Mixing of powder

Although the discussion in this section is qualitative, it must be concluded that the mixing and grinding procedures in preparation of powder mixtures for catalyst synthesis via solid/solid wetting and spreading undoubtedly play an important role and must be carried out carefully and under controlled and reproducible conditions. 

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Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

These mixers are particularly suited for rapid mixing of powders and granules with liquids, for dissolving resins or solids in liquids, or for removal of volatiles from pastes under vacuum. Scale-up is usually on the basis of maintaining constant peripheral velocity of the impeller. 

Ashton and Valentin, Mixing of Powders and Particles in Industrial Mixers, Trisni. Inst. Chem. Eng. (London), 44, tl65-tl88 (1966). 

At the outset it is useful to consider some common examples of problems encountered in industrial mixing operations, since this will not only reveal the ubiquitous nature of the process, but will also provide an appreciation of some of the associated difficulties. Several attempts have been made to classify mixing problems and, for example, REAVELL(1) used as a criterion for mixing of powders, the flowability of the final product. HARNBY et at.(2) base their classification on the phases present that is liquid-liquid, liquid-solid and so on. This is probably the most useful description of mixing as it allows the adoption of a unified approach to the problems encountered in a range of industries. This approach is now followed here. 

All mixing of powdered admixture must be done in a mixing tank which is separate from the admixture storage tank from which the admixture is fed into the dispenser. The density of the admixture mixed in this manner should... 

Muller W, Rumpf H. Mixing of powders in mixers with axial motion (in German). Chem Ing Tech 1967 39 365-373. 

J. R. Bourne, The Mixing of Powders, Pastes, and Non-Newtonian Fluids, Industrial Research Fellow Report No. 2, The Institute of Chemical Engineers, London, 1969. 

Without doubt, the method of sampling is more important than the statistical method used to draw a conclusion. The samples taken from the mixture must accurately represent the system. A large number of statistical analyses have been applied to the mixing of powders.913 Determination of blend homogeneity and sampling methodology will be further discussed later in this chapter. 

Uniform mixing of powdered materials occurs if they have similar particle size distributions and particle shapes. Spherical particles mix least well while plate and fiber shapes also do not mix well because they tend to clump. The more cohesive the material, the more difficult it is to mix that material with other materials. Similarly, cohesiveness between drug and excipient or among excipients may hinder the... 

Engineers at Mobil Oil Corporation are satisfied that a one-dimensional analysis is suitable for treating reaction kinetics in these beds, simply using an appropriate Peclet number to represent the effective axial gas diffusivity (Avidan, 1982 Krambeck et al, 1987). Inputs for Mobil s analysis are two (1) the Peclet number expected for a commercial fluid bed in question—they estimate this to be 7 for beds they contemplate for carrying out Mobil s methanol-to-gasoline or methanol-to-olefin reactions—and (2) kinetic data from a pilot fluid bed, which can be expected to reflect, reasonably well, whatever top-to-bottom mixing of powder will occur in the commercial bed. 

The efficiency of mixing of powdered solids in a process stream is being investi ... 

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