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Grading of Abrasive Fertilizers

All life needs nitrogen. Even plants. Compounds, containing nitrogen, phosphorous and potassium atoms, are particularly important for their growth. However, lots of soils are low in nitrogen, meaning they must be fertilized. Due to its high nitrogen content, urea is the most important nitrogen fertilizer in the world. Urea-based fertilizers tend to be very free-flowing – but also very abrasive, which means the highest demands are placed on the machine technology used for their production, as a current project by screening machine manufacturer J. Engelsmann AG shows.

The use of screening machines in fertilizer production is subject to a multitude of requirements: As well as the highest possible degree of selectivity and vast throughput rates, a high level of material resistance against the abrasive fertilizer products is a particular challenge, which must be overcome. A practical example from the industry shows how economic machine use can be achieved at the highest level of productivity using a screening technology solution which has been tailored individually to the product-specific issues.

For example, a global fertilizer manufacturer based in Germany was faced with the problem that the grading machinery used in the production of nitrogen fertilizer did not achieve the necessary throughput rates. In addition, the ongoing operating costs were increasing constantly, which was connected with the highly abrasive urea product. Due to its 46% nitrogen content, urea is the most important nitrogen fertilizer in the world. Pure urea is a white, crystalline, non-toxic and hygienically safe solid that smells vaguely of ammonia. It also has a high water-binding capability, which is why it is also used as a moisturizing factor in cosmetics, among other things. The granulated urea product being processed is very free-flowing but also highly abrasive, which leads to abrasive material wear. For this reason, it is particularly important to take protection against wear into account when designing and producing screening machines for fertilizers.

A demand that is not unfamiliar to screening machine manufacturer Engelsmann, as the screening technology experts from Ludwigshafen have already produced and supplied a number of grading machines for the fertilizer industry in the past. The Freischwinger type long-stroke screening machine, which was developed specifically for this, was used in this case.

As well as low material abrasion and gentle grading of the 2 to 5 mm granules, special focus was given to its productivity and cost-effectiveness when designing the machine solution.

Horizontal screening motion guarantees a high degree of selectivity

Due to the high product concentration of the nitrogen fertilizer, selectivity of over 98% was required together with a throughput rate of 52 t/h as part of the planned production increase. Other important factors for the client included simpler machine handling, a more energy-efficient drive and fewer cleaning and maintenance cycles in order for the machine to be used more efficiently and productively than before. To best fulfil these requirements as a whole, two machines from the Freischwinger FW 1.200/4 series were selected as the basic model, which were then designed for the specific scales. This meant the 7.80 metre and 2.85 metre wide long-stroke screening machines have a single-deck design with twelve successively arranged screening inserts and a total screening area of 12 square metres each.

The special drive system, which moves the entire screening trough in a horizontal motion, achieves the best possible layering of the fertilizer. With minimal, vertical force components, the lifting movement prevents a “carpet formation”, which causes grains to jump and get mixed together. The fine-grain proportion moves directly above the screening cloth, which leads to a high selectivity of 99% and means that it is not necessary to resieve. Another advantage of this drive system is the lower energy requirement. This is achieved through the exact mass compensation between the drive unit and the screening trough. For low-energy operation, the 7.5 tonne Freischwinger screening machines are operated using a 5.5 KW electric motor, which passes its energy to a centrifugal mass equipped with driving plates using a V-belt. This is tailored exactly to the dimensions of the screening trough. The motor drives the mass with full nominal flow rate for approx. 15 seconds and then is kept in momentum with just 10 to 20 % of the nominal current. In this way, the electrical energy is converted into rotational energy, saved in the flywheels and gradually released.

Gentle wear protection against abrasive wear

The inside of the Freischwinger machines is designed in such a way that the granulated urea is only stored in the screening deck for a short period of time in order to minimise the abrasion of the product on critical parts. At the critical belt reversal points of the screening machine, wear protecting sheets made of specially hardened materials are used. This prevents excessively quick wear-out of sensitive areas, considerably increasing the lifetime of the machine.?

In the same way, the screening inserts are covered with specially hard-drawn fabric made of high-strength stainless steel wire. Due to the abrasive product, the product distribution plates have been screwed in so that these can easily be replaced. On one hand, the product distributors guarantee gentle material feeding and product supply, while optimally distributing the screening product over the entire surface of the screening fabric.

To ensure gentle screening, the long-stroke screening machine including all the parts that come into contact with the product are produced from etched and passivated stainless steel (V4A 1.4571) so that transitions are avoided and a high proportion of smooth surfaces enable easy cleaning. So that the screening fabric is not clogged with near-mesh grain during the screening process, the installed dust removal system was fitted with silicone balls to avoid any contamination of the granulate caused by material abrasion. The horizontal movements of the screening trough toss the impacted balls to the mesh, ensuring the screening fabric is clear. The two long-stroke screening machines can each separate four fractions (sortings) in one screening process and each have four outlets to discharge the screened material. The filtered undersized particles are fed back into the process for recycling purposes. The fine grain is sent to the granulator. The right-sized grain is packaged as ready-to-sell product and the oversized material is also fed back into the granulator through a crusher.

Lower operation costs with higher machine availability

For screening machines with high throughput rates and larger screening surfaces, the quick and simple replacement of the screen is essential. For this reason, the screening surface is completely accessible after removing a covering sheet with lifting eyes. Then, you simply need to open the eccentric tensioners and the screening inserts can be replaced. This reduces downtime and increases machine availability. The screening machines can be installed directly onto the steel construction or concrete floor, as machine suspension and similar are unnecessary due to the drive principles of the mass compensation. Only mild vibrations are transferred from the machine. The operating noise of just 78 dB(A) contributes to a low-noise production operation, taking into account the machine size and in comparison with conventional products.

To achieve the highest possible machine availability in a three-shift operation over 7 days of the week, the oscillating drive is only subject to slight wear-and-tear in comparison with the previously used screening machines with oil-lubricated motor drives, which in turn results in low material use and maintenance costs. Furthermore, replacement parts, such as replacement V-belts, are found in direct proximity to the drive belt, so that the belt can be changed as quickly as possible in an emergency.

The replacement of existing screening machines with two Engelsmann Freischwinger models has been worth it for the fertilizer producer in many ways. As well as the gentle, selective grading, the throughput rate per square metre of screening surface has also been considerably increased, meaning the same production capacity could be achieved as before with half the screening surface size. In addition, the operating costs could be considerably reduced due to the lower energy consumption of the oscillating drive of 5.5 KW in comparison with 11 KW of rival products and as a result of the reduced replacement part and maintenance costs due to the designed wear protection. The easier machine handling with the option of quickly and easily replacement the screening insert by removing the covering sheet, also minimised downtimes for the machinery, as the heavy machine lid previously needed lifting by crane when the screen needed changing.