Speciation of Cr and V within BOF steel slag reused in road constructions (original) (raw)
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Journal of Hazardous Materials, 2007
Basic oxygen furnace (BOF) steel slag is a residue from the basic oxygen converter in steel-making operations, and is partially reused as an aggregate for road constructions. Although BOF slag is an attractive building material, its long-term behaviour and the associated environmental impacts must be taken into account. Indeed BOF slag is mainly composed of calcium, silicon and iron but also contains trace amounts of potential toxic elements, specifically chromium and vanadium, which can be released. The present research focuses (i) on the release of Cr and V during leaching and (ii) on their speciation within the bearing phase. Indeed the mobility and toxicity of heavy metals strongly depend on their speciation. Leaching tests show that only low amounts of Cr, present at relatively high concentration in steel slag, are released while the release of V is significantly high. X-ray absorption near-edge structure (XANES) spectroscopy indicates that Cr is present in the less mobile and less toxic trivalent form and that its speciation does not evolve during leaching. On the contrary, V which is predominantly present in the 4+ oxidation state seems to become oxidized to the pentavalent form (the most toxic form) during leaching.
2007
The speciation of chromium (Cr) and vanadium (V), two potential pollutants, present as traces in a residue from steel-making operations (BOF slag) and their leaching behavior were studied to assess the environmental compatibility of this waste as a recycled material for road making. A multi techniques approach (XAS, XANES, micro-XANES, micro-XRF and micro-XRD) was required to assess the molecular mechanisms of Cr and V release. Leaching tests have shown that Cr is little released and moreover remains at its initial trivalent form, less toxic, even during leaching. One part of octahedral Cr(III) ions replaces structurally the octahedral Fe(III) in a dicalcium aluminoferrite phase and the other part is associated to a solid solution (Fe, Mn, Mg)O. Spatially resolved techniques allowed showing that Cr is tightly bound to a secondary phase formed during leaching which controls its solubility. V present in BOF slag is environmentally more critical because its release is relatively high and it is oxidized to its most toxic form (V(V)) during leaching. Micro-XRF coupled with chemometric analysis (SIMPLISMA) indicates that V is also associated to a dicalcium aluminoferrite.
Basic oxygen furnace (BOF) steelmaking slag is enriched in potentially toxic V which may become mobilized in high pH leachate during weathering. BOF slag was weathered under aerated and air-excluded conditions for 6 months prior to SEM/EDS and μXANES analysis to determine V host phases and speciation in both primary and secondary phases. Leached blocks show development of an altered region in which free lime and dicalcium silicate phases were absent and Ca−Si−H was precipitated (CaCO 3 was also present under aerated conditions). μXANES analyses show that V was released to solution as V(V) during dicalcium silicate dissolution and some V was incorporated into neo-formed Ca−Si−H. Higher V concentrations were observed in leachate under aerated conditions than in the air-excluded leaching experiment. Aqueous V concentrations were controlled by Ca 3 (VO 4) 2 solubility, which demonstrate an inverse relationship between Ca and V concentrations. Under air-excluded conditions Ca concentrations were controlled by dicalcium silicate dissolution and Ca−Si−H precipitation, leading to relatively high Ca and correspondingly low V concentrations. Formation of CaCO 3 under aerated conditions provided a sink for aqueous Ca, allowing higher V concentrations limited by kinetic dissolution rates of dicalcium silicate. Thus, V release may be slowed by the precipitation of secondary phases in the altered region, improving the prospects for slag reuse.
Environmental assessment of a BOF steel slag used in road construction : The ECLAIR research program
Environnement, Ingénierie & Développement, 2010
Steel production generates great amounts of by-products as steel slag. Unlike blast furnace slag, the use of Basic Oxygen Furnace slag (BOF slag) has been restrained due to insufficient volume stability and to the lack of environmental regulations. This study aimed at investigating the potential release and impact of pollutants, especially Cr and V that are present in rather high concentrations in slag, from a BOF slag used in a civil engineering structure (an industrial platform), using a multi-scale approach. The oneyear follow up of the experimental platform showed that concentrations of Cr and V were generally low in seepage waters, and in leachates from leaching test. Microanalyses carried out on slag allowed us to confirm the location of these metals in rather stable ferrous mineral phases, but V was also bound to more reactive silicates. No real toxicity effect of seepage waters has been revealed from eco-toxicological tests carried out with earthworms. La fabrication de l’ac...
