Presets
Playback
Furnace and gas
The gas must be reducing enough to make magnetite, but not so reducing that it makes weakly magnetic wustite.
Grain and playback
Coarse grains are slowed by diffusion through the layer they have already made.
Model checks
The science
Turning hematite into magnetite
Hematite (Fe2O3) is only weakly magnetic. Roasting it in a gas that is slightly reducing converts it to strongly magnetic magnetite: 3 Fe2O3 + CO → 2 Fe3O4 + CO2. Which oxide is stable depends on the gas ratio CO2/CO and the temperature (the Baur-Glaessner diagram). Using ΔG° fits, the equilibrium ratio is K = exp(−ΔG°/RT): hematite needs only a trace of CO to turn into magnetite, while magnetite itself turns into wustite (FeO) once the CO share rises above the second equilibrium line. The window between the two lines is the target.
Shrinking core
The reaction front moves inward from the surface. With chemical control the front radius falls as r = R − u t; once a product layer exists, gas must diffuse through it. The combined time to reach conversion X is t = τc [1 − (1 − X)1/3] + τd [1 − 3(1 − X)2/3 + 2(1 − X)]. The rates follow Arrhenius laws and scale with the distance from equilibrium. Large grains are diffusion-limited because τd ∝ R².
Honest simplifications
The thermodynamic fits are textbook values, but the kinetic constants are illustrative, not fitted to a real ore. Grains are single spheres with no pores or impurities, the gas composition is constant, density changes are ignored, and the magnetic recovery weights (0.97, 0.08 and 0.15 for magnetite, hematite and wustite) are an assumption. The page demonstrates the trade-off, not a plant design.
Checks in the page
Check 1 compares the closed-form conversion with an RK4 integration of the rate equation. Check 2 verifies the wustite equilibrium at the temperature where its free energy change is zero. Check 3 compares the mass loss for full conversion with the stoichiometric 3.34 percent.
What to try
- Load Good magnetite window and watch the dark magnetite shell grow inward while the red hematite core shrinks.
- Load Too hot and too reducing: a brown wustite skin forms on the outside and the magnetic recovery falls.
- Load Coarse grain and compare the time to convert the core with the fine grain: diffusion through the shell dominates.
- Move the operating point on the equilibrium diagram into the wustite region to see over-reduction.