Factors affecting feed pelleting quality and efficiency
by Zheng Chang, China
The 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 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.
The influence of protein content on pelleting efficiency
High protein raw materials (generally with high density), such as soybean powder and cottonseed powder, are extruded by die and roller in the process of pelleting, resulting in friction heat, which is conducive to pelleting. However, for high protein feed (such as calf feed and concentrated feed), a large amount of powder or urea should be added during pelleting, to obtain optimised results.
The influence of fat content on pelleting efficiency
Fat is a great lubricant for pelleting. It can increase output, reduce abrasion and prolong the service life of both roller and die. The feed itself contains fat, and we can also add fat before pelleting. Too much fat content, such as sic percent fat content, will affect pelleting, since the particles may become soft and difficult to be shaped. If the particles with high fat content (3% or more) are required, one-to-two percent fat can be added during mixing, and the rest can be added by spraying after pelleting.
The influence of fibre content on pelleting efficiency
Fiber has adverse effects on pelleting. It may reduce the output and accelerate the abrasion of die hole. There are two types of fibres, one is stringy fibre such as alfalfa, sweet bud stem, sweet potato stem, etc, the other is shelled fibre such as oat, soybean, cottonseed, peanut shell, etc. The stringy fibre can absorb steam and soften during conditioning, and play a role of binder in the particles, improving the particle strength; while the fibre with shell cannot absorb steam, and play a role of dispersion in the particles, affecting the particle quality.