Slurries and pastes

This group of materials offers the most problems in colour measurement terms, because of the influence of translucency and light trapping effects on the colour results. Baardseth et al. (1988) reported that the prediction index for yam colour measurements was the least precise in the red/green parameter than for all the other foods evaluated in the paper, such as mashed potato, raw potato and raw cauliflower. The authors suggested that homogeneity was a key factor in obtaining the most precise prediction index, and that in the case of yams, therefore, variations in the fruit pieces may have been a factor in the loss of precision. 

Nagle et al. (1979) homogenised pepper pods, the seeds and stems having been removed, prior to introduction of the sample into a cup provided with a white backing. Care was taken to allow the purees to settle they were then 

Reference Sample Preparation Temper- Presentation Illumination/ Standard Instru- Measure- 

Baardseth et al. Mashed Commercially available NM Plexiglass cell, 50/50 White tile H(l) L, (3 b  

Nagle et al. (1979) Red pepper Milled whole fruit NM Sample cell with NM Colour plate 1(4) L, a, b 

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Liquids. Liquids (33) are common forming additives in plastic, paste, and slurry processing. In plastic forming operations, the Hquid aids forming and serves as the binder/plasticizer for the system. In pastes and slurries, other additives are also dissolved or dispersed in the Hquid/solvent. Water is a good, inexpensive solvent that can be recycled. Organic Hquids such as alcohols are used to process water-sensitive materials and to dissolve water-insoluble forming additives, however, at considerably more expense. 

Ceramic forming iavolves consoHdation and mol ding of ceramic powders to produce a cohesive body of the desired size and shape. Ceramic forming operations (38,40—66) are conducted with dry powders, plastic bodies, pastes, and slurries. 

Drum dryers for pastes and slurries operate with contact times of 3-12 sec, produce flakes 1-3 mm thick with evaporation rates of 15-30kg/m2hr. Diameters are 1.5-5.0ft the rotation rate is 2-10 rpm. The greatest evaporative capacity is of the order of 3000 Ib/hr in commercial units. 

For pastes and slurries, some form of pretreatment equipment will normally be needed, such as extrusion or granulation. 

Kilns. Reactions involving free-flowing solid, paste and slurry materials can be carried out in kilns. In a rotary kiln, a cylindrical shell is mounted with its axis making a small angle to the horizontal and rotated slowly. The solid material to be reacted is fed to the elevated end of the kiln and it tumbles down the kiln as a result of the rotation. 

Suitable feeds to a spray dryer are solutions or pumpable pastes and slurries. Such a material is atomized in a nozzle or spray wheel, contacted with heated air or flue gas and conveyed out of the equipment with a pneumatic or mechanical type of conveyor. Collection of fines with a cyclone separator or filter is a major aspect of spray dryer operation. Typical equipment arrangements and flow patterns are shown in Figure 9.14. 

P. V. Danckwerts, Theory of Mixtures and Mixing, Research (London), 6, 355-361 (1953) see also P. V. Danckwerts, General Aspects of Mixtures and Mixing, paper presented at the Symposium on Mixing of Thick Liquids, Pastes, and Slurries, British Society of Rheology, London, September 1953 cf. N. Wookey, Mixing of Thick Liquids, Pastes and Slurries , Nature, 172, 846-847 (1953). 

The attractiveness of the network-of-zone method to compute solid-liquid mixing flows resides in its relative simplicity while being capable of capturing the main flow phenomena for a wide range of concentrations. The objective of the present work is to assess the real capability of this approach in the case of a complex mixing problem involving a coaxial mixer. Coaxial mixers are very popular for the preparation of pastes and slurries in the chemical, food, and coating industries. Another mixer setup is also tested. 

Often it is advisable to cover the specimen eell with a black-lined box in order to prevent extraneous room light from passing into the specimen. Care should be taken when introducing pastes and slurries into test cells that no air is entrapped in the sample. When pastes and slurries are prepared by mineing followed by diluting with water, the mincing conditions must be standardised as must the dilution factor. Samples should be well stirred and then measured quickly to reduce the effects of sedimentation and oxidation. Finally, the thermochromic properties of certain food materials should not be forgotten, and sample temperatures should be standardised. 

Uses Wetting agent, dispersant tor mfg. of pigment pastes and slurries, optical brightening agents, anthraquinone, paints, and inks latex stabilizer and emulsifier... 

Heuristic 20 For pastes and slurries offlne solids, use a drum dryer with indirect heat. For a liquid or pumpable slurry, use a spray dryer with direct heat. 

Spouted beds for drying of granular materials, pastes, and slurries are an emerging technology. Although there are few large-scale industrial applications reported in the literature, the modified spouted beds display significant potential for the future applications. 

Synthesized diamond is available in the size range from submicron to about one centimeter. Submicron and micron size diamond is used as loose abrasive in pastes and slurries for lapping and polishing applications. Depending on the needs of the application, diamond powders can be provided in several types that differ in aggressiveness or sharpness of cutting points, in shape, and in toughness. 

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