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

Hematite single crystal reduction into magnetite with CO-CO 2

Influence of sintering conditions on the reduction behaviour of pure hematite compacts

Fired compacts made from pure Fe2O3 and Fe2O3 doped with 1·0%CaO and/or 1·0%MgO were isothermally reduced in a stepwise manner at 1173–1473 K. At each temperature, hematite was first reduced to magnetite in 10CO–90CO2, then to wustite (Fe0·934O) in the appropriate mixture of CO–CO2, and finally to iron in pure CO. The influence of doping with CaO and/or MgO on structural changes and the reduction kinetics and mechanism was intensively studied. The reduction rate of Fe2O3 to Fe3O4 was increased in the order CaO–MgO–Fe2O3 > MgO–Fe2O3 > CaO–Fe2O3 > Fe2O3, and this transformation step was controlled by gaseous diffusion in the early stages and by interfacial chemical reaction at later stages. The reduction of magnetite to wustite was controlled by a combined gaseous diffusion and interfacial chemical reaction mechanism. The reduction of wustite was also promoted, in the early stages, in the presence of CaO and/or MgO. At a level of ≥40%, the presence of MgO retarded reduction, resulting in a slowing d...

Influence of doping with CaO and/or MgO on stepwise reduction of pure hematite compacts

The reduction kinetics and mechanisms of hematite ore with various particle sizes with hydrogen at low temperature were studied using the thermogravimetric analysis. At the same temperature, after the particle size of powder decreases from 107.5 μm to 2.0 μm, the surface area of the powder and the contact area between the powder and gas increase, which makes the reduction process of hematite accelerate by about 8 times, and the apparent activation energy of the reduction reaction drops to 36.9 kJ/mol from 78.3 kJ/mol because the activity of ore powder is improved by refining gradually. With the same reaction rate, the reaction temperature of 6.5 μm powder decreases by about 80 °C compared with that of 107.5 μm powder. Thinner diffusion layer can also accelerate the reaction owing to powder refining. The higher the temperature, the greater is the peak of the reduction rate; at the same temperature, the greater the particle size, the smaller is the peak value of the reduction rate; both inner diffusion and interface chemical reaction play an important role in the whole reaction process.

Influence of Size of Hematite Powder on Its Reduction Kinetics by H2 at Low Temperature

High manganese containing iron ore samples were isothermally reduced with a CO–CO2 gas mixture at 600–1000°C. The course of reduction was followed by a weight loss technique. The influence of reducing gas composition and temperature on the reduction kinetics was investigated. The different phases formed during reduction were identified by X-ray phase analysis, while their structures were microscopically examined. The reduced samples were magnetically tested by means of a Davis tube tester. The effect of grain size, drum speed, and cleaning conditions on the efficiency of magnetic separation was studied using a Box-Mag wet low intensity magnetic separator. The separation efficiency was determined by analysing total iron, manganese, and acid insoluble contents in both magnetic and non-magnetic fractions. Best testing results were obtained on separation of the sample reduced with 80CO–20CO2 (vol.-%) at 800°C. The optimum grain size for magnetic separation is below 0·15 mm while that of the drum spee...

Behaviour of manganese oxides during magnetising reduction of baharia iron ore by CO-CO2 gas mixture

Carol Lake hematite particles were reduced to magnetite at 600 and 1000 °C using CO + CO2 mixtures. The rate of reduction was measured in gravimetric tests and structural changes followed by optical microscopy, BET surface area, and mercury porosimetry. At 600 °C, the reduction of each grain approximately followed the shrinking core model, and fine pores were created in the magnetite produced. The volume of pores was constant at a porosity level of 8.8 pct, but the average pore size depended on the rate of oxygen removal, and finer pores were obtained under conditions of fast reduction. The reaction followed a different path at 1000 °C and proceeded by sideway thickening of a finite number of magnetite lamellae formed parallel to each other in each grain. Reduced particles showed reentrant surface depressions and cracks, but no porosity. Reaction mechanisms were postulated to relate these structural features to the progress of the reaction. Examination of the reaction steps indicated that the separation of oxygen from solid surfaces was likely to be the rate determining step at both temperatures. This was reflected in rate measurements by a strong dependence on CO pressure while the influence of oxygen activity (as represented by CO2/CO ratio) was of secondary importance at 1000 °C and negligible at 600 °C. A detailed analysis of reaction rates could not be made, however, because the particles were of a wide range of sizes and their structure changed during reduction.

A. Ünal

Papers

Rate processes and structural changes in gaseous reduction of hematite particles to magnetite