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2. Article: Dedusting on a large scale: Screening off undersize particles in dried fodder

06.11.2017

Dedusting on a large scale: Efficient sifting of fine dust with dry food pellets

Today in the animal feed industry around 80% of the demand for all mixed feed is for pellets. The pressed pellet form has many advantages for the end user and the mixed feed customer: a better feed intake, easier handling, no separation of feed ingredients, better pourability that is, mostly, dust-free, or an increased bulk weight and with it a lower need for storage and carrying capacities.

Pelletizing is a dedicated procedure of the agglomeration. Here the raw materials that correspond to each specific recipe are crushed and compacted into pellets in a press. One particular aspect with the manufacturing of animal feed pellets is the dust particles - the smallest particles and debris from the raw materials that are formed during the process and intermix with the pellets. A level of dust that is as low as possible is one of the most important signs of a good quality product for the customer. The following practical example shows how an optimal dedusting can be achieved through the use of screening technology and which aspects facility operators must consider when choosing a suitable screening process.

The feed manufacturer in this example is specialized in the manufacturing of high-quality dry food for dogs and cats. Their branded products are well-established throughout the EU in retailers such as discount stores or in specialist shops, where they are mostly marketed as pellets in different forms of packaging. The company produces special extrudates according to different recipes, that are pressed into pellets and, in the case of one product, coated as well.

At the beginning of the continuous manufacturing process, all raw materials are crushed and mixed according to the recipe. The dry mix goes through an optimal particle size distribution, where the particle size of the dry mix plays a large role in the appearance and digestibility. When it comes from the mixer, the feed mixture is pressed into form in the pellet press. Some varieties of pellet are additionally coated, with vitamins for example, before they are dedusted and packaged. When they come from the press, the finished pellets are finally sent to the packaging station in up to 14 lines. In each line the product is first put into interim storage in collection containers such as silos. Between the storage containers and the respective packaging stations a screening stage was to be integrated for the dedusting of pellets.

For the feed production project managers, the screening solution had to fulfil certain requirements:

• In continual operation, two sizes of pellet are produced: 2 mm for cat food, 5 mm for dog food
• The new screening process was supposed to be as gentle on the product as possible, so that the pellets (in particular the coated variety) would not be damaged.
• High sifting degree of dust
• Throughput rate of 10-12 tons per hour per line
• Due to the frequently changing recipes, the screen insert change was to be carried out as quickly as possible
• As efficient as possible to purchase and to operate

The company Baumgarten Silo- und Anlagentechnik GmbH in Porta Westfalica was commissioned for the planning and execution of this stage of processing. As bulk solid professionals with over 100 years of experience, Baumgarten offers a wide spectrum of components for the handling of bulk solids. For this task, in addition to the screening machines, trapezoid floor conveyers, valves and discharge devices from Baumgarten were used, amongst other things.

Because Baumgarten designed part of the system environment, the engineers worked out a proposed procedure for the integration of a screening stage. Because a conveyer section of 2 m had to be bridged between the collection container and the tube chain conveyer for each of the 14 lines the screening machines used had to be adjusted so that they would work as a conveying element at the same time. The screening machines were positioned directly under the outlets of the storage containers and are loaded with pellets through the opening of a gate valve. After the fine particles have been separated off by screening, the dedusted pellets, with the help of a tube chain conveyer, are conveyed as good material to several screening channels. In this case, they assume the function of a dosing device before the products are actually packaged. The material feed and the dosing rate can be adjusted via the oscillation amplitude of the vibration motor at the screening channels so that there is always a sufficient amount of pellets available for the packaging process.

For Baumgarten it was particularly important to choose the optimal screening process and a competent partner for the subcontracting of the screening solution. When it came to making their choice J. Engelsmann AG stood out. The screening technology professionals from Ludwigshafen am Rhein are one of the best known manufacturers of screening technology in the world and, like Baumgarten, can look back on over a hundred years of experience in the bulk solids industry. With their long-stroke, vibration and centrifugal screening solutions, Engelsmann covers almost all screening processes. They also have their own technical center where they can test these processes with a variety of different products, which Baumgarten also used to carry out tests and compare results for different processes. Finally, they chose to go with the JEL EasyVib vibration screening machine.

With its affordable price and simplified operability, the JEL EasyVib vibration screening machine is specifically tailored towards facility operators for whom efficient machine use is the highest priority but without cutting corners with reductions in quality, reliability or screening results. The screening machine is made from stainless steel and easy to clean and maintain thanks to its modular construction, the quick release clamps on the housing cover and the especially smooth surfaces. The high-quality folded sieve mesh can be removed from the machine from above without any problems, simply by opening the retaining device. What's more, the JEL EasyVib has an inclination adjustment device for the screening deck, as well as adjustment options for both vibration motors and the oscillation amplitude. This allows the screening machine to be quickly adjusted to different product properties.

Although the vibration screening machines are not necessarily the most gentle screening processes for products, the project managers at the Engelsmann technical center found a screening machine configuration that, through the combination of an especially low oscillation amplitude and a specific thickness of the layer of product on the screen mesh, allows the feed pellets to gently glide over the screen mesh whilst the fine particles are separated off. The screen surface of the JEL EasyVib in this case of application was 1.125 m2 and the tests in the technical center yielded a throughput rate of 10 tons per machine. Furthermore, the desired separation rate of the fine particles could be attained. The screen trough is completely sealed, which ensures that no dust particles escape during the screening process. The maintenance requirements are reduced thanks to the low number of wear parts and the low cost of replacement parts.

However, the JEL EasyVib in the largest standard model was slightly too short for the 2-m long conveyer section that had to be covered. Therefore, the engineers at Engelsmann constructed a longer version that would fit exactly between the collection container and the tube chain conveyer. The in- and outlets of the JEL EasyVib were fitted with wear-resistant flexible V sleeves to connect the screening machine with the gate valves of the collection containers and to the tube chain conveyer on the outlet side so that it is dust-tight.

The specific challenge in this example was to facilitate a high throughput rate and an optimal separation rate at low purchasing and operating costs. The sound process and procedural knowledge of Baumgarten, and of course the end customer, paired with the screening technology know-how of Engelsmann led to success and a suitable solution to the set task was found. Optimal problem solutions not only deliver the desired screening results, but they are also smoothly integrated into the facility environment of the operator and work reliably and with minimum system standstills, especially in continuous production processes. Ideally, alternative processes can be tested or compared on a small scale during the solution finding. The knowledge and process parameters gained through that can then be safely scaled to the entire facility and this way the operator receives more process safety.