Comminuting mill

The ability of comminuting mills to scale-up from lab to production scale is one of the most important characteristics size reduction equipment must offer. Scale-up in conical screen mills is achieved through the maintaining of the tip velocity of the tooling as applied in the pilot scale (Fig. 17). 

The raw ROM (run of mine) ore is reduced in size from boulders of up to 100 cm in diameter to about 0.5 cm using jaw cmshers as weU as cone, gyratory, or roU-type equipment. The cmshed product is further pulverized using rod mills and ball mills, bringing particle sizes to finer than about 65 mesh (230 p.m). These size reduction (qv) procedures are collectively known as comminution processes. Their primary objective is to generate mineral grains that are discrete and Hberated from one another (11). Liberation is essential for the exploitation of individual mineral properties in the separation process. At the same time, particles at such fine sizes can be more readily buoyed to the top of the flotation ceU by air bubbles that adhere to them. 

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. 

The basic manufacturing process involves thorough blending of the components, especially the pigments, and comminution with the aid of a variety of mills to reduce the particle size. Loose powders ate filled without additional processing. 

Ceramics fill a speciahzed niche in comminution where metallic contamination cannot be tolerated. Therefore ceramics are used for miUiug cements and pigments. Ceramic tiles have been used for hning roller mills and chutes and cyclones, where there is a minimum of impac t. 

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. 

Many crystalline products, including fine chemicals, foodstuffs and pharmaceuticals, require a final particle size that is significantly smaller than that produced during the crystallization or precipitation step. One way of achieving the required particle size is to employ a subsequent size-reduction step using some form of comminution device, frequently a mill. 

A colorless, colloidal precipitate was formed and stirred thoroughly for about 15 minutes, whereupon it was filtered by suction. The raw product thus obtained was washed with water until It contained only about Va% water-soluble salts. After drying for 12 hours In a vacuum apparatus at 60°C and under a pressure of 12 mm Hg, the product had the form of hard pieces. The pieces were comminuted to powder in a ball mill and the powder was passed through a sieve (3,600 meshes per cm ). The small residue on the sieve was again pulverized and passed through the same sieve. The yield was 870 g, or 99% of theoretical, calculated on the assumed formula... 

Ambient grinding involves the comminution of scrap rubber, including tires, at ambient temperatures, typically in a toothed-wheel mill, bench grinder, shear mill, or two-roll miU, to give particles... 

Comminution is mechanically a very wasteful process. It has been seen that most of the energy is lost, and this is so because of the difficulty in transmitting the applied forces to the particles in the mill contents that most need to be broken down. The culprit in this... 

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. 

Polymorphs are different crystal forms of the same compound [55], They are usually prepared by crystallization of the drug from different solvents under diverse conditions. However, exposure to changes in temperature, pressure, relative humidity, and comminution which are encountered in processes such as drying, granulation, milling, and compression may also lead to polymorphic transformations. 

The S-form can be obtained by treating Copper Phthalocyanine Blue in benzene or toluene with aqueous sulfuric acid in the presence of a surfactant [21], The e-phase is produced by comminution of the a-, 7-, or 8-modification, for instance in a planetary ball mill. The mill base is then aftertreated in an organic solvent at elevated temperature. It is important to realize that the temperature, depending on the solvent, must be kept below the transition temperature at which the e-phase converts to the (3-modification (30 to 160°C). The e-modification is made best from the 7-phase, and the most preferred solvents are alcohols [22], For the industrially hitherto insignificant tt, X, and R-forms of Copper Phthalocyanine Blue (see [1], Vol. II, 34-35). 

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