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; relevant to current and future operations • Three faculty; 6-10 PhD students; visiting researchers / postdoctoral fellows

• 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 • Scale growth (oxidation) during steel processing • Mold fluxes for continuous casting

Scale growth on steel studied in situ

worldsteel.org

Current topics

(AP Photo)

Production of pellets from taconite Milling, magnetic separation, flotation

Strip mining of taconite ore: ~50% magnetite

3 cm

Fine magnetite powder (below 50µm)

Pelletized 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: - 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 pellets

grate

kiln cooler

indurated pellets

Grate-kiln process (Forsmo, 2007)

Thermal profile: grate-kiln process

Pellet temperature ((°C)

1400 grate

kiln

cooler

1000 (after Young et al., 1979)

top 600

30 min top

200

bottom bottom ~50% oxidation

~50% oxidation

time

Liquid metal

Liquid oxide

δ-Fe γ-Fe Fe3O4 Fe2O3

air

α-Fe

Research question: Will increasing the oxygen content in the furnace atmosphere improve pellet properties? Fundamental question: magnetite oxidation kinetics and mechanism

Possibly rate-determining: O2 mass transfer to & into pellets; nucleation of Fe2O3; O2- diffusion through Fe2O3 product

nucleation; 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 temperature

Air

Sintering temperature

Neutral (Cooke & Ban, 1952)

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