| 1. |
- Pourghahramani, Parviz, et al.
(författare)
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Changes in the structure of hematite by extended dry grinding in relation to imposed stress energy
- 2007
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Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 178:1, s. 30-39
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Tidskriftsartikel (refereegranskat)abstract
- The effect of extended dry milling in different mills on the structural changes of hematite concentrate has been investigated using a combination analysis of XRD line broadening, BET and particle size measurements. Structural changes were followed by XRD line broadening analysis using integral breadth method and Warren–Averbach approach. For analysis, the stress energy was estimated by considering different grinding variables in different mills and changes in the structure discussed in terms of stress energy. Within comparable range of stress energy, lower BET surface area was produced by grinding in the vibratory mill. The maximum surface area increased to 18,400 m2/kg in the vibratory mill after releasing 51,300 kJ/kg energy. The conversion of the 80% of initial hematite to amorphous phase during extended dry grinding by tumbling, planetary and vibratory mills, needs 4000, 8500 and 50,000 kJ/kg energy respectively. It was understood that vibratory mill introduces the minimum lattice strain and gives the largest crystallites when applying the same level of stress energy. The smallest crystallites with grinding in tumbling, vibratory and planetary mills were obtained about 17.3, 13.5 and 5.6 nm after releasing 5230, 51,300 and 15,600 kJ/kg respectively. For these levels of stress energy, in turn, the microstrain <εL=10 nm2>1/2 exceeds 4.4 × 10− 3, 3.9 × 10− 3 and 5.3 × 10− 3. It was further revealed that higher concentrations of defects (Amorphization and excess energy) per unit surface area were induced by grinding in the planetary and tumbling mills. A theoretical calculation of the energy contribution to the long-lived defects indicated that products from tumbling and planetary mills have higher excess energy compared to the products from vibratory mill for the same stress energy. The maximum theoretical excess energy was estimated about 75.4, 80.0 and 81.3 kJ per mole of the ground hematite with tumbling, vibratory and planetary mills after releasing 5230, 51,300 and 15,600 kJ/kg of stress energy respectively. Grinding in vibratory mill needs much more energy to reach the same effect as the other used mills. A comparison of specific energy input and stress energy among the used mills points out that for generation of the same levels of stress energy, the planetary mill consumes more energy than the other used mills.
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| 2. |
- Pourghahramani, Parviz, et al.
(författare)
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Comparative study of microstructural characteristics and stored energy of mechanically activated hematite in different grinding environments
- 2006
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Ingår i: International Journal of Mineral Processing. - : Elsevier BV. - 0301-7516 .- 1879-3525. ; 79:2, s. 120-139
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Tidskriftsartikel (refereegranskat)abstract
- 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, (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
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| 3. |
- Pourghahramani, Parviz, et al.
(författare)
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Effects of mechanical activation on the reduction behavior of hematite concentrate
- 2007
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Ingår i: International Journal of Mineral Processing. - : Elsevier BV. - 0301-7516 .- 1879-3525. ; 82:2, s. 96-105
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Tidskriftsartikel (refereegranskat)abstract
- The effect of mechanical activation on the reduction behavior of a hematite concentrate has been examined using a combination of simultaneous thermal analysis (STA), X-ray diffraction (XRD), scanning electron microscope (SEM) analysis, and laser diffraction. The samples were activated in vibratory and planetary mills. Differential thermal analysis (DTA) and thermogravimetric (TG) analysis revealed that reduction of mechanically activated and initial hematite proceeds stepwise (Fe2O3 → Fe3O4 → Fe). The hydrogen reduction of mechanically activated samples initiates at low temperatures compared with the initial sample. The beginning temperature (onset) of the reduction decreases from 421 °C in the initial sample to 330 °C in the mechanically activated sample, depending on the grinding intensity. Further, the reduction of hematite to magnetite in the activated samples is more pronounced due to mechanical activation. At low temperatures, the activated samples give a higher degree of conversion than the initial samples regardless of which milling device is used. Hematite reduces completely to iron metal. A comparison of mill-type effect based on stress energy (specific grinding work) suggests that the mill-type effect is confined by a stress energy of 4300 kJ/kg. After releasing 4300 kJ/kg energy, mechanical activation by the planetary mill brings about a larger decrease in onset temperature and a slightly higher degree of conversion at lower temperatures than does activation by the vibratory mill for a given stress energy. A direct relationship between the reaction characters at lower temperatures and structure sensitivity character (S/X) and stored energy can be identified after releasing 4300 kJ/kg energy. However, partial sintering of material at higher temperatures during the reduction of the mechanically activated hematite became active, and the effects of disordering of the hematite structure vanished and subsequently the reduction reaction was retarded.
