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The Auckland Harbour Bridge is made up of a lot of steel (Photo: Brendon O’Hagan).
The Auckland Harbour Bridge is made up of a lot of steel (Photo: Brendon O’Hagan).

ScienceDecember 28, 2019

How to reduce the carbon cost of building our world with steel

The Auckland Harbour Bridge is made up of a lot of steel (Photo: Brendon O’Hagan).
The Auckland Harbour Bridge is made up of a lot of steel (Photo: Brendon O’Hagan).

Lauire Winkless speaks to the New Zealand scientists working to clean up the final frontier and how to make steel in a zero-carbon future.

In today’s urbanised world, steel is everywhere. It’s used in everything from critical infrastructure like roads and railways, through to earthquake-resilient buildings, wind turbines and electric vehicles. But making steel comes with a significant environmental cost. 

In 2018, the IPCC reported that the global iron and steel industry was responsible for producing 2.6 Gt of carbon dioxide (CO2) per year; that’s 7 % of the world’s total CO2 emissions. New Zealand’s contribution to that, according to a recent report from the Ministry for the Environment, was equivalent to about 1,750 kilotonnes of CO2 per year.

These numbers go some way to explaining why New Scientist magazine in November described heavy industries as the final frontier in the fight against climate change. As countries race to reduce their environmental impact, they’re beginning to re-evaluate how they manufacture essential materials. New Zealand researchers are working to clean up the production of steel and vanadium, by moving away from carbon.

“The source of all this CO2 is the chemical reduction of iron ore,” says MacDiarmid researcher Dr Chris Bumby. “Modern ironmaking is an industry based on the incremental development of a 2000-year-old process.” So while a time-traveller from the Iron Age would be astonished by the scale and complexity of a modern smelting plant, they’d certainly recognise the fundamental chemistry that supports it. 

The process starts with iron oxide ore, which is combined with a carbon-rich fuel – usually coal – in a furnace that runs at incredibly high temperatures. The reaction between them produces CO2 and a liquid metal alloy called pig iron, which is 4.5% carbon. Turning pig iron into high-strength steel involves a further process that removes almost all of that remaining carbon. 

Here in New Zealand, ironmaking is slightly different, because our iron ore doesn’t come in the form of solid rock. Rather, we use a dark, dense, grainy material called ironsand that is found all along the west coast of the North Island. In addition to iron and oxygen, ironsand contains small amounts of other compounds, like titanium oxide. This means that it must be put through a more specialised process. Colloquially known as the ‘NZ Steel process’, this approach still relies on coal to drive the reaction. 

Regardless of the route, carbon is still central to large-scale steel production, both in NZ and worldwide. In fact, on average, the production of one tonne of steel emits 1.8 tonnes of CO2. Finding a new way to make iron – one that could eliminate the use of coal – could be a significant step towards NZ’s climate targets.

That’s been the goal of Dr Bumby and his colleagues at Victoria University’s Robinson Research Institute since 2014.  He says, “The aim of our first project with the University of Wollongong was to use methane (CH4) in the reduction of ironsand. But along the way, we switched to hydrogen, which took the carbon entirely out of the process.” 

For the past 18 months, the team have been looking exclusively at hydrogen reduction of iron ores. “The results have been fantastic,” he says, “We have an entirely new, zero-carbon way to make iron, and it works especially well for New Zealand ironsand.” Their one-step process produces very high purity (up to 99.85%) iron in under 20 minutes. Bumby continues, “For context, NZ Steel’s equivalent process takes about 10.5 hours, and the product contains more than 4% carbon.” 

To make their carbon-free iron, Bumby uses a system called a fluidised bed reactor. He explains, “It’s effectively a tube containing ironsand sitting on top of a porous plug. We push hydrogen gas through it.” The unique chemical makeup of NZ ironsand has proved to have an added benefit, “As the reaction starts, we’ve found that the titanium content migrates out of the sand to form a very thin protective skin on the outside of each grain.” This stops the grains from sticking to one another, allowing the reaction to happen more quickly. 

The process, previously carried out only at laboratory scale, looks set to expand, thanks to a $6.5 million grant from the MBIE Endeavour programme. Over the next five years, Bumby and his trans-Tasman team of collaborators will scale up production of their carbon-free iron, from hundreds of grams to tens of kilograms. 

They’ll also extend their research to include another valuable component of NZ ironsand – vanadium. “Vanadium is an exceptionally useful metal,” says Bumby, “it is used in lightweight alloys, and is the basis of the best energy storage solution for electricity grids. And it’s expensive; in the range of $50,000 per tonne. So even though ironsand contains less than 1% vanadium, it’s still an incredibly valuable resource. We want to find cleaner, more efficient ways to extract it from the ore.”  

Leading this part of the work are two other members of the MacDiarmid Institute – Associate Professor Aaron Marshall (University of Canterbury) and Professor Jim Johnston (Victoria University of Wellington). They will be taking a new approach to vanadium extraction, “one that is much more careful about waste streams than today’s processes,” says Bumby. Another key figure is Dr John Kennedy from GNS, who will develop specialised microwave heaters for the new reactor. “Ironsand absorbs microwave radiation really well,” explains Bumby. “This will let us reach much higher temperatures, and make the whole reaction faster and more efficient.”

New Zealand’s relatively green electricity grid provides an important opportunity for the team – the hydrogen gas so central to their work could be generated using renewable energy. “We’re in discussions with electricity providers at the moment, and we’re optimistic,” Bumby says. “We have to look at the big picture. Demand for steel is growing all the time, and importing it will simply result in more CO2 entering the atmosphere, produced by overseas factories. So, as a country, if we’re serious about becoming a zero-carbon economy, then we need to look very seriously at our domestic steel industry.”

This content was created in paid partnership with The MacDiarmid Institute. Learn more about our partnerships here

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