3.2. Balling drum

3. Balling technologies

46 Iron Ores and Iron Oxide Materials

3.1. Balling disc or disc pelletizer

size distribution.

The balling equipment that can be either a disc or a drum produces green pellets in a variable

Figure 3 schematically shows a laboratory disc pelletizer. The balling disc basically consists of a pan with a peripheral wall, which rotates with a certain inclination to the horizontal [1]. In

When the disc rotates, the feed material with less moisture than that required for the pellets formation, is charged to the bottom of the disc, where it get in contact with the water from the sprayers, initiating the nucleation stage. At this stage, the nuclei begin to take the form of small pellets, which by rolling action, occur in the lower section, to the left of the disc, toward the top. As the added ore aggregates unto the surface of the pellets, they increase in size and the coefficient of friction is reduced causing the pellets to acquire a centrifugal force that carries them out of the nucleation zone. This movement takes the pellets to the top of the disc

general, in a balling disc, the pan angle can be adjusted between 40 and 60 [11].

following a semicircular trajectory before returning to the base of the disc [1, 12].

Figure 3. Schematic diagram of laboratory balling disc. Source: Modified by the authors with data from Ball [1].

The balling drum equipment fundamentally consists of an inclined rotating cylindrical shell with water sprays in its inlet end, where the feed material is introduced to make balls. All formed pellets are discharged, regardless of particle size, which is different from the disc where only balls larger than a certain size are discharged. Because of this, the product has to be screened by a roller screen, which has increasingly replaced vibrating screen to extract undersize and oversize [4].

The small pellets in the undersize will serve as seeds forming rightly sized pellets [4], the oversize is shredded, and both return to the feed addition point, forming a closed circuit. The water sprays promote nucleation and seed growth in the feed addition zone, while the assimilation process, responsible for the ball growth, occurs along the length of the drum [1].

The drums usually have a length-to-diameter ratio of 2.5–3.5 and very low slope with angles of inclination of the drum axis to the horizontal between 6 and 10�. The optimum rotating speed is generally between 25 and 35% of the critical speed that is the speed in which balls will centrifuge causing their degradation [1]. Speed control is necessary to develop a correct rolling and tumbling action to produce balls.

The rotation speed, the depth of the material in the drum (controlled by scrappers), and the time required for balling are constraints that need to be satisfied. The residence time in a drum is given by Eq. (1):

$$T = \frac{0.0037(\alpha + 24)L}{nDS} \tag{1}$$

where: T is the residence time (minutes).

L is the drum length (feet).

D is the drum diameter (feet).

n is the speed of revolution (revolutions per minute).

S is the slope of the drum (inches/foot length).

α is the angle of repose of the feed material.

Figure 4 shows a balling drum arrangement.

Figure 4. Typical balling drum arrangement. Source: Modified by the authors with data from Ball [1].
