Mad About Science: Aluminum

By Brenden Bobby
Reader Columnist

Have you spent any time arguing with someone about how to spell aluminum? Is it “aluminum” or “aluminium”? It turns out that both are correct, and though the world at large tends to use aluminium, the American version may have come first.

Aluminum is a metallic element with an atomic number of 13. It has a density that is about one-third that of steel, which makes it lightweight. You’ve undoubtedly encountered aluminum in your daily life on several occasions — from foil to parts of your vehicle — though one of its greatest uses is in aircraft. Being lightweight and malleable means it’s great for forming specialized parts that can generate lift when forward energy is applied. 

While steel components are used in aircraft, the importance of aluminum in the field of flight cannot be overstated.

Aluminum is an abundant element in the universe and on Earth, but extracting enough of it to do something meaningful is an energy intensive process that requires multiple stages to achieve. The bulk of aluminum is extracted from the mineral bauxite, a sedimentary rock with a dull red sheen attributed to the oxidized iron also found within the mineral. 

Row of rolls of aluminum lie in production shop of plant.

Metals like copper, iron, gold and silver are often extracted by applying heat directly to the ore. This causes the elemental metals to melt into a liquid, which gives metallurgists a way to extract impurities by separating them from the rock. Aluminum, however, requires a considerably more complicated series of processes, beginning with the “Bayer process.”

This process begins when the bauxite is ground up and blended into a slurry, which is then mixed with a hot solution of lye. The mixture is fed into a pressurized digestion chamber, where steam is generated and released to cool it down and allow for an easy extraction of the bauxite and other impurities. During the cooling process, aluminum hydroxide that had evaporated begins to precipitate for collection.

Another, even more energy intensive process, is called the “Hall-Héroult process.” This process involves mixing aluminum oxide in a solution of molten cryolite and calcium fluoride, and then subjected to an electrical current in a process called electrolysis. The aluminum sinks to the bottom of the solution, where it is cast off in large rounded billets that are then used for manufacturing.

As you can imagine, these processes eat up an immense amount of energy. That’s worth thinking about the next time you’re trying to bank that crushed soda can off your trash bin’s back stop. Recycling aluminum requires much less energy, which means less carbon going into our atmosphere, which means less crazy hot summers in our future, more plants and animals in our present, and cleaner air for everyone. Those are some pretty great rewards for something as simple as not throwing a can in the garbage.

Aluminum is nonmagnetic, so it becomes more difficult to sort out of landfills. Metals like iron and copper can be pulled from waste with large magnets, but aluminum will sit right where it’s at and have to be sorted out through different mechanical means, which will consume even more energy and generate more pollution. While current recycling methods may not be perfect, it’s certainly better than poisoning the environment because a waste bin was easier to use at the moment.

You may be wondering what the recycling process for aluminum looks like. It’s fairly straightforward: simply heating the material up to a melting point and casting it back into billets for manufacturing. However, it’s not a perfect system and does produce a waste material called dross. Dross is an ashy substance of oxides mixed with bits of aluminum — this aluminum can be extracted industrially, but the resulting waste from the process is extremely volatile and reacts explosively with the air. 

Interestingly, dross is frequently mixed with asphalt and concrete, partially because there are very few other uses for it that don’t involve highly volatile chemicals, and partially to lock up the waste mixture in a substance where it can do the least amount of harm while still being a beneficial building material.

Aluminum in the home is almost exclusively an alloy — a metal mixed with other metals. Copper, magnesium, manganese and zinc are the most common elements mixed with aluminum to create alloys for aircraft, automobiles, cans and foil used in the kitchen. Aluminum as an alloy is more pliable and far more lightweight than steel, while remaining nonmagnetic, corrosion resistant and nontoxic.

Despite the fact that aluminum is nontoxic and nonreactive to most biology, the presence of acids can cause detrimental environmental effects to fish and plants. The presence of aluminum in acidic soils causes slow growth in plants, and aluminum in acidic water can cause disorders among gill-breathing animals like fish. Unfortunately, both of these are at equal risk in areas near aluminum smelters that produce sulfur dioxide, a precursor to acid rain that ends up in water and soils, in addition to particulate aluminum released into the air during the process of refinement.

The next time you crack open an ice cold beer, stop and ponder for a moment where the metal may have come from. Chances are, it has already been recycled more than once.

Stay curious, 7B.