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