Man cleaning die ahead of liquid metal injection

LiME aims to close the loop for metal recycling

E&T visits the LiME centre for liquid metal engineering, where researchers are turning low-grade recycled metal into top quality alloys.

Swaddled in insulated padding and with cables dangling from its nozzle, the long helical mixer-pump would not look out of place on the set of a Bond movie, as the latest villainous attempt to deliver our hero to a painful death.

And painful it would be - the twin-screw pump may be derived from food-mixing technology, but this version is designed to deliver finely-blended liquid aluminium alloy at temperatures in excess of 500C.

Fortunately, it is being developed as a force for good, not evil. We are at LiME, the EPSRC Research Centre for Innovative Manufacturing in Liquid Metal Engineering, where the aim is to minimise - and perhaps even eliminate - the need for primary refining of metals such as aluminium by instead turning existing low-grade metal, such as recycled drinks cans, into high-grade alloys.

E&T toured LiME, which is a partnership between three universities - Birmingham, Brunel and Oxford - and 14 industrial sponsors, with Brunel metallurgist Professor Geoff Scamans. An expert on light metals and a background as a scientist in industry, he is one of the lead investigators on the liquid metal engineering team.

According to Scamans, LiME's technologies could close the loop for recirculating non-ferrous metals. 'The aluminium can is a wonderful invention,' he says, 'but aluminium is a more interesting material than that! We are a developed country with resources, the key thing is to value and re-use those resources, not throw them away. If you look at every metallic element we use, we are very profligate.'

The problem for recyclers is that it takes a lot of energy to make recycled metal as pure as primary metal, so LiME is looking for other ways to reprocess scrap. In particular, this means those twin-screw helical mixers, invented at Brunel by LiME's director Professor Zhongyun Fan, and used as what Scamans calls 'melt conditioners'.

'There's a lot of commonality with the polymer industry, but this is a higher temperature: 500-600C. The liquid metal gets 20 to 30 seconds in the melt conditioner, it's a combination of pressure and shear,' Scamans explains.

'When I first got involved it was almost a mystery what the high shear was doing. Now there's the beginning of some understanding of what melt conditioning does.'

That understanding is already the subject of several academic papers, with many more sure to follow, but in brief what the melt conditioner appears to do is refine and smooth out the metal's structure. In particular, it disperses - or blends in - the oxide impurities, turning them from a quality disadvantage into the opposite.

'We realised that instead of filtering oxides out, we can turn them to our advantage,' says Scamans. He adds: 'The twin-screw mixers are built to our spec, they are the first prototypes, going out to check how they work.'

While the first melt conditioners were based on helical mixers used in food and polymer production, for the second generation the LiME team has studied other kinds of mixing technology in the food and paint industries.

'The next generation will be quite different - we've already learnt a lot,' Scamans explains. 'This version is over-designed for what it does. In the next generation we're using a much simpler mixing device - the patent went in last week.'

He adds: 'We're also doing fundamental work on the viscosity of metal to help with the understanding of what shear is doing to metal. The problem is we're going against conventional wisdom here, so we have to be very careful and sure of the evidence.'

And summing up LiME's ambition, he concludes: 'It's about fundamentally changing people's view of what a metal is - understanding that it is entirely appropriate to re-use metal again and again, and entirely wrong to rely on primary production of metal. It is a fundamental industrial change and a fundamental cultural change.'

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