Effect of the Heat Treatment on the Chromium Partition in CaO-MgO-SiO 2 -Cr 2 O 3 Synthetic Slags

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METALLURGICAL slag from stainless steel production process containing environmentally harmful elements, such as chromium, is a continuously growing problem. Utilization of high alloy steelmaking slags is restricted due to the high content of harmful elements. In particular, slags from stainless steel production process contain considerable amounts of chromium. A number of mineralogical phases present in the slag can contain chromium. Chromium in the slag if not GALINA JELKINA ALBERTSSON, Ph.D. Student, and LIDONG TENG, Docent, are with the Division of Materials Process Science, Royal Institute of Technology (KTH), 100 44, Stockholm, Sweden. Contact e-mail: [email protected] FREDRIK ENGSTRO¨M, Doctor, is with the Division of Sustainable Process Engineering, Lulea˚ University of Technology (LTU), 971 87, Lulea˚, Sweden. SESHADRI SEETHARAMAN, Professor, Mercator Visiting Professor, is with the Institute of Iron and Steel Technology, TU-Bergakademie, 09596 Freiberg Germany and currently with the Royal Institute of Technology. Manuscript submitted November 23, 2012. Article published online September 10, 2013. 1586—VOLUME 44B, DECEMBER 2013

stabilized, could get oxidized to the hexavalent state (Cr6+), and thus leach out if exposed to acidic and oxygen-rich environment.[1,2] Recently, elution behavior of elements from chromium-containing phases into sea water was investigated by Samada et al.[3] These authors found that the presence of dicalcium silicate (Ca2SiO4) in the slag enhanced the dissolution of chromium into seawater. Ca2SiO4 is also strongly attacked by acidified water. In order to avoid the presence of Ca2SiO4, attempts have been made to modify the slag composition by decreasing the slag basicity.[4–9] Silica addition was earlier suggested in order to control the slag composition and prevent slag disintegration.[4–9] However, low basicity slags cause rapid refractory degradation and low chromium yields in the steel during steelmaking process.[9] Thus, the slag composition must be optimized with regard to slag/ metal separation.[9] Presence of free CaO and MgO as well as merwinite phases would also be undesirable in view of their high solubility in water, the latter in the entire pH range. Thus, chromium dissolved in merwinite, CaO and MgO can easily end up in water.[9] METALLURGICAL AND MATERIALS TRANSACTIONS B

Mineralogical phases present in a slag system,[1–9] are highly dependent on slags composition and heattreatment history. Such phases considered in the current work, are divided into spinel, monoxide solid solution, and the silicate matrix phases. Silicate matrix phases can consist of glass phase, merwinite, dicalcium silicate, pseudo-wollostonite, and other silicate phases. Tossavainen et al.[9] studied the influence of rapid cooling by water granulation and its effect on chromium leachability. The differences between granulated slag samples and conventional slag were low. Loncnar et al.[10] studied the impact of the cooling rate of hot EAF slag on the leaching behavior. It was concluded that the cooling