One-hundred-percent recycling of blast furnace sludge seems possible

Both the focus and the heat were tangible during the trial campaign in one of Swerim's demo halls. To the reactor were added 15.5 tonnes of black blast furnace sludge, a residue from SSAB, Swerim's neighbour in Luleå.

The main product from the reactor was lifted out regularly with large cranes. Once cooled and solidified, the glowing hot, high-iron-content agglomerate can be broken into pieces and recirculated to steel production. Researchers followed the process with keen interest and great satisfaction.

“The results are very positive,” says Fredrik Nyman, AGA Linde, provider of the so-called Oxyfines technology. A reactor vaporizes the zinc particles present in the blast furnace sludge, while the non-vaporizable components, such as iron, form a bottom residue that can be recirculated back to the steelmaking process. The zinc dust that is vaporized in the process can be used in zinc production at, for example, Boliden.

The whole process has been stable and easily regulated. With the knowledge we gained from a trial campaign we ran in the spring we have been able to refine the process.

The research project is part of SSAB's efforts to increase utilization of residual materials in order to realize both economic and environmental gains.

Daniel Söderström, development manager at SSAB Merox, is participating in the project, which concludes in June 2020.

“Today, the blast furnace sludge is deposited in sludge ponds. In Luleå alone it's a matter of 11,000 tonnes each year. The key issue is if there is zinc in the sludge. As things are today, there is so much zinc that it can't be recycled in the process. This is because zinc sticks to the walls of the blast furnace, due to its low melting point and the great temperature differences between the top and bottom of the blast furnace. But this trial demonstrates a method that could make recycling possible.”

Annually, Swedish ore-based steelmaking generates about 20,000 tonnes of blast furnace sludge, which is a residue of ironmaking in the blast furnace. If that sludge could be recirculated to the blast furnace, it would correspond to approximately 12,000 tonnes of iron ore pellets per year in Sweden.

After this trial, we have great confidence that, in future, we can reduce the amount of depositioned material while also saving on virgin raw materials. 

The research project budget amounts to 15 million kronor, of which five million kronor is funded by the Swedish Energy Agency. Other project partners include AGA Linde, SSAB Europe, SSAB Merox and Boliden Mineral.

Photo: It cools, solidifies and is broken into pieces that can be recirculated to steel production.