Mr. Algermissen, FEhS-Institut für Baustoff-Forschung e. V. about NuKoS

New utilization of iron and steel slag

CO₂-WIN Connect: In Germany, the iron and steel industry produce around 14 million tons of iron and steel slag per year. Currently, 95% of the slag produced is already reused, for example in the cement industry. How do you come into this with "NuKoS" and what is the aim of the project?

Herr Algermissen: In "NuKoS", we aim to use the remaining 5% of the slag produced in a valuable way. But at the same time, we focus on the grain sizes whose application is increasingly difficult, as there is only little demand in fine-grained products, for example in traffic route construction. Thus, the objective of "NuKoS" is to generate a new product from this material stream together with CO₂ from waste gases of the steel or cement industry.

CO₂-WIN Connect: The first tests of the new processes will take place in a gas-tight sealable pressure vessel. Can you explain to us how the tests are carried out, what test conditions (temperature, pressure, and gas composition) are necessary for carbonation, and what special features have to be taken into account with respect to the carbonation process?

Herr Algermissen: So far, numerous tests for carbonation, i.e. cement-free bonding, have been carried out using different test conditions. Especially the temperature, pressure, and carbonation time were varied. It showed that temperatures above 50 °C are not necessary, which in turn saves energy. The maximum carbonation time is 24 hours, as longer times do not bring any further benefit. Thus, an attempt is already being made to optimize the small-scale test in terms of energy and to use as large quantities of slag as possible.

CO₂-WIN Connect: After the lab-scale experiments, you want to transfer the results to the pre-industrial demonstration scale. What is the order of magnitude here and what obstacles must be considered during the transition to the demonstration scale? Can you rely on existing plant types for these purposes or do you have to specifically adjust the demonstration plants for your purposes?

Herr Algermissen: So far, the carbonation tests have taken place in small-scale trials on a 2-liter scale. The first large-scale trials are planned for April. Based on the parameters determined previously, we aim to generate larger quantities of material and obtain important findings. Due to the very practice-oriented consortium, both the small-scale and large-scale trials can be fully covered by our partners. It was important to us that we are not only strongly positioned on the research side. Instead, we are able to cover the process chain from the producer to the user as well.

Regarding the other process routes, we also plan large-scale trials, e.g. the foaming of LD-slag. Here, on-going work includes the scale-up from the laboratory scale to an operational slag bucket, which usually holds up to 25 t of material.

CO₂-WIN Connect: The transfer to the demonstration scale is realized with industrial partners. Do you already have a specific site for the demonstration plants in mind and what does a suitable site have to bring to the table, also in term of the industrial scale?

Herr Algermissen: As described at the beginning, there are currently different paths and products as well as different concepts, where a corresponding plant can be located. As soon as the results from the large-scale tests are available, the economic and ecological views will be started. Therein, the advantages and disadvantages of the different locations will be examined and evaluated in detail.

CO₂-WIN Connect: Thinking of the potential industrial use of your new processes, where do you see these plants being located? Centralized at the iron and steel mills or decentralized? And if they are to be decentralized, where does the carbon dioxide-containing gas for the carbonation process come from and how would you transport and store it?

Herr Algermissen: We imagine that the plants are rather located centralized close to the producers or users. On the one hand, the advantage of being close to a steel mill is that the two necessary products, slag and CO₂, are directly available and long transport routes of the products can be avoided. Thereafter, only the products have to be transported to the user. On the other hand, the location near a cement plant offers many advantages as well. Although the slag would have to be transported there first, the CO₂ from the cement plant could be used directly. Additionally, the finished product would then be directly at the place of further processing, i.e. that further transport is not necessary.

Thus, a decentralized solution is currently not preferred due to the lack of proximity to the CO₂ emitter, as described previously.

CO₂-WIN Connect: In steel mills, a switch from the blast furnace route to direct reduction using synthesis gas/hydrogen is forecast to reduce the emissions generated. However, this is expected to reduce the amount of slag produced in steel mills. In this case, is there still a need for further slag utilization options in addition to the current fields of application?

Herr Algermissen: Absolutely, if not precisely because of that! On the one hand, 100% of the blast furnace slag produced today is already used, mainly in the cement industry but also in smaller quantities in road construction and earth work. For this reason, we have not focused on this slag in "NuKoS", as there is no need for any further application.

On the other hand, slag produced at further processing stages, e.g. secondary metallurgy, but also from the scrap-based electric steel route will continue to be produced with the same challenges as today. In addition, there is still no direct application for "future slags", as many properties are still largely unknown. Here, "NuKoS" could directly offer another possible application, if these slags fit within the defined limits of the parameters determined.

CO₂-WIN Connect: If you had to speculate, when do you estimate that one of your processes could reach industrial scale? What has to be achieved before reaching industrial scale and what is the timeline for this?

Herr Algermissen: The various application paths investigated in "NuKoS" are deliberately chosen very differently to always have an alternative. At the same time, the paths themselves are also very different with respect to their time horizon. Currently, the substitution with slags of different materials in the lime-sandstone production is certainly the highest in terms of TRL. This means that an implementation within five years after the end of the project can be considered realistic. Regarding the other paths, it will also strongly depend on the conditions as well as on the results, which are still pending. After all, we still have one year to go in the project.

CO₂-WIN Connect: Mr. Algermissen, thank you very much for this interview!

Do you want to learn more about the BMBF-funded project NuKoS?

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