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

Abstract

Boron oxide is frequently applied in modification of stainless steelmaking slag to mitigate the disintegration of slag. In this work, the effect of B 2 O 3 on mineralogical phases and hexavalent chromium leaching behavior of synthetic CaO-SiO 2-MgO-Al 2 O 3-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% B 2 O 3 addition is sufficient to eliminate the disintegration of synthetic slag by suppressing the phase transition to γ-Ca 2 SiO 4. The size of spinel phase increases with increasing B 2 O 3 , which could be well interpreted by enhanced Ostwald ripening. The amount of Ca 2 SiO 4 phase in slag was reduced by addition of B 2 O 3 ; however, excess B 2 O 3 (> 2%) addition would significantly increase chromium concentration and overall chromium distribution in Ca 2 SiO 4 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.

Effect of binary basicity on chromium occurrence in stainless steel slag

“…In our previous study, the leaching experiments were performed on SSS with different basicity (the ratio of CaO% and SiO2% is 1.25~1.89). It was confirmed that the Cr 6+ leaching was higher for the sample with low Cr2O3 content and high basicity than for the sample with high Cr2O3 content and low basicity, which implied a good agreement between Cr stability and the occurrence state [24]. CaO-Al2O3-MgO-SiO2-CrOx slag with varied CaO/ SiO2 (1.1, 1.3, 1.5) was studied at 1673K by Shu [25], the results showed the biggest size of spinel crystal at 1.5 basicity and Cr is enriched in spinel phase and liquid phase.…”

“…It was confirmed that the Cr 6+ leaching was higher for the sample with low Cr2O3 content and high basicity than for the sample with high Cr2O3 content and low basicity, which implied a good agreement between Cr stability and the occurrence state [24]. CaO-Al2O3-MgO-SiO2-CrOx slag with varied CaO/ SiO2 (1.1, 1.3, 1.5) was studied at 1673K by Shu [25], the results showed the biggest size of spinel crystal at 1.5 basicity and Cr is enriched in spinel phase and liquid phase. Liu found that the presence of solid particles in slag has a critical influence on the viscosity at high basicity [26].…”

Hydration Activity and Carbonation Characteristics of Dicalcium Silicate in Steel Slag: A Review

“…Slow cooling is beneficial to the crystallization of silicate minerals, and the grain size can grow to 300 µm [10,26,43]. When steel slag contains a certain amount of Cr, Cr 2+ will replace Ca 2+ and enter its crystal when dicalcium silicate undergoes crystal transformation [44]; P will cause obvious phase segregation in steel slag and form C 2 S−C 3 P solid solution [45]; when the steel slag contains a certain amount of CaF 2 , CaF 2 reacts with CaO and SiO 2 to form gun spar (Ca 4 Si 2 O 7 F 2 ). As an early−formed mineral, gun spar will inhibit the formation of Ca 2 SiO 4 [46,47].…”