The chemical, pharmaceutical, food and mining industries all rely on size reduction. Its uses include grinding polymers for recycling, improving extraction of a valuable constituent from ores, facilitating separation of grain components, boosting the biological availability of medications, and producing particles of an appropriate size for a given use. There are many types of size-reduction equipment, which are often developed empirically to handle specific materials and then are applied in other situations.
Knowing the properties of the material to be processed is essential. Probably the most important characteristic governing size reduction is hardness because almost all size-reduction techniques involve somehow creating new surface area, and this requires adding energy proportional to the bonds holding the feed particles together. A common way of expressing hardness is the Mohs scale, on which talcum is a 1 and diamond is a 10. Also important is whether a material is tough or brittle, with brittle materials being easier to fracture.
Other characteristics include particle-size distribution, bulk density, abrasiveness, moisture content, toxicity, explosiveness and temperature sensitivity. Flow properties can be major factors, too, because many size-reduction processes are continuous, but often have choke points at which bridging and flow interruption can occur. For instance, most size-reduction equipment is fed by chutes, which might constrict flow. Often, the feed flows adequately, but the crushed product will compact and flow with difficulty. Intermediate storage bins might aggravate flow issues by causing compaction and bridging.
For a given feed material, it is important to determine the desired particle-size distribution of the product. In mining, for example, very fine particles can interfere with separation processes, such as froth flotation, and might result in loss of valuable components. In other operations, the objective might be to produce very fine particles. Sometimes, as in sugar grinding, very fine particles are agglomerated to increase the share of larger particles.
Many particle-size distributions can be represented by the Gaudin-Schuhmann equation:
y = 100 (x/xm)Âª where y is the cumulative percentage of material that is finer than size x, xm is the theoretical maximum size, and Âª is the distribution modulus, which is related to hardness and has lower values for softer materials (0.9 for quartz and 0.3 for gypsum, for instance). The equation indicates that softer materials produce more fines .
Nearly all size-reduction techniques result in some degree of fines. So unless producing very fine particles is the objective, it usually is more efficient to perform size reduction in stages, with removal of the desired product after each operation.
Size-reducing equipment relies on compression or impact. Compression is applied via moving jaws, rolls or a gyratory cone. The maximum discharge size is set by the clearance, which is adjustable. Impact-based equipment commonly uses hammers or media. The pros and cons of several types of size-reduction equipment are shown in the table.
Rolls, in particular, can produce very fine particles. Rolls are used in flour milling, where crushing yields different-sized particles, allowing separation of purified flours. Moisture content is important so that, for example, the bran is soft and remains in large pieces, whereas the endosperm is brittle and fractures into small granules. Corn germ can be separated from starch and fiber by roller milling because the germ selectively absorbs water and is made into flakes, whereas the starch fractures.
Impact mills use revolving hammers to strike incoming particles and to break or fling them against the machine case (Figure 1). The hammers might be fixed or, more commonly, pivoted. Typically, the hammers can be reversed to provide added life before they need to be replaced.
In jet mills, particles strike each other as they are transported in a stream of air or steam. For the initial reduction of large materials, a rotating drum propels the feed into the air where the pieces strike each other and fracture.
Ball, pebble and rod mills are rotating cylinders that are partially filled with metal or ceramic balls, flint pebbles or rods. The units are capable of producing very fine powders, such as pigments for inks and paints, but are quite energy inefficient. The crushing mechanism is a combination of impact with the grinding media and shearing between the media and the cylinder walls (Figure 2). A variation is a jar mill, in which relatively small ceramic containers holding some grinding media are rotated on a common machine frame. It is used for small batches of valuable chemicals and in laboratories.