SLAG: WHAT IS IT GOOD FOR? PHOSPHATE REMOVAL AND ACID-NEUTRALIZING CAPACITY OF STEELMAKING SLAG FROM THE CALUMET AREA, IL AND IN
Modern and legacy slags (n=13) generally have comparable chemical compositions with 10 - 44 wt. % CaO, 0.3 - 28 wt. % FeO, 10 - 44 wt. % SiO2, 1 – 15 wt. % Al2O3, 2 – 11 wt. % MgO, and 0.3 – 9 wt. % MnO. Commonly identified mineral phases include: larnite, srebrodolskite, melilite, wustite, spinel, calcite, quartz, and Fe metal. Samples yield high net neutralization potentials (400 – 830 kg CaCO3/t) and produce alkaline paste pH values (pH 10 – 13), indicating they are appropriate for use as an acid neutralizing agent. Only one slag from a legacy site fell out of this range with only 65 kg CaCO3/t. In addition, benchtop experiments with a solid to solution ratio of 1:20 document efficient removal of phosphate from a synthetic solution containing 10 mg P/L. The most effective materials for removing phosphate were fine-grained (<10 mm) modern iron blast furnace and steel slags. Modern granulated iron slag, modern coarse iron and steel slag (>10 mm), and the legacy crushed slag (<10 mm) all removed phosphate but to a lesser degree. The phosphate removal efficiency generally correlated with the final solution pH (9.4 to 12.4) and its alkalinity (20 to 1,375 mg CaCO3/L). The formation of Ca phosphates (e.g., carbonate-hydroxylapatite, brushite) suggests phosphate is being removed through precipitation.
Several trace elements in the slag may be of environmental concern such as Cr (up to 0.8 wt. %) and Pb (up to 570 mg/kg). However, mineralogical studies indicate most of the elements occur in weathering-resistant phases and simulated weathering experiments (USEPA Method 1312) suggest that these elements are leached at concentrations well below aquatic toxicity levels. This observation is particularly relevant in terms of using ferrous slag for treating waters.