Integrating solvent extraction with the processing of advanced ceramic materials

Abstract

Many advanced technological applications require ceramic materials with specific mechanical, thermal, electrical or magnetic properties. The chemical composition and microstructure of these materials must be controlled precisely, by using appropriate processing techniques. Chemical routes generally give much better control than can be achieved using the more traditional physical and mechanical ceramics processing methods. One of the most versatile chemical processing routes is the hydrolysis of metal-organic complexes, such as alkoxides (loosely described as sol-gel methods). Under different conditions, sub-micron, monosized powders, monolithic components, aerogels, films and coatings can be prepared by hydrolyzing metal alkoxides dissolved in alcohols. However, these methods are relatively costly, and give rather low product yields. The solvent extraction systems used routinely in hydrometallurgy, particularly cation-exchangers, are somewhat similar to alkoxide solutions, and provide a technically promising, economically competitive route for synthesizing oxide ceramics. A wide range of oxide powders, including magnetic ferrites and zirconia, have been precipitated by direct hydrolysis of carboxylate solutions at elevated temperatures. This behavior is discussed in terms of thermodynamics, kinetics, the structure and composition of the metal carboxylate complexes and other factors influencing precipitate nucleation, growth and agglomeration. Rare-earth oxalate powders, which can be calcined to oxides, have been precipitated from carboxylate and D2EHPA extractants, using emulsions to control the size and morphology of the powders. Metal-loaded solvent extractants might also be used to prepare oxide thin-films, or to coat whiskers with thick layers of oxides to facilitate the fabrication of composite materials. In some cases these materials preparation routes could be integrated with primary solvent extraction circuits. For other applications, the initial solvent extraction stage could be used to prepare high-purity materials from cheaper feedstocks.