Types of Comminution Equipment

TYPES OF COMMINUTION EQUIPMENT 12.4.1 Factors Affecting Choice of Machine... 

Characteristics of the main common types of size reduction equipment are listed in Table 12.3, including size of feed, size of product, capacity, power consumption, and average reduction ratio. Coarse comminuters perform with reduction ratios less than 10, fine ones with ratios of 100 or more. From very large to very fine may require several operations in series, as in the flowsketch of Figure 12.4(b), where three stages of crushing and two of classification are shown. 

Classification can be performed by centrifuges, cyclones, or air classifiers. In this diapter, the fundamentals of comminution and classification will be discussed in addition to a review of the different types of equipment used for these two steps. Comminution and classification are the most important methods of transformation of minerals to ceramic powders. They are also used for synthetic ceramic powders, because... 

Comminution and size separation are characterized by the variety of equipment devised for them. Examples of the main types are described in this chapter with a few case studies. For equipment, it is essential to consult manufacturers catalogs for details of construction, sizes, capacities, space, and power requirements. Textbooks give general information for these operations but there are few generalizations for the prediction of equipment characteristics. A list of manufacturers of this equipment may be found in the Chemical Engineering Equipment Buyers Guide 2004 (2003) as well as on the Internet. 

The most important factors that determine the selection of the size-reduction equipment are the mechanical characteristics (shear strength, ductility, etc.) of the feed material, as well as the size distribution of feed and comminuted product. From the aforementioned analysis, it is clear that the mechanical characteristics determine the acting force for size reduction and, consequently, the selection of the proper equipment. The size distribution of the feed stream and product determines the type of the corresponding equipment as well as the dimensions of feed and discharge openings. 

The capacities of comminuting machines are best discussed when the individual types of equipment are described. The fundamentals of product size and shape and of energy requirements are, however, common to most machines and can be discussed more generally. 

The size distribution of products from various types of size-reduction equipment can be predicted by a computer simulation of the comminution process. This makes use of two basic concepts, that of a grinding-rate function S and a breakage function A5 a. The material in a mill or crusher at any time is made up of partides of many different sizes, and they all interact with one another during the size-reduction process, but for purposes of computer simulation the material is imagined to be divided into a number of discrete fractions (such as the ones retained on the various standard screens) and that particle breakage occurs in each fraction more or less independently of the other fractions. 

The diversity of designs of the above-described machinery implies that their operating variables differ quite considerably. While energy requirements are generally governed by the comminution laws previously discussed, as illustrated in Example 4.1, some other features such as capacity and rotational velocity are particular for each imit and may be related, in some way, to the predominant force performing the reduction action. Two types of equipment with critical particularities of operation would be roller mills and tumbling mills. 

A(p) is the rate constant for attrition. The rate of attrition depends on the material to be granulated, the binder, the type of equipment used for granulation and the intensity of agitation. The rate of breakage may also be accounted for by the use of selection and breakage function as used in the simulation of population balances in comminution (see Chapter 12). 

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. 

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