Tumbling Granulators

In tumbling granulators, particles are set in motion by the tumbling action caused by the balance between gravity and centrifugal forces. The most common types of tumbling granulators are disc and drum granulators. Their use is widespread, including the iron-ore industry (where the process is sometimes called balling or wet pelletization), fertilizer manufacture, agricultural chemicals and pharmaceuticals.

Tumbling granulators generally produce granules in the size range 1 to 20 mm and are not suitable for making granules smaller than 1

mm. Granule density generally falls between that of fluidized-bed and mixer granulators (Fig. 20-71), and it is difficult to produce highly porous agglomerates in tumbling granulators. Tumbling equipment is also suitable for coating large particles, but it is difficult to coat small particles, as growth by coalescence of the seed particles is hard to control.

Drum and disc granulators generally operate in continuous feed mode. A key advantage to these systems is the ability to run at large scale. Drums with diameters up to four meters and throughputs up to 100 ton/hr are widely used in the mineral industry.

Disc Granulators Figure 20-81 shows the elements of a disc granulator. It is also referred to as a pelletizer in the iron-ore industry or a pan granulator in the agricultural chemical industry. The equipment consists of a rotating, tilted disc or pan with a rim. Solids and wetting agents are continuously added to the disc. A coating of the feed material builds up on the disc and the thickness of this layer is controlled by scrapers or a plow, which may oscillate mechanically. The surface of the pan may also be lined with expanded metal or an abrasive coating to promote proper lifting and cascading of the particulate bed, although this is generally unnecessary for fine materials. Solids are typically fed by either volumetric or gravimetric feeders. Gravimetric feeding generally improves granulation performance due to smaller fluctuations in feed rates which act to disrupt rolling action in the pan. Wetting fluids which promote growth are generally applied by a series of single-fluid spray nozzles distributed across the face of the bed. Solids feed and spray nozzle locations have a pronounced effect on granulation performance and granule structure.

Variations of the simple pan shape include (1) an outer reroll ring which allows granules to be simultaneously coated or densified without further growth, (2) multistepped sidewalls, and (3) a pan in the form of a truncated cone (Capes, Particle Size Enlargement, Elsevier, 1980).

The required disc-rotation speed is given in terms of the critical speed, i.e., the speed at which a single particle is held stationary on

Disc Granulator Diagram

the rim of the disc due to centripetal forces. The critical speed Nc is given by:

2n2D

where g is the gravitational acceleration, ^ is the angle of the disc to the horizontal, and D is the disc diameter. The typical operating range for discs is 50 to 75 percent of critical speed, with angles ^ of 45-55°. This range ensures a good tumbling action. If the speed is too low, sliding will occur. If the speed is too high, particles will be thrown off the disc or openings develop in the bed, allowing spray blowthrough and uneven buildup on the disc bottom. Proper speed is influenced by flow properties of the feed materials in addition to granulation performance.

Discs range in size from laboratory models 30 cm in diameter up to production units of 10 meters in diameter with throughputs of 100 ton/hr. Figure 20-82 shows throughput capacities for discs of varying diameter for different applications and formulation feed densities. When scaling up from laboratory or pilot tests it is usual to keep the i00

Q = 0.5D2 Mg/hr

Dry feed density o Manufacturer A i.i2 Mg/m3 • " A 2.00 " " a " C 0.94 Mg/m3 □ " D+ Various »Includes mixing, pelletizing and micropelletizing applications

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