Comparative study of microstructural characteristics and stored energy of mechanically activated hematite in different grinding environments

Hematite concentrate was mechanically treated using different milling machines and experimental conditions in air atmosphere. The changes in phase constitution, particles size, specific surface area, lattice parameters and X-ray amorphous phase fraction of activated hematite were determined. It was found that the agglomeration of the particles take place during extended milling with accessible pores for Nitrogen gas. The higher media surface brought about the largest specific surface area whatever milling devices used. After 9 h of grinding with higher media surface, the maximum and minimum specific surface area resulted from the grinding in the tumbling and vibratory mills, accounting for 6.83 m(2)/g and 18.42 m(2)/g, respectively. For the same grinding condition, tumbling mill produced the lowest X-ray amorphous phase. The maximum X-ray amorphous material estimated around 85% from the grinding in the planetary mill with higher media surface for 9 h of milling. Structural changes were followed by XRD line broadening analysis (LPA) using the integral breadth method and Warren-Averbach approach. From the Williamson-Hall plots, it was understood that strain and size contributions exist simultaneously in the milled samples. Besides, the physical broadening increases as milling time and media surface increase regardless of milling types. Besides, it was found that hematite crystal is 'soft' between (024) and other crystallographic directions. From the Warren-Averbach approach, it was observed that the higher grinding media surface and prolonged milling favor the generation of small crystallite, higher microstrain, limited crystallite length and subsequently uniform activation of hematite. After 9 h of milling with higher media surface in tumbling, vibratory and planetary mills, the surface weighted crystallite size reached 17.3, 12.2 and 5.6 nnn respectively. The maximum lattice strain, <epsilon(2)(L=10) (nm)>(1/2), in the grinding with tumbling, vibratory and planetary mills was found about 4.44 x 10(-3), 3.95 x 10(-3) and 5.23 x 10(-3), respectively. The maximum dislocation density accounted for 46.3 x 10(14) m/m(3) in the planetary milling with higher media surface after 9 h of milling. The evaluation of energy contributions of structural defects suggested that the energy contribution of the amorphization was dominant and amounted to 92-98% of the overall stored energy in hematite, depending on milling conditions. Finally, for a given stress energy, the products of tumbling mill represent higher reactivity potential. (c) 2006 Elsevier B.V. All rights reserved.