In mechanical process technology, grading describes the separation of solid mixtures by particle size. The goal of the grading is to separate the original screening product into at least two groups according to size criteria which is provided in advance. How precisely the individual groups are separated from each other is determined by the selectivity, which is measured in percent. Ideally, the selectivity should be 100%, and it is the most important evaluation criterion of a graded screening machine. However, in industrial practice, selectivity of one hundred percent can only be approximated. The selectivity of a screen is predominantly influenced by two factors: the screen movement and the dwell time of the products on the screen mesh.
Screen movement and dwell time
The screen movement describes the way in which the screen housing, including the inserts, is put into motion. In practice, this is done in three different ways: The screen is moved horizontally and vertically, with a low oscillation amplitude, by vibration motors. An oscillating weight drive ensures clean horizontal screen movement, which is either circular or straight. A third screen variation is centrifugal screening machines, where a screening basket is rotated with help from a drive motor.
The selection of the correct screening movement ultimately depends on the specific material characteristics. For example, it is best for delicate bulk solids to be graded with a gentle, linear screen movement. In this case, a vibration screening machine would be unsuitable, as the housing moves both horizontally and vertically, which would cause delicate bulk solids to jump up and down on the screen mesh and break easily. As a result, small fragments, which no longer correspond to the given size criteria, may be produced and separated off as undersized particles – therefore also falsifying the selectivity.
However, for a selective grading it is not sufficient to simply select the right type of screening movement, the movement intensity must also be adjusted to the screening product in order to achieve an optimal dwell time of the product on the screen mesh. Whether it be a vibration or a linear oscillating screen, the screen movement is regulated by the vibration or oscillation amplitude. For a selective grading, the screen must be set so that the product is spread equally over the entire screen surface, ensuring that all particles come into contact with the screen mesh so that they can be graded. If the screen housing moves too quickly for the screening product, it may be that fine particles, which have to be screened off, could end up reaching the good or large particle outlet, for example. In addition, vibration screening machines can easily vibrate too strongly which can start to make the screening product jump up and down on the screen mesh. When this happens, the product has almost no contact with the screen mesh, and can even jump out of open screening machines.
A further important setting which influences the dwell time is the screen inclination. For example, poorly flowing or sticking products often require a steeper screen inclination than a more easily flowing screening product to get it to flow over the screen surface. As can happen if the screen movement is set to be too quick, if the screen inclination is set too high then the product can move so quickly over the screen surface that is doesn’t have enough contact with the screen mesh.
Due to their linear movement, long-stroke screening machines from Engelsmann automatically achieve a very high level of selectivity with graded screening. Thanks to the solely horizontal movement of the screen, the product always remains in contact with the screen mesh. In addition, the dwell time on long-stroke screening machines can be easily increased by placing several screen inserts with the same mesh size behind or on top of one another. Thus increasing the course that the product must cover on the screen surface. Return plates can also be easily integrated. These plates are located beneath the screen insert and guide the screened bulk solid to the beginning of the screen insert below. By doing this, the dwell time is increased and it is ensured that the entire screening surface is used.
Alongside the screen inclination and the type of screen movement (vibration, momentum drive, etc.), the distribution of the product on the screen mesh also influences the degree of selectivity. For example, if the bulk solid is poorly granulated and is guided directly vertically onto the screen, then it can already build up too high on the screen insert during the feeding, which means that the particles lying on the top cannot come into contact with the screen surface. A “screen carpet” which is too high will just move over the mesh without being able to pass through it. In cases such as this, it is sensible to fit the grading screen with a distributing aid.
Engelsmann offers two different solutions for product distribution. Distribution plates are slightly inclined and placed directly underneath the screen inlet. The screening product is guided through the inlet and onto the plate. Thanks to the beveled angle, the product is directly put into motion and is distributed over the entire width of the insert. Again, the screen rails function as “stoppers” and create a barrier on the screen mesh just behind the product inlet which means that the product can only flow through at a certain (adjustable) layer height. This ensures that all of the screening product comes into contact with the screening surface and can be optimally screened off.
Custom-fit tensioned screen inserts
Precisely tensioned screen inserts also play an important role when it comes to graded screening, as they also influence the selectivity. If the mesh is not evenly tensioned, the individual meshes can sag or warp. Screen inserts with uneven machine widths inevitably lead to lower selectivity and poorer screening results. Thanks to Engelsmann’s new testing procedure, screen inserts can be measured after they have been tensioned, in accordance with DIN ISO 9044 and DIN ISO 3310-1, and the homogeneity of the screen mesh can be checked. Customers that buy a screening machine or replacement screen insert from Engelsmann can also receive a corresponding test report upon request and therefore increase process safety.
In practice, graded screening is carried out with widely different products and particle size distributions. The range of applications of the grading screening machines from Engelsmann ranges from dust particles with a diameter of just 40 µm, up to coarse products with a diameter of 20 mm. Even the number of desired groups varies depending on the task. In many cases, over- and under-sized particles are formed during manufacture and should be separated off as ‘out of place’ particles. The correctly sized particles can then be graded further into different particle sizes as needed. With grading screening machines from Engelsmann, up to 8 different groups can be screened off.
Conclusion: A selective grading is achieved by a combination of optimally tensioned screen inserts, the ideal screen movement for the product, the product distribution and the dwell time of the screening product. Should different bulk solids needs to be screened or if the composition of the formulas varies, it is advantageous for the operator if these parameters can still be adjusted retrospectively. Grading screening machines from Engelsmann are equipped with an adjustable oscillation or vibration amplitude as well as a regulable screen inclination as standard to ensure consistently optimal screening results.Click here for related articles and newsSearch for screener manufacturers in our equipment guide
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J. Engelsmann AG disposes of 140 years of experience in developing and realizing solutions for process technology.Our engineers design and construct both process machines and complete plants according to the requirements specified by the customers.Our main customers are leading companies of the chemical, pharmaceutical, plastic processing ...