To recycle old gadgets, crush them into nanodust

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Researchers have an idea to simplify electronic waste recycling: Crush it into nanodust.

Specifically, they want to make the particles so small that separating different components is relatively simple compared with processes used to recycle electronic junk now.

Chandra Sekhar Tiwary, a postdoctoral researcher at Rice University and a researcher at the Indian Institute of Science in Bangalore, uses a low-temperature cryo-mill to pulverize electronic waste—primarily the chips, other electronic components, and polymers that make up printed circuit boards (PCBs)—into particles so small that they do not contaminate each other. Then they can be sorted and reused, he says.

Tiwary and his coauthors intend their idea to replace current processes that involve dumping outdated electronics into landfills, or burning, or treating them with chemicals to recover valuable metals and alloys. None is particularly friendly to the environment, Tiwary says.

“In every case, the cycle is one way, and burning or using chemicals takes a lot of energy while still leaving waste,” he says. “We propose a system that breaks all of the components—metals, oxides, and polymers—into homogenous powders and makes them easy to reuse.”

A billion tons by 2030

The researchers estimate that so-called e-waste will grow by 33 percent over the next four years, and by 2030 will weigh more than a billion tons. Nearly 80 to 85 percent of often-toxic e-waste ends up in an incinerator or a landfill, Tiwary says, and is the fastest-growing waste stream in the United States, according to the Environmental Protection Agency.

The answer may be scaled-up versions of a cryo-mill designed by the Indian team that, rather than heating them, keeps materials at ultra-low temperatures during crushing.

Cold materials are more brittle and easier to pulverize, Tiwary says. “We take advantage of the physics. When you heat things, they are more likely to combine: You can put metals into polymer, oxides into polymers. That’s what high-temperature processing is for, and it makes mixing really easy.

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“But in low temperatures, they don’t like to mix. The materials’ basic properties—their elastic modulus, thermal conductivity, and coefficient of thermal expansion—all change. They allow everything to separate really well,” he says.

Very cold crushing

As reported in Materials Today, the test subjects in this case were computer mice—or at least their PCB innards. The cryo-mill contained argon gas and a single tool-grade steel ball. A steady stream of liquid nitrogen kept the container at 154 kelvins (minus 182 degrees Fahrenheit).

Shaking makes the ball smash the polymer first, then the metals, and then the oxides just long enough to separate the materials into a powder, with particles between 20 and 100 nanometers wide. That can take up to three hours, after which a water bath separates the particles.

“Then they can be reused,” Tiwary says. “Nothing is wasted.”

Source: Rice University