Changes in mineralogical and leaching properties of converter steel slag resulting from accelerated carbonation at low CO2 pressure (original) (raw)
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Journal of CO2 Utilization, 2019
Accelerated carbonation is a treatment for converting alkaline industrial residues into added-value products and storing CO2 in solid form. This work investigated the influence of reacting phases and CO2 uptake on microstructure development, mechanical properties and the environmental behavior of carbonated compacts produced from Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) slags, characterized by a different mineralogy. The compacts were cured under a 100% CO2 atmosphere at 50°C and pressure of 1.3 or 10 bar for 15 min to 4 h. The BOF slag reacted very fast in the first 30-60 min due to the complete conversion of portlandite to calcite, amorphous calcium carbonate and aragonite, and continued to react over time due to the presence of slower reacting Ca-silicate phases. For the EAF slag, rich in Ca-silicates, the CO2 uptake was lower, and increased only slightly over time at 1.3 bar and became almost stable after 15 minutes at 10 bar; the EAF slag products however presented a higher compressive strength than the BOF slag ones, because of the different phases involved in the carbonation reaction. For the BOF slag, portlandite dissolution caused the formation of voids, only partially filled up by the reaction products. For the EAF slag, formation of a carbonate and amorphous silica layer around the reacting silicates yielded a denser matrix. pH and Ba leaching decreased for both types of slag, whereas V release increased due to the dissolution of reactive phases such as dicalcium silicates, which initially contained this element.
Mineral carbonation of metallurgical slags
Mineralogia, 2015
Due to increasing emissions of greenhouse gases into the atmosphere number of methods are being proposed to mitigate the risk of climate change. One of them is mineral carbonation. Blast furnace and steel making slags are co-products of metallurgical processes composed of minerals which represent appropriate source of cations required for mineral carbonation. Experimental studies were performed to determine the potential use of slags in this process. Obtained results indicate that steel making slag can be a useful material in CO2 capture procedures. Slag components dissolved in water are bonded as stable carbonates in the reaction with CO2 from ambient air. In case of blast furnace slag, the reaction is very slow and minerals are resistant to chemical changes. More time is needed for minerals dissolution and release of cations essential for carbonate crystallisation and thus makes blast furnace slags less favourable in comparison with steel making slag.
Controls on the Carbonation of Steel Slag
Steel production is currently the largest industrial source of atmospheric CO 2 . As annual steel production continues to grow, the need for effective methods of reducing its carbon footprint increases correspondingly. The carbonation of the calcium-bearing phases in steel slag generated during basic oxygen furnace (BOF) steel production, in particular its major constituent, larnite {Ca 2 SiO 4 }, which is a structural analogue of olivine {(MgFe) 2 SiO 4 }, the main mineral subjected to natural carbonation in peridotites, offers the potential to offset some of these emissions. However, the controls on the nature and efficiency of steel slag carbonation are yet to be completely understood. Experiments were conducted exposing steel slag grains to a CO 2 -H 2 O mixture in both batch and flow-through reactors to investigate the impact of temperature, fluid flux, and reaction gradient on the dissolution and carbonation of steel slag. The results of these experiments show that dissolution and carbonation of BOF steel slag are more efficient in a flow-through reactor than in the batch reactors used in most previous studies. Moreover, they show that fluid flux needs to be optimized in addition to grain size, pressure, and temperature, in order to maximize the efficiency of carbonation. Based on these results, a two-stage reactor consisting of a high and a low fluid-flux chamber is proposed for CO 2 sequestration by steel slag carbonation, allowing dissolution of the slag and precipitation of calcium carbonate to occur within a single flow-through system.
Hydrogeochemistry of alkaline steel slag leachates in the UK
Water, Air, & Soil Pollution, 2008
Drainage from steel slag disposal sites can be extremely alkaline and a source of pollution to surface and ground waters. Data is presented detailing the hydrogeochemistry of seven highly alkaline (pH > 10) steel slag surface discharges in the UK. While there is the consistent presence of Ca–OH type groundwater in all the discharges, there are clear disparities in hydrochemical facies within and between sites, reflecting native hydrochemistry, source material and hydrogeological setting. The longevity of the pollution problem from steel slag disposal sites is highlighted at one site where the water quality records date back three decades. The consistent presence of Al, B, Ba, Fe, Sr, V and occasional presence of Cr, Mo, Ni, Pb were found at concentrations typically below surface water quality standards in the leachates. Some of the monitored metals (Al, Fe, Ni, V) were found to be lost from solution downstream of emergence in calcite-dominated precipitates which rapidly form at all sites at rates up to 100 g m−2 day−1. The low concentrations of potentially problematic trace elements in both solution and the sediments are discussed with regard development of economically viable passive treatment wetlands for highly alkaline industrial discharges.
