National Geographic/Getty Images
The vats used in the Hall-Heroult process are known as pots. A large pot can produce more than 2 tons of aluminum each day. But companies can and do multiply that output by connecting several pots together in potlines. One smelting plant may contain one or more potlines, each with 200 to 300 pots. Inside these pots, aluminum production continues day and night to make sure the metal remains in its liquid form.
Once a day, workers siphon aluminum from the potlines. Much of the metal is set aside to become fabricating ingots. To make a fabricating ingot, molten aluminum proceeds to large furnaces where it can be mixed with other metals to form alloys. From there, the metal undergoes a cleaning process known as fluxing. Fluxing uses gases such as nitrogen or argon to separate impurities and bring them to the surface so they can be skimmed away. The purified aluminum is then poured into molds and cooled rapidly by spraying cold water over the metal.
Some of the aluminum siphoned from the potlines isn't alloyed or cleaned. Instead, it's poured directly into molds, where it cools slowly and hardens to form foundry (or remelt) ingots. Primary aluminum plants sell remelt ingots to foundries. Foundries return the aluminum to its liquid state and proceed with the alloying and fluxing themselves. They then turn the aluminum into various parts -- for appliances, automobiles and other applications -- by using the following fabricating techniques.
- Casting: Aluminum can be cast into an infinite variety of shapes by pouring the molten metal into a mold. As the aluminum cools and hardens, it takes the shape of the mold. Casting is used to make solid, uniquely shaped objects, such as parts for car engines, aluminum hammers and the bottoms of electric irons.
- Rolling: By repeatedly passing heated aluminum ingots through heavy rollers, the metal can be flattened into thin sheets or even wafer-thin foils. It takes about 10 to 12 passes to make the thinnest foils, which can be a mere 0.15-millimeter thick.
- Extruding: Extrusion involves forcing softened aluminum through a die. The shape of the die opening determines the shape of the extruded aluminum.
- Forging: Forging, a process whereby aluminum is hammered or pressed, results in superstrong metal. This method makes forged aluminum ideal for stress-bearing parts of aircraft and automobiles.
- Drawing: To make wire, an aluminum rod is pulled through a series of successively smaller dies, a process known as drawing. Drawing aluminum can produce wire that is less than 10 millimeters in diameter.
- Machining: Traditional machining operations, such as turning, milling, boring, tapping and sawing, are easily performed on aluminum and its alloys. Machining is often used to produce bolts, screws and other small pieces of hardware.
A Beverage Can Is Born
A beverage can starts with a circular piece of metal punched from an aluminum sheet. This circle, which is 5.5 inches (14.0 cm) in diameter, is called a blank. One machine draws the blank into a cup with a diameter of 3.5 inches (8.9 cm). A second machine redraws the cup, elongating it, ironing it and thinning out the sides. Finally, the can is cleaned, decorated and "necked" to accommodate the lid.
Aluminum is an attractive metal and often requires no finish. But it can be polished, painted and electroplated. For example, beer and soda makers use a printing process to affix their labels on aluminum cans (see sidebar). Typical printing formulations are often lacquer coatings that both adhere well to the aluminum and provide aesthetic appeal. Of course, such finishes are a concern when it comes to recycling because they must be removed. In the next section, we'll explore how aluminum is recycled in detail.