Aluminium scrap charging and melting
Understand the complexity of charging aluminium scrap into a melting furnace to produce secondary aluminium alloys. Get insights into aluminum remelting scrap, impurities & their effect on the production process.
Aluminium charge
When the charge is aluminium remelting scrap consists of, which are likely on the ground, than under the ground. However, an analogy between these two types of charge much more, than it might seem at first glance. Aluminum scrap comes with their “waste rock” in the form mounted thereon various parts of other materials, as well as various coatings. This leads to the presence of aluminum charge a large number of impurities, that have a significant impact on the production of secondary aluminum alloys. To combat these impurities in the production of secondary aluminum alloys have to apply additional technologies and processes.
Various aluminum alloys – wrought and casting – differently affect the range and quality of secondary aluminum alloys. The difficulty of recycling of aluminum alloys is, after remelting to remain within the tolerances for the presence in the alloy of different impurity elements for him.
There are numerous sources of aluminum scrap in various industries, which is available for recycling. The three largest sources of aluminum scrap in the world are transport engineering, construction and packaging industry (table 1). Due to the relatively short service life of aluminum products in transport and, special, packaged, these two sources give the maximum amount of aluminum scrap.
Table 1 – The use of aluminium alloys in transport engineering, construction and packaging industry [2]
Control of aluminium scrap
The aluminum charge is carefully selected to ensure that it does not contain any undesirable alloys or elements that will contaminate or distort the final product, the given aluminum alloy. This is achieved by careful control of the supply of aluminum scrap, as well as constant monitoring of the charge for the absence of foreign materials, for example, steel packaging tapes or cans that may contain liquids.
No water at all
If aluminum scrap contains any amount of water, especially in closed containers, there is a high risk of explosions. Such explosions can pose a threat to the health and life of foundry workers, as well as destroy the furnace and other equipment. It is necessary to ensure that there are no liquids in the aluminum scrap loaded into the furnace. Drink cans and other metal containers are especially dangerous.
To reduce the risk of loading problematic, for example wet, scrap into the furnace, it is recommended to pre-dry it at the entrance to the furnace before immersing it in a bath of molten metal. Waiting for 10-15 minutes is usually sufficient.
Тo weigh the scrap
Aluminum scrap loaded into the furnace must be weighed to prevent excessive loading of the furnace and splashing out of liquid metal, as well as to accurately calculate alloying additives. It is impossible to estimate the amount of loaded scrap by eye, since its volume can vary significantly for different types of scrap.
No steel tape
Aluminum pigs are usually loaded in whole bundles. They are usually placed using a forklift at the entrance to the furnace and then pushed into a bath of molten metal. Scrap extruded profiles are usually loaded in bundles of approximately 150 kg. All steel packaging tapes must be removed before loading into the furnace to prevent iron contamination of the melt.
Aluminium scrap charging machines
Scrap can be charged into the side door of reverberatory furnaces very simply using a forklift with the scrap on wooden pallets, or with purpose-built charging machines (Figure 1). For secondary smelters with the charge being ∼100% scrap, purpose-built systems are favoured to be able to charge the scrap quickly.
The recovery of scrap charged depends on the amount of oxidation taking place during melting. This in turn depends on the coating on the scrap if painted or lacquered and the surface area per volume; finely divided material is prone to burn away if charged to a dry hearth. Scrap such as UBC is shredded and then delaquered before charging directly to the melt.
The sequence of solid material charged into the furnace plays an important
role in both the efficiency and the preservation of the furnace:
- First, primary aluminium ingots are loaded on the furnace floor
- Then, large pieces of scrap and secondary aluminium.
- Finally, fine pieces of scrap and master alloys are loaded.
In most cases, however, the solid material is loaded into the liquid bath. In this case, first, the master alloys are loaded, followed by large and then fine scrap. The necessary precautions taken are preheating and dry scrapping, to avoid explosion.
Some small scrap charger options are shown in Figures 2-4.
Figure 1 – A side charging of scrap machine for a 75-tonne stationary reverberatory side-well melter
with a side charging of scrap machine (CASTconsult) [3]
Figure 2 – Simple aluminium scrap charging machine for a small 3500 kg melting furnace [1].
Figure 3 – The charger exits the furnace after charging aluminium scrap [1]
Figure 4 – Semi-automatic aluminium scrap charger [1]
Acceptance and storage of aluminium scrap
Preventing water explosions and other hazards is the most important task in the receiving and handling of scrap aluminum [2]:
- Visual inspection of incoming scrap to determine if a load is especially wet.
- Storing scrap in a covered location is recommended.
- Policy for charging must be a FIFO (first in, first out).
- Shredding the baled scrap is strongly recomended to give undesired fluids a chance to drain away.
- Inspection of incoming scrap for removing other hazards.
- Training personnel to recognize hazards is an important part of a safety program.
- Visual inspection of aluminium scrap with a radiation detection station, since radioactive material is the hardest to spot and the most damaging if not caught.
- Magnetic separators can remove most of the ferrous material (and the rust) from incoming scrap.
Sources:
1. Материалы фирмы Ashford Engineering Services, 1997
2. Aluminum recycling / Mark Schlesinger – 2007