Experimental Study on Phosphorus Partitions Between Liquid Iron and Liquid Slags Based on DRI
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NTRODUCTION
THE importance of direct reduced iron (DRI) as a raw material for virgin steel production will likely enjoy an upswing globally and particularly in Sweden, as efforts are being made to delink the steel industry from its fossil dependence. A possible transition could be to direct reduction of iron ore by hydrogen, thus making hydrogen reduced DRI the principle raw material in ore-based steel production. Such a transition is likely to face economic challenges, especially during an initial period of development. Cost-minimizing measures will have to be taken, to carry the additional direct costs associated with a fossil-free value chain. One cost bearer, that could be optimized, is the removal of impurities. Sulphur impurities in steels are largely attributed to the use of fossil compounds, such as PCI, natural gas and coke, both during reduction and as addition in subsequent refining steps. A process route using little or even no fossil-based agents would largely limit sulphur contamination. Phosphorus in contrast, is to a significant extent carried to the liquid metal by the
JOAR HUSS and MARTIN BERG are with the Department of Materials Science and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden. NIKLAS KOJOLA is with the Group of research and innovation, SSAB AB, Box 70, 10121 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted March 26, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
iron ore. A process utilizing hydrogen reduced DRI should, therefore, focus on the removal of phosphorus whilst keeping close control of costs.[1] During the melting of the DRI a slag phase is formed. The slag consists of oxides that have been added during pelletization and residual gangue. The property, that DRI pellet forms an autogenous slag layer, is described by Selin as self-fluxing.[2,3] Furthermore, these slags display a CaO/SiO2 ratio close to parity.[4] A substantial amount of work has been conducted on the dephosphorization capabilities of slags in the CaOSiO2-MgO-FeOx system.[1–3,5–16] These earlier works have commonly reported a positive effect of increasing the CaO/SiO2 ratio, a negative effect of an increase in temperature and the duality of FeOx on phosphorus removal for a range of compositions. Several empirical descriptions attempting to predict the distribution of phosphorus between slag and liquid iron have been proposed. The majority of these studies, however, place their focus on high CaO/SiO2 ratio systems, typically from 2 to 4. Lower ratios are not used in conventional steelmaking, as the concentration of phosphorus in commercially viable steels needs to be kept to a minimum. On the other hand, the dephosphorization power of slags containing a CaO/SiO2 ratio below 1.3 has hardly been studied. Only three publications[3,9,15] have reported data on slags with low basicity. Note that experimental studies of low basicity slags would become essential when DRI is used and plays a substantial role in steelmaking, since removing phosphorus by utilizing the self-flu
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