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| 4. |
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| 5. |
- Pourghahramani, Parviz, et al.
(författare)
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Microstructural characterization of hematite during wet and dry millings using Rietveld and XRD line profile analyses
- 2008
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Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 186:1, s. 9-21
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Tidskriftsartikel (refereegranskat)abstract
- The effects of extended milling in a stirred media mill and a tumbling mill on the structural changes in hematite have been examined using a combination of particle size analysis, BET surface areas, X-ray diffraction (XRD), thermal (TG and DSC) and FTIR measurements. Rietveld's whole profile fitting based on crystal structure refinement and the Warren-Averbach's method of X-ray line profile analysis were applied to monitor the microstructural evolution of the hematite phase. It is found that the BET surface area, X-ray amorphization phase content and XRD line breadths increase over the specific energy input. The use of stirred media mill exhibits larger surface areas, smaller particle sizes, more XRD line broadening and subsequently greater structural distortions compared to the tumbling mill for a given energy input; although the X-ray amorphous phase content remains unaffected by the grinding environments. The maximum X-ray amorphization degree of about 80 and 95% were calculated at a specific energy of 22,400 and 82,000 kJ/kg in the tumbling and stirred media mills respectively. The maximum specific BET surface area in the stirred media and tumbling milling increases to about 72.5 and 6.8 m2/g after 82,000 and 22,400 kJ/kg energy consumption respectively. As a result of structural refinement during milling, the surface-weighted crystallite size in ground hematite are 17 and 4 nm after consuming 22,400 and 82,000 kJ/kg in the tumbling and stirred media mills respectively, corresponding to the volume-weighted crystallite size of 17 and 11 nm. For the same energy consumption in the mills, in turn, the root mean square strain, 〈 εL = 10 nm2 〉1 / 2, increases to about 4.4 × 10- 3 and 4.9 × 10- 3. The results of the two applied methods are compared and discussed in details. In addition, thermogravimetric analysis of the wet ground samples reveals that the TG curves represent two weight loss steps. A weight loss is observed around 100 °C in the sample which is attributed to the removal of adsorbed water due to the wet milling operations. A strong weight loss step starting from 100 to 400 °C is attributed to surface/bulk dehydroxilation of iron hydroxides. The weight loss increases with extending of the milling. In contrast, the dry milled samples yield negligible weight losses comparing with the wet ground samples.
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| 6. |
- Pourghahramani, Parviz, et al.
(författare)
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Microstructure characterization of mechanically activated hematite using XRD line broadening
- 2006
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Ingår i: International Journal of Mineral Processing. - : Elsevier BV. - 0301-7516 .- 1879-3525. ; 79:2, s. 106-119
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Tidskriftsartikel (refereegranskat)abstract
- The effect of dry milling in a vibratory mill on the structural changes and microstructural characteristics of hematite using different methods was investigated. We have described the line profile analysis (LPA) to extract the size of coherently diffracting domains and the lattice strain of activated hematite in a vibratory mill. The Warren-Averbach and Williamson-Hall methods were used as the main tools for characterization. The changes in the particle size, surface area and new phase formation of hematite concentrate were also investigated. It was concluded that the breakage and agglomeration of particles take place mainly at lower and higher levels of specific energy input, respectively. The pores in agglomerates remain accessible for the nitrogen gas. Milling of hematite increased specific surface area up to 18.4 m(2)/g. The hematite milled under various levels of specific energy input did not undergo a significant reaction or phase transformation during milling. 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 specific energy input was revealed. Higher level of specific energy input favors the generation of small crystallite size, higher microstrain, BET surface area, amorphization and line breadth. The Warren-Averbach method suggested that the nanocrystalline hematite with grain sizes of 73.5-12.2 nm was formed by mechanical treatment using different milling intensities in the vibratory mill. The root mean square strain (RMSS) at L = 10 nm varies between 1.7 x 10(-3) and 4.0 x 10(-3) depending on the level of energy input. Limits in the applicability of Williamson-Hall method and reliability of the results are discussed in detail.
