Sifting in its most elementary definition is the separation of a thin material from a coarse material with a mesh or perforated surface. This technique was used in the early Egyptian days as a method of grain size. These early sieves were made from woven reeds and grasses. Today, a sieve test is the method most often used to analyze particle size distribution.
Although at first glance the screening process seems elementary, in practice there is science and art involved in creating reliable and consistent results. To better understand sifting, it is necessary to explain several areas of sieve specifications, including:
1. What are test sieves?
Test sieves are measuring instruments used to determine the size and size of the distribution of particles in a material sample using a wire mesh of various openings to separate particles of different sizes.
Test sieves usually consist of wire mesh held in a frame. In most laboratory applications, the frame is round and made of stainless steel or brass. The standard frame sizes are three, six, eight, ten, or twelve inches and metric equivalents. Woven mesh can be made of stainless steel, brass or bronze. For most applications, stainless steel is the most common material.
2. What are the limitations of the test sieve procedure?
The main limitation in the design of test sieves is the inherent nature of the woven product, including the control of sagging during installation and the uniformity of the design of the retaining frame. It is also important to maintain consistent calibration across all openings in the grid piece.
Due to the inherent variations of the holes in any woven product, there are limitations on the degree of uniformity that can be achieved with the size of the hole on the mesh in the sieve. This leads to the practical limitation of the range of holes and the accuracy of the results from a particular sieve.
The screen test requires particles to pass through the screen. The practical limit for using the test sieve procedure is a particle size of 20 (micron).
3. What are the test sieve standards?
The first screening test standards were developed by WS Tyler before the 1920s. This original work preceded any published activities of standardization organizations, and Tyler design is the de facto standard in many industries. In 1925, ASTM International prepared an official standard for test sieve size, test sieve design and test sieve mesh in the United States. European standards were developed by the German university group in 1977 and are known for the design of DIN 4188. They are followed by British standards (BS 410). International Standards (ISO 565) have been developed by the International Organization for Standardization in Europe. It has been developed as a universal international standard. However, in practice, all standards are valid.
The sieve test standards refer to the sieve frame design and grid mounting, as well as the tolerances allowed in the variability of the holes in the grid. The basic principles are common to all standards and variations in terminology and are small in details. However, these small differences often lead to confusion. The following is a brief overview of the principles underlying these standards.
Test sieve frame standards include the following:
1. Rigid construction
2. Fabric (mesh) mounted without distortion, friability or waviness
3. The connection between the mesh and the frame must be filled or constructed so that the particles are not trapped.
4. The frame will have non-aggressive material and a seamless
5. The lower part of the frame, the size of which is easily inserted into the upper part of the sieve of the same size, which allows you to stack
6. The hole for the fabric should be at least 0.5 inches smaller than the nominal diameter.
Wire mesh standards include the following list of holes of nominal size in inches, millimeters (µm) and sieve quantity. The following specific size examples are given in ASTM E11:
1. Permissible variation of middle holes (depending on the size of the hole and from ± 2.9% of the nominal size for a mesh of 125 mm to ± 15% for a mesh of 20 m)
2. No more than 5% of the holes may exceed 1.04 of the nominal size for the net 125 mm to 1.45 times the nominal opening for the net 20 m
3. The maximum individual opening (for any opening) ranges from 1.0472 times from the nominal size for a mesh of 125 mm to 1.75 times a conventional mesh for a mesh of 20 m
4. The diameters of the wire are indicated and vary from 8 mm for a mesh of 125 mm to 0.020 mm for a mesh of 20 m
More recently, methods based on laser and energy technologies, methods of sedimentation, image analysis and centrifugal-type methods have been recognized. However, procedures using test sieves are still widely used. The result of the sieve test remains the basis or standard against which new methods are tested. In addition, the cost of equipment for the test sieve procedure is significantly lower than the capital investment required for new methods.
4. What are sieve certificates?
Sieve certificates are claims that a test sieve meets or issues published criteria. This is a guarantee that the new sieve will work in a predictable way. The tighter the tolerance required in the production process, the higher the level of certification. Similarly, a master set of test sieves for which working sieves (sieves in daily use) are tested for wear and projected performance requires a high level of certification. When test sieves are part of a process that is necessary to meet the prerequisites of traceability, such as a specific ISO level, certification will document the required traceability.
Many sieve manufacturers provide a certificate stating that the sieve was manufactured in accordance with a special standard (for example, ASTM, ISO). This certificate of conformity to manufacturing does not refer and does not confirm the conformity of the grid. Most manufacturers who supply a certificate of conformity will analyze the grid and provide grid certification for an additional fee.
A mesh-certified sieve will be supplied with a certificate stating that the sieve has been manufactured in accordance with a specific standard, and has been submitted for laboratory analysis and certified in accordance with this particular specification / standard (for example, ASTM, ISO).
There is a third level of tolerance, which confirms the compliance of the production standard and that the grid was submitted for laboratory analysis. It also confirms that its holes fall in the middle of a specific standard / specification (for example, ASTM, ISO). This is effectively 30% better than a fully certified screen. This is called an intermediate sieve. These three sieve certification levels allow you to compare the performance of one sieve with another size of the same size.
