Magnetite oxidation in North American iron ore pellet ...
Transcript of Magnetite oxidation in North American iron ore pellet ...
Magnetite oxidation
in North American iron ore pellet production
Chris Pistorius
Department of Materials Science & Engineering
Carnegie Mellon University
Outline
• Center for Iron and Steelmaking Research:
brief introduction
• Project overview:
oxidation of magnetite in pellet production
• Electron microscopy facilities in MSE
Center for Iron and Steelmaking Research,
Carnegie Mellon University
• Center members:
US and international steel companies
Service providers to the steel industry
• Research focus:
Fundamentals of ironmaking and steelmaking;Fundamentals of ironmaking and steelmaking;
relevant to current and future operations
• Three faculty;
6-10 PhD students;
visiting researchers / postdoctoral fellows
Current topics
• Carbon footprint of ironmaking:
Producing 1 ton of steel
causes ~2 tons of CO2 emissions
• Better use of ironmaking raw materials
• Better control of ceramic impurities
(inclusions) in steel
worl
dst
eel.
org
(inclusions) in steel
• Scale growth (oxidation) during
steel processing
• Mold fluxes for continuous casting
Scale growth on steel studied in situ
Milling, magnetic
separation,
flotation
Strip mining
(AP
Ph
oto
)
Fine magnetite
Production of pellets from taconite
Strip miningof taconite ore:~50% magnetite
Fine magnetite
powder
(below 50µm)
3 cmPelletized
before
ironmaking
Project:
Effect of oxygen enrichment
on magnetite pellet oxidation
Use oxygen enrichment to increase magnetite → hematite oxidation rate during pellet induration
Possible advantages of more rapid oxidation:Possible advantages of more rapid oxidation:- pellet quality
(strength, absence of internal cracks)- throughput
Hardening of pellets:
Oxidizing heat treatment, heated from room temperature
to 1350°C, then cooled; reaction 2Fe3O4 + O2 → 3Fe2O3
Processes: Grate-kiln-cooler; straight-grate
green
pelletsgrate
kiln
Grate-kiln process (Forsmo, 2007)
indurated
pellets
kiln
cooler
Pel
let
tem
per
atu
re (
°C)
1400
1000
grate kiln cooler
top
Thermal profile: grate-kiln process
(after Young et al.,
1979)
time
Pel
let
tem
per
atu
re (
600
200
30 min
bottom
top
bottom
~50%oxidation
~50%oxidation
Fe3O4
Liquid metal Liquid oxide
γ-Fe
δ-Fe
Fe3O4
α-Fe
air
Fe2O3
Research question:
Will increasing the oxygen content in the furnace
atmosphere improve pellet properties?
Fundamental question:
magnetite oxidation kinetics and mechanism
nucleation;
phase changes
Possibly rate-determining:
O2 mass transfer to & into pellets;
nucleation of Fe2O3;
O2- diffusion through Fe2O3 product
phase changes
(maghemite vs. hematite)
Binding mechanisms in pellets:Hematite-hematite bond by oxidation (grate);
bond strengthened by sintering at T > 1100°C (kiln)
Magnetite sintering at T > 900°C if incomplete oxidation
(undesirable – causes core to shrink away from shell)
Strength after
30min at 30min at
temperature
Sintering temperature
(Cooke & Ban,
1952)
NeutralAir
Possible rate-determining steps:Gaseous diffusion (of O2) into pellet:
cored structure develops; fully controlling only at T>~1100°C
Solid-state diffusion (of O2- or Fe3+ or both) through hematite
product layer around each particle:
fully controlling at T<~400°C
⇒ mixed control over most of temperature range
(Kokal, 1970)
800°C
Oxidation temperature:
effect on product morphology
(dark: magnetite;
bright: hematite)
Forsmo, 20071100°C
Implications of mixed control:Role of gaseous diffusion:
pellet size and porosity important;
partial pressure of O2 important
Role of solid-state diffusion:
taconite structure (origin)
& grain size important
(Zetterstrom, 1950)
Oxidation of
different magnetites
at 600°C (air)
(Zetterstrom, 1950)
Hematite product layer
around individual magnetite particles:Strongly limits oxidation
in lower-temperature region
- effect not equally strong
for all magnetites
- possibly also affected by
pO2, water vapor?
700°C
800°C
time
% oxidation
pO2, water vapor?
400°C
500°C
600°C
Experimental work:Measure isothermal oxidation kinetics
Unagglomerated concentrate
(different particle sizes screened out)
Mass change measured
Gases: pure O2 and 90% O2 – 10% H2O
Sample characterization: optical microscopy, XRD, SEM
Results:Plateau effect confirmed;
decrease in rate sharper than shown in literature
Expanded view: initial oxidation
Water vapor: small effect
Water vapor: small effect
Particle size: smaller particles oxidize more
-325 mesh (-45µm)
230-325 mesh (45-63µm)
600°C
50 µm
Optical microscopy:
Reflected light, polarized
- hematite brighter and magnetite dark
No clear product layer around particle edges
800°C600°C
25.5
26.5
Vol
ume,
Å3
magnetite
hematite
Difference in volume per iron atom
24.50 200 400 600 800 1000
T, °C
Vol
ume,
Å
magnetite
Unoxidized magnetite
Oxidized at 460°C
Oxidized at 670°C
Oxidized at 850°C
Oxidized at 1000°C
Observed hematite whiskers are similar to "nanowires"
reported to form when Fe is oxidized:
Whiskers on Fe oxidized
in O2 at 600°C
(Voss et al., 1982)Whiskers on Fe oxidized in
19%CO2 -81%NO2-0.14%SO2
at 600°C (Fu et al., 2003)
Conclusions
Sharp decrease in oxidation rate
after initial oxidation confirmed;
temperature effect confirmed
Reason for sharp drop in rate not clear:
no obvious continuous product layer
Oxidation-induced roughening of particle surfaces likely Oxidation-induced roughening of particle surfaces likely
contribute to strength of pellets
- likely role of hematite whisker formation
Electron microscopy facilities, MSE
SEMs:• FEI Quanta 200 Field Emission Environmental
Scanning Electron Microscope; EDAX/TSL EBSD system (Hikari high speed camera)
• FEI Quanta 600 Field Emission Environmental Scanning Electron Microscope with Heating Stage and Cooling stage; EDAX/TSL EBSD system
• Philips XL-30 FEG-SEM with Oxford Instruments INCA analysis system (EDX and WDX); HKL EBSD system (NordlysF high speed camera)system (NordlysF high speed camera)
Electron microscopy facilities, MSE
TEMs:• FEI Titan 80-300; Energy Dispersive X-ray and Tridiem Electron Energy
Loss Imaging Filter• FEI Tecnai Super Twin 200 kV FEG; Energy Dispersive X-ray and
Gatan Electron Energy Loss Imaging Filter; ACT Orientation Imaging System
• Philips CM-12; BF STEM, SEI and BSEI detectors; ACT Orientation Imaging Microscopy system
• JEOL 2000EX High Resolution Transmission Electron Microscope; heating and cooling stages.heating and cooling stages.
Electron microscopy facilities, MSE
Focused ion beam microscope:• Novalab 600 Dual Beam with Gallium Focused Ion Beam System and
Field Emission Electron Beam Microscope, with EDAX/TSL EBSD system (Hikari High Speed camera) and Auto Probe lift out tool. Software for serial section and 3D imaging.
MSE Newsletter, Spring 2007