Papers by Keyword: Fayalite

Abstract: Bioleaching is applied mainly for copper recovery from low-grade sulfide ores via heap leaching. The main copper processing route includes pyrometallurgy and the remaining copper slag from smelting may still contain copper in amounts found in the ore. Here bioleaching of copper slag material with a copper content of about 1 % (grain size < 63 µm) and fayalite (Fe₂SiO₄) und magnetite (Fe₃O₄) as main mineral phases was tested in aerobic shake flask experiments with a mixed culture of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Acidiphilium spp..To additionally test for reductive bioleaching, experiments under anaerobic conditions (80% N₂, 20% CO₂, v/v) with or without addition of elemental sulfur were run. The pH was adjusted to < 3 by addition of sulfuric acid. After the incubation period of more than 50 days at 30°C cell growth was observed in all biological assays. The redox potential was above 800 mV SHE in the aerobic biological assays and dropped to around 500 mV in the chemical control assays as well as in the anaerobic biological and chemical assays. A significant copper bioleaching was observed in the aerobic experiments with 91 % copper release (max. 35 % in the chemical controls). Anaerobic bioleaching experiments did not show a significant copper release, however the release of iron (as iron(II)) and sulfate was much higher than in the abiotic assays and several fold higher than in the classical aerobic bioleaching experiments. Overall the results show that copper bioleaching from slag material is possible, however the economic feasibility needs to be demonstrated.

Abstract: Slags from three iron smelting and processing sites were investigated. The composition of the slags is connected to the metallurgical processes and the investigation should provide an indication of the process conditions.A slag from a small bloomery furnace at the archaeological site Waschenberg (Hallstatt period) was investigated. Slags from the medieval period were found near Eisenerz and at this time iron was produced in developed bloomery furnaces (Stuckofen). These slags are termed fayalitic slags and they consist of wustite (FeO), fayalite (Fe₂SiO₄) and glass-phase (amorphous silicates) in various concentrations. Three solidification-types were described and correlated with the microstructures and the FeO-SiO₂-CaO₂ phase diagram.During the 19^th century steel was produced in puddling furnaces by remelting iron products from Eisenerz. Such slags, from this process, found in the surroundings of Palfau, were investigated and contain FeO and Ca rich glass-phase, but no fayalite.Due to the different compositions of the slags the solidification and the microstructures are different.

Abstract: Bloomery furnaces were the first units for iron smelting. In the Hallstatt period small bowl-type furnaces were used and until the medieval period the size of such furnaces was increasing continuously. Experimental archaeologists reconstruct bloomery furnaces to study the processes of bloom production. In a small bowl-type furnace (Hallstatt period) at Asparn and in a larger shaft-type furnace (medieval period) at Ybbsitz smelting experiments were performed, The samples contained metallic iron and slag. Various amounts of iron in different stages of conglomeration up to larger iron pieces were found. The slag belongs to a fayalitic-type, consisting of wustite (FeO), fayalite (Fe₂SiO₄) and glass-phase (amorphous Ca-, Al-silicates) in various concentrations. The yield of metallic iron was highly different for the various experiments. In general, more metallic iron was formed in the larger shaft-type furnaces. A large bloom was not obtained.

Abstract: The boundary constants between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were calculated in order to clarify the formation mechanism of fayalite scale (Fe₂SiO₄) or chromite scale (FeCr₂O₄), which can form as a “sub-scale” in Si or Cr containing steels. The diffusion coefficient of oxygen in the alloy, Do, and the oxygen concentration at the specimen surface, N_O^(s), which are constituents of the internal oxidation rate constant, (2D_ON_O^(s)/N_B^(O)n), were calculated for various oxidation conditions, and the rate equation for internal oxidation was derived. By comparing the calculated and measured values of (2D_ON_O^(s)/N_B^(O)n), we confirmed that the rate equation determined for internal oxidation was reasonable. The boundary condition between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were also calculated by substituting the calculated values of D_O and N_O^(s) into the rate equation.

Abstract: Carbonthermic method was adopted to reduce the iron in the form of fayalite in copper slag to metal iron in this study. The reduction was undertaken under the conditions of roasting reduction at 1423 K for 4 h with specified parameters that the ratio of copper slag versus anthracite was 2∶1, and that the ratio of calcium carbonate to silicon dioxide was 1.43∶1. It was demonstrated that the metallization efficiency of iron could reach 88.26%. Moreover it was showed that the reduction process of fayalite by carbon could be routed as: Fe₂SiO₄(or 2FeO·SiO₂)→ FeO + 2CaO·SiO₂→ Fe. In addition it was proved that the combination ability between calcium positive ion and silicon oxygen anion was bigger than that between iron positive ion and silicon oxygen anion. As a result，SiO₂ compacted in fayalite could decompose easily and could combine with CaO at the reaction temperature, therefore FeO could exist with a form of freedom state which could be reduce easily by carbon or monocarbon produced in carbonthermic method.

Abstract: Exposure of metals and alloys to high temperatures leads to the formation of oxide scales, with a large impact on surface quality. The most important features of the oxide layer are its thickness, composition, structure, adherence and coherence. Temperature, time, gas atmosphere and chemical composition determine the growth of oxide layers. In this paper, the characteristics of the high temperature oxidation properties of Fe-Si alloys are discussed in terms of oxide growth mechanism, kinetics and phase morphology. The oxidation kinetics of different Fe-Si alloy steels in air, its scale structure and composition were investigated over the temperature range 900-1250°C. Oxidation experiments were performed in air, to analyse the oxidation process. Experiments were carried out in an electric furnace at temperatures ranging from 900 to 1250°C, for times between 16 and 7200s. Thus treated specimens were characterised by metallography and their scale thickness was measured by optical microscopy. Scale morphology was studied and scale composition confirmed by EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscattered Diffraction) analysis. Results show that high temperature oxidation of Si- alloys presents the classic three layered oxide scale. On the grain boundaries in the scale, iron-silicate was found. Observations show a Sienrichment at the scale –metal interface. This enrichment is present in the form a mixed wustite-iron silicate (fayalite) phase, FeO-Fe2SiO4. A very rapid build-up of oxide occurs when a liquid phase, due to the wustite-fayalite eutectic, is present in the surface of the steel.

Abstract: An oxide scale layer always forms at the strip surface during the hot rolling process. As a consequence, de-scaling and pickling operations must be performed prior or after hot rolling. Many surface defects caused by hot rolling are related to oxidation in the reheating furnace. One of these is the melting of eutectic FeO/Fe2SiO4 during reheating over 1170°C giving as a result red scale defects in Si-added steel. On the other hand, steel strip surface oxidation during hot rolling causes an industrial and environmental problem: secondary oxide is removed after roughing, but tertiary oxide scales already start to form before entering the finishing stands. Their properties affect the final steel surface quality and its response to further processing. Furthermore, the addition of alloying elements has an important impact on scale properties. In particular the alloying of silicon effects the region between scale and substrate. It causes peculiar surface properties inherited from its specific oxidation characteristics. Conventional oxidation experiments in air of silicon steels are a valuable tool to study the influence of Si on steel oxidation. After oxidation in air in the temperature range of 900-1250°C it has been observed that Si enhance markedly scale adhesion, especially above 1177°C (the eutectic temperature of FeO-Fe2SiO4 ) and also at lower temperatures. Special attention has been paid on the investigation of the effects of alloying Si on the high-temperature oxidation of steel, for a better understanding of the behaviour of modern steels during hot rolling.