Simpler E-Waste Recycling Method Might Be Greener, Too
(Inside Science) -- It's estimated that more than 50% of critical materials in electronics -- such as gold or rare earth metals -- end up in landfills. That's partly because common methods to extract and purify the metals are often difficult and expensive. In addition, the techniques use either corrosive chemicals or extremely high temperatures of around 1,800 degrees Fahrenheit (980 degrees Celsius), which both have a negative impact on the environment.
In a paper published in the journal Materials Horizons in March, materials scientist Martin Thuo and his collaborators at Iowa State University demonstrated a new method of extracting valuable metals from recycled electronics: one that requires only air and relatively low temperatures of 500-700 degrees Fahrenheit (260-370 degrees Celsius).
"Our biggest advantage is in simplicity, or ease of use," Thuo said. "My lab works on frugal technologies, and this method is one of a set that we are developing to recover metals from mixed-metal waste."
The method uses the relative reactivity of the waste metals. The more reactive a metal, the faster it will oxidize -- bind with oxygen to form an oxide -- when exposed to air. The oxidation starts at the surface, where atoms of the most reactive component will bind to oxygen first. Eventually a protective layer of oxidized metal will form that hinders further oxidation. So, Thuo and his colleagues use a balance between the amount of air in the environment and the temperature of the environment to encourage the different metal ions in the sample to diffuse: When the most reactive reach the surface, they oxidize and stay there. The oxides can then be physically separated from the rest of the waste sample due to their much larger density.
"If you are increasing the temperature, you're encouraging diffusion," Thuo said. "Materials are not static -- their ions are always diffusing -- in an amount proportional to temperature. All you need to do is help them move around, and then you can pull them out."
A strength of this new method is its tunability, allowing different metals to be extracted from a sample one by one. The balance of air and temperature controls which component in the waste reacts fastest. Once the most reactive component is entirely oxidized, it is separated from the rest of the sample, and the process continues with the next reactive component. The least reactive metals are all that remain at the end.
The tunability of the process also means that the waste must be "profiled" first, to determine the quantity of each component and design the air and temperature changes accordingly. Thuo said fine-tuning these profiles, as well as characterizing the method's environmental friendliness and cost, will be a priority as his team begins to commercialize the technology.
Ikenna Nlebedim, a materials scientist at Ames Laboratory in Iowa, believes the work has "great potential." "It provides a different perspective in the field of recycling and separation in which harsh chemicals are typically used," he said. "I look forward to the team applying it to actual waste material -- that will be great."
"We are also pursuing other approaches that will complement the tunable thermal-oxidative process reported here," Thuo said. "It is our desire to push the boundaries of conventional recycling and up-cycling methods through frugal innovation."