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After iron and the different steels, aluminum is one of the most important metals for the human species. It is element 13 of the periodic table and corresponds to one of the few metals in the p block. In addition to being a very light metal, aluminum is extremely malleable, allowing it to be worked into very thin sheets, thinner even than a sheet of paper.
On the other hand, many aluminum alloys can also be tempered to obtain a very strong structure, which allows its use as a low-weight structural building material. Finally, unlike iron and other metals, aluminum is considerably resistant to oxidation, which allows it to be used in a variety of applications, including its widespread use as a container for food and beverages.
Finally, in addition to its abundance, aluminum is a metal that can be easily recycled, mainly by melting and solidifying parts made of aluminum, such as cans for soft drinks or spirits. In fact, this process can be carried out at home in a simple way with the use of a blowtorch and, if you have one, with a special oven. This is due to the thermal properties of this ubiquitous material in our society.
Thermal properties of aluminum
The thermal properties of aluminum that are relevant to its recycling through melting and solidification are melting point, heat of fusion, and heat capacity .
Melting point: 660.323 °C
Compared to iron, titanium, and many other transition metals, aluminum’s melting point is considerably low. For example, the melting point of iron is 1,538°C, and that of titanium is 1,670°C, while many other metals melt at temperatures close to 2,000°C or higher.
This makes it relatively easy to melt aluminum, since it does not require the use of special or overly complex equipment and the liquid aluminum can be contained in iron or steel vessels.
While a home kitchen oven won’t work for melting aluminum (the maximum temperature of a home oven rarely exceeds 300°C), an electric ceramic kiln would do just fine. These reach temperatures ranging from 650 °C to 1,350 °C. On the other hand, a homemade oven made of refractory blocks or cement or concrete poured into a mold and heated with butane or propane gas burners, or even with coal, can reach the right temperature and more.
Finally, we can easily melt aluminum by heating it directly with the flame of a butane (1,430 °C) or propane (1,995 °C) torch, so it is not even necessary to use an acetylene flame as it is necessary to melt iron using a torch.
Heat capacity (900 J/kg.°C)
The heat capacity determines the amount of heat that must be supplied to a system to increase its temperature. In the case of aluminum, the heat capacity is relatively high. For example, it is almost twice that of iron at 20 °C. This means that it would take twice as much heat to heat the same mass of aluminum as iron to the same initial and final temperatures.
However, since aluminum’s melting point is very low, it doesn’t matter that its heat capacity is higher than many other metals. This is because we only need to heat up to 660°C instead of up to 1,500 or 2,000°C.
Fusion heat: 322 – 394 kJ/kg
Another important physical property to consider when recycling aluminum is its heat of fusion. This represents the amount of energy in the form of heat that is required to melt a given amount of substance at its normal melting point.
Melting or melting a substance is not just about heating it to its melting point. Once this temperature is reached, an additional amount of heat is required to carry out the melting process itself. This heat is called latent heat, since it does not cause an increase in temperature, but is spent on breaking the forces that bind the particles of the substance. For this reason, this is an important part of the energy analysis of the recycling process.
The heat of fusion of aluminum, which is between 322 and 394 kJ/kg, is greater than that of other important metals such as iron, which is 293 kJ/kg. This means that it is easier to melt iron, once its melting temperature has been reached, than to melt the same amount of aluminum. However, since iron is nearly three times as dense as aluminum, we get a much larger volume of molten aluminum from the same amount of heat than iron if we expend the same amount of heat.
Also, again since iron has a much higher melting point, likewise more energy is spent melting iron than aluminum.
Aluminum recycling versus aluminum production
It should be noted that, given the thermal properties of aluminum, the total amount of energy that must be invested to recycle it is much less than the amount necessary to produce it from the different aluminum ores or minerals. Between the mineral extraction process, its crushing, dissolution and subsequent electrolytic reduction, almost 20 times more energy is consumed producing aluminum than recycling it.
How to melt aluminum at home
As we can see from the information provided above, it is not difficult to melt aluminum at home. All you need is an oven capable of reaching a temperature of at least 660°C (although you really need to be higher) or, alternatively, a butane or propane torch.
Torches used in the kitchen for flambéing would do the trick, although one that can be kept burning for a long time is required.
WARNING!
It goes without saying that working with a foundry furnace, or its equivalent, and with molten metals is extremely dangerous , and can cause very serious burns . On the other hand, spilling molten aluminum on different surfaces can cause damage to them.
Additionally, working with compressed flammable gases such as butane or propane near a smelting furnace also poses a considerable explosion hazard if care is not taken. For all these reasons, the casting of aluminum cans for recycling should only be carried out by a responsible adult with experience in carrying out this type of process. A boy or girl should never do it, much less without the company of an adult.
In fact, it is recommended to avoid having children present whenever you work with these materials that will be at extreme temperatures.
For the rest, adults must, in the same way, observe some basic safety rules to avoid accidents and minimize their effects, should they occur:
- Never work without the proper protective equipment (see the section on safety equipment below).
- Work slowly and carefully. From the hurry, only fatigue remains, as the saying goes, and doing things in a hurry only increases the risk of having an accident.
- Work in a well ventilated place, preferably open.
