Effect of K and Na on reduction swelling performance of oxidized roasted briquettes

The presence of potassium oxide (K2O) and sodium oxide (Na2O) causes high reduction swelling of pellets of Bayan Obo iron concentrate during reduction and thus affects the permeability of blast gases during blast furnace operations. The influencing mechanism of K2O and Na2O on the swelling behavior of reduction reactions (1) Fe2O3 → Fe3O4, (2) Fe3O4 → Fe x O, and (3) Fe x O → Fe was researched by adding (K2O + Na2O) to Australian fine ore briquettes. The mineral composition and structure of the briquettes, as well as the reduction swelling after the three reactions coupled with the morphology and lattice parameters of the reduced products, were studied. From the results, the swelling index with 0.6% (K2O + Na2O) added was 8.52%, 7.91%, and 33.81%, respectively, and without were 12.36%, 3.27%, and 12.61%, respectively, for the three reactions. The swelling index of the first reaction (Fe2O3 → Fe3O4) is reduced because alkali metal suppresses crystal cracking. The swelling mainly occurs at the third stage (Fe x O → Fe), because K2O and Na2O enhance the oriented growth of iron whiskers, as well as make them smaller. Crystal transformation does not occur at the second stage (Fe3O4 → Fe x O) and the reduction swelling is small, but the swelling index of the briquettes with added with K2O and Na2O increases (7.91% compared to 3.27%). The main reason is that the alkali metal reduces the melting point of the slag phase and promotes the cascade crystallization of FeO. Therefore, the abnormal swelling of briquettes caused by K and Na is mainly caused by the growth of iron whiskers at the third stage.

Effect of basicity on the reduction swelling properties of iron ore briquettes

The influence mechanism of basicity on the reduction swelling index (RSI) of iron ore briquettes was investigated using the SEM analysis and Factsage 7.3 thermodynamic calculations based on the addition of pure CaO to Bayan Obo iron concentrate. The results revealed that the solid solution of Ca2+ in the FeO lattice increased with the basicity of the briquettes, whereas the diffusion channels of Fe2+ ions increased during the reduction process from FeO to Fe and resulted in the formation of a great number of slender and anisotropic iron whiskers, which consequently increased the RSI. Furthermore, the melting point of the slag phase decreased as the CaO content increased; this reduced its ability to resist the reduction swelling of iron oxides. When the basicity was increased from 0.3 to 0.8, the RSI reached a maximum of 69.85%. However, due to the saturated solid solution of Ca2+ in FeO lattice, as the basicity further increased from 0.8 to 1.2, excess CaO melting into the slag phase promoted the precipitation of spinel minerals with high melting points and difficult reduction properties. Thus, the diffusion of Fe2+ and the growth of the iron whiskers were hindered, and the RSI was reduced.

On the absorption properties of metallic needles

ABSTRACT Needle-like metallic particles have been suggested to explain a wide variety of astrophysical phenomena, ranging from the mid-infrared interstellar extinction to the thermalization of starlight to generate the cosmic microwave background. These suggestions rely on the amplitude and the wavelength dependence of the absorption cross-sections of metallic needles. On the absence of an exact solution to the absorption properties of metallic needles, their absorption cross-sections are often derived from the antenna approximation. However, it is shown here that the antenna approximation is not an appropriate representation, since it violates the Kramers–Kronig relation. Stimulated by the recent discovery of iron whiskers in asteroid Itokawa and graphite whiskers in carbonaceous chondrites, we call for rigorous calculations of the absorption cross-sections of metallic needle-like particles, presumably with the discrete dipole approximation. We also call for experimental studies of the formation and growth mechanisms of metallic needle-like particles as well as experimental measurements of the absorption cross-sections of metallic needles of various aspect ratios over a wide wavelength range to bound theoretical calculations.

Crystallography of γ′-Fe4N formation in single-crystalline α-Fe whiskers

Gaseous nitriding of steel and iron can significantly improve their properties, for example corrosion resistance, fatigue endurance and tribological properties. In order to obtain a better understanding of the early stages of formation of the initial cubic primitive γ′-Fe4N, the mechanism and crystallography of the α–γ′ phase transformation was investigated under simplified conditions. Single-crystal α-Fe whiskers were nitrided at 823 K and a nitriding potential of 0.7 atm−1/2 for 20 min. The resulting microstructure and phases, as well as the crystallographic orientation of crystallites belonging to a particular phase, were characterized by scanning electron microscopy coupled with electron backscatter diffraction. The habit planes were investigated by single- and two-surface trace analysis. The α-Fe whiskers partly transform into γ′-Fe4N, where γ′ grows mainly in a plate-like morphology. An orientation relationship close to the rational Pitsch orientation relationship and {0.078 0.432 0.898}α and {0.391 0.367 0.844}γ′ as habit planes were predicted by the phenomenological theory of martensite crystallography (PTMC), adopting a {101}α〈101〉α shear system for lattice invariant strain, which corresponds to a {1 1 1}γ′〈1 12〉γ′ shear system in γ′. The encountered orientation relationship and the habit planes exhibit excellent agreement with predictions from the PTMC, although the transformation definitely requires diffusion. The γ′ plates mainly exhibit one single internally untwinned variant. The formation of additional variants due to strain accommodation, as well as the formation of a complex microstructure, was suppressed to a considerable extent by the fewer mechanical constraints imposed on the transforming regions within the iron whiskers as compared to the situation at the surface of bulk samples.

Nucleation and Growth of Iron Whiskers during Gaseous Reduction of Hematite Iron Ore Fines

A high-temperature confocal scanning laser microscope and an online reduction–water quenching experiment system were used to systematically study the generation of iron whiskers during the reduction of hematite ore particles with CO/CO2 gas. The "blooming" phenomenon of the surface during the reduction of iron ore particles was found in this experiment. The orientation of the grain on the longitudinal section of an iron whisker was measured to be uniform by applying the electron back-scattered diffraction technique, which proved that the iron whiskers are most likely to exist in single crystal form. According to the in-situ online experimental video, the average diffusion flux of iron atoms when the layered iron completely covers the surface of the ore particle is about 0.0072 mol/(m2·s). While the iron atom diffusion flux at the root of the iron whisker during the pre-growth process is much larger than the flux when the layered iron is produced, which are defined to be 0.081 mol/(m2·s), 0.045 mol/(m2·s), 0.013 mol/(m2· s), and 0.0046 mol/(m2·s), respectively during the four stages of the growth of an iron whisker. The quantitative relationship between the chemical driving force and the whisker growth is established as Δ G θ + R T ln p CO 2 p CO + 2 n 0.056 r ρ E s T = 0 .