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Factors affecting feed pelleting quality and efficiency
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by Zheng Chang, China
he quality of pelleted feed is mainly reflected in the vulnerability of the pellet, the hardness of pellets, the appearance and quality of pellets (whether the surface is cracked, the gloss, the colour, etc), and the length of the pellet. The factors affecting the quality of pellet are various. The influence of raw material quality on pelleting quality and efficiency includes the following aspects: • The influence of particle size of raw materials on pelleting quality and efficiency • Raw materials can be divided into coarse, medium and fine particles. Generally, coarse particles have diameters of 3mm or more. Medium and fine particles have good pelleting properties, involve lower than average energy consumption and less abrasion towards die and rollers. Steam easily penetrates to the core of small particles and causes physical and chemical changes, which improve the quality of pelleting. As shown in Figure 1, the cores of the medium and fine particles (the two small cubes on the left of Figure 1) are easily penetrated by steam. The steam, however, cannot penetrate through the core of the coarse particle (the larger cube on the right of Figure 1), leaving a dry core. In addition, coarse-particle raw materials are more likely to crack after pelleting (as shown in Figure 2). Medium and fine materials have a high density of pressed particles (as shown in Figure 3), and are better able to pass through the die holes (as shown in Figure 4), with less abrasion to die holes and higher output. However, if the materials are crushed too fine then the production cost will be increased. The ideal particle size is a reasonable combination of coarse, medium and fine materials. Taking the production of Ф6mm livestock and poultry pellet feed as an example, the reasonable particle size distribution is shown in Figure 5. For Ф2mm prawn feed, 85 percent of the raw materials should pass through 40 mesh sieve. Now the minimum
50 | April 2020 - Milling and Grain
diameter of ring die hole is 0.8mm, so the maximum diameter of raw materials for pelleting should not be above one-third of the diameter of die hole, that is to say, the raw materials must pass through the 60 mesh sieve (0.267mm) after crushing to meet the particle size requirement
The influence of feed density on pelleting efficiency
The pelleting efficiency is related to the density of raw materials. Light materials have a density of less than 0.33t/m³; heavy materials have a density rating of more than 0.4t/m³. When pelleting, lighter materials have low output, but heavy materials showcase higher output. For example, when using alfalfa with density of 0.22t/m³, the output is 4t/h. When using cottonseed powder with density of 0.53t/m³, the output is 16t/h. Figure 1
Figure 2