Each year, more than 7 gigatonnes of man-made carbon dioxide is released into the atmosphere, much of which comes from steel manufacturing plants around the world. In 2007, global steel production stood at 1.34 billion tonnes; producing one tonne of steel can release up to one tonne of carbon dioxide.

Now, Mourad Kharoune and colleagues at the Ecole de Technlogie Supérieure (ETS) in Montreal have put forward a new way to sequester this carbon dioxide using steel slags. These are complex mixtures of calcium, iron, silicon, aluminium, magnesium and manganese oxides produced when molten steel is separated from impurities in the furnace.

There are four types of steel slag, named after the processes used to produce them: blast-furnace slag; basic oxygen-furnace slag; electric arc-furnace (EAF) slag; and ladle-furnace (LF) slag. And fresh steel slags contain three major phases of calcium: portlandite (Ca(OH)2), Ca-(Fe)-silicate and Ca-Fe-O, and several mineral phases, including Mg-Fe-O, Fe-O, and traces of calcite (CaCO3). The large amount of basic oxides means that steel slags are highly alkaline – with a pH of around 12 – and, based on the total calcium content, can theoretically sequester around 0.25 kg of carbon dioxide per kg of slag.

Kharoune and colleagues suggest that EAF and LF slag suspensions are especially suited to carbon-dioxide capture.

The carbon sequestration technique they propose involves transforming steel slags into calcium and magnesium oxides. In turn, these are transformed into calcium and magnesium carbonates, capturing carbon dioxide. The method is simple and inexpensive, works at ambient temperature and pressure, and can neutralize 15% of emitted carbon dioxide at a cost of just $10 per tonne. Other similar technologies cost up to $100 per tonne.

The researchers say that the process could be improved so that it neutralizes up to 40% of carbon-dioxide emissions.

According to the team, the LF slag suspension's capacity to sequester emissions (24.7 g of carbon dioxide/100 g of slag) is 14 times higher than that of the EAF suspension. This could be because the LF suspension contains more of the rare mineral portlandite, which is highly alkaline.

The work was published in Industrial and Engineering Chemistry Research.