist mainly of oxides, Although there are other types of chemical compounds. There are two main classes of inclusions - exogenous and indigenous.
Exogenous inclusions in aluminum
The exogenous inclusions - a particle, that already existed as a separate phase to the melting of the charge. Examples of exogenous inclusions are small pieces of lining, which fall into the melt. Another example can be oxide particles or contaminants, who were on the charge of aluminum scrap. Exogenous inclusions are oxides almost completely and have a considerable size, than indigenous inclusion. Therefore, their presence in the aluminum is considered more harmful, than the presence of indigenous impurities. The large size of exogenous inclusions allow more easily extract them from the melt.
Indigenous inclusions in aluminum
Indigenous inclusions are formed in the course of chemical reactions "on the spot" - in the aluminum melt. An example is the reaction of dissolved oxygen with liquid aluminum to form alumina:
2Al + 3O = Al2O3.
In alloys, containing magnesium, magnesium oxide (magnesia) and spinel are formed:
Mg + O = MgO
Mg + 2Al + 4O = MgAl2O4
If nitrogen is used for degassing the melt, the particles may be formed AlN, using as a flux MgCl2 - the inclusion of chlorides.
Particular problem borides TiB2, formed by the melt modification rod Ti-B. These inclusions are much less, than any exogenous inclusions and they are very difficult to remove.
Inclusion in primary and secondary aluminum
The inclusion in the secondary aluminum are different from those, which are located in the primary aluminum. Aluminum scrap initially contains many impurities and oxides, than the primary metal, and the formation of an oxide film upon melting of the scrap makes maintenance more inclusions. As a result,, removal of inclusions is more important in the production of secondary aluminum. Remelting scrap aluminum major produces fewer inclusions, than the remelting of used beer cans or light scrap. Therefore, a large scrap price is higher.
The requirements on the content of inclusions in aluminum
The figure below shows the significance of the various types of inclusions in molten aluminum. The shaded area shows the limit on informal concentration of inclusions of varying sizes in the cast aluminum - a deformable aluminum requirements must be stiffer.
Curve C shows the inclusion, which formed during aluminum casting. It - oxide films and slag inclusions. Exogenous inclusions also fall on the curve. curve B – for indigenous inclusions, which were formed during the melting and processing of metal. In general, the larger oxide inclusions, than those, which consist of carbides and borides - that they should be removed in the first place.
Source: Mark E. Schlesinger, Aluminum Recycling, 2013.