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Steelmaking slags are a by-product of steel production, that are currently used primarily in road construction, earthwork, and hydraulic engineering. In this use, the iron bound in the steelmaking slags (< 30 wt.%) is lost.
Recovery of iron from steelmaking slags is possible by thermochemical reductive treatment. The reductive treatment of liquid steelmaking slags causes iron oxides to be reduced to metallic iron, which separates from the mineral phase due to its higher density. The chemical composition of the mineral phase is thus adapted to that of the Portland cement clinker and the mineral alite, the most important component of Portland cement, is formed. This way, crude iron can be recovered, and at the same time a hydraulic binder can be produced.
This process, however, is uneconomical due to the high temperatures required (~1800 °C). In the current project, the process is to be adapted so that the reduction of liquid steelmaking slag can be carried out at ~1600 °C. The chemical composition is to be modified in such a way that the melting temperature of the slags as well as their viscosity are in a technically suitable range and still a product with good cementitious properties is obtained.
Basic oxygen furnace slag (BOFS) is a by-product of the steelmaking process, of which about 10.4 Mt are produced annually in the European Union. Besides its predominant use in road construction, earthwork, and hydraulic engineering, it is also possible to use BOFS as a source material for Portland cement clinker.
The main difference in the chemical composition of BOFS from the chemical composition of Portland cement clinker is its high content of iron oxides (7-50 wt.%). In recent decades, many researchers have investigated the production of both Portland cement clinker and crude iron from BOFS via thermochemical reductive treatment. Carbothermal treatment of liquid BOFS causes reduction of iron oxides to metallic iron, which separates from the mineral phase due to its higher density.
In this study, German BOFS was reduced in a small-scale electric arc furnace using petrol coke as reducing agent. The produced low-iron mineral product was chemically similar to Portland cement clinker and contained the most important Portland cement mineral alite (Ca3SiO5) as main component. Besides alite, the mineral product contained other Portland cement clinker constituents such as belite (β-Ca2SiO4) and tricalcium aluminate (Ca3Al2O6).
The production of Portland cement clinker and crude iron from BOFS has economic and ecological benefits for both the cement and steel industry. Cement clinker from reduced BOFS may be used as a substitute for cement clinker from conventional cement production, thereby CO2 emissions will be reduced. The steel industry benefits from a high-value application for its by-products that avoids cost expensive landfilling and may even bring economic advantages. However, reductive treatment requires high temperatures and, for economic reasons, has to be carried out immediately after casting of the liquid BOFS, which is a logistical challenge for most steel plants. A cost-benefit analysis is therefore essential.