Paste mixer

Ainong the first TFIz mixers to be constructed were those based on room-temperature Schottky diodes [11]. Over the past decade, new mixers based on superconducting tunnel junctions have been developed that have effective noise levels only a few tunes the quantum limit of [12]. Flowever, certain conditions... 

Convection. In these mixers an impeller operates within a static shell and particles are moved from one location to another within the bulk. bbon Tjpe. Spiral or other blade styles transfer materials from one end to the other or from both ends to the center for discharge (Fig. 37b). This mixer can be used for dry materials or pastes of heavy consistency. It can be jacketed for heating or cooling. Blades can be smoothly contoured and highly pohshed when cleanliness is an important process requirement. 

Planeta-iy Type. Paddles or whips of various configurations are mounted in an off-center head that moves around the central axis of a bowl or vessel. Material is mixed locally and moved inward from the bowl side, causing intermixing. This mixer handles dry materials or pastes. 

Pastes. For blending of viscous pastes, mixers are classified as batch or continuous. Most convection-type mixers for dry soHds are also used for thick pastes. 

Fig. 38. Various mixer types for viscous pastes (a) the change-can mixer, (b) rake mixer, (c) double-rake mixer, and (d) sigma-blade kneader.

Paste Mixing. The active materials for both positive and negative plates are made from the identical base materials. Lead oxide, fibers, water, and a dilute solution of sulfuric acid are combined in an agitated batch mixer or reactor to form a pastelike mixture of lead sulfates, the normal, tribasic, and tetrabasic sulfates, plus PbO, water, and free lead. The positive and negative pastes differ only in additives to the base mixture. Organic expanders, barium sulfate [7727-43-7] BaSO carbon, and occasionally mineral oil are added to the negative paste. Red lead [1314-41 -6] or minium, Pb O, is sometimes added to the positive mix. The paste for both electrodes is characterized by cube weight or density, penetration, and raw plate density. 

Refining. The next stage in chocolate processing is a fine grinding in which a coarse paste from the mixer is passed between steel rollers and converted to a drier powdery mass. Refining breaks up crystalline sugar, fibrous cocoa matter, and milk soflds. 

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. 

Bulk Blenders Vlanv of the mixers used for solids blending (Sec, 19) are also suitable for some liquid-solids blending. Ribbon blenders can be used for such tasks as wetting out or coating a powder, When the final paste product is not too fluid, other solids-handling equipment finds frequent use. 

Heat Transfer Pastes are often heated or cooled by heat transfer through the walls of the container or hollow mixing arms. Good agitation, a large ratio of transfer surface to mixer volume, and frequent removal of material from the surface are essential for high rates of heat transfer. Sometimes evaporation of part of the mix is used for cooling. 

In most mixers, the metal wall has a negligible thermal resistance. The paste film, however, usually has high resistance. It is important, therefore, while minimiziug the resistance of the heating or coohng medium, to move the paste up to and away from the smooth wall surface as steadily and rapidly as possible. This is best achieved by having the paste flow so as to follow a close-fitting scraper which wipes the film from the wall with each rotation. Typical overall heat-transfer coefficients are between 25 and 200 J/(m -s-K) [4 to 35 Btu/(h-fF-°F)j. 

Since paste mixing is not within the scope of this section, such widely used paste mixers as the sigma blade and banbury types will not be covered here but instead are discussed in Sec. IS. 

When it is deleterious, laminar flow can be avoided by mixing over the cross section. For this purpose static mixers in line can be provided. For very viscous materi s and pastes, screws of the type used for pumping and extrusion are used as reactors. 

Wastewater from the grid paste application process is entirely recycled within the process after treatment in a muiti-stage settler. Wastewater from the grid paste mixer wet scrubber is... 

Figure 6. Vacuum mixer for lead paste 1, mixing compartment 2, fast-rotating mixing tools 3, material-deflecting plate 4, discharge opening 5, static, vacuum-sealed enclosure [21J.

For the batch mixing of viscous pastes and doughs using ribbon impellers and Z-blade mixers, the tanks may be mounted horizontally. In such units, the working volume of pastes and doughs is often relatively small, and the mixing blades are massive in construction. 

Kneaders, Z- and sigma-blade, and Banbury mixers as shown in Figure 7.20, are generally used for the mixing of high-viscosity liquids, pastes, rubbers, doughs, and so on. The tanks are usually mounted horizontally and two impellers are often used. The impellers... 

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