| 1. |
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
- Bhuiyan, Iftekhar Uddin, et al.
(författare)
-
Microstructure of Bentonite in Iron Ore Green Pellets
- 2014
-
Ingår i: Microscopy and Microanalysis. - 1431-9276 .- 1435-8115. ; 20:1, s. 33-41
-
Tidskriftsartikel (refereegranskat)abstract
- Sodium-activated calcium bentonite is used as a binder in iron ore pellets and is known to increase strength of both wet and dry iron ore green pellets. In this article, the microstructure of bentonite in magnetite pellets is revealed for the first time using scanning electron microscopy. The microstructure of bentonite in wet and dry iron ore pellets, as well as in distilled water, was imaged by various imaging techniques (e.g., imaging at low voltage with monochromatic and decelerated beam or low loss backscattered electrons) and cryogenic methods (i.e., high pressure freezing and plunge freezing in liquid ethane). In wet iron ore green pellets, clay tactoids (stacks of parallel primary clay platelets) were very well dispersed and formed a voluminous network occupying the space available between mineral particles. When the pellet was dried, bentonite was drawn to the contact points between the particles and formed solid bridges, which impart strength to the solid compact.
|
|
| 5. |
|
|
| 6. |
- Forsmo, Seija
(författare)
-
Influence of green pellet properties on pelletizing of magnetite iron ore
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetite iron ore green pellets are produced by balling moist concentrates to green pellets, which are then dried, oxidized to hematite, sintered, cooled and transported to steelmaking plants. The existing theory for balling is based on the capillary theory, but its applicability under industrial balling conditions is unclear. The aim of this study has been to clarify the principal mechanisms controlling the properties of iron ore green pellets. Special attention has been paid to studying how variations in raw material fineness influence green pellets behaviour during balling, oxidation and sintering. This knowledge of the principal mechanisms is needed to provide a sound basis for a successful process control strategy. The applied approach was to further develop the laboratory methods used in green pellet characterization. Oxidation in green pellets was measured by thermogravimetry and sintering was followed by dilatometry. A new measuring device for the characterization of green pellet strength was built and a new measuring method for green pellet plasticity was developed. The optimum moisture content in balling was defined as the moisture content resulting in a given degree of plasticity in green pellets. Pellet feeds with steeper particle size distributions required a higher moisture content in balling. Properties of green pellets prepared from different raw materials should be compared at constant plasticity (under realistic balling conditions), not at constant moisture content, as has been done earlier. At constant plasticity and with 0.5% bentonite binder, variations in the fineness of the magnetite concentrate did not influence the green pellet wet strength, within the limits studied in this work. This is because in the presence of the bentonite binder, green pellet wet strength was mainly controlled by the viscous forces of the binder liquid. A marked degradation in green pellet mechanical strength both in wet and dry states was found in the presence of a surface-active flotation collector reagent. This loss in green pellet quality was explained by a strong attachment of air bubbles in the green pellet structure. High-speed camera images showed multi-breakage patterns due to crack propagation between the air bubbles. This explains the increased generation of dust observed at the pellet plant. The negative effects of the flotation collector reagent on balling diminished during storage of the pellet feed. The results emphasize the importance of minimizing the reagent dosages in flotation and maximizing the residence time of the pellet feed in the homogenizing storage before balling. When a pellet starts to oxidize, a shell of hematite is formed while the pellet core is still magnetite. Thermal volume changes in these two phases were studied. Sintering in the magnetite phase started earlier (950°C) compared to the hematite phase (1100°C). Therefore, the difference in sintering rates between the magnetite and hematite phases was largest at around 1100°C. The sintering rate increased in both phases with increasing fineness in the magnetite concentrate. A finer grind in the raw material would, therefore, promote the formation of the unwanted duplex structures with a more heavily sintered core pulling off from the shell. At constant original porosity in green pellets, the oxidation rate decreased as the magnetite concentrate became finer, because of the enhanced sintering. However, in practical balling, finer raw materials would necessitate the use of more water in balling, which results in an increase in green pellet porosity. These two opposite effects levelled out and the oxidation time became constant. Under process conditions, differences in the duplex structure would still be expected. This is because only partial oxidation takes place before sintering in the kiln. Olivine, which is used as an additive in LKAB blast furnace pellets, was found to initiate the dissociation of hematite back to magnetite already at temperatures that can occur during oxidation in the PH zone. The rate of dissociation was largely influenced by the olivine fineness. If the dissociation temperature is exceeded, the resulting decrease in the oxidation rate increases the size of the un-oxidized core exposed to sintering before oxidation. Also, dilatometer measurements showed opposite thermal volume changes in the oxidized hematite shell and in the magnetite core in the presence of olivine. Dissociation caused a large volume increase in the oxidized hematite shell, while the olivine addition further enhanced the sintering of the magnetite core. These mechanisms lead to increased structural stress between the hematite shell and the magnetite core. This knowledge was applied at the LKAB Svappavaara pelletizing plant. Coarser grinding of the olivine additive resulted in a marked improvement in the low- temperature reduction strength (LTD) in pellets. The final conclusion, then, is that excessive grinding of the pelletizing raw materials, both the magnetite concentrate and the additives, can cause severe problems and step-wise changes in the oxidation and sintering mechanisms without resulting in any additional gain in terms of green pellet mechanical strength. The capillary theory failed to describe the properties of wet green pellets under industrial balling conditions. The results also clearly point out that continuous variations in raw material properties would cause complex fluctuations in both balling and induration.
|
|
| 7. |
- Forsmo, Seija, et al.
