RESycling for a resource-efficient circular economy

In the "RESycling" research project, thyssenkrupp Materials Services colleagues are taking up the challenge of making hot metal desulfurization slag fully recyclable. Instead of going to landfill, the by-product of steel production is to be used in the value chains of various industries in the future. We spoke to project manager Dr. Michael Dohlen about the ambitious research project.

Slag is produced in steel production, among other things through the desulfurization of pig iron, and is still often disposed of without secondary use. This is set to change. Project manager Dr. Michael Dohlen and his team at thyssenkrupp MillServices & Systems have set themselves the task of making hot metal desulfurization slag completely recyclable under the project name "RESycling".

The project started on January 1, 2022. It will run for four years and is funded by the German Federal Ministry of Education and Research (BMBF) under the measure Avoidance of climate-relevant process emissions in industry (KlimPro-Industrie).

Less dependence on raw materials: the circular economy makes it possible

RESycling pursues the goal of developing a resource-efficient circular economy and reducing Germany's dependence on raw materials. Together with the partners Fraunhofer Institute for Building Physics, the Fraunhofer Center for Chemical-Biotechnological Processes, the southern Bavarian Portland cement plant Gebr. Wiesböck & Co. GmbH and Weihenstephan-Triesdorf University of Applied Sciences, Dr. Dohlen's team is tackling this challenge.

RESycling - a promising research project

In Germany, an estimated 450,000 tons of hot metal desulfurization slag are produced annually. More than half is disposed of unused. Dohlen's research project is pursuing a zero-waste approach - with great potential for industry: "We want to recycle almost one hundred percent of the slag. 200,000 tons could be reused in the iron and steel industry, 180,000 tons in the cement industry and 22,000 tons in the fertilizer industry," says Michael Dohlen.

How is hot metal desulfurization slag formed?

To understand the potential of hot metal desulfurization slag and how it is created, we need to look at the principle of the blast furnace: "A classic blast furnace is charged in layers from above with iron ore, sulfurous coke and aggregates such as limestone," explains Dohlen. "Hot air is blown in from below. The resulting carbon monoxide reduces the iron oxides in the iron ore." Liquid pig iron is produced. 

At regular intervals, the pig iron is tapped off - the classic blast furnace slag is produced. Before the liquid pig iron can be further processed in the steel mill, the sulfur content of the slag must be reduced. "In a desulfurization plant, calcium is added to the pig iron. The calcium reacts with the iron sulfide. The sulfur becomes calcium sulfide," says Dohlen.

Part of the resulting hot metal desulfurization slag is returned to the blast furnace and recycled. "But only part of it. The rest is disposed of," says the expert. This is a waste of raw materials that Dohlen and his team now want to make usable for various industrial sectors, such as the cement industry and agriculture.

Use in the cement industry

"In order to recycle as much of the hot metal desulfurization slag as possible, our goal is to extract as pure iron as possible from the slag," explains Dohlen. "By means of so-called electrodynamic fragmentation, the slag is subjected to lightning discharges and thereby broken down into its mineral components and iron."

This so-called mineral aggregate can be used in the construction industry as a sand substitute, e.g. as an aggregate in concrete or as a cement substitute. thyssenkrupp's experts are working closely with the Fraunhofer Center for Chemical-Biotechnological Processes CBP in Leuna and the South Bavarian Portland cement plant Gebr. GmbH.

Sulfur - a popular fertilizer

In addition to the construction industry, the sulfur-containing waste product can also be used in other industries, as sulfur is also a sought-after fertilizer in the agricultural sector. In view of the growing world population, agriculture is subject to enormously high productivity pressure.

The hot metal desulfurization slag recovered can meet this growing demand. The partner in the thyssenkrupp Materials Services research project in this specialist area is Weihenstephan-Triesdorf University of Applied Sciences.

Preparations for use on an industrial scale

"Following the successful elaboration of the process chain in the laboratory, we will set up a test plant," says Dohlen. "It will form the basis for further development of the processing and use of hot metal desulfurization slag." Close involvement of the other industrial companies is expected to create realistic conditions. "We want to reduce the energy input and CO2 emissions of the recycling processes to such an extent that ecological and economic viability is ensured," says the expert, explaining further planning.

In this way, the research team is preparing the RESyling project for use on an industrial scale that is both economical and sustainable. After all, the raw materials resulting from the recycling processes not only reduce Germany's dependence on resources, they also save many kilometers of transport routes and the associated emissions. Not to mention avoiding 450,000 tons of waste pollution per year in Germany alone.