Steel slag is a major byproduct of one of the world’s largest industries – but its upcycling and circular use is currently limited. CarbonBlue’s EcoLime technology can transform it from a nuisance used as filler in the construction industry into a high-value source of lime.

Slag: a cornerstone of the emerging circular economy

Steel slag, a byproduct of the steel-making process, is transitioning from an industrial waste product to a vital mineral resource within the circular economy. Today, on a global scale, its primary importance lies in its role as an alternative to natural aggregates in infrastructure and construction. By substituting virgin stone and gravel with processed slag, the international market is attempting to reduce the environmental degradation associated with quarrying. The material’s durability and hydraulic properties make it a suitable component for road surfaces and hydraulic engineering, turning this potential pollutant into a commodity in the construction materials industry – but is it enough?

Today, for every tonne of steel produced, around 200-400 kg of slag is generated; with the steel industry producing nearly 2 billion tonnes of steel annually, that’s around a billion tonnes of slag produced each year. As resource scarcity increases and global carbon-neutrality mandates intensify, the ability to repurpose millions of tonnes of slag annually represents a multi-billion dollar opportunity to decouple economic growth from resource exhaustion, solidifying its status as a cornerstone of industrial sustainability.

Not all slag is created equal

As the global economy struggles to find new uses for slag, a distinction between types of slags emerges. Today, the main distinction is between Blast Furnace Slag (BFS), which accounts for roughly 53% of total ferrous slag, and Steelmaking Slag, which includes Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) varieties. BFS is considered high value, since it can be usesd in the cement industry, but since BOF and EAF don’t have this capacity, they are typically used as aggregates and fillers, and sold cheaply.

But BOF and EAF have lime locked away inside of them.

Lime is used to purify the steel ore in the steelmaking process, and as it bonds with impurities, it creates slag, which then needs to be disposed of. This makes lime an expensive, single-use commodity. But if a way is found for that lime to be extracted from the slag, it can the be re-used, for a fraction of the original price. Essentially, its use can become circular, with the same lime used over and over in repeating processes in the steel industry – or simply sold as a feedstock for other lime-utilizing industrial processes and products.

Harnessing slag as a source of lime

CarbonBlue’s EcoLime technology has the unique ability to extract minerals, including lime, from cheap BOF and EAF slags, in a high state of purity. This allows us to transform these slags from inert, nonreactive materials used as fillers, into materials with high reactivity potential, like lime, which are essential for countless industrial processes.

Whether this technology is implemented as part of the steel production cycle, extending the use of purchased lime from a one-time application into a “self-regenerating” resource, or used to cheaply produce high-value lime for the global market, the ability to transform slag from waste and into a source of high-demand minerals is a gamechanger in the circular economy.

The environmental impact

Today, lime production accounts for nearly 8% of all global CO₂ emissions. This is because lime is produced using limestone – a mineral which is essentially lime bonded with CO₂. When lime is produced, the bonded CO₂ is released back into the atmosphere, alongside further emissions generated by the heating process required to break their chemical bond. By turning to carbon-free slag as an alternative source of lime, we can completely bypass the CO₂ emissions associated with the traditional limestone feedstock, as well as significantly reduce the emissions associated with the industrial process, as CarbonBlue’s EcoLime technology does not rely on fuel combustion, but rather, on electricity, which can be generated from sustainble sources.

In addition to this emission reduction, the circular use of lime EcoLime unlocks for industries like the steel industry also reduces the environmental impact of quarrying, as well as shortens supply chains and their associated ecological footprints.

CarbonBlue’s industrial optimization decarbonizes, boosts resource efficiency, and breathes new life into existing resources and waste.

Read more about EcoLime here.

Sources:

Aziz, M. M. A., Shokri, M., Ahsan, A., Liu, H. Y., Tay, L., & Muslim, N. H. (2020). An Overview on Performance of Steel Slag in Highway Industry. Journal of Advanced Research in Materials Science, 67(1), 1–10. https://doi.org/10.37934/arms.67.1.110

Liu, X., Zhang, C., Yu, H., Qian, G., Zheng, X., Zhou, H., et al. (2024). Research on the Properties of Steel Slag with Different Preparation Processes. Materials, 17(7), 1555. https://doi.org/10.3390/ma17071555

Wang, S., Wang, M., Liu, F., Song, Q., Deng, Y., Ye, W., et al. (2024). A Review on the Carbonation of Steel Slag: Properties, Mechanism, and Application. Materials, 17(9), 2066. https://doi.org/10.3390/ma17092066

Kurecki, M., Meena, N., Shyrokykh, T., Korobeinikov, Y., Jarnerud Örell, T., Voss, Z., Pretorius, E., Jones, J., & Sridhar, S. (2024). Recycling Perspectives of Electric Arc Furnace Slag in the United States: A Review. Steel Research International. https://doi.org/10.1002/srin.202300854

Lee, M.-Y., Kang, J.-H., Hwang, D.-G., Yoon, Y.-S., Yoo, M.-S., & Jeon, T.-W. (2021). Environmental Assessment of Recycling (EAoR) for Safe Recycling of Steelmaking Slag in the Republic of Korea: Applications, Leaching Test, and Toxicity. Sustainability, 13(16), 8805. https://doi.org/10.3390/su13168805