Page 372 - Chemical Process Equipment - Selection and Design

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>ISINTEGRATION, AGGLOMERATION, AND
SEPARATION OF PARTICULATE SOLIDS

rom the standpoint of chemical processing, size handling, or to reduce dust nuisances, or to densify the
reduction of solids is most often performed to make product for convenient storage or shipping, or to prepare
them imore reactive chemically or to permit recovery products in final form as tablets, granules, or prills.
of valuable constituents. Common examples of Comminution and size separation are characterized by
comminution are of ores for separation of valuable minerals the variety of equipment devised for them. Examples of the
from gangue, of limestone and shale for the manufacture of main types can be described here with a few case studies.
cement, of coal for combustion and hydrogenation to liquid For real, it is essential to consult manufacturers’ catalogs for
fuels, of cane and beets for recovery of sugar, of grains for details of construction, sizes, capacities, space, and power
recovery of oils snd fiour, of wood for the manufacture of requirements. They are properly the textbooks for these
paper, of some fr’ora for recovery of natural drugs, and so on. operations, since there are few generalizations in this area for
Since the pnxess of disintegration ordinarily is not highly prediction of characteristics of equipment. A list of about
selective with respect to size, the product usually requires 90 U.S. and Canadian manufacturers of size separation
separation into size ranges that are most suitable to their equipment is given in the Encyclopedia of Chemical
subsequent processing. Very small sizes are necessary for Technology [21, 137 (7983)1, together with identification of
some applications, but in other cases intermediate sizes are nine equipment types. The Chemical Engineering Equipment
preferred. Thus the byproduct fines from the crushing of coal Buyers Guide (McGraw-Hill, New York) and Chemical
are briquetted with pitch binder into 3-4-in. cubes when Engineering Catalog (Penton/Reinhold, New York) also
there is a demand for coal in lump form. Agglomeration in provide listings of manufacturers according to kind of
general is practilced when larger sizes are required for ease of equipment.

12.1. ~C~EENI~~~ sized perforations may be in series as shown on they may be on
concentric surfaces. They are suitable for wet or dry separation in
Separation of mixtures of particulate solids according to size may be the range of 60-10 mm. Vertically mounted centrifugal screens run
accomplished with a series of screens with openings of standard at 60-80 rpm and are suitable for the range of 12-0.4 mm.
sizes. Table 12.8 compares several such sets of standards. Sizes Examples of performance are: (1) a screen 3 ft dia by 8 ft long
smaller than the 38pm in these tables are determined by with 5-mesh screen at 2 rpm and an inclination of 2” has a capacity
elutriatian, microscopic examination, pressure drop measurements, of 600 cuft/hr of sand; (2) a screen 9 ft dia by 8 ft long at 10 rpm and
and other indirect means. The distribution of sizes of a given an inclination of 7” can handle 4000 cuft/hr of coke.
mixture often is of importance. Some ways of recording such data Flat screens are vibrated or shaken to force circulation of the
are illustrated in Figure 16.4 and discussed in Section 16.2. bed of particles and to prevent binding of the openings by oversize
The distribution of sizes of a product varies with the kind of
disintegration equipment. Typical distribution curves in normalized particles. Usually several sizes are arranged vertically as in Figures
12.2(b) and (c), but sometimes they are placed in line as in the
form are presenlted in Figure 12.1, where the size is given as a cylindrical screen of Figure 12.2(a). Inclined screens vibrate at
percentage of the maximum size normally made in that equipment. 600-7000 strokes/min. They are applicable down to 38pm or so,
The more concave the curves, the greater the proportion of fine but even down to 200 mesh at greatly reduced capacity. Horizontal
material. According to these correlations, for example, the screens have a vibration component in the horizintal direction to
percentages of material greater than 50% of the maximum size are convey the material along; they operate in the range of 300-3000
50% from rolls, 15% from tumbling mills, and only 5% from closed strokes/min.
circuit conical ball mills. Generalization of these curves may have Shaking or reciprocating screens are inclined slightly. Speeds
led to some loss of accuracy since the RRS plots of the data shown are in the range of 30-1000 strokes/min; the lower speeds are used
in Figure 12.l(c) deviate much more than normally from linearity. for coal and nonmetallic minerals down to 12mm, and higher
In order to handle large lumps, separators are made of sturdy
parallel bars called grizzlies. Punched plates are used for speeds may size down to 0.25mm. The bouncing rubber balls of
Figure 12.2(c) prevent permanent blinding of the perforations.
intermediate sizes and woven screens for the smallest sizes. Rotary sifters are of either gyratory or reciprocating types.
Screening is best perfarmed dry, unless the feed is the product of
wet grinding or is overly dusty and an equipment cover is not They operate at 500-600rpm and are used for sizes of 12mm-
50 pm, but have low capacity for fine sizes.
feasible. Wetting sometimes is used to prevent particles from
sticking together. Types of screens and other classifiers to cover a
range of sizes are shown in Figure 12.2. Usually some kind of CAPACITY OF SCREENS
movement of the stock or equipment is employed to facilitate the
separations. For coarse screening, the required area per unit of hourly rate may
be taken off Figure 12.3. More elaborate calculation procedures
REV~LVIN~ SCREENS OR TROMMELS that take into account smaller sizes and design features of the
equipment appear in the following references:
One type is shown in Figure 12.2(a). They are perforated cylinders
rotating at 15-20 rpm, below the critical velocity. The different- Mathews, Chem. Eng. 76 (10 July 1972) and presented in Chemical

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