Prior to the development of a medium-special sieve, a high level of comparability was achieved by providing sieves that were optically matched to the user's standard sieve. To achieve this level of comparability, a time-consuming and costly procedure was required, and the results were not significantly better than the results achieved using mid-season sieves.
Mesh-certified sieves, medium-size sieves and screens with a manufacturing conformity certificate are manufactured with a mesh that already meets official standards. However, there are three levels of a lower degree of lattice grid when tolerance levels are not so strict.
The first is the Market Grade. These sieves have a weave that uses larger wire diameter, which results in a high-strength square mesh suitable for general screening. There are no official standards for sieves to assess the market. The second, Mill Grade, is a class of woven mesh using smaller wires, which leads to an increase in open space in the screen mesh. There is also Twill Weave, in which weft and strain wires alternately pass over and under two wires than over and under alternative wires, as in a standard grid. Since none of them have official standards by which the expected performance can be measured, none of them have a grid certificate.
5. Sieve Calibration
The quality control of the screening process is necessary, and for people processing materials and characterizing particles, sieve calibration can be a confusing topic. It is helpful to understand what sieve calibration is, why you need to calibrate a working sieve, and how to calibrate a sieve.
A. What is sieve calibration?
Sieve calibration is the process of testing the operation of a working sieve. (The working sieve is a test sieve that is regularly used to analyze particle size).
B. Why calibrate the working sieve?
Since working sieves are used daily for testing, they are also regularly cleaned. Although frequent use by itself can cause changes in the mesh holes, much of the damage done to hard work occurs during cleaning. Often the operator rushes to clean the mesh of residual particles by tapping hard on the frame. This click may distort the grid. Operators also use brushes to remove residual particles after the test. This process often distorts portions of the screen. These sieve changes will change the results obtained in subsequent tests, hence the need for calibration.
Excessive damage, such as tears or large mesh weave distortions, can be detected by visual inspection. Damaged sieves can be taken out of service when damage is observed. When the change is small, visual observation cannot detect a change in the test results associated with a change in sieve. The way to determine if a change has occurred is to compare the sieve efficiency with a known standard. This is a sieve calibration.
In addition, for operations with stringent particle size specifications, new test sieves are calibrated to establish a baseline performance level for the sieve.
C. How is a test sieve calibrated?
The base point of the sieve calibration process is the use of a fixed standard and a number of approaches are used. The most common is the use of a master stack of sieves, the main sample or calibration spheres or balls.
The main stack of screens includes one of the screens used in the processes. The master stack must consist of sit-certified screens. In the case of tight tolerances, it is recommended to use medium sieves for screen tests. The following steps are used for this method:
1. Prepare two samples of material selected for the calibration process.
2. Place the master stack of sieve onto the sieve shaker.
3. Load one of the samples into the upper sieve.
4. Run the sieve shaker for the specified time.
5. Prepare a percentage analysis of the result.
6. Place a stack of working sieve (sieve with dimensions in accordance with the master stack)
7. Repeat steps three to five for the second sample of material.
8. Compare the results of the two analyzes.
9. Check the deviation from the master stack with tolerances
10. Replace working sieves that do not withstand tolerance.
Some users only calibrate one sieve at a time and compare it with one sieve from the main kit. This procedure can be performed prior to the commissioning of new work screens.
Some processes store basic samples of all materials subject to screening. The results expected from working sieves were set using a master sieve stack or other calibration methods. This method uses a sample from the master and performs the following steps:
1. Place a stack of working sieves to check on the sieve shaker.
2. Load the selected sample from the base sample into the upper sieve.
3. Run the sieve shaker for the specified time.
4. Prepare a percentage of saved result analysis.
5. Compare the results with acceptable tolerances for the screens in this stack.
6. Replace working sieves that do not meet the requirements.
The used sample can be returned to the original sample sample. Depending on the type of material, a deterioration may occur during the screening test. When this happens, the test sample is discarded after use.
As with the master stack, some users only calibrate one sieve at a time and compare it with a performance tolerance chart for this sieve size. This procedure can also be used for new work screens, before putting them into operation.
The calibration spheres in size for each of the calibrated sieves are used to determine the actual results obtained by each test sieve. This method is simple and gives a clean result in the average size of the aperture. The result is traceable to the NIST and NPL standards. This is a good test for standards reporting and for setting internal standards. The procedure for this calibration is:
in the following way:
1. Select a sieve to calibrate.
2. Empty the contents of the bottle containing the appropriate standard on the sieve.
3. Shake evenly over the surface for one minute.
4. Calculate the skip percentage and read the average aperture for the calibration graph.
The method specified by ASTM is to optically test sample openings, measure apertures and wires, and compare results with ASTM E11. Traditionally this is done visually with a microscope. However, there are new computer-based image analysis systems that are beginning to be limited to using for sieve calibration.
6. Summary
Sieves have a long history as a basis for measuring and analyzing particle size in a material. Despite the advent of technology-based methods, screen-based procedures are still the main basis for determining particle size. To ensure reliable and consistent results, it is obvious that screening requires an understanding of not only one, but also a number of integral factors, such as test sieves, limitations of the test sieve procedure, test sieve standards, sieve certificates and sieve calibration,
