Geochemical Characterization of Iron and Steel Slag and Its Potential to Remove Phosphate and Neutralize Acid

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.

Global Research Progress and Trends on Critical Metals: A Bibliometric Analysis

“…The nonferrous slag includes copper slag, zinc slag, lead-zinc slag, and nickel slag. Several studies highlighted the importance of reprocessing waste piles for secondary metal recovery, thus, generating revenues [64,65]. Of course, one of the keys issues to be addressed is that the slag is generally characterized by an uneven distribution of the target components throughout the slag dump [66].…”

“…However, the laboratory tests performed by Kasikow et al demonstrated that a recovery process for large volumes of nickel-cobalt technogenic raw materials is valid to the total volume of slag [66]. On the other hand, bio-assisted extraction of metals from historical slag by use of Acidithiobacillus thiooxidans was demonstrated as being technical feasible and, at the same time, economically efficient [65]. Another already in practice example of metal recovery is the modern smelter of Zambian Copperbelt where the historical copper tailing was revisited for extraction of cobalt recovery [64].…”

Environmental and Socioeconomic Impact of Copper Slag—A Review

“…The slag can be characterized using Xray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS), and light optical microscopy (LOM) in transmitted and reflected light [45][46][47]. Previous studies have indicated that copper slag behaves differently when subjected to different temperatures, thus exhibiting different properties [42,48]. BCL slag was characterized to analyze its composition.…”

“…Direct reduction is one method used to recover iron from the slag by reducing Fe 3 O 3 and Fe 2 SiO 4 found in copper slag to metallic iron using coal. This involves using reduction temperatures of about 1300 • C over the course of 30 min, giving an output of 98.13% Fe [48]. Iron is a vital metal element that encourages the growth of humanity and national economies [62,63].…”

Editorial for Special Issue “Metallurgical Slags”

“…Another example of how to reuse metallurgical slags is presented in the contribution of Piatak et al [7]. Through the study of ferrous slags from the Chicago-Gary area (Illinois and Indiana, USA), the effective removal of phosphates from solution was documented.…”