Alkalinity generation and toxic trace metal (such as vanadium) leaching from basic oxygen furnace (BOF) steel slag particles must be properly understood and managed by pre-conditioning if beneficial reuse of slag is to be maximised. Water leaching under aerated conditions was investigated using fresh BOF slag at three different particle sizes (0.5–1.0, 2–5 and 10 × 10 × 20 mm blocks) and a 6-month pre-weathered block. There were several distinct leaching stages observed over time associated with different phases controlling the solution chemistry: (1) free-lime (CaO) dissolution (days 0–2); (2) dicalcium silicate (Ca 2 SiO 4) dissolution (days 2–14) and (3) Ca– Si–H and CaCO 3 formation and subsequent dissolution (days 14–73). Experiments with the smallest size fraction resulted in the highest Ca, Si and V concentrations, highlighting the role of surface area in controlling initial leaching. After ~2 weeks, the solution Ca/ Si ratio (0.7–0.9) evolved to equal those found within a Ca–Si–H phase that replaced dicalcium silicate and free-lime phases in a 30-to 150-μm altered surface region. V release was a two-stage process; initially, V was released by dicalcium silicate dissolution, but Valso isomorphically substituted for Si into the neo-formed Ca–Si–H in the alteration zone. Therefore, on longer timescales, the release of V to solution was primarily controlled by considerably slower Ca–Si–H dissolution rates, which decreased the rate of V release by an order of magnitude. Overall, the results indicate that the BOF slag leaching mechanism evolves from a situation initially dominated by rapid hydration and dissolution of primary dicalcium silicate/free-lime phases, to a slow diffusion limited process controlled by the solubility of secondary Ca–Si–H and CaCO 3 phases that replace and cover more reactive primary slag phases at particle surfaces.
We here report the chemical composition of (a) Fe–Ni laterite ores from central and northern Greece and bauxite ores from Parnassos-Ghiona deposit, (b) the corresponding high volume residues (slag and red mud) produced during the metallurgical processing of ferronickel and alumina, (c) experimentally produced water (natural and/ or seawater) leachates of these ores and metallurgical leachates and (d) chromium isotope composition (expressed as δ 53 Cr values) of the leachates, seawater and of Cr (VI) contaminated natural water, in an attempt to assess the role of industrial mining processes on the Cr(VI) contamination of groundwater in ultramafic rock-dominated terrain. The five week lasting laboratory leaching experiments, performed under atmospheric conditions, showed that only small amounts of Cr(VI) (b~2 μg·L −1) from Fe–Ni laterites of Lokris (Central Greece) were mobilized into the aqueous phase. In contrast, Cr(VI) concentrations in leachates from Fe–Ni laterites of Kastoria reached values as high as 1300 μg·L −1. In addition, unexpectedly high Cr(VI) concentrations were measured in water leachates from red mud samples (2100 μg·L −1). The leachates from red mud are also characterized by relatively high V (6200 μg·L −1) and Ga (90 μg·L −1) concentrations. In contrast to leachates from red mud, the Cr(VI) concentrations in leachates from slag remained below ~2 μg·L −1. The δ 53 Cr values measured in natural water leachates of Fe–Ni-laterite from Kastoria range between +0.03 ± 0.06‰ and −0.21 ± 0.08‰ and differ from the more positively fractionated value of +1.01 ± 0.05 ‰ obtained for the laterite from Lokris. The δ 53 Cr values in natural water and seawater leachates from red mud returned δ 53 Cr values of +0.83 ± 0.05‰ and +0.73 ± 0.08‰, respectively, and thus are statistically indistinguishable. Although there is no clear-cut relationship between Cr(VI) concentrations and δ 53 Cr values in the diverse leachates, there is an indication that the laterite leachates from Kastoria with high Cr(VI) concentrations yielded less positively frac-tionated δ 53 Cr values, compared to the laterite leachates from Lokris. The agreement between chromium isotope compositions of artificial leachates of red mud and that of contaminated seawater from the Assopos river extrusion (estuary) forming a heavily Cr(VI) contaminated point source, suggests that the release of Cr(VI) from the red mud rather than from the primary bauxite ore is a major source of toxic Cr(VI), and that insignificant back-fractionation (reduction) of this mobile and toxic chromium along its pathway to the sea takes place.
