Classification or Size Separation

CLASSIFICATION OR SIZE SEPARATION

In the introduction to this chapter, the influence of particle size on several processes was described. The operation in which particles of a suitable size are selected and others rejected because they are too small or too large is called classification or size separation. This process is also important in closed-circuit grinding, removal of fine powders to promote flow, and restriction of particle size distribution to prevent segregation or to enhance appearance.

Although a number of particle properties can be used to classify a powder, only two are important. The first is based on the ability of a particle to pass through an aperture. This is sieving or screening. The second employs the drag forces on a particle moving through a fluid. The term classification is sometimes restricted to this method of separation, but in this text elutriation and sedi-mentation will be used. In general, screening is applied to the separation of coarse particles, and elutriation and sedimentation to fine particles.

Sieving and Screening

Sieves and screens are widely used for the classification of relatively coarse materials. For very large particles, greater than half an inch, a robust plate perforated with holes is used. However, the pharmaceutical applications of screening are with much smaller particles, and screens are in the form of woven meshes. Unless special methods are used to prevent clogging and powder aggregation, the lower useful limit resides in a cloth woven with 7900 meshes/m.

This corresponds to a mesh spacing of between 7.0 x 10^-5 and 8.0 x 10^-5 μm. Fine screens of this type are extremely fragile and must be used with great care.

A series of suitable sieve cloths is described in the Fine Mesh series of British Standard (BS) 410:1962. This specifies the gauge of the wire and the permitted weaving tolerances. In successive meshes of this series, the mesh space alters by a factor of 4√2. In the mesh series commonly chosen for size analysis, 16-22-30-44-60-85-120-170, alternate screens are selected so that the mesh spacing decreases by √2 and the area of the apertures is halved. For classification, one or more meshes of suitable weave can be chosen from this series and mounted in a frame.

In operation, the mesh should be lightly loaded so that all particles capable of passing the mesh (undersize) have a chance to do so. The mesh must, therefore, be agitated both to ensure access of particles to the holes and to clear holes blocked by particles just unable to pass. Under these conditions, the rate of sieving is proportional to the number of undersize particles on the screen. It therefore decreases exponentially.

Most screening, particularly that of coarse materials, is carried out dry. The wet screening of dilute slurries is adopted for powders that aggregate strongly, clog the mesh, or become electrostatically charged by the vibrations of the screen. Sieving errors arising from the cohesion of small and large particles and the retention of the former on a coarse mesh are avoided. Wet screening is particularly useful if the subsequent process is wet and drying is unnecessary.

For small-scale classification, test sieves with meshes conforming to BS 410 can be used. The mesh is mounted on a circular brass frame, 8, 12, or 18 in. in diameter, a rim on the lower edge enabling it to “nest” with the sieve below. When the chosen sieves are equipped with a lid and a receiving pan, the agi-tated assembly becomes an effective small-scale grading unit. Sieving is stopped when the rate at which particles pass the mesh has reached some low value or after some predetermined time at which the rate is known to be low.

As the scale of the operation increases, it becomes, in general, less precise. For continuous screening, the feed material is made to move across the screen to a point of discharge. The residence time on the screen is usually short, and many undersize particles traverse it without falling through. With increase of sieving area, the meshes become more fragile and the finest meshes must be supported with a coarser wire. An example of a large-scale separator utilizes a circular screen of up to 5 ft in diameter and is vibrated in a horizontal plane, the gyratory movement being imparted by an out-of-balance fly wheel connected to the assembly. In other machines, the mesh is rectangular and inclined at a shallow angle (5^○–30^○). A gyratory movement is developed, and the material to be classified is fed to the top end. These machines may bear more than one deck, thus allowing the separation of the powder into several fractions at one time.

Elutriation and Sedimentation

The balance of the drag force on the particle and the forces promoting move-ment occurs at the terminal velocity. This velocity depends, among other things, on the size of the particle, and it is the property on which several classifiers are based. The fluid is either air or a liquid. The latter affords a higher precision because dispersion can be more thorough. High shear forces cannot be developed and dispersing agents cannot be used in air.

FIGURE 12.7 (A) An elutriator and (B) grade efficiency curve.

The simplest classifier is a rising current of fluid in which the particles are suspended. In this case, the force opposing the upward drag is gravitational. If the opposition gives a terminal velocity greater than the current speed, the particle will fall. This is the principle of elutriation, and the particle size, d, at which the separation is made follows from a rearrangement of equation (2.24) in chapter 2 for conditions in which Stokes’ law is valid.

where ρ_s - ρ is the density difference between solid and fluid, η is the viscosity of the fluid, and m is the speed of the upward current.

The elutriator shown in Figure 12.7A consists of three tubes. The first is smallest in diameter and offers the highest upward liquid velocity. Coarse particles with a high terminal velocity settle in this tube, while the remainder are swept to the bottom of the second. The diameter of the second tube exceeds that of the first, and elutriation speeds are lower. Only fine particles are swept into the third tube where the process is repeated at a finer size. In this way, the original slurry is divided into four fractions.

In practice, fluctuations in flow conditions due to natural convection and a violation of the conditions for which Stokes’ law is valid blur the point of sep-aration. The evaluation of the separation must, therefore, take account of the fine particles that fall with the coarse particles and the coarse particles that move to the fine fraction. This is best expressed by a grade efficiency curve. Returning to equation (24) in chapter 2, a particle of size d should be stationary in the elu-triation tube. Because of fluctuating conditions, it eventually resides with either the coarse or the fine fractions, the chances being equal. 

FIGURE 12.8 Cyclone separator.

The weight fraction in each is, therefore, 0.5 at this particular size. We shall assume that of the particles that are twice this size (2d), virtually all appear in the coarse product.

The weight fraction here is 1. Similarly, all particles of size 0.2d move to the fine product so that the weight fraction in the coarse product is 0. As shown in Figure 12.7B, a sigmoid curve, passing through 0.5 at size d, links these extremes. The closer these extremes and the steeper the curve, the more efficient the separation. A grade efficiency curve of this type can be used as an appraisal of any sedimentor or elutriator.

Gravitational sedimentation is not of great importance in small-scale classification. Sedimentation in a spinning fluid stream is, however, widely used. The most common classifier of this type is the cyclone separator shown in Figure 12.8. The fluid enters tangentially and acquires an intense spinning motion, spiraling downward into the cone before rising to the outlet as a central core. The inlet speed is very high so that large angular velocities are developed. Because of centrifugal force, particles move radially across the spinning stream to fall at the wall into the cone. Operated in this way, complete separation of solids occurs, and the cyclone is, therefore, an effective air cleaner. Operated with lower centrifugal forces, the cyclone transports the finest particles to the exhaust, leaving the coarser particles to fall into the cone. Cyclone classifiers are designed for use with either liquid or air.

The centrifuge is normally operated to completely separate two phases. If, however, the rate at which the feed passes through does not allow all particles to settle, the action of a classifier is developed. This is illustrated by a solid bowl centrifuge, which consists of a steel shell in the form of a frustrum mounted horizontally. It contains a conveying screw at the wall, which rotates at a slightly higher speed than the shell. Particles that settle at the wall are conveyed to the narrow end of the shell and discharged. Fine particles are entrained with the overflow to the other end. Further details of this and other centrifugal classifiers have been given by Treasure (Treasure, 1965).