Inhibition of domain wall formation and its effects on the bulk magnetic properties of certain spinels

It is shown that hysteresis and susceptibility studies at various temperatures could provide an idea whether a magnetic sample contains multidomain, single-domain or superparamagnetic particles. Our results on titanomagnetites with those of others on cobalt substituted magnesium ferrites suggest that domain wall formation is inhibited in some of them whereby only single-domain or superparamagnetic particles occur irrespective of the physical grain size of such materials. At high concentration of titanium in titanomagnetites, the magnetic behaviour is similar to a spin glass, which we interpret as a transition of optimum single-domains going over to superparamagnetic state.     

Raman spectroscopy of laser‐induced oxidation of titanomagnetites

Titanomagnetites are important carriers of magnetic remanence in nature and can track redox conditions in magma. The titanium concentration in magnetite bears heavily on its magnetic properties, such as saturation moment and Curie temperature. On land and in the deep ocean, however, these minerals are prone to alteration which can mask the primary magnetic signals they once recorded. Thus, it is essential to characterize the cation composition and oxidation state of titanomagnetites that record the paleomagnetic field. Raman spectroscopy provides a unique tool for both purposes. Nonetheless, the heat generated by the excitation laser can itself induce oxidation. We show that the laser power threshold to produce oxidation decreases with increasing titanium content. With confocal Raman spectroscopy and magnetic force microscopy (MFM) on natural and synthetic titanomagnetites, a non‐destructive Raman imaging protocol was established. We applied this protocol to map out the composition and magnetization state within a single ex‐solved titanomagnetite grain in a deep‐sea basalt. Copyright © 2011 John Wiley & Sons, Ltd.     

钒钛磁铁精矿直接还原过程中金属铁颗粒长大特性

通过钒钛磁铁矿精矿直接还原实验,研究了不同还原剂和添加剂对还原过程金属铁颗粒长大的影响.提高还原温度能促进还原产物中金属铁颗粒的长大,金属铁颗粒中V含量也显著增加.与用无烟煤和褐煤还原产物相比,用烟煤还原产物中金属铁颗粒明显长大,这是由烟煤中高灰分含量所引起的.金属铁颗粒长大机理的研究表明:Na2CO3和Na2SiO3的熔点较低,且能破坏铁橄榄石和铁尖晶石的结构,并生成一些低熔点物质,而SiO2能与铁橄榄石形成低共熔混合物.这些低熔点物质都有助于改善金属铁相的扩散,从而促进金属铁颗粒长大.     

Phase transformation behavior of titanium during carbothermic reduction of titanomagnetite ironsand

The reduction of titanomagnetite (TTM) ironsand, which contains 11.41wt% TiO₂ and 55.63wt% total Fe, by graphite was performed using a thermogravimetric analysis system under an argon gas atmosphere at 1423–1623 K. The behavior and effects of titanium in TTM ironsand during the reduction process were investigated by means of thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. During the reduction procedure, the titanium concentrated in the slag phase, where the phase transformation followed this sequence: FeO + FeTiO₃ → Fe₂TiO₄ → FeTiO₃ → FeTi₂O₅ → TiO₂. The calculated results for the reduction kinetics showed that the carbothermic reduction was controlled by the diffusion of ions through the product layer. Furthermore, the apparent activation energy was 170.35 kJ·mol-1.     

Leaching of vanadium,sodium, and silicon from molten V-Ti-bearing slag obtained from low-grade vanadium-bearing titanomagnetite

The water leaching process of vanadium, sodium, and silicon from molten vanadium-titanium-bearing (V-Ti-bearing) slag obtained from low-grade vanadium-bearing titanomagnetite was investigated systematically. The results show that calcium titanate, sodium aluminosilicate, sodium oxide, silicon dioxide and sodium vanadate are the major components of the molten V-Ti-bearing slag. The experimental results indicate that the liquid-solid (L/S) mass ratio significantly affects the leaching process because of the respective solubilities and diffusion rates of the components. A total of 83.8% of vanadium, 72.8% of sodium, and 16.1% of silicon can be leached out via a triple counter-current leaching process under the optimal conditions of a particle size below 0.074 mm, a temperature of 90°C, a leaching time of 20 min, an L/S mass ratio of 4:1, and a stirring speed of 300 r/min. The kinetics of vanadium leaching is well described by an internal diffusion-controlled model and the apparent activation energy is 11.1 kJ/mol. The leaching mechanism of vanadium was also analyzed.     

