An electrochemical investigation of the formation of CoSx and its effect on the anodic dissolution of iron in ammoniacal-carbonate solutions

It has been found that the co-presence of cobalt (II) and thiosulphate ions in ammoniacal-carbonate solutions promotes the passivation of iron, under conditions in which it would otherwise continue to dissolve anodically. Electrochemical experiments have shown a relationship between the immersion time required for passivation and the formation of a solid species on the iron surface, which is thought to be implicated in the mechanism of passivation, whilst not being itself the protective species. Based on a combination of electrochemical, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and grazing incidence X-ray diffraction (GIXRD) characterisation techniques, the said species has been identified as CoSx, resulting from the interaction of cobalt (II) and thiosulphate ions. It is thought to form as a product of the cathodic reactions taking place on the iron surface during its active dissolution. These findings are particularly relevant to the Caron process, in which the ammoniacal-carbonate solutions containing dissolved cobalt and thiosulphate ions are used to leach nickel and cobalt from pre-reduced laterite ores rich in metallic iron. Both the loss of cobalt into the CoSx layer and the passivation of iron and of its alloys with nickel and cobalt, are potential contributing factors to the low cobalt and nickel recoveries, which are typical of the Caron process. This study provides a better understanding of the conditions under which the CoSx layer forms and promotes the passivation of iron, and may therefore provide useful information to help minimise the effect this may have on the extraction efficiency of the process. In particular, at the cobalt and thiosulphate ion concentrations usually encountered at a Caron plant, the passivation of iron was found to be prevented by maintaining a high enough concentration of ammonia

Ultrastructural changes of collagen and elastin in human gingiva during orthodontic tooth movement

After 15 days of mesializing or distalizing orthodontic treatment, 10 permanent premolars of young patients were extracted with the interdental gingiva. The connective tissues of the compressed or stretched interdental papillae were compared to that of untreated samples by light and transmission electron microscope.Large collagen fibres bundles represented by fibrils with a banding pattern of 64 nm and a mean diameter of 75 nm were observed in compressed interdental gingiva. Several elastic fibres with a mean diameter of 950 nm were also present. In some central areas of compressed gingiva collagen fibrils longitudinally split into widely spaced microfibrils were often observed in proximity to the elastic fibres.In stretched and untreated interdental papillae the collagen fibrils presented a mean diameter of 66 nm and 57 nm respectively. In both groups, few elastic fibres ranging in diameter 600 nm were seen. The increased size of the gingival collagen fibrils undergoing pressure and tension is indicative of remodelling of the fibrous collagen system.The fair increase in number and size of elastic fibres in compressed gingiva suggests that the elastic fibre system takes over the place whenever a collapse of the collagenous framework occurs.Après 15 jours de traitement orthodontique par mésialisation ou distalisation, 10 prémolaires permanentes ont été extraites chez de jeunes patients, ainsi que la gencive interdentaire adjacente. Le tissu conjonctif des papilles interdentaires comprimées ou étirées a été comparé avec celui d’échantillons de dents n’ayant pas subi de traitement. L’observation a été faite en microscopie classique et en microscopie électronique à transmission. Dans la gencive interdentaire comprimée, on observe de longs faisceaux de fibres de collagène faits de fibrilles qui présentent une périodicité de 64 nm et un diamètre inférieur à 75 nm. Ont été observées également de fibres élastiques d’une diamètre inférieur à 950 nm. Dans certaines zones centrales de gencive comprimée, on peut voir, à proximité dess fibres élastiques, des fibrilles de collagène réparties longitudinalement en microfibrilles très espacées.Dans la gencive interdentaire étirée et dans celle non soumise au traitement orthodontique, les fibrilles de collagène présentent un diamètre de 66 nm et 57 nm respectivement. Dans ces deux groupes, on note un très petit nombre de fibres élastiques de 600 nm de diamètre. L’augmentation du nombre et de la taille des fibres élastiques dans la gencive comprimée indique que le système de fibres élastiques remplace la trame collagène qui a subi un collapsus