- If working with a gas oven, torch or burner, be sure to protect gas hoses or pipes from the heat of the oven or flame and locate the gas cylinder as far away from the fire or heat as possible.
- Handle the crucible with the molten aluminum very carefully and without haste.
materials and instruments
In addition to the aluminum cans that we will use as starting material, the essential implements that we will need to recycle aluminum by melting are:
- A clay oven, electric ceramic kiln, or a propane or butane torch to heat or melt the aluminum samples. We can also make our own smelting furnace using a mixture of plaster, gray cement and sand and two pots of different sizes to create a mold and fill the space between the two with the concrete mix. Then, by drilling a hole near the bottom to inject air, we can fill the furnace with burning coal and use this as a heat source to melt the aluminum.
- A crucible with a lid or container to heat and melt the aluminum. In principle, this can be made of any material that has a considerably higher melting point than aluminum. An iron or steel crucible works, but one made of earthenware, ceramic, porcelain, or some similar refractory material could also be used.
- Long metal tongs to manipulate the hot crucible with the molten aluminum.
- Casting molds where to pour the molten aluminum to let it solidify. These can also be made of any strong ceramic or metallic material. One option is to use a wooden box filled with silica sand or foundry sand. This will allow us to print the desired shape to the mold by pressing another object on its surface and thus obtain a copy of the object made of aluminum.
Safety equipment
Recycling aluminum requires the use of high temperatures and the handling of very hot molten metal. This type of work requires taking safety measures to protect our skin against burns and other accidents. For this, we require the following safety equipment:
- Thermal gloves to protect us from the heat. Kitchen gloves are not enough. It is preferable to use leather gloves like those used by blacksmiths.
- Safety glasses to protect the eyes from any accident.
- Long clothing that covers our arms and legs well.
- Closed shoes , preferably leather.
- A fire extinguisher is never superfluous when we work with fire and high temperatures.
Procedure to recycle aluminum at home
Step 1
Preheat the oven to at least 670°C to ensure the temperature is high enough to melt the aluminum. If you plan to work with a torch, it’s also a good idea to preheat the crucible with the torch for faster melting.
Step 2
If the crucible is small, while the oven is heating up, it is recommended to crush or cut the aluminum cans into small pieces with the help of metal scissors or steel pliers. You should also prepare the molds where you will empty the recently recycled molten aluminum in a place near the oven. If the crucible is large enough to hold several crushed cans, this may not be necessary.
Pro Tip: Aluminum beverage cans are generally made from at least two different aluminum alloys. The cap is usually made of one alloy while the body is made of another. It’s a good idea to separate these parts of the can and recycle them separately to get two different kinds of aluminum with different physical properties.
Step 3
Place all the aluminum in the steel pot or crucible and, with the help of metal tongs, introduce them into the oven. If you are working with a torch, then you can place the crucible or steel container with the aluminum on a hot burner to heat it while you directly heat the aluminum with the torch.
Step 4
Wait a few minutes while the furnace heats up again (it cools down a bit when you open it to insert the crucible) and to give the aluminum time to heat up and melt. You should check that all the aluminum is molten by using some type of long metal rod as a stirrer.
step 5
Once the entire sample is melted, allow to heat for a few more minutes to raise the temperature above the melting point. This is done to prevent the aluminum from solidifying before it is transferred to the molds. It is important to remove the solid slag that forms on the surface of the molten aluminum. This contains the impurities and carbon from the plastic coating of the can and the printed labels.
step 6
Carefully and using the metal tongs, pour the molten aluminum into the molds and let stand until solid.
step 7
Once the aluminum is completely solid, it can be quickly cooled by immersing the mold in cold water.
step 8
Once cool to the touch, it is time to demold the aluminum block. This can be accomplished by inverting it and tapping it on the ground or another hard surface.
References
Cleanipedia. (2021, September 8). How to recycle aluminum . https://www.cleanipedia.com/ar/sustentabilidad/como-se-recicla-el-aluminio.html
Physical Didactics. (nd). heat capacities . https://didactica.fisica.uson.mx/tablas/capcalorificas.htm
Fluke. (2021, May 9). Obtaining the correct temperature over time . https://www.fluke.com/en-us/information/blog/temperature/obtaining-the-correct-temperature-with-the-pass-of-time
Franco Grace, A. (2010). Latent heat of fusion . Physics with computer. http://www.sc.ehu.es/sbweb/fisica/estadistica/otros/fusion/fusion.htm
Merckel, G. (2021, April 10). How to recycle aluminum at home . Youtube. https://www.youtube.com/watch?v=0v5CuqWVASQ
Olmo, M., & Narvae, R. (nd). Table of Specific Heats . Hyper Physics. http://hyperphysics.phy-astr.gsu.edu/hbasees/Tables/sphtt.html
Portillo, S.R. (2020, May 25). How to recycle aluminum . greenecology.com. https://www.ecologiaverde.com/como-reciclar-aluminio-2786.html
Salillas, RP (2020, May 12). What temperature does a flame reach? Mercor Tecresa. https://mercortecresa.com/blog/que-temperatura-alcanza-una-llama
Weerg. (2020, June 23). Aluminum: characteristics, properties and advantages . https://www.weerg.com/es/es/blog/aluminio-character%C3%ADsticas-propiedades-y-ventajas
