Segregation an issue during bulk transportSegregation an issue during bulk transport
August 4, 2016
*Thomas Winowiski is technical applications manager-animal feed at LignoTech USA in Rothschild, Wis.
FINES will always be present in pelleted feed, but their degree of acceptance varies by market — e.g., dairy pellets intended for use in robotic feeders should be virtually free of fines, while the integrated poultry industry is more tolerant of fines.
Still, almost everyone would be upset to find that their feed sample contained more than 50% fines. This not only creates physical problems, but there is potential for nutrient segregation and a loss of animal performance.
Kansas State University researchers conducted an extensive study to understand where fines are generated in the feed mill. They found that fines increased from 9% off the pellet mill to 14% after the fat coater and then to 20% going into the bulk truck (De Jong et. al., 2015). The discussion that follows examines how feed containing 20% fines might look as it moves through the transport system and shows how fines can quickly increase from 20% to more than 50%.
Pellets typically come off the pellet press, drop into the counter-flow cooler and are discharged into a bulk storage bin. Figure 1 shows a bin that has been filled by a continuous stream of pellets containing 20% fines (left). Note how the fines stay where they initially land, forming a column up the center of the bin. When feed is removed, a channel forms directly above the outlet. By the time 40% of the feed has been taken out, the center is removed completely. The dark shaft that then forms down the middle is due to the pellets that are rolling down the slope into the vortex. Thus, once the center has been removed, the bin unloads from the top.
Silo No. 1 represents the bulk storage bin in the plant. From there, the feed could be sacked off, and the first bags are likely to have more fines. More often, the feed is loaded into a bulk truck and then transferred again to a farm silo. It would seem that all of that transferring should probably mix the fines and pellets together.
The primary silo discharges into a secondary container, and as it does so, the first feed it releases contains most of the fines. This secondary silo can be seen in Figure 2. Note the high concentration of fines in the bottom of Silo No. 2. When this silo discharges, it once again forms a shaft up the center and pulls the core of fines out first. This is followed by relatively clean pellets that roll down the slope and continue to feed into the vortex.
Samples were collected as feed was removed from both of these silos. Each sample represented about 5% of the total amount. The samples were screened, and the percentage of pellets was determined (Figure 3). Note that, in Silo No. 1, the percentage of pellets is fairly constant at about 65% during the first 40% of removal; this represents the central column that has a high concentration of fines. The proportion of pellets in the last 50% of material removed is, again, relatively stable, with an average of about 93% pellets.
Fines have been sorted and concentrated in the bottom of Silo No. 2. Thus, when this silo is emptied, the first few samples have less than 50% pellets, while the later samples have more than 95% pellets. This segregation can have nutritional implications.
Studies by West Virginia University discovered that when phytase is applied post-pelleting, it will be concentrated in the fines. They found that pellets contained 160 phytase units (FTU) per kilogram, while the fines contained 860 FTU/kg (Moritz, 2013). Post-pelleting application of fat or other enzymes will similarly be higher in the fines due to their greater surface area. Segregation during bulk transport can further concentrate these nutrients and cause an unbalanced formulation.
These results are from a small silo for the purpose of illustration. However, real-world experience suggests that the story these pictures tell is correct. Some of the lessons learned in this study include:
* Segregation will occur unless care is taken to distribute fines and pellets evenly in the bulk silo.
* Reducing fines by improving pellet quality will help by reducing the total amount of fines.
* Silos empty from the top and, therefore, should be completely empty before a new feed is added.
De Jong, J., et. al. 2015. Kansas State University, Midwest Amer. Soc. of Anim. Sci., March 17, 2015. Abstract 140.
Moritz, J. 2013. West Virginia University, Arkansas Nutrition Conf., Sept. 3-5.
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