Basic oxygen furnace (BOF) slag, a byproduct of a steel-making process, is reused as construction materials such as land banking and road base. When exposed to water, however, BOF slag generates highly alkaline leachate (pH≈12) by dissolved free calcium oxide (CaO) located on the slag surface. Because this highly alkaline leachate may pose adverse effects on the surrounding environments BOF slag should be appropriately treated prior to reuse. In this study, addition of CO32- or Al3+ was tested whether or not the ions could mitigate the alkaline leachate generation. A batch test was conducted with BOF slag and a pH-mitigating chemical solution (i.e., NaHCO3 or Al(NO3)39H2O solution) with the ratio of 1:5 (g-slag:mL-solution) for 48 hours. The pH of the leachate was then measured, and the free CaO content in the residual BOF slag was also determined by titration. The results showed that both treatments were effective to decrease the pH of the BOF slag leachate, but the extents were different. The pH values of the leachates after 0.05, 0.1, 0.5, and 1.0 M of CO32- solution treatment were 12.4, 12.4, 11.1, and 9.6, respectively. After 0.05, 0.1, and 0.15 M of Al3+ solution treatment, the pH values of 10.6, 9.5, and 8.8 was obtained, respectively. Interestingly, the CO32- treatment did not significantly reduce the free CaO content in BOF slag, whereas free CaO was barely detected in the BOF slag treated with Al3+. This result suggests that the mechanisms of pH mitigation involved may be different. The pH reduction in the CO32- treated BOF slag was probably due to the prevention of additional free CaO dissolution from slag surface by forming CaCO3 precipitates on the slag surface, which was confirmed by SEM-EDS analysis. In contrast, Al3+ addition seemed to directly remove OH- generated from free CaO dissolution, which in turn expedited the dissolution of free CaO from slag surface. Al(OH)3 precipitates were found in the Al3+ treated BOF slag by XRD analysis.

Mitigation of alkaline leachate generated from basic oxygen furnace slags by chemical precipitation

251st National Meeting (2016)

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