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The in situ speciation of metals in hydrothermal fluids was studied using UV/VIS and NMR spectroscopies. The measurment of pH of aqueous solutions was achieved using NMR spectroscopy, and a zirconia-based sensor was developed for directly measuring the pH of hydrothermal fluids up to 200 C and 200 bar. Aspects of the Partial Charge Model were extended to hydrothermal conditions, to provide a theoretical framework against which the spectroscopic data can be tested.

Zirconia (both monoclinic and stabilized), aluminosilicates (mullite) and Al and Fe hydroxides were synthesized hydrothermally under a variety of conditions. New synthesis flow-sheets were established. The morphological and structural properties of the zirconia powders were determined as a function of the synthesis conditions and chemistry of the hydrothermal medium. In order to up-scale the processes, the synthesis of batches in the 100 gr range of zirconia powders was carried out.

The alkaline dissolution of bauxite was studied experimentally using simple analogue system. Solubility date on diaspore and goethite were obtained under the temperature and chemical conditions of the Bayer process. The data allow to quantitatively model the behaviour of Fe and Al in Bayer-like solutions at equilibrium with boehmite and Al-bearing goethite. Laboratory simulations of the addition of additives during bauxite digestion have been performed.

Hydrothermally synthesized powders were characterized for their textural and mechanical properties. Powders were pressed, both uniaxially and isostatically, then sintered, and their properties determined. Various applications of the hydrothermal zirconia powders, including plasma spraying of zirconia coatings, have been carried out.

Applications mainly concern the ceramic industry (production of ceramic powders, improvements of the Bayer Process). There are also less important applications for other industrial sectors, such as nuclear industry, geothermal energy and waste management.
Hydrothermal synthesis is a direct route to high purity monodispersed oxide powders. Compared to other processing techniques, hydrothermal synthesis is of low cost, allows control of powder morphology and granulometry and produces powders with definite mechanical properties and behaviour during sintering. This technique is also general for simple and complex oxides. For raw oxides, the Bayer process of alumina production (1.2 106 tons/year in EEC countries) belongs to this group of techniques. For advanced oxides, the high quality of oxide powders formed by hydrothermal synthesis makes these of great interest for powder processing and sintering, isostatic pressing and plasma-spray coatings.

In order to improve existing processes (e.g. Bayer) and to provide the scientific background for a future development of hydrothermal synthesis, processes of nucleation and growth in aqueous media at high temperature and pressures must be studied. One critical aspect is speciation in the hydrothermal fluid, which controls the structure of the dissolved metal complex that may act as a precursor for the oxide phase. Speciation will be studied by using optical cells for vibrational spectroscopic and NMR studies at high temperatures and pressures. Results will be cross-checked with predictions from the Partial Charge Model. From this step 1, synthesis of oxides having specific structure, morphology, granulometry and mechanical properties will be carried out (step 2). The powders will be characterized and tested for sintering and thermal spraying (step 4). The isostatic pressing of these powders will be studied for their commercial use (step 5). In parallel, steps 1 and 2 will serve as inputs for step 3, aimed to model the alkaline dissolution of bauxite in the Bayer process. Step 6 will consist of the application of the results of step 3 at the scale of the production plant.

This project gathers 3 academic institutions from France and Germany, 1 german research centre, one french industrial and a german SME. It is characterized by 3 levels of participation, i.e. fundamental research (3 partners), applied research (2 partners) and production (2 partners). The join participation of industrial organizations of different countries (France, Germany), sizes (Industry, SME) and strategies (respectively major producer of raw ceramic oxides and manufacturer of advanced ceramic components), may contribute to a better industrial collaboration inside EEC countries and also between industrial "suppliers" and "users" of ceramic materials.

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Centre National de la Recherche Scientifique (CNRS)
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1a rue de la Ferollerie
45071 Orléans

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