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Based on recently published research on leaching control mechanisms in electric arc furnace (EAF) slags, it is assumed that a FeO/SiO2 ratio of around one leads to low leached V and Cr concentrations. This ratio influences the mineral phase composition of the slag toward higher amounts of spinel and a lower solubility of calcium silicate phases by suppressing the formation of magnesiowuestite and highly soluble calcium silicate phases. To evaluate this hypothesis, laboratory and scaled up tests in an EAF pilot plant were performed on slag samples characterized by elevated V and Cr leaching and a high FeO/SiO2 ratio. Prior to the melting experiments, the optimum FeO/SiO2 ratio was calculated via FactSageTM. In the melting experiments, the ratio was adjusted by adding quartz sand, which also decreased the basicity (CaO/SiO2) of the slag. As a reference, remelting experiments without quartz sand addition were conducted and additionally, the influence of the cooling rate of the slag was examined. The remelted (without quartz sand) and the remelted modified slags (with quartz sand) were analyzed chemically and mineralogically and the leaching behavior was investigated. The modification of the slags yielded a minimized release of V and Cr, supporting the hypothesis that the FeO/SiO2 ratio influences the mineralogy and the leaching behavior.
Based on recently published research on leaching control mechanisms in electric arc furnace (EAF) slags, it is assumed that a FeO/SiO2 ratio of around one leads to low leached V and Cr concentrations. This ratio influences the mineral phase composition of the slag toward higher amounts of spinel and a lower solubility of calcium silicate phases by suppressing the formation of magnesiowuestite and highly soluble calcium silicate phases. To evaluate this hypothesis, laboratory and scaled up tests in an EAF pilot plant were performed on slag samples characterized by elevated V and Cr leaching and a high FeO/SiO2 ratio. Prior to the melting experiments, the optimum FeO/SiO2 ratio was calculated via FactSageTM. In the melting experiments, the ratio was adjusted by adding quartz sand, which also decreased the basicity (CaO/SiO2) of the slag. As a reference, remelting experiments without quartz sand addition were conducted and additionally, the influence of the cooling rate of the slag was examined. The remelted (without quartz sand) and the remelted modified slags (with quartz sand) were analyzed chemically and mineralogically and the leaching behavior was investigated. The modification of the slags yielded a minimized release of V and Cr, supporting the hypothesis that the FeO/SiO2 ratio influences the mineralogy and the leaching behavior.
Basic oxygen furnace slags (BOFS) are by-products of the steelmaking process. Several researchers have studied the production of Portland cement clinker and metallic iron from BOFS via a reductive treatment.
In this study, we applied a carbothermal reduction of BOFS in a technical-scale electric arc furnace and characterised the clinker-like products. Those clinker-like non-metallic products (NMPs) had a chemical and mineralogical composition comparable to clinker for ordinary Portland cement (OPC) and contained large elongated alite crystals as major component. The pure NMPs reacted more slowly and achieved a lower degree of hydration compared with commercial OPC.
If the reactivity of the products can be further increased by employing specific adaptations, it can be used as a full clinker substitute for OPC. Nevertheless, it is also an option to use the material without further modifications as a cement component or concrete addition, which contributes to the strength development in both cases.
The recycling of refractory materials in the chemical, metallurgical and glass industry has only had minor relevance until now because of the high performance required from these materials (stability against corrosion, inert when coming into contact with the glass melt, etc.). Refractory lifetimes are limited because of corrosion at higher temperatures. The best furnace lifetimes has been achieved with chromium oxide and chromium oxide corund refractories. Waste from the production and use of refractory material is generated in three main forms: as residues from cutting and grinding, as refractories which have limited or no contact with glass melts, and as refractories with infiltration of sublimation residues or condensates. The waste material is classified as hazardous waste due to its chromium content. In this study, arc furnace technology was used for thermal treatment of the waste materials to enable recycling of refractory materials. (author abst.)
The material properties of recycled concrete aggregates (RCA) are
correlated to the sorting accuracy of the former demolition waste. Impurities like
wood, clay bricks or gypsum can lead to inferior building material properties. Harmful
substances like heavy metals or organic pollutants should be minimised as well.
Hence the non-concrete materials have to be separated from the concrete material
stream. This can be done during the demolition process by using selective
dismantling techniques. Alternatively a variety sorting and classifying techniques to
purify the crushed concrete can be utilized.
A research project, funded by the Federal Ministry for the Environment, Nature
Conservation and Nuclear Safety, investigated the possibilities of gaining recycled
concrete aggregates of a high quality for the reuse in the production of concrete. The
work focuses especially on how gypsum respectively sulphates in the crushed
concrete can be reduced. Sulphates can impair the setting behaviour of concrete
and also damage set concrete by causing expansion. Therefore the content of
sulphates in RCA is restricted by guidelines and standards in Germany.
Generally, gypsum in construction waste originates from interior fittings like gypsum
walls, floor screeds, plaster boards and also plaster. Most of these materials can be
reconstructed using selective dismantling techniques. This can be achieved either by
manual labour or by using mechanical equipment e.g. to remove floor screeds by
milling. Depending on the specific deconstruction site it can be more effective and/or
more environmentally compatible to remove sulphates by treating the crushed
concrete. Applicable treatments for sulphate reduction include dry processes like
manual sorting of gypsum wall blocks as well as wet treatments e.g. jigging.
In addition this research project an environmental performance evaluation was
undertaken to assess different techniques for reducing sulphates in recycled
concrete aggregates.