Evolution of the Cathode Spot Distribution in an Axial Magnetic Field Controlled Vacuum Arc at Long Contact Gap

The distribution of cathode spots in a CuCr25 vacuum arc controlled by an axial magnetic field and ignited on the lateral surface of the cathode is investigated for long gap distances, from the processing of high-speed video images. The processing method includes also estimating the current carried by a single spot and reconstructing the distribution of the current density at the cathode. Various distributions depending partly on the arc current are described

Utilisation des plans d'expériences pour évaluer la robustesse d'une méthode d'analyse quantitative par Chromatographie en Phase Liquide

The ruggedness of an analytical method is defined by its capacity to remain unaffected (e.g. to maintain its performances) when small changes in experimental conditions happen. It can be studied by applying changes within the same order of magnitude of those which could occur when running routinely the method. In the case of a L.C. method, given the high number of factors (temperature, flow rate, mobile phase composition, detection wavelength, injected sample quantity...) susceptible to vary and the possible interactions between them, varying each factor independently would be awkward. The experimental design method enables to model the effects and interactions of a high number of factors varying simultaneously with a limited number of runs. After having estimated the order of magnitude of the potential changes in the factors level, a central composite design, allowing independent estimations of model coefficients, must be carried out around the working point defined during the method optimisation. Such a design allows the use of a second order model and gives the possibility to have an extremum inside the experimental domain. So it is possible to predict the consequences of small variations in operating conditions on the responses. A global function is proposed to account for ruggedness in terms of capacity for the method to deliver steady and reproducible assays for a product, which represents the aim of quantitative analysis. A statistical treatment of the design results reveals that the percentage of acetonitrile in the mobile phase, the column temperature and the pH of the mobile phase are the factors that have a significant influence on the method ruggedness and are critical. Hence their identification leads to recommendations in the operating protocol ; the ruggedness of the analytical method will be considered as maintained only when the tolerances defined for those parameters judged critical are respected

2-D modelling of longitudinal and radial segregations in vacuum arc remelting of titanium alloys

A model of heat and mass transfers in vacuum arc remelting process is used to simulate the remelting of TA6V ingots on an industrial scale. The model enables to calculate the shape and depth of the liquid pool, as well as the oxygen segregation caused by liquid/solid partition. Computed results are presented for different preparations of electrode

Effect of electromagnetic stirring on melt pool free surface dynamics during vacuum arc remelting

International audienc

Fluid flow and mass transfer in a vacuum induction melting furnace

In order to control the composition and the inclusion content of a vacuum induction melted maraging steel, an experimental and theoretical study was undertaken to characterize the hydrodynamic behaviour and the transport of elements dissolved in the liquid metal. The bath movement due to stirring forces was calculated with the aid of a model which then determines the transfer characteristics of a solute, due to molecular diffusion, convection and turbulence. Particular attention was given to the phenomenon of volatilization at the free surface. The results obtained using this approach are presented and discussed

Elimination of arsenic traces contained in liquid effluents by chromatographic treatment

Our present work is included in a development plan of a two-step detoxication process of arsenical by-products originated from chemical warfare destruction effluents [1]. The first method studied was classical anion exchange chromatography. Less than 1% of the anion exchanger capacity is used to fix arsenate ions for a Cl$^-$/AsO$_4^{3-}$ concentration ratio equal to 100. Because of the lack of anion exchanger selectivity, the use of more selective adsorbents has been investigated, namely a macroporous cation exchange resin (BioRad AGMP-50 resin). We confirm that the As$^{\rm III}$ and As$^{\rm V}$ retention does not depend on the nature of the resin functional groups and that only the ferric hydroxide precipitate is involved in the retention. We also show that the arsenic available exchanger capacity is not significantly affected by high concentrations in chloride ions (1 mol L$^{-1}$), and it seems that the kinetics of arsenite and arsenate ions fixation is better in presence of chlorides. The use of the exchanger for the complete detoxication of arsenic trace (10 ppm) solutions in presence of high contents in chlorides (1 to 3 mol L$^{-1}$) leads to an effluent volume equal to 280 and 540 times the resin bed volume, for AsIII and AsV respectively. The effluent thereby obtained has an arsenic concentration lower than the M.C.L. (100 ppb)