The shape of DTA endothermic peak, associated with the dehydration of goethite FeOOH into hematite, depends strongly on the size of the goethite particles. With samples of low specific surface, two distinct peaks were observed. The dehydration process of a low-specific surface goethite was followed by X-ray diffraction and lowtemperature nitrogen adsorption. The results are in good agreement with the hypothesis of the formation of an impermeable hematite layer on each grain which temporarily stops the dehydration process of the internal part of the grain.Synthetic goethite is usually prepared by slow oxidation of an aqueous suspension of iron(II) hydroxide by air bubbling, either in alkaline medium, or in the presence of an excess of iron(II) sulfate in solution.By undergoing the following reactions:2 c~FeOOH ~ c~Fe20 z + H20 3 aFe20 z + Ha ~ 2 Fe304 + H20 4 FeaO4 + 02 ~ 6 ?Fe203(ca 250 ~ ) (ca 300 ~ ) (ca 250 ~ ) goethite is transformed into maghemite, ?Fe203, which is the commonest magnetic material for recording tape applications. If the transformation reactions are made Jin mild temperature conditions (not above 300~ the acicular form of the goethite particles remains unaltered in the final maghemite. The first of these reactions, the dehydration of goethite into hematite, never gives a simple endothermal DTA ]peak: in some cases the peak is unusually, broad, in other cases, two more or less distinct peaks are observed. No satisfactory explanation of this anomaly can be found in the literature. The present work is a systematic study of the influence of the particle size on the dehydration process, as followed by DTA and X-ray diffraction.
ExperimentalA series of samples of different specific surfaces have been chosen among the industrial products available, and among some goethites made in the laboratory. ,All these products were thoroughly washed with warm water, in order to eliminate most of the sulfate ions adsorbed on the surface.
Cette étude consacrée à l'oxydation d'un gel d'hydroxyde de fer(II), par aération, en milieu chlorure, et en présence de Fe++ en solution, complète nos travaux antérieurs en milieu sulfate.
Nous avons étudié l'influence de différents paramètres chimiques et physiques sur la cinétique réactionnelle. Le mécanisme d'oxydation a été étudié en portant une attention particulière à l'étape intermédiaire. l'ensemble des résultats permet de constater que la produit intermédiaire d'oxydation, le green rust I, est un composé chloruré où l'état d'oxydation du fer varie entre 2,25 et 2,55. Dans les conditions expérimentales usuelles, le produit final d'oxydation en milieu chlorure est uniquement constitué de lépidocrocite. l'apparition de la goethite pour des durées très longues provient vraisembl‐ablement d'une transformation γ FeOOH → α FeOOH thermodynamiquement possible.
Oxidation of Fe (OH)2 by Air in Chloride‐containing Medium
We studied the oxidation of an aqueous suspension of Fe(OH)2, by air, in the presence of sodium and iron(II) chloride in solution. The experimental techniques were the same as those used in a previous work on the oxidation of Fe(OH)2 in sulfate medium.
We determined the influence of some physico‐chemical factors, such as temperature, concentration, air flow rate, on the oxidation kinetics; we draw a particular attention to the nature and the composition of the intermediate oxidation product, the green rust I. It is a chloride‐containing compound, in which the oxidation state of iron is comprised between 2.25 and 2.55. In the usual experimental conditions, the final oxidation product ist lépidocrocite only. For very long oxidation times, goethite may also be formed, but this is probably due to a subsequent transformation of γ‐into α‐FeOOH, via the aqueous phase.
Ce travail est consacré à l'étude de l'influence de la température et de l'addition des ions étrangers sur la cinétique de formation de la goethite en milieu acide par aération. La température jusqu' à 70°C n'influence pas la cinétique de cette oxydation, ce qui prouve que le transfert d'oxygène se fait par la phase liquide.
Les cations manganèse et cobalt ralentissent la formation et la croissance de αFeOOH. Les anions citrate et phosphate qui augmentent la solubilité du fer(III) en solution sont défavorables à la transformation de l'hydroxyde de fer(II) en goethite.
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