Hydrothermal processing

Process name Temperature range, °C Pressure range, MPa Conditions Apphcation Commercial use 

A commercial process which uses hydrothermal leaching on a large scale is the Bayer process for production of aluminum oxide (see Aluminum compounds). This process is used to extract and precipitate high grade alurninum hydroxide (gibbsite [14762-49-3]) from bauxite [1318-16-7] ore. The hydrothermal process step is the extraction step in which concentrated sodium hydroxide is used to form a soluble sodium aluminate complex  

Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

Nickel and cobalt are recovered by processes that employ both pressure leaching and precipitation steps. The raw materials for these processes can be sulfide concentrates, matte, arsenide concentrates, and precipitated sulfides. Typically, acidic conditions are used for leaching however, ammonia is also effective in leach solutions because of the tendency for soluble cobalt and nickel ammines to form under the leach conditions. 

Most hydrothermal synthesis processes are carried out at moderate (in the range of 100 to 300°C) temperatures and at the corresponding solution 

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Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

Greater dimensional control and thinner tapes in multilayer ceramics are the driving forces for techniques to prepare finer particles. Metal organic decomposition and hydrothermal processing are two synthesis methods that have the potential to produce submicrometer powders having low levels of agglomeration to meet the demand for more precise tape fabrication. 

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