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Non-Topochemical reduction of iron oxides

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Abstract

Non-topochemical behavior was studied during reduction of porous spheres of hematite by stages through the intermediate oxides and also continuously to iron by CO/CO2 mixtures at temperatures of 600 to 900°C (873 to 1173 K). The behavior became more nearly topochemical as temperature increased. Shrinking occurred during the reduction of hematite to magnetite and of magnetite to wüstite, whereas swelling was observed during the reduction of wiistite to iron. Shrinking was greater, and swelling less, at higher temperatures. The total surface area of the solid decreased with increasing extent of reduction during each of the three stages. A non-topochemical model was developed which satisfies, better than previously proposed models, the reduction data for the single reactions and the three reactions occurring simultaneously. The model provides for variation in particle size and local changes in porosity and effective diffusivity. An empirical “sintering exponent” was introduced to describe changes in reacting surface area.

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Abbreviations

a :

Sintering exponent for hematite-magnetite reaction

b :

Sintering exponent for magnetite-wüstite reaction

c :

Sintering exponent for wüstite-iron reaction

Ci :

Moles of gas i/cm3 of gas (106 mol.m-3)

Ci :

Moles of solid j/cm3 of particle

Co j :

Initial concentration of solid j, mol/cm3 of particle

Di :

Effective diffusivity of i, cm2/s (10-4 m2S-1)

DCO-CO 2 :

Binary diffusion coefficient of CO in CO2 cm2/s

kG :

Mass transfer coefficient of CO, cm/s (10-2 m.s-1)

k 1,k-1 :

Rate constants for hematite-magnetite reaction, cm/s

K2,K-2 :

Rate constants for magnetite-wüstite reaction, cm/s

k 3,k-3 :

Rate constants for wüstite-iron reaction, cm/s

K2 :

Equilibrium constant for magnetite-wüstite reaction,k 2/k-2

K3 :

Equilibrium constant for wüstite-iron reaction,k 3/k- 3

m°:

Initial mass of particle, g (10-3 kg)

Mj :

Molecular weight of solid species j

p:

Total pressure, atm (1.013×105 Pa)

Pi :

Partial pressure of gas i, atm

pb i :

Partial pressure of i in bulk gas phase, atm

q:

Radial sintering factor for magnetite-wüstite reaction, Eq. [24]

R:

Universal gas constant, 82.06 atm cm3/mol K (8.314 J mol-1 K-1)

r:

Radial position, cm (10-2 m)

ro :

Particle radius, cm

ro :

Initial radius of particle, cm

r∞ :

Final radius of particle, cm

rj :

Radius of particle of pure j, cm

Ŕi :

Generation rate of gas i per unit volume of particle, mol/cm3 s (106 mol.m-3s-1)

Ŕj :

Generation rate of solid j per unit volume of particle, mol/s cm3

Sj :

Surface area of solid reactant j per unit volume of particle, cm-1 (102 m-1)

So j :

Initial surface area of pure solid reactant j per unit volume of particle, cm-1

so j :

Initial surface area of pure solid reactant j per unit mass, m2/g (103 m2.kg-1)

t:

Time, s

T:

Temperature, K

u:

Velocity of solid, cm/s (10-2 m.s-1)

v*:

Molar average velocity of gas, cm/s

x :

Stoichiometric factor for variable wüstite, FexO, composition

Xj :

Fraction of iron atoms as component j locally

•Xj :

Average fraction of iron atoms as j

ε:

Local void fraction of solid

ε°:

Initial void fraction of particle

Pi:

Density of pure solid component j, g/cm3 (103 Kg.m-3)

h:

hematite

m:

magnetite

w:

wüstite

Fe:

iron

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Authors and Affiliations

  1. Amoco Production Co., Tulsa, Okla

    S. P. Trushenski (Senior Research Engineer)

  2. Carnegie-Mellon University, 15213, Pittsburgh, Pa.

    Kun Li (Professor of Chemical Engineering) & W. O. Philbrook (Professor of Metallurgy and Materials Science)

Authors
  1. S. P. Trushenski
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  2. Kun Li
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  3. W. O. Philbrook
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Trushenski, S.P., Li, K. & Philbrook, W.O. Non-Topochemical reduction of iron oxides. Metall Trans 5, 1149–1158 (1974). https://doi.org/10.1007/BF02644326

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