Size reduction equipment Ball mill

The raw ROM (run of mine) ore is reduced in size from boulders of up to 100 cm in diameter to about 0.5 cm using jaw cmshers as weU as cone, gyratory, or roU-type equipment. The cmshed product is further pulverized using rod mills and ball mills, bringing particle sizes to finer than about 65 mesh (230 p.m). These size reduction (qv) procedures are collectively known as comminution processes. Their primary objective is to generate mineral grains that are discrete and Hberated from one another (11). Liberation is essential for the exploitation of individual mineral properties in the separation process. At the same time, particles at such fine sizes can be more readily buoyed to the top of the flotation ceU by air bubbles that adhere to them. 

The selection guides given by Lowrison (1974) and Marshall (1974), which are reproduced in Tables 10.12 (see p. 465) and 10.13, can be used to make a preliminary selection based on particle size and material hardness. Descriptions of most of the equipment listed in these tables are given in Volume 2, Chapter 2 or can be found in the literature Perry et al. (1997), Hioms (1970), Lowrison (1974). The most commonly used equipment for coarse size reduction are jaw crushers and rotary crushers and for grinding, ball mills or their variants pebble, roll and tube mills. 

The apparent efficiency of the size reduction operation depends on the type of equipment used. Thus, for instance, a ball mill is rather less efficient than a drop weight type of crusher because of the ineffective collisions that take place in the ball mill. 

The second class of grinding equipment is used to prepare dispersions. Typical of this class are ball and pebble mills, ultrasonic mills, and attrition mills. Solids, eg, sulfur, antioxidants, accelerators, and zinc oxide, are generally ground on this equipment (see Size reduction). Ball mill action is assisted in some mills by a combination of dispersion circulation by an external pump and mechanical oscillation of an otherwise fixed nonrotary mill chamber. Where ball mill chambers are rotated it is necessary to experimentally establish an optimum speed of rotation, the size and weight of the ball charge, and ensure the mills do not overheat during the grinding period. 

Figure 12.5. Jaw, roll, impact, and tumbling equipment for size reduction, (a) Blake-type jaw crusher operates at 200-300 strokes/min (Allis-Chalmers Co.), (b) Smooth roll crusher, for which operating data are in Table 12.8(b). (c) Swing hammer mill operating data in Table 12.7(a). (d) View of a conical ball mill, showing distributions of balls and material and crusting ranges data in Table 12.6(b) (Hardinge Co.). (e) Tube mill with three compartments, length to diameter ratio 3-5. (f) Rod mill in a cylindrical tumbler, LID = 1.2-1.6 data in Table 12.6(d).

Open-circuit grinding occurs when no attempt is made to return the oversize material to the mill for further size reduction. Closed-circuit grinding employs a means whereby only material smaller than a specified size appears in the product. A less precise mode of operation employs an air stream through the equipment at such a rate that only the appropriately fine material is withdrawn and the rest remains until it is crushed to size. Ball mills sometimes are operated in this fashion, and also the ring-roller mill of Figure 12.5(a). For closer size control, all of the crushed material is withdrawn as it is formed and classified externally into product and recycle. The other examples of Figure 12.5 illustrate several such schemes. 

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