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.
Speciation of Cr and V within BOF steel slag reused in road constructions
2006
Basic Oxygen Furnace (BOF) steel slag is a residue from the basic oxygen converter in steel-making operations, which is partially reused as an aggregate for road constructions. It is essentially composed of calcium, silicon and iron but also contains potential toxic elements present as traces, like chromium (Cr, 2600 mg kg À 1 ) and vanadium (V, 690 mg kg À 1 ), which can be released. The linked results of chemical analysis, XRD and SEM-EDX enabled to identify the main mineral phases composing BOF slag and EDX micro-analyses indicated that V and Cr were associated to dicalciumferrite. A 47-days static leaching test at a laboratory scale with a controlled pH of 5 (pH stat leaching test) showed that Cr was little released, while V was significantly released. Finally, X-ray absorption near-edge structure (XANES) spectra of 3 BOF slag samples were recorded (brawQ, leached 47 days at pH 5 and aged 2 years in a lysimeter). XANES spectra showed that Cr is present at octahedral coordination in the trivalent form, the less mobile and less toxic one, and that its speciation does not evolve during natural ageing and leaching at pH 5. They also indicated that V is predominantly present in the + 4 oxidation state and seems to become oxidized to the pentavalent form (the most toxic form) during natural ageing. D
The electric arc furnace (EAF) slag could be exploited in several fields of application, such as land filling, road constructions and concrete production. However, their use is limited by the presence of polluting chemical elements (chromium (Cr), barium (Ba), Vanadium(V), etc.) that can be dangerous to humans and the environment. Thus, chemical and structural stability is a fundamental requirement, especially when the slag may come in contact with water. Therefore, the interaction between slag and water is key, in order to classify the slag as a safe raw material. In this work, the effect of slag chemical composition on the chemical leaching of about seventy EAF carbon steel slags of different production steel grades was investigated. Standard leaching tests (24 h at 10 l/kg) in deionized water on slag bulks were performed and the results were correlated with the slag chemical composition. The survey has made possible defining the safe chemical composition areas on the main ternary diagrams, able to transfer stability to the slag. The results obtained indicate the water/slag ratio as the most important factor in the release of polluting substances, also identifying a critical scenario for slag recycling.
Chemical Engineering Journal, 2014
Please cite this article as: M. Salman, Ö. Cizer, Y. Pontikes, R.M. Santos, R. Snellings, L. Vandewalle, B. Blanpain, K. van Balen, Effect of accelerated carbonation on AOD stainless steel slag for its valorisation as a CO 2 -sequestering construction material, Chemical Engineering Journal (2014), doi: http://dx.
Kinetics of steel slag leaching: Batch tests and modeling
Waste Management, 2011
Reusing steel slag as an aggregate for road construction requires to characterize the leaching kinetics and metal releases. In this study, basic oxygen furnace (BOF) steel slag were subjected to batch leaching tests at liquid to solid ratios (L/S) of 10 and 100 over 30 days ; the leachate chemistry being regularly sampled in time. A geochemical model of the steel slag is developed and validated from experimental data, particularly the evolution with leaching of mineralogical composition of the slag and trace element speciation. Kinetics is necessary for modeling the primary phase leaching, whereas a simple thermodynamic equilibrium approach can be used for secondary phase precipitation. The proposed model simulates the kineticallycontrolled dissolution (hydrolysis) of primary phases, the precipitation of secondary phases (C-S-H, hydroxide and spinel), the pH and redox conditions, and the progressive release of major elements as well as the metals Cr and V. Modeling indicates that the dilution effect of the L/S ratio is often coupled to solubility-controlled processes, which are sensitive to both the pH and the redox potential. A sensitivity analysis of kinetic uncertainties on the modeling of element releases is performed.