This project was the first step in developing a control system for the open slag bath furnace at Highveld Steel, in order to maximize production levels and product quality. In this dissertation, available knowledge from literature and practical experience is included. This is then processed to propose a realistic control system with the current resources available at Highveld Steel. Through research and practical experience it is shown that these furnaces are extremely sensitive to raw material quality and consistency of decision making. This is a result of the open bath configuration that necessitates the careful control of the excess energy, and the raw material composition supplied to the furnace. Through this project it was shown that both the productivity and the product quality (vanadium recovery) can be increased by simply controlling the feed-topower ratio and correct use of correction material. This has the effect of stabilizing the process energy balance which results in smaller slag temperature variations and therefore also less product quality variability. Through controlling the feed-power-ratio, energy consumption was also shown to be reduced, since excess power is minimized without negatively impacting on productivity. In this project a proposal is made to control the flow of material between the two stages in the process, which is based on either being kiln or furnace constrained. A better tapping scheduling is also proposed, based on both material accumulations inside the furnace and the charging condition of the furnace. This results in having sufficient pressure to tap iron and slag at good rates and should also yield consistent tapping weights and add to general process stability. However, engineering availability strongly impacts on both process control and furnace productivity. Consistent availability of the furnace and its supporting systems is hence essential for optimal operation. / Dissertation (MEng)--University of Pretoria, 2009. / Materials Science and Metallurgical Engineering / unrestricted

This study focused on the dynamic behaviour of the freeze lining and slag bath, and the interactions between these components in an ilmenite-smelting furnace process. The purpose of the work was to gain a better understanding of these issues and to ultimately contribute to an improved understanding of the ilmenite-smelting process in its entirety, and to future improvements in the design, operation and control of these processes. A mathematical model of the freeze lining and furnace sidewall was developed. This model was used in isolation for focused characterisation of the dynamic behaviour and interactions of the freeze lining and slag bath. The influences of net power input and slag composition were studied and various aspects of the freeze lining and slag bath were considered. These aspects included freeze lining thickness, temperature distribution through the freeze lining and furnace sidewall, composition distribution through the freeze lining, slag bath temperature and slag bath composition. The thermal response of thermocouples installed in the furnace sidewall to changing conditions on the inside of the furnace was also investigated. A mathematical model of the crust that forms on the slag bath surface was developed. This model was not used in isolation, and was only incorporated into a complete model of the process. A mathematical model of the entire ilmenite-smelting furnace process was constructed. This model incorporated the two models mentioned above and was able to describe the metal bath, slag bath, furnace atmosphere, freeze lining, furnace sidewall and the crust that is sometimes present on top of the slag bath. The model was used to study the influence of changes in operational parameters on the slag bath and freeze lining. The operational parameters that were studied included electrical power and reductant feed rate, both relative to ilmenite feed rate. The influence of severe operational errors and furnace down time were also investigated. Operational errors included loss of all feed while maintain electrical power input, and loss of reductant feed while maintaining power input and ilmenite feed. The above-mentioned studies were conducted by executing numerous experiments with two of the mathematical models. The experimental results were processed into sets of graphs displaying variations in the aspects that were considered. Many valuable insights resulted from the interpretation of these results. One specific aspect that formed part of the scope of this work was the origin of the compositional invariance of the slag close to the stoichiometric M3O5composition. This invariance was studied and a mechanism was proposed that explains the observed behaviour. The proposed mechanism created some questions about other mechanisms in the process. These mechanisms were also considered and elaborated on. The models and results produced in this study provide valuable insights into the behaviour of the ilmenite-smelting process. It also represents a useful foundation for future modelling work, and finally, it presents numerous opportunities for organisations operating ilmenite-smelting furnaces to improve their understanding and even the performance of their processes. / Thesis (PhD (Metallurgical Engineering))--University of Pretoria, 2004. / Materials Science and Metallurgical Engineering / unrestricted