Prédiction de l'évolution granulométrique et morphologique d'une poudre dans un four tournant

National audienceThe dry conversion process for making UO2 nuclear fuel pellets consists of two steps: hydrolysis of UF6 to UO2F2, followed by reducing pyrohydrolysis to UO2 in a rotary kiln. The physical characteristics (morphology, particle size distribution) of the powder obtained at the kiln end determine the final properties (sinterability, flow-ability, green strength). We developed a mathematical model describing the morphological evolution of the powder in the rotary kiln, which enables us to predict morphological characteristics of UO2 powder as functions of the processing conditions. Firstly, the powder flow in the kiln was modelled, including the exchanges between a dense phase (powder bed) and an airborne phase (particles showering down). An original feature of this model is to consider the effect of lifters for calculating the dynamic variables. Secondly, the phenomena responsible for the changes in morphology and grain size were identified and modelled. A population of fractal agglomerates was considered, whose number and size change due to Brownian and sedimentation agglomeration, sintering, breaking up, and chemical reactions. The model is based on population balances and the particle size distribution is divided into sections. Results of both dynamic and morphological calculations are compared to available measurements. Lastly, the influence of the different mechanisms of morphological evolution on the final size distribution is analysed

Thermodynamic study of heavy metals behavior during municipal waste incineration

The incineration of municipal solid waste (MSW) contributes significantly to the presence of heavy metals in urban area aerosols. It is thus important to ascertain the quantities and chemical forms of the heavy metals (HM) that are emitted from the incineration plant stacks. The behaviour of HM, which depends strongly on the thermal and chemical environments, was investigated herein with a modelling approach, consisting of several parts. First, a refuse bed combustion model was developed for simulating on-grate MSW incineration. It describes most of the physico-chemical and thermal phenomena occurring during waste combustion. Second, results from the bed model were taken as boundary conditions to perform 3D simulations of the post-combustion zone and of the boiler. The case studied was of the Strasbourg incineration plant. Finally, the local thermal conditions and the local elementary compositions of gas and solid phases obtained from these simulations were used to carry out thermodynamic calculations of the speciation of HM at each point in the incinerator. The results for four metals (Cd, Zn, Pb, Cr) are presented, discussed and compared to available data. Predicted species are in agreement with observations for volatile metals, except lead, whose volatilization seems overestimated

Transport phenomena and modelling in melting and refining processes

Increasing requirements for ultra pure metallic materials with greater reliability and more reproducible mechanical properties have led over the years to a continual endeavour to improve the control of composition and inclusion contents (nitrides, oxides, carbides, etc.) in cast products. This is illustrated by melting and refining processes such as Vacuum Induction Melting (VIM), Vacuum Arc Remelting (VAR), Electro Slag Remelting (ESR), Electron Beam Melting (EBM) and Electron Beam Cold Hearth Remelting (EBCHR). The materials involved are nickel-base superalloys, specialty steels, refractory metals such as tantalum, niobium, tungsten and molybdenum, and the reactive metals hafnium, zirconium and titanium, together with their alloys. The mastery of these refining processes requires a perfect knowledge of the physical-chemical phenomena involved, including both thermodynamic aspects and transport mechanisms within the liquid metal and at the interfaces (particularly that between the melt and the vacuum or gaseous phase), together with the behaviour of inclusions. Examples are described to show how this knowledge of the basic phenomena can be combined with process modelling to improve the control of product quality

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

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