INTRODUCTION
2.3. TYPES OF CRUSHING EQUIPMENT
2.4.3. Size enlargement processes
Processes commonly used for size enlargement are listed in Table 2.4, taken from Perry(30). For comprehensive overall reviews, reference may be made to PERRY and to the work of BROWNING(31).
(a) Spray drying (as discussed in Chapter 16).
In this case, particle size is largely determined by the size of the droplet of liquid or suspension, which may be controlled by a suitably designed spray nozzle. The aggregates of dried material are held together as a result of the deposition of small amounts of solute on the surface of the particles. For a given nozzle, the drop sizes will be a function of both flowrate and liquid properties, particularly viscosity, and to a lesser extent of outlet temperature. In general, viscous liquids tend to form large drops yielding large aggregates.
(b) Prilling in which relatively coarse droplets are introduced into the top of a tall, narrow tower and allowed to fall against an upward flow of air. This results in somewhat larger particles than those formed in spray dryers.
(c) Fluidised beds (as discussed in Chapter 6). In this case, an atomised liquid or suspension is sprayed on to a bed of hot fluidised particles and layers of solid build up to give enlarged particles the size of which is largely dependent on their residence time, that is the time over which successive layers of solids are deposited.
Spouted beds (as discussed in Chapter 6). These are used, particularly with large particles. In this case, the rapid circulation within the bed gives rise to a high level of inter-particle impacts. These processes are discussed by MORTENSEN and HOVMAND(32)and by MATHERand EPSTEIN(33).
(d) Drum and pan agglomerators.
Indrum agglomerators, particles are ‘tumbled’ in an open cylinder with roughened walls and subjected to a combination of gravitational and centrifugal forces. In order to aid agglomeration, liquid may be sprayed on to the surface of the bed or introduced through distribution pipes under the bed. Mean retention times in the equipment are in the range 60 to 120 s. A similar action is achieved in apaddle mixer where centrifugal forces dominate. In the pan agglomerator, a classifying action may be achieved which results in the fines having a preferentially longer retention time. Larger, denser and stronger agglomerates are produced as compared with those from the drum agglomerator.
(e) Pug mills and extruders.
Pug mills impart a complex kneading action that is a combination of ribbing and shearing and mixing. Densification and extrusion are both achieved in a single operation. The feed, which generally has only a small water content, is subjected to a high energy input which leads to a considerable rise in temperature. The action is similar to that occurring in anextruder. High degrees of compaction are achieved, leading to the production of pellets with low porosity with the result that less binder is required.
(f) Elevated temperatures.
With many materials, agglomeration may be achieved by heating as a result of which softening occurs in the surface layers. For the formation of porous metal sheets and discs, high temperatures are required.
PARTICLESIZEREDUCTIONANDENLARGEMENT141 Product size
Method (mm) Granule density Scale of operation Additional comments Typical application
Tumbling granulators 0.5 to 20 Moderate 0.5– 800 tonne/h Very spherical granules Fertilisers, iron ore,
Drums ferrous ore, agricultural
Discs chemicals
Mixer granulators
Continuous high shear 0.1 to 2 Low to high Up to 50 tonne/h Handles very cohesive Chemicals, detergents,
(e.g. Shugi mixer) materials well, both clays, carbon black
Batch high shear 0.1 to 2 High Up to 500 kg batch batch and continuous Pharmaceuticals, ceramics
(e.g. paddle mixer)
Fluidised granulators Flexible, relatively easy Continuous: fertilisers,
Fluidised beds 0.1 to 2 Low (agglomerated) 100– 900 kg batch to scale, difficult for inorganic salts,
Spouted beds Moderate (layered) 50 tonne continuous cohesive powders, detergents
Wurster coaters good for coating Batch: pharmaceuticals,
applications agricultural chemicals, nuclear wastes Centrifugal granulators 0.3 to 3 Moderate to high Up to 200 kg batch Powder layering and Pharmaceuticals,
coating applications agricultural chemicals Spray methods
Spray drying 0.05 to 0.5 Low Morphology of spray Instant foods, dyes,
dried powders can detergents, ceramics
Prilling 0.7 to 2 Moderate vary widely Urea, ammonium nitrate
Pressure compaction High to very high Very narrow size Pharmaceuticals, catalysts,
Extrusion >0.5 Up to 5 tonne/h distributions, very inorganic chemicals,
Roll press >1 Up to 50 tonne/h sensitive to powder organic chemicals
Tablet press 10 Up to 1 tonne/h flow and mechanical plastic performs, metal
Molding press properties parts, ceramics, clay
Pellet mill minerals, animal feeds
Thermal processes
Sintering 2 to 50 High to very high Up to 100 tonne/h Strongest bonding Ferrous & non-ferrous ores, cement clinker minerals, ceramics Liquid systems
Immiscible wetting <0.3 Low Up to 10 tonne/h Wet processing based Coal fines, soot and oil
in mixers on flocculation removal from water
Sol– gel processes properties of Metal dicarbide, silica
particulate feed hydrogels
Pellet flocculation Waste sludges and slurries
Reprinted fromGranulation and Coating Technologies for High-Value-Added Industries, Ennis and Litster (1996) with permission of E & G Associates. All rights reserved.
(g) Pressure compaction.
If a material is subjected to very high compaction forces, it may be formed into sheets, briquettes or tablets. In the tableting machines used for producing pills of pharmaceuticals, the powder is compressed into dies, either with or without the addition of a binder.
Powder compaction may also be achieved in roll processes, including briquetting, in which compression takes place between two rollers rotating at the same speed — that is without producing any shearing action. In pellet mills, a moist feed is forced through die holes where the resistance force is attributable to the friction between the powder and the walls of the dies.
A commercial pelleting process, used for powdery, lumpy and pasty products, is illustrated in Figure 2.35.
Product inlet
Hydraulic roller adjustment device
Pan grinder roller
For powdery, lumpy, or pasty products.
Pellet diameter:
< 2 mm slipping agent required
> 2 mm no slipping agent required
Gear Roller bearing
with slip ring seal Scraper
Die
Roller gap
Cutting device
Main bearing Pellet discharge
Figure 2.35. The KAHL pelleting press