Size reduction

Size Reduction The dismantlement of nuclear weapons results in many thousands of pounds of plastic bonded explosive (PBX) waste in an assortment of hemisphere sizes and odd pieces. The PBX types of interest are PBX-9404, PBX-9501, LX-10, and LX-04. Size reduction is important in the subsequent destruction or recycling of this PBX waste. 

Size reduction is an extremely important unit operation, whereby materials are subjected to stress in order to reduce the size of individual pieces. The stress is apphed by transmitting mechanical force to the soHd. 

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. 

Stress concentration Kis defined (1) as local stress/mean stress in a particle and calculated according to if = 1 + 2 LR) length and R is the radius of the crack tip. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

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. 

These are used to make two product size ranges, oversize and undersize, with some overlap. The break commonly is between 28 and 200 mesh. A considerable variety of equipment of this nature is available, and some IS kinds are described by Kelly and Spot-tiswood (1982, pp. 200-201). Two of the most important kinds, the drag rake classifier and the hydrocyclone, will be described here. 

Crushing is applied to large lumps of feed stock and grinding to smaller lumps, often the products of crushing, but the size distinction is not overly sharp. The process of size reduction results in a range of product sizes whose proper description is with the complete cumulative size distribution, but for convenience a characteristic diameter corresponding to 80% pass in the cumulative distribution curve is commonly quoted. 

Some devices employ impact (hammers) and others employ crushing by nipping (roils or jaws). Within limits, kinetic energy and dimensions of crushing elements can be selected to give a desired reduction ratio. Because of the deformability of solid materials, however, a theoretical limit does exist to the size of particles that can be crushed. These limits are 1 jum for quartz and 3-5 jum for limestone. The products of crushing these sizes, of course, can be very much smaller, so that really there is no practical lower limit to grinding. 

In practical operations, only about 1% of the input energy to the mill appears as new surface energy of the product. Nevertheless, empirical relations for power consumption based on the extent of size reduction have been developed. One such relation is 

The kinds of equipment used for certain materials are identified in Table 12.4. Usually several kinds are more or less equally suited. Then the choice may be arbitrary and based on experience or on marginal considerations. Table 12.5 presents a broader range of materials that are being ground in four of the principal kinds of fine 

Bond (1952) postulated that the work required to reduce large feed particles, 4, to particles of diameter, d, is given by Equation 12.6. He also proposed a work index, IT, which is defined as the gross energy requirement in KWH/ton of feed needed to reduce the feed, 4, to a size that 80% of the material passes a 100 p.m screen. Table 12.4 is a list of typical work index values. Example 12.2 compares a result from this equation with direct data from a manufacturer s catalog. 

A hydrocyclone assembly is required to handle 10,000 gpm of slurries of a solid with specific gravity 2.9 with a cutoff point of dsQ = 100 nm. The allowable pressure is AP = 5 psi. Several slurry concentrations V will be examined. Substituting into Eq. 12.4 with z the function of F in parentheses. 

Pretreatment is, simply, breaking, crushing, and screening of the ROM coal in order to provide a uniform raw coal feed of predetermined top size thereby minimizing the production of material of ultrafine size by excessive crushing or handling. Thns, pretreatment and size reduction are terms that are often used synonymously. 

Most conventional coal cleaning facilities utilize gravity methods for the coarser size fractions and surface treatment methods for the finest particle sizes (Riley and Firth, 1993). The selection of equipment, especially for the finer sizes, depends on the mining method, coal hardness, and size distribution and amounts thereof. 

Most commercial circuits utilize dense-media vessels of jigs for the coarsest size usually +3/8 in., dense-media cyclones, concentrating tables or jigs for the 3/8 in. x 28-mesh size, water-only cyclones or spirals, and sometimes flotation for the 28 x 100-mesh size and flotation for the -100 mesh. 

For example, coal which is destined for power generation may undergo size reduction to produce a product with a top size of 0.04 in. (1 mm). On the other hand, the size of the coal needed for a coking operation is coarse and the number of stages of size reduction involved in preparing a coal feed for a coking is somewhat less than required to prepare coal as the feedstock for power generation utilization. 

Because the types of mining processes are varied, and size reduction actually begins at the face in the mining operation, it is quite understandable that the characteristics of the products from these 

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