Sulfonic acid–functionalized magnetic Fe3-xTixO4 nanoparticles: New recyclable heterogeneous catalyst for one-pot synthesis of tetrahydrobenzo[b]pyrans and dihydropyrano[2,3-c]pyrazole derivatives

In this work, titanomagnetite nanoparticles (Fe_3-xTi_xO₄) have been used as a novel suppport for the synthesis of a magnetic acidic catalyst. These nanoparticles were functionalized with sulfonic acid groups to prepare the Fe_3-xTi_xO₄@SO₃H nanoparticles. The synthesized acidic nanoparticles have been explored as new and efficient recyclable heterogeneous catalyst for a one-pot, three-component synthesis of tetrahydrobenzo[b]pyrans known as 4H-chromenes and 1,4-dihydropyrano[2,3-c]pyrazoles. The structure of the catalyst was established by infrared, energy dispersive x-ray (EDX), and scanning electron microscopy analyses. The reactions proceed smoothly to furnish the respective products in excellent yields and short reaction times. The facile reaction conditions, easy isolation of the products, versatility, and easy magnetic separation and reusability of the catalyst with no significant loss of activity are the main merits of the present method.     

Effects and mechanisms of fluorite on the co-reduction of blast furnace dust and seaside titanomagnetite

The co-reduction roasting and grinding magne -tic separation of seaside titanomagnetite and blast furnace dust was investigated with and without fluorite addition at a reduction roasting temperature of 1250℃ for 60 min, a grinding fineness of -43 μm accounting for 69.02wt% of the total, and a low-intensity magnetic field strength of 151 kA/m. The mineral composition, microstructure, and state of the roasted products were analyzed, and the concentrations of CO and CO₂ were analyzed in the co-reduction roasting. Better results were achieved with a small fluorite dosage (≤ 4wt%) in the process of co-reduction. In addition, F- was found to reduce the melting point and viscosity of the slag phase because of the high content of aluminate and silicate minerals in the blast furnace dust. The low moisture content of the blast furnace dust and calcic minerals inhibited the hydrolysis of CaF₂ and the loss of F-. Compared with the blast furnace dust from Chengdeng, the blast furnace dusts from Jiugang and Jinxin inhibited the diffusion of F- when used as reducing agents, leading to weaker effects of fluorite.     

Comparison of reduction disintegration characteristics of TiO2-rich burdens prepared with sintering process and composite agglomeration process

To reveal the impact of the composite agglomeration process (CAP) on the reduction disintegration properties of TiO2-rich iron-making burden for a blast furnace, the reduction disintegration indices (RDIs), mineral constituents, and microstructure of the products pre-pared by the CAP and the traditional sintering process (TSP) were investigated. The results showed that, compared to the sinter with a ba-sicity of 2.0 prepared by the TSP, the RDI+6.3 and the RDI+3.15 of the CAP product with the same basicity increased by 28.2wt%and 13.7wt%, respectively, whereas the RDI?0.5 decreased by 2.7wt%. The analysis of the mineral constituents and microstructure of the products indicated that the decreasing titanohematite content decreased the volume expansion during reduction. Meanwhile, the decreasing perovskite content decreased its detrimental effect on the reduction disintegration properties. In addition, the higher silicoferrite of calcium and aluminum (SFCA) content improved the strength of the CAP product. Together, these factors result in an improvement of the RDI of the CAP products. In addition, compared to the sinter, the reduced CAP products clearly contained fewer cracks, which also led to mitigation of reduction disintegration.     

A novel process for the recovery of iron,titanium, and vanadium from vanadium-bearing titanomagnetite:sodium modification-direct reduction coupled process

A sodium modification-direct reduction coupled process was proposed for the simultaneous extraction of V and Fe from vanadium-bearing titanomagnetite. The sodium oxidation of vanadium oxides to water-soluble sodium vanadate and the transformation of iron oxides to metallic iron were accomplished in a single-step high-temperature process. The increase in roasting temperature favors the reduction of iron oxides but disfavors the oxidation of vanadium oxides. The recoveries of vanadium, iron, and titanium reached 84.52%, 89.37%, and 95.59%, respectively. Moreover, the acid decomposition efficiency of titanium slag reached 96.45%. Compared with traditional processes, the novel process provides several advantages, including a shorter flow, a lower energy consumption, and a higher utilization efficiency of vanadium-bearing titanomagnetite resources.