Investigations on the Physico-Chemical Phenomena during Induration of a Magnetite Pellet

Sammanfattning: In the era of globalization with increasing environmental awareness, sustainable utilization of the available resources is a necessity; iron ores are no exception. Pelletization, being one of the increasingly practiced agglomeration techniques for the iron ore fines as well as other solid wastes from steel plants. The pellets produced are further fed into the metallurgical furnaces for subsequent processing. Induration is the vital cog in the process of pelletization, especially for magnetite ore fines. Induration of magnetite pellet is a complex physico-chemical phenomenon involving oxidation, sintering and the associated heat transfer.Rates of these processes not only depend on the thermal and gaseous environment the pellet is exposed to in the induration reactor but are also interdependent on each other. A doctorate project was undertaken to systematically understand these processes in isolation to the extent possible and seek their physics by quantifying them with the overall objective of creating a single pellet model. Isothermal experiments on single pellet scale were designed to understand the sintering and oxidation behavior of magnetite pellets independently.Sintering behavior of oxidized magnetite and non-oxidized magnetite pellets have been evaluated by continuously capturing their in-situ shrinkage using an Optical Dilatometer. The pellets were exposed to different thermal profiles in the defined range of temperature. The kinetics of sintering phenomenon was estimated with the help of power law and Arrhenius equations. The values of activation energy and time exponent derived suggests that sintering of oxidized magnetite (hematite) is dominated by a single diffusion mechanism, whereas sintering of magnetite showed two distinct mechanisms; one operating at lower temperatures and the other at higher temperatures. Further, in order to predict the sintering state of pellets during induration in plant scale operations, the isothermal sintering kinetic equation is also extended to predict the non-isothermal sintering. Thereafter, the predicted profiles were validated with the laboratory experiments, and found to be in fair agreement. Subsequently, these macroscopic sintering behaviors is correlated to quantitative microstructural characterization. This was done by quantifying the mosaic optical microstructures of pellet using the principle of distance transform.Oxidation behavior of magnetite was studied by investigating the kinetics at both at particle and pellet scales. Isothermal experiments were designed with Thermo Gravimetric Analysis (TGA) at sufficiently low enough temperatures so that sintering effects are minimized. The experimental results were analyzed by Shrinking Core Model (SCM) and Avrami Kinetic Model (AKM). It was found from the fit that oxidation at particle scale suitably follows Avrami mechanism, which infers that the rate of oxidation is primarily determined by the rate of nucleation initially followed by the rate of growth. The activation energy of 226 kJ/mol suggests solid-state diffusion mechanism. These findings were corroborated by the microstructural evaluation of particles, where, the hematite crystals were seen growing in some preferred directions.Further, the Pellet Oxidation Model is developed on the principles of grain model for gas-solid reactions by incorporating the derived particle oxidation kinetics with gaseous diffusion. Interestingly, it was found from the experiments as well as from the model that there exist two peaks in oxidation rate curves for magnetite pellet oxidation. The intensity of the peaks increases with the temperature and shifts towards lower times. After investigating different cases, it was found that these peaks were attributed to the initial thermal transient as the pellet is lowered from room temperature into the isothermal zone of the reactor as well as initial high rates of oxidation at the particle scale. The earlier rise of peaks in the rate of oxidation curves for the pellets determined experimentally as compared to the model output could be because of the presence of large fraction of fine particles (size distribution) in the pellet instead of mono-sized particles. These findings were substantiated by microstructural investigations at pellet scale and particle scale. Thereafter, pellet oxidation model is used to predict the oxidation behavior of the pellets treated at higher oxidation temperature or enriching the oxygen content in the oxidizing gas. It is further intended to integrate the oxidation models and sintering models along with the associated heat transfer to develop the comprehensive Single Pellet Induration Model (SPIM). SPIM can be used as a tool to simulate the induration behavior of magnetite pellet at any stage during processing. In future, SPIM can be incorporated into the reactor scale models improving their efficiency, considering the raw material variability.