Selecting optimal mixer options in feed milling
by Detlef Bunzel, Evonik, Germany
Dosing 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.
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.
A paddle mixer may achieve a comparable level of homogeneity in less than 60 seconds, while it takes more than 200 seconds to achieve a good Coefficient of Variation in a double ribbon mixer.
Considering time requirements for mixer filling and discharging, total cycle times of two-to-six minutes may result. Consequently, approximately 10-30 batches may be produced per hour.
Time sequence of batch process
The time sequence of the batch mixing process can roughly be structured in four blocks:
1 - Dosing time
2 - Mixer fill and discharge times
3 - Mixing time
4 - Discharge time of bin after mixer
A close look at the time sequence improves the understanding of the batch process:
• Dosing time is shorter than mixing time; so, the subsequent batch will be ready to be filled into the mixer as soon as the mixer is discharged
• The liquid addition system is designed to allow for sufficient mixing of dry components before spraying of liquids begins and provides for a final mixing time after spraying stops
• The conveying line after the mixer is designed to discharge the batch from the surge hopper after the mixer within the mixing time of the following batch.
Process parameters and batch size
Changing the batch size affects the size of the equipment involved:
• Mixer size increases proportionally with the batch size, just as the output per hour will increase, assuming constant number of batches per hour
• Dosing time increases proportionally with batch size and dosing equipment remaining unchanged
• If mixing time is shortened, in order to increase output (rather than increasing batch size), this will result in shortened dosing time. To dose the same amounts in a shorter time, dosing equipment must be upsized, and dosing accuracy will decrease relative to batch size. Doubling the diameter of a screw feeder would increase output and dosing error approximately by the factor four
• Weighing accuracy is related directly to batch size. E.g., for a scale with 3,000 digits resolution and with a weighing range of three tonnes, the smallest read out will be 1kg. With a weighing range of six tonnes the according read out will be 2kg
• Mixing time is much more influenced by mixer design (besides product related parameters) than by batch size. With the same basic mixer design shortening mixing time may have a negative effect on mix homogeneity.
• Spraying time – just as dosing time – increases proportionally with batch size with the same spraying equipment. Liquid addition has its restrictions as liquids can only be sprayed onto the surface of the mash inside the mixer, while batch size increases with mixer volume. With batch size spraying time will increase relative to mixing time with negative effects on mix quality. Such effects are stronger with a modern mixer design and shorter mixing time
• Fill and discharge time is relatively short and isn't greatly affected by batch cycle time. Generally, with an increasing number of batches per hour time losses by changing batches will increase in relation to total production time.
Another quality related aspect is the 'product carry-over'. Depending on equipment design and maintenance, whenever changing batches, product from the one being discharged will remain and be carried over into the following batch.
While keeping output-per-hour constant, there is a tendency for increased carry-over when batch size decreases and the number of batches per hour increases.
A further consideration is that, with an increasing number of batches per hour, wear and tear will increase on parts that are used when changing batches. These include slide gates, pneumatic pistons, and drives and electric drives.
Batch size and cycle time
The most critical consequence of increasing the number of batches per hour is the shortened mixing time, relative to batch cycle time.
If the fill and discharge time of the mixer is 30 seconds in total, then increasing the number of batch cycles would have a negative impact on net-mixing time. With 10 batches per hour, 300 seconds, or five minutes per hour, would be needed to fill and discharge the mixer. 55 minutes would be net mixing time to produce good mix homogeneity.
With 30 batches per hour, 15 minutes each hour would be needed to change batches, meaning only 45 minutes would remain to produce a good quality mixture.
Increasing batch frequency above 15 batches per hour causes excessive downtime for changing batches at the expense of productive dosing, spraying and mixing time.
Segmentation of mixing time
In order to get good mix homogeneity with a balanced batch cycle segmentation, it is important to consider the order and timing in which ingredients are added. Mixing time starts after all dry ingredients have been added.
When filling the mixer, macro ingredients should be added first. This ensures a good distribution in the mash. In some mixers there are dead zones that don't mix close to the bottom. These would fill up with micros if those were added first. Losses of micros per batch, due to mixer gates not being 100 percent tight when closed, will also be reduced if they are added on top of macro ingredients.
For micro ingredients, like amino acids and vitamins, a good distribution, as well as exact dosing and avoidance of carry-over, is critical for high feed quality, to ensure good growth performance and the health status of animals.
Adding liquids too early within the mixing cycle will affect the homogeneity of all the ingredients. Once liquids are sprayed into the mixer particle size increases, due to adhesion between liquid droplets and dry particles.
When adding fluids, water-soluble liquids should be sprayed before fat-soluble liquids. The fat-soluble liquids will coat the surface of dry mash granules and, thus, prevent the absorption of further liquids. This means liquids will stay on the surface and form lumps and caking on the mixer surface and mixing tools (paddles and ribbons).
Mixer filling rate
Mixer manufacturers generally recommend a filling rate of 70-to-85 percent. Specifically, with double ribbon mixers, it is important to note that the inner ribbon must always be covered with mash. If the filling rate falls below this level, the mixing dynamics of the inner ribbon is negatively affected, and product will accumulate to one side of the mixer by the outer ribbon.
If mixers are overfilled, the distance from the spraying nozzles to the mash is diminished and the liquids will cover a smaller surface. Lumping will occur because the volume of liquids will exceed the specific absorption capacity of the mash.
With a low filling rate, liquids sprayed onto mixer tools and side walls will cause caking that will eventually be carried over into following batches.
Therefore, it is recommended a minimum filling rate of over 60 percent is used, even though manufacturers may claim that such mixers perform well below 50 percent. The mash will only absorb the liquids and keep mixer tools clean, if mixer tools are well covered beneath the spraying array.
Working precision test of the feed production
Compound feed ensures animals are supplied with adequate levels of energy and nutrients. In modern animal production, compound feed is also used to apply prophylactic medical treatments to maintain animal health.
Consumers give food safety a lot of attention and, as part of the food chain, feed production has to live up to certain standards and comply with basic rules and best practices. Many countries have issued specific regulations with additional guidelines for the feed industry, to safeguard this compliance. Traceability, hygiene and working precision are the key words addressed by these guidelines.
Different concepts have evolved to meet customers' needs and to cope with regulatory requirements to not only produce an adequate quality but also verify it on a regular basis. The focus of such concepts is to evaluate mix homogeneity of the batch mixer by measuring the 'coefficient of variation' of a tracer added into the mash. For example, our AMINOBatch® Working Precision Test uses supplemental amino acids as tracers to evaluate the coefficient of variation in the batch mixing process.
Dosing and mixing are core processes in the feed mill with high impact on output and feed quality. Conflicting aspects of equipment and process design must be reconciled to find an acceptable balance between cost effectiveness and good quality production.
With batch cycle times below four minutes, the ratio between time needed for changing batches and net mixing time gets out of balance at the expense of dosing accuracy, mix homogeneity and process stability, causing higher risks for feed quality.
The industry trend of speeding up the batching process requires special attention regarding the segmentation of the dosing and mixing cycle. Namely liquid addition systems may become a bottleneck: shorter batch time results in shorter mixing and spraying time. The limiting factor for liquid spraying is the capability of the mash to absorb liquids added at high dose rates.
With liquid addition, it is very important to check and maintain a good filling rate of the mixer. Otherwise, lumping and caking on mixer walls and tools will result.
In feed production it is important to check working precision of the dosing and mixing process to ensure quality production. If well executed, such tests give opportunities to identify optimisation potential in the process and equipment operation as well as in maintenance.