Coarse comminution

The main purpose of coarse comminution in sample preparation is to reduce the maximum particle size of samples to allow effective mixing prior to subsampling. The ratio of feed size before reduction to the product size after reduction is known as the reduction ratio, and for the first crushing or milling stage this ratio should be as large as possible (Smith James 1981). Ratios of 40/1 for coarse crushers and 15/1 for coarse milling equipment are useful minimum specifications for laboratory preparation of coarse contaminated soil samples of up to 2kg. 

If highly resistant items (e.g. metal bars, rubbers or plastics) are to be included in the sample, they require preparation by a more appropriate method. Metals may be carefully cut down to 3 mm using a water-cooled saw or metal shear followed by cleaning to remove any contaminating swarf. Milling using 

Sample particles are loaded between die puck and die ring, and die ring and die vessel wall 

Milling vessel Percentage of sample passing test sieve mesh  

By using modified milling equipment it is possible to collect the gases liberated during comminution of a known amount of solid (Fig. 3.5). Routine 

---

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. 

Figure 21.5. Possible structures of coarse comminute meats, showing the influence of process and temperature.

Mechanical comminution may be used to form metal powders. Relatively coarse particles are produced by machining, whereas ball mills, impact mills, gyratory cmshers, and eddy mills give fine powders of britde materials. 

A novel comminution device, the roll press, has achieved commercial success [Schoenert, International J. Mineral Processing, 22, 401-412 (1988)]. It is used for fine crushing, replacing the function of a coarse-baU mill or of tertiary crushers. Unlike ordinary roll crushers which crush individual particles, the roll press is choke fed and acts on a thick stream or ribbon of feed. Particles are crushed mostly against other particles, so wear is veiy low. Energy efficiency is also greater than in ball mills. 

Herbal drugs are available in coarsely to finely chopped forms, as cut or concis" drugs leaves often appear on the market cut into four-sided fragments woods, roots, and barks as chips most fruits and seeds usually unbroken, in the whole or toto " condition, and they are sometimes crushed before being used. The extent of comminution plays an important part in the preparation of herbal infusions (see below Making the Tea). 

The efficiency of a classifier is represented by a performance or partition curve, similar to that used for screens, which relates to the particle size to the percentage of each size in the feed that reports to the underflow. The most important industrial application of classifiers is to reduce overgrinding in a mill by separating the milled product output into coarse and fine fractions in order that the fines can be removed as they are produced and the coarse returned for further comminution. 

Textured Soy Proteins. Textured vegetable proteins, primarily textured flours and concentrates (50% protein and 70% protein, dry basis, respectfully) are widely used in the processed meat industry to provide meat-like structure and reduce ingredient costs (3-6, 9-10). Available in a variety of sizes, shapes, colored or uncolored, flavored or unflavored, fortified or unfortified, textured soy proteins can resemble any basic meat ingredient. Beef, pork, seafood and poultry applications are possible 03, 4-7, 15, 19) Proper protein selection and hydration is critical to achieving superior finished product quality. Textured proteins have virtually no solubility and, thus, no ability to penetrate into whole muscle tissue Therefore, textured soy proteins are inherently restricted to coarse ground (e.g. sausage) or fine emulsion (e.g. weiners and bologna) products, and comminuted and reformed (i.e. restructured) meat products. None are used in whole muscle absorption or injection applications (2-4, 6, 11). 

Crushers of different designs are large-scale machines for coarse grinding. They have throughputs of up to 35001 h-1, with a specific comminution work of 0.15-2.2 kWh H. The product particle size lies between 5 and 250 mm. 

To use natural minerals it is necessary to grind them down to a desired particle size distribution. Grinding can be performed with the minerals dry or slurried in liquid. In most laboratories, this process is performed in a batch jar mill while on an industrial scale, continuous comminution equipment is used in conjunction with size classification equipment to recycle the coarse material. Figure 4.1 shows a typical comminution circuit with classification and recycle steps, as well as separation of the mineral from the conve3ring fluid. 

The effects of bypass on size selectivity are shown in Figure 4.15. Bypass reporting to the fine stream changes curve b to curve c. Bj ass reporting to the coarse stream moves from curve b to curve c. Comminution of the particles in a classifier will give rise to a size selectivity curve such as d. 

Figure 2.31 shows a log-normal distribution of geometric mean size 10 pm. The distribution is deficient in sub 6 pm particles, probably due to classification during a comminution operation, and is asymptotic to this size. If ( c-6) is taken as the particle size the parent distribution is obtained. A similar sort of plot occurs when there is a deficiency of coarse particles. 

Comminution gives particles smaller than that coarse division. The following factors may influence the choice of the appropriate device ... 

Compression is the most usual comminution force for brittle materials. Materials are compressed between two heavy-duty metal surfaces. Crushers based on compaction force are jaw, cone, gyratory, and roll crushers. These are most widely used to reduce the size of coarse rocks and minerals particles. In soUd waste treatment, their use is not widespread, because compression force, in most cases, simply deforms the materials but does not change their size (e.g., metals, paper, plastic, organics). 

---