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Selecting optimal
mixer options IN FEED MILLING
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by Detlef Bunzel, Evonik, Germany
osing and mixing are two of the major procedures in feed milling. Getting them right can have a significant impact on feed quality and the cost effectiveness of the process. The design of this process depends on the number and properties of ingredients, dose rates and required output. Feed production ingredients are usually mixed in a batch, due to the amount of ingredients and frequent changes in feed formulation. Even though the number of ingredients may vary, different groups can be classified: • Main ingredients (soy, corn and wheat etc., typically > five percent of the formulation) • Minerals and major additives (limestone, salt, phosphorus etc., one-to-five percent of the formulation) • Micro ingredients (amino acids, vitamins, < one percent of the formulation) • Medication (<< 0.1 percent) Most of these ingredients (typically > 95 percent) are added as dry bulk. Dry bulk ingredients are weighed on scales per batch and the accuracy of scales depends on their weighing ranges. Therefore, individual scales with corresponding and appropriate weighing ranges are needed to weigh main, minor and micro ingredients (Regulation (EC) No 183/2005 of the European Parliament, Annex II). Dry bulk ingredients are loaded into the mixer after dosing and weighing at the beginning of the mixing cycle. Certain ingredients are added as liquid (oil, fat, molasses, water, acids and other additives, usually < five percent. These are dosed via flow meters or on scales and sprayed onto the dry mash during the mixing cycle.
74 | April 2019 - Milling and Grain
In order to ensure continuous operation of the pellet mills, the capacity of the batch mixing line will be defined based on the design capacity of the pelletising line.
Mixing line capacity
Production capacity of the feed mill is defined based on continuous processes in tonnes per hour. For the mixing line equipment, this must be transferred into batches per hour as mixing is a batch process. Each given output can be met by varying batch size and batch frequency. Batch size is defined in weight units as bulk batches are assembled on scales. Nevertheless, equipment for transport, storage, dosing and mixing of these bulk batches is primarily defined by batch volume: • Dosing equipment must be designed to dose the volume of bulk ingredients in the assigned dosing time • Scale hoppers must be designed to accommodate batch volume according to their weighing capacities • Hoppers before and after the mixer and the mixer itself must be designed to accommodate the total batch volume • Mixers must be designed to allow for an appropriate filling rate at full batch volume, in order to get optimal mix homogeneity within the assigned mixing time • Conveying equipment after the mixer must be designed to convey the total batch volume within the cycle time While mixer sizes in feed mills range from less than one tonne (2,000 litres) to 10 tonnes (20,000 litres), mixing time may vary from around 60 seconds to four-to-five minutes (or even longer) depending on mixer design and on quality expectations.