The mineral grades contained in premixed plaster are usually extracted at quarries and then crushed in a jaw crusher. After that the produced bulk materials are classified and made available for further production. The moisture of the feed material may vary widely due to rainfall and temperature changes and can be as high as 10 wt% – which makes screening difficult. Also, the concentration of the moisture in the fine fractions and the not inconsiderable clay content cause trouble in the processing.
On linear motion vibrating screens the indicated task led to immediate blinding of the screening cloth. Accordingly, extensive tests were performed in the process circuit plant at RHEWUM GmbH. The only practicable screening solution was a screening machine with direct excitation of the screen cloth driven by unbalanced motors.
These provide the user with the advantage of an extremely high acceleration of the screening surface. This is achieved thanks to the low masses that must be accelerated. Unlike with linearmotion vibrating screens where the entire machine is set in motion, in this type of screen the machine frame remains static. Only the screening surface is vibrated, but these vibrations are of a higher frequency, leading to higher acceleration at the screening surface. This has advantages in the separation of critical materials, in respect of particle size as well as keeping the screening surface open. Furthermore, no dynamic loads are transmitted to the building. The aggregate can consequently be equipped with fixed flange connections and without compensators.
As due to the drive principle no immediate blinding of the screening surface with the naturally moist material was observed, a higher material moisture was simulated artificially with the additional spraying of water. Even at a moisture content of 4.5 wt% the screening surface was still reliably kept clear. But with this water content, chain cleaning was necessary.
Subsequently another test was performed, also with a directly excited screen, but this time with an electromagnetic drive. The background to this drive concept change is the useful inharmonic oscillation of the screening surface by electromagnets. This drive with electromagnets has considerable advantages for materials heavily prone to agglomeration.
In addition, a cleaning impulse ensures that the screening surface remains free. During the cleaning cycle, the magnet draws the hammer to the final position of the screen casing at maximum acceleration, then the hammer hits the casing. This generates additional excitation of the screening surface that keeps the cloth clean.
The motivation behind these tests was the maintenance friendliness of the unbalance motors available today in contrast to the high investment for electromagnetic drives. After three months of continuous operation, the screening surface of the electromagnetically driven machines were open and ready for operation whereas that of the unbalance driven screen was blind .
The optimal solution in this case was a combination of different processes: The feed material is separated at 10.0 mm in the feed section on co-vibrating strainer bars. The feed is sized at 3.5 and 2.25 mm on a screen driven by an unbalance motor – a simple and cost-effective alternative to electromagnetic drives. Critical cuts at 1.6 and 1.0 mm are performed on a directly excited screen with electromagnetic drive.
An additionally installed electrical screening surface heating with a required maximum power of 30 kW permits the heating of the screening surface to above 100 °C. This temperature prevents the caking of moist fines to the screening surface. To adapt the energy consumption to the varying water content of the feed material, the heating system can be switched on or off. As only the critical part of the feed is dried, the consumption of primary energy is lower than at common drying solutions. Additionally, an automatic chain cleaning system was installed. This reduces the inevitable wear of the screening surface to a minimum. Thereby, an optimal solution was found for screening this critical material.
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RHEWUM is an independent family owned company which was founded in 1927 as „Rheinische Werkzeug- und Maschinenfabrik“. RHEWUM have been developing and manufacturing screening machines and vibrating feeders since the beginning of the 1950’s.