Effect of B 2 O 3 addition on mineralogical phases and leaching behavior of synthetic CaO–SiO 2 –MgO–Al 2 O 3 –CrO x slag

Boron oxide is frequently applied in modification of stainless steelmaking slag to mitigate the disintegration of slag. In this work, the effect of B2O3 on mineralogical phases and hexavalent chromium leaching behavior of synthetic CaO–SiO2–MgO–Al2O3–CrOx slag was investigated. Di-calcium silicate, merwinite, spinel, akermanite, and matrix phase were observed as main minerals in slags by scanning electron microscope (SEM) equipped with energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD) techniques. It was found that 2% B2O3 addition is sufficient to eliminate the disintegration of synthetic slag by suppressing the phase transition to γ-Ca2SiO4. The size of spinel phase increases with increasing B2O3, which could be well interpreted by enhanced Ostwald ripening. The amount of Ca2SiO4 phase in slag was reduced by addition of B2O3; however, excess B2O3 (> 2%) addition would significantly increase chromium concentration and overall chromium distribution in Ca2SiO4 phase. Leaching results according to US-EPA-3060A method indicated that excess boron oxide addition (> 2%) leads to a significant increase of hexavalent chromium leaching concentrations and should be avoided for stabilizing stainless steel slag.