The Sample

The major concern with sieving operations is the efficiency and time of each sieving step. This becomes particularly important when we use a nest of sieves to perform several separations simultaneously. The use of words such as effective or nominal diameters with sieves is in recognition of the imperfect separation that may occur. What is less readily recognized is that the sample may contribute to imperfect separation.

Placement of a soil sample on a sieve does not result in instantaneous separation. Several factors influence the time to achieve a fixed level of segregation. These factors include sample size, shaking intensity, particle shape, particle size and hole geometry. No one set of sieving times applies for all conditions, but for many soils with small samples (& 100 g) a waiting time of about 3-5 min for coarse fractions and 10 min for fine fractions gives acceptable results. A typical nest of sieves (with 3 in (7.5 cm) diameter) operating for at least 15 min is desirable for separation. Since samples vary in their sieving characteristics, it is best to run a trial sample at several times. This is particularly true with a nest of sieves to ensure adequate separation past the smallest sieve opening.

The single most important factor changing the efficiency of sieving is the initial sample mass. It is faster and more efficient to split a large sample into several smaller ones. A useful rule of thumb is to keep the depth of material on the sieve to less than 1 cm. A better rule is to run a test sieving curve. In general, this will show that as the sieve opening decreases smaller masses are needed. For 8 in (20 cm) diameter sieves these typically range from 200 to 30 g for sieve openings varying from 2 mm to 45 |im.

Particle shape also influences the efficiency of sieving. Rougher surfaces with elongated shapes are expected to require longer sieving times than smooth surfaces with a more spherical shape. It may be possible to use equations like the one presented earlier to predict the sieving rate constant. However, the inverse problem of determining the particle shape from sieving curves appears to be ambiguous.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment