|Area||Particle size analysis|
|title||Particle size analysis; Sieve analysis; Part 1: Basics, Part 2: Implementation|
|Brief description:||Determination of the grain distribution of bulk material|
The sieve analysis (rarely sieve analysis ) is a method of particle measurement for determining the particle size distribution of bulk materials . It is an important process in quality assurance and is described in the German standard DIN 66165. The standard consists of two parts; DIN 66165-1 defines the basics, DIN 66165-2 the implementation of the sieve analysis.
The most common method of sieve analysis is dry sieving with a sieve tower that is attached to a sieving machine . Wet sieving and air jet sieving are also used for difficult sieving goods that tend to agglomerate and finer sieving goods .
When sieving with a sieve tower, several test sieves (seldom also called test sieves ) are arranged on top of one another and clamped onto a sieving machine. The test sieves each consist of a sieve bottom and the sieve frame. The mesh sizes of the individual test sieves are descending from top to bottom. When performing the sieve analysis, the sample to be analyzed is placed on the coarsest test sieve and subjected to a defined movement for a specified time - how this looks exactly depends on the sieving method selected. The particle size distribution of the sample is then determined by weighing the residues on the individual test sieves.
The sieving makes it possible to characterize particle mixtures with sizes in the range from 20 µm to several centimeters. In order to cover as many areas of application as possible, different sieving methods have been developed over time.
Sieving can be carried out using different techniques, most of which differ in the movement that is transferred to the material to be sieved.
Dry sieving is a machine type of sieve analysis in which the material sample is dried in a drying oven at a temperature of 105 ° C for preparation. After cooling, the sample is weighed and then placed in the sieve system. The sieve system has smaller mesh sizes from top to bottom.
If there are no smaller portions than 0.063 mm, the sample can be examined with a dry sieve. Otherwise the fine parts under 0.063 mm must first be separated wet. During the sieving process, that portion remains in each sieve that has passed through the upper sieve, but has not passed through the sieve under consideration. The individual proportions are now weighed and displayed as a percentage in the form of a grain distribution (in a table and / or graphically as a grain distribution curve / sieve line).
Throw screening is a machine type of sieve analysis in which the material to be screened is exposed to a throwing motion. Here, vertical throwing movements are superimposed with slight rotating movements. This means that the sample is distributed over the entire surface of the sieve bottom and the particles are simultaneously accelerated in the vertical direction (thrown up). They can rotate freely in the air and are compared with the meshes of the sieve fabric when they fall back onto the sieve. If the particles are smaller than these, they pass the sieve, if they are larger, they are thrown up again. The turning movement ensures that the next time they hit the screen mesh, they have a different orientation and so they might get through a mesh opening.
Modern screening machines work with an electromagnetic drive in which a spring-mass system is set in motion and the resulting vibrations are transmitted to the screening tower. The oscillation amplitude and sieving time should be digitally adjustable and constantly monitored by an integrated control unit - this is the only way to reproduce and precisely reproduce sieve results (important prerequisites for a meaningful analysis). The flexible setting of parameters such as amplitude and sieving time allows the degree of sieving to be optimized for each sample individually.
With this type of sieving, the sieve tower performs a horizontal circular motion in one plane. As a result, the particles largely retain their orientation on the screen mesh. Flat sieve machines are preferably used for needle-shaped, platelet-shaped, elongated or fibrous material to be screened (º deviating significantly from a spherical geometry), where throwing the sample material up is not necessarily an advantage. Since test sieves with a large diameter of up to 400 mm can be used with this method, it is also suitable for large quantities such as B. occur in the grain analysis of building materials and aggregates.
A horizontal circling movement is superimposed with a vertical movement, which is triggered by a knocking pulse. These movement processes, which are characteristic of manual sieving, achieve a higher sieving rate for heavier particles (e.g. abrasives) than can be achieved when sieving with throwing sieve machines.
Sieve analyzes are mostly carried out dry. However, there are also applications that make wet sieving unavoidable. This is e.g. B. the case when the material to be tested is a suspension that must not be dried; or if the powder is very fine and tends to agglomerate (usually less than 45 µm) - with dry sieving this would lead to clogging of the sieve mesh and make further sieving impossible.
With wet sieving, the sieve tower is clamped onto the sieving machine just like with dry sieving. The sample material is placed as a suspension on the uppermost sieve and a water spray nozzle is placed over it, which supports the sieving process in addition to the sieve movement. Rinsing takes place until the liquid discharged through the sieve bottom (lowest element of the sieve tower) is clear and no longer has any turbidity due to the solid particles that have been rinsed out. The sample residues must be dried on the sieves and then weighed. It is important for wet sieving that the water must not change the volume of the material being sieved (no swelling, dissolving or reacting with the liquid).
Air jet sieving
Air jet sieving machines are often used to sieve fine powders, since particles smaller than 100 µm tend to agglomerate and often do not pass through the sieve mesh. Due to their working principle, these devices offer the highest level of effectiveness with such materials. The reason for this effectiveness is based on two components: A slot nozzle rotating in the sieve chamber and connected to the ambient air and an industrial vacuum cleaner with high suction power connected to the sieve chamber. If the vacuum cleaner is switched on, it creates a vacuum inside the sieve space. The resulting suction allows ambient air to flow in through the slot nozzle. The extremely narrow gap in the slot nozzle ensures that the subsequent air masses are accelerated and flow at high speed from below against the screen mesh. The particles lying on the sieve mesh are dispersed by the air jet for a short time before the suction of the vacuum cleaner grabs them and pulls them down through the sieve mesh.
Hand sieving is the oldest method for performing sieve analysis. The procedure here is described by a VDI guideline . However, it should be noted that manual sieving only plays a very subordinate role in the sieving process used, since both reproducibility and accuracy of the results are very poor. It is mostly used to get a first rough overview of the particle size distribution of bulk materials.
Favored by the advancing development in the field of computer technology, various alternative methods for particle size analysis have been developed since the beginning of the 1980s . These include laser diffraction particle size analysis (ISO 13320) as well as static (ISO 13322-1) and dynamic image processing (ISO 13322-2). The significantly lower expenditure of time as well as the often larger amount of information (particle shape) and the non-contact measurement of the particles have meant that the importance of sieve analysis has been reduced somewhat compared to the years before 1980.
- Paul Schmidt, Rolf Körber, Matthias Coppers: Sieving and sieving machines: Basics and application . Wiley-VCH Verlag , 2003, ISBN 978-3-527-30207-9 , Chapter 4.4: Analytical sieving.
- Jörg Hoffmann: Handbook of measurement technology . Hanser Verlag, 2007, ISBN 978-3-446-40750-3 , chapter 188.8.131.52.1. Example of sieving using digital image processing
- DIN 66165-1: 1987-04 particle size analysis ; Sieve analysis; Basics. Beuth Verlag , Berlin.
- DIN 66165-2: 1987-04 particle size analysis ; Sieve analysis; Execution. Beuth Verlag, Berlin.
- Felton, PG: Measurement of Particle or Droplet Size Distribution by Laser Diffraction. Lasers and their Analytical Applications , 1980.
- ISO 13320: 2009, Particle size analysis - Laser diffraction methods , Beuth Verlag, 2009.