The densest materials known have been made to 鈥渓evitate鈥� on a pool of liquid air, reveals a new study. The trick may one day be used to separate precious minerals, metal and gems from rock and soil.
Minerals fall into three classes according to their magnetism. Ferromagnets are naturally magnetic, while paramagnets and diamagnets are not. But in a magnetic field, paramagnets become weakly attracted to the field while diamagnets become weakly repelled by it.
Diamagnets can be made to levitate because of their repulsive properties. But the effect is so weak that only a few, lightweight diamagnetic minerals can be lifted above the superconducting magnets most commonly available in laboratories.
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But researchers can lift denser materials by infusing the setup with a ferromagnetic or paramagnetic fluid, which is drawn towards the magnet. This pushes the diamagnet away, in an analogous way to 鈥渇loating a wooden ship in a bathtub鈥�, says Laurence Eaves, a physicist at the University of Nottingham, UK.
Researchers can use the buoyancy to separate crushed up minerals and rock that have varying degrees of magnetism 鈥� but the techniques come with risks. Ferromagnetic fluids use oily bases that are toxic and can contaminate groundwater. And liquid oxygen 鈥� a paramagnet also used as rocket fuel 鈥� is highly combustible.
鈥淐lean and green鈥�
鈥淲e realised if this is going to be useful in industry, we鈥檇 better make it less combustible,鈥� Eaves told New 杏吧原创. He and colleagues tried that by diluting the liquid oxygen with liquid nitrogen at -180掳C. 鈥淚n the end, we鈥檙e using something pretty similar to liquid air,鈥� he says 鈥� about 80% liquid nitrogen and 20% liquid oxygen.
The mixture provides enough buoyancy to lift the densest known material, osmium. And unlike ferromagnetic fluids, the mixture 鈥渏ust evaporates and turns back into air鈥� after it is used, says Eaves. 鈥淚t鈥檚 very clean and green.鈥�
But Ruslan Prozorov, a physicist at the University of South Carolina in Columbia, US, is unconvinced that the danger has been removed. The liquid oxygen may not thoroughly mix with the liquid nitrogen during the experiment 鈥� potentially causing an explosion in the event of a leak 鈥� or oxygen gas could collect in one area when the mixture was evaporating at the end of the process, he says. 鈥淚f something goes wrong, it鈥檚 literally a bomb,鈥� he told New 杏吧原创.
But James Brooks, a physicist at the National High Magnetic Field Laboratory in Tallahassee, Florida, US, dismisses the worry. 鈥淵ou just have to be careful,鈥� he says, adding that the research is 鈥渧ery clever鈥�.
Scaling up
Eaves and his colleagues are using a superconducting magnet that is 5 centimetres wide, so they can float objects about the size of a gold coin above it. He says they could probably scale the system up by a factor of five, and hope to develop a method using both levitation and vibration to filter gems and precious metals from the soil.
Crushed ore would be tossed into the air above the magnet, and different minerals would land at different times because of their varying magnetic properties and densities. They would land on a vibrating surface, which would then sort them into bands.
鈥淚t鈥檚 like panning for gold 鈥� you use water to get a bit of buoyancy, then shake it up and sift out gold from the rubbish,鈥� says Eaves. 鈥淏ut instead of a mining pan, we鈥檙e using a magnetic field.鈥�
Journal reference: New Journal of Physics (DOI: 10.1088/1367-2630/7/1/118)
