The need for more efficient blast furnaces is even greater now that there are stricter environmental regulations on greenhouse gas (GHG) emissions. Coke within the blast furnace not only supports the furnace bed and allows gas flow, it also carburises liquid iron. The carburisation of iron is one of the most important reactions and must be better understood if the ironmaking process in the blast furnace is to be made more sustainable. By understanding what coke properties influence the rate at which coke dissolves in iron we can predict a coke?s performance and use it to determine its quality. As carbon dissolution rates have only been determined for a few cokes, a systematic and comprehensive study was conducted on the dissolution of carbon from nine Australian cokes into liquid iron. The kinetics of carbon dissolution from Cokes A to I was measured and a range of experimental techniques were used to elucidate the dominant rate influencing factors. The role of coke structure, coke inorganic matter composition and yield and temperature were investigated. Furthermore, the influence of interfacial products and dynamic wettability studies were also conducted. The carburiser cover method was used to measure carbon pick-up as a function of time over the temperature range of 1450-1550 ??C. Fundamental data on the apparent carbon dissolution rate constant (K) in molten iron at 1550 ??C for Cokes A to I were obtained and ranged from K (x 103 s-1) = 0.47 to K (x 103 s-1) = 14.7. The wide variation in K showed that not all cokes dissolve at similar rates. In fact one of the nine cokes in this investigation dissolved at a rate comparable to graphite dissolution rates. The apparent carbon dissolution activation energy, Ea, for two of the nine cokes plus synthetic graphite (SG) was also determined. The Ea obtained for SG (Ea = 54 kJ / mol) was in agreement with literature values and was consistent with a diffusion controlled mechanism. The observed Ea values for Cokes D and F (313 kJ / mol and 479 kJ / mol respectively) are an order of magnitude larger than the Ea obtained for SG. The difference in Ea between cokes and SG does not appear to be solely due to differences in the structure of the carbon source. The difference in Ea between the cokes was attributed to differences in their inorganic matter composition. The interfacial contact area is a function of inorganic matter yield and composition, which in turn is a function of temperature. Therefore, as temperature decreases the slag / ash layer produced at the carbon / iron interface can increase in area and viscosity and thus hinder carbon dissolution and transfer, and increasing the apparent activation energy for carbon dissolution. Thus, the differences in viscosity and melting temperature of the interfacial product play a key role. Wettability experiments were carried out using the sessile drop technique. The wettability of Cokes D, F and G with liquid iron at 1550 ??C was measured as a function of time. All three coke samples showed non-wetting behaviour with contact angles ranging between 123-129?? in the initial stages and between 109-114?? after two hours of contact. The differences in the wettability of the three coke samples could not explain the large differences in dissolution rates observed between these cokes. Thus, the wettability of these coke samples was not considered a dominant factor in influencing the rate of carbon dissolution. The sessile drop technique was also used to study the interfacial products formed at the coke / iron interface. The interfacial products formed on the underside of the iron droplet after contact with Cokes F and G were initially different in regards to the morphology and chemical composition. The interfacial product formed with Coke G had a network or mesh like structure that seemed to wet the iron droplet much better than the interfacial product formed with Coke F. In contrast, Fe globules and discrete interfacial products were observed in Coke F. It was suggested that this was due to differences in inorganic matter content, especially in calcium (Ca) and sulfur (S) content in the coke. Formation of interfacial products containing sulfides, such as calcium sulfide (CaS) and manganese sulfide (MnS), were observed on the iron side of the interface of both Cokes F and G. As a result, the interfacial products can act as a physical barrier blocking iron and coke contact, thus reducing the contact area for carbon dissolution and decreasing the rate of carbon dissolution. The presence of MnS may act to lower the liquidus temperature of the interfacial product, which in turn can affect the overall viscosity of the interfacial layer. Thus, the deposition of reaction products at the interfacial region can have a significant effect on carbon dissolution rates. The mineral pyrrhotite was also identified as a significant factor in influencing the rate of carbon dissolution. Electron dispersive X-ray analyses of Coke F identified iron to be in close association with sulfur. These Fe / S species have atomic ratio similar to pyrrhotite (Fe1-xS) or troilite (FeS). Pyrrhotite in coke can decompose to release gaseous sulfur and metallic iron, which can be carburised by carbon in the surrounding area to form Fe-C particles. Thus, carburisation of liquid iron can occur via Fe-C particles. There was little difference in structure between the nine coke samples and therefore the high dissolution rates of Coke F, cannot be explained on the basis of crystallite size or anisotropic carbon content. Inorganic matter yield and composition were identified as the dominant rate influencing factors on carbon dissolution. More specifically: - High content of iron phases, such as iron oxides and pyrrhotite, can lead to an increase in carbon dissolution rates. This maybe due to increased amounts of Fe-C particles that are formed upon the reduction of magnetite and decomposition of pyrrhotite, and carried through the slag layer to carburise the bulk liquid iron. - High aluminium oxide content can lead to a decrease in carbon dissolution rates. This maybe due to higher ash fusion / melting temperature or decrease in wettability, both of which lead to a decrease in carbon / iron contact area. - Formation of interfacial products, such as CaS and MnS, can lead to a decrease in carbon dissolution rates. Such products can act as a physical barrier blocking iron and coke contact, thus reducing the contact area for carbon dissolution. However, the presence of MnS may act to lower the liquidus temperature of the interfacial product. - An increase in temperature increases the rate of carbon dissolution. This dependence is predominantly due to the composition of the inorganic matter present in cokes, which influences the viscosity and melting point of the interfacial product formed and hence contact area between coke and iron.
| Identifer | oai:union.ndltd.org:ADTP/276252 |
| Date | January 2007 |
| Creators | Cham, S. Tsuey, Materials Science & Engineering, Faculty of Science, UNSW |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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