Working at the Century Aluminum Co. smelter in Hawesville, Ky., can be like having a job in an oven. The interior temperature hovers around 140F, which isn’t necessarily hotter than, say, your typical steel mill. What’s especially hellish about an aluminum smelter is how close you have to stand to bubbling vats of molten, electrified metal.

Workers wear helmets, masks, and heavy, fire-retardant clothing. They look like smoke jumpers. Over a 12-hour shift they’ll lose several pounds of water weight as they peer over cauldrons, occasionally stirring 1,700-degree liquid aluminum with long metal rods. They wear earplugs against the hum of 170,000 amps surging through the mixture, which chemically breaks down ore. The air itself feels charged — and smells like the blended aromas of an overheated car engine and a sweaty fistful of coins. If you breathe through your mouth, you can taste the metal on your tongue.

The Hawesville smelter makes some of the world’s highest-purity aluminum, and it’s the only one in the U.S. that mass-produces the military-grade kind needed for fighter jets and tanks. Yet the method it uses isn’t that different from how aluminum was made in 1886, when Charles Martin Hall, an Alcoa Corp. co-founder, first shot an electrical current through a mineral bath of aluminum oxide. The process has become more efficient over time, but no one’s figured out a better way to separate oxygen atoms from aluminum atoms. The business is stubbornly dirty, expensive, and dependent on human labor.

It’s also incredibly energy-intensive. At full capacity, the Hawesville plant uses as much electricity as the entire city of Louisville (population 620,000), about 70 miles away. Not that the plant is at full capacity these days. This smelter was built in 1969, on the banks of the Ohio River, in a region that used to be the aluminum industry’s equivalent of Silicon Valley. Cheap coal dug out of the Appalachians and barged down the river was used to power half a dozen smelters clustered around Kentucky, Ohio, and Western Pennsylvania.

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