"We are setting impulses for a circular economy"
Ambitious research project by thyssenkrupp MillServices & Systems: Hot metal desulfurization slag is to be fully recycled and reused in several industries. An interview with project leader Dr. Michael Dohlen.
Dr. Dohlen, you want to fully recycle the slag produced during the desulfurization of pig iron. What exactly do you have in mind?
That's important: We're not talking about the classic blast furnace slag, but the slag that is produced during the subsequent desulfurization of the pig iron. Some of this has so far been returned to the blast furnace and recycled there. But only part of it. Most of the rest is disposed of, which is a waste of raw materials. We want to recover these and make them usable for various sectors of industry, for example in the cement industry and in agriculture.
How is hot metal desulfurization slag produced?
For this we have to look at the principle of the blast furnace. A blast furnace is charged in layers from above with iron ore, sulfurous coke and aggregates such as limestone. Hot air is blown in from below. The resulting carbon monoxide reduces the iron oxides. Liquid pig iron collects at the bottom and is tapped off at regular intervals. This also produces the classic blast furnace slag. However, before the liquid pig iron goes to the steel mill, the sulfur content must be reduced. This process produces hot metal desulfurization slag.
How does the process work?
In the desulfurization plants, calcium compounds are added to the pig iron. The calcium reacts with the iron sulfide, turning sulfur into calcium sulfide. The resulting slag contains calcareous minerals, sulfur and iron. It floats on top in the molten iron and is skimmed off. It is then cooled and physically broken. Then the various fractions are separated out with the aid of magnets and sieves.
What are the fractions made of?
We call fractions the different grain sizes. Based on the grain size, we divide into three fractions: fine iron, crushed iron and heap iron. Briquette iron and heap iron can easily be reused in the blast furnace. However, fine iron with a grain size of less than 10 millimeters is very difficult to recycle due to its high sulfur content. Partial quantities are used in the sinter plant, the rest usually goes to landfill.
This is where your research project comes in. What orders of magnitude are we talking about?
We estimate that around 450,000 tons of hot metal desulfurization slag are produced in Germany every year. More than half of this ends up in landfills. We are pursuing a zero-waste approach, we want to recycle almost one hundred percent. 200,000 tons could be reused in the iron and steel industry, 180,000 tons in the cement sector and 22,000 tons in the fertilizer industry.
That sounds ambitious. Are there any examples?
In itself, recycling in iron and steel production is nothing new. Iron mill slags, in particular blast furnace and steel mill slags, are already used via many recycling paths in the interests of a resource-conserving circular economy. And that's how we intend to proceed with hot metal desulfurization slag: Everything is to be reused.
To this end you have sought partners from industry and research. How are the tasks distributed?
Firstly, there is the Fraunhofer Institute for Building Physics IBP in Valley. We want to recover the purest possible iron from the slag. This is done by means of so-called electrodynamic fragmentation, as used at the Fraunhofer Institute there. The material is subjected to lightning discharges. The resulting pressure waves break down the composite material into its components: Mineral and iron.
What about the other components?
We want to use the mineral components in the construction industry, for example as aggregates in concrete or as a cement substitute. Sand is already in short supply due to the high level of construction activity in the country. We are working closely with the Fraunhofer Center for Chemical-Biotechnological Processes CBP in Leuna and the South Bavarian Portland cement works Gebr. Wiesböck & Co. GmbH.
And what happens to the sulfur?
That is the third pillar. We want to use it to develop a sulfur-containing fertilizer for use in agriculture. Our partner here is Weihenstephan-Triesdorf University of Applied Sciences, which is conducting research in this area. Sulfur is a fertilizer in high demand in agriculture. Older people will remember the acid rain of the 1980s. Back then, too much sulfur from power plants and industry entered the atmosphere. In the meantime, filter systems have become so effective that farmers have to fertilize with sulfur. That's what we want to supply.
Where do we go from here?
After successfully working out the process chain in the laboratory, we will set up a test plant. 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 will create realistic conditions. We want to reduce the energy consumption and CO2 emissions of the recycling processes to such an extent that ecological and economic viability is ensured. Of course, we will also examine the legal framework of such an undertaking.
The project is not only technically exciting, but also emotional. Behind all this is the question of whether industry can actually manage to reduce its greenhouse gas emissions in line with the German government's plan. How do you live with that?
Good, because the climate targets in our company are far more ambitious than those of the German government. The thyssenkrupp Materials Services segment, which includes thyssenkrupp MillServices & Systems, has set itself the goal of becoming climate-neutral worldwide by 2030 rather than 2050. In addition, as part of our research project we aim to avoid greenhouse gas emissions in key sectors such as the steel and cement industries. In this way we are contributing to the German government's climate protection plan. The real strength of our project, however, is that the different partners will illustrate the processes from the generation of slag to its conditioning and application, thus enabling new collaborations between different sectors and providing impetus for a circular economy.
The research project entitled "Reduced process emissions in steel and cement production - treatment and use of hot metal desulfurization slag" started on January 1, 2022. It will run for four years and is funded by the German Federal Ministry of Education and Research under the measure "Prevention of climate-relevant process emissions in industry". Its aim is to develop a resource-efficient circular economy and reduce dependence on raw materials.
The project manager
Dr. Michael Dohlen is Head of Research & Development at thyssenkrupp MillServices & Systems. He is also a publicly appointed and sworn expert for substitute building materials