Page 388 - Chemical Process Equipment - Selection and Design

12.5. PARTICLE SIZE ENLARGEMENT
TABLE 12.10. Nlill Performance Data for Grinding of Specific agglutination:
PrOdRlCUS
1. Superphosphate, sulfates, and NPK fertilizers.
Handbook 2. Carbamide and diammophosphate.
Material Equipment Table No.
3. Ammonium phosphate, potassium chloride, potassium bicarbon-
Anthracite ball mill CC 46 ate, and salt.
Barite wet Hardinge ball mill 35
Cement clinker three-compartment 42
wet tube mill Adhesion of any mass of particles can be developed by sufficiently
Fertilizers hammer mill 41 high pressure, but lower pressures suffice upon addition of liquid or
Fuller’s earth roller 48 syrupy binders. Table 12.12 is a list of some commercial
Grain attrition 32 agglomerations and the binders that they employ, and Table 12.13
Gypsum rock ring-roller 45 shows how much moisture is needed.
Iron oxide ring-roller 47 The main types of processes used industrially for particle size
Limestone ring-roller 34
Limestone wet Hardinge ball mill 35 enlargement are five in number, defined as foIlows:
Metai stearates hammer mill 50
Oyster shells hammer mill 38 1. Compaction is achieved either by compression or extrusion.
Phosphates ball mill 39-40 Compression is done either into a mold to give a final shape or
Quicklime ball mill CC 44 into a sheet or block that is later broken up to proper sizes.
Rubber roller mill 51 Extrudates are formed under pressure in dies of a variety of cross
Seed cake hammer mill 33 sections; as they leave the die they are broken up or cut to size.
Siliceous pebble mill 36 2. Agglomeration is accomplished under tumbling or otherwise
refractories agitated conditions, with or without binding agents. Size is
Slate three-compartment 43 controlled by adjusting the residence time and by gradual
wet tube mill
Sodium carbonate roller 48 addition of feed and binder, slurry or solution.
Sulfur ring-roller 49 3. Globulation is the formation of droplets of solution, slurry, or
melt followed by solidification by prilling, spray drying, or
Note: CC is closed circuit grinding; the ring-roller mill has built-in fluidized bed operation. Control of particle size is best achieved
air classification. in fluidized beds.
(From Chemikal Engineers’ Handbook, IWcGraw-Hill, New York, 4. Heat bonding is of two types: nodulization in which material is
1984, pp. 8.48-8.60).
tumbled while heated to give hard rounded granules and
sintering in which the product is an integrated mass that is
subsequently broken to size.
5. Flocculation, coagulation and growth of particles in dilute
flatter surface. When the rotating surface is cylindrical and the slurries, to assist in subsequent sedimentation and liltration.
flat surface is horizontal, the equipment is called a dry pan mill.
The equipment shown throws the crushed material outwards A particular industry may employ more than one of these
where it is picked up and removed with an air stream. Table techniques, for instance the manufacture of solid catalysts.
12.5(c) is a list of materials that are being ground in roller mills. Spherical catalysts are made in rotating pan granulators (Fig. 12.7).
The ring-roller mill of Figure 12.4(a) is in this class. If the rheological properties are suitable, the material can be
Fluid jet pulverizers have opposed high speed gas jets that cause extruded (Fig. 12.8), then cut into short cylinders, and subsequently
collision and disintegration of the particles. A size classifier and tumbled (Fig. 12.9) into rounded shapes. Smaller spherical beads,
fan return larger sizes to the jet stream. The “Majac” jet mill for instance, of catalysts for moving bed processes, are made by
of Figure 12.h(g) is a related kind of device; it has a horizontal precipitation or coagulation in an immiscible fluid. Pellets or rings
section in which high speed gas jets act on the particles. These are made on tabletting machines (Fig. 12.10). Although the process
mills are used primarily for specialty fine grinding of high-value is more expensive than extrusion, the produce is more nearly
materials. Performance data of Micronizers are in Table 12.9(c); uniform. Both extrusion and tabletting result in diffusion resistant
those of the PAajac puliveiizer are expected to be similar. skins that, however, usually are eIiminated on drying or calcination
of the catalyst. Ammonia synthesis catalyst is made by sintering
(Fig. 12.11) or fusion of the several ingredients, then crushed and
lCLE SEE ENLARGEMENT used as irregular lumps of size ranges 1.5-3, 6-10, and 12-21 mm.
In the following, the main equipment €or particle size enlarge-
For many purposes, lumps of materials of intermediate sizes are the ment will be illustrated and discussed.
most desirable forms, neither too small nor too large. For instance,
beds of overly small granules of catalysts exhibit too great resistance TUMBLERS
to flow of reacting fluids, and too small particles in suspensions
settle out or filter too slowly. Other situations that benefit from size The particles of a granular mass will cohere when they are tumbled
enlargement of particles are listed in Table 12.11. and sprayed lightly with a liquid binder which often is water or a
Because of adhesive forces, particles tend to stick together, concentrated solution of the material being agglomerated. The
particularly small particles that have a large ratio of surface to mass. growth may be due to agglomeration of small particles or to
If a mass is vibrated or shaken lightly, for instance, smaller particles layering of material evaporated from the sprayed solution. Rotary
penetrate the interstices between larger ones with increase of kilns of the kind used for drying or chemical reaction (cement or
contact area and adhesion of the mass. Substances differ naturally in lime burning, for instance) are adapted to size enlarging service.
their tendency to agglutinate; as examples, the following groups Usually the tumbling action is less intense, only enough to expose
of materials are listed in the order of increasing tendency to the material to sprays. The sprays are fine and are applied to the