Change search
Refine search result
1 - 14 of 14
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1. Pourghahramani, Parviz
    Effects of grinding variables on structural changes and energy conversion during mechanical activation using line profile analysis (LPA)2006Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 2. Pourghahramani, Parviz
    Mechanical activation of hematite using different grinding methods with special focus on structural changes and reactivity2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 3. Pourghahramani, Parviz
    et al.
    Altin, Erguen
    Institute of Particle Technology, Friedrich-Alexander University, Cauerstr. 4, D-91058 Erlangen.
    Mallembakam, Madhusudhan Rao
    Institute of Particle Technology, Friedrich-Alexander University, Cauerstr. 4, D-91058 Erlangen.
    Peukert, Wolfgang
    Institute of Particle Technology, Friedrich-Alexander University, Cauerstr. 4, D-91058 Erlangen.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Microstructural characterization of hematite during wet and dry millings using Rietveld and XRD line profile analyses2008In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 186, no 1, p. 9-21Article in journal (Refereed)
  • 4. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Changes in the structure of hematite by extended dry grinding in relation to imposed stress energy2007In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 178, no 1, p. 30-39Article in journal (Refereed)
    Abstract [en]

    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.

  • 5. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Comparative study of microstructural characteristics and stored energy of mechanically activated hematite in different grinding environments2006In: International Journal of Mineral Processing, ISSN 0301-7516, E-ISSN 1879-3525, Vol. 79, no 2, p. 120-139Article in journal (Refereed)
    Abstract [en]

    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

  • 6. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Effects of mechanical activation on the reduction behavior of hematite concentrate2007In: International Journal of Mineral Processing, ISSN 0301-7516, E-ISSN 1879-3525, Vol. 82, no 2, p. 96-105Article in journal (Refereed)
    Abstract [en]

    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.

  • 7. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Mechanical activation of hematite in different grinding mills: plenary lecture2007In: XII Balkan Mineral Processing Congress: (BMPC 2007) : 10 - 14 June 2007, Delphi, Greece / [ed] Georgios N. Anastassakis, Athens: National technical university of Athens (NTUA) , 2007, p. 3-10Conference paper (Refereed)
  • 8. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Microstructure characterization of mechanically activated hematite using XRD line broadening2006In: International Journal of Mineral Processing, ISSN 0301-7516, E-ISSN 1879-3525, Vol. 79, no 2, p. 106-119Article in journal (Refereed)
    Abstract [en]

    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.

  • 9. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Reduction kinetics of mechanically activated hematite concentrate with hydrogen gas using nonisothermal methods2007In: Thermochimica Acta, ISSN 0040-6031, E-ISSN 1872-762X, Vol. 454, no 2, p. 69-77Article in journal (Refereed)
    Abstract [en]

    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.

  • 10. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Review of applied particle shape descriptors and produced particle shapes in grinding environments: Part 1: Particle shape descriptions2005In: Mineral Processing and Extractive Metallurgy Review, ISSN 0882-7508, E-ISSN 1547-7401, Vol. 26, no 2, p. 145-166Article in journal (Refereed)
    Abstract [en]

    The various types of particle shape, morphology, texture, and particle angularity descriptors that are applicable and useful in different grinding methods with respect to various geometric measures existing in image analysis software are presented and reviewed. In addition, some disadvantages of such descriptors are discussed.

  • 11. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Review of applied particle shape descriptors and produced particle shapes in grinding environments: Part II: The influence of conninution on the particle shape2005In: Mineral Processing and Extractive Metallurgy Review, ISSN 0882-7508, E-ISSN 1547-7401, Vol. 26, no 2, p. 167-186Article in journal (Refereed)
    Abstract [en]

    In this article the literature on the shape of particles produced by comminution methods and the grinding condition is reviewed and presented. Some agreement and disagreement among workers for the shapes that are produced by comminution are presented. In addition, the important factors affecting the shape of particles in different comminution methods are also described.

  • 12. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Structural changes and reactivity of hematite subjected to extended milling2008In: Proceedings of XXIV International Mineral Processing Congress: Beijing, China 24-28 September 2008 / [ed] Wang Dian Zuo; Sun Chuan Yao; Wang Fu Liang; Zhang Li Cheng; Han Long, Beijing: American Science Press Inc., 2008, p. 274-282Conference paper (Refereed)
  • 13. Pourghahramani, Parviz
    et al.
    Forssberg, Eric
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    The characterization of structural changes in hematite ground in a confined particle bed using Rietveld analysis2007In: International Journal of Mineral Processing, ISSN 0301-7516, E-ISSN 1879-3525, Vol. 83, no 1-2, p. 47-59Article in journal (Refereed)
    Abstract [en]

    The interparticle breakage of fine feed fraction of hematite concentrate was investigated by stressing two particle beds with a pressure between 255 and 1000 MPa. The experiments were conducted in such a way that the wall friction effects during compression were eliminated. The effects of interparticle breakage in a confined bed on the structural changes of hematite concentrate were studied using a combination analysis of XRD line broadening, BET and particle size measurements. The specific energy comminution was estimated using loading and de-loading hysteresis curves. It was found that energy absorption by the particle bed varies between 6 and 31 J/g depending on the applied pressures and bed heights. The experiments indicated that energy absorption was a major factor for the interparticle breakage of hematite. In addition, it was revealed that an increasing bed height brought about a higher stiffness and hence reduced energy absorption and subsequently declined the surface area, solid content as well as structural changes. The linear energy-force relationship stands well even if the particle-bed heights are changed. The maximum BET surface area was measured about 1.4 m2/g after energy absorption of 31 J/g by the particle bed. Structural changes were followed by XRD line broadening analysis using Rietveld refinement and Warren-Averbach approach. It was found that the intensity and the broadening of XRD diffraction patterns decreased and increased, respectively, by increasing energy absorption with a first approximation. With increasing absorbed energy by the bed up to 15 J/g the degree of amorphization increased sharply and afterwards continued to change slightly. The maximum X-ray amorphization was calculated at maximum energy absorption, accounting for 31%. The volume and surface weighted crystallite sizes reduced to about 108 and 53 nm, respectively, after releasing 31 J/g specific grinding energy. For the same energy, the root mean square strain (RMSS), <εL2 = 10 nm>1/2, and maximum lattice strain, e, increased to 9.4 × 10- 4 and 4.1 × 10- 3 respectively. The comparison of bed grinding with tumbling milling revealed that the grinding in tumbling mill needs much more energy to induce the same structural changes as in bed grinding. The results obtained from the two methods are discussed and compared in details.

  • 14. Pourghahramani, Parviz
    et al.
    Pålsson, Bertil
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Forssberg, Eric
    Multivariate projection and analysis of microstructural characteristics of mechanically activated hematite in different grinding mills2008In: International Journal of Mineral Processing, ISSN 0301-7516, E-ISSN 1879-3525, Vol. 87, no 3-4, p. 73-82Article in journal (Refereed)
    Abstract [en]

    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.

1 - 14 of 14
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf