Mixing of pastes

The mixing of pastes is discussed in the section on Non-Newtonian Technology in Volume 1, Chapter 7. 

The degree of uniformity of a mixed product, as measured by analysis of a number of spot samples, is a valid quantitative measure of mixing effectiveness. Mixers act on two or more separate materials to intermingle them, nearly always in a random fashion. Once a material is randomly distributed through another, mixing may be considered to be complete. Based on these concepts, a statistical procedure for measuring mixing of pastes is as follows. 

Conventional mechanical mixers are batch type. The paste viscosity will rise at the beginning of paste mixing, but will then gradually decrease. Mixing of paste... 

Iron Browns. Iron browns are often prepared by blending red, yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. After mixing, the paste has to be dried at temperatures around 100°C, as higher temperatures might result in the decomposition of the temperature-sensitive iron yellows and blacks. Iron browns can also be prepared directiy by heating hydrated ferric oxides in the presence of phosphoric acid, or alkaU phosphates, under atmospheric or increased pressure. The products of precipitation processes, ie, the yellows, blacks, and browns, can also be calcined to reds and browns. 

Calcined aluminas are also used for polishing appHcations by mixing into polishing compounds in the form of paste or suspensions. Polishing aluminas are used to alter the surfaces of metals, plastics, glass, and stones in the manufacture of cutlery, automobiles, computers, furniture, eyewear, semiconductors, and jewelry. Polishing aluminas are also used to coat surfaces, such as video tapes (1). 

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. 

The AIG is used to uniformly inject diluted ammonia into the reactor. Uniform mixing of the ammonia into the flue gas is necessary to maintain catalyst performance at its highest level and to minimize ammonia leakage (ammonia sHp) past the catalyst. 

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. 

Valentin, Mixing of Powders and Pastes Basic Concepts, Chem. Eng. (London), 45, CE99-CE106 (1967). 

The plate heat exchanger consists of a frame in which closely spaced metal plates are clamped between a head and follower. The plates have corner ports and are sealed by gaskets around the ports and along the plate edges. A double seal forms pockets open to atmosphere to prevent mixing of product and service liquids in the rare event of leakage past a gasket. 

It is well known, that under industrial conditions a method of introducing filler into the polymer mixture is used. In this case, the filler is introduced in the form of paste containing up to 60 per cent water in order to reduce viscosity, As heating is affected by viscous friction, the temperature conditions are not stable on mixing and, therefore, conductivity of the conducting polymer composite becomes unreproducible. Up to now this factor has not been taken into consideration. 

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... 

Zinc phosphate cement is prepared by introducing small incremental amounts of powder into the liquid and mixing the paste over a large area on a glass slab in order to dissipate heat because of the excessive exotherm... 

These materials are prepared by mixing two strips of paste of equal length, and when mixed form fluid pastes. Some set rapidly, but setting depends on the availability of water (Plant Wilson, 1970 Bryant Wing, 1976a,b). In contact with water one example set in 2 minutes, and in the absence of water another did not set at all (Bryant Wing, 1976b). Normally they set in 3-5 to 8 minutes (Plant, Jones Wilson, 1972). 

---