Hematite single crystal reduction into magnetite with CO-CO₂

Abstract

In view of the striking discrepancies among previous authors as regards the transition between porous and lamellar magnetites, we have carried out a wide series of experiments with single crystals prepared by chemical vapor transport. In addition to the classical temperature and CO pct parameters, which varied over a large range (400 ≤T ≤ 1000 ≤C and 2 ≤ CO pct ≤ 50), we also investigated the influence of the crystal size and of the fractional weight change. Through observation of a great number of cross-sections of partially reduced crystals, we established that lamellar magnetite is favored by high temperature and low CO pct. This is explained by consideration of the conditions governing the competition between cation diffusion in the semi-coherent hematite-magnetite interface and chemical reaction rate. At low temperatures, the crystals are severely fractured, because hematite is not plastic enough, especially at a high CO₂ pct. The kinetic data are analyzed with the shrinking-core model, where the reaction interface is topochemical. The chemical rate constant thus obtained is ϕ = 69 exp(−8950/T), in mol(CO) · m⁻² · s⁻¹, for crystals in the range 50 to 150 μm andT varying from 500 to 900 °C. Bigger crystals yield a slightly higher preexponential term, confirming that porous diffusion does not rule the kinetics. The nucleation frequency has also been evaluated; it tends toward a kind of saturation at around 700 °C with a value of 10 to 10⁹ s⁻¹. The nuclei growth rate is in reasonable agreement with direct measurements.