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A study of micro-particles in the dust and melt at different stages of iron and steelmaking
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.ORCID iD: 0000-0002-2377-2383
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dust particles generated due to mechanical wear of iron ore pellets and clusters formed in molten stainless steel alloyed with rare earth metals (REM) are considered in this study. Firstly, the influence of the characteristics of iron ore pellets, applied load on a pellet bed and partial reduction of the pellets on the size distribution of the generated dust was investigated. Secondly, REM clusters are investigated to evaluate the size distribution of the clusters. Also, an extreme value distribution (EVD) analysis has been applied for the observed REM clusters.

The large sized pellets showed 10-20% higher wear rate than small sized pellets during wear in a planetary mill. Moreover, an increase of ~67% was observed in the friction and dust generation in the pellet bed as the applied load increased from 1 to 3 kg. Also, it was observed that a higher friction in the pellet bed can lead to an increased amount of airborne particles. The mechanical wear experiments of pellets reduced at 500 °C (P500) and 850 °C (P850) showed that P500 pellets exhibit ~16-35% higher wear rate than unreduced pellets. For the P850 pellets, the wear is inhibited by formation of a metallic layer at the outer surface of the pellets. The mechanism of dust generation has been explained using the obtained results.

A reliable cluster size distribution of REM clusters was obtained by improving the observation method and it was used to explicate the formation and growth mechanism of REM clusters. The results show that the growth of clusters is governed by different types of collisions depending on the size of the clusters. For EVD analysis three different size parameters were considered. Moreover, using the maximum length of clusters results in a better correlation of EVD regression lines compared to other size parameters. Moreover, a comparison of predicted and observed maximum lengths of clusters showed that further work is required for the application of EVD analyses for REM clusters.

Abstract [sv]

Studien fokuserar på två olika typer av mikropartiklar som är valda från olika delar av järn- och ståltillverkningsprocessen. Dessa partiklar är dels stoft som genereras på grund av mekanisk nötning av partiklar och dels klusters som bildas i flytande rostfria stål legerade med sällsynta jordartsmetaller (REM). Inledningsvis så undersöktes inverkan av tre faktorer på storleksfördelningen hos stoft som bildas vid hantering av järnoxidpellets. De undersökta faktorerna inkluderade karakteristiken hos järnoxidpellets, det applicerade trycket på pelletsbädden och den partiella reduktionen av järnoxidpellets. Därefter så utfördes tredimensionella undersökningar av REM kluster som extraherats med hjälp av elektrolytisk extraction för att bestämma storleksfördelningen hos klustren. Dessutom så utfördes en extremvärdesdistribution (EVD) studie för de studerade klustren.

En planetkvarn användes för att undersöka inverkan of karakeristiken hos pellets på stoftbildningen. Resultaten visade att storleken på pellets kan påverka nötningshastigheten under dessa försöksförhållanden. Pellets som hade en större storlek (13.5< Deq <15.0 mm) uppvisade en 10 till 20% högre nötningshastighet i jämförelse med mindre pellets (9.5< Deq <12.5 mm). Baserat på analyserna av stoftet som genererades under nötningsexperimenten så konstaterades att nötningsmekanismerna för dessa pellets var abrasions- och kollisionsnötning.

En pelletsbädd skapades för att möjliggöra studier av inverkan av ett applicerat tryck på stoftbildningen och friktionskrafterna i en pelletsbädd. Ett varierat tryck på mellan 1 till 3 kg applicerades på pelletsbädden. Resultaten visade att en ökning på ~67% av friktionskraften och stoftbildningen ägde rum när det applicerade trycket ökades från 1 till 3kg. Dessutom så visade resultaten att en högre friktionskraft i pelletsbädden kan resultera in en ökad mängd luftburna partiklar. Den mekaniska nötningen av pellets som reducerats vid 500 °C (P500) och 850 °C (P850) studerades också genom användande av en planetkvarn. Resultaten visade att P500 pellets uppvisade en ~ 16 till 35% högre nötningshastighet i jämförelse med oreducerade referenspellets.  Resultaten för P850 pellets visade att den mekaniska nötningen motverkades genom bildningen av ett metalliskt skikt på den yttre delen av pelletsen. Resultaten visade också att stoftet som bildats pga mekanisk nötning av reducerade pellets innehöll 3 till 6 gånger mer grova partiklar  (>20µm) i jämförelse med stoft som bildats från oreducerade pellets. Slutligen så diskuterades hur dessa resultat kan relateras till industriella förhållanden med avseende på mekanismerna som är involverade i den mekaniska nötningen av pellets samt med avseende på relationen mellan hastigheten av de utgående gaserna och storlken och morfologin hos stoftpartiklarna.

