Effect of B₂O₃ addition on mineralogical phases and leaching behavior of synthetic CaO–SiO₂–MgO–Al₂O₃–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–CrO_x 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.