Thermally Reductive Conditioning Procedures of Basic Oxygen Furnace Slag

Over 111 years, ‘LOESCHE’ has always been associated with a great innovative capability that has significantly contributed to the success of the company worldwide.

For decades, LOESCHE has stood in particular for mill technology developments. According to the market demands, LOESCHE is increasingly enhancing this field by developing complete processes. Providing complete solutions is becoming an ever larger part of LOESCHE’s portfolio, which is also made clear in the new LOESCHE theme.

Our technology is traditionally found in the cement, steel, ore and mineral industry, as well as power plant construction. In some of these fields, LOESCHE is the global market leader.

In particular, since the introduction of the patented M + S roller technology in 1992/1993, the number of granulated blast furnace slag and cement mills sold has increased, even exceeding the 400 grinding units sold recently.

The past successes can only be perpetuated and carried on into the future if the increasing market demands, which today also increasingly include sustainability and ecology, are taken into account by increased efforts in research and development.

LOESCHE today does not just have a comprehensively equipped test centre for grinding-related tests. Thanks to the company’s farsighted vision, there is also a strong research team, which uses this centre for the practical implementation of creative ideas. This research team processes complex tasks by combining profound knowledge of machine and process technology and extensive competence in the field of materials.

One outstanding process development that LOESCHE has initiated and that has been processed in close cooperation with the F. A. Finger-Institut für Baustoffkunde (FIB) at Bauhaus-Universität Weimar is the thermally reductive modification of steel slags for recycling iron and manufacturing ‘steelworks clinker’. The results achieved by LOESCHE and the FIB impressively reinforce the great success of this approach.

In contrast to the granulated blast furnace slag, original steel slag shows no appreciable binding behaviour after normal finish grinding. That is why the resulting slags have not been used previously in cement but instead are used in road construction in unbound base layers or bituminous-bound base and cover layers. A significant portion must also be deposited. Against the backdrop of the preservation of resources, the current debate regarding reducing CO2 and LOESCHE’s experiences of being able to recycle higher metal content from a siliceous matrix, new research began to produce an innovative conditioning process for steel slags.

In a thermochemical procedure in a reducing atmosphere, the iron that is initially permanently bonded in mineral phases is transferred to an elemental, metallic form; this allows this iron to be separated and fed back into production.

By transferring the iron to its elemental form, the chemical composition of the remaining molten metal is changed such that the optimum range for the formation of cement clinker phases is achieved in many cases without the addition of any corrective components. Consequently, after the molten metal cools down, a material similar to clinker with a high reactivity, which shows an alite content (tricalcium silicate) of up to 70m, is produced. Another important part of the procedure is the insight from LOESCHE and the FIB that MnO2 reduced to MnO can be integrated into the alite grid, thus leading to stabilisation of this phase, which prevents the decay of alite into belite (dicalcium silicate) and CaO largely independently of the cooling.

The hydraulic mineral binder manufactured can be used as a composite material for cement or as an independent clinker material in accordance with our experience. Two conditioning procedures have been registered for a patent, which largely differs in the respective cooling process for the remaining molten metal. The first procedure provides for slow cooling over 15min, which can be implemented primarily as passive, dormant cooling. A second process is based on fast cooling through additional measures in less than 15min.

The individual procedure stages have already been tried and tested on an industrial scale. LOESCHE’s partner for the entire procedure is Primetals Technologies, which has industrial-scale plants for reduction and fast cooling based on patented procedures. The remaining iron still in the ‘steelwork clinker’ can be separated in our mill. The patented separation procedure has been successfully in operation for six years to recycle stainless-steel from stainless-steel slags in a Belgian recycling plant.

To verify the idea of thermally reductive treatment of LD slags, smelting trials with test material from a German steelworks in the arc furnace of the German Federal Office for Material Research and Testing (BAM) were ordered in 2011 by LOESCHE/FIB. All chemical and mineralogical tests were carried out at the FIB, which also managed further development of the procedure in terms of materials.

LOESCHE has made international patent applications for the two conditioning procedures for the formation of a hydraulic mineral binder with fast and slow cooling. These two conditioning procedures have already led to national patents.

Based on the activities within the framework of the smelting trials, the BAM brought a lawsuit against LOESCHE for the complete transfer of all rights to the patent applications made and the patents already granted. The subsequent legal uncertainty led to the suspension of further development and implementation of these two conditioning procedures for more than three years. The legal dispute was conclusively decided in December 2017 in front of Munich’s Oberlandesgericht (Higher Regional Court). That decision confirmed that the fast cooling procedure was the sole property of LOESCHE. In relation to the slow cooling procedure, a third of the ownership was conceded to the BAM.

After the end of the dispute, the activities relating to the implementation of the two procedures on an industrial scale could be resumed by LOESCHE. The second procedure is considered to be more economical since the fast cooling can be carried out in facilities, in which heat recovery is also possible.

We aim to successfully implement the procedure into industrial practice. These innovations lead to use of high-quality though previously unsuitable secondary material and ensure LOESCHE’s international competitiveness, as well as local jobs.

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