Klusters innehållande REM-oxider som extraherats från en 253MA rostfri stålsort undersöktes med användande av en tredimensionell teknik. En trovärdig storleksfördelning av klusters (CSD) erhölls genom att förbättra undersökningsmetoden och denna användes för att studera bildningen och tillväxten av REM oxider. Dessutom så användes cirkularitetsfaktorn hos klusters för att dela in klustren i två olika grupper, vilka bildas och tillväxer enligt olika mekanismer. Resultaten visade också att tillväxten av klusters gynnas av olika typer av kollisioner som beror av av storleken på klusters. För REM-klusters så drogs slutsatsen att turbulenta kollisioner är den huvudsakliga mekanismen som påverkar tillväxten.

Avhandlingen behandlar även problemet om hur det är möjligt att hantera synfält där det inte förekommer kluster vid en extremvärdesdistribution (EVD) analys. Tre olika parametrar undersöktes i EVD analysen. Resultaten visar att om den maximala längden på kluster (LC) används i analysen så erhålls den bästa korrelationen gällande regressionslinjen för en EVD analys. Specifikt så var R2 värdet upp till 0.9876 i jämförelse med de andra storleksparametrarna som har värden i intervallet 0.9656 – 0.9774. Slutligen så visar resultaten från en jämförelse mellan beräknade och observerade maximala klusterlängder att EVD analyser för studier av REM kluster behöver undersökas ytterligare i framtiden.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 55 p.
Keyword [en]
Particle size distribution, ironmaking, iron ore pellets, dust generation, mechanical wear, friction, reduction, REM clusters, Electrolytic extraction, cluster size distribution, growth mechanism, collisions, statistical analysis.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering; Metallurgical process science
Identifiers
URN: urn:nbn:se:kth:diva-196805ISBN: 978-91-7729-217-3OAI: oai:DiVA.org:kth-196805DiVA: diva2:1048845
Public defence
2016-12-20, B2, Brinellvägen 23, Stockhoolm, 10:00 (English)
Opponent
Supervisors
Note

QC 20161128

Available from: 2016-11-28 Created: 2016-11-22 Last updated: 2016-11-28Bibliographically approved
List of papers
1. Evaluation of Dust Generation during Mechanical Wear of Iron Ore Pellets
Open this publication in new window or tab >>Evaluation of Dust Generation during Mechanical Wear of Iron Ore Pellets
2016 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 56, no 6, 960-966 p.Article in journal (Refereed) Published
Abstract [en]

Iron ore pellets undergo mechanical wear during handling, transportation and use in a blast furnace. This results in a loss of valuable raw materials and causes environmental problems in form of dust in off-gases from a blast furnace. Thus, this study is focused on the investigation of the mechanical wear of iron ore pellets and the dust formation. The characteristics of industrial pellets (such as size, weight, density and hardness) have been investigated. Moreover, the influence of pellet characteristics on the wear mechanism (sliding/abrasion and impact/collisions wear) and the characteristics of generated dust have been investigated. It was observed that the size of pellets can influence the wear rate under the given experimental conditions. The pellets with larger size (13.5 < d(eq) < 15.0 mm) showed 10-20% higher wear rate as compared to small sized pellets (9.5 < d(eq) < 12.50 mm). SEM studies of the dust generated during wear experiments inferred that larger contribution of impact/collisions in wear of pellets is the reason for the higher wear rate of large size pellets. Further, a relationship between the critical diameter of dust particles, which can be removed with off-gases from the blast furnace, and the velocity of off-gases in top part of blast furnace was developed.

Keyword
pellets, mechanical wear, dust, particle size distribution
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-189831 (URN)10.2355/isijinternational.ISIJINT-2015-680 (DOI)000378189600007 ()
Funder
Swedish Energy Agency
Note

QC 20160718

Available from: 2016-07-18 Created: 2016-07-15 Last updated: 2016-11-22Bibliographically approved
2. Friction forces and mechanical dust generation in an iron ore pellet bed subjected to varied applied loads
Open this publication in new window or tab >>Friction forces and mechanical dust generation in an iron ore pellet bed subjected to varied applied loads
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Iron ore pellets degrade and generate dust during transportation and handling as well as during the iron making process. This leads to material losses and effects the process efficiency in a negative manner. In order, to reduce the generation of dust it is important to understand the influence of process parameters on the dust formation. An experimental setup was used to measure the dust generation and friction forces caused by abrasion of iron ore pellets in a closed pack bed. A varied load of 1 to 3 kg was applied on the pellet bed but at a constant air flow rate to capture the airborne dust particles. It was observed that an increase of ~67% is observed in the friction and the dust generation in the bed as the applied load increased from 1 to 3 kg. Moreover, the evaluation of the particle size distribution of the generated dust showed that a higher friction in the pellet bed can lead to an increased amount of airborne particles. Moreover, it has been shown that in an air flow the morphology and the orientation of dust particles can influence the air velocity required to transport the particles upwards.   

