Methods of sieving

Sieving procedures are standardized in BS 1796 [14], Dry sieving by machine is used for coarse separation but other procedures are necessary as the powder becomes finer and more cohesive. Conventional dry sieving is not recommended for brittle material since attrition takes place and an endpoint is difficult to define. If the rate of passage of particles does not decrease with sieving time, it may be due to particle attrition, deagglomeration or a damaged sieve [72]. 

Some materials tend to form granules when sieved. Coating the particles in order to reduce cohesiveness can often reduce this effect. Powders may, for example, be shaken in a container with 1% fatty acid (stearic acid is often used) or fumed silica. Alternatively, the powder may be sieved wet. The addition of 0.1% sub-sieve carbon black has been found useful (although rather messy) for eliminating electrostatic charge. 

Brittle powders are best sieved using a gentle sieving action as is found, for example, with the Air Jet Sieve or the Sonic Sifter. 

For reliable data, around 5% of the sample should be retained on the coarsest sieve and a similar amount should pass through the finest. Unless the size distribution is narrow, alternate sieves (a root two progression of aperture sizes) is recommended. 

With machine sieving, sieve motion should minimize the risk of aperture blockage and preferably include a jolting action to remove particles that are wedged in the sieve mesh. 

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Wet sieving method of sieving coal that uses water as a medium for facilitating segregation of a sample into particle size (ASTM D-4749). 

Due to the differences between sieve analysis methods, the method of sieve analysis used must be indicated according to USP <786> as the different of types and magnitude of agitation produced by different sieving instruments will yield different results (7). 

Physical properties of the panicles Method of sieving motion Panicle dimension and shape Types, size, and geometry of the sieves... 

The main factors that affect the particle passage through sieve aperture are the method of sieve shaking, the ratio of open area of sieve to total area, particle size distribution, the number of particles on the sieve (sieve loading), and the dimension and shape of the particle. Friability and cohesiveness of solid particles can also affect the sieving operation. Difficulty can also arise with high aspect ratio particles (i.e., needle-shaped or flaky particles). 

B. H. Kaye, Investigations into the Possibilities of Developing a Rate Method of Sieve Analysis, Powder Met. (1962) 199—217. 

Current design practice is based on methods of sieving, for which manufacturers report values in their technical literature. 

An alternate method for predicting the flood point of sieve and valve plates has been reported by Kister and Haas [Chem. Eng. Progi , 86(9), 63 (1990)] and is said to reproduce a large data base of measured flood points to within 30 percent. It applies to entrainment flooding only (values of Flc less than about 0.5). The general predictive equation is... 

Screen Ejficiency There is confusion concerning the meaning of screen efficiency, as a uniform method for figuring efficiency has never been established. A sound method of evahiating screen performance is given by W. S. Tyler, Inc., Mentor, Ohio, in its Sieve Handbook, no. 53. In this formula, when material put through the screen is the desired product, efficiency is the ratio of the amount of undersize obtained to the amount of undersize in the feed. 

Graphical methods of evaluating efficiency, using sieve analyses, are also employed and are recommended when serious research on screening is done. 

Sieving Methods and Classification Sieving is probably the most frequently used and abused method of analysis because the equipment, an ytical procedure, and basic concepts are deceptively simple. In sieving, the particles are presented to equal-size apertures that constitute a series of go-no-go gauges. Sieve analysis presents three major difficulties (1) with woven-wire sieves, the weaving process produces three-dimensional apertures with considerable tolerances, particularly for fine-woven mesh (2) the mesh is easily damaged in use (3) the particles must be efficiently presented to the sieve apertures. 

CH3OCH2OCH3 TsOH, CH2CI2, mol. sieves, N2, reflux, 12 h, 60-80% yield. This method of formation avoids the use of the carcinogen chloromethyl methyl ether. 

Klein [201] has developed the correlation based on published data of others (his citations) as shown in Figure 8-149. Because the method of aeration between sieve trays [205] and valve trays is different [201], the same aeration correlation cannot be used, because valve trays have lower... 

The particle size of powder most often used for consolidation is —200 mesh (74 /xm sieve aperture), and the most widely practised method of consolidation is hot pressing in vacuo. 

The old dry process enamelling of cast iron (baths etc.) is no longer widely used. The method consisted of sieving finely powdered frit onto the preheated casting and inserting the casting back into a furnace at about 900°C to produce the smooth finish. 

The carbon raw material in the form of coke, coal or natural or synthetic graphite is ground and sieved (following calcination at 700-1300°C to control volatiles, if necessary) to give a desired particle size distribution. The distribution depends upon the size of the artifact to be formed and the method of forming. 

Sieving is probably the easiest and certainly the most popular method of size analysis but is restricted to powders having the greater1 portion coarser than 75 y, For finer powders the method is limited by the high cost of producing sieves with uniform, small apertures... 

Shiflett, Particle Size Investigation of Polyox WSR 301 Polymer Powders Using a Turbidity Technique", NSRDC 28-621 (1973) 45) Anon, Standard Method of Test for Fineness of Hydraulic Cement by the No 325 (45 p m) Sieve", ASTM C430-75 (1975) 46) D.J. 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

Tetrahydrofuran was distilled from lithium aluminum hydride by the submitters and collected in a flask containing molecular sieves. For a warning regarding this method of purifying tetrahydrofuran, see Org. Sytt., Coll. Vol. 5, 976 (1973). 

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