Mechanical activation of hematite using different grinding methods with special focus on structural changes and reactivity
Abstract (Summary)The mechanical treatment of solids is one of the most common and widely used operations which man has been concerned from the very beginning of the history of civilization. Nowadays, mechanical activation has a wide range of application potentials. Mechanical activation processes are used to modify the properties of materials, to enhance the reactivity of materials and to produce advanced materials and to separate composite materials into its constituents. When materials are subjected to intensive grinding, the structure and microstructure characters of material change widely. These structural changes determine the reactivity of materials and/or minerals and may play an important role in subsequent processes. The objective of this study is to investigate the influence of the grinding techniques on the microstructure and structural changes of natural hematite. The influences of the five grinding methods with various grinding variables have been investigated: (1) three types of loose media mills in dry mode, (2) interparticle comminution in a confined piston-die press and (3) a stirred media mill in wet mode. A variety of microstructural characterization methods based on X-ray diffraction line profile analysis such as Warren-Averbach, Williamson-Hall and Rietveld methods associated with other characterizations methods have been employed in the present study. In addition, the effects of mechanical activation on the thermal reactivity of hematite concentrate have been studied using hydrogen reduction of activated samples. The results reveal that mechanical activation of hematite causes great changes in geometrical and microstructural characteristics with increased grinding intensity, whatever milling methods were applied. In the case of dry grinding with loose media mills, the results show that the particles show a tendency to form agglomerates during prolonged milling. The expansions of hematite lattice and volume cell were identified. The Williamson-Hall method provides itself to be a technique for a rapid overview of the X-ray line broadening effects and facilitates the understanding of the influence of grinding processes on the material structures. The anisotropic character of line broadening for deformed hematite as a function of grinding variables was revealed. From the Warren- Averbach method, it has been found that the planetary mill products yield the smallest crystallites and the maximum root mean square strain (RMSS) with one exception. The products of the vibratory mill yield approximately lower X-ray amorphization degree with regard to the grinding time and media surface variables. The approximation of the energy contribution to the long- lived defects demonstrated that the amorphization character is the most important energy carrier in the activated hematite, accounting for more than 93% of overall stored energy in hematite. The comparison of the loose media mills based on stress energy revealed that the vibratory mill brings about less distortion in the hematite than other mills for the same level of stress energy. In addition, the variance analysis revealed that the media surface and grinding time significantly influence the five main response variables at 95% confidence level. Multivariate techniques are successfully applied for projection of microstructure characters to identify the salient features underlying the data. Principal component analysis (PCA) makes it possible to predict easily which condition leads to production of similar properties or microstructure characters and opens a new window for prediction of microstructure characteristics based on changes in the grinding variables for further investigations. Partial least square discrimination analysis (PLS-DA) analysis suggested that mills could be differentiated from each other. From the interparticle comminution investigations, it has been found that the energy absorption is the dominating factor for the size reduction, surface area and induced structural changes in the particle bed comminution. It was also found that the interparticle breakage causes plastic deformation in the material and subsequently induces changes in the structure of the ground hematite and thus provides evidences for the activation potentiality of this method. The comparison with loose media mill (tumbling) in terms of net grinding energy indicated that the interparticle breakage has high energy transfer efficiency to the particles being ground and subsequently favor in the structural changes for a given energy. The comparison of the dry tumbling milling with wet stirred media milling showed that the stirred media mill is more effective in producing structural changes compared to the dry operation; although the X-ray amorphous phase content remained unaffected by the grinding environments, but a large difference was observed in the production of BET surface area. The milling process has been shown to have a pronounced influence on the reduction behavior and kinetic scheme of hematite especially at lower temperature or conversion degrees. Mechanical activation of hematite concentrate lead to the initiation of reduction at lower temperatures. The starting temperature of the reduction was decreased to from 420 about 330?C depending on grinding intensity. Moreover, the pretreatment resulted in improved resolution of overlapping reduction events and the activation energy of the first step of reduction decreased with increasing grinding time. The study showed that the activation energy of the two steps of the reduction depends greatly on the extent of conversion implicating that the reduction processes of hematite to magnetite and magnetite to iron features multi-step characteristics. To investigate the influence of other milling variables in detail, more investigations are recommended, especially as the experiment design and progress in the knowledge to-day provide possibilities to use advanced methods for characterization and analysis. In our opinion, the investigation of the effects of various defects formed during mechanical activation on the reactivity of the minerals are currently only at the beginning of their developments. Systematic investigations are recommended to explore what defects are formed in the crystal of the substances under various types of mechanical action and how these defects influence the reactivity.
School:Luleå tekniska universitet
Source Type:Doctoral Dissertation
Date of Publication:01/01/2007