Shear comminution

Size reduction may be described in many terms, such as crushing, grinding, cutting, shearing, comminution, breaking, disintegration, or pulverization. The choice of term depends on the machine used to... 

Ambient grinding involves the comminution of scrap rubber, including tires, at ambient temperatures, typically in a toothed-wheel mill, bench grinder, shear mill, or two-roll miU, to give particles... 

Comminution, or particle size reduction of solids, is considerably different from that of the breakup of one liquid by dispersal as small droplets in another. Particle size reduction is generally achieved by one of four mechanisms (1) compression, (2) impact, (3) attrition and (4) cutting or shear. Equipment for particle size reduction or milling includes crushers (which operate by compression, e.g., crushing rolls), grinders (which operate principally by impact and attrition, although some compression may be involved, e.g.,... 

Particle comminution occurs owing to a compression-shear action produced by the rotating disks and grinding media. 

Products are comminuted with cutting tools such as shears, knives, or razor blades. Drilling, milling, etc. are also suitable. A smaller particle diameter can also be obtained by grinding precooled samples in mills. Depending on the elasticity characteristics of the sample, either dry ice or liquid nitrogen can be effectively used. 

M. Marktscheffel, K. Schonert, Liberation of composite particles by single particle compression, shear and impact loading, in Preprints of 6th European Symposium on Comminution, Nuremberg, Germany, 1986, pp. 29-45. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

A surfactant used in froth flotation to adsorb onto solid particles, make them hydrophobic, and thus facilitate their attachment to gas bubbles. See also Froth Flotation. A high-shear mixing device used to prepare colloidal dispersions of particles or droplets by size reduction (comminution). Also termed dispersion mill . 

Recently, the simultaneous application of pressure and elongational shear by extrusion, allows the production of elongated particles, lightly adhered to their neighbours. This produces a remarkably meatlike aligned flake structure (Cheftel et al. 1992). For fine comminutes, the adhesion occurs in the batter itself, and casings are used to form the final product shape. 

Suspension of finely ground hard fat or emulsifier in a cool liquid oil with subsequent comminution using a homogenizer, colloid mill, or shear pump. 

The strength of most materials is greater in compression than in tension. It is therefore unfortunate that technical difficulties prevent the direct application of tensile stresses. The compressive stresses commonly used in comminution equipment do not cause failure directly but generate by distortion sufficient tensile or shear stress to form a crack tip in a region away from the point of primary stress application. This is an inefficient but unavoidable mechanism. Impact and attrition are the other basic modes of stress application. The distinction between impact and compression is referred to later. Attrition, which is commonly employed, is difficult to classify but is probably primarily a shear mechanism. 

The most important factors that determine the selection of the size-reduction equipment are the mechanical characteristics (shear strength, ductility, etc.) of the feed material, as well as the size distribution of feed and comminuted product. From the aforementioned analysis, it is clear that the mechanical characteristics determine the acting force for size reduction and, consequently, the selection of the proper equipment. The size distribution of the feed stream and product determines the type of the corresponding equipment as well as the dimensions of feed and discharge openings. 

Despite their insubstantial nature, plant materials are also subject to contamination by sample containers and milling equipment. Paper sample bags may act as sources of contamination including boron (Standing Committee of Analysts 1986). Normally, vegetation is comminuted in high shear cyclone mills or blenders fitted with stainless steel or titanium-coated blades. As with other more resistant materials the amount of contamination from the mill can be related to the abrasiveness of the sample (Allan etal. 1999). 

Forces commonly used in food processes for particle size reduction are compressive, impact, attrition (or shear), and cutting forces. More than one force usually participates in the comminution operation in industrial size-reduction equipment. In particular, crushing rolls use mainly compressive forces, hammer mills are based on impact, disc mills cause particle attrition through shear force application, and rotary knife cutters use cutting forces. 

Colloid Mill A high-shear mixing device used to prepare colloidal dispersions of particles or droplets by size reduction (comminution). Also termed dispersion mill. 

The most dilTicult powder mixtures to repeatedly restructure will have some submicron constituents, will have interlocking or plate-like shapes, will have some moisture present, and will have been subjected to compaction at some stage. To mix such a system the mixer is likely to need high shearing or impaction characteristics and could well be a particle comminuter rather than a conventional mixer . 

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