Researchers in the UK report that, with a bit of management, a largely untended slag heap could absorb significant amounts of CO₂, partially offsetting the emissions from steel production. Their open-access paper is published in the ACS journal Environmental Science & Technology.

Pullin et al.

Steel slag is a by-product of steel making and is produced during the separation of the molten steel from impurities in steel-making furnaces. The slag manifests as a molten liquid melt and is a complex solution of silicates and oxides that solidifies upon cooling. Several different types of steel slag are produced during the steel-making process: furnace or tap slag, raker slag, synthetic or ladle slags, and pit or cleanout slag.

[Furnace] Slags are named from the furnace from which they are generated, and there are three predominant forms: blast furnace (BF) slag from the production of iron and basic oxygen furnace (BOF) and electric arc furnace (EAF) slags, both of which are from the production of steel. The mineralogy of these slags depends on the raw minerals used, production methods, and postprocessing practices, and iron and steelworks carefully manipulate these factors to tailor the physical and chemical characteristics of their products. More than 60 different minerals have been identified in iron and steel making slags.

—Pullin et al.

Slag is rich in calcium silicates that dissolve in water; when atmospheric CO₂ dissolves in water to form carbonic acid, it reacts with the dissolved silicates to form stable carbonate minerals such as calcite, which nearly permanently sequesters the carbon.

Steelmaking produces two tons of CO₂ emissions per ton of steel but the researchers estimate that CO₂ absorption by the resulting slag could offset around 10% of those emissions.

Legacy iron (Fe) and steel wastes have been identified as a significant source of silicate minerals, which can undergo carbonation reactions and thus sequester carbon dioxide (CO₂). In reactor experiments, i.e., at elevated temperatures, pressures, or CO₂ concentrations, these wastes have high silicate to carbonate conversion rates. However, what is less understood is whether a more “passive” approach to carbonation can work, i.e., whether a traditional slag emplacement method (heaped and then buried) promotes or hinders CO₂ sequestration.

In this paper, the results of characterization of material retrieved from a first of its kind drilling program on a historical blast furnace slag heap at Consett, U.K., are reported. The mineralogy of the slag material was near uniform, consisting mainly of melilite group minerals with only minor amounts of carbonate minerals detected. Further analysis established that total carbon levels were on average only 0.4% while average calcium (Ca) levels exceeded 30%. It was calculated that only ∼3% of the CO₂ sequestration potential of the >30 Mt slag heap has been utilized. It is suggested that limited water and gas interaction and the mineralogy and particle size of the slag are the main factors that have hindered carbonation reactions in the slag heap.

—Pullin et al.

Resources

• Huw Pullin, Andrew W. Bray, Ian T. Burke, Duncan D. Muir, Devin J. Sapsford, William M. Mayes, and Phil Renforth (2019) “Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste” Environmental Science & Technology 53 (16), 9502-9511 doi: 10.1021/acs.est.9b01265