| 1. |
- Jonsson, Carrie, et al.
(author)
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Comparison of particle and deposit formation between a full-scale grate-kiln plant (40 MW) and a pilot-scale pulverised coal-fired furnace (400 kW)
- 2013
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Conference paper (peer-reviewed)abstract
- The iron ore pelletizing industry utilizes the grate-kilnprocess to dry and sinter the pellets into finished product.The grate-kiln process has a known deposit formation issuethat needs to be further understood. Combustion ofpulverised coal in the rotary kiln generates fly ash particles;in addition to that, particles generated from disintegratediron ore pellets are also entrained in the process gas stream.The combined effect of both sources of particles cantherefore contribute to the deposit formation in the process.In this work, particle- and deposit formation were studiedboth from a full-scale grate-kiln plant (40 MW) and from apilot-scale pulverised coal fired furnace (400 kW). Particleswere collected with a water-cooled probe with nitrogen gasas dilution medium at the tip of the probe. The particleswere separated simultaneously with a pre-cyclone and a 13stages low-pressure impactor during samplings. Depositswere collected with a refractory plate which was attachedat the tip of a water-cooled probe, exposed to the hightemperature (>1100 °C) process gas stream. Particles anddeposits were characterized with an environmentalscanning electron microscope and a scanning electronmicroscope that equipped with energy dispersivespectroscopy detector. A comparison of particle and depositcharacteristics between the grate-kiln plant and the pilotscale pulverised coal fired furnace is presented in this paper,with focus on the potential influence of disintegrated ironore pellets on the particle- and deposit formation process.
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| 2. |
- Jonsson, Carrie, et al.
(author)
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Deposit formation in a grate-kiln plant for iron-ore pellet production. : Part 1: Characterization of process gas particles
- 2013
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In: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 27:10, s. 6159-6170
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Journal article (peer-reviewed)abstract
- Slag formation in the grate-kiln process is a major problem for iron-ore pellet producers. It is therefore important to understand the slag formation mechanism in the grate-kiln production plant. This study initiated the investigation by in situ sampling and identifying particles in the flue gas from a full-scale 40 MW grate-kiln production plant for iron-ore pelletizing. Particles were sampled from two cases of combustion with pulverized coal and heavy fuel oil. The sampling location was at the transfer chute that was situated between the traveling grate and the rotary kiln. The particle-sampling system was set up with a water-cooled particle probe equipped with nitrogen gas dilution, cyclone, and low-pressure impactor. Sub-micrometer and fine particles were size-segregated in the impactor, while coarse particles (>6 μm) were separated with a cyclone before the impactor. Characterization of these particles was carried out with environmental scanning electron microscopy (ESEM), and the morphology of sub-micrometer particles was studied with transmission electron microscopy (TEM). The results showed that particles in the flue gas consisted principally of fragments from iron-ore pellets and secondarily of ashes from pulverized coal and heavy fuel oil combustions. Three categories of particle modes were identified: (1) sub-micrometer mode, (2) first fragmentation mode, and (3) second fragmentation mode. The sub-micrometer mode consisted of vaporized and condensed species; relatively high concentrations of Na and K were observed for both combustion cases, with higher concentrations of Cl and S from heavy fuel oil combustion but higher concentrations of Si and Fe and minor P, Ca, and Al from coal combustion. The first fragmentation mode consisted of both iron-ore pellet fines and fly ash particles; a significant increment of Fe (>65 wt %) was observed, with higher concentrations of Ca and Si during heavy fuel oil combustion but higher concentrations of Si and Al during coal combustion. The second fragmentation mode consisted almost entirely of coarse iron-ore pellet fines, predominantly of Fe (∼90 wt %). The particles in the flue gas were dominantly iron-ore fines because the second fragmentation mode contributed >96 wt % of the total mass of collected particles.
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| 3. |
- Ramanenka, Dmitrij, et al.
(author)
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FEM investigation of global mechanisms affecting brick lining stability in a rotary kiln in cold state
- 2016
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In: Engineering Failure Analysis. - : Elsevier BV. - 1350-6307 .- 1873-1961. ; 59, s. 554-569
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Journal article (peer-reviewed)abstract
- Severe degradation of refractory lining in a rotary kiln often leads to very costly production delays. Use of finite element analysis for understanding the mechanisms behind the failure of the lining is poorly reported in this field. To increase the knowledge and to update the field a simplified model of a kiln and a new methodology for studying stability of the lining are suggested. Behaviour of the lining in cold state – in static and dynamic cases – is studied. Influence of ovality, brick's Young's modulus and friction coefficient on stress and brick displacement are evaluated at two rotational speeds. It was found that the induced loads in the lining are harmless regardless of the tested conditions — challenging the traditional beliefs. On the other hand, recorded brick displacements were found to be significantly affected by rotational speed and ovality. Gaps as large as 80 mm could be observed between the bricks and the casing in a worst case scenario. An integrity coefficient was defined in order to quantify overall integrity of the lining.
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| 4. |
- Ramanenka, Dmitrij, et al.
