Microstructural characterization of iron ore green pellets

Sammanfattning: The aim of this thesis work was to develop new methodologies to characterize iron ore green pellets, in wet and dry state. The new characterization methods applied and developed in this work were mainly based on scanning electron microscopy (SEM) to gather both qualitative and quantitative data on different components of the pellets, i.e. mineral particles, water, bentonite and entrapped bubbles. In a first attempt to preserve the structure of wet iron ore green pellets by freezing before investigation by cryogenic SEM, wet pellets were frozen in liquid nitrogen by direct plunging or a new method developed in the present work denoted unidirectional freezing. The former method was found useful to study the degree of water filling at the outer surface of the pellet but led to artifacts in the interior of the pellet. The latter method was developed to confirm that the spherical cavities observed in dry pellets were related to entrapped bubbles in wet pellets. Capillaries were observed at the outer surface of the pellets and fine particles were lacking within a layer of approximately 100 µm from the outer surface and also in the direct vicinity of the air bubbles in the interior of the pellets. More advanced freezing methods were subsequently employed to reveal the artifact free microstructure of bentonite in wet pellets. In order to verify the observations made on a slice of a wet pellet frozen by plunging in liquid ethane, SEM investigations were also carried out on a bentonite suspension and a bentonite-iron ore slurry, which could be cryo-fixed by the most reliable freezing method, i.e. high pressure freezing. All microstructures were comparable and consisted in a voluminous network of well-dispersed clay platelets. This network was found to collapse upon drying. Bentonite was drawn to the contact points between the particles and formed what appeared as bridges, which may impart strength to the dry pellets. A combination of energy dispersive spectroscopy (EDS) and imaging by low-loss backscattered electronsat low voltage evidenced the presence of very finely divided silicate species on the magnetite particles. In order to visualize the three dimensional structure of dispersed bentonite clay with unprecedented resolution, a method based on SEM imaging with a monochromatic and decelerated beam was used for the first time. The recorded images showed very well-dispersed clay platelets forming a fine network of Y shaped contacts, which is quite different from earlier reports of much coarser structures formed as a result of poor sample preparation. Finally, in order to gain quantitative data about the porosity due to bubble entrapment in dry pellets, the entire cross-section of dry epoxy embedded and polished pellets were recorded by SEM. The three-dimensional bubble size distribution was unfolded from 2D SEM data using image processing, image analysis and stereological principles. The same type of pellets was also investigated by X-ray micro-tomography (XMT). The resulting three-dimensional dataset allowed the validation of the unfolding procedure based on stereology. However, the lack of resolution obtained by XMT was shown to lead to slight discrepancies with the SEM data for small bubble sizes. Entrapped air bubbles due to the addition of extra flotation reagent in pellets were shown to be responsible for additional porosity observed by mercury intrusion porosimetry (MIP). In summary, useful characterization methods for iron ore pellets based on SEM have been developed in this work, which opens up new possibilities to for instance study agglomeration processes in more detail.