Direct excited polishing screens for fertilizer

M‌odification on the existing concrete steel beams was very limited and had to be respected in the new machine design. Installed at the top of the building with limitation in height, respecting on and off going connections and footprint, a tailor made design was the only solution which could be fitted, so the fertilizer producer decided for a customized flexible solution from Remscheid. To analyze the installed machine RHEWUM visited Agropolychim AD in Devnya/Bulgaria to check the available area and provide fitting dimensions with the new machine. The available height for the screening machine was limited to 2.8 m. Therefore, the distribution and outlet chutes were specially designed and integrated between the concrete floors.  

Fertilizer types

Agropolychim AD produces a wide range of fertilizer that includes:

Ammonium nitrate

Urea ammonium nitrate

Urea ammonium nitrate with sulfur

Urea, triple superphosphate

Mono-ammonium phosphate

Diammonium phosphate

NP and

NPK

This variations of fertilizers are produced in different granulation, prilling and compacting processes. Agroplychim AD has an annual production capacity of:

Ammonium nitrate – 400 000 Mt or Urea Ammonium Nitrate – 800 000 Mt

Mono/Diammonium phosphate – 300 000 Mt or Triple superphosphate – 330 000 Mt

Further to its own production the company procures from its partners – about 300 000 Mt of nitrogen and phosphorus, mono/di-component and complex fertilizers annually.  All these fertilizer types should be sieved by only one polishing screening machine. Therefore, the very flexible direct excited screening machine type WAF has been proposed for handling all the different screening tasks and save up unnecessary costs for additional equipment.   

Decision for the RHEWUM WAF screening machine

The screening machine type WAF is a combination of a direct excited screen and a linear motion vibrating conveyor. This polishing screen is characterized by low down times, low space requirements as well as fast and easy maintenance. Additionally, it allows a maximum feed rate with maximum yield at a given purity which is the perfect combination for polishing fertilizer. In the range of fine and finest particles much better sieving results are achieved. 

The linear motion is generated by two counter-rotating out-of-balance motors and ensures the transport of the fertilizer. The additional direct excitation of the screen cloth prevents the mesh from being blinded by clogged particles. By this combination of drive and excitation difficult to sieve fertilizers, which normally cannot be screened on linear motion sieves, are classified precisely and at high feed rates. Furthermore, the automatic self-cleaning cycle with high accelerations of up to 15 g helps to keep the cloth free reliably even when screening sticky materials.

Reaching high product purities of at least 85 % in all major fractions, especially the highly valuable fractions, amplified the arguments for the type WAF. Screening fine materials with a high efficiency at high feed rates requires a precise adjustment of all machine parts. This is only possible on precisely working, high quality equipment. The setting of the amplitude for each screen deck has to be adjusted individually to ensure and ideal screening process for each fraction.

One other major reason the client decided in favor of RHEWUM with the supply of the screening machines was that the screening plant can be retrofitted in the original space to keep the construction cost economical. As a result the WAF will come up to the requirements regarding pureness and yield of the project and therewith guarantee a trouble-free and reliable screening process.

Due to the higher number of small drives compared to conventional screens with a lower number of larger motors, RHEWUM‘s direct excited screens are highly reliable. Each inlet and outlet half of each screen deck can have its individual amplitude and self-cleaning interval to maximize the efficiency for every fraction. Vibration is imparted into the screen cloth by high speed movement of a transversally mounted knocker shafts, located beneath the screen cloth. Thereby the velocity of the material down the screen is determined by the angle of the screen deck. Outside the screen housing rugged electromagnetic drives excite the knocker shafts, conveying the high frequent oscillation directly into the screen mesh. The electromagnetic drives create high accelerations on the screening cloth of up to 15 g that guarantee a correct screening even of adhesive products. The electrical control of the vibrating heads allows infinite control of the vibrator oscillation during operation and thus a perfect adaption of vibration amplitude to product variations.

Every screen cloth can be changed individually without removing any of the others, which is very useful considering that the wear of the screen cloths can lead to individual maintenance intervals. The screen cloths are tensioned by tensioning ledges with only four clamping nuts. Between the tensioning ledges and clamping nuts Belleville spring packages are installed to make the screen mesh more flexible and self-tension itself in case of wear. A protection sleeve keeps the dust away from each screw and ensures easy replacing. In total there are only four screen cloths installed and each can be replaced within just 5 to 10 minutes.

Trials

When designing and optimizing production plants, it is nowadays inevitable to carry out trials despite all new developments in this field. The RHEWUM testing facilities offer the possibility to compare the laboratory results with the respective product data. These results have a significant role in the design process of the respective machine guaranteeing the later success in the production.

The screen will be fed with 100 t/h of granulated fertilizer with a bulk density of 0.92 t/m³. Water content of the feed material is maximum 1.0 %_wt. Due to space limitations at customer‘s site a screening machine type WAF was tested with similar material to optimize the screening area as much as possible.

Several mesh types have been tested in order to find the best solution for the customer. Square meshes showed a slight clogging, but rectangular meshes were tested successfully. The reason for the clogging was the high specific load in the compact machine. The high material layer pushed the round particles into the square opening and clogged the mesh which was not the case with rectangular meshes. The difference concerning the clogging of square and rectangular meshes can be seen in Fig. 6 and Fig. 7.

Outlook

After the convincing performance at the RHEWUM test facilities the screening machine type WAF will prove its high-grade applicability for screening fertilizer in the Agropolychim plant in Devnya. The tests conducted by RHEWUM demonstrate that customers‘ demands on purity and yield will be reached and possibly exceeded.

Due to the high product quality which will be reached with the RHEWUM WAF screens these machines are perfect answers to the requirements in the screening of different products in the modern fertilizer industry. They are able to support the customers‘ success in ensuring them process guarantees regarding product yield and purity as well as machine reliability even in limited areas.

Summary

Together with its customer RHEWUM designed, supplied and commissioned a highly efficient and flexible screen for polishing of various types of fertilizer at high capacities.

The type WAF was chosen due to the good experiences with previously supplied machines to several fertilizer plants. Upscaling of the test results shows that minimum dimensions of the screen chosen have to be 1750 mm width at a length of 3250 mm with two screening decks to ensure the desired process quality and feed rate of 100 t/h. Thus, the decision was made to use a WAF 175 x 325/2.