Size reduction comminuted materials

See also Minerals recovery/processing Minerals recovery/processing, 16 595-668. See also Minerals concentration Minerals processing classification in, 16 618-622 economic aspects of, 16 606-609 environmental aspects of, 16 609—610 flow sheets in, 16 603-605 materials handling in, 16 660-663 from ocean waters, 17 695—697 ores, 16 598-603 process control in, 16 663-665 size reduction (comminution) in, 16 610-615... 

A machine for the comminution, or size reduction, of materials. Such machines use a rotating shaft on which is mounted a series of cutting knives that interleave with a series of separately mounted stationary knives. Cutting mills can reduce materials to particles on the order of 100 tm. 

Breaking is the term applied to size operations on large material (say +75 mm) and crushing to particle size reduction below 75 mm the term grinding covers the size reduction of material to below about 6 mm. However, these terms are loosely employed. A general term for all equipment is size reduction equipment, and because the term comminution means size reduction, another general term for the equipment is comminution equipment. 

Flow Sheets. AH minerals processing operations function on the basis of a flow sheet depicting the flow of soHds and Hquids in the entire plant (6,13,14). The complexity of a flow sheet depends on the nature of the ore treated and the specifications for the final product. The basic operations in a flow sheet are size reduction (qv) (comminution) and/or size separation (see Separation, size), minerals separation, soHd—Hquid separation, and materials handling. The overaH flow sheet depends on whether the specification for the final mineral product is size, chemical composition, ie, grade, or both. Products from a quarry, for example, may have a size specification only, whereas metal concentrates have a grade specification. 

Size reduction (qv) or comminution is the first and very important step in the processing of most minerals (2,6,10,20—24). It also involves large expenditures for heavy equipment, energy, operation, and maintenance. Size reduction is necessary because the value minerals are intimately associated with gangue and need to be Hberated, and/or because most minerals processing/separation methods require the ore mass to be of certain size and/or shape. Size reduction is also required in the case of quarry products to produce material of controlled particle size (see Size measurement of particles). In some instances, hberation of valuables or impurities from the ore matrix is achieved without any apparent size reduction. Scmbbers and attritors used in the industrial minerals plants, eg, phosphate, mtile, glass sands, or clay, ate examples. 

Institution of Chemical Engineers, 1975. In Comminution, Institution of Chemical Engineers Working Party concerned with the theory and practice of the size reduction of solid materials. Ed. V.C. Marshall, Institution of Chemical Engineers, Rugby, 83pp. 

The major purposes of comminution or size reduction operations can be summarized in four categories as shown in Table 2.2. Comminution is almost invariably performed in two or more stages. It starts with the crude mined or quarried product this is progressively taken to the desired final size by a step-by-step process. As the process of fracture is involved throughout this procedure, a brief discussion on the fracture of materials is appropriate at this point. 

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... 

A machine for the comminution, or size reduction, of minerals. Such machines crush the input material between a stationary ring and vertical rollers revolving inside the ring. Particle sizes of as low as about 30 im can be produced. 

Impact is the simplest size-reduction force. It is also appUed for the comminution of brittle materials. As feed particles fall free on a high-speed solid rotor, the massive rotor impacts and breaks them into smaller particles. Secondarily, the material is struck onto a compact plate, where particle size is further reduced. Crushers based on impact force are impactors and hammer mills. They are characterized by high size reduction ratio, reaching up to 20 1. 

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 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. 

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