Particle breakage

Particle board adhesives Particle bonding Particle breakage Particle distribution Particle filtration... 

Types of Equipment Solid Product Dryness Washing Liquid Product Quality Crystal Breakage Particle Size (pm) Solids in Feed (vol.%)... 

Briquettes must be transported carefliny to avoid breakage. They ate usually dumped into piles in sheds and frequently ate screened to remove smaller broken particles. Hand-stacking significantly improves storage quaHty, and permits mote material to be stored in a limited volume. 

Screw Feeders. Screws are primarily used when feed over a slotted outlet is requited. Screws are a good choice when an enclosed feeder is requited, when space is restricted, when handling dusty or toxic materials, or when attrition (particle breakage) is not a problem. A screw is composed of a series of flights that are wound around a common shaft. The flights have a particular diameter and pitch (the distance between flights). Some screws have constant pitch flights others vary. The screw shaft has to be sized to prevent deflection (12). 

Size reduction causes particle breakage by subjecting the material to contact forces or stresses. The apphed forces cause deformation that generates internal stress in the particles and when this stress reaches a certain level, particle breakage occurs. 

Table 1 summarizes typical data the specific breakage rate, 3), for particles in size class 1 is 0.6, and the breakage product mass in size classes 2—6 is as shown. The corresponding values of b and B are given. 

Using the above concepts, models have been developed to predict size distribution from comminution devices. An assumption is that the rate of breakage of material of a particular size is proportional to the mass of that size present in the comminution zone of a machine. If the mass size distribution in the machine is where is the mass of particles in size class /, then rate of breakage is given by equation 2. 

The summation term is the mass broken into size interval / from all size intervals between j and /, and S is the mass broken from size internal i. Thus for a given feed material the product size distribution after a given time in a mill may be deterrnined. In practice however, both S and b are dependent on particle size, material, and the machine utilized. It is also expected that specific rate of breakage should decrease with decreasing particle size, and this is found to be tme. Such an approach has been shown to give reasonably accurate predictions when all conditions are known however, in practical appHcations severe limitations are met owing to inadequate data and scale-up uncertainties. Hence it is stiH the usual practice to carry out tests on equipment to be sure of predictions. 

Whereas an explosion from methane tends to be localized, it may start coal dust explosions resulting in more widespread injury and loss of life. AH coal breaking operations result in formation of fine coal particles some are controlled with water during the mining operation. Breakage associated with hauling disperses dust, and dust accumulations can be made safe by rockdusting. Powdered limestone is spread over the mine surfaces to cover the dust. 

For a steady-state ciystallizer receiving sohds-free feed and containing a well-mixed suspension of ciystals experiencing neghgible breakage, a material-balance statement degenerates to a particle balance (the Randolph-Larson general-population balance) in turn, it simplifies to... 

From this it can be concluded that the wide distribution of fragment sizes from milhng is inherent in the breakage process itself and that attempts to improve grinding efficiency by weakening the particles will result in coarser fragments which may reqiiire a further break to reach the desired size. 

The energy laws of Bond, Kick, and Rittinger relate to grinding from some average feed size to some product size but do not take into account the behavior of different sizes of particles in the mill. Computer simulation, based on population-balance models [Bass, Z. Angew. Math. Phys., 5(4), 283 (1954)], traces the breakage of each size of particle as a function of grinding time. Furthermore, the simu-... 

In the investigations mentioned earlier the breakage function was assumed to be normahzable i.e., the shape was independent of Xq. Austin and Luclde [Powder Technol., 5(5X 267 (1972)] allowed the coefficient A to vary with the size of particle breaking when grinding soft feeds. 

In wet ball milling the grinding rate increases with sohds content up to 70 wt % (35 vol%), as Fig. 20-33 shows, due to pulp rheology. Examination of gouge marks indicated that most breakage was by impact of balls on particles rather than by abrasion. 

Bond [Min. Eng. (London), 60(1), 63-64 (1968)] reviewed attempts to induce breakage without wastefuUy applying pressure and concluded that inherent practical limitations have been found for the following methods spinning particles, resonant vibration, electro-hydrauhe crushing, induction heating, sudden release of gas pressure, and chisel-effect breakers. For a review of more recent efforts, see edition 6 of this handbook. 

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