Li Zimin, Chanéac Corinne, Berger Gilles, Delaunay Sophie, Graff Anaïs, Lefèvre Grégory
Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris, PSL Research University France
Department of Materials and Mechanics of Components, EDF R&D France.
RSC Adv. 2019 Oct 18;9(58):33633-33642. doi: 10.1039/c9ra03234g.
The stability of magnetite under oxidizing hydrothermal conditions was evaluated at temperatures of 120, 150, 180 and 275 °C. A well-characterized sample of commercially-available magnetite with a particle size of approximately 690 nm was oxidized by dissolved oxygen (DO) under alkaline hydrothermal conditions in titanium autoclaves. In these trials, the DO was always in equilibrium with the gas phase oxygen that was air-derived and was located above the hydrothermal solution, which contained ammonium hydroxide at a pH of approximately 9.5. Samples recovered by filtration were analysed by X-ray diffraction and scanning electron microscopy, while Fe(ii)/Fe ratios were determined by titration in conjunction with spectrophotometry. Oxidation between 120 and 180 °C was found to generate high concentrations of maghemite and hematite in the product, with the latter compound having either a hexagonal bipyramidal or rhombohedral morphology. The oxidation kinetics was consistent with a diffusion controlled process. The reaction probably proceeded the outward diffusion of ferrous ions from the magnetite, forming a magnetite/maghemite core/shell structure in conjunction with the dissolution of maghemite and reprecipitation of hematite. Oxidation at 275 °C presented different characteristics from those observed at the lower temperatures. Negligible amounts of maghemite were found, and the primary oxidation product was hematite with no specific morphologies. Moreover, the kinetics was slower than at 180 °C. This unexpected temperature effect is attributed to the rapid growth, at 275 °C, of a dense layer of hematite on the surface of the magnetite that impeded the oxidation of magnetite.
在120、150、180和275℃的温度下评估了磁铁矿在氧化水热条件下的稳定性。在钛高压釜中，在碱性水热条件下，用溶解氧（DO）对粒径约为690nm的市售磁铁矿的一个特征明确的样品进行氧化。在这些试验中，DO始终与源自空气且位于水热溶液上方的气相氧处于平衡状态，水热溶液中含有pH约为9.5的氢氧化铵。通过过滤回收的样品用X射线衍射和扫描电子显微镜进行分析，而Fe（ii）/Fe比率通过滴定结合分光光度法测定。发现在120至180℃之间的氧化会在产物中产生高浓度的磁赤铁矿和赤铁矿，后一种化合物具有六方双锥体或菱面体形态。氧化动力学与扩散控制过程一致。反应可能是亚铁离子从磁铁矿向外扩散，形成磁铁矿/磁赤铁矿核/壳结构，同时磁赤铁矿溶解和赤铁矿再沉淀。275℃下的氧化呈现出与较低温度下观察到的不同特征。发现磁赤铁矿的量可以忽略不计，主要氧化产物是没有特定形态的赤铁矿。此外，动力学比180℃时慢。这种意外的温度效应归因于在275℃时磁铁矿表面迅速生长出一层致密的赤铁矿层，这阻碍了磁铁矿的氧化。