(author)
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Modelling of refractory brick furniture in rotary-kiln using finite element approach
- 2014
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In: 11th World Congress on Computational Mechanics (WCCM XI) 5th European Conference on Computational Mechanics (ECCM V) 6th European Conference on Computational Fluid Dynamics (ECFD VI). - Barcelona : International Center for Numerical Methods in Engineering (CIMNE). - 9788494284472 ; , s. 1199-1210
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Conference paper (peer-reviewed)
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| 5. |
- Stjernberg, Jesper
(author)
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Degradation mechanisms in refractory lining materials of rotary kilns for iron ore pellet production
- 2012
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Doctoral thesis (other academic/artistic)abstract
- Iron is one of the most important resources in the lithosphere; 90% of all metal ores extracted are based on iron. Many steps are included in the extraction of iron from ore, with extraction processes varying among different producers. Iron ore pellets are a prepared burden material for ironmaking in the blast furnace or by direct reduction. Such pellets can be sintered in a grate-kiln furnace system, in which the kiln is usually lined with mullite-containing bricks. These bricks degrade by various mechanisms, and need to be replaced regularly, which causes expensive production stops. Materials deposit in chunks on the lining in the kiln. These deposits comprise ~95% hematite with oxides of alkali metals, alkaline earth metals and others, which originate mainly from pellets that have disintegrated, but also from fly-ash from the burner fuel used to heat the kiln.This study investigated the interaction of deposit materials with refractory lining bricks in kilns during the sintering process in iron ore pellet production. Results are obtained from laboratory scale experiments, and from samples collected from industrial kilns, both in-situ and during production stops. Refractory/deposit compatibility tests were performed in a laboratory furnace at various temperatures, for different holding times, and in a number of atmospheres. Deposit materials collected from three commercial lining bricks in production kilns were analysed in both powder and solid forms. Deliberate additions of alkali species (carbonates of potassium and sodium) were made in order to evaluate their influence on degradation mechanisms.Analysis using scanning electron microscopy, QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy), x-ray diffraction, differential scanning calorimetry, thermogravimetry and in-situ mass spectrometry confirmed that alkali additions in the deposit materials dissolve mullite in the liner bricks, which accelerates degradation. Phases such as nepheline (Na2O·Al2O3·2SiO2), kalsilite (K2O·Al2O3·2SiO2), leucite (K2O·Al2O3·4SiO2) and potassium β-alumina (K2O·11Al2O3) were formed. Moreover, it was observed that potassium penetrates deeper into the lining material, and in larger amounts, than sodium, both on the laboratory scale and in industrial furnaces. Formations of alkalicontaining phases such as the feldspathoid minerals kalsilite and nepheline are coupled with an expansion in the lining material, observed by dilatometry, which causes structural spalling that appears as cracks in some of the refractory/deposit compatibility tests. Grains of hematite with sizes between 50-100 μm remain on the original surface of the brick, whereas micrometer-scale hematite migrates through capillary infiltration (in pores, brick joints and cracks) and diffusion, and appears in finer grains deeper in the lining material. The degradation mechanisms of the bricks in an iron ore pellet producing kiln are shown to involve these chemical reactions in combination with thermomechanical stresses. Recommendations are given regarding the choice of materials, the design of refractory liners in the kilns to extend the time between production stops necessary for repair.
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| 6. |
- Stjernberg, Jesper
(author)
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Degradation of mullite based materials by alkali containing slags
- 2008
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Licentiate thesis (other academic/artistic)abstract
- Iron is one of the most important resources that can be found in the lithosphere; 90 % of all metal ores extracted are iron ore. Many steps are included in the extraction from iron ore to metallic iron, where the processes vary between different producers. Iron ore pellets, are a prepared burden material for ironmaking in the blast furnace. Such pellets are commonly sintered in a grate-kiln furnace system, where the kiln usually is insulated with mullite containing bricks. Different mechanisms wear these bricks and they need to be replaced regularly and this causes production stops. The slag present in the kiln consists of ~95 % hematite, alkali-, alkaline earth- and other oxides, mainly from pellets that have disintegrated and adheres in chunks on the bricks. This study is focusing on the interaction between refractories and slags that occurs in kilns during the sintering process in the iron ore pellet production. Results are shown from lab scale experiments, and from samples collected in industrial furnaces, commonly called rotary kilns. Slag/brick compatibility tests were performed in a laboratory furnace at various temperatures, holding times and atmospheres. Slag collected from a production kiln and three commercial bricks, in powder or solid form, were used. Deliberate additions of alkali species were included in order to evaluate their influence. XRD, DSC, TG and in-situ mass spectrometry confirm that addition of alkali dissolves the mullite in the bricks, and forms the phase nepheline (Na2O•Al2O3•2SiO2), which disintegrate to an amorphous phase at elevated temperature. QEMSCAN were used to view mineralogical mappings of different chemical phases by field image scans. It was found that when alkali penetrates the surface of the brick, besides formation of nepheline, phases as kalsilite (K2O•Al2O3•2SiO2), leucite (K2O•Al2O3•4SiO2) and potassium â-alumina (K2O•11Al2O3) are formed. Also seen is that potassium penetrates deeper, and in larger amounts than sodium in the lining material. Formations of alkali containing phases as the feldspathoid minerals kalsilite and nepheline are coupled to an expansion in the lining material, observed by dilatometry, causing structural spalling observed as cracks in some of the slag/brick compatibility tests. Grains of hematite with sizes between 50- 100 ìm stay on the original surface of the brick, while micrometer sized hematite migrates through the dissolved brick by capillary infiltration and diffusion, and nucleates in needle formations deeper in the lining material. We propose a wear mechanism of the bricks in an iron ore pellet producing kiln that involves these chemical reactions in combination with erosion by the continuously flowing slag.
