D. Inman

The Production of Ti–Mo Alloys from Mixed Oxide Precursors via the FFC Cambridge Process

Ti-10 wt. % W alloys were produced via the electrochemical deoxidation of mixed TiO2+WO3 sintered precursors in a molten CaCl2 electrolyte at 1173 K. The reduction of these ceramic precursors was characterised by analysing several partially reduced samples taken periodically through the deoxidation process. Fully metallic samples were retrieved after 15 h of reduction. This reduction time was longer than that observed by the authors for metallisation of (Ti,Mo)O2 sintered precursors. This was believed to occur as a result of significant differences in the reduction pathway, despite tungsten and molybdenum possessing similar interactions with titanium (group VI elements). It was found that reduction initiated with the rapid reduction of WO3 to a W-Ti particulate. TiO2 then proceeded to reduce sequentially through the lower oxides, with the formation of Ca(Ti,W)O3. Between 1 h and 3 h of reduction the sample is believed to be composed of Ca(Ti,W)O3 and TiO. A comproportion reaction between the two phases is then observed with the formation of W-Ti and CaTi2O4, which then proceeds to reduce to titanium. However homogenisation between the product titanium and W-Ti does not take place until the titanium is sufficiently deoxidised, thus β Ti forms late in the reduction process. It is believed that the lack of formation of β Ti early on in the reduction process, coupled with the lack of formation of a conductive (Ti,Mo)O network, accounts for the relatively slow reduction time.

Production of NiTi via the FFC Cambridge Process

The FFC Cambridge process is a direct electrodeoxidation process used to reduce metal oxides to their constituent metals in a molten CaCl2 salt bath. NiTiO3 was used as a precursor (the first stable oxide to form upon blending and sintering NiO and TiO2 powders) and was successfully reduced using the FFC Cambridge process at 1173 K and a constant cell voltage of -3.1 V to produce a NiTi alloy. This work builds on the literature work [Chinese Science Bulletin, 51, 2535 (2006)] through: (i) a predominance diagram calculated to show the regions of phase stability throughout the usable potential window of the CaCl2 salt; (ii) the investigation of a wide range of reduction times for a fixed cell voltage, elucidating several additional stable phases, to yield a complete and detailed reduction pathway. The reduction pathway for NiTiO3 was identified through the analysis of a series of partial reductions, with fully reduced NiTi formed after a period of 24 h. The first stage of the reaction involved the rapid formation of Ni and CaTiO3. The reduction then proceeded via the formation of the intermediate compounds Ni3Ti and Ni2Ti4O. All the NiTiO3 and Ni were consumed after a period of 6 h, while the intermediate compounds remained until the reaction was near completion. The experimental results related well to the thermodynamic predictions of the predominance diagram. A small variation in stoichiometry of the produced NiTi observed from the edge to the core of the samples was attributed to redeposition of Ti on the sample surface from the salt and a slightly Ti-rich NiTiO3 precursor material. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2987739] All rights reserved.

Characterization of the FFC Cambridge Process for NiTi Production Using In Situ X-Ray Synchrotron Diffraction

To date, the characterization of the reduction pathway for the Fray Farthing Chen (FFC) Cambridge process has been achieved through ex situ studies, leading to some ambiguities. This study employs a synchrotron X-ray diffraction technique to monitor in situ the FFC reduction of NiTiO3 to NiTi, yielding an unmatched level of detail on the electrochemical and chemical reactions involved. The reduction pathway consists of rapid initial reduction of NiTiO3 to form CaTiO3 and Ni, then the transformation of Ni to Ni3Ti, and finally the consumption of CaTiO3 and Ni3Ti to produce NiTi. The phases observed agree with thermodynamic predictions [J. Electrochem. Soc., 155, E171 (2008)] and allow the mapping of the reduction pathway on an electrochemical predominance diagram. Ni3Ti is a short-lived transient phase in the reduction pathway. Ni3Ti was found in significant quantities in ex situ studies [J. Electrochem. Soc., 155, E171 (2008); Chin. Sci. Bull., 51, 2535 (2006)]. The authors propose that Ni3Ti forms during furnace cooling to ambient temperature if extracted before a complete reduction. Neither Ni2Ti4O nor CaO was observed. The progression of the reduction front is clearly in line with existing observations and models [Metall. Mater. Trans., B, Process Metall. Mater. Proc. Sci., 35, 223 (2004); J. Phys. Chem. B, 109, 14043 (2005)].

The electroreduction of molten phosphates

Summary The electroreduction of phosphates has been examined by voltammetry andchronopotentiometry in molten sodium trimetaphosphates and as solutions in sodium chloride-potassium chloride over the temperature range 400°–800°C. Three electroactive species, reducing at potentials more anodic than the solvent, were found by chronopotentiometry; these were identified as the ortho, pyro and tripoly-phosphate ions. The electroreduction appears to be that of pentavalent to trivalent phosphorus; the overall products probably result from further chemical reactions.

Voltammetric studies of iron in molten MgCl2+NaCl+KCl: Part I. The reduction of Fe(II)

Abstract The electrochemical reduction of Fe2+ ions in the MgCl2+NaCl+KCl eutectic at temperatures between 485 and 550°C has been successfully investigated using the techniques of cyclic voltammetry and chronoamperometry. The reduction was diffusion controlled and involved a single two-electron transfer process at tungsten and vitreous carbon electrodes as well as at platinum electrodes where Fe/Pt alloy formation occurred. Chronoamperometric studies suggested that nucleation and crystal growth phenomena were important.

The anodic dissolution of chalcopyrite in water + acetonitrile solutions

Abstract The characteristics of chalcopyrite anodic electrodissolution in aqueous acetonitrile solutions have been studied. Experiments with a high-grade sample, of massive chalcopyrite have been carried out at 25°C in solutions containing 50 g l −1 NaCl and different concentrations of sulphuric acid and acetonitrile. Galvanostatic and potentiostatic conditions were employed and the results obtained in solutions with and without acetonitrile compared. The characteristics of the mechanism involved in the chalcopyrite electrodissolution precluded acetonitrile having an influence at low potentials. However its influence is more noticeable at higher potentials where solvent decomposition reactions are involved. Chronopotentiometric experiments in acetonitrile aqueous solutions indicate the existence of diffusional (in the solid) and coulombic phenomena in originating the potential jump.

Optimization of the FFC Cambridge Process for NiTi Production

The FFC Cambridge process is ideal for NiTi production, as it avoids the Ni segregation associated with pyrometallurgical routes to manufacture NiTi, and is able to produce a homogenous low-oxygen product. The process involves the direct electrodeoxidation of NiTiO3 (first stable oxide to form upon mixing and sintering of NiO and TiO2) to NiTi in a molten CaCl2 salt bath. This work builds on previous literature that has elucidated the reduction pathway by investigating the effect of reduction temperature and current collector material on the microstructure of the final reduction product. It was found that the use of a Ni current collector caused Ni-enrichment at the surface of the product, stabilizing the high temperature B2 cubic form to room temperature. Ni-rich phases Ni4Ti3 and Ni3Ti were observed after 24h reductions. The former is favoured at higher reduction temperatures, and the latter at lower temperatures. Mechanisms for formation are proposed.