Effects of grinding variables on structural changes and energy conversion during mechanical activation using line profile analysis (LPA)

Abstract: The mechanical treatment of solids is one of the most common and widely used operations with which man has been concerned from the very beginnings of history of civilization. At the present, mechanical activation has a wide range of application potential. Mechanical activation processes are used to modify the properties of materials, enhance the reactivity of materials and produce advanced materials. 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 a proper subsequent process. The use of X-ray diffraction line broadening measurements has been proved to be useful in the characterization of microstructure and structural characteristics. The objective of this study is to investigate the influence of the milling operation variables on the microstructure and structural changes of natural hematite. The influence of the three variables, mill type, grinding time and media surface, through an experimental design was investigated using different methods of characterization by XRD line profile analysis (LPA). The results revealed that mechanical activation of hematite brings about great changes in geometrical and microstructural characteristics with increased the grinding intensity, whatever milling methods are applied. The measurements of the BET surface area, granulometric surface area and particle size show a tendency of the particles to form agglomerates during prolonged milling; in particular with grinding under higher media surface. The agglomeration stage of particles appears to be related to the milling operation conditions. The results indicated that the pores of the agglomerates remain accessible for Nitrogen gas, which addresses the formation of relatively weaker (soft) agglomerates. With a first approximation, the vibratory mill yielded the maximum BET specific surface area, accounting for 18.4 m2/g after 9 hours of milling with higher media surface. The expansions of hematite lattice and volume cell, especially in the initial stages of milling, were identified. The Williamson-Hall method confirms its merit for a rapid overview of the line broadening effects and possible understanding of the main causes. The anisotropic character of line broadening for deformed hematite as a function of grinding variables was revealed. From the Williamson-Hall plots, it was understood that strain and size contributions exist simultaneously in the milled samples. It was found that the hematite crystal is ‘soft' between (024) and other reflections. As seen by the Warren-Averbach method, the planetary mill products yield the smallest crystallites and the maximum root mean square strain (RMSS) (with the exception of the ground sample within one hour and low media surface). The final products contain crystallites sizes between 73.5 and 5.6 nm and its lattice strain (RMSS) at L=10 nm varies from 0.06 up to 5.32 , depending on the milling performance. With a first approximation, the products of the vibratory mill yielded 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. For a given stress energy, the activated hematite in the tumbling mill contains the largest excess energy and has in vibratory mill the smallest amount of excess energy. Generally, the vibratory mill brings about less distortion in the hematite than other mills for the same level of stress energy. However, to produce an identical stress energy in different mills, the planetary mill is needed a specific energy input much higher than the other mills. To investigate the influence of other milling variables in detail, more investigations are recommended, especially as the experiment design and progress in the knowledge nowadays provide possibilities to use advanced methods for the characterization. In our opinion, the investigation of the effect of various defects formed during mechanical activation on the reactivity of the minerals are now only at the beginning of their development. Systematic investigations are recommended to explore what defects are formed under various types of mechanical action in the crystal of the substances of different types and how these defects influence reactivity.