Particle size reduction purposes

Particle size reduction can be achieved using a variety of methods. Sometimes it is helpful to carry out the particle size reduction at reduced temperatures, such as at 4°C or at liquid nitrogen temperature, -196°C. In other instances, grinding with an excipient has been employed as a means of obtaining amorphous materials. Cyclodextrins and microcrystalline cellulose have been used for this purpose. It is also possible that the use of polymeric excipients may inhibit crystal growth when the amorphous solid is dissolved in water. Table 5 contains a list of compounds that have been obtained in amorphous, or partly amorphous, form by milling. 

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

The method employs a cylindrical porcelain jar mill fitted with three lifters that assist in tumbling the coal. A sample (usually 1000 g) of sized coal is tumbled in the mill for a specified time at a specified number of revolutions per minute. The coal is then removed and screened and the friability is reported as the percentage reduction in the average particle size during the test. For example, if the average particle size of the tumbled coal was 75% that of the original sample, the friability would be 25%. As with several other tests, the test parameters can be adjusted to suit the purpose of the investigation, but the precise parameters must be reported with the data. 

Researchers at the University of Illinois utilize a small-gauge needle that vibrates at an ultrasonic frequency for this purpose. A jet of a polymer solution passing the needle breaks up into uniform droplets, which becomes solidified micromatrices after solvent removal. The droplet size can be precisely controlled as a function of orifice size, solution flow rate, and vibration frequency. An optional carrier stream enables further reduction of the particle size. In vitro release studies using microparticles of different mean diameters demonstrated the dependence of the diffusion-dependent release profiles on the particle size. ... 

Size reduction is sometimes necessary, either for liberation purposes or simply for particle volume reduction. The size of the soUd waste is considerably larger than the minerals or rocks. 

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. 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

The tenn size reduction is applied to all the ways in which particles of solids are cut or broken into smaller pieces. Throughout the process industries solids are reduced by different methods for different purposes. Chunks of crude ore arc crushed to workable size synthetic chemicals are ground into powder sheets of plastic are cut into tiny cubes or diamonds. Commercial products must often meet stringent specifications regarding the size and sometimes the shape of the particles they contain. Reducing the particle size also increases the reactivity of solids it permits separation of unwanted ingredients by mechanical methods it reduces the bulk of fibrous materials for easier handling and for waste disposal. 

The size distribution of products from various types of size-reduction equipment can be predicted by a computer simulation of the comminution process. This makes use of two basic concepts, that of a grinding-rate function S and a breakage function A5 a. The material in a mill or crusher at any time is made up of partides of many different sizes, and they all interact with one another during the size-reduction process, but for purposes of computer simulation the material is imagined to be divided into a number of discrete fractions (such as the ones retained on the various standard screens) and that particle breakage occurs in each fraction more or less independently of the other fractions. 

There are many cases where the initial particle size of the sample requires preliminary reduction. Classical machines for this purpose are jaw crushers, normally used with samples of medium to extreme hardness as well as brittle to hard-tenacious samples, and cutting mills, used with samples of soft and tough make-up. 

Size separation is a process which often follows size reduction its purpose is to separate and classify the solid particles according to their size. The main methods are screening, magnetic separation, and mechanical classifying. Magnetic separation is limited to iron and similar metals, and is often used to separate iron impurities from other materials to eliminate damage to chemical plant or discoloration of the products. 

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