Post on 28-Oct-2021
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