Mechanical comminution

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

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. 

Mechanical comminution of the sample is performed because the composition of polymers frequently shows inhomogeneities despite good processing. Furthermore, some tests (e.g., monomer content and H20 content) depend on the sample. 

Waltersson J, Lundgren P. The effect of mechanical comminution on drug stability. Acta Pharm Suec 1985 22 291-300. 

Among low-temperature procedures widely used in the powder production of ductile materials are various kinds of cryodispersion or cryomilling methods based on mechanical comminution of materials at cryogenic temperatures. A decrease of the comminution temperature is accompanied by a corresponding... 

Figure 6. Relationship between compacting pressure and sintered density for mineral source (as-received MCW UO2) and mechanically comminuted (hammer-milled MCW UO2) powders. (After Ref. 2.)...

Coldstream Process [11.7] (see the flow sheet Fig. 11.5). This process is a mechanical comminution and employs air blasts to accelerate particles against a target with sufficient energy to cause fracture (particle velocities near Mach 2). The air is cooled by expansion from the nozzles, protecting the scrap from oxidation. After screening or air classification, oversize material is recycled. 

Petrovich, R. 1981. Kinetics of dissolution of mechanically comminuted rock-forming oxides and silicates - I. Deformation and dissolution of quartz under laboratory conditions. Geochim. Cosmochim. Aclu 45 1665-1674. 

J. Waltersson and P. Lundgren, The effect of mechanical comminution on drag stability, Acta Phann. Suec. 22, 291-300 (1985). 

Mechanical comminution of lignocellulosic materials beyond that achieved by animals tends to increase the digestion ceiling in vitro. This is illustrated by the percentage of the forage carbohydrate fermented... 

Small metal particles are produced by mechanical comminution, chemical reaction, electrolytic deposition, and liquid-metal atomization ... 

Increase of surface area by mere mechanical comminution may sometimes lead to self-flammable products. Such is the case in the ball-milling of certain stainless steels and of misch metal (cerium alloy) under a hydrocarbon. The process leads to formation of minute flakes with the character of bronzing powders. Spontaneous ignition after removal of the liquid in the case of the stainless steels is due to the absence of a passivating protective layer upon the newly created surfaces. 

The coke, the solid product of pyrolysis, contains the mineral and nonoutgas-sable components of the starting material. It is brittle when cooled, so that mineral moieties can be separated out by means of mechanical comminution and classifying methods. Utilization of this material is still being worked out [16]. 

The advantages of steam explosion pretreatment include the low energy requirement compared with mechanical comminution and no recycling or environmental costs. 

The disintegrating behavior of substances with any such characteristics can be substantially improved by addition of carbon dioxide ice or by embrittlement with liquid nitrogen. The use of direct or indirect cooling, in combination with mechanical comminution techniques, facilitates adequate processing of many types and grades of plastic, rubber, organic tissue, pharmaceutical products, spice, herbs, and food samples. 

Petrovich, R. (1981b). Kinetics of dissolution of mechanically comminuted rock-... 

Thermoset composite materials can be recycled through mechanical comminution techniques, to reduce the size of the scrap to produce lecyclates, using thermal processes to break the scrap down into materials and energy (Pickering, 2006). 

Particle size reduction. Typically, the hair segment (say, 1—3 cm long) is cut into 1—2-mm pieces using scissors. As an alternative to this laborious process, mechanical comminution of the hair sample in various types of mills, including ball, bead, bullet, or freeze mills, has been described [118—123]. The dramatic increase in surface area achieved has the added advantage of reducing the time required for the subsequent extraction step. A procedure has been developed that combines the micropulverization and extraction steps, reducing the time for the entire process to minutes [122]. 

Technologically, electrodeposition competes with other preparation techniques including (t) chemical reaction (2) mechanical comminution and (3) atomization. 

As the current density is increased, however, the deposit generally becomes rougher in texture and higher in the active area (Chapter 8, Fig. 8.10) as mass transport control begins to exert an influence. Under conditions of significant mass transport control, the roughened deposit may become loosely adherent and disperse, i.e a metal powder is formed. This is the direct electrolytic method for metal-powder formation. Alternatively, for certain metals, the electrolysis conditions may be chosen to give a relatively coherent but brittle deposit which may be transformed to a powder by mechanical comminution. 

The choice of technique is important not only from the standpoints of economics and convenience but also the properties of the metal powder are very dependent upon the production method. Chemical reactions generally yield cheap, porous, readily-compressible powders but alloy production is not possible. Mechanical comminution is a convenient technique for brittle metals but results in an irregular particle shape. 

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