Abstract
Iron and steel slags from legacy and modern operations in the Chicago-Gary area of Illinois and Indiana, USA, are predominantly composed of Ca (10–44 wt. % CaO), Fe (0.3–28 wt. % FeO), and Si (10–44 wt. % SiO2), with generally lesser amounts of Al (<1–15 wt. % Al2O3), Mg (2–11 wt. % MgO), and Mn (0.3–9 wt. % MnO). Mineralogy is dominated by Ca ± Mg ± Al silicates, Fe ± Ca oxides, Ca-carbonates, and high-temperature SiO2 phases. Chromium and Mn concentrations in most samples may be environmentally significant based on comparison with generic soil contaminant guidelines. However, simulated weathering tests suggest these elements are present in generally insoluble phases making their use in water treatment applications possible; however, the generation of high pH and alkaline solutions may be an issue. As for possible water treatment applications, batch and flow-through experiments document effective removal of phosphate from synthetic solutions for nearly all slag samples. Air-cooled fine fractions (<10 mm) of modern slag were most effective; other types, including modern granulated, modern air-cooled coarse fractions (>10 mm), and legacy slag removed phosphate, but to a lesser degree. An additional water treatment application is the use of slag to neutralize acidic waters. Most slag samples are extremely alkaline and have high net neutralization potentials (NNP) (400–830 kg CaCO3/t), with the highest approximately equivalent to 80% of the neutralization potential of calcite. Overall, phosphate removal capacity and NNP correlate positively with total Ca content and the dissolution of Ca minerals facilitates secondary Ca phosphate formation and consumes acid during hydrolysis. Utilizing locally available slag to treat waste or agricultural waters in this region may be a higher value alternative than use in construction, potentially offsetting restoration costs to degraded legacy areas and decreasing steel manufacturers’ current waste footprint.
Highlights
Ferrous slags are byproducts generated after smelting iron ore in a blast furnace to make pig iron or after smelting pig iron and scrap metal most commonly in aBasic Oxygen Furnace (BOF) or Electric Arc Furnace
Total Fe is reported as Fe2 O3 or Fe depending on the analytical technique used in Hoppe et al [31]
The final set of batch experiments using a 1085 mg P/L leachant solution resulted in only partial removal of the phosphate for all tested samples. These results provided data for the calculation of a maximum phosphate removal capacity (PRC in mg P/g slag): Phosphate removal capacity (PRC) = ((Pin − Pef )V)/M
Summary
Basic Oxygen Furnace (BOF) or Electric Arc Furnace (referred to as steel slag). In the U.S alone, it is estimated that 15 to 20 million tons of ferrous slag are produced annually; it is considered a domestic commodity for its value in various applications [2]. The most common use of ferrous slag is in construction. Minerals 2019, 9, 468 dense nature of air-cooled slag makes it suitable for its primary use as aggregates in concrete, asphalt paving, fill and road bases. Almost all granulated blast furnace slag (e.g., cooled with high-pressure sprays of water) is used in concrete mixes or in blended cements as a partial substitute for Portland cement [2]. In addition to construction use, ferrous slag has various environmental applications
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