Improving the dry magnetic separation of BOF-slag by applying an innovative pre-treatment process

To approach circular economy by enhancing the internal recycling potential of BOFslag, an innovative pre-treatment process for the dry magnetic separation is investigated. It consists of slow-cooling to enlarge the slag crystals, followed by microwave irradiation to generate intergranular cracks to enhance the mineral liberation during comminution. It can be shown that the pre-treatment leads to an increase in the separation degree by at least four percent. This can be further enhanced by a factor of five by using solid-state oxidation.

Basic oxygen furnace (BOF-)slag, with an amount of about 8 Mt produced in 2018, is a major residual material in the steel production process of European manufacturers. The main end use is road construction, while less than 15 % of the annually produced slag is recycled internally (Euroslag Statistics 2018 2020). Since BOF-slag contains a considerable amount of iron (Fe), which could be used metallurgically to save natural resources and to move towards the European circular economy goals, an increase of the recycling rate of BOF-slag would be beneficial. However, the internal recycling is limited, as BOF-slag contains unwanted elements with respect to steel production, e.g., phosphorous (P). Therefore, the Fe must be recovered from the BOF-slag first. The majority of the BOF-slag’s Fe is contained within wuestite crystals and the P in larnite

(dicalcium-silicate, C2S) crystals (Kubo et al. 2010). Thus, if the Fe bearing crystal phases are separated from the C2S phases, a recyclable material can be obtained. An added benefit would be the utilization of the remaining C2S-rich fraction in cement production or the recovered P for fertilizer production. As part of the ongoing RFCS project Slagreus an innovative pre-treatment process is investigated to try to achieve the separation of Fe-rich and C2S-rich fractions.
To improve the magnetic separation of the BOF-slag into the desired fractions an innovative pretreatment process is set up. The first step is the pre-treatment of the liquid slag. The aim of this is to enlarge the formed crystals and limit intricate intergrowth, which would be beneficial for liberation.
The main influence on the crystallisation is a reduced cooling rate (Gautier et al., Ono et al. 1981). After the slow cooling, the slag is crushed and subjected to microwave irradiation. This microwave treatment generates cracks at the grain boundaries of the crystals; in natural mineral this leads to an improved liberation during comminution (Omran et al. 2015). The cracks are formed due to the thermal stress, induced by the selective heating of the slag, as Fe bearing phases, unlike C2S, absorb the microwave irradiation (Morita et al. 2001). The pre-treated slag is then ground in order to liberate the mineral phases before being subjected to magnetic separation, as the Fe bearing phases show magnetic susceptibility in contrast to C2S (Kubo et al. 2010). Dry magnetic separation methods (DMS) are employed, so the C2S may retain its chemical activity (Alanyali et al. 2009).



Copyright: © Lehrstuhl für Abfallverwertungstechnik und Abfallwirtschaft der Montanuniversität Leoben
Quelle: Recy & Depotech 2022 (November 2022)
Seiten: 6
Preis: € 3,00
Autor: Simon Wölfelschneider
H. Schmid
D.Sc. Mamdouh Omran
Monika Häuselmann
Dr. rer. nat. Lars Gronen
 
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