Sieving times

Whitby (Ref 7) discovered that in the nonsteady-state with mechanical sieve shakers, the percentage passing versus sieving time curve could be divided into two regions with a transition between (Fig 4). Region 1 exists when there are many particles much less than the mesh size still on the sieve, while region 2 exists when the residue on the sieve consists entirely of nearmesh or larger particles... 

It was concluded that approximately three minutes sieving time was adequate with the air-jet sieve even with waxlike materials such as cadmium stearate whereas, 20 minutes or longer was required for vibrating or shaking screens. The data presented (Refs 19 35) for AP by the Alpine, Tyler Ro-Tap, and Micromerograph agree remarkably well... 

Sieving Time Because sieving time depends upon the type of sieving motion and the provision for continuous movement of the sample over all parts of the sieving surface, the only requirement for sieving time appears to be that shaking be continued until equilibrium conditions are reached... 

R(h) — R(fc) = = weight of particles passing. For determining dR(t) above, it is convenient to have the sieving time t2 adjusted so that... 

Fig 4.5 Effective aperture width distribution with increasing sieving time. 

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. 

Table 4.4 The effect of sieving time and load on the amount passing a...

Duration of sieving. Sieving time is closely related to sieve loading, a reduction in the latter resulting in a reduction in the former. 

Heywood [57] also investigated the effect of sample weight on sieving time using 20, 50 and 100 g samples of coal dust on 60, 100, 150 and 200-mesh sieves. He found that neither the time required to attain the endpoint of 0.1% per min. nor the residual percentage on the sieve was affected by the weight of the sample. 

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]. 

Sieving is more protracted with finer mesh sieves, and a sieving time of 20min. is usual with a 1 g load on a 3in diameter 20 pm sieve. Similar instruments are marketed by Alpine, Micron Powder Systems and by MicroPul. Jones [98] has presented a discussion of this technique and Lauer [83,99] has appraised it by microscopic examination of the powder fractions. 

Though sieving analysis is a simple and reliable particle size analysis method, there are some data collection issues that one must be aware of and control in order to obtain reproducible results. Factors that may affect data accuracy include sieve properties, sieve motion and powder properties, sieve time and load, and errors of experimental method and environment. This section will discuss the effects and methods to reduce data collection errors. 

This behavior is critical to understand and to obtain accurate results. As one can see in Figure 29, that if the sieving time was set in the region where any slight variations in either sieve time or sample loading would affect the passage of the particles and yield drastically different results as the particle size distribution is very much dependent upon the sieving time. 

Sieving separates powders solely based on their size—it is independent of density or surface properties (although the sieving time may be a function of density). Table 9.8 summarizes the sizes of the US Mesh from 31 tim to 4760 p.m. 

Best reproducibility was observed using moderate sieve vibration amplitudes. A sieving time of 5 min was sufficient to get constant oversize mass (Fig. 4). The maximum standard deviation SD was 5 %. 

To allow for these practical difificulties it is usually necessary to rigidly specify and adhere to sieve fractionation study protocols. One also may fix limitations on the permitted variations in sieve apertures and sieving times according to empirical limits. In the manufacturing of sieve surfaces using wire woven cloth most major industrial nations have established specifications for sieves used in test studies [3, 7—10]. See Table 3.1. 

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