It can seem that every steelmaker on the planet brags that steel is ‘infinitely recyclable’ – but is it? Well, yes and no.
In one sense steel – or more accurately, iron – really is infinitely recylable. Nuclear reactions aside iron atoms are indestructable. It doesn’t matter how many times you roll, cut or cast iron, the atoms will be unchanged. So yes, you can recycle them for ever. That’s different to, say, plastic or paper, which will deteriorate every time they are recycled.
But suppose I sell you a nice white T-shirt, and I tell you that it is infinitely washable. And you wear it once, and you wash it, and you find it’s shrunk by 5%. What’s more, it’s not white any more, it’s a bit grey. And every time you wash it it shrinks by another 5%, and the colour gets a bit darker. You might complain that it isn’t really infinitely washable at all.
And this is the problem with claiming that steel is infinitely recyclable. Every time you recycle it there’s a yield loss – it could be 5% or more. That’s not so significant if your product lasts 40 or 50 years, but it is hugely significant if it is recovered and recycled repeatedly, like a tin can, or steel packaging.
And what about the T-shirt getting darker and darker grey? There are two problems here. One is that steel is commonly mixed with other metals in the form of an alloy, to achieve specific properties: hardness, strength, corrosion resistance and such like. The other is that steel is often used in composite products along with other metals such as copper and tin, and when those products are recovered for recycling the copper or tin may get mixed in with it.
The consequence of these two factors is that every time the steel is recycled it ends up containing a bit more copper, a bit more tin, or a bit more of those other alloying elements. Once they are in, it is extremely hard to get them out again. And so the proportion of these so-called ‘tramp elements’ builds up over time.
For some products that doesn’t matter. The steel for rebar doesn’t need to have an especially high level of purity to do its job. But for other products purity is paramount, and the alloy content is controlled to fractions of a percent. For engineering applications for cars or critical components of a structure, contamination could be lethal.
To stretch the analogy, it doesn’t matter if your T-shirt is getting rather grey, as long as you only use it for jogging. But if it’s your best white shirt, it’s a problem.
What can be done about it? To date, steelmakers have been able to concentrate their use of recycled steel in low specification applications, and dilute it by adding primary iron as required. But neither of those approaches can achieve circularity in the long run.
Scrap could be separated and sorted better. If you can keep unwanted elements out of your scrap stream entirely, then it won’t build up in recycled products. And if you can match scrap quality to the product specification – for example only end-of-life scrap rebar is recycled into new rebar – then you avoid the need to dilute tramp elements with new primary material.
Both those solutions are easier said than done. If you’re recycling complex products it may not be so easy to separate out the steel from all the copper wires and non-steel parts. And given the number of different steel alloys that could be in something like a car, it may be simply impossible to separate them all and direct them to bespoke recycling streams.
How much does it matter? Two research papers have modelled steel recycling to understand the scale of the problem, looking at steel production in the US1, and globally2. To summarise their findings: i) as ever, there’s a lot of uncertainty; ii) it’s essential to allow free international trade in scrap, so that scrap with relatively high levels of tramp elements can be directed to countries with the greatest demand for high tolerance steel products – which will be developing countries; and iii) if trade is not impeded, and if action to improve scrap separability and sorting takes place urgently, then at the global level contamination may not constrain recycling rates until 2050.
You can’t stop your T-shirt shrinking (yield loss), but you can at least separate your jogging kit from your best whites and keep both clean (managing tramp elements).
And one more recycling lesson from the infinitely washable T-shirt: it’s really important to make sure your clothes don’t get lost in the laundry.
Footnotes
- Cooper et al (2020) The potential for material independence and circularity in the US Steel Sector, in the Journal of Industrial Ecology, January 2020 ↩︎
- Daehn et al (2018) How will copper contamination constrain future global steel recycllng? Environmental Science and Technology, 2017, 51, 6599-6606 ↩︎