Coarse solids

Peripheral Discharge Peripheral discharge sometimes is used to permit the reduced installation cost of a tlat-bottorn tank on compacted soil. Because more torque is required to rake the solids to the perimeter of the tank, this arrangement is not suitable for seivice in ol ing coarse solids or solids that become nontliiid at high concentrations. 

The classifying operation is carried out in a pool of fluid pulp confined in a tank arranged to allow the coarse solids to settle out, whereupon they are removed by gravity, mechanical means, or induced pressure. Solids which do not settle report as overflow. Mesh of sepa-... 

Coarse solids are discharged by siphons extending to the bottom of the hindered-settling zone. Siphon control is obtained by a novel hydrostatically actuated valve which makes or breaks the siphon to flow only when the teeter zone is in correct condition. Discharge by an intermediate fraction from the upper column is by means of additional siphons. Hydraulic-water consumption is considerably lower than required for multipocket sizers. 

Drum Separators Very coarse solids, up to 0.3 m (12 in), are often processed in a drum separator of the type shown in Fig. 19-32. This is similar to a ball-mill shell with hfters permanently attached to the wall. Medium and feed enter at one end, and the float product flows out through the discharge trunnion, while the sink is lifted by the rotation of the drum to a stationaiy launder, through which it is flushed out. Modifications of this type include division of the shell into two compartments, which permits simultaneous operation at two different piup densities resulting in various grades of products. The two-compartment revolving drum is illustrated in Fig. 19-32. 

A final note is with regard to flat bottom cyclones which have gained acceptance in recent years. The installation of a flat bottom in place of the conical section will coarsen the separation by more than twofold. Additionally, the sharpness of the recovery curve will decrease significantly. As such, flat bottom cyclones should be restricted to those applications in which coarse separations are required. The flat bottom cyclone does produce a very clean underflow but at the expense of a large amount of misplace coarse solids in the overflow. An illustration of a flat bottom cyclone is shown in Figure 58. The reader should refer to the reference section of this chapter for citations that provide more in-depth coverage of this equipment, as well as design case studies and example. 

Body-aid (i.e., the filter-aid) and precoating are often mentioned in connection with pressure filtration and the difference in their application is (1) Body aid is used when the slurry is low in solids content with fine and slimy particles that are difficult to filter. To enhance filtration coarse solids with large surface area are added to the slurry and serve as a body-aid that captures and traps in its interstices... 

Coarse solid particles Any solid particle larger than 50 xm, and solid particles contained in or on any liquid particle. 

In a recent study of the transport of coarse solids in a horizontal pipeline of 38 mrrt diameter, pressure drop, as a function not only of mixture velocity (determined by an electromagnetic flowmeter) but also of in-line concentration of solids and liquid velocity. The solids concentration was determined using a y-ray absorption technique, which depends on the difference in the attenuation of y-rays by solid and liquid. The liquid velocity was determined by a sail injection method,1"1 in which a pulse of salt solution was injected into the flowing mixture, and the time taken for the pulse to travel between two electrode pairs a fixed distance apart was measured, It was then possible, using equation 5.17, to calculate the relative velocity of the liquid to the solids. This relative velocity was found to increase with particle size and to be of the same order as the terminal falling velocity of the particles in the liquid. 

The classifier described above works very well with coarse particles where exact splits are not needed. Typical applications are in connection with ball or rod mills for reduction to particle sizes between 8- and 20-mesh. These classifiers have high capacities they lift coarse solids for return to the mill, so that auxiliary conveyors and elevators are not required. There are other types of classifier that must be usual for close separations with fine particles. One such device is the centrifugal classifier its action bears a strong resemblance to that of the crossflow classifier, but the settling is greatly accelerated by the substitution of centrifugal force for gravitational force. 

Geldart, D., Behavior of Fine Particles in a Fluidized Bed of Coarse Solids, EPRI Report CS-2094, Electric Power Research Institute, Palo Alto, CA. (1981)... 

Kono, H., Attrition Rates of Relatively Coarse Solid Particles in Various Types of Fluidized Beds , AIChE Symp. Ser., 205(77) 96 (1981)... 

To accommodate efficient heating of coarse solids, it would be desirable to break the fall of these particles during their descent by means of baffles in order to prolong their residence time, as shown schematically in Fig. 13 (Kwauk, 1979b). In this respect, a good baffle needs to cover up to 100% of the cross-sectional area traversed by vertical flow, and yet permit oblique passage as near to 100% as possible. Also, baffles should distribute solids laterally in order to give uniform solids population in the heat transfer apparatus. Thus, conceptually, an ideal baffle plate should consist of a cellular array, structurally robust, of deflectors made of infinitesimally thin sheet materials. 

This result follows from the Richardson-Zaki equation. In their original work, Richardson and Zaki (1954) studied batch sedimentation, in particular the settling of coarse solid particles through a liquid in a vertical cylinder with a closed bottom. Richardson and Zaki found that the settling speed uc of the equal-sized particles in the concentrated suspension was related to the terminal settling speed u, of a single particle in a large expanse of liquid by the equation... 

The change in the free energy AG involved in subdividing a coarse solid suspended in aqueous solution into a finely divided one of molar surface S is given by... 

The influence of pressure, over the range 100-1600 kN/m2, on the fluidisation of three grades of sand in the particle size range 0.3 to 1 mm has been studied by Olowson and Almstedt(50) and it was showed that the minimum fluidising velocity became less as the pressure was increased. The effect, most marked with the coarse solids, was in agreement with that predicted by standard relations such as equation 6.14. For fine particles, the minimum fluidising velocity is independent of gas density (equation 6.5 with Ps >> P), and hence of pressure. 

It is possible substantially to eliminate the fluctuations which are characteristic of beds of coarse solids by incorporating baffles into the bed. The nature and arrangement of the baffles is critical, and it is generally desirable to avoid downward-facing horizontal surfaces because these can give rise to regimes of defluidisation by blocking the upward... 

A simple batch plant used for coarse solids consists of a cylindrical vessel in which the solids rest on a perforated support. The solvent is sprayed over the solids and, after extraction is complete, the residue is allowed to drain. If the solid contains a high proportion of solute such that it disintegrates, it is treated with solvent in a tank and the solution is decanted. 

Mathur, K.B. and Gishler, P.E., A technique for contacting gases with coarse solid particles, A.l.Chem.E.]., 1 (1955) 157-164. 

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