Sieve analysis (also known as sieving analysis or test sieving) is used to determine the particle size distribution of various bulk materials. Its handling and evaluation is described in a variety of international standards. It is also considered an important and indispensable quality assurance procedure to this day.
Sieve analysis is divided into dry sieving and wet sieving. The sieving motion can be based on the principles of throw sieving, plan sieving, tapping sieving, air jet sieving and ultrasonic sieving. Manual sieving is not easily reproducible due to the individual influences of the operator (stamina, speed, strength).
Application of reproducible sieving analysis
Reproducible sieving analysis is considered particularly important in industry and science so that the results obtained can be independently verified or checked at any time. As a rule, a sieving stack is used in combination with a vibratory sieving machine.
These devices generate a strong and consistent vibration that can be reproduced at any time, which is why the results obtained from the analysis itself can be reproduced. As already mentioned, manual sieving is also possible as a method, but this option is rarely or never used in science because the results and accuracy of the analysis depend on the individual performance.
Dry sieving
Dry sieving is the most popular method of reproducible sieve analysis, including vibration, horizontal and tap sieving. Air jet sieving is also considered a dry sieving method, but it is a special process (see below). If necessary, the sample is dried in advance to avoid clumping. Before sieving, the sample is weighed, then placed in the sieving system and weighed again at a later point in time.
Sieving is used to determine the percentage of the sample that remains on the sieve or is smaller than the selected mesh size. If a particle size determination of the various fractions is to be carried out (set sieving), a sieve stack is used that contains several sieves with different mesh sizes (40 µm – 125 mm).
However, to ensure that the results are reproducible beyond doubt, the machine should be set up completely digitally. Furthermore, the integrated control unit should be constantly monitored to avoid unintentional changes and deviations during the test.
Wet sieving
Wet sieving is used to determine particle sizes in moist, greasy or oily samples. It is also the method of choice when the material to be analyzed is already present as a suspension and cannot be dried, as well as for particles that tend to agglomerate (usually < 45 µm), which would otherwise clog the sieve openings.
The material to be sieved is suspended and, as with dry sieving, applied to the uppermost sieve and then rinsed with water under vibration until the liquid emerging from below the sieve stack is unclouded. Wet sieving is carried out in the range 20 µm - 20 mm.
Air jet sieving
In air jet sieving, only a single sieve is used at a time, and it is not moved during the sieving process. A rotating nozzle below the sieve directs a jet of air onto the material to be sieved, causing particles to deagglomerate and then be sucked through the sieve. Air jet sieving is suitable for size ranges from 10 µm to 4 mm.