Page 378 - Chemical Process Equipment - Selection and Design

12.4. EQUIPMENT FOR SIZE REDUCTION 341

EXAMPLE ELI The results are tabulated following at several values of V:
Siing a Hydrocyclone
A hydrocyclone assembly is required to handle 10,000gpm of D
slurries of a soliid with specific gravity 2.9 with a cutoff point of No. Units
d,, = 100 pm. The allowable pressure is AP = 5 psi. Several slurry v ep in. mm o in Parallel
concentrations 'ir will be examined. Substituting into Eq. (12.1), 5 1.0953 57.7 1466 5214 2
with z the functiion of V in parentheses, 10 1.4282 39.0 989 2375 4
20 2.0385 23.0 584 828 12
30 3.2440 11.6 293 209 48

whence From Figure 12.3(b), with 5 psi a 660 mm unit will handle 1000 gpm
and have a cutoff between 50 and 150 pm. This corresponds to the
D = (16.92/ez)l 4815.
calculated data with V about 19 ~01%. For a more detailed study of
The correspondung capacity of one hydrocyclone is hydrocyclone sizing, the article of Mular and Jull (in Mular and
Bhappu, 1980, pp. 376-403) may be consulted. The pressure drop
Q = 0.7(5)0 5D2. can be adjusted to compensate for changes in slurry concentration.

grinders. Performances of attrition, cutter and jet mills with some ROLL CRUSHERS
materials are given with Table 12.7. Additional operating data
arranged by material are referred to in Table 12.10. Toothed rolls such as those of Table 12.8(b) can handle rela-
Closed circiilit grinding employs a means whereby only material tively large lumps, for example, 14 in. maximum with 24 in. rolls
smaller than a specified size appears in the product. A less precise according to the table. To smooth rolls, the feed size is limited by
mode of operatiion employs an air stream through the equipment at the angle of nip which depends on the surface conditions but often
such a rate that only the appropriately fine material is withdrawn is approximately 16" or arccos 0.961. Accordingly the relation
and the rest remains until it is crushed to size. Ball mills sometimes between the diameters of the roll d, and feed df and the gap do
are operated in this fashion, and also the ring-roller mill of Figure between the rolls is given by
12.4(a). For closer size control, all of the crushed material is
withdrawn as if is formed and classified externally into product and d, = (0.961df - d0)/0.039. (12.4)
recycle. The other examples of Figure 12.4 illustrate several such
schemes. For example, with df = 1 in. and do = 0.25 in., the roll diameter is
Wet grinding with water is practiced when dusting is a problem, figured as 18 in. Table 12.8(b) lists 16 in. as the smallest size suitable
or when subsequent processing is to be done wet, as of ores that are for this service, which appears to be somewhat marginal in
later subjected to separation by flotation or sink-float processes. comparison with the calculated result. According to the formula,
Removal of a slurry from a ball mill is easier than of dry material; 1 in. lumps could be nipped by 16 in. rolls with a spacing of 0.34 in.
there are cases where this advantage is controlling. Because of the It is not possible to state who is smarter, the formula or the
lubricating effect of the water, power consumption of wet milling is manufacturer.
less per ton, but this advantage may be outweighed by corrosion of Figure 12.5(b) shows a smooth roll assembly. Usually only one
the equipment. of the rolls is driven and one is spring mounted to prevent damage
by uncrushable material in the feed. Reduction ratios shown in
Table 12.8(c) range only between 2: 1 and 4: 1. The proportion of
12.4. EQUIPMENT FOR SIZE REDUCTION fines is comparatively small. Sets of rolls in series with decreasing
settings are used to achieve overall high reduction ratios. The rolls
Some of the many available kinds of size reduction equipment can
be described here. Manufacturers' catalogs have the most complete of a pair can be driven at the same or different speeds, within a
descriptions of the equipment and almost always provide typical or range of 50-900rpm. The capacity generally is about 25% of the
expected performance data. Useful compilations of such informa- maximum corresponding to a continuous ribbon of material passing
tion are by Taggart (1945) and the Chemical Engineers Handbook between the rolls. A sample listing of materials that are ground in
(1984, Section 8, as well as older editions). roll mills is in Table 12.5(a). In the arrangement of Figure 12.4(c),
the upper pair of rolls is the primary crusher whereas the lower pair
works on recycle of the oversize.
Hammer mills employ rotating elements that beat the material
CRUSHERS
until it is small enough to fall through the screen at the bottom of
Lumps as large as several feet in diameter are crushed in gyratory the casing. Product size is determined by the speed of the hammers
or jaw crushers. Figure 12.5(a) shows a type of crusher that is made and the size of the screen openings. Table 12.9(a) shows the former
in widths from about 5 to 70in. and with gaps from 4 to 60in. effect. The units of this table operate at speeds up to 900rpm and
Stroke rates vary from 300 to 100/min. The average feed is make size reductions of 40:l or so. The smaller units of Table
subjected to 8-10 strokes before it becomes small enough to escape. 12.9(b) operate at speeds to 16,000rpm and make very fine
The jaw crusher is suited to hard, abrasive, and also sticky feeds; it powders. Because of the heating effect, they often are operated
makes minimum fines but the product may be slabby because of the with a stream of ambient or refrigerated air for cooling. Under
iong, narrow exit. Gyratory crushers are more suited to slabby feeds these conditions even heat softening materials such as natural resins
and make a more rounded product. or chicle can be ground satisfactorily. Hammer mills are the