Particle reduction ratios

Some equipment employs impact, as in hammer mills and others employ nipping (i.e., the crushing of a feed between rolls or jaws). Within limits, kinetic energy as well as the dimensions and design of the crushing elements can be selected to obtain the desired particle reduction ratio. Because of the deformability of solid... 

Many attempts have been made to develop models which predict the behavior of materials undergoing size reduction. One proposal is that the energy expended in size reduction is proportional to the new surface formed (5). Another theory is that the energy required to produce a given reduction ratio (feed size product size) is constant, regardless of initial feed particle size (6). Practical results show, however, that both these theories are limited in their usehilness. 

Smooth jaws produce more numerous flaky pieces than corrugated jaws. Curved jaws produce less fines but more numerous flaky particles. Crusher speed has little effect. The presence of material too small to be crushed has a deleterious effect on the shape of products. Secondary crushers with a small reduction ratio can improve the shape of primaiy crushed material, but secondaiy crushers are not inherently different from primaiy crushers. Slotted screens can remove flaky particles from the product. Impact crushers produce fewer flaky particles than any other type. 

The strong influence of morphology and mixing is well illustrated with the composite particle investigation. These particles were composed of a nickel shell coated on spherical aluminum particles by hydrogen reduction in aqueous metal salt solution. The overall ratio of material in a particle was about 80 wt% Ni and 20 wt% aluminum. With these particles, the ratio of reactants was approximately the same as in the mixed powders, but the morphology of the reactants is radically different. 

Ni-YSZ cermet anodes satisfy most of the basic requirements for SOFC anodes. The effective conductivity of a Ni-YSZ cermet anode increases with the Ni to YSZ volume ratio, relative density, and decreasing the particle size ratio of NiO to YSZ. While coarse YSZ powders may result in poor mechanical strength and low stability, coarse NiO powders may lead to poor effective conductivity. The effective conductivity increases with the temperature at which the NiO is reduced to Ni metal in a reducing atmosphere. Further, very low reduction temperatures (e.g., below 400°C) may result in not only low electrical conductivity, but also poor mechanical strength. 

The desirability of using a number of size reduction units when the particle size is to be considerably reduced arises from the fact that it is not generally economical to effect a large reduction ratio in a single machine. The equipment used is usually divided into classes as given in Table 2.1, according to the size of the feed and the product. 

Particle mixing is caused by the bubbles, partly be shear displacement or drift but also by the bulk transport of particles in the bubble wake. Bubbles may also cause segregation if there are different kinds of particles present. Unlike other kinds of mixers, segregation is insensitive to particle size difference but particularly sensitive to density difference. In a binary system of particles segregation increases approximately as particle density ratio to the power 5/2 but with particle size ratio only to the power 1/5 (11). This can cause problems in, for example, coal combustion where char has a markedly lower density than ash and also in some ore reduction processes using coke. 

Most of these quantities can be measured by using either BET (Bmnauer, Emett, Teller surface area measurement) or X-ray diffraction methods. The fact that the necessary amount of energy to comminute solids depends on the particle size, the size reduction ratio and the solid can be expressed by empirical formulas. According to Beddow [18] three of the most common types are the equations by ... 

PTFE Resin Reduction Ratio Range Specific Gravity Average Particle Size, pm Thermal Instability Index Extrusion Pressure, MPa Reduction Ratio Bulk Density... 

Initial Feed Particle Size and Degree of Size Reduction Desired. Each device has a reduction ratio it can accept materials of sizes in a certain range and will provide particle of a set size. The particle size distribution of the obtained powder must be as even as possible to... 

Kick s law essentially states that the work required to obtain a given reduction ratio is the same irrespective of starting size. According to Rittinger s law work is proportional to surface created. Rittinger s and Kick s laws are only useful over a limited particle size range and are not utilized today. 

These tertiary crushers employ smooth or toothed heavy-duty impact and abrasion-resistant steel-rimmed rolls. The rolls are mounted inline in a horizontal manner and turn toward each other at equal speeds to create a nip into which a friable feed material is introduced (Fig. 4). Heavy-duty compression springs with automatic reset are used to dampen crushing shock and to protect the crusher from tramp iron and oversize material. An adjustable screw that adjusts spring tension changes the crusher opening. A flywheel is used to even out pulses and economize on power consumption. These crushers have a theoretical maximum reduction ratio of 4 1 and will only crush materials to about 10 mesh. Roll crushers produce a controlled product size distribution without a lot of fines. The narrow particle size distribution is achieved by controlling a combination of variables including roll speed, gap measure, differential speed, feed rate, and roll surface. 

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. 

The main purpose of coarse comminution in sample preparation is to reduce the maximum particle size of samples to allow effective mixing prior to subsampling. The ratio of feed size before reduction to the product size after reduction is known as the reduction ratio, and for the first crushing or milling stage this ratio should be as large as possible (Smith James 1981). Ratios of 40/1 for coarse crushers and 15/1 for coarse milling equipment are useful minimum specifications for laboratory preparation of coarse contaminated soil samples of up to 2kg. 

The optical and scanning electron micrographs presented in this chapter show that the particle size of solid materials, such as polymers, monomers, and intermediate chemicals, can be altered by precipitation from a supercritical fluid solution. The only requirement for carrying out the SCF particle reduction process is that the compound must exhibit some solubility in a supercritical fluid. Because the pressure reduction rates are so rapid during the expansion of the solution, supersaturation ratios can be achieved that are much, much greater than can be achieved by thermal, chemical, or antisolvent precipitation processes. Furthermore, it is conjectured that such rapid nucleation rates can result in the particle formation of some materials with a size distribution or morphology that cannot now be achieved by any other process. 

In 1885 Kick proposed another law, based on stress analysis of plastic deformation within the elastic limit, which states that the work required for crushing a given mass of material is constant for the same reduction ratio, that is, the ratio of the initial particle size to the final particle size. This leads to the relation... 

If in Eq. (29.13) is constant, this equation says that the particle-size distribution of the crushed material is the same for all sizes of the initial material. The value of A5 in crushing 4/6-mesh material to 8/10-raesh will be the same as in crushing 6/8-mesh particles to 10/14-mesh, since the size-reduction ratio is the same. 

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