Hungarian mud remedy also captured carbon

Hungarian mud remedy also captured carbon

8 March 2012, by Tom Marshall

Red mud impacted surface waters of the Torna Creek in Kolontár, approximately 1km downgradient of the collapsed tailings dam.

Researchers were surprised to find that neutralising a tonne of dangerously alkaline red mud also absorbed and locked up around 220kg of CO₂.

The findings, published in Science of the Total Environment, could mean industrial by-products like red mud become a useful resource and not just dangerous waste.

'A few people had looked at the possibility of using red mud for carbon capture in the past, but it was a real surprise that adding gypsum turned out to have this effect,' says Dr Phil Renforth, a geochemist at the University of Oxford and the paper's lead author. 'Looking at the geochemistry of the deposits it left behind showed a direct relationship between how much gypsum was added and how much carbon was captured.'

The October 2010 disaster at the Ajkai Timföldgyár aluminium plant in Western Hungary sent around a million cubic metres of hyper-alkaline red mud spilling across the Hungarian countryside. It killed at least nine people, injured more than 100, and caused enormous environmental damage. A number of studies led by Dr Will Mayes of the University of Hull and Dr Ian Burke of the University of Leeds have explored the disaster's environmental impact.

Beyond trying to contain the spill as much as possible, emergency services' immediate priority was to neutralise its pH. They did this by adding acid directly, and by dosing the mud with crushed gypsum - a soft rock that's used in plaster and cement.

The mud has now largely been cleared up and stored in a more secure facility. But researchers investigating the effects of the second countermeasure have found it had carbon-capture benefits beyond stopping the immediate environmental catastrophe.

They arrived at this conclusion after analysing the different isotopes of carbon and oxygen in the mineral deposits left behind where gypsum had been used to neutralise the mud. (Isotopes are different forms of the same element, with the same basic atomic structure but different numbers of neutrons.)

Examining these isotopes showed that the carbon now locked up in the deposits formed when the gypsum reacted with the red mud had come recently from the atmosphere. And the amount of it stored there depended on how much gypsum had been added during the cleanup.

Red mud is an inevitable by-product of refining aluminium from its ore, bauxite. It's dangerous in several ways; it's extremely alkaline, with a pH of more than 10, and contains lots of potentially toxic metals. The first is the bigger and more immediate problem; the red mud that spilled in Hungary was so corrosively alkaline that it killed whole rivers and caused severe chemical burns to any people or animals unlucky enough to touch it.

Gypsum and red mud certainly aren't going to solve the problem of human carbon emissions wholesale. There's a lot of red mud around – all of it that's ever been produced is still stockpiled somewhere – and not enough gypsum to deal with it. To neutralise just one year's worth of red mud would take between 33 and 69 per cent of total global gypsum production. And this would counteract just 3-4 per cent of the aluminium industry's annual carbon emissions.

But the technique could nevertheless prove helpful to efforts to reduce global CO₂ emissions, particularly if more gypsum can be made cheaply by capturing the flue gases of coal power stations - emissions that are themselves environmentally damaging. The gypsum technique could also be applied to other industries with dangerously alkaline waste products, such as paper manufacture.

'It's important we stop thinking of these technologies as magic bullets that will solve all our problems at once,' says Renforth. 'We're going to need a whole portfolio of technologies, and many of them will have benefits beyond carbon capture, such as increasing biological productivity or, as in this example, dealing with what's otherwise a dangerous industrial waste product. If we can show that these things have a carbon-capture function, they start to have some intrinsic value. We can begin to build carbon capture into the way we process waste.'

Renforth is working on a variety of other possible ways to use minerals to capture atmospheric carbon, in particular on the potential of adding crushed limestone to the oceans, or enhanced weathering of rocks on the land surface.