Drying of granular ceramics

Moist granular ceramics produced by using the common forming methods, such as tape casting and extrusion, should be dried before the binder burnout and sintering. Drying is also a key step in solid firee-form fabrication techniques that involve layer-by-layer deposition of colloidal suspensions. 

The moisture content of the green bodies in casting or extrusion is usually in the range of 20-35 %. 

During the CRP, the rate of evaporation is independent on the content of moisture. As evaporation starts, a dry surface region is formed, so that liquid tends to cover the dry region. As a result, tension is developed in the liquid, which could be balanced by the formation of compressive stresses in the solid phase. Because of the compressive stresses, the body tends to shrink, while the liquid meniscus remains at the surface. As drying proceeds, the particles form a denser packing, so that the body becomes stiffer. The liquid meniscus at the surface deepens and the tension in the liquid is increased. Eventually, the particles are surrounded by a thin layer of water and then the shrinkage stops. The moisture content at this point is also known as the leatherhard moisture content. 

As the shrinkage stops, further evaporation pushes the meniscus into the body and the rate of evaporation starts to decrease, which corresponds to the so-called critical moisture content. It is found that the critical moisture content and the leatherhard moisture content are nearly the same. In the FRPl, the evaporation of liquid from the surface continues. There are pathways for the liquid to flow to the surface. Eventually, the liquid near the surface of the body is isolated into pockets. As the liquid stops flowing to the surface, it is removed by the diffusion of the vapor, so that the drying process enters the FRP2. 

If the evaporation rate is very high, the tension in the liquid can reach its maximum value. In this case, the total stress at the surface of the body is given by  

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R. Chui and M. Cima. Drying of Granular Ceramic Films 2 Drying Stress and Saturation Uniformity. Submitted to J. Am. Ceramic Soc. 

The experimental method reviewed here is focused on the compaction of granular ceramic powders it has application in the characterization of ceramic granules as well as dry-forming operations. A later section discusses the extension of this technique to other types of granular solids (e.g., agglomerates made of pharmaceutical excipient materials). Other methods of compaction analysis are also discussed, and an empirical relationship between the compaction curve and the Kawakita analysis is proposed. 

Flash dryers are suitable for drying granular, powdery, pasty, and crystalline products. As mentioned earlier, the residence time of the particles in the dryer is very short that leads to the fact that only products containing surface moisture can be dried in flash dryers. Drying of heat-sensitive products in this type of dryers is very useful. Flash dryers are successfully used in the chemical, food, pharmaceutical, mining, ceramic, and wood industries. 

Fluidized bed dryers have found widespread applications for drying of particulate or granular materials in the chemical, food, ceramic, pharmaceutical, polymer, and similar industries. More recently, they also found special applications in drying of suspensions, sludges, and soft pastes which are atomized into a fluidized bed of already dry material (back-mixing) or the bed of inert particles (Mujumdar, 1995 Devahastin, 2000). Among the advantages of the fluidized bed dryers, one may cite... 

The wax patterns are cleaned with water or organic solvent and coated with a wetting agent, which helps the ceramic slurry to adhere to the wax. The cluster is then dipped in a liquid ceramic slurry, stuccoed with granular silica, zircon or alumina/silica refractories and then dried before the application of the next coat. The coating process is continued until a sufficiently thick shell is established. 

High rates of evaporation in flash dryers lead to low temperatures of the dried material and that means that flash dryers are particularly useful for drying granular, crystalline, pasty and powdery products, etc. Flash dryers are used in various branches of the chemical, pharmaceutical, ceramic, gypsum, wood, and mining industries. Flash dryers are simple in construction and have low capital cost and they are almost trouble free. 

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