Field lines
2 December 2020

An Attractive Way to Recycle

Left: In magnetic density separation, a magnet magnetizes a magnetic fluid and produces a field gradient, resulting in an effective density gradient used to separate plastics of different densities. Right: A powerful neodymium magnet sits between two different ferrofluid solutions. The solution on the right remained stable in the magnetic field. In the solution on the left, however, the iron oxide particles clumped together where the field was strongest. Left: In magnetic density separation, a magnet magnetizes a magnetic fluid and produces a field gradient, resulting in an effective density gradient used to separate plastics of different densities. Right: A powerful neodymium magnet sits between two different ferrofluid solutions. The solution on the right remained stable in the magnetic field. In the solution on the left, however, the iron oxide particles clumped together where the field was strongest. Alex van Silfhout

Magnetic fields could lead to a cost-effective solution for recycling plastics.

Story by KRISTEN COYNE

The world is awash in plastic. Hundreds of tons of the stuff is produced every year worldwide, most of which never makes it to the recycling center.

Part of the reason why is that plastic is cheap to produce. In order to make recycling it more attractive, the process needs to be cheaper at a commercial scale. Recent research on ferrofluids, a liquid with small particles of iron oxide mixed in, has brought that goal closer to reality.

Scientists from the High Field Magnet Laboratory (HFML) and Utrecht University, both in the Netherlands, tested ways to optimize the iron-packed solutions so they could be used in large-scale magnetic density separation (MDS) facilities.

Hans EngelkampHans Engelkamp
Image credit: Dick van Aalst

MDS is a technique used to sort items by their density. It has been used in niche applications, such as separating diamonds from less dense materials, for some time, and a few recyclers have tried it in small-scale pilot projects for plastics.

In MDS, a mix of plastics is chopped into small pieces then poured into a large container of ferrofluid positioned above a powerful magnet. Because the field strength of that magnet tapers off with distance, the particles in the ferrofluid spread out across a gradient, with more of them in the zone closest to the magnet. As a result, the effective density of any given layer in the fluid varies depending on its proximity to the magnet. The plastic bits float at the level that corresponds to their density, allowing recyclers to sort them into different grades.

Although this works fairly well at a small scale, ramping up production introduces a new set of problems, explains Hans Engelkamp, an assistant professor at the HFML and a co-author on the publication resulting from the research project.

"The problem with large scale here would be that you need a magnetic field gradient across a large distance," Engelkamp says. "That means you need big magnets."

But a stronger field causes the 10-nanometer wide particles in ferrofluid to clump together, ruining the gradient.

"One of the dangers is that they start to attract each other," explains Engelkamp. "So the project here was really to stabilize those nanoparticles."

To discourage the clumping, the research team added a citric acid to the ferrofluid that boosted the negative charge of the particles. The stronger repulsion between the particles was enough to overcome the magnetic attraction causing them to clump.

This more stable ferrofluid could make MDS useful for treating heavier recyclables, like those in electronics.

"If the ferrofluids are more stable, it also means you can use a higher concentration, and that means that you can have a higher magnetization per volume," says Engelkamp. "And that means that you could separate higher density materials."

Engelkamp won't be involved in those projects, however. Although the research team used relatively high fields to develop the better-performing ferrofluid, they also demonstrated that future R&D can happen at lower fields.

That's exactly as it should be, Engelkamp says. "We should discover stuff at high fields — that's what we do. But then later we should not be needed anymore. We want to pave the way for further development that no longer needs these expensive, very high magnetic fields."

Last modified on 3 December 2020