Effects of basicity and mesh on Cr leaching of EAF carbon steel slag
2019
The slag's chromium leaching is one of the most pressing concerns in Electric Arc Furnace (EAF) carbon and stainless steels production. In recent years, many studies have aimed at understanding which properties (basicity, cooling speed) and phases (spinels, wustite, silicates) determine the Cr leaching, defining different indices (sp-factor, cs-factor) in order to forecast the slag's behavior compared to the leaching of this toxic metal. The literature suggests that spinel formation is usually a good way to fix Cr and prevent its leaching. However, in some conditions (high basicity, low amount of spinel-forming species) soluble Cr-bearing phases can be formed, i.e., Ca-chromite or unstable spinel. In these conditions, Cr can be leached easily, even if it is bound in a spinel structure. In this paper, the effects of basicity and impurities (Ca, Si) on the instability of Cr-spinel was investigated, with respect to slag mesh. The influence of basicity was also studied on Mg-wustite stability, which might contribute to the leaching of Cr. Different samples of carbon steel slag, suspected of forming unstable spinels, belonging to different steel grade production, were also investigated. Both granulated (4 mm) and milled (< 100 μm) slag were analyzed. Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS) analyses were carried out to measure the local chemical composition of Cr-bearing phases. This data was correlated with slag basicity (by X-Ray Fluorescence: XRF), spinel fraction (by X-Ray Diffraction: XRD), and Cr leaching (by Inductively Coupled Plasma Mass Spectrometry: ICP-MS). The main results indicate that the increase of the slag basicity implies an increase of the impurity content (Ca, Si) in the spinel, also over-saturated by Cr. This aspect, coupled with spinel geometrical features, seems to justify the unexpected Cr leaching of some slag samples. Basicity does not influence the chemistry of wustite, thus excluding it as an additional Cr leaching source.
The Journal of Physical Chemistry B, 2007
This paper presents a comparison between several methods dedicated to the interpretation of V K-edge X-ray absorption near-edge structure (XANES) features. V K-edge XANES spectra of several V-bearing standard compounds were measured in an effort to evaluate advantages and limits of each method. The standard compounds include natural minerals and synthetic compounds containing vanadium at various oxidation state (from +3 to +5) and in different symmetry (octahedral, tetrahedral, and square pyramidal). Correlations between normalized pre-edge peak area and its centroid position have been identified as the most reliable method for determining quantitative and accurate redox and symmetry information for vanadium. This methodology has been previously developed for the Fe K edge. It is also well adapted for the V K edge and is less influenced by the standard choice than other methods. This methodology was applied on an "environmental sample," i.e., a well-crystallized leached steel slag containing vanadium as traces. Micro-XANES measurements allowed elucidating the microdistribution of vanadium speciation in leached steel slag. The vanadium exhibits an important evolution from the unaltered to the altered phases. Its oxidation state increases from +3 to +5 together with the decrease of its symmetry (from octahedral to tetrahedral).
Chemical, Mineralogical, and Morphological Properties of Steel Slag
Advances in Civil Engineering, 2011
Steel slag is a byproduct of the steelmaking and steel refining processes. This paper provides an overview of the different types of steel slag that are generated from basic-oxygen-furnace (BOF) steelmaking, electric-arc-furnace (EAF) steelmaking, and ladle-furnace steel refining processes. The mineralogical and morphological properties of BOF and electric-arc-furnace-ladle [EAF(L)] slag samples generated from two steel plants in Indiana were determined through X-Ray Diffraction (XRD) analyses and Scanning Electron Microscopy (SEM) studies. The XRD patterns of both BOF and EAF(L) slag samples were very complex, with several overlapping peaks resulting from the many minerals present in these samples. The XRD analyses indicated the presence of free MgO and CaO in both the BOF and EAF(L) slag samples. SEM micrographs showed that the majority of the sand-size steel slag particles had subangular to angular shapes. Very rough surface textures with distinct crystal structures were observed...