Pelletization and Sintering of New Zealand Titanomagnetite Ironsand

Direct reduction (DR) of iron ore with hydrogen is a promising alternative ironmaking process with near-zero CO2 emissions. This approach is applicable for the reduction of New Zealand (NZ) titanomagnetite (TTM) ironsand, which offers the potential for establishing a clean steelmaking industry in NZ. However, iron ore fines must first be agglomerated into pellets to provide a porous medium for reducing gases in a DR shaft reactor. Pelletization is currently the most widely used process for agglomeration of fine concentrates. In this process, iron ore particles are balled into green pellets that are strengthened through induration at high temperature. The main challenges with pelletization are the selection of a binder that provides the required pellet strength without causing contamination, and the large energy consumption associated with induration. The optimum pelletizing conditions also differ between iron ore types as they have distinct physical and chemical characteristics. This work investigates into the pelletization and sintering behaviour of NZ TTM ironsand. Ironsand pellets were initially bonded with sodium bentonite clay and carboxymethyl-cellulose (Peridur) to analyse the effects of various ironsand particle size, sintering temperature, and sintering duration. It was found that ironsand with an average particle size of 65 µm was optimal in producing green pellets with high strength. The compressive strength of these pellets after sintering at 1200 °C in air for 2 hr was measured to be 976 N, which is well above the strength requirement for pellet feedstock in a shaft reactor. This strength was attributed to the recrystallisation of titanohematite grains from oxidation of TTM, and formation of a liquid bonding phase by the melted binders in the interparticle regions. Building on the results, alternative binders and additives were then explored to potentially lower the sintering temperature requirement. A combination of both organic binder and inorganic additive was found essential in meeting the target strength at each stage of pelletization. Organic binders such as carboxymethyl-cellulose enhanced the strength of green pellets by increasing the liquid viscosity between the particles which led to greater interparticle forces. Inorganic additives, such as bentonite, calcium borate and ground glass, promoted high sintered strengths by melting and forming additional bonding bridges. Microwave heating was then investigated as a potential low-energy alternative to a conventional induration process. NZ TTM ironsand pellets were sintered in a prototype microwave furnace. It was found that the sintered strength could be increased by incorporating alumina balls with the pellet feed. The microwave transparent alumina balls allowed the uniform distribution of heat amongst the pellets and reduced the thermal load along the length of the cavity. Plasma production occurred due to the high power concentrated in the cavity which also increased the strength of sintered pellets. The high temperature from the plasma allowed the binder to melt into a liquid bonding phase. However, excessive exposure to plasma caused the ironsand particles to form a highly dense structure that affected the reducibility of pellets in hydrogen. In summary, the findings in this thesis provide a general understanding of the pelletizing and subsequent sintering behaviour of NZ TTM ironsand. The production of ironsand pellets that meet the target strength is entirely feasible. This knowledge can be applied in the future where a feedstock of NZ ironsand pellets is required in an industrial hydrogen-based DR process. The induration of pellets in a prototype microwave furnace was also demonstrated, and the results show promise for further development of microwave heating as an alternative approach to pellet induration.