Design and optimisation of a laboratory scale microwave furnace for heating titanomagnetite ironsand
TTM (Titanomagnetite) ironsand is an abundant source of iron oxide on the western beaches of the Waikato and Auckland regions in New Zealand, with chemical formula TixFe3-xO4 (x=0.27). This ironsand has been used for the last four decades to produce steel in New Zealand, but the reduction process releases large amounts of carbon dioxide. This is because coal is used as the primary reducing agent. Using hydrogen gas instead as a reducing agent, it is possible to reduce ironsand while avoiding the excessive production of carbon dioxide. In addition, standard electrical heating methods are generally limited by low power transfer rates to the steelmaking reactants. Microwave heating is an alternative heating method, shown to be good candidate for ironsand heating via direct power transfer to the NZ ironsand itself due to its excellent microwave absorbing features. This research focuses on modelling the dynamics of microwave power transfer in a resonant microwave cavity, and refines a computational model to improve the modelling capability of such a process.
Microwave heating is known to demonstrate high direct power transfer rates to microwave absorbing materials, such as TTM ironsand. The microwave heating of ironsand as a candidate heating method is shown in this work by the optimisation of the resonant performance of a custom-built laboratory scale microwave cavity to heat ironsand to over 1000 C. The microwave furnace developed in this work was specifically designed to mimic the contemporary steelmaking standards (i.e., a continuous throughput furnace). A microwave furnace was designed with a computational model, then built and tested in a laboratory. Measurements tracking various energy fluxes throughout the furnace refined an initial transient microwave heating simulation of the computational model with a simple experimental procedure, which exploits the significant variation of the real permittivity (εr) of the ironsand at T = 430 C.
From that experimental procedure, a significant variation in microwave absorption was observed as the ironsand passed through its Curie temperature. This effect was reproduced in an initial transient microwave heating simulation. Experimental results were then used to improve simulation accuracy. This encourages further refinement of the computational model as an avenue for future work in this field.
In summary, this research demonstrates the feasibility of designing a microwave furnace for efficiently heating TTM ironsand. It also exhibits the feasibility of simulating microwave heating of TTM ironsand with a computational model. The results from this thesis show promise for further study on the hydrogen reduction of TTM ironsand within a microwave furnace. The findings therein present results which ‘set the scene’ for for larger-scale zero-CO2 production of techno-economically essential materials via microwave heating.