A Study of Parameters that Influence the Kinetics of the AOD Decarburisation Process
Abstract: This thesis focuses on the AOD process, which is an important metallurgical reactor in stainless steel production. To be more specific, the thesis is limited to study the decarburisation step, which is the first of three process steps in the AOD converter. The main research questions is to increase the knowledge of reasons for random differences in decarburisation rates during the process. In the first part of the study, physical modeling is used to study the mixing in AOD converters. Parameters that were studied included, bath heights, gas flow rates and chemical reactions. The results showed that the mixing time decreased with an increased gas flow rate or an increased bath height. In addition, the influence of the top slag on the fluid flow and mixing time was studied. The results showed that the flow field was influenced by the slag phase and that it is of importance to account for the solid slag fractions to simulate the fluid flow and mixing time to resemble AOD converters. However, the results from this first part of the thesis illustrates that mixing is not the rate-limiting step for decarburisation in AOD converters. Instead, the focus was shifted to study if the slag was the cause for random differences in the decarburistaion rate. Slag samples were collected from an industrial AOD reactor. These slags are quite unique since they contain mainly solids and a small liquid fraction. Thus, petrography was used to study the samples and a new methodology was developed to characterize the slag samples. Methods for characterising the top slag samples from the AOD process were established, including combinations of different techniques. The common slag phases in decarburisation slag were identified. The results showed good agreement with samples analysed with SEM and EDS compared to calulations made in Thermo-Calc. Overall, it was shown that the slag characteristics changes during the decarburization period and that these changes can be determined using the new methodology. In the last part of the thesis, the commercial AOD process control model TimeAOD2 was used in combination with Thermo-Calc calculations to study how the process could be improved so that the slag composition became most beneficial for improving the kinetics of the decarburisation part of the AOD converter process. The results show that it is possible to predict the slag composition and especially the amount of liquid slag in the sample. This in turn, makes it possible to better estimate the optimal lime addition depending on the silicon content in steel and the amount of carry-over slag from the electric arc furnace. Furthermore, it is shown that to large lime additions will lead to an increased heating time while not improving the decarburization rate.
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