Comminution reduction mechanism

The top-down approaches to the production of sub-micrometric particulate systems are mainly based on comminution by mechanical size reduction techniques, which require minimal use of... 

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. 

Comminution, or particle size reduction of solids, is considerably different from that of the breakup of one liquid by dispersal as small droplets in another. Particle size reduction is generally achieved by one of four mechanisms (1) compression, (2) impact, (3) attrition and (4) cutting or shear. Equipment for particle size reduction or milling includes crushers (which operate by compression, e.g., crushing rolls), grinders (which operate principally by impact and attrition, although some compression may be involved, e.g.,... 

Impact Milling Particle size reduction by high-speed mechanical impact or impact with other particles (also known as milling, pulverizing, or comminuting)... 

In industrial and laboratory settings the subdivision process more commonly involves the comminution of large particles or aggregates into smaller sizes, either dry with subsequent dispersion (size reduction to the order of a few pm) or directly in a slurry (size reduction to as small as a few tenths of pm). Examples of comminution machines include agitator ball mills, colloid mills, cutting mills, disk mills, homogenizers, jet mills, mechanical impact mills, ring-roller mills, and roll crushers. 

A kind of mechanical impact mill or jet mill for size reduction (comminution) that also incorporates a particle classifier. 

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. 

Mechanical Impact Mill A machine for the comminution, or size reduction, of mineral or other particles. Such machines pulverize feed particles (typically about 10 mm initially) by causing them to strike a surface at high speed. Very small-sized particles can be produced with these mills. 

The top size upper limit of the crushed (sized) material is defined by the smallest sieve opening upon which is retained a total of less than 5% of the sample (ASTM, 2011c). The term nominal top size is used to describe the product of a size reduction operation and is the smallest sieve opening upon which is retained a total of 5%-20% of the sample. The reduction ratio is the ratio between the feed top size and the product top size or the ratio between the feed and product sizes at which a determined percentage of the material passes. The relative ease with which a coal can be comminuted (reduced in top size) by mechanical action affects the design of a size reduction unit or operation. The term used to refer to this relative ease is the grind ability index of the coal. 

The conventional machines for the size reduction, or comminution, of materials make use of the following types of mechanical action applied to the particles compression, shear, percussion and impact. As a rule, there are no clear-cut divisions between these various actions, and in most machines two or more of them occur simultaneously, i.e., the particles are subjected to a combination of actions. 

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