Keyword
friction, dust generation, iron ore pellets, ironmaking, mechanical wear
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-196919 (URN)
Note

QC 20161129

Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-29Bibliographically approved
3. Characterization of dust generated during mechanical wear of partially reduced iron ore pellets
Open this publication in new window or tab >>Characterization of dust generated during mechanical wear of partially reduced iron ore pellets
(English)Manuscript (preprint) (Other academic)
Abstract [en]

During reduction in a blast furnace (BF), the iron ore pellets undergo structural changes which facilitate dust generation due to the mechanical wear / disintegration of the pellets. The generated dust decreases the permeability and productivity of the BF process. Thus, this study investigates the mechanical wear of iron ore pellets reduced at 500 °C (P500) and 850 °C (P850) and compares the results to the wear of unreduced pellets (P25). Moreover, the dust generated during the wear experiments is also characterized. It was found that pellets reduced at 500 °C exhibit a ~ 16 to 35% higher wear rate than reference unreduced pellets. For the pellets reduced at 850 °C, the mechanical wear is inhibited by a formation of a metallic layer at the outer surface of the pellets. Further, the dust generated due to mechanical wear of reduced pellets contained 3 to 6 times higher amount of coarse particles (>20µm) as compared to the dust from unreduced pellets. The obtained results are explained on the basis of the structural changes which take place during the reduction of pellets. 

Keyword
wear, pellets, dust, reduction, blast furnace
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-196927 (URN)
Note

QC 20161129

Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-29Bibliographically approved
4. Formation and Growth Mechanism of Clusters in Liquid REM-alloyed Stainless Steels
Open this publication in new window or tab >>Formation and Growth Mechanism of Clusters in Liquid REM-alloyed Stainless Steels
2015 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 55, no 11, 2358-2364 p.Article in journal (Refereed) Published
Abstract [en]

REM-oxide clusters extracted from 253MA stainless steel grade samples from a pilot trial were investigated using a 2%TEA electrolyte. The samples were taken from liquid steel at different holding times after an addition of an appropriate amount of mischmetal. Thereafter, SEM in combination with EDS was deployed for three dimensional (3D) investigations of the characteristics of the extracted REM-oxide clusters. A reliable cluster size distribution (CSD) was obtained by improving the observation method and it was used to explicate the formation and growth mechanism of REM-oxide clusters. A correlation between morphology of clusters and their growth rate was found. This was used to divide the clusters into two different groups, which form and grow in accordance to different mechanisms. The results also show that the growth of clusters is governed by different types of collisions dependent up on size of the clusters. It has been concluded that for REM-oxide clusters turbulent collisions are the main controlling mode for the growth rate.

Keyword
REM clusters, electrolytic extraction, cluster size distribution, growth mechanism, collisions
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-180180 (URN)10.2355/isijinternational.ISIJINT-2015-293 (DOI)000365372400012 ()2-s2.0-84948751922 (ScopusID)
Note

QC 20150111

Available from: 2016-01-11 Created: 2016-01-07 Last updated: 2016-11-22Bibliographically approved
5. Extreme Value Distribution of clusters in REM-alloyed stainless steels
Open this publication in new window or tab >>Extreme Value Distribution of clusters in REM-alloyed stainless steels
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An extreme value distribution (EVD) analysis has been applied for three dimensional (3D) investigations of clusters observed in REM alloyed stainless steel samples. The presence of observed unit areas without any clusters has been discussed. It has been shown that an increase of the observed unit area (AO) significantly improves the correlation of EVD regression lines. Moreover, three different size parameters were considered for EVD analysis. The results show that using the maximum length of clusters (LC) results in a better correlation of EVD regression lines by improving R2 value up to 0.9876 as compared to 0.9656 – 0.9774 for other size parameters. Moreover, a comparison of predicted and observed maximum lengths of clusters showed that there is need of further work on validation of EVD analysis. 

National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-196928 (URN)
Note

QC 20161129

Available from: 2016-11-25 Created: 2016-11-25 Last updated: 2016-11-29Bibliographically approved

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