The passivation of iron in ammoniacal solutions containing copper (II) ions

In the present study it was found that the presence of millimolar amounts of copper (II) ions in ammoniacal solutions leads to the spontaneous formation of a stable passive layer on metallic iron and iron alloys with nickel and cobalt, under conditions in which they would otherwise remain in active dissolution. This finding may be of significance to industrial processes which employ ammoniacal solutions to leach metal values from materials rich in metallic iron, such as, for example, laterite ores which have undergone a reductive pre-treatment. The spontaneous passivation is found to take place regardless of the presence or practical absence of dissolved oxygen, and occurs more readily the higher the copper concentration and the lower the ammonia-ammonium bicarbonate concentration, though it does not take place in ammonia-free aqueous solutions of similar copper (II) ion concentrations. Electrochemical and scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM/EDX) investigations conducted as part of this study showed that the observed passivation is promoted by the cementation of copper onto the actively dissolving iron surface, and subsequently by its re-dissolution. Such process is thought to create favourable conditions which promote the formation of a stable passive layer on the iron surface

Heterogeneous electrochemical reactions taking place on metallic iron in ammoniacal-carbonate solutions containing dissolved nickel, cobalt, copper and thiosulfate ions

In the Caron process, nickeliferous laterite ores undergo a pyrometallurgical pre-treatment step, in which their nickel and cobalt content is reduced to the metallic state. This is followed by leaching in ammoniacal-carbonate solutions, where metallic nickel and cobalt dissolve by forming complexes with ammonia. Since the reductive pre-treatment step also results in some metallic iron being formed, nickel and cobalt are present in the pre-reduced ore mainly as ferro-alloys. As a result, the dissolution behaviour of metallic nickel and cobalt is strongly influenced by the behaviour of metallic iron. In particular, the passivation of iron has been identified as a potential factor contributing to the relatively low metal value recoveries suffered by the process (80-82% nickel and 50-60% cobalt). The present study consists of an investigation into various heterogeneous reactions involving metallic iron and ions commonly found in Caron leach liquors, namely nickel(II), cobalt(II), copper(II) and thiosulfate, in order to identify interactions which may adversely affect the extraction efficiency. The study was carried out using a combination of electrochemical and surface characterisation techniques, as well as thermodynamic calculations. Metallic iron was found to generally dissolve in ammoniacal-carbonate solutions by forming ferrous ammine complexes, but a few specific conditions were found to promote its spontaneous passivation: the presence of copper(II) ions and the presence of both cobalt(II) and thiosulfate ions. In the presence of copper(II) ions, the passivation process was promoted by the cementation and subsequent redissolution of metallic copper, as confirmed by surface characterisation studies. However, when thiosulfate ions were also present, passivation was not observed and the formation of a partially adherent layer containing metallic copper dendrites and cuprous sulfide took place on the iron surface. In the presence of both cobalt(II) and thiosulfate ions, the passivation process, which was not observed when either species was present on its own, was promoted by the formation of an amorphous cobalt sulfide or polysulfide species. The loss of cobalt from the leach solution into this layer was identified as another potential factor contributing to the poor cobalt extractions suffered by the process

The anodic dissolution of iron in ammoniacal–carbonate–thiosulfate–copper solutions with formation of Cu2S and dendritic copper

The presence of thiosulfate ions in ammoniacal-carbonate solutions containing copper(II) ions was found to prevent the passivation of iron, even though iron passivation is observed in solutions with no thiosulfate at very low copper(II) concentrations. The prolonged anodic dissolution of iron resulted in the formation of a partly crystalline sulfide layer on its surface, which based on Grazing Incidence X-Ray Diffraction (GI-XRD) analysis is thought to consist mainly of Cu2S. The effect of the sulfide layer was investigated using rotating disk cyclic voltammetry. Unlike the formation of an amorphous CoSx layer, which took place in similar solutions containing cobalt ions, the cuprous sulfide layer was not found to promote passivation of the iron. A significant amount of solid precipitate also became detached from the iron surface, remaining undissolved in the solution. This was analysed by X-Ray Diffraction (XRD) and scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDX). Dendritic copper was observed both in the solid separated from the solution and in the precipitate still attached to the iron surface. The absence of iron passivation is thought to be due to both to the non-adherent nature of the cuprous sulfide layer, and to a disrupting effect caused by the cementation of copper

"Reform of the CAP in a changing Europe"

[From the Introduction]. The Common Agricultural Policy (CAP) has received much negative publicity as the most expensive, complex and problematic of the European Economic Community's policies. Suggestions for its reform, however, have met with limited success, owing either to entrenched political factors or to the intellectual inadequacy of the ideas themselves. A short-sighted perspective of European agriculture has inevitably prevailed

Evolution and Trends of the European Common Agricultural Policy

Agricultural and Food Policy, International Relations/Trade,