Selective Ammoniacal Extraction Process for Valorizing Zn-rich BOF Sludges (SAMEX)

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Photo: ArcelorMittal – Bremen

For each million tonne of steel ArcelorMittal (AM) produces, it also generates on average 10,000 tonne of zinc-rich, fine Basic Oxygen Furnace (BOF) steelmaking sludge. In contrast with the coarse BOF sludge fraction, which is already internally recycled by AM via the sinter plant and then to the Blast Furnace, the fine BOF sludge fraction cannot be fed to the Blast Furnace (BF), as the zinc content would lead to prohibitive refractory failure and disturbances in the BF process. As a result, AM either internally stores these sludges or is forced to landfill them. To avoid excessive landfilling and to create an industrial symbiosis system, AM developed in 2017-2018 an ammoniacal leaching process, in collaboration with KU Leuven, first on lab-scale, subsequently on a small pilot level (TRL5). The developed process selectively extracts zinc from the sludge (76% leaching yield) while leaving behind most iron. The cleaned, iron-rich residue can be fed to the BF, representing major iron cost savings, while the leached zinc in the pregnant leach solution can be recovered as a zinc sulphide-precipitate product, as a feed for the zinc industry.

In the SAMEX project, a knowledge triangle consortium – consisting of Tecnalia (Spain), AM (Spain) and KU Leuven (Belgium) – shall upscale the AM process to TRL7, aiming to engineer and build a pilot plant, which will consist of several units: (a) dewatering (incl. filter pressing), (b) leaching reactor incl. air injection, (c) solid/liquid (filter press) separation, (d) ZnS precipitation). The lixiviants will be regenerated. The pilot plant will be used to demonstrate and validate the flowsheet, using distinct BOF sludges from different AM plants in Europe. If successful, AM foresees to implement the process in at least one third of its EU-plants by 2025 (i.e. treatment of 120,000 tonne/year BOF fine sludge). Furthermore, other sludge producers and steelmaking companies will be able to benefit from the results generated in the project.