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Introduction to Aluminum Recycling
The aluminum scrap for recycling has numerous used components such as old pots and pans, aluminum engines, alloy wheels, etc., which all have to be processed before re-melting, this being the most labor intensive part of the recycling process.
Once this has been completed, the scrap is loaded into a furnace along with a quantity of pure aluminum, and melted at a fraction of the cost of original smelting processing. The molten aluminum is then tapped of the furnace and poured into molds being left to cool down and solidify into ingots.
This is another article in my present series on recycling metals. In this article, we will see how aluminum is produced from the ore, comparing the energy expended in aluminum smelting to that used in recycling scrap aluminum.
We shall begin then, with the current methods used in aluminum smelting………
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Aluminum Smelting Overview
Aluminum is produced from bauxite, which is extracted mostly by open mining. It is processed into a white powder known as alumina, which is shipped to aluminum smelters. Here it is fed into cells, these being fabricated rectangular steel boxes lined with carbon or graphite.
Carbon blocks attached to rods hang downwards into the cells from overhead supports, leaving a gap between them and the cell lower carbon linings. A cryolite flux is added and alumina is fed into the cell effectively filling this gap. Then, an electric current is applied, passing through the cryolite causing it to become molten, as more alumina is added.
The molten aluminum sinks down to the bottom of the cell, where it is siphoned off into a crucible and transported to gas holding furnaces. From here, it is cast into billets (similar to telegraph poles) or into long rectangular blocks. These are cut into manageable sections and strapped onto pallets for transportation to various aluminum extrusion or component manufacturers.
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Domestic and Industrial Applications of Aluminum and Component Recycling Inititives
These include the following components:
- Pots and pans
- Foil for cooking
- Drink cans
- Solar panels
- Cosmetics, paints and varnishes
- Patio furniture
- Aircraft frames and fuselages
- Railway carriages
- Truck bodywork
- Car and truck engines and gearboxes
Why Recycle Aluminum?
All the above components can be produced from aluminum that has been collected and treated for recycling. The domestic input to this being from drink cans, old pots and pans, used cooking foil, broken or redundant garden furniture, cosmetics, and old paint or varnish. Without the domestic input, more land and energy would be used in mining and smelting, producing more greenhouse gas emissions and using up dwindling landfill area.
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Sorting and Pre-treatment of Aluminum Scrap
The recycling process begins with the collection of scrap aluminum components and aluminum cans. These are gathered at various recycling sites, and non-ferrous specialist collection centers where they undergo sorting, cleaning and pretreatment processes
These pretreatment processes involve conveying the segregated aluminum metals under an electromagnetic screen to remove any ferrous metals. Once this has been accomplished, the remaining material on the conveyor is then subjected to an eddy current, which literally ejects aluminum material from the rest of the materials on the conveyor into a suitably located container.
The contents of the container are then washed, dried, shredded and compacted, ready for shipment to an aluminum recycling center.
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Reverberatory Furnace used in Recycling Aluminum
The reverberatory furnace consists of a steel box lined with firebricks, with two gas burners on the sides above the molten metal level. It has a vertical sliding door at the front which is used to load the scrap aluminum.
The pre-treated aluminum compressed bales are loaded into the furnace along with an amount of pure aluminum ingots. The melt engineer will have calculated the amount of pure aluminum needed to produce the required properties of the finished product.
The door is shut and the furnace fired up to a temperature of 700oC then reduced to 660oC when the charge has become molten. Any dross is then removed from the top of the molten aluminum and several samples taken to check for excessive levels of impurities.
These impurities are reduced to acceptable levels by injecting an inert gas such as argon or nitrogen into the molten aluminum, the resultant slag being removed from the surface.
Further samples are taken to check the required properties of the aluminum, and once these are acceptable, the furnace is tapped and the molten aluminum run into molds. Once cooled and solidified, the ingots are removed from the molds and pelleted, ready for transport.
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Aluminum smelting from bauxite ore is a very energy consuming process, from the extraction of the ore to the final casting of the aluminum, which is used in numerous components ranging from aluminum pots and pans to aircraft fuselages and car engines.
Recycling aluminum saves energy and aluminum can be recycled any number of times without any detrimental effects on the base metal properties. This process provides huge aluminum energy savings in the process. Recycling aluminum consumes only a twentieth or 5% of the energy used in the initial smelting of the alumina.
There is a requirement for pretreatment of scrap aluminum which involves chemical, electrical and mechanical processes before melting it in a furnace. The energy expended during these processes coupled with that of collection and separation activities are difficult to quantify. Not withstanding these treatments, aluminum recycling energy savings are still very significant.
There are also savings made to landfill sites, along with that of greenhouse gas emissions. These coupled with the land area saved by not requiring as much aluminum ore with its inherent processing and shipping costs, should encourage us all to recycle as much of our used aluminum components as possible.
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