Impact, size-reduction force

Impact is the simplest size-reduction force. It is also appUed for the comminution of brittle materials. As feed particles fall free on a high-speed solid rotor, the massive rotor impacts and breaks them into smaller particles. Secondarily, the material is struck onto a compact plate, where particle size is further reduced. Crushers based on impact force are impactors and hammer mills. They are characterized by high size reduction ratio, reaching up to 20 1. 

Fig. 3. Stressing mechanisms (a) single particles or (b) a bed of particles cmshed between two solid surfaces impact of a particle against (c) a solid surface or (d) another particle (e) cutting (f) shearing forces or pressure waves and (g) plasma reaction, an example of size reduction by nonmechanical energy.

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

Impact - Particle size reduction by applying an instantaneous force perpendicular to the particle/agglomerate surface. The force can result from particle-to-particle or particle-to-mill surface collision. 

Compression - Particle size reduction by applying a force slowly (as compared lo Impact) to the particle surface in a direction toward the center of the particle. 

Size reduction involves the decrease in size of a panicle or granule by fracturing the material using, generally, one of the four forces shear, compression, impact, and tension. Single or combination of forces being applied to the material affect the level of size reduction that will be achieved, but also the magnitude and duration of the applied force(s) will help determine the overall resultant panicle size distribution. 

For brittle materials, shear force is restricted to hammer mills, acting along with impact and attrition, as well as to toothed crushing rolls. The prerequisite for successful size reduction of particles is the existence of macro- or microstructure weaknesses (e.g., layered structure, schistosity) or differences in the characteristics of adjacent minerals (e.g., hardness, friability). Contrary to rocks and minerals, shear force is widely applied in soUd waste size reduction, as most of its constituents are soft, nonbrittle, plastic, and ductile, and, consequently, shear forces are effective. The various forms of shredders use shear force to reduce the size of solid waste constituents. 

Although the term shredders is sometimes used as a general term to include the equipment that causes size reduction of solid waste, it fits best to the equipment that uses mainly shear force to reduce the size of the materials. In such a sense, hammer mills are sometimes included in the category of shredders, as, in addition to impact force, materials are also subjected to shear force for their size reduction, in case that they are also equipped with stationary knives fixed at the wall. In this chapter, shredders are considered only the equipment that use shear... 

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. 

Knowledge of the compression properties of feed materials can indicate the type of force most likely needed in size reduction. A friable or crystalline structure can be reduced through fracture along cleavage planes using compressive forces. However, if new crack tips must be formed, impact and shear forces may be more effective. For food materials of fibrous structures, shredding or cutting should be considered for the desired size reduction. 

For the size reduction of smaller particles, hammer mills are often employed. In these machines, free-swinging, hammers are rotated at high speed so that they are thrown out by the centrifugal force, hit the feed, and break it down by the force of impact. In many cases the discharge from this mill is through a sieve or screen to control the output particle size. 

The magnitude of the mechanical force applied the duration the type offeree, such as compression, shear, and impact and other factors affect the extent and efficiency of the size-reduction process. The important factors in the size-reduction process are the amount of energy or power used and the particle size and new surface formed. 

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