Coarse bulk solids

The unconfined yield strength of a bulk solid is the main property associated with arching and rathole formation. In fine or sticky powders, arching can result from cohesive forces, while in the case of coarse bulk solids, arching is caused by either the interlocking or the frictional force of single particles. Consolidation time is crucial in both cases, especially in rathole formation. A rathole can collapse if the stress imposed on the material exceeds its yield strength. 

Coarse bulk solids are those in which the particle size range is such that the air permeability is sufficiently high to allow air to percolate easily through... 

Funnel flow bins are only suitable for bulk solids that are coarse, free flowing, and do not degrade, and for use when segregation is not important. For such materials, the principal benefits of funnel flow bins are reduced headroom and lower initial cost for the bin (excluding feeders or dischargers). Examples of funnel flow bins are shown in Figure 2. 

The following variables can affect wall friction values of a bulk solid. (/) Pressure as the pressure acting normal to the wall increases, the coefficient of sliding friction often decreases. (2) Moisture content as moisture increases, many bulk solids become more frictional. (3) Particle size and shape typically, fine materials are somewhat more frictional than coarse materials. Angular particles tend to dig into a wall surface, thereby creating more friction. (4) Temperature for many materials, higher temperatures cause particles to become more frictional. (5) Time of storage at rest if allowed to remain in contact with a wall surface, many solids experience an increase in friction between the particles and the wall surface. (6) Wall surface smoother wall surfaces are typically less frictional. Corrosion of the surface obviously can affect the ability of the material to slide on it. 

A bulk solid is defined as a collection of discrete solid particles. A "powder is an example of a fine bulk solid, and this term will be used predominantly throughout this chapter. The concepts discussed in this chapter apply to many types of bulk solids, whether fine or coarse, such as dust, granulations, and granules, either as a single substance or a multi-component blend. 

The bulk solid must be coarse. This is usually interpreted as a material having a mean particle size of at least 6 mm with a minimal amount of fines. 

Momentum. This mechanism occurs when a flowing bulk solid impacts a pile that is formed below the bulk sohd. Because of differences in the momentum and in some cases the cohesion between the coarse and fine fractions, the coarse particles tend to gather at the outer periphery of the pile while the fines are deposited directly below the point of impact. This mechanism is quite common and has been shown to occur when the ratio of particle diameters is as small as 1.3. This mechanism would probably occur when, for example, a batch mixer is discharged into a hopper below it. Fortunately, the use of mass flow hoppers can effectively combat this radial segregation. 

Here fine particles sift in between the coarse as the bulk solid avalanches down toward the center of the silo. This usually manifests itself by showing an increase in fines content in the material exiting the silo at the end of the discharge of the silo. Another common cause of sifting is vibration. In this case, the finer fraction can sift into the interstices of the coarse. This occurs frequently during product shipment. 

Specific gravity is direcdy related to the bulk density of waste fuels prepared ia a variety of ways. Solid oven-dry (OD) wood, for example, has a typical bulk density of 48.1 kg/m (301b/ft ). In coarse hogged form, eg, <1.9-cm minor dimension, this bulk density declines to about24kg/m (151b/ft ). In pulverized form, at a particle size <0.16 cm, this bulk density declines to 16—19 kg/m (10—12 Ib/ft ). Similar relationships hold for municipal waste, agricultural wastes, and related fuels. 

Usually the rate, Q, is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles. Slowing down the discharge rate requires a feeder. Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a mass flow bin, because of the interaction between air (or gas) and solid particles as reflected in the permeabiUty of the material. 

Once a bulk sample is selected, a laboratory sample must be prepared for analysis (Figure 28-2). A coarse solid sample should be ground and mixed so that the laboratory sample has the same composition as the bulk sample. Solids are typically dried at 110°C at atmospheric pressure to remove adsorbed water prior to analysis. Temperature-sensitive samples may simply be stored in an environment that brings them to a constant, reproducible moisture level. 

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