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| 7. |
- Pourghahramani, Parviz, et al.
(författare)
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Multivariate projection and analysis of microstructural characteristics of mechanically activated hematite in different grinding mills
- 2008
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Ingår i: International Journal of Mineral Processing. - : Elsevier BV. - 0301-7516 .- 1879-3525. ; 87:3-4, s. 73-82
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Tidskriftsartikel (refereegranskat)abstract
- A statistical analysis was done to investigate the relationship between grinding variables and structural changes during mechanical activation of hematite concentrate. Experiments were carried out according to a statistical design by varying the grinding time, media surface and mill type. Several multivariate techniques are applied to interpret the grinding processes.The variance analysis revealed that the media surface and grinding time significantly influence the five main response variables at 95% confidence level. The use of multivariate analysis allows the projection of high-dimensional data to a low subdimensional subspace. An overview of principal component analysis (PCA) on 27 variables yielded a three component model explaining 89% and predicting 76% of the total variance. It was found that the observations belonging to low and high levels of media surfaces fall into two groups. Most of the microstructural characteristics such as microstrain, dislocation and amorphization and granulometric surface area, BET specific surface area, specific energy input, stored energy, portion of smaller particles and stress energy coincide with high level of media surface group. The variables crystallite size, peak intensity and mean particle size appear with lower media surface. The PLS-DA (partial least squares discrimination analysis) made it possible to discriminate the three types of mills. From the projection of dummy variables, it was concluded that the vibratory mill caused comparatively less structural changes in hematite than the other mills in spite of releasing higher stress energy. The planetary mill introduced relatively higher dislocation defects and generated higher lattice strain. The hematite ground in the tumbling and planetary mills had comparatively higher X-ray amorphization degree and subsequently higher excess energy than the hematite ground in the vibratory mill. The tumbling mill produced relatively lower specific surface than the others. It was concluded that the products of the tumbling mill represented higher defect concentration (amorphization) per unit surface area despite releasing lower stress energy level. From the PLS modeling of the five main response variables, it was found that the X-variables specific energy input and stress energy are the most influencing factors.
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| 8. |
- Pourghahramani, Parviz, et al.
(författare)
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Reduction kinetics of mechanically activated hematite concentrate with hydrogen gas using nonisothermal methods
- 2007
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Ingår i: Thermochimica Acta. - : Elsevier BV. - 0040-6031 .- 1872-762X. ; 454:2, s. 69-77
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Tidskriftsartikel (refereegranskat)abstract
- The reduction kinetics of both non-activated and mechanically activated hematite concentrate in a vibratory mill for different grinding periods have been studied using themogravimetry (TG). Changes in the structure of hematite were studied using X-ray diffraction analysis. The isoconversional method of Kissinger-Akahira-Sunose (KAS) was used to determine the activation energy of the different reactions. The Vyazovkin model-free kinetic method was also used for prediction of kinetic behavior of the samples for a given temperature. Fe2O3 was found to reduce to Fe in a two-step via Fe3O4. Intensive grinding resulted in improved resolution of overlapping reduction events. It was also established that the mechanical activation had a positive effect on the first step of reduction. With increasing the grinding time, the activation energy at lower extent of conversion (alpha <= 0.11) decreased from 166 to 106 kJ mol(-1) range in the initial sample to about 102-70 kJ mol(-1) in the sample ground for 9 h. The complexity of the reduction of hematite to magnetite and magnetite to iron was illustrated by the dependence of E on the extent of conversion, alpha(0.02 <= alpha <= 0.95). The values of E decreased sharply with alpha for 0.02 <= alpha <= 0.11 range in the initial sample and mechanically activated samples, followed by a slight decrease in the values of E during further reduction by alpha <= 0.85 in the ground samples up to 3 h. A slight increasing dependence of E on alpha for mechanically activated sample within 9 h in the second step of reduction was observed due to the finely agglomerated particles during intensive milling and subsequently the formation of a dense layer during the reduction processes. In addition, the dependence of ln A(alpha) on alpha was detected and it was found that the ln A(alpha) shows the same dependence on alpha as the apparent activation energies.
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| 10. |
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