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| 7. |
- Stjernberg, Jesper, et al.
(author)
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Degradation of refractory bricks used as thermal insulation in rotary kilns for iron ore pellet production
- 2009
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In: International Journal of Applied Ceramic Technology. - : Wiley. - 1546-542X .- 1744-7402. ; 6:6, s. 717-726
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Journal article (peer-reviewed)abstract
- Degradation of bricks in an iron ore pellet producing kiln has been investigated. Lab-scale tests of brick/slag interaction performed under different temperatures, atmospheres, and alkali additions show that addition of alkali dissolves the mullite in the brick and leads to formation of the phase nepheline (Na2O·Al2O3·2SiO2). At a high temperature, the grain boundary where nepheline is formed disintegrates due to volume expansion. At increased temperature, the nepheline transforms to an amorphous phase. Thus, a wear mechanism is proposed in the kiln using these bricks that involves these chemical reactions in combination with erosion by the continuously flowing slag.
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| 8. |
- Stjernberg, Jesper, et al.
(author)
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Deposit formation in a grate-kiln plant for iron-ore pellet production : Part 2: Characterization of deposits
- 2013
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In: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 27:10, s. 6171-6184
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Journal article (peer-reviewed)abstract
- Buildup of deposit material in chunks on refractory linings caused by combustion of various fuels is a well-known problem. This study characterizes the short-term deposits on refractory material in a grate-kiln process, carried out through in situ measurements using a water-cooled probe with a part of a refractory brick mounted in its end. Sampling was carried out during combustion of both oil and coal. A significant difference in deposition rates was observed; deposition during oil firing was negligible compared to coal firing. The deposits are mainly hematite particles embedded in bonding phase, mainly comprising Si, Al, Fe, Ca, and O. Moreover, it was found that the prevailing flue-gas direction determines the formation of the deposits on the probe and that inertial impaction controls the deposition rate. However, this rate can also be affected by the amount of air-borne particles present in the kiln.
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| 9. |
- Stjernberg, Jesper, et al.
(author)
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Evaluation of refractory castables and coatings used in pre-heat zone of grate-kiln for iron ore pellet production
- 2015
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In: Ironmaking & steelmaking. - : Maney Publishing. - 0301-9233 .- 1743-2812. ; 42:4, s. 274-281
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Journal article (peer-reviewed)abstract
- Iron ore pellets are a prepared burden material for ironmaking that are commonly sintered in a grate kiln furnace system. Deposited material can adhere in chunks to localised regions of the lining of such furnaces, causing lining depletion. Such deposits consist mainly of iron oxides, but alkali, alkaline earth and other oxides formed from disintegrated pellets and fly ash may also be present. In order to investigate methods of preventing such formations, tests have been performed using several different castables and coatings. Samples of liner castables with and without coatings, installed in an industrial furnace, were collected after 6 and 24 months in production use, and characterised using scanning electron microscopy to identify the materials, characterise morphological changes at the deposit-coating-lining interface, and to determine the active chemical reactions. An Al2O3 based coating applied to one of the fields was found to be in good condition after 6 months in use, but no traces of the coating were observed after 24 months in use. A carbon phosphate based coating was not intact after 6 months in use; it was probably burned-up as a result of the oxidising atmosphere. Hematite grains from the deposit material remained at the original deposit-lining interface, while calcium migrated further into the lining.
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| 10. |
- Stjernberg, Jesper, et al.
(author)
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Extended studies of degradation mechanisms in the refractory lining of a rotary kiln for iron ore pellet production
- 2012
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In: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 32:8, s. 1519-1528
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Journal article (peer-reviewed)abstract
- Changes, over a period of 8 years, in the chemical composition and morphology of deposit and lining materials in a production rotary kiln for iron ore pellet manufacture are described. The following have been studied: two types of refractory brick used as lining material; deposited chunk materials from the lining; the interaction zones between deposits and linings. Morphological changes at the deposit/lining interface, and the active chemical reactions, are established. Larger hematite grains in the deposit material (5-50 mu m) primarily remain at the original deposit/lining interface. The remainder penetrates fissures, voids and brick joints, forms a laminar structure with corundum from the bricks, and migrates in grains in the lining material. Potassium penetrates more deeply into the bricks than hematite, resulting in the formation of kalsilite, leucite and potassium beta-alumina, which contribute to degradation of the lining.
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