Sieve operation

The sieving operation starts by mounting the sieves in a nest, the coarsest at the top. The sample is placed on the top screen, and the screens are shaken for several minutes. It is generally impossible to screen a sample completely the longer one shakes the screens, the more material comes thru, although to a continu-... 

Mardlly, C.R. (2000) Where and how shape selectivity of molecular sieves operates in refining and petrochemistry catalytic processes. Top. Catai, 13, 357-366. 

In conventional vapor phase molecular sieve operations, the operating temperature must be high and the operating pressure must be in the vacuum range in order to get the high molecular weight molecules into the vapor phase for contacting with the molecular sieves. The use of volatility amplification with supercritical fluid obviates these requirements. 

Data obtained from a sieving operation are commonly presented as the weight per cent of crystals associated with each crystal size range. From these data a cumulative plot, showing the total weight per cent of crystals, finer or coarser than a given size, may be constructed. 

Using the conventional rate test, the sieving operation is terminated some time during region 2. The true end-point, when every particle capable of passing through a sieve has done so, is not reached unless the sieving time is unduly protracted. 

In practice, this value of R is of limited practical value, since it cannot apply to the nominal aperture of the sieve. As sieving progresses, the smaller apertures become ineffective since all the particles finer than these apertures will have passed through the sieve. The largest aperture in the sieve therefore controls the sieving operation and the final particle to pass through the sieve will only do so when presented to this aperture in its most favorable orientation, i.e. for a 75 pm sieve, the true end-point could be 100 pm or more. 

It is not widely realized that analyses of the same sample of material, by different sieves of the same nominal aperture size, are subject to discrepancies that may be considerable. These discrepancies may be due to non-representative samples, differences in the time the material is sieved, operator errors, humidity, different sieving actions and differences in the sieves themselves which may be due to wear. 

Whether or not the particle will pass the sieve when it is presented at the sieving surface will then depend upon its dimensions and the angle at which it is presented. The size distribution given by the sieving operation depends also on the following variables. 

The results may be expressed in terms of the nominal size, although it is preferable to use calibrated sieves. A reference set of sieves should be used after every fiftieth analysis for comparison purposes in order to detect wear. In essence, the smaller the sieve loading, the more rapid is the sieving operation. The low weights however lead to errors in weighing and intolerable percentage losses. 

In BS 1796 [14] which applies to the sieving of material from 3350 to 53 pm in size, it is suggested that the sieving operation be carried out in 5 min. stages at the end of which the sieves should be emptied and brushed in order to reduce aperture blockage. This procedure can however lead to excessive powder loss. 

For new sieves the system isolates the effects of extremes of aperture size to give a distribution more nearly related to nominal size than a conventional sieving operation. The better mechanical support should also reduce wear and tear. Kaye suggests the reduction of an 8 in diameter sieve to a honeycomb of 0.5 in square partitions. 

Often the process design requirements for applications of ion exchange prescribe more rigorous particle size constraints which are met by either further sieving operations, or as is recently the case with some principal resin manufacturers, the ability to produce directly a uniformly sized product ( 50 /xm) at the resin copolymerization... 

Optical microscopy indicates that the average size of particles in each SDDP fraction corresponds to the size aimed by the sieving operation. In all cases, fine particles (5-20pm) are stuck on the surface of the large ones. 

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). 

After comminution, sieving operations separate large-, mainly poorly-ground particles from smaller wellground particles. Particles that are too large are reground. 

Errors incurred during the sieving operation may include ... 

Crystalline products are usually sized by sieving, so ideally the length term to be used should be that derived directly from the sieving operation, i.e. the second largest dimension of the particle Figure 2.14). 

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