(författare)
-
Influence of the olivine additive fineness on the oxidation of magnetite pellets
- 2003
-
Ingår i: International Journal of Mineral Processing. - 0301-7516 .- 1879-3525. ; 70:1-4, s. 109-122
-
Tidskriftsartikel (refereegranskat)abstract
- Olivine is used as an additive in Luossavaara-Kiirunavaara AB (LKAB) blast furnace pellets. The LKAB iron ore is magnetite which oxidizes to hematite during the sintering process. Olivine retards the oxidation of magnetite pellets if the threshold temperature of magnesioferrite formation is exceeded. In this study, we have developed a thermogravimetric measuring method to study the relationship between the olivine reactivity in green pellets and the olivine fineness. A less reactive olivine makes the pelletizing process more tolerant of excessively high temperatures in the green pellets during oxidation. The reactivity of olivine decreases when the amount of fine tail in olivine is decreased. The top size is limited by balling. Grinding tests in pilot scale show that if the olivine additive is ground in closed circuit with a ball mill keeping good control over the top size, the olivine fineness can be appreciably decreased without disturbing balling. The olivine reactivity can be decreased by 40% compared to the reference olivine in the production plant. Also, savings can be made on both the grinding energy and grinding media consumption. The results in both pilot scale and full production scale show that better oxidation in the grate due to a coarser olivine additive improves the low temperature reduction strength (LTD, ISO 13930) in pellets. The measuring method developed for the olivine reactivity enabled evaluation of different particle-sizing methods for olivine characterisation. The screening size fraction % - 45 mum showed a good correlation to olivine reactivity in green pellets within a very large measuring range. The correlation of laser diffraction analysis to the olivine reactivity showed, however, that the laser diffraction measuring method is affected too much by variations in the large particles.
|
|
| 8. |
- Forsmo, Seija, et al.
(författare)
-
The determination of porosity in iron ore green pellets by packing in silica sand
- 2005
-
Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 159:2, s. 71-77
-
Tidskriftsartikel (refereegranskat)abstract
- Porosity is a very important property of iron ore green pellets. At LKAB, the mercury porosimeter was replaced some years ago by the GeoPyc instrument, in which the sample volume is measured by packing in silica sand. Some critical features of the GeoPyc measurement are pointed out in this study. The green pellets need to be strengthened by spraying with a fast-drying lacquer before measurement. A system for continuous instrument control is needed to detect any wear on the measuring tools that might lead to erroneous measurement results. The reproducibility measured on steel spheres was +/- 1% (2 sigma), which is equal to the reproducibility given by the instrument manufacturer. The reproducibility measured on lacquer-strengthened green pellets used as in-house reference samples was, however, much better, 0.3% (2 sigma). The calibration error against mercury porosimetry was improved from +/- 0.6% to +/- 0.2% (2 sigma) after introducing the changes specified in the text. The GeoPyc instrument is easy to use and the problematic handling of mercury is avoided.
|
|
| 9. |
- Mouzon, Johanne, et al.
(författare)
-
Cryo-SEM method for the observation of entrapped bubbles and degree of water filling in large wet powder compacts
- 2011
-
Ingår i: Journal of Microscopy. - : Wiley. - 0022-2720 .- 1365-2818. ; 242:2, s. 189-196
-
Tidskriftsartikel (refereegranskat)abstract
- There are generally two problems associated with cryogenic scanning electron microscopy (cryo-SEM) observations of large wet powder compacts. First, because water cannot be vitrified in such samples, formation of artefacts is unavoidable. Second, large frozen samples are difficult to fracture but also to machine into regular pieces which fit in standard holders, especially if made of hard materials like ceramics. In this article, we first describe a simple method for planning hard cryo-samples and a low-cost technique for cryo-fracture and transfer of large specimens. Subsequently, after applying the entire procedure to green pellets of iron ore produced by balling, we compare the influence of plunge- and unidirectional freezing on large entrapped bubbles throughout the samples as well as the degree of water filling at the outer surface of the pellets. By carefully investigating the presence of artefacts in large areas of the samples and by controlling the orientation of the sample during freezing and preparation, we demonstrate that unidirectional freezing enables the observation of large entrapped bubbles with minimum formation of artefacts, whereas plunge freezing is preferable for the characterization of the degree of water filling at the outer surface of wet powder compacts. The minimum formation of artefacts was due to the high packing density of the iron ore particles in the matrix
|
|
| 10. |
- Nellros, Frida, et al.
(författare)
-
Automated measurement of sintering degree in optical microscopy through image analysis of particle joins
- 2015
-
Ingår i: Pattern Recognition. - : Elsevier BV. - 0031-3203 .- 1873-5142. ; 48:11, s. 3451-3465
-
Tidskriftsartikel (refereegranskat)abstract
- In general terms, sintering describes the bonding of particles into a more coherent structure, where joins form between packed particles, usually as a result of heating. Characterization of sintering is an important topic in the fields of metallurgy, steel, iron ore pellets, ceramics, and snow for understanding material properties and material strength. Characterization using image analysis has been applied in a number of these fields but is either semi-automatic, requiring human interaction in the analysis, or based on statistical sampling and stereology to characterize the sample. This paper presents a novel fully automatic image analysis algorithm to analyze and determine the degree of sintering based on analysis of the particle joins and structure. Quantitative image analysis of the sintering degree is demonstrated for samples of iron ore pellets but could be readily applied to other packed particle materials. Microscope images of polished cross-sections of iron ore pellets have been imaged in their entirety and automated analysis of hundreds of images has been performed. Joins between particles have been identified based on morphological image processing and features have been calculated based on the geometric properties and curvature of these joins. The features have been analyzed and determined to hold discriminative power by displaying properties consistent with sintering theory and results from traditional pellet diameter measurements on the heated samples, and a statistical evaluation using the